Currently, materials used for the bone tissue engineering scaffolds are porous bioactive ceramics, polymer foams and their composites. A major limitation of these materials is their uncompetitive mechanical property. Therefore, development of the scaffolds with appropriate mechanical properties and good biodegradation is an important issue in the tissue engineering research field. Compared with other commonly used biomaterials, magnesium (Mg) is attractive as a bone tissue engineering scaffold material due to its good biocompatibility, suitable biomechanical compatibility as well as biodegradation. In the present work, the focus is concentrated on the feasibility of porous Mg-based bone tissue engineering scaffold material, including pure Mg and AZ31 alloy.
Considering the configuration of bone and excellent machining properties of metals, a type of novel porous Mg-based scaffold was successfully manufactured by laser perforation technique in this study, which has uniform pore distribution and interconnecting pores of three-dimensional honeycomb-like structure. The pore size is from 200μm to 500μm and the porosity is between 42% and 52%. The porous Mg-based scaffold fabricated by laser perforation technique possesses much more attractive strength than those produced by other methods. When the porosity of AZ31 alloy was 42%, the corresponding compressive strength could attain 28.88MPa. The mechanical properties of porous Mg-based scaffold were analyzed by computer simulation technique and it was found that the mechanical properties of this type of porous Mg-based scaffold significantly depended on the dimension of pore strut.
To control the biodegradation rate of Mg and AZ31 alloy and improve the surface bioactivity, a biodegradable Ca-P coating was successfully precipitated on the surface of porous Mg-based scaffold by a low-temperature soft chemical approach with two steps. Firstly, the phosphate coating produced by pre-treatment improved the corrosion resistance of Mg-based materials, which could trigger the subsequent precipitation of calcium phosphate. Then, the bioactive Ca-P coatings of different thickness, such as 10-20μm，40-50μm，70-90μm, can be adapted to the different parts of human body, and can be produced on the surface of porous Mg-based scaffold with complex shapes from the result of orthogonal arrays. The Ca-P coating prepared on the Mg-based scaffold can further improve its corrosion resistance and bioactivity, which can further induce the formation of apatite.
The Mg-based materials before and after surface treatment were soaked in Hank’s、SBF、DMEM solutions, respectively, in order to evaluate the biodegradation behaviors. The results showed that the biodegradations were obviously different. Therefore the biodegradable property of Mg-based materials in vitro can not be analyzed by only a certain experiment. The Ca-P coating could slow down the corrosion rate of Mg-based materials for 1-3 times in the initial stage of soaking in those solutions and effectively slacken the rise of pH of the extracted solutions. The continuous formation of bonelike apatite showed good bioactivity with the degradations of Mg substrate and Ca-P coating in simulated body solution.
The biocompatibility of extracted solution of Ca-P coated Mg and AZ31 alloy was studied, respectively. The results indicated nontoxicity of Mg and AZ31 with Ca-P coating. The osteoblast-like MG63 cells seeded on Mg-based scaffolds could grow well and a plenty of extracellular matrix proteins were synthesized and had good ALP activity. Compared to the β-TCP ceramic and PLA polymer scaffolds, the surface treated Mg and AZ31 alloy showed the same excellent biocompatibility.
The above results proved that porous Mg-based materials with bioactive Ca-P coatings are expected to be used as bone tissue engineering scaffolds.
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