WO2025111669A1

WO2025111669A1 - Microfluidic chip

Info

Publication number: WO2025111669A1
Application number: PCT/AU2024/051293
Authority: WO
Inventors: Javad TAVAKOLI; Joanne Lisbeth TIPPER
Original assignee: Royal Melbourne Institute of Technology Ltd; University of Technology Sydney; Melbourne Institute of Technology
Current assignee: University of Technology Sydney; RMIT University; Melbourne Institute of Technology
Priority date: 2023-12-01
Filing date: 2024-12-02
Publication date: 2025-06-05
Anticipated expiration: 2026-06-01

Abstract

The present invention relates to an intervertebral disc (IVD)-on-a-chip device for use as an in-vitro model in applications such as tissue engineering, biomechanics and mechanobiology and the method of preparing and using said IVD-on-a-chip device. The IVD-on-a-chip device can mimic the IVD structural complexity and structural organisation of IVD components of the vertebral column for use an in-vitro organ model. Particularly, the methods use a facile strategy for forming the IVD-on-a-chip device by using templates to recapitulate the ultrastructural organisation (such as size, orientation and distribution) of IVD components (such as collagen and elastin fibres).

Description

"Microfluidic chip" Technical Field [0001] Embodiments relate to a microfluidic device configured to be loaded with biological cells to form part of an organ model, such as an intervertebral disc model, for example, as well as associated methods of manufacture and use and fabrication. Background [0002] In-vitro organ models are often used, when available, to research the effects of various stimuli on modelled biological cells and tissues. In some cases, microfluidic devices have been used for culturing biological cells and tissues for this purpose. [0003] The intervertebral disc (IVD) is a complex structure positioned between two adjacent vertebrae in a vertebral (spinal) column. The IVD can protect the spinal cord and segment spinal nerves while also providing flexibility, multi-axial spinal motion and load transmission to the spine. The IVD can be injured or suffer from disease leading to degeneration which is often associated with back pain. Lower back pain and IVD disorders are the third most common long-term health condition reported by individuals aged 25-69 years, impacting up to 70% of the adult population, with 6.9 million people affected in Australia in 2014-2015 alone. Claims in Australia annually for lower back pain direct expenditures reached $AU1.2 billion, with indirect costs being estimated to be ten times more. Accordingly, an understanding of the biology of the IVD is desirable for diagnosing disc-related injury/disease and to develop therapeutic strategies (such as IVD tissue engineering). [0004] It is desired to address or ameliorate one or more shortcomings or disadvantages associated with existing organ models, microfluidic chips, and/or associated methods of fabrication and use, particularly in relation to intervertebral disc models, or to at least provide a useful alternative. [0005] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the appended claims. [0006] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Summary [0007] The IVD is a multi-component structure. At the macro scale, the IVD consists of a central, gelatinous nucleus pulposus (NP) and surrounding annulus fibrosus (AF). At the microscale, the AF is a lamellar structure (having a lamella width of about 150 μm) with adjacent lamellae (having both cross-section (CS) and in-plane (IP) lamellae) connected via the interlamellar matrix (ILM) and partition boundaries (PB), both with a width of about 30 μm. The partition boundaries cross through each lamella, while the interlamellar matrix is parallel to the lamellae. The interface between the AF and NP is known as the transition zone (TZ) having a width of about 50-200 μm. At the microscale, the organization of collagen and elastic fibres along with tissue stiffness gradient controls the biological and biomechanical properties of the IVD. [0008] The AF comprises highly packed collagen fibres, which are aligned in opposite directions between adjacent lamellae (about ±30°, relative to the transverse plane of the disc). The AF collagen fibers can be observed in both parallel and in cross-sectional directions. Within the nucleus pulposus collagen fibers are mainly orientated toward the top and bottom vertebra (endplate) whilst they create a network, with their overall orientation towards the AF, at the transition zone. The present inventors have previously identified that the average diameter of the NP collagen fibres/bundles is bigger at the centre with a decreasing trend toward the AF. They found that the NP’s elastic fibres are likely to create a natural scaffold that determines the orientation and size of the collagen fibres/bundles within the NP { Structure-function characterization of the transition zone in the intervertebral disc, Acta Biomaterialia, Volume 160, April 2023, Pages 164-175}. [0009] The present inventors have previously identified that the IVD comprises a modular assembly consisting of collagen bundle units that are surrounded by continuous, well- organised, and integrated network of elastic fibres. The inventors previously identified that the elastic fibres are the main component of the interlamellar matrix and partition boundaries. Further, the inventors observed a loose elastic fibre network in the AF lamellae and the NP compared to the interlamellar matrix, partition boundaries and transition zone regions. The elastic fibres have a fibre thickness of about 2 μm and are typically parallel to the AF lamellae, while being orientated radially in the NP (i.e., oriented towards the AF) and form a criss-cross network (angle of orientation of about 0° and ±45°) in the interlamellar matrix, partition boundaries and transition zone regions. In addition, the present inventors have performed numerous micromechanical tests to fully determine the biomechanical and material properties of the AF, NP, and TZ regions in the IVD. This has paved the way to suggest the use of different gel (hydrogel) compositions in the IVD-on-a-chip channels to mimic the natural IVD material stiffness gradient characteristics { Stress relaxation behavior of the transition zone in the intervertebral disc, Acta Biomaterialia, Volume 189, November 2024, Pages 366-376}. [0010] Currently, Intervertebral disc (IVD) tissue engineering research typically involves extensive, expensive, and time-consuming in-vitro experiments to demonstrate safety, efficacy, and commercial viability validation. In-vitro IVD studies typically begin with 2D or 3D cell cultures that are oversimplified and are not representative of the IVD structural organisation and material stiffness gradient. Bioreactors use IVDs derived from animals or humans and are generally better models compared to 2D/3D cell culture models. However, using human IVDs impose a strong bias depending on the donor (gender, age, medical history, etc.). Similarly, animal IVDs have serious limitations regarding their recapitulation of the human IVD in terms of size, mechanics, and biology and the need for specific animal facilities. Further, an issue with the use of naturally derived IVDs are the research ethics guidelines which highlight the need to reduce, refine, and replace the use of animals in research. [0011] Currently, there are limited suitable in-vitro IVD models which allow precise tuning of material and mechanical properties in a controlled environment to perform IVD research with targeting of precise research questions and objectives. The lack of a suitable in-vitro IVD model to date is a limitation affecting the progress of IVD studies in a range of fields including tissue engineering, biomechanics, mechanobiology. [0012] Accordingly, it is desirable to develop a suitable in-vitro IVD model which can recapitulate the relevant IVD structural complexity at the microscale and substantially represent the ultrastructural organisation (such as size, orientation, and distribution) and material stiffness gradient of IVD components in different regions of IVD which can replace naturally derived IVDs. [0013] The present disclosure relates to an intervertebral disc (IVD)-on-a-chip device for use as an in-vitro model in applications such as tissue engineering, biomechanics and _{mechanobiology and the method of preparing and using said IVD-on-a-chip device. I n} _{s o m e e m b o d i m e n t s , the IVD- on-a-chip device may substantially recapitulate or mimic the} biological complexity of the native IVD for use an in-vitro organ model. Particularly, in some embodiments, the methods use a facile strategy for forming the IVD-on-a-chip device by using micro-features that recapitulate the ultrastructural organisation (such as size, orientation and distribution) of IVD components (such as collagen and elastin fibres) and material stiffness gradient (through being modular incorporating separate but still connected channels for each IVD region). [0014] Continual research and development for identifying suitable in-vitro IVD models for tissue engineering, biomechanics and mechanobiology has been driven by the desire to recapitulate the structural features and functions of the IVD. However, recapitulating the IVD structural complexity at the microscale and ultrastructural organisation of IVD components remains a challenge. To date, most models have been investigated using 2D/3D cell culture or use of animal and human IVDs. [0015] Some embodiments relate to a microfluidic chip comprising a body defining: a channel extending between an inlet and an outlet; and a microstructure formed by a plurality of protrusions extending away from a surface of the channel, wherein the microstructure is configured to substantially recapitulate one or more structural characteristics of a target natural biological tissue. [0016] In some embodiments, the channel is a first channel, the inlet is a first inlet, the outlet is a first outlet, the microstructure is a first microstructure, the plurality of protrusions is a first plurality of protrusions, the body further defines: a second channel extending between a second inlet and a second outlet; and a second microstructure formed by a second plurality of protrusions extending away from a surface of the second channel; and the first and second microstructures are configured to substantially recapitulate one or more structural characteristics of different target natural biological tissues. [0017] The body may define any suitable number of channels for different applications, and corresponding microstructures in each channel. The microstructures may be configured to substantially recapitulate or mimic one or more structural characteristics of various target natural biological tissues to assist in preparing organ models to substantially recapitulate the biological tissues. [0018] In some embodiments, the microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in part of the target natural biological tissue. The protrusions of the microstructure may include relatively wider collagen fibre bundle protrusions recapitulating collagen fibre bundles, and relatively narrower elastic fibre protrusions recapitulating elastic fibres. [0019] In some embodiments, the average width of the protrusions recapitulating collagen fibre bundles is at least 50%, 60%, 70%, 80%, 90%, or 100% wider than the average width of the protrusions recapitulating the elastic fibres, for example. [0020] In some embodiments, the microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in part of the annulus fibrosus of a natural intervertebral disc in a plane extending through a radial axis of the disc, wherein the microstructure includes a lamella zone corresponding to a lamella of the annulus fibrosus, and wherein the lamella zone includes an array of collagen fibre bundle protrusions with elastic fibre protrusions in the form of ridges extending between different regions of the array of collagen fibre bundle protrusions to form partition boundary zones. [0021] In some embodiments, at least some of the partition boundary zones comprise elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons separating different regions of the array of collagen fibre bundle protrusions. [0022] In some embodiments, the lamella zone is configured to recapitulate part of the lamella in a plane extending through the radial axis of the disc and intersecting the collagen fibre bundles, and the collagen fibre bundle protrusions comprise elliptical prisms with the eccentricity of the elliptical prisms selected based on the angle of intersection with the plane. [0023] In some embodiments, the lamella zone is configured to recapitulate part of the lamella in a plane extending through the radial axis of the disc and inclined relative to the transverse plane of the disc to extend parallel to collagen fibre bundles, and the collagen fibre bundle protrusions comprise parallel ridges. [0024] In some embodiments, the lamella zone is a first lamella zone and the microstructure further includes a second lamella zone and an interlamellar matrix zone between the first and second lamella zones, corresponding to first and second lamellae, and interlamellar matrix of the disc, respectively, wherein the protrusions of the second lamella zone include an array of collagen fibre bundle protrusions with elastic fibre protrusions in the form of ridges extending between different regions of the array of collagen fibre bundle protrusions to form partition boundary zone, and wherein the protrusions of the interlamellar matrix zone include elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons extending across the interlamellar matrix zone between the first and second lamella zones. [0025] In some embodiments, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95% (or optionally 100%) of the ridges of the interlamellar matrix zone are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° (or optionally within 20°, 15° or 5°) of parallel relative to the reference axis; within 10° (or optionally within 20°, 15° or 5°) of 45° relative to the reference axis (i.e., 45° in either direction relative to the reference axis, or ±45°); and within 10° (or optionally within 20°, 15° or 5°) of perpendicular relative to the reference axis. [0026] In some embodiments, the plane represented by the microstructure of the microfluidic chip extends through the radial axis of the disc and is inclined relative to the transverse plane of the disc to extend parallel to collagen fibre bundles of the first lamella of the annulus fibrosus (e.g., approximately 30° inclined relative to transverse plane). The collagen fibre bundle protrusions of the first lamella zone may comprise elongate ridges extending parallel to the interlamellar matrix zone, the ridges substantially recapitulating the in-plane oriented collagen fibre bundles of the first lamella of the disc. The collagen fibre bundle protrusions of the second lamella zone may comprise an array of pillars substantially recapitulating the out-of-plane oriented collagen fibre bundles of the second lamella of the disc. [0027] In some embodiments, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95% (or optionally 100%) of the elastic fibre ridges in the partition boundary zones are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° (or optionally within 20°, 15° or 5°) of parallel relative to the reference axis; within 10° (or optionally within 20°, 15° or 5°) of 45° relative to the reference axis (i.e., 45° in either direction relative to the reference axis, or ±45°); and within 10° (or optionally within 20°, 15° or 5°) of perpendicular relative to the reference axis. [0028] In some embodiments, the channel is a first channel, the inlet is a first inlet, the outlet is a first outlet, the microstructure is a first microstructure, the plurality of protrusions is a first plurality of protrusions, wherein the body further defines: a second channel extending between a second inlet and a second outlet; and a second microstructure formed by a second plurality of protrusions extending away from a surface of the second channel; and wherein the second microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in the nucleus pulposus of a natural intervertebral disc in a plane extending through a radial axis of the disc and inclined relative to the central axis of the disc. [0029] In some embodiments, the second microstructure comprises similar features to the first microstructure defined in any one of the described embodiments with the protrusions arranged in closer proximity to each other forming a relatively higher density of protrusions compared with the first microstructure. [0030] In some embodiments, the second microstructure includes a core zone on one side of the channel and a peripheral zone extending partially around the core zone on another side of the channel, wherein the protrusions of the core zone comprise an array of pillars corresponding to out-of-plane oriented collagen fibre bundles of the nucleus pulposus, and wherein the protrusions of the peripheral zone comprise an array of pillars corresponding to out-of-plane oriented elastic fibres of the nucleus pulposus. [0031] In some embodiments, the second microstructure further includes elastic fibre ridges radiating away from the core zone at different angles relative to each other, corresponding to in-plane elastic fibres. The elastic fibre ridges may radiate away from the core zone at different angles with angles between adjacent elastic fibre ridges being in the range of 15° to 30°, 20° to 25°, about 20°, or about 25°, for example. [0032] In some embodiments, the body further defines a plurality of microvalves arranged between and configured to selectively allow fluid communication between the first and second channels. The microvalves may comprise active microvalves or passive microvalves, such as capillary valves, for example, which resist flow due to surface tension until a threshold pressure gradient is exceeded and flow through the valves is allowed. [0033] In some embodiments, the body further defines: a third channel extending between a third inlet and a third outlet, and located between the first and second channels; and a third microstructure formed by a third plurality of protrusions extending away from a surface of the third channel; wherein the third microstructure is configured to substantially recapitulate one or more structural characteristics of the natural intervertebral disc. [0034] In some embodiments, the body further defines a plurality of microvalves arranged between and configured to selectively allow fluid communication between the first and third channels, and between the second and third channels. [0035] In some embodiments, the third microstructure is configured as a transition zone to substantially recapitulate the size, distribution and orientation of elastic fibres in the transition zone of the natural intervertebral disc between the nucleus pulposus and the annulus fibrosus. [0036] In some embodiments, the protrusions of the third microstructure include elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons extending across the transition zone. [0037] In some embodiments, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95% (or optionally 100%) of the ridges of the transition zone are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° (or optionally within 20°, 15° or 5°) of parallel relative to the reference axis; within 10° (or optionally within 20°, 15° or 5°) of 45° relative to the reference axis (i.e., 45° in either direction relative