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Screen sensors are the most commonly used human-machine interfaces in our everyday life, which have been extensively applied in personal electronics like cellphones. Touchless screen sensors are attracting growing interest due to their distinct advantages of high interaction freedom, comfortability, and hand hygiene. However, the material compositions of current touchless screen sensors are rigid and fragile, hardly meeting the needs of wearable and stretchable on-skin electronics development. Additionally, these touchless screen sensors are also restricted by high power consumption, limited gesture types of recognition, and the requirement of light conditions. Here, we report a stretchable on-skin touchless screen sensor (OTSS) enabled by an ionic hydrogel-based triboelectric nanogenerator (TENG). Compared with current touchless screen sensors, the OTSS is stretchable, self-powered, and competent to recognize diverse gestures by making use of charges naturally carried on fingers without the need of sufficient light conditions. An on-skin noncontact screen operating system is further demonstrated on the basis of the OTSS, which could unlock a cellphone interface in touchless operation mode on the human skin. This work for the first time introduces the on-skin touchless concept to screen sensors and offers a direction to develop new-generation screen sensors for future cellphones and personal electronics.

The application of stretchable strain sensors in human movement recognition, health monitoring, and soft robotics has attracted wide attention. Compared with traditional electronic conductors, stretchable ionic hydrogels are more attractive to organization‐like soft electronic devices yet suffer poor sensitivity due to limited ion conduction modulation caused by their intrinsic soft chain network. This paper proposes a strategy to modulate ion transport behavior by geometry‐induced strain concentration to adjust and improve the sensitivity of ionic hydrogel‐based strain sensors (IHSS). Inspired by the phenomenon of vehicles slowing down and changing lanes when the road narrows, the strain redistribution of ionic hydrogel is optimized by structural and mechanical parameters to produce a strain‐induced resistance boost. As a result, the gauge factor of the IHSS is continuously tunable from 1.31 to 9.21 in the strain range of 0–100%, which breaks through the theoretical limit of homogeneous strain‐distributed ionic hydrogels and ensures a linear electromechanical response simultaneously. Overall, this study offers a universal route to modulate the ion transport behavior of ionic hydrogels mechanically, resulting in a tunable sensitivity for IHSS to better serve different application scenarios, such as health monitoring and human–machine interface.

Ionic hydrogels possess great advantages over traditional electronic devices due to their notable properties such as excellent flexibility, high conductivity, and tunable gel structure, making them widely employed in electronic skins for electrophysiological monitoring. However, conventional electronic skins are usually easy to detach from the skin and suffer from stress fatigue due to frequent body movements, impeding user experience in health monitoring. Therefore, the fabrication of ionic hydrogels with certain unique properties could address these issues, offering broad prospects for ionic hydrogels. This review first introduces the fabrication materials for ionic hydrogels. Then the unique properties of ionic hydrogels in adhesiveness, self-healing, and recyclability are discussed. Subsequently, the applications of ionic hydrogels in electrophysiological monitoring are summarized. Finally, we give an outlook on the current status and future prospects of ionic hydrogel-based electronic skins. Hopefully this work will contribute insights into the advance of ionic hydrogel-based electronic skins. Graphical abstract This review introduces the fabrication materials for ionic hydrogels. The unique properties of ionic hydrogels in adhesiveness, self-healing, and recyclability are discussed. Subsequently, the applications of ionic hydrogels in electrophysiological monitoring are summarized. Finally, we give an outlook on the current status and future prospects of ionic hydrogel-based electronic skins. It is expected that continued efforts in material design, device integration will shape the future of ionic hydrogel advancements, enabling new possibilities in healthcare monitoring.

Piezoionic materials consisting of a polymer matrix and mobile ions can produce an electrical output upon an applied pressure inducing an ion concentration gradient. Distinct from charges generated by the piezoelectric or triboelectric effects, the use of generated mobile ions to carry a signal closely resembles many ionic biological processes. Due to this similarity to biology, the piezoionic effect has great potential to enable seamless integration with biological systems, which accelerates the advancement of medical devices and personalized medicine. In this review, a comprehensive description of the piezoionic mechanism, methods, and applications are presented, with the aim to facilitate a dialogue among relevant scientific communities. First, the piezoionic effect is briefly introduced, then the development of mechanistic understanding over time is surveyed. Next, different types of piezoionic materials are reviewed and methods to enhance the piezoionic output via materials properties, electrode interfaces, and device architectures are detailed. Finally, applications, challenges, and outlooks are provided. With its novel properties, piezoionics is expected to play a key role in the overcoming of grand challenges in the areas of sensing, biointerfaces, and energy harvesting. Comprehensive understanding of piezoionics is summarized, mainly on its mechanism, methodology, and applications. This review is expected to facilitate further exploration, discussion, and collaboration among different fields.

Wearable strain sensor prepared with ionic conductive hydrogel holds great promises in a variety of engineering fields. In this work, we introduce sodium casein (SC) into a dual network hydrogel system made of polyvinyl alcohol (PVA) and polyacrylamide (PAM), to prepare an ionic hydrogel sensor. Compared to the PAM/PVA dual network hydrogel, the introduction of SC plays a significant synergistic role. Such dual network PAM/PVA/SC hydrogels exhibit excellent mechanical properties (a maximum strain of 719%, a maximum stress of 444.3 kPa), low hysteresis, and rapid recovery after uni-axial stretching. Since SC drives a large number of free ions, PAM/PVA/SC hydrogels present good conductivity while maintaining high physical stability, to enable an excellent sensitivity in a comparatively large strain range (Gauge factor, GF = 2.17 under 400% strain). The unique properties allow the generation of stable and accurate electrical signals transduced from different locations of the human body. As such, the PAM/PVA/SC hydrogel has the potential to be used as human–machine interface for continuous, real-time physiological monitoring.

