Wearable Tactile Sensor Research Articles

High-performance and robust biomimetic triboelectric nanogenerators for energy harvesting and self-powered wearable tactile sensing

High-performance and robust Triboelectric nanogenerators (TENGs) with biomimetic micro-structured PDMS and nanostructured aluminum as main tribo-layers have been proposed. Leaf of Calathea Zebrina plant is used as a mold to prepare PVA template which is further used to obtain micro-structured PDMS layer whereas nanostructures on aluminum surface have been created by water-assisted oxidation process. Corresponding TENG assembled in vertical contact-separation mode produces VOC ∼430 V and ISC ∼45 μA with high power density of 8.89 W m−2. Excellent performance stability observed at least over 20,000 cycles and up to 120 days is another remarkable feature of the proposed TENG. Further, these TENGs are found highly viable as self-powered wearable tactile sensors for several hand and foot-based tactile activities, which can precisely record spatial and temporal details of the activity. Such TENG development strategies, which involve mimicking of natural micro-patterns have huge performance potential and may play an integral role to augment commercial prospects of TENGs in their wide application spectrum.

A self-powered flexible tactile sensor utilizing chemical battery reactions to detect static and dynamic stimuli

Flexible and wearable tactile sensors have become increasingly important in various fields, including healthcare monitoring and human-computer interaction. However, traditional resistive or capacitive sensors require an external power supply to continuously consume energy, while piezoelectric and triboelectric sensors respond only to dynamic mechanical stimuli. The development of tactile sensors that do not require an external power source and are capable of monitoring dynamic/static mechanical stimuli could complement the shortcomings of existing sensing devices. In this paper, we propose a simple, convenient, and environmentally friendly paper-based sandwich solid Zn-MnO2 battery, which can be used as a self-powered flexible tactile sensor through simple structural design. As a battery, it exhibited an open circuit voltage of 1.29 V, the highest short circuit current density of 2.8 mA/cm2, and a capacity of 2.2 mAh/cm2 at a current density of 0.1 mA/cm2, which is sufficient to power small electrical appliances. As a self-powered pressure sensor, it exhibits an average sensitivity of 1.27 μA/kPa in the 0–25 kPa and 0.14 μA/kPa within the 25–500 kPa pressure range, a response time of 42 ms and a recovery time of 41 ms, a minimum pressure resolution of tens of Pascals, and long-period repeatability and stability. This work will significantly contribute to the further advancement of intelligent, self-powered tactile sensors for a wide range of applications in fields such as human healthcare and human-machine interfaces for detecting static and dynamic stimuli.

An implantable, wireless, battery-free system for tactile pressure sensing

The sense of touch is critical to dexterous use of the hands and thus an essential component of efforts to restore hand function after amputation or paralysis. Prosthetic systems have addressed this goal with wearable tactile sensors. However, such wearable sensors are suboptimal for neuroprosthetic systems designed to reanimate a patient’s own paralyzed hand. Here, we developed an implantable tactile sensing system intended for subdermal placement. The system is composed of a microfabricated capacitive pressure sensor, a custom integrated circuit supporting wireless powering and data transmission, and a laser-fused hermetic silica package. The miniature device was validated through simulations, benchtop assessment, and testing in a primate hand. The sensor implanted in the fingertip accurately measured applied skin forces with a resolution of 4.3 mN. The output from this novel sensor could be encoded in the brain with microstimulation to provide tactile feedback. More broadly, the materials, system design, and fabrication approach establish new foundational capabilities for various applications of implantable sensing systems.

Anti‐Freezing Wearable Tactile Sensors Prepared by Laser Processing of Crumpled Xanthan Gum‐Based Hydrogels

AbstractWearable piezoresistive tactile sensors are becoming increasingly popular in fields related to human motion/health monitoring, and artificial intelligence. However, piezoresistive materials are still limited in terms of safety and biocompatibility in medical settings, as well as availability under harsh conditions. Herein, elastic and biocompatible hydrogels are prepared by cross‐linking of xanthan gum, polyvinyl alcohol, carbon nanotubes, and glycerol. The presence of glycerol results in hydrogel exhibiting an outstanding freezing resistance (−40 °C). Meanwhile, the controllable, large‐scale, and facile two‐step direct laser writing process allows the fabrication of hydrogels with double‐rough surface structures. By tuning surface roughness, the sensors with the resulting hydrogels show a wide detection range with high sensitivities of 27.5, 5.4, and 2.1 kPa−1 under pressures of 0.1–6, 6–25, and 25–80 kPa. The sensors also display fast response time (≈126 ms), low detection limit (2.26 Pa), good reproducibility of sensing performance (1000 cycles), relevant freezing tolerance (−40 °C), and decent environmental stability (over 30 days). This work provides a low‐cost, ease of manipulation, high reproducible, and large‐scale way for the microstructure engineering of natural‐polymer hydrogels and promotes their widespread applications.

A soft tactile sensing array with high sensitivity based on MWCNTs-PDMS composite of hierarchically porous structure

The advancement of wearable tactile sensors that involves with high sensitivity under ultra-low pressures is crucial for varieties of human-machine interactive applications, like smart phones, healthcare monitoring, and electronic skins. Here in this paper, a soft capacitive tactile sensing array is introduced based on hierarchically porous multi-walled carbon nanotubes (MWCNTs)-polydimethylsiloxane composite, which leads to sensitivity improvement attributing to a synergistic effect of the hierarchically porous elastomer and conductive MWCNTs supplements. The proposed device exhibits superior pressure-sensing performances, with high sensitivity (3.58 kPa−1) under small mechanical stimuli (<80 Pa), broad measuring range (0–265 kPa), fast response time (<45 ms), good repeatability, minimum limit of detection (<10 Pa), as well as low-hysteresis, allowing efficient sensing of pressure from all types of sources, from vulnerable signals such as human breathing, artery and venous pulses, and soft human finger touch to possible brutal variations such as sudden change of object weight or prompt collide. Moreover, extensive body attached experiments confirm that the soft tactile sensing array is fully human compatible and capable for a variety of human-machine interfaces and health monitoring applications.

A Self-Powered Piezoelectric Nanofibrous Membrane as Wearable Tactile Sensor for Human Body Motion Monitoring and Recognition.

The online version contains supplementary material available at 10.1007/s42765-023-00282-8.

Recent Advances in Wearable Tactile Sensors Based on Electrospun Nanofiber Platform

AbstractWearable electronics have triggered the great development of flexible tactile sensors for promising applications such as healthcare monitoring, motion detection, and human‐machine interaction. However, most of the flexible sensors are constructed on compact and airtight polymer films with inferior flexibility and no breathability, hindering the wearability and comfortability for long‐time continuous operation usage. To address such challenges, flexible sensors based on the electrospun polymer platform with comprehensive advantages of ultrathin thickness, superior flexibility, excellent stretchability, high porosity, low density, and surface functionality are emerging. It has become a hot research direction, and considerable progress has been made. Therefore, it is necessary to timely review the latest findings on the rapidly developing electrospun nanofiber‐based tactile sensors. Firstly, the principle of the electrospinning technique, the factors affecting the nanofiber morphology, and the engineering of the nanofibers are briefly introduced, and the key material and structural factors affecting the sensing performance are analyzed. Secondly, representative work on the electrospun nanofiber‐based tactile sensors is discussed in detail according to the sensing mechanism. Thirdly, unique properties of electrospun nanofibers, such as superior flexibility, breathability, hydrophobicity, anti‐bacterial, and self‐cleaning, are highlighted. Finally, the remaining challenges and future development trends are outlined.

Conjugated Polymer-Based Nanocomposites for Pressure Sensors.

Flexible sensors are the essential foundations of pressure sensing, microcomputer sensing systems, and wearable devices. The flexible tactile sensor can sense stimuli by converting external forces into electrical signals. The electrical signals are transmitted to a computer processing system for analysis, realizing real-time health monitoring and human motion detection. According to the working mechanism, tactile sensors are mainly divided into four types-piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. Conventional silicon-based tactile sensors are often inadequate for flexible electronics due to their limited mechanical flexibility. In comparison, polymeric nanocomposites are flexible and stretchable, which makes them excellent candidates for flexible and wearable tactile sensors. Among the promising polymers, conjugated polymers (CPs), due to their unique chemical structures and electronic properties that contribute to their high electrical and mechanical conductivity, show great potential for flexible sensors and wearable devices. In this paper, we first introduce the parameters of pressure sensors. Then, we describe the operating principles of resistive, capacitive, piezoelectric, and triboelectric sensors, and review the pressure sensors based on conjugated polymer nanocomposites that were reported in recent years. After that, we introduce the performance characteristics of flexible sensors, regarding their applications in healthcare, human motion monitoring, electronic skin, wearable devices, and artificial intelligence. In addition, we summarize and compare the performances of conjugated polymer nanocomposite-based pressure sensors that were reported in recent years. Finally, we summarize the challenges and future directions of conjugated polymer nanocomposite-based sensors.

Shear-Thickening Covalent Adaptive Networks for Bifunctional Impact-Protective and Post-Tunable Tactile Sensors

Shear-thickening materials have been widely applied in fields related to smart impact protection due to their ability to absorb large amounts of energy during sudden shock. Shear-thickening materials with multifunctional properties are expanding their applications in wearable electronics, where tactile sensors require interconnected networks. However, current bifunctional shear-thickening cross-linked polymer materials depend on supramolecular networks or slightly dynamic covalently cross-linked networks, which usually exhibit lower energy-absorption density than the highly dynamic covalently cross-linked networks. Herein, we employed boric ester-based covalent adaptive networks (CANs) to elucidate the shear-thickening property and the mechanism of energy dissipation during sudden shock. Guided by the enhanced energy-absorption capability of double networks and the requirements of the conductive networks for the wearable tactile sensors, tungsten powders (W) were incorporated into the boric ester polymer matrix to form a second network. The W networks make the materials stiffer, with a 13-fold increase in Young's modulus. Additionally, the energy-absorption capacity increased nearly 7 times. Finally, we applied these excellent energy-absorbing and conductive materials to bifunctional shock-protective and strain rate-dependent tactile sensors. Considering the self-healable and recyclable properties, we believe that these anti-impact and tactile sensing materials will be of great interest in wearable devices, smart impact-protective systems, post-tunable materials, etc.

Wearable, ultrathin and breathable tactile sensors with an integrated all-nanofiber network structure for highly sensitive and reliable motion monitoring

Wearable tactile sensors are under high pursuit in healthcare monitoring. However, most of previous sensors were assembled by non-binding multilayers and built on airtight substrates, hindering the wearability with comfort. Herein, we develop an ultrathin and breathable tactile sensor based on an integrated all-nanofiber network via a facile electrospinning and spray-drying technique. The sensor is constructed by directly ink-patterning of conductive Co3O4/PEDOT:PSS/AgNW electrodes on both sides of one single ionic IL/TPU fabric electrolyte mat, the overall thickness of which is only 40 µm for a conformal attachment on any curved surface. The highly porous nanofiber network endows the sensor with both high air and moisture permeabilities, ensuring an excellent comfortability for a long-time wearing. Due to the pseudocapacitive sensing mechanism and nanofiber-based microstrucutre, the sensor exhibits ultrahigh sensitivity of 79.5 kPa−1, fast response time of 40 ms, and a wide detection range of 38 kPa. Reliability tests show that the sesnor is capable of outputting stable and repeatable sensing signals for over 10,000 loading/unloading cycles of 5 kPa on testing machine and for over 5,000 s of continuous motion detection of foot stamping and elbow bending. Monitoring of arm swinging and elbow bending by sensor units and recognition of hand gesture by sensor array are also demonstrated towards practical wearable sensing applications.

Sweeping-responsive interface using the intrinsic polarity of magnetized micropillars for self-powered and high-capacity human-machine interaction

The rapid development of wearable tactile sensors in human-machine interaction (HMI) is revolutionizing the way that people communicate with intelligent terminals. Compared with “tapping”, another daily operation, “directional sweeping”, is rarely explored in tactile sensors for HMI due to the limits from device configuration. Herein, we designed and optimized the in-plane magnetized flexible micropillars as the self-powered interface that can perceive the sweeping with distinguishable signals according to the operation directions. Based on Faraday’s law of induction, the intrinsic magnetic polarity was applied as an avenue to rapidly produce diverse electromotive forces of “+ /-” and “-/+ ” through the bi-directional micropillar deformation. With the self-powered interface, several interesting HMI applications, e.g., smartphone page flip, Morse code communication, and game playing, were demonstrated via habitual finger sweeping with high efficiency, accuracy, intuitive experience, and control diversity. Furthermore, the unique behavior of “+ /-” and “-/+ ” enables the build-up of ternary system with broader command capacity of 3 n if n devices were integrated in parallel. Along with the merits such as high sensitivity, applicable diversity (humid condition), mechanical robustness, and coding accuracy, we believe that the study would bring inspiration of future HMI design for a more fascinating, effective and intelligent living.

