High-Performance Graphene Sponges Reinforced with Polyimide for Room-Temperature Piezoresistive Sensing

Synthesis of Superelastic, Highly Conductive Graphene Aerogel/Liquid Metal Foam and its Piezoresistive Application

Graphene aerogel (GA) has important application potential as piezoresistive sensors due to its low density, high conductivity, high porosity, and good mechanical properties. However, the fabrication of GA-based sensors with good mechanical properties and excellent sensing performance is still challenging. Herein, liquid- metal-modified GAs (GA/LM) are proposed for the development of an excellent GA-based sensor. GA/LM with three-dimensional interconnected layered structure exhibits excellent compressive stress of 41 KPa and fast response time (<20 ms). While generally flexible GA composites cannot be compressed beyond 80 % strain without plastic deformation, GA/LM demonstrates a high compressive strength of 60 kPa under a strain of 90 %. A real-time pressure sensor was fabricated based on GA/LM-2 to monitor swallowing, pulse beating, finger, wrist and knee bending, and even plantar pressure during walking. These excellent features enable potential applications in health detection.

Monitoring and analysis of cardiovascular pulse waveforms using flexible capacitive and piezoresistive pressure sensors and machine learning perspective

The growing interest in flexible electronics for physiological monitoring, particularly using flexible pressure sensors for cardiovascular pulse waveforms monitoring, has potential applications in cuffless blood pressure measurement and early diagnosis of cardiovascular disease. High sensitivity, fast response time, good pressure resolution and a high signal-to-noise ratio are essential for effective pulse waveform detection. This review focuses on flexible capacitive and piezoresistive pressure sensors, which have seen significant enhancements due to their simple operation, superior performance, wide range of materials, and easy fabrication. The comparison of sensing methods for acquiring pulse waveforms from the wrist artery, device integration configurations, high-quality pulse waveforms collection, and performance analysis of capacitive and piezoresistive sensors are discussed. The review also covers the use of machine learning for analyzing pulse waveforms for cardiovascular disease diagnosis and cuff-less blood pressure monitoring. Lastly, it provides perspectives on current challenges and further advancements in the field.

A resilient and lightweight cellulose/graphene oxide/polymer-derived multifunctional carbon aerogel generated from Pickering emulsion toward a wearable pressure sensor

In the new era of competitive smart electronics, the development of compressible multifunctional carbon aerogels is highly needed, but still faces enormous challenges. Here we demonstrate a robust strategy to fabricate multifunctional carbon aerogel via freeze-drying of cellulose nanofibers (CNF) and graphene oxide (GO) co-stabilized Pickering emulsion gel followed by high-temperature annealing. The resulting carbon aerogel exhibits tunable mechanical, hydrophilic and hydrophobic properties due to varying the elemental composition and the pyrolysis of introduced polymers. The carbon aerogel is resilient against high compression strain up to 99 % and has ultralow density (1.82 mg/cm³). The CNF/GO/acrylonitrile butadiene styrene-derived carbon aerogel (CRA)-based sensor has shown desirable sensitivity (17.65 kPa^-1), ultralow detection limit of pressure (60 Pa), and fast responsive time (130 ms), which is capable of detecting human activity, identifying spatial pressure distribution, and communicating with smartphones via Wi-Fi. Moreover, the carbon aerogel reveals effective thermal insulation and photothermal conversion performance. These results suggest the great potentials for developing lightweight and compressible carbon aerogels with multiple functions to meet various applications.

