No abstract

Cellulose microfibrils (CMFs) in plant cell walls are a major load-bearing component in plant primary cell walls, and their collective orientational alignment is known to be a key factor to determine the mechanical properties of the cell wall. Plant epidermis has been widely used as a model system for the primary cell wall to study the cellulose structure and tissue mechanics because of its ease of access for characterization. However, the structural information of CMFs in epidermal walls and their mechanics have often been interpreted assuming that CMFs are uniformly distributed in the whole tissue. Here, we report distinct CMF assembly patterns in the flat face region of the epidermal cell and the edge region of the cell where two cells meet. The vibrational sum frequency generation (SFG) imaging analysis found that the CMF orientation in the cell edges is preferentially aligned perpendicular to the anticlinal walls. Finite element analysis (FEA) was employed to test if the cell geometry and the discovered inhomogeneous CMF assemblies could explain the previously observed anisotropic mechanical properties of epidermal cell walls. Our study resolves discrepancies in microfibril structure obtained with different techniques and suggests that the distinct CMF assemblies in the edge region may contribute to tissue-level mechanical anisotropy of epidermal cell walls.

Skin is the largest organ of many animals. Its protective function against hostile environments and predatorial attack makes high mechanical strength a vital characteristic. Here, we measured the mechanical properties of bass fish skins and found that fish skins are highly ductile with a rupture strain of up to 30–40% and a rupture strength of 10–15 MPa. The fish skins exhibit a strain-stiffening behavior. Stretching can effectively eliminate the stress concentrations near the pre-existing holes and edge notches, suggesting that the skins are highly damage tolerant. Our measurement determined a flaw-insensitivity length that exceeds those of most engineering materials. The strain-stiffening and damage tolerance of fish skins are explained by an agent-based model of a collagen network in which the load-bearing collagen microfibers assembled from nanofibrils undergo straightening and reorientation upon stretching. Our study inspires the development of artificial skins that are thin, flexible, but highly fracture-resistant and widely applicable in soft robots.