WO2025083707A1

WO2025083707A1 - Novel borosilicate based bioactive glass for coating on ti-6al-4v implants and its process thereof

Info

Publication number: WO2025083707A1
Application number: PCT/IN2024/052069
Authority: WO
Inventors: Anustup CHAKRABORTY; Subhadip Bodhak; Kaushik Biswas
Original assignee: Council of Scientific and Industrial Research CSIR
Current assignee: Council of Scientific and Industrial Research CSIR
Priority date: 2023-10-16
Filing date: 2024-10-15
Publication date: 2025-04-24
Anticipated expiration: 2026-04-16

Abstract

While the Ti-6A1-4V alloy is widely preferred for orthopaedic implants, it is crucial to address the potential risks of genotoxicity and implant loosening caused by released metal ions and the absence of osseointegration, respectively. In this context, the present invention describes novel borosilicate based bioactive glass compositions, suitable for coating application on Ti-6A1-4V implants. The assessed characteristics of these glasses demonstrate high thermal stability (>145°C), fast in vitro bioactivity(24-72h) and similar linear thermal expansion coefficient (8.5-12.19x 10^-6/°C) to that of the alloy. The favorable combination of properties enables achievement of a well adherent and amorphous coating on the alloy surface, ensuring osseointegration capability and protection against the risk of genotoxicity. Additionally, this invention presents a commercially viable coating technique that involves cold-spraying followed by thermal treatment, resulting in desired amorphous coating on the implant surface. Overall, this invention is anticipated to greatly enhance the performance of Ti-6A1-4V based orthopaedic implants.

Description

Novel borosilicate based bioactive glass for coating on Ti-6A1-4V implants and its process thereof

FIELD OF INVENTION:

The present invention relates to novel borosilicate based bioactive glass for coating on Ti-6A1-4V implants and its process thereof. In particular, the novel glass compositions yielding fast in vitro biomineralization ability, high thermal stability, and a similar thermal expansion coefficient to that of Ti-6A1-4V implants are suitable for obtaining a well adherent and crack-free coating on such implants through simple enameling technique. More specifically, the high thermal stability of the novel glasses helps in retaining amorphous nature even in the form of a sintered coating and thus, the biological functionalities of the parent glasses can be preserved in the applied coating.

BACKGROUND OF INVENTION:

According to current scenario, metallic implants (such as Ti and its alloys, stainless steel, Co-Cr etc.) are inevitably used in load bearing applications. Among the different biocompatible metals and alloys, Ti-6A1-4V is highly advantageous because of high corrosion resistance, low density, and favorable mechanical properties relevant to orthopaedic implants. However, the released metal ions from the implant can induce genotoxicity and carcinogenesis while the lack of osseointegration can cause loosening of the implant. To address this challenge, bioactive glass coating on the surface of the metallic implant has been found very effective for improving corrosion resistance and osseointegration. Bioactive glasses are well-known for their bone bonding ability while those are also capable of stimulating new bone growth. The bone regeneration capability of bioactive glasses is mediated by apatite crystallization on the surface which enhance bone cell proliferation. Moreover, the released ions from bioactive glass to the physiological medium may result in many beneficial functionalities such as antibacterial property, osteoinductive effect, stimulated vascularization etc. Despite the efficacy of bioactive glasses for such applications, it is challenging to obtain an adherent coating on the metallic substrate due to an imbalance in the thermal expansion coefficient (Table 1). Additionally, the commercial bioactive glasses suffer from high crystallization tendency or devitrification tendency during processing at higher temperatures. Bioactive glass coating deposited on metallic implants often needs to be consolidated through viscous flow sintering at a temperature higher than the glass transition temperature. A crystallization event during sintering results in incomplete densification because the development of crystalline phases hinders the viscous flow sintering process. Moreover, the crystalline phase dissolves at a much lower rate compared to the glassy phase when in contact with a physiological medium. The slower dissolution of the crystalline phase leads to deterioration of the osseointegration process as it depends on the concentration of the released ions in the surrounding medium.

Table 1: Comparison of linear thermal expansion coefficient ((0150-300) x 10~⁶/°C) of some commercial bioactive glasses and Ti alloys

PRIOR ART AND DRAWBACKS:

Following description contains the details of some relevant hitherto known prior art documents and their major drawbacks relevant to field of invention.

