WO2008127438A2

WO2008127438A2 - Parallel flow control (pfc) approach for active control, characterization, and manipulation of nanofluidics

Info

Publication number: WO2008127438A2
Application number: PCT/US2007/085630
Authority: WO
Inventors: Stephen J. Fonash; Wook Jun Nam; Huinan Liang
Original assignee: Penn State Research Foundation
Current assignee: Penn State Research Foundation
Priority date: 2006-11-27
Filing date: 2007-11-27
Publication date: 2008-10-23
Anticipated expiration: 2009-05-27
Also published as: WO2008127438A3

Abstract

A method of active nanofluidic flow control (parallel flow control-PFC) includes providing a nanofluidic channel and a pressure-driven microfluidic channel connected in parallel and actively controlling flow through the nanofluidic channel by using the pressure-driven microfluidic channel. A method of nanofluidic flow measurement includes providing a nanofluidic channel, a pressure-driven microfluidic channel connected in parallel for flow control, and an additional measurement microfluidic channel connected in series for flow measurement and measuring the nanofluidic flow rate by measuring the filling rate in the measurement microfluidic channel. A method of nano-scale volume fluid manipulation includes providing a nanofluidic channel and a pressure-driven microfluidic channel connected in parallel and manipulating nano-scale volume fluid through the nanofluidic channel by using the pressure-driven microfluidic channel. A method of fabricating a fluidic system is provided. The method includes forming a nanofluidic channel and a pressure-driven microfluidic channel connected to the nanofluidic channel in parallel.

Description

TITLE: PARALLEL FLOW CONTROL (PFC) APPROACH FOR ACTIVE

CONTROL, CHARACTERIZATION, AND MANIPULATION OF NANOFLUIDICS

RELATED APPLICATION

This applications claims priority to U.S. Provisional Patent Application No. 60/867, 293, entitled Active Nanofluidic Flow Control through Microfluidic Flow, filed November 27, 2006, herein incorporated by reference in its entirety.

GRANT REFERENCE

This application was funded in part under NSF Grant No. DMI-0615579. The Government may have certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to nanofluidics, including the novel parallel flow control (PFC) approach for nano-scale fluidics control, characterization, and manipulation thereof.