Sponge-Hosting Polyaniline Array Microstructures for Piezoresistive Sensors with a Wide Detection Range and High Sensitivity

The development of highly elastic spongy conductors is attractive for use in piezoresistive sensors due to their low cost, easy fabrication, and wide sensing range. However, the combination of high sensitivity and broad sensing range in a single piezoresistive sensor, though highly demanded, is challenging because they are contradictory in principle. Herein, a highly elastic spongy conductor with sponge-hosting polyaniline (PANI) fluff-like array microstructures is fabricated through cryopolymerization. The as-obtained sponge-hosting conductors exhibited an excellent elasticity under high compression strain and stress of up to 80% and 101 kPa, respectively, covering typical deformation ranges for detecting complex human activities. Benefiting from the presence of the sponge-hosting fluff-like PANI arrays, the as-prepared sponge-hosting conductors for piezoresistive sensors possessed a high sensitivity of 0.54 kPa^-1 in a broad pressure range of 0.1-101 kPa, ascribing to the formation of the sponge-hosting fluff arrays with graded conductive network structures of array contacts and sponge-skeleton contacts at low and high compressions, respectively. As a result, the as-assembled piezoresistive sensors were demonstrated for tactile sensing and human-motion monitoring. This work reveals new approaches for tailored fabrication of sponge-hosting conducting polymers with tunable fluff-like microstructures for highly sensitive and wide-range wearable piezoresistive sensors.

A bioinspired porous-designed hydrogel@polyurethane sponge piezoresistive sensor for human-machine interfacing

Conductive coating sponge piezoresistive pressure sensors are attracting much attention because of their simple production and convenient signal acquisition. However, manufacturing sponge-structure pressure-sensing materials with high compressibility and wide pressure detection ranges is difficult because of the instability of rigid and brittle conductive coatings at large strains. Herein, a tough conductive hydrogel@polyurethane (PU) sponge with a porous design is prepared via immersion of a polyurethane sponge in a low-cost and biocompatible polyvinyl alcohol (PVA)/glycerin (Gl)/sodium chloride (NaCl) solution. The sensor based on the hydrogel/elastomer sponge composite material exhibits a compressible range of 0-93%, a pressure detection range of 100 Pa-470.2 kPa, and 10 000-cycle stability (80% strain) because of the compressibility, flexibility, and toughness of the porous hydrogel coating. Benefiting from the resistance change mechanism of microporous compression, the sensor also exhibits a wide range of linear resistance changes, and the corresponding sensitivity and gauge factor (GF) are -0.083 kPa^-- (100 Pa-10.0 kPa) and -1.33 (1-60% strain), respectively. Based on its flexibility, compressibility, and wide-ranging linear resistance changes, the proposed sensor has huge potential application in human activity monitoring, electronic skin, and wearable electronic devices.

Detecting subtle yet fast skeletal muscle contractions with ultrasoft and durable graphene-based cellular materials

Human bodily movements are primarily controlled by the contractions of skeletal muscles. Unlike joint or skeletal movements that are generally performed in the large displacement range, the contractions of the skeletal muscles that underpin these movements are subtle in intensity yet high in frequency. This subtlety of movement makes it a formidable challenge to develop wearable and durable soft materials to electrically monitor such motions with high fidelity for the purpose of, for example, muscle/neuromuscular disease diagnosis. Here we report that an intrinsically fragile ultralow-density graphene-based cellular monolith sandwiched between silicone rubbers can exhibit a highly effective stress and strain transfer mechanism at its interface with the rubber, with a remarkable improvement in stretchability (>100%). In particular, this hybrid also exhibits a highly sensitive, broadband-frequency electrical response (up to 180 Hz) for a wide range of strains. By correlating the mechanical signal of muscle movements obtained from this hybrid material with electromyography, we demonstrate that the strain sensor based on this hybrid material may provide a new, soft and wearable mechanomyography approach for real-time monitoring of complex neuromuscular–skeletal interactions in a broad range of healthcare and human–machine interface applications. This work also provides a new architecture-enabled functional soft material platform for wearable electronics.

