Transition from viscous to elastic-based dependency of mechanical properties of self-assembled type I collagen fibers

Fibrous collagen networks are the major elements that provide mechanical in tegrity to tissues; they are composed of fiber forming collagens in combina tion with proteoglycans and elastic fibers. Using uniaxial incremental tens ile stress-strain tests we have studied the viscoelastic mechanical propert ies of self-assembled collagen fibers formed at pHs between 5.5 and 8.5 and temperatures of 25 and 37 degreesC. Fibers formed at pH 7.5 and 37 degrees C and crosslinked by aging at 22 degreesC and 1 atmosphere pressure were al so tested. Analysis of the mechanical tests showed that the ultimate tensil e strength (UTS), and slopes of the total, elastic and Viscous stress-strai n curves were related directly to the volume fraction of polymer. Further a nalysis suggested that the UTS, and slopes of the total, elastic, and visco us stress-strain curves showed the highest correlation coefficient with the calculated effective fibril length and axial ratio. The mechanical data su ggested that at low levels of crosslinking the mechanical properties were d ominated by the viscous sliding of collagen molecules and fibrils by each o ther, which appears to be dependent on the collagen fibril length and axial ratio, while at higher levels of crosslinking the mechanical behavior is d ominated by elastic stretching of the nonhelical ends, crosslinks, and coll agen triple helix. The latter behavior appears to be dependent on the prese nce of crosslinks that stabilize fibrillar units. These results lead to the hypothesis that early in development viscous sliding of fibrils plays an i mportant role in the mechanical response of animal tissues to forces experi enced in utero, while later in development when locomotion is required, mec hanical stability is primarily a result of elastic deformation of the diffe rent parts of the collagen molecule within crosslinked fibrils. (C) 2000 Jo hn Wiley & Sons, Inc.