Self-powered Tactile Sensor Research Articles

All electrospun fabrics based piezoelectric tactile sensor

Tactile sensors have been widely used in the areas of health monitoring and intelligent human-machine interface. Flexible tactile sensors based on nanofiber mats made by electrospinning can meet the requirements of comfortability and breathability for wearing the body very well. Here, we developed a flexible and self-powered tactile sensor that was sandwich assembled by electrospun organic electrodes and a piezoelectric layer. The metal-free organic electrodes of thermal plastic polyurethane (PU) nanofibers decorated with multi-walled carbon nanotubes were fabricated by electrospinning followed by ultrasonication treatment. The electrospun polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) mat was utilized as the piezoelectric layer, and it was found that the piezoelectric performance of PVDF-TrFE nanofiber mat added with barium titanate (BaTiO3) nanoparticles was enhanced about 187% than that of the pure PVDF-TrFE nanofiber mat. For practical application, the as-prepared piezoelectric tactile sensor exhibited an approximative linear relationship between the external force and the electrical output. Then the array of fabricated sensors was attached to the fingertips of a glove to grab a cup of water for tactile sensing, and the mass of water can be directly estimated according to the outputs of the sensor array. Attributed to the integrated merits of good flexibility, enhanced piezoelectric performance, light weight, and efficient gas permeability, the developed tactile sensor could be widely used as wearable devices for robot execution end or prosthesis for tactile feedback.

A skin-inspired, self-powered tactile sensor

Self-powered pressure- and temperature-sensing abilities are essential for the skin-inspired tactile perception ability of artificial electronic skins (e-skins). Recent research on e-skins has focused on simultaneous detection of pressure and temperature with a single device, whereas sensor operation without energy consumption remains a challenge. For genuine skin-like extrasensory perception, this study demonstrates a self-powered multifunctional ionic tactile sensor (SMITS) through lamination of a single-electrode triboelectric nanogenerator (SETENG) and a capacitive ionic sensor (CIS) sharing a common electrode. For this new type of electronic mechanoreceptor, a single touch not only enables identification of the kind of material but also electrifies the CIS, with the voltage varying under different pressures from 0 to 60 kPa and temperatures from 25° to 45°C without any additional energy supply.

Flexible Self-Powered Tactile Sensors Based on Hydrothermally Grown ZnO Nanorods

Flexible tactile sensors with high sensitivity and good flexibility are urgently required in modern biomimetic robots’ electronic skin, and artificial intelligence interaction. In this work, a flexible piezoelectric tactile sensor was fabricated using hydrothermally grown ZnO nanorods (ZnO NRs) on Cu-coated polyimide (PI) substrate. Structural and morphological properties of the ZnO NRs were investigated by SEM, XRD. The piezoelectric response properties of the sensors by using the pasted Cu/PI electrode and directly screen-printed Ag electrode were compared. The results show the sensor with the screen-printed Ag electrode exhibits a high sensitivity of 974 mV/N, which is higher than the pasted Cu/PI electrode, and the response of all sensors to the applied pressure is almost linear in the range of 0.1-1 N. Meanwhile, the sensor can be employed to detect human movements such as bending/stretching motion of fingers and the vibration of the vocal cords and pulse, and realize the conversion from mechanical energy to electrical energy. It is demonstrated that the sensor has great potential applications in wearable health monitoring systems and self-powered devices.

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 UV patterning for wearable one-terminal tactile sensor array to perceive dynamic contact motions

Triboelectric nanogenerators (TENGs) have attracted intensive attention for the self-powered tactile signalling because strong potential signals are generated even by weak mechanical stimuli. The TENG-based sensors typically request one or two interconnecting wires in each pixel, hence a large number of interconnections are necessary to acquire multiple position information. Here, we introduce innovative triboelectric UV patterning on a single stretchable elastomer dielectric film and fabricate a position-encoded tactile sensor requiring only one interconnection line. We investigate controllable change of triboelectric potentials simply adjustable by UV irradiation time on saturated thermoplastic block copolymers. The one-terminal triboelectrically-patterned tactile sensor (TPTS) perceives the position and time of dynamic contact motions such as sliding, ball rolling, sequential or simultaneous touches, and grasping. We believe that this study will pave the way for the novel design of self-powered tactile sensors for various applications such as artificial skin.