to the reference axis, or ±45°); and within 10° (or optionally within 20°, 15° or 5°) of perpendicular relative to the reference axis. [0038] In some embodiments, the third microstructure may include collagen fibre bundle protrusions in the form of pillars. The pillars may be elliptical prisms, for example. The collagen fibre bundle protrusions may be interspersed between the elastic fibre ridges within the network of ridges. The eccentricity of the collagen fibre bundle protrusions in the transition zone may higher than the eccentricity of the collagen fibre bundle protrusions in the core zone by at least 20%, at least 30%, at least 40%, between 25% and 35%, or about 30%, for example. The collagen fibre bundle protrusions of the transition zone may be randomly oriented, or oriented at 4, 5, 6, 7, 8, 9, 10, or more angles relative to each other, [0039] Throughout the microstructures and various zones of the microstructures, the collagen fibre bundle protrusions and elastic fibre ridges may comprise any suitable dimensions, and may have dimensions in the following ranges. Dimensions may vary or be similar within or between different zones and microstructures. The depth or height of the protrusions relative to the surface they extend from may be in the range of 0.5 µm to 5 µm, 1 µm to 3 µm, or about 2 µm, for example. [0040] The collagen fibre bundle protrusions may have an average width or diameter in the range of 5 µm to 35 µm, 5 µm to 20 µm, or 5 µm to 10 µm, for example. The collagen fibre bundle ridges may have a length in the range of 10 µm to 200 µm, for example. The collagen fibre bundle pillars may have a minimum diameter of about 5 µm and a maximum diameter of about 10 µm, for example, the elliptical pillars may have a minor axis diameter of about 5 µm and a major axis diameter of about 10 µm. Depending on the angle of cross-section of the reference plane in the natural intervertebral disc being modelled, the elliptical prism pillars may have different eccentricities. For example, the major axis diameter may be larger than the minor axis diameter by a factor of 1.2, 1.5, 1.8, 2, 2.5, 3, 3.5, 4, 4.5, 5 or any other suitable factor. [0041] The collagen fibre bundle pillars may vary in size in different zones of the microstructure(s). For example, the average diameter of the pillars in the core zone may be larger than the average diameter of the pillars in the peripheral zone by 50%, 80%, 100%, 120%, 150%, 200%, 300% or more. The elliptical pillars in the core zone and peripheral zone may be oriented at different angles, which may be randomly oriented, or randomly oriented at 25° increments relative to each other. The collagen fibre bundle pillars in the transition zone may be more elongated (eccentric) than the pillars in the core zone by at least 20%, at least 30%, at least 40%, between 25% and 35%, or about 30%, for example. [0042] The elastic fibre ridges may have a constant width or varying widths in the range of 0.2 µm to 20 µm, 0.5 µm to 10 µm, 1 µm to 10 µm, 1 µm to 5 µm, 2 µm to 3 µm, about 1 µm, about 1.5 µm, about 2 µm, or about 2.5 µm. The length of the elastic fibre ridges may be in the range of 5 µm to 500 µm, 100 µm to 500 µm, 100 µm to 300 µm, 50 µm to 150 µm, 5 µm to 50 µm, 5 µm to 35 µm, 5 µm to 10 µm, 10 µm to 20 µm.20 µm to 40 µm. or any other suitable dimensions for a given application. [0043] The number density of protrusions may vary in different zones. [0044] For example, the number of collagen fibre bundle pillars in a 40 µm x 40 µm area may be in the range of 2 to 20, 3 to 15, 9 to 13, about 3, about 9, or about 13, or any other suitable density. For example, in a lamella zone corresponding to an outer lamella of an annulus fibrosus, the density may be about 13 pillars per 40 µm x 40 µm area; in a lamella zone corresponding to an inner lamella of an annulus fibrosus, the density may be about 9 pillars per 40 µm x 40 µm area; in a transition zone corresponding to a transition zone between nucleus pulpopus and an annulus fibrosus, the density may be about 9 pillars per 40 µm x 40 µm area; in a peripheral zone corresponding to a peripheral zone of a nucleus pulpopus, the density may be about 9 pillars per 40 µm x 40 µm area; in a core zone corresponding to a core zone of a nucleus pulpopus (with larger diameter pillars), the density may be about 3 pillars per 40 µm x 40 µm area. [0045] The density (spacing) of elastic fibre ridges may be considered in different ways. [0046] Considering the average number of ridge crossings per straight line distance measured in various angles and zones, the number/mm may be in the range of 5 mm^-1 to 200 mm^-1, 10 mm^-1 to 30 mm^-1, 50 mm^-1 to 150 mm^-1, 80 mm^-1 to 140 mm^-1, 100 mm^-1 to 130 mm- ¹, about 5 mm^-1, about 10 mm^-1, about 50 mm^-1, about 100 mm^-1, about 120 mm^-1, about 130 mm^-1, about 140 mm^-1, or about 150 mm^-1, for example. The number density of elastic fibre ridges per millimetre may be about 5 mm^-1 to 10 mm^-1 in the lamella zones in a direction along the lamella; about 20 mm^-1 to 50 mm^-1 across the lamella zones; about 100 mm^-1 to 150 mm^-1 in the interlamellar matrix zone and transition zone; about 10 mm^-1 to 30 mm^-1 in the peripheral zone; and about 30 mm^-1 to 80 mm^-1 in the core zone. [0047] In some embodiments, the body is formed of an elastomeric material. The body may be formed of any suitable material, including polymers, elastomers, two-part curable elastomers, Polydimethylsiloxane (PDMS), for example. [0048] Some embodiments relate to a method of forming an in-vitro organ model, the method comprising: loading biological cells onto the protrusions of a microfluidic chip according to any one of the described embodiments; and perfusing the biological cells with a biological medium. [0049] In some embodiments, the method further comprises loading a gel onto the protrusions of the microfluidic chip, such as a hydrogel or collagen gel, for example. [0050] The biological cells may comprise healthy or degenerated intervertebral disc cells selected from the group consisting of annulus fibrosus cells, nucleus pulposus cells, notochordal cells and combinations thereof, for example. The pH, glucose levels, oxygen levels and/or other nutrient levels of the perfusion medium may be adjusted appropriately to recapitulate the native environment of degenerated cells in a degenerated intervertebral disc, for example. [0051] Some embodiments relate to a method of forming an in-vitro intervertebral disc model, the method comprising: loading annulus fibrosus cells into the first channel of a microfluidic chip according to any one of the described embodiments; loading nucleus pulposus cells into the second channel of the microfluidic chip; and perfusing the annulus fibrosus cells and nucleus pulposus cells with a biological medium. [0052] In some embodiments, the method further comprises: loading a first gel into the first channel; and loading a second gel into the second channel, wherein the stiffness of the first gel is at least 50% higher than the stiffness of the second gel. [0053] The first and second gels may comprise any suitable gels, such as a hydrogel or collagen gel, for example. The stiffness of the first gel may be higher than the stiffness of the second gel by at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 100%, at least 110%, at least 120%, or in the range of 50% to 100%, 80% to 120%, or 80% to 100%, for example. [0054] In some embodiments, the method further comprises exposing the microfluidic chip to an external stimuli and measuring a corresponding biological response. [0055] The external stimuli may include any external stimuli of interest, alone or in combination with other stimuli, including: applying a mechanical load to the microfluidic chip; applying a temperature variations to the microfluidic chip; introducing wear particles to the first or second channel; and introducing an active agent (e.g., a pharmaceutical agent) to the first or second channel. [0056] Substances may be introduced to the perfusion medium through the channel inlet(s) to study the likely impact of such substances in patients. For example, adding wear particles to study the biological impact of spinal implant wear particles on the health of an intervertebral disc, or injecting a growth factor (or transplant cells, or stem cells, or other agents) in the NP channel to examine its efficacy to promote cell viability in the AF channel. [0057] The biological response may be measured using any suitable technique, including but not limited to: mass spectrometry, chromatography, gel electrophoresis, fluorescence spectroscopy, flow cytometry, electron microscopy, atomic force microscopy, UV absorbance spectroscopy, and rheology. [0058] Any suitable method may be employed to fabricate the microfluidic chip. Some conventional manufacturing techniques may not achieve sufficiently high resolution to form the microstructures at suitable dimensions for recapitulating the dimensions of collagen fibre bundles and elastic fibres in intravertebral discs; however, some conventional manufacturing techniques may be suitable for larger microstructures. [0059] Some embodiments relate to a method of fabricating the microfluidic chip of any one of the described embodiments, the method comprising: casting a first part of the body defining the or each microstructure in a microstructure template defining recesses corresponding to the protrusions of the or each microstructure; forming a second part of the body defining the or each channel, inlet and outlet; and connecting the first part to the second part to form the microfluidic chip. [0060] The microstructure template may be formed using one or more techniques selected from the group comprising: lithography, photolithography, etching, laser ablation, focused ion beam, machining, 3D printing, and Digital Light Processing (DLP) 3D printing. [0061] In some embodiments, the microstructure template has been formed by 3D printing using two-photon polymerisation. [0062] The first and second parts may be cast in a curable elastomeric material or any other suitable material, such as described herein, for example. The first and second parts may then be bonded together by welding or adhesive bonding, for example, as described herein. [0063] Some embodiments relate to an intervertebral disc (IVD)-on-a-chip device comprising: an elastomer body having at least a nucleus channel and an annulus channel in fluid communication, wherein the nucleus channel comprises at least one surface having a first micropatterned structure; wherein the annulus channel comprises at least one surface having a second micropatterned structure; and wherein the device is adapted to comprise an inlet and an outlet for perfusion. [0064] Advantageously, in some embodiments, the IVD-on-a-chip device provides a reproducible and adaptable microfluidic platform which is a 3D IVD-on-a-chip model that is capable of substantially recapitulating the relevant IVD structural complexity at the microscale and the ultrastructural organization of IVD components in different regions of the IVD. The device may provide the first highly efficient and multi-functional IVD-like organ model. The IVD-on-a-chip device may provide a universal 3D and physiologically relevant in-vitro IVD platform. The device may be capable of providing fine tuning of material stiffness gradient (modularity) and facilitate the application of external loads which can be important for highly accurate IVD mechanobiological and tissue engineering studies. [0065] Accordingly, in some embodiments, the first and/or second micropatterned structure comprises one or more predefined micropatterns. In certain embodiments, the first and/or second micropatterned structure is derived from a template. In some embodiments, the template which is a ‘negative’ pattern, transfers the micropatterned structure to the respective surface of the nucleus channel and/or annulus channel. In some embodiments, the first and/or second micropatterned structure is derived from a template formed by 3D printing . [0066] In some embodiments, the annulus channel comprises an inner annulus channel and an outer annulus channel in fluid communication. In some embodiments, the device further comprises a transition zone channel in fluid communication between the nucleus channel and the annulus channel. In further embodiments, the transition zone channel comprises at least one surface having a third micropatterned structure. [0067] As with the first and/or second micropatterned structure, the third micropatterned structure can comprise a micropattern. In certain embodiments, the third micropatterned structure is derived from a template. The third micropatterned structure may be derived from a template formed by 3D printing. [0068] Advantageously, in some embodiments, the configuration of the annulus and nucleus channels and their respective micropatterned structures have been adapted to provide a world first 3D in vitro IVD-on-a-chip device that can substantially recapitulate the relevant IVD function, structural complexity, and stiffness gradient of a biological IVD which offers a more realistic, and cost- effective IVD model. This may provide users with the capability of real- time monitoring to establish highly physiologically relevant in-vitro IVD studies that are currently difficult, if not impossible, to achieve. In some embodiments, the device can be used for establishing different high-precision in-vitro IVD experiments (i.e. mechanobiology) to explore novel therapeutic and tissue engineering strategies. [0069] In some embodiments, the IVD-on-a-chip device is a closed-channel device. Advantageously, the closed-channel design facilitates the application of external loads for highly accurate IVD mechanobiological studies. [0070] Some embodiments relate to a method of forming an IVD-on-a-chip device comprising the steps of: casting a first elastomer on a channel template to form a channel layer; casting a second elastomer on a patterning template to form a micropatterned structure layer; curing the channel layer and the micropatterned structure layer; and mating the channel layer and micropatterned structure layer to form an IVD-on-a- chip device comprising a nucleus channel having a surface comprising a first micropatterned structure and an annulus channel having a surface comprising a second micropatterned structure. [0071] In some embodiments, the patterning template is formed by 3D printing on a substrate. In some embodiments, the 3D printing is a two-photon polymerisation process. Advantageously, the use of two-photon polymerisation 3D printing can provide high resolution and fidelity of the micropatterns printed on the templates. The resolution can be in the nanometer scale, for example, in the order of about 100 nm resolution. [0072] In certain embodiments, the casting steps is performed by mixing a curing agent with an elastomer base. In certain embodiments, the channel template has been configured to form the device as described herein. In certain embodiments, the patterning template has been configured to form the device as described herein. In preferred embodiments, the method further comprises the step of forming an inlet and an outlet for perfusion. [0073] Some embodiments relate to a method of using an IVD- on-a-chip device as described herein for measuring a biological response, comprising the steps of: seeding cells in the annulus channel and nucleus channel; loading the annulus channel with a first collagen gel ; loading the nucleus channel with a second collagen gel having a different Young’s modulus to the different tissues or gels (hydrogels) with different stiffness, e.g. collagen gel; performing perfusion with a biological medium through an inlet and an outlet; and exposing the device to an external stimuli and measuring a biological response. The biological medium may also include cell culture media, growth factors, pharmaceutics for cell growth and stimulation, other chemicals to stimulate IVD disease and degeneration state (such as those changing/tunning pH, glucose level, oxygen level, or other nutrient levels) as well as metal and polymer debris to simulate spinal implant- IVD cells interaction and mechanobiological assessments. [0074] In some embodiments, the external stimulus is selected from the group consisting of mechanical load, wear particles, active agent, temperature and combinations thereof. In some embodiments, the biological response is selected from the group consisting of a change in cell morphology, change in metabolite concentration, change in cell proliferation, change in cell viability, change in protein concentration and combinations thereof. [0075] When the external stimulus is a mechanical load, this may be configured to simulate various micromechanical loading conditions on the IVD-on-a-chip device to replicate different daily activities with concomitant monitoring of the effect on IVD cell viability, morphology, and fibrous tissue formation. [0076] The method of using the device as an in-vitro IVD model may contribute to the development of regulations for the appraisal of the efficacy and safety of IVD tissue- engineering practices (such as drug and biomolecules screening) with low capital outlay. [0077] Some embodiments relate to use of an IVD-on-a-chip device as described herein for an in-vitro organ model. [0078] Some embodiments relate to a kit comprising: a channel template adapted to form a channel layer; and a patterning template adapted to form a micropatterned structure layer; such that the templates can be used to form an IVD-on-a-chip device comprising a nucleus channel having a surface comprising a first micropatterned structure and an annulus channel having a surface comprising a second micropatterned structure. Definitions [0079] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains. [0080] Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. [0081] As used herein, the phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phrase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter. [0082] With respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of”. [0083] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %”, “ratio” will mean “weight ratio” and “parts” will mean “weight parts”. [0084] The term ‘substantially’ as used herein shall mean comprising more than 50% by weight, where relevant, unless otherwise indicated. [0085] The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). [0086] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention. [0087] It must also be noted that, as used in the specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. [0088] The prior art referred to herein is fully incorporated herein by reference. [0089] Although exemplary embodiments of the disclosed technology are explained in detail herein, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the disclosed technology be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The disclosed technology is capable of other embodiments and of being practiced or carried out in various ways. Brief Description of Drawings [0090] Embodiments are described below, for illustrative purposes only, with reference to the accompanying drawings in which: [0091] Figure 1 shows an embodiment of an IVD-on-a-chip device. The inset shows a configuration of the microvalve structure. [0092] Figure 2 shows an alternative embodiment of an IVD-on-a-chip device having an inner and outer annulus channel. [0093] Figure 2a shows a cross section of the device having a top layer and a bottom layer in its component form before being mated to form the device. [0094] Figure 