BACKGROUND OF THE INVENTION A major research interest in analytical technology is to develop a highly integrated and miniaturized platform incorporating fluid manipulation. In the past two decades, significant research efforts have been put towards the development of micro-total analysis systems (wTAS) [1, 2] and, more recently, toward the development of miniaturized chemical process systems [3, 4, 5]. These systems offer "on-chip" processing, sensing, or both which integrate micro fluidics thereby giving higher throughput and control [6]. With the rapid development of nanotechnology, these systems have evolved to incorporating nanofluidics in which the nanofluidic system fabrication has been realized through various patterning techniques [7]. Nanofluidics, which is generally ascribed to fluidic flow in channels with at least one cross-sectional dimension of about 100 nm or less, offers even more precise fluid control over the traditional microfluidic systems. With nanofluidics, samples used in sensing, chemical reactions, or both, can now even be in the pL range or smaller. Therefore, chemical, medical, and biological analysis/detection systems as well as chemical processing systems are now rapidly moving toward the use of nanofluidics to exploit smaller sample size, potentially more precise flow control, and new sensing and chemical reaction approaches [8-12]. However, there are key issues that must be addressed to reach full control of nanofluidics for the successful integration and application of nano-scale fluidic systems into sensing and chemical processing platforms. One of the key issues is communicating with "outside world" and developing the actual means of nano fluidic flow control and characterization. Due to the extremely small nano-scale (and below) volumes of nanofluidic systems, this interfacing becomes a main bottleneck to further development. At present, molecular flow and exchange in most of the nanofluidic systems are based on an inactive diffusion mode set up by concentration gradients [13, 14] which will be impracticable for driving large molecules with extremely small diffusion coefficients. In such systems, a small trapped air bubble has the possibility of blocking all effective flow. Active flow overcomes the drawbacks of diffusion flow by using some other means to drive flow. The general approach for nano-scale active flow is to have a nanochannel in communication with a microchannel which is, in turn, in communication with a reservoir. With proper surface treatments, fluid in the reservoir wets and fills the microchannel and capillary action then allows this fluid to fill the nanochannel. After the filling of the full fluidic system, active flow can be established by one of three ways [15, 16, 17]: (1) electroosmosis, (2) electrophoresis, or (3) pressure gradients. Of these, pressure-driven flow is more general since it does not necessitate the presence of charged entities and therefore does not rely on the complexities of their motion. The use of a direct, pump-imposed, pressure gradient across a nanochannel has been tried (See FIG. IA) but tends to be unstable for the pumps commonly used (e.g., syringes) and leads to very long sample residence times in the series-connected microchannels [17]. A different configuration using independent flow in each of connecting microchannels has been explored recently to create a microchannel flow-caused pressure gradient across a nanochannel, as seen in FIG. IB [18]. This approach allows the residence times in the microchannels to be independent of the pressure gradient across the nanochannel, permitting very short residence times in the microchannels. The up-stream reservoir and microchannel, then the nanochannel, and finally the down-stream microchannel are sequentially filled to avoid air bubble problems in this configuration. However, the approach requires multiple syringe pumps and back pressure control may be needed resulting in system complexity. Another key issue beyond interfacing and establishing active flow is having the ability to screen and measure the actual flow rate through the nanofluidic channel in a fluidic system. This is fundamental for full nanofluidics control and application. Current efforts at achieving this characterization (i.e., measurement) accuracy have resulted in a reported measuring resolution of nano liters per minute [19]. Means to improve this characterization capability are needed and must be attained in a direct, accessible, and straightforward way without involving system complexity issues. Attempts to achieve this characterization to-date have included methods such as extremely sensitive balance approaches [20], or sophisticated particle imaging velocity (PIV) techniques [21]. To reach the full potential of nanofluidics, systems must be as simple as possible and yet be able to drive, characterize, and manipulate nano-scale volumes of fluid through the system while taking advantage of the opportunities available through the ability to cope with limited sample volumes.

This invention addresses these needs required to attain active control, characterization and manipulation for nanofluidics. In particular this invention gives a method and a system that provides for interfacing between nanofluidic systems and external macro fluidic systems, provides for screening and measuring the actual flow rate through a nanofluidic channel, and provides for the ability to manipulate nano-scale volumes of fluid through a system.

SUMMARY OF THE INVENTION The present invention provides an active nanofluidic flow control approach which we term parallel flow control (PFC). PFC employs a microfluidic flow which interfaces with the "outside world" to set up the pressure gradient across a nanofluidic channel and to thereby realize fine nanofluidic flow control. In addition, the present invention provides two direct, accessible, and straightforward approaches to measuring nanofluidic flow rate with high accuracy. The present invention allows for manipulating nano-scale (or smaller) volumes of fluid through a fluidic system for applications in technical, medical, and scientific areas that can profit from the ability to use limited sample volumes in analysis/detection (sensing), chemical reactions, or both.

According to one aspect of the present invention, a method of active nanofluidic flow control (parallel flow control-PFC) includes providing a nanofluidic channel and a pressure- driven microfluidic channel connected in parallel and actively controlling flow through the nanofluidic channel by using the pressure-driven microfluidic channel. A method of nanofluidic flow measurement is provided using a geometrically determined flow rate ratio. According to another aspect of the present invention, an additional method of nanofluidic flow measurement is provided. This option includes providing a nanofluidic channel, a pressure-driven microfluidic channel connected in parallel for flow control, and an additional measurement microfluidic channel connected in series for direct flow measurement of the nanofluidic flow rate by achieved by measuring the flow rate in the measurement microfluidic channel. This flow in the measurement microfluidic channel is monitored using detection approaches such as direct optical or electrical observation.

According to another aspect of the present invention, a method of nano-scale volume fluid manipulation is provided. The method includes providing a nanofluidic channel and a pressure-driven microfluidic channel connected in parallel and manipulating nano-scale volume fluid through the nanofluidic channel by using the pressure-driven microfluidic channel.

