High Piezoresistive Sensitivity Research Articles

Smart Carbon Fiber-Reinforced Polymer Composites for Damage Sensing and On-Line Structural Health Monitoring Applications.

High electrical conductivity, along with high piezoresistive sensitivity and stretchability, are crucial for designing and developing nanocomposite strain sensors for damage sensing and on-line structural health monitoring of smart carbon fiber-reinforced polymer (CFRP) composites. In this study, the influence of the geometric features and loadings of carbon-based nanomaterials, including reduced graphene oxide (rGO) or carbon nanofibers (CNFs), on the tunable strain-sensing capabilities of epoxy-based nanocomposites was investigated. This work revealed distinct strain-sensing behavior and sensitivities (gauge factor, GF) depending on both factors. The highest GF values were attained with 0.13 wt.% of rGO at various strains. The stability and reproducibility of the most promising self-sensing nanocomposites were also evaluated through ten stretching/relaxing cycles, and a distinct behavior was observed. While the deformation of the conductive network formed by rGO proved to be predominantly elastic and reversible, nanocomposite sensors containing 0.714 wt.% of CNFs showed that new conductive pathways were established between neighboring CNFs. Based on the best results, formulations were selected for the manufacturing of pre-impregnated materials and related smart CFRP composites. Digital image correlation was synchronized with electrical resistance variation to study the strain-sensing capabilities of modified CFRP composites (at 90° orientation). Promising results were achieved through the incorporation of CNFs since they are able to form new conductive pathways and penetrate between micrometer-sized fibers.

Self‐Repairable Hybrid Piezoresistive‐Triboelectric Sensor Cum Nanogenerator Utilizing Dual‐Dynamic Reversible Network in Mechanically Robust Modified Natural Rubber

AbstractThe greener alternatives to tactile‐integrated multimodal sensors with self‐powered and self‐healing abilities are highly desirable for all‐in‐one autonomous sensing systems, particularly impressive in diverse application ranges including smart home, healthcare, and e‐skin. The dynamically self‐healable, stretchable piezoresistive sensors, and triboelectric nanogenerators (TENGs) reported herein are constructed by a facile, industrially viable method of grafting imidazolium ions on epoxidized natural rubber (ENR) backbone. Owing to cation‐π and π–π interaction between the percolated carbon nanotubes (CNTs)‐network and the imidazolium ions formed by non‐covalent interactions, the interfacial adhesion between the filler and elastomer is shown to improve considerably. The sensors show high piezoresistive strain sensitivity, reversible ionic network‐assisted self‐healability (efficiency ≈80%) and wide‐ranging detectability for precise monitoring of human movements. Both the healed and pristine sensors feature low hysteresis and stable electrical outputs over a wide strain range (≤200%). While achieving rapid self‐healing efficiency, the substrates are shown to exhibit remarkable robustness for harsh climates owing to significant mechanical toughness. Supported by excellent triboelectric tactile sensitivity (2.12 V N−1), the multifunctional TENG‐enabled sensor yields superior power density (0.16 mW cm−2). Moreover, the TENG module exhibits high force sensitivity and ease of operation that are considered versatile for all‐weather integrated tactile solutions for future technology.

Engineered structural carbon aerogel based on bacterial Cellulose/Chitosan and graphene Oxide/Graphene for multifunctional piezoresistive sensor

Piezoresistive sensors are highly desired in a wide variety of fields in health monitoring, human–machine interaction, robot sensing, and so on. Developing engineering-architected piezoresistive sensors with biocompatibility, high sensitivity, and mechanical robustness is one of the hot research fields. Here, a high-performance hybrid carbon aerogel is constructed from bacterial cellulose, chitosan, graphene oxide, and graphene based on synergistic electrostatic interaction and hydrogen bonding through unidirectional freeze-casting and carbonization. The carbon aerogel, characterized by its unique lamellar and fibrous alternating structure, mirrors the stability of a well-constructed “bridge”. In this analogy, the layer functions as a steady “girder”, while the fiber emulates indispensable “stay cable”. The ordered microstructure contributes to uniform stress transfer, yielding a carbon aerogel-based sensor that exhibits high piezoresistive linear sensitivity (150 kPa−1), outstanding mechanical stability (3000 compressing cycles), and a quick response/recovery time (120/90 ms). The piezoresistive sensor detects real-time human physiological signals and effectively distinguishes subtle and different acoustic signals, enabling sound visualization. Also, it can work well in extreme conditions (200 °C and −196 °C). The prepared piezoresistive sensor is expected to have versatile applications in next-generation spatial pressure monitoring, wearable electronics, and sound visualization.

