Human Motion Detection Research Articles

Highly stretchable and room-temperature self-healing sheath-core structured composite fibers for ultrasensitive strain sensing and visual thermal management

Highly stretchable and self-healing wearable electronics for strain sensing and Joule heating are highly desirable for future emerging applications of wearable devices, smart robots, human-machine interface and artificial intelligence, etc. Herein, the highly stretchable and room-temperature self-healing sheath-core structured composite fibers are fabricated via the feasible in-situ polymerization modification followed by electroless silver-plating approach. Benefitting from the simultaneous incorporation of dual dynamic reversible chemical networks and construction of sheath-core structures, the composite fibers show ultrasensitive strain sensing and visual thermal management performances with excellent room-temperature self-healing capacity at ultralow Ag loadings. The composite fibers after cutting and self-healing also exhibit outstanding strain sensing performances with a high gauge factor (GF) of 64.0 and visual thermal management performances with tailorable Joule heating temperatures. Furthermore, they possess excellent working stability and reliability in practical applications of human motion detection and personal thermal management. This work demonstrates the fabrication of highly stretchable and room-temperature self-healing composite fibers for next-generation wearable devices, smart robots, human-machine interface and artificial intelligence, etc.

Ionic Liquid/Water Binary Solvent Anti-Freezing Hydrogel for Strain and Temperature Sensors.

Hydrogels are widely applied in the flexible wearable electronic devices field owing to their skin-like stretchability, superb biocompatibility, and high conductivity retention under mechanical deformations. Nevertheless, hydrogels are prone to freezing at low temperatures and losing water at high temperatures, which seriously limits their practical applications. Herein, a binary solvent system of ionic liquid (1-ethyl-3-methylimidazolium chloride) and water was prepared to endow the ionic hydrogel high ionic conductivity (0.28 S m-1 at 25 °C), high transparency (94.26%), and superior freezing tolerance (-50 °C). The multiple hydrogen bonds formed among polymer chains, water, and ionic liquids significantly improved the mechanical properties of the ionic hydrogel, enabling excellent tensile properties (strain >1800%) and durability (1000 times at 100% strain). Moreover, the ionic hydrogel was further assembled into a dual-response sensor, which exhibited satisfactory sensitivity to both tension (gauge factor = 2.15 at 200% strain) and temperature (temperature coefficient of resistance = -1.845%/°C) and can be applied for human motion and body temperature monitoring. This study provides a versatile method for preparing multifunctional hydrogels with a wide range of applications and lays the groundwork for human movement detection and smart health care.

Breathable, durable, flexible, and battery-free full action response electronic textiles toward simply achieving the function of human skin

Recently, the advantages of electronic textiles (e-textiles) for electronic skin have attracted great attention. As epidermal electronic products, they need to possess characteristics such as portability, comfortable to wear, durability, and easy to manufacture. However, in practical application scenarios, wearable electronics need to make trade-offs between various functions. Here, we propose a lightweight and breathable multi-functional electronic skin based on the idea of e-textiles, which employs an approach using nanogenerators (NGs), a simple electrospinning and three-dimensional printing (3D-print) process to fabricate a scalable, porous, and triboelectric-conductive-piezoelectric (T-C-P) multilayer structure-based electronic skin (e-skin) called TPSA. The piezoelectric-triboelectric hybrid nanogenerator exhibits remarkable electrical behavior due to its unique coupling enhancement effect. TPSA achieves excellent generating capacity (output of up to 105 kV/m2, 1.87 mA/m2, 29.4 mW/m2 in single-electrode working mode), enables full-scale human motion detection, achieves EMI shielding performance of 46.9 dB, possesses a high killing rate of Escherichia coli up to 99%, and exhibits good air permeability up to 35.453 cm3/(m2 *d*Pa). This electronic skin can perform some of the functions of the skin, quickly and without power monitoring of physiological signals and joint movements throughout the body, while ensuring comfort and protection. This work provides a strategy for easy access to versatile e-skins with excellent utility.

