Qiyi Zhang

Dissolution and mineralization behaviors of HA coatings.

The dissolution and mineralization behavior of HA coatings are two of the main factors governing the bioactivity of coatings. After different post treatment operators, the plasma-sprayed HA coatings have different characteristics, including different chemical composition, crystallinity, crystallite size and dissolution behavior. In this study, HA coatings were characterized by X-ray diffraction, scanning electron microscope, and Fourier transform infrared spectra before and after immersion in simulated body fluid (SBF). When immersed in SBF, both dissolution and precipitation occurred at the same time, but the kinetics of dissolution was quite different from that of precipitation. The former was dominated by ion exchange, while the latter was controlled by the ion concentration product and the solubility of the particles. Therefore, the dissolution behaviors of phosphate ions partly depended on the dissolution behaviors of calcium ions. With the increase of ions concentration in solution by dissolution, more nucleation sites appeared on the surface of coatings. Crystalline grains gradually grew up on the nucleation sites and developed into biomineral layers. The biomineral layers were the results of the precipitation of the ions in the solution; and the carbonates partially substituted phosphates to form bone-like apatite. The different dissolution characters resulted in quite different morphology of the biomineral layers: the coatings with low solubility induced biomineral layers of large grains; on the contrary, the biomineral layers of network structure were observed on the more soluble coatings.

Bioactive films on metallic surfaces for osteoconduction

A fast and effective electrochemical method was developed to make a dense calcium phosphate films on titanium and stainless steel for hard tissue replacement. The surfaces of titanium and stainless steel were cathodically treated in an electrochemical cell. By controlling the treatment parameters, a film of 100-nm thickness was deposited on the metal surface in several minutes. The thin film was amorphous calcium phosphate containing octacalcium phosphate nuclei, and also dense and ductile. The treated metals were able to induce bioactive calcium phosphate deposition after immersion in simulated body fluid (SBF) for only 1 and 2 days. In vivo study was conducted by implanting the treated specimens of titanium and stainless steel in dogs femur cavity. The treated metallic surfaces showed good ability of osteoconduction. This surface treatment method can be potentially used to enhance bioactivity of any type of metallic surfaces.

Effects of porosity and crystallinity of glass ceramics on the in vivo bioactive response

Two types of glass ceramics with different porosities and crystallinities were produced by sintering 45S5 Bioglass particulates. The in vivo bioactive response of the glass ceramics was evaluated by implanting them in rabbit muscle sites. After six weeks implantation, silicon-rich networks were observed on both types of ceramics, but the number of networks was much larger in the poorly crystallized dense ceramics than in the well crystallized porous ones. This indicated that the crystallization of glass ceramics retarded the formation of networks on surfaces. Apatite precipitation was found in pore areas of the porous ceramics, but not on the dense ones, suggesting that porosity was necessary for glass ceramics to induce apatite precipitation in a rabbit model.