Stretchable ionic skins are intriguing in mimicking the versatile sensations of natural skins. However, for their applications in advanced electronics, good elastic recovery, self-healing, and more importantly, skin-like nonlinear mechanoresponse (strain-stiffening) are essential but can be rarely met in one material. Here we demonstrate a robust proton-conductive ionic skin design via introducing an entropy-driven supramolecular zwitterionic reorganizable network to the hydrogen-bonded polycarboxylic acid network. The design allows two dynamic networks with distinct interacting strength to sequentially debond with stretch, and the conflict among elasticity, self-healing, and strain-stiffening can be thus defeated. The representative polyacrylic acid/betaine elastomer exhibits high stretchability (1600% elongation), immense strain-stiffening (24-fold modulus enhancement), ~100% self-healing, excellent elasticity (97.9 ± 1.1% recovery ratio, <14% hysteresis), high transparency (99.7 ± 0.1%), moisture-preserving, anti-freezing (elastic at −40 °C), water reprocessibility, as well as easy-to-peel adhesion. The combined advantages make the present ionic elastomer very promising in wearable iontronic sensors for human-machine interfacing. Ionic skins are of interest for a range of electronic, sensing and interfacing applications but often have trade-offs in properties. Here, the authors report on the creation of a dual network ionic skin using a supramolecular zwitterionic competing network to produce a strain-stiffening, self-healing adhesive sensor.

/pdf/skin-like-mechanoresponsive-self-healing-ionic-elastomer-395hyak827.pdf

Skin-like mechanoresponsive self-healing ionic elastomer from supramolecular zwitterionic network.

Traditional electronic skin (e‐skin), due to the lack of human‐brain‐like thinking and judging capability, is powerless to accelerate the pace to the intelligent era. Herein, artificial intelligence (AI)‐motivated full‐skin bionic (FSB) e‐skin consisting of the structures of human vellus hair, epidermis–dermis–hypodermis, is proposed. Benefiting from the double interlocked layered microcone structure and supercapacitive iontronic effect, the FSB e‐skin exhibits ultrahigh sensitivity of 8053.1 kPa−1 (<1 kPa), linear sensitivity of 3103.5 kPa−1 (1–34 kPa), and fast response/recovery time of <5.6 ms. In addition, it can realize the evolution from tactile perception to advanced intelligent tactile cognition after being equipped with a “brain”. First, static/dynamic contactless tactile perception is achieved based on the triboelectric effect of the vellus hair bionics. Second, the supercapacitive iontronic effect based structural bionics of the epidermis–dermis–hypodermis and a five‐layer multilayer perception (MLP) enable the general intelligent tactile cognition of gesture cognition and robot interaction. Most importantly, by making full use of the FSB e‐skin with a six‐layer MLP neural network, an advanced intelligent material cognition system is developed for real‐time cognition of the object material species and locations via one contact, which surpasses the capability of humans.

Perception‐to‐Cognition Tactile Sensing Based on Artificial‐Intelligence‐Motivated Human Full‐Skin Bionic Electronic Skin

Past research aimed at increasing the sensitivity of capacitive pressure sensors has mostly focused on developing dielectric layers with surface/porous structures or higher dielectric constants. However, such strategies have only been effective in improving sensitivities at low pressure ranges (e.g., up to 3 kPa). To overcome this well-known obstacle, herein, a flexible hybrid-response pressure sensor (HRPS) composed of an electrically conductive porous nanocomposite (PNC) laminated with an ultrathin dielectric layer is devised. Using a nickel foam template, the PNC is fabricated with carbon nanotubes (CNTs)-doped Ecoflex to be 86% porous and electrically conductive. The PNC exhibits hybrid piezoresistive and piezocapacitive responses, resulting in significantly enhanced sensitivities (i.e., more than 400%) over wide pressure ranges, from 3.13 kPa-1 within 0-1 kPa to 0.43 kPa-1 within 30-50 kPa. The effect of the hybrid responses is differentiated from the effect of porosity or high dielectric constants by comparing the HRPS with its purely piezocapacitive counterparts. Fundamental understanding of the HRPS and the prediction of optimal CNT doping are achieved through simplified analytical models. The HRPS is able to measure pressures from as subtle as the temporal arterial pulse to as large as footsteps.