Fabrication, characterization, and repair of nanocarbon-loaded aircraft paint-based sensors for real-world SHM: studies at the laboratory scale

There has been considerable interest in piezoresistive nanocarbon-loaded polymer films for structural health monitoring, including damage detection and strain monitoring. While good performance has been demonstrated, issues related to practical implementation have received less attention. Here we present sensors made from exfoliated graphite nanoplatelets (xGnP) incorporated into a commercial paint that is applied to Sikorsky aircraft. A formulation and a fabrication method are developed that deliver high piezoresistive strain sensitivity alongside mechanical integrity. At approximately 7 wt% xGnP, the gauge factor in tension is in the range of 30–55, and the effectiveness of the sensors for damage monitoring is demonstrated by the detection of perforations. To obtain a paintable solution, key considerations in choosing the solvent employed for introducing the nanocarbon are compatibility and the ability to keep the nanocarbon suspended, which is achieved using ethyl acetate. The ability to form sensors in situ on aircraft structures requires an uncomplicated method of making robust electrical connections, which is demonstrated here using embedded copper mesh. The strong, often nonlinear, environmental sensitivity of polymer-nanocarbon materials must also be considered in applications; here, increasing temperature and humidity both raise sensor resistance. This work shows that a second, unstrained reference sensor would work well for automatic compensation. Lastly, a method for effecting a repair that employs standard processes and maintains the high gauge factor is demonstrated. With these advances, the paint-xGnP sensors are ready for in-the-field testing on aircraft.

Ultrathin Mo2S3 Nanowire Network for High-Sensitivity Breathable Piezoresistive Electronic Skins.

Flexible piezosensing electronic skins (e-skins) have attracted considerable interest owing to their applications in real-time human-health monitoring, human-machine interactions, and soft bionic robot perception. However, the fabrication of piezosensing e-skins with high sensitivity, biological affinity, and good permeability at the same time is challenging. Herein, we designed and synthesized Mo2S3 nanowires by inserting ∞1[Mo2+S] chains between MoS2 interlayers. The resulting Mo2S3 nanowires feature high conductivity (4.9 × 104 S m-1) and a high aspect ratio (∼200). An ultrathin (∼500 nm) Mo2S3 nanowire network was fabricated using a simple liquid/liquid interface self-assembly method, showing high piezoresistive sensitivity (5.65 kPa-1), a considerably low pressure detection limit (0.08 Pa), and gratifying air permeability. Moreover, this nanowire network can be directly attached to human skin for real-time human pulse detection, finger movement monitoring, and sign language recognition, exhibiting excellent potential for health monitoring and human-machine interactions.

An Ultra-Low-Power Strain Sensing Node for Long-Range Wireless Networks in Carbon Nanotube-Based Materials

The application of flexible electronics to device-engineering technologies has enabled the manufacture of low-cost, lightweight, extensible, and foldable sensors. However, despite the efficient response of composite materials for the detection of signal stimuli reported in the state-of-art literature, there is an area of opportunity for the implementation of ultra-low-power wireless sensor networks (WSN) with novel flexible materials in precision medicine, structural health monitoring, environmental sensing, wearables and applications based on the Internet of Things (IoT). This article proposes a portable strain sensing node based on multiwall carbon nanotubes (MWCNTs)/polypropylene(PP) composites for Long-Range (LoRa) wireless networks with IoT connectivity. The nanostructured sensor node with 4 wt% MWCNTs shows high piezoresistive sensitivity with gage factors of ~ 4.5. A system identification problem is conducted following a constrained optimization model, which estimates the sensor errors due to nonlinearity and system parameters, providing a stable strain response with high accuracy. An energy management strategy is also implemented for the efficient integration of a microcontroller unit in combination with a wireless LoRa communication transceiver, enabling the utilization of flexible composite materials into a wireless nanostructured sensing network with ultra-low-power consumption of 0.8705 mW. The experimental results of the MWCNT/PP strain sensing node show the feasibility for a variety of IoT-WSN applications.

