Flexible Tactile Sensor Research Articles

Waterproof and Flexible Aquatic Tactile Sensor with Interlocked Ripple Structures for Broad Range Force Sensing

AbstractUnderwater perception of a broad range of force holds great importance in aquatic explorative activities, while flexible tactile sensors face technical challenges to realize. This paper presents a novel flexible aquatic tactile sensor based on waterproof graphene (GR)/carbon nanotube (CNT)/Polydimethylsiloxane (PDMS) composites. The prepared GR/CNT/PDMS composites possess excellent hydrophobic and electromechanical properties with a water contact angle over 134° and an ultrahigh gauge factor of 2296, making them an ideal piezoresistive sensing material for underwater broad‐range force sensing. The proposed tactile sensor has 3 × 3 sensing units and uses dual interlocked water‐ripple structures to improve its sensitivity and force detection range. The fabricated tactile sensor is characterized by two distinct sensitivities: a high sensitivity of 0.0338 kPa−1 at 0.062–150 kPa and a low sensitivity of 0.00357 kPa−1 at 150–450 kPa. Further, the sensor exhibited excellent resistance response, fast dynamic recovery, and both mechanical and electrical stability in aquatic environments. Then, the aquatic tactile sensor is worn on the palm of the human hand to detect the distribution and variation of contact forces when grasping objects with different shapes and hardness, demonstrating the potential underwater applications of the developed aquatic tactile sensor for broad‐range force sensing.

Electrospun Cellulose Nanocrystals Reinforced Flexible Sensing Paper for Triboelectric Energy Harvesting and Dynamic Self-Powered Tactile Perception.

The technical synergy between flexible sensing paper and triboelectric nanogenerator (TENG) in the next stage of artificial intelligence Internet of Things engineering makes the development of intelligent sensing paper with triboelectric function very attractive. Therefore, it is extremely urgent to explore functional papers that are more suitable for triboelectric sensing. Here, a cellulose nanocrystals (CNCs) reinforced PVDF hybrid paper (CPHP) is developed by electrospinning technology. Benefitting from the unique effects of CNCs, CPHP forms a solid cross-linked network among fibers and obtains a high-strength (25MPa) paper-like state and high surface roughness. Meanwhile, CNCs also improve the triboelectrification effect of CPHP by assisting the PVDF matrix to form more electroactive phases (96% share) and a higher relative permittivity (17.9). The CPHP-based TENG with single electrode configuration demonstrates good output performance (open-circuit voltage of 116V, short-circuit current of 2.2µA and power density of 91mWm-2) and ultrahigh pressure-sensitivity response (3.95mVPa-1), which endows CPHP with reliable power supply and sensing capability. More importantly, the CPHP-based flexible self-powered tactile sensor with TENG array exhibits multifunctional applications in imitation Morse code compilation, tactile track recognition, and game character control, showing great prospects in the intelligent inductive device and human-machine interaction.

Normal contact mechanism of flexible film/substrate bilayer structure: Experimental and numerical insight

The film/substrate bilayer structure design plays a crucial role in enhancing the adaptability of flexible tactile sensors in complex environments. Nonetheless, as the cornerstone for machine tactility, the normal contact mechanism of the film/substrate bilayer structure remains elusive. Herein, the normal contact characteristics of polydimethylsiloxane film/sponge bilayer structure (PF/SBS) with different structural parameters, including substrate porosity, film mass ratio, and relative thickness between film and substrate, are investigated by experimental and numerical methods. The roles of the film and substrate structure parameters of PF/SBS in the normal contact process are analyzed by a cohesive contact model from the energy perspective. The results demonstrate that PF/SBS not only modulates the maximum normal force by substrate porosity but also exerts a substantial impact on the critical contact state by the film mass ratios. The contribution of film thickness to the critical separation state can be enhanced up to 90% with increasing relative thickness.

