Conductive Sponge Research Articles

Polypyrrole (PPy) attached on porous conductive sponge derived from carbonized graphene oxide coated polyurethane (PU) and its application in pressure sensor

A porous conductive sponge derived from carbonized graphene oxide (GO) coated polyurethane (PU) is made and polypyrrole (PPy) attached on the walls then. The as-obtained PPy/rGO@carbonized PU nanocomposite reserves the three-dimensional porous morphology of original PU sponge with PPy that the forms are nanospheres attached on the skeleton walls. After graphene oxide reduced to graphene, the conductivity and mechanical strength of carbonized PU increased. The as-made PPy/rGO@carbonized PU nanocomposite with good mechanical property was used as pressure sensor. Due to introduce PPy in the rGO@carbonized PU that could provide more contact sites which facilitating electron transfer and suffer more spatial deformation, the gauge factor (GF) of PPy/rGO@carbonized PU is 0.171 in compression strain range below 33.8% and 0.770 at larger compression strain region. Furthermore, the value of the other sensitivity of PPy/rGO@carbonized PU based pressure sensor S is 0.635 kPa−1. With the excellent performances, PPy/rGO@carbonized PU shows potential applications in pressure sensors under the deformations of compression.

Graphite nanopowder functionalized 3-D acrylamide polymeric anode for enhanced performance of microbial fuel cell

3-D highly conductive polyvinyl formaldehyde sponges functionalized with acrylamide are fabricated using polyvinyl alcohol with varying concentrations of graphite nanopowder. The properties of the fabricated anodes are analyzed and its application in microbial fuel cells is evaluated. A comparative study with Graphite felt is also performed to evaluate its commercial viability. The presence of Hydroxyl and Amine functional groups enhanced the hydrophilic and biocompatible nature of the synthesized anodes. The phylogenetic analysis substantiated the biocompatible nature and mercury porosimetry showed macroporous nature of the fabricated anode. The highest power density of ~8 W/m2 is recorded for C10 establishing solid biofilm formation. A ~94% COD removal revealed the versatility of the anode for MFC based wastewater treatment. The MFC performance was twice than that of control and was also highest among the most reported modified 3-D anodes. The durability study displayed the commercial opportunity of the anode for real-time MFC operation.

Hydrophilic Conductive Sponge Sensors for Fast Setup, Low Impedance Bio-potential Measurements.

Low electrode-skin impedance can be achieved if the interface has an electrolytic medium that allows the movement of ions across the interface. Maintaining good physical contact of the sensor with the skin is imperative. We propose a novel hydrophilic conductive sponge interface that encapsulates both of these fundamental concepts into an effective physical realization. Our implementation uses a hydrophilic polyurethane prepolymer doped with conductive carbon nanofibers and cured to form a flexible sponge material that conforms to uneven surfaces, for instance, on parts of the scalp with hair. Our results show that our sponges are able to stay in a hydrated state with a low electrode-skin impedance of around 5kΩ for more than 20 hours. The novelty in our conductive sponges also lies in their versatility: the carbon nanofibers make the electrode effective even when the electrode dries up. The sensors remain conductive with a skin impedance on the order of 20kΩ when dry, which is substantially lower than typical impedance of dry electrodes, and are able to extract alpha wave EEG activity in both wet and dry conditions.

Towards emerging EEG applications: a novel printable flexible Ag/AgCl dry electrode array for robust recording of EEG signals at forehead sites

Objectives. With the rapid development of EEG-based wearable healthcare devices and brain–computer interfaces, reliable and user-friendly EEG sensors for EEG recording, especially at forehead sites, are highly desirable and challenging. However, existing EEG sensors cannot meet the requirements, since wet electrodes require tedious setup and conductive pastes or gels, and most dry electrodes show unacceptable high contact impedance. In addition, the existing electrodes cannot absorb sweat effectively; