Animal Hairs as Water-stimulated Shape Memory Materials: Mechanism and Structural Networks in Molecular Assemblies

Abstract

Animal hairs consisting of α-keratin biopolymers existing broadly in nature may be responsive to water for recovery to the innate shape from their fixed deformation, thus possess smart behavior, namely shape memory effect (SME). In this article, three typical animal hair fibers were first time investigated for their water-stimulated SME, and therefrom to identify the corresponding net-points and switches in their molecular and morphological structures. Experimentally, the SME manifested a good stability of high shape fixation ratio and reasonable recovery rate after many cycles of deformation programming under water stimulation. The effects of hydration on hair lateral size, recovery kinetics, dynamic mechanical behaviors and structural components (crystal, disulfide and hydrogen bonds) were then systematically studied. SME mechanisms were explored based on the variations of structural components in molecular assemblies of such smart fibers. A hybrid structural network model with single-switch and twin-net-points was thereafter proposed to interpret the water-stimulated shape memory mechanism of animal hairs. This original work is expected to provide inspiration for exploring other natural materials to reveal their smart functions and natural laws in animals including human as well as making more remarkable synthetic smart materials.

Water adsorption with relative humidity changes for keratin and collagen as studied by infrared (IR) micro‐spectroscopy

“…They are assembled into protofilament groups called protofibril with a diameter of about 7 nm . The double helices are forming crystalline phases, while others are in amorphous states bound either by strong S‐S bonds or by weak H bonds among peptides . Water sorption properties of α keratin have been studied by sorption isotherms, thermal analyses, Raman and IR spectroscopy .…”

“…The double helices are forming crystalline phases, while others are in amorphous states bound either by strong S‐S bonds or by weak H bonds among peptides . Water sorption properties of α keratin have been studied by sorption isotherms, thermal analyses, Raman and IR spectroscopy . Although different natures of water (bound water and free water) adsorbed to α‐keratin have been proposed mainly by thermal analyses, detailed natures of water molecules and their interactions with keratin functional groups still remain unclear.…”

“…Although infrared (IR) spectroscopy has been used for studying amide structures of keratins, it has not been employed extensively for studying water bound to keratins except for some limited studies mentioned above. For investigating water molecules bound to biomaterials, infrared (IR) micro‐spectroscopy equipped with a humidity control system has been combined with quartz crystal microbalance (QCM) .…”

Shape Memory Investigation of α-Keratin Fibers as Multi-Coupled Stimuli of Responsive Smart Materials

“…For example, recently, peacock tail convert feathers and spider silk, both containing β-folded protein molecular chains, were reported to have obvious water-sensitive SME [18,23]. Referring to α-keratin animal hairs, an evident indication has shown them to have purely four types of stimulus-induced SME perspective [24,25]. In textiles, silks and animal hairs both show higher moisture regain values (10%~15%), indicating a large amount of hydrophilic/polar groups in either intra-molecules or inter-molecules of fiber compared with cellulose-type fibers [26,27].…”

Facile control of high temperature shape memory polymers

“…Stimuli‐responsive polymers are playing an important role in various applications as they are smart materials that can respond to external stimuli . Among them, shape memory polymers (SMPs) have attracted more and more attentions as they can recover to initial shape from contemporary shape under external stimuli such as heat, electric, magnetic and light .…”