Motion Detection Sensors Research Articles

Superhydrophobic and flame-retardant chitosan/ammonium polyphosphate/montmorillonite aerogel-based piezoresistive pressure sensor for human motion detection

Flexible sensors have great application prospects in wearable electronics, electric skin and smart robot. However, the lack of functionalities such as superhydrophobicity and flame retardancy limits the practical applications of the sensors under complex conditions. Herein, a flame-retardant chitosan (CS)/ammonium polyphosphate (APP)/montmorillonite (MMT) aerogel was prepared through freeze-drying method, and then the superhydrophobic and flame-retardant piezoresistive pressure sensor was obtained after coating silver nanowires and spraying silica/PDMS. The sensor demonstrated superhydrophobicity even after 1500 compression cycles. APP and MMT effectively improved the flame retardancy of the aerogel, and the self-extinguishing time was only 1.3 s. The sensor showed real-time electrical response in a wide pressure range and was able to detect not only large-scale limb activities but also small-scale muscle movements. The prepared superhydrophobic and flame-retardant piezoresistive pressure sensor has great potential in wearable electronics, healthcare monitoring and smart robots even under watery or flammable conditions.

Thin and high conductive multi‐walled carbon nanotubes/polydimethylsiloxane composite film prepared via solvent method as strain sensors

AbstractFlexible strain sensors based on conductive fillers and flexible polymers possessed significant advantages in human motion detection. Preparing a strain sensing layer with high electrical conductivity and excellent mechanical property under high content of conductive filler contributed to the stability of flexible strain sensors. In this study, MWCNTs/PDMS composite film was prepared by the organic solvent method. The microstructure, electrical conductivity, mechanical property, and piezoresistive characteristics of the composite film at different MWCNTs contents were characterized and discussed. When the mass fraction of MWCNTs in the composite film was 5%, the composite film exhibited a high electrical conductivity of 9.56 S/m while maintaining ideal mechanical properties, and the film thickness was just about 180 μm. The relationship between electrical signals and film strain was performed. The piezoresistive characteristic results demonstrated that the prepared composite film could be used as flexible strain sensor for human motion detection. The prepared thin MWCNTs/PDMS composite film in this paper illustrated high conductive and desired flexibility, and was an alternative material for human motion detection.

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.

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

Development of an IoT-Based Drowning Detection System for Private Swimming Pools

Drowning is among the top three leading causes of unintentional death worldwide, accounting for over 8% of all injury-related deaths. Annually, over 22,000 deaths occur due to drowning in adults, teenagers, and children. The highest rate is among children aged 0–15 years. Available solutions to this murder (drowning) involve traditional fencing of swimming pools, the use of wrist bands with RF connections, heartbeat sensors, monitoring systems using LASER and LDR, etc. These methods are not sufficient to solve the problems, as past methods involve wearable buildings systems. This paper proposes a method that is dependent on the swimming pool (the swimmer does not need to wear any device). The proposed system utilizes multiple sensor technology and IoT technology; PIR sensor for motion detection and Raspberry Pi module v3 for capturing and detecting possible drowning events via a trained model using yolo v5 architecture integrated into Raspberry Pi 4 8GB RAM. The system sends an email message to the authorized user if it senses motion around the swimming pool environment, detects a human in an un-save zone, and sends an alert of possible drowning.

Ultrahigh moisture resistance, highly sensitive and flame retardancy wearable strain sensor for agile water rescue, fire alarm and human motion detection

