Finger Bending Research Articles

High-performance amorphous carbon/PDMS flexible strain sensors by introducing low interfacial mismatch

It is important and challenging to develop environment-tolerant stretchable strain sensors under some harsh environments. Here, amorphous carbon (a-C)/Polydimethylsiloxane (PDMS) sensors were fabricated with the direct-current magnetron sputtering technology, their sensing performances were optimized by adjusting the elastic mismatch between the top a-C film and used PDMS substrate, and their application under a simulated marine atmospheric environment was evaluated. The results showed that the combination of low stress a-C and low elastic modulus PDMS was favorable to improve the sensing performance, the optimal sensor exhibited the maximum gauge factor (GF) of 3026.9, large tensile strain range to 50 %, rapid response (loading delay time 92.3 ms and unloading delay time 24.6 ms), as well as high durability (>5000 cycles). After 120 h salt spray test, the sensor was quite stable and can detect various human movements with both high sensitivity and stability, such as wrist pulse, wrist bending, and finger bending. This work pioneers the elastic mismatch strategy for high-performance a-C/PDMS sensors and confirms their great potential in the practical applications under harsh environments.

Ultra-highly sensitive graphene/polyaniline@epoxidized natural rubber strain sensors for human motion monitoring

Flexible strain sensors are one of the core devices in wearable electronics. At present, it is still difficult to develop flexible strain sensors with high stretchability, sensitivity, and long-term stability. In this work, polyaniline (PANI) was coated on the surface of epoxidized natural rubber (ENR) nanospheres to obtain the [email protected] nanospheres, and then mechanically blended with graphene (GN) sheets to prepare the GN/[email protected] composites. The composite conductive networks composed of PANI and GN endow the composites with excellent mechanical, electromechanical and strain-sensing performances. The strain-sensing tests show that the ENR composites possess an ultra-high gauge factor (GF) of 29671.7 within 270% of strain. Meanwhile the composites exhibit excellent cycling stability of over 2500 cycles. In addition, the GN/[email protected] strain sensors can monitor the large-strain (finger, wrist, elbow and knee joints bending) and small-strain (strains of vocal cord vibration, pulse, facial muscle movements) motions of the human body in real time, indicating a wide range of potential applications in wearable flexible electronics.

2D Shape Estimation of a Pneumatic‐Driven Soft Finger with a Large Bending Angle Based on Learning from Two Sensing Modalities

A shape estimation method that utilizes two sensing modalities of a customized fiber Bragg grating (FBG) sensor and a commercial air pressure sensor for a pneumatically driven soft finger with an extensive bending angle range based on an artificial neural network model is proposed. The proposed FBG sensor utilizes two tiny nitinol rods as a backbone to attach the long‐grating FBG element fiber, enabling high strain transfer, shape sensing for large bending deformation, and preventing chirping failure and fiber sliding when bending. Its distal end is set free to slide and synchronizes with the extended length and reflects shapes for large bending deformation (up to 320° with a linearity of 99.96%), while its proximal end is fixed. The small packaged sensor unit enables modular design, easy assembly, and high repeatability with negligible effects on the soft finger's bending performances. The artificial neural network model is utilized to process the input of two sensing modalities, reducing errors from material nonlinearity, fabrication, and assembly of soft fingers while improving shape estimation's accuracy and transferability with average errors of 0.90 mm (0.69%) and 1.55 mm (1.19%) for whole shape and distal end position, respectively. Preliminary experiments also verify the potential for pressing force prediction and hardness recognition.

Chiral 1D Hybrid Metal Halides with Piezoelectric Energy Harvesting and Sensing Properties

Chiral hybrid metal halides have been widely studied in the field of photondetectors, chiral optics, and spintronics due to their great structural flexibility and suitable bandgaps. Inspired by such great progress made in the abovementioned fields, continuous efforts need to be paid to seek other interesting applications for chiral hybrid metal halides. Herein, the synthesis of a pair of one‐dimensional enantiomorphic hybrid metal halide piezoelectrics, R/SMPCdCl4(R/SMP = R/S‐2‐methylpiperazine), and their applications in piezoelectric energy harvesting and sensing are reported. Density functional theory calculations show that these chiral piezoeletrics possess low elastic properties and high piezoelectric constants (16.71, 8.39, and 7.35 pC N−1). In addition, devices made of RMPCdCl4/PDMS (PDMS = polydimethylsiloxane) composite films are fabricated for piezoelectric energy harvesting and sensing. The piezoelectric energy harvesters with optimized content of 15 wt% RMPCdCl4/PDMS show not only excellent performance with an open‐circuit voltage of 2.57 V, short‐circuit current of 0.37 μA, and power density of 0.55 μW cm−2 but also outstanding stability of more than 3500 cycles. Meanwhile, these piezoelectric energy harvesters exhibit prominent sensing properties for detecting tapping, finger bending, walking, and running. It is demonstrated that chiral hybrid metal halides hold great potential in intelligent wearable and sensing devices.

A Flexible Multifunctional Sensor Based on Triboelectric Nanogenerator for Measuring Variable Stiffness of Soft Grippers and Recognizing Item Size

AbstractThe monitoring of the grasping behavior of variable stiffness grippers has emerged as a challenge due to the increasing use of soft grippers in logistics sorting applications. In this research, a flexible multifunctional sensor is developed by combining a liquid metal microchannel structure with a granular single electrode which detects the bending angle and stiffness of the flexible gripper in real‐time. The novel works by integrating triboelectric nanogenerator with particle interference variable stiffness and detects the stiffness of the soft finger. It is found that increasing the loading force on the gripper by a factor of 2.39 does not affect the kinematic characteristics. The size of the grasped object is determined with 96% accuracy by detecting the resistance offered to the stretched liquid metal during the bending of the soft finger. A soft grasping system is built with multifunctional flexible sensors and the real‐time signal feedback during the grasp‐move‐release process of objects of different diameters and weights is demonstrated. Besides providing a theoretical and experimental foundation of variable stiffness and nondestructive grasping of the soft gripper, the anthropomorphism of the soft grasping gripper is implemented by employing a novel sensor.

Facile fabrication of carbon nanocolloids reinforced hydrogels for the application of high-performance strain sensors

Conductive hydrogels have attracted extensive attention in the application of wearable devices due to their excellent flexibility, biocompatibility, and stability. However, it is still a significant challenge to facilely fabricate the conductive hydrogels with high strength, high sensitivity and stable conductivity for wearable strain sensors. Herein, carbon nanoparticles based conductive hydrogels were prepared by a facile one-pot method of combining dispersant-free carbon nanocolloids with PVA/TA/PAM. When a small amount of carbon nanoparticles (0.45 wt%) were added into the hydrogel, the conductivity was significantly enhanced. Due to the synergetic effects from the PVA/TA/PAM polymers and carbon nanoparticles, the as-synthesized composites exhibited large mechanical strength (11.9 MPa) and failure strain (476 %), good sensitivity (GFmax = 1.41), great stability (over 500 cycles). In addition, the strain sensor made from the conductive hydrogel could sensitively detect human movements such as finger bending, arm bending, wrist bending, and leg bending. This hydrogel preparation strategy exhibits potential applications in low-cost wearable devices with high mechanical strength and sensitivity.

Development of Flexion Posture Formation Mechanism in Wearable Type of Flexor Tendon Rehabilitation Equipment

Rehabilitation robots and rehabilitation braces are in demand, as they aid in reducing a patient’s need for therapist’s attendance and guidance during therapy. This study focuses on the bending of fingers under the tension of the fingertips using clinical equipment and demonstrates the development of a small and versatile rehabilitation brace.