Fully 3D printed flexible, conformal and multi-directional tactile sensor with integrated biomimetic and auxetic structure

Tactile sensors play a crucial role in the development of biologically inspired robotic prostheses, particularly in providing tactile feedback. However, existing sensing technology still falls short in terms of sensitivity under high pressure and adaptability to uneven working surfaces. Furthermore, the fabrication of tactile sensors often requires complex and expensive manufacturing processes, limiting their widespread application. Here we develop a conformal tactile sensor with improved sensing performance fabricated using an in-house 3D printing system. Our sensor detects shear stimuli through the integration of an auxetic structure and interlocking features. The design enables an extended sensing range (from 0.1 to 0.26 MPa) and provides sensitivity in both normal and shear directions, with values of 0.63 KPa−1 and 0.92 N−1, respectively. Additionally, the sensor is capable of detecting temperature variations within the range of 40−90 °C. To showcase the feasibility of our approach, we have printed the tactile sensor directly onto the fingertip of an anthropomorphic robotic hand, the proximal femur head, and lumbar vertebra. The results demonstrate the potential for achieving sensorimotor control and temperature sensing in artificial upper limbs, and allowing the monitoring of bone-on-bone load.

Review: Progress on 3D printing technology in the preparation of flexible tactile sensors

Review: Progress on 3D printing technology in the preparation of flexible tactile sensors

Nonlinear Response Analysis of a Polymer-Based Piezoresistive Flexible Tactile Sensor at Low Pressure

Nonlinear Response Analysis of a Polymer-Based Piezoresistive Flexible Tactile Sensor at Low Pressure

Flexible, Breathable, and Hydrophobic Iontronic Tactile Sensors Based on a Nonwoven Fabric Platform for Permeable and Waterproof Wearable Sensing Applications

Flexible, Breathable, and Hydrophobic Iontronic Tactile Sensors Based on a Nonwoven Fabric Platform for Permeable and Waterproof Wearable Sensing Applications

Capacitive flexible haptic sensor based on micro-cylindrical structure dielectric layer and its decoupling study

With the advancement of robotics, flexible sensors have undergone significant development. Extensive research has been conducted on the sensor structure to enhance its sensing capabilities. However, due to the absence of a definitive theory on the relationship between input force and output signal, and the unclear mechanism of force and deformation, decoupling the force remains challenging. To address this, we propose a capacitive flexible tactile sensor that utilizes a dielectric layer with a microcylindrical structure for detecting external loads. The micro-cylindrical structure of the dielectric layer enhances the strain capacity, thereby improving the sensor's performance. We also present a decoupling model based on the deformation mechanism by analyzing the sensor coupling. Furthermore, we validate the sensor's application in tangential detection and object grasping by integrating it into a soft rehabilitation glove and a soft gripper. This research lays the foundation for future self-aware operations.

Flexible and breathable iontronic tactile sensor with personal thermal management ability for a comfortable skin-attached sensing application

Wearable sensors are tightly attached on skin of different body parts for physiological monitoring and motion detection. In addition to sensing performance and flexible formation, maintaining of the normal mass microexchange between the covered skin and external environment with appropriate thermal dissipating ability for a comfortable wearing is highly important. Herein we report the design and fabrication of a new type of flexible and breathable tactile sensor with unique personal thermal management ability. The sensor adopts a supercapacitive iontronic sensing structure with microstructures by sandwiching one porous polyurethane/ionic liquid sponge with two electrospun thermoplastic polyurethane/MXene nanofiber electrodes for an enhanced sensitivity (∼105.77 kPa−1). In addition, the intrinsic elastic nature of polymers endows the whole device with excellent mechanical flexibility and the fabric and porous internal open-ended structure of substrates provide well air and moisture permeability (∼48 mm s−1) to maintain a normal thermal dissipating ability (∼1 ℃ lower than that covered by airtight film) without causing skin redness or inflammation. Furthermore, highly solar-reflective (∼0.98) silica microparticles are incorporated into the emissive (∼0.94) thermoplastic polyurethane nanofibers to achieve an effective passive cooling effect in the outdoor scene (∼ 3 ℃ lower than that covered by fabric clothes). The developed flexible and breathable tactile sensor as well as its design and fabrication strategy provide a promising route to develop advanced wearable electronics with unique personal thermal management ability for a comfort skin-attached sensing application.

