Refinement of Nanocomposite Coating and Magnesium Substrate Surface Properties for Improved Degradation Resistance and Bone Cell Adhesion

The mechanical and biological properties of magnesium (Mg) alloys are ideal for degradable bone implant applications, but excessively rapid Mg degradation in vivo limits their clinical translation. This rapid degradation may be prevented by controlling the bulk properties of Mg through methods such as alloy composition and processing, or by controlling the surface properties of Mg through methods such coatings and surface treatments.The effects of alloy composition were investigated by the comparison of commercially pure Mg and a Mg alloy containing yttrium (Y), both of which had oxidized or polished surfaces. The Y alloy component could decrease degradation by forming a more stable passivation layer or increase degradation by forming a galvanic couple with Mg. These results demonstrated that the effects of the alloy parameters depended upon their interactions with each other and the working environment.Nanocomposite coatings have the potential to control the degradation rate of Mg substrates while simultaneously performing additional functions, such as increasing the adhesion of bone cells. However, the duration over which such nanocomposite coatings remain effective on Mg substrates may be limited by delamination caused by gas evolution and surface instability. Improved engineering of nanocomposite coatings can increase their barrier properties and extend the duration of their effectiveness. Nanohydroxyapatite (nHA)/Poly(caprolactone) (PCL), nHA/poly(lactic-co-glycolic acid) (PLGA), and nHA/poly(L-lactic acid) (PLLA) coatings were compared for their ability to reduce Mg degradation. The coating deposition method was refined, and the effects of post-deposition processing upon the coatings were characterized. All of the coatings significantly reduced Mg degradation and did not delaminate.The nHA/PCL coatings reduced Mg degradation more significantly than the other coatings, and thus were tested for the adhesion of bone marrow stromal cells (BMSCs). The nHA/PCL coatings increased the density of adhered cells and their spreading areas, and prevented changes to their aspect ratios. As an additional benefit from this work, guidelines for the rational design of nanocomposite coatings were elucidated, which will aid in the clinical translation of Mg-based degradable biomaterials for bone implant applications