Carbon fibre based flexible piezoresistive composites to empower inherent sensing capabilities for soft actuators

Microstructured Polyfluoroacrylate Elastomeric Dielectric Layer for Highly Stretchable Wide-Range Capacitive Pressure Sensors

Capacitive pressure sensors capable of replicating human tactile senses have garnered tremendous attention. Introducing microstructures into the dielectric layer is an effective approach to improve the sensitivity of the sensors. However, most reported processes to fabricate microstructured dielectric layers are complicated and time-consuming and usually have adverse effects on the mechanical properties. Herein, we report a mechanically strong and highly stretchable dielectric layer fabricated from a microstructured fluorinated elastomer with a high dielectric constant (5.8 at 1000 Hz) via a simple and low-cost thermal decomposition process. Capacitive pressure sensors based on this microstructured fluorinated elastomer dielectric layer and soft ionotronic electrodes illustrate an impressing stretchability (>300%), a high pressure sensitivity (17 MPa-1), a wide detection range (70 Pa-800 kPa), and a fast response time (below 300 ms). Moreover, the multipixel capacitive pressure sensors sensing array maintains the unique spatial tactile sensing performance even under significant tensile deformation. It is believed that our microstructured fluorinated elastomer dielectric layer might find wide applications in stretchable ionotronic devices.

Self-Assembly of Multiwalled Carbon Nanotubes on a Silicone Rubber Foam Skeleton for Durable Piezoresistive Sensors

Conductive nanomaterial/flexible polymer composite foams are of great interest in the field of flexible and wearable piezoresistive pressure sensors. However, the existing composite foam sensors are faced with stability issues from conductive nanomaterials, which tends to decrease their long-term durability. Herein, we developed a solvent evaporation-induced self-assembly strategy, which could significantly improve the stability of multiwalled carbon nanotubes (MWCNTs) on a silicone rubber foam skeleton. The process for self-assembly of MWCNTs was straightforward. Aqueous MWCNT dispersion droplets were first hierarchically enclosed in silicone rubber via water-in-oil (W/O) Pickering high internal phase emulsions (HIPEs). Then, the high pressure generated by fast evaporation of the solvent from the droplets could break the thinnest pore walls to form interconnected pores. As a result, very dense and firm MWCNT layers were self-assembled on the pore wall surface. Due to the excellent stability of MWCNTs and tetramodal interconnected porosity, our MWCNTs/silicone rubber composite foam showed the following "super" properties: low density of 0.26 g/mL, high porosity of 76%, and excellent mechanical strength (the maximum stress loss of 8.3% at 80% strain after 100 compression cycles). In addition, excellent piezoresistive performance, including superior discernibility for different amplitudes of compressive strain (up to 80%), rapid response time (150 ms), and high sensitivity (gauge factor of 1.44), was demonstrated for such foams, together with prominent durability (39,000 compression cycles at 60% strain in air) and excellent stability of resistance response in water and organic solvents (5000 compression cycles at 30% strain in water and ethanol). Regarding its application, a wearable piezoresistive sensor, which was assembled from the as-prepared conductive silicone rubber composite foam, could capture various movements from tiptoeing and finger bending to small deformations resulting from human pulse.

An End-to-End Dynamic Posture Perception Method for Soft Actuators Based on Distributed Thin Flexible Porous Piezoresistive Sensors

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.

Research status of polysiloxane-based piezoresistive flexible human electronic sensors

Flexible human body electronic sensor is a multifunctional electronic device with flexibility, extensibility, and responsiveness. Piezoresistive flexible human body electronic sensor has attracted the extensive attention of researchers because of its simple preparation process, high detection sensitivity, wide detection range, and low power consumption. However, the wearability and affinity to the human body of traditional flexible human electronic sensors are poor, while polysiloxane materials can be mixed with other electronic materials and have good affinity toward the human body. Therefore, polysiloxane materials have become the first choice of flexible matrixes. In this study, the research progress and preparation methods of piezoresistive flexible human electronic sensors based on polysiloxane materials in recent years are summarized, the challenges faced in the development of piezoresistive flexible human electronic sensors are analyzed, and the future research directions are prospected.

