Wearable Piezoresistive Sensors Research Articles

Superelastic carbon aerogels with anisotropic and hierarchically-enhanced cellular structure for wearable piezoresistive sensors

Elastic carbon aerogels have promising applications in the field of wearable sensors. Herein, a new strategy for preparing carbon aerogels with excellent compressive strength and strain, shape recovery, and fatigue resistance was proposed based on the structure design and carbonization optimization of nanocellulose-based precursor aerogels. By the combination of directional freezing and zinc ion cross-linking, bacterial cellulose (BC)/alginate (SA) composite aerogels with high elasticity and compressive strength were first achieved. The existance of zinc ions also significantly improved the carbon retention rate and inhibited structural shrinkage, thus making the carbon aerogels retain ultra-high elasticity and fatigue resistance after compression. Moreover, the carbon aerogel possessed excellent piezoresistive pressure sensing performance with a wide detection range of 0–7.8 kPa, high sensitivity of 11.04 kpa-1, low detection limit (2 % strain), fast response (112 ms), and good durability (over 1,000 cycles). Based on these excellent properties, the carbon aerogel pressure sensors were further successfully used for human motion monitoring, from joint motion to and speech recognition.

Antibacterial highly sensitive eco-friendly wearable piezoresistive sensor for monitoring multiple physiological parameters

Recent progress in wearable biomedical devices has made real-time monitoring possible through non-invasive wearable strain sensors, which can transform physical or biological signals into electrical responses. This work has fabricated multifunctional wearable strain sensors by coating composites (SiE-µG) of electrochemically exfoliated micro-sized multi-layer graphene flakes (µG) and Silicone elastomer (SiE) onto stretchable textile-based orthopedic bandage having ZnO adhesive (SiE-µG@Bandage). The composite coated textile sensors exhibited a remarkable sensitivity of 18330, large stretchability of 35%, superior mechanical durability of 50000 cycles, minute detection limit of 0.04% strain, and fast response time of 400 and recovery time 900 ms. The SiE-µG@Bandage also showed antibacterial properties against Escherichia coli (E. coli) bacteria. Additionally, zinc oxide-based skin-friendly and non-toxic adhesive on the backside of the bandage allowed stable and conformal attachment with the skin. Owing to the above merits, the sensors have shown applicability in real-time monitoring of various physical activities, physiological signals, joint movements, yoga postures, and meditation. Furthermore, the relevance of the sensors in the man-machine interaction has been demonstrated via real-time monitoring of traffic signals by finger movements. Moreover, the exceptional performance and specific features of the SiE-µG@Bandage sensors have enabled their application as electronics skins in health care and preventive care.

Design of a stable wearable piezoresistive sensor with a laminated pattern for simultaneous anti-wetting and self-power

Wearable piezoresistive sensors have great potential in motion monitoring, health detection, and virtual reality. However, such sensors have difficulty maintaining long-term and stable sensing performance in complicated conditions such as wetting, corrosion and without external power supply. Herein, we integrated a stable wearable piezoresistance sensor (SWPS) with sandwich-like laminated structure, which consists of a flexible superhydrophobic conductive material, a rigid superhydrophobic material, conductive coil and magnetic composite materials which is used to provide magnetic field. SWPS achieves simultaneously effective pressure sensing and self-power supply. Importantly, such sensor maintains outstanding superhydrophobicity even when exposed to mechanical or chemical damage, such as cyclic compression, impact and wetting erosion, ensuring the reliable and long-lasting sensing performance. Our design strategy could also guide the development of other wearable sensor materials that need to retain long-term sensing performance in harsh operating conditions.

