In vitro fibrillogenesis of tropocollagen type III in collagen type I affects its relative fibrillar topology and mechanics

Tropocollagen types I and III were simultaneously fibrilized in vitro, and the differences between the geometric and mechanical properties of the heterotypic fibrils with different mixing ratios of tropocollagen III to I were investigated. Transmission electron microscopy was used to confirm the simultaneous presence of both tropocollagen types within the heterotypic fibrils. The incorporation of collagen III in I caused the fibrils to be thinner with a shorter D-banding than pure collagen I. Hertzian contact model was used to obtain the elastic moduli from atomic force microscope indentation testing using a force volume analysis. The results indicated that an increase in the percentage of tropocollagen III reduced the mechanical stiffness of the obtained fibrils. The mechanical stiffness of the collagen fibrils was found to be greater at higher loading frequencies. This observation might explain the dominance of collagen III over I in soft distensible organs such as human vocal folds.