According to another aspect of the present invention, a method of fabricating a fluidic system is provided. The method includes forming a nanofluidic channel with an optional measurement microchannel in series and a pressure-driven microfluidic channel connected to the nanofluidic channel in parallel using micro-/nano-fabrication.

BRIEF DESCRIPTION OF THE FIGURES

FIG. IA is a side view of a nanofluidic flow control system using the approach of a direct, pump-imposed, pressure gradient across a nanochannel.

FIG. IB is a top view of a nanofluidic flow control system using the approach of flow in independent microchannels causing a pressure gradient across a nanochannel.

FIG. 2A is a schematic of the parallel flow control (PFC) approach for nanofluidic flow control where the pressure gradient across the nanofluidic channel is controlled by the pressure-driven microfluidic flow at points A and B (Top and side views).

FIG. 2B is a schematic of the parallel flow control (PFC) approach for nanofluidic flow control where the pressure gradient across the nanofluidic channel is controlled by the pressure-driven microfluidic flow at A and by a fixed pressure (e.g., atmospheric pressure) at B (Top view). FIG. 3 A Schematic of the approach of FIG. 2B with an additional measurement microfluidic channel designed to allow the direct measurement of flow through the nanofluidic channel. FIG. 3B Schematic of the approach of FIG. 2B used to manipulate nano-scale volumes of fluid using another fluid (e.g., air).

FIG. 4A is a schematic of fluidic flow model.

FIG. 4B illustrates an electric circuit model corresponding to the nanofluidic flow control system. R_n and R_m represent the flow resistance of nanofluidic channel and pressure- driven micro fluidic channel, respectively. The current source represents the flow rate source controlled by a syringe pump. The flow through the nanofluidic channel can be controlled by the flow through the pressure-driven microfluidic channel with the flow rate ratio decided by Eq. (5). FIG. 5 A illustrates a nanofluidic channel patterned in glass by drying etching.

FIG. 5B illustrates a microfluidic channel patterned in glass by wet etching.

FIG. 5C illustrates access ports drilled through glass.

FIG. 5D is a schematic of the active nanofluidic flow control system after glass bonding. FIG. 6 A illustrates bonded nanofluidic flow control device as seen under optical microscopy.

FIG. 6B illustrates cross-sectional FESEM picture of bonded nanofluidic channel. The channel is about 1 OOnm high and 15 μ m wide.

FIG. 7A illustrates an additional measurement microfluidic channel in series with the nanofluidic channel, the bonded nanofluidic flow rate measurement system under optical microscopy.

FIG. 7B illustrates the same system with water infused by a syringe pump and the water-air interface visible in the measurement microfluidic channel.

FIG. 8 A is an optical fluorescence picture of the active nanofluidic flow control system. The flow through the nanofluidic channel is controlled by pressure-driven microfluidic flow. (Inset: device on silicon to enhance contrast).

FIG. 8B is an optical fluorescence picture of the fluidic system after infusing DI water for 5 mins.

FIG. 9A illustrates optical fluorescence spectra produced in the nanofluidic channel region with fluorescein solution infused through the system.

FIG. 9B illustrates a temporal fluorescence spectrum (integrated between 505nm and 525nm) behavior in the nanochannel region on syringe infusion of a fluorescein solution at constant rates of 0.5 ml/hr or 1 ml/hr. FIG. 9C illustrates temporal fluorescence spectrum (integrated between 505nm and 525nm) behavior in the nanochannel region after stopping the fluorescein syringe infusion. The previously infused fluorescein molecules bleach under the incident laser causing a 25% light intensity decrease within 20 mins. FIG. 10 illustrates nanochannel flow rates as a function of syringe, or equivalently, pressure-driven microchannel flow rates. Shown are the experimental values obtained by tracking flow in the measurement microchannel, the calculated values from the product of

Q

— — and the syringe flow rates, and the same curves adjusted for fabrication-caused

dimension changes.

DETAILED DESCRIPTION

A detailed description of particular embodiments of the present invention is provided. The present invention is not to be limited merely to the specific embodiments described herein. To assist in describing the invention, several definitions are set forth. As used herein, the term "nanofluidic channel" generally means a fluidic channel having a cross-section with at least one of the dimensions being on the order of about 1 to about 100 nm.

