High-performance Piezoresistive Sensors Research Articles

Microstructured CNTs/Cellulose Aerogel for a Highly Sensitive Pressure Sensor.

Flexible sensors have been applied in human health monitoring and biomedical research, but producing high-performance piezoresistive sensors at low cost is still challenging. To address these shortcomings, we proposed a microstructured carbon nanotube (CNT)/cellulose aerogel-based pressure sensor. The sensor consists of three parts, i.e., cellulose/poly(vinyl alcohol)/CNT aerogel-based sensing layer and top and bottom thermoplastic polyurethane elastomer (TPU)/silver nanowire (Ag NW) nanofiber electrode. The aerogel is fabricated using a simple freeze-drying method and an easy electrospinning method to obtain the nanofiber-based electrode. Two TPU/Ag NW nanofiber electrodes sandwiched the aerogel with a microstructure in the middle. Benefiting from the microcone and micropore structures on the nanofiber electrode, the assembled sensors show a high sensitivity of 66.4 kPa-1, a significant detection boundary of 50 kPa, and an excellent response speed of 10 ms. The high sensing performance enables the sensor to monitor physiological signals, Morse code interactions, and gesture recognition. With the help of machine learning, the success rate of gesture recognition is as high as 98.8%. The preparation of this pressure sensor based on an aerogel shows excellent health and environmental monitoring potential as an artificial skin.

Nanofiber-Reinforced MXene/rGO Composite Aerogel for a High-Performance Piezoresistive Sensor and an All-Solid-State Supercapacitor Electrode Material.

The assembly of two-dimensional (2D) nanomaterials into a three-dimensional (3D) aerogel can effectively prevent the problem of restacking. Here, nanofiber-reinforced MXene/reduced graphene oxide (rGO) conductive aerogel is prepared via the hydrothermal reduction of GO using pyrrole and in situ composite with MXene. Combined with low-content 2D conductive nanosheets (MXene and rGO) as "brick", conductive polypyrrole as "mortar", and one-dimensional (1D) nanofiber as "rebar", a strong interfacial cross-linking of MXene and rGO nanosheets is realized through covalent and noncovalent bonds to synergistically improve its mechanical performance. Based on the prepared MXene/rGO aerogel, a high-performance piezoresistive sensor with a sensitivity of up to 20.80 kPa-1 in a wide pressure range of 15.6 kPa is obtained, and it can withstand more than 5000 cyclic compressions. Besides, the sensor shows a stable output and can be applied to monitor various human motion signals. In addition, an all-solid-state supercapacitor electrode is also fabricated, which shows a high area-specific capacitance of up to 274 mF/cm2 at a current density of 1 mA/cm2.

Graphene Nanoplatelets/Polydimethylsiloxane Flexible Strain Sensor with Improved Sandwich Structure.

In engineering measurements, metal foil strain gauges suffer from a limited range and low sensitivity, necessitating the development of flexible sensors to fill the gap. This paper presents a flexible, high-performance piezoresistive sensor using a composite consisting of graphene nanoplatelets (GNPs) and polydimethylsiloxane (PDMS). The proposed sensor demonstrated a significantly wider range (97%) and higher gauge factor (GF) (6.3), effectively addressing the shortcomings of traditional strain gauges. The microstructure of the GNPs/PDMS composite was observed using a scanning electron microscope, and the distribution of the conductive network was analyzed. The mechanical behavior of the sensor encapsulation was analyzed, leading to the determination of the mechanisms influencing encapsulation. Experiments based on a standard equal-strength beam were conducted to investigate the influence of the base and coating dimensions of the sensor. The results indicated that reducing the base thickness and increasing the coating length both contributed to the enhancement of the sensor's performance. These findings provide valuable guidance for future development and design of flexible sensors.

