Although soft electronic materials are of significant importance for flexible electronic devices, most of them are derived from commercial polymer elastomers, such as polydimethylsiloxane, polyurethane and Ecoflex. In this work, citric acid-based degradable polyester elastomers are prepared by a melt polycondensation process, utilizing citric acid, 1,8-octanediol and poly(ethylene glycol) (PEG) as monomers. Furthermore, poly(1,8-octanediol citrate acid) (POC)-PEG/silver nanowire (AgNW) conductive polyester elastomers (CPEs) are prepared by introducing a AgNW layer on the surface of the POC-PEG films. Scanning electron microscopy images reveal that the thickness of the AgNW layer is on the scale of several micrometers and the AgNWs form a continuous conductive network. Upon mechanical stimuli, POC-PEG exhibits recoverable deformation and induces variation in the AgNW conductive network, resulting in a conversion of strain to detectable resistance. When tensile strain is applied, the POC-PEG/AgNW CPEs achieve a gauge factor of 231.6, a response range of 0%-50%, a low response time of 35 ms and high stability. Moreover, the POC-10PEG/AgNW CPE also responds to bending deformation with a gauge factor of 3667.5, a response range of 0%-8.4%, a low response time of 62 ms and high stability. On the basis of strain sensitivity, wireless sensors are further assembled by integrating the POC-PEG/AgNW CPEs into a Bluetooth signal transmission system. Various human motions and physiological activities are successfully monitored using the wireless sensors. The results demonstrate that degradable citric acid-based polyester elastomers/AgNW CPEs are promising materials for next-generation sustainable and flexible electronic devices.

Gels, as typical flexible and soft materials, possess the intrinsic merits of transparent bionic structures, superior mechanical properties and excellent elasticity and viscosity. Recently, gel-based materials have attracted significant attention as a result of their broad and promising applications in biomedical, energy storage, light emission, actuator, military and aerospace devices, especially the intelligent sensing for human-related applications. Among the various flexible and soft pressure sensors, gel-based ones have been gradually studied as an emerging hot research topic. This review focuses on the latest findings in the rapidly developing field of gel-based pressure sensors. Firstly, the classification and properties of the three types of gels and their corresponding fabrication methods are introduced. Secondly, the four basic working principles of pressure sensors are summarized with a comparison of their advantages and disadvantages, followed by an introduction to the construction of pressure sensors based on gel structures. Thirdly, the latest representative research on the three types of gel-based materials towards various wearable sensing applications, including electronic skin, human motion capture, healthcare and rehabilitation, physiological activity monitoring and human-machine interactions, is comprehensively reviewed. Finally, a summary of the remaining challenges and an outline of the development trend for this field are presented.

With the increasing popularity of wearable devices, lightweight electronic skin (e-skin) has attracted significant attention. However, current fabrication technologies make it difficult to directly fabricate sensing materials on flexible substrates at low temperatures. Hence, we propose a flexible graphene nanosheet-embedded carbon (F-GNEC) film, which is directly grown on a flexible substrate using an electron cyclotron resonance low-temperature sputtering system. The direct batch manufacturing of e-skin is obtained by the unique plasma generation mode of electron cyclotron resonance and the polariton energy transfer mode between the plasma and substrate surface. The F-GNEC film contains a large number of graphene nanosheets grown vertically and the graphene edges can serve as electron capture centers, thereby enabling the multi-response properties. We achieve a high gauge factor of 14,699 under a tensile strain of ε = 0.5% and the changing rate of the resistance reaches to 113.2% when the e-skin is bent to 120°. Furthermore, the e-skin achieves a photocurrent of 1.2 μA under 532 nm laser illumination. The F-GNEC film exhibits a sensitive temperature response and achieves a coefficient of -0.58%/°C in a wide temperature range (30-100 °C). The directly fabricated F-GNEC film-based e-skin is stable and firm and exhibits multi-response detection capabilities, which enable its potential application in virtual reality technology and flexible robots.

