A composite coating by electrolysis-induced collagen self-assembly and calcium phosphate mineralization

A composite coating that is composed of collagen protein and calcium phosphate minerals is considered to be bioactive and may enhance bone growth and fixation of metallic orthopedic implants. In this study, we have successfully developed a uniform collagen fibril/octacalcium phosphate composite coating on silicon substrate by electrolytic deposition (ELD). The coating deposition was done through applying a constant potential to the cathode in a three-electrode electrochemistry cell that contain a mild acidic (pH 4.8-5.3) aqueous solution of collagen molecules, calcium and phosphate ions. The coating process involved self-assembly of collagen fibrils and the deposition of calcium phosphate minerals as a result of cathode reaction and local pH increase. The two steps could be synchronized to form a bone-like composite at nanometer scale through proper adjustment of the solution and deposition parameters. Coating morphology, crystal structure and compositions were analyzed by optical and fluorescence microscopy, scanning and transmission electron microscopy, energy dispersive X-ray analysis, inductively coupled argon plasma optical emission spectrophotometry, and Fourier-transformed infrared spectroscopy. Under typical deposition conditions, the cathode (Si) surface formed a thin (100 nm) layer of calcium phosphate coating, on top of which a thick (approximately 100 microm) composite layer formed. The porous composite layer consists of a collagen fibril network on which clusters of octacalcium phosphate crystals nucleate and grow. By combining photolithography and ELD, we were also able to pattern the composite coating into regular arrays of squares. Preliminary results by nanoindentation tests showed that properly prepared composite coating may have higher elastic modulus and scratch resistance than monolithic porous calcium phosphate coating. The results not only provide a novel bioactive coating for biomedical implants, but also establish a new experimental protocol for studying biomineralization mechanisms of collagen based biological tissues.