As used herein, the term "micro fluidic channel" generally means a fluidic channel having a cross-section with dimensions being on the order of about 0.2 micrometers or larger. The present invention focuses on the active flow control, flow rate measurement, and nano-scale volume manipulation through a nanofluidic channel which is a component in analytical, chemical reaction or both systems. To achieve these objectives, we introduce a novel approach to addressing the nanofluidics interfacing issue based on employing pressure- driven microchannel flow which is depicted in FIGS. 2A-2B. We term this approach parallel flow control (PFC). PFC uses flow in the microfluidic channel, which interfaces with the

"outside world", to set up the pressure gradient across the nanofluidic channel. The residence time in this pressure-driven microfluidic channel is established solely by its flow which has been set up using a pump (e.g., a syringe pump). Such a parallel flow control (PFC) approach makes it possible to actively control flow through a nanofluidic channel. With this novel configuration, active controlled molecule flow and exchange through a nanofluidic system can be realized. PFC has two basic versions as seen in FIG. 2A and FIG. 2B. In FIG. 2A a nanofluidic flow control system 10 is shown. A pressure-driven microfluidic channel 12 is shown positioned between a first port 16 and a second port 18. The pressure gradient across the nanochannel 14 is controlled by the microfluidic channel flow at points A and B. In FIG. 2B, the pressure gradient is controlled by the microfluidic channel flow established pressure at A and a fixed pressure at point B (e.g., point B is at atmospheric pressure), the same pressure being maintained at ports 18A and 18B. Air bubble avoidance is easily attained by first filling the up-stream reservoir and then the pressure-driven microfluidic channel 12 in both versions. As we demonstrate, the PFC approach of either the first embodiment (FIG. 2A) or the second embodiment (FIG. 2B) can provide essentially infinitely fine nanofluidic channel flow control by using one pump such as a syringe pump, the most universal fluid handling instrument. While the flow rate through the pressure-driven microfluidic channel 12 can be varied for example, from 1 μ L/hr to lL/hr in either embodiment by using a syringe pump, for example, the flow rate through the nanofluidic channel 14 can be adjusted by changing the dimensions of the two channel types. Based on the size-scale differences between nano- and microfluidic channels, flow rate ratios of 10^~4: 1 and lower value are easily attainable with the PFC approach thereby allowing the attainment of a broad range of fine nanofluidic flow control. Both version 1 and 2 of Fig 2 allow the

nanochannel flow rate to be determined and thereby controlled by using the product of — - ,

the geometrically determined flow rate ratio of the nanochannel flow rate to the microchannel flow rate, and the pressure-driving (e.g., syringe) flow rates.

An option in the PFC configuration also allows the direct observation and therefore additional measurement of the actual nanofluidic flow rate which is controlled by the pressure-driven microfluidic flow. This PFC option is based on the embodiment of FIG. 2B and is shown in FIG. 3A. This option is seen to have an additional measurement microfluidic channel 20, which may be serpentine to extend the time of observation, which is connected with the nanofluidic channel 14 in series to work as a real-time, nanochannel flow monitor. Since in this option both the pressure-driven microfluidic channel 12 and the measurement microfluidic channel 20 are open to the same pressure (e.g., atmosphere) at one end, and since the pressure across the measurement (e.g., serpentine) microfluidic channel 20 is essentially negligible compared with the pressure across the nanofluidic channel 14, the same pressure exists across both the pressure-driven microfluidic channel 12 and nanofluidic channel 14. Flow in the nanochannel can be directly monitored by measuring flow in the series connected measurement microchannel in this arrangement. For example the filling rate of fluid into measurement microfluidic channel 20 can be observed from the position of fluid- air interface and by knowing the cross-section area of the measurement microfluidic channel 20. By neglecting fluid wetting effects, interactions with the walls, and evaporation effects in the measurement microfluidic channel 20, the filling rate in measurement microfluidic channel 20 is equal to the flow rate though the nanofluidic channel 14 connected in series. As we show, the PFC system also allows one to assess the impact of neglecting the above mentioned effects for any given fluid. It does so since the flow rate in the nanochannel 14 in

any PFC system can always be calculated from the product of — — and the pressure-driving

(e.g., syringe) flow rates.