Strain Sensors for Human Movement Detection Based on Fibrous Membranes Comprising Thermoplastic Polyurethane, Ag Nanoparticles, and Carbon Nanotubes

Strain Sensors for Human Movement Detection Based on Fibrous Membranes Comprising Thermoplastic Polyurethane, Ag Nanoparticles, and Carbon Nanotubes

Wide strain range and high sensitivity sandwich structure CNTs/AgNWs/CNTs/TPU strain sensors for human motion detection

Nowadays, designing a flexible strain sensor that offers both high sensitivity and a wide operating range has been a significant challenge. However, this paper demonstrates the successful preparation of a sandwich-structured, flexible strain sensor with high sensitivity and wide operating range through vacuum-assisted filtration. The substrate of sensor is composed of electrospinning TPU. The three conductive layers of the sensor consist of CNTs, AgNWs, and CNTs. The AgNWs provide extremely low initial resistance and serve to bridge the individual CNTs islands under large strains. The CNTs/AgNWs/CNTs/TPU flexible strain sensor boasts a high sensitivity (GF=1005.8), a wide strain range (up to 350%), and fast response time (39 ms). It also maintains a long working range and long-term usage stability at 2500 cycles. Additionally, the electrothermal and photothermal performance of the sensor were investigated, which revealed a rapid temperature rise to 93.1 °C at 5 V and 80.8 °C at 500 mW/cm2 irradiation intensity. This sensor can accurately monitor the full range of human motion, from tiny pulse and breathing to large-scale finger joint flexion. These findings suggest that our sensor could have great potential for application in wearable devices as well as body health monitoring.

Hydrogen Bonds Reinforced Ionogels with High Sensitivity and Stable Autonomous Adhesion as Versatile Ionic Skins.

Flexible wearable sensors have demonstrated enormous potential in various fields such as human health monitoring, soft robotics, and motion detection. Among them, sensors based on ionogels have garnered significant attention due to their wide range of applications. However, the fabrication of ionogels with high sensitivity and stable autonomous adhesion remains a challenge, thereby limiting their potential applications. Herein, we present an advanced ionogel (PACG-MBAA) with exceptional performances based on multiple hydrogen bonds, which is fabricated through one-step radical polymerization of N-acryloylglycine (ACG) in 1-ethyl-3-methylimidazolium ethyl sulfate (EMIES) in the presence of N,N'-methylenebis(acrylamide) (MBAA). Compared with the ionogel (PAA-MBAA) formed by polymerization of acrylic acid (AA) in EMIES, the resulting ionogel exhibits tunable mechanical strength (35-130 kPa) and Young's modulus comparable to human skin (60-70 kPa) owing to the multiple hydrogen bonds formation. Importantly, they demonstrate stable autonomous adhesion to various substrates and good self-healing capabilities. Furthermore, the ionogel-based sensor shows high sensitivity (with a gauge factor up to 6.16 in the tensile range of 300-700%), enabling the detection of both gross and subtle movements in daily human activities. By integration of the International Morse code, the ionogel-based sensor enables the encryption, decryption, and transmission of information, thus expanding its application prospects.

Conductive Cross-Linkable PDMS-MXene Nanosheet Networks on Polyurethane Sponges as Robust Superhydrophobic Sensors for Human Motion Detection

Conductive Cross-Linkable PDMS-MXene Nanosheet Networks on Polyurethane Sponges as Robust Superhydrophobic Sensors for Human Motion Detection

Hierarchical Spatial Confinement Unlocking the Storage Limit of MoS2 for Flexible High-Energy Supercapacitors.

Molybdenum sulfide (MoS2) is a promising electrode material for supercapacitors; however, its limited Mo/S edge sites and intrinsic inert basal plane give rise to sluggish active electronic states, thus constraining its electrochemical performance. Here we propose a hierarchical confinement strategy to develop ethylene molecule (EG)-intercalated Co-doped sulfur-deficient MoS2 (Co-EG/SV-MoS2) for efficient and durable K-ion storage. Theoretical analyses suggest that the intercalation-confined EG and lattice-confined Co can enhance the interfacial K-ion storage capacity while reducing the K-ion diffusion barrier. Experimentally, the intercalated EG molecules with mildly reducing properties induced the creation of sulfur vacancies, expanded the interlayer spacing, regulated the 2H-1T phase transition, and strengthened the structural grafting between layers, thereby facilitating ion diffusion and ensuring structural durability. Moreover, the Co dopants occupying the initial Mo sites initiated charge transfer, thus activating the basal plane. Consequently, the optimized Co-EG/SV-MoS2 electrode exhibited a substantially improved electrochemical performance. Flexible supercapacitors assembled with Co-EG/SV-MoS2 delivered a notable areal energy density of 0.51 mW h cm-2 at 0.84 mW cm-2 with good flexibility. Furthermore, supercapacitor devices were integrated with a strain sensor to create a self-powered system capable of real-time detection of human joint motion.

