Time-resolved X-ray diffraction from tendon collagen during creep using synchrotron radiation

In order to understand the molecular mechanism of relaxation phenomena in collagenous tissue, time-resolved, small-angle X-ray diffraction measurements were performed on bovine Achilles tendon collagen under creep. A tension-induced increase in the 67 nm period (D-period) was observed, and the strain in the D-period, εD, was found to be almost proportional to the external force per unit cross-sectional area (average stress) of the specimen. With an increase in εD, a change in the ratio of intensities of the third-order reflection peak of the D-period to that of the second-order peak was also observed. The increase in εD was decomposed into three elementary processes of D-period deformation, which are presented on the basis of the Hodge–Petruska model: (1) molecular elongation, (2) increase in gap region, and (3) relative slippage of lateral adjoining molecules. Up to 8 MPa of average stress, the contribution to εD originated mostly from only mode (1). At more than 10 MPa of average stress, modes (2) and (3) also contributed to fibril elongation. For εD by molecular elongation (mode (1)), the time dependence of the D-period change in the immediate response region is a sharply shaped step function, while the contribution to εD by molecular rearranging modes gives a slight creep nature at the immediate response region in the time dependence of εD. Because this creep nature is observed at the immediate response, it is related qualitatively to the KWW function in a stress-relaxation modulus of collagenous tissue observed in an immediate response region (Sasaki et al. (1993) Journal of Biomechanics 26, 1369–1376). The elementary process of KWW-type relaxation is concluded to be related to the tension-induced molecular rearrangement within a D-period.