Bioprocessing of Silk Proteins - Design Rules from Nature

초록

A model for silk processing in silkworms and spiders is proposed and is based on the unique domain structure in the sequences of silk proteins. A hypothesis for the silk spinning process begins with chain folding at lower concentrations of protein, proceeds through formation of micelles and then micellar aggregates (globules) through water loss and increasing protein concentration, and finally leads to fiber formation due to the physical shear process during fiber spinning. Many aspects of this process can be mimicked in vitro and the all-aqueous environment used is instructive as a model for polymer processing in general. The overall process is controlled by the content and location of water in the system (protein polymers and gland) and the intra- and inter-chain interactions among the silk proteins. This system represents an important intersection between an evolutionarily optimized complex hydrophobic polymer design/sequence chemistry to deal with an all-aqueous processing environment. The result of this process is a product (silk fiber) that is water insoluble - required for environmental stability. The engineering designs also must accommodate high-end mechanical performance for the required functions of these fibers. Together, these factors provide a complex intersection among polymer chemistry, constraints in processing environment and evolutionary pressure to achieve a remarkable outcome of engineering suitable as a model for green chemistry, polymer processing and polymer designs in general.