The PFC concept also provides the ability to manipulate small (e.g., nano-scale or smaller) volumes of fluid through the nanofluidic channel 14 in case of limited sample volumes. This is also shown in FIG. 3. Firstly, a fluid sample can be transported down the pressure-driven microfluidic channel 12 to access the nanofluidic channel 14 at point A (FIG. 3B) as seen in the enlargement in this figure. As seen some fluid (e.g., a gas such as air) is used in front of and behind the sample to drive this sample. Thereafter, the fluid sample can be driven through the nanofluidic channel 14 by impacting pressure at left port 16.

THEORETICAL ANALYSIS We have established our assessment of the flow rate ratios available using PFC by using the simple flow model sketched in FIG. 4A. Assuming a single one phase Newtonian

fluid in the channel shown and a pressure gradient — — along the flow direction, it follows dz from mass conservation and momentum conservation that the velocity field u{x,y) of fully developed flow can be written as [22]

where η is viscosity and the other parameters are defined in FIG. 4A. The total flow rateg deduced from this equation is r 2 LZDJ 1 \ a tanh(nm 12b) (2)

L nπ ^{' ~4} Neglecting any entry effects, we take the pressure gradient to be constant through the whole channel, whether it is a nanofluidic channel or microfluidic channel. Hence, we have

dz ~ L ^{K )} where Ap is the pressure drop along a given channel from point A to B and L is the channel length from point A to B. In both versions of the PFC approach the microfluidic and nanofluidic channels are in parallel with the length L_m for the microfluidic channel and the length L_n for the nanofluidic channel. Therefore, using Eq (3) we can now write the flow rate

ratio ^²- for micro-channel flow to nano-channel flow for both PFC versions of FIG. 2. This

Q_n ratio is

Eq (4) can be re-written as

Q_m c_m L_n

(5)

Q_n C_n L_m where C_m and C_n are seen from Eq (4) to be geometric factors, for the micro- and nano- channels, respectively. These geometric factors have been discussed previously [23]. For the case where the channel height is much smaller than channel width (b«a), the ratio C_m I C_n

can be simply expressed as [22]. Since b«a is very valid for our nanochannels and

approximately true for some of the microchannels we used, we were often able to evaluate Eq. (5) using

In any case, we can see from the ratio of Eq. (5) that the pressure-driven microchannel flow rate ratio can be used to control a very precise (fine) flow through a nanofluidic channel in parallel to the pressure-driven microfluidic channel. This is analogous to controlling the electric current through one large resistor by connecting a smaller resistor in parallel (FIG. 4B). FABRICATION PROCESS

The devices used in our demonstration and preliminary evaluation of the PFC concept were fabricated in a class- 10/100 cleanroom. The fabrication processes are CMOS compatible allowing potential application in future integrated micro-/nano-total analysis systems. The overall fabrication processes for the first embodiment is shown in FIG. 5. As seen, the nanofluidic channel (1 OOnm high, 15 μ m wide, and 200 μ m long) was first dry etched into a glass wafer in a magnetically enhanced reactive ion etching (MERIE) tool using photoresist as the etching mask. Then the microfluidic channel (2 μ m high, 50 μ m wide, and 300 μ m long) was constructed in alignment with the nanofluidic channel using wet chemical etching (6:1 buffered oxide etch solution) with photoresist as the etching mask. Subsequently, the reservoir/accessing ports were mechanically drilled through the glass wafer. Finally, the patterned glass wafer was bonded with another blank glass wafer in furnace at 615 ⁰C for 1.5 hrs to complete the sealed flow control devices. The specific processing steps for the first embodiment PFC devices are seen in FIGS. 6A-6B. FIG. 6 A shows an actual PFC device under optical microscopy after glass bonding. FIG. 6B is the FESEM picture of the nanofluidic channel cross section showing the approximate channel dimensions of 1 OOnm deep and 15 μ m wide.