2b shows a cross section of the device having the top layer and the bottom layer mated together via plasma bonding to form the device. [0095] Figure 2c shows an embodiment of the micropatterned structures of the bottom layer for each region/channel of the IVD-on-a-chip device. The micropatterned structures can also be separated in domains as shown in Figure 2c. [0096] Figures 2d show the schematic of an embodiment of the top layer having square/rectangular shaped channels. The top layer of the device includes individual channels representing the IVD regions, microvalves to separate channels, and outlet and inlet chambers for perfusion of cells and biomaterials. [0097] Figures 2e show the schematic of an alternative embodiment of the top layer having trapezoidal shaped channels. The alternative design for the top layer of the chip includes individual channels representing the IVD regions, microvalves to separate channels, and outlet and inlet chambers with trapezoidal shape allowing perfusion of cells and biomaterials with minimal dead zone at the corners of each channel. [0098] Figure 3 provides representations of embodiments of the first, second and/or third micropatterned structures. [0099] Figure 3a is a 3D design of the inner and outer micropatterned structure of the annulus channel (i.e., cross section lamella). [0100] Figure 3b is a 3D design of the inner and outer micropatterned structure of the annulus channel (i.e., in-plane lamella). [0101] Figure 3c is a 3D design of the interlamellar matrix inner and outer annulus layers and transition zone channel (between the annulus and nucleus channel). [0102] Figure 4 shows an embodiment of the device for use as an in-vitro model. Active agents such as implant wear particles and disc cells can be used as an input to design and fabricate improved and safer spinal implants, for example. Further, stem and IVD cells can be used as an input for disc regeneration. [0103] Figure 5A is a diagram of an intervertebral disc for reference. [0104] Figure 5B is a diagram illustrating an inclined cross-section plane parallel to collagen fibre bundles of an in-plane lamella of the disc, and resultant cross-section. [0105] Figure 5C is a diagram illustrating a transverse cross-section plane parallel to collagen fibre bundles of an in-plane lamella of the disc, and resultant cross-section. [0106] Figure 6 is a schematic diagram of a single channel microfluidic chip and microstructure, according to some embodiments. [0107] Figure 7 is a schematic diagram of a single channel microfluidic chip and microstructure, according to some embodiments. [0108] Figure 8 is a schematic diagram of a microstructure of part of a microfluidic chip, according to some embodiments. [0109] Figure 9 is a schematic diagram of a microstructure of part of a microfluidic chip, according to some embodiments. Description of Embodiments [0110] Continual research and development for identifying suitable in-vitro IVD models for tissue engineering, biomechanics and mechanobiology has been driven by the desire to recapitulate the structural features, material stiffness gradient and functions of the IVD. However, recapitulating the IVD structural complexity at the microscale and ultrastructural organisation of IVD components remains a challenge. [0111] The inventors have developed a simple and cost-effective protocol for visualization and isolation of the elastic fibres network in the annulus fibrosus (AF) of the disc with a method that can be applied in disc ultra-structural analysis, biomechanical assessment of elastic fibre and tissue engineered scaffold fabrication. This protocol was developed based on simultaneous sonication and alkali digestion of tissue that eliminates all matrix constituents except for elastic fibres, which is applicable for different species including human. Thin samples harvested from ovine, bovine, porcine and human, which are commonly used in disc research, were exposed to 0.5^M sodium hydroxide solution along with sonication (25^kHz) for defined periods of time at room temperature. Post heat treatment removed collagen fibres via the gelatinization process, for visualization of elastic fibres using SEM technique. Later this methodology was adopted and modified to further isolate the elastic network at different regions in the IVD including NP, TZ, etc. [0112] Related research papers can be found at: https://www.sciencedirect.com/science/article/pii/S0928493118300286 https://www.sciencedirect.com/science/article/abs/pii/S1751616117301248 https://www.sciencedirect.com/science/article/abs/pii/S1742706120303299 https://www.sciencedirect.com/science/article/abs/pii/S1742706120304074 https://www.sciencedirect.com/science/article/pii/S174270612300096X https://www.sciencedirect.com/science/article/pii/S1742706117303306 https://www.sciencedirect.com/science/article/pii/S1742706118301351 https://www.sciencedirect.com/science/article/pii/S1742706118304173 https://www.sciencedirect.com/science/article/pii/S1742706122001337 https://www.mdpi.com/1422-0067/23/16/8931 https://www.sciencedirect.com/science/article/pii/S1742706124005476 https://www.sciencedirect.com/science/article/pii/S0167779923001300 [0113] This technique allowed the inventors to examine the structural characteristics of the elastic fibre network, and to design microfluidic chips with different microstructures to substantially recapitulate or mimic the size, distribution and orientation of the elastic fibres as well as collagen fibre bundles in different regions of the intervertebral disc. [0114] Figure 5A is a diagram of an IVD showing a closeup perspective view of a cutaway portion of the AF. The collagen fibre bundles are arranged in parallel in each lamella layer, and the collagen bundles of each adjacent lamella are angled relative to each other, so that the collagen bundles of each lamella are angled at approximately ±30° relative to the transverse (horizontal) plane of the disc. That is, one lamella at +30° and the next at -30°. [0115] The microstructures of the microfluidic device represent a cross-section through the disc (or part thereof) and may be arranged differently depending on the angle of the cross- section under consideration. [0116] For example, Figure 5B shows a cross-section plane which is angled relative to the transverse plane so as to be parallel with the collagen bundles of one of the lamella. Therefore, in the cross-section view, the collagen bundles of one lamella appear as parallel rectangles (in-plane) and the collagen bundles of the next lamella are shown in cross-section as round pillars (out-of-plane). [0117] Figure 5C shows a transverse cross-section, where the collagen bundles of both lamella are shown in cross-section, and due to the angle, the cut faces are elongated. [0118] The microstructures of the microfluidic chip may be designed to recapitulate either of these views, or indeed any other cross-sectional view through any tissues to be modelled. [0119] The microfluidic chip may comprise any suitable number of channels and microstructures, each substantially recapitulating one or more structural characteristics of a target natural biological tissue. [0120] Throughout the specification, it should be understood that substantial recapitulation does not require perfect replication of the structures of the target tissue. Instead, it is intended to mimic one or more aspects of the structure, such as dimensions, distribution, density, stiffness, orientation, for example, with sufficient accuracy to provide a useful model for in- vitro research, such as applying a stimuli and monitoring a biological response, for example. [0121] Figure 6 illustrates a microfluidic chip 600 comprising a body 601 defining: a channel 606 extending between an inlet 607 and an outlet 608; and a microstructure 610 formed by a plurality of protrusions 620 extending away from a surface of the channel, wherein the microstructure is configured to substantially recapitulate one or more structural characteristics of a target natural biological tissue. [0122] The protrusions 620 include collagen fibre bundle protrusions 622 and elastic fibre protrusions 624. [0123] The chip 600 corresponds with the angled cross-section shown in Figure 5B, and only shows a single in-plane lamella, so in Figure 6 the collagen fibre bundle protrusions 622 comprise elongate rectangles, corresponding to the in-plane bundles. [0124] Figure 7 illustrates a similar microfluidic chip 700 with similar features, corresponding to a single out-of-plane lamella with collagen fibre bundle protrusions 622 comprising elliptical prism pillars, corresponding to the out-of-plane bundles. [0125] In both chips 600, 700, the elastic fibre protrusions 624 comprise narrower elongate ridges, corresponding to partition boundaries between adjacent collagen bundles in the lamella. Some of the partition boundaries extend entirely through the lamella, and others only extend part way through. [0126] Figure 8 shows another microstructure 800 (similar to that shown in Figure 2c), which also corresponds to the inclined plane cross-section shown in Figure 5B, and includes protrusions corresponding to two adjacent lamella. A first in-plane lamella 801 with collagen bundle ridges 622a, similar to Figure 6, and a second out-of-plane lamella 802 with collagen bundle pillars 622b, similar to Figure 7. [0127] The microstructure 800 also has elastic fibres 624 forming partition boundaries, which in some areas are single ridges, and in other areas comprise interconnected networks of ridges forming polygons and taking up more space between adjacent collagen bundles. [0128] The microstructure 800 also has an interlamellar matrix zone 803, with and interconnected network of elastic fibre ridges 624 forming polygons and extending between the first and second lamellae 801, 802. [0129] In some embodiments, the channel is a first channel, the inlet is a first inlet, the outlet is a first outlet, the microstructure is a first microstructure, the plurality of protrusions is a first plurality of protrusions, the body further defines: a second channel extending between a second inlet and a second outlet; and a second microstructure formed by a second plurality of protrusions extending away from a surface of the second channel; and the first and second microstructures are configured to substantially recapitulate one or more structural characteristics of different target natural biological tissues. [0130] The body may define any suitable number of channels for different applications, and corresponding microstructures in each channel. The microstructures may be configured to substantially recapitulate or mimic one or more structural characteristics of various target natural biological tissues to assist in preparing organ models to substantially recapitulate the biological tissues. [0131] In some embodiments, the microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in part of the target natural biological tissue. The protrusions of the microstructure may include relatively wider collagen fibre bundle protrusions recapitulating collagen fibre bundles, and relatively narrower elastic fibre protrusions recapitulating elastic fibres. [0132] In some embodiments, the average width of the protrusions recapitulating collagen fibre bundles is at least 50%, 60%, 70%, 80%, 90%, or 100% wider than the average width of the protrusions recapitulating the elastic fibres, for example. [0133] In some embodiments, the microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in part of the annulus fibrosus of a natural intervertebral disc in a plane extending through a radial axis of the disc, wherein the microstructure includes a lamella zone corresponding to a lamella of the annulus fibrosus, and wherein the lamella zone includes an array of collagen fibre bundle protrusions with elastic fibre protrusions in the form of ridges extending between different regions of the array of collagen fibre bundle protrusions to form partition boundary zones. [0134] In some embodiments, at least some of the partition boundary zones comprise elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons separating different regions of the array of collagen fibre bundle protrusions. For example, as shown in Figure 8. [0135] In some embodiments, the lamella zone is configured to recapitulate part of the lamella in a plane extending through the radial axis of the disc and intersecting the collagen fibre bundles, and the collagen fibre bundle protrusions comprise elliptical prisms with the eccentricity of the elliptical prisms selected based on the angle of intersection with the plane. [0136] In some embodiments, the lamella zone is configured to recapitulate part of the lamella in a plane extending through the radial axis of the disc and inclined relative to the transverse plane of the disc to extend parallel to collagen fibre bundles (Figure 5B), and the collagen fibre bundle protrusions comprise parallel ridges (Figures 6 and 8). [0137] In some embodiments, the lamella zone is a first lamella zone 801 and the microstructure further includes a second lamella zone 802 and an interlamellar matrix zone 803 between the first and second lamella zones, corresponding to first and second lamellae, and interlamellar matrix of the disc, respectively (as shown in Figure 8), wherein the protrusions of the second lamella zone include an array of collagen fibre bundle protrusions 622 with elastic fibre protrusions 624 in the form of ridges extending between different regions of the array of collagen fibre bundle protrusions to form partition boundary zone, and wherein the protrusions of the interlamellar matrix zone include elastic fibre protrusions 624 arranged to form a network of ridges extending in different directions to form interconnected polygons extending across the interlamellar matrix zone between the first and second lamella zones. [0138] In some embodiments, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95% (or optionally 100%) of the ridges of the interlamellar matrix zone are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° (or optionally within 20°, 15° or 5°) of parallel relative to the reference axis; within 10° (or optionally within 20°, 15° or 5°) of 45° relative to the reference axis (i.e., 45° in either direction relative to the reference axis, or ±45°); and within 10° (or optionally within 20°, 15° or 5°) of perpendicular relative to the reference axis. [0139] In some embodiments, the plane represented by the microstructure of the microfluidic chip extends through the radial axis of the disc and is inclined relative to the transverse plane of the disc to extend parallel to collagen fibre bundles of the first lamella of the annulus fibrosus (e.g., approximately 30° inclined relative to transverse plane, as shown in Figure 5B). The collagen fibre bundle protrusions of the first lamella zone may comprise elongate ridges extending parallel to the interlamellar matrix zone, the ridges substantially recapitulating the in-plane oriented collagen fibre bundles of the first lamella of the disc. The collagen fibre bundle protrusions of the second lamella zone may comprise an array of pillars substantially recapitulating the out-of-plane oriented collagen fibre bundles of the second lamella of the disc. [0140] In some embodiments, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95% (or optionally 100%) of the elastic fibre ridges in the partition boundary zones are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° (or optionally within 20°, 15° or 5°) of parallel relative to the reference axis; within 10° (or optionally within 20°, 15° or 5°) of 45° relative to the reference axis (i.e., 45° in either direction relative to the reference axis, or ±45°); and within 10° (or optionally within 20°, 15° or 5°) of perpendicular relative to the reference axis. [0141] In some embodiments, the channel is a first channel, the inlet is a first inlet, the outlet is a first outlet, the microstructure is a first microstructure, the plurality of protrusions is a first plurality of protrusions, wherein the body further defines: a second channel extending between a second inlet and a second outlet; and a second microstructure formed by a second plurality of protrusions extending away from a surface of the second channel; and wherein the second microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in the nucleus pulposus of a natural intervertebral disc in a plane extending through a radial axis of the disc and inclined relative to the central axis of the disc. [0142] In some embodiments, the second microstructure comprises similar features to the first microstructure defined in any one of the described embodiments with the protrusions arranged in closer proximity to each other forming a relatively higher density of protrusions compared with the first microstructure. For example, Figure 2C shows an inner AF microstructure 106a and an outer AF microstructure 106b with similar features, but a higher density of protrusions (i.e., a higher number of protrusions per unit area in each zone). [0143] In some embodiments, the second microstructure includes a core zone on one side of the channel and a peripheral zone extending partially around the core zone on another side of the channel, wherein the protrusions of the core zone comprise an array of pillars corresponding to out-of-plane oriented collagen fibre bundles of the nucleus pulposus, and wherein the protrusions of the peripheral zone comprise an array of pillars corresponding to out-of-plane oriented elastic fibres of the nucleus pulposus. [0144] This type of microstructure corresponds to the NP structure of an IVD, as shown in Figure 2c (item 108). [0145] In some embodiments, the second microstructure further includes elastic fibre ridges 624 radiating away from the core zone at different angles relative to each other, corresponding to in-plane elastic fibres. The elastic fibre ridges may radiate away from the core zone at different angles with angles between adjacent elastic fibre ridges being in the range of 15° to 30°, 20° to 25°, about 20°, or about 25°, for example. [0146] In some embodiments, the body further defines a plurality of microvalves arranged between and configured to selectively allow fluid communication between the first and second channels. The microvalves may comprise active microvalves or passive microvalves, such as capillary valves, for example, which resist flow due to surface tension until a threshold pressure gradient is exceeded and flow through the valves is allowed. [0147] In some embodiments, the body further defines: a third channel extending between a third inlet and a third outlet, and located between the first and second channels; and a third microstructure formed by a third plurality of protrusions extending away from a surface of the third channel; wherein the third microstructure is configured to substantially recapitulate one or more structural characteristics of the natural intervertebral disc. [0148] In some embodiments, the body further defines a plurality of microvalves arranged between and configured to selectively allow fluid communication between the first and third channels, and between the second and third channels. [0149] In some embodiments, the third microstructure is configured as a transition zone to substantially recapitulate the size, distribution and orientation of elastic fibres in the transition zone of the natural intervertebral disc between the nucleus pulposus and the annulus fibrosus. As shown in Figure 2c (item 118). [0150] In some embodiments, the protrusions of the third microstructure include elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons extending across the transition zone. [0151] In some embodiments, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95% (or optionally 100%) of the ridges of the transition zone are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° (or optionally within 20°, 15° or 5°) of parallel relative to the reference axis; within 10° (or optionally within 20°, 15° or 5°) of 45° relative to the reference axis (i.e., 45° in either direction relative to the reference axis, or ±45°); and within 10° (or optionally within 20°, 15° or 5°) of perpendicular relative to the reference axis. [0152] In some embodiments, the third microstructure may include collagen fibre bundle protrusions in the form of pillars. (as shown in Figure 9) The pillars may be elliptical prisms, for example. The collagen fibre bundle protrusions may be interspersed between the elastic fibre ridges within the network of ridges. The eccentricity of the collagen fibre bundle protrusions in the transition zone may higher than the eccentricity of the collagen fibre bundle protrusions in the core zone by at least 20%, at least 30%, at least 40%, between 25% and 35%, or about 30%, for example. The collagen fibre bundle protrusions of the transition zone may be randomly oriented, or oriented at 4, 5, 6, 7, 8, 9, 10, or more angles relative to each other, [0153] Figure 9 illustrates a small area of a transition zone 900, according to some embodiments, showing the collagen fibre bundle pillars 622b and elastic fibre ridges 624. [0154] Throughout the microstructures and various zones of the microstructures, the collagen fibre bundle protrusions and elastic fibre ridges may comprise any suitable dimensions, and may have dimensions in the following ranges. Dimensions may vary or be similar within or between different zones and microstructures. The depth or height of the protrusions relative to the surface they extend from may be in the range of 0.5 µm to 5 µm, 1 µm to 3 µm, or about 2 µm, for example. [0155] The collagen fibre bundle protrusions may have an average width or diameter in the range of 5 µm to 35 µm, 5 µm to 20 µm, or 5 µm to 10 µm, for example. The collagen fibre bundle ridges may have a length in the range of 10 µm to 200 µm, for example. The collagen fibre bundle pillars may have a minimum diameter of about 5 µm and a maximum diameter of about 10 µm, for example, the elliptical pillars may have a minor axis diameter of about 5 µm and a major axis diameter of about 10 µm. Depending on the angle of cross-section of the reference plane in the natural intervertebral disc being modelled, the elliptical prism pillars may have different eccentricities. For example, the major axis diameter may be larger than the minor axis diameter by a factor of 1.2, 1.5, 1.8, 2, 2.5, 3, 3.5, 4, 4.5, 5 or any other suitable factor. [0156] The collagen fibre bundle pillars may vary in size in different zones of the microstructure(s). For example, the average diameter of the pillars in the core zone may be larger than the average diameter of the pillars in the peripheral zone by 50%, 80%, 100%, 120%, 150%, 200%, 300% or more. The elliptical pillars in the core zone and peripheral zone may be oriented at different angles, which may be randomly oriented, or randomly oriented at 25° increments relative to each other. The collagen fibre bundle pillars in the transition zone may be more elongated (eccentric) than the pillars in the core zone by at least 20%, at least 30%, at least 40%, between 25% and 35%, or about 30%, for example. [0157] The elastic fibre ridges may have a constant width or varying widths in the range of 0.2 µm to 20 µm, 0.5 µm to 10 µm, 1 µm to 10 µm, 1 µm to 5 µm, 2 µm to 3 µm, about 1 µm, about 1.5 µm, about 2 µm, or about 2.5 µm. The length of the elastic fibre ridges may be in the range of 5 µm to 500 µm, 100 µm to 500 µm, 100 µm to 300 µm, 50 µm to 150 µm, 5 µm to 50 µm, 5 µm to 35 µm, 5 µm to 10 µm, 10 µm to 20 µm.20 µm to 40 µm. or any other suitable dimensions for a given application. [0158] The number density of protrusions may vary in different zones. [0159] For example, the number of collagen fibre bundle pillars in a 40 µm x 40 µm area may be in the range of 2 to 20, 3 to 15, 9 to 13, about 3, about 9, or about 13, or any other suitable density. For example, in a lamella zone corresponding to an outer lamella of an annulus fibrosus, the density may be about 13 pillars per 40 µm x 40 µm area; in a lamella zone corresponding to an inner lamella of an annulus fibrosus, the density may be about 9 pillars per 40 µm x 40 µm area; in a transition zone corresponding to a transition zone between nucleus pulpopus and an annulus fibrosus, the density may be about 9 pillars per 40 µm x 40 µm area; in a peripheral zone corresponding to a peripheral zone of a nucleus pulpopus, the density may be about 9 pillars per 40 µm x 40 µm area; in a core zone corresponding to a core zone of a nucleus pulpopus (with larger diameter pillars), the density may be about 3 pillars per 40 µm x 40 µm area. [0160] The density (spacing) of elastic fibre ridges may be considered in different ways. [0161] Considering the average number of ridge crossings per straight line distance measured in various angles and zones, the number/mm may be in the range of 5 mm^-1 to 200 mm^-1, 10 mm^-1 to 30 mm^-1, 50 mm^-1 to 150 mm^-1, 80 mm^-1 to 140 mm^-1, 100 mm^-1 to 130 mm- ¹, about 5 mm^-1, about 10 mm^-1, about 50 mm^-1, about 100 mm^-1, about 120 mm^-1, about 130 mm^-1, about 140 mm^-1, or about 150 mm^-1, for example. The number density of elastic fibre ridges per millimetre may be about 5 mm^-1 to 10 mm^-1 in the lamella zones in a direction along the lamella; about 20 mm^-1 to 50 mm^-1 across the lamella zones; about 100 mm^-1 to 150 mm^-1 in the interlamellar matrix zone and transition zone; about 10 mm^-1 to 30 mm^-1 in the peripheral zone; and about 30 mm^-1 to 80 mm^-1 in the core zone. [0162] In some embodiments, the body is formed of an elastomeric material. The body may be formed of any suitable material, including polymers, elastomers, two-part curable elastomers, Polydimethylsiloxane (PDMS), for example. [0163] Some embodiments relate to a method of forming an in-vitro organ model, the method comprising: loading biological cells onto the protrusions of a microfluidic chip according to any one of the described embodiments; and perfusing the biological cells with a biological medium. [0164] In some embodiments, the method further comprises loading a gel onto the protrusions of the microfluidic chip, such as a hydrogel or collagen gel, for example. [0165] The biological cells may comprise healthy or degenerated intervertebral disc cells selected from the group consisting of annulus fibrosus cells, nucleus pulposus cells, notochordal cells and combinations thereof, for example. The pH, glucose levels, oxygen levels and/or other nutrient levels of the perfusion medium may be adjusted appropriately to recapitulate the native environment of degenerated cells in a degenerated intervertebral disc, for example. [0166] Some embodiments relate to a method of forming an in-vitro intervertebral disc model, the method comprising: loading annulus fibrosus cells into the first channel of a microfluidic chip according to any one of the described embodiments; loading nucleus pulposus cells into the second channel of the microfluidic chip; and perfusing the annulus fibrosus cells and nucleus pulposus cells with a biological medium. [0167] In some embodiments, the method further comprises: loading a first gel into the first channel; and loading a second gel into the second channel, wherein the stiffness of the first gel is at least 50% higher than the stiffness of the second gel. [0168] The first and second gels may comprise any suitable gels, such as a hydrogel or collagen gel, for example. The stiffness of the first gel may be higher than the stiffness of the second gel by at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 100%, at least 110%, at least 120%, or in the range of 50% to 100%, 80% to 120%, or 80% to 100%, for example. [0169] In some embodiments, the method further comprises exposing the microfluidic chip to an external stimuli and measuring a corresponding biological response. [0170] The external stimuli may include any external stimuli of interest, alone or in combination with other stimuli, including: applying a mechanical load to the microfluidic chip; applying a temperature variations to the microfluidic chip; introducing wear particles to the first or second channel; and introducing an active agent (e.g., a pharmaceutical agent) to the first or second channel. [0171] Substances may be introduced to the perfusion medium through the channel inlet(s) to study the likely impact of such substances in patients. For example, adding wear particles to study the biological impact of spinal implant wear particles on the health of an intervertebral disc, or injecting a growth factor (or transplant cells, or stem cells, or other agents) in the NP channel to examine its efficacy to promote cell viability in the AF channel. [0172] The biological response may be measured using any suitable technique, including but not limited to: mass spectrometry, chromatography, gel electrophoresis, fluorescence spectroscopy, flow cytometry, electron microscopy, atomic force microscopy, UV absorbance spectroscopy, and rheology. [0173] Any suitable method may be employed to fabricate the microfluidic chip. Some conventional manufacturing techniques may not achieve sufficiently high resolution to form the microstructures at suitable dimensions for recapitulating the dimensions of collagen fibre bundles and elastic fibres in intravertebral discs; however, some conventional manufacturing techniques may be suitable for larger microstructures. [0174] Some embodiments relate to a method of fabricating the microfluidic chip of any one of the described embodiments, the method comprising: casting a first part of the body defining the or each microstructure in a microstructure template defining recesses corresponding to the protrusions of the or each microstructure; forming a second part of the body defining the or each channel, inlet and outlet; and connecting the first part to the second part to form the microfluidic chip. [0175] The microstructure template may be formed using one or more techniques selected from the group comprising: lithography, photolithography, etching, laser ablation, focused ion beam, machining, 3D printing, and Digital Light Processing (DLP) 3D printing. [0176] In some embodiments, the microstructure template has been formed by 3D printing using two-photon polymerisation. [0177] The first and second parts may be cast in a curable elastomeric material or any other suitable material, such as described herein, for example. The first and second parts may then be bonded together by welding or adhesive bonding, for example, as described herein. [0178] Additional embodiments are described below with specific examples, for illustrative purposes only. Intervertebral disc (IVD)-on-a-chip device [0179] Accordingly, in some embodiments, there is provided a intervertebral disc (IVD)-on- a-chip device comprising: an elastomer body having at least a nucleus channel and an annulus channel in fluid communication, wherein the nucleus channel comprises at least one surface having a first micropatterned structure; wherein the annulus channel comprises at least one surface having a second micropatterned structure; and wherein the device is adapted to comprise an inlet and an outlet for perfusion. [0180] In some embodiments, these micropatterns (microfeatures) are adapted to mimic the size, distribution and orientation of elastic and collagen fibers in the native IVDs. Therefore, they are pre-defined patterned structures that are specific to each region of the IVD. For example, the pattern of the TZ is different to the NP and always will be the same for each IVD- on-a-chip. In other embodiments, the micropatterns (microfeatures) may be designed to mimic the size, distribution and orientation of other tissues or organ structures. [0181] In some embodiments, the first micropatterned structure comprises a micropattern, preferably, the first micropatterned structure is derived from a template. As would be understood by a skilled addressee, the template provides a “negative” pattern which can function as a mold which transfers the resulting micropatterned structure to the device using any suitable method such as casting. [0182] In preferred embodiments, the first micropatterned structure is derived from a template formed by 3D printing. [0183] In some embodiments, the second micropatterned structure comprises a micropattern, preferably, the second micropatterned structure is derived from a template. In preferred embodiments, the second micropatterned structure is derived from a template formed by 3D printing. [0184] In certain embodiments, the device further comprises a transition zone channel in fluid communication between the nucleus channel and the annulus channel. In preferred embodiments, the transition zone channel comprises at least one surface having a third micropatterned structure. In certain embodiments, the third micropatterned structure comprises a micropattern, preferably, the third micropatterned structure is derived from a template. In preferred embodiments, the third micropatterned structure is derived from a template formed by 3D printing. [0185] As would be appreciated by a skilled addressee, the first, second and third micropatterned structures can be configured to substantially recapitulate the elastic and collagen fibres of an IVD and provides an in-vitro organ model without requiring donor animal or human IVDs. [0186] For example, the first micropatterned structure of the nucleus channel can be adapted to substantially recapitulate the elastic and collagen fibres of the nucleus pulpous observed in an IVD, the second micropatterned structure of the annulus channel can be adapted to substantially recapitulate the elastic and collagen fibres of the annulus fibrosus and the third micropatterned structure of the transition zone channel can be adapted to substantially recapitulate the elastic and collagen fibres of the transition zone of an IVD. [0187] The present inventors surprisingly found a similar structural organisation for elastic fibres for all regions of IVD (annulus, nucleus, and transition zone); hence, the IVD-on-a-chip device made to represent the similar structural organisation. The elastic fibre orientation was identified using open-access software (FIJI- DirectionJ plugin). [0188] In certain embodiments, the first micropatterned structure is selected from the group consisting of pillars, a cross and derivatives thereof, a lamella and combinations thereof. In some embodiments, the cross and derivatives thereof of the first micropatterned structure further comprises one or more lines extending from the intersection of the cross. In some embodiments, the cross and derivatives thereof of the first micropatterned structure is enclosed. In some embodiments, the cross and derivatives thereof of the first micropatterned structure is enclosed in the form selected from the group consisting of a square, quadrilateral, circle and irregular shape. In preferred embodiments, the first micropatterned structure are pillars. [0189] In certain embodiments, the second micropatterned structure is selected from the group consisting of pillars, a cross and derivatives thereof, a lamella and combinations thereof. In some embodiments, the cross and derivatives thereof of the second micropatterned structure further comprises one or more lines extending from the intersection of the cross. In some embodiments, the cross and derivatives thereof of the second micropatterned structure is enclosed. In some embodiments, the cross and derivatives thereof of the second micropatterned structure is enclosed in the form selected from the group consisting of a square, quadrilateral, circle and irregular shape. [0190] In certain embodiments, the third micropatterned structure is selected from the group consisting of pillars, a cross and derivatives thereof, a lamella and combinations thereof. In some embodiments, the cross and derivatives thereof of the third micropatterned structure further comprises one or more lines extending from the intersection of the cross. In some embodiments, the cross and derivatives thereof of the third micropatterned structure is enclosed. In some embodiments, the cross and derivatives thereof of the third micropatterned structure is enclosed in the form selected from the group consisting of a square, quadrilateral, circle and irregular shape. In preferred embodiments, the third micropatterned structure is in the form of a cross and derivatives thereof. In preferred embodiments, the cross and derivatives thereof is enclosed. [0191] As would be understood by a skilled addressee, each surface (the surface for each first, second and third micropatterned structure) can independently comprise a plurality of domains. Each domain can independently comprise a micropatterned structure selected from the group consisting of pillars, a cross and derivatives thereof, a lamella, and combinations thereof. In some embodiments, the domain consists of a single form of the micropatterned structure, for example, pillars or lamella only. [0192] For at least one of the first, second and third micropatterned structures, when the micropatterned structure is in the form of pillars, the pillars can have a cross-section having a shape selected from the group consisting of a circle, ellipse, quadrilateral, trapezoid, square, rectangular, triangular, star-shaped, scutoid, irregular shaped and combinations thereof. In preferred embodiments, the pillars have a cross-section having a shape of an ellipse. [0193] In certain embodiments, the annulus channel comprises an inner annulus channel and an outer annulus channel in fluid communication. [0194] The micropatterned structure of the inner annulus channel and the micropatterned structure can have any relative orientation. In some embodiments, the orientation of the micropatterned structure of the inner annulus channel and the micropatterned structure of the outer annulus channel is between about 5° to about 175° about an axis of rotation. In some embodiments, the orientation of the micropatterned structure of the inner annulus channel and the micropatterned structure of the outer annulus channel is between about 30° to about 120° about an axis of rotation. In some embodiments, the orientation of the micropatterned structure of the inner annulus channel and the micropatterned structure of the outer annulus channel is between about 60° to about 120° about an axis of rotation. In some embodiments, the orientation of the micropatterned structure of the inner annulus channel and the micropatterned structure of the outer annulus channel is between about 80° to about 110° about an axis of rotation. In some embodiments, the orientation of the micropatterned structure of the inner annulus channel and the micropatterned structure of the outer annulus channel is about 90° about an axis of rotation. [0195] In preferred embodiments, the orientation of the pillars of the micropatterned structure of the inner annulus channel and the pillars of the micropatterned structure of the outer annulus channel is between about 30° to about 120° about an axis of rotation. In preferred embodiments, the orientation of the pillars of the micropatterned structure of the inner annulus channel and the pillars of the micropatterned