Elastic Electrically Conductive Composites Based on Vapor-Grown Carbon Fibers for Use in Sensors

Elastic electrically conductive composites with an ethylene octene copolymer matrix (EOC) and vapor-grown carbon fibers (VGCF) were prepared by ultrasonication in a toluene solution, and their morphology, mechanical and electrical properties were also evaluated. EOC/CF composites were estimated for their mechanical and viscoelastic properties. The morphology of the composites was analyzed using scanning electron microscopy (SEM), and stress-strain curves were generated to measure the stress and tensile modulus of the composites. The experimental results were compared with various theoretical models, including the Burgers model, which separates viscoelastic behavior into several components. A dynamic mechanical analysis was also used to measure the composites' storage modulus, loss modulus, and damping factor at different frequencies. The composites' complex viscosity and storage modulus were increased with higher wt.% of CF, which enhances the elastic response. Electrical resistivity measurements were conducted on the composites and it was found that the resistivity decreased as the sample was loaded and increased as it was unloaded. Overall, the study provides insights into the mechanical and viscoelastic properties of EOC/CF composites, which could be helpful in developing sensors such as pressure/strain sensors.

Piezoresistive Fibers with Large Working Factors for Strain Sensing Applications

Piezoresistive fibers with large working factors remain of great interest for strain sensing applications involving large strains, yet difficult to achieve. Here, we produced strain-sensitive fibers with large working factors by dip-coating nanocomposite piezoresistive inks on surface-modified polyether block amide (PEBA) fibers. Surface modification of neat PEBA fibers was carried out with polydopamine (PDA) while nanocomposite conductive inks consisted of styrene-ethylene-butylene-styrene (SEBS) elastomer and carbon black (CB). As such, the deposition of piezoresistive coatings was enabled through nonconventional hydrogen-bonding interactions. The resultant fibers demonstrated well-defined piezoresistive linear relationships, which increased with CB filler loading in SEBS. In addition, gauge factors decreased with increasing CB mass fractions from ∼15 to ∼7. Furthermore, we used the fatigue theory to predict the endurance limit (C_e) of our fibers toward resistance signal stability. Such a piezoresistive performance allowed us to explore the application of our fibers as strain sensors for monitoring the movement of finger joints.

Bio-inspired flapping wing robots with foldable or deformable wings: a review

Traditional flapping-wing robots (FWRs) obtain lift and thrust by relying on the passive deformation of their wings which cannot actively fold or deform. In contrast, flying creatures such as birds, bats, and insects can maneuver agilely through active folding or deforming their wings. Researchers have developed many bio-inspired foldable or deformable wings (FDWs) imitating the wings of flying creatures. The foldable wings refer to the wings like the creatures' wings that can fold in an orderly manner close to their bodies. Such wings have scattered feathers or distinct creases that can be stacked and folded to reduce the body envelope, which in nature is beneficial for these animals to prevent wing damage and ensure agility in crossing bushes. The deformable wings refer to the active deformation of the wings using active driving mechanisms and the passive deformation under the aerodynamic force, which functionally imitates the excellent hydrodynamic performance of the deformable body and wings of the creatures. However, the shape and external profile changes of deformable wings tend to be much smaller than that of folding wings. FDWs enable the FWRs to improve flight degree of flexibility, maneuverability, and efficiency and reduce flight energy consumption. However, FDWs still need to be studied, and a comprehensive review of the state-of-the-art progress of FDWs in FWR design is lacking. This paper analyzes the wing folding and deformation mechanisms of the creatures and reviews the latest progress of FWRs with FDWs. Furthermore, we summarize the current limitations and propose future directions in FDW design, which could help researchers to develop better FWRs for safe maneuvering in obstacle-dense environments.

Printable Carbon Nanotube-Liquid Elastomer-Based Multifunctional Adhesive Sensors for Monitoring Physiological Parameters

Developing a printed elastomeric wearable sensor with good conformity and proper adhesion to skin, coupled with the capability of monitoring various physiological parameters, is very crucial for the development of point-of-care sensing devices with high precision and sensitivity. While there have been previous reports on the fabrication of elastomeric multifunctional sensors, research on the printable elastomeric multifunctional adhesive sensor is not very well explored. Herein, we report the development of a stencil printable multifunctional adhesive sensor fabricated in a solvent-free condition, which demonstrated the capability of having good contact with skin and its ability to function as a temperature and strain sensor. Functionalized liquid isoprene rubber was selected as the matrix while carboxylated multiwalled carbon nanotubes (c-CNTs) were used as the nanofiller. The selection of the above model compounds facilitated the printability and also helped the same composition to demonstrate stretchability and adhesiveness. A realistic three-dimensional microstructure (representative volume element model) was generated through a computational framework for the current c-CNT-liquid elastomer. Further computational simulations were performed to test and validate the correlation between electrical responses to that of experimental studies. Various physiological parameters like motion sensing, pulse, respiratory rate, and phonetics detection were detected by leveraging the electrically resistive nature of the sensor. This development route can be extended toward developing different innovative adhesives for point-of-care sensing applications.

Soft Robots' Dynamic Posture Perception Using Kirigami-Inspired Flexible Sensors with Porous Structures and Long Short-Term Memory (LSTM) Neural Networks

Soft robots can create complicated structures and functions for rehabilitation. The posture perception of soft actuators is critical for performing closed-loop control for a precise location. It is essential to have a sensor with both soft and flexible characteristics that does not affect the movement of a soft actuator. This paper presents a novel end-to-end posture perception method that employs flexible sensors with kirigami-inspired structures and long short-term memory (LSTM) neural networks. The sensors were developed with conductive sponge materials. With one-step calibration from the sensor output, the posture of the soft actuator could be calculated by the LSTM network. The method was validated by attaching the developed sensors to a soft fiber-reinforced bending actuator. The results showed the accuracy of posture prediction of sponge sensors with three kirigami-inspired structures ranged from 0.91 to 0.97 in terms of R2. The sponge sensors only generated a resistive torque value of 0.96 mNm at the maximum bending position when attached to a soft actuator, which would minimize the effect on actuator movement. The kirigami-inspired flexible sponge sensor could in future enhance soft robotic development.

Comparison between Piezoelectric and Piezoresistive Wearable Gait Monitoring Techniques

Insole plantar stress detection (PSD) techniques play an important role in gait monitoring. Among the various insole PSD methods, piezoelectric- and piezoresistive-based architectures are broadly used in medical scenes. Each year, a growing number of new research outcomes are reported. Hence, a deep understanding of these two kinds of insole PSD sensors and state-of-the-art work would strongly benefit the researchers in this highly interdisciplinary field. In this context, this review article is composed of the following aspects. First, the mechanisms of the two techniques and corresponding comparisons are explained and discussed. Second, advanced materials which could enhance the performance of current piezoelectric and piezoresistive insole prototypes are introduced. Third, suggestions for designing insole PSD prototypes/products for different diseases are offered. Last, the current challenge and potential future trends are provided.

Green Manufacturing of Flexible Sensors with a Giant Gauge Factor: Bridging Effect of CNT and Electric Field Enhancement at the Percolation Threshold

Toxic organic solvents are commonly used to disperse nanomaterials in the manufacturing of flexible conductive composites (e.g., graphene-PDMS). The dry-blended method avoids toxic organic solvent usage but leads to poor performance. Here, we proposed an innovative manufacturing method by adapting the traditional dry-blended method, including two key steps: minor CNT bridging and high-frequency electric field enhancement at the percolation threshold of graphene-PDMS. Significant improvement was achieved in the electrical conductivity (1528 times), the giant gauge factor (>8767.54), and the piezoresistive strain range (30 times) over the traditional dry-blended method. Further applications in measurements of culturing rat neonatal cardiomyocytes and mouse hearts proved that the proposed method has great potential for the manufacturing of nontoxic flexible sensors.

Sustainable-Macromolecule-Assisted Preparation of Cross-linked, Ultralight, Flexible Graphene Aerogel Sensors toward Low-Frequency Strain/Pressure to High-Frequency Vibration Sensing

Ultralight and highly flexible aerogel sensors, composed of reduced graphene oxide cross-linked by sustainable-macromolecule-derived carbon, are prepared via facile freeze-drying and thermal annealing. The synergistic combination of cross-linked graphene nanosheets and micrometer-sized honeycomb pores gives rise to the exceptional properties of the aerogels, including superior compressibility and resilience, good mechanical strength and durability, satisfactory fire-resistance, and outstanding electromechanical sensing performances. The corresponding aerogel sensors, operated at an ultralow voltage of 0.2 V, can efficiently respond to a wide range of strains (0.1-80%) and pressures (13-2750 Pa) even at temperatures beyond 300 °C. Moreover, the ultrahigh-pressure sensitivity of 10 kPa^-1 and excellent sensing stability and durability are accomplished. Strikingly, the aerogel sensors can also sense the vibration signals with ultrahigh frequencies of up to 4000 Hz for >1 000 000 cycles, significantly outperforming those of other sensors. These enable successful demonstration of the exceptional performance of the cross-linked graphene-based biomimetic aerogels for sensitive monitoring of mechanical signals, e.g., acting as wearable devices for monitoring human motions, and for nondestructive monitoring of cracks on engineering structures, showing the great potential of the aerogel sensors as next-generation electronics.

Ultrasensitive airflow sensor prepared by electrostatic flocking for sound recognition and motion monitoring

Recently, airflow sensors have attracted great attention due to their unique characteristics. However, the preparation of high-performance airflow sensors via extraordinarily simple, controllable and cost-effective methods remains a great challenge. Herein, inspired by the fluff system of the spider, an ultrasensitive fluffy-like airflow sensor with carbon fibers (CFs) uniformly and firmly planted on the surface of a polyvinyl alcohol (PVA) fibrous substrate has been easily fabricated using electrostatic flocking technology. The fluffy-like structure endows the airflow sensor with superior properties including ultra-sensitivity, fast response time (0.103 s), low airflow velocity detection limit (0.068 m s^-1), ultra-sensitive detection in a wide airflow range (0.068-16 m s^-1), and multi-directional consistent response to airflow. This sensor can be used to accurately recognize sound waves and voiceless speech and detect human and object motions in different postures and speeds. This work presents insights into designing and preparing high-performance airflow sensors on a large-scale for sound recognition, motion monitoring, and assisting the disabled.

All-Nanofiber Network Structure for Ultrasensitive Piezoresistive Pressure Sensors

Sensing materials with fiber structures are excellent candidates for the fabrication of flexible pressure sensors due to their large specific surface area and abundant contact points. Here, an ultrathin, flexible piezoresistive pressure sensor that consists of a multilayer nanofiber network structure prepared via a simple electrospinning technique is reported. The ultrathin sensitive layer is composite nanofiber films composed of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) and polyamide 6 (PEDOT:PSS/PA6) prepared by simultaneous electrospinning. PEDOT:PSS conductive fibers and PA6 elastic fibers are interwoven to form a multilayer network structure that can achieve ultrahigh sensitivity by forming a wealth of contact points during loading. In particular, gold-deposited PA6 fibers as upper and lower flexible electrodes can effectively increase the initial resistance. Due to this special fiber electrode structure, the sensor is able to generate a large electrical signal variability when subjected to a weak external force. The devices with different sensing properties can be obtained by controlling the electrospinning time. The sensor based on the PEDOT:PSS/PA6 nanofiber network has high sensitivity (6554.6 kPa^-1 at 0-1.4 kPa), fast response time (53 ms), and wide detection range (0-60 kPa). Significantly, the device maintains ultrahigh sensitivity when cyclically loaded over 10,000 cycles at 5 kPa, which makes it have great prospects for applications in human health monitoring and motion monitoring.

Beyond Chemistry: Tailoring Stiffness and Microarchitecture to Engineer Highly Sensitive Biphasic Elastomeric Piezoresistive Sensors

Carbon-based nanoparticles and conductive polymers are two classes of materials widely used in the production of three-dimensional (3D) piezoresistive sensors. One conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) has excellent stability and conductivity yet is limited in its application as a sensor, often existing upon a base, limiting its performance and potential. Despite much progress in the field of materials chemistry and polymer synthesis, one aspect we consider worthy of exploration is the impact that microstructure and stiffness may have on the sensitivity of 3D sensors. In this study, we report a strategy for fabricating biphasic electroactive sponges (EAS) that combine 3D porous PEDOT:PSS scaffolds possessing either an isotropic or anisotropic microarchitecture, infused with insulating elastomeric fillers of varying stiffness. When characterizing the electromechanical behavior of these EAS, a higher stiffness yields a higher strain gauge factor, with values as high as 387 for an isotropic microarchitecture infused with a stiff elastomer. The approach we describe is cost-effective and extremely versatile, by which one can fabricate piezoresistive sensors with adaptable sensitivity ranges and excellent high strain gauge factor with the underlying microarchitecture and insulant stiffness dictating this performance.

Tough Bioinspired Composites That Self-Report Damage

The increasing use of lightweight composite materials in structural applications requires the development of new damage monitoring technologies to ensure their safe use and prevent accidents. Although several molecular strategies have been proposed to report damage in polymers through mechanochromic responses, these approaches have not yet been translated into lightweight bioinspired composites for load-bearing applications. Here, we report on the development of bioinspired laminates of alternating polymer and nacre-like layers that combine optical translucency, high fracture toughness, and damage-reporting capabilities. The composites signal damage via a fluorescence color change that arises from the force activation of mechanophore molecules embedded in the material's polymer phase. A quantitative correlation between the applied strain and the fluorescence intensity was successfully established. We demonstrate that optical imaging of mechanically loaded composites allows for the localized detection of damage prior to fracture. This fluorescence-based self-reporting mechanism offers a promising approach for the early detection of damage in lightweight structural composites and can serve as a useful tool for the analysis of fracture processes in bulk transparent materials.

