Flexible sensors have great potential for human motion sensing, medical monitoring, electronic skin and display devices. Flexible hydrogel sensors with transparent characteristics have a good visual effect, which can be used for “invisible” devices and visual optical monitoring, as well as to improve the aesthetics of devices for daily usage, etc. However, there are no systematic reviews on the working principle, characteristics and application prospects of transparent stretchable hydrogel sensors. In this review, we summarize recent advances in wearable strain sensors, pressure sensors, temperature sensors, humidity sensors and gas sensors based on transparent stretchable hydrogels, emphasizing materials, designs and applications. We also discuss the combinations and applications of sensors with emerging technologies such as wearable devices, artificial intelligence, human–computer interaction and 3D printing. Finally, this review presents a brief summary, challenges, and perspectives of transparent stretchable hydrogel sensors.

Windows are the least energy efficient part of the buildings, as building accounts for 40% of global energy consumption. Traditional smart windows can only regulate solar transmission, while all the solar energy on the window is wasted. Here, for the first time, the authors demonstrate an energy saving and energy generation integrated smart window (ESEG smart window) in a simple way by combining louver structure solar cell, thermotropic hydrogel, and indium tin oxides (ITO) glass. The ESEG smart window can achieve excellent optical properties with ≈90% luminous transmission and ≈54% solar modulation, which endows excellent energy saving performance. The outstanding photoelectric conversion efficiency (18.24%) of silicon solar cells with louver structure gives the smart window excellent energy generation ability, which is more than 100% higher than previously reported energy generation smart window. In addition, the solar cell can provide electricity to for ITO glass to turn the transmittance of hydrogel actively, as well as the effect of antifreezing. This work offers an insight into the design and preparation together with a disruptive strategy of easy fabrication, good uniformity, and scalability, which opens a new avenue to realize energy storage, energy saving, active control, and antifreezing integration in one device.

Transition metal dichalcogenides (TMDs) show promising electrical and electronic properties and potential catalytic applications in the hydrogen evolution reaction (HER). However, most TMDs suffered from low conductivity and scarcity of active edge sites. Herein, new ternary vanadium-based TMD, VSSe alloy single crystals were synthesized for the first time by agent-assisted chemical vapor transport and used as an excellent HER electrocatalyst due to the pure conductive phase and abundant active sites induced by ternary structures. Different transport agents were employed to tune the interlayer spacing, and it was found that the expansion of the (001) interlayer spacing and the co-existence of V³⁺ could introduce more S or Se vacancies. The exfoliated VSSe sample exhibits a lower overpotential of −180 mV at a current density of 10 mA cm⁻², a Tafel slope of 87 mV dec⁻¹, and better durability, which are comparable to or better than those of most previously reported ternary TMD systems. Consistent with the theoretical calculation results, more defects controlled by the transport agent of VCl₃ give an enhanced conductivity and a lower H* adsorption energy, and thereby higher HER performance. By simply varying the transport agents, high-conductivity, high-purity, and high-performance TMDs can be fabricated in one step without the conventional post-treatment.