The fabrication for the second embodiment of devices was carried out following the same basic processes illustrated in FIG. 5. The particular device to be discussed here is that of FIG. 3 A; i.e., it had a pressure-driven microfluidic channel (10 μ m high, 10 // m wide, and 4350// m long) and an additional measurement microfluidic channel (10// m high, 20// m wide) in series with the nanofluidic channel. The serpentine feature, or any other length extending feature, may be used for the measurement microchannel to extend the real-time observation of nanochannel flow. FIG 7A shows this flow rate measurement device under optical microscopy. Since the pressure drop across the measurement (e.g. serpentine) microfluidic channel length is negligible compared with the pressure drop across the nanofluidic channel length, our flow rate analysis above is still valid for this configuration. FIG. 7B shows the device with DI water infused into the system by a syringe pump. As seen, the water-air interface progression with time can be used to measure the filling rate of the measurement (e.g., serpentine) microfluidic channel thereby showing flow through the nanofluidic channel. For steady- state measurements, a purposefully introduced marker such as a bubble can also be used to measure the flow rate. In all our flow experiments, whether with the device types seen in FIG. 6 or FIG. 7, needles were attached into the reservoir/accessing ports and connected to the fluidic source controlled by a syringe pump via plastic tubing. While optical microscope detection was used to observe (and film) the interface motion in the measurement microchannels discussed herein, electrical (e.g., capacitance, resistance) or other optical detection methods such as CCD, laser or LED/sensor, etc detection can also be utilized.

RESULTS AND DISCUSSIONS The fabrication and operation of the PFC concept was initially evaluated using the first embodiment. This evaluation was undertaken using optical fluorescence microscopy by infusing an Alexa Fluor fluorescein solution into the pressure-driven microchannel with a syringe pump (Figure 8A). As seen, this results in the clear presence of fluorescein molecules in the nanofluidic channel. These fluorescein molecules could be removed by infusing DI water through the system showing the system's ability for fast molecule flow and exchange (FIG. 8B). This molecule flow and exchange can be ascribed to two different mechanisms: diffusion and active pressure-driven flow. To differentiate between these, a study of steady- state PFC flow in the nanofluidic channel was undertaken by a focusing laser beam on the nanofluidic channel region in a first embodiment device using the optical fluorescence mode of a WITec Raman spectroscopy system. Specifically, we used optical fluorescence to confirm that active steady-state molecule flow through the nanofluidic channel was induced by the steady-state pressure across the pressure-driven microfluidic channel and not by diffusion. To establish this point we note that FIG. 9A shows the optical fluorescence spectra produced with fluorescein solution infused through the system. FIG. 9B shows the temporal fluorescence spectrum (integrated between 505nm and 525nm) behavior on infusing fluorescein solution at syringe pump flow rates of 0.5 ml/hr or 1 ml/hr. As can be seen, the emission light intensity remained at a constant level for both these steady-state flow rates implying the molecules traversed the nanochannel measuring region before bleaching for both flow rates. FIG. 9c shows the temporal fluorescence spectrum behavior in the nanochannel after stopping fluorescein solution infusion into the microchannel. The previously infused fluorescein molecules in the nanochannel bleach under the incident laser light and, therefore, the total emission light intensity decays as seen. About 25% light intensity decrease is detected within 20 minutes. These results establish that molecular diffusion is not dominant enough to replace the bleached molecules. However, the active pressure-driven flow can overcome the bleaching effect and sustain the light intensity at a

constant level. Since the calculated flow rate ratio — - is 1 :6x10^~5 from Eq. (6) for this

device, and since a microchannel flow rate of 0.5 ml/hr or more can stop bleaching, we deduce that a nanochannel flow rate at least as low as 30 nl/hr with an average fluorescein molecule speed of 0.5 cm/s can stop bleaching.