Reference may be made to the European patent EP0154513B1 wherein the glass compositions suitable for coating on Co-Cr-Mo alloy, and belonging to the SiOi-NaiO- CaO-PiOs-CaFi-BiOs based system are disclosed. The glasses have thermal expansion coefficient which is similar to that of the substrates (difference is less than 2%). However, no information has been provided on the nature (amorphous or crystalline) of the fired coating and the in vitro biological performance of the glasses or the coated substrate. More importantly, the thermal expansion coefficients of Co-Cr-Mo alloys are widely different than that of Ti-6A1-4V alloy while the later has gained huge importance as orthopedic implants in recent times. The proportions of different ingredients in the reported glass compositions are also widely different than those of the present invention.

Reference may be made to the Chinese patent CN114558177B wherein the preparation of a multi-level hole structure on the metallic surface for better anchorage of the coating and fabrication of bioactive glass coating on the implant’s surface are described. Furthermore, the coating is applied in a textured manner through laser spray followed by partial conversion of the glass coating to hydroxy carbonate apatite through chemical treatment. The glass comprises 10-25% of Sit , 10-20% of CaO, and 1-3% of P2O5., 3-8% of Na₂O, 6-10% of K₂O, 5-10% of MgO, 5-8% of SrO, and 30-50% of B2O3. The process is quite complex and the benefit of ionic dissolution product on the tissue healing process cannot be realized completely as glass is partially converted to HCA. The composition of the glasses and relevant properties described in the patent significantly differ from the present invention.

RO 130068 describes a method of producing alkali free bioactive glass coated dental implants through radio frequency magnetron field sputtering technology. The bioactive glass composition belongs to SiOi-CaO-MgO-PiOs-ZnO-SrO system and maintains the thermal expansion coefficient close to that of titanium and its alloys. The details regarding the composition range, thermal expansion coefficient values and thermal stability are not disclosed in the patent document. Moreover, the compositions are widely different than the present invention while having some additional components like MgO, ZnO and SrO.

W02009081120A1 relates to bioactive glass coatings for Ti-6A1-4V alloys and chrome cobalt alloys, wherein the thermal expansion coefficient of the glass coating is matched to that of the alloy. The bioactive glass comprises SiC , NaiO, CaO, MgO, K2O, ZnO, B2O3 and P2O5. Thermal Expansion coefficient of the claimed compositions varies between 8.8 x 10’⁶ K ¹ and 12 x 10’⁶ K ¹. The said composition exhibits a processing temperature window of at least 90°C when measured with glass powder of mean particle size of less than 100 pm. The said composition includes MgO and ZnO which should result in delayed bioactivity unlike present invention. Furthermore, no information has been provided on whether the coating remain amorphous or partially crystalline after the coating has been sintered. The present invention includes a glass system which is free of MgO or ZnO and demonstrates considerably fast in vitro bioactivity in simulated body fluid.

Reference may be made to the Chinese patent CN102258432B wherein, a mix of bioactive glass, solid acidic granule, glycerine, polyethelene glycol and resin showcasing pH buffer activity is reported. The reported glass mix is able to prevent pH increase in the surrounding physiological medium and thereby, helps in eliminating irritation of the patient. Nevertheless, the compositions of the glasses are widely different than the present invention and the thermal stability, thermal expansion coefficient and in vitro bioactivity of the glasses are not reported. Furthermore, the reported glasses are not designed for coating application on metallic implants. Reference may be made to Chinese patent CN105582571B, wherein preparation of a bio-ink comprising mixture 45S5 bio-vitric powder and calcium silicate in a PVA based gel solution for preparation of a mechanically strong porous construct through 3D printing technique is disclosed. The 45S5 bio-vitric powder which contains SiC , NaiO, CaO, P2O5, B2O3 is prepared by sol-gel method followed by sintering at a temperature range of 950- 1000 °C. However, this glass composition range, preparation technique and application are completely different than our present invention.

Reference may be made to world patent W02004031086A1 wherein, bioactive glass and glass-ceramics bone substitutes in granular form is described. Granular material can be utilized for filling any irregular shaped bone defects and therefore, offers advantage over bulk implants. However, the large surface area to volume ratio of granular bone substitute requires a less reactive material than commercial 45S5 for rapid healing. The disclosed glass compositions are of intermediate reactivity (less than 45S5 composition), and thus, can be used effectively in granular form circumventing the effect of large surface area to volume ratio. The glass composition is quite different that the present invention and the thermal expansion coefficient or the thermal stability of the material is not at all disclosed in the description.

