| The designs of highly extensible soft materials in nature are of a fundamental interest to engineers so that insights into the production of modern, synthetic materials can be gleaned. Marine gastropods of the genus Busycotypus produce a protein based elastomer which possesses a unique combination of stiffness and extensibility. Furthermore, this material displays shape-memory/self-healing properties that are unmatched in synthetic engineering systems. Four variants of the precursor protein components of the egg capsules are highly unique in their amino acid sequences, showing no homology to any known protein families. These proteins are strongly a-helical in nature, and can self-assemble into nanometer scaled fibers in vitro. The remarkable reversibly extensibility of the bulk material is dictated not by entropic forces, but rather by a crystalline phase transition within the protein components when the material is put under tension. Proteins shift from alpha-helix to beta-sheet, and it is this uncoiling of helices within the polymer backbone that allows for the extensibility of the egg capsules. This transition is reversible, as when loads are removed and the material is allowed to relax, it returns to its original alpha-helical conformation. When examined more closely, it is shown that this alpha ↔ beta transition is a multi-step transformation which involves first the uncoiling of crystalline alpha-helices into non-crystalline random coils before these then lock into beta-sheets. These different steps dictate changes in the mechanical properties of the material as this transition is occurring. Furthermore, the supramolecular structure of how the individual proteins interact in the intact material also changes throughout tension/relaxation cycles. These structural changes also have effects on the bulk mechanical properties of the material. This work explores in detail the structure-function relationships of Busycotypus egg capsule material mentioned above. |
