Hydrogels and shape memory polymers (SMPs) possess excellent and interesting properties that may be harnessed for future applications. However, this is not achievable if their mechanical behaviors are not well understood. This paper aims to discuss recent advances of the constitutive models of hydrogels and SMPs, in particular the theories associated with their deformations. On the one hand, constitutive models of six main types of hydrogels are introduced, the categorization of which is defined by the type of stimulus. On the other hand, constitutive models of thermal-induced SMPs are discussed and classified into three main categories, namely, rheological models; phase transition models; and models combining viscoelasticity and phase transition, respectively. Another feature in this paper is a summary of the common hyperelastic models, which can be potentially developed into the constitutive models of hydrogels and SMPs. In addition, the main advantages and disadvantages of these constitutive modes are discussed. In order to provide a compass for researchers involved in the study of mechanics of soft materials, some research gaps and new research directions for hydrogels and SMPs constitutive modes are presented. We hope that this paper can serve as a reference for future hydrogel and SMP studies.

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Hydrogel-based epidermal electrodes attract widespread attention in health monitoring and human-machine interfaces for their good biocompatibility, skin-matched Young's modulus, and stable in situ electrophysiological recording performance. However, it is difficult to make the exact conformal attachment between skin and electrodes because of the hair, wrinkles, as well as complex, curved contours of the skin. This also results in signal distortion and large noise. Here, a body temperature enhanced skin-adhesive epidermal electrode is proposed based on non-covalent cross-linked network ionic hydrogel. The ionic hydrogel is fabricated by the polyvinyl alcohol, branched polyethyleneimine, and calcium chloride (CaCl 2 ), which demonstrates impressive performances including ultra-stretchability of 1291%, great adhesion to skin and other non-biological materials, stable conductivity of 3.09 S m −1 , recyclability, and outstanding antibacterial ability, simultaneously. Specifically, the adhesion of the ionic hydrogel behaves as temperature-sensitive and could be enhanced by body temperature. Furthermore, the ionic hydrogel is utilized as epidermal electrodes, which display seductive capability to record multifarious electrophysiological signals with high signal-to-noise ratio and ultra-low detection limit, including electrocardiogram, electromyogram, and electroencephalogram. The as-proposed body temperature enhanced skin-adhesive ionic hydrogel brings intelligent functions and broadens the way for epidermal electronics, promoting the development of healthcare electronics.

The double-K fracture model can well describe the development of cracks undergoing during the entire fracture process in concrete. Therefore, it has been selected as the theoretical basis of the "Norm for fracture test of hydraulic concrete" (China) and the double-K fracture parameters proposed in this model are regarded as the essential fracture parameters needed to be measured. Therefore, the primary objection of this paper is to present an overall report on the determination of the double-K fracture parameters of concrete involving the fundamental theory, calculation formulas, experimental arrangement and identification of initial cracking load in order to provide a reference for the specifi cation committee. The fracture tests are carried out on the total of 43 concrete specimens where the small-size aggregates with the maximum size of 10 mm are chosen. Two typical widelyused geometries, i.e. three-points bending beam and wedge splitting specimen are adopted. The initial cracking load is determined by means of graph method or electrical resistance strain gauge method. Then, the initial fracture toughness and unstable fracture toughness are measured. It is found that the ratio of initial cracking load to maximum load is 0.67-0.71 and the ratio of initial fracture toughness to unstable fracture toughness is 0.45-0.50. It is also found that, for the initial fracture toughness and unstable fracture toughness, the values are approximately close to the constants when the depth of specimens is larger than 200 and 400 mm for three-points bending beams and wedge splitting specimens respectively. Besides, the critical crack tip opening displacement of each specimen is calculated, too. The big difference between two geometries is noticed.

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