Canadian patent CA2210070C discloses glass compositions with a large working range, controlled durability and chemical bonding ability with hard and soft tissues. The disclosed glass compositions contain additional oxides such as K2O and MgO which are not used in the glass composition related to the present invention. Furthermore, the thermal expansion coefficient and the suitability of the glass compositions for coating on metallic alloys have not been addressed in the patent document. Our present invention mainly focuses on the thermal expansion behavior, devitrification resistance and in vitro bioactivity of the glass compositions that make them suitable for coating applications on Ti-6A1-4V implants.

Reference may be made to BRPI1014607B1, wherein glass compositions useful for prevention and treatment of bone infections are disclosed. The invented glass composition has been successfully utilized for treatment of chronic osteomyelitis disease in multiple patients as disclosed in the patent. However, there is no mention of the thermal expansion behavior, devitrification resistance or in vitro bioactivity of the invented glasses in the patent document.

JP2008518650A relates to the development of glass compositions for treatment of lesions associated with damaged or insufficient angiogenesis and for preventing avascular fibrosis. The glass composition belongs to SiOi-NaiO-CaO-KiO-MgO-BiCh-PiOs system and used in the unsintered form or as fibers. The patent lacks any information regarding the thermal expansion behavior and thermal stability while the glass composition is also completely different than our present invention.

Peddi et al. (Peddi L, Brow RK, Brown RF. Bioactive borate glass coatings for titanium alloys. Journal of Materials Science: Materials in Medicine. 2008 Sep; 19:3145- 52.) reported bioactive borate glass compositions for coating application on Ti-6A1-4V implants. The glass containing NaiO, CaO, B2O3, SiCh, AI2O3, and P2O5 possess a thermal expansion coefficient close to that of Ti alloy. The coating with this glass composition was achieved through enamelling technique resulting in good adhesion strength. However, the thermal stability or the amorphous nature of the coating was not addressed while the in vitro bioactivity results highlight a delayed bioactivity (2 weeks). The compositions of the reported glasses were also substantially different than the present invention.

Khalil et al. (Khalil EM, Youness RA, Amer MS, Taha MA. Mechanical properties, in vitro and in vivo bioactivity assessment of Na2O-CaO-P2O5-B2O3-SiO2 glass-ceramics. Ceramics International. 2018 May l;44(7):7867-76) reported the synthesis and property evaluation of bioactive glass ceramics containing Na2O, CaO, B2O3, SiO2 and P2O5. The examined properties highlight that the crystallization process involved in the preparation of glass-ceramic caused a deterioration of both the in vitro bioactivity and in vivo bone bonding ability. However, the favorable mechanical properties indicated potential of such glass ceramic materials for dental restoration related applications. Although the reported parent glass compositions contain similar ingredients like present invention, the proportion of different ingredients is significantly different. For example, the reported glasses comprise 25-45 mol% P2O5 and 15 mol% SiCh while glasses from the present invention contain 1-2 mol% P2O5 and 30-32 mol % SiCh. Moreover, the report mostly focuses on the derived glass ceramics without mentioning the important properties (linear thermal expansion coefficient, processing window or thermal stability) of parent glasses pertinent to coating application on metallic implants.

In summary, the bioactive glasses from prior art are having substantially different compositions compared to the present invention. The usage of additional reactants like MgO, ZnO, CaFi, K2O, SrO can be noticed in most of the prior art documents. The usage of MgO, ZnO, CaF2 and SrO are known to cause a delay in their in vitro biomineralization ability. Furthermore, in vitro bioactivity and thermal stability factor of the glasses from prior art are not discussed in most of the cases. More importantly, the nature of coating (amorphous or crystalline) is not reported in any of the prior art document. Therefore, it is technologically significant for commercial success to develop a bioactive glass composition with superior devitrification resistance, fast in-vitro bioactivity and a thermal expansion coefficient comparable to that of Ti-6A1-4V alloy.

