Protein filaments exhibit unique material properties. Most synthetic polymers are highly flexible and act as linear springs. As a result, such networks exhibit a linear elastic response under shear.  In contrast, the mechanics of crosslinked biopolymer networks of protein filaments is highly nonlinear and very sensitive to the nature and concentration of crosslinks. The reason is that protein filaments are many orders of magnitude stiffer than synthetic polymers. As a results, when a cross-linked network of biopolymers is subjected to a deformation, the individual filaments either buckle or stretch in the direction of the applied shear. This results in an increase of the overall network elasticity, a phenomenon  known as strain-stiffening behavior.
Using a special shear cell device designed and built in house we are able to observe the behavior of the individual filaments inside biopolymer networks, such as actin, fibrin, collagen, and intermediate filaments, under an applied shear. The bulk non-linear stiffening of those networks can be further quantified using a rheometer.

More information: Izabela Piechocka