Soft biological tissues perform various functions. Sensory nerves bring sensations of light, voice, touch, pain, or temperature variation to the central nervous system. Animal senses have inspired tremendous sensors for biomedical applications. Following the same principle as photosensitive nerves, we design flexible ionic hydrogels to achieve a biologic photosensor. The photosensor allows responding to near-infrared light, which is converted into a sensory electric signal that can communicate with nerve cells. Furthermore, with adjustable thermal and/or electrical signal outputs, it provides abundant tools for biological regulation. The tunable photosensitive performances, high flexibility, and low cost endow the photosensor with widespread applications ranging from neural prosthetics to human–machine interfacing systems.

The transduction from mechanical to electrical signal or energy, and vice versa, has wide applications spanning wearable electronics, medical devices, human‐machine interfaces, prosthetics, synthetic biology, soft robotics, augmented reality (AR), internet‐of‐things (IoT), smart cities, environmental monitoring, energy conversation, and energy harvesting. Although many biological processes such as those in sensory receptors and cell communication uses ions, much of existing artificial intelligent sensory and processing systems operate based solely on electrons as signal or energy carriers. Therefore, the realization of natively ionic devices can significantly advance the frontiers of both capabilities and applications. This invited paper introduces the piezoionic effect, surveying the historical evolution, scientific description, methodology, application, future outlook, and current debates. This work complements existing ones in that it is an in‐depth analysis of key attributes of the piezoionics field but, unlike a conventional review, does not emphasize the categorization of individual works, though is also provided. This work is also aimed at facilitating the understanding and evaluation of the piezoionics field in terms of its key capabilities, promises, fundamental limitations, technical challenges, and practical applications. Piezoionics as a biomimetic mechanical‐electrical transduction mechanism is expected to grow rapidly. Through in‐depth analysis and working out implications for the future, it is hoped that this paper provides a foundation for further exploration, discussion, and collaboration across different fields. The community looks forward to the piezoionic effect evolving into a mainstream paradigm for the ubiquitous realization of sensors, actuators, interfaces, as well as energy harvesters and beyond. Piezoionic effect: Although many biological processes such as those in sensory receptors and cell communication uses ions, much of existing artificial intelligent systems operate based solely on electrons as signal or energy carriers. Therefore, the realization of natively ionic devices is critical. This paper introduces the piezoionic effect, surveying the historical evolution, scientific description, methodology, application, outlook, and current debates.

The formation of self-crosslinking chitosan hydrogels using carboxylic acids has a number of limitations. Chitosan dissolves in oxalic, malonic, and succinic acids at a ratio of 1 amino group to 2 carboxyl groups into viscous solutions (G′ < G′′), but does not dissolve with lower amounts of the acid. Mixing chitosan hydrochloride with disodium carboxylates does not afford gels, but only a coacervate in the case of disodium oxalate, which dissolves upon dialysis. In the homologous series of N-carboxyalkyl derivatives (alkyl = methyl, ethyl, propyl), all members form gels (G′ > G′′). At approx. 50% of substitution, the storage modulus increases from 40 Pa (methyl) to 30,000 Pa (propyl) indicating the increasing strength of intermolecular interactions with the increasing length of the alkyl spacer. This could indicate that a sufficiently long spacer is required to properly connect the chitosan helices. N-succinyl chitosan, where the spacer is attached to the backbone as an amide, also forms polymer gels across all degrees of N-acylation. When compared to N-carboxypropyl chitosan, the latter forms significantly stiffer gels that swell less. This indicates that one covalent bond, a sufficient length, and the conformational flexibility of the spacer are important for gelation.

Robust adhesion is crucial for ionic hydrogels in flexible electronic pressure and strain sensors. Herein, we design a transparent and strong adhesion electronic pressure and strain sensor based on a multifunctional ionic hydrogel with double network using polyacrylamide (PAM) and gum arabic (GA). First, the effect of ionic type on the mechanical properties of hydrogels was measured. The GA/PAM–Fe 3+ hydrogel with high extensibility (1835%) and toughness 1 . 47 MJ / m 3 was selected as the best sensor material. Moreover, the GA/PAM–Fe 3+ hydrogel has good adhesion property with various substrates, and the maximum peel strength of the hydrogel to skin could reach up to 123.9 N/m. Furthermore, the resistance of GA/PAM–Fe 3+ hydrogel is sensitive to a wide strain window and the gauge factor shows stable and reliable change during deformation. Due to its compliant and excellent adhesive properties, strain sensors based on this hydrogel can be well fixed on the epidermis without adhesive tape, and can perceive large and gentle body movements. These characteristics demonstrate that GA/PAM–Fe 3+ hydrogel is promising for a broad range of practical applications in wearable sensors.

The engineering applications of hydrogels are generally limited by the common problem of their softness and brittlness. In this study, a composite double network ionic hydrogel (CDN-gel) was obtained by the facile visible light triggered polymerization of acrylic acid (AA), polyvinyl alcohol (PVA), and hydrolyzed triethoxyvinylsilane (TEVS) and subsequent salt impregnation. The resulting CDN-gels exhibited high toughness, recovery ability, and notch-insensitivity. The tensile strength, fracture elongation, Young’s modulus, and toughness of the CDN-gels reached up to ~21 MPa, ~700%, ~3.5 MPa, and ~48 M/m3, respectively. The residual strain at a strain of 200% was only ~25% after stretch-release of 1000 cycles. These properties will enable greater application of these hydrogel materials, especially for the fatigue resistance of tough hydrogels, as well as broaden their applications in damping.