Facile strategy to prepare polyimide nanofiber assembled aerogel for effective airborne particles filtration

Polyimide nanofiber (PINF) aerogel materials have received extensive attention as heat insulation, sensors and filtration media due to their excellent thermodynamic properties and unique porous structure. However, PINF must be difficult to disperse in organic solvents (dioxane or dimethyl sulfoxide) and dimensional instability has been regarded as issues that limits the preparation of PINF aerogels, especially in the water. So, it is of great significance to prepare polyimide aerogels with stable structure using water as a dispersant. In this work, the electrospun polyimide nanofiber precursor (polyamic acid (PAA) nanofiber (PAANF)) is uniformly dispersed in water, and triethylamine is added to terminated PAA oligomer as a binder. The resultant PINF aerogel has excellent mechanical properties with outstanding elasticity and a maximum compressive stress of 7.03 kpa at 50% strain. Furthermore, due to the extremely high porosity (98.4%) and hierarchical porous structure, the aerogel exhibits a high filtration efficiency (99.83%) for PM2.5, while the pressure drop is lower than that of the corresponding nanofiber membrane materials, which will facilitate its application in high temperature filtration and other fields.

Flexible pressure sensors with high pressure sensitivity and low detection limit using a unique honeycomb-designed polyimide/reduced graphene oxide composite aerogel

It is still a challenge to fabricate flexible pressure sensors that possess high sensitivity, ultralow detection limit, wide sensing range, and fast response for intelligent electronic devices. We here demonstrate superelastic and highly pressure-sensitive polyimide (PI)/reduced graphene oxide (rGO) aerogel sensors with unique honeycomb structure, which were designed and fabricated using a bidirectional freezing technique. This unique honeycomb structure with large aspect ratio is composed of aligned thin lamellar layers and interconnected bridges. The combination of the aligned lamellar layers and the bridges endows the aerogel sensors with high pressure sensitivity (1.33 kPa^-1), ultralow detection limit (3 Pa), broad detection range (80% strain, 59 kPa), fast response time (60 ms), and excellent stability during cycling (over 1000 cycles). Remarkably, the aerogel sensors maintain stable piezoresistive performance at -50 °C, 100 °C, and 200 °C in air, indicating promising potential applications in harsh environments. Owing to the high sensitivity and wide sensing range, the aerogel sensors have been used to detect a full-range of human motion including small-scale motion monitoring (wrist pulse, blowing, puffing) and large-scale movement monitoring (finger bending, elbow bending, walking, running). These advantages make the composite aerogels attractive for high-performance flexible pressure sensors and wearable electronic devices.

Superelastic, Fatigue-Resistant, and Flame-Retardant Spongy Conductor for Human Motion Detection against a Harsh High-Temperature Condition

The construction of wearable piezoresistive sensors with high elasticity, large gauge factor, and excellent durability in a harsh high-temperature environment is highly desired yet challenging. Here, a lightweight, superelastic, and fatigue-resistant spongy conductor was fabricated via a sponge-constrained network assembly, during which highly conductive graphene and flame-retardant montmorillonite were alternatively deposited on a three-dimensional melamine scaffold. The as-obtained spongy conductor exhibited a highly deformation-tolerant conductivity up to 80% strain and excellent fatigue resistance of 10,000 compressive cycles at 70% strain. As a result, the spongy conductor can readily work as a piezoresistive sensor and exhibited a high gauge factor value of ∼2.3 in a strain range of 60-80% and excellent durability under 60% strain for 10,000 cycles without sacrificing its piezoresistive performance. Additionally, the piezoresistive sensor showed great thermal stability up to 250 °C for more than 7 days and sufficient flame-retardant performance for at least 20 s. This lightweight, superelastic, and flame-retardant spongy conductor reveals tremendous potential in human motion detection against a harsh high-temperature environment.