Smart fabrics provide considerable inspiration for the construction of wearable electronic devices due to their excellent skin affinity and breathability. However, the limited tolerance to high humidity environments and the flammable nature of fabrics impedes broader practical applications. In this paper, by an advanced and effective method, phytic acid doped (PA-) polyaniline (PANI) was polymerized in-situ on cotton fabric deposited by carbon black (CB), and then modified by non-fluorine hydrophobic particles, a flexible breathable fabric-based strain sensor (HPCF) with ultrahigh moisture resistance, flame retardancy and environmental resistance was successfully prepared. The obtained sensor shows high sensitivity (16.14 kPa−1) in the detection range of 0–2 kPa, fast response/recovery time (82/40 ms) and high cycle stability (>10,000 cycles). Thanks to its excellent hydrophobicity (WCA = 153°) and flame retardancy properties, the material enables the detection of human movement (such as finger/wrist/elbow/knee/foot movement) under a variety of harsh conditions (extremely high temperatures, underwater, acid, alkali, salt and blown sand environments). More importantly, it can be further applied to remote detection of potentially dangerous scenarios. HPCF can successfully transmit the Morse code signals of “SOS” and “HELP” sent by drowning people, and realize water rescue. In case of fire, it serves as a fire warning sensor to realize agile (1 s) fire alarming. The highly sensitive and versatile HPCF fabric sensor enables physical condition monitoring for fire and rescue personnel working in high temperature and humidity environments, which is of great value for all human activities carried out in harsh environments.

Strong and Tough Antifreezing Hydrogel Sensor via the Synergy of Coordination and Hydrogen Bonds.

Hydrogel sensors are fascinating as flexible sensors and electronic skin due to their excellent biocompatibility and structure controllability. However, developing conductive hydrogels possessing both excellent mechanical and antifreezing properties for environmental-adaptive sensors remains a challenge. Herein, a strategy of combining betaine and metal ions to construct poly(acrylic acid) (PAA)-based high-conductive hydrogels has been reported. PAA-Al3+/betaine hydrogels with high toughness and antifreezing property were prepared by a one-step UV curing method. Their high toughness is attributed to the coordination of metal ions with the carboxylic groups in PAA, the interaction of betaine with PAA, and the formation of hydrogen bonds between them and water molecules. Moreover, the significant antifreezing property is due to the reduction of free water in the hydrogel. This, in turn, is attributed to the hydration of metal ions and the synergistic hydrogen bonding between betaine and water. The experiments demonstrate that the hydrogel has excellent mechanical property, high conductivity, superior transparency, antiswelling property, antipuncture as well as shape memory properties, and especially, low cytotoxicity. It can be used as a sensor for motion detection and information recognition. This work provides new insights into the application of flexible sensors and human-machine interfaces in multienvironmental conditions.

Stretchable and high sensitive ionic conductive hydrogel for the direction recognizable motion detection sensor

Due to the good conductivity and adjustable mechanical properties, ionic conductive hydrogel is considered an ideal candidate for use in wearable electronic devices. However, maintaining good flexibility, conductivity, and high sensitivity of electromechanical performance, though highly desirable, remains a great challenge. In this work, a zwitterionic ionic conductive hydrogel (CMHZ: chitosan/polyacrylic acid-acrylamide/ZnSO4) was prepared with excellent mechanical properties, ionic conductivity, and high sensitivity. The CMHZ showed excellent adjusted mechanical properties (up to 1210% tensile strain under 0.15 MPa, toughness 5.42 MJ/m3) and good shape-recovery ability. Meanwhile, the free-moving metal ions in the hydrogel guaranteed the CMHZ with good conductivity (33.73 mS/cm). More importantly, the unique zwitterionic structure made the hydrogel have a wide working voltage range (0 - 2.0 V), which endowed the hydrogel with a significant improvement of sensitivity up to GF = 4.22. Based on these obvious advantages, the CMHZ hydrogel was used as a stretchable sensor demonstrated high-sensitivity physiological signal detection and motion direction recognition, suggesting wide potential in flexible electronic devices.

3D printable, stretchable, anti-freezing and rapid self-healing organogel-based sensors for human motion detection

Self-healing hydrogels have promising applications in sensors and wearable devices. However, self-healing hydrogels prepared with water as the dispersion medium inevitably freeze at sub-zero temperature, resulting in a loss of the self-healing and sensing ability. The black phosphorene / ethylene glycol / polyvinyl alcohol / sodium tetraborate / sodium alginate (BP/EG-SPB) organogels were prepared by 3D printing technology and solvent displacement method. The organogel exhibits high stretchability (1900 % strain), excellent self-healing property (25 s) and outstanding anti-freezing property (lower than −120 °C freezing point). Furthermore, the organogel can rapidly self-healed (150 s) at a low temperature (−80 °C) without any external stimulation. Additionally, this organogel-based flexible sensor possesses excellent sensitivity (gauge factor: 28.66 at 1900 % strain) and fast response capability, allowing for effective detection of human motion. This work provides a novel method for preparing multifunctional organogel-based sensors for use in harsh climates.

