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Multifunctional Soft Robotic Finger Based on a Nanoscale Flexible Temperature–Pressure Tactile Sensor for Material Recognition
... [1][2][3][4] The technology enables simultaneous acquisition of multiple environmental information using a single sensor and decoupling measurements, resulting in more comprehensive, accurate and reliable data. [5][6][7] Single parameter flexible sensors have been deeply developed. [8][9][10][11][12][13][14] Compared with single-parameter sensor which only needs to improve the sensitivity and measuring range, multi-parameter sensor signal decoupling is the focus and difficulty of its research. ...
... The addition of MWCNTs can reduce the influence of PSS on PEDOT chain and improve the overall conductivity of PEDOT:PSS. 6 This is mainly due to the following mechanism: Due to the non-covalent interaction between MWCNT molecules and PEDOT molecules, the delocalized π bond network of MWCNT interacts with the thiophene ring of the PEDOT skeleton, and some nanotubes overlap into a network structure, which is equal to embedding some continuous channels into the PEDOT:PSS molecules, thus improving the transport efficiency of carriers. At the same time, PEDOT:PSS can effectively stabilize and disperse MWCNTs in water, thus enhancing the electrical conductivity of the solution by preventing sedimentation and aggregation, which is the reason why the conductivity of PEDOT:PSS/MWCNTs composites is greatly improved. ...
... Compared with the published literature, as shown in Figure 4i, the temperature measurement of the double-parameter sensor designed in this paper has better sensitivity and detection range. 6,38,[43][44][45] ...
Multi-parameter comprehensive sensing, as data source for measuring and evaluating the physical state, has become one of the important development directions of flexible electronics. Temperature and pressure are two common physical parameters and are usually coupled with each other, while it is of great value but challenge to decouple them simultaneously. In this paper, a flexible double-parameter sensor is proposed to realize the completely decoupling measurement of the temperature and pressure based on the thermal-resistance effect and piezocapacitive effect. PEDOT:PSS/MWCNTs serpentine electrode is prepared by dispensing process to measure the temperature, while PVA/H3PO4 ionic film dielectric layer is prepared on porous conductive fabric by electrostatic spinning process to sense the pressure. The advantage of the proposed sensor is that the double dielectric layer capacitance for measuring pressure has a relatively large value and is sensitive to pressure, but not to temperature, which can achieve direct decoupling measurement of pressure and temperature in conventional measurements. The sensor layers are innovatively designed so that the serpentine electrode for measuring temperature can be used as one electrode of piezocapacitive sensor. Finite element analysis is conducted to compare the sensitivity of pressure measurement, which gets an optimized sensor configuration of upper piezocapacitive and lower thermal-resistance. The designed sensor has been proved to have an extremely wide measuring range and high measuring sensitivity. For temperature measurement, it can achieve the measurement of 15-80 ℃, and the sensitivity below 50 ℃ is as high as 0.032 ℃-1. For pressure measurement, a wide measurement range of 0-600 kPa is provided, with an extremely high sensitivity of 1249.34 kPa−1 for low pressure measurements below 10 kPa. The above excellent performance proves that the proposed flexible sensor has a significant potential application in the simultaneous measurement of temperature and pressure.
... In this study, we utilized commercially available thin conductive sponges as the sensor material, which were attached to the lateral surface of a three-degree-of-freedom (DoF) soft actuator. Previous research has demonstrated that the piezoresistive properties of thin conductive sponges enable the detection of deformation and tactile information of attached objects [22,[27][28][29][30][31][32][33]. Due to their flexible and lightweight nature, sensors developed with sponge materials have been widely employed in human-robot interactions, including tactile sensing [22,27,29,30] and human motion detection [28]. ...
... Although recent research work has explored the feasibility of collecting the tactile information of robots using conductive sponge sensors [31,34,35], only a few studies have employed sensors developed with conductive sponge material to detect or estimate the deformation of soft robots. In our previous work [36], we proposed a kirigami-inspired flexible sponge sensor that could estimate the bending angle of a single degree-of-freedom flexible fiber-reinforced bending actuator. ...
This paper proposes a method for accurate 3D posture sensing of the soft actuators, which could be applied to the closed-loop control of soft robots. To achieve this, the method employs an array of miniaturized sponge resistive materials along the soft actuator, which uses long short-term memory (LSTM) neural networks to solve the end-to-end 3D posture for the soft actuators. The method takes into account the hysteresis of the soft robot and non-linear sensing signals from the flexible bending sensors. The proposed approach uses a flexible bending sensor made from a thin layer of conductive sponge material designed for posture sensing. The LSTM network is used to model the posture of the soft actuator. The effectiveness of the method has been demonstrated on a finger-size 3 degree of freedom (DOF) pneumatic bellow-shaped actuator, with nine flexible sponge resistive sensors placed on the soft actuator's outer surface. The sensor-characterizing results show that the maximum bending torque of the sensor installed on the actuator is 4.7 Nm, which has an insignificant impact on the actuator motion based on the working space test of the actuator. Moreover, the sensors exhibit a relatively low error rate in predicting the actuator tip position, with error percentages of 0.37%, 2.38%, and 1.58% along the x-, y-, and z-axes, respectively. This work is expected to contribute to the advancement of soft robot dynamic posture perception by using thin sponge sensors and LSTM or other machine learning methods for control.
... These sensors enable the monitoring and control of the shape and position of different parts of the soft robot. Additionally, the sensors can enhance a soft robot's awareness of external stimuli such as temperature, 6 pH, 7 chemicals, 8 pressure, 9 light, 10 and sound, 11 which significantly widens the scope of the application of soft robots. With the help of sensors, soft robots can perform complex tasks in diverse fields such as healthcare 12,13 agriculture, 14,15 and warehouse management, among others. ...
Soft robotics is an exciting field of science and technology that enables robots to manipulate objects with human-like dexterity. Soft robots can handle delicate objects with care, access remote areas, and offer realistic feedback on their handling performance. However, increased dexterity and mechanical compliance of soft robots come with the need for accurate control of the position and shape of these robots. Therefore, soft robots must be equipped with sensors for better perception of their surroundings, location, force, temperature, shape, and other stimuli for effective usage. This review highlights recent progress in sensing feedback technologies for soft robotic applications. It begins with an introduction to actuation technologies and material selection in soft robotics, followed by an in-depth exploration of various types of sensors, their integration methods, and the benefits of multimodal sensing, signal processing, and control strategies. A short description of current market leaders in soft robotics is also included in the review to illustrate the growing demands of this technology. By examining the latest advancements in sensing feedback technologies for soft robots, this review aims to highlight the potential of soft robotics and inspire innovation in the field.
... Research in the field of flexible pressure sensors (FPS) and strain sensors has gained tremendous attention in recent years (Xiang et al., 2019;Xiang et al., 2022b;Hou et al., 2022;Kim et al., 2019;Nabeel et al., 2022Nabeel et al., , 2021Tang et al., 2020;Yang et al., 2021). More and more research is focused on insulating polymers containing a conductive phase to create piezoresistive FPS (L. ...
Pressure sensors based on nitrogen-doped bamboo-shaped carbon nanotubes (N-BCNT) and carbon black (CB) as nanofillers, polyurethane foam (PU) as supporting substrate, and silicone rubber (SR) as a matrix were prepared. Dip coating was used to coat PU with 0.44 wt% nanofiller, including different mixing ratios of N-BCNT and CB (5:5; 6:4; 7:3; 8:2; 9:1). Then, the coated PU is impregnated in SR to fill the pores. Due to the higher aspect ratio of the N-BCNT, it contributes more to improving the electrical conductivity in the composites, while the CB fills the smaller gaps. The prepared sensors were tested in various applications, and it was found that the optimal mixing ratio of nanofillers was 7:3 N-BCNT:CB. Thus, a multifunctional pressure sensor has been developed successfully with excellent flexibility and good resilience, suitable for motion detection and finger touch applications. The pressure sensor showed high sensitivity, and the ability to detect a wide range of pressures. The sensor exhibited success in a range of applications, paving the way for its potential use in various fields in the future, such as wearable devices, prosthetics, robotic devices, and medical devices.
... Most conventional tactile sensors have inherent flaws, such as poor resistance to fatigue and incompatibility with biological interfaces. In addition, sensor bodies with stiff substrates are easily damaged by external loads [12,13]. Besides, overloading stimuli applied directly to the surface without buffering or preprocessing may destroy the interior components [14]. ...
The sense of touch events, achieved by artificial tactile sensory systems (ATSSs), is a milestone in the progress of human-machine interactions. However, it has been a challenge for ATSSs to serve functions comparable with the human tactile perception system (HTPS). The biomimetic strategies and technologies inspired by HTPS are considered an optimal solution to this challenge. Recent studies have reported bioinspired strategies for improving specific aspects of ATSS performance, such as feature collection, signal conversion, and information computation. Here, we present a systematic interpretation of biomechanisms for HTPSs, and correspondingly, address biomimetic strategies and technologies contributing to ATSSs as an integral system. This review will benefit the development and application of ATSSs in the future.