Conformable and Compact Multiaxis Tactile Sensor for Human and Robotic Grasping via Anisotropic Waveguides

AbstractRich and accurate tactile perceptive capability is important for both humans and robots. Soft materials exhibit unique characteristics to construct high‐performance tactile sensors such as high sensitivity and high resistance to overload. In this work, a multiaxis tactile sensor based on a soft anisotropic waveguide that can distinguish normal force and shear force, which can greatly expand its potential uses in practice, is reported. First, the anisotropy of the waveguide sensor's response to vector forces is validated numerically and experimentally, and then two of those anisotropic units are embedded into one cladding with a crossed‐over layout, to form a multiaxis sensor. Then, a calibration algorithm is implemented on this sensor and the reconstruction of vector forces is achieved at an average accuracy of 28.0 mN for normal forces and 81.1 mN for shear forces, both with the sensing range of 1 N. Using this device, three demonstrations are shown to give outlooks of its potential application in human and robotic grasping tasks: a wearable tactile sensor for collecting human's haptic data during operation, a uniaxial gyroscope, and a robotic gripper's tactile sensor for assisting an unlocking task with a key. This work is a step toward more functional soft waveguide‐based force sensors.

Self-Powered Tactile Sensor for Gesture Recognition Using Deep Learning Algorithms.

A multifunctional wearable tactile sensor assisted by deep learning algorithms is developed, which can realize the functions of gesture recognition and interaction. This tactile sensor is the fusion of a triboelectric nanogenerator and piezoelectric nanogenerator to construct a hybrid self-powered sensor with a higher power density and sensibility. The power generation performance is characterized with an open-circuit voltage VOC of 200 V, a short-circuit current ISC of 8 μA, and a power density of 0.35 mW cm-2 under a matching load. It also has an excellent sensibility, including a response time of 5 ms, a signal-to-noise ratio of 22.5 dB, and a pressure resolution of 1% (1-10 kPa). The sensor is successfully integrated on a glove to collect the electrical signal output generated by the gesture. Using deep learning algorithms, the functions of gesture recognition and control can be realized in real time. The combination of tactile sensor and deep learning algorithms provides ideas and guidance for its applications in the field of artificial intelligence, such as human-computer interaction, signal monitoring, and smart sensing.

Triboelectric Nanogenerators as Active Tactile Stimulators for Multifunctional Sensing and Artificial Synapses.

The wearable tactile sensors have attracted great attention in the fields of intelligent robots, healthcare monitors and human-machine interactions. To create active tactile sensors that can directly generate electrical signals in response to stimuli from the surrounding environment is of great significance. Triboelectric nanogenerators (TENGs) have the advantages of high sensitivity, fast response speed and low cost that can convert any type of mechanical motion in the surrounding environment into electrical signals, which provides an effective strategy to design the self-powered active tactile sensors. Here, an overview of the development in TENGs as tactile stimulators for multifunctional sensing and artificial synapses is systematically introduced. Firstly, the applications of TENGs as tactile stimulators in pressure, temperature, proximity sensing, and object recognition are introduced in detail. Then, the research progress of TENGs as tactile stimulators for artificial synapses is emphatically introduced, which is mainly reflected in the electrolyte-gate synaptic transistors, optoelectronic synaptic transistors, floating-gate synaptic transistors, reduced graphene oxides-based artificial synapse, and integrated circuit-based artificial synapse and nervous systems. Finally, the challenges of TENGs as tactile stimulators for multifunctional sensing and artificial synapses in practical applications are summarized, and the future development prospects are expected.

Inkjet printing aided patterning of transparent metal mesh for wearable tactile and proximity sensors

Fabrication of an electronic device involves patterning that contributes a substantial cost to it. Development of an unconventional and low-cost patterning technique is thus the goal-post. Herein, an exclusive, simple and large-scale patterning technique for metal mesh is established. The steps involve the formation of a crackle template on a transparent substrate by spray coating, inkjet printing polyvinyl alcohol (PVA) solution followed by drying, metal deposition and template lift-off. An interdigitated electrode (IDE) with a finger-gap of ∼ 300 µm over a large electrode area (30 × 30 cm2) is accomplished with such a simple process. The patterning technique is further utilized to fabricate an IDE on flexible poly(ethylene terephthalate) (PET) substrate for the application of a transparent multifunctional capacitive sensor with proximity (up to 9 cm) and touch sensing ability.