Flexible Tactile Sensors Using AlN and MOSFETs Based Ultra-Thin Chips

This paper presents ultrathin chips (UTCs) based flexible tactile sensing system for dynamic contact pressure measurement. The device comprises of an AlN piezocapacitor based UTCs tightly coupled with another UTCs having metal-oxide-semiconductor field-effect transistors (MOSFETs). In this arrangement the AlN piezocapacitor forms the extended gate of MOSFETs. Both AlN piezocapacitor and MOSFET based UTCs are obtained by post-process reduction of wafer thicknesses to ~35μm using backside lapping. The performances of both UTCs were evaluated both before and after thinning and there was no noticeable performance degradation. The UTC-based AlN piezocapacitor exhibited six times higher sensitivity (43.79mV/N) than the thin filmbased AlN sensors. When coupled with MOSFETs based UTC, the observed sensitivity was 0.43N-1. The excellent performance, flexible form factor and compactness shows the potential of presented device in applications such minimal invasive surgical instruments where high-resolution tactile feedback is much needed.

Mechanically robust, flexible hybrid tactile sensor with microstructured sensitive composites for human-cyber-physical systems

Advances in human-cyber-physical system (HCPS) requires multiple and multimodal sensory network to address the challenge of human-machine relationships, posing substantial potential for human health monitoring and human-centric smart manufacturing. However, combining electrical sensitivity and mechanical robustness of flexible sensors with simple device structure is still challenging. This work presents a flexible porous bimodal pressure sensor (PBPS) by fabricating 3D microstructured sensitive porous carbon-nanotubes composites (PCNC). The multi-layered PBPS can offer the ability to switch between piezoresistive and piezocapacitive sensing modes without altering the device structure, enhancing mechanical stability of device under loading/unloading cycles. This versatility allows for the detection of hardness discrimination, grabbing sensing, and dynamic human physiological signal recognition, which possess superior pressure sensing performances with a high sensitivity (0.1059 kPa−1 in piezoresistive mode, 0.2054 kPa−1 in capacitive mode), wide measuring range (0–25 kPa), rapid response time (<130 ms in piezoresistive mode, <55 ms in piezocapacitive mode), and long-term durability (>3000 cycles). Furthermore, we proposed a mechanical constitutive model to elucidate the sensing mechanism of microstructured PCNC by 3D micro-CT based modeling and experimental data. Finally, we employed multiple PBPSs as sensing elements to real-time monitor 3D printing manufacturing stress, structure assembling stress, and human physiology signals in HCPS, indicating the potential applications of PBPS ranging from monitoring human health to facilitating intelligent manufacturing.

Flexible tactile sensor with an embedded-hair-in-elastomer structure for normal and shear stress sensing

Endowing robots with multi-directional tactile sensing capabilities has long been a challenging task in the field of flexible electronics and intelligent robots. This paper reports a highly sensitive, flexible tactile sensor with an embedded-hair-in-elastomer structure, which is capable of decoupling normal stress and shear stress. The flexible tactile sensor is fabricated on a thin polyimide substrate and consists of four self-bending piezoresistive cantilevers in a cross-shaped configuration, which are embedded in an elastomer. The sensor can decouple the tactile information into a normal stress and a shear stress with simple summation and differencing algorithms, and the measurement error is kept within 3%. Moreover, the sensitivity and detection threshold of the sensor can be adjusted by simply changing the elastic material. As a demonstration, the flexible tactile sensor is integrated into a robotic manipulator to precisely estimate the weight of the grasped objects, which shows great potential for application in robotic systems.

Templated Laser-Induced-Graphene-Based Tactile Sensors Enable Wearable Health Monitoring and Texture Recognition via Deep Neural Network.

Flexible tactile sensors show great potential for portable healthcare and environmental monitoring applications. However, challenges persist in scaling up the manufacturing of stable tactile sensors with real-time feedback. This work demonstrates a robust approach to fabricating templated laser-induced graphene (TLIG)-based tactile sensors via laser scribing, elastomer hot-pressing transfer, and 3D printing of the Ag electrode. With different mesh sandpapers as templates, TLIG sensors with adjustable sensing properties were achieved. The tactile sensor obtains excellent sensitivity (52260.2 kPa-1 at a range of 0-7 kPa), a broad detection range (up to 1000 kPa), a low limit of detection (65 Pa), a rapid response (response/recovery time of 12/46 ms), and excellent working stability (10000 cycles). Benefiting from TLIG's high performance and waterproofness, TLIG sensors can be used as health monitors and even in underwater scenarios. TLIG sensors can also be integrated into arrays acting as receptors of the soft robotic gripper. Furthermore, a deep neural network based on the convolutional neural network was employed for texture recognition via a soft TLIG tactile sensing array, achieving an overall classification rate of 94.51% on objects with varying surface roughness, thus offering high accuracy in real-time practical scenarios.

