Flexible, Permeable, and Recyclable Liquid-Metal-Based Transient Circuit Enables Contact/Noncontact Sensing for Wearable Human-Machine Interaction

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

Proximity Sensing Electronic Skin: Principles, Characteristics, and Applications

The research on proximity sensing electronic skin has garnered significant attention. This electronic skin technology enables detection without physical contact and holds vast application prospects in areas such as human-robot collaboration, human-machine interfaces, and remote monitoring. Especially in the context of the spread of infectious diseases like COVID-19, there is a pressing need for non-contact detection to ensure safe and hygienic operations. This article comprehensively reviews the significant advancements in the field of proximity sensing electronic skin technology in recent years. It covers the principles, as well as single-type proximity sensors with characteristics such as a large area, multifunctionality, strain, and self-healing capabilities. Additionally, it delves into the research progress of dual-type proximity sensors. Furthermore, the article places a special emphasis on the widespread applications of flexible proximity sensors in human-robot collaboration, human-machine interfaces, and remote monitoring, highlighting their importance and potential value across various domains. Finally, the paper provides insights into future advancements in flexible proximity sensor technology.

Recent progress in multifunctional, reconfigurable, integrated liquid metal-based stretchable sensors and standalone systems

Possessing a unique combination of properties that are traditionally contradictory in other natural or synthetical materials, Ga-based liquid metals (LMs) exhibit low mechanical stiffness and flowability like a liquid, with good electrical and thermal conductivity like metal, as well as good biocompatibility and room-temperature phase transformation. These remarkable properties have paved the way for the development of novel reconfigurable or stretchable electronics and devices. Despite these outstanding properties, the easy oxidation, high surface tension, and low rheological viscosity of LMs have presented formidable challenges in high-resolution patterning. To address this challenge, various surface modifications or additives have been employed to tailor the oxidation state, viscosity, and patterning capability of LMs. One effective approach for LM patterning is breaking down LMs into microparticles known as liquid metal particles (LMPs). This facilitates LM patterning using conventional techniques such as stencil, screening, or inkjet printing. Judiciously formulated photo-curable LMP inks or the introduction of an adhesive seed layer combined with a modified lift-off process further provide the micrometer-level LM patterns. Incorporating porous and adhesive substrates in LM-based electronics allows direct interfacing with the skin for robust and long-term monitoring of physiological signals. Combined with self-healing polymers in the form of substrates or composites, LM-based electronics can provide mechanical-robust devices to heal after damage for working in harsh environments. This review provides the latest advances in LM-based composites, fabrication methods, and their novel and unique applications in stretchable or reconfigurable sensors and resulting integrated systems. It is believed that the advancements in LM-based material preparation and high-resolution techniques have opened up opportunities for customized designs of LM-based stretchable sensors, as well as multifunctional, reconfigurable, highly integrated, and even standalone systems.

Two-Stage Micropyramids Enhanced Flexible Piezoresistive Sensor for Health Monitoring and Human-Computer Interaction

High-performance flexible piezoresistive sensors are becoming increasingly essential in various novel applications such as health monitoring, soft robotics, and human-computer interaction. The evolution of the interfacial contact morphology determines the sensing properties of piezoresistive devices. The introduction of microstructures enriches the interfacial contact morphology and effectively boosts the sensitivity; however, the limited compressibility of conventional microstructures leads to rapid saturation of the sensitivity in the low-pressure range, which hinders their application. Herein, we present a flexible piezoresistive sensor featuring a two-stage micropyramid array structure, which effectively enhances the sensitivity while widening the sensing range. Owing to the synergistic enhancement effect resulting from the sequential contact of micropyramids of various heights, the devices demonstrate remarkable performance, including boosting sensitivity (30.8 kPa-1) over a wide sensing range (up to 200 kPa), a fast response/recovery time (75/50 ms), and an ultralong durability of 15,000 loading-unloading cycles. As a proof of concept, the sensor is applied to detect human physiological and motion signals, further demonstrating a real-time spatial pressure distribution sensing system and a game control system, showing great potential for applications in health monitoring and human-computer interaction.

Magnetically Recyclable Nanoscale Zero-Valent Iron-Mediated PhotoRDRP in Ionic Liquid toward Smart, Functional Polymers

A facile method based on recyclable nanoscale zero-valent iron (nZVI)-mediated photoinduced reversible deactivation radical polymerization in ionic liquid (IL) leads to the synthesis of narrow disperse poly(tert-butyl methacrylate) (PTBMA), amphiphilic PTBMA-block-poly(poly(ethylene glycol)methacrylate) diblock copolymer and double hydrophilic poly(methacrylic acid)-block-poly(poly(ethylene glycol)methacrylate) (PMAA-b-PPEGMA) diblock copolymers thereof. Stimuli response of the synthesized PMAA-b-PPEGMA diblock copolymer against variation in pH and temperature is assessed. Recyclability of the nZVI (catalyst) and IL (solvent) is established. Polymerization may be switched ON or OFF, simply by turning the UVA light irradiation ON or OFF, offering temporal control. The diblock copolymer self-aggregates into spherical nanoaggregates which are employed for encapsulation of coumarin 102 (C102, a typical hydrophobic dye), describing their potential application in drug delivery applications. The facile synthesis strategy may open up new avenues for the preparation of intelligent functional polymers for engineering and biomedical applications.

