Nonlinear elasticity in biological gels
Storm, C., Pastore, J.J., et al., Nature 435 (2005).
elasticity, polymer gel
This paper deals with elucidating the nonlinear elastic properties common to biological gels. As shown in Fig. 1, the shear moduli of various biological networks vary over orders of magnitude as a function of applied strain. The molecular structures responsible for the nonlinear elasticity are unknown, but the paper reports a molecular theory that accounts for the strain-stiffening in these biological networks.
In polymer theory, there are three types of filaments, characterized by the persistence length and the contour length. The persistence length is a measure of stiffness: for pieces of polymer shorter than the persistence length, the molecules can be approximated as a flexible rod; for pieces longer than the persistence length, a random walk is more appropriate to describe the mechanics of the polymer. The contour length is simply the length of the polymer if stretched to its full length. The first two types of filaments are filaments for which the persistence length dominates the contour length, and vice verse. The third is one for which the two length scales are comparable. These filaments do not form loops and knows, but they are flexible enough to have thermal bending fluctuations.