Bioactive coatings for direct skeletal attachment applications for lower limb prostheses

Abstract

To tackle the current drawbacks with metallic implants used in direct skeletal attachment, novel bioactive glasses are considered as implant coatings in order to reduce bacterial infections and promote bone cell growth. Silica-based and borate-based glasses, with increasing amounts of titanium dioxide at the expense of either silica (for the silica-based glasses) or borate (for the borate-based glasses), respectively, were synthesized and characterized to determine the parameters that define a glass capable of inhibiting bacterial growth, stimulating cell proliferation and offering mechanical stability when enameled into a surgical alloy. The effect of substituting the glass backbone with titanium dioxide, in both glass series, is also investigated with respect to its effect on both biocompatibility and mechanical properties of the resultant glass/implant constructs. Borate-based glasses exhibited greater processing windows compared to the silica-based glasses, making them more desirable for coating applications. They also exhibited superior performance in terms of their in vitro bioactivity and biocompatibility, over their silica-based counterparts, due to their higher solubility and greater ability to inhibit S. epidermidis and E. coli bacteria. Specifically, glass BRT0 (control borate-based glass, with no titanium incorporated) exhibited an inhibition zone against S. epidermidis of 17.5 mm and a mass loss of 40% after 30 days, with BRT3 (borate-based glass, with 15 mol% titanium incorporated) exhibiting an inhibition zone against S. epidermidis of 7.6 mm and a mass loss of 34% after 30 days. Furthermore, borate-based glasses with greater titanium dioxide contents exhibited superior mechanical properties (e.g. bulk hardness, and critical strain energy release rates), which could be attributed to their more closely matched coefficients of thermal expansion with the titanium alloy substrate, Ti6Al4V, to which they were adhered. The critical strain energy release rates in mode I for the silica-based coating/substrate system ranged from 6.2 J/m2 (for SRT0, control silica-based glass with no titanium) to 12.08 J/m2 (for SRT3), whereas for the borate-based systems they ranged from 10.86 J/m2 (for BRT0) to 18.5 J/m2 (for BRT3), with the increase for the borate-based glasses being attributed to the presence of compressive residual stresses in the coating after application.