Modelling Collagen Fibrils

Synopsis

Collagen is the most abundant type of protein in animal bodies, and is a key component of diverse tissues such as bone, skin, tendon, and the transparent cornea of the eye. Collagen assembles into hierarchical structures, ranging from molecule to fibril to tissue. Collagen fibrils can be studied in isolation, and exhibit rich structural and mechanical variety. We have been modelling collagen fibrils with the tools of soft-matter physics.  Key observables include fibril radius, the radial “double-twist” field of molecular orientation, the axial “D-band”, and mechanical properties. A cholesteric liquid crystal model exhibits equilibrium radius control, even with an axial D-band. We model slower in vivo fibril assembly with non-equilibrium growth that is ratcheted by ongoing molecular cross-linking. Mechanical properties are dominated by cross-linking, and so we model mature fibrils as liquid-crystalline elastomers. We observe strain-coexistence on compression and strain-straightening on extension. If we allow fibril rotation, we observe a novel structural bifurcation at small extensional strains.  Future directions include fibril hydration and fibril damage.