... Benefiting from its flexibility and certain stretchability, flexible pressure sensors had been widely used in electronic skin, 1,2 intelligent clothing, 3 medical care, 4 soft robots, 5 and other fields. Among the common flexible pressure sensors, such as capacitive sensors, 6,7 piezoresistive sensors, 8,9 piezoelectric sensors, 10,11 and friction sensors, 12,13 capacitive flexible pressure sensors had been widely concerned because of their simple mechanism, good stability, fast response speed, and high sensitivity. ...
Microstructure plays an important role in improving the performance of flexible sensors. Changing the shape of the dielectric layer microstructure is an effective countermeasure to promote the sensitivity of capacitive sensors. Nevertheless, traditional microstructure fabrication methods have high manufacturing costs, cumbersome manufacturing processes, and single structure manufacturing, which restrict the development of flexible sensors. In this work, electro-hydro-dynamic (EHD) printing method and aerosol jet (AJ) printing method were applied to fabricate 3D microstructures, in a manner of printing the same pattern in multiple layers. The height and morphology of 3D microstructures, under different printing parameters, were compared by changing the number of printing layers and printing speed. Additionally, the printing effects of the two printing methods were compared. The results demonstrated that various shapes and highly controllable 3D microstructures could be fabricated by both methods. The EHD printing method had higher manufacturing precision, whereas the AJ printing method had higher stacking efficiency. The height and morphology of 3D microstructures could be effectively controlled by changing the number of printed layers and the printing speed of the microstructures. It is indicated that the EHD printing method and the AJ printing method both have great potential in the fabrication of 3D microstructures and that both methods had their own advantages.
... Recognizing different contact patterns applied to the tactile sensors plays a very important role in human-machine interaction. A flexible tactile sensor is an essential tactile information acquisition medium of the robot sensor system, and it has special advantages in detecting target surface texture and physical properties [1][2][3], which is conducive to establishing a more secure and reliable humanmachine interaction system [4,5]. ...
Recognizing different contact patterns imposed on tactile sensors plays a very important role in human–machine interaction. In this paper, a flexible tactile sensor with great dynamic response characteristics is designed and manufactured based on polyvinylidene fluoride (PVDF) material. Four contact patterns (stroking, patting, kneading, and scratching) are applied to the tactile sensor, and time sequence data of the four contact patterns are collected. After that, a fusion model based on the convolutional neural network (CNN) and the long-short term memory (LSTM) neural network named CNN-LSTM is constructed. It is used to classify and recognize the four contact patterns loaded on the tactile sensor, and the recognition accuracies of the four patterns are 99.60%, 99.67%, 99.07%, and 99.40%, respectively. At last, a CNN model and a random forest (RF) algorithm model are constructed to recognize the four contact patterns based on the same dataset as those for the CNN-LSTM model. The average accuracies of the four contact patterns based on the CNN-LSTM, the CNN, and the RF algorithm are 99.43%, 96.67%, and 91.39%, respectively. All of the experimental results indicate that the CNN‑LSTM constructed in this paper has very efficient performance in recognizing and classifying the contact patterns for the flexible tactile sensor.
The growing demand for soft intelligent systems, which have the potential to be used in a variety of fields such as wearable technology and human‐robot interaction systems, has spurred the development of advanced soft transducers. Among soft systems, sensor–actuator hybrid systems are considered the most promising due to their effective and efficient performance, resulting from the synergistic and complementary interaction between their sensor and actuator components. Recent research on integrated sensor and actuator systems has resulted in a range of conceptual and practical soft systems. This review article provides a comprehensive analysis of recent advances in sensor and actuator integrated systems, which are grouped into three categories based on their primary functions: i) actuator–assisted sensors for intelligent detection, ii) sensor‐assisted actuators for intelligent movement, and iii) sensor‐actuator interactive devices for a hybrid of intelligent detection and movement. In addition, several bottlenecks in current studies are discussed, and prospective outlooks, including potential applications, are presented. This categorization and analysis will pave the way for the advancement and commercialization of sensor and actuator‐integrated systems.
This study presents an intelligent soft robotic system capable of perceiving, describing, and sorting objects based on their physical properties. This work introduces a bimodal self‐powered flexible sensor (BSFS) based on the triboelectric nanogenerator and giant magnetoelastic effect. The BSFS features a simplified structure comprising a magnetoelastic conductive film and a packaged liquid metal coil. The BSFS can precisely detect and distinguish touchless and tactile models, with a response time of 10 ms. By seamlessly integrating the BSFSs into the soft fingers, this study realizes an anthropomorphic soft robotic hand with remarkable multimodal perception capabilities. The touchless signals provide valuable insights into object shape and material composition, while the tactile signals offer precise information regarding surface roughness. Utilizing a convolutional neural network (CNN), this study integrates all sensing information, resulting in an intelligent soft robotic system that accurately describes objects based on their physical properties, including materials, surface roughness, and shapes, with an accuracy rate of up to 97%. This study may lay a robotic foundation for the hardware of the general artificial intelligence with capacities to interpret and interact with the physical world, which also serves as an interface between artificial intelligence and soft robots.
Bio‐inspired cilium‐based mechanosensors offer a high level of responsiveness, making them suitable for a wide range of industrial, environmental, and biomedical applications. Despite great promise, the development of sensors with multifunctionality, scalability, customizability, and sensing linearity presents challenges due to the complex sensing mechanisms and fabrication methods involved. To this end, high‐aspect‐ratio polycaprolactone/graphene cilia structures with high conductivity, and facile fabrication are employed to address these challenges. For these 3D‐printed structures, an “inter‐cilium contact” sensing mechanism that enables the sensor to function akin to an on‐off switch, significantly enhancing sensitivity and reducing ambiguity in detection, is proposed. The cilia structures exhibit high levels of customizability, including thickness, height, spacing, and arrangement, while maintaining mechanical robustness. The simplicity of the sensor design enables highly sensitive detection in diverse applications, encompassing airflow and water flow monitoring, braille detection, and debris recognition. Overall, the unique conductive cilia‐based sensing mechanism that is proposed brings several advantages, advancing the development of multi‐sensing capabilities and flexible electronic skin applications in smart robotics and human prosthetics.
Electrically conductive polymer nanocomposites have been the subject of intense research due to their promising potential as piezoresistive biomedical sensors, leveraging their flexibility and biocompatibility. Although intrinsically conductive polymers such as polypyrrole (PPy) and polyaniline have emerged as lucrative candidates, they are extremely limited in their processability by conventional solution-based approaches. In this work, ultrathin nanostructured coatings of doped PPy are realized on polyurethane films of different architectures via oxidative chemical vapor deposition to develop stretchable and flexible resistance-based strain sensors. Holding the substrates perpendicular to the reactant flows facilitates diffusive transport and ensures excellent conformality of the interfacial integrated PPy coatings throughout the 3D porous electrospun fiber mats in a single step. This allows the mechanically robust (stretchability > 400%, with fatigue resistance up to 1000 cycles) nanocomposites to elicit a reversible change of electrical resistance when subjected to consecutive cycles of stretching and releasing. The repeatable performance of the strain sensor is linear due to dimensional changes of the conductive network in the low-strain regime (ε ≤ 50%), while the evolution of nano-cracks leads to an exponential increase, which is observed in the high-strain regime, recording a gauge factor as high as 46 at 202% elongational strain. The stretchable conductive polymer nanocomposites also show biocompatibility toward human dermal fibroblasts, thus providing a promising path for use as piezoresistive strain sensors and finding applications in biomedical applications such as wearable, skin-mountable flexible electronics.
The electronic skin has accurate digital sensing capability and human-like skin structure to realize in-situ measurement and perception. It has broad application prospects such as artificial intelligence, health monitoring, and robotics. However, multifunction integration, structure design, and signal demodulation are still the key points that hinder the development of e-skin. To solve this problem and for the application of robotic fingers, we propose a new multifunctional electronic skin in this paper with five sensing functions including temperature, thermosensation, pressure, wind speed and wind direction. It also has the characteristics of softness and smallness, which is suitable for fingers. Considering the geometry and application scenario of the “finger-sensor-object” touch, a new transient heat conduction model is developed and validated, which can achieve quantitative sensing and calculating of the material’s thermal parameters. Six objects were quantitatively identified in the experiments. Moreover, an interactive control logic between sensors and smart robots is innovatively proposed to improve the utilization of sensor-sensitive units. The proposed flexible multifunctional electronic skin provides a new idea for the application of wearable smart devices and intelligent sensing of robots.