OBJECTIVE OF THE INVENTION

• An objective of the present invention is to provide bioactive glass compositions which have similar thermal expansion coefficient to that of Ti-6A1-4V implants and thus, eliminate or minimize the problems described in the background and prior art section.

• Another objective of the present invention is to provide bioactive glass compositions which show considerably fast in vitro bioactivity and high enough thermal stability for retaining amorphous nature even in the sintered coating form.

• Yet another objective of the present invention is to provide bioactive glass compositions for coating on Ti-6A1-4V implants with a suitable balance between in vitro bioactivity, thermal stability or devitrification resistance.

• Still another objective of the present invention is to provide an easy and economic method for coating the above-mentioned glasses on to Ti-6A1-4V substrate.

NOVELTY AND NON-OBVIOUS INVENTIVE STEP(S):

The present invention discloses a novel range of bioactive glass compositions exhibiting a unique combination of relevant properties like thermal expansion coefficient, thermal stability and bioactivity pertinent to coating on Ti-6A1-4V based medical implants retaining the amorphous nature. The bioactive glass compositions comprise 30-32 molar % SiCh, 21-23 molar % B2O3, 1-2 molar % P2O5, 0-23 molar % Na2O, and 22-45 molar % CaO. Glasses belonging to this composition range exhibit a unique combination of properties which make them more suitable and advantageous for coating application on Ti- 6A1-4V implants compared to the compositions reported in prior art. The examined properties which make these glasses stand out are: Linear thermal expansion coefficient (0150-300) in the range of 8.5-12.19 xlO⁶/ °C which matches with Ti-6A1-4V alloy, overcoming the thermal expansion coefficient mismatch problem. thermal stability factor (AT) of more than 145°C, ensuring good processability at high temperatures avoiding crystallization. In vitro bioactivity within 24-72 h of immersion in simulated body fluid when tested according to TC-04 protocol.

Such unique combination of properties ensures a well adherent, crack-free and amorphous coating on Ti-6A1-4V implants with the ability to integrate with the host tissue. A commercially viable and simple process for coating the said glass compositions on the metallic substrate is also described. The coating has been applied through cold spraying followed by sintering technique. This coating technique is commercially viable and very simple to adopt, resulting in an amorphous coating layer on the implant. The process parameters involved in slurry preparation (slurry recipe, solid loading, particle size of powder, milling parameters) and coating process (spraying parameters and sintering schedule) are very crucial since the desired coating can only be obtained by strictly adhering to the optimized process parameters.

DESCRIPTION OF DRAWINGS:

The present invention is illustrated in figure 1 to figure 4 of the drawing(s) accompanying this specification. In the drawings like reference numbers/letters indicate corresponding parts in the various figures. Figure 1 represents DSC thermograms of the glasses from example 1 to example 5.

Figure 2 represents the dilatometric curves of the glasses from example 1 to example 5.

Figure 3 represents the XRD plots after 3 days of in vitro bioactivity study of glasses from example 1 to example 5.

Figure 4 represents the XRD plots of the sintered coating with commercial 45S5 glass and the glass from example 5.

Figure 5 represents the optical image of the sintered coating on Ti-6A1-4V substrate with the 45S5 glass (example 6) (a) and glass from example 5 (b).

The drawings are included to provide better understanding of the invention, illustrate the embodiments of the invention and together with the description serve to explain the importance of the invention.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides novel borosilicate based bioactive glass for coating on Ti-6A1-4V implants and its process thereof which comprises 30-32 molar % SiOi, 21-23 molar % B2O3, 1-2 molar % P2O5, 0-23 molar % Na2O, and 22-45 molar % CaO as chemical constituents of the glass.

In an embodiment of the present invention, the invented glasses (compositions presented in table 2) have a linear thermal expansion coefficient (0150-300) in the range of 8.5- 12.19 xlO⁶/ °C. The coating of commercial bioactive glasses (45S5, S53P4) on Ti-6A1-4V based medical implants comes with a spalling problem as the linear thermal expansion coefficient of those glasses varies in the range of 15-13.5 xlO⁶/ °C which is a huge mismatch with the substrate. The thermal expansion coefficients of novel glass compositions are similar to the substrate and hence a well adherent coating can be obtained.