Self-assembled gel-assisted preparation of high-performance hydrophobic PDMS@MWCNTs/PEDOT:PSS composite aerogels for wearable piezoresistive sensors

The conductive polymer poly(3,4-thylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) exhibits potential in the development of flexible devices due to its unique conjugated structure and water-solubility characteristics. To address the incompressibility of the original PEDOT:PSS aerogel without compromising its high conductivity, a stable interpenetrating polymer network (IPN) was self-assembled by guiding the molecular motion within PEDOT:PSS and introducing multi-walled carbon nanotubes (MWCNTs). By combining critical surface removal, directional freeze-drying, and polydimethylsiloxane (PDMS) reinforcement processes, a hydrophobic PDMS@MWCNTs/PP aerogel with a highly oriented porous structure and high strength was prepared. Under the synergistic effect of MWCNTs/PEDOT:PSS electroactive scaffold, the composite aerogel exhibited a high sensitivity of up to 16.603 kPa−1 at 0–2 kPa, a fast response time of 74 ms, and excellent repeatability. Moreover, the sensor possessed hydrophobicity with a good water contact angle of 137° The sensor could serve as a wearable electronic monitoring device to achieve accurate and sensitive detection of human motion including large-scale human activities and tiny muscle movements. Therefore, our findings provide a new direction to fabricate high-performance piezoresistive sensors based on three-dimensional (3D) conductive polymer active scaffolds, demonstrating their great potential for flexible electronics, human-computer interaction, and a wide range of applications under special working conditions.

Flexible and Wearable Piezoresistive Sensors Based on Double Wrinkled Layers for Motion Monitoring and Human Physiological Signal Monitoring

Flexible and Wearable Piezoresistive Sensors Based on Double Wrinkled Layers for Motion Monitoring and Human Physiological Signal Monitoring

Flexible germanium monotelluride phase change films with ultra-high bending stability for wearable piezoresistive sensors

Although non-stoichiometric chalcogenide phase change materials (e.g., Ge2Sb2Te5, Sb2Te3) have been used in wearable electronics, their non-stoichiometric vacancies will inevitably degrade their bending stability and electrical contrast. In this study, we demonstrated that stoichiometric germanium monotelluride films deposited on flexible polyimide substrates exhibited excellent phase transition characteristics and ultra-high bending stability under different bending states. The GeTe films showed improved bending stability under compressive strain than under tensile strain, and the GeTe films still exhibited high conductivity contrast (6 orders of magnitude) after 2000 bending cycles, with potential use in flexible memory devices. As a proof-of-concept, the bending stability of the stoichiometric GeTe films was shown to be significantly better than the non-stoichiometric Ge2Sb2Te5 films. Notably, we fabricated a flexible piezoresistive sensor based on the GeTe film, which exhibited good sensitivity (gauge factor is 38), low detection limit (0.1%), short response time (83.3 ms), good reliability and repeatability (more than 1200 cycles) and high heat resistance (200 ℃), and was used to track human pulse, as well as finger and face movements. The results showed that the piezoresistive sensor had good sensitivity and a stable time-resolved electrical response, and could be used to detect the movement of different human body parts and the small signals of different pressure points. These findings indicated that GeTe-based wearable piezoresistive sensors hold significant promise for medical health monitoring and biosensing applications.

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.

Recent progress in graphene-based wearable piezoresistive sensors: From 1D to 3D device geometries

Electronic skin and flexible wearable devices have attracted tremendous attention in the fields of human-machine interaction, energy storage, and intelligent robots. As a prevailing flexible pressure sensor with high performance, the piezoresistive sensor is believed to be one of the fundamental components of intelligent tactile skin. Furthermore, graphene can be used as a building block for highly flexible and wearable piezoresistive sensors owing to its light weight, high electrical conductivity, and excellent mechanical. This review provides a comprehensive summary of recent advances in graphene-based piezoresistive sensors, which we systematically classify as various configurations including one-dimensional fiber, two-dimensional thin film, and three-dimensional foam geometries, followed by examples of practical applications for health monitoring, human motion sensing, multifunctional sensing, and system integration. We also present the sensing mechanisms and evaluation parameters of piezoresistive sensors. This review delivers broad insights on existing graphene-based piezoresistive sensors and challenges for the future generation of high-performance, multifunctional sensors in various applications.