Sandpaper-assisted preparation of microstructured multifunctional PANI/PVA composite hydrogel film for high-sensitivity pressure sensors with accurate pressure identification and motion monitoring

Bio-inspired microstructure surface flexible piezoresistive sensors offer an attractive opportunity for a variety of wearable device applications. Unfortunately, the realization of high-performance piezoresistive sensors still faces great challenges owing to the balance between responsitivity and sensing capability of them, as well as being limited by the level of deformation of their surface structures. Here, we propose a simple and efficient, low-cost sandpaper-assisted solution casting-scraping strategy to fabricate PANI/PVA hydrogel film with the reverse random height distribution (RHD) spinosum microstructure on the surface that is simultaneously conductive, flame retardant, and EMI shielding. The flexible piezoresistive sensor designed based on this film can detect pressure from 0.01 N to 20 N with a responsitivity of 77.7263 kPa−1 in the region of 0.03183–1.27324 kPa, a quick response/recovery delay of 18 ms/30 ms, and a good reliability within 2000 load-unload cycles. Notably, this piezoresistive sensor can accurately identify objects of different masses and monitor physiological signals of human motion, and can also produce significant current responses to extremely slight mouse clicks and handwriting styles. The successful preparation of multifunctional PANI/PVA microstructured film provides a new idea to explore the potential applications of piezoresistive sensors in flexible wearable electronic devices.

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.

Preparation of Elastic Macroporous Graphene Aerogel Based on Pickering Emulsion Method and Combination with ETPU for High Performance Piezoresistive Sensors.

High-performance pressure sensors provide the necessary conditions for smart shoe applications. In this paper, the elastic Macroporous Graphene Aerogel (MGA) was synthesized via the modified Hummers' method, and it was further combined with Expanded-Thermoplastic polyurethane (ETPU) particles to assemble MGA-ETPU flexible sensors. The MGA-ETPU has a low apparent density (3.02 mg/cm3), high conductivity (0.024 S/cm) and fast response time (50 ms). The MGA-ETPU has a large linear sensing range (0-10 kPa) and consists of two linear regions: the low-pressure region (0 to 8 kPa) and the high-pressure region (8 to 10 kPa), with sensitivities of 0.08 kPa-1, and 0.246 kPa-1, respectively. Mechanical test results show that the MGA-ETPU sensor showed 19% reduction in maximum stress after 400 loading-unloading compression cycles at 40% strain. Electrical performance tests showed that the resistance of MGA-ETPU sensor decreased by 12.5% when subjected to sudden compression at 82% strain and returned to its original state within 0.05 s. Compared to existing flexible sensors, the MGA-ETPU sensors offer excellent performance and several distinct advantages, including ease of fabrication, high sensitivity, fast response time, and good flexibility. These remarkable features make them ideally suited as flexible pressure sensors for smart shoes.

Ultra-high-sensitivity micro-accelerometer achieved by pure axial deformation of piezoresistive beams

Microfabricated piezoresistive accelerometers with purely axially deformed piezoresistive beams have demonstrated high performance. However, the conventional design of such accelerometers requires complex theoretical calculations and specific dimensional conditions to achieve purely axial deformation, which inevitably increases the difficulty and cost of the design and manufacturing. We propose an innovative structure that can simply realize pure axial deformation of piezoresistive beams by eliminating the transverse displacement at both ends without tedious calculations. An accelerometer based on the structure was fabricated; both static and dynamic performances were tested. The results showed that the accelerometer had high sensitivity (2.44 mV g−1 with 5 V bias, without circuit amplification), low cross-axis sensitivity (1.56% and 0.49 %, respectively), and high natural frequency (11.4 kHz), with a measurement range of 0–100 g. This design method provides an easy approach for designing high-performance piezoresistive sensors.

