Junjie Wei

The hydrogel ionic diode is regarded as a promising self‐powered sensor, capable of harvesting energy from low‐frequency stimuli human motions and converting it into electrical signals. However, the sensitivity of the reported conventional bilayer hydrogel ionic diodes are relatively low, due to the single heterojunction interface and high interface resistance, making it challenging to meet the demands of high‐precision sensing. Here, a universal method for fabricating dual‐gradient hydrogel ionic diodes without bilayer structure through the induction of anionic and cationic polymer gradient distribution via a direct current electric field is developed. Due to the dual‐gradient distribution, numerous heterogeneous microstructures (i.e., microdiodes) with low interface resistance are formed in the bulk phase of hydrogel, and these series‐connected microdiodes demonstrate a significantly increase in open circuit voltage in response to mechanical pressure. The dual‐gradient hydrogel ionic diode exhibits ultra‐high sensitivity (1247.3 mV/MPa) and ultralow detection limit (0.8 Pa), enabling the smart prosthetic hand to non‐destructive grasp ultrasoft tofu. This work is expected to pave the way for novel high‐precision self‐powered sensors in intelligent wearable electronics.

Flexible wearable electronic devices based on hydrophobic, conductive hydrogels have attracted widespread attention in the field of underwater sensing. However, traditional homogeneous hydrogels tend to compromise their conductivity and sensing performance when achieving hydrophobicity, and the high complexity of marine environments further reduces their sensing performance and service life. Here, we develop a seawater-resistant conductive hydrogel with ultrahigh sensitivity and self-healing ability by the introduction of a skin-like heterogeneous structure, consisting of a hydrophobic outer layer that protects against seawater and a conductive internal layer that senses. Based on a heterogeneous structure obtained through surface hydrophobic modification of confined nitrogen-alkylation reaction, the conductive hydrogel simultaneously achieves satisfying seawater resistance (contact angle of 123.2°), high ionic conductivity (2.86 S m^-1), and excellent sensing sensitivity in seawater (GF = 6.15), harmonizing the contradiction between water resistance and sensing of traditional hydrophobic hydrogels. In addition, abundant hydrogen-bonding and dipole-dipole interactions endow the heterogeneous hydrogel with an outstanding self-healing ability, exhibiting high-efficiency self-healing behavior in seawater. Underwater strain sensors constructed with the heterogeneous hydrogel can be used for detecting human motion in simulated seawater environments and real-time signal transmission, showcasing their great potential as wearable electronic devices in the marine sensing field.

Coelenterates, such as Atolla jellyfish, are capable of integrating color, communication, and motion in a sophisticated manner, thereby enabling them to function as intelligent biological systems that can adapt to the challenges of the underwater environment. Extensive efforts have been dedicated to exploiting underwater visual, sensory, actuating, or combined systems. However, current biomimetic soft systems are still limited by the lack of comprehensive, integrated functional skins that can automatically deform, dynamically sense, and further send color signals when diving into underwater conditions. Here, we propose the synthetic soft skins composed of assembled entangled carbon nanotube networks and fluorescent unit-embedded elastomers which can be applied in a suspended form to allow autonomic 3D deformation, real-time perception, and dynamic fluorescence color transformation. The capabilities of the sensory and color display thresholds were controlled through the entanglement density of carbon nanotubes and the suspended area. As a demonstration, the soft thin skin was integrated into an artificial jellyfish robot, enabling the realization of a closed-loop feedback system for dynamic sensory processing, signal processing, and further 3D morphing-induced fluorescent color change, demonstrating significant potentials in underwater visual display, danger warning, and environmental exploration.

In the past decades, remarkable progress has been made in the field of human-machine interaction. The need for accurate sensing devices with satisfactory user experiences has propelled the development of flexible, stretchable, biocompatible, and imperceptible hydrogel-based interfaces. These innovative interfaces facilitate direct interactions between humans and machines while receiving detected input signals from sensors and giving output commands to controllers, thus motivating accurate real-time responsiveness. This Perspective discusses the sensing mechanisms for the two categories of hydrogel-based sensors and summarizes the recent progress in the development of different representations of human-machine interactions, including intelligent identification, information secrecy, interactive control, and virtual reality and augmented reality technologies. The advantages of hydrogel-based systems over conventionally used rigid electrical components are explicitly discussed. The conclusion provides a perspective on current challenges and outlines a future roadmap for the realization of state-of-the-art hydrogel-based smart systems.

Hydrophobic ionogel has attracted much attention in underwater sensing as the artificial electronic skins and wearable sensors. However, when the low conductive ionogel‐based sensor works in the marine environment, the salty seawater weakens its sensing performance, which is difficult to recognize. Herein, a salt‐adaptively conductive ionogel with high submarine strain sensitivity is reported. Based on the preliminary improvement via the proton conduction mechanism, the conductivity of the ionogel further increases with the surrounding salinity rising up since the salt‐induced dissociation phenomenon, which is described as the environmental salt‐adaptive feature. In seawater, the conductivity of the ionogel is as high as 2.90 × 10⁻¹ S m⁻¹. Significantly, with its long‐term underwater stability and adhesion, the resultant ionogel‐based sensor features prominent strain sensing performance (gauge factor: 1.12) while combining with various soft actuators in the marine environment. The ionogel‐based sensor is capable of monitoring human breath frequency, human actions, and the locomotion of soft actuators, demonstrating its great potential in diving detection and intelligent preceptive soft robotics for marine environmental protection and exploration.

The energy crisis is one of the most critical and urgent problems in modern society; thus, harvesting energy from ubiquitous low-grade heat energy with thermoelectric (TE) materials has become an available strategy in sustainable development. Recently, emerging ionic TE materials have been widely used to harvest low-grade heat energy, owing to their excellent performance in high ionic Seebeck coefficient, low thermal conductivity, and mechanical flexibility. However, the instability of ionic conductive materials in the underwater environment seriously suppresses underwater energy-harvesting, resulting in a waste of underwater low-grade heat energy. Herein, we developed a water-resistant TE ionogel (TEIG) with excellent long-term underwater stability utilizing a hydrophobic structure. Due to the hydrophobic polymer network and hydrophobic ionic liquid (IL), the TEIG exhibits high hydrophobicity and antiswelling capacity, which meets the requirement of environment stability for underwater thermoelectric application. Furthermore, the water resistance endows the TEIG with great thermoelectric performances in the underwater environment, including satisfactory ionic Seebeck coefficient, outstanding durability, and superior salt tolerance. Therefore, this investigation provides a promising strategy to design water-resistant TE materials, enabling a remarkable potential in harvesting low-grade heat energy under water.

To make soft robotics intelligent, dazzling artificial skin and actuators have been created. However, compared to rigid commercial robots, the sophisticated demands of raw materials become a key challenge for autonomous soft actuators to realize manufacturing repeatability and reproducibility. Inspired by the stem cell, which has the potential to differentiate into multifunctional cells with the same original compositions, a potential multifunctional gel is presented. With well-designed polymer chains, the gel has a salt-induced regulating module, conductivity, and other bionic properties. Making use of this advantage, the gel acts as a double-duty electrode for both the actuator and sensor. An artificial reflex arc is therefore formed by their tight integration via an e-brain: a computing unit that specifically responds to organism intervention. This efficient strategy to obtain diverse components with minimal raw materials is promising for effortlessly fabricating fully soft robotics.

Stimuli-responsive hydrogel is regarded as one of the most promising smart soft materials for the next-generation advanced technologies and intelligence robots, but the limited variety of stimulus has become a non-negligible issue restricting its further development. Herein, we develop a new stimulus of “touch” (i.e., spatial contact with foreign object) for smart materials and propose a flytrap-inspired touch-responsive polymeric hydrogel based on supersaturated salt solution, exhibiting multiple responsive behaviors in crystallization, heat releasing, and electric signal under touch stimulation. Furthermore, utilizing flytrap-like cascade response strategy, a soft actuator with touch-responsive actuation is fabricated by employing the touch-responsive hydrogel and the thermo-responsive hydrogel. This investigation provides a facile and versatile strategy to design touch-responsive smart materials, enabling a profound potential application in intelligence areas.

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