Vincenzo Fazio, Ph.D. Candidate 36th cycle, University of Trento, DICAM

The behavior of macromolecular structured biological materials is often significantly affected by environmental conditions such as humidity and temperature. However, due to the great variability of composition and response, the modelling of such materials is currently matter of debate among scientists of very different fields.
 
We focus on a specific material, and consider the spider silk, one of the most interesting biological materials for the striking mechanical properties [1]. Specifically, we propose to study such protein material starting from a detailed description at the macromolecular scale. Thus, the fiber silk thread is modelled as a composite material with a hard crystalline and a soft amorphous region. Following the experimental literature, the main humidity effect is the decreasing of the percentage of crosslinks in the softer region that induces a variation of the natural configuration of the macromolecules. The model is phenomenologically extended to consider the dependence of the mechanical behavior on the temperature. The predictivity of the model is demonstrated by quantitatively reproducing the experimentally observed behavior.

A further extension of the model is in progress, with the aim of predicting the so called supercontraction stress arising from a spider silk fiber when restrained at both the ends and exposed in a humid or high temperature environment. Moreover, in the spirit of biomimetics, the model may be employed in the design of artificial spider silk thanks to its capability in relating the microscopic quantities describing the structure of the spider silk with the macroscopic mechanical response. Others ongoing works include the modelling of the torsion that a spider silk fiber may undergo when exposed to wet environments and the study of the humidity and temperature induced soft-hard phase transition of the spider silk in the framework of the statistical mechanics.