Piezoionic SnSe Nanosheets‐Double Network Hydrogel for Self‐Powered Strain Sensing and Energy Harvesting

AbstractFlexible strain sensors have enormous potential in wearable devices, robots, and health monitoring equipment. However, the poor stretchability of strain sensors based on semiconductors and the low sensitivity of resistance change‐based hydrogel strain sensors hinders the comprehensive application. Herein, a flexible piezoionic SnSe‐hydrogel composite with an optimized structure and improved performance is designed. The piezoionic output rises nonlinearly as the applied force increases, with the piezoionic coefficient up to 1780 nV Pa−1 and −7.21 nA Pa−1. The composite can realize the continuous positioning in 1D space based on the piezoionic effect. It also demonstrates self‐powered characteristics, an ultrafast response speed of 6–8 ms, and a high gauge factor of 95.89. The sensor is exemplified to monitor fist clenching and finger bending, which has the potential to discriminate different joint movements. Meanwhile, the device can light up a light–emitting diode under pressure and bending. The as‐prepared piezoionic SnSe‐hydrogel device, having both high stretchability and sensitivity, may shed light on developing high‐performance flexible strain sensors and generators.

A novel topographically patterned MXene@MnO2/PVDF piezo-active hybrid for flexible real-time and sensitive force sensor

Flexible force sensors capable of real-time and sensitive analysis have gained immense popularity because of their great potential to be integrated into potable and wearable electronics. Herein, a novel flexible force sensor constructing from topographically patterned piezo-active MXene@MnO2/poly(vinylidene fluoride) (PVDF) hybrid that allows for real-time precise pressure detection is proposed. Benefiting from the synergistic effect of topographical patterning and nano-reinforcement filling, the piezo-active hybrids with convex microarrays patterns demonstrate outstanding piezoelectric coefficient d33 (49 pC/N) and output upon mechanical stimuli, enabling the resultant force sensors with high voltage sensitivity of 0.450 V/N, fast response time of 16–19 ms and excellent operation stability. Furthermore, the feasibility of sensors in effectively detecting water droplets, gas flow and finger bending is demonstrated, confirming their viability for real applications.

Bilayer PVA composite film with structural color for high-performance and multifunctional sensing

Intelligent E-skins attract lots of attention for their potential in health monitoring, human-computer interaction, and soft-body robotics. In this work, a colorful multifunctional E-skin sensor with a Janus structure was constructed by solution casting of polyvinyl alcohol (PVA) with clay and carbon nanoparticles on a patterned PDMS template. The halloysite nanotubes (HNTs) and carbon nanoparticles (C) derived from Chinese ink were introduced into PVA to obtain a nano-reinforced layer (top) with structural color and multi-functional layer (bottom) for detection of pressure, temperature, and humidity. Uniformly dispersed HNTs greatly improves the mechanical properties of PVA (up to 46.8 MPa at 303.9% strain). The difference in water absorption of the two layers provides excellent sensitivity to humidity, so it can be used as contactless password unlocking, closing and blooming of bionic flowers, and humidity alarm. The device can achieve object capture and release within 12 s by turning on and off the xenon light. Furthermore, the sensor responds distinctly to pressure and can track human activities such as drinking motion and finger bending, which can also detect the temperature by resistance change. This work provides a materials design strategy for E-skin aimed to applications in robotics, artificial intelligence, prosthetics, and health monitoring.

Fabrication of styrene–butadienestyrene (SBS) matrix-based flexible strain sensors with brittle cellulose nanocrystal (CNC)/carbon black (CB) segregated networks

Strain sensors based on conductive polymer composites (CPCs) filled with carbon black (CB) are potential as wearable detection devices for mass production due to their low cost. However, it is still challenging to design sensors with low CB loading and the conductivity and sensitivity guaranteed. Herein, a Pickering emulsion template method was conducted to design the styrene–butadienestyrene (SBS) matrix-based strain sensors, in which a conducting structure of segregated cellulose nanocrystal (CNC)/carbon black (CB) networks was constructed, improving the conductivity and reducing the percolation threshold (φc) to ∼0.28 wt%. Besides, the CNC incorporation and defect construction in the networks greatly promote the sensitivity of the sensor, the maximum gauge factor (GF) determined is beyond 260 within the 200% strain. Moreover, the sensor can exhibit remarkable working durability and reproductivity. All these above favorable properties have empowered the strain sensor with superior applicability in detecting various human motions (finger bending, elbow bending, knee bending, neck rotation, cheek bulging, and swallowing).

