Piezoresistive Tactile Sensor Research Articles

PVA/PANI-DBSA Nanomesh Tactile Sensor for Force Feedback

Touch serves as an important medium for human–environment interaction. The piezoresistive tactile sensor has attracted much attention due to its convenient technology, simple principle, and convenient signal acquisition and analysis. In this paper, conductive beads-on-string polyvinyl alcohol (PVA)/polyaniline doped with dodecyl benzene sulfonic acid (PANI-DBSA) nanofibers were fabricated via the electrospinning technique. Due to the special nanostructure of PVA-coated PANI-DBSA, the tactile sensor presented a wide measuring range of 12 Pa–121 kPa and appreciable sensitivity of 8.576 kPa−1 at 12 Pa~484 Pa. In addition, the response time and recovery time of the sensor were approximately 500 ms, demonstrating promising prospects in the field of tactile sensing for active upper limb prostheses.

Research on multiwalled carbon nanotubes spray method for preparing flexible piezoresistive tactile sensor

AbstractFlexible piezoresistive tactile sensor (FPTS) is widely used in wearable devices, e‐skin, and so on. However, the performance consistency and durability hinder its widespread application at a larger scale. It is still a challenge to prepare conductive composites with high performance consistency and high durability. In this research, a multiwalled carbon nanotubes (MWCNTs)/polydimethylsiloxane FPTS with high performance consistency and high durability was prepared by a combination of spray and spin coating method. The spray parameters of MWCNTs solution were optimized by numerical simulation, and a self‐developed high pressure spray device was adopted to realize the high‐pressure spray. In the 6000 tensile cycles experiments, the fluctuation of FPTS resistance is less than 5%. The range of prepared FPTS can reach 1.2 kPa, and the sensitivity is 0.8 kPa−1. The results of mouse click and wrist bending tests confirmed that the prepared FPTS can satisfy the detection requirements of wearable devices.

Recent Advances in Tactile Sensory Systems: Mechanisms, Fabrication, and Applications.

Flexible electronics is a cutting-edge field that has paved the way for artificial tactile systems that mimic biological functions of sensing mechanical stimuli. These systems have an immense potential to enhance human-machine interactions (HMIs). However, tactile sensing still faces formidable challenges in delivering precise and nuanced feedback, such as achieving a high sensitivity to emulate human touch, coping with environmental variability, and devising algorithms that can effectively interpret tactile data for meaningful interactions in diverse contexts. In this review, we summarize the recent advances of tactile sensory systems, such as piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. We also review the state-of-the-art fabrication techniques for artificial tactile sensors. Next, we focus on the potential applications of HMIs, such as intelligent robotics, wearable devices, prosthetics, and medical healthcare. Finally, we conclude with the challenges and future development trends of tactile sensors.

Enhanced braille recognition based on piezoresistive and piezoelectric dual-mode tactile sensors

Human Tactile perception can be divided into pressure perception and slip perception according to the mode of contact. At present, however, tactile sensors can not have the same good performance as human pressure and slip perception. In this paper, a piezoresistive and piezoelectric dual-mode tactile sensing scheme is proposed based on human finger tactile skin structure. PVDF polymer is used as the sensitive material in piezoelectric layer, which has good dynamic response and high sensitivity. Graphene and carbon black are used as the sensitive material in piezoresistive layer, the array process is simple and high resolution can be achieved. The fast response of piezoelectric signal can be used to segment the piezoresistive signal and improve the recognition accuracy when the tactile sensor is used for slip sensing. At the same time, the piezoelectric layer can also reflect the velocity information and direction information. In recognition method, according to the feature that the continuous tactile signal has both temporal and spatial features in the process of sliding, CNN-LSTM is proposed in this paper. The tactile sensors is integrated into the robot arm to detect 20 kinds of braille. At the same speed and the same force, the recognition rate of bimodal braille recognition is 90.58%. In addition, the model can be well recognized at different speeds and sliding directions, and the recognition rate is 84.2%. Finally, the tactile sensor is wore on the index finger of human hand, the average recognition rate of touch braille is 77.5%, which shows that the algorithm has good robustness, and the speed of recognition is twice as fast as that of the normal blind. In the future, it can be applied to blind reading, surface detection, texture identification and other fields.

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

Design, Fabrication, and Characterization of Inkjet-Printed Organic Piezoresistive Tactile Sensor on Flexible Substrate.