MXene/Polylactic Acid Fabric-Based Resonant Cavity for Realizing Simultaneous High-Performance Electromagnetic Interference (EMI) Shielding and Efficient Energy Harvesting.

Proliferation in telecommunications and integrated/intelligent devices entails an intense concern for electromagnetic interference (EMI) shielding and versatility. It remains an activated passion to launch infusive EMI shielding materials integrated with self-powered peculiarities. Herein, a double-layered MXene/polylactic acid (PLA) fabric resonance cavity (D-MPF-RC) comprised of two MXene/PLA fabrics (MPFs) with alternating MXene and PLA structures that are separated by a poly(tetrafluoroethylene) (PTFE) frame is developed. The D-MPF-RC achieved 48.5 and 74.8% improvement in SET and SEA, and 24.6% reduction in SER by introducing the double-layered structure and increasing the resonance cavity (RC) distance without varying the material composition and cost. A high shielding efficiency (SE) of 92.3 dB was obtained at an RC distance of 6 mm owing to the synergetic effects of multiple reflections and destructive EM wave interference. The tribopolarity difference between PLA and MXene and the RC structure made the D-MPF-RC a readily available triboelectric nanogenerator (TENG) that could convert mechanical energy into electricity. The D-MPF-RC TENG demonstrated an open-circuit voltage of 88 V and achieved a peak power density of 35.4 mW m-2 on a 6.6 MΩ external resistor, which made it possible to charge capacitors and serve as a self-powered tactile sensor. This report offers new insights into the design of high-performance EMI shielding shields with a resonance cavity and proposes a feasible pathway to integrate them with energy harvesting capabilities.

Weaved piezoresistive triboelectric nanogenerator for human motion monitoring and gesture recognition

Self-powered tactile sensors based on triboelectric nanogenerators (TENGs) can be sustainable and intelligent for dynamically monitoring various physiological signals and motions. Nevertheless, it is still very challenging for the TENG-based sensors to be simultaneously sensitive to deformation caused by stress and strain. Herein, a piezoresistive fiber-base triboelectric sensor (PRF-TES) is designed by using nylon thread, silver nanowires (Ag NWs), carbon nanotubes (CNTs), Parafilm and polydimethylsiloxane (PDMS) as source materials. The elaborate hierarchical structure of the textile PRF-TES ranges from one-dimensional macroscopic fibers to elastic polymer with an internal nanoscale CNTs percolation network, which presents a relatively large contact area, multiple contact points and large deformation space for both mechanical energy collection and force sensing. Dynamically, the textile TENG enables the monitoring of physically changes of human body (for example, joint movement, gesture, etc.), but also the information on the contacted object, such as material texture, type and shape. Statically, it can sensitively detect the stress and strain itself as well as the mass and position of the touched object in the two-dimensional space. This work enlightens the development of highly sensitive and multimodal sensors for broad application in personal health monitoring and human-computer interaction.

A Skin-Inspired, Self-Powered Tactile Sensor

A Skin-Inspired, Self-Powered Tactile Sensor

PVDF-bismuth titanate based self-powered flexible tactile sensor for biomechanical applications

Flexible piezoelectric tactile sensors have attracted tremendous attention towards bio-healthcare monitoring and wearable applications. For piezoelectric tactile sensors, a careful selection of materials is important to maximize the piezoelectric effect and improve the measurement sensitivity. In this work, we synthesized a bismuth titanate, Bi4Ti3O12 (BiTO), and polyvinylidene fluoride (PVDF) composite thin film-based self-powered tactile sensor. The electrical output was measured by alteration of the BiTO composition in the PVDF and the composite film thickness. The PENG device based on PVDF-10 wt% BiTO and 40 μm thickness deliver the highest voltage output of 41 V and maximum power density of 4.3 mW/m2. The sensor was further attached to the fingers or vocal cords demonstrating the detection of the different human motions without an external power source. These results pave the way towards the development of portable self-powered biomechanical sensors for rehabilitation, health care monitoring, and human–machine interface.

