Hydrogel-based wearable devices have attracted tremendous interest due to their potential applications in electronic skins, soft robotics, and sensors. However, it is still a challenge for hydrogel-based wearable devices to be integrated with high conductivity, a self-healing ability, remoldability, self-adhesiveness, good mechanical strength and high stretchability, good biocompatibility, and stimulus-responsiveness. Herein, multifunctional conductive composite hydrogels were fabricated by a simple one-pot method based on poly­(vinyl alcohol) (PVA), sodium alginate (SA), and tannic acid (TA) using borax as a cross-linker. The composite hydrogel network was built by borate ester bonds and hydrogen bonds. The obtained hydrogel exhibited pH- and sugar-responsiveness, high stretchability (780% strain), and fast self-healing performance with healing efficiency (HE) as high as 93.56% without any external stimulus. Additionally, the hydrogel displayed considerable conductive behavior and stable changes of resistance with high sensitivity (gauge factor (GF) = 15.98 at a strain of 780%). The hydrogel was further applied as a strain sensor for monitoring large and tiny human motions with durable stability. Significantly, the healed hydrogel also showed good sensing behavior. This work broadens the avenue for the design and preparation of biocompatible polymer-based hydrogels to promote the application of hydrogel sensors with comfortable wearing feel and high sensitivity.

Ultraviolet (UV) detectors have attracted considerable attention in the past decade due to their extensive applications in the civil and military fields. Wide bandgap semiconductor-based UV detectors can detect UV light effectively, and nanowire structures can greatly improve the sensitivity of sensors with many quantum effects. This review summarizes recent developments in the classification and principles of UV detectors, i.e., photoconductive type, Schottky barrier type, metal-semiconductor-metal (MSM) type, p-n junction type and p-i-n junction type. The current state of the art in wide bandgap semiconductor materials suitable for producing nanowires for use in UV detectors, i.e., metallic oxide, III-nitride and SiC, during the last five years is also summarized. Finally, novel types of UV detectors such as hybrid nanostructure detectors, self-powered detectors and flexible detectors are introduced.

radiatively at the active emissive layer driven by constant-voltage or direct current (DC). However, the DC-driven mode for OLEDs and QLEDs limits their practical applications. One reason is that the unidirectional DC flow may lead to unfavorable charges accumulation at high current density. Furthermore, the power losses are unavoidable as the DC-driven devices require power converters and rectifiers when connected to the 110/220 V at 50/60 Hz alternating current (AC) power sources. OLEDs are also particularly sensitive to dimensional variations accompanied by the generation of leakage currents at such imperfections, which is unfavorable for their application in flexible electronics. Consequently, AC-driven EL devices have attracted attention as promising alternatives to DC-driven EL devices for a variety of applications. [25][26][27][28][29][30][31][32][33] AC-driven EL devices are primarily composed of electrodes, an emissive layer and a single-or multilayer of insulating dielectric without the critical requirement for energy band matching, which facilitates their application in large-scale displays and flexible devices. [34] The device structure of AC-driven EL devices can be mainly divided into three kinds: i) AC-driven thin film electroluminescent (AC-TFEL) devices; ii) AC-driven lightemitting devices (AC-LEDs), and iii) AC-driven light-emitting field effect diodes (AC-LEFETs). Under the AC electric field, light generation is based on either the hot-electron impact excitation mechanism or the exciton recombination mechanism, depending on the device configuration. Despite the different operation principles, AC-driven EL devices have shown specific Alternating current (AC)-driven electroluminescent (EL) devices have recently attracted attention as potential alternatives to direct current (DC)-driven organic light-emitting diodes (OLEDs), as they have the great advantage of easy integration into the AC power system of 110/220 V at 50/60 Hz without complicated back-end electronics. However, the high driving voltage and low power efficiency inherent to AC-driven EL devices limit their widespread application. While researchers have made some remarkable progress in this field, the underlying causes during the development process remain to be explored. The strategies for improving the performance of AC-driven EL devices with different configurations, such as the conventional sandwiched structure and multilayer-based light-emitting devices, are summarized in this review. For example, it is crucial to enhance the effective electric field around the emitters for AC-driven thin film electroluminescent (AC-TFEL) devices, while the unbalanced generation/injection of charge carriers is the main limiting factor for the performance of AC-driven light-emitting devices (AC-LEDs). The recent advances in AC-driven EL devices, with some new configurations or new-type emitting materials, are presented by category. The challenges and opportunities for the further development of AC-driven EL devices are also discussed.

Hydrogel-based flexible strain sensors have shown great potential in body movement tracking, early disease diagnosis, noninvasive treatment, electronic skins, and soft robotics. The good self-healing, biocompatible, sensitive and stretchable properties are the focus of hydrogel-based flexible strain sensors. Dual network (DN) hydrogels are hopeful to fabricate self-healing hydrogels with the above properties. Here, multifunctional DN hydrogels are prepared via a combination of host−guest interaction of β-cyclodextrin and ferrocene with dynamic borate ester bonds of poly(vinyl alcohol) and borax. Carbon nanotubes are used to endow the DN hydrogels with good conductivity. The obtained DN composite hydrogels possess good biocompatibility, stretchability (436%), fracture strength (41.0 KPa), self-healing property (healing efficiency of 95%), and high tensile strain sensitivity (gauge factor of 5.9). The DN composite hydrogels are used as flexible strain sensors to detect different human motions. After cutting, the healed hydrogels also can monitor human motions and have good stability. In addition, the hydrogel sensors may track the respiratory movement of a pig lung in vitro. This work exhibits new ideas and approaches to develop multifunctional self-healing hydrogels for constructing flexible strain sensors.

Tough, biocompatible, and conductive hydrogel-based strain sensors are attractive in the fields of human motion detection and wearable electronics, whereas it is still a great challenge to simultaneously integrate underwater adhesion and self-healing properties into one hydrogel sensor. Here, a highly stretchable, sensitive, and multifunctional polysaccharide-based dual-network hydrogel sensor was constructed using dialdehyde carboxymethyl cellulose (DCMC), chitosan (CS), poly(acrylic acid) (PAA), and aluminum ions (Al 3+ ). The obtained DCMC/CS/PAA (DCP) composite hydrogels exhibit robust mechanical strength and good adhesive and self-healing properties, due to the reversible dynamic chemical bonds and physical interactions such as Schiff base bonds and metal coordination. The conductivity of hydrogel is 2.6 S/m, and the sensitivity (gauge factor (GF)) is up to 15.56. Notably, the DCP hydrogel shows excellent underwater repeatable adhesion to animal tissues and good self-healing properties in water (self-healing rate > 90%, self-healing time < 10 min). The DCP hydrogel strain sensor can sensitively monitor human motion including finger bending, smiling, and wrist pulse, and it can steadily detect human movement underwater. This work is expected to provide a new strategy for the design of high-performance intelligent sensors, particularly for applications in wet and underwater environments.