Tri-Level Wrinkle-Vein-Sheet Structured Reduced Graphene Oxide/Nanofiber Films for Flexible Piezoresistive Sensors

For practical applications, it is critically necessary to develop piezoresistive sensors with high sensitivity, low detection limit, extensive detection range, short response and recovery time, and good durability. In this paper, a novel high-performance piezoresistive sensor is tactfully fabricated based on a tri-level wrinkled-vein-sheet structured reduced graphene oxide (rGO)/nanofiber film. The rGO/nanofiber film is prepared by a combination of high-conductivity rGO, polyacrylonitrile (PAN) nanofibers, and elastic VHB tape, in which rGO is first coated on short PAN nanofibers to form a flexible film and then combined with the bidirectional prestretching method to build a composite film with a random wrinkled structure. The influence of a short nanofiber addition amount and prestretching ratio on the device performance is investigated in detail. Due to the contribution of nanofibers and wrinkled structures to the contact sites, the obtained piezoresistive sensor shows excellent performance, including an ultrahigh cycling durability of 30,000 s, a wide detection range of 2.83 Pa–23.81 kPa, and a high sensitivity of 318.14 kPa–1 at 0–2.83 kPa. More importantly, the prepared sensor can accurately recognize human activities such as touching, chewing, vocalizing, and fist clenching, which is expected to exhibit significant potential for application in future flexible wearable electronic devices.

Ambilateral convergent directional freeze casting meta-structured foams with a negative Poisson’s ratio for high-performance piezoresistive sensors

Flexible piezoresistive sensors show broad application value in wearable electronic skin, health monitoring and motion detection. The practical range of conventional piezoresistive sensors, however, is often severely limited by their sensitivity, measuring span, mechanical properties, and lateral expansion caused by positive Poisson's ratio (PPR) effects. Herein we introduce a generally applicable methodology, ambilateral convergent directional freeze casting technique, for the fabrication of anisotropic porous foams with a negative Poisson's ratio (NPR). Different from traditional freeze casting technique, this new design methodology enables precise control of pore types and pore orientations, thereby endowing artificial materials with special mechanical properties. Under uniaxial compression, synthetic foams exhibit macroscopic lateral shrinkage, with a Poisson's ratio below – 4.2% at the strain of 25%. These conductive meta-structured foams are used to construct piezoresistive sensors. Compared with traditional structures, metastructures with NPR effects significantly increase the number of wall-to-wall contacts and the formation of electrical paths, thereby enhancing the sensitivity and sensing reliability of sensors. In addition, the NPR effect markedly enhances the mechanical elasticity and compression resistance of sensors, enabling it to have excellent signal stability even in the face of reiterative compressions. These meta-structured sensors also exhibited smart resistivity repair behavior in the face of overload damage. Integrated sensors can be used for monitoring of pressure distribution and athlete's gait. Lateral contractions caused by NPR effects can avoid the contact of adjacent sensors and narrow the gap between two sensors, thus improving sensing stability and pressure distribution resolution of distributed sensors.

Ultrahigh ionic conductivity and alkaline tolerance of poly(amidoxime)-based hydrogel for high performance piezoresistive sensor

The preparation of polymer hydrogels with excellent mechanical stability and high conductivity still remains huge challenges in practical applications as wearable electronics and energy storage. Herein a new strategy is proposed to prepare uniform poly(amidoxime)/polyethyleneimine (PAO/PEI) hydrogel through hydrogen bond interactions. The as-prepared PAO/PEI hydrogels demonstrate good compressive strength (7.07 kPa) and transparency. Due to the excellent ionic adsorption properties of PAO containing the amidoxime groups, it is firstly found that the PAO/PEI hydrogels show a ultrahigh ionic conductivity of 19.1 S m−1 in 6 M LiCl, and the hydrogels also present an excellent alkaline tolerance with ionic conductivity as high as 22.35 S m−1 in 6 M KOH. Furthermore, graphene oxide (GO) is introduced into the PAO/PEI system to form PAO/PEI/reduced GO (rGO) hydrogels which exhibit higher mechanical strength, and can endow electronic conductivity. Moreover, both the as-prepared PAO/PEI/LiCl1 (1 M LiCl) and PAO/PEI/rGO/LiCl1 (1 M LiCl) hydrogels as pressure sensors, present high sensitivity (5.09 and 8.55 kPa−1 respectively), ultra-low detection limit (20 Pa), and excellent cycle stability (1000 compression cycles). It can be expected that the PAO-based hydrogels with high ionic conductivity developed firstly will have a broad application prospects in wearable sensing devices and energy storage/conversion devices.