Highly Sensitive Capacitive Pressure Sensors over a Wide Pressure Range Enabled by the Hybrid Responses of a Highly Porous Nanocomposite

Although Ti3C2Tx MXene sheets are highly conductive, it is still a challenge to design highly stretchable MXene electrodes for flexible electronic devices. Inspired by the high stretchability of kirigami patterns, we demonstrate a bottom-up methodology to design highly stretchable and conductive polydimethylsiloxane (PDMS)/Ti3C2Tx MXene films for electromagnetic interference (EMI) shielding and pressure sensing applications by constructing wrinkled MXene patterns on a flexible PDMS substrate to create a hierarchical surface with primary and secondary surface wrinkles. The self-controlled microcracks created in the valley domains of the hierarchical film via a nonuniform deformation during prestretching/releasing cycles endow the hierarchical PDMS/MXene film with a high stretchability (100%), strain-invariant conductivity in a strain range of 0%-100%, and stable conductivities over an 1000-cycle fatigue measurement. The stretchable film exhibits a highly stable EMI shielding performance of ≈30 dB at a tensile strain of 50%, and its EMI shielding efficiency increases further to 103 dB by constructing a two-film structure. Furthermore, a highly stretchable and sensitive iontronic sensor array with integrated MXene-based electrodes and circuits is fabricated by a stencil printing process, exhibiting high sensitivity (66.3 nF kPa-1), excellent dynamic cycle stability over 1000 cycles under different frequencies, and sensitive pressure monitoring capability under a tensile strain of 50%.

Kirigami-Inspired Highly Stretchable, Conductive, and Hierarchical Ti3C2Tx MXene Films for Efficient Electromagnetic Interference Shielding and Pressure Sensing.

Touch sensing has a central role in robotic grasping and emerging human-machine interfaces for robot-assisted prosthetics. Although advancements in soft conductive polymers have promoted the creation of diverse pressure sensors, these sensors are difficult to be employed as touch skins for robotics and prostheses due to their limited sensitivity, narrow pressure range, and complex structure and fabrication process. Here, a highly sensitive and robust soft touch skin is presented with ultracapacitive sensing that combines ionic hydrogels with commercially available conductive fabrics. Prototypical designs of the capacitive sensors through facile manufacturing methods are introduced and a high sensitivity up to 1.5 kPa-1 (≈44 times higher than conventional parallel-plate capacitive counterparts), a broad pressure detection range of over four orders of magnitudes (≈35 Pa to 330 kPa), ultrahigh baseline of capacitance, fast response time (≈18 ms), and good repeatability are demonstrated. Ionogel skins composed of an array of cutaneous mechanoreceptors capable of monitoring various physiological signals and shape detection are further developed. The touch skin can be integrated within a soft bionic hand and provide an industrial robot and an amputee with robust tactile feedback when handling delicate objects, illustrating its potential applications in next-generation human-in-the-loop robotic systems with tactile sensing.

Cutaneous Ionogel Mechanoreceptors for Soft Machines, Physiological Sensing, and Amputee Prostheses.

Over the past decade, a brand-new pressure- and tactile-sensing modality, known as iontronic sensing has emerged, utilizing the supercapacitive nature of the electrical double layer (EDL) that occurs at the electrolytic-electronic interface, leading to ultrahigh device sensitivity, high noise immunity, high resolution, high spatial definition, optical transparency, and responses to both static and dynamic stimuli, in addition to thin and flexible device architectures. Together, it offers unique combination of enabling features to tackle the grand challenges in pressure- and tactile-sensing applications, in particular, with recent interest and rapid progress in the development of robotic intelligence, electronic skin, wearable health as well as the internet-of-things, from both academic and industrial communities. A historical perspective of the iontronic sensing discovery, an overview of the fundamental working mechanism along with its device architectures, a survey of the unique material aspects and structural designs dedicated, and finally, a discussion of the newly enabled applications, technical challenges, and future outlooks are provided for this promising sensing modality with implementations. The state-of-the-art developments of the iontronic sensing technology in its first decade are summarized, potentially providing a technical roadmap for the next wave of innovations and breakthroughs in this field.

First Decade of Interfacial Iontronic Sensing: From Droplet Sensors to Artificial Skins

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First Decade of Interfacial Iontronic Sensing: From Droplet Sensors to Artificial Skins