Embedded Pressure Sensing Metamaterials Using TPU-Graphene Composites and Additive Manufacturing

Disabilities impacting mobility are a global concern requiring gait rehabilitation, where monitoring foot pressure distribution is fundamental. Wearable systems provide an alternative to stationary equipment eliminating space limitations. However, wearable sensors present challenges in the calibration, sensitivity, and human-sensor interface, requiring application-specific sensors. This study aimed to develop wearable sensors where the structural and material properties can characterise the sensitivity and range of measurement during the design phase. We developed wearable piezoresistive sensors using additive manufacturing to create mechanical metamaterials with embedded pressure-sensing capabilities. Three structural designs were developed for different measuring ranges (0 – 50 N, 0 – 100 N, and 0 – 150 N) using body-centred cubic lattices constructed via pyramid unit cells. In addition, two graphene infusion processes were evaluated. We analysed the influence of structural dimensions and the graphene infusion process on the piezoresistive response of the sensors. The measuring range was affected mainly by tuneable structural dimensions, while the infusion process influenced the piezoresistive sensitivity and the linear response. The outcomes in characterising the piezoresistive sensors based on structural and material properties could allow the development of wearables with embedded pressure sensing with a predictive response solely based on design parameters using additive manufacturing and graphene inks.

Soft Wearable Piezoresistive Sensors Based on Natural Rubber Fabricated with a Customized Vat-Based Additive Manufacturing Process.

Piezoresistive sensors for monitoring human motions are essential for the prevention and treatment of injury. Natural rubber is a material of renewable origin that can be used for the development of soft wearable sensors. In this study, natural rubber was combined with acetylene black to develop a soft piezoresistive sensing composite for monitoring the motion of human joints. An additive manufacturing technique based on stereolithography was used, and it was seen that the sensors produced with the method could detect even small strains (<10%) successfully. With the same sensor composite fabricated by mold casting, it was not possible to detect low strains reliably. TEM microscopy revealed that the distribution of the filler was not homogeneous for the cast samples, suggesting a directionality of the conductive filler network. For the sensors fabricated through the stereolithography-based method, a homogeneous distribution could be achieved. Based on mechano-electrical characterization, it was seen that the samples produced with AM combined the ability to endure large elongations with a monotonic sensor response. Under dynamic conditions, the sensor response of the samples produced by 3D printing showed lower drift and lower signal relaxation. The piezoresistive sensors were examined for monitoring the motion of the human finger joints. By increasing the bending angle of the sensor, it was possible to increase the sensitivity of the response. With the renewable origin of natural rubber and manufacturing method, the featured sensors can expand the applicability of soft flexible electronics in biomedical applications and devices.

3D wearable piezoresistive sensor with waterproof and antibacterial activity for multimodal smart sensing

High sensitivity, excellent stability and rapid response have been gradually satisfied with the rapid development of 3D wearable piezoresistive sensors. Nevertheless, advanced parameters such as waterproof and antibacterial property, which are critical to wearable electronics, have received inadequate attention. Herein, a 3D wearable piezoresistive sensor with waterproof and antibacterial activity was fabricated with the utilization of graphene, silver nanoparticles (Ag NPs), polyurethane (PU) and thermoplastic polyurethanes (TPU). This 3D TPU@rGO-Ag NPs@PU sensor exhibits competitive sensing performance, achieving a remarkable sensitivity (GFmax=152.97kPa−1), reliable dynamic stability (over 10,000 cycles), fast response/recovery time (50 ms/40 ms), and satisfactory sensitivity retention (∼90%). Furthermore, this sensor achieves outstanding waterproof and high antibacterial activity (>99.9%), providing ample convenience for the realization of wearable. Besides, fingering habits, typing strength and typing speed can be captured by this sensor, which overcomes the security deficiencies of password authentication system and improves the security of information. This 3D piezoresistive sensor shows huge potential in multimodal smart sensing, which provides opportunities for developing intelligent wearable sensors and future broad commercial applications.