Despite the extensive developments of flexible capacitive pressure sensors, it is still elusive to simultaneously achieve excellent linearity over a broad pressure range, high sensitivity, and ultrahigh pressure resolution under large pressure preloads. Here, we present a programmable fabrication method for microstructures to integrate an ultrathin ionic layer. The resulting optimized sensor exhibits a sensitivity of 33.7 kPa⁻¹ over a linear range of 1700 kPa, a detection limit of 0.36 Pa, and a pressure resolution of 0.00725% under the pressure of 2000 kPa. Taken together with rapid response/recovery and excellent repeatability, the sensor is applied to subtle pulse detection, interactive robotic hand, and ultrahigh-resolution smart weight scale/chair. The proposed fabrication approaches and design toolkit from this work can also be leveraged to easily tune the pressure sensor performance for varying target applications and open up opportunities to create other iontronic sensors.
Ultrasensitive flexible pressure sensors with excellent linearity are essential for achieving tactile perception. Although microstructured dielectrics have endowed capacitive sensors with ultrahigh sensitivity, the compromise of sensitivity with increasing pressure is an issue yet to be resolved. Herein, a spontaneously wrinkled MWCNT/PDMS dielectric layer is proposed to realize the excellent sensitivity and linearity of capacitive sensors for tactile perception. The synergistic effect of a high dielectric constant and wrinkled microstructures enables the sensor to exhibit linearity up to 21 kPa with a sensitivity of 1.448 kPa-1 and a detection limit of 0.2 Pa. Owing to these merits, the sensor monitors subtle physiological signals such as various arterial pulses and respiration. This sensor is further integrated into a fully multimaterial 3D-printed soft pneumatic finger to realize material hardness perception. Eight materials with different hardness values are successfully discriminated, and the capacitance of the sensor varies linearly (R2 > 0.975) with increasing hardness. Moreover, the sensitivity to the material hardness can be tuned by controlling the inflation pressure of the soft finger. As a proof of concept, the finger is used to discriminate pork fats with different hardness, paving the way for hardness discrimination in clinical palpation.
Lossless and efficient robotic grasping is becoming increasingly important with the widespread application of intelligent robotics in warehouse transportation, human healthcare, and domestic services. However, current sensors for feedback of grasping behavior are greatly restricted by high manufacturing cost, large volume and mass, complex circuit, and signal crosstalk. To solve these problems, here, we prepare lightweight distance sensor-based reduced graphene oxide (rGO)/MXene-rGO coaxial microfibers with interface buffer to assist lossless grasping of a robotic manipulator. The as-fabricated distance microsensor exhibits a high sensitivity of 91.2 m-1 in the distance range of 50-300 μm, a fast response time of 116 ms, a high resolution of 5 μm, and good stability in 500 cycles. Furthermore, the high-performance and lightweight microsensor is installed on the robotic manipulator to reflect the grasp state by the displacement imposed on the sensor. By establishing the correlation between the microsensing signal and the grasp state, the safe, non-destructive, and effective grasp and release of the target can be achieved. The lightweight and high-powered distance sensor displays great application prospects in intelligent fetching, medical surgery, multi-spindle automatic machines, and cultural relics excavation.
Tactile sensors can transform the environmental stimuli into electrical signals to perceive and quantify the environmental information, which show huge application prospects. The development of bionic robots and wearable devices towards intelligence has put high demands on the performance of tactile sensor arrays. Herein, the current state-of-the-art tactile sensor arrays over recent years have been summarized, from sensor array fabrication to advanced applications. The main preparation methods of patterned array including screen printing, 3D printing, laser microprocessing, and textile technology are discussed in detail. Strategies to optimize the signal crosstalk caused by flexible high-density sensor arrays are systematically introduced from the perspective of structure design and circuit design. Furthermore, advanced tactile sensors are not limited to a single pressure sensing function, and hence the development of multimodal detection for sensors has been discussed. In order to promote the adaptability in applications, stretchable and self-powered versatile integration scheme for advanced sensing are briefly described. Then, by means of machine learning and neural networks, it is possible to deeply explore the information embedded in the tactile acquisition signal with enriched application scenarios. Finally, the current challenges and the future perspectives for flexible tactile sensor arrays towards practical use are provided.
Although flexible sensors have been widely used in areas such as human-computer interaction and electronic skin with their sensitive response and excellent robustness, their fabrication process remains complex and expensive. In addition, the flexible pressure sensor can only detect a single pressure. This paper presents a dual-mode pressure and temperature sensor with complementary layers based on stretchable electrodes. The templates required for sensor production are simple, easy to make and inexpensive. Comparing pressure sensors without complementary layers improves the sensitivity of devices with complementary layers more than 2.5 times. In addition, the pressure sensor enables dual-mode testing of pressure and temperature by modifying the complementary layer's material while keeping the structure unchanged. The temperature sensor demonstrates pressure insensitivity and cyclic stability. The sensor shown some practical applications, such as finger pressing, gesture recognition, arm bending, and Morse code. Finally, sensors were integrated into the mechanical gripper to detect pressure and temperature during gripping. This work provides a promising way to improve the performance of flexible pressure sensors with easily prepared structures and to increase the integration of pressure and temperature dual-mode sensors, with great potential for applications in human-computer interaction and electronic skin.
Over the past decade, soft robot research has expanded to diverse fields, including biomedicine, bionics, service robots, human–robot interaction, and artificial intelligence. Much work has been done in modeling the kinematics and dynamics of soft robots, but closed‐loop control is still in its early stages due to limited sensory feedback. Thanks to the advancement in functional materials, structures, and manufacturing techniques for flexible electronics, flexible and stretchable sensors are developing rapidly. These sensors provide feedback for closed‐loop control tasks and enable soft robots to effectively explore the unknown and safely interact with humans and the environment. Herein, recent advances in perceptive soft robots that utilize flexible/stretchable sensors and functional materials are outlined. The perceptive functions of soft robots from two different aspects, that is, proprioception and exteroception, are summarized. Furthermore, the constructions of autonomous soft robots by integrating both proprioceptive and exteroceptive capabilities for closed‐loop control tasks and other challenging tasks in the real world are discussed. Soft robots have shown potential in bionics, human–robot interaction, and artificial intelligence. To improve their interactivity and adaptability, developing closed‐loop control systems is essential. Herein, a review of the proprioceptive and exteroceptive functions and closed‐loop control systems is presented to provide readers with a better understanding of recent advances in perceptive intelligence for soft robotics.
Fiber-based pressure/temperature sensors are highly desired in wearable electronics because of their natural advantages of good breathability and easy integrability. However, it is still a great challenge to fabricate reliable and highly sensitive fiber-based pressure/temperature sensors via a scalable and facile strategy. Herein, a novel fiber-based iontronic sensor with excellent pressure- and temperature-sensing capabilities is designed by assembling two crossed hollow and porous ionogel fibers filled with liquid metal. Serving as a pressure sensor, a high detection resolution (1.16 Pa), a high sensitivity of 13.30 kPa-1 (0-2 kPa), and a wide detection range (∼207 kPa) are realized owing to its novel hierarchical structure and the selection of deformable liquid electrodes. As a temperature sensor, it exhibits a high temperature sensitivity of 25.99% °C-1 (35-40 °C), high resolution of 0.02 °C, and good repeatability and reliability. On the basis of these excellent sensing capabilities, the as-prepared sensor can detect not only pressure signals varied from weak pulse to large joint movements but also the proximity of different objects. Furthermore, a large-area fiber array can be easily woven for acquiring the pressure mapping to intuitively distinguish the location, magnitude, and shape of the loaded object. This work provides a universal strategy to design fiber-shaped iontronic sensors for wearable electronics.
Commonly encountered problems in the manipulation of objects with robotic hands are the contact force control and the setting of approaching motion. Microelectromechanical systems (MEMS) sensors on robots offer several solutions to these problems along with new capabilities. In this review, we analyze tactile, force and/or pressure sensors produced by MEMS technologies including off-the-shelf products such as MEMS barometric sensors. Alone or in conjunction with other sensors, MEMS platforms are considered very promising for robots to detect the contact forces, slippage and the distance to the objects for effective dexterous manipulation. We briefly reviewed several sensing mechanisms and principles, such as capacitive, resistive, piezoresistive and triboelectric, combined with new flexible materials technologies including polymers processing and MEMS-embedded textiles for flexible and snake robots. We demonstrated that without taking up extra space and at the same time remaining lightweight, several MEMS sensors can be integrated into robotic hands to simulate human fingers, gripping, hardness and stiffness sensations. MEMS have high potential of enabling new generation microactuators, microsensors, micro miniature motion-systems (e.g., microrobots) that will be indispensable for health, security, safety and environmental protection.