3D printable, anti-freezing, and rapid self-healing violet phosphorene incorporated hydrogel-based sensors for human motion detection

Conductive self-healing hydrogels, as an emerging flexible material, have been extensively studied in flexible sensors which have great application potential in the field of wearable electronic devices. However, hydrogels are prone to freezing at low temperatures, resulting in reduced or even lost flexibility, conductivity and self-healing properties. Besides, manufacturing method, processing time and cost of hydrogel-based sensors remain extremely challenging, hindering their application in complex and extreme environments. The hydrogel with excellent strain-sensing properties and outstanding anti-freezing properties was fabricated by 3D printing technology and solvent displacement method. The VP-SPB-EG hydrogel has excellent anti-freezing performance (−80 °C). In particular, this hydrogel exhibits excellent self-healing performance (120 s) at −80 °C. In addition, VP-SPB-EG hydrogel-based has high stretchability (1850 %) and sensitivity (GF: 27.82) can be used to detect strain changes caused by human joint movements. Therefore, this flexible hydrogel-based sensor has broad application prospects for wearable devices in extreme environments.

Polydopamine functionalized stellate mesoporous silica using mussel inspired chemistry for ultrastretchable, conductive and self-healing hydrogel on wearable strain sensors

Conductive hydrogels hold great promise in wearable strain sensor for human motion detection. Nevertheless, the poor mechanical properties, inferior sensitivity and the deficiency of multifunctionalities of conventional hydrogels tremendously constrain their applications. Herein, inspired by mussel chemistry, polydopamine (PDA) and Au nanoparticles (Au NPs) were used to modify stellate mesoporous silica (STMS), which could provide conductivity and multiple non-covalent interactions. The synthesized STMS@PDA-Au NPs were further used to prepare composite polyacrylic acid (PAA) hydrogel (STMS@PDA-Au NPs/PAA) by a one-pot strategy for the application of motion sensors. The resulting hydrogel exhibited satisfactory stretchability (663%), remarkable tensile strength (2.18 MPa), high sensitivity (Gauge Factor = 2.86) and distinguished self-healing properties (90.8%), which are believed to be contributed by the existence of coordination complexations and multiple hydrogen-bonding interactions in the hydrogel network. Additionally, with a rapid dynamic response time (180 ms) and fatigue resistance, this work provides useful insights as a wearable sensor for epidermal sensing applications.

A Mobile Health Application Using Geolocation for Behavioral Activity Tracking.

The increasing popularity of mHealth presents an opportunity for collecting rich datasets using mobile phone applications (apps). Our health-monitoring mobile application uses motion detection to track an individual's physical activity and location. The data collected are used to improve health outcomes, such as reducing the risk of chronic diseases and promoting healthier lifestyles through analyzing physical activity patterns. Using smartphone motion detection sensors and GPS receivers, we implemented an energy-efficient tracking algorithm that captures user locations whenever they are in motion. To ensure security and efficiency in data collection and storage, encryption algorithms are used with serverless and scalable cloud storage design. The database schema is designed around Mobile Advertising ID (MAID) as a unique identifier for each device, allowing for accurate tracking and high data quality. Our application uses Google's Activity Recognition Application Programming Interface (API) on Android OS or geofencing and motion sensors on iOS to track most smartphones available. In addition, our app leverages blockchain and traditional payments to streamline the compensations and has an intuitive user interface to encourage participation in research. The mobile tracking app was tested for 20 days on an iPhone 14 Pro Max, finding that it accurately captured location during movement and promptly resumed tracking after inactivity periods, while consuming a low percentage of battery life while running in the background.

Multimodal E-Textile Enabled by One-Step Maskless Patterning of Femtosecond-Laser-Induced Graphene on Nonwoven, Knit, and Woven Textiles.