Wearable and high-performance piezoresistive sensor based on nanofiber/sodium alginate synergistically enhanced MXene composite aerogel

The emerging wearable electronics promote the rapid development of flexible piezoresistive sensors. However, it remains a big challenge to develop a high-performance piezoresistive sensor with high sensitivity, wide pressure range, and stable output. Here, an lightweight (17.33 mg cm−3) cellulose acetate nanofiber/sodium alginate synergistically enhanced MXene composite aerogel is fabricated through a liquid nitrogen-assisted unidirectional-freezing strategy. The conductive MXene-based aerogel possesses excellent mechanical performance and high compressive strength (16 kPa). Importantly, the unique synergies endow the aerogel assembled piezoresistive sensor with an ultra-high sensitivity up to 114.55 kPa−1 in a wide pressure range up to 21.78 kPa, and it can withstand more than 24,000 cycles of compression. More interestingly, the prepared sensor can be used to realize real-time monitoring of various human activities and developed as a wireless device to transmit information. The unparalleled sensing performance endows the MXene composite aerogel with a broad application prospect in the field of flexible intelligent wearable devices.

Highly sensitive and stable 3D flexible pressure sensor based on carbon black and multi-walled carbon nanotubes prepared by hydrothermal method

High-performance piezoresistive sensors have the potential for human health monitoring applications. However, the conductive ink should add adhesive to ensure the hydrophobic carbon-based conductive material uniformly, which decrease the sensitivity. We fabricated a piezoresistive sensor without any adhesive polymer by introduced multi-walled carbon nanotubes (MWCNTs) and carbon black (CB) into the skeletons of the melamine sponge (MS) via a simple hydrothermal method. Due to the crosslinking effect, the microporous structure and strong adsorption of the of conductive MWCNTs and CB, the MWCNTs@CB sensor exhibits excellent performance: a sensitivity of 48.26 kPa−1 in the working range of 12.5 kPa–20 kPa, a response time of 15 ms, an low detection limit of 20 Pa, and performance stability (>250,000 loading cycles). This MWCNTs@CB flexible pressure sensor can detect physiological motions.

Near-Zero Hysteresis Ionic Conductive Elastomers with Long-Term Stability for Sensing Applications.

Soft conductive elastomers with low hysteresis over a wide range of stretchability are desirable in various applications. Such applications include soft sensors with a long measurement range, motion recognition, and electronic skin, just to name a few. Even though the measurement capability of the sensors based on soft materials has been greatly improved compared to the traditional ones in recent years, hysteresis in the loading and unloading states has limited the applications of these sensors, thereby negatively affecting their accuracy and reliability. In this work, conductive elastomers with near-zero hysteresis have been formulated and fabricated using 3D printing. These elastomers are made by combining highly stretchable dielectric elastomer formulations with a polar hydrophobic ionic liquid and polymerizing under ultraviolet light. High-performance piezoresistive sensors have been fabricated and characterized, with a 10-fold stretchability and low hysteresis (1.2%) over long-term stability (more than 10 000 cycles under cyclic stress) with a 20 ms response time. Additionally, the current elastomers displayed fast mechanical and electrical self-healing properties. Using 3D printing in conjunction with some of our structural innovations, we have fabricated smart gloves to show this material's wide range of applications in soft robots, motion detection, wearable devices, and medical care.

Epidermis-Inspired Wearable Piezoresistive Pressure Sensors Using Reduced Graphene Oxide Self-Wrapped Copper Nanowire Networks.