Creatures in nature have evolved fascinating functions in the natural selection of superiority and inferiority to adapt to complex and harsh survival environments. Some special and excellent functions have been found to originate from the multilayered physical microstructure and chemical composition of biological surfaces, providing humans with a wide range of bionic references and inspirations. Inspired by the biological surfaces, numerous novel functional materials have sprung up to achieve unexpected results by mimicking the physical and chemical structures. Herein, some typical applications for healthcare advances of various biomimetic microstructured surfaces including sensors, adhesives, and superhydrophobic surfaces are reviewed, which are associated with the working mechanism around biological surfaces. The techniques and methods to fabricate biomimetic microstructured surfaces are also reviewed. The future challenges and opportunities for biomimetic microstructures are presented. Nature provides humans with inspiration and inspiration for a wide range of biological surfaces. This review overviews various biosurface‐inspired biomimetic microstructured surfaces for applications such as sensors, adhesives, and superhydrophobisc surfaces, which are fabricated by methods including template methods, photolithography, 3D printing, and vapor phase deposition. The development and challenges of biomimetic microstructures are outlined.
Flexible bifunctional sensors that mimic the function of human skin are essential as they provide critical interacting information with human and environment for intelligentization. However, the problem of interference between sensing signals from different stimuli and the deficiency of comfortability after long-time skin-attaching wearing still exist. Herein, we present an ultrathin and flexible bifunctional sensor based on two measurable parameters of pressure-induced supercapacitance and temperature-induced resistance with neglectable crosstalk between the two. The sensor consists of a planar iontronic supercapacitor underneath a serpentine resistor with assembly of total seven layers integrated on an electrospun thermoplastic polyurethane (TPU)-based nanofiber platform. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based sensing electrodes are patterned by direct ink writing with addition of graphene nanoflakes and Co3O4 nanoparticles to enhance the sensing performance. A high pressure sensitivity (147.19 kPa⁻¹, 0-7 kPa; 4.41 kPa⁻¹, 25-85 kPa) and a high temperature sensitivity (0.040 ℃⁻¹, 25-50 ℃; 0.002 ℃⁻¹, 50-100 ℃) are simultaneously achieved. The sensor also has the advantages of humidity inertness, waterproof ability and air permeability, which is breathable to obtain a wearing comfortability. Decoupled pressure-temperature sensing applications of detecting various subtle pressures with objects of different temperatures are comparatively demonstrated in a wireless and real-time mode. The proposed bifunctional sensor is potential in wearable healthcare monitoring and human-machine interaction.
Despite the extensive developments of flexible capacitive pressure sensors, it is still elusive to simultaneously achieve excellent linearity over a broad pressure range, high sensitivity, and ultrahigh pressure resolution under large pressure preloads at low cost. This work presents a facile and low-cost fabrication method to integrate an ultrathin ionic layer with gradient microstructures with programmable profiles and heights created by a simple CO 2 laser. The coupled electrical and mechanical simulations provide a route to optimize the design of iontronic pressure sensors based on the electric double layer to address the existing challenges for significantly improved pressure sensing performance. The resulting optimized sensor exhibits a high sensitivity of 33 kPa − 1 over an ultra-board linear sensing range of 1700 kPa, an ultralow detection limit of 0.36 Pa, and a pressure resolution of 0.00725% under ultrahigh pressure of 2000 kPa. Taken together with a rapid response/recovery time of 4/16 ms and excellent repeatability over 4,500 cycles, the sensor has been applied to subtle pulse detection from the fingertip, interactive control on the robotic hand, and a smart weight scale/chair with ultrahigh pressure resolution. The simple fabrication approaches and design toolkit from this work can also be leveraged to easily tune the pressure sensor performance for varying target applications and open up the opportunities to create other iontronic sensors for the next-generation flexible devices.
Flexible electronics plays important roles in many research fields, including but not limited to biomedicine, energy devices, robotics, and virtual/augmented reality. Multilayer configurations stack different flexible electronic components in a vertical layer-by-layer format, thereby providing multimodality, high density and many other advanced features. This review picks representative works of multilayer flexible electronics in recent years, with a focus on introducing the manufacturing approaches and applications. Specifically, schemes in constructing multilayer flexible electronic systems mainly involve direct multilayer microfabrication and layer-by-layer transfer printing, with additional options in forming three-dimensional geometries and multilayer interconnections; applications of multilayer flexible electronic systems span from optoelectronic and robotics to biomedicine and energy devices. As a concluding remark, this article summarizes the challenges and prospects of multilayer flexible electronics.
Wearable devices developed with flexible electronics have great potential applications for human health monitoring and motion sensing. Although material softness and structural flexibility provide a deformable human-machine interface to adapt to joint bending or tissue stretching/compression, flexible sensors are inconvenient in practical uses as they usually require calibration every time they are installed. This article presents an approach to design and fabricate a flexible curvature sensor to measure human articular movements for amphibious applications. This flexible sensor employs the capacitive sensing principle, where the dielectric layer and electrodes are made from the polyurethane resin and eutectic gallium-indium (EGaIn) liquid metal; and the fabrication process is implemented with shape deposition molding for batch production. The sensing method for articular rotation angles employs the Euler beam model to make the sensor reusable after one-time calibration by compensating for the unpredicted manual installation error. The illustrative application to ankle sensing in amphibious gaits shows that the root-mean-square error is within 5° for different walking speeds (0.7-1.1 m/s) in treadmill tests and the maximum error is within 3° for underwater sensing with quasi-static measurements. It is expected that the proposed waterproof flexible sensor can push the boundaries of wearable robotics, human locomotion, as well as their related applications.
Achieving highly accurate responses to external stimuli during human motion is a considerable challenge for wearable devices. The present study leverages the intrinsically high surface‐to‐volume ratio as well as the mechanical robustness of nanostructures for obtaining highly‐sensitive detection of motion. To do so, highly‐aligned nanowires covering a large area were prepared by capillarity‐based mechanism. The nanowires exhibit a strain sensor with excellent gauge factor (≈35.8), capable of high responses to various subtle external stimuli (≤200 µm deformation). The wearable strain sensor exhibits also a rapid response rate (≈230 ms), mechanical stability (1000 cycles) and reproducibility, low hysteresis (<8.1%), and low power consumption (<35 µW). Moreover, it achieves a gauge factor almost five times that of microwire‐based sensors. The nanowire‐based strain sensor can be used to monitor and discriminate subtle movements of fingers, wrist, and throat swallowing accurately, enabling such movements to be integrated further into a miniaturized analyzer to create a wearable motion monitoring system for mobile healthcare. A highly sensitive flexible strain sensor that is based on large‐area of aligned nanowires is developed. The sensor can be used as a wearable device to accurately detect, discriminate and monitor subtle movements linked with human motion.
Low dimension poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT: PSS) has been applied as resistor-type devices for temperature sensing applications. However, their response speed and thermal sensitivity is still not good enough for practical application. In this work, we proposed a new strategy to improve the thermal sensing performance of PEDOT: PSS by combined micro/nano confinement and materials doping. The dimension effect is carefully studied by fabricating different sized micro/nanowires through a low-cost printing approach. It was found that response speed can be regulated by adjusting the surface/volume (S/V) ratio of PEDOT: PSS. The fastest response (<3.5 s) was achieved by using nanowires with a maximum S/V ratio. Besides, by doping PEDOT: PSS nanowires with Graphene oxide (GO), its thermo-sensitivity can be maximized at specific doping ratio. The optimized nanowires-based temperature sensor was further integrated as a flexible epidermal electronic system (FEES) by connecting with wireless communication components. Benefited by its flexibility, fast and sensitive response, the FEES was demonstrated as a facile tool for different mobile healthcare applications.
Facile fabrication and high ambient stability are strongly desired for the practical application of temperautre sensor in real-time wearable healthcare. Herein, a fully printed flexible temperature sensor based on cross-linked poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was developed. By introducing the crosslinker of (3-glycidyloxypropyl)trimethoxysilane (GOPS) and the fluorinated polymer passivation (CYTOP), significant enhancements in humidity stability and temperature sensitivity of PEDOT:PSS based film were achieved. The prepared sensor exhibited excellent stability in environmental humidity ranged from 30% RH to 80% RH, and high sensitivity of −0.77% °C−1 for temperature sensing between 25 °C and 50 °C. Moreover, a wireless temperature sensing platform was obtained by integrating the printed sensor to a printed flexible hybrid circuit, which performed a stable real-time healthcare monitoring.
Flexible and wearable pressure sensors have attracted tremendous attention due to their wider applications such as human-interfacing and healthcare monitoring. However, it is a great challenge to achieve accurate pressure detection and stability against external stimuli (in particular, bending deformation) over a wide range of pressure from tactile to body weight levels. Here, we introduce an ultrawide-range, bending-insensitive, and flexible pressure sensor based on a carbon nanotube (CNT) network-coated thin porous elastomer sponge for use in human interface devices. The integration of the CNT networks into three dimensional (3D) microporous elastomers provides high deformability and a large change of contact between the conductive CNT networks due to the presence of micropores, thereby improving the sensitivity compared with that obtained using CNTs-embedded solid elastomers. As electrical pathways are continuously generated up to high compressive strain (~80%), the pressure sensor shows an ultrawide pressure-sensing range (10 Pa-1.2 MPa) while maintaining favorable sensitivity (0.01-0.02 kPa-1) and linearity (R2 ~0.98). Also, the pressure sensor exhibits excellent electromechanical stability and insensitivity to bending-induced deformations. Finally, we demonstrate that the pressure sensor can be applied in a flexible piano pad as an entertainment human interface device and a flexible foot insole as a wearable healthcare and gait monitoring device, respectively.
The highly toxic hydrogen sulphide (H 2 S) present in air can cause negative effects on human health. Thus, monitoring of this gas is vital in gas leak alarms and security. Efforts have been devoted to the fabrication and enhancement of the H 2 S-sensing performance of gas sensors. Herein, we used electron beam evaporation to decorate nickel oxide (NiO) nanoparticles on the surface of tin oxide (SnO 2 ) nanowires to enhance their H 2 S gas-sensing performance. The synthesised NiO-SnO 2 materials were characterised by field-emission scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy analysis. H 2 S gas-sensing characteristics were measured at various concentrations (1-10 ppm) at 200-350 °C. The results show that with effective decoration of NiO nanoparticles, the H 2 S gas-sensing characteristics of SnO 2 nanowires are significantly enhanced by one or two orders compared with those of the bare material. The sensors showed an effective response to low-level concentrations of H 2 S in the range of 1-10 ppm, suitable for application in monitoring of H 2 S in biogas and in industrial controls. We also clarified the sensing mechanism of the sensor based on band structure and sulphurisation process.
Recent work has begun to explore the design of biologically inspired soft robots composed of soft, stretchable materials for applications including the handling of delicate materials and safe interaction with humans. However, the solid-state sensors traditionally used in robotics are unable to capture the high-dimensional deformations of soft systems. Embedded soft resistive sensors have the potential to address this challenge. However, both the soft sensors—and the encasing dynamical system—often exhibit nonlinear time-variant behavior, which makes them difficult to model. In addition, the problems of sensor design, placement, and fabrication require a great deal of human input and previous knowledge. Drawing inspiration from the human perceptive system, we created a synthetic analog. Our synthetic system builds models using a redundant and unstructured sensor topology embedded in a soft actuator, a vision-based motion capture system for ground truth, and a general machine learning approach. This allows us to model an unknown soft actuated system. We demonstrate that the proposed approach is able to model the kinematics of a soft continuum actuator in real time while being robust to sensor nonlinearities and drift. In addition, we show how the same system can estimate the applied forces while interacting with external objects. The role of action in perception is also presented. This approach enables the development of force and deformation models for soft robotic systems, which can be useful for a variety of applications, including human-robot interaction, soft orthotics, and wearable robotics.
The fabrication of nanowire (NW)‐based flexible electronics including wearable energy storage devices, flexible displays, electrical sensors, and health monitors has received great attention both in fundamental research and market requirements in our daily lives. Other than a disordered state after synthesis, NWs with designed and hierarchical structures would not only optimize the intrinsic performance, but also create new physical and chemical properties, and integration of individual NWs into well‐defined structures over large areas is one of the most promising strategies to optimize the performance of NW‐based flexible electronics. Here, the recent developments and achievements made in the field of flexible electronics composed of integrated NW structures are presented. The different assembly strategies for the construction of 1D, 2D, and 3D NW assemblies, especially the NW coassembly process for 2D NW assemblies, are comprehensively discussed. The improvements of different NW assemblies on flexible electronics structure and performance are described in detail to elucidate the advantages of well‐defined NW assemblies. Finally, a short summary and outlook for future challenges and perspectives in this field are presented. Directional assembly of nanowires into 1D, 2D, and 3D assemblies toward flexible electronic devices benefits many potential applications. 1D assemblies with fiber structures can be used as flexible electronics for textiles, 2D assemblies can be used as transparent electrodes or units for logic circuits, and 3D assemblies can be used in the fabrication of pressure sensors or high‐performance energy storage devices.
Gas nanosensors, comprised of arrays of nanoelectrodes with finger-widths of ~100 nm developed by electron beam lithography and aerosol assisted chemical vapor deposited non-functionalized and Pt-functionalized tungsten oxide nanowires (<100 nm) subsequently integrated across the pairs of electrodes via dielectrophoresis method, are developed in this work. The functionality of these devices is validated towards various concentrations of NO2 and C2H5OH. Results demonstrate reproducible and consistent responses with better sensitivity and partial selectivity for the non-functionalized systems to NO2, as opposed to the Pt-functionalized systems, which display better sensing properties towards C2H5OH with a loss of response to NO2, that, in turn, increases the cross-sensitivity between these gases. These results are explained on the basis of the additional chemical and electronic interactions at the Pt/tungsten oxide interface, which increase the pre-adsorption of oxygen species and make the functionalized surface rather more sensitive to C2H5OH than to NO2, in contrast to the non-functionalized surface.
Resistive devices composed of one dimensional nanostructures are promising candidate for next generation gas sensors. However, the large-scale fabrication of nanowires is still a challenge, restricting the commercialization of such type of devices. Here, we reported a highly efficient and facile approach to fabricate poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) nanowire chemiresistive type of gas sensor by nanoscale soft lithography. Well-defined sub-100 nm nanowires are fabricated on silicon substrate which facilitates the device integration. The nanowire chemiresistive gas sensor is demonstrated for NH3 and NO2 detection at room-temperature and shows a limit of detection at ppb level which is compatible with nanoscale PEDOT:PSS gas sensors fabricated with conventional lithography technique. In comparison with PEDOT:PSS thin film gas sensor, the nanowire gas sensor exhibits a higher sensitivity and much faster response to gas molecules.
Some materials feel colder to the touch than others, and we can use this difference in perceived coldness for material recognition. This review focuses on the mechanisms underlying material recognition based on thermal cues. It provides an overview of the physical, perceptual, and cognitive processes involved in material recognition. It also describes engineering domains in which material recognition based on thermal cues have been applied. This includes haptic interfaces that seek to reproduce the sensations associated with contact in virtual environments and tactile sensors the aim for automatic material recognition. The review concludes by considering the contributions of this line of research in both science and engineering.
Epidermal electronic systems (EESs) are skin-like electronic systems, which can be used to measure several physiological parameters from the skin. This paper presents materials and a simple, straightforward fabrication process for skin-conformable inkjet-printed temperature sensors. Epidermal temperature sensors are already presented in some studies, but they are mainly fabricated using traditional photolithography processes. These traditional fabrication routes have several processing steps and they create a substantial amount of material waste. Hence utilizing printing processes, the EES may become attractive for disposable systems by decreasing the manufacturing costs and reducing the wasted materials. In this study, the sensors are fabricated with inkjet-printed graphene/PEDOT:PSS ink and the printing is done on top of a skin-conformable polyurethane plaster (adhesive bandage). Sensor characterization was conducted both in inert and ambient atmosphere and the graphene/PEDOT:PSS temperature sensors (thermistors) were able reach higher than 0.06% per degree Celsius sensitivity in an optimal environment exhibiting negative temperature dependence.
Despite the emergence of flexible and stretchable actuators, few possess sensing capabilities. Here, we present a facile method of integrating a flexible pneumatic actuator with stretchable strain sensor to form a soft sensorized actuator. The elastomeric actuator comprises a microchannel connected to a controlled air source to achieve bending. The strain sensor comprises a thin layer of screen-printed silver nanoparticles on an elastomeric substrate to achieve its stretchability and flexibility while maintaining excellent conductivity at ≈8 Ω sq–1. By printing a mesh network of conductive structures, our strain sensor is able to detect deformations beyond 20% with a high gauge factor beyond 50 000. The integration of a pneumatic soft actuator with our sensing element enables the measurement of the extent of actuator bending. To demonstrate its potential as a rehabilitation sensing actuator, we fit the sensorized actuator in a glove to further analyze finger kinematics. With this, we are able to detect irregular movement patterns in real time and assess finger stiffness or dexterity.
The excellent compliance and large range of motion of soft actuators controlled by fluid pressure has lead to strong interest in applying devices of this type for biomimetic and human-robot interaction applications. However, in contrast to soft actuators fabricated from stretchable silicone materials, conventional technologies for position sensing are typically rigid or bulky and are not ideal for integration into soft robotic devices. Therefore, in order to facilitate the use of soft pneumatic actuators in applications where position sensing or closed loop control is required, a soft pneumatic bending actuator with an integrated carbon nanotube position sensor has been developed. The integrated carbon nanotube position sensor presented in this work is flexible and well suited to measuring the large displacements frequently encountered in soft robotics. The sensor is produced by a simple soft lithography process during the fabrication of the soft pneumatic actuator, with a greater than 30% resistance change between the relaxed state and the maximum displacement position. It is anticipated that integrated resistive position sensors using a similar design will be useful in a wide range of soft robotic systems.