structure of the outer annulus channel is between about 60° to about 120° about an axis of rotation. In preferred embodiments, the orientation of the pillars of the micropatterned structure of the inner annulus channel and the pillars of the micropatterned structure of the outer annulus channel is about 90° about an axis of rotation. [0196] For each of the first, second and third micropatterned structures, when the structure comprises a lamella, the lamella can be independently planar or non-planar. For each of the first, second and third micropatterned structures, when the structure comprises a plurality of crosses and derivatives thereof, the crosses and derivatives thereof can be interconnected. For each of the first, second and third micropatterned structures, when the structure comprises a plurality of crosses and derivatives thereof, the crosses and derivatives thereof can be arranged in a 2D array. [0197] As would be appreciated by the skilled addressee, each pillar can be any suitable dimension which can substantially recapitulate the IVD components. In some embodiments, the cross-sectional diameter of the pillar is between about 5 to about 35 µm, between about 5 to about 30 µm, between about 10 to about 35 µm, between about 10 to about 30 µm, between about 25 to about 35 µm, between about 5 to about 15 µm, between about 6 to about 14 µm, between about 8 to about 12 µm, between about 9 to about 11 µm, about between about 10 µm or about between about 30 µm. In some embodiments, the height of the pillar is between about 5 to about 15 µm, between about 6 to about 14 µm, between about 8 to about 12 µm, between about 9 to about 11 µm or about between about 10 µm. [0198] As would be appreciated by the skilled addressee, each ridge can be any suitable dimension which can substantially recapitulate the IVD components. In some embodiments, the width of the ridges is between about 0.5 to about 10 µm, between about 0.5 to about 8 µm, between about 0.5 to about 6 µm, between about 1 to about 6 µm, between about 2 to about 5 µm, about 2 µm or about 5 µm. In some embodiments, the height of the ridges is between about 0.5 to about 10 µm, between about 0.5 to about 8 µm, between about 0.5 to about 6 µm, between about 1 to about 6 µm, between about 2 to about 5 µm, about 2 µm or about 5 µm. [0199] As would be appreciated by the skilled addressee, each cross and derivative thereof can be any suitable dimension which can substantially recapitulate the IVD components. In some embodiments, the width of the cross (i.e., the lines forming the cross) is between about 0.5 to about 10 µm, between about 0.5 to about 8 µm, between about 0.5 to about 6 µm, between about 1 to about 6 µm, between about 2 to about 5 µm, about 2 µm or about 5 µm. [0200] The IVD-on-a-chip device may further comprise a microvalve to separate each channel to provide control of the flow of fluid between the channels. In preferred embodiments, the device comprises a plurality of microvalves. In some embodiments, a plurality of microvalves is disposed between each channel in fluid communication. Preferably, the microvalves separates the nucleus channel and the annulus channel. In some embodiments, the microvalves separates the inner annulus channel and the outer annulus channel. In preferred embodiments, the transition zone channel comprises a plurality of microvalves. The plurality of microvalves provides control over the gelation process and supporting fluid transfer during cell culture and perfusion during use of the device. [0201] In some embodiments, the arrangement of the microvalves can be extended to the interlamellar matrix (ILM) and partition boundaries (PB) regions to separate the ILM and/or PB from the adjacent lamella. This represents the addition of separate channels for these regions (PB and ILM) in some embodiments. [0202] In some embodiments, the microvalve is an active microvalve. In some embodiments, the microvalve is a passive microvalve. In preferred embodiments, the microvalve is a capillary microvalve. In preferred embodiments, the microvalve is in the form of a post. [0203] The post (microvalve) can have any suitable cross-sectional shape. In some embodiments, the post has a cross-section having a shape selected from the group consisting of a circle, ellipse, quadrilateral, trapezoid, square, rectangular, triangular, star-shaped, irregular shaped and combinations thereof. In preferred embodiments, the cross-section of the post is a trapezoid. [0204] The post can be any suitable dimension sufficient to control the flow of fluid between the channels. In some embodiments, the post has a length along at least one dimension of between about 10 to about 50 µm, between about 15 to about 40 µm, between about 20 to about 35 µm, between about 25 to about 35 µm or about 30 µm. In some embodiments, the post has a height of between about 5 to about 35 µm, between about 10 to about 30 µm, between about 15 to about 25 µm or about 20 µm. [0205] The nucleus channel and annulus channel can be configured to have any suitable shape and dimension. In some embodiments, the nucleus channel and annulus channel is independently configured to be in the shape of a circle, ellipse, quadrilateral, trapezoid, square, rectangular, triangular, star-shaped, irregular shaped and combinations thereof. In preferred embodiments, the nucleus channel and annulus channel is independently configured to be in the shape of a trapezoid. In this embodiment, a trapezoidal shaped channel minimises “dead” zones. Advantageously, the shape of a trapezoidal channel allows the fluid injected into the channels during use which can comprise a gel-cell mixture to efficiently flush the entire volume of the channels and minimises any dead zones, particularly, at the corners of the channels. In other words, the trapezoidal shape allows perfusion of cells and biomaterials with minimal dead zone at the corners of each channel. [0206] In preferred embodiments, each channel is independently adapted to provide an inlet and an outlet for perfusion. In further preferred embodiments, the inlet and outlet have been adapted to be provided at opposite ends of the channel. In some embodiments, the inlet and outlet is configured by providing inlet and outlet chambers for fluid flow, respectively. [0207] In some embodiments, the IVD-on-a-chip device is a closed-channel device. [0208] The annulus channel and nucleus channel can be adapted to have any suitable volume suitable for cell culture. In some embodiments, the annulus channel has a volume of between about 0.05 to about 1 mm³, between about 0.05 to about 0.8 mm³, between about 0.1 to about 0.8 mm³, between about 0.05 to about 0.6 mm³, between about 0.05 to about 0.5 mm³, between about 0.05 to about 0.4 mm³, between about 0.1 to about 0.3 mm³, between about 0.2 to about 0.3 mm³ or about 0.24 mm³. [0209] In some embodiments, the nucleus channel has a volume of between about 0.05 to about 1 mm³, between about 0.05 to about 0.8 mm³, between about 0.1 to about 0.8 mm³, between about 0.05 to about 0.6 mm³, between about 0.05 to about 0.5 mm³, between about 0.05 to about 0.4 mm³, between about 0.1 to about 0.3 mm³, between about 0.2 to about _{0.3 mm} ³ _{or about 0.24 mm} ³ _. [0210] The elastomeric body of the IVD-on-a-chip device can be made from any suitable elastomer. Suitable elastomers include epoxy, polystyrene, polycarbonate, polymethyl methacrylate, poly(ethylene glycol) diacrylate, cyclic olefin copolymer (COP), cyclic olefin, polyisoprene, polybutadiene, chloroprene, butyl rubber, styrene-butadiene, nitrile, ethylene propylene, epichlorohydrin, polyacrylic, fluorosilicone, silicone, polyethylene, polyurethane, neoprene, polysulfide and combinations thereof. In preferred embodiments, the elastomer is a silicone elastomer. In further preferred embodiments, the silicone elastomer is polydimethylsiloxane (PDMS). [0211] In some embodiments, the IVD-on-a-chip device further comprises sensors suitable for quantitative analysis. In certain embodiments, the device can be made in the form of a tray that has a few lines of devices on it. Method of fabricating IVD-on-a-chip device [0212] Some embodiments relate to a method of forming an IVD-on-a-chip device comprising the steps of: -casting a first elastomer on a channel template to form a channel layer; -casting a second elastomer on a patterning template to form a micropatterned structure layer; -curing the channel layer and the micropatterned structure layer; and -mating the channel layer and micropatterned structure layer to form an IVD-on-a-chip device comprising a nucleus channel having a surface comprising a first micropatterned structure and an annulus channel having a surface comprising a second micropatterned structure. [0213] As would be appreciated by a skilled addressee, the method of forming the device can be formed using any known method for forming elastomeric microfluidic devices. For example, exemplary methods of fabricating a microfluidic device is described in Friend and Yeo, Fabrication of microfluidic devices using polydimethylsiloxane, Biomicrofluidics, 2010, 026502 and Scott and Ali, Fabrication Methods for Microfluidic devices: An Overview, Micromachines, 2021, 319, the contents of which are incorporated herein by reference. [0214] In some embodiments,, the casting step in the method is performed by mixing a curing agent with an elastomer base. In certain embodiments, the first elastomer is a different material to the second elastomer. In preferred embodiments, the first elastomer and second elastomer are the same material. [0215] Typically, the curing step can be performed by placing the casted layers at elevated temperature for a prolonged duration. For example, the casted layers can be placed in an oven above 50 °C (such as 65 °C) for over 6 hours (typically for 12 hours or 24 hours). [0216] Any suitable technique can be used to fabricate the template (for example, patterning template and channel template) as described herein. As would be understood by a skilled addressee, the template is a mold which has a ‘negative’ pattern which when cast with an elastomer provides the desired structure of the resulting fabricated microfluidic device. The template can be formed using lithography (such as photolithography), etching, laser ablation, focused ion beam, machining, 3D printing and combinations thereof. In preferred embodiments, the patterning template is formed by 3D printing on a substrate. In some embodiments, the 3D printing is Digital Light Processing (DLP) 3D printing. In particularly preferred embodiments, the 3D printing technique is two-photon polymerisation. In preferred embodiments, the template is formed by 3D printing on a substrate. [0217] Briefly, two-photon polymerisation (2PP), also referred to as multiphoton polymerisation, is a multiphoton lithography or direct laser writing technique.2PP is an additive manufacturing technique that uses light (typically in the near-infrared range) to cure a liquid photoresin to create digitally defined 3D micropattern. The light causes the photoresin used to solidify only if its molecules simultaneously absorb the energy of two photons, a process known as two-photon absorption. Light and a molecular component, the photoinitiator, trigger a chemical reaction in the photoresin. Upon excitation by the light, the monomers in the liquid photoresin convert to a cross-linked solid state. This produces a solid, insoluble thermoset polymer forming the micropattern. [0218] Typically, thermosetting polymers are the primary material for 2PP. Any suitable material can be used in 2PP. For example, the photoresin used can be a monomer comprise a methacrylate group, acrylate group, vinyl group and combinations thereof. [0219] The substrate to form the each of the patterning template and channel template can be any suitable material. Exemplary materials to be used as the substrate for the patterning template and channel template include silicon (wafer), glass, ceramic, polymer and combinations thereof. The preferred substrate material is silicon wafer. [0220] There may be circumstances wherein the channel layer and micropatterned structure layer do not bond together due to their surface properties. For example, when the elastomer is PDMS, the elastomer can be hydrophobic in certain embodiments. In these embodiments, the elastomer can be pre-treated to assist bonding. In these embodiments, the channel layer and micropatterned structure layer are pre-treated such that the layers are bonded together when the mating step is performed. In certain embodiments, the pre- treatment step comprises plasma treatment. In preferred embodiments, the pre-treatment step comprises oxygen plasma treatment. [0221] In preferred embodiments, the channel template has been configured to form the device as described herein. In preferred embodiments, the patterning template has been configured to form the device as described herein. [0222] In some embodiments,, the method further comprises the step of forming an inlet and an outlet for perfusion. In particularly preferred embodiments, each channel is independently adapted to provide an inlet and an outlet for perfusion. In further preferred embodiments, the inlet and outlet have been adapted to be provided at opposite ends of the channel. In some embodiments, each inlet and outlet is configured by providing inlet and outlet chambers for fluid flow. [0223] Some embodiments relate to a kit comprising: a channel template adapted to form a channel layer; and a patterning template adapted to form a micropatterned structure layer; such that the templates can be used to form an IVD-on-a-chip device comprising a nucleus channel having a surface comprising a first micropatterned structure and an annulus channel having a surface comprising a second micropatterned structure. Method of using the IVD-on-a-chip device [0224] Some embodiments relate to a method of using an IVD- on-a-chip device as described herein for measuring a biological response, comprising the steps of: -seeding cells in the annulus channel and nucleus channel; -loading the annulus channel with a first collagen gel; -loading the nucleus channel with a second collagen gel having a different Young’s modulus to the first collagen gel; -performing perfusion with a biological medium through an inlet and an outlet; and -exposing the device to an external stimuli and measuring a biological response. Biological media/medium is also known as culture media or growth media commonly used for cell culture applications as would be known to a skilled addressee. [0225] Some embodiments relate to use of an IVD-on-a-chip device as described herein for an in-vitro organ model. [0226] Any suitable cells can be used in the IVD-on-a-chip device to substantially recapitulate the IVD. In some embodiments, the cells are selected from the group consisting of intervertebral disc cells (primary cells), mesenchymal stem cells, cancerous cell lines, fibroblast cells, any cell lines (e.g. L929 mouse fibroblast cells) and combinations thereof. In some embodiments, the intervertebral disc cells are selected from the group consisting of annulus fibrosus cells, nucleus pulposus cells, notochordal cells and combinations thereof. In preferred embodiments, the cells are derived from an animal, human and combinations thereof. In certain embodiments, the animal cells are derived from an ovine, a bovine, a canine, a mouse, and combinations thereof. [0227] In some embodiments, the ratio of cells in the annulus channel and nucleus channel is between about 1:1 to about 3:1, between about 1.5:1 to about 2.5:1, between about 2:1 to about 2.5:1 or about 2.25:1. [0228] In some embodiments, the amount of cells seeded in the annulus channel is between _{about 1.0×10} ⁴ _{to about 3.0×10} ⁴ _{cells, between about 1.5×10} ⁴ _{to about 3.0×10} ⁴ _cells, between about 2.0×10⁴ to about 3.0×10⁴ cells, between about 2.25×10⁴ to about 2.75×10⁴ cells or about 2.5×10⁴ cells. [0229] In some embodiments, the amount of cells seeded in the nucleus channel is between about 0.5×10⁴ to about 2×10⁴ cells, between about 0.8×10⁴ to about 1.5×10⁴ cells, between about 1.0×10⁴ to about 1.5×10⁴ cells, between about 1.1×10⁴ to about 1.3×10⁴ cells or about 1.2×10⁴ cells. IVD is a sparsely populated organ with 4 and 9×10⁶ cells.cm^-3 for the nucleus and annulus, respectively. To prevent nutrient depletion and channel occlusion by the growing cell mass, initial cell numbers to be seeded in each channel can be any number/density; however, the ratio of the seeded cells for annulus to nucleus regions should preferably remain at 2.25 (9×10⁶/4×10⁶). [0230] As would be appreciated by a skilled addressee, any suitable first and second collagen gels can be independently selected to substantially recapitulate the collagen fibres of the IVD. In preferred embodiments, the first and second collagen gels can be independently derived from an animal, human, synthetic source and combinations thereof. In certain embodiments, the first and second collagen gels are independently derived from an ovine, a bovine, a canine and combinations thereof. In preferred embodiments, the first and second collagen gels are derived from a bovine. The annulus-nucleus and annulus-transition zone tissue stiffness ratios are ≈1.88 and 1.53, respectively. Therefore, the use of any gels or combination of any gels that mimic the stiffness ratio is possible. The preferred gel is collagen. [0231] In some embodiments, the first collagen gels can further comprise cells (i.e., gel- cell mixture). In some embodiments, the second collagen gels can further comprise cells (i.e., gel- cell mixture). [0232] In some embodiments, the concentration of the first and second collagen gels are each independently between about 1 to about 15 mg/mL, between about 1 to about 12 mg/mL, between about 1 to about 10 mg/mL, between about 1 to about 10 mg/mL, between about 3 to about 10 mg/mL, between about 3 to about 8 mg/mL, between about 5 to about 7 mg/mL or about 6 mg/mL. [0233] In some embodiments, the Young’s modulus ratio of the first collagen gel and the second collagen gel is between about 1.5 to about 2.2, between about 1.6 to about 2.0, between about 1.7 to about 1.9, between about 1.8 to about 1.9, about 1.81 or about 1.88. [0234] Any suitable external stimuli can be used in the method. For example, the external stimuli can be selected from the group consisting of mechanical load, wear particles, active agent, temperature and combinations thereof. In other embodiments, the external stimuli may include cell culture media, growth factors, pharmaceutics for cell growth and stimulation, other chemicals to stimulate IVD disease (such as those changing/tunning pH, glucose level, oxygen level, or other nutrients level) as well as metal and polymer debris to simulate spinal implant- IVD cells interaction and mechanobiological assessments. [0235] The biological response during use of the IVD-on-a-chip device can be selected from the group consisting of a change in cell morphology, change in metabolite concentration, change in cell proliferation, change in cell viability, change in protein concentration, change in tissue formation (density of collagen and ECM to be produced by cells) and combinations thereof. In preferred embodiments, the biological response is measured using a biological assay. [0236] The biological response can be measured using a method selected from the group consisting of mass spectrometry, chromatography, gel electrophoresis, fluorescence spectroscopy, electron microscopy, atomic force microscopy, UV absorbance spectroscopy, rheology and combinations