Bioactive glass–apatite composite coating for titanium implant synthesized by electrophoretic deposition
Introduction
Titanium and titanium alloys are widely used as materials for implants, because of their mechanical properties and nontoxic behaviour. Unfortunately, metallic implants are bioinert, which could be attached to the bone through form fit or frictional connections. This can lead to encapsulation by dense fibrous tissue in the body. As a consequence, non-appropriate stress distribution at the bone–implant interface appear, which can lead to interfacial failure and loosening of the implant.
In order to solve this problem, the metal implant could be coated with bioactive materials with good adhesion to metal and which could be also bonded interfacially to the bone.1, 2, 3 One approach is to use hydroxyapatite (HAP) coating obtained by sputtering,4 plasma spray processing technique,5 sol–gel6 or aerosol–gel method,7 hydrothermal reaction8 or electrophoretic deposition.9, 10, 11
Another approach is to coat the implant with bioactive glass that could provide interfacial attachment to the bone. Although glass coatings based on Bioglass® have an excellent in vitro behaviour, the thermal stresses generated as a consequence of differences in the thermal expansion between the glass and the metal, resulted in cracks appearing at the glass–metal interface. In order to solve this problem, Tomsia and coworkers2, 12, 13 developed the glasses with composition with a similar thermal expansion to the Ti6Al4V substrate. The softening point of these glasses is lower then the temperature of α → β transformation of titania. At the same time, glass coatings with silica contents lower than 60 wt.% has shown good in vitro behaviour in SBF,3 and other with silica content higher than 60 wt.% had a better mechanical stability and adhesion to the substrate but are no longer bioactive. Functionally graded bioactive glass coating developed by the same researchers14, 15, 16 provided all of mentioned requirements. Using conventional enameling technique or dip-coating method, controlling the gradient in the glass composition along the coating or incorporating HAP particles into the glass, they performed a coating with excellent adhesion to the substrate and bioactivity of the surface.
Electrophoretic deposition (EPD),17, 18 which is a colloidal process wherein ceramic bodies or coatings are shaped directly from a stable colloid suspension by a dc electric field, seem to be very promising in developing glass or ceramics functionally graded coating with different thickness on the substrate of complex shapes. The electrophoretic deposition conditions such as deposition voltage, current, concentration and time are very important if the thickness, morphology or composition profile is to be controlled.
The aim of this work is to investigate the possibility of developing the functionally graded bioactive coating on Ti6Al4V substrate by using electrophoretic deposition. The influence of EPD parameters such as deposition voltage and time of deposition on the coating morphology and deposit weight were examined.
Section snippets
Experimental
Synthesis of glass 6P61 in the system SiO2–Na2O–K2O–CaO–MgO–P2O5 were performed according to the procedure described by Tomsia and coworkers.17 SiO2 (99%) (Silica Gel, Redondo Beach), CaCO3 (99%) (Kemika, Cro), MgCO3 (99%) (Kemika, Cro), NaH2PO4 (99%) (Centrohem, SCG), Na2CO3 (Zorka, SCG), K2CO3 (99%) (Kemika, Cro) were used as reagents. The mixtures were fired in air at 850 °C for 1 h and further at 1500 °C for 2 h in a Pt crucible and quenched. The glass was milled during 1 h at radial velocity 396
Results and discussion
The SEM micrograph of milled glass powder is shown in Fig. 1.
It is obvious that glass particles (shown in Fig. 1), milled in planetary mill under the described conditions, have an irregular shape and particle size distribution ranging from 0.3 to 0.8 μm.
The SEM micrograph of synthesized HAP particles is shown in Fig. 2. By observing this figure, it is obvious that the synthesized HAP particles are spherical having the size in the range from 3 to 5 μm. Each particle consists of great number of
Conclusion
In this work the synthesis of functionally graded glass–apatite coating on Ti6Al4V alloys by the electrophoretic method is presented. The influence of synthesis conditions such as deposition voltage and time on the coating properties and deposit weights were examined. It was shown that by controlling the deposition voltage and time, the deposit weight and coating thickness could be controlled. The non-linear dependence of deposit weight on applied voltage could be explained by different
Acknowledgement
The authors are grateful to the Ministry of Science and Environmental Protection, Republic of Serbia for the financial support through the projects No. 142070B and Eureka E!3033.
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