Biomimetic hydroxyapatite coating on metal implants

The combination of the high mechanical strength of metals with the osteoconductive properties of calcium phosphates make hydroxyapatite coatings on titanium implants widely used in orthopedic surgery. However, the most popular coating method, plasma spraying, exhibits some important drawbacks: the inability to cover porous implants and to incorporate biologically active agents, delamination, and particle release. The aim of this study was to elaborate a dense, strong, and thick calcium-phosphate coating on titanium and poroustantalum implants using a two-step biomimetic procedure. In the first step, the implants were soaked in a solution that was 5 times more concentrated than regular simulated body fluid (SBF-A solution). A thin but uniform amorphous calcium-phosphate coating was deposited on the metal. Then, the implants were immersed in the SBF-B solution, which had a similar composition as the SBF-A solution, but with decreased contents of crystal growth inhibitors (i.e., Mg2+ and HCO3-). This resulted in the fast precipitation of a 30 mum thick crystalline calcium-phosphate coating. The pH of the SBF-B solution and the thickness of the crystalline coating layer were studied as a function of time. The Fourier transform infrared spectra and X-ray diffraction patterns showed that this new coating closely resembles bone mineral. Our biomimetic coating should facilitate rapid bone formation around the implant, reducing therewith the patient's recovery time after surgery.