Self-driving flexible piezoresistive sensors integrated with enhanced energy harvesting sensor of ZnO/Ti3C2Tx nanocomposite based on 3D structure

Wearable piezoresistive sensors (PRS) based on detecting small resistance variation have broad applications. Aiming at solving self-driving issue, a self-driving PRS based on piezoelectric energy harvesting sensor is proposed. Therefore, the piezoresistive sensor was prepared by the Ti3C2Tx nanosheets coated 3D sponge. The piezoelectric energy harvesting sensor based on ZnO nanorods growing on the surface of the Ti3C2Tx nanosheets coated sponge was designed by perfusing Polydimethylsiloxane (PDMS). The piezoresistive sensor with the detection range of 0–61.12 kPa and the sensitivity of 6.40 kPa−1 was obtained, and further constructed the detection array with the 4 × 4 PRSs. The piezoelectric energy harvesting sensor illustrated an output voltage of 101 V, a power of 10 μW, and a sensitivity of 4.37 V·kPa−1, which can drive the piezoresistive sensor and monitor human motions. Meanwhile, the demonstration of self-driving and detecting human motions of the piezoresistive sensor by the piezoelectric energy harvesting sensor was illustrated. Therefore, the piezoresistive and piezoelectric devices based the Ti3C2Tx modified 3D sponge can be constructed and achieve self-powered pressure detection.

Mechanically durable and amphiphobic rubber foam composites for strain/pressure sensors

Wearable piezoresistive sensors possessing outstanding flexibility and breathability have aroused growing interests of researchers due to their promising applications in wearable electronics, health monitoring, etc. The development of waterproof and oil-proof piezoresistive sensors that respond to both tensile and compressive strains is still a great challenge. Herein, a two-step dip-coating method, i.e., ion reduction assisted phase separation and subsequent fluorination, is used to prepare amphiphobic rubber foam composites (ARFC). The ARFC shows excellent photothermal and electro-thermal conversion performance. The ARFCs with high electrical conductivity can be served as both strain and pressure sensors, demonstrating a dual sensing mode. The ARFC exhibits a relatively low detection limit (1%), wide working range (0–150%) and a gauge factor of 34.2 under 100%–150% strain. Also, the strain/pressure sensor exhibits outstanding cyclic sensing performance and can realize real-time monitoring of different body motions. Its amphiphobicity and dual sensing mode further broaden the application in smart wearable electronics.

Ti3C2-MXene ionogel with long-term stability and high sensitivity for wearable piezoresistive sensors

Wearable piezoresistive sensors have attracted tremendous attention recently, which can be applied for health monitoring and human-computer interaction. MXenes, possessing high metallic conductivity and large specific surface area, are often used as the conductive filler in hydrogels and organohydrogels for piezoresistive sensors. However, the environmental instability of MXene composites caused by the easy oxidation of MXene is a long-standing issue which needs to be solved urgently. Herein, taking advantage of phase transfer achieved by cationic surface active ionic liquid 1-vinyl-3-dodecylimidazolium bromide ([VC12Im]Br), we combine Ti3C2Tx with hydrophobic ionic liquid to fabricate Ti3C2-MXene ionogel successfully, which exhibits high compressibility (2.97 MPa), high conductivity (0.908 mS/cm) and long-term stability (40 days). And the wearable piezoresistive sensor based on Ti3C2-MXene ionogel shows rapid response time (100 ms), high sensitivity (6.47 kPa−1) and wide detection range, thus can be used to monitor the tiny movements of human body parts, recognize vocalizations and handwriting. This preparation method develops a new path to fabricate Ti3C2-MXene ionogel with long-term stability and high sensitivity for wearable electronic devices.

Reduced graphene oxide/polyurethane coatings for wash-durable wearable piezoresistive sensors

Graphene-based functional coatings for cotton textiles were realized through an easy dip-coating procedure. Cotton fabrics were coated with a reduced graphene oxide (rGO) layer and then protected with a very thin polyurethane (PU) layer that does not affect the flexibility and the hand of the pristine cotton. The application of the rGO coating induces electrical conductivity to the fabric and the application of the PU phase increases the durability of the coatings, that show very stable surface resistivity after 10 washing cycles performed at temperatures up to 40 °C. Furthermore, the rGO and rGO/PU coated fabrics show good comfort properties, increased thermal conductivity and breathability with respect to cotton. In particular, the realized coatings allow to confine the heat transfer in correspondence of a localized heating source, which is very interesting for thermal therapy applications. Finally, the rGO/PU coated fabrics present a piezoresistive behaviour characterized by very stable electrical response to applied stretching up to 50% deformation, high sensitivity especially at low deformations with gauge factor values up to 11.7 and fast response time down to 500 ms when stretched at 100 mm/min rate at 2.5% strain. Overall, the results demonstrate that rGO/PU coated fabrics are very promising wash-durable electrically conductive e-textiles with improved comfort, enhanced thermal conductivity for possible thermal therapy applications, and piezoresistive properties for sensing applications as human motion monitoring.