A stretchable self-powered triboelectric tactile sensor with EGaIn alloy electrode for ultra-low-pressure detection

Self-powered tactile and pressure sensor is one of the key components for new smart electronic devices which can sense various external physical stimuli without external power supplies. With the potential application in detection of health conditions and sport performances, self-powered tactile sensors have attracted broad attentions. Still, fabrication of self-powered tactile sensors with stretchability, flexibility and low detection limit remains challenge. In this work, a self-powered triboelectric tactile sensor with multi-level structured polydimethylsiloxane (PDMS) triboelectric layer and PDMS/Eutectic Gallium-Indium (EGaIn) alloy composite electrode is demonstrated with simple fabrication process and low-cost. The detection limits of 0.23 Pa (experimental) and 7 mPa (calculated) are the state of art for reported self-powered triboelectric tactile sensor. With this excellent characteristic, monitoring of human wrist pulses is demonstrated by this self-powered triboelectric tactile sensor. Moreover, due to the working mechanism of the triboelectric nanogenerator, this self-powered triboelectric tactile sensor exhibits uncompromising resilience to stretching and even significant damage.

MXene-enhanced β-phase crystallization in ferroelectric porous composites for highly-sensitive dynamic force sensors

Piezoelectric polyvinylidene fluoride (PVDF) has been widely utilized in flexible and self-powered tactile sensors, which require high ferroelectricity of polar phase PVDF. Herein, we demonstrate self-powered piezoelectric e-skins with high sensitivity and broad sensing range based on 3D porous structures of MXene (Ti 3 C 2 T x )/PVDF. MXene was used as a nucleation agent to increase the ferroelectric properties of PVDF. This was carried out considering its 2D geometry and abundant surface functional groups that facilitate intermolecular hydrogen bonding between the surface functional groups of MXene and the CH 2 group of PVDF. In addition, porous structures can increase the variation in contact area and localized stress concentration in response to applied pressure. This further enhances the piezoelectric sensitivity. Owing to structural deformation and localized stress concentration, the piezoelectric sensitivity of porous MXene/PVDF e-skin is 11.9 and 1.4 nA kPa −1 for low (< 2.5 kPa) and high (2.5–100 kPa) pressure ranges, respectively. These are 31 and 3.7 times higher, respectively, than that of planar MXene/PVDF e-skin (0.4 nA kPa −1 for <100 kPa). In addition, porous MXene/PVDF e-skin exhibits a broad sensing range of up to 100 kPa, and stable sensing performance (5000 repetitions). Our piezoelectric porous MXene/PVDF e-skins enable the monitoring of high-frequency dynamic signals such as acoustic sound waves as well as low-frequency radial artery pulses. In particular, the detection of high-frequency vibrations from sliding friction enables our sensor array to perceive various surface textures with different roughness and moduli, as well as the spatial distribution of words embossed on surfaces. This demonstrates its substantial potential for application in wearable devices, prosthetic limbs, robotics, and healthcare monitoring devices. MXene with a large specific surface area and abundant functional groups can be used as a heterogeneous nucleation agent to induce crystallization of the polar β-phase and enhance the ferroelectric properties of PVDF, which enables the monitoring of high-frequency dynamic signals such as acoustic sound waves as well as low-frequency radial artery pulses. In particular, the detection of high-frequency vibrations from sliding friction enables our sensor array to perceive various surface textures with different roughness and moduli, as well as the spatial distribution of words embossed on surfaces. • High-performance piezoelectric skin sensor is demonstrated using 3D porous structures of MXene (Ti 3 C 2 T x )/PVDF composites. • MXene is used as a nucleation agent to induce polar β-phase and enhance the ferroelectric properties of PVDF. • MXene/PVDF sensor perceives various surface textures as well as the spatial distribution of words embossed on surfaces.