thereof. [0237] Suitable biological assays include using colorimetric assays, for example, MTT (3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay, XTT (2,3-bis-(2- methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5- carboxanilide) assay, MTS (3-(4,5- dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4- sulfophenyl)-2H-tetrazolium) assay, WST (Water-soluble Tetrazolium salts) assays or the like. Alternatively, the biological assays may be assessed using microscopy techniques with cell staining to differentiate between live and dead cells. [0238] The active agent may be any active agent that has a desired biological activity. The active agent may be a pharmaceutically active agent or a veterinary active agent. Potential active agents may include proteins or protein crystals, peptides, DNA, polymer-drug conjugates, drugs, nanoparticles e.g. magnetite, and quantum dots. [0239] As used herein the term “drug” refers a molecule, group of molecules, complex, substance or derivative thereof administered to an organism for diagnostic, therapeutic, preventative medical, or veterinary purposes. Other suitable drugs can include anti-viral agents, different growth factors, hormones, antibodies, or therapeutic proteins. Other drugs include prodrugs, which are agents that are not biologically active when administered but, upon administration to a subject are converted to drugs through metabolism or some other mechanism. [0240] Drugs can also specifically include nucleic acids and compounds comprising nucleic acids that produce a bioactive effect, for example deoxyribonucleic acid (DNA), ribonucleic acid (RNA), or mixtures or combinations thereof, including, for example, DNA nanoplexes. [0241] In certain embodiments, the perfusion step and exposing step is performed simultaneously. In certain embodiments, the perfusion step and exposing step is performed sequentially. [0242] The perfusion step can be performed at any suitable flow rate, cycle and volume per cycle. In some embodiments, the perfusion step is performed at a flow rate of between about 5 to about 150 µL/h, between about 5 to about 130 µL/h, between about 5 to about 110 µL/h, between about 5 to about 100 µL/h, between about 5 to about 80 µL/h, between about 5 to about 50 µL/h, between about 10 to about 50 µL/h, between about 10 to about 50 µL/h, between about 10 to about 40 µL/h, between about 20 to about 40 µL/h, between about 25 to about 35 µL/h or about 30 µL/h. [0243] In some embodiments, the perfusion step is performed at a cycle of between about 1 to about 15 perfusions per day, between about 3 to about 12 perfusions per day. In some embodiments, the perfusion step is performed at a cycle of 3 perfusions per day. In some embodiments, the perfusion step is performed at a cycle of 6 perfusions per day. In some embodiments, the perfusion step is performed at a cycle of 9 perfusions per day. In some embodiments, the perfusion step is performed at a cycle of 12 perfusions per day. [0244] In some embodiments, the perfusion step is performed at a volume per cycle of between about 5 to about 150 µL, between about 5 to about 130 µL, between about 5 to about 110 µL, between about 5 to about 100 µL, between about 5 to about 80 µL, between about 5 to about 50 µL, between about 10 to about 50 µL, between about 10 to about 50 µL, between about 10 to about 40 µL, between about 20 to about 40 µL, between about 25 to about 35 µL or about 30 µL. Rationale for the design of the IVD-on-a-chip device [0245] Modular design. The modular design (considering different separate but still connected channels for each region of the IVD-on a chip including AF, NP, TZ, ILM and PB) of the IVD-on-a-chip device can provide for single parameters to be adjusted independently of each other. This can make it possible, for example, to embed juvenile cells in the annulus or nucleus channels (or both) with the mechanical properties of older or degenerative tissues, or to embed cells from older donors in ‘young’ IVDs, allowing targeted investigations of the finely regulated interplay between mechanics, biomaterials, and cell biology. The IVD-on-a- chip device can make it possible to generate a variety of hypotheses and perform a range of proof- of-concept studies in-vitro. [0246] The IVD-on-a-chip device substantially represents the structural complexity of native IVDs. This innovative, reproducible and adaptable microfluidic model developed by the present inventors is a breakthrough in IVD research. By substantially recapitulating the structural complexity of the IVD (size, orientation, and distribution of collagen and elastic fibres) at the microscale, it can significantly improve the physiological relevance of experimental data and reduces associated costs. The IVD-on-a-chip device of another embodiment substantially represents the material stiffness gradient of native IVDs. This is through the perfusion of different gels into different channels with similar stiffness ratios often observed in native IVDs. The annulus-nucleus and annulus-transition zone tissue stiffness ratios are ≈1.88 and 1.53, respectively. [0247] Method for fabricating a closed-channel system. The conventional method to fabricate closed-channel microfluidic systems is to combine features and channels into one component and simply seal the channels with a microscope slide or a flat, silicon-based sheet. This method may be unsuitable to fabricate the present device (unless future innovations allow it) because it has small structural features (micropatterned structure) and high channel height (high aspect ratio), which creates a specific challenge of peeling silicon from 3D printed silicon wafers. Instead, this disclosure provides an innovative approach to create channels in a top layer (separated by microvalves to address specific challenge of fabricating distinct but continuous channels) and features in a bottom layer and their assembly. [0248] Method of fabrication. In preferred embodiments, high resolution 3D printing and 2- photon polymerisation. [0249] Biomechanically active model. The IVD mechanobiological studies using microfluidics require models that transmit key dynamic mechanical stimuli directly. To date, biomechanically active IVD-on-a-chip device models do not exist. Mechanical stimulation in organ-on-a-chip models (in general) is often implemented by fluid shear, acoustic waves and air pressure, which do not replicate the true dynamic micromechanical environment of tissues. These methods are often expensive (e.g. require highly-accurate micro-pumps) and unable to precisely replicate a wide range of loading scenarios or control load magnitudes. In some embodiments, the device may allow the application of direct micromechanical loading, offering a range of control not currently possible (i.e. displacement and load control, static and cyclic modes, various input waveforms such as ramp and sinusoidal, different strain rates, and load directions). [0250] The IVD-on-a-chip device can significantly contribute to the development of regulations (FDA, TGA) for the appraisal of the efficacy and safety of IVD tissue-engineering practices (i.e. drug and biomolecules screening) with low capital outlay. [0251] The development of an IVD-on-a-chip device as an in-vitro model with specific features (recapitulating the IVD intrinsic complexity, mechanical properties, and stiffness gradient) offers a unique in-vitro platform for IVD studies which is independent of availability, donor variability, comorbidities, and degeneration state. The present inventors believe this is a paradigm shift in IVD tissue engineering research. Examples [0252] Further details will now be described with reference to the following examples for illustrative purposes only. Example 1: IVD-on-a-chip device [0253] Referring to Figure 1, there is shown a representation of an embodiment of the intervertebral disc (IVD)-on-a-chip device. Figure 1 shows an intervertebral disc (IVD)-on-a- chip device (100) comprising an elastomer body (102) having at least a nucleus channel (104) and an annulus channel (106) in fluid communication. The nucleus channel (104) comprises at least one surface having a first micropatterned structure (108). The annulus channel (106) comprises at least one surface having a second micropatterned structure (110). The device (100) is a closed-channel device formed by bonding a top layer (112) with a bottom layer (114) to provide the annulus channel (106) and the nucleus channel (104) having the first micropatterned structure (108) and second micropatterned structure (110), respectively. [0254] The device has also been configured to provide a transition zone channel (116) in fluid communication between the nucleus channel (104) and the annulus channel (106). The transition zone channel (116) comprises at least one surface having a third micropatterned structure (118). [0255] The nucleus channel (104) and the annulus channel (106) of the device (100) has been configured to be in the shape of a trapezoidal channel such that the injected biological media and/or collagen gel or gel-cell mixtures can efficiently flush the entire volume of the nucleus channel (104) and the annulus channel (106) and minimise ‘dead’ zones (mainly at the four corners of the channels). Each channel has been configured to provide an inlet and outlet for perfusion, preferably, in the form of an inlet (120) and outlet chamber (122). [0256] The embodiment of Figure 1 provides a device (100) having four different regions with variable widths (width = 150, 150, 30, and 30 μm for the lamellae (L1 and L2), the ILM, and the PB, respectively). The volume of the annulus (106) and nucleus (104) channels are [500 (h) × 600 (w) × 800 (l) μm = 0.24 mm³], which is suitable for cell culture. [0257] In some embodiments, the nucleus channel (104) and the annulus channel (106) of the device (100) is separated by 50 μm. [0258] The transition zone channel (116) has dimensions of 50-100 (w) × 20 or 500 (h) μm having capillary microvalves (124) shaped in the form of a post. In other embodiments, channels can be various in terms of dimension, such as various channel widths while the chip maintain a similar ration between NP, TZ, and AF width in the native IVDs. The post (124) has a cross-sectional shape in the form of a trapezoid as shown in Figure 1 with a 20 to 30 μm gap width, or any dimension such that the ration between NP, TZ, and AF is substantially preserved, in the native IVDs between each post that connect channels while providing control over the gelation process and supporting fluid transfer during cell culture. This can provide control over the gelation process as different gels (collagen gels) with different stiffness can be injected into the nucleus channel (104) and the annulus channel (106) to resemble the stiffness gradient similar to those of the native IVD counterpart. Further, the transition zone channel (116) can facilitate fluid transfer after gelation and during cell culture. [0259] Typically, the size of the device and channels may vary as required from μm to mm ranges depending on the application. [0260] The bottom layer (114) has been configured to substantially recapitulate the structural features of the IVD including organisation, size, and distribution of tissue fibres with respect to the corresponding zones in the top layer (112). [0261] The closed-channel and modular design of the device (100) provides two separate channels to represent the IVD main regions, namely, the (AF) annulus fibrosus and (NP) nucleus pulposus. Each channel (the nucleus channel (104) and the annulus channel (106)) has individual inlet and outlet chambers for perfusion of different biomaterials, cells, and cell- biomaterial mixtures. The modular design allows mimicking of the stiffness gradient similar to the native counterpart of an IVD. [0262] Referring to Figure 2, there is shown a representation of an alternative embodiment of the intervertebral disc (IVD)-on-a-chip device. The numbering for Figure 2 is the same for that of Figure 1. Figure 2a shows a cross section of the device (100) having the top layer (112) and bottom layer (114) in its component form before being mated to form the device (100). In this embodiment, the annulus channel (106) has been configured to provide an inner annulus channel (106a) and an outer annulus channel (106b). Figure 2b shows a cross section of the device (100) having the top layer (112) and bottom layer (114) mated together via plasma bonding to form the device (100). [0263] Figure 2c shows an embodiment of the micropatterned structures of the bottom layer (114). The micropatterned structures can also be separated in domains as shown in Figure 2c. [0264] The differences as shown in Figure 2c between the outer and inner annulus are: a. Inner annulus has less compact micropatterned structures compared to the outer annulus (for both CS and IP lamella). The density of micropatterned structures is lower for the inner AF compared to the outer AF. b. Less complete (connecting one interlamellar matrix (ILM) to the adjacent ILM) partition boundaries is found in the inner compared to the outer annulus. [0265] Figures 2d and 2e show the schematic of the top layer (112) having square/rectangular shaped channels or trapezoidal shaped channels. Example 2: Micropatterned structures [0266] Figure 3 provides representations of embodiments of the first, second and/or third micropatterned structures. [0267] Figure 3a is a 3D design of the inner and outer micropatterned structure of the annulus channel (i.e., cross section lamella). The difference between the outer and inner annulus micropatterned structure in the form of pillars is the rotation (+45° and -45°, respectively) of the pillars as shown in Figure 2c. In this embodiment, the height of the pillars are 10 μm, the cross-sectional diameter of the pillars is 10 μm and the height and width of the lines as shown is 2 × 2 μm (up to 5 × 5 μm), respectively. [0268] In some embodiments, the size of the pillars for the nucleus channel is similar to the inner and outer pillars of the annulus channel (i.e., cross section lamella). The size of the larger pillars can be up to 25 μm for example as shown in Figure 2c. [0269] Figure 3b is a 3D design of the inner and outer micropatterned structure of the annulus channel (i.e., in-plane lamella). In this embodiment, the height and width of the lines as shown is 2 × 2 μm (up to 5 × 5 μm), respectively. [0270] Figure 3c is a 3D design of the interlamellar matrix inner and outer annulus layers and transition zone channel (between the annulus and nucleus channel). In this embodiment, the height and width of the lines as shown is 2 × 2 μm (up to 5 × 5 μm), respectively. [0271] Figure 4 shows an embodiment of the device for use as an in-vitro model. Active agents such as implant wear particles and disc cells can be used as an input to design and fabricate improved and safer spinal implants, for example. Further, stem and IVD cells can be used as an input for disc regeneration. Example 3: Method of fabricating device [0272] The IVD-on-a-chip device was prepared using 3D printing. The patterning template (channel template and micropatterned structure templates) was prepared by using high resolution 3D printing (Nanoscribe Photonic Professional GT) and two-photon polymerisation technology at the Australian National Fabrication Facility (Sydney). The 3D design for IVD features was prepared using ACAD and ACE3000 conversion software and the height of the features was 2 μm. Using IPS, a Nanoscribe commercial resin, 3D printing was performed on the surface of an oxygen plasma cleaned silicon wafer. The final step was the salinisation of 3D-printed silicon wafers for PDMS casting. [0273] The top and bottom layer was prepared using any suitable fabrication method as known to a skilled addressee (such as PDMS casting). [0274] The top and bottom layers are developed on the surface of separate silicon wafers to allow PDMS casting. The top and bottom layers are then plasma-bonded and a microfluidic chip aligning platform (for example but not limited to WH-AM-01, WenHao Ltd) is used to create a sealed closed-channel device. The use of the aligning platform (sealing parts under a microscope with a fine-tuning XYZ stage) ensures a proper sealing process with minimal risk of misalignment. The final area of the device (PDMS) will be set to 25 mm² (5 × 5 mm²). Example 4: Cell extraction [0275] Primary AF and NP cells were extracted using healthy ovine lumbar IVDs. It is important to note that the number of notochordal cells decreases rapidly after birth in human IVD and they are completely absent from the IVD by early adulthood. Ovine IVDs were selected in this example because they are among the few animals to lose the notochordal cells rapidly following birth similar to humans. In addition, the structural and biochemical similarities of ovine to human IVD make them a suitable candidate for in-vitro studies. [0276] To isolate primary cells, ovine IVD tissue (healthy) were dissected from the NP and AF regions separately, and digested initially in 2.5% (w/v) Pronase E (Sigma) solution for 1 h at 37 °C with subsequent overnight (16 h) digestion in 0.125% (w/v) collagenase (Worthington) solution in serum-free DMEM (Gibco) containing antibiotics. Isolated cells were labelled by CD166 marker and characterised using flow cytometry. [0277] Using the device and healthy ovine cells “NP and AF”, microfluidic perfusion cell culture was employed to identify the optimum conditions and develop a physiologically- relevant 3D IVD-on-a-chip model with cell morphologies being similar to the IVD. The impact of mechanical (strain rate), biomaterial (gel stiffness), and cell culture (perfusion flow rate, duration, cycle, and media volume) parameters can be investigated on the cell culture process. [0278] Two cell culture strategies (cell only and gel-cell mixture perfusions) were used having two loading scenarios (with and without load applications). Example 5: Cell culture optimisation [0279] The native IVD is a sparsely populated organ with 4 and 9×10⁶ cells/cm³ for the NP and AF, respectively. The AF cells are mostly fibroblast-like cells with a volume of ≈2700 _μm ³ _. [0280] The volume of the annulus channel (800×600×500 μm³) of the device is 240×10⁶ μm³ and, therefore, can accommodate approximately 8.8×10⁴ cells. However, to prevent nutrient depletion and channel occlusion by growing cell mass, seeding was performed with 2.5×10⁴ cells in the annulus channel(s) (scaled down to 30% of channel volume). Since the ratio of the AF to NP cells in human IVD is 2.25, the nucleus channel was seeded initially with 1.2×10⁴ cells, which occupies less than 3% of the channel volume (NP cells are spherical with approximately 10 μm diameter, making the volume of a single NP cell ≈ 530 μm³). [0281] Media perfusion is commenced after 12 hours of initial cell culture allowing cell adhesion and spread. A micropump is used to maintain the flow rate low enough (30 μL/h) to avoid generating high shear stress (<10^–3 dynes cm^–2) and the media volume per cycle is 30 μL. The cell culture process based on the number of cell culture media perfusion/day was set to 3, 6, 9, and 12 cycles/day, as required. [0282] A similar strategy is used for cell culture using gel-cell mixtures and the volume of gel and cells together is 144×10⁶ μm³ (60% of the channel volume). [0283] To optimise the cell culture process under different scenarios, cell viability, morphology, and fibrous tissue formation (for both the annulus and nucleus channels) can be evaluated for 7 days (days 1, 3, and 7). Test Procedure C o r d C T

