Stiffness by design in biological fibers: the influence of microstructure and temperature

Abstract

Biological fibers exhibit exceptional mechanical properties such as high stiffness, toughness, and elasticity. The stiffness of bio-fibers is governed by a hierarchical microstructure that is highly sensitive to the extraction method, post-processing, and environmental factors such as temperature. Commonly, the microstructure features an amorphous matrix comprising polypeptide chains that interact through weak intermolecular bonds and interconnect via crystalline domains. In this work, we develop a microscopically motivated energy-based model that sheds light on the underlying mechanisms governing the stiffness of bio-fibers. The initial deformation is driven by (1) the entropic extension of polypeptide chains, (2) the elastic stretching and rotation of rigid crystalline domains, and (3) the distortion of weak intermolecular interactions, leading to the relative sliding of polypeptide chains. The model captures the influence of key physical microstructural quantities such as chain alignment, chain stretch, intermolecular bond strength, and crystallite size on the overall stiffness. The merit of the model is demonstrated through a comparison to spider silk and cocoon silk fibers. We also employ the model to show that the reeling speed during the extraction of spider silk fibers governs the microstructure and, therefore, leads to different stiffnesses. We follow with a parametric analysis that sheds light on how different microstructural quantities affect the stiffness. Lastly, the framework is extended to account for thermally-induced microstructural changes and the model predictions are compared to experimental data on the stiffness of cocoon silk fibers as a function of temperature. The findings from this work delineate the role of microstructure on the overall stiffness and offer a pathway for the efficient design of tunable and optimized biomimetic fibers for target applications.