Collagen mineralization with lepidocrocite via Fe(OH)₂ addition

The mineralization of collagen in vitro has been extensively investigated for hydroxyapatite, silica, calcium carbonate and lepidocrocite (γ-FeOOH). Henceforth, it is interesting to investigate whether collagen also could serve as a generic mineralization template for other minerals, like magnetite. To this end, and inspired by the partial oxidation approach, first a ferrous hydroxide (Fe(OH)₂) intermediate is synthesized via the titration of base to a solution of Fe²⁺. Subsequently, the Fe(OH)₂ is mixed with collagen fibrils and poly(aspartic acid) is added to promote the formation of intrafibrillar crystals. Platelet-shaped lepidocrocite crystals being present throughout the entire thickness of the collagen fibrils can be realized, as was confirmed with electron tomography. The formation of lepidocrocite, which is an Fe³⁺ compound, is hypothesized to be induced via oxidation of the Fe²⁺ species and, therefore, the oxygen concentration during titration, TEM sample preparation and drying of TEM samples are investigated. Although the reaction is sensitive to small changes in experimental conditions, highly mineralized collagen fibers can be realized.