Assembly of type I collagen: fusion of fibril subunits and the influence of fibril diameter on mechanical properties

Structural stability of the extracellular matrix is primarily a consequence of fibrillar collagen and the extent of cross-linking. The relationship be tween collagen self-assembly, consequent fibrillar shape and mechanical pro perties remains unclear. Our laboratory developed a model system for the pr eparation of self-assembled type I collagen fibers with fibrillar substruct ure mimicking the hierarchical structures of tendon. The present study eval uates the effects of pH and temperature during self-assembly on fibrillar s tructure, and relates the structural effects of these treatments on the uni axial tensile mechanical properties of self-assembled collagen fibers. Resu lts of the analysis of fibril diameter distributions and mechanical propert ies of the fibers formed under the different incubation conditions indicate that fibril diameters grow via the lateral fusion of discrete similar to 4 nm subunits, and that fibril diameter correlates positively with the low s train modulus. Fibril diameter did not correlate with either the ultimate t ensile strength or the high strain elastic modulus, which suggests that lat eral aggregation and consequently fibril diameter influences mechanical pro perties during small strain mechanical deformation. We hypothesize that sel f-assembly is mediated by the formation of fibrillar subunits that laterall y and linearly fuse resulting in fibrillar growth. Lateral fusion appears i mportant in generating resistance to deformation at low strain, while linea r fusion leading to longer fibrils appears important in the ultimate mechan ical properties at high strain. (C) 2000 Elsevier Science B.V./Internationa l Society of Matrix Biology. All rights reserved.