Bioinspired Multifunctional Self-Sensing Actuated Gradient Hydrogel for Soft-Hard Robot Remote Interaction

HighlightsThe bioinspired self-sensing actuated gradient hydrogel was developed by a wettability-based method via precipitation of MoO2 nanosheets.Self-sensing actuated gradient hydrogel combined ultrafast thermo-responsive actuation (21° s–1), exceptional photothermal efficiency (3.7 °C s–1) and high sensing properties (GF = 3.94).The first self-sensing remote interaction system based on gradient hydrogel actuators and robotic hands was constructed.

An anti-swelling chitosan-based hydrogel driven by balance of hydrophilic segment and hydrophobic segment strategy for underwater detection of human motion

Hydrogel-based flexible strain sensors have gained rapid development. However, most of traditional hydrogels were prone to undergo swelling behavior underwater as solvent molecules continuously penetrated, which limited their application scenarios. To make up this deficiency, a chitosan-based anti-swelling hydrogel cross-linked by ionic liquid are presented. The hydrogel is consisted of chitosan (CS) and copolymer of acrylic acid (AA), 2-hydroxyethyl methacrylate (HEMA), butyl acrylate (BA) and dimethylaminoethyl methacrylate maleate (DMAEMA-MA). The P(HEMA-BA-AA)/ILs/CS hydrogel exhibited excellent anti-swelling properties driven by balance of hydrophilic segment and hydrophobic segment strategy (equilibrium swelling rate of −3.79 % in water for 100 days). The ionic liquid was introduced into the hydrogel system as cross-linking agent, which could effectively prevent the loss of conductivity of the hydrogel after soaking in water. The synergy effect chain entanglement, H-bonds and electrostatic interactions endowed the hydrogel with excellent mechanical property. Surprisingly, the modulus and toughness of the hydrogel were significantly boosted after a week of submersion. Thus, this anti-swelling hydrogel was employed as wireless strain sensor, which implemented underwater motion monitoring and safety alarms. It is expected that the anti-swelling hydrogel driven by balance of hydrophilic and hydrophobic effects would inspire the development of underwater sensors.

A Flexible and Stretchable MXene/Waterborne Polyurethane Composite-Coated Fiber Strain Sensor for Wearable Motion and Healthcare Monitoring.

Fiber-based flexible sensors have promising application potential in human motion and healthcare monitoring, owing to their merits of being lightweight, flexible, and easy to process. Now, high-performance elastic fiber-based strain sensors with high sensitivity, a large working range, and excellent durability are in great demand. Herein, we have easily and quickly prepared a highly sensitive and durable fiber-based strain sensor by dip coating a highly stretchable polyurethane (PU) elastic fiber in an MXene/waterborne polyurethane (WPU) dispersion solution. Benefiting from the electrostatic repulsion force between the negatively charged WPU and MXene sheets in the mixed solution, very homogeneous and stable MXene/WPU dispersion was successfully obtained, and the interconnected conducting networks were correspondingly formed in a coated MXene/WPU shell layer, which makes the as-prepared strain sensor exhibit a gauge factor of over 960, a large sensing range of over 90%, and a detection limit as low as 0.5% strain. As elastic fiber and mixed solution have the same polymer constitute, and tight bonding of the MXene/WPU conductive composite on PU fibers was achieved, enabling the as-prepared strain sensor to endure over 2500 stretching-releasing cycles and thus show good durability. Full-scale human motion detection was also performed by the strain sensor, and a body posture monitoring, analysis, and correction prototype system were developed via embedding the fiber-based strain sensors into sweaters, strongly indicating great application prospects in exercise, sports, and healthcare.

Cold-resistant, highly stretchable ionic conductive hydrogels for intelligent motion recognition in winter sports.

Conductive hydrogels have attracted much attention for their wide application in the field of flexible wearable sensors due to their outstanding flexibility, conductivity and sensing properties. However, the weak mechanical properties, lack of frost resistance and susceptibility to microbial contamination of traditional conductive hydrogels greatly limit their practical application. In this work, multifunctional polyvinyl alcohol (PVA)/carboxymethyl cellulose (CMC)/poly(acrylamide-co-1-vinyl-3-butylimidazolium bromide) (P(AAm-co-VBIMBr)) (PCPAV) ionic conductive hydrogels with high strength and good conductive, transparent, anti-freezing and antibacterial properties were constructed by introducing a network of chemically crosslinked AAm and VBIMBr copolymers into the base material of PVA and CMC by in situ free radical polymerization. Owing to the multiple interactions between the polymers, including covalent crosslinking, multiple hydrogen bonding interactions, and electrostatic interactions, the obtained ionic conductive hydrogels exhibit a high tensile strength (360.6 kPa), a large elongation at break (810.6%), good toughness, and fatigue resistance properties. The introduction of VBIMBr endows the PCPAV hydrogels with excellent transparency (∼92%), a high ionic conductivity (15.2 mS cm-1), antimicrobial activity and good flexibility and conductivity at sub-zero temperatures. Notably, the PCPAV hydrogels exhibit a wide strain range (0-800%), high strain sensitivity (GF = 3.75), fast response, long-term stability, and fantastic durability, which enable them to detect both large joint movements and minute muscle movements. Based on these advantages, it is believed that the PCPAV-based hydrogel sensors would have potential applications in health monitoring, human motion detection, soft robotics, ionic skins, human-machine interfaces, and other flexible electronic devices.