Wearable piezoresistive sensors are being developed as electronic skins (E-skin) for broad applications in human physiological monitoring and soft robotics. Tactile sensors with sufficient sensitivities, durability, and large dynamic ranges are required to replicate this critical component of the somatosensory system. Multiple micro/nanostructures, materials, and sensing modalities have been reported to address this need. However, a trade-off arises between device performance and device complexity. Inspired by the microstructure of the spinosum at the dermo epidermal junction in skin, a low-cost, scalable, and high-performance piezoresistive sensor is developed with high sensitivity (0.144kPa-1 ), extensive sensing range ( 0.1-15 kPa), fast response time (less than 150ms), and excellent long-term stability (over 1000 cycles). Furthermore, the piezoresistive functionality of the device is realized via a flexible transparent electrode (FTE) using a highly stable reduced graphene oxide self-wrapped copper nanowire network. The developed nanowire-based spinosum microstructured FTEs are amenable to wearable electronics applications.

Highly Elastic and Fatigue-Resistant Graphene-Wrapped Lamellar Wood Sponges for High-Performance Piezoresistive Sensors

Three-dimensional (3D) conductive aerogels with structural robustness and mechanical resilience are highly attractive for sensitive and stable pressure sensing. However, the fabrication of such 3D aerogels often relies on complicated bottom-up assembly processes that involve costly raw materials or intensive energy consumption or directly coating synthetic polymer sponges (e.g., polyurethane) with conductive materials, which may pose environmental concerns for their disposal. Herein, a simple and sustainable strategy is proposed to fabricate a reduced graphene oxide-coated wood sponge (RGO@WS) with a lamellar structure for high-performance piezoresistive sensors. The introduced RGO nanosheets endow the RGO@WS not only with high conductivity but also with high elasticity and excellent fatigue resistance. These features make it an ideal piezoresistive sensor with a high sensitivity of 0.32 kPa–1 (superior to most polymeric sponge-based sensors), high working stability over 10 000 cycles, and excellent sensing reproducibility at ultralow temperatures. Thanks to its prominent sensing performance, the RGO@WS-based sensor can serve as a wearable device for detecting human motions and physiological signals and allows for spatially resolved pressure mapping via integrating the sensors into a large-area sensing array. The developed highly elastic and fatigue-resistant RGO@WS represents a promising and sustainable alternative to the synthetic polymer-based piezoresistive sensors.

Flexible GO/Nb2CT x hybrid films for high-performance piezoresistive sensors

High-performance flexible piezoresistive sensors have attracted more and more scientific attention due to their promising applications in robotics and portable electronic devices. However, most flexible piezoresistive sensors require a fine structure design at the nanoscale, which makes the manufacturing process very complex and expensive. Two-dimensional (2D) transition metal carbides named MXene have been regarded as an important candidate material for piezoresistive sensors due to their good conductivity, large interlayer spacing, and the inherent nature of 2D materials. The interlayer spacing between MXene nanosheets would be varied under an external pressure, resulting in the conductivity modulation. To improve the sensitivity of MXene-based piezoresistive sensors, the interlayer spacing of MXene-based films should be further tuned and optimized. In this contribution, flexible Nb2CT x -based films were developed using a simple filtration method and evaluated for piezoresistive sensors. 2D GO nanosheets were introduced between the Nb2CT x layers to tune the interlayer spacing to enhance piezoresistive performance. The fabricated flexible piezoresistive sensors based on GO/Nb2CT x films possess high sensitivity up to 228 kPa−1 and rapid response time as fast as 101 ms. This study would promote the application of MXenes-based 2D nanomaterials in the field of piezoresistive sensors.

A high-performance piezoresistive sensor based on poly (styrene-co-methacrylic acid)@polypyrrole microspheres/graphene-decorated TPU electrospun membrane for human motion detection