Personal wearable devices are considered important in advanced healthcare, military, and sports applications. Among them, e-textiles are the best candidates because of their intrinsic conformability without any additional device installation. However, e-textile manufacturing to date has a high process complexity and low design flexibility. Here, we report the direct laser writing of e-textiles by converting raw Kevlar textiles to electrically conductive laser-induced graphene (LIG) via femtosecond laser pulses in ambient air. The resulting LIG has high electrical conductivity and chemical reliability with a low sheet resistance of 2.86 Ω/□. Wearable multimodal e-textile sensors and supercapacitors are realized on different types of Kevlar textiles, including nonwoven, knit, and woven structures, by considering their structural textile characteristics. The nonwoven textile exhibits high mechanical stability, making it suitable for applications in temperature sensors and micro-supercapacitors. On the other hand, the knit textile possesses inherent spring-like stretchability, enabling its use in the fabrication of strain sensors for human motion detection. Additionally, the woven textile offers special sensitive pressure-sensing networks between the warp and weft parts, making it suitable for the fabrication of bending sensors used in detecting human voices. This direct laser synthesis of arbitrarily patterned LIGs from various textile structures could result in the facile realization of wearable electronic sensors and energy storage.

Wearable strain sensors for human motion detection and health monitoring based on hybrid graphite-textile flexible electrodes

Facile and inexpensive assembly of graphite coatings through pencil drawing method has become a fascination for variety of electronic and optoelectronic applications. Here, we report the successful assembly of graphite coatings on flexible and stretchable textiles for the fabrication of electronic devices. Graphite pencils are used for directly coating the graphite on commercial textiles through drawing method. The graphite-coated textiles are used as electrodes for the fabrication of stretchable multifunctional sensors. A fruitful pre-strain strategy is used for assembly of graphite coatings which not only can tune the sensitivity of sensors, but also increases the resistance stability of devices. The fabricated sensor is used for human motion detection and health monitoring; it efficiently detects the strains provided by human organs during their movements, and being multifunctional, adeptly senses the changes in humidity and human sweating, notices the changes in temperature and responds to changes in light concentration. The sensor can work soundly for uniaxially applied strain up to 50% and multiaxially up to 50 × 50%, it is washable, low-cost, possessed a stable response, worked proficiently for 5000 stretching cycles, delivered a markable sensitivity of 3247 for applied strain 50%, and offered a response time of 0.11 s.

Highly Sensitive and Wearable ZnO–Graphene Nanocomposite-Based Strain Sensors for Human Motion Detection

Flexible electronic sensors have garnered considerable interest in wearable health-monitoring devices and electronic skin. In this work, a simple method for the fabrication of flexible zinc oxide-graphene nanoplatelets (ZnO-GNP) nanocomposite based strain sensors has been proposed. The sensing element was deposited on polydimethylsiloxane (PDMS) substrate by a facile spin and peel strategy to yield flexible sandwiched sensors. Effect of varying the blending ratio of the constituents on the electromechanical responses of the sensors was studied. The sensors exhibited high stretchability, good sensitivity, high reversibility and superior stability under tensile and bending loads. The flexible nanocomposite sensors were able to detect extensive human movements such as bending of the elbow, wrist and finger and also subtle motions such as eye blinking, wrist pulse and phonation. Owing to the facile and economical fabrication method, high sensitivity and good reversibility, the ZnO-GNP sensors have high prospect of application in wearable health monitoring devices, robotics and various forms of human-machine interface.

Highly Stretchable, Ultra-Sensitive, and Self-Healable Multifunctional Flexible Conductive Hydrogel Sensor for Motion Detection and Information Transmission.