In another embodiment of the present invention, the invented glasses exhibit a thermal stability factor (AT) of more than 145°C. Notably, the thermal stability factor of commercial bioactive glasses developed so far varies in the range of 50-100°C leading to incomplete densification and deteriorated biological functionalities due to early crystallization during sintering process. The higher thermal stability factor of the novel glass compositions indicates the ability of these glasses to be sintered retaining completely amorphous nature.

In yet another embodiment of the present invention, the in vitro bioactivity of the glass is comparable to that of commercial S53P4 composition which demonstrates formation of surface apatite layer within 24-72 h of immersion in stimulated body fluid when tested according to TC04 protocol.

In still another embodiment of the present invention, the glass can be processed in to a slurry containing 30-45 vol % glass powder of 10-45 pm particle size in distilled water, ethanol or isopropanol.

In still another embodiment of the present invention, the glass slurry can be deposited on the metal substrate by simple and commercially viable cold spraying technique using a spray gun followed by sintering at (50-70) °C above the glass transition temperature for 1-2 h.

In still another embodiment of the present invention, the final coating was observed to be completely amorphous in nature and free from cracks.

DETAILED DESCRIPTION OF THE INVENTION

According to the present innovation, bioactive glass compositions with multiple advantages pertinent to coating application on Ti-6A1-4V implants have been developed. The glass composition comprises 30-32 molar % SiOi, 21-23 molar % B2O3, 1-2 molar % P2O5, 0-23 molar % Na2O, and 22-45 molar % CaO.

The bioactive glass compositions reported here possess linear thermal expansion coefficient similar to that of Ti-6A1-4V implants and thereby coating those glasses on the metallic substrate will result in a well-adherent and crack free coating. The coating applied with conventional bioactive glasses generally contain micro cracks which from during the cooling process following the sintering of coating. The micro cracks result from the different shrinkage behavior of the coating and substrate due to the mismatch in linear thermal expansion coefficients of the two materials. Any dislodged particles from such coating can lead to inflammation and other complexities. The glasses from present innovation are quite well suited to mitigate such problems. Furthermore, a well adherent coating can prevent the corrosion of metallic implants to a larger extent than a coating which is having cracks and a spalling issue. Therefore, the novel glasses in the form of coating on metallic implants can restrict the release of metal ions into the physiological medium and thus, minimize the risk of complex medical conditions such as genotoxicity or carcinogenesis. The borosilicate glasses reported here, are also more advantageous for coating application on Ti-alloys compared to the silicate compositions. Such advantage stems from the possibility of formation of a Ti-boride layer at the glass-metal interface which acts as a barrier to interdiffusion and thus prevent weakening of the bonding between two materials. Another aspect of the innovation is that these glasses are quite well resistant to crystallization or devitrification during processing at high temperatures. The high value (>145°C) of thermal stability factor indicates these glasses can be sintered without any hindrance from crystallization. The conventional glasses are prone to crystallization and the crystallization process often interferes with the sintering process. The crystallization of glasses during sintering comes with a few negative impacts on the processing and properties such as requirement of a much higher temperature for proper densification, and the loss or degradation of many biological functionalities. The Ti-6A1-4V alloy undergoes phase transformation (a«->P) at such high temperatures required for proper densification of those crystallization prone glasses and thereby the alloy can get damaged. On the other hand, the dissolution rate of glasses in physiological medium slows down when it is partially or fully converted to crystalline phase. Since all the biological functionalities are mainly governed by the released ions from the glass to the physiological medium, a deterioration of the biological functionalities can be experienced. Moreover, the residual glass composition does not remain same as the parent glass due to selective portioning of elements into the crystalline phase. The glasses of our present invention showcase high devitrification resistance and thus, minimize the possibility of such alteration of functionalities. Apart from that, the bioactive glass coating using the compositions of the present invention can lead to fast osseointegration which prevents loosening of the implant. The in vitro bioactivity study works as an indicator for in vivo osseointegration ability. In general, a faster in vitro apatite forming ability or bioactivity is a marker of faster in vivo bone bonding ability. The glasses in the present invention showcase formation of apatite layer within 1-3 days of immersion in simulated body fluid. The commercial glasses like 45S5 and S53P4 take 1 day and 3 days, respectively, to exhibit the same. However, those glasses get partially crystallized during sintering which will in turn cause delayed bioactivity when applied as a coating. The glasses reported here are able to retain their apatite forming ability even in the form of a sintered coating. The coating of the glasses on the metallic substrate can be attained through simple cold spraying technique followed by sintering of the coating. This commercially viable process is rather simple to adopt while the sintering conditions can also be maintained well below the phase transformation temperature of Ti-6A1-4V alloy.

