Compressing collagen fibrils attached to an elastic substrate: buckling and torsional instabilities*

Collagen fibrils are nanoscale biological ropes with a diameter between 50 and 500 nm. They are the main constituent of load bearing tissues and are known to be strong under tension. Fibrils can also be very long in some tissues, up to millimeters in length, which means that they are in principle prone to buckling and torsional instabilities. One way to study buckling of a rigid rod is to attach it to a stretched elastic substrate that is then returned it to its original length. We extract single collagen fibrils from bovine tendons and absorb them on a pre-stretched silicon rubber. We characterize the shape of the fibrils after and during strain release using atomic force microscopy, optical microscopy and second harmonic generation microscopy. We observe both the well-characterized short wavelength sinewave buckling mode and two types of torsional instabilities, writhing and bird-caging. The onset of buckling and bird-caging occurs at substrate pre-stretch around 1 to 3%. This high susceptibility to compression has implications on the way collagen fibrils are organized within tissues.