Figure 3.

Force-displacement and stress-strain relationships at different levels of the ligament and tendon hierarchy. Each scale exhibits non-linear behavior and the behavior of higher level structures is due to a combination of the behavior of subscale features directly and their interactions. A) force-extension of a single tropocollagen molecule, stretched by optical tweezers. Since, the optical tweezer method is not capable of achieving failure loads for a collagen molecule, this curve only displays behavior of the molecule at low strains. The elongated toe region is a result of stretching the molecule to its contour length, estimated at 315 ± 44nm. While this toe region appears long with respect to the molecule force-extension curve, it is still short compared to the micron to millimeter scale toe regions observed for higher level structures (reprinted with permission, from Sun et al.⁶⁶). B) Stress-strain curves from human patellar tendon (HPT) and native rat tail tendon (N-RTT) fibrils, loaded in tension using an atomic force microscope (reprinted with permission, from Svensson et al.⁷²). C) Stress-strain curve of a single fascicle from rat tail tendon, loaded by an electro-mechanical test system. This shape of this curve holds strong similarity to that observed for collagen fibrils. The toe region at the fascicle level is likely the result of molecules stretching to their contour length and uncrimping at the fiber level.