Submillimeter-Scaled PEDOT:PSS/PPy Piezoresistive Pressure Sensor Array and Its Applications in Biomedicine

Developing a small, smart, and sensitive pressure sensor array is necessary for establishing a comfortable and safe environment in daily life. Herein, the miniaturization effects of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 1$ </tex-math></inline-formula> poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) piezoresistive pressure sensors with a cross-point electrode (CPE) structure have been investigated and found to suffer from response delays owing to the serious current crowding effect. To eliminate the adverse effects caused by the device scaling, polypyrrole (PPy) was blended into PEDOT:PSS to obtain a high piezoresistive sensitivity of 0.58 kPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> and a fast relaxation time of 0.74 s for the miniaturized PEDOT:PSS/PPy piezoresistive pressure sensors with a PPy concentration of 60 ppm. Subsequently, the optimized PEDOT:PSS/PPy composites were implemented to fabricate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times 3$ </tex-math></inline-formula> CPE piezoresistive sensor arrays with an electrode size of 0.2 mm, and a readout circuit was designed to instantaneously measure the resistance of each pixel of the sensor arrays under the application of pressure. Using the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times 3$ </tex-math></inline-formula> PEDOT:PSS/PPy piezoresistive sensor arrays with a readout circuit, the pressure distribution of the finger pressing and an occlusal pressure monitoring of a molar before and after the dental filling process were successfully demonstrated, suitable for prospective applications in biomedicine.

Prediction of piezoresistive sensitivity and percolation probability of synergetic CNT-GNP conductive network composite

A novel one-step Monte-Carlo approach considering the orientation effect of carbon nanotubes (CNTs) in synergism with uniformly distributed ellipsoidal graphene nanoplatelets (GNPs) teamed up with percolation model is developed to study the percolation probability and gauge factor of hybrid CNT-GNP piezoresistive conductive network composite. A representative volume element is generated based on the randomly oriented rod-like CNTs and uniformly distributed disk-shape GNPs to examine the piezoresistive sensitivity developed by strain under tension. The model predictions are compared with experimental studies related to electrical conductivity and piezoresistivity of hybrid CNT-GNP nanocomposites and a good agreement is achieved. A parametric investigation of the influences of CNT volume fraction, degree of orientation, GNP diameter and volume fraction is performed on the piezoresistive sensitivity of nanocomposite. High piezoresistive sensitivity is achieved for scattered short aligned CNTs distributed in the matrix with sparse low aspect ratio GNPs. The results also demonstrated that high percolation probability is achievable for nanocomposite with longer CNTs oriented in random directions and perfect dispersion of GNPs with higher surface area.

Fatigue Resistant Aerogel/Hydrogel Nanostructured Hybrid for Highly Sensitive and Ultrabroad Pressure Sensing.

Achieving high sensitivity over a broad pressure range remains a great challenge in designing piezoresistive pressure sensors due to the irreconcilable requirements in structural deformability against extremely high pressures and piezoresistive sensitivity to very low pressures. This work proposes a hybrid aerogel/hydrogel sensor by integrating a nanotube structured polypyrrole aerogel with a polyacrylamide (PAAm) hydrogel. The aerogel is composed of durable twined polypyrrole nanotubes fabricated through a sacrificial templating approach. Its electromechanical performance can be regulated by controlling the thickness of the tube shell. A thicker shell enhances the charge mobility between tube walls and thus expedites current responses, making it highly sensitive in detecting low pressure. Moreover, a nucleotide-doped PAAm hydrogel with a reversible noncovalent interaction network is harnessed as the flexible substrate to assemble the aerogel/hydrogel hybrid sensor and overcome sensing saturation under extreme pressures. This highly stretchable and self-healable hybrid polymer sensor exhibits linear response with high sensitivity (Smin >1.1kPa-1 ), ultrabroad sensing range (0.12-≈400kPa), and stable sensing performance over 10000cycles at the pressure of 150kPa, making it an ideal sensing device to monitor pressures from human physiological signals to significant stress exerted by vehicles.