Integrated and shape-adaptable multifunctional flexible triboelectric nanogenerators using coaxial direct ink writing 3D printing

Triboelectric nanogenerators (TENGs), particularly those with high flexibility and sophisticated geometry, have shown great application foreground in portable and wearable electronics. However, traditional fabrication approaches remain complicated and non-versatile, which are also impractical in the preparation of complex shapes and structures. Herein, a one-pot coaxial direct ink writing (DIW) 3D printing technique has been proposed to fabricate a fully flexible single-electrode TENG (FFTENG) with sophisticated shapes and 3D structures for the purpose of harvesting and utilizing biomechanical energy. This FFTENG is made up of a silicone elastomer shell as the triboelectric layer and an inner silicone/carbon black (CB) core as the flexible electrode. Various factors that affect the output electric performance, including the ratio of the inner and outer diameter of the printed fiber, CB content, loading frequency, applied contact force, specimen size, and external load resistance, are investigated in detail via the contact-separation test. The results show that a standard square FFTENG with a size of 30 × 30 mm2 can yield an open-circuit voltage (Voc), a short-circuit current (Isc) and a short-circuit transferred charge (Qsc) of as high as 60 V, 0.23 μA and 58 nC, respectively, and a maximum output peak power density of 15.59 mW m−2 at a matched resistance of 80 MΩ. More importantly, various shape-adaptable FFTENGs can be designed and tailored to meet the diverse needs of different applications, such as self-powered LED systems, self-charging power systems, LED control devices, self-powered tactile sensors, flexible self-monitoring grip exercisers, and flexible self-powered keyboards, and open up new avenues for use in multifunctional self-powered electronic systems, including biomechanical energy harvesting and utilization, self-powered sensing and human-machine interaction.

A 3D-printed, alternatively tilt-polarized PVDF-TrFE polymer with enhanced piezoelectric effect for self-powered sensor application

Abstract This work reports a 3D-printed PVDF-TrFE piezoelectric film (PF) coated with one pair of dislocated interdigital electrode (ID) to produce multiple, alternatively tilt-polarized regions in the cross-sectional area, which exhibits quite high sensitivity to an external stress stimulation. The theoretical analysis further reveals that the enhanced piezoelectric effect of the film can be attributed to the larger electric dipole moment due to tilt dipole orientation in PVDF-TrFE polymer. The experimental measurements show that the ID tilt-polarized PVDF-TrFE film (ID-TPPF) can produce a stable peak voltage of 73.5 V under dynamic compression stress of 50 kPa at 1 Hz, corresponding to a pressure sensitivity of 1.47 V/kPa, which are 14.7 times and 3.6 times those of a PVDF-TrFE film with conventional thickness-polarization and in-plane polarization, respectively. Correspondingly, the peak power density of ID-TPPF is as high as 478 μW/cm2, while at a load of 3 MΩ, the load power is still 207 μW/cm2, which even instantly lights up eight LEDs connected in series without using any charging capacitor. This work confirms that the 3D-printed ID-TPPF shows great potential in self-powered tactile sensors and artificial skin applications in the future.

Nanofibrous Grids Assembled Orthogonally from Direct-Written Piezoelectric Fibers as Self-Powered Tactile Sensors.