Anisotropic and Highly Sensitive Flexible Strain Sensors Based on Carbon Nanotubes and Iron Nanowires for Human-Computer Interaction Systems

Flexible strain sensors for multi-directional strain detection are crucial in complicated hman-computer interaction (HCI) applications. However, enhancing the anisotropy and sensitivity of the sensors for multi-directional detection in a simple and effective method remains a significant issue. Therefore, this study proposes a flexible strain sensor with anisotropy and high sensitivity based on a high-aspect-ratio V-groove array and a hybrid conductive network of iron nanowires and carbon nanotubes (Fe NWs/CNTs). The sensor exhibits significant anisotropy, with a difference in strain detection sensitivity of up to 35.92 times between two mutually perpendicular directions. Furthermore, the dynamic performance of the sensor shows a good response rate, ranging from 223 ms to 333 ms. The sensor maintains stability and consistent performance even after undergoing 1000 testing cycles. Additionally, the constructed flexible strain sensor is tested using the remote control application of a trolley, demonstrating its high potential for usage in practical HCI systems. This research offers a significant competitive advantage in the development of flexible strain sensors in the field of HCI.

Recent progress in construction methods and applications of perovskite photodetector arrays

Metal halide perovskites are considered promising materials for next-generation optoelectronic devices due to their excellent optoelectronic performances and simple solution preparation process. Precise micro/nano-scale patterning techniques enable perovskite materials to be used for array integration of photodetectors. In this review, the device types of perovskite-based photodetectors are introduced and the structural characteristics and corresponding device performances are analyzed. Then, the typical construction methods suitable for the fabrication of perovskite photodetector arrays are highlighted, including surface treatment technology, template-assisted construction, inkjet printing technology, and modified photolithography. Furthermore, the current development trends and their applications in image sensing of perovskite photodetector arrays are summarized. Finally, major challenges are presented to guide the development of perovskite photodetector arrays.

Superelastic Radiative Cooling Metafabric for Comfortable Epidermal Electrophysiological Monitoring

Epidermal electronics with superb passive-cooling capabilities are of great value for both daytime outdoor dressing comfort and low-carbon economy. Herein, a multifunctional and skin-attachable electronic is rationally developed on a porous all-elastomer metafabric for efficient passive daytime radiative cooling (PDRC) and human electrophysiological monitoring. The cooling characteristics are realized through the homogeneous impregnation of polytetrafluoroethylene microparticles in the styrene-ethylene-butylene-styrene fibers, and the rational regulation of microporosity in SEBS/PTFE metafabrics, thus synergistically backscatter ultraviolet-visible-near-infrared light (maximum reflectance over 98.0%) to minimize heat absorption while efficiently emit human-body midinfrared radiation to the sky. As a result, the developed PDRC metafabric achieves approximately 17 °C cooling effects in an outdoor daytime environment and completely retains its passive cooling performance even under 50% stretching. Further, high-fidelity electrophysiological monitoring capability is also implemented in the breathable and skin-conformal metafabric through liquid metal printing, enabling the accurate acquisition of human electrocardiograph, surface electromyogram, and electroencephalograph signals for comfortable and lengthy health regulation. Hence, the fabricated superelastic PDRC metafabric opens a new avenue for the development of body-comfortable electronics and low-carbon wearing technologies.

Electrospun Nanofiber-Based Bioinspired Artificial Skins for Healthcare Monitoring and Human-Machine Interaction

Artificial skin, also known as bioinspired electronic skin (e-skin), refers to intelligent wearable electronics that imitate the tactile sensory function of human skin and identify the detected changes in external information through different electrical signals. Flexible e-skin can achieve a wide range of functions such as accurate detection and identification of pressure, strain, and temperature, which has greatly extended their application potential in the field of healthcare monitoring and human-machine interaction (HMI). During recent years, the exploration and development of the design, construction, and performance of artificial skin has received extensive attention from researchers. With the advantages of high permeability, great ratio surface of area, and easy functional modification, electrospun nanofibers are suitable for the construction of electronic skin and further demonstrate broad application prospects in the fields of medical monitoring and HMI. Therefore, the critical review is provided to comprehensively summarize the recent advances in substrate materials, optimized fabrication techniques, response mechanisms, and related applications of the flexible electrospun nanofiber-based bio-inspired artificial skin. Finally, some current challenges and future prospects are outlined and discussed, and we hope that this review will help researchers to better understand the whole field and take it to the next level.