Over the past few decades, flexible sensors have been developed from the “electronic” level to the “iontronic” level, and gradually to the “ionic” level. Ionic flexible sensors (IFS) are one kind of advanced sensors that are based on the concept of ion migration. Compared to conventional electronic sensors, IFS can not only replicate the topological structures of human skin, but also are capable of achieving tactile perception functions similar to that of human skin, which provide effective tools and methods for narrowing the gap between conventional electronics and biological interfaces. In this review, the latest research and developments on several typical sensing mechanisms, compositions, structural design, and applications of IFS are comprehensively reviewed. Particularly, the development of novel ionic materials, structural designs, and biomimetic approaches has resulted in the development of a wide range of novel and exciting IFS, which can effectively sense pressure, strain, and humidity with high sensitivity and reliability, and exhibit self‐powered, self‐healing, biodegradability, and other properties of the human skin. Furthermore, the typical applications of IFS in artificial skin, human‐interactive technologies, wearable health monitors, and other related fields are reviewed. Finally, the perspectives on the current challenges and future directions of IFS are presented.

Ionic Flexible Sensors: Mechanisms, Materials, Structures, and Applications

Breathable Kirigami-Shaped Ionotronic e-Textile with Touch/Strain Sensing for Friendly Epidermal Electronics

Conductive hydrogels have been extensively used in wearable skin sensors owing to their outstanding flexibility, tissuelike compliance, and biocompatibility. However, the dehydration and embrittlement of hydrogels can result in sensitivity loss or even invalidation, restraining their wearable applications in external environments, especially at low temperatures and in arid environments. Herein, an environment-resistant organohydrogel is developed for multifunctional sensors. A double-network organohydrogel based on hyaluronic acid and poly(acrylic acid-co-acrylamide) is developed, and glycerol is introduced into the organohydrogel network via a solvent displacement strategy. Owing to the water-locking effects of glycerol and tough polymeric backbone, the resultant organohydrogel not only exhibits stable tensibility but also maintains excellent flexibility and stable conductivity with the environment-resistant properties, including freezing resistance against -30 °C and moisture retention at 4% relative humidity in a high temperature of 60 °C. Moreover, a series of organohydrogel-based sensors and an array device are developed to achieve highly sensitive strain, temperature, and humidity responses and exhibit a high gauge factor of 10.79 in the strain-sensitive test. This work develops a universal ionic skin based on organohydrogels to be applied to wearable sensors for health monitoring.

Environment-Resistant Organohydrogel-Based Sensor Enables Highly Sensitive Strain, Temperature, and Humidity Responses.

Iontronic sensors represent a novel class of soft electronics which not only replicate the biomimetic structures and perception functions of human skin but also simulate the mechanical sensing mechanism. Relying on the similar mechanism with skin perception, the iontronic sensors can achieve ion migration/redistribution in response to external stimuli, promising iontronic sensing to establish more intelligent sensing interface for human-robotic interaction. Here, a comprehensive review on advanced technologies and diversified applications for the exploitation of iontronic sensors toward ionic skins and artificial intelligence is provided. By virtue of the excellent stretchability, high transparency, ultrahigh sensitivity, and mechanical conformality, numerous attempts have been made to explore various novel ionic materials to fabricate iontronic sensors with skin-like perceptive properties, such as self-healing and multimodal sensing. Moreover, to achieve multifunctional artificial skins and intelligent devices, various mechanisms based on iontronics have been investigated to satisfy multiple functions and human interactive experiences. Benefiting from the unique material property, diverse sensing mechanisms, and elaborate device structure, iontronic sensors have demonstrated a variety of applications toward ionic skins and artificial intelligence.