Flexible Strain Sensor with Tunable Sensitivity via Microscale Electrical Breakdown in Graphene/Polyimide Thin Films

Carbon-based piezoresistive nanomaterials are widely used for the fabrication of flexible sensors. Although our previous work demonstrated that an electrical breakdown (EBD) process can endow a graphene/polyimide (G/PI) composite with piezoresistivity, the formation of EBD-induced electrical traces with high consistency in bulk nanocomposites remains a technical challenge. With the aim of developing highly sensitive flexible strain sensors using a batch fabrication process, we introduce herein a microscale EBD (μEBD) method to form localized piezoresistors with diverse shapes in a G/PI thin film. The results of scanning electron microscopy, Raman spectroscopy, and electromechanical tests indicate that high piezoresistivity is derived from the high porosity of the carbonized conductive traces generated by the μEBD process. The gauge factor of the μEBD-treated G/PI strain sensor is over 20 times greater than that of the as-prepared G/PI film, and the sensitivities of the strain sensors can be tuned by varying the applied current in the μEBD process. We also demonstrate the potential applications of μEBD-treated G/PI strain sensors in the fields of finger gesture detection, sound pressure measurement, and airflow sensing.

Polyimide-Based Foams: Fabrication and Multifunctional Applications

Because of their unique three-dimensional cellular structure and intrinsic properties, polyimide foam materials have bright prospects for development in multiple functional equipment, which arouses extensive concern. In this Spotlight on Applications, several typical fabrication methods of polyimide foams and the related synthesis mechanism have been systematically described. The advantages and disadvantages of the preparation methods have been compared with each other. Representative functions and the corresponding mechanism models have been concluded, which involve thermal, mechanical, sensing, electromagnetic, environmental, and electrical fields. In the end, the severe tasks and challenges of polyimide foam materials have been summarized, and their promising future development is worth expecting.

Development of Fully Flexible Tactile Pressure Sensor with Bilayer Interlaced Bumps for Robotic Grasping Applications

Flexible tactile sensors have been utilized in intelligent robotics for human-machine interaction and healthcare monitoring. The relatively low flexibility, unbalanced sensitivity and sensing range of the tactile sensors are hindering the accurate tactile information perception during robotic hand grasping of different objects. This paper developed a fully flexible tactile pressure sensor, using the flexible graphene and silver composites as the sensing element and stretchable electrodes, respectively. As for the structural design of the tactile sensor, the proposed bilayer interlaced bumps can be used to convert external pressure into the stretching of graphene composites. The fabricated tactile sensor exhibits a high sensing performance, including relatively high sensitivity (up to 3.40% kPa-1), wide sensing range (200 kPa), good dynamic response, and considerable repeatability. Then, the tactile sensor has been integrated with the robotic hand finger, and the grasping results have indicated the capability of using the tactile sensor to detect the distributed pressure during grasping applications. The grasping motions, properties of the objects can be further analyzed through the acquired tactile information in time and spatial domains, demonstrating the potential applications of the tactile sensor in intelligent robotics and human-machine interfaces.

Tough, Ultralight, and Water-Adhesive Graphene/Natural Rubber Latex Hybrid Aerogel with Sandwichlike Cell Wall and Biomimetic Rose-Petal-Like Surface

Graphene aerogel (GA) as a rising multifunctional material has demonstrated great potential for energy storage and conversion, environmental remediation, and high-performance sensors or actuators. However, the commercial use of GA is obstructed by its fragility and high cost. Herein, by a simple stirring-induced foaming of the mixed aqueous solutions of natural rubber latex (NRL) and graphene oxide liquid crystal (GOLC), we obtained tough, ultralight (4.6 mg cm^-3), high compressibility (>90%), and water-adhesive graphene/NRL hybrid aerogel (GA/NRL). Of particular note, the NRL particles are conformally wrapped by graphene layers to form a sandwichlike cell wall with a biomimetic rose-petal-like surface. These distinct hierarchical structures endow GA/NRL not only with high toughness to bear impact, torsion (>90°), and even ultrasonication but also with strong adhesion to water. As proof of concept, the utilization of the as-prepared GA/NRL for collecting water droplets suspended in moist air and its improved solar-thermal harvest capacity have been demonstrated. This facile, green, and cost-effective strategy opens a new route for tailoring the microstructure and functionality of GA, which will facilitate its large-scale production and commercial application.

