Sensing of both temperature and strain is crucial for various diagnostic and therapeutic purposes. Here, we present a novel hydrogel-based electronic skin (e-skin) capable of dual-mode sensing of temperature and strain. The thermocouple ion selected for this study is the iodine/triiodide (I⁻/I₃⁻) redox couple, which is a common component in everyday disinfectants. By leveraging the thermoelectric conversion in conjunction with the inherent piezoresistive effect of a gel electrolyte, self-powered sensing is achieved by utilizing the temperature difference between the human body and the external environment. The composite hydrogels synthesized from polyvinyl alcohol (PVA) monomers using a simple freeze‒thaw method exhibit remarkable flexibility, extensibility, and adaptability to human tissue. The incorporation of zwitterions further augments the resistance of the hydrogel to dehydration and low temperatures, allowing maintenance of more than 90% of its weight after 48 h in the air. Given its robust thermal current response, the hydrogel was encapsulated and then integrated onto various areas of the human body, including the cheeks, fingers, and elbows. Furthermore, the detection of the head-down state and the monitoring of foot movements demonstrate the promising application of the hydrogel in supervising the neck posture of sedentary office workers and the activity status. The successful demonstration of self-powered on-body temperature and strain sensing opens up new possibilities for wearable intelligent electronics and robotics.

Thermoelectric cells (TEC) directly convert heat into electricity via the Seebeck effect. Known as one TEC, thermogalvanic hydrogels are promising for harvesting low-grade thermal energy for sustainable energy production. In recent years, research on thermogalvanic hydrogels has increased dramatically due to their capacity to continuously convert heat into electricity with or without consuming the material. Until recently, the commercial viability of thermogalvanic hydrogels was limited by their low power output and the difficulty of packaging. In this review, we summarize the advances in electrode materials, redox pairs, polymer network integration approaches, and applications of thermogalvanic hydrogels. Then, we highlight the key challenges, that is, low-cost preparation, high thermoelectric power, long-time stable operation of thermogalvanic hydrogels, and broader applications in heat harvesting and thermoelectric sensing.

The water covering the Earth’s surface not only supports life but also contains a tremendous amount of energy. Water energy is the most important and widely used renewable energy source in the environment, and the ability to extract the mechanical energy of water is of particular interest since moving water is ubiquitous and abundant, from flowing rivers to falling rain drops. In recent years, triboelectric nanogenerators (TENGs) have been promising for applications in harvesting kinetic energy from water due to their merits of low cost, light weight, simple structure, and abundant choice of materials. Furthermore, TENGs can also be utilized as self-powered active sensors for monitoring water environments, which relies on the output signals of the TENGs caused by the movement and composition of water. Here, TENGs targeting the harvest of different water energy sources have been systematically summarized and analyzed. The TENGs for harvesting different forms of water energy are introduced and divided on the basis of their basic working principles and modes, i.e., in the cases of solid–solid and solid–liquid. A detailed review of recent important progress in TENG-based water energy harvesting is presented. At last, based on recent progresses, the existing challenges and future prospects for TENG-based water energy harvesting are also discussed.

Collecting ambient energy to power various wearable electronics is considered a prospective approach to addressing their energy consumption. Mechanical and thermal energies are abundantly available in the environment and can be efficiently converted into electricity based on different physical effects. Hydrogel-based energy harvesters have turned out to be a promising solution, owing to their unique properties including flexibility and biocompatibility. In this review, we provide a concise overview of the methods and achievements in hydrogel-based energy harvesters, including triboelectric nanogenerators, piezoelectric nanogenerators, and thermoelectric generators, demonstrating their applications in power generation, such as LED lighting and capacitor charging. Furthermore, we specifically focus on their applications in self-powered wearables, such as detecting human motion/respiration states, monitoring joint flexion, promoting wound healing, and recording temperature. In addition, we discuss the progress in the sensing applications of hydrogel-based self-powered electronics by hybridizing multiple energy conversion in the field of wearables. This review analyzes hydrogel-based energy harvesters and their applications in self-powered sensing for wearable devices, with the aim of stimulating ongoing advancements in the field of smart sensors and intelligent electronics.

Most traffic accidents are caused by driver fatigue, so achieving effective monitoring of fatigue status is particularly important. However, current detection methods mainly rely on complex biosensors that lack independence from power supply and adaptive adhesion, and have a risk of ingestion, especially for infants. Here, we propose an edible thermogalvanic hydrogel with I⁻/I₃ ⁻ as a redox pair for self-powered physiological monitoring by coupling thermogalvanic and piezoresistive effects. The gelatin/glycerol (Gel/GL) hydrogel exhibits excellent thermo-reversible gelling and temperature-responsive adhesion that can ensure on-demand reliable adhesion to skin for capturing physiological signals under hot compress and facilitate easy removal under cold compress due to the decomplexation and re-entanglement of gelatin chains. Moreover, a low-cost passive patch based on this self-adaptable thermogalvanic gel has been invented to simultaneously monitor breathing, blinking, yawning, and pulse at the canthus, philtrum and radial artery of the human body by acquiring thermoelectric signals correlated with physiological action, achieving a self-powered accurate surveillance of fatigue status, which can provide timely reminders based on fatigue levels. This work is beneficial for improving driving safety and exhibits the potential of the designed hydrogel for application in intelligent transportation.

Thermogalvanic hydrogel, an environmentally friendly power source, enable the conversion of low-grade thermal energy to electrical energy and powers microelectronic devices in a variety of scenarios without the need for additional batteries. Its toxicity, mechanical fragility and low output performance are a hindrance to its wide application. Here, we demonstrate thermoelectric gels with safe non-toxic, recyclable, highly transparent and flexible stretchable properties by introducing gelatin as a polymer network and SO${{}_{3/4}}^{2-}$ as a redox electric pair. When the temperature difference is 10 K, the gel-based thermogalvanic cell achieves an open-circuit voltage of about 16.2 mV with a maximum short-circuit current of 39 $\mathsf{\mu }$A. Furthermore, we extended the application of the Gel-SO${{}_{3/4}}^{2-}$ gel to monitor the temperature of hot or cold food, enabling self-powered sensing for food temperature detection. This research provides a novel concept for harvesting low-grade thermal energy and achieving safe and harmless self-driven temperature monitoring.