In this paper, we propose a novel tactile sensor with a "fingerprint" design, named due to its spiral shape and dimensions of 3.80 mm × 3.80 mm. The sensor is duplicated in a four-by-four array containing 16 tactile sensors to form a "SkinCell" pad of approximately 45 mm by 29 mm. The SkinCell was fabricated using a custom-built microfabrication platform called the NeXus which contains additive deposition tools and several robotic systems. We used the NeXus' six-degrees-of-freedom robotic platform with two different inkjet printers to deposit a conductive silver ink sensor electrode as well as the organic piezoresistive polymer PEDOT:PSS-Poly (3,4-ethylene dioxythiophene)-poly(styrene sulfonate) of our tactile sensor. Printing deposition profiles of 100-micron- and 250-micron-thick layers were measured using microscopy. The resulting structure was sintered in an oven and laminated. The lamination consisted of two different sensor sheets placed back-to-back to create a half-Wheatstone-bridge configuration, doubling the sensitivity and accomplishing temperature compensation. The resulting sensor array was then sandwiched between two layers of silicone elastomer that had protrusions and inner cavities to concentrate stresses and strains and increase the detection resolution. Furthermore, the tactile sensor was characterized under static and dynamic force loading. Over 180,000 cycles of indentation were conducted to establish its durability and repeatability. The results demonstrate that the SkinCell has an average spatial resolution of 0.827 mm, an average sensitivity of 0.328 mΩ/Ω/N, expressed as the change in resistance per force in Newtons, an average sensitivity of 1.795 µV/N at a loading pressure of 2.365 PSI, and a dynamic response time constant of 63 ms which make it suitable for both large area skins and fingertip human-robot interaction applications.

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.

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.

Piezoresistive Composites in Tension and Compression Using a Cantilevered Beam for Spot Testing and Tactile Sensing

Abstract Piezoresistive soft composite materials exhibit a change in resistance when undergoing deformation. This combined with their optical, thermal, and mechanical properties makes these composites good candidates for force sensors. Tactile force sensors have long been studied for applications in healthcare, robot–human interactions, and displacement monitoring. The main goal in this work is to characterize a soft piezoresistive layer in both tension and compression to enable a model system for a piezoresistive tactile force sensor and a characterization platform. However, the mechanisms by which these composites exhibit piezoresistivity are complex and must be characterized before use not only in bulk but at the exact locations where contact is expected. In this paper, a cantilevered beam is proposed as a base-mounted force-sensing mechanism. This mechanism allows for characterization of the composites at multiple locations across the sample using a two-probe technique. Multiwalled carbon nanotubes (MWCNTs) are mixed by weight with a soft polyurethane in 15, 16, and 17 wt. % concentrations. Because the elastic modulus of the piezoresistive layer is not known, indentation tests using Hertz theory and numerical calculations are used to simulate the effective elastic modulus and average strain. These results are then compared with the experimental stress results. In general, these tests show a greater sensitivity in tension than in compression. However, the difference lessens as the concentration increases. A linear fit is applied to the ΔR/R versus strain graphs to calculate the gauge factors. Each sensor exhibits a positive and negative gauge factor over two different ranges. ΔR/R versus strain graphs for tension and compression show gauge factors between −19 and 24 with the range decreasing with increasing MWCNT percentage.

Recent Progress in Advanced Tactile Sensing Technologies for Soft Grippers

AbstractTactile sensing technology is crucial for soft grippers. Soft grippers equipped with intelligent tactile sensing systems based on various sensors can interact safely with the unstructured environments and obtain precise properties of objects (e.g., size and shape). It is essential to develop state‐of‐the‐art sensing technologies for soft grippers to handle different grasping tasks. In this review, the development of tactile sensing techniques for robotic hands is first introduced. Then, the principles and structures of different types of sensors normally adopted in soft grippers, including capacitive tactile sensors, piezoresistive tactile sensors, piezoelectric tactile sensors, fiber Bragg grating (FBG) sensors, vision‐based tactile sensors, triboelectric tactile sensors, and other advanced sensors developed recently are briefly presented. Furthermore, sensing modalities and methodologies for soft grippers are also described in aspects of force measurement, perception of object properties, slip detection, and fusion of perception. The application scenarios of soft grippers are also summarized based on these advanced sensing technologies. Finally, the challenges of tactile sensing technologies for soft grippers that need to be tackled are discussed and perspectives in addressing these challenges are pointed out.

Development of flexible tactile sensing arrays for hardness recognition

Hardness sensation is an essential function of human skin, which is significant for enhancing the objects perception ability of intelligent manipulators in various scenarios. Although progress has been made in mimicking this function of human hands, the hardness identification still remains a great challenge. In this study, inspired by the Hertz model and the static pressure response of Merkel cells (SA-I) mechanoreceptors, we develop a membrane-like piezoresistive flexible tactile sensor array (PFTSA) that consists of a flexible electrode array layer and a sensitive material layer with a micropyramidal structure, which can effectively identify the hardness values of different objects. The sensor can be a part of the manipulator’s system due to its conformal ability and large deformation, which shows high pressure response sensitivity (4.432 mA/N in the 0–1 N pressure range) and good cycling stability (2100 cycles at 5 N), enabling the consistent and precise detection of external information. As a proof-of-concept illustration, we integrate the PFTSA on a finger of the manipulator to discriminate six objects with different hardness values. The results demonstrate the great potential of PFTSA in the application of human-machine interactions, paving the way for the development of robotic diagnostics, mechanical sorting operations and virtual reality technology.

