Thermal assembly of a biomimetic mineral/collagen composite

A strategy is described for exploiting temperature driven self-assembly of collagen and thermally triggered liposome mineralization to form a mineralized collagen composite from an injectable precursor fluid. Optical density and rheological experiments demonstrated the formation of a collagen gel when acid-soluble type I collagen solutions (1–7 mg/ml) were heated to 24–30°C. Scanning calorimetry experiments demonstrated that mixtures of calcium- and phosphate-loaded liposomes composed of dipalmitoylphosphatidylcholine (90 mol%) and dimyristoylphosphatidylcholine (10 mol%) were stable at room temperature but formed calcium phosphate mineral when heated above 35°C, a consequence of the release of entrapped salts at the lipid chain melting transition. The formation of calcium phosphate mineral induced by triggered release of calcium and phosphate was detected as an endothermic transition (ΔH=6.2±1.1 kcal/mol lipid) near the lipid chain melting transition (Tm=37°C). Combining an acid-soluble collagen solution with calcium- and phosphate-loaded liposomes resulted in a liposome/collagen precursor fluid, which when heated from room temperature to 37°C formed a mineralized collagen gel. The dynamic storage modulus of the collagen scaffold increased upon mineralization, and direct nucleation of mineral from the collagen scaffold was detected by electron microscopy.