Preparation of multifunctional self-healing MXene/PVA double network hydrogel wearable strain sensor for monitoring human body and organ movement

In this work, a kind of conductive, self-healing hydrogel was prepared. Then it is assembled into a flexible wearable sensor for human motion detection and human-computer interaction. MXene/PVA-CBA hydrogel has super mechanical properties and excellent self-healing ability (1.8 s). It is assembled into a flexible sensor with high sensitivity, which can accurately detect various movements of the human body (ranging from frowning, speaking, and coughing on the face to bending of fingers and wrists, and body movements). Furthermore, it can be used for handwriting recognition. When it is installed on the artificial limb, it can realize the function of touching the capacitive screen. It solves the problem of using silicone prostheses to control the screen and has broad research potential in the field of intelligent robots. Therefore, the flexible wearable sensor composed of MXene/PVA-CBA hydrogel has great potential in human motion detection, bionic intelligent robot, and intelligent detection.

A Flexible Piezocapacitive Pressure Sensor with Microsphere-Array Electrodes.

Flexible pressure sensors that emulate the sensation and characteristics of natural skins are of great importance in wearable medical devices, intelligent robots, and human-machine interfaces. The microstructure of the pressure-sensitive layer plays a significant role in the sensor's overall performance. However, microstructures usually require complex and costly processes such as photolithography or chemical etching for fabrication. This paper proposes a novel approach that combines self-assembled technology to prepare a high-performance flexible capacitive pressure sensor with a microsphere-array gold electrode and a nanofiber nonwoven dielectric material. When subjected to pressure, the microsphere structures of the gold electrode deform via compressing the medium layer, leading to a significant increase in the relative area between the electrodes and a corresponding change in the thickness of the medium layer, as simulated in COMSOL simulations and experiments, which presents high sensitivity (1.807 kPa-1). The developed sensor demonstrates excellent performance in detecting signals such as slight object deformations and human finger bending.

Preparation of fabric-based electronic device via surface topological modification and enzymatic polymerization for personal thermal management, subtle motion detection, and ultraviolet protection

With the increasingly severe problem of population aging, human motion detection and personal thermal management have attracted widespread attention in the field of smart wearable electronics. However, the current multifunctional flexible devices face problems, such as lower stability and poor comfort, which greatly hinder their further development. In this work, an improved fabrication strategy combining in situ enzymatic polymerization and surface topological modification is proposed to facile and efficiently prepare poly(3,4-ethylenedioxythiophene)/polyacrylamide (PEDOT/PAAm) smart fabric. To increase the conductivity and the bonding force between fabric and PEDOT particles, the PAAm was used to pre-construct a microgel layer on the surface of cotton fabric to make PEDOT uniformly coated on the fabric, thus forming a highly electrically conductive interconnection network. The resulting fabric samples show outstanding duel-driven thermal performance (88.6 °C for 2 sun, 84.5 °C for 10 V), satisfactory detection capability for subtle and rapid motion (drops of water and paper pill, bending of fingers), and remarkable ultraviolet protection performance (24.6 times than the UPF value of unmodified fabric sample). All in all, multifunctional fabric-based devices with remarkable performance exhibited great application potential in rehabilitation therapy and personal home care, especially for people with disabilities and older people.