Selective laser sintering of carbon nanotube–coated thermoplastic polyurethane: Mechanical, electrical, and piezoresistive properties

Flexible and electrically conductive carbon nanotube/thermoplastic polyurethane (CNT/TPU) nanocomposites have been herein fabricated by selective laser sintering (SLS) for the applications of wearable electronics and strain sensors. CNT/TPU nanocomposite powders were systematically developed by a latex-based technique. The effects of CNT content on the SLS processability of nanocomposite powders and the mechanical, electrical, and piezoresistive properties of SLS-printed specimens were investigated. It was found that the CNT improved the electrical conductivity and strain sensing performance of the printed parts, but excessively high CNT content led to high melt viscosity, deteriorated sintering behavior, and hence degraded mechanical properties. The 2.0 wt% CNT/TPU nanocomposite fabricated by SLS exhibited enhanced electrical conductivity (seven orders of magnitude higher than the neat TPU), high piezoresistive sensitivity (gauge factor of 60 at a tensile strain of 20%), and a wide sensing range (0‒130% strain). A mathematical model based on the tunneling theory was established to describe and predict the strain sensing performance of the printed nanocomposites. This study provides profound insights into the development of multi-functional nanocomposites by SLS for their applications in flexible strain sensors.

Super-elastic ultrasoft natural rubber-based piezoresistive sensors for active sensing interface embedded on soft robotic actuator

Piezoresistive soft composites are ubiquitous in strain sensing that manifests in a dramatic increment of electrical resistivity upon elongation. A piezoresistive strain sensor embedded-soft robotic arm has been a challenging task in terms of surface compatibility, shape and dynamics of the soft robotic components. We present a super-elastic, ultrasoft natural rubber composite containing multiwalled carbon nanotubes in presence of a hydrofinished oil-based softener. The resulting conducting elastomer offered a line-up of compelling characteristics such as low electrical percolation (<2 vol%), ultra-softness (Shore A harness ∼19), elastic modulus in the kPa range (∼350 kPa at 100% elongation), ultra-stretchability (∼800%) and high tensile strength (∼10.5 MPa). In addition, the sensor exhibited low hysteresis (3.5%), high piezoresistive sensitivity (gage factor≈472) and switching response over wide strain range (70%) and stable sensing performance for multiple test cycles (>1000). On account of the excellent responses mentioned, the sensor could detect human motion and has also been demonstrated in this paper. Furthermore, the sensing strip embedded on a soft robotic pneumatic actuator mounted on a test rig showed excellent movement detection response upon actuation. This proof-of-concept sensor-integrated soft robotic interface could be instrumental in the future development of proprioceptive sensing robots and soft robotic segments.

Polyimide Nanofiber-Reinforced Ti3C2Tx Aerogel with "Lamella-Pillar" Microporosity for High-Performance Piezoresistive Strain Sensing and Electromagnetic Wave Absorption.

Assembling two-dimensional MXenes into 3D macroscopic structures is an applicable method to give full play to its excellent electrical and mechanical properties toward multi-functionality. Considering the weak interfacial interaction and poor gelation ability of MXenes, short polyimide nanofibers (PINFs) are utilized as cross-linking and supporting building blocks in this work to construct a lightweight, robust, and elastic PINF/Ti3C2Tx MXene composite aerogel (PINF/MA) via a simple synergistic assembly strategy. Taking advantage of its unique 3D "lamella-pillar" microporous architecture, the designed PINF/MA composite aerogel exhibits excellent piezoresistive sensing performance in terms of a wide pressure range of 0-8 kPa (50% strain), a high piezoresistive sensitivity of 22.32 kPa-1, an ultra-low detection limit of 0.1% strain, and great compression/rebound stability (signal remained stable after 1500 cycles). These remarkable piezoresistive sensing properties enable the PINF/MA with intriguing capability to detect small and large human activities in real time (wrist and finger bending, pulse, and vocal cord vibration). More interestingly, the parallelly aligned leaf vein-like lamellae also empower the PINF/MA with prominent wave absorption performance [RLmin is -40.45 dB at 15.19 GHz, with an effective absorption bandwidth of 5.66 GHz (12.34-18 GHz)], making the multi-functional PINF/MA composite aerogels promising candidates for wearable strain sensors and microwave absorbers.

