a, Hierarchical structure of collagen. b, Force–strain response of a single tropocollagen molecule under uniaxial stretching (top) and under compression (bottom). Insets show snapshots at various levels of deformation. The maximum load that can be sustained by the tropocollagen molecule under pure compression is predicted to be much lower than that under tension because of buckling. The simulations also reveal the underlying mechanisms of strain-hardening of the tropocollagen molecule under stretching. c, Deformation map of collagen fibrils obtained with a multiscale modelling scheme based on atomistic and molecular simulations. The results reveal that the mechanical response is governed by two critical length scales. The physiological collagen range is also shown (L 300 nm). (a and c are reprinted in part with permission from ref. 62; b is reprinted in part with permission from ref. 80.)]