The detection of biological and chemical species is central to many areas of healthcare and the life sciences, ranging from uncovering and diagnosing disease to the discovery and screening of new drug molecules. Hence, the development of new devices that enable direct, sensitive, and rapid analysis of these species could impact humankind in significant ways. Devices based on nanowires are emerging as a powerful and general class of ultrasensitive, electrical sensors for the direct detection of biological and chemical species.
This paper presents a feasibility study of a central pattern generator-based analog controller for an autonomous robot. The operation of a neuronal circuit formed of electronic neurons based on Hindmarsh–Rose neuron dynamics and first order chemical synapses is modeled. The controller is based on a standard CMOS process with 2 V supply voltage. In order to achieve low power consumption, CMOS subthreshold circuit techniques are used. The controller generates an excellent replica of the walking motor program and allows switching between walking in different directions in response to different command inputs.The simulated power consumption is 4.8 mW and die size including I/O pads is 2.2 mm by 2.2 mm. Simulation results demonstrate that the proposed design can generate adaptive walking motor programs to control the legs of autonomous robots.
The objective of these two experiments was to determine the role of thermal cues in material discrimination and localization, using materials that spanned a range of thermal properties. In the first experiment, the subjects were required to select the cooler of two materials presented to the index fingers. In the second, the finger that was in contact with a material that was different from that presented to the other two fingers on the same hand had to be identified. The results indicated that the subjects were able to discriminate between materials, using thermal cues, when the differences in their thermal properties were large. The changes in skin temperature when the fingers were touching the materials were, however, smaller than those predicted by the theoretical model. The ability to localize the thermal changes when three fingers on the same hand were stimulated was poor and depended on both the thermal properties of the target and the distractor materials.
The ability to produce distributed sensors by tailoring materials readily available on the market is becoming an emerging strategy for Internet of Things applications. Embedding sensors into functional substrates allows one to reduce costs and improve integration and gives unique functionalities inaccessible to silicon or other conventional materials used in microelectronics. In this paper, we demonstrate the functionalization of a commercial polyurethane (PU) foam with the conductive polymer PEDOT:PSS: the resulting material is a modified all-polymeric foam where the internal network of pores is uniformly coated with a continuous layer of PEDOT:PSS acting as a mechanical transducer. When an external force causes a modification of the foam microstructure, the conductivity of the device varies accordingly, enabling the conversion of a mechanical pressure into an electric signal. The sensor provides a nearly linear response when stimulated by an external pressure in the range between 0.1 and 20 kPa. Frequency-dependent measurements show a useful frequency range up to 20 Hz. A simple micromechanical model has been proposed to predict the device performance based on the characteristics of the system, including geometrical constrains, the microstructure of the polymeric foam, and its elastic modulus. By taking advantage of the simulation output, a flexible shoe in sole prototype has been developed by embedding eight pressure sensors into a commercial PU foam. The proposed device may provide critical information to medical teams, such as the real-time bodyweight distribution and a detailed representation of the walking dynamic.
Robot hands with tactile perception can improve the safety of object manipulation and also improve the accuracy of object identification. Here, we report the integration of quadruple tactile sensors onto a robot hand to enable precise object recognition through grasping. Our quadruple tactile sensor consists of a skin-inspired multilayer microstructure. It works as thermoreceptor with the ability to perceive thermal conductivity of a material, measure contact pressure, as well as sense object temperature and environment temperature simultaneously and independently. By combining tactile sensing information and machine learning, our smart hand has the capability to precisely recognize different shapes, sizes, and materials in a diverse set of objects. We further apply our smart hand to the task of garbage sorting and demonstrate a classification accuracy of 94% in recognizing seven types of garbage.
Discrimination of surface textures and their surface roughness using tactile sensors have attracted increasing attention. Highly sensitive tactile sensors with the ability to recognize and discriminate the surface textures and roughness of grasped objects are crucial for intelligent robotics. This paper presents a methodology by using the developed WMB model (W-M function and Beam-Bundle Theory) and an algorithm based on artificial neural network to study the performance of a flexible tactile sensor for surface texture classification. For the WMB model, the quasi-3D surfaces of specific objects are reconstructed based on W-M function and basic statistical theory. A simplified Beam-Bundle Model is utilized to represent the cover layer of the sensor and simulates the normal force fluctuations during sliding movements. According to the simulation results, surface textures can be classified by the characteristic frequency cluster (CFC) existing in the fluctuation of curve’s spectrum. As an experiment, an artificial neural network is established to classify surface textures based on voltage signals from the tactile sensor. An MAF array represents the CFC information and improves the classification accuracy from 78 % to 82 %. The results demonstrate the effectiveness of the proposed WMB model and that it provides a new method of analysis involving robotic tactile interactions.
There has been a great deal of interest in designing soft robots that can mimic a human system with haptic and proprioceptive functions. There is now a strong demand for soft robots that can sense their surroundings and functions in harsh environments. This is because the wireless sensing and actuating capabilities of these soft robots are very important for monitoring explosive gases in disaster areas and for moving through contaminated environments. To develop these wireless systems, complex electronic circuits must be integrated with various sensors and actuators. However, the conventional electronic circuits based on silicon are rigid and fragile, which can limit their reliable integration with soft robots for achieving continuous locomotion. In our study, we developed an untethered, soft robotic hand that mimics human fingers. The soft robotic fingers are composed of a thermally responsive elastomer composite that includes capsules of ethanol and liquid metals for its shape deformation through an electrothermal phase transition. And these soft actuators are integrated fully with flexible forms of heaters, with pressure, temperature, and hydrogen gas sensors, and wireless electronic circuits. Entire functions of this soft hand, including the gripping motion of soft robotic fingers and the real-time detections of tactile pressures, temperatures, and hydrogen gas concentrations, are monitored or controlled wirelessly using a smartphone. This wireless sensing and actuating system for somatosensory and respiratory functions of a soft robot provides a promising strategy for next-generation robotics.
Development of wearable devices for continuous respiration monitoring is of great importance for evaluating human health. Here, we propose a new strategy to achieve rapid respiration response by confining conductive polymers into 1D nanowires which facilitates the water molecules absorption/desorption and maximizes the sensor response to moisture. The nanowires arrays were fabricated through a low-cost nanoscale printing approach on flexible substrate. The nanoscale humidity sensor shows a high sensitivity (5.46%) and ultrafast response (0.63 s) when changing humidity between 0 and 13% and can tolerate 1000 repetitions of bending to a curvature radius of 10 mm without influencing its performance. Benefited by its fast response and low power assumption, the humidity sensor was demonstrated to monitor human respiration in real time. Different respiration patterns including normal, fast and deep respiration can be distinguished accurately.
Multisensory tactile systems play an important role in enhancing robot intelligence. A competent robotic tactile system needs to be simple in structure and easily operated, especially with multiple sensations, and have good coordinate ability like human skin. A novel multisensory tactile system for humanoid robotic hands is proposed, allowing the hand to identify objects by grasping and manipulating them. Robotic multifunction sensors based on skin‐inspired thermosensation and structured with micro platinum ribbons partially covered with piezo‐thermic silver‐nanoparticle‐doped porous polydimethylsiloxane membrane simultaneously and independently detect contact pressure, local ambient temperature, and thermal conductivity and temperature of an object. Multiple tactile information which is obtained by the robotic sensors is fused comprehensively based on neural network classification to identify diverse objects in uncertain and dynamic gripping with an object recognition accuracy of 95%. This multisensory system enables robotic hand to better interact with its environment, enhances robotic intelligence, and makes various complex tasks feasible for robots, such as sorting material or rescuing from fire or other disasters. A multisensory tactile system enables a robotic hand to intelligently recognize objects. Robotic multifunction sensors based on thermosensation and structured with platinum ribbons partially covered with piezo‐thermic membrane are used to simultaneously and independently measure contact pressure, temperature, and thermal conductivity of object are simultaneously and independently detected objects. Multi‐sensory information is fused based on neural‐network classification to achieve precise recognition in uncertain and dynamic gripping.
Flexible and wearable pressure sensors are of paramount importance for development of personalized medicine and electronic skin. However, the preparation of easily disposed pressure sensors is still facing with pressing challenges. Herein, we have developed an all paper-based piezoresistive (APBP) pressure sensor through a facile, cost-effective and environment-friendly method. This pressure sensor was based on tissue paper coated with silver nanowires (AgNWs) as sensing material, nanocellulose paper (NCP) as bottom substrate for printing electrodes, and NCP as top encapsulating layer. The APBP pressure sensor showed high sensitivity of 1.5 kPa-1 in the range of 0.3-30.2 kPa and retained excellent performance in bending state. Furthermore, the APBP sensor has been mounted on the human skin to monitor human physiological signals (such as arterial heart pulse and pronunciation from throat) and successfully applied as soft electronic skin to response to external pressure. Due to the use of common tissue paper, NCP, AgNWs and conductive nanosilver ink only, the pressure sensor has low-cost, facile-craft and fast-preparation advantages and can be disposed easily by incineration. We believe that the developed sensor will propel the advance of easily disposed pressure sensor and green paper-based flexible electronic devices.
We develop a flexible and multifunctional resistive sensor integrating uniform conductive coating layers with interlaced nanofibrous structure through a large-scale and cost-efficient strategy. Elastomer nanofiber framework not only endows superior flexibility for the multi-mode sensor, but also provides abundant contact sites and contact area which can significantly enhance the sensitivity and operating range of obtained sensor. More impressively, the multilevel sensing paths comprised of both interlaminar and intrastratal signal transmissions fulfill the simultaneous and precise detection of pressure-temperature stimuli without interference to each other. The achieved sensor ultimately shows an ultrahigh pressure sensitivity of 1185.8 kPa-1 and superior reliability, enabling the rapid detection of subtle stimulus as low as 2.4 Pa and superior response behavior under 5000 cyclic loading tests. Besides, high linearity and stability are achieved for temperature sensing characteristic even under various pressure loadings. These outstanding performances are further evaluated by preparing 4×5 bimodal sensor array to synchronously monitor multiple signals, consequently demonstrating precise sensing capability with negligible interference and providing effective approach for developing multiparametric sensing platforms and wearable devices.
Glucose metabolism plays an important role in cell energy supply, and quantitative detection of intracellular glucose level is particularly important for understanding many physiological processes. Glucose electrochemical sensors are widely used for blood and extracellular glucose detection. However, intracellular glucose detection cannot be achieved by these sensors owing to their large size and consequent low spatial resolution. Herein, we developed a single nanowire glucose sensor for electrochemical detection of intracellular glucose by depositing Pt nanoparticles (Pt NPs) on a SiC@C nanowire and further immobilizing glucose oxidase (GOD) there on. Glucose was converted by GOD to an electroactive product H2O2 which was further electro-catalyzed by Pt NPs. The glucose nanowire sensor is endowed with high sensitivity, high spatial-temporal resolution and enzyme specificity due to its nanoscale size and enzymatic reaction. This allows the real-time monitoring of the intracellular glucose level, and the increasement of intracellular glucose level induced by a novel potential hypoglycemic agent, reinforcing its potential application in lowering the blood glucose level. This work provides a versatile method for the construction of enzyme-modified nanosensors to electrochemically detect intracellular non-electroactive molecules, which is of great benefit to the physiological and pathological studies.
Recently, cellulose paper based materials are emerging for applications in wearable “green” electronics due to their earth-abundance, low cost, light weight, flexibility, and sustainability. Herein, for the first time, we develop an almost all cellulose paper based pressure sensor through a facile, cost-effective, scalable, and environment-friendly approach. The screen-printed interdigital electrodes on the flat printing paper and the carbonized crepe paper (CCP) with a good conductivity are integrated into a flexible pressure sensor as substrates and active materials, respectively. The porous and corrugated structure of the CCP endows the pressure sensor with high sensitivity (2.56-5.67 KPa⁻¹ in the range of 0-2.53 KPa), wide workable pressure range (0-20 KPa), fast response time (<30 ms), low detection limit (~0.9 Pa), and good durability (>3000 cycles). Additionally, we demonstrate the practical applications of the CCP pressure sensor in detection of finger touching, wrist pulse, respiration, phonation, and acoustic vibration, etc., and real-time monitoring of spatial pressure distribution. The proposed CCP pressure sensor has great potentials in various applications as wearable electronics. Moreover, the subtle fabrication of desired materials based on commercially available products provides new insights into the development of green electronics.
Flexible and wearable pressure sensor may offer convenient, timely, and portable solutions to human motion detection, yet it is a challenge to develop cost-effective materials for pressure sensor with high compressibility and sensitivity. Herein, a cost-efficient and scalable approach is reported to prepare highly flexible and compressible conductive sponge for piezoresistive pressure sensor. The conductive sponge, PEDOT:PSS@MS, is prepared by one-step dip coating the commercial melamine sponge (MS) in an aqueous dispersion of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Due to the interconnected porous structure of MS, the conductive PEDOT:PSS@MS has high compressibility and stable piezoresistive response at the compressive strain up to 80%, as well as good reproducibility over 1000 cycles. Thereafter, versatile pressure sensors fabricated using the conductive PEDOT:PSS@MS sponges are attached to the different parts of human body; the capabilities of these devices to detect a variety of human motions including speaking, finger bending, elbow bending, and walking are evaluated. Furthermore, prototype tactile sensory array based on these pressure sensors is demonstrated.
Humans possess manual dexterity, motor skills, and other physical abilities that rely on feedback provided by the somatosensory system. Herein, a method is reported for creating soft somatosensitive actuators (SSAs) via embedded 3D printing, which are innervated with multiple conductive features that simultaneously enable haptic, proprioceptive, and thermoceptive sensing. This novel manufacturing approach enables the seamless integration of multiple ionically conductive and fluidic features within elastomeric matrices to produce SSAs with the desired bioinspired sensing and actuation capabilities. Each printed sensor is composed of an ionically conductive gel that exhibits both long-term stability and hysteresis-free performance. As an exemplar, multiple SSAs are combined into a soft robotic gripper that provides proprioceptive and haptic feedback via embedded curvature, inflation, and contact sensors, including deep and fine touch contact sensors. The multimaterial manufacturing platform enables complex sensing motifs to be easily integrated into soft actuating systems, which is a necessary step toward closed-loop feedback control of soft robots, machines, and haptic devices.
In this study, we demonstrate a fabrication of highly sensitive flexible temperature sensor with a bioinspired octopus-mimicking adhesive. A resistor-type temperature sensor consisting of a composite of poly(N isopropylacrylamide) (pNIPAM)-temperature sensitive hydrogel, poly(3,4 ethylenedioxythiophene) polystyrene sulfonate, and carbon nanotubes, exhibits a very high thermal sensitivity of 2.6%•ºC⁻¹ between 25 and 40 °C so that the change in skin temperature of 0.5 °C can be accurately detected. At the same time, the polydimethylsiloxane adhesive layer of octopus-mimicking rim structure coated with pNIPAM, is fabricated through the formation of a single mold by utilizing undercut phenomenon in photolithography. The fabricated sensor shows stable and reproducible detection of skin-temperature under repeated attachment/detachment cycles onto skin without any skin irritation for a long time. This work suggests a high potential application of our skin-attachable temperature sensor to wearable devices for medical and healthcare monitoring.
Identification of the material from which an object is made is of significant value for effective robotic grasping and manipulation. Characteristics of the material can be retrieved using different sensory modalities: vision based, tactile based or sound based. Compressibility, surface texture and thermal properties can each be retrieved from physical contact with an object using tactile sensors. This paper presents a method for collecting data using a biomimetic fingertip in contact with various materials and then using these data to classify the materials both individually and into groups of their type. Following acquisition of data, principal component analysis (PCA) is used to extract features. These features are used to train seven different classifiers and hybrid structures of these classifiers for comparison. For all materials, the artificial systems were evaluated against each other, compared with human performance and were all found to outperform human participants’ average performance. These results highlighted the sensitive nature of the BioTAC sensors and pave the way for research that requires a sensitive and accurate approach such as vital signs monitoring using robotic systems.
Working together, heated and unheated temperature sensors can recognize contact with different materials and contact with the human body. As such, distributing these sensors across a robot's body could be beneficial for operation in human environments. We present a stretchable fabric-based skin with force and thermal sensors that is suitable for covering areas of a robot's body, including curved surfaces. It also adds a layer of compliance that conforms to manipulated objects, improving thermal sensing. Our open hardware design addresses thermal sensing challenges, such as the time to heat the sensors, the efficiency of sensing, and the distribution of sensors across the skin. It incorporates small self-heated temperature sensors on the surface of the skin that directly make contact with objects, improving the sensors’ response times. Our approach seeks to fully cover the robot's body with large force sensors, but treats temperature sensors as small, point-like sensors sparsely distributed across the skin. We present a mathematical model to help predict how many of these point-like temperature sensors should be used in order to increase the likelihood of them making contact with an object. To evaluate our design, we conducted tests in which a robot arm used a cylindrical end effector covered with skin to slide objects and press on objects made from four different materials. After assessing the safety of our design, we also had the robot make contact with the forearms and clothed shoulders of 10 human participants. With 2.0 s of contact, the actively-heated temperature sensors enabled binary classification accuracy over 90% for the majority of material pairs. The system could more rapidly distinguish between materials with large differences in their thermal effusivities (e.g., 90% accuracy for pine wood vs. aluminum with 0.5 s of contact). For discrimination between humans vs. the four materials, the skin's force and thermal sensing modalities achieved 93% classification accuracy with 0.5 s of contact. Overall, our results suggest that our skin design could enable robots to recognize contact with distinct task-relevant materials and humans while performing manipulation tasks in human environments.