Highly Tunable Piezoresistive Behavior of Carbon Nanotube-Containing Conductive Polymer Blend Composites Prepared from Two Polymers Exhibiting Crystallization-Induced Phase Separation

Conductive polymer composites (CPCs) are suitable as piezoresistive-sensing materials. When using CPCs for strain sensing, it is still a big challenge to simultaneously improve the piezoresistive sensitivity and linearity along with the electrical conductivity and mechanical properties. Here, highly tunable piezoresistive behavior is reported for multiwalled carbon nanotube (CNT)-filled CPCs based on blends of two semicrystalline polymers poly(vinylidene fluoride) (PVDF) and poly(butylene succinate) (PBS), which are miscible in the melt. When cooling the homogeneous mixture of the blend components, successive crystallization of PVDF and PBS occurs, creating complex crystalline structures in a mixed amorphous phase. The morphology of the blend matrix, the crystallinity of the blend components, and the dispersion and location of the CNTs in the blend depend on the CNT content and the blend composition. Compared with PVDF/CNT composites, the substitution of 10 to 50 wt % PVDF by PBS in the composites shifts the electrical percolation concentration Φc from 0.79 wt % to filler contents as low as 0.50 wt % while improving the stretchability. The piezoresistive behavior is highly tunable by changing the PVDF/PBS ratio. The ternary composites with matrix compositions of PVDF (90 wt %)/PBS (10 wt %) and PVDF (50 wt %)/PBS (50 wt %) show either higher piezoresistive sensitivity or linearity, respectively, caused by the differences in the microstructure of the CPCs. For example, the crystallinity of PBS in the ternary composites increased from 19.8% to 52.0% as the PBS content increased from 10 wt % to 50 wt %, which is connected with altered CNT distribution and conductive network structure and substantial improvement of the linearity of the electrical response to strains up to >20%. Our findings highly contribute to the understanding of the piezoresistive properties of CPCs based on two semicrystalline polymers and are important for future studies to tune the piezoresistive behavior to achieve simultaneously improved sensitivity and linearity.

Bioinspired Gradient Conductivity and Stiffness for Ultrasensitive Electronic Skins.

Hierarchical and gradient structures in biological systems with special mechanical properties have inspired innovations in materials design for construction and mechanical applications. Analogous to the control of stress transfer in gradient mechanical structures, the control of electron transfer in gradient electrical structures should enable the development of high-performance electronics. This paper demonstrates a high performance electronic skin (e-skin) via the simultaneous control of tactile stress transfer to an active sensing area and the corresponding electrical current through the gradient structures. The flexible e-skin sensor has extraordinarily high piezoresistive sensitivity at low power and linearity over a broad pressure range based on the conductivity-gradient multilayer on the stiffness-gradient interlocked microdome geometry. While stiffness-gradient interlocked microdome structures allow the efficient transfer and localization of applied stress to the sensing area, the multilayered structure with gradient conductivity enables the efficient regulation of piezoresistance in response to applied pressure by gradual activation of current pathways from outer to inner layers, resulting in a pressure sensitivity of 3.8 × 105 kPa-1 with linear response over a wide range of up to 100 kPa. In addition, the sensor indicated a rapid response time of 0.016 ms, a low minimum detectable pressure level of 0.025 Pa, a low operating voltage (100 μV), and high durability during 8000 repetitive cycles of pressure application (80 kPa). The high performance of the e-skin sensor enables acoustic wave detection, differentiation of gas characterized by different densities, subtle tactile manipulation of objects, and real-time monitoring of pulse pressure waveform.

