Construction of multilayered molecular reservoirs on a titanium alloy implant for combinational drug delivery to promote osseointegration in osteoporotic conditions.

In this study, β-cyclodextrin (β-CD) molecules are used as molecular reservoirs and grafted onto chitosan molecules for calcitriol (VD3) loading, which is a hormonally active metabolite of vitamin D. The resultant molecular complex is co-assembled with an antiosteoporosis drug calcitonin (CT) to form bio-functional multilayer structure on Ti6Al7Nb substrate via layer-by-layer self-assembly, which is capable of releasing VD3 and calcitonin in a sustained manner to modulate osteoblasts, osteoclasts, and macrophages at the bone-implant interface. In vitro results show that the released VD3 and CT individually upregulated the expression of calcium-binding protein (including Calbindin D9k and Calbindin D28k) and BMP2 in osteoblasts in peri-implant regions to stimulate their Ca deposition and differentiation. RAW264.7 cells (a murine macrophage) on the biofunctional implant displayed improved M2 phenotypical differentiation and expression of BMP2 and VEGF genes, but M1 phenotypical differentiation potential and MCF and TRAP gene expression levels are evidently lower. Results from in vivo micro-CT and histological analysis also demonstrate that VD3/CT co-loaded implant can dramatically enhance the bone remodeling under osteoporotic conditions with significantly enhanced interfacial shear strength and improved osseointegration as compared to other groups. The insights in this study offer new avenues for the rational functionalization of titanium implants to effectively repair osteoporotic fractures. STATEMENT OF SIGNIFICANCE: A promising strategy to enhance the recovery rate of osteoporotic fractures is to immobilize antiosteoporotic drugs onto the surface of titanium-based implants. In this study, we grafted beta-cyclodextrin (β-CD) onto chitosan (Chi) molecules to load VD3, which was co-assembled with calcitonin (CT) onto Ti6Al7Nb implants by the layer-by-layer assembly technique. The obtained functional titanium alloy implant (Ti6Al7Nb/LBL/Chi-CD@VD3/ CT) could stably release VD3 and calcitonin agents in a sustained manner. RAW264.7 cells grown on Ti6Al7Nb/LBL/Chi-CD@VD3/CT showed superior M2 phenotypical differentiation efficiency, but lower MCF/TRAP gene expression levels. In vitro and in vivo results showed that the released VD3 and CT individually upregulated the expression of calcium binding proteins and BMP2 in osteoblasts, promoting new bone formation in the peri-implant region.