Engineering Osteoinductive Biomaterials by Bioinspired Synthesis of Apatite Coatings on Collagen Hydrogels with Varied Pore Microarchitectures

Biomaterial controlled osteoinduction is influenced by the porous microenvironment and the composition of incorporated calcium orthophosphate (CaPi) polymorphs, however, for the design of materials that rival the efficacies of natural grafts a systematic approach to assessing the physicochemical properties that affects cellular differentiation is needed. In this research, we introduce a bioinspired synthetic approach to the mineralization of preformed porous collagen hydrogel scaffolds with tunable apatite coatings. Initially, collagen scaffolds are mineralized with dicalcium phosphate dihydrate (DCPD) by alternate immersion in Ca2+ and HPO42- salt solutions. Utilizing classic DCPD conversion chemistry, the surface coatings are selectively transformed to apatite by immersion of the DCPD-collagen substrate in Tris buffer at pH 7.4, 37 degrees C, for 5 days. The composition and morphology of the deposited mineral coatings are characterized by XRD, SEM, and AFM. Variations in the porous microarchitecture of the collagen hydrogel substrate, pore size (9.5 +/- 5m, 165 +/- 50m) and pore alignment altered the morphology of the deposited apatite particles. Intrafibrillar and extrafibrillar mineralization of the collagen templates were observed for both investigated pore sizes. However, templates with aligned pores of both sizes were observed to restrict intrafibrillar mineralization resulting in the exclusive deposition of surface coatings. The osteoinductive activity of the apatite-collagen materials with varied pore microarchitectures was evaluated by in vitro culture of human mesenchymal stem cells for 28 days based on cellular proliferation, alkaline phosphatase activity, and the expression of RUNX2. The combined effects of apatite coatings, reduced pore size, and pore alignment contributed to reductions in cellular proliferation. However, the apatite mineral coating was determined to induce high levels of RUNX2 expression in the absence of additional osteoinductive agents, indicative of biomaterial-induced osteogenesis. This work establishes a versatile synthetic platform for the preparation of bone-like apatite collagen materials with osteoinductive activity.