Wearable sensors for real-time physiological monitoring based on self-assembled diphenylalanine peptide nanostructures

Self-assembled diphenylalanine short peptides show great potential in wearable and flexible biomedical devices due to its excellent biocompatibility and piezoelectric nature. Herein, the piezoelectric activity of self-assembled diphenylalanine analogues by rationally selecting chemical linkers such as ethylene diamine, succinic acid and terephthalaldehyde. Our results show that the self-assembled diphenylalanine with terepthalaldehyde linker has a favorable effect on the piezoelectric activity. The output of an optimized SP3 PNG can reach 29 V and 480 nA, and, it has exhibited excellent stability, repeatability, and charging-discharging capacity. Such a device can be used to harness energy or detect biomechanical actions such as swallowing, finger bending, coughing, posture angle monitoring and alphabet recognition. The PNG has been used to monitor arterial pulses as a biomedical application model. The obtained results are steppingstones towards developing robust bio-implantable PNGs for powering pacemakers and implantable cardioverter-defibrillators (ICDs) in future.

Flexible microfluidic triboelectric sensor for gesture recognition and information encoding

Flexible triboelectric sensor has a broad application prospects in self-powered human-machine interface. However, due to the complexity of real environment, the output signal is easily disturbed, thus resulting in low reliability. Here, a flexible microfluidic triboelectric sensor (FMTS) was proposed by counting the number of wave peaks in output waveform. With a high transmittance of 82%, the FMTS shows bendable, twistable and conformable characteristics so that can be operated attached to skin. Based on triboelectrification and electrostatic induction between liquid stream, microchannel and interdigital electrodes, it enables to generate quantifiable voltage wave peaks. The FMTS has a maximum sensitivity of 0.418 kPa−1 and a wide detection range from 2.38 to 58.12 kPa with the microchannel width of 500 µm. For demonstration, a FMTS is attached to the finger, and the number of peaks is used to determine the angle of finger bending with a resolution of 10°. The machine learning approach is employed to achieve accurate recognition of five gestures with an accuracy of 99.2%. The information can also be defined by using waveforms generated by different combinations of movements. An encoding system that recognizes eight types of information with an accuracy of up to 98.8% is finally demonstrated.

An effective DLP 3D printing strategy of high strength and toughness cellulose hydrogel towards strain sensing

Photocurable 3D printing technology has outperformed extrusion-based 3D printing technology in material adaptability, resolution, and printing rate, yet is still limited by the insecure preparation and selection of photoinitiators and thus less reported. In this work, we developed a printable hydrogel that can effectively facilitate various solid or hollow structures and even lattice structures. The chemical and physical dual-crosslinking strategy combined with cellulose nanofibers (CNF) significantly improved the strength and toughness of photocurable 3D printed hydrogels. In this study, the tensile breaking strength, Young's modulus, and toughness of poly(acrylamide-co-acrylic acid)D/cellulose nanofiber (PAM-co-PAA)D/CNF hydrogels were 375 %, 203 % and 544 % higher than those of the traditional single chemical crosslinked (PAM-co-PAA)S hydrogels, respectively. Notably, its outstanding compressive elasticity enabled it to recover under 90 % strain compression (about 4.12 MPa). Resultantly, the proposed hydrogel can be utilized as a flexible strain sensor to monitor the motions of human movements, such as the bending of fingers, wrists, and arms, and even the vibration of a speaking throat. The output of electrical signals can still be collected through strain even under the condition of energy shortage. In addition, photocurable 3D printing technology can provide customized services for hydrogel-based e-skin, such as hydrogel-based bracelets, fingerstall, and finger joint sleeves.

Fabrication of Eco-Friendly Wearable Strain Sensor Arrays via Facile Contact Printing for Healthcare Applications.