Example 6: Gel-cell mixture culture [0284] Based on shear elastography, the average Young’s moduli for the NP and AF of native IVD is equal to 0.34 and 0.64 MPa, respectively. Two extracellular matrix (ECM) bovine collagen gels (TeloCol-6® and Nutragen® from Advanced Biometrix) were used to resemble similar Young’s modulus ratio for the annulus and nucleus channels of the device. [0285] Preliminary results showed that the average Young’s moduli for TeloCol-6® and Nutragen® gels (both 6mg/ml) measured by AFM (nano-indentation) were 3201±28 and 1759±16 kPa, respectively. [0286] Accordingly, the perfusion of Nutragen® in the nucleus channel and TeloCol-6® in the annulus channels create a composition gradient approximately similar to human IVD, since Young’s moduli ratios for the human IVD and the IVD-on-a-chip device are 1.88 and 1.81, respectively. [0287] Another rationale for selecting these collagen gels is gel formation under physiological conditions (37 °C for less than 30 min). Changes in gel moduli can occur during cell culture and Brillouin spectroscopy (non-contact) can be used for real-time measurement of gel-cell mixture stiffness in the device. Example 7: Mechanical Loading [0288] To simulate mechanical loading on the IVD-on-a-chip device, the following physiologically relevant mechanical loading scenarios were implemented (using Electroforce, TA instrument Ltd) while the device is stretched to 40% (320 μm) of the initial length (800 μm). S_{t i t %} ^{-1 Fr n L d tim nd tt rn D r ti n} 1 1