Tactile feedback is of great significance for amputees to improve the controllability of prosthetic hands and obtain tactile information regarding the interacting objects, which remains a significant challenge for neuroprosthetic hands. In this study, we present a method to restore finger-specific tactile sensations on the projected finger map of a unilateral forearm amputee with a sensory soft neuroprosthetic hand through electrotactile stimulation. On this basis, five soft touch sensors embedded in the fingertips are first adopted to measure the pressure changes of the soft neuroprosthetic hand with the touched objects. The measured pressure information is then accordingly encoded into electrotactile stimulation patterns to trigger an electrical stimulator that outputs programmable electrical pulses on the projected finger map of the amputee. In this manner, the finger tactile sensation can be elicited, which can help the amputee to distinguish the finger press state and discriminate the curvature and hardness of the touched objects. Experimental results show that, based on the different stimulation regions, the amputee subject can instantaneously distinguish the tactile sensation of a single finger or multiple fingers with an accuracy of 98.57% and 91.71%, respectively. By programming the frequencies of the electrical pulses, the amputee subject can successfully discriminate the touching objects with different curvatures and hardnesses with an accuracy of 97.26% and 97.93%, respectively. Finally, we demonstrate that the amputee subject can achieve closed-loop control of the sensory soft neuroprosthetic hand by integrating a myoelectric control interface and electrotactile feedback to achieve multilevel perception.

Soft actuators have been receiving tremendous attention as a result of their excellent adaptability to the environment. However, due to their inherently low stiffness, soft actuators are difficult to adapt to high-load tasks. Despite previous efforts in developing stiffness-tunable actuators by utilizing variable stiffness materials, they still suffer from limitations, including relatively low load and locking capacity to grasp weights and difficulties regarding their fabrication with complex structures. This work reports a novel stiffness-tunable and shape-locking soft (Tri-S) actuator using hybrid multi-material 3D printing. The Tri-S actuator consists of polylactic acid, thermoplastic polyurethane and a flexible carbon fiber heating wire. Its stiffness can be effectively tuned by Joule heating. A soft robotic gripper equipped with three Tri-S actuators demonstrates their stiffness-tunable and shape-locking capability by grasping and holding objects of various shapes and weights. The gripper can grasp weights up to 2.2 kg with an external driving force by tuning the stiffness and hold weights up to 310 g depending on its own shape locking without an external driving power source.

Remarkable flame-retardant and integrated mechanical properties are essential requirements for the potential applications of bio-based films in industrial areas. Unfortunately, the design and fabrication of such film materials that possess a good trade-off between mechanical properties and flame-retardant performance remain significant challenges. Here, phosphorylated cellulose nanofibril-based films (PCNFs) integrated with chitosan (CS) and MXene (PCNF/CS-M) are fabricated via a facile water evaporation-induced self-assembly method. An evident reinforcement of the mechanical performance can be achieved by constructing additional interactions (i.e., hydrogen bonding and nanoreinforcement) among the hybrid network, which endows the optimized films with highly improved and balanced mechanical performance (i.e., tensile strength of 172.1 MPa, tensile strain of 8.0%, Young’s modulus of 4.4 GPa and toughness of 8.5 MJ/m^-3). Furthermore, the resultant films also exhibit outstanding flame resistance, as clearly illustrated by their structural integrity after cyclic testing using a butane lamp flame at 700-800 °C. The synergistic reinforcing and flame-retardant mechanisms of the films are clarified based on structural evolution and performance variation. The strategy developed herein provides an innovative concept for designing and developing advanced bio-based film materials for fireproof coatings.

Human-machine interactions are becoming increasingly required for intelligent sensing and effective manipulation. Recent developments in flexible piezoelectric sensors with short response time and high force-electric interconversion efficiency present a tendency toward facilitating diverse human-machine interactive applications. Here, we review the development of flexible piezoelectric human-machine interactions in the context of robotic control, the Internet of Things, sports coaching and acoustic therapeutics. The synthesis of unique materials, the distinct design of device structures, the typical applications of piezoelectric human-machine interactions and the integration of cutting-edge technologies are elaborated in detail based on recent research. Finally, we highlight the current challenges and directions for the development of piezoelectric human-machine interactions for more advanced application scenarios.