Wearable flexible strain sensors with spatial resolution enable the acquisition and analysis of complex actions for noninvasive personalized healthcare applications. To provide secure contact with skin and to avoid environmental pollution after usage, sensors with biocompatibility and biodegradability are highly desirable. Herein, wearable flexible strain sensors composed of crosslinked gold nanoparticle (GNP) thin films as the active conductive layer and transparent biodegradable polyurethane (PU) films as the flexible substrate are developed. The patterned GNP films (micrometer- to millimeter-scale square and rectangle geometry, alphabetic characters, and wave and array patterns) are transferred onto the biodegradable PU film via a facile, clean, rapid and high-precision contact printing method, without the need of a sacrificial polymer carrier or organic solvents. The GNP-PU strain sensor with low Young's modulus (≈17.8MPa) and high stretchability showed good stability and durability (10000 cycles) as well as degradability (42% weight loss after 17 days at 74°C in water). The GNP-PU strain sensor arrays with spatiotemporal strain resolution are applied as wearable eco-friendly electronics for monitoring subtle physiological signals (e.g., mapping of arterial lines and sensing pulse waveforms) and large-strain actions (e.g., finger bending).

Highly flexible phase-change film with solar thermal storage and sensitive motion detection for wearable thermal management

The utilisation of phase-change materials (PCMs) for the thermal management of wearable devices remains a significant challenge because of intrinsic leakage and rigidity issues. Therefore, this study develops a wearable phase-change film (FSPCF) that addresses these issues while providing high flexibility, superior solar thermal conversion efficiency and sensitive motion detection. The FSPCFs were fabricated using n-eicosane, vinyl-dimethylsiloxane, hydrosilicone oil and multiwall carbon nanotubes (MWCNTs) via salt-template-assisted and dip-coating methods. The interconnected 3D polysiloxane networks provided ample support for n-eicosane and effectively prevented liquid leakage during 500 thermal storage/release cycles. Increasing the amount of the deposited MWCNTs considerably improved the thermal conductivity of the FSPCF, from 0.314 W·m−1·K−1 (FSPCF-0) to 0.719 W·m−1·K−1 (FSPCF-1.2). The synthesised FSPCFs exhibited high thermal-energy-storage density (109.5–117.8 J/g), and a suitable phase-change temperature (about 36℃) for human thermal regulation and desirable thermal stability. Furthermore, FSPCF-added MWCNTs demonstrated excellent solar thermal conversion and storage efficiency (92.3%), making FSPCFs an attractive heat source for integration into wearable devices. Notably, the FSPCFs also demonstrated sensitive motion detection (GF = 46.3) and could monitor various human movements in real time, including curvature movements (bending of fingers, wrists, elbows, knees and neck), facial expressions and vocal vibrations. Overall, the developed film exhibits tremendous potential for use in the thermal management of wearable devices.

Electrically conductive and highly compressible anisotropic MXene-wood sponges for multifunctional and integrated wearable devices

Although piezoresistive sensors have wide applications in human motion detection and wearable electronics, it is still a challenge to produce multifunctional and integrated sensors with wide working ranges and linear sensing responses. Herein, electrically conductive, highly compressible, and structurally anisotropic MXene-wood (MW) sponges are fabricated by converting balsa wood blocks to compressible wood sponges with extremely low density and highly anisotropic porous structure, followed by decorating with conductive MXene sheets. The MW sponge is utilized to assemble a piezoresistive sensor with a large working range of 0-25 kPa, wide linear sensing ranges of 5%-50% and 0-180o, and excellent long-term reliability under a bending angle of 30° for 5000 cycles. Additionally, the sensor is suitable for monitoring practical human physiological signals including pulse beating, and finger and wrist bending. An all-MW sponge-based multifunctional and integrated sensing system is constructed by integrating the MW sponge piezoresistive sensor, the MW sponge-based solid-state supercapacitor, and the MW sponge electrophysiological electrodes. The sensing system can concurrently detect surface electromyogram and tactile pressures, and hence realize real-time closed-loop controls. The multifunctional and integrated MW sponge sensing system would have great potentials in real-time pressure recognition and human-machine interfaces.