Progress and prospects in flexible tactile sensors.

Flexible tactile sensors have the advantages of large deformation detection, high fault tolerance, and excellent conformability, which enable conformal integration onto the complex surface of human skin for long-term bio-signal monitoring. The breakthrough of flexible tactile sensors rather than conventional tactile sensors greatly expanded application scenarios. Flexible tactile sensors are applied in fields including not only intelligent wearable devices for gaming but also electronic skins, disease diagnosis devices, health monitoring devices, intelligent neck pillows, and intelligent massage devices in the medical field; intelligent bracelets and metaverse gloves in the consumer field; as well as even brain-computer interfaces. Therefore, it is necessary to provide an overview of the current technological level and future development of flexible tactile sensors to ease and expedite their deployment and to make the critical transition from the laboratory to the market. This paper discusses the materials and preparation technologies of flexible tactile sensors, summarizing various applications in human signal monitoring, robotic tactile sensing, and human-machine interaction. Finally, the current challenges on flexible tactile sensors are also briefly discussed, providing some prospects for future directions.

Machine Learning-Enabled Tactile Sensor Design for Dynamic Touch Decoding.

Skin-like flexible sensors play vital roles in healthcare and human-machine interactions. However, general goals focus on pursuing intrinsic static and dynamic performance of skin-like sensors themselves accompanied with diverse trial-and-error attempts. Such a forward strategy almost isolates the design of sensors from resulting applications. Here, a machine learning (ML)-guided design of flexible tactile sensor system is reported, enabling a high classification accuracy (≈99.58%) of tactile perception in six dynamic touch modalities. Different from the intuition-driven sensor design, such ML-guided performance optimization is realized by introducing a support vector machine-based ML algorithm along with specific statistical criteria for fabrication parameters selection to excavate features deeply concealed in raw sensing data. This inverse design merges the statistical learning criteria into the design phase of sensing hardware, bridging the gap between the device structures and algorithms. Using the optimized tactile sensor, the high-quality recognizable signals in handwriting applications are obtained. Besides, with the additional data processing, a robot hand assembled with the sensor is able to complete real-time touch-decoding of an 11-digit braille phone number with high accuracy.

Design and tactile classification of flexible tactile sensor for soft gripper

Tactile object recognition is very important in robot sorting and handling system. The existing researches on tactile recognition mainly focus on rigid grippers, which cannot be directly applied to soft grippers with better safety and applicability. In this paper, a tactile sensor for soft gripper is proposed. The tactile sensor consists of a carbon composite Velostat and a flexible printed circuit designed for soft gripper. The sensor is equipped with signal scanning, amplifying and processing systems. A number of performance tests were carried out on the designed tactile sensor to prove that it has good performance in sensing sensitivity, loading stability, repeatability. In addition, tactile sensors are applied to tactile classification. The two fingers of the flexible gripper were loaded with a sensor and nine different objects were contacted. The tactile information of the objects was obtained through the stable gripper, and the tactile data was trained and classified by the support vector machine. Finally, the tactile recognition accuracy of the double-exponent data was 91.1%.

An Aerial–Aquatic Hitchhiking Robot with Remora‐Inspired Tactile Sensors and Thrust Vectoring Units

Hybrid aerial–aquatic robots can operate in both air and water and cross between these two. They can be applied to amphibious observation, maritime search and rescue, and cross‐domain environmental monitoring. Herein, an aerial–aquatic hitchhiking robot is proposed that can fly, swim, and rapidly cross the air–water boundaries (0.16 s) and autonomously attach to surfaces in both air and water. Inspired by the mechanoreceptors of the remora (Echeneis naucrates) disc, the robot's hitchhiking device is equipped with two flexible bioinspired tactile sensors (FBTS) based on a triboelectric nanogenerator for tactile sensing of attachment status. Based on tactile sensing, the robot can perform reattachment after leakage or adhesion failure, enabling it to achieve long‐term adhesion on complex surfaces. The rotor‐based aerial–aquatic robot, which has two thrust vectoring units for underwater locomotion, can maneuver to pitch, yaw, and roll 360° and control precision motion position. The field tests show that the robot can continuously cross the air–water boundary, attach to the rough stone surface, and record video in both air and underwater. This study may shed light on future autonomous robots capable of intelligent navigation, adhesion, and operation in complex aerial–aquatic environments.