An assessment of PFC nanofluidic channel steady state flow rate and flow control possibilities was undertaken using the PFC second embodiment configuration with a series- connected measurement microfluidic channel (FIG. 3A). The nanochannel steady-state flow rate was monitored by using the filling rate of the measurement microfluidic channel. This filling rate was determined by optically tracking the position, as a function of time, of the water-air interface defining the filling front in the measurement microfluidic channel. We then assumed that the filling rate of the measurement microchannel determines the steady- state flow rate of the nanochannel with which it is in series. For this flow, DI water was infused through the system at different flow rates controlled by a syringe pump (FIG. 7B). The results of these measurements are labeled "experimental values" in FIG. 10 and are plotted there against the syringe (and, equivalently, the pressure-driven microchannel) flow rate. As seen, the smallest flow rate we can currently measure in our PFC system is ~0.5 pL/s. While this is not the smallest flow rate possible with the PFC approach, we note that this measured rate is much smaller than the smallest, reported, measured nanochannel flow rate of 30 pL/s [19]. In FIG. 10 we also plot what we term "calculated values" for the nanochannel

flow rate. These values are determined by Eq. (5), which gives a — — ratio of 1 :7.7xlO^"5 for

the device used, and by the known, pressure-driven microchannel (equivalently, syringe) flow rate. As seen in FIG. 9, the experimental nanofluidic flow rate is about 22 times slower than the theoretical nanochannel flow rate. However, the calculated curve has been developed

Q using a — — ratio calculated using design dimensions.

The impact of dimension inconsistencies we experienced in this demonstration will be briefly assessed here. These inconsistencies can arise, for example, from the processing steps of isotropic wet etching of the pressure-driven microfluidic channel and of the measurement microfluidic channel. This wet etching can produce an undercut profile and thereby make the widths of these microchannels larger than that designed. For example, based on the isotropic etching geometry and etching depth into the glass substrate (10 μ m), the actual measurement micro fluidic channel cross-section area is estimated to actually be 78.5% larger than what was designed. This larger measurement microchannel width dimension resulting from isotropic wet etching means that the actual nanochannel flow is larger than that given by the experimental curve of FIG. 10. Using this increased cross-sectional area value leads to the "adjusted experimental values" curve seen in FIG. 10. A processing caused increase in the pressure-driven microchannel cross-section has a corresponding impact on the theoretical

Q curve of FIG. 10. To be specific, the — — ratio which is multiplied times the syringe flow rate

to get the theoretical nanochannel flow rate is decreased to 2.37xlO^~5 based on the corrected etching geometry. Using this ratio results in the "adjusted calculated values" curve also shown in FIG. 10. As seen, the adjusted experimental flow rate is about 4 times slower than the adjusted theoretical value. The plots of FIG. 10 indicate that accounting for the correct microchannel dimensions is of primary concern in making precise nanochannel flow rate measurements. Corrections due to wetting effects, interactions with the walls [17], and evaporation effects [19] are bracketed by this assessment and are seen to be more subtle, at least for the case of DI water. This demonstration shows the importance of dimension control, which is fully attainable with well known technology. This demonstration also shows that the PFC approach also has the unique benefit of allowing the assessment of wetting effects, interactions with the walls, and evaporation effects to be obtained for any fluid used in a PFC system. As seen above, this is done by comparing the calculated and experimental nanochannel flow rates obtained using carefully controlled channel dimensions.

We point out that wetting effects, interactions with the walls, and evaporation effects, which can enter into the filling rate of the measurement microchannel, can be totally avoided with the PCF approach. This can be done by using steady state (i.e., not filling) flow in the measurement channel and following a procedure similar to that of Fig 3B. To be specific, steady state flow is set up everywhere in the system and then a marker fluid region surrounded by the system fluid in use is driven through the system. Part of the marker goes down the nanochannel (see Fig 3B) and thereby through the measurement microchannel where it is observed optically or electrically giving rise to steady state flow measurement in the nanochannel. APPLICATIONS

Nanofluidics with its advantages of nano-scale or smaller volume use and manipulation offers great potential for integration and application in different types of analytical/detection and chemical processing systems. Applications include sample preparation, biochemical reactions, and analysis on one single "chip". The analysis systems are now rapidly moving towards the use of nanofluidics to exploit smaller sample size, potentially precise flow control, and new sensing approaches. Chemical processing work is also rapidly moving toward the use of nanofluidics to exploit mixing and reaction effects at nano-scale. Our novel active nanofluidic flow control configuration allows the primary problems of fluidic systems including interfacing, measuring, and manipulating, which have been plaguing nanofluidics, to be successfully solved. This invention has established the effectiveness and utility of our PFC configuration. We believe this novel nanofluidic flow control approach points the way to significant advances in nano-total analysis system (nTAS) and miniaturized chemical process systems. It is to be understood that the scope of the invention is not to be limited by the description of a particular embodiment of the invention but is to be limited only by the recitations of the appended claims.

