Controllable Electrochemical Preparation And Characterization Of Minerzalied Collagen Coatings

The promotion or acceleration of bone repair is the most important area of biomaterials. Titanium (Ti) and its alloys are the most commonly used metals for orthopedic and dental implants, due to their good biocompatibility, excellent mechanical properties and good corrosion resistance. However, the cell response for Ti and its alloys is unsatisfactory because of lack of biological activity. Thus, the research on the surface modification of metallic implants to improve the biological response and create the loading-release capacity for biological factors/drug is one of the most attractive aspects of biomaterials.The extracellular matrix of natural bone is composed mainly of collagen, and bone tissue has hydroxyapatite (HA) as the major inorganic phase. Therefore, Ti surface modification with collagen/HA composite (mineralization) coating can promote the attachement, growth and proliferation of bone cells, as well as to increase the biological factor/drug loading-release capacity, which could effectively improve osseointegration capacity and surgery quality of implantation. This research focuses on the controllable preparation of mineralized collagen coating on the Ti implant by using electrochemical deposition method (ELD) with regulation of electrolyte composition and deposition parameters.According to the results, a porous oxide layer was synthesized on Ti substrate after alkali heat treatment, which became more electronic conductive, can effectively improve the uniformity and adhesion strength of the mineralized collagen coating. Deposition temperature affected coating morphology and calcium phosphate crystalline phase, when deposited at 37℃or even higher, HA/collagen coatings were obtained. Deposition potential significantly influenced the morphology of the coatings, leading to porous structure at high potential and dense structure at low potential. As the deposition potential changed, the generation rate of OH" ions near cathode changed, consequently this could influence the mineralization and deposition of collagen. H2O2 could change the cathode reaction during ELD process. When added to electrolyte, H2O2 accelerated the generation of OH- ions near cathode, so the coating could be achieved in a shorter deposition time and lower deposition potential. When deposited at the concentration of H2O218 mM, pH of electrolyte 4.5, temperature 37℃, a dense mineralized collagen coating could be obtained at 1.3-1.9 V and a porous mineralized collagen coating at 2-2.7 V.The collagen fibrils in both porous coating and dense coatings were mineralized. The collagen fibrils in porous coating had a higher degree of mineralization than that of dense coating. The dense coating had a low crystallinity of HA and the collagen fibrils tended to be dense accumulation. The coatings were quasi-three dimensional structure with thickness of 20-30μm. The coating had a multilayer structure, the bottom layer was a HA layer, the middle layer was composed of HA and mineralized collagen, the top layer was mineralized collagen. The HA layer played a significant role in bonding the top mineralized collagen layer with the substrate, leading to the coating with a reasonably good adherent strength.In vitro evaluation showed the mineralized collagen coatings were stable in Kokubo’s simulated body fluid. Cell experiments showed mineralized collagen coatings promoted cell attachment and proliferation, and displayed a good cytocompatibility. The mineralized collagen coatings loaded with vancomycin showed a sustained drug release and an inhibitory effect on the growth of S. aureus, which demonstrated that the coatings could be used as a coating platform for loading biological factors/drugs.Based on the general ELD considerations and the present results, a comprehensive ELD model of mineralized collagen coatings was proposed. The deposition model includes is three steps:Firstly, the deposition of calcium phosphate onto cathode at the beginning of the electrolytic process, simultaneously, the self-assembly of collagen fibrils takes place at its isoelectric point. Secondly, the mineralization of collagen fibrils or self-assembled collagen fibrils occurs process. Finally, the mineralized collagen moved to the Ti substrate under the impetus of the electrical force and deposited onto the calcium phosphate layer. From the model, it can be concluded that the self-assembly and mineralization behavior played a vital role in ELD of the mineralized collagen coatings, so the key to controllable preparation of mineralized collagen is the regulation of isoelectric point of collagen and the pH gradient near cathode.