Nonlinear elasticity in biological gels

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Storm, C., Pastore, J.J., et al., Nature 435 (2005).


elasticity, polymer gel


Figure 1. a. Diagram of experimental setup. b. Actual picture of particle structuring. c. In-layer particle structure inside wedge film.
Figure 2. a. Disjoining pressure versus film thickness. b. Spreading coefficient as a function of film thickness.

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.

The proposed model starts with the force of a single filament. To get from there to the bulk elastic properties of a network of filaments, several assumptions are made. First, the network is assumed to be isotropic, in which a pair of nodes are connected by independent semi-flexible filaments. It is also assumed that no torques are exerted at nodes, so that filaments can stretch or compress but cannot bend. Lastly, the deformations are assumed to be affine.