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Claims

What is claimed is:

1. A method of controlling active nanofluidic flow (parallel flow control-PFC), comprising: providing a nanofluidic channel and a pressure-driven microfluidic channel connected in parallel; and actively controlling flow through the nanofluidic channel by using flow in the pressure-driven microfluidic channel.

2. The method of claim 1 wherein the actively controlling flow through the nanofluidic channel by using the pressure-driven microfluidic channel comprises actively controlling a pressure gradient across the nanofluidic channel by using a microfluidic flow in the pressure- driven microfluidic channel.

3. The method of claim 2 wherein the pressure gradient is controlled using the microfluidic flow in the pressure-driven microfluidic channel to establish the pressure at both ends of the nanofluidic channel.

4. The method of claim 2 wherein the pressure gradient is controlled using the microfluidic flow in the pressure-driven microfluidic channel to establish the pressure at a first end of the nanofluidic channel while having a fixed pressure at a second end of the nanofluidic channel.

5. The method of claim 4 wherein the fixed pressure is an atmospheric pressure.

6. The method of claim 4 further comprising measuring the flow rate through the nanofluidic channel.

7. The method of claim 6 further comprising measuring the flow rate through the nanofluidic channel using the flow rate ratio.

8. The method of claim 6 further comprising providing a measurement microfluidic channel at the second end of the nanofluidic channel and wherein the step of directly measuring nanofluidic flow uses said measurement microfluidic channel.

9. The method of claim 8 wherein said measurement microfluidic channel is serpentine to extend real-time observation of nanofluidic flow.

10. The method of claims 3 and 4 further comprising manipulating nano-scale volumes of fluid through the nanofluidic channel.

11. The method of claims 3 and 4 wherein the step of manipulating volumes of about the nano-scale or less of fluid through the nanofluidic channel uses the pressure-driven microfluidic channel to transport a fluid sample to access the nanofluidic channel and drive the fluid sample through the nanofluidic channel.

12. The method of claim 1 wherein the active nanofluidic flow control is achieved by adjusting the dimensions of the pressure-driven microfluidic, nanofluidic, or both channel structures.

13. The method of claim 1 wherein the active nanofluidic flow control is achieved by adjusting the flow rate in the pressure-driven microfluidic channel.

14. The method of claim 1 wherein the microfluidic and nanofluidic flow includes flow of molecules.

15. The method of claim 1 wherein the step of actively controlling flow uses a syringe pump.

16. A fluidic system, comprising: a nanofluidic channel; a pressure-driven microfluidic channel connected to the nanofluidic channel in parallel; wherein the fluidic system being configured to actively control flow through the nanofluidic channel by using flow through the pressure-driven microfluidic channel.

17. The fluidic system of claim 16 wherein the nanofluidic channel is connected with the pressure-driven microfluidic channel in parallel.

18. The fluidic system of claim 16 wherein the fluidic system is configured to control the flow through the nanofluidic channel by using the pressure-driven microfluidic channel.

19. The fluidic system of claim 16 further comprising a measurement microfluidic channel connected to the nanofluidic channel in series to thereby directly measure the flow through the nanofluidic channel.

20. The fluidic system of claim 16 further comprising manipulating volumes of about the nano-scale or less of fluid through the nanofluidic channel using the pressure-driven microfluidic channel.

21. A method of fabricating a fluidic system, comprising: forming a nanofluidic channel on a substrate; forming a pressure-driven microfluidic channel using micro-/nano-fabrication; and connecting the nanofluidic channel and the pressure-driven microfluidic channel in parallel.