3D Printing of Flexible and Stretchable Parts Using Multiwall Carbon Nanotube/Polyester-Based Thermoplastic Polyurethane

Abstract This paper reports on the 3D printing of flexible and stretchable parts based on multiwall carbon nanotube (MWCNT)/polyester-based thermoplastic polyurethane (TPU) nanocomposites. The rheological properties of the WCNT/TPU nanocomposites with different wt% of MWCNTs (0.1–3) were determined and used as guidance for the extrusion and 3D printing processes. MWCNT/TPU filaments were extruded and used for 3D printing of different flexible and stretchable parts. The mechanical, electrical, and piezoresistive response of the MWCNT/TPU nanocomposite filaments and 3D printed parts under static and monotonic loading was studied. The experimental results show that with increasing temperature and shear rate, respectively, the shear viscosity of the MWCNT/TPU nanocomposite decreases, whereas the viscosity increases with increasing wt% of MWCNTs. With the addition of MWCNTs, the elastic modulus and tensile strength of the feedstock filament all increase, enhancing the printability of TPU by increasing the buckling resistance and the stability of the 3D printed layer. The electrical conductivity of the 3D printed MWCNT/TPU nanocomposites increases with increasing wt% of MWCNTs and exceeds the conductivity of the filaments. The 3D printed MWCNT/TPU nanocomposites with 3 wt% show an electrical conductivity about 10 S/m, irrespective of the printing direction. Moreover, the 3D printed MWCNT/TPU nanocomposites exhibit good mechanical properties and high piezoresistive sensitivity with gauge factor (50–600) dependent on both strain and printing direction.

Self-Sealing Carbon Patterns by One-Step Direct Laser Writing and Their Use in Multifunctional Wearable Sensors.

A combined photothermal simulation and experimental study leads to a novel internal reflection-assisted direct laser writing carbonization method (IR-DLWc), which enables in situ fabrication of carbon features/patterns that are self-sealed in the interior of a thin polyimide (PI) film in one step without additional packaging procedures. With this new method, carbon line patterns that are fully contained in a 50 μm PI film are fabricated, characterized, and evaluated for their electrical and piezoresistive performance. The self-sealing character of the carbon features created by IR-DLWc imparts them unprecedented mechanical stability/robustness as compared to those fabricated by the conventional DLWc method. Upon applying a double-writing scheme and strain-engineering treatment, the IR-DLWc-created carbon lines show significantly improved piezoresistive sensitivity with a gauge factor evaluated to be 428 in tension and 107 in compression. The high piezoresistive sensitivity, excellent dynamic response, reasonably good durability, self-sealing character, and compliant nature of the IR-DLWc generated carbon patterns make them suitable for a variety of wearable sensing applications. In this work, we demonstrated their use as a tactile sensor for sensing contact force; a functional bandage for monitoring physiological activities like swallowing, pulsing, and breathing; and a glove sensing system for finger gesture recognition.

1D/2D Nanomaterials Synergistic, Compressible, and Response Rapidly 3D Graphene Aerogel for Piezoresistive Sensor

AbstractGraphene‐based aerogels have been widely studied for their high porosity, good compressibility, and electrical conductivity as piezoresistive sensors. However, the fabrication of graphene aerogel sensors with good mechanical properties and excellent sensing properties simultaneously remains a challenge. Therefore, in this study, a novel nanofiber reinforced graphene aerogel (aPANF/GA) which has a 3D interconnected hierarchical microstructure with surface‐treated PAN nanofiber as a support scaffold throughout the entire graphene network is designed. This 3D interconnected microporous aPANF/GA aerogel combines an excellent compressive stress of 43.50 kPa and a high piezoresistive sensitivity of 28.62 kPa−1 as well as a wide range (0–14 kPa) linear sensitivity. When aPANF/GA is used as a piezoresistive sensor, the compression resilience is excellent, the response time is fast at about 37 ms at 3 Pa, and the structural stability and sensing durability are good after 2600 cycles. Indeed, the current signal value is 91.57% of the initial signal value at 20% compressive strain. Furthermore, the assembled sensors can monitor the real time movement of throat, wrist pulse, fingers, wrist, and knee joints of the human body at good sensitivity. These excellent features enable potential applications in health detection.

