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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.

Flexible strain sensors have attracted much attention due to their good flexibility, high sensitivity, superior repeatability, and great potentials for application in physiological signal detection. Carbon materials, including carbon nanotubes, graphene, carbon black, graphite, and natural-bioderived carbon materials are often used as active materials for the fabrication of flexible strain sensors because of their superior electrical conductivity and flexibility. Among them, carbon nanotubes and graphene can be prepared into flexible sensors in various forms, such as fibers, films, or textiles. Therefore, carbon material flexible sensors used for physiological signal detection have been sufficiently studied. Herein, the sensing mechanism of flexible strain sensors and the recent advances are reviewed. Sensor characteristics and functions of fibers/films with carbon nanotubes, graphene, and other carbon materials are described in terms of materials, preparation, and properties. From the aspect of sensor application, the sensors with different materials in large- and small-amplitude physiological signals are introduced in detail. Eventually, the superiorities and disadvantages of various carbon-based flexible strain sensors are summarized, and the challenges and opportunities of them in the future are also presented.