Recent Progress in Essential Functions of Soft Electronic Skin

Flexible pressure sensors that have high sensitivity, high linearity, and a wide pressure-response range are highly desired in applications of robotic sensation and human health monitoring. The challenge comes from the incompressibility of soft materials and the stiffening of microstructures in the device interfaces that lead to gradually saturated response. Therefore, the signal is nonlinear and pressure-response range is limited. Here, we show an iontronic flexible pressure sensor that can achieve high sensitivity (49.1 kPa-1), linear response (R2 > 0.995) over a broad pressure range (up to 485 kPa) enabled by graded interlocks of an array of hemispheres with fine pillars in the ionic layer. The high linearity comes from the fact that the pillar deformation can compensate for the effect of structural stiffening. The response-relaxation time of the sensor is <5 ms, allowing the device to detect vibration signals with frequencies up to 200 Hz. Our sensor has been used to recognize objects with different weights based on machine learning during the gripper grasping tasks. This work provides a strategy to make flexible pressure sensors that have combined performances of high sensitivity, high linearity, and wide pressure-response range.

Graded Interlocks for Iontronic Pressure Sensors with High Sensitivity and High Linearity over a Broad Range.

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

As an indispensable branch of wearable electronics, flexible pressure sensors are gaining tremendous attention due to their extensive applications in health monitoring, human –machine interaction, artificial intelligence, the internet of things, and other fields. In recent years, highly flexible and wearable pressure sensors have been developed using various materials/structures and transduction mechanisms. Morphological engineering of sensing materials at the nanometer and micrometer scales is crucial to obtaining superior sensor performance. This review focuses on the rapid development of morphological engineering technologies for flexible pressure sensors. We discuss different architectures and morphological designs of sensing materials to achieve high performance, including high sensitivity, broad working range, stable sensing, low hysteresis, high transparency, and directional or selective sensing. Additionally, the general fabrication techniques are summarized, including self-assembly, patterning, and auxiliary synthesis methods. Furthermore, we present the emerging applications of high-performing microengineered pressure sensors in healthcare, smart homes, digital sports, security monitoring, and machine learning-enabled computational sensing platform. Finally, the potential challenges and prospects for the future developments of pressure sensors are discussed comprehensively. 

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Morphological Engineering of Sensing Materials for Flexible Pressure Sensors and Artificial Intelligence Applications

Despite the extensive developments of flexible capacitive pressure sensors, it is still elusive to simultaneously achieve excellent linearity over a broad pressure range, high sensitivity, and ultrahigh pressure resolution under large pressure preloads. Here, we present a programmable fabrication method for microstructures to integrate an ultrathin ionic layer. The resulting optimized sensor exhibits a sensitivity of 33.7 kPa-1 over a linear range of 1700 kPa, a detection limit of 0.36 Pa, and a pressure resolution of 0.00725% under the pressure of 2000 kPa. Taken together with rapid response/recovery and excellent repeatability, the sensor is applied to subtle pulse detection, interactive robotic hand, and ultrahigh-resolution smart weight scale/chair. The proposed fabrication approaches and design toolkit from this work can also be leveraged to easily tune the pressure sensor performance for varying target applications and open up opportunities to create other iontronic sensors. 

/pdf/iontronic-pressure-sensor-with-high-sensitivity-over-ultra-1rsoz11n.pdf

Iontronic pressure sensor with high sensitivity over ultra-broad linear range enabled by laser-induced gradient micro-pyramids

Electronic skins and flexible pressure sensors are important devices for advanced healthcare and intelligent robotics. Sensitivity is a key parameter of flexible pressure sensors. Whereas introducing surface microstructures in a capacitive-type sensor can significantly improve its sensitivity, the signal becomes nonlinear and the pressure response range gets much narrower, significantly limiting the applications of flexible pressure sensors. Here, we designed a pressure sensor that utilizes a nanoscale iontronic interface of an ionic gel layer and a micropillared electrode, for highly linear capacitance-to-pressure response and high sensitivity over a wide pressure range. The micropillars undergo three stages of deformation upon loading: initial contact (0–6 kPa) and structure buckling (6–12 kPa) that exhibit a low and nonlinear response, as well as a post-buckling stage that has a high signal linearity with high sensitivity (33.16 kPa−1) over a broad pressure range of 12–176 kPa. The high linearity lies in the subtle balance between the structure compression and mechanical matching of the two materials at the gel-electrode interface. Our sensor has been applied in pulse detection, plantar pressure mapping, and grasp task of an artificial limb. This work provides a physical insight in achieving linear response through the design of appropriate microstructures and selection of materials with suitable modulus in flexible pressure sensors, which are potentially useful in intelligent robots and health monitoring.

Iontronic pressure sensor with high sensitivity and linear response over a wide pressure range based on soft micropillared electrodes

Interfacial solar evaporators (ISEs) for seawater desalination have garnered enormous attention in recent decades due to global water scarcity. Despite the progress in the energy conversion efficiency and production rate of ISE, the poor portability of large-area ISE during transportation as well as the clogging of water transport pathways by precipitated salts during operation remain grand challenges for its fielded applications. Here, we designed an ISE with high energy conversion efficiency and shape morphing capability by integrating carbon nanotube (CNT) fillers with a light-responsive shape memory polymer (SMP, cross-linked polycyclooctene (cPCO)). Utilizing the shape memory effect, our ISE can be folded to an origami with 1/9 of its original size to save space for transportation and allow for on-demand unfolding upon sunlight irradiation when deployed in service. In addition, the ISE is equipped with a real-time clogging monitoring function by measuring the capacitance of the electric double layer (EDL) formed at the evaporator/seawater nanointerface. Due to its good energy conversion efficiency, high portability, and clogging monitoring capability, we envisage our ISE as a promising selection in solar evaporation technologies.

Shape-Programmable Interfacial Solar Evaporator with Salt-Precipitation Monitoring Function.

Stretchable, transparent and imperceptible supercapacitors based on Au@MnO2 nanomesh electrodes.

Direct oxidation of methane to methanol (DMTM) remains an economically tantalizing but fundamentally challenging goal because of the highly stable C-H bonds. By using density functional theory calculations, we investigated the catalytic properties of single transition metals (Fe, Pd, Ni) supported on O-doped BN in different coordination environments for DMTM. The results indicated that embedding Ni into O-doped BN via two N atoms and one O atom coordination (Ni1/O1N2-BN) was an efficient option for DMTM. Ni1/O1N2-BN was capable of effectively activating the strong C-H bonds of CH4 by generating key Ni-O intermediates. Besides, Ni1/O1N2-BN also exhibited high selectivity for CH3OH owing to the inhibition of CH2 competitive species and low desorption energy of CH3OH. Furthermore, the excellent thermal stability of Ni1/O1N2-BN was verified via ab initio molecular dynamics calculations at 500 K for 10 ps. This work provides a new insight into the fundamental understanding and materials design of high-efficiency catalysts for DMTM.

A DFT study of boron nitride-confined nickel single atoms for the oxidation of methane to methanol.

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Iontronic pressure sensor with high sensitivity and linear response over a wide pressure range based on soft micropillared electrodes

Shape-Programmable Interfacial Solar Evaporator with Salt-Precipitation Monitoring Function.

Stretchable, transparent and imperceptible supercapacitors based on Au@MnO2 nanomesh electrodes.

A DFT study of boron nitride-confined nickel single atoms for the oxidation of methane to methanol.