Table 2: Composition of novel borosilicate bioactive glasses

The glass compositions in the present invention were synthesized through conventional melt-quench technique using pure platinum crucible. The thoroughly mixed batches of analytical grade raw materials such as SiCh (99.8%, Sipur Al Bremthelar Quartz - itwerk), NaiCCh (99.5%, Sigma Aldrich), CaCCh (99.5%, Sigma Aldrich), CaHPC .SHiO (98% Sigma Aldrich), and H3BO3(99.5%, Fluka) were melted at 1150-1450 °C range inside a raising hearth furnace (Kanthal). The homogeneity of the glass melt was ensured by intermittent stirring with a fused silica rod during the 1.5-2.5 h melting time. Thereafter, glass melt was cast in a stainless-steel mould and the cast block was immediately transferred to a muffle furnace for annealing. Annealing was carried out for 2-3 h followed by gradual cooling to room temperature for obtaining a stress-free glass block. Finally, these annealed glass blocks were crushed and further milled in a planetary ball mill to obtain fine glass powder (<45 microns).

The thermal stability of the glasses was measured using differential scanning calorimetry (DSC) (STA 449F3, Netzsch GmbH, Selb, Germany). The DSC thermograms of glass powders with below 45 pm particle size were recorded at 10 °C min ¹ heating rate. The temperature window in between the glass transition temperature (T_g) and crystallization onset temperature (T_x) gives the measure of thermal stability (AT= T_x-T_g). A higher value of AT indicates more resistance against devitrification. Thermal stability of the glass compositions described in this invention are listed in the table 3.

Linear thermal expansion coefficient (0150-300) of the glasses was measured utilizing a pushrod dilatometer (Netsch, DIL402PC). The Cylindrical samples having 6 mm diameter and 25 mm length was heated at 5°C/min rate and the ratio of change in length/original length (thermal strain) was recorded as a function of temperature. Linear thermal expansion coefficient of glass samples was measured from the slope of the thermal expansion versus temperature curve and the obtained values for different glasses in the 50-300 °C temperature range is reported in table 3.

Table 3: Properties of borosilicate bioactive glasses

The in vitro bioactivity indicates the time taken for formation of a crystalline apatite layer on the material’s surface when immersed in aqueous medium such as simulated body fluid, tris buffer etc. Generally, a faster in vitro bioactivity of a material is a marker of faster bone bonding ability in vivo condition. The in vitro bioactivity of the glasses was examined according to the TC04 protocol. TC04 stands for technical committee 04 of international commission on glass (ICG) that is concerned with the glasses for medicine and biotechnology. TC04 committee members conducted a round robin study across several different laboratories to develop this reliable and simple protocol for comparing bioactivity of different glasses. This protocol is frequently adopted to compare bioactivity of different samples by the bioactive glass researchers. Briefly, 150 mg glass powder of below 45 pm particle size was immersed in 100 ml simulated body fluid, and thereafter kept inside an incubator (37 °C) with orbital shaking at 120 rpm. After different time points (Ex: 24 h, 72 h etc.) glass powder was filtered out from the solution and investigated by the means of x- ray diffraction and Fourier transform infrared spectroscopy to check the formation of apatite layer. The in vitro bioactivity of different glass samples described in this invention are enlisted in table 3.

For the coating initially substrate preparation was carried out. First, Ti-6A1-4V substrate was grit blasted using alumina powder for providing a rough surface which will allow better anchorage of the coating. The grit blasted substrate was ultrasonically cleaned with ethanol. The glass powder has to be taken in the form of a slurry to apply coating on the prepared substrate through the cold spraying technique. The slurry was prepared by mixing fine glass powder (30-45 vol%) of 10-45 pm particle size with a solution of distilled water and poly vinyl alcohol (0.5- 1.5 weight % of glass powder). The mixing operation to obtain a stable dispersion was carried out by milling the mixture in a planetary ball mill for 1-3 h at 200-300 rpm. The slurry was then sprayed on to the prepared substrate with the help of a spraying gun. The coating thickness on the substrate can be controlled by changing the number of passes. Following the spraying process, coated sample was dried at 110 °C in an oven. Finally, the dried samples were subjected to sintering at a temperature of 550 - 720 °C (50-70 °C higher than T_g) for 1-2 h in a muffle furnace depending on their composition. The sintered sample was cooled to room temperature maintaining a 2-4 °C/min cooling rate.

