Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing.

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Effect of phase coarsening under melt annealing on the electrical performance of polymer composites with a double percolation structure.

The effect of phase coarsening on the evolution of the carbon black (CB) nanoparticle network under quiescent melt annealing and the electrical performance of polypropylene/polystyrene/carbon black (PP/PS/CB) composites with a double percolation structure was investigated. The results showed that when the CB content is low, the coarsening process of PP/PS/CB blends can be divided into two stages. In the first stage, the coarsening rate is fast before the formation of the CB nanoparticle network, and after annealing for a certain time, the evolution of the co-continuous morphology can drive the CB nanoparticles to self-assemble into a complete nanoparticle network. In the second stage, the coarsening rate is slow after the formation of the CB nanoparticle network. When the CB content is high, the CB nanoparticle network can be maintained throughout the whole annealing process, so that the conductivity and morphology of the PP/PS/CB composites are stable. Moreover, the electrical conductivity of the PP/PS/CB composites greatly increases after annealing for a certain time, and a percolation threshold as low as 0.07 vol% can be obtained. These results reveal the relationship between the evolution of the morphology and the conductivity in the conductive polymer composites with a double percolation structure, and provide a more in-depth and comprehensive understanding of the double percolation structure.

Nanofibrillar Poly(vinyl alcohol) Ionic Organohydrogels for Smart Contact Lens and Human-Interactive Sensing.

Hydrogel bioelectronics as one of the next-generation wearable and implantable electronics ensures excellent biocompatibility and softness to link the human body and electronics. However, volatile, opaque, and fragile features of hydrogels due to the sparse and microscale three-dimensional network seriously limit their practical applications. Here, we report a type of smart and robust nanofibrillar poly(vinyl alcohol) (PVA) organohydrogels fabricated via one-step physical cross-linking. The nanofibrillar network cross-linked by numerous PVA nanocrystallites enables the formation of organohydrogels with high transparency (90%), drying resistance, high toughness (3.2 MJ/m3), and tensile strength (1.4 MPa). For strain sensor application, the PVA ionic organohydrogel after soaking in NaCl solution shows excellent linear sensitivity (GF = 1.56, R2 > 0.998) owing to the homogeneous nanofibrillar PVA network. We demonstrate the potential applications of the nanofibrillar PVA-based organohydrogel in smart contact lens and emotion recognition. Such a strategy paves an effective way to fabricate strong, tough, biocompatible, and ionically conductive organohydrogels, shedding light on multifunctional sensing applications in next-generation flexible bioelectronics.

Flexible Anti-Biofouling MXene/Cellulose Fibrous Membrane for Sustainable Solar-Driven Water Purification.

Solar-driven interfacial water evaporation is regarded as an effective, renewable, and environment-friendly technology for clean water production. However, biofouling caused by the nonspecific interaction between the steam generator and biofoulants generally hinders the efficient application of wastewater treatment. Herein, this work reports a facile strategy to fabricate flexible anti-biofouling fibrous photothermal membrane consisting of a MXene-coated cellulose membrane for highly efficient solar-driven water steam evaporation toward water purification applications. The as-prepared MXene/cellulose photothermal membrane exhibits light absorption efficiency as high as ∼94% in a wide solar spectrum range and a water evaporation rate up to 1.44 kg m-2 h-1 under one solar illumination. Also, the MXene/cellulose membrane shows very high antibacterial efficiency (above 99.9%) owing to the MXene coating as a highly effective bacteriostatic agent. Such a flexible, anti-biofouling, and high-efficiency photothermal membrane sheds light on practical applications in long-term wastewater treatments.

Human Skin-Inspired Electronic Sensor Skin with Electromagnetic Interference Shielding for the Sensation and Protection of Wearable Electronics.

Increasingly serious electromagnetic radiation pollution puts higher demands on wearable devices. Electronic sensor skin capable of shielding electromagnetic radiation can provide extra protection in emerging fields such as electronic skins, robotics, and artificial intelligence, but combining the sensation and electromagnetic shielding performance together remains a great challenge. Here, inspired by the structure and functions of the human skin, a multifunctional electronic skin (M-E-skin) with both tactile sensing and electromagnetic radiation shielding functions is proposed. The tactile sensing of human skin is mimicked with irregular dermislike rough surfaces, and the electromagnetic shielding performance not available on natural skin is introduced by mimicking the ultraviolet electromagnetic radiation absorption of melanin in epidermis. The M-E-skin shows superior sensitivity (9.8 × 104 kPa-1 for the pressure range 0-0.2 kPa and 3.5 × 103 kPa-1 within 0.2-20 kPa), broad operating range (0-20 kPa), fast response and relaxation times (<62.5 ms), great pressuring-relaxing stability (10 kPa, 1000 cycles), low operating voltage (0.1 V), low power consumption (1.5 nW), and low detection limit (5 Pa). Besides, a broad range of electromagnetic wave (0.5-7.5 GHz) is shielded more than 99.66% by the M-E-skin. This work holds great potential to enlarge the application scope of current electronic skins.

2D end-to-end carbon nanotube conductive networks in polymer nanocomposites: a conceptual design to dramatically enhance the sensitivities of strain sensors.

New generation wearable devices require mechanically compliant strain sensors with a high sensitivity in a full detecting range. Herein, novel 2D end-to-end contact conductive networks of multi-walled carbon nanotubes (MWCNTs) were designed and realized in an ethylene-α-octene block copolymer (OBC) matrix. The prepared strain sensor showed a high gauge factor (GF) of 248 even at a small strain (5%) and a linear resistance response throughout the whole strain range. The sensors also exhibited very good stretchability up to 300% and high cycling durability. This novel design solved the intrinsic problem of sensors based on carbon nanotube bundles, i.e., a long sliding phase before the disconnection of CNTs in a cost-effective and scalable way. This study rationalizes the 2D end-to-end contact concept to improve the sensitivity of the existing sensors and has great potential to be used in a wide variety of polymer based sensors.