An implantable and versatile piezoresistive sensor for the monitoring of human-machine interface interactions and the dynamical process of nerve repair

Flexible wearable and implantable piezoresistive sensors have attracted lots of attention in the applications of healthcare monitoring, disease diagnostics, and human-machine interactions. However, the restricted sensing range, low sensing sensitivity at small strains, limited mechanical stability at high strains, and sophisticated fabrication processes restrict the far-reaching applications of these sensors for ultrasensitive full-range healthcare monitoring. In this work, an implantable and versatile piezoresistive sensor was developed from a series of conductive composites. The conductive composites, hydroxyethyl cellulose (HEC)/soy protein isolate (SPI)/polyaniline (PANI) sponges (HSPSs), were prepared by lyophilization of HEC/SPI solution and then in situ polymerization of aniline. The sensitivity, response time, and mechanical robustness of the HSPS sensors were characterized, and they can achieve a gauge factor of -0.29, a response time of 0.14 s, and sensing stability for at least 100 cycles. The HSPS sensors could efficiently work in vivo for 4 weeks for the measurement of stimuli, without severe inflammatory reaction. When the versatile HSPS sensors were attached to different parts of the human body, they could detect a variety of human motions including coughing, bending of fingers and elbow, abdominal breathing and walking. Notably, the HSPS sensors could be used to monitor the nerve repair in rats and the results are highly consistent with the electrophysiological data. At the same time a new score system was developed to evaluate rat nerve repair. These results indicate that the HSPS sensors exhibit good biocompatibility, sensitivity, sensing stability and fast response time. The HSPS sensors can be used not only as implantable sensors in vivo but also for analyzing human body motions. Furthermore, they provide an effective sensor device and a real-time, dynamic method for evaluating nerve repair without damage and death of animals. Hence, HSPSs might have great potential in in vivo detection, monitoring of human-machine interfacing interactions and the nerve tissue engineering field.

Highly Compressible and Robust Polyimide/Carbon Nanotube Composite Aerogel for High-Performance Wearable Pressure Sensor

Wearable pressure sensors are in great demand with the rapid development of intelligent electronic devices. However, it is still a huge challenge to obtain high-performance pressure sensors with high sensitivity, wide response range, and low detection limit simultaneously. Here, a polyimide (PI)/carbon nanotube (CNT) composite aerogel with the merits of superelastic, high porosity, robust, and high-temperature resistance was successfully prepared through the freeze drying plus thermal imidization process. Benefiting from the strong chemical interactions between PI and CNT and stable electrical property, the composite aerogel exhibits versatile and superior brilliant sensing performance, which includes wide sensing range (80% strain, 61 kPa), ultrahigh sensitivity (11.28 kPa^-1), ultralow detection limit (0.1% strain, <10 Pa), fast response time (50 ms) and recovery time (70 ms), remarkable long-term stability (1000 cycles), and exceptional detection ability toward different deformations (compression, distortion, and bending). Furthermore, the composite aerogel also shows stable sensing performance after annealing under different high temperatures and good thermal insulation property, making it workable in various harsh environments. As a result, the composite aerogel is suitable for the full-range human motion detection (including airflow, pulse, vocal cord vibration, and human movement) and precise detection of the pressure distribution when it is assembled into E-skin, demonstrating its great potential to serve as a high-performance wearable pressure sensor.

