body motion monitoring, covering subtle and large-strain ranges.Capacitive strain sensor has a simple structure with two electrodes separated by a dielectric material. Metals [19] and semiconductors [20] can be used as electrodes for constructing capacitive sensors. However, these electrode materials have poor mechanical properties, limiting the sensing range of capacitive strain sensors. [21] To solve the problem, geometrical engineering of rigid materials into buckled, [22][23][24] wrinkled, [16,25] and kirigami [26] forms is widely employed. For example, Someya's group shaped gold film into a wrinkled form for developing capacitive strain sensors, with a stretchability up to 140%. [16] In addition to the geometrical engineering, the usage of intrinsic stretchable materials as electrode for soft capacitive strain sensors is the other strategy widely adopted. These intrinsic stretchable materials include liquid metal, [27] nanomaterial/elastomer composites, [28] and conductive textiles. [29] Dickey and co-workers developed a liquid-metal-based capacitive strain sensor having a high stretchability (≈100%). [8] A capacitive strain sensor using conductive textile electrode was developed by Walsh's group, with a stretchability of 100% and a sensitivity of 1.23. [29] Cohen et al. reported a carbon nanotube (CNT)/elastomer-based capacitive sensor, owning a stretchability of 100% and a sensitivity of 0.99. [7] Although the above strategies can provide capacitive sensors with high stretchability, most of the electrode materials are not intrinsically self-healable. To fully mimic the functionality of human skin, the self-healability for sensory devices is highly demanded. [30] Hydrogels which consist of 3D networks with large quantity of water or ionic liquid have been proposed as electrode materials for soft electronics, due to their large stretchability, self-healability, and biocompatibility. [31,32] For sensing applications, hydrogel-based resistive strain sensors have been widely studied. [33][34][35][36] These devices have achieved large sensing limits, but the hydrogels have relatively long self-healing time and low self-healing efficiency. [33,36,37] Additionally, some of them needs external heating to accelerate the self-healing process. [35,38] Through the dynamic complexing interaction between metal ion and OH group, polyvinyl alcohol (PVA) based hydrogel has been investigated as active materials for constructing sensors. [39,40] CNT/PVA-based resistive strain Capacitive strain sensors could become an important component of electronic skin (E-skin) due to their low hysteresis and high linearity. However, to fully mimic the functionality of human skin, a capacitive strain sensor should be stretchable and self-healable. The development of such a sensor is limited by electrode materials which generally lack self-healability and/or stretchability. A highly stretchable and self-healing MXene (Ti 3 C 2 T x )/polyvinyl alcohol (PVA) hydrogel electrode is developed for use in capacitive strain sensors for E-skin. The...

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Electronic skins (e-skins) have been widely investigated as important platforms for healthcare monitoring, human/machine interfaces, and soft robots. However, mask-free formation of patterned active materials on elastomer substrates without involving high-cost and complicate processes is still a grand challenge in developing e-skins. Here, SiC-based strain sensor arrays are fabricated on elastomer for e-skins by a laser direct writing (LDW) technique, which is mask-free, highly efficient, and scalable. The direct synthesis of active material on elastomer is ascribed to the LDW-induced conversion of siloxanes to SiC. The SiC-based devices own a highest sensitivity of ≈2.47 × 10 5 achieved at a laser power of 0.8 W and a scanning velocity of 1.25 mm s −1 . Moreover, the LDW-developed device provides a minimum strain detection limit of 0.05%, a small temperature drift, and a high mechanical durability for over 10 000 cycles. When it is mounted onto human skins, the SiC-based device is able to monitor external stimuli and human health conditions, with the capability of wireless data transmission. Its potential application in e-skins is further proved by an LDW-fabricated device having 3 × 3 SiC sensor array for tactile sensing.

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Recently, studies on the synthesis, modification, and functionalization of polyesters derived from 2,5-furandicarboxylic acid (2,5-FDCA) have attracted widespread attention. Among them, poly­(ethylene 2,5-furanoate) (PEF) has been in the spotlight due to its greener production process, higher glass transition temperature (T g), good mechanical performance, and excellent barrier properties, which have qualified it as a new generation of packaging material to be considered for replacement of polyethylene terephthalate (PET). Currently it is a promising rising star in the field of biobased polymers, yet there has been no review entirely focused on it. This article focuses on the research related PEF, discussing its mainstream synthetic methods including optimization of catalysts with solutions of coloration and side reactions, and its comprehensive properties such as thermal, crystallization, mechanical, and barrier performances. Other aspects highlighted are modifications for better properties and the introduction of biodegradability as well as the construction of composites materials. This article aims to reveal the development path of PEF and help us to understand it more deeply and widely in a short time.