Wearable Capacitive Tactile Sensor Based on Porous Dielectric Composite of Polyurethane and Silver Nanowire.

In recent years, the implementation of wearable and biocompatible tactile sensing elements with sufficient response into healthcare, medical detection, and electronic skin/amputee prosthetics has been an intriguing but challenging quest. Here, we propose a flexible all-polyurethane capacitive tactile sensor that utilizes a salt crystal-templated porous elastomeric framework filling with silver nanowire as the composite dielectric material, sandwiched by a set of polyurethane films covering silver nanowire networks as electrodes. With the aids of these cubic air pores and conducting nanowires, the fabricated capacitive tactile sensor provides pronounced enhancement of both sensor compressibility and effective relative dielectric permittivity across a broad pressure regime (from a few Pa to tens of thousands of Pa). The fabricated silver nanowire-porous polyurethane sensor presents a sensitivity improvement of up to 4-60 times as compared to a flat polyurethane device. An ultrasmall external stimulus as light as 3 mg, equivalent to an applied pressure of ∼0.3 Pa, can also be clearly recognized. Our all-polyurethane capacitive tactile sensor based on a porous dielectric framework hybrid with conducting nanowire reveals versatile potential applications in physiological activity detection, arterial pulse monitoring, and spatial pressure distribution, paving the way for wearable electronics and artificial skin.

High-performance flexible tactile sensor enabled by multi-contact mechanism for normal and shear force measurement

Flexible tactile sensors are essential components of pressure monitoring in the fields of electronic skin, health care, and robotic hands. Currently, there is a great demand for tactile sensors that exhibit high sensitivity, linearity, and a wide dynamic range, as they play an increasingly vital role in information exchange. However, it is still challenging to achieve a synchronous improvement in performance through a simple strategy. Theoretically, we present the multi-contact mechanism for sensing enhancement by changing the contact mode and the initial state. Specifically, the curved polydimethylsiloxane (PDMS)/multi-walled carbon nanotubes (MWCNTs) surface ensures the continuity of contact deformation, leading to a wide dynamic range. Besides, the discrete resistor pillars on the micro-honeycomb electrodes (MHEs) depress the initial current, and thus enhance the sensitivity. Experimentally, we utilize microelectromechanical systems (MEMS) technology to fabricate the MHEs, mainly including patterning and micro-electroforming processes. By adjusting the resistor distribution density and the curvature of PDMS contact, the extraordinary sensitivity is tuned from 25.88 kPa−1 to 64.68 kPa−1, and the maximum detection pressure switches between 500 kPa and 1400 kPa, which is consistent with the physical model. Furthermore, a proof-of-concept of the flexible three-axis tactile sensor demonstrates the possibility of realizing normal and force measurement. These results reveal the great application prospect of tactile sensors based on multi-contact mechanism in future wearable electronics.

Fully sprayed MXene-based high-performance flexible piezoresistive sensor for image recognition

High-performance flexible pressure sensors provide comprehensive tactile perception and are applied in human activity monitoring, soft robotics, medical treatment, and human-computer interface. However, these flexible pressure sensors require extensive nano-architectural design and complicated manufacturing and are time-consuming. Herein, a highly sensitive, flexible piezoresistive tactile sensor is designed and fabricated, consisting of three main parts: the randomly distributed microstructure on T-ZnOw/PDMS film as a top substrate, multilayer Ti3C2-MXene film as an intermediate conductive filler, and the few-layer Ti3C2-MXene nanosheet-based interdigital electrodes as the bottom substrate. The MXene-based piezoresistive sensor with randomly distributed microstructure exhibits a high sensitivity over a broad pressure range (less than 10 ​kPa for 175 ​kPa−1) and possesses an out-standing permanence of up to 5000 cycles. Moreover, a 16-pixel sensor array is designed, and its potential applications in visualizing pressure distribution and an example of tactile feedback are demonstrated. This fully sprayed MXene-based pressure sensor, with high sensitivity and excellent durability, can be widely used in, electronic skin, intelligent robots, and many other emerging technologies.