Self-healable flexible sensing materials are extensively investigated for their potential use in human motion detection, healthcare monitoring, and other fields. However, the existing self-healable flexible sensing materials have limited their application in real life due to the weak stability of the conductive network and the difficulty in balancing stretchability and self-healing performances. In this paper, a flexible sensor with skin-like properties was prepared by composing a polymer composite hydrogel with a multiple network structure consisting of polyaniline, polyvinyl alcohol, chitosan, and phytic acid. The composite hydrogel was tested and proved to own high mechanical properties (stretchability ≈ 565%, strength ≈ 1.4 MPa), good electrical conductivity (0.214 S cm-1), excellent self-healing properties (>99% healing efficiency in a 4 h healing period), and antibacterial properties. It had high sensitivity and a wide sensing range for strain and pressure, making it possible to manufacture multifunctional flexible sensors with comprehensive performance exceeding that of most flexible sensing materials. Notably, this polymer composite hydrogel can be manufactured in a large area and at a low cost, which is beneficial for its further application in many fields.

Self-Adhesive, Anti-Freezing MXene-Based Hydrogel Strain Sensor for Motion Monitoring and Handwriting Recognition with Deep Learning.

Flexible strain sensors based on self-adhesive, high-tensile, super-sensitive conductive hydrogels have promising application in human-computer interaction and motion monitoring. Traditional strain sensors have difficulty in balancing mechanical strength, detection function, and sensitivity, which brings challenges to their practical applications. In this work, the double network hydrogel composed of polyacrylamide (PAM) and sodium alginate (SA) was prepared, and MXene and sucrose were used as conductive materials and network reinforcing materials, respectively. Sucrose can effectively enhance the mechanical performance of the hydrogels and improve the ability to withstand harsh conditions. The hydrogel strain sensor has excellent tensile properties (strain >2500%), high sensitivity with a gauge factor of 3.76 at 1400% strain, reliable repeatability, self-adhesion, and anti-freezing ability. Highly sensitive hydrogels can be assembled into motion detection sensors that can distinguish between various strong or subtle movements of the human body, such as joint flexion and throat vibration. In addition, the sensor can be applied in handwriting recognition of English letters by using the fully convolutional network (FCN) algorithm and achieved the high accuracy of 98.1% for handwriting recognition. The as-prepared hydrogel strain sensor has broad prospect in motion detection and human-machine interaction, which provides great potential application of flexible wearable devices.

Copolymer-grafted cellulose nanocrystal induced nanocomposite hydrogels with enhanced strength, high elasticity and adhesiveness for flexible strain and pressure sensors

Recently, the application of cellulose nanocrystals (CNCs) in the field of hydrogel sensors has attracted much attention. However, it remains challenging to construct CNC-reinforced conductive hydrogels with a combination of enhanced strength, low hysteresis, high elasticity and remarkable adhesiveness. Herein, we present a facile method to prepare conductive nanocomposite hydrogels with the above-mentioned properties by reinforcing chemically crosslinked poly(acrylic acid) (PAA) hydrogel with rational-designed copolymer-grafted CNCs. The copolymer-grafted CNCs interact with PAA matrix to form carboxyl-amide conventional hydrogen bonds and carboxyl-amino ionic hydrogen bonds, among which the ionic hydrogen bonds with rapid recovery capability are critical to the low hysteresis and high elasticity of hydrogel. The introduction of copolymer-grafted CNCs endowed the hydrogels with enhanced tensile/compressive strength, high resilience (>95 %) during tensile cyclic loading, rapid self-recovery during compressive cyclic loading and improved adhesiveness. Thanks to the high elasticity and durability of hydrogel, the assembled hydrogel sensors exhibited good cycling repeatability and durability in detecting various strains, pressures and human motions. The hydrogel sensors also showed satisfying sensitivity. Hence, the proposed preparation method and the obtained CNC-reinforced conductive hydrogels would open new avenues in flexible strain and pressure sensors for human motion detection and beyond.

Stretchable kirigami-inspired conductive polymers for strain sensors applications

Kirigami metamaterials can be exploited in stretchable electronics owing to their architecture, which can be leveraged to amplify stretchability, bendability and deformability. Herein, we report a stretchable kirigami-structured poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/polydimethylsiloxane (PDMS) polymer composite. The electromechanical response and mechanical behavior of kirigami PEDOT:PSS-coated PDMS and polymer composite specimens were investigated and compared with their non-kirigami counterparts. The kirigami structure exhibited improved electromechanical properties owing to its characteristic architecture. This study illustrates the application of a kirigami polymer composite as a strain sensor for human motion detection.

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.