Biological liquid crystal elastomers

Abstract

Liquid crystal elastomers (LCEs) have recently been described as a new class of matter. Here we review the evidence for the novel conclusion that the fibrillar collagens and the dragline silks of orb web spiders belong to this remarkable class of materials. Unlike conventional rubbers, LCEs are ordered, rather than disordered, at rest. The identification of these biopolymers as LCEs may have a predictive value. It may explain how collagens and spider dragline silks are assembled. It may provide a detailed explanation for their mechanical properties, accounting for the variation between different members of the collagen family and between the draglines in different spider species. It may provide a basis for the design of biomimetic collagen and dragline silk analogues by genetic engineering, peptide- or classical polymer synthesis. Biological LCEs may exhibit a range of exotic properties already identified in other members of this remarkable class of materials. In this paper, the possibility that other transversely banded fibrillar proteins are also LCEs is discussed.

Silk fibroin as an organic polymer for controlled drug delivery

“…Due to the highly repetitive primary domains, fibrous proteins and especially silk fibroin (SF) assemble into regular structures during materials formation and can be considered as Nature's equivalents to synthetic block copolymers [1][2][3][4][5]. Silks are naturally produced by spiders or insects, such as Nephila clavipes and Bombyx mori, respectively [6,7]. The primary sequence of SFs have achieved wide evolutionary adaptation to such diverse needs as spinning underwater nets to trap air for underwater breathing, lifelines, and prey capture, common features associated with the formation of robust and stable material structures.…”

Mineralization and non-ideality: on nature’s foundry

“…With similarities to silk fibroin, crowding might have similar effects on these two functional classes originating from distinct sources. In the case of silk fibroin, liquid crystal phases can develop by microphase separation under conditions of self-crowding, which subsequently form filaments with defined structural periodicity (Hamley 2010;Knight and Vollrath 2002). From this perspective, biomineral-bound proteins with RLCDs are not functionally well characterized due to an aggregation-prone nature; however, a cholesteric liquid-crystalline phase of chitin is implicated in spiral geometries and patterns present in developing nacre (Cartwright et al 2009).…”

Morphing in nature and beyond: a review of natural and synthetic shape-changing materials and mechanisms

“…This well-understood state of matter exhibits a shift in stiffness [47], volume and optical properties as a result of changes in the internal order of rod-like sub-unit mesogens. LCs are found throughout nature, from spider silk to retinal proteins [27], although as yet the reason for the prominence of this state is not clear. However, studies of insect wing muscles have yielded crystalline forms under X-ray [48], demonstrating that at least some kinds of muscle structures benefit from high levels of order.…”

“…In synthetic competitors, both shape memory polymer and hydrogel are comparable to the best plant and animal material energy storages shown, with only dragline spider silk-a highly anomalous material which has resisted many attempts to analyse and reproduce-exceeding the capacity of our most successful synthetic by this (admittedly artificial) metric, Kevlar. Both Kevlar and spider silk are liquid crystalline in solution and achieve alignment during the process of spinning extrusion, from which their properties derive [27].…”