Human‐machine interfaces (HMIs) play important role in the communication between humans and robots. Touchless HMIs with high hand dexterity and hygiene hold great promise in medical applications, especially during the pandemic of coronavirus disease 2019 (COVID‐19) to reduce the spread of virus. However, current touchless HMIs are mainly restricted by limited types of gesture recognition, the requirement of wearing accessories, complex sensing platforms, light conditions, and low recognition accuracy, obstructing their practical applications. Here, an intelligent noncontact gesture‐recognition system is presented through the integration of a triboelectric touchless sensor (TTS) and deep learning technology. Combined with a deep‐learning‐based multilayer perceptron neural network, the TTS can recognize 16 different types of gestures with a high average accuracy of 96.5%. The intelligent noncontact gesture‐recognition system is further applied to control a robot for collecting throat swabs in a noncontact mode. Compared with present touchless HMIs, the proposed system can recognize diverse complex gestures by utilizing charges naturally carried on human fingers without the need of wearing accessories, complicated device structures, adequate light conditions, and achieves high recognition accuracy. This system could provide exciting opportunities to develop a new generation of touchless medical equipment, as well as touchless public facilities, smart robots, virtual reality, metaverse, etc.

Deep‐Learning‐Assisted Noncontact Gesture‐Recognition System for Touchless Human‐Machine Interfaces

As an on‐skin electronic device, artificial skin shows great potential in medical monitoring and personal electronics, which also holds promise to develop human‐machine merging interfaces. However, merging artificial skins with human bodies is largely restricted by the dissimilarity of material compositions in existing artificial skins and biological tissues. Naturally conductive protein is a potential material candidate for artificial skins, nevertheless, it suffers from the critical issue of dehydration which harms its proton conductivity. Inspired by the sebum membrane of human skin, herein, a protein‐based bioprotonic hydrogel (PBH) with reliable water retention ability is reported for artificial skins. The bovine serum albumin with natural proton conductivity is utilized in the PBH, and the glycerol that originally presents on human skin surface is used as an artificial sebum membrane to retain water. The PBH can act as a bioprotonic skin (B‐skin) for collecting electrophysiological signals and self‐powered sensing. Based on the B‐skin, intelligent robot and cellphone control systems are demonstrated. Compared with present artificial skins, this B‐skin is all made out of biological materials that are consistent with material components of human skin tissues including proteins, endogenous glycerol, and water. Such a B‐skin may enable the development of next‐generation human‐machine merging interfaces.

Sebum‐Membrane‐Inspired Protein‐Based Bioprotonic Hydrogel for Artificial Skin and Human‐Machine Merging Interface

Stretchability is an essential property for wearable devices to match varying strains when interfacing with soft tissues or organs. While piezoelectricity has broad application potentials as tactile sensors, artificial skins, or nanogenerators, enabling tissue-comparable stretchability is a main roadblock due to the intrinsic rigidity and hardness of the crystalline phase. Here, an amino acid-based piezoelectric biocrystal thin film that offers tissue-compatible omnidirectional stretchability with unimpaired piezoelectricity is reported. The stretchability was enabled by a truss-like microstructure that was self-assembled under controlled molecule-solvent interaction and interface tension. Through the open and close of truss meshes, this large scale biocrystal microstructure was able to endure up to 40% tensile strain along different directions while retained both structural integrity and piezoelectric performance. Built on this structure, a tissue-compatible stretchable piezoelectric nanogenerator was developed, which could conform to various tissue surfaces, and exhibited stable functions under multidimensional large strains. In this work, we presented a promising solution that integrates piezoelectricity, stretchability and biocompatibility in one material system, a critical step toward tissue-compatible biomedical devices. 

Stretchable piezoelectric biocrystal thin films.

Stretchable on-skin touchless screen sensor enabled by ionic hydrogel

As personal portable devices, wearable sensors supply a leading-edge pathway to diagnose various diseases through actuating biological, physical, and chemical sensing capabilities. This could be commonly carried out via recording continuous and real-time of the patient's physiological statuses, as well as pathophysiological information. Although wearable sensor technology is in the infancy stage, tremendous attempts have been devoted to approaching flexible polymeric sensors. Among various polymer candidates applicable for synthesizing flexible and wearable sensors, the bio-based ones have piqued more interest due to their biocompatibility, biodegradability, eco-friendly features, and cost-effectiveness. Additionally, several fabrication techniques have been professed to architect efficient frameworks, such as films, hydrogels, aerogels, ferrogels, 3D layers, electrospun mats, and textiles. In this review, different mechanisms declared to engineer wearable sensors are overviewed. Then, regarding the advantages observed for bio-based polymers, the focused studies on the fabrication of natural-based wearable sensors are described. Notably, cellulose, chitosan, silk, gelatin, and alginate's role in sensing functionality is highlighted. Accordingly, this review has opened a new window to ahead opportunities for wearable sensors based on natural polymers. It is hoped that the new generation of sensors will be launched by combining emerging achievements obtained from employing sustainable and green elements and miniaturized sensor structures. 

Recent progress of bio-based smart wearable sensors for healthcare applications

Intrinsically stretchable polymer semiconductor based electronic skin for multiple perceptions of force, temperature, and visible light


 Ionic hydrogels outperform existing rigid and bulky electronics with many remarkable advantages including great flexibility, high conductivity, exceptional biocompatibility, and transparency, making them ideal materials for wearable human-machine interfaces (HMIs). However, traditional HMIs typically rely on external power sources, which impose limitations in terms of device size and weight, thereby compromising the user experience in HMIs. The advent of triboelectric nanogenerators (TENGs) employing ionic hydrogels has introduced a sustainable energy solution for self-powered HMIs. These TENGs can harvest the electrical energy resulting from the migration of ions induced by mechanical motion, thereby offering a sustainable energy solution for applications in wearable HMIs. Hence, the development of ionic hydrogels-based TENGs holds immense potential for the advancement of self-powered HMIs. This review first introduces the latest achievements in the fabrication of ionic hydrogel-based TENGs using diverse materials, including synthetic polymers, natural polymers, and low-dimensional materials. Then different working principles and modes of the ionic hydrogel-based TENGs are elucidated. Subsequently, the applications of these TENGs in self-powered HMIs are discussed, such as robot control, medical applications, electronic device control, and other applications. Finally, the current status and future prospects of ionic hydrogel-based TENGs in self-powered HMIs are summarized. We hope that this review will provide inspiration for the future development of self-powered human-machine interfaces utilizing ionic hydrogels-based TENGs.

Ionic Hydrogels-based Triboelectric Nanogenerators for Self-Powered Human-Machine Interfaces

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Intrinsically stretchable polymer semiconductor based electronic skin for multiple perceptions of force, temperature, and visible light

Ionic Hydrogels-based Triboelectric Nanogenerators for Self-Powered Human-Machine Interfaces