Jiaxin
Guo†
ab,
Yan
Du†
ab,
Zhonglin
Wang
ac and
Di
Wei
*ad
aBeijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 101400, China
bSchool of Nanoscience and Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
cBeijing Key Laboratory of Micro-Nano Energy and Sensor, Center for High-Entropy Energy and Systems, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 101400, P. R. China
dCentre for Photonic Devices and Sensors, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK. E-mail: dw344@cam.ac.uk
First published on 6th March 2025
With the rapid advancement of embodied perception technologies, the demand for enhanced interaction versatility, extended perceptual reach, and heightened sensitivity in human-machine interfaces (HMI) continues to grow. Triboelectric nanogenerator (TENG) based non-contact sensors have emerged as a transformative solution, offering exceptional adaptability while mitigating challenges such as mechanical degradation and potential health risks. However, achieving superior sensitivity and extending sensing ranges remain critical bottlenecks. Addressing these limitations, researchers have pioneered the concept of tele-perception, a groundbreaking innovation that breaks the limitations of traditional non-contact sensors by enabling precise, long-range perceptual capabilities. This review explores the paradigm shift from traditional non-contact sensors to tele-perception, highlighting the foundational principles, representative system architectures, and cutting-edge optimization strategies that define this new approach to sensing without physical interaction. Particular emphasis is placed on the integration of advanced charge-trapping mechanisms to enhance electrostatic charge stability and the deployment of intelligent algorithms and deep learning (DL) techniques to advance tele-perception functionalities. Concluding with an analysis of the challenges and future opportunities in tele-perception systems development, this review offers critical insights to guide next-generation research and applications in this transformative field.
Traditional non-contact sensing technologies, including visual,12–15 ultrasonic,16–19 light detection and ranging (LiDAR),20–22 infrared,23–27 electromagnetic,28,29 and capacitive sensors,30–34 have advanced significantly to meet modern demands. Visual sensors deliver detailed image and video data but are limited by lighting conditions. Ultrasonic and LiDAR sensors excel in long-distance detection, with LiDAR offering wide-range scanning and high precision, though at a high cost. Infrared sensors provide fast thermal detection but are sensitive to temperature fluctuations. Electromagnetic sensors, such as magnetic and capacitive types, offer high precision and adaptability in complex environments. Magnetic sensors detect magnetic field changes via the Hall effect, while capacitive sensors deliver high-precision, short-range proximity detection with adjustable sensitivity. However, significant challenges remain. Reliance on external power sources restricts energy efficiency and operational lifespan. Sensitivity and accuracy are influenced by material properties, design limitations, and environmental variables, which compromise performance under complex conditions. Furthermore, limited integration capabilities, lack of multifunctionality, and high production costs impede scalability and broader adoption.35–39 TENGs provide a promising solution by converting ambient mechanical energy into electrical signals, eliminating the need for external power. In addition, TENG exhibits simplicity, cost-effectiveness, diverse application scenarios and high conversion efficiency under low-frequency conditions, making them suitable for non-contact sensing applications.40–45 When integrated with sensors, TENGs enable self-powered, portable systems for applications like gesture recognition,46–48 non-contact interfaces,49,50 joint motion51 and gait monitoring.52–54 These systems perform reliably in dynamic environments, offering scalable, sustainable solutions for next-generation intelligent sensing technologies.
Traditional non-contact sensors based on TENG, despite their advantages, face significant challenges in output performance, primarily due to the limited charge generated by electrostatic induction. A key difficulty lies in charge-trapping, as the retention of triboelectric charges becomes increasingly difficult. As the separation distance grows, the inability to effectively trap charges leads to a further decline in output performance, thereby restricting the sensing range.55–58 To address these challenges, research has primarily focused on material composite design, chemical modification and structural optimization. For instance, Park et al.59 improved output performance by incorporating laser-carbonized MXene/ZiF-67 nanocomposites as a charge trapping layer, while Xu et al.60 achieved long-distance sensing using MXene/silicone composites. However, these advancements still face limitations in sensing range and require larger devices for long-distance detection.61 Additionally, challenges persist in sensitivity adjustment and threshold setting, which are critical for accurate detection across varying distances. Recent work by Wei et al.62 demonstrated human presence detection at distances of up to 155 cm by structured doping of inorganic nanoparticles to create charge traps. This design achieved a sensitivity of ΔV/Δd = 142 V cm−1, significantly surpassing current technological standards. The key innovation involved doping inorganic non-metallic nanoparticles (SiO2, TiO2, BaTiO3 and SrTiO3) into the structure of TENGs to improve electrostatic charge-trapping ability and stabilize charge retention, thereby addressing the challenge of maintaining charge stability in traditional non-contact TENGs. In addition, the integration of adaptive pulse identification technology and long short-term memory (LSTM) networks significantly enhanced the robustness, adaptability, and accuracy of the tele-perception system. Two-dimensional sensor matrix was employed to integrate real object scanning data into a convolutional neural network (CNN), enabling the multi-receptor skin to effectively function in real-world applications, thereby improving target recognition and overall system performance. This breakthrough highlights the necessity of extending sensing capabilities beyond physical contact, laying a strong foundation for the continued evolution of tele-perception. Fully realizing the potential of tele-perception necessitates a fundamental paradigm shift, from traditional non-contact sensors to adaptive embodied intelligent systems. Tele-perception itself embodies this transformation, redefining the landscape of interaction by enabling long-range, high-precision sensing and facilitating more dynamic context awareness HMI.
This review explores the paradigm shift63 from traditional non-contact sensors to tele-perception. We begin by presenting two primary types of traditional non-contact sensors based on TENG: electronic receptors and ionic receptors, with a focus on their operating principles, typical architectures, performance optimization strategies, and applications. The concept of tele-perception is then introduced, emphasizing the critical role of charge-trapping strategies in enhancing charge capture, transport efficiency, and long-term stability, alongside a particular focus on algorithm integration and optimization. Subsequently, recent advancements in traditional non-contact sensors and tele-perception systems are examined, highlighting their applications in HMI, intelligent monitoring, and machine learning assistant systems, as shown in Fig. 1. Moreover, various properties of non-contact sensors, including the latest advancements in tele-perception technologies, are compared in Table 1. This comparison highlights the improvements in sensitivity, charge retention, and sensing range, and underscores the integration of cutting-edge techniques. The review concludes by addressing the current technical challenges and outlining potential future research directions for tele-perception, particularly in the areas of advanced electrostatic charge-trapping strategies, collaborative algorithm optimization, and multifunctional integrated designs. By synthesizing these developments, we aim to provide a comprehensive resource for advancing tele-perception and promoting its broader integration into embodied intelligence systems.
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Fig. 1 An overview of the latest advancements of traditional non-contact sensors and tele-perception systems. |
Dielectric material | Electrode material | Approaching object | Sensing range (cm) | Sensitivity (V cm−1) | Reference | |
---|---|---|---|---|---|---|
1 | Ionic hydrogel | Ionic hydrogel/PDMS | Finger | 4 | — | 64 |
2 | Printing paper | Copper | People | 100 | 0.15 | 61 |
3 | CNF–Mxene | Cu | Finger | 50 | 0.9 | 65 |
4 | FEP | ITO/PET | Human hand | 250 | 1 | 66 |
5 | F-TiO2 | Cu | Robot arm | 300 | 1.4 | 67 |
6 | PTFE | Cu | Human hand | 11 | 3.15 | 68 |
7 | MXene/silicone, LC-MXene/ZiF-67 nanocomposites | Ag coated conductive fabric | Human hand | 2 | 3.5 | 59 |
8 | Siloxene/Ecoflex | Cu | Nitrile glove | 14 | 4.38 | 69 |
9 | PDMS/SiO2 | Organohydrogel | PTFE | 9 | 4.7 | 70 |
10 | PVDF/Ti3C2Tx | Cu | Nylon film | 70 | 5 | 71 |
11 | MXene/silicone | Conductive sponge | Nylon film | 200 | 5 | 60 |
12 | PDMS–PTFE and TPU–SiO2 composite film | Al | PDMS–PTFE and TPU–SiO2 | 0.3 | 5.1 | 72 |
13 | Co–NPC/Ecoflex | Silver-coated conducting fabric | Human hand | 20 | 5.6 | 73 |
14 | PI-b-C60 | Ag/PDMS/Au and Al | Finger | 0.4 | 8.2 | 74 |
15 | Fish bladder film (FBF) | Cu | FEP | 2.7 | 34.9 | 75 |
16 | PET | EIPG | Human hand | 10 | 43 | 76 |
17 | PTFE | Ag/polyamide | Polyamide | 18 | 56 | 77 |
18 | Bi2Se3/S.R. and carbonized textile | Ag NWs | Nylon | 1 | 58 | 78 |
19 | PDMS/PTFE/SrTiO3 | Ag NWs | Nylon/PTFE/FEP | 155 | 142 | 62 |
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Fig. 2 The operating principles of traditional non-contact sensors including electronic receptors and ionic receptors. |
The detailed working mechanism of the sensors can be outlined in the following stages. In electronic receptors, the finger initially maintains a specific distance from the device surface, inducing a small amount of positive charge on the device due to electrostatic induction. As the finger approaches the device, additional positive charges accumulate to neutralize the negative charge on the finger. This results in a potential difference that drives free electrons from the electrode to the ground through an external circuit, generating an output current signal. As the finger draws closer, the induced positive charges on the device surface fully screen the negative charges, achieving electrostatic equilibrium. Conversely, as the finger moves away, the process reverses, causing the charge flow to change direction and completing a full signal cycle. In contrast to traditional electronic devices based on electronic conduction, ionic receptors primarily function through ion movement, making them more akin to the electrical signaling mechanisms found in biological systems.81–83 In this case, the electret layer in ionic receptors is initially pre-charged with negative charges. As a result, cations accumulate in the ionic electrode layer beneath the electret, neutralizing the electric field generated by the electret and achieving electrostatic equilibrium. Simultaneously, an electrical double layer (EDL)84 forms at the interface between the metal wire and the ion electrode due to the polarization effect, inducing an equal amount of anions. As the finger approaches the device, the cation concentration on the ion electrode surface increases, creating an electric field that neutralizes the negative charge on the finger, achieving balance. Concurrently, the anion concentration in the EDL rises, driving electron flow from the metal wire to the ground through an external circuit. As the finger draws closer, the cation concentration continues to rise until the finger's negative charge is fully neutralized, reaching electrostatic equilibrium. Conversely, as the finger moves away, the cation concentration gradually decreases, disrupting the charge balance and generating a reverse current signal. Through this cyclic process, periodic electrical signal fluctuations are induced as the finger moves closer or farther, enabling highly sensitive mechanical signal detection.
The most common planar structure consists of a triboelectric charge generation layer and an electrode layer. This simple design enables devices to respond sensitively to external mechanical energy. Reducing the number of layers makes the devices lighter, thinner and more flexible, significantly improving its bendability. These features make it ideal for applications requiring high flexibility, such as wearable devices and electronic-skin (e-skin). For example, Cao et al.75 developed a fish bladder film-based TENG that integrated the advantages such as multifunctionality, ultra-flexibility and high sensitivity. In addition, the device demonstrated non-contact sensing capabilities within a range of 0–2.7 cm, making it ideal for applications such as HMI, smart systems, and prosthetics control (Fig. 3a). The planar structure forms the foundation for other device designs based on TENG, serving as a basis for the development and optimization of diverse structures and applications. In contrast, the stacked structure includes an additional intermediate layer, which typically serves as a charge-generating surface to enhance charge collection, while also functioning as both a charge transfer and storage layer. The charge transfer layer mitigates charge accumulation on the triboelectric surface and captures electrons within the space charge region, thereby extending the detection range. For example, Park et al.59 employed laser-carbonized MXene/ZiF-67 nanocomposite as an intermediate layer, achieving a charge density of 15.3 μC m−2 and stable output performance up to 2 cm (Fig. 3b). This approach represents an innovative and effective strategy for designing high-performance devices in self-powered systems and sensing applications. Sensors with spring structures, in particular, demonstrate heightened sensitivity to subtle environmental disturbances or object displacements, owing to their exceptional flexibility and recoverability. In non-contact mode, even minor displacements or mechanical stimuli can be detected in real-time through variations in electrical signals, showcasing excellent sensitivity and rapid response times. For example, Cao et al.68 developed a non-contact, self-powered positioning and motion tracking system based on the spring structure, capable of detecting small movements at heights of 1–11 cm, with a sensitivity of approximately 3.15 V cm−1 (Fig. 3c). The origami structure, an innovative variation of the planar design, incorporates the human body as a functional component, with other parts made of printed paper and metal electrodes. This design integrates the advantages of light weight, low cost, and ease of fabrication, providing an optimal solution for lightweight and cost-effective TENG applications. For example, Shi et al.61 proposed the non-contact paper-based TENG, demonstrating that the output voltage signal directly reflected the walking gait cycle (leg raising and falling), movement direction, walking or running speed and movement path (Fig. 3d). Overall, these structural innovations have advanced TENG technology and paved the way for more efficient and intelligent sensor systems.
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Fig. 3 Typical structures of electronic receptors. (a) Representative planar structure diagram of electronic receptors and the response curve with different starting points (0–2.7 cm). Reproduced from ref. 75 with permission from American Chemical Society, copyright 2020. (b) Representative stacked structure diagram of electronic receptors embedded with the laser-carbonized Mxene/ZiF-67 nanocomposite and the electrical output performance with a varying distance. Reproduced from ref. 59 with permission from Elsevier, copyright 2022. (c) Representative spring structure diagram of electronic receptors and application in position monitoring. Reproduced from ref. 68 with permission from American Chemical Society, copyright 2018. (d) Representative origami structure diagram of electronic receptors and principle of human motion monitoring. Reproduced from ref. 61 with permission from Elsevier, copyright 2020. |
Each structure offers distinct advantages: the planar structure is simple, lightweight, and well-suited for wearable applications; the stacked structure enhances the detection range; the spring structure delivers high sensitivity and rapid response, making it ideal for positioning and motion tracking; and the origami structure incorporates the human body as part of the device, offering low cost and the ability to directly capture motion data. These innovative structural designs provide strong support for the diverse applications of non-contact sensors based on TENG across a wide range of scenarios.
Material composite design focuses on enhancing charge capture capabilities by selecting appropriate triboelectric materials to increase charge density and precisely tuning the interfacial electrical and chemical properties through doping. This approach significantly enhances device performance by optimizing charge generation and collection efficiency.90 Among various triboelectric materials, MXene is notable for its high electronegativity, decent conductivity and superior charge capture capability, leading to widespread applications.91–96 A novel laser-carbonized MXene/ZiF-67 nanocomposite fabricated via a simple laser processing technique was developed by Park et al.,59 which integrated the unique porous structure of ZiF-67, enabling high output performance and stable operation in both contact and non-contact modes (Fig. 4a). Nie et al.67 proposed a hierarchical spatial assembly strategy for a double-layer functionalized triboelectric material, which significantly improves humidity stability. Electrophoretic deposition was used to achieve the layered spatial assembly, incorporating MXene filler as an intermediate charge storage layer, enhancing the signal intensity and accuracy of the triboelectric material (Fig. 4b). In addition, a polyvinylidene fluoride (PVDF)@Ti3C2Tx triboelectric material with a spherical multi-physical network was developed to efficiently capture and accumulate negative charges.71 The highly conductive Ti3C2Tx was uniformly distributed within the microspheres, forming multiple interfaces and continuous conductive paths, significantly improving output performance and stability (Fig. 4c). Xu et al.60 proposed a self-powered non-contact TENG (SNC-TENG) comprising MXene/silicone nanocomposites and a fully embedded conductive sponge. The complete embedding of the conductive sponge within the dielectric layer increased the effective electrode area in the electrostatic field, thereby enhancing the electrostatic induction effect. This design endowed the non-contact sensor with high sensitivity and an ultra-long detection range, enabling it to detect human activities at distances of up to 200 cm (Fig. 4d). The selection of triboelectric materials is fundamentally the most effective approach to enhance output performance.97,98 However, it is constrained by the limited availability of high-performance materials. To further enhance performance, functional treatments such as doping can be applied to specific materials, optimizing their triboelectric properties. For example, a cobalt nanoporous carbon (Co–NPC)/Ecoflex with MXene/Ecoflex nanocomposite layer was developed for meeting the ongoing demand of tribo-negative materials, where Co nanoparticles uniformly distributed throughout the NPC matrix's spatial network.73 The porous structure of the Co–NPC offered a high surface area for the nanocomposite, while the charge storage layer of the MXene/Ecoflex nanocomposite accumulated more negative charge, significantly enhancing the device output performance (Fig. 4e). Additionally, chemical modification through functional groups grafting and fluorination processes offers effective approaches to enhance triboelectric charge density, aiming to regulate charge distribution on material surfaces, manipulate charged groups or binding energy and enhance interface properties, achieving high output performance. Lee et al.74 reported a non-contact device based on a novel dielectric of C60-containing block polyimide (PI-b-C60), achieving high output power and reliable operation due to the excellent charge retention properties and highly negative electrostatic potential of C60 (Fig. 4f). Baik et al.72 functionalized PDMS–PTFE composites by etching and fluorination to create amphiphobic surfaces. The etching process introduced micro/nano hierarchical structures to create rough surface with air bubbles, while the subsequent fluorination further incorporated low-surface-energy fluorine groups, endowing the material with excellent water and oil repellency. These surface characteristics effectively suppressed charge dissipation caused by surface conductivity under high humidity or contaminated environments, significantly enhancing the material's charge retention capability and environmental adaptability, thereby improving the device's stability under harsh conditions (Fig. 4g).
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Fig. 4 Performance optimization of electronic receptors. Material composite design: (a) laser carbonized Mxene/ZiF-67 nanocomposites as an intermediate layer to improve the output performance of fabric-based TENG. Reproduced from ref. 59 with permission from Elsevier, copyright 2022. (b) Schematic diagram of CMF-TENG-based sensor in a humid environment with MXene filler as an intermediate charge storage layer. Reproduced from ref. 67 with permission from Wiley-VCH, copyright 2024. (c) Non-contact sensor based on a polyvinylidene fluoride@Mxene (Ti3C2Tx) composite film with sphere multi-physical network structure. Reproduced from ref. 71 with permission from Wiley-VCH, copyright 2022. (d) Ultra-long distance detection and high sensitivity self-powered non-contact sensor based on MXene composites. Reproduced from ref. 60 with permission from Wiley-VCH, copyright 2023. (e) Cobalt nanoporous carbon functionalized nanocomposites for non-contact sensing. Reproduced from ref. 73 with permission from Wiley-VCH, copyright 2021. Chemical modification: (f) sustainable high-charge C60-functionalized polyimide for non-contact sensing. Reproduced from ref. 74 with permission from Royal Society of Chemistry, copyright 2021. (g) The schematic diagram of a sustainable charging composite material with amphiphilic surface prepared by reactive ion etching and fluorination treatment. Reproduced from ref. 72 with permission from Elsevier, copyright 2023. |
Surface structure optimization is a universal and direct strategy that focuses on creating surface microstructures to enhance the effective triboelectric contact area.99–101 This approach facilitates charge accumulation and distribution, thereby improving electrical performance, output efficiency, and stability across a wide range of materials and applications. Etched PTFE nanoparticle arrays68 were employed to significantly enhance the effective contact surface and triboelectric charge density of the non-contact sensors, achieving an output of 222 V and 18 μA with improved sensitivity for non-contact sensing (Fig. 5a). Zhang et al.65 employed a freeze-drying assisted 3D printing technique to construct a deep-trap layered structure of cellulose nanofiber (CNF) and MXene. Incorporating MXene nanosheets created open pores in the printed scaffold and enhanced the electrical performance of non-contact sensor (Fig. 5b). Chen et al.77 proposed a bionic-antennae-array (BAA) sensor inspired by cockroach antennae, representing another innovative structural design. By leveraging the amplification effect of the antenna array, the BAA sensor achieved a detection range of 18 cm, a displacement resolution of 0.1 cm, and a sensitivity of 56 V cm−1, demonstrating excellent performance in detecting the movement of non-contact objects (Fig. 5c). Zhong et al.66 designed a megascopic air-bubble structure to enhance charge storage stability and recovery capability in devices using the heterocharge-synergy effect in electrets. And the multilayer electret film strengthened the electric field via the electrostatic field superposition effect, enabling an ultra-wide sensing range of over 250 cm in harsh environments (Fig. 5d). Yang et al.76 demonstrated a flexible, transparent sensor with layered micro–nano textures, which increased the output signal by over 200%, significantly enhancing the device sensitivity (Fig. 5e).
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Fig. 5 Performance optimization of electronic receptors. Surface structure optimization: (a) the nano-particle array structure of PTFE formed by etching improves the sensitivity of the non-contact sensor to about 3.15 V cm−1, and the sensing range is from 1 to 11 cm. Reproduced from ref. 68 with permission from American Chemical Society, copyright 2018. (b) A deep trap hierarchical structure based on CNF/MXene was fabricated by freeze-drying assisted 3D printing method, showing excellent sensing performance in multi-directional motion monitoring. Reproduced from ref. 65 with permission from Elsevier, copyright 2022. (c) The BAA sensor for non-contact motion recognition with a maximum sensing distance of 18 cm, a displacement resolution of 0.1 cm and a maximum sensitivity of approximately 56 V cm−1. Reproduced from ref. 77 with permission from Wiley-VCH, copyright 2019. (d) The self-powered electret non-contact sensor with a large bubble structure achieves an ultra-wide sensing range of more than 250 cm. Reproduced from ref. 66 with permission from American Chemical Society, copyright 2023. (e) The flexible transparent sensor with layered micro–nano texture achieves higher sensitivity, and the output signal is increased by more than 200%. Reproduced from ref. 76 with permission from Elsevier, copyright 2021. |
The three aforementioned strategies, material composite design, chemical modification and structural optimization, effectively enhance traditional non-contact sensors' performance. Each method has distinct advantages and inherent limitations. Selecting appropriate triboelectric materials to fundamentally enhance performance is among the most effective strategies for improving output performance. However, this strategy is typically limited to specific material systems. Not all materials satisfy the requirements for high electronegativity and charge capture capability, making it challenging to identify perfectly suitable materials for specific applications. While modification moderately expands the material selection range and enhances design flexibility, its complexity significantly increases preparation costs and technical challenges. For example, uneven fluorination during the process may result in inconsistent surface characteristics, adversely affecting output stability and consistency. Additionally, chemical modification may introduce side effects, such as adverse impacts on the material's mechanical or electrical properties, requiring careful evaluation and optimization. Creating a surface microstructure efficiently increases the contact area of the triboelectric layer and reduces the air gap, preventing charge deposition and accumulation on the surface. This common and direct method improves charge capture ability and charge density, and is not limited by material characteristics, making it broadly applicable. However, improper structural design may affect the device long-term stability and service life. In summary, these three methods have distinct advantages and can complement each other in suitable application scenarios. In practice, method selection should comprehensively consider the requirements and constraints of specific application scenarios to achieve optimal performance and meet diverse field needs.
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Fig. 6 Typical structures of ionic receptors. Representative laminated structure of ionic receptors: (a) the structure and the applicability for different materials of the artificial receptor. Reproduced from ref. 70 with permission from the American Association for the Advancement of Science, copyright 2022. (b) Diagram of the wearable ECG monitoring system and different signals detected in sleep heart detection. Reproduced from ref. 112 with permission from Wiley-VCH, copyright 2024. Representative parallel structure of ionic receptors: (c) schematic illustration of the working mechanism for the on-skin touchless screen sensor (OTSS) and application of screen operating system based on the OTSS units during the unlocking process. Reproduced from ref. 64 with permission from Tsinghua University Press, copyright 2023. (d) Schematic illustration of the fabricated PSBMA–clay–LiCl (PSCL) hydrogels and applications and performance that varies with finger proximity. Reproduced from ref. 113 with permission from Elsevier, copyright 2022. |
Material selection and design focus on constructing conductive nanochannels and achieving molecular synergies. When combined with dynamic cross-linked networks, these materials create synergistic effects that prevent liquid leakage or evaporation, thereby enhancing structural integrity and environmental adaptability. Wu et al.123 introduced an intrinsically stretchable conductor in which small-molecule liquid electrolytes, such as ionic liquids, served as charge carriers. Polymers with ionic structures, such as polyzwitterions, enhanced ionic synergy with liquid electrolytes, forming conductive nanochannels and reducing electrolyte aggregation or leakage risks (Fig. 7a). Zhou et al.112 employed molecular design to introduce hydrogen bonds and electrostatic interactions at the matrix–hydrogel interface, imparting excellent self-adhesive properties to the ionic hydrogel system (Fig. 7b). Additionally, dynamically crosslinked dry ICE was introduced for soft ionic electronics.124 Without liquid molecules, ICE and related ionic electronic devices demonstrated excellent stability and functioned across a broad temperature range (Fig. 7c). Jia et al.125 introduced another type of ICE, consisting of copolymer networks that hosted lithium cations and associated anions through lithium bonds and hydrogen bonds. Consequently, these materials exhibited inherent resistance to leakage and evaporation (Fig. 7d). Enhancing adhesion depends on interface modification, a direct approach emphasizing chemical anchoring via covalent crosslinking interactions to establish reliable interfacial bonding. Sun et al.126 established covalent and robust anchoring between hydrogels and elastomers via benzophenone treatment (Fig. 7e). Zhao et al.127 used benzophenone to modify cured elastomer surfaces, forming covalent crosslinks in the elastomeric polymer network and integrating hydrogels and elastomers into structures with strong interfaces (Fig. 7f). Suo et al.128 developed a method to print integrated hydrogel and elastomer structures in arbitrary sequences. By incorporating a crosslinking initiator into one of the inks, covalent bonds were formed between hydrogel and elastomer networks, achieving adhesion energy exceeding 5000 J m−2 (Fig. 7g). Chen et al.113 addressed these challenges by designing a composite hydrogel incorporating polyzwitterions, which provide self-adhesion through ion–dipole and dipole–dipole interactions with various substrates. Additionally, LiCl, a key solute with high moisture capacity and low molar mass, was utilized to simultaneously enhance the hydrogel's antifreeze properties, water retention, and thermal tolerance (Fig. 7h). These methods have partially alleviated issues such as liquid leakage and poor self-adhesion in ionic receptors. However, several challenges persist. Firstly, material selection and design are often tailored to specific substances, and the crosslinking reactions are typically irreversible. Once the crosslinked structure is established, the material's shape and properties become difficult to modify, complicating processing technology and increasing demands. Additionally, the complexity and sensitivity of the interface modification process present challenges in precisely controlling dopant proportions, making it difficult to ensure consistent quality and effectiveness. Therefore, more convenient and effective strategies must be explored in future research to further enhance the performance and broaden the application range of ionic conductors.
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Fig. 7 Performance optimization of ionic receptors. Material selection and design: (a) the schematic diagram of a strategy for using ionic liquids that can form hydrogen bonds with polymer chains. Reproduced from ref. 123 with permission from the Author(s), copyright 2019. (b) The new hydrogel system based on an ionic liquid imidazolium salt with a ureido backbone (UL) to enhance mechanical properties and water retention. Reproduced from ref. 112 with permission from Wiley-VCH, copyright 2024. (c) Avoiding liquid leakage by using dry ion conductive elastomers (ICE). Reproduced from ref. 124 with permission from Wiley-VCH, copyright 2021. (d) Molecular design schematic of liquid-free ICE consisting of a copolymer network carrying lithium cations and related anions through lithium bonds and hydrogen bonds. Reproduced from ref. 125 with permission from Wiley-VCH, copyright 2021. Interface modification: (e) the robust interfaces between PDMS and hydrogel based on the chemical anchoring of benzophenone-treated (hexadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (HDFS) coating. Reproduced from ref. 126 with permission from the Author(s), copyright 2018. (f) The skin-inspired hydrogel–elastomer mixture with a robust interface and functional microstructure. Reproduced from ref. 127 with permission from the Author(s), copyright 2016. (g) A method for printing the integrated structure of hydrogel and elastomer in any order and having strong adhesion. Reproduced from ref. 128 with permission from Wiley-VCH, copyright 2019. (h) The environmentally adaptable multifunctional hydrogel with strong adhesion strength by in situ polymerization of polyzwitterions in the presence of inorganic nanoclays and lithium chloride. Reproduced from ref. 113 with permission from Elsevier, copyright 2022. |
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Fig. 8 Multi-receptor skin perception system. (a) Schematic diagram of intelligent perception system based on the multi-receptor skin and DL algorithms. (b) The working principle of the multi-receptor skin in response to proximity of positively or negatively charged objects. (c) Experimental verification of the effectiveness of inorganic non-metallic nanoparticles and microporous structure doping on signal acquisition enhancement. (d) The sensitivity comparison between the bionic inductor and some related inductors representative work. Reproduced from ref. 62 with permission from the American Association for the Advancement of Science, copyright 2024. |
Free radical anion transfer serves as a direct and efficient mechanism for selective migration, compensating for charge loss to the surrounding environment by facilitating the movement of free radical ions. It enhances charge stability and ensures the effective retention of charge within the system. Chen et al.130 employed naphthalene radical anion as a wet chemical treatment agent for polytetrafluoroethylene (PTFE), producing ionized PTFE (I-PTFE) with a high concentration of free radical ions. These free radical ions compensated for charge escape via selective transfer induced by mechanical force, achieving ultra-high charge density (Fig. 9a). Building on this, they proposed the acid-ion complexation method to fabricate positively charged triboelectric materials with enhanced charge retention. This approach leverages the formation of acid-ion complexes to increase the activation energy required to release stored charges, thereby improving charge stability.131 It can also induce selective anion migration to compensate for the escape of polarized charges, achieving a charge density of 596 μC m−2 and a charge retention rate of 49.7% (Fig. 9b). The primary approaches for creating deep trap structures include functional group grafting, which involves attaching specific functional groups to the material surface to enhance charge trapping; quenching polarization (QP), which generates ultrahigh and long-lasting triboelectric charge density through molecular deformation and deep trapping; and molecular structure regulation, which directly tailors the material's structure to form deep traps, thereby improving charge stability. For example, Wang et al.132 introduced sulfonic acid groups into cellulose nanofibers (CNF) to produce CNF–SO3Na. Sulfonic acid functional groups with high electrostatic potential and uniform distribution acted as deep traps, enhancing the triboelectric charge capture ability of modified CNF and increasing the electrical output performance (Fig. 9c). Qu et al.133 increased the number of interfaces between crystalline and amorphous regions on weak polar polymer and generated numerous interface deep traps for charge storage by QP. The sensor prepared using this method achieved a maximum charge density of 391 μC m−2, 200% higher than that achieved by conventional polarization methods (Fig. 9d). In addition, self-assembled molecules with large energy gaps onto commercial PET fibers was introduced to create deep carrier traps.134 Manipulating the trap distribution within the triboelectric layer reduces triboelectric charge dissipation, thereby enhancing the electrical output (Fig. 9e). In polymers, the dihedral angle between adjacent conjugated planes (conjugated dihedral angle) is a crucial factor in modulating molecular structure, thereby affecting trap distribution and ultimately determining performance.135,136 Variations in this angle can significantly affect the electronic properties of the polymer, including band structure and charge transfer behavior. Recognizing this, Li et al.137 systematically tailored the conjugated dihedral angle in polyimide (PI) chains by incorporating different monomers into the polymer backbone. By increasing the dihedral angle, they weakened the conjugation between molecular chains, thereby expanding the energy gap between molecular orbitals and suppressing the formation of charge transfer complexes (CTCs). This approach highlights the importance of controlling molecular geometry to optimize polymer performance. Chen et al.138 effectively modified the molecular structure of triboelectric polymers by RRF process, including molecular orientation, crystallinity and dielectric constant. In ethylene propylene fluoride (FEP) films with high crystallinity, numerous deep traps formed at the interface between microcrystalline and amorphous regions, enhancing charge storage. The intrinsic surface charge density was 1.46 times higher than the previously reported record. Additionally, the sensor's output charge density, achieved using a 30 μm thick thin film, reached 352 μC m−2 (Fig. 9g). Collectively, these chemical modification techniques offer valuable insights for optimizing charge retention, thereby enhancing the long-term performance and reliability of tele-perception systems.
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Fig. 9 Selective migration: (a) the method to achieve ultra-high charge density in the open air by using free radical anion transfer as a compensation for charge escape. Reproduced from ref. 130 with permission from Elsevier, copyright 2023. (b) A method to suppress charge escape in the open air by acid-ion complexation. Reproduced from ref. 131 with permission from American Chemical Society, copyright 2024. Deep trap structure creation: (c) ultra-high electrical output device with sulfonated cellulose nanofibers as a positively charged tribolayer. Reproduced from ref. 132 with permission from American Chemical Society, copyright 2022. (d) A method to induce a large number of interface deep traps by QP. Reproduced from ref. 133 with permission from Wiley-VCH, copyright 2023. Molecular structure regulation: (e) a strategy of introducing self-assembled molecules with large energy gaps to form deep carrier traps. Reproduced from ref. 134 with permission from Wiley-VCH, copyright 2024. (f) The strategy of dihedral angle adjustment in molecular engineering to prevent charge escape. Reproduced from ref. 137 with permission from Wiley-VCH, copyright 2024. (g) A strategy to effectively tune the molecular structure of triboelectric polymers by repeated rheological forging (RRF) process. Reproduced from ref. 138 with permission from the Author(s), copyright 2022. |
In terms of the electron blocking layer or charge storage layer, Lai et al.78 developed graphitic carbonized textiles to achieve the long-term retention of triboelectric charges. Sensors constructed with this approach exhibited excellent output performance during non-contact operations over various separation distances, maintaining triboelectric charges for up to 5000 minutes (Fig. 10a). Wen et al.142 integrated a solution-processed high-permittivity electron blocking layer composed of LaZrO into the device to form an energy barrier (ΔE > 1.3 eV) between the transparent indium tin oxide (ITO) and PDMS, blocking electron transfer to the bottom electrode to reduce the surface potential decline rate of PDMS and extend the charge decay time by approximately 3.1 times (Fig. 10b). Additionally, a MoO3 electron blocking layer was introduced between the ITO electrode and the fluorinated ethylene propylene (FEP) polymer triboelectric layer.143 This layer effectively mitigates charge shielding effects at the interface, polarizes the FEP layer, and consequently improves and stabilizes charge trapping (Fig. 10c). For the electron transmission layer, Tang et al.144 enhanced charge storage by designing rhombus-shaped all-inorganic perovskite nanoarrays functioned as a conductive transmission layer sandwiched between the triboelectric layer and an inserted dielectric layer, facilitating the transfer of surface charges to deeper regions, thereby enabling enhanced charge storage (Fig. 10d). In terms of the high permittivity materials incorporation, Wang et al.145 introduced a dual-dielectric layer that effectively suppressed dielectric charge leakage, enhancing the output performance to 2200 μC m−2 (Fig. 10e). Similarly, LIG doped with MoS2 was integrated to achieve a fourfold increase in surface potential.69 The synergistic interaction between MoS2 and LIG enhanced interlayer dielectric properties and polarization, ultimately improving the output performance (Fig. 10f).
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Fig. 10 Methods of interface layer incorporation. Electron blocking layer or charge storage layer: (a) method for extending the triboelectric charge retention time by using graphitic carbonized textiles as a charge storage layer. Reproduced from ref. 78 with permission from Wiley-VCH, copyright 2024. (b) A method of embedding a solution-processed high-permittivity electron blocking layer of lanthanum zirconium oxides (LaZrO) to prevent the transmission of electrons to the bottom electrode. Reproduced from ref. 142 with permission from Elsevier, copyright 2022. (c) A method to increase the surface charge density by adding a MoO3 as electron blocking layer with high dielectric constant and wide band gap. Reproduced from ref. 143 with permission from American Chemical Society, copyright 2022. Electron transmission layer: (d) a method to enhance and store charge by customizing a diamond-shaped all-inorganic perovskite nanoarray as a electron transmission layer. Reproduced from ref. 144 with permission from Elsevier, copyright 2020. High permittivity materials incorporation: (e) a strategy to suppress the charge decay of high surface charge density device by using a double dielectric layer. Reproduced from ref. 145 with permission from Wiley-VCH, copyright 2023. (f) A method to enhance dielectric properties and polarization by introducing MoS2/laser-induced graphene (LIG). Reproduced from ref. 69 with permission from Wiley-VCH, copyright 2022. |
For metal nanoparticles, Meng et al.149 designed a Co–NPC/Ecoflex composite layer with a microstructured square-loop surface to develop a single-electrode mode non-contact sensor, layering over a dielectric copper calcium titanate/Ecoflex nanocomposite with an ultra-high dielectric constant. The fine Co nanoparticles with high specific surface area uniformly disperse in the NPC matrix, which benefits in capturing generated charges and stabilize output energy. Through extensive optimization of material selection, component composition and structural design, the sensor, with an effective electrode area of 6.25 cm2, generated a high output voltage of 29 V at a 1 cm distance and detected motion up to 6 m away (Fig. 11a). For metal oxide nanoparticles, Zhai et al.150 improved the dielectric constant by doping BTO nanoparticles and further enhanced the output with an external charge pump module, achieving a total transferred charge density of 3500 μC m−2 using an external capacitor with a capacitance of 20 nF (Fig. 11b). And for non-metallic inorganic nanoparticles, orderly mesoporous SiO2 nanoparticles (OMS) were utilized as charge storage sites to improve the output performance.151 The enhancement effect of OMS nanoparticles on output demonstrated a linear relationship with their specific surface area. When the concentration of OMS nanoparticles increased to 1 wt%, the transferred charge rose sharply from 21 nC to 60 nC due to the additional bulk charges stored by OMS nanoparticles. Furthermore, OMS–PDMS exhibited superior charge retention capability after contact–separation motions ceased, with its voltage remaining stable for an extended period, while the voltage of pure PDMS rapidly dropped to near zero. The OMS–PDMS device achieved an instantaneous output power density of 5.26 W m−2, representing a 25-fold improvement (Fig. 11c). In terms of ferroelectric nanoparticles, Khare et al.152 proposed a sensor with a bilayer negatively charged structure. Incorporating ferroelectric NaNbO3 nanoparticles into the PDMS bottom layer enhanced internal polarization and created additional trap sites, enabling longer retention of generated surface charges. The synergistic interaction between these two layers significantly enhanced the overall performance (Fig. 11d).
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Fig. 11 Methods of nanoparticles doping. (a) A triboelectric Co–NPC/Ecoflex nanocomposite layer with a microstructured square-ring surface was constructed by using functional nanofillers, extending the non-contact sensing capability of the sensor to a few meters for the first time. Reproduced from ref. 149 with permission from Elsevier, copyright 2024. (b) A method to further enhance the triboelectric properties by doping BaTiO3 (BTO) nanoparticles with high dielectric constant. Reproduced from ref. 150 with permission from Elsevier, copyright 2024. (c) A strategy to enhance the output performance by adding ordered mesoporous SiO2 nanoparticles as charge storage sites. Reproduced from ref. 151 with permission from Elsevier, copyright 2023. (d) A method of introducing NaNbO3 ferroelectric nanoparticles into the PDMS matrix to enhance internal polarization and increase trap sites, thereby maintaining the generated surface charge for a longer time. Reproduced from ref. 152 with permission from American Chemical Society, copyright 2024. |
Although these approaches have not yet been widely adopted in non-contact sensing devices, they offer highly effective solutions for enhancing charge-trapping capabilities. Improving charge retention is crucial for achieving high-performance tele-perception. As related technologies continue to advance and optimize, these methods are expected to significantly boost tele-perception performance. Potential advancements may include enhanced sensing sensitivity, extended sensing range, and improved signal output stability, driving technological upgrades and innovative tele-perception applications across diverse fields, such as environmental monitoring, intelligent security systems, and industrial automation.
Multi-data fusion techniques focus on achieving consistent interpretation of the observed environment by leveraging multi-level and spatially diverse information through complementary and optimized processing, significantly improving environmental perception accuracy and reliability. Analogous to cognitive functions in the human brain, cross-modal sensory information interactions enable learning frameworks to make more precise and continuous predictions about stimulus location, depth, and temperature. For example, Clemens et al.166 achieved biomimetic sensory capabilities by integrating piezoresistive sensing elements with learning algorithms, enabling continuous perception of light touch, depth, and temperature (Fig. 12a). Similarly, Mo et al.167 developed a highly stretchable cross-reactive sensor matrix with artificial perception capabilities that enables the detection, classification, and differentiation of various tactile and thermal stimuli using machine learning. Specifically, multimodal sensing was achieved through the learning algorithm based on the bag-of-words (BoW) model, which identified distinct 2D image patterns generated by mixed tactile and thermal stimuli, enabling differentiation of each stimulus. This approach offered a versatile pathway to simplify e-skin systems, reduce architectural complexity and enhance adaptability to diverse environments, surpassing the limitations of traditional “lock-and-key” methods (Fig. 12b). Additionally, neuromorphic sensing building blocks exhibit significant potential in intelligent sensing, emphasizing the mimicry of neural sensing and data processing mechanisms observed in biological systems to enhance sensory performance. For instance, Dahiya et al.168 proposed a neural architecture for computational e-skin, comprising a sensory neuron layer and a cuneate neuron layer interconnected by synapses. Analogous to mechanoreceptors with diverse receptive fields, stimulation of a single neuron influenced its surrounding neurons. This mechanism mapped incoming tactile signals into a higher-dimensional space in the first layer for processing, and then fed the output into the second layer for further processing, thereby supporting robots to perform diverse tasks, including localized tactile feature recognition and contact/sliding detection. Notably, the plasticity-based learning rule (STDP) proved particularly suitable for hardware applications, especially in synaptic devices. Under this rule, synaptic weights were determined only by the time correlation between paired spikes from the pre- and post-neurons, and operated according to various pairing schemes, including “nearest-neighbor takes all”, “nearest-neighbor takes more”, or “all-spike pairs count equally”, providing promising avenues for further enhancing the sensory capabilities of e-skin.
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Fig. 12 (a) Response input tactile stimulation prediction based on learning algorithm. Reproduced from ref. 166 with permission from Wiley-VCH, copyright 2023. (b) A schematic diagram using the bag-of-words (BoW) algorithm to distinguish mixed stimuli. Reproduced from ref. 167 with permission from Wiley-VCH, copyright 2020. (c) The neural network structure and adaptive learning rules for the proposed computational e-skin. Reproduced from ref. 168 with permission from the American Association for the Advancement of Science, copyright 2022. |
In HMI, non-contact sensors present a novel interaction mode. Compared to traditional methods relying on physical buttons and touchscreens, they enable more natural and seamless interactions, particularly excelling in humanoid robots, touch line screens and AR/VR systems. In humanoid robot applications, Chen et al.77 utilized BAA sensors to successfully implement hand-avoidance functionality, as shown in Fig. 13a. Pu et al.70 developed a virtual distance alarm robot interface with indicator lights controlled by calibrated threshold voltage, displaying corresponding color changes based on varying distances (Fig. 13b). Similarly, Cao et al.75 designed humanoid robots equipped with distance sensors capable of exhibiting human-like perceptual behavior (Fig. 13c). For touch line screens, non-contact sensors provide broad application prospects, including remote password verification,61 contactless control panels to reduce virus risk70 and keyless electronic door lock systems.74 In the AR/VR field, innovative breakthroughs continue to emerge. One example is a non-contact keyboard powered by a resonant triboelectric nanogenerator (R-TENG), comprising a self-powered transmission unit and a non-contact coupling unit169 (Fig. 13g). Another notable example is the intelligent, contactless wearable keyboard system developed by Park et al.59 When users operated the keyboard with their fingers at a specific distance, the system produced text such as “ASER” (Fig. 13h). Additionally, Yuce et al.170 developed a comprehensive HMI system for cursor control, leveraging a self-powered eye motion sensor based on triboelectric interaction and near-field electrostatic induction, significantly improving interaction flexibility in wearable assistive technologies (Fig. 13i).
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Fig. 13 Applications of traditional non-contact sensors in HMI. (a) Interactive interface for robot hand control. Reproduced from ref. 77 with permission from Wiley-VCH, copyright 2019. (b) Virtual distance alarm robot based on distance sensing inductor. Reproduced from ref. 70 with permission from the American Association for the Advancement of Science, copyright 2022. (c) Perceptual behavior of humanoid robot based on distance sensor. Reproduced from ref. 75 with permission from American Chemical Society, copyright 2020. (d) Non-contact device for password verification. Reproduced from ref. 61 with permission from Elsevier, copyright 2019. (e) Touchless control panel for preventing virus risk. Reproduced from ref. 70 with permission from the American Association for the Advancement of Science, copyright 2022. (f) Non-contact sensing for keyless electronic door lock systems. Reproduced from ref. 74 with permission from Royal Society of Chemistry, copyright 2021. (g) Non-contact receiving keyboard based on R-TENG. Reproduced from ref. 169 with permission from Elsevier, copyright 2018. (h) Wearable keyboard based on CNM-TENG. Reproduced from ref. 59 with permission from Elsevier, copyright 2022. (i) Comprehensive HMI system for cursor control. Reproduced from ref. 170 with permission from Elsevier, copyright 2020. |
In intelligent monitoring, non-contact sensors facilitate real-time monitoring of human physiological parameters, behaviors and industrial conditions, providing a robust foundation for medical diagnosis, health management and industrial automation. For intelligent healthcare, non-contact monitoring of vital signs such as breathing and heartbeat ensures continuous, reliable data collection and aids in the timely detection of potential health issues.171 In daily care, the device autonomously detects and differentiates complex human movements without human intervention,172 facilitating multi-site, multi-dimensional motion monitoring65 and providing essential support for rehabilitation via postoperative gait analysis.173 During the COVID-19 pandemic, the device was adapted as a non-contact gesture recognition system to enable robot-assisted throat swab collection.174 In addition, the advancements of intelligent industry are closely linked to non-contact devices. A non-contact sensing system for robot collision avoidance is illustrated Fig. 14f, with sensors installed at the front and rear of the vehicle to detect and avoid obstacles.73 Furthermore, as shown in Fig. 14g, the system can serve as a highly sensitive and reliable speed sensor for commercial vehicles.72 A limit alarm device173 that utilized a non-contact motion vector sensor with multi-channel signal acquisition capabilities was shown in Fig. 14h. During operation, the device prevented collisions between the positioning block and the limit block as the positioning block approached the alarm position, thereby prolonging the machine's service life. Additionally, as shown in Fig. 14i, non-contact sensors are applicable in diverse motion environments, including reverse engineering, grasping detection, and intelligent control.173 Notably, these sensors exhibit high effectiveness in non-contact hand sanitizer devices69 (Fig. 14j). In industrial health management and public spaces with high personnel traffic, they detect hand proximity to trigger automatic hand sanitizer dispensers, preventing cross-contamination issues associated with traditional shared devices, enhancing hygiene standards, and highlighting the critical role of non-contact sensors in smart industry applications. Furthermore, non-contact devices demonstrate significant potential in hydrological measurement and geological resource exploration175 (Fig. 14k). Moreover, non-contact devices are widely used for position monitoring. Single-electrode TENG (STENG) sensors detect the charge type of charged objects, while freestanding TENG (FTENG) sensors identify their motion direction. Based on this principle, STENG-based sensors were installed at the entrance of a complex space, and FTENG-based sensors were placed at various positions within the space, enabling effectively monitoring of the moving direction and trajectory of charged objects,176 as shown in Fig. 14l. Another sensor capable of identifying human motion states and indoor positions, such as walking, falling, approaching and running is illustrated in Fig. 14m, making it valuable for assisting the blind and elderly.71 Additionally, Peng et al.177 designed a user-friendly intelligent navigation sensor for the visually impaired, enabling effective obstacle avoidance (Fig. 14n).
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Fig. 14 Applications of traditional non-contact sensors in intelligent monitoring. (a) Non-contact heartbeat and respiration monitoring system. Reproduced from ref. 171 with permission from American Chemical Society, copyright 2018. (b) A non-contact sensing platform that can monitor and distinguish complex human movements. Reproduced from ref. 172 with permission from Elsevier, copyright 2021. (c) Non-contact multi-directional motion monitoring sensor based on 3D printing deep trap layered architecture. Reproduced from ref. 65 with permission from Elsevier, copyright 2022. (d) Non-contact sensor for gait monitoring of patients after surgery. Reproduced from ref. 173 with permission from Wiley-VCH, copyright 2022. (e) A non-contact gesture recognition system for controlling the robot to collect throat swabs. Reproduced from ref. 174 with permission from Wiley-VCH, copyright 2022. (f) A non-contact sensing system for obstacle prevention in robotic vehicles. Reproduced from ref. 73 with permission from Wiley-VCH, copyright 2021. (g) A commercial vehicle speed sensor system. Reproduced from ref. 72 with permission from Elsevier, copyright 2023. (h) Non-contact limit alarm with multi-channel signal acquisition function. Reproduced from ref. 173 with permission from Wiley-VCH, copyright 2022. (i) A non-contact motion vector sensor. Reproduced from ref. 173 with permission from Wiley-VCH, copyright 2022. (j) Application of non-contact hand sanitizer. Reproduced from ref. 69 with permission from Wiley-VCH, copyright 2022. (k) Non-contact sensors for hydrological measurements and geological resource exploration. Reproduced from ref. 175 with permission from Elsevier, copyright 2024. (l) Non-contact monitoring of human motion trajectories. Reproduced from ref. 178 with permission from IEEE, copyright 2022. (m) Time domain and frequency domain analysis of walking for indoor navigation. Reproduced from ref. 71 with permission from Wiley-VCH, copyright 2022. (n) Blind navigation sensor based on triboelectrification and electrostatic induction. Reproduced from ref. 177 with permission from Elsevier, copyright 2022. |
Integrating ML algorithms with non-contact sensors provides an innovative solution to the challenges of computing power and complex circuitry in high-resolution devices, enabling multifunctional capabilities with fewer sensors by leveraging efficient data analysis and processing and achieving more efficient interactions and intelligent response mechanisms.179 This synergy enhances motion recognition accuracy and promotes the application and development of non-contact sensors in fields such as smart homes, health monitoring, and AR, assisting them to address user needs more effectively and providing intelligent, adaptive functions supported by continuously optimized data processing capabilities. Firstly, ML algorithms have significantly enhanced the accuracy of non-contact gesture recognition. To address the challenge of distinguishing similar voltage sequences, Yang et al.180 utilized PCA to extract features from collected data, demonstrating clear divisibility in two-dimensional space. They employed six algorithms—KNN, SVM, RF, DT, MLP, and XGB—to train the gesture recognition model, achieving accuracies of 0.82, 0.92, 0.94, 0.94 and 0.98 for different gestures, demonstrating excellent performance (Fig. 15a). Using the KNN learning algorithm, Zhao et al.113 successfully trained a model to classify data for non-contact digital recognition, achieving an 88% overall accuracy in identifying ten digits after 20 training cycles (Fig. 15b). Moreover, ML algorithms enhanced the accuracy of material shape recognition. Pu et al.70 developed an artificial proximal somatosensory system for 3D recognition by inputting simulation results from an electric receptor matrix into a CNN, achieving a recognition accuracy of 97% after 75 training cycles (Fig. 15c). Wen et al.181 integrated a CNN into an intelligent soft robot system and enhanced its performance through iterative optimization. After training on 270 original datasets, the robot system achieved 97% recognition accuracy. By accurately perceiving and describing the object's features, the soft robot deformed and grasped objects based on recognition results from multi-modal sensors (Fig. 15d). Furthermore, ML algorithms also enhance human motion pattern recognition. Different motion patterns, such as walking, running and jumping, generate distinct responses in non-contact sensors. Wan et al.182 applied RF algorithms to train motion recognition models to extract and identify features from human motion signals, facilitating the development of multimodal integrated e-skin, demonstrating significant potential for diverse applications (Fig. 15e).
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Fig. 15 Applications of traditional non-contact sensors in machine learning assistant system. (a) Real-time non-contact gesture recognition system utilized PCA. Reproduced from ref. 180 with permission from Donghua University, copyright 2024. (b) Proximity sensor in non-contact digital recognition based on the KNN. Reproduced from ref. 113 with permission from Elsevier, copyright 2022. (c) The 3D object recognition system based on the bioinspired electroreceptor matrix (21 × 21 units) and the CNN. Reproduced from ref. 70 with permission from the American Association for the Advancement of Science, copyright 2022. (d) An intelligent soft robot system integrated CNN that described the physical properties (material, shape and roughness) of objects in a linguistic way. Reproduced from ref. 181 with permission from Wiley-VCH, copyright 2023. (e) Non-contact sensors based on RF for motion pattern recognition. Reproduced from ref. 182 with permission from Elsevier, copyright 2023. |
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Fig. 16 Applications of tele-perception systems. (a) The application of bionic electroreceptor in HMI including virtual distance alarm robots and operating robot arms to wave and shake hands when adults are approaching. (b) The application of bionic electroreceptor in ML assistant system: the 3D object tele-perception system based on the bionic electroreceptor matrix (20 × 20 units) and the CNN. Reproduced from ref. 62 with permission from the American Association for the Advancement of Science, copyright 2024. |
ML algorithms further improved the system's ability to effectively interact with the real-world. As shown in Fig. 16b, a 3D tele-perception somatosensory system employed a CNN to efficiently identify and perceive the shapes and materials of various 3D objects while offering detailed 3D visualization. Using common indoor geometric configurations (humans, chairs, and tables) as test examples, the system achieved 100% recognition accuracy after only three training cycles, establishing a robust foundation for future practical applications. Additionally, the system utilized spectral data to learn the unique color features of various materials and associated them with corresponding material types through ML algorithms and DL models, enabling accurate material identification, even for distant or partially obscured objects. This technological advancement created more opportunities in intelligent robotics, autonomous vehicles and industrial manufacturing. The core value of tele-perception lies in its ability to extend human perception, offering a more intelligent perception method for machines and accelerating advancements in HMI, automatic control and related fields.
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2025 |