The flexible piezoresistive sensor (FPS) with typical three-dimensional (3D) structure is a key component for many wearable products because of its excellent performance. We report a high-performance FPS based on micro-nano 3D networked conductive electrospun membrane. This membrane is framed by thermoplastic polyurethane electrospun membrane (TPUEM) coated with reduced graphene oxide(rGO) bridged conductive poly (styrene-co-methacrylic acid)@polypyrrole (P(St-MAA)@PPy) microspheres. Interestingly, P(St-MAA)@PPy microsphere is adsorbed on the surface of rGO/TPU fiber, leading to the formation of a hierarchical conductive surface that enhances the sensitivity to external pressure. The obtained P(St-MAA)@PPy/rGO/TPUEM pressure sensor can effectively detect small pressure (0.94 Pa). The pressure sensor shows a low operating voltage (1.0 V), high sensitivity (13.65 kPa−1), short response time (37 ms), and excellent cycle stability (over 1650 cycles). As a result, this pressure sensor is used to monitor the body's physiological signals and motions, such as the human pulse, facial muscles and joint movements, etc. This novel P(St-MAA)@PPy/rGO/TPUEM pressure sensor can meet the performance requirements of wearable electronic products, demonstrating great potential in human health monitoring and human-computer interaction.

Coating of multi-wall carbon nanotubes (MWCNTs) on three-dimensional, bicomponent nonwovens as wearable and high-performance piezoresistive sensors

Wearable piezoresistive sensors have attracted increasing attention because of their important applications in wearable electronics, smart textiles and human–machine interaction. However, existing piezoresistive sensors still suffer from problems including expensive starting materials, weak bonding fastness between conductive substances and substrates, limited sensing range and poor comfort. Herein, we report a three-dimensional (3D) piezoresistive sensor based on a nonwoven substrate composed of highly crimped bicomponent fibers. The unique core-sheath structure of these fibers in combination with a thermally-assisted coating method ensures the swift and tight adhesion of multi-walled carbon nanotubes (MWCNTs) on fiber surface, and thus making the nonwoven conductive. With the multi-layered fibrous structure, the resultant sensor exhibits good flexibility, high sensitivity (5.57% and 0.113% kPa−1 in the range of 0–7 and 30–131.32 kPa, respectively), broad dynamic sensing range (0–131.32 kPa), fast response/relaxation (105/156 ms), as well excellent breathability and long-term reliability. Versatility of this sensor is proved by real-time detection of both small and large human motions (such as pulse, speech recognition and joint motion). A sensor network containing 16 sensors is further developed to precisely map and recognize the position, shape and local distribution of pressure. Moreover, the sensor is fabricated by using affordable materials and scalable methods. Taken together, we believe this work may open up a new perspective for the design of high-performance wearable piezoresistive sensors.

Facilely constructed two-sided microstructure interfaces between electrodes and cellulose paper active layer: eco-friendly, low-cost and high-performance piezoresistive sensor

The microstructure plays an important role in improving the sensing performance of pressure sensor. However, the design of microstructural active layer of pressure sensor usually involves complex process and expensive raw materials. Herein, the common polyester conductive electrodes and cellulose paper that both have inherent microstructure surface are combined to form two-sided microstructure interfaces for low-cost, eco-friendly and high-performance flexible piezoresistive pressure sensor. In order to obtain conductive and low-cost active layer paper, daily carbon ink, which is usually used for writing, is preferred as a conductive material. Meanwhile, we experimentally confirm that the proposed structure is also suitable for other conductive materials, such as carbon nanotubes. The results show that as-fabricated piezoresistive sensor has high pressure sensitivities of 5.54 and 1.61 kPa−1 in the wide linear ranges of 0.5 − 5 and 5 − 60 kPa, respectively, and good durability (5000 cycles under 2 kPa). The sensing mechanism of the piezoresistive sensor is analyzed by combining the characterization results and finite element simulation. Benefitting from the high sensing performance and good flexibility, the piezoresistive sensor is demonstrated for multiple wearable applications (e.g., wrist pulse, speech recognition, finger bending, abdominal respiration, counting steps, and pressure distribution). This work provides a simple and effective strategy for the design of piezoresistive sensor from the microstructure interfaces between electrodes and active layer.Graphic abstract

An in situ polymerized polypyrrole/halloysite nanotube–silver nanoflower based flexible wearable pressure sensor with a large measurement range and high sensitivity

In this paper, a high-performance piezoresistive sensor based on PPy/HNTs/AgNFs is prepared and has great potential for application in artificial electronic skin, real-time motion detection and medical health monitoring.