Easily scalable and highly flexible machine knitted resistive pressure sensor for smart textile applications

This study presents textile based resistive pressure sensor that incorporates off-the-shelf PES/stainless steel conductive yarn and nonconductive acrylic yarn. The fabrication process involves integrating the resistive pressure sensors directly into the base fabric during one knitting operation through computerized machine knitting technology with little human involvement. This work also explores opportunities for manufacturing of soft, highly flexible, and easily controllable double-layered out-of-plane sensors in an easily scalable way. A custom resistance measuring circuit was built to characterize the sensors and the resulting equivalent resistance under various loads, i.e., (0 g–900 g). Reproducibility was confirmed by developing and testing several sensors with the same structural characteristics. The results obtained from the experiments showed that the conductive yarn types and the design parameters significantly affect the sensing properties of knitted sensors. It has been found that sensor types 2, 5, and 6 show better stability, repeatability, high response and recovery time, dynamic ranges, and sensitivity when subjected to various loadings, compared to other developed sensors in this work. As a proof of concept, this sensor demonstrates various smart textile applications, including interactive sleeves for wearable user interface, soft controllable switch, and human movement detection.

A "Mesh Scaffold" that regulates the mechanical properties and restricts the phase transition-induced volume change of the PNIPAM-based hydrogel for wearable sensors.

Poly(N-isopropylacrylamide) (PNIPAM) is capable of improving the reversibility and responsiveness of flexible electronics. However, its phase transition-induced volume variation and poor adhesiveness remain limitations for expending its applications. Herein, a pressure-sensitive adhesive (PSA), which is a type of mesh scaffold, is constructed inside the network of PNIPAM, providing the hydrogel with a constant volume in response to different temperatures, in situ tunable mechanical properties, and superior adhesiveness. The reversible density of the mesh scaffold adjusts the aggregation state of the hydrogel chains, whereupon it is capable of changing its mechanical modulus from 6.7 kPa to 45.3 kPa. This mechanical mechanism contributes to hydrogel-based flexible devices for multiple applications, especially in pressure-related sensors. The mesh scaffold restricts the phase-transition-induced volume variation, which allows the hydrogel sensor to stably monitor the external pressure at various temperatures. The high adhesion enables the effective interfacial interaction with the skin, avoiding the loss of sensing signals during the detection of human body movements. When it is assembled into an electronic device, it can transmit information and recognize sign language via Morse code. Thus, herein, we report a hydrogel sensor that is promising for pressure detection in temperature-unstable environments, especially for managing the health of patients who require emergency medical care through sign language recognition.

A Long Short-Term Memory-Based Interconnected Architecture for Classification of Grasp Types Using Surface-Electromyography Signals

Reliable classification of grasp types from human limbs has become an important aspect used by applications with humanoid robotic systems, because of their high accuracy implementations in human movement replication and detection. Biomedical features extracted from muscular signals are commonly used for this purpose, however, their extraction and usage have been targeted independently, with time series features not even considered in the classification stage. Recently, studies show Deep Neural Networks could obtain the signal's features in their internal architecture and use them directly over a classification task, avoiding all pre-processing steps and improving the obtained accuracy. Therefore, the current study proposes a deep architecture based on LSTM Networks for the classification of 6 grasp types as an end-to-end deep model approach, working with raw surface electromyography signals. Classification accuracy of 99.12% was obtained and compared with previous studies which use different machine learning techniques over the same dataset. Results obtained showed that our model's architecture improves previous results as well as provides a robust solution avoiding overfitting, with an F1-score higher than 99% for all grasp types.

A biodegradable, highly sensitive and multifunctional mechanical sensor based on rGO-silk fibroin hydrogel for human motion detection and gesture recognition

Hydrogel based flexible sensors have attracted tremendous attention in the fields of artificial skin, motion detection and health management due to their excellent characteristics such as portability and wearability.

High-strength fibrous sensors with an enhanced aggregate state for biomechanical monitoring of the Achilles tendon.

Continuous monitoring of biomechanical signals generated from the injured Achilles tendon is essential for the deep understanding of the recovery or rehabilitation process, thus decreasing the risk of secondary injuries. With tissue-like components and adjustable properties, hydrogel-based biomechanical sensors are considered promising materials for human motion detection. However, existing hydrogels are characterized by inferior mechanical properties with strength and modulus typically lower than 1 MPa, as well as poor stability under physiological conditions, which hampers their applications in implantable devices. Moreover, acquiring the stress signal from collected electrical signals remains challenging. Herein, based on the regulation of polymer aggregation, a high-strength fibrous sensor composed of polyvinyl alcohol (PVA) and reduced graphene oxide (rGO) for in vivo monitoring is prepared through a two-step procedure, including freeze-thaw and freeze-soak. Benefiting from the synergy of crystallization, Hofmeister effect and nanocomposite, the hydrogel fibers feature high tensile strength (8.34 ± 0.66 MPa) and elastic modulus (1.15 ± 0.10 MPa). Meanwhile, the removal of salt ions during fabrication improves the water content (69.18 ± 1.47%) and anti-swelling performance of such fibers and minimizes side effects after implantation. It is demonstrated that the fibrous sensor could record the relative resistance changes upon stretching with ideal sensitivity (GF = 1.57) and convert them into bearing stress through formula derivation and calculations. In vitro and in vivo assays further confirm its feasibility for real-time monitoring of joint motion, providing important references for medical diagnosis and treatment.

Influence of Soft Materials on Ultrasonic Waveguide Signal Quality for Wearable Human Motion Detection Across the Legs, Arms, and Face

Influence of Soft Materials on Ultrasonic Waveguide Signal Quality for Wearable Human Motion Detection Across the Legs, Arms, and Face

A stretchable conductive elastomer sensor with self-healing and highly linear strain for human movement detection and pressure response.

Expanding the detection information of wearable smart devices in applications has practical implications for their use in daily life and healthcare. Damage and breakage caused by mechanical injuries and continuous use are unavoidable for polymer matrices so self-healing properties are expected to be conferred on flexible sensors to extend their life and durability. In addition, a good linearity of relative resistance change vs. strain (gauge factor, GF) facilitates the streamlined conversion of electrical signals to 3D information of human motion, whereas existing works on sensors neglect the quantitative analysis of signals. This letter reports a self-healable flexible electronic sensor based on hydrogen bonding and electrostatic interaction between maleic acid-grafted natural rubber (MNR), polyaniline (PANI), and phytic acid (PA). MNR is the flexible matrix and the template for aniline (ANI) polymerization, and PA acts as the dopant and crosslinking agent. The MNR-PANI-PA sensor shows easy self-healing at room temperature, enhanced mechanical behaviour (∼2.5 MPa, 1000% strain), and excellent linearity (GF of 13.8 over 250% strain and GF of 32.0 over 250-100% strain). Due to the highly linear relationship between ΔR/R and bending angle, the electrical signals of human limb movement can output relevant information on bending angle and frequency. By constructing a sensing array, changes in the position and magnitude of applied pressure could also be detected in real-time. Based on these advantages, the MNR-PANI-PA composite sensor is expected to have potential applications in health monitoring, body motion detection, and electronic skins.

Elastomers with ultrahigh mechanical properties for flexible sensing and triboelectric nanogenerators

With the continuous development of technology, flexible wearable electronic devices have received widespread attention. However, flexible substrate materials with high strength and toughness and flexible stretchable electrode materials that can meet practical needs are still full of challenges. Here, we successfully prepared elastomeric substrate materials with high tensile strength (30.95 MPa), high toughness (134.16 MJ/m3), solvent resistance and puncture resistance by modulating the reaction conditions of maleic anhydride and polystyrene-b-polybutadiene-b-polystyrene. Subsequently, the preparation of flexible and stretchable electrodes was achieved by mixing an elastomeric substrate with carbon nanotubes. The capacitive sensor designed and assembled based on this stretchable electrode has fast response characteristics and can be used for human movement detection. Additionally, the triboelectric nanogenerator (TENG) fabricated using this material for the friction layer exhibits a high open-circuit voltage of 848 V along with a short-circuit current of 20 μA. Its excellent stability is evidenced by its ability to withstand up to 10,000 cycles, and it also possesses an impressive power density of 3.8 W/m2. These characteristics provide a solid foundation for advancing research in intelligent sensing technologies and lay the groundwork for its future incorporation into energy-harvesting devices.