Investigating the Electromechanical Sensitivity of Carbon-Nanotube-Coated Microfibers.

The piezoresistance of carbon nanotube (CNT)-coated microfibers is examined using diametric compression. Diverse CNT forest morphologies were studied by changing the CNT length, diameter, and areal density via synthesis time and fiber surface treatment prior to CNT synthesis. Large-diameter (30-60 nm) and relatively low-density CNTs were synthesized on as-received glass fibers. Small-diameter (5-30 nm) and-high density CNTs were synthesized on glass fibers coated with 10 nm of alumina. The CNT length was controlled by adjusting synthesis time. Electromechanical compression was performed by measuring the electrical resistance in the axial direction during diametric compression. Gauge factors exceeding three were measured for small-diameter (<25 μm) coated fibers, corresponding to as much as 35% resistance change per micrometer of compression. The gauge factor for high-density, small-diameter CNT forests was generally greater than those for low-density, large-diameter forests. A finite element simulation shows that the piezoresistive response originates from both the contact resistance and intrinsic resistance of the forest itself. The change in contact and intrinsic resistance are balanced for relatively short CNT forests, while the response is dominated by CNT electrode contact resistance for taller CNT forests. These results are expected to guide the design of piezoresistive flow and tactile sensors.

Wide Range Strain Distributions on the Electrode for Highly Sensitive Flexible Tactile Sensor with Low Hysteresis.

Flexible piezoresistive tactile sensors are widely used in wearable electronic devices because of their ability to detect mechanical stimuli. However, achieving high sensitivity and low hysteresis over a broad detection range remains a challenge with current piezoresistive tactile sensors. To address these obstacles, we designed elastomeric micropyramid arrays with different heights to redistribute the strain on the electrode. Furthermore, we mixed single-walled carbon nanotubes in the elastomeric micropyramids to compensate for the conductivity loss caused by random cracks in the gold film and increase the adhesion strength between the gold film (deposited on the pyramid surface) and the elastomer. Thus, the energy loss of the sensor during deformation and hysteresis (∼2.52%) was effectively reduced. Therefore, under the synactic effects of the percolation effect, tunnel effect, and multistage strain distribution, the as-prepared sensor exhibited a high sensitivity (1.28 × 106 kPa-1) and a broad detection range (4.51-54837.06 Pa). The sensitivity was considerably higher than those of most flexible pressure sensors with a microstructure design. As a proof of concept, the sensors were successfully applied in the fields of health monitoring and human-machine interaction.

Cicada-Wing-Inspired Highly Sensitive Tactile Sensors Based on Elastic Carbon Foam with Nanotextured Surfaces.

Electronic devices with tactile and pressure-sensing capabilities are becoming increasingly popular in the automatic industry, human motion/health monitoring, and artificial intelligence applications. Inspired by the natural nanotopography of the cicada wing, we propose here a straightforward strategy to fabricate a highly sensitive tactile sensor through nanotexturing of erected polyaniline (PANI) nanoneedles on a conductive and elastic three-dimensional (3D) carbon skeleton. The robust and compressible carbon networks offer a resilient and conducting matrix to catering complex scenarios; the biomimetic PANI nanoneedles firmly and densely anchored on a 3D carbon skeleton provide intimate electrical contact under subtle deformation. As a result, a piezoresistive tactile sensor with ultrahigh sensitivity (33.52 kPa-1), fast response/recovery abilities (97/111 ms), and reproducible sensing performance (2500 cycles) is developed, which is capable of distinguishing motions in a wide pressure range from 4.66 Pa to 60 kPa, detecting spatial pressure distribution, and monitoring various gestures in a wireless manner. These excellent performances demonstrate the great potential of nature-inspired tactile sensors for practical human motion monitoring and artificial intelligence applications.

Selection of optimal hyperparameter for detecting multiple contacts from large-area tactile sensors based on electrical impedance tomography

Large-area tactile sensors based on the technique of electrical impedance tomography (EIT) has drawn considerable interest in human-robot interactions. However, due to the ill-posed condition, it is challenging to differentiate between the real contacts and the artifacts from the reconstructed image. To address this issue, a new method to select an optimal hyperparameter that tunes the amount of regularization is developed in the context of tactile sensing. The optimal hyperparameter is determined to be the minimum value to obtain a stabilized number of sub-regions in the reconstructed image. The proposed method not only guarantees a correct detection on the number of multiple contacts at the minimum amount of regularization, but also provides a proper range of hyperparameters. The optimal hyperparameter is found in a chair-shape relation with the boundary signal-to-noise ratio (SNR), by varying the noise level of the hardware in simulation. The optimal hyperparameter decreases significantly when the boundary SNR increases between 5 ∼ 10 dB and 25 ∼ 35 dB, and keeps almost unchanged when SNR is between 10 ∼ 25 dB. The chair-shape relation also holds for contact conditions with varied intensities and sizes. Experimental validations on the proposed method are conducted on a compliant piezoresistive tactile sensor made of exfoliated graphite polymer composites. By varying the number of contacts in experiments, the relation between the optimal hyperparameter and the boundary SNR is consistent with the chair-shape curve. The investigation made in this work helps improve the performance of identifying multiple contacts from tactile sensors based on electrical impedance tomography.

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.

A Smart Tactile Sensing System Based on Carbon Nanotube/Polypropylene Composites for Wearable Applications

Recent advances in nanomaterials render the possibility of fabricating tactile sensors suitable for electronic skin, haptic interfaces, and biomedical applications. Furthermore, the problem of performing complex inertial measurements generated during the tactile process of polymer composites can be tackled using artificial intelligence (AI). However, the implementation of AI-based signal processing on embedded devices still represents an open area of opportunity for the design of the next generation of smart wearable sensor systems. A novel nanostructured smart tactile sensing system for wearable applications is proposed, using a 3-D-printed structure with embedded electronic devices. The highly sensitive piezoresistive tactile sensor is based on multiwall carbon nanotube/polypropylene (MWCNT/PP) composites. The integration of electronic circuits for signal processing of an artificial neural network (NN) on a digital controller unit improves the tactile interpretation in the wearable embedded device. Experiments show that the pressure classification results on MWCNT/PP composites with 98% accuracy.

Black phosphorous-based human-machine communication interface

Assistive technology involving auditory feedback is generally utilized by those who are visually impaired or have speech and language difficulties. Therefore, here we concentrate on an auditory human-machine interface that uses audio as a platform for conveying information between visually or speech-disabled users and society. We develop a piezoresistive tactile sensor based on a black phosphorous and polyaniline (BP@PANI) composite by the facile chemical oxidative polymerization of aniline on cotton fabric. Taking advantage of BP’s puckered honeycomb lattice structure and superior electrical properties as well as the vast wavy fabric surface, this BP@PANI-based tactile sensor exhibits excellent sensitivity, low-pressure sensitivity, reasonable response time, and good cycle stability. For a real-world application, a prototype device employs six BP@PANI tactile sensors that correspond to braille characters and can convert pressed text into audio on reading or typing to assist visually or speech-disabled persons. Overall, this research offers promising insight into the material candidates and strategies for the development of auditory feedback devices based on layered and 2D materials for human-machine interfaces.

Rationally designed cellulose hydrogel for an ultrasensitive pressure sensor.

Flexible pressure sensors with high sensitivity are required in fields such as human-machine interactions, electronic skin, and health tracking. In this work, we reported cellulose ion-conductive hydrogel (ICH) rationally designed from both nano and micron perspectives for ultrasensitive pressure sensors, via a zero-waste approach, without involving soft components. By introducing low molecular weight cellulose and using the idea of a rough surface, the piezocapacitive sensitivity of the ICH was increased from 0.04 kPa-1 to 89.81 kPa-1 in increments of 2245, which also has a high degree of transparency, excellent durability, and good electrical transmission. Moreover, the ICH demonstrated great potential as sensors and arrays practicable in various industries, including medical treatment and motion recognition. The design is also applicable for piezoresistive tactile sensors, which realize enhanced sensitivity. This affordable, effective, and environmentally friendly technology definitely offers novel perspectives and the potential to enhance the functionality of flexible pressure sensors.

Tactile Pressure Sensors Calibration with the Use of High Pressure Zones.

A simple and cost-effective calibration procedure for piezoresistive ink tactile pressure sensors is crucial for their use in geotechnical research applications. Such a procedure should be applicable in field conditions and require a minimum amount of equipment. The paper describes a new method for calibrating tactile pressure sensors with 8-bit sensels’ output. The method is based on the approximation of a single sensel output and consideration of multiple calibration patches. The advantage of the developed method is using local high-pressure zones in calibration patches. The developed method has been successfully applied in calibration of two 5051-350 Tekscan sensors by means of three dead weights: 2 kg, 5 kg and 10 kg. One calibrated sensor was new, and another one had been previously used in the harsh environment of the ice tank in the experiment with model ice. The calibration curves for these two sensors did not reveal a significant difference. For 72% of the 150 obtained load patches in calibration, the absolute discrepancy of actual and calibrated load occurred to be less than 5%.