EXAMPLES

The following examples are given by way of illustration of the working of the invention in actual practice and should not be construed to limit the scope of the present invention in any way.

Example 1

The glass comprises 31.43 molar % SiCh, 22.45 molar % B2O3, 22.21 molar % CaO, 22.21 molar % NaiO and 1.7 molar % P2O5. For glass preparation, appropriate quantity of each raw materials was mixed before charging in a platinum crucible inside raising hearth furnace. The glass melting was carried out at 1150 °C and the cast block was annealed at 485 °C. The linear thermal expansion coefficient (0150-300) of the glass was estimated to be 12.19 xlO’⁶/°C. The thermal stability of glass powder having below 45 pm particle size was measured to be 180 °C at 10 °C min ¹ heating rate. The in vitro bioactivity results highlighted formation of apatite layer after 1 day (24h) of immersion in simulated body fluid.

Example 2

The glass comprises 31.43 molar % SiCh, 22.45 molar % B2O3, 27.76 molar % CaO, 16.66 molar % Na2O and 1.7 molar % P2O5. For glass preparation, appropriate quantity of each raw materials was mixed before charging in a platinum crucible inside raising hearth furnace. The glass melting was carried out at 1200 °C and the cast block was annealed at 515 °C. The linear thermal expansion coefficient (0150-300) of the glass was estimated to be 10.96 xlO’⁶/°C. The thermal stability of glass powder having below 45 pm particle size was measured to be 149°C at 10 °C min ¹ heating rate. The in vitro bioactivity results highlighted formation of apatite layer after 3 days (72h) of immersion in simulated body fluid.

Example 3

The glass comprises 31.43 molar % SiCh, 22.45 molar % B2O3, 33.31 molar % CaO, 11.11 molar % Na2O and 1.7 molar % P2O5. For glass preparation, appropriate quantity of each raw materials was mixed before charging in a platinum crucible inside raising hearth furnace. The glass melting was carried out at 1250 °C and the cast block was annealed at 545 °C. The linear thermal expansion coefficient (0150-300) of the glass was estimated to be 10.07 xlO’⁶/°C. The thermal stability of glass powder having below 45 pm particle size was measured to be 169°C at 10 °C min ¹ heating rate. The in vitro bioactivity results highlighted formation of apatite layer after 3 days (72h) of immersion in simulated body fluid.

Example 4

The glass comprises 31.43 molar % SiCh, 22.45 molar % B2O3, 38.87 molar % CaO, 5.55 molar % NaiO and 1.7 molar % P2O5. For glass preparation, appropriate quantity of each raw materials was mixed before charging in a platinum crucible inside raising hearth furnace. The glass melting was carried out at 1300 °C and the cast block was annealed at 575 °C. The linear thermal expansion coefficient (0150-300) of the glass was estimated to be 9.44 xlO’⁶/°C. The thermal stability of glass powder having below 45 pm particle size was measured to be 167°C at 10 °C min ¹ heating rate. The in vitro bioactivity results highlighted formation of apatite layer after 3 days (72h) of immersion in simulated body fluid.