Nanoarchitectured porous carbons derived from ZIFs toward highly sensitive and selective QCM sensor for hazardous aromatic vapors

Metal-organic frameworks (MOFs) are a versatile source of carbon nanoarchitectures in gas sensing applications (Torad et al., 2019). Herein, several types of nanoporous carbons (NPCs) have been prepared by in-situ carbothermal treatment of zeolitic imidazolate frameworks (ZIFs) under different inert atmospheres to achieve a highly sensitive discrimination of vaporized aromatic compounds. In this study, we demonstrate how different carbonization conditions under the flow of N₂ or H₂ gases affect the surface area and the degree of graphitization of the resulting NPCs polyhedrons, and their consequent effect on the sensing performance in terms of sensitivity and selectivity toward toxic volatile hydrocarbons. A growth of carbon nanotubes (CNTs) is observed on the surface of polyhedral NPCs after careful carbonization of ZIF crystals under H₂ atmosphere. The fabricated quartz crystal microbalance (QCM) sensor with CNT-containing NPCs demonstrates increased sensitivity and selectivity towards toxic volatile aromatic hydrocarbons over the aliphatic analogues, suggesting the rich growth of hairy graphitic-like CNTs on the surface of carbon framework act as highly selective sensing antennae for vapor molecular discrimination of toxic aromatic hydrocarbons. Despite of increased selectivity towards volatile aromatic compounds, however, the surface area of CNT-rich NPCs derived from hybrid ZIFs and ZIF-67 is greatly sacrificed as compared to CNT-free NPCs from ZIF-8 polyhedron. In the case of Co-containing ZIF-67, the rich growth of hair-like CNTs, which is induced by the presence of Co, is observed during carbothermal reduction under a flow of H₂ gas, thus allowing ultra-selective detection of aromatic hydrocarbons in the vapor phase, such as benzene (C₆H₆) and toluene (C₆H₅CH₃) over their aliphatic analogue, c-hexane (c-C₆H₁₂) of same molecular mass, size and vapor pressure.

Soft Controllable Carbon Fibre-based Piezoresistive Self-Sensing Actuators

Soft robots and devices exploit deformable materials that are capable of changes in shape to allow conformable physical contact for controlled manipulation. While the use of embedded sensors in soft actuation systems is gaining increasing interest, there are limited examples where the body of the actuator or robot is able to act as the sensing element. In addition, the conventional feedforward control method is widely used for the design of a controller, resulting in imprecise position control from a sensory input. In this work, we fabricate a soft self-sensing finger actuator using flexible carbon fibre-based piezoresistive composites to achieve an inherent sensing functionality and design a dual-closed-loop control system for precise actuator position control. The resistance change of the actuator body was used to monitor deformation and fed back to the motion controller. The experimental and simulated results demonstrated the effectiveness, robustness and good controllability of the soft finger actuator. Our work explores the emerging influence of inherently piezoresistive soft actuators to address the challenges of self-sensing, actuation and control, which can benefit the design of next-generation soft robots.

Multi-Arch-Structured All-Carbon Aerogels with Superelasticity and High Fatigue Resistance as Wearable Sensors

Compressible and ultralight all-carbon materials are promising candidates for piezoresistive pressure sensors. Although several fabrication methods have been developed, the required elasticity and fatigue resistance of all-carbon materials are yet to be satisfied as a result of energy loss and structure-derived fatigue failure. Herein, we present a two-stage solvothermal freeze-casting approach to fabricate all-carbon aerogel [modified graphene aerogel (MGA)] with a multi-arched structure, which is enabled by the in-depth solvothermal reduction of graphene oxide and unidirectional ice-crystal growth. MGA exhibits supercompressibility and superelasticity, which can resist an extreme compressive strain of 99% and maintain 93.4% height retention after 100 000 cycles at the strain of 80%. Rebound experiments reveal that MGA can rebound the ball (367 times heavier than the aerogel) in 0.02 s with a very fast recovery speed (∼615 mm s^-1). Even if the mass ratio between the ball and aerogel is increased to 1306, the ball can be rebound in a relatively short time (0.04 s) with a fast recovery speed (∼535 mm s^-1). As a result of its excellent mechanical robustness and electrical conductivity, MGA presents a stable stress-current response (10 000 cycles), tunable linear sensitivity (9.13-7.29 kPa^-1), and low power consumption (4 mW). The MGA-based wearable pressure sensor can monitor human physiological signals, such as pulses, sound vibrations, and muscular movements, demonstrating its potential practicability as a wearable device.