Preparation And Properties Of The Biomedical Porous Magnesium And Bioglass Ceramic Cement Coatings On The Surface Of Pure Magnesium

Although traditional biomedical metals (e.g.316L stainless steels, Ti and its alloys, etc.) possess excellent mechanical properties, their applications for tissue engineering and orthopedic implants are still limited by the low bioactivity and release of certain toxic elements into the human body. In contrast, as a kind of the potential bone tissue scaffold material, porous metallic Mg may exhibit no toxicity, biodegradability and mechanical compatibility (e.g., low Young’s modulus). However, the rapid corrosion of magnesium and its alloys in chloride containing solutions including human body fluid or plasma has limited their clinical applications. Meanwhile, hydrogen gas could yield collectively around the Mg implants as a result of fast corrosion. Therefore, it is very important to improve the corrosion resistance of Mg and its alloys in order that they can be applied clinically. In this way, it is necessary to find the efficacious surface modification technologies for reducing its degradation rate and introducing a highly biocompatible bone-implant interface. In the present work, the preparation, microstructures, mechanical properties, in vitro and corrosion resistant properties of the biocompatible porous magnesium and bioglass ceramic cement (BGCC) coatings on the surface of pure magnesium have been studied systematically, in order to develop novel porous Mg scaffolds and BGCC/Ca-P coated Mg, which would have a good potential for biomedical applications.Pure magnesium powders were used as a starting material, and salt particles were chosen as space holder. A novel magnesium based scaffold with a specific two-layer structure was prepared by powder metallurgical process. The outer layer of the scaffold shows an interconnected porous structure, which can meet the demands for the fresh fluid to be sent into the material and thus allow the ingrowth of new bone tissues, while the inner compact structure reinforced by the salt particles can bring about a higher strength of the material. Experimental results indicate that the porous Mg specimens with a higher porosity could be prepared by using irregular shaped powders, while the porous Mg specimens with higher compressive strength and Young’s modulus could be prepared by using spherical powders.A novel porous Mg scaffold with the porosity of 33-54% was successfully produced by a method of fiber deposition followed by hot pressing. The porous Mg scaffold had a 3D interconnected network structure, and the pore size, porosity and pore distribution were controlled by fiber size, fiber spacing and hot pressing parameters. Compression tests show that the mechanical properties of the prepared porous Mg samples exhibit an anisotropy. In particular, for the porous Mg specimen oriented towards the axial direction (parallel to the direction of hot pressing), which has a porosity of 48%, with pore sizes ranging from 180 μm to 300 μm, its compressive strength and Young’s modulus reach 17.6 MPa and 0.22 GPa, respectively, which are fairly comparable to those of cancellous bone.To control the biodegration rate and improve the biocompatibility and osteointegration of pure Mg, the BGCC coating, Ca-P coating and BGCC/Ca-P coating were prepared on the pure Mg ribbons in the present work. The bioglass ceramic powder (BGC) was prepared by sol-gel method, and its main phase composition are composed of Ca2SiO4·0.05Ca3(PO4)2 and Ca2P2O7. Subsequently, pure Mg ribbons were immersed into the BGCC slurry, which was prepared by the mix of the BGC powders and the phosphate liquid ((NH4)2HPO4 and NH4H2PO4) with different liquid-to-solid ratios (L/S), to obtain BGCC coatings (abbreviated as B1-B5) by a dipping-pulling method. As the L/S ranges from 0.8 to 1.2, the main phase composition is HA. As the L/S ranges from 1.5 to 2.0, the phase composition primarily comprises Ca(NH4)2(HPO4)2-H2O. The in vitro assessments of the B1-, B2-and B3-coated Mg specimens indicated that the L/S has significant effects on the morphologies of new precipitated crystals on the surface of BGCC coating. The petal-like crystals with a small size were precipitated as the L/S is 0.8, while the ball-like crystals were precipitated as the L/S is 1.0. However, when the L/S is increased to 1.2, the surface of the coating becomes more smooth and densified, and meanwhile, the plate-like crystals have been precipitated.In order to improve the biocompatibility of pure Mg, the Ca-P coating was chosen as an intermediate transition layer to enhance bonding interfaces both with Mg substrate and outer BGCC coating. In doing so, H3PO4 and HNO3 were added into the coating solution to adjust the pH value, respectively. Two kinds of coatings called Ca-P1 and Ca-P2 with Ca/P molar ratio being-1.00 and<1.00, respectively, were thus prepared by aqueous solution method. The Ca-P2 coating has a better bonding with Mg substrate than Ca-P1 coating, and it could keep stable in SBF solution and effectively induce the formation of apatite layer on its surface However, it is found that the both Ca-P coatings have peeled from the surface of the Ca-P coated Mg in some local areas after immersion in SBF solution for 15 days. This implies that a simple Ca-P coating could not provide an effective corrosion resistance for Mg substrate if the material is implanted into human body for a long-time.Three kinds of slurries of BGCC1, BGCC2 and BGCC3 were prepared by the mix of the BGC powders and the phosphate liquid with different concentration ratios of (NH4)2HPO4 and Ca(H2PO4)2 with an L/S ratio of 1.6. The deposition mechanism of BGCC coating on Mg substrate in coating solutions can be described as follows:the BGCC coating was obtained by the BGCC slurry setting, and the deposition of the BGCC coating is mainly governed by the crystalline phase of Ca2SiO4·0.05Ca3(PO4)2 in the BGC powder. On the one hand, the hydration reaction of Ca2SiO4 occurs, inducing a large number of ions of Ca2+, and increasing the pH value of the solution. Then, in this enhanced alkaline environment, the transformation of the H2PO4- and HPO42- into PO43- was promoted. Finally, the CDHA was produced by the reaction of Ca2+ with PO43-. On the other hand, the hydration reaction of Ca3(PO4)2 also leads to the formation of CDHA.In order to find a BGCC/Ca-P coating with optimum properties, BGCCl/Ca-P1、 BGCC2/Ca-P1 and BGCC3/Ca-P2 coatings have been prepared. The phase compositions are composed of HA for both BGCC1/Ca-P1 and BGCC2/Ca-P1 coatings, while the BGCC3/Ca-P2 coating consists mainly of CDHA. The in vitro assessments indicated that three kinds of BGCC/Ca-P coatings possess excellent biocompatibility and osteoconductivity. The potentiodynamic polarization tests indicated that the corrosion current densities of the BGCC1/Ca-P1, BGCC2/Ca-P1 and BGCC3/Ca-P2 coated Mg are 3 times,5 times and 10 times lower than that of uncoated Mg. Therefore, the BGCC3/Ca-P2 coated Mg exhibits the superior biocompatibility and corrosion resistant properties.