Tactile sensors are indispensable to wearable electronics, but still lack self-powering, high resolution, and flexibility. Herein, we present direct-written piezoelectric poly(vinylidene difluoride) fibers that are orthogonally assembled into nanofibrous grids (NFGs) as self-powered tactile sensors. Five nanofibrous strips (NFSs) are written on a polyurethane film via a uniform-field electrospinning (UFES) process, and two polyurethane films are orthogonally assembled into 5 × 5 NFGs with 25 pixels. Benefited from the mechanical flexibility and helical architecture of UFES fibers, stable piezoelectric outputs have been detected according to different locations or different pressures on an NFS, and a sensitivity of 7.1 mV/kPa is detected from the slope of voltage-pressure curves. In the orthogonally assembled NFGs, the pressure on a pixel of an NFS causes corresponding deformations of neighboring NFSs. The piezoelectric outputs vary with the distance from the pressing point, enabling us to position the pressing points and track the pressing trajectory in real time. Through judging piezoelectric outputs of all NFSs, precise locations of any pressed pixel with a resolution of 1 mm are presented vividly via luminous light-emitting diodes (LED), and the mapping profiles are displayed by pressing metal letters (S, W, J, T, and U) on multiple pixels. Furthermore, the coordinates of pressure either on an NFS or between NFSs with a resolution of 0.5 mm are reported digitally on a liquid crystal display (LCD). Thus, we developed novel self-powered tactile sensors with orthogonal NFGs to achieve real-time motion tracking, accurate spatial sensing, and location identification with high resolutions, which provide potential applications in electronic skin, robotics, and interface of artificial intelligence.

A flexible ultra-sensitive triboelectric tactile sensor of wrinkled PDMS/MXene composite films for E-skin

Abstract Electronic skin (E-skin) based on tactile sensors plays an important role in monitoring daily health status and artificial intelligence (AI). High sensitivity and flexibility are highly demanded to simulate human skins. In this work, a flexible TENG, based on wrinkled PDMS/MXene composite films prepared by facile ultraviolet ozone (UVO) irradiation, was constructed as self-powered tactile sensor. The best sensitivity reaches 0.18 V/Pa during 10–80 Pa and 0.06 V/Pa during 80–800 Pa, respectively, higher than most of the other self-powered tactile sensors. The different sensitivity under low pressure and high pressure is related to wrinkles, illustrated by a simulation using COMSOL software combined with theory analysis. Our tactile sensors show good application prospects in monitoring complicated human physiological signal and imitating human touch sensation.

Trampoline inspired stretchable triboelectric nanogenerators as tactile sensors for epidermal electronics

Abstract Recent advances in flexible electronics have brought great promotions to the next generation of wearable devices, like electronic skin (E-skin). Nevertheless, the powering issue still limits the size, weight and cost for E-skin. Here, trampoline inspired mechanics design and processing techniques in epidermal electronics are combined together to develop a thin, soft, stretchable self-powered tactile sensors based on triboelectric effect for E-skin. With the assistance of microstructure modifying by sandpapers, the TENG sensor exhibits great improved electrical performance and capable of distinguishing a broad range of pressure, with a great sensitivity of 0.367 mV Pa−1. The resulted TENG sensor exhibits excellent stretchability and sensing stability with accurate unchanged signal outputs even under a high-level strain up to 35%. Demonstrations of the sensors associating with the integration with a glove for human-machine interfaces and the development of a 4 × 4 tactile array for pressure mapping and recognition of contact objects, offers great opportunity for next generation self-powered E-skin.