The mechanical actuation system is placed in a CO₂ incubator during the cell culture process. [0289] The IVD-on-a-chip in-vitro model can be used understand how cell viability and morphology are affected during different daily activities (at different spine positions). A reference for IVD biomechanical studies is provided in the table as follows: P L R S S L w

[0290] In the present Example, two loading scenarios (isolated and combined activities) were used to investigate the impact of spine positions (reflecting different daily activities) on IVD cell viability and morphology. Forces were measured in an IVD with an approximate volume of 2011×10 mm³; however, the volume of the device is 25×1 mm³. Accordingly, corresponding IVD-on-a-chip device forces are scaled down by a factor of 1.2 ×10^-3 (normalizing by volume) to resemble similar native IVD forces. [0291] Loading Scenario 1 (Isolated activity): Compressive IVD-on-a-chip device forces, relevant to each position are applied to the device for 250 s. [0292] Loading Scenario 2 (Combined activities - most common daily spine positions) as follows: C a p R ⁰ s s C ₀ p s ( N R C 0 p s ( N R L

ying prone) For both loading scenarios, cell viability and morphology will be evaluated [0293] Mechanical stimulation in organ-on-a-chip models often implemented by fluid shear, acoustic waves, and air pressure do not replicate the true dynamic micromechanical environment of tissues. Despite their success, these conventional methods are often expensive (i.e. require highly-accurate micro-pumps) and are unable to precisely control different loading scenarios. [0294] Of particular importance, these conventional approaches can only control strain magnitudes. The method of the present Example offers a wide range of control for mechanical loading (displacement and load control, static and cyclic modes, various input waveforms such as ramp, sinusoidal, different strain rates, and load directions) which allows the application of different loading scenarios for numerous applications. [0295] Some embodiments relate to the following clauses: Clause 1: An intervertebral disc (IVD)-on-a-chip device comprising: an elastomer body having at least a nucleus channel and an annulus channel in fluid communication, wherein the nucleus channel comprises at least one surface having a first micropatterned structure; wherein the annulus channel comprises at least one surface having a second micropatterned structure; and wherein the device is adapted to comprise an inlet and an outlet for perfusion. Clause 2: An IVD-on-a-chip device according to clause 1, wherein the first micropatterned structure comprises a micropattern. Clause 3: An IVD-on-a-chip device according to clause 1 or 2, wherein the first micropatterned structure is derived from a template. Clause 4: An IVD-on-a-chip device according to any one of clauses 1 to 3, wherein the first micropatterned structure is derived from a template formed by 3D printing. Clause 5: An IVD-on-a-chip device according to any one of clauses 1 to 4, wherein the second micropatterned structure comprises a micropattern. Clause 6: An IVD-on-a-chip device according to any one of clauses 1 to 5, wherein the second micropatterned structure is derived from a template. Clause 7: An IVD-on-a-chip device according to any one of clauses 1 to 6, wherein the second micropatterned structure is derived from a template formed by 3D printing. Clause 8: An IVD-on-a-chip device according to any one of clauses 1 to 7, wherein the annulus channel comprises an inner annulus channel and an outer annulus channel in fluid communication. Clause 9: An IVD-on-a-chip device according to any one of clauses 1 to 8, wherein the device further comprises a transition zone channel in fluid communication between the nucleus channel and the annulus channel. Clause 10: An IVD-on-a-chip device according to claim 9, wherein the transition zone channel comprises at least one surface having a third micropatterned structure. Clause 11: An IVD-on-a-chip device according to claim 10, wherein the third micropatterned structure comprises a micropattern. Clause 12: An IVD-on-a-chip device according to claim 10 or 11, wherein the third micropatterned structure is derived from a template. Clause 13: An IVD-on-a-chip device according to any one of clauses 10 to 12, wherein the third micropatterned structure is derived from a template formed by 3D printing. Clause 14: An IVD-on-a-chip device according to any one of clauses 1 to 13, wherein the micropatterned structure is selected from the group consisting of pillars, a cross and derivatives thereof, a lamella and combinations thereof. Clause 15: An IVD-on-a-chip device according to claim 14, wherein the cross and derivatives thereof further comprises one or more lines extending from the intersection of the cross. Clause 16: An IVD-on-a-chip device according to claim 15, wherein the cross and derivatives thereof is enclosed. Clause 17: An IVD-on-a-chip device according to any one of clauses 14 to 16, wherein the pillars have a cross-section having a shape selected from the group consisting of a circle, ellipse, quadrilateral, trapezoid, square, rectangular, triangular, star-shaped, irregular shaped and combinations thereof. Clause 18: An IVD-on-a-chip device according to any one of clauses 1 to 17, wherein each surface independently comprises a plurality of domains. Clause 19: An IVD-on-a-chip device according to claim 18, wherein each domain can independently comprise a micropatterned structure selected from the group consisting of pillars, a cross and derivatives thereof, a lamella, and combinations thereof. Clause 20: An IVD-on-a-chip device according to any one of clauses 8 to 19, wherein the orientation of the micropatterned structure of the inner annulus channel and the micropatterned structure of the outer annulus channel is between about 5° to about 175° about an axis of rotation. Clause 21: An IVD-on-a-chip device according to any one of clauses 1 to 20, wherein the device comprises a microvalve. Clause 22: An IVD-on-a-chip device according to claim 21, wherein a plurality of microvalves is disposed between each channel in fluid communication. Clause 23: An IVD-on-a-chip device according to any one of clauses 20 to 22, wherein the microvalve is in the form of a post. Clause 24: An IVD-on-a-chip device according to claim 23, wherein the post has a cross- section having a shape selected from the group consisting of a circle, ellipse, quadrilateral, trapezoid, square, rectangular, triangular, star-shaped, irregular shaped and combinations thereof. Clause 25: An IVD-on-a-chip device according to any one of clauses 1 to 24, wherein the nucleus channel and annulus channel is independently configured to be in the shape of a circle, ellipse, quadrilateral, trapezoid, square, rectangular, triangular, star-shaped, irregular shaped and combinations thereof. Clause 26: An IVD-on-a-chip device according to any one of clauses 1 to 25, wherein the elastomer is epoxy, polystyrene, polycarbonate, polymethyl methacrylate, poly(ethylene glycol) diacrylate, cyclic olefin copolymer (COP), cyclic olefin, polyisoprene, polybutadiene, chloroprene, butyl rubber, styrene-butadiene, nitrile, ethylene propylene, epichlorohydrin, polyacrylic, fluorosilicone, silicone, polyethylene, polyurethane, neoprene, polysulfide and combinations thereof. Clause 27: An IVD-on-a-chip device according to claim 26, wherein the elastomer is a silicone elastomer. Clause 28: An IVD-on-a-chip device according to claim 27, wherein the silicone elastomer is polydimethylsiloxane (PDMS). Clause 29: An IVD-on-a-chip device according to any one of clauses 1 to 28, wherein each channel is independently adapted to provide an inlet and an outlet for perfusion. Clause 30: A method of forming an IVD-on-a-chip device comprising the steps of: -casting a first elastomer on a channel template to form a channel layer; -casting a second elastomer on a patterning template to form a micropatterned structure layer; -curing the channel layer and the micropatterned structure layer; and -mating the channel layer and micropatterned structure layer to form an IVD-on-a- chip device comprising a nucleus channel having a surface comprising a first micropatterned structure and an annulus channel having a surface comprising a second micropatterned structure. Clause 31: A method according to claim 30, wherein the patterning template is formed using lithography (such as photolithography), etching, laser ablation, focused ion beam, machining, 3D printing and combinations thereof. Clause 32: A method according to claim 31, wherein the patterning template is formed by 3D printing on a substrate. Clause 33: A method according to claim 32, wherein the 3D printing is two- photon polymerisation. Clause 34: A method according to any one of clauses 30 to 33, wherein the channel template is formed using lithography (such as photolithography), etching, laser ablation, focused ion beam, machining, 3D printing and combinations thereof. Clause 35: A method according to claim 34, wherein the channel template is formed by 3D printing on a substrate. Clause 36: A method according to claim 35, wherein the 3D printing is two- photon polymerisation. Clause 37: A method according to any one of clauses 30 to 36, wherein the channel layer and micropatterned structure layer are pre-treated such that the layers are bonded together when the mating step is performed. Clause 38: A method according to claim 37, wherein the pre-treatment step comprises oxygen plasma treatment. Clause 39: A method according to any one of clauses 30 to 38, wherein the casting steps is performed by mixing a curing agent with an elastomer base. Clause 40: A method according to any one of clauses 30 to 39, wherein the channel template has been configured to form the device according to any one of clauses 1 to 29. Clause 41: A method according to any one of clauses 30 to 40, wherein the patterning template has been configured to form the device according to any one of clauses 1 to 29. Clause 42: A method according to any one of clauses 30 to 41, further comprising the step of forming an inlet and an outlet for perfusion. Clause 43: A method of using an IVD-on-a-chip device according to any one of clauses 1 to 29 for measuring a biological response, comprising the steps of: -seeding cells in the annulus channel and nucleus channel; -loading the annulus channel with a first collagen gel; -loading the nucleus channel with a second collagen gel having a different Young’s modulus to the first collagen gel; -performing perfusion with a biological medium through an inlet and an outlet; and -exposing the device to an external stimuli and measuring a biological response. Clause 44: A method according to claim 43, wherein the cells are selected from the group consisting of intervertebral disc cells, mesenchymal stem cells and combinations thereof. Clause 45: A method according to claim 44, wherein the intervertebral disc cells are selected from the group consisting of annulus fibrosus cells, nucleus pulposus cells, notochordal cells and combinations thereof. Clause 46: A method according to any one of clauses 43 to 45, wherein the cells are derived from an animal, human and combinations thereof. Clause 47: A method according to claim 46, wherein the cells are derived from an ovine, a bovine, a canine and combinations thereof. Clause 48: A method according to any one of clauses 43 to 47, wherein the Young’s modulus ratio of the first collagen gel and the second collagen gel is between about 1.7 to 1.9. Clause 49: A method according to any one of clauses 43 to 48, wherein the external stimuli is selected from the group consisting of mechanical load, wear particles, active agent, temperature and combinations thereof. Clause 50: A method according to any one of clauses 43 to 49, wherein the biological response is selected from the group consisting of a change in cell morphology, change in metabolite concentration, change in cell proliferation, change in cell viability, change in protein concentration and combinations thereof. Clause 51: A method according to any one of clauses 43 to 50, wherein the biological response is measured using a biological assay. Clause 51: A method according to any one of clauses 43 to 50, wherein the biological response is measured using a method selected from the group consisting of mass spectrometry, chromatography, gel electrophoresis, fluorescence spectroscopy, flow cytometry, electron microscopy, atomic force microscopy, UV absorbance spectroscopy, rheology and combinations thereof. Clause 52: A method according to any one of clauses 43 to 52, wherein the perfusion step and exposing step is performed simultaneously. Clause 53: Use of an IVD-on-a-chip device according to any one of clauses 1 to 29 for an in-vitro organ model. Clause 54: A kit comprising: a channel template adapted to form a channel layer; and a patterning template adapted to form a micropatterned structure layer; such that the templates can be used to form an IVD-on-a-chip device comprising a nucleus channel having a surface comprising a first micropatterned structure and an annulus channel having a surface comprising a second micropatterned structure. [0296] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS: 1. A microfluidic chip comprising a body defining: a channel extending between an inlet and an outlet; and a microstructure formed by a plurality of protrusions extending away from a surface of the channel, wherein the microstructure is configured to substantially recapitulate one or more structural characteristics of a target natural biological tissue.

2. The microfluidic chip of claim 1, wherein the channel is a first channel, the inlet is a first inlet, the outlet is a first outlet, the microstructure is a first microstructure, the plurality of protrusions is a first plurality of protrusions, wherein the body further defines: a second channel extending between a second inlet and a second outlet; and a second microstructure formed by a second plurality of protrusions extending away from a surface of the second channel; and wherein the first and second microstructures are configured to substantially recapitulate one or more structural characteristics of different target natural biological tissues.

3. The microfluidic chip of claim 1, wherein the microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in part of the target natural biological tissue, and wherein the protrusions of the microstructure include relatively wider collagen fibre bundle protrusions recapitulating collagen fibre bundles, and relatively narrower elastic fibre protrusions recapitulating elastic fibres.

4. The microfluidic chip of claim 3, wherein the average width of the protrusions recapitulating collagen fibre bundles is at least 50% wider than the average width of the protrusions recapitulating the elastic fibres.

5. The microfluidic chip of claim 3 or 4, wherein the microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in part of the annulus fibrosus of a natural intervertebral disc in a plane extending through a radial axis of the disc, wherein the microstructure includes a lamella zone corresponding to a lamella of the annulus fibrosus, and wherein the lamella zone includes an array of collagen fibre bundle protrusions with elastic fibre protrusions in the form of ridges extending between different regions of the array of collagen fibre bundle protrusions to form partition boundary zones.

6. The microfluidic chip of claim 5, at least some of the partition boundary zones comprise elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons separating different regions of the array of collagen fibre bundle protrusions.

7. The microfluidic chip of claim 5 or 6, wherein the lamella zone is configured to recapitulate part of the lamella in a plane extending through the radial axis of the disc and intersecting the collagen fibre bundles, and the collagen fibre bundle protrusions comprise elliptical prisms with the eccentricity of the elliptical prisms selected based on the angle of intersection with the plane.

8. The microfluidic chip of any one of claims 5 to 6, wherein the lamella zone is configured to recapitulate part of the lamella in a plane extending through the radial axis of the disc and inclined relative to the transverse plane of the disc to extend parallel to collagen fibre bundles, and the collagen fibre bundle protrusions comprise parallel ridges.

9. The microfluidic chip of any one of claims 5 to 8, wherein the lamella zone is a first lamella zone and the microstructure further includes a second lamella zone and an interlamellar matrix zone between the first and second lamella zones, corresponding to first and second lamellae, and interlamellar matrix of the disc, respectively, wherein the protrusions of the second lamella zone include an array of collagen fibre bundle protrusions with elastic fibre protrusions in the form of ridges extending between different regions of the array of collagen fibre bundle protrusions to form partition boundary zone, and wherein the protrusions of the interlamellar matrix zone include elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons extending across the interlamellar matrix zone between the first and second lamella zones.

10. The microfluidic chip of claim 9, wherein at least 50% of the ridges of the interlamellar matrix zone are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° of parallel relative to the reference axis; within 10° of 45° relative to the reference axis; and within 10° of perpendicular relative to the reference axis.

11. The microfluidic chip of claim 9 or 10, wherein the plane represented by the microstructure of the microfluidic chip extends through the radial axis of the disc and is inclined relative to the transverse plane of the disc to extend parallel to collagen fibre bundles of the first lamella of the annulus fibrosus, wherein the collagen fibre bundle protrusions of the first lamella zone comprise elongate ridges extending parallel to the interlamellar matrix zone, the ridges substantially recapitulating the in-plane oriented collagen fibre bundles of the first lamella of the disc, and wherein the collagen fibre bundle protrusions of the second lamella zone comprise an array of pillars substantially recapitulating the out-of-plane oriented collagen fibre bundles of the second lamella of the disc.

12. The microfluidic chip of any one of claims 5 to 11, wherein at least 50% of the elastic fibre ridges in the partition boundary zones are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° of parallel relative to the reference axis; within 10° of 45° relative to the reference axis; and within 10° of perpendicular relative to the reference axis.

13. The microfluidic chip of any one of claims 5 to 12, wherein the channel is a first channel, the inlet is a first inlet, the outlet is a first outlet, the microstructure is a first microstructure, the plurality of protrusions is a first plurality of protrusions, wherein the body further defines: a second channel extending between a second inlet and a second outlet; and a second microstructure formed by a second plurality of protrusions extending away from a surface of the second channel; and wherein the second microstructure is configured to substantially recapitulate the size, distribution and orientation of collagen fibre bundles and elastic fibres in the nucleus pulposus of a natural intervertebral disc in a plane extending through a radial axis of the disc and inclined relative to the central axis of the disc.

14. The microfluidic chip of claim 13, wherein the second microstructure comprises similar features to the first microstructure defined in any one of claim 5 to 12 with the protrusions arranged in closer proximity to each other forming a relatively higher density of protrusions compared with the first microstructure.

15. The microfluidic chip of claim 13, wherein the second microstructure includes a core zone on one side of the channel and a peripheral zone extending partially around the core zone on another side of the channel, wherein the protrusions of the core zone comprise an array of pillars corresponding to out-of-plane oriented collagen fibre bundles of the nucleus pulposus, and wherein the protrusions of the peripheral zone comprise an array of pillars corresponding to out-of-plane oriented elastic fibres of the nucleus pulposus.

16. The microfluidic chip of claim 15, wherein the second microstructure includes elastic fibre ridges radiating away from the core zone at different angles relative to each other, corresponding to in-plane elastic fibres.

17. The microfluidic chip of claim 2 or any one of claims 13 to 16, wherein the body further defines a plurality of microvalves arranged between and configured to selectively allow fluid communication between the first and second channels.

18. The microfluidic chip of any one of claims 14 to 16, wherein the body further defines: a third channel extending between a third inlet and a third outlet, and located between the first and second channels; and a third microstructure formed by a third plurality of protrusions extending away from a surface of the third channel; wherein the third microstructure is configured to substantially recapitulate one or more structural characteristics of the natural intervertebral disc.

19. The microfluidic chip of claim 18, wherein the body further defines a plurality of microvalves arranged between and configured to selectively allow fluid communication between the first and third channels, and between the second and third channels.

20. The microfluidic chip of claim 18 or 19, wherein the third microstructure is configured as a transition zone to substantially recapitulate the size, distribution and orientation of elastic fibres in the transition zone of the natural intervertebral disc between the nucleus pulposus and the annulus fibrosus.

21. The microfluidic chip of claim 20, wherein the protrusions of the third microstructure include elastic fibre protrusions arranged to form a network of ridges extending in different directions to form interconnected polygons extending across the transition zone.

22. The microfluidic chip of claim 21, wherein at least 50% of the ridges of the transition zone are oriented at angles selected from the following ranges relative to a reference axis corresponding to the radial axis of the disc: within 10° of parallel relative to the reference axis; within 10° of 45° relative to the reference axis; and within 10° of perpendicular relative to the reference axis.

23. The microfluidic chip of any one of claims 1 to 22, wherein the body is formed of an elastomeric material.

24. A method of forming an in-vitro organ model, the method comprising: loading biological cells onto the protrusions of a microfluidic chip according to any one of claims 1 to 23; and perfusing the biological cells with a biological medium.

25. The method of claim 24, further comprising loading a gel onto the protrusions of the microfluidic chip.

26. The method of claim 24 or 25, wherein the biological cells comprise healthy or degenerated intervertebral disc cells selected from the group consisting of annulus fibrosus cells, nucleus pulposus cells, notochordal cells and combinations thereof.

27. A method of forming an in-vitro intervertebral disc model, the method comprising: loading annulus fibrosus cells into the first channel of a microfluidic chip according to any one of claims 13 to 23; loading nucleus pulposus cells into the second channel of the microfluidic chip; and perfusing the annulus fibrosus cells and nucleus pulposus cells with a biological medium.

28. The method of claim 27, further comprising: loading a first gel into the first channel; and loading a second gel into the second channel, wherein the stiffness of the first gel is at least 50% higher than the stiffness of the second gel.

29. The method of claim 27 or 28, further comprising: exposing the microfluidic chip to an external stimuli and measuring a corresponding biological response.

30. The method of claim 29, wherein the external stimuli comprises one or more stimuli selected from the group consisting of: applying a mechanical load to the microfluidic chip; applying a temperature variations to the microfluidic chip; introducing wear particles to the first or second channel; and introducing an active agent to the first or second channel.

31. The method of claim 29 or 30, wherein the biological response is measured using one or more methods selected from the group consisting of: mass spectrometry, chromatography, gel electrophoresis, fluorescence spectroscopy, flow cytometry, electron microscopy, atomic force microscopy, UV absorbance spectroscopy, and rheology.

32. A method of fabricating the microfluidic chip of any one of claims 1 to 23, the method comprising: casting a first part of the body defining the or each microstructure in a microstructure template defining recesses corresponding to the protrusions of the or each microstructure; forming a second part of the body defining the or each channel, inlet and outlet; and connecting the first part to the second part to form the microfluidic chip.

33. The method of claim 32, wherein the microstructure template has been formed using one or more techniques selected from the group comprising: lithography, photolithography, etching, laser ablation, focused ion beam, machining, 3D printing, and Digital Light Processing (DLP) 3D printing.

34. The method of claim 33, wherein the microstructure template has been formed by 3D printing using two-photon polymerisation.

35. The method of any one of claims 32 to 34, wherein the first and second parts are cast in a curable elastomeric material.