Molecular scale insights into initial apatite growth to enhance bone regeneration

Abstract

Bone mainly consists of hydroxyapatite (Ca10(PO4)6(OH)2) and collagen. So far, the interplay of hydroxyapatite with body fluids during the mineralization process, is unclear due to experimental difficulties unraveling dynamic molecular scale effects. We hypothesize that the structure of the Electrical Double Layer (EDL) influences the initial growth of apatite, possibly decisively steering nucleation. For detailed characterization, growth of calcium hydrogen phosphate (CaHPO4) on atomically smooth mica surfaces was studied as a function of different solution conditions using three approaches: (1) layer-by-layer deposition through dipcoating, (2) in situ growth using Atomic Force Microscopy (AFM) and (3) in situ growth monitored within the Quartz Crystal Microbalance with Dissipation (QCM-D). In addition, a 3D-AFM method was developed to characterize the solid/liquid interface structure before, during and after growth initiation. Growth of CaHPO4 was initially accompanied by the undesired formation of Calcite (CaCO3) due to presence of environmental carbon dioxide (CO2). Interestingly, this process was induced by the first surface hydration layer. Specifically, irreversible calcium adsorption leads to charge reversal that can result in local carbonate concentrations above the solubility level, in turn initiating growth. Calcite growth is eliminated by environmental control. Current work on apatite growth indicates similar effects, implying the importance of the EDL on crystal growth initiation.