Example 5

The glass comprises 31.43 molar % SiCh, 22.45 molar % B2O3, 44.42 molar % CaO, and 1.7 molar % P2O5. For glass preparation, appropriate quantity of each raw materials was mixed before charging in a platinum crucible inside raising hearth furnace. The glass melting was carried out at 1350 °C and the cast block was annealed at 605 °C. The linear thermal expansion coefficient (0150-300) of the glass was estimated to be 8.5xl0^-6/°C. The thermal stability of glass powder having below 45 pm particle size was measured to be 187 °C at 10 °C min ¹ heating rate. The in vitro bioactivity results highlighted formation of apatite layer after 3 days (72h) of immersion in simulated body fluid. Furthermore, the glass from example 5 was applied as a coating on the Ti-6A1-4V substrate. The coating process has already been mentioned earlier. Briefly, an aqueous slurry containing 30-45 volume % glass powder of 10-45 pm particle size and small amount (1 weight % of glass powder) of polyvinyl alcohol as binder was prepared. The prepared slurry was then coated on to the grit blasted Ti-6A1-4V substrate through a spray gun. Finally, applied coating was sintered at 710 °C for Ih and then slowly cooled to room temperature to obtain consolidated coating. Both visual and microscopic examinations confirmed the coating is free of cracks (figure 5). The sintered coating obtained from the glass composition in example 5 was noticed to be completely amorphous in nature from the x-ray diffraction technique (figure 4). Mechanical properties of the coating were investigated by the means of nano-indentation technique. The hardness and modulus of the coating comprising glass from example 5 are 99.5 GPa and 1.83 GPa, respectively.

Example 6

Additionally, commercial 45S5 bioactive glass (composition is mentioned in Table 4) which is available as prior art (Brauer DS. Bioactive glasses — structure and properties. Angewandte Chemie International Edition. 2015 Mar 27;54(14):4160-81) was also synthesized and coated on the same metallic substrate (Ti-6A1-4V) under similar coating technique and parameters (as given in Table 4) as used for Example 5 to understand the efficacy of present invention over the said commercially available 45S5 bioactive glass composition. However, 45S5 glass coating was found to be full of cracks visible to the naked eye whereas our invention demonstrates a well adherent and crack-free coating (figure 5). The 45S5 coating exhibited crystalline nature and the crystalline phase was combeite or sodium-calcium-silicate (figure 4) whereas the sintered coating obtained from the glass composition in example 5 was noticed to be completely amorphous in nature (figure 4).

Table 4: Properties of the glass from example 6 and the process parameters utilized for coating on Ti-6A1-4V substrate

ADVANTAGES OF THE INVENTION

The main advantages of the present invention are:

• The described compositions can be applied as a well adherent coating on Ti-6A1-4V implants which will not show any kind spalling problems due to thermal expansion mismatch.

• The developed compositions described in this invention will be useful for coating application on Ti-6A1-4V implants. The metal implants coated with the novel glasses will be able to show proper osseointegration which will in turn circumvent the issue of implant loosening in the long run.

• The novel glass compositions when applied as a coating on the metallic implants will also help in reducing the release of metal ions into the physiological medium, and thus, the risk of carcinogenesis and genotoxicity can be reduced.

• The novel compositions also exhibit high thermal stability values which indicate those will retain their amorphous nature even after sintering. The amorphous coating helps in retaining the biological functionalities originating from the dissolution behavior of the parent glasses.

• The developed glasses exhibit fast in vitro bioactivity which indicates the well adherent coating will also have fast osseointegration ability. The glass composition described in this invention have been achieved through usage of less number reactants as the reactants like MgO, K2O, CaFi etc. described in the glass compositions of prior art are not incorporated.

• The coating on the metallic substrate can be achieved through an economic and commercially viable cold-spraying technique followed by sintering.

Claims

WE CLAIM:

1. A bioactive glass composition for coating on Ti-6A1-4V implants comprising of: a. SiOi 30-32 molar %, b. B2O3 21-23 molar %, c. P2O5 1-2 molar %, d. Na2O 0-23 molar %, and e. CaO 22-45 molar %.

2. The glass composition as claimed in claim 1 wherein, the linear thermal expansion coefficient (0150-300) varies in the range of 8.5-12.19 xlO⁶/ °C.

3. The glass composition as claimed in claim 1 wherein, the particle size of the glass powder is in the range of 10 to 45 pm.

4. The glass composition as claimed in claim 1 exhibits thermal stability in the range of 149 to 187 °C.

5. The composition as claimed in claim 1, wherein the in vitro bioactivity or biomineralization can be observed within 24-72 h of immersion in simulated body fluid when tested according to TC04 method.

6. The process of coating the composition as claimed in claim 1, comprising the steps of: a. Preparation of slurry containing 30-45 vol % glass powder of 10 to 45 pm particle size in distilled water, ethanol or isopropanol, b. Spray coating of glass slurry on grit blasted Ti-6A1-4V substrate using a spray gun, and c. Sintering of the sprayed coating wherein the sintering temperature is in the range of 550 to 720 °C.