Versatile Aerogels for Sensors

Aerogels are unique solid-state materials composed of interconnected 3D solid networks and a large number of air-filled pores. They extend the structural characteristics as well as physicochemical properties of nanoscale building blocks to macroscale, and integrate typical characteristics of aerogels, such as high porosity, large surface area, and low density, with specific properties of the various constituents. These features endow aerogels with high sensitivity, high selectivity, and fast response and recovery for sensing materials in sensors such as gas sensors, biosensors and strain and pressure sensors, among others. Considerable research efforts in recent years have been devoted to the development of aerogel-based sensors and encouraging accomplishments have been achieved. Herein, groundbreaking advances in the preparation, classification, and physicochemical properties of aerogels and their sensing applications are presented. Moreover, the current challenges and some perspectives for the development of high-performance aerogel-based sensors are summarized.

Multilayered Ag NP-PEDOT-Paper Composite Device for Human-Machine Interfacing

Flexible pressure sensors have attracted increasing interest because of their potential applications on wearable sensing devices for human-machine interface connections, but challenges regarding material cost, fabrication robustness, signal transduction, sensitivity improvement, detection range, and operation convenience still need to be overcome. Herein, with a simple, low-cost, and scalable approach, a flexible and wearable pressure-sensing device fabricated by utilizing filter paper as the solid support, poly(3,4-ethylenedioxythiophene) to enhance conductivity, and silver nanoparticles to provide a rougher surface is introduced. Sandwiching and laminating composite material layers with two thermoplastic polypropylene films lead to robust integration of sensing devices, where assembling four layers of composite materials results in the best sensitivity toward applied pressure. This practical pressure-sensing device possessing properties such as high sensitivity of 0.119 kPa^-1, high durability of 2000 operation cycles, and an ultralow energy consumption level of 10^-5 W is a promising candidate for contriving point-of-care wearable electronic devices and applying it to human-machine interface connections.

Recent Advances in Flexible and Wearable Pressure Sensors Based on Piezoresistive 3D Monolithic Conductive Sponges

High-performance flexible strain and pressure sensors are important components of the systems for human motion detection, human-machine interaction, soft robotics, electronic skin, etc., which are envisioned as the key technologies for applications in future human healthcare monitoring and artificial intelligence. In recent years, highly flexible and wearable strain/pressure sensors have been developed based on various materials/structures and transduction mechanisms. Piezoresistive three-dimensional (3D) monolithic conductive sponge, the resistance of which changes upon external pressure or stimuli, has emerged as a forefront material for flexible and wearable pressure sensor due to its excellent sensor performance, facile fabrication, and simple circuit integration. This review focuses on the rapid development of the piezoresistive pressure sensors based on 3D conductive sponges. Various piezoresistive conductive sponges are categorized into four different types and their material and structural characteristics are summarized. Methods for preparation of the 3D conductive sponges are reviewed, followed by examples of device performance and selected applications. The review concludes with a critical reflection of the current status and challenges. Prospects of the 3D conductive sponge for flexible and wearable pressure sensor are discussed.

Extremely elastic and conductive N-doped graphene sponge for monitoring human motions

Due to the extraordinary properties of great elasticity, ultralow density, and good electrical conduction, macroporous graphene bulk materials are the promising candidate as the sensing materials for flexible electronics. Although a large number of progresses have been made, the piezoresistive effect of graphene does not completely meet requirements for various applications yet. Herein, the nitrogen-doped graphene sponge (NGS) material with well-ordered and uniform macroscopic porous structure in long range has been developed by a simple and low-cost strategy by combining hydrothermal and thermal annealing processes. The resulting NGS possesses superior properties of a low density, large elasticity with 80% of the reversible compressibility and outstanding linear elastic region (up to 30%-40%), favorable structural stability, good electrical conductivity (1.4 S m-1), as well as the outstanding sensing performances of a fine sensitivity (1.33 kP-1), low detection limit (2% strain), excellent linear sensing range (nearly 20%), long term stability (3000 cycles), and fast response (72.4 ms). Based on these prominent performances of NGS, a new type of piezoresistive sensor is fabricated, which can successfully detect the human motions from subtle deformations, including the finger bending with the bending degree of approximately 30° and the tiny pulse perturbation of the wrist joint.