Optical micro/nanofiber enabled tactile sensors and soft actuators:A review

As a combination of fiber optics and nanotechnology,optical micro/nanofiber(MNF)is considered as an important multifunctional building block for fabricating various miniaturized photonic devices.With the rapid progress in flexible optoelectronics,MNF has been emerging as a promising candidate for assembling tactile sensors and soft actuators owing to its unique optical and mechanical properties.This review discusses the advances in MNF enabled tactile sensors and soft actuators,specifically,focusing on the latest research results over the past 5 years and the applications in health monitoring,human-machine interfaces,and robotics.Future prospects and challenges in developing flexible MNF devices are also presented.

MATERIAL SURFACE THERMAL PROPERTY IDENTIFICATION USING HEAT FLUX TACTILE SENSOR

Eased on the mechanism of temperature tactile sensing of human finger,a heat flux tactile sensor com- posed of a thermostat module and a heat flux sensor is designed to identify material thermal properties. The ther- mostat module maintains the sensor temperature invariable, and the heat flux sensor（Peltier device） detects the heat flux temperature difference between the thermostat module and the object surface. Two different modes of the heat flux tactile sensor are proposed, and they are simulated and experimented for different material objects. The results indicate that the heat flux tactile sensor can effectively identify different thermal properties.

Optical Fiber Type Slide Tactile Sensor Used for Underwater Robot

Because of the special underwater environment, many sensors used well in robots working in space or on the land can not be used in the underwater. So an optical fiber type slide tactile sensor is designed by the inner modulation mechanism of the intensity type optical fiber. The principle and structure of the sensor are introduced in detail. The static and dynamic characteristics are analyzed theoretically and experimentally. The dynamic characteristic model is built and the simulation is made by using genetic algorithm based on neural network. In order to use the sensor perfectly, the recognition model of the sensor is built on the basis of the principle of “inverse solution” using neural networks. The control precision and sensitivity of the manipulator are improved.

A Liquid–Solid Interface-Based Triboelectric Tactile Sensor with Ultrahigh Sensitivity of 21.48 kPa-1

Traditional triboelectric tactile sensors based on solid–solid interface have illustrated promising application prospects through optimization approach.However,the poor sensitivity and reliability caused by hard contact-electrification still poses challenges for the practical applications.In this work,a liquid–solid interface ferrofluid-based triboelectric tactile sensor(FTTS)with ultrahigh sensitivity is proposed.Relying on the fluidity and magnetism of ferrofluid,the topography of microstructure can be flexibly adjusted by directly employing ferrofluid as triboelectric material and controlling the position of outward magnet.To date,an ultrahigh sensitivity of 21.48 k Pa;for the triboelectric sensors can be achieved due to the high spike microstructure,low Young’s modulus of ferrofluid and efficient solid–liquid interface contact-electrification.The detection limit of FTTS of 1.25 Pa with a wide detection range to 390 k Pa was also obtained.In addition,the oleophobic property between ferrofluid and poly-tetra-fluoro-ethylene triboelectric layer can greatly reduce the wear and tear,resulting in the great improvement of stability.Finally,a strategy for personalized password lock with high security level has been demonstrated,illustrating a great perspective for practical application in smart home,artificial intelligence,Internet of things,etc.

MEMS-based ZnO Piezoelectric Tactile Sensor for Minimally Invasive Surgery

This paper reports the design and fabrication of a MEMS-based ZnO piezoelectric tactile sensor,which can be integrated on to the endoscopic grasper used in minimally invasive surgery (MIS).The sensor includes a silicon substrate, platinum bottom electrode,platinum top electrode,and a ZnO piezoelectric thin film,which is sandwiched between the two-electrode layers.The sensitivity of the micro-force sensor is analyzed in theory and the sensor exhibits high sensitivity about 7pc/uN.The application of this tactile sensor to MIS will allow the surgeon feeling the touch force between the endoscopic grasper and tissue in real-time,and manipulating the tissue safely.

Optical fiber based slide tactile sensor for underwater robots

In the underwater environment, many visual sensors don’t work, and many sensors which work well for robots working in space or on land can not be used underwater. Therefore, an optical fiber slide tactile sensor was designed based on the inner modulation mechanism of optical fibers. The principles and structure of the sensor are explained in detail. Its static and dynamic characteristics were analyzed theoretically and then simulated. A dynamic characteristic model was built and the simulation made using the GA based neural network. In order to improve sensor response, the recognition model of the sensor was designed based on the ‘inverse solution’ principle of neural networks, increasing the control precision and the sensitivity of the manipulator.

Piezoresistive Characteristic of Conductive Rubber for Flexible Tactile Sensor

In the research of 2D flexible tactile sensor matrix,pressure-sensitive conductive rubber was developed and tested in which carbon black was used as its conductive phase and silicon rubber as its matrix layer.Experiments were undertaken and the resultant data were used for its piezoresistive characteristics investigation for two kinds of electrode connection configurations,the surface directive connection and embedded connection.It is found that due to the rather strong nonlinearity of the piezoresistive characteristic curves obtained,a higher correlation relationship can be obtained by means of quadratic polynomial fitting.It also showed that the embedded electrode assembling has higher fitting accuracy while the surface directive connection has better mechanical sensitivity.

Advances in advanced solution-synthesis-based structural materials for tactile sensors and their intelligent applications

Intelligent applications,with tactile sensors at their core,represent significant advancement in the field of artificial intelligence.However,achieving perception abilities in tactile sensors that match or exceed human skin remains a formidable challenge.Consequently,the design and implementation of hierarchical structural materials are considered the optimal solution to this challenge.In contrast to conventional methods,such as complicated lithography and three-dimensional printing,the cost-effective and scalable nature of advanced solution-synthesis methods makes them ideal for preparing diverse tactile sensors with hierarchical structural materials.However,the process and applicability of advanced solution synthesis methods have yet to form a seamless system.Accordingly,the development and intellectualization of tactile sensors based on advanced solution synthesis methods are still in their early stages,and require a comprehensive and systematic review to usher in progress.This study delves into the advantages and disadvantages of various advanced solution synthesis methods,providing detailed insights.Furthermore,the positive effects of hierarchical structural materials constructed using these methods in tactile sensors and their intelligent applications are also discussed in depth.Finally,the challenges and future opportunities faced by this emerging field are summarized.

Fast-response,high-sensitivity multi-modal tactile sensors based on PPy/Ti_(3)C_(2)T_(x) films for multifunctional applications

In recent years,multi-modal flexible tactile sensors have become an important direction in the development of electronic skin because of their excellent sensitivity,flexibility and wearable properties.In this work,a humidity-pressure multi-modal flexible sensor based on polypyrrole(PPy)/Ti_(3)C_(2)T_(x) sensitive film packaged with porous polydimethylsiloxane(PDMS)is investigated by combining the sensitive structure generation mechanism of in situ polymerization to achieve the simultaneous detection of humidity and pressure,which has a sensitivity of 89,113.4Ω/%RH in a large humidity range of 0%-97%RH,and response/recovery time of 2.5/1.9 s.The tactile pressure sensing has a high sensitivity,a fast response of 67/52 ms,and a wide detection limit.The device also has excellent performance in terms of stability and repeatability,making it promising for respiratory pattern and motion detection.This work provides a new solution to address the construction of multi-modal tactile sensors with potential applications in the fields of medical health,epidemic prevention.

Field effect transistor-based tactile sensors: From sensor configurations to advanced applications

The past several decades have witnessed great progress in high-performance field effect transistors(FET)as one of the most important electronic compo-nents.At the same time,due to their intrinsic advantages,such as multiparameter accessibility,excellent electric signal amplification function,and ease of large-scale manufacturing,FET as tactile sensors for flexible wear-able devices,artificial intelligence,Internet of Things,and other fields to per-ceive external stimuli has also attracted great attention and become a significant field of general concern.More importantly,FET has a unique three-terminal structure,which enables its different components to detect external mechanics through different sensing mechanisms.On one hand,it provides an important platform to shed deep insights into the underlying mechanisms of the tactile sensors.On the other hand,these properties could in turn endow excellent components for the construction of tactile matrix sensor arrays with high quality.With special emphasis on the configuration of FETs,this review classified and summarized structure-optimized FET tactile sensors with gate,dielectric layer,semiconductor layer,and source/drain electrodes as sensing active components,respectively.The working principles and the state-of-the-art protocols in terms of high-performance tactile sensors are detail discussed and highlighted,the innovative pixel distribution and integration analysis of the transistor sensor matrix array concerning flexible electronics are also intro-duced.We hope that the introduction of this review can provide some inspiration for future researchers to design and fabricate high-performance FET-based tactile sensor chips for flexible electronics and other fields.

A machine learning-assisted multifunctional tactile sensor for smart prosthetics

The absence of tactile perception limits the dexterity of a prosthetic hand and its acceptance by amputees.Recreating the sensing properties of the skin using a flexible tactile sensor could have profound implications for prosthetics,whereas existing tactile sensors often have limited functionality with cross-interference.In this study,we propose a machine-learning-assisted multifunctional tactile sensor for smart prosthetics,providing a human-like tactile sensing approach for amputations.This flexible sensor is based on a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)-melamine sponge,which enables the detection of force and temperature with low cross-coupling owing to two separate sensing mechanisms:the open-circuit voltage of the sensor as a force-insensitive intrinsic variable to measure the absolute temperature and the resistance as a temperature-insensitive extrinsic variable to measure force.Furthermore,by analyzing the unsteady heat conduction and characterizing it using real-time thermal imaging,we demonstrated that the process of open-circuit voltage variation resulting from the unsteady heat conduction is closely correlated with the heatconducting capabilities of materials,which can be utilized to discriminate between substances.Assisted by the decision tree algorithm,the device is endowed with thermal conductivity sensing ability,which allows it to identify 10 types of substances with an accuracy of 94.7%.Furthermore,an individual wearing an advanced myoelectric prosthesis equipped with the above sensor can sense pressure,temperature,and recognize different materials.We demonstrated that our multifunctional tactile sensor provides a new strategy to help amputees feel force,temperature and identify the material of objects without the aid of vision.

Highly stable capacitive tactile sensors with tunable sensitivity facilitated by electrostatic interaction of layered double hydroxide,MXene,and Ag NWs

A capacitive tactile sensor(CTS)has been developed by assembling a double-sided patterned dielectric layer and novel electrodes.The patterned aluminium foil-supported electrode comprises zinc-aluminium layered double hydroxides(Zn AlLDH),MXene,and Ag NWs via electrostatic interaction.This flexible device enables greater structural deformation,thereby enhancing sensitivity to a wide range of pressure.The sensitivity of the CTS can be customized to meet specific requirements by matching the microstructured electrodes with the patterned dielectric layer.An optimized sensor exhibits a sensitivity of 2.752 kPa^(–1)within 30 kPa,a response time of approximately 100 ms,and a wide detection range of 0–300 kPa.The strong physical interaction among the electrode materials ensures a reliable conductive network,ensuring the long-term stability of the sensor,even after 7500 loading and unloading cycles.Furthermore,the fabricated CTS device presents a promising prospect for the integration into wearable electronics,with the ability to effectively respond to both human activities and external physical stimuli.

Recent progress in tactile sensors and their applications in intelligent systems

With the rapid development of intelligent technology,tactile sensors as sensing devices constitute the core foundation of intelligent systems.Biological organs that can sense various stimuli play vital roles in the interaction between human beings and the external environment.Inspired by this fact,research on skin-like tactile sensors with multifunctionality and high performance has attracted extensive attention.An overview of the development of high-performance tactile sensors applied in intelligent systems is systematically presented.First,the development of tactile sensors endowed with stretchability,selfhealing,biodegradability,high resolution and self-powered capability is discussed.Then,for intelligent systems,tactile sensors with excellent application prospects in many fields,such as wearable devices,medical treatment,artificial limbs and robotics,are presented.Finally,the future prospects of tactile sensors for intelligent systems are discussed.

MODELING CONTACTS OF IONIC POLYMER METAL COMPOSITES BASED TACTILE SENSORS

We report the first attempt to model the contacts of an ionic polymer metal composite（IPMC） based tactile sensor. The tactile sensor comprises an IPMC actuator, an IPMC sensor and the target to be detected. The system makes use of multiple contacts to work： the actuator comes into contact with the sensor and pushes the movement of sensor; the contact between the sensor and the object detects the existence and the stiffness of the target. We integrate modeling of various physical processes involved in IPMC devices to form a simulation scheme. An iteration and optimization strategy is also described to correlate the experimental and simulation results of an IPMC bending actuator to identify the two key parameters used in electromechanical transduction. Modeling the multiple contacts will aid the design and optimization of such IPMC based soft robotics.

PDMS/MWCNT-based tactile sensor array with coplanar electrodes for crosstalk suppression

The severe crosstalk effect is widely present in tactile sensor arrays with a sandwich structure.Here we present a novel design for a resistive tactile sensor array with a coplanar electrode layer and isolated sensing elements,which were made from polydimethylsiloxane(PDMS)doped with multiwalled carbon nanotubes(MWCNTs)for crosstalk suppression.To optimize its properties,both mechanical and electrical properties of PDMS/MWCNT-sensing materials with different PDMS/MWCNT ratios were investigated.The experimental results demonstrate that a 4 wt% of MWCNTs to PDMS is optimal for the sensing materials.In addition,the pressure-sensitive layer consists of three microstructured layers(two aspectant PDMS/MWCNT-based films and one top PDMS-based film)that are bonded together.Because of this three-layer microstructure design,our proposed tactile sensor array shows sensitivity up to−1.10 kPa^(−1),a response time of 29 ms and reliability in detecting tiny pressures.

Control of myoelectric prosthetic hand with a novel proximity-tactile sensor

Currently, prosthetic hands can only achieve several prespecified and discrete hand motion patterns from popular myoelectric control schemes using electromyography(EMG) signals. To achieve continuous and stable grasping within the discrete motion pattern, this paper proposes a control strategy using a customized, flexible capacitance-based proximity-tactile sensor. This sensor is integrated at the fingertip and measures the distance and force before and after contact with an object. During the pregrasping phase, each fingertip’s position is controlled based on the distance between the fingertip and the object to make all fingertips synchronously approach the object at the same distance. Once contact is established, the sensor turns to output the tactile information, by which the contact force of each fingertip is finely controlled. Finally, the method is introduced into the human-machine interaction control for a myoelectric prosthetic hand. The experimental results demonstrate that continuous and stable grasping could be achieved by the proposed control method within the subject’s discrete EMG motion mode. The subject also obtained tactile feedback through the transcutaneous electrical nerve stimulation after contact.

3D Printing of Liquid Metal Based Tactile Sensor for Simultaneously Sensing of Temperature and Forces

Tactile sensors have been used for haptic perception in intelligent robotics,smart prosthetics,and human-machine interface.The development of multifunctional tactile sensor remains a challenge and limit its application in flexible electronics and devices.We propose a liquid metal based tactile sensor for both temperature and force sensing which is made by 3D printing.The structural design and working principle of liquid metal based tactile sensor are firstly described.A digital light processing-based printing process is developed to print two kinds of photosensitive resins with different hardness,and used to fabricate the tactile sensor.A Wheatstone bridge circuit is designed for decoupling the temperature and forces from the measured output voltages.Characterization tests show that the tactile sensor has relatively high force sensing sensitivity of 0.29 N^(-1),and temperature sensing sensitivities are 0.55%°C−1 at 20~50°C and 0.21%°C^(−1)at 50~80°C,respectively.Then,the fabricated tactile sensor is mounted onto hand finger to measure the contact force and temperature during grasping.Results show that the 3D printed tactile sensor has excellent flexibility and durability and can accurately measure the temperature and contact forces,which demonstrate its potential in robotic manipulation applications.

Detection of Stiff Nodules Embedded in Soft Tissue Phantoms,Mimicking Cancer Tumours,Using a Tactile Resonance Sensor

Background: Prostate cancer (PCa) is the most common form of cancer among males in Europe and in the USA and the most common curative treatment is removal of the prostate, i.e. prostatectomy. After the removal, the prostate is histopathologically analysed. One area of interest is to examine the perifery of the prostate, as tumours on and near the surface can indicate that the PCa has spread to other parts of the body. There are no current methods to examine the surface of the prostate at the time of surgery. Tactile resonance sensors can be used for detecting areas of different stiffness in soft tissue. Human prostate tissue affected by cancer is usually stiffer than healthy tissue, and for this purpose, a tactile resonance sensor was developed. The aim of this study was to investigate the depth at which embedded stiffer volumes could be detected, using soft tissue phantoms. Methods: With the tactile resonance sensor used in this study, the shift of the resonance frequency and the force at contact with tissue can be measured, and combined into a tissue stiffness parameter. The detection sensitivity of the sensor at impression depths, 0.4 and 0.8 mm, was measured for detection of inserted nodules of stiff silicone in softer silicone and in chicken muscle tissue, mimicking prostate tissue with cancer tumours. Results: Measurements on the silicone samples detected the hidden stiffer object at a depth of 1 - 4 mm with a difference in the stiffness parameter of 80 - 900 mN/kHz (p < 0.028, n = 48). At the depth 5 - 6 mm the difference was smaller but still significant < 30 mN/kHz (p < 0.05, n = 24). For the measurements on chicken muscle, the detectable depth was 4 mm (p < 0.05, n = 24). Conclusion: This model study suggests that, with only a small impression depth of ≤1 mm, the resonance sensor system described here can detect stiffness variations located at least 4 mm in silicone and chicken muscle, mimicking tumours in prostate tissue.

A thermally flexible and multi-site tactile sensor for remote 3D dynamic sensing imaging

A flexible,multi-site tactile and thermal sensor(MTTS)based on polyvinylidene fluoride(resolution 50×50)is reported.It can be used to implement spatial mapping caused by tactile and thermal events and record the two-dimensional motion trajectory of a tracked target object.The output voltage and current signal are recorded as amapping by sensing the external pressure and thermal radiation stimulus,and the response distribution is dynamically observed on the three-dimensional interface.Through the mapping relationship between the established piezoelectric and pyroelectric signals,the piezoelectric component and the pyroelectric component are effectively extracted from the composite signals.The MTTS has a good sensitivity for tactile and thermal detection,and the electrodes have good synchronism.In addition,the signal interference is less than 9.5%and decreases as the pressure decreases after the distance between adjacent sites exceeds 200μm.The integration of MTTS and signal processing units has potential applications in human-machine interaction systems,health status detection and smart assistive devices.

Tactile-sensing-based robotic grasping stability analysis

Tactile signals play a crucial role in enabling robots to successfully manipulate unfamiliar objects.For robots to grasp unknown objects securely and without causing damage,it is essential that they can analyze grasping stability in real time through tactile signals and respond promptly.This study introduces a novel method for analyzing the stability of robotic hand grasping,utilizing the Wilcoxon signed rank test.The efficacy of this method is demonstrated through its static and dynamic performance,and evaluated across a series of experiments.The findings of this research highlight the method’s ability to accurately detect when an object begins to slip from the robot’s grasp.Employing this method allows the gripper to maintain a secure hold on objects by applying the minimal necessary force.It also enables the gripper to dynamically adjust the force it applies in real time,thus preventing the object from slipping during the movement of the robotic arm.Moreover,the gripper demonstrates the ability to stably grasp objects of varied weights and with unknown characteristics,showcasing the versatility and effectiveness of the proposed method.