Double-Network Hierarchical-Porous Piezoresistive Nanocomposite Hydrogel Sensors Based on Compressive Cellulosic Hydrogels Deposited with Silver Nanoparticles

Hydrogels have attracted growing attention in electronics due to their soft, flexible nature and ion-rich physiological environment. Herein, we reported a strategy to not only enhance a traditional polyanion hydrogel—poly(acrylic acid) (PAAc)—but also improve its piezoresistive sensitivity by synthesizing and introducing a precompressed double-cross-linked (DC) lignocellulosic hydrogel with uniformly and regularly distributed big pores (with diameters of 0.6–3.3 μm) in the compact PAAc, producing double-network (DN), hierarchical-porous composite ionic hydrogels. The lignocellulosic hydrogel was coated with polydopamine (pDA) followed by in situ deposition of silver nanoparticles (AgNPs) via reduction under the assistance of pDA prior to compositing. The resulting composite ionic hydrogels exhibited excellent properties, including high water content (around 82%) and thus abundant hydration ions, outstanding mechanical properties (2.13–3.20 MPa with PAAc concentrations from 2.8–3.2 wt %), high softness (76.8%–94.8% maximum strains), and high piezoresistive sensitivity (up to 9.34 MPa–1). The assembled hydrogel sensors can detect a wide range of pressures quickly and accurately in both gentle and hard physical pressing modes, demonstrating great potential application in various human–machine interfaces in the fields of electronics, sports, recreation, and so on.

Drop-on-demand high-speed 3D printing of flexible milled carbon fiber/silicone composite sensors for wearable biomonitoring devices

Three-dimensionally (3D) printed flexible piezoresistive composite sensors have provided valuable solutions for the personalized therapeutic development due to their promising capability in biomonitoring applications. Silicone rubber (SR) matrix is an important candidate to enable flexibility to the 3D printed devices. However, 3D printing of silicone inks blended with conductive fillers is limited due to the high viscosity, long curing time, and high percolation threshold. In the present study, a novel high-speed material jetting (MJ) 3D printing of high-viscosity conductive inks based on the mixture of a UV crosslinkable silicone rubber and milled carbon fibers (MCF) is demonstrated. The MCF content was optimized for printability, UV curability, and electrical conductivity. The sensors (with 30 wt. % MCF content) show high flexibility and foldability as well as a high resistance sensitivity to sever bending tests. The stretchability of 3D printed sensors was further improved by sandwiching the MCF/SR sensing layer between the SR layers. The electromechanical evaluation of the sandwiched MCF/SR sensors (S-MCF/SR) confirmed the high piezoresistive sensitivity of sensors (gauge factor in order of ∼400). Finally, the 3D printed sensors were employed for monitoring human joint motions to demonstrate the potential application in monitoring biosignals.

Influence of concentration, length and orientation of multiwall carbon nanotubes on the electromechanical response of polymer nanocomposites

Multiwall carbon nanotube (MWCNT)/polymethyl methacrylate (PMMA) composites were fabricated by solution casting with a range of MWCNT concentrations, two groups of MWCNT lengths and with or without the application of electric field. As-received (CNTs) and fragmented (FCNTs) MWCNTs were used to fabricate composites with MWCNT concentrations between 0.05 wt% and 0.7 wt%. A well-controlled procedure to systematically fragment the MWCNTs based on the application of high-density ultrasonic energy was implemented, and MWCNT length distributions were obtained by a formal statistical analysis. The relationship between the sparseness of the MWCNT network (macroscale morphology), electrical conductivity and piezoresistive sensitivity was evaluated. In order to evaluate the effect of the MWCNT alignment on the composite properties, specific MWCNT concentrations were selected for composites with CNTs and FCNTs under the application of an electric field. Composites with FCNTs exhibited a sparser network, higher transparency, lower electrical conductivity, higher percolation threshold and higher piezoresistive sensitivity than their counterparts with CNTs. On the other hand, composites with electric field-aligned MWCNT bundles presented higher electrical conductivity and piezoresistive sensitivity than their non-aligned counterparts, for both fragmented and non-fragmented cases. The evaluation of the combined effect of carbon nanotube length and alignment on the electromechanical properties of the composites represents an important step on the understanding of the structure-property relation of polymer nanocomposites.