/pdf/emerging-iontronic-sensing-materials-mechanisms-and-1tpaci3e.pdf

Emerging Iontronic Sensing: Materials, Mechanisms, and Applications

Porous poly-ether-ether-ketone (PEEK) scaffolds have a good application prospect in orthopedics because of their excellent biocompatibility, chemical stability, and permeability. However, the porous PEEK scaffolds fabricated by conventional manufacturing methods are restricted to the uncontrollable and weak mechanical properties. In this work, porous PEEK scaffolds with highly-tunable elastic modulus are designed using typical triply periodic minimal surface (TPMS) structures, Gyroid, Diamond, I-WP, and then printed by laser powder bed fusion (LPBF). The effects of unit cell type and volume fraction on the manufacturing deviation, stress distribution, and mechanical properties of LPBF-printed PEEK scaffolds are investigated. The results illustrate that the Gyroid scaffolds present the lowest volume fraction deviation, followed by the I-WP scaffolds. By adjusting the volume fraction, the elastic modulus of the TPMS scaffolds is in the range of 42.17–512.12 MPa, which is comparable with those of human trabecular bones. Using the finite element (FE) simulation method, the experimental mechanical properties of the TPMS scaffolds can be predicted. In summary, the Gyroid structure is the optimal choice for the design and LPBF fabrication of porous PEEK scaffolds due to the best manufacturing fidelity and relatively good mechanical properties. • PEEK TPMS scaffolds were successfully fabricated by LPBF technology. • A FE method was proposed and it could accurately predict the mechanical responses of the PEEK TPMS scaffolds. • The elastic moduli of PEEK TPMS scaffolds can match those of human trabecular bones. 

Comparative evaluation of printability and compression properties of poly-ether-ether-ketone triply periodic minimal surface scaffolds fabricated by laser powder bed fusion

Traditional wearable devices are mainly fabricated using chemically synthetic organic and inorganic materials, which require high-cost manufacturing processes and consume limited resources, resulting in environmental problems such as electronic waste and plastic pollution. Consequently, sustainable and eco-friendly materials have been considered as potential candidates for fabricating renewable and green human machine interface devices. In this work, we designed wearable skin-like iontronics by using an edible cassava hydrogel to replace unrecoverable sensing components. The prepared capacitive and resistive iontronics can be used to supervise large as well as subtle human motions such as finger bending, micro-expression reading, language recognition and disease monitoring. Moreover, the skin-like hydrogel-based iontronics showed good cyclic repeatability within an appropriate deformation range. In addition, the good film-forming and self-healing properties of the hydrogel and touch sensitivity of the iontronics made it possible to design “artificial skin”. The skin-like iontronics might show great potential in future human–machine interface devices. This work explored the opportunities of an eco-friendly hydrogel in wearable skin-like sensors.

Eco-friendly biogenic hydrogel for wearable skin-like iontronics

It is challenging to prepare ultralow-density microcellular foams based on high-performance polymers due to their low gas solubility and rigid polymer matrix. In this study, by applying microcellul...

Poly(ether imide)/Epoxy Foam Composites with a Microcellular Structure and Ultralow Density: Bead Foam Fabrication, Compression Molding, Mechanical Properties, Thermal Stability, and Flame-Retardant Properties.

Fabrication of multifunctional porous fibers with excellent mechanical properties has attracted abundant attention in the fields of personal thermal management textiles and smart wearable devices. However, the high cost and harsh preparation environment of the traditional solution-solvent phase separation method for making porous fibers aggravates the problems of resource consumption and environmental pollution. Herein, a microextrusion process that combines environmentally friendly CO2 physical foaming with fused deposition modeling technology is proposed, via the dual features of high gas uptake and restricted cell growth, to implement the continuous production of porous polyetheretherketone (PEEK) fibers with a production efficiency of 10.5 cm s-1 . The porous PEEK fiber exhibits excellent stretchability (234.8% strain) and good high-temperature thermal insulation property. The open-cell structure on the surface is favorable for the adsorption to achieve superhydrophobicity (154.4°) and high-efficiency photocatalytic degradation of rhodamine B (90.4%). Moreover, the parameterized controllability of the cell structure is beneficial to widening the multifunctional window. In short, the first porous PEEK physical foaming fiber, which opens up a new avenue for the application expansion, especially in the medical field, is realized.

Robust and Multifunctional Porous Polyetheretherketone Fiber Fabricated via a Microextrusion CO2 Foaming

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Eco-friendly biogenic hydrogel for wearable skin-like iontronics

Poly(ether imide)/Epoxy Foam Composites with a Microcellular Structure and Ultralow Density: Bead Foam Fabrication, Compression Molding, Mechanical Properties, Thermal Stability, and Flame-Retardant Properties.

Robust and Multifunctional Porous Polyetheretherketone Fiber Fabricated via a Microextrusion CO2 Foaming