Electronic Skin to Feel "Pain": Detecting "Prick" and "Hot" Pain Sensations

A flexible capacitive pressure sensor with a hybrid porous PDMS/SA hydrogel structure for touch/pain detection

Exploring highly sensitive flexible electronic skins (e-skins) that can mimic the tactile and pain perception of human skin is an important prerequisite for achieving biomimetic robots and intelligent prosthetics. However, it is still difficult to realize both touch and pain sensing using a single pressure sensor. Herein, a novel flexible capacitive pressure sensor that can distinguish noxious pressure stimuli is proposed for detecting touch and pain, which is composed of a porous polydimethylsiloxane (PDMS) skeleton and a sodium alginate (SA) hydrogel core. The sensor employs two different working mechanisms depending on the range of external pressure, determining the mechanism of operation for transducing the sense of touch or pain. Such a unique structural design plays a crucial role in enhancing pain perception, leading to maximum sensitivity (14.25 kPa-1) in a large pressure regime (up to 400 kPa) and an adjustable pressure threshold. Moreover, the sensor also exhibits a fast response (45 ms) and recovery speed (70 ms), ensuring a sufficiently fast response to noxious pressure stimuli. Finally, we demonstrate the capabilities of a robotic hand based on the pressure sensor for precisely detecting both touch and pain, which shows great promise in developing intelligent robots and prosthetic limbs to prevent possible damage under external noxious stimuli.

Skin-Inspired Thermoreceptors-Based Electronic Skin for Biomimicking Thermal Pain Reflexes

Electronic systems possessing skin-like morphology and functionalities (electronic skins [e-skins]) have attracted considerable attention in recent years to provide sensory or haptic feedback in growing areas such as robotics, prosthetics, and interactive systems. However, the main focus thus far has been on the distributed pressure or force sensors. Herein a thermoreceptive e-skin with biological systems like functionality is presented. The soft, distributed, and highly sensitive miniaturized (≈700 µm2 ) artificial thermoreceptors (ATRs) in the e-skin are developed using an innovative fabrication route that involves dielectrophoretic assembly of oriented vanadium pentoxide nanowires at defined locations and high-resolution electrohydrodynamic printing. Inspired from the skin morphology, the ATRs are embedded in a thermally insulating soft nanosilica/epoxy polymeric layer and yet they exhibit excellent thermal sensitivity (-1.1 ± 0.3% °C-1 ), fast response (≈1s), exceptional stability (negligible hysteresis for >5 h operation), and mechanical durability (up to 10 000 bending and twisting loading cycles). Finally, the developed e-skin is integrated on the fingertip of a robotic hand and a biological system like reflex is demonstrated in response to temperature stimuli via localized learning at the hardware level.

Tactile Avatar: Tactile Sensing System Mimicking Human Tactile Cognition

As a surrogate for human tactile cognition, an artificial tactile perception and cognition system are proposed to produce smooth/soft and rough tactile sensations by its user's tactile feeling; and named this system as "tactile avatar". A piezoelectric tactile sensor is developed to record dynamically various physical information such as pressure, temperature, hardness, sliding velocity, and surface topography. For artificial tactile cognition, the tactile feeling of humans to various tactile materials ranging from smooth/soft to rough are assessed and found variation among participants. Because tactile responses vary among humans, a deep learning structure is designed to allow personalization through training based on individualized histograms of human tactile cognition and recording physical tactile information. The decision error in each avatar system is less than 2% when 42 materials are used to measure the tactile data with 100 trials for each material under 1.2N of contact force with 4cm s-1 of sliding velocity. As a tactile avatar, the machine categorizes newly experienced materials based on the tactile knowledge obtained from training data. The tactile sensation showed a high correlation with the specific user's tendency. This approach can be applied to electronic devices with tactile emotional exchange capabilities, as well as advanced digital experiences.

Triboelectric and Piezoelectric Nanogenerators for Future Soft Robots and Machines

The triboelectric nanogenerator (TENG) and piezoelectric nanogenerator (PENG) are two recently developed technologies for effective harvesting of ambient mechanical energy for the creation of self-powered systems. The advantages of TENGs and PENGs which include large open-circuit output voltage, low cost, ease of fabrication, and high conversion efficiency enable their application as new flexible sensors, wearable devices, soft robotics, and machines. This perspective provides an overview of the current state of the art in triboelectric and piezoelectric devices that are used as self-powered sensors and energy harvesters for soft robots and machines; hybrid approaches that combine the advantages of both mechanisms are also discussed. To improve system performance and efficiency, the potential of providing self-powered soft systems with a degree of multifunctionality is investigated. This includes optical sensing, transparency, self-healing, water resistance, photo-luminescence, or an ability to operate in hostile environments such as low temperature, high humidity, or high strain/stretch. Finally, areas for future research directions are identified.