Evidence for Interfibrillar Shear Load Transfer Between Sliding Fibrils in Tendon

While collagen fibrils are understood to be the primary tensile load bearing components in tendon, how loads applied at the tissue level are transmitted across each element within the tissue hierarchical structure is unclear. A central unresolved question is whether collagen fibrils bear load independently or if the applied load is transferred across the fibrils through interfibrillar shear forces. Relative sliding between fibrils is suggested by findings that fibril strains within rat tail tendon fascicles do not agree with the applied tissue tensile strains [1]. Other studies using confocal microscopy have directly measured sliding behavior [2,3]; however, the impact that interfibrillar sliding has on tendon macroscale mechanics and whether sliding is associated with interfibrillar shear stresses are unknown. Therefore, the objective of this work is to quantify the contribution of interfibrillar sliding on tendon macroscale mechanics by simultaneously measuring the tissue behavior at both length-scales and interpreting the results with a micro-structural shear lag model directly incorporating interfibrillar shear stresses. We hypothesize that the reduced stiffness and increased viscosity observed in the tissue macroscale properties at higher strains are due to increases in interfibrillar sliding and that this behavior is consistent with a shear lag model involving interfibrillar shear stress.