Self-powered supercapacitor-mode tactile sensor based on polygonal litchi–like nanospheres decorated three-dimensional reduced graphene oxide aerogel for wearable electronics device

Rapidly growing improvement of portable and wearable devices stimulates requirements for multifunction tactile electronics. Herein, we report a novel electrochemical-driving supercapacitor-mode self-powered tactile sensor based on polygonal litchi–like nanospheres embedded 3D reduced graphene oxide aerogel framework, which displays outstanding compressibility to accommodate complex external conditions such as large-scale mechanical deformations. The sensor has the power to realize function of both strain compression sensing and energy storage capabilities simultaneously. The porosity of aerogels and pseudocapacitance property of polygonal litchi–like CoV2O6 nanospheres endows the composite aerogel with superior electrochemical performance with energy density of 139.18 Wh kg−1 at 500.0 W kg−1. Taking advantage of splendid elasticity of aerogel structure, self-powered supercapacitor-mode sensor response to external pressure signal sensitively. The self-powered sensor not only presents promising potentials in wireless charging system, but also expands the practical application areas of electrochemical energy storage device greatly.

High-performance Sb-doped p-ZnO NW films for self-powered piezoelectric strain sensors

Abstract Novel wide-bandgap piezoelectric semiconductor materials, with wurtzite structure, set a great impetus to research, which outreach the promising applications in optoelectronic devices, wearable electronic devices and medical monitoring systems. Here, we studied the piezotronic effect of p-ZnO NW films under strain which were suitable for self-powered tactile sensors. Typically, the doping level was 2 × 1017 cm−3 at the 2% Sb doping in solution. Based on different devices design, it was verified that the piezoelectric charges induced by applied strain could significantly affect the charge carrier separation and transport at the interface/junction. For p-ZnO NW thin film devices, the negative piezoelectric charge reduces Schottky barrier height, while the positive piezoelectric charge increases it. Self-powered strain sensors were then developed based on the changes of piezoelectric potential under different strain, which have a potential candidate for applications in man-machine interaction interface and biomedical sciences.

Self-Powered Tactile Sensor with Learning and Memory.

Fabrication of human-like intelligent tactile sensors is an intriguing challenge for developing human-machine interfaces. As inspired by somatosensory signal generation and neuroplasticity-based signal processing, intelligent neuromorphic tactile sensors with learning and memory based on the principle of a triboelectric nanogenerator are demonstrated. The tactile sensors can actively produce signals with various amplitudes on the basis of the history of pressure stimulations because of their capacity to mimic neuromorphic functions of synaptic potentiation and memory. The time over which these tactile sensors can retain the memorized information is alterable, enabling cascaded devices to have a multilevel forgetting process and to memorize a rich amount of information. Furthermore, smart fingers by using the tactile sensors are constructed to record a rich amount of information related to the fingers' current actions and previous actions. This intelligent active tactile sensor can be used as a functional element for artificial intelligence.

Flexible PDMS-based triboelectric nanogenerator for instantaneous force sensing and human joint movement monitoring

Flexible wearable sensors with excellent electric response and self-powered capability have become an appealing hotspot for personal healthcare and human–machine interfaces. Here, based on triboelectric nanogenerator (TENG), a flexible self-powered tactile sensor composed of micro-frustum-arrays-structured polydimethylsiloxane (PDMS) film/copper (Cu) electrodes, and poly(vinylidenefluoride–trifluoroethylene) (P(VDF-TrFE)) nanofibers has been demonstrated. The TENG-based self-powered tactile sensor can generate electrical signals through the contact-separation process of two triboelectric layers under external mechanical stimuli. Due to the uniform and controllable micro-frustum-arrays structure fabricated by micro-electro-mechanical system (MEMS) process and the P(VDF-TrFE) nanofibers fabricated by electrostatic spinning, the flexible PDMS-based sensor presents high sensitivity of 2.97 V kPa−1, stability of 40,000 cycles (no significant decay), response time of 60 ms at 1 Hz, low detection pressure of a water drop (~4 Pa, 35 mg) and good linearity of 0.99231 in low pressure region. Since the PDMS film presents ultra-flexibility and excellent-biocompatibility, the sensor can be comfortably attached on human body. Furthermore, the tactile sensor can recognize various types of human body movements by the corresponding electrical signals. Therefore, the as-prepared TENGs are potential on the prospects of gesture detection, health assessment, human-machine interfaces and so on.