Theoretical and computational hierarchical nanomechanics of protein materials: Deformation and fracture

Proteins constitute the building blocks of biological materials such as tendon, bone, skin, spider silk or cells. An important trait of these materials is that they display highly characteristic hierarchical structures, across multiple scales, from nano to macro. Protein materials are intriguing examples of materials that balance multiple tasks, representing some of the most sustainable material solutions that integrate structure and function. Here we review progress in understanding the deformation and fracture mechanisms of hierarchical protein materials by using a materials science approach to develop structure-process-property relations, an effort defined as materiomics. Deformation processes begin with an erratic motion of individual atoms around flaws or defects that quickly evolve into formation of macroscopic fractures as chemical bonds rupture rapidly, eventually compromising the integrity of the structure or the biological system leading to failure. The combination of large-scale atomistic simulation, multi-scale modeling methods, theoretical analyses combined with experimental validation provides a powerful approach in studying deformation and failure phenomena in protein materials. Here we review studies focused on the molecular origin of deformation and fracture processes of three types of protein materials. The review includes studies of collagen - Nature's super-glue; beta-sheet rich protein structures as found in spider silk - a natural fiber that can reach the strength of a steel cable; as well as intermediate filaments - a class of alpha-helix based structural proteins responsible for the mechanical integrity of eukaryotic cells. The article concludes with a discussion of the significance of universally found structural patterns such as the staggered collagen fibril architecture or the alpha-helical protein motif.