Retracted: Development of a Highly Sensitive Wearable Tactile Sensor on Fabric by Using Conductive Inks Based on Electrical Contact Resistance (ECR) Change Mechanism

Macromol. Mater. Eng. 2021, 306, 2100130https://doi.org/10.1002/mame.202100130The above article, published online on 17 June 2021 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the journal's Editor‐in‐Chief, David Huesmann, and Wiley‐VCH GmbH. The retraction has been agreed due to a non‐disclosed previous publication of data in this article that, had the editorial team been made aware of upon submission, would have affected the likelihood of the article being considered further. As a result, the journal has made the decision to retract this article.

A breathable and reliable thermoplastic polyurethane/Ag@K2Ti4O9composite film with an asymmetrical porous structure for wearable piezoresistive sensors

Hierarchical porous TPU/Ag@K2Ti4O9hybrid membranes with qualities of durable, breathable, easy to recycle, as well as the sensing properties were successfully prepared by a modified-NIPS method for wearable tactile sensors.

Wearable human-machine interaction device integrated by all-textile-based tactile sensors array via facile cross-stitch

Novel HMI devices have shown great potential application in somatosensory games, intelligent robot systems and artificial intelligence, attracting lots of researcher interests. However, the lack of suitable flexible sensors for signal input, reasonable signal processing and feedback schemes extremely restricts practical applications of the novel HMI devices. In this paper, a flexible wearable tactile sensors array based on textiles was prepared. A wearable HMI device was fabricated by setting finger tapings and sound as signal input as well as feedback. The textile based-sensor exhibits a high sensitivity of 0.84 kPa−1, fast response time less than 4 ms, outstanding cycle stability and washability after optimizing sensors configurations and woven structures. Collecting as well as processing of finger tapping data, and sound feedback were realized based on ESP32-DevKitC circuit, Bluetooth module and Android APP. A strategy of predicting signal trend before the maximum tapping value was proposed to reduce the response time of wearable HMI device. In addition, we also made a series of adjustments to the wearable HMI device feedback referring to the characteristics of electronic drums, which fully proved the huge application potential of this wearable HMI device.

Development of a Highly Sensitive Wearable Tactile Sensor on Fabric by Using Conductive Inks Based on Electrical Contact Resistance (ECR) Change Mechanism

Front Cover: In article number 2100130 by Yen-Wen Lu and co-workers, a fabric tactile sensor, based on the fact that contact resistance between two surfaces changes when pressure is applied, is developed by using a screenprinting technique for subtle pressure sensing (<10 kPa) at high sensitivity. Wrist pulse measurements and spatial pressure mapping using the sensors with wireless modules are demonstrated for wearable health monitoring and internet-of-things applications.

Superhydrophobic and elastic 3D conductive sponge made from electrospun nanofibers and reduced graphene oxide for sweatproof wearable tactile pressure sensor

Herein, we report an innovative method to produce three-dimensional (3D), elastic, and conductive nanofibrous sponges with high sensitivity, great mechanical properties, and sweatproof feature for wearable tactile pressure sensor. The 3D sponges were prepared by assembling the shortened polyacrylonitrile/polyimide (PAN/PI) electrospun nanofibers and reduced graphene oxide (rGO) nanosheets. In specific, the PAN/PI nanofibers and graphene oxide (GO) nanosheets were mixed in aqueous circumstance first and then freeze dried to form a 3D porous sponge. Subsequently, the sponge was thermally treated at 100 °C and 210 °C successively to reduce the GO nanosheets to conductive rGO nanosheets. Thereafter, the polydimethylsiloxane (PDMS) treatment was carried out to provide the sponge with elasticity/robustness and hydrophobicity. Upon varying the amount of rGO, three types of sponges were fabricated to study their mechanical and electrical properties. The obtained 3D conductive sponge with the most rGO amount (3-rGO/NF) possessed light weight (11.50 mg/cm3) and high porosity (99.23%). The mechanical strength and current change ratio during the cyclic compression process were investigated, and the results were closely correlated with the amount of rGO existed in the sponge. The 3-rGO/NF had the largest current change ratio when compressed, leading to the highest sensitivity even at a relatively small compressive strain. It is important to note that the prepared 3D conductive sponges were sensitive, bendable, and sweatproof, which would be particularly suitable for making wearable tactile pressure sensors.