Collagen gel fibrillar density dictates the extent of mineralizationin vitro

Abstract

Current in vitro models have provided insights into collagen mineralization. However, the role of the collagen fibrillar framework in the formation of carbonated hydroxylapatite (CHA) has not been clarified due to difficulties in processing biomimetic fibrillar collagen gels with tailored properties. In this work, reconstituted collagen gels underwent various degrees of plastic compression (PC) to produce sheets, rolls and strips of controlled collagen fibrillar densities (CFDs, 0.2, 3.5, 14.1 weight%). Increasing levels of PC corresponded with a decrease in the thickness of the gels and their roughness, indicating a more compacted nano-fibrillar structure. Immersion in simulated body fluid resulted in the bulk mineralization of the various matrices. Increased CFD was positively correlated with the extent of CHA formation in the gels and with the volume fraction of mineralized fibrils. Tensile mechanical testing demonstrated that the apparent modulus and ultimate tensile strength of the gels increased with CFD and mineralization extent. Exposing collagen gels to cationic and anionic dye solutions indicated that an increase in CFD also corresponded with an increase in the extent and concentration of dyes bound to the collagenous framework. The results suggested that the increase in CFD altered gel electrostatic properties and generated a microenvironment more favorable to crystal formation.