Polymer–Mineral Interaction Influences the Mineralization of Hydroxyapatite in Hydrogels

Bone mineralization relies on the interaction between collagen and minerals to control bone growth and multiscale hierarchical structure. Urged by the increasing need for bone defect repairs, tissue engineering searches for biocompatible materials to assist and enhance repairs. One potential avenue is to use hydrogels as organic scaffolds to control nucleation and growth of bonelike minerals. Here, two biocompatible polymers, polyacrylamide and agarose, were selected for the mineralization of hydroxyapatite, and the mineralization kinetics was investigated in the presence of calcium carbonate (to simulate early bone formation conditions) and in its absence. The results of this work show that agarose and polyacrylamide lead to different polymer–mineral interactions, which influence the stabilization of carbonate and phosphate precursors and thereby the onset of the crystallization of hydroxyapatite and more so in the presence of carbonate. In both hydrogels, amorphous calcium carbonate and hydroxyapatite are noncongruent, and amorphous calcium phosphate forms as a precursor. This distinct interaction between the mineral and agarose vs polyacrylamide leads to different microstructures and thereby mechanical responses. This research not only advances our understanding of the influence of mineral–polymer interactions on hydroxyapatite mineralization but also provides new opportunities for designing biomaterials for specific applications.