3d Wearable Piezoresistive Sensor with Waterproof and Antibacterial Activity for Multimodal Smart Sensing

3d Wearable Piezoresistive Sensor with Waterproof and Antibacterial Activity for Multimodal Smart Sensing

Lightweight, superelastic, and temperature-resistant rGO/polysulfoneamide-based nanofiber composite aerogel for wearable piezoresistive sensors

The piezoresistive sensor assembled by rGO/PSAN composite aerogel is characterized by wide pressure detection range and high sensitivity, and have a broad application prospect in the field of high and low-temperature sensing.

Tracheid-inspired nanoarchitectured carbon-based aerogels with ultra-compressibility for wearable piezoresistive sensors

Carbon-based aerogels are difficult to accomplish satisfactory mechanical strength and fatigue resistance due to the brittleness of framework, limiting their application in piezoresistive sensors. In this work, inspired by tracheid structure in plants, flexible piezoresistive sensors were prepared based on stable and elastic carbon-based aerogels with a long-range oriented multiscale matrix structure. The carbon-based aerogel was generated by bidirectional freezing and further assembled to i) 2D morphology Ti3C2Tx MXene as conductive framework and ii) cellulose nanofibers (CNF) offering abundant hydroxyl groups as flexible substrate. Besides, positively charged chitosan was introduced to serve as “bridging junction reinforcement element”, which tightly connects CNF and Ti3C2Tx, contributing to improved strength and stability of the carbon-based aerogel. According to finite element simulation, the bonding and supporting effect of chitosan is confirmed, and the aligned carbon layer and elastic supporting microstructure can be directly modulated. Interestingly, the piezoresistive sensor exhibits super-compressibility which can withstand 10,000 cycles under 50% strain, showing a fast response time (6.3 ms) and high sensitivity (177.08 kPa−1). Briefly, this work provides an innovative strategy to obtain nanoarchitecture functional materials integrating multiscale microstructures into carbon-based aerogels, demonstrating their potential to improve the sensing characteristics and functionality of wearable piezoresistive sensors.

Multiwalled Carbon Nanotube/Graphite Powder Film for Wearable Pressure Sensors with High Sensing Performance.

With the continuous progress of artificial intelligence and other manufacturing technologies, there is promising potential for wearable piezoresistive sensors in human physiological signal detection and bionic robots. Here, we present a facile solution-mixing process to fabricate a multiwalled carbon nanotube/graphite powder (MWCNT@Gp) film, which has high sensitivity and great linearity and is more oriented to flexible piezoresistive sensors. The sensor consists of two parts: a spinosum microstructure shaped by a sandpaper template and polydimethylsiloxane (PDMS) as the top substrate and interdigital electrodes as the bottom substrate. The experiments we have conducted show that these two parts provide good protection to the MWCNTs@Gp film and improve sensor sensitivity. Additionally, the sensitivity of the optimal ratio of multiwalled carbon nanotubes and graphite powder is analyzed. The 5%MWCNT@5%Gp composites were found to have relatively good conductivity, which is convenient for the fabrication of conductive films of piezoresistive sensors. Finally, we conducted application experiments and found that the flexible piezoresistive sensor can detect minute signals of human motion and different pressure points. This indicates the feasibility of portable sensors in electronic skin and smart devices.

Iron-Adjustable Compressible Elastic Chitosan-Derived Carbon Aerogel with Wide-Range Linear Sensitivity and Super Sensing Performances for Wearable Piezoresistive Sensors

Iron-Adjustable Compressible Elastic Chitosan-Derived Carbon Aerogel with Wide-Range Linear Sensitivity and Super Sensing Performances for Wearable Piezoresistive Sensors