Biological materials possess some remarkable mechanical properties. Cells and tissues can adjust, remodel, stiffen, soften, in some cases even pack up and leave when circumstances require action. Surprisingly, most systems that exhibit this stunningly complex response, such as the cytoskeleton inside cells and the extracellular matrix, share a common design: under a microscope, they are crosslinked, hierarchical networks of biological polymers. Even more surprisingly, many of the in vivo behaviors can be reproduced in vitro in reconstituted proteinaceous polymer gels. Many of these systems, most notably collagen, play a purely structural role in living organisms. In other words, their function _is_ their mechanical response. Biopolymer networks are therefore particularly suited to begin to understand the complex relationship between structural design and functionality in living systems.

In this lecture, I introduce the general theory of nonlinear elasticity, and I will discuss our efforts to bridge the gap from microscopic structure to macroscopic mechanical response of such nonlinear systems using collagen as an example. Time permitting, I will discuss our first steps towards controlling the mechanical properties of biomimetic synthetics.