Approaching to the super-sensitivity, we herein develop a pressure sensor based on fully bubbled ultralight graphene block through a simple sparkling strategy. The obtained sparking graphene block (SGB) exhibits excellent elasticity even at 95% compressive strain and rebounds a steel ball with an ultra-fast recovery speed (~1085 mm s−1). Particularly, the SGB based sensor reveals a record pressure sensitivity of 229.8 kPa−1, much higher than other graphene materials due to the highly cavity-branched internal structure. Impressively, the pressure sensor can detect the extremely gentle pressures even beyond the real human skin, promising for ultrasensitive sensing applications.
In this review, PEDOT-PSS is mainly a commercially available PEDOT-PSS, which is a water-dispersible form of the intrinsically conducting PEDOT doped with the water-soluble PSS, including its derivatives, copolymers, analogs (PEDOT:PSSs), even their composites via the chemical or physical modification toward the structure of PEDOT and/or PSS. First, we will focus on discussing the scientific importance of PEDOT-PSS in conjunction with its extraordinary properties and broad multidisciplinary applications in organic/polymeric electronics and optoelectronics from the viewpoint of the historical development and the promising application of representative ECPs. Subsequently, versatile film-forming techniques for the preparation of PEDOT-PSS film electrode were described in details, including common coating approaches and printing techniques, and many emerging preparative methods were mentioned. Then challenges (e.g., conductivity, stability in Water, adhesion to substrate electrode) of PEDOT-PSS film electrode for devices under the high humidity/watery circumstances, especially electrochemical devices are discussed. Fourth, we take PEDOT-PSS film electrode for a relatively new application in sensors as an example, mainly summarized advances in the development of various sensors based on PEDOT-PSSs and their composites in combination with its preparative methods and extraordinary properties. Finally, we give the outlook of PEDOT-PSS for possible applications with the emphasis on PEDOT-PSS film electrode for electrochemical devices, including sensors. (c) 2016 Wiley Periodicals, Inc.
Bending and pressure sensors are very essential for evaluating external stimuli in human motions; however, most of them are separate devices. Here, two orthogonal carbon nanotube–polyurethane sponge strips (CPSSs) are used, each of which has different resistances when bent or pressed, to fabricate a multi-functional stretchable sensor capable of detecting omnidirectional bending and pressure independently. Due to the shape of the strip, the resistance of CPSS changes differently when bent along different directions. Based on this feature, two perpendicular CPSSs can reflect information of both bending distance and bending direction. After basic measurement data are obtained, a function set can be formulated to calculate bending distance and bending direction simultaneously. The errors of bending distance and bending angle can be controlled to less than 4%. With the help of the triboelectric effect, which only happens when the device is pressed, the sensor can differentiate bending and pressure effectively, ensuring the device works in complex situations.
An ultra-stretchable and force-sensitive hydrogel with surface self-wrinkling microstructure is demonstrated by in situ synthesizing polyacrylamide (PAAm) and polyaniline (PANI) in closely packed swollen chitosan microspheres, exhibiting ultra-stretchability (>600%), high sensitivity (0.35 kPa(-1) ) for subtle pressures (<1 kPa), and can detect force in a broad range (10(2) Pa-10(1) MPa) with excellent electrical stability and rapid response speed, potentially finding applications for E-skin.
Stretchable and multifunctional sensors can be applied in multifunctional sensing devices, safety forewarning equipment, and multiparametric sensing platforms. However, a stretchable and multifunctional sensor was hard to fabricate until now. Herein, a scalable and efficient fabrication strategy is adopted to yield a sensor consisting of ZnO nanowires and polyurethane fibers. The device integrates high stretchability (tolerable strain up to 150%) with three different sensing capabilities, i.e., strain, temperature, and UV. Typically achieved specifications for strain detection are a fast response time of 38 ms, a gauge factor of 15.2, and a high stability of >10 000 cyclic loading tests. Temperature is detected with a high temperature sensitivity of 39.3% °C−1, while UV monitoring features a large ON/OFF ratio of 158.2. With its fiber geometry, mechanical flexibility, and high stretchability, the sensor holds tremendous prospect for multiparametric sensing platforms, including wearable devices.
Wearable human-interactive devices are advanced technologies that will improve the comfort, convenience, and security of humans, and have a wide range of applications from robotics to clinical health monitoring. In this study, a fully printed wearable human-interactive device called a “smart bandage” is proposed as the first proof of concept. The device incorporates touch and temperature sensors to monitor health, a drug-delivery system to improve health, and a wireless coil to detect touch. The sensors, microelectromechanical systems (MEMS) structure, and wireless coil are monolithically integrated onto flexible substrates. A smart bandage is demonstrated on a human arm. These types of wearable human-interactive devices represent a promising platform not only for interactive devices, but also for flexible MEMS technology.
Soft robots actuated by inflation of a pneumatic network (a “pneu-net”) of small channels in elastomeric materials are appealing for producing sophisticated motions with simple controls. Although current designs of pneu-nets achieve motion with large amplitudes, they do so relatively slowly (over seconds). This paper describes a new design for pneu-nets that reduces the amount of gas needed for inflation of the pneu-net, and thus increases its speed of actuation. A simple actuator can bend from a linear to a quasi-circular shape in 50 ms when pressurized at ΔP = 345 kPa. At high rates of pressurization, the path along which the actuator bends depends on this rate. When inflated fully, the chambers of this new design experience only one-tenth the change in volume of that required for the previous design. This small change in volume requires comparably low levels of strain in the material at maximum amplitudes of actuation, and commensurately low rates of fatigue and failure. This actuator can operate over a million cycles without significant degradation of performance. This design for soft robotic actuators combines high rates of actuation with high reliability of the actuator, and opens new areas of application for them.
This paper reports on the preliminary experimental results of using polydimethylsiloxane (PDMS) to manufacture a visual pulsating heat pipe with length, width and internal diameter of 56mm, 50mm and 2mm, respectively, including the manufacturing process, the vacuuming management for filling and packaging. The experiment used methanol and ethanol as working fluids. A fix filled ratio (about 60%) and different heating power values (3–8W) were used to test the thermal performance. A high-speed video camera was used to record the working situation of the working fluid inside the channel. The results are discussed and analyzed.The experiment shows that methanol, in a vertical orientation, shows the most efficient results. When the heating power is 3W, the thermal resistance is more than 4.5°C/W below the value for ethanol as the working fluid. For a heating power of 4W, the average temperature decreases to 15°C in the evaporator. Also, gravity will have an impact on the PHP performance: the vertical orientation is better as compared to the horizontal orientation.
silicon nanowires (SiNWs) with well orientation and crystallization are synthesized by the vapor-liquid-solid (VLS) process, and doped as n-type by an ex situ process using spin on dopant (SOD) technique. The ex situ doping process using SOD was based on solid-state diffusion, which comprised two stages: pre-coating and drive in. The phosphorous concentration in SiNWs was controlled by appropriate selections of the drive in temperature and the time period, which are 950 o C and 5-60 min in the present studies. The doped nanowire can be readily made into a temperature sensor with much better resolution and response. Calibration of the SiNW temperature sensor at different doping level has been performed. With a concentration of 4 x 10 15 atoms/cm 3 the SiNW sensor has the best temperature resolution (6186 :� / o C) and sensitivity in this study. Keywords-SiNWs; VLS synthesis; Ex situ doping; SOD;
We demonstrated the first application of a pyroelectric nanogenerator as a self-powered sensor (or active sensor) for detecting a change in temperature. The device consists of a single lead zirconate titanate (PZT) micro/nanowire that is placed on a thin glass substrate and bonded at its two ends, and it is packaged by polydimethylsiloxane (PDMS). By using the device to touch a heat source, the output voltage linearly increases with an increasing rate of change in temperature. The response time and reset time of the fabricated sensor are about 0.9 and 3 s, respectively. The minimum detecting limit of the change in temperature is about 0.4 K at room temperature. The sensor can be used to detect the temperature of a finger tip. The electricity generated under a large change in temperature can light up a liquid crystal display (LCD).
Electron microscopy studies are used to explore the morphology of thin poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT:PSS) films. The figures show that the films are composed of grains with diameters in the range of about 50 nm. Energy dispersive X-ray spectroscopy analysis reveals that individual grains have a PEDOT-rich core and a PSS-rich shell with a thickness of about 5–10 nm. Atomic force microscopy (AFM) is then used to analyze the topography of fracture surfaces of ruptured PEDOT:PSS tensile specimens. These AFM scans also show that the films are composed of grains dispersed in a matrix. The investigations presented herein yield a picture of PEDOT:PSS morphology with unprecedented clarity.
