  Spider capture silk is a biomaterial with both high strength and high
elasticity, but the structural design principle underlying these remarkable
properties is still unknown. It was revealed recently by atomic force
microscopy that, an exponential force--extension relationship holds both for
capture silk mesostructures and for intact capture silk fibers [N. Becker et
al., Nature Materials 2, 278 (2003)]. In this Letter a simple hierarchical
chain model was proposed to understand and reproduce this striking observation.
In the hierarchical chain model, a polymer is composed of many structural
motifs which organize into structural modules and supra-modules in a
hierarchical manner. Each module in this hierarchy has its own characteristic
force. The repetitive patterns in the amino acid sequence of the major
flagelliform protein of spider capture silk is in support of this model.
