Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576, Singapore Center for Intelligent Sensors and MEMS, National University of Singapore, Singapore, 117608, Singapore Hybrid-Integrated Flexible (Stretchable) Electronic Systems Program, National University of Singapore, Singapore, 117608, Singapore NUS Suzhou Research Institute (NUSRI), Suzhou, 215123, China NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Singapore, 117456, Singapore

Progress in wearable electronics/photonics—Moving toward the era of artificial intelligence and internet of things

The fabrication of highly durable skin-mimicking sensors remains challenging because of the unavoidable fatigue and physical damage that sensors are subjected to in practical applications In this study, ultra-durable ionic skins (I-skins) with excellent healability and high sensitivity are fabricated by impregnating ionic liquids (ILs) into a mechanically robust poly(urea-urethane) (PU) network The PU network is composed of crystallized poly(e-caprolactone) and flexible poly(ethylene glycol) that are dynamically cross-linked with hindered urea bonds and hydrogen bonds Such a design endows the resultant ionogels with high mechanical strength, good elasticity, Young's modulus similar to that of natural skin, and excellent healability The ionogel-based I-skins exhibit a high sensitivity to a wide range of strains (01-300%) and pressures (01-20 kPa) Importantly, the I-skins show a highly reproducible electrical response over 10 000 uninterrupted strain cycles The sensing performance of the I-skins stored in open air for 200 days is almost the same as that of the freshly prepared I-skin The fractured I-skins can be easily healed by heating at 65 °C that restores their original ultra-durable sensing performance The long-term durability of the I-skins is attributed to the combination of non-volatility of the ILs, excellent healability, and well-designed mechanical properties

Mechanically Robust, Elastic, and Healable Ionogels for Highly Sensitive Ultra-Durable Ionic Skins

Recent advances in the design and implementation of wearable resistive, capacitive, and optical strain sensors are summarized herein. Wearable and stretchable strain sensors have received extensive research interest due to their applications in personalized healthcare, human motion detection, human–machine interfaces, soft robotics, and beyond. The disconnection of overlapped nanomaterials, reversible opening/closing of microcracks in sensing films, and alteration of the tunneling resistance have been successfully adopted to develop high‐performance resistive‐type sensors. On the other hand, the sensing behavior of capacitive‐type and optical strain sensors is largely governed by their geometrical changes under stretching/releasing cycles. The sensor design parameters, including stretchability, sensitivity, linearity, hysteresis, and dynamic durability, are comprehensively discussed. Finally, the promising applications of wearable strain sensors are highlighted in detail. Although considerable progress has been made so far, wearable strain sensors are still in their prototype stage, and several challenges in the manufacturing of integrated and multifunctional strain sensors should be yet tackled.

Wearable and Stretchable Strain Sensors: Materials, Sensing Mechanisms, and Applications

Artificial skin-like materials have wide applications, particularly in flexible electronics. However, developing intelligent skin-like soft materials with a remarkable range of properties is still a challenge. Herein, a versatile biomimetic hydrogel inspired by natural skin was fabricated based on Ag/TA@CNC (cellulose nanocrystals (CNCs) decorated with tannic acid (TA) and Ag nanoparticles) nanohybrids and polyvinyl alcohol via dynamic borate ester bond crosslinking, which realized the combination of superstretchability (>4000%), efficient (within 10 min, 98.6%) and repeatable self-healing property, conformability and mechano-stimuli sensitivity within a single structure. This skin-inspired hydrogel can be assembled as a self-healing flexible capacitive sensor to track human body motions with a relatively broad sensible range of strain (up to 400%). Meanwhile, the hydrogel can be further used for repairing circuits, constructing switches, programmed electrical circuit assembly, as electronic skin, and in touch screen pens. Moreover, the modified CNCs could be used as a functional trigger to endow the nanocomposite hydrogels with excellent antibacterial properties and repeatable self-adhesiveness, which also play an important role in bionic skin materials. Compared with previous skin-like materials, these nanocellulose biomimetic hydrogels achieve more favorable functionalities in mimicking natural skin, which can meet versatile application needs ranging from flexible/wearable/self-healable electronic devices to artificial electronic skin.

Natural skin-inspired versatile cellulose biomimetic hydrogels

Hydrogel soft robotics

Fabricating a strain sensor that can detect large deformation over a curved object with a high sensitivity is crucial in wearable electronics, human/machine interfaces, and soft robotics. Herein, an ionogel nanocomposite is presented for this purpose. Tuning the composition of the ionogel nanocomposites allows the attainment of the best features, such as excellent self-healing (>95% healing efficiency), strong adhesion (347.3 N m-1 ), high stretchability (2000%), and more than ten times change in resistance under stretching. Furthermore, the ionogel nanocomposite-based sensor exhibits good reliability and excellent durability after 500 cycles, as well as a large gauge factor of 20 when it is stretched under a strain of 800-1400%. Moreover, the nanocomposite can self-heal under arduous conditions, such as a temperature as low as -20 °C and a temperature as high as 60 °C. All these merits are achieved mainly due to the integration of dynamic metal coordination bonds inside a loosely cross-linked network of ionogel nanocomposite doped with Fe3 O4 nanoparticles.

Li Mei Zhang

Papers

Self-Healing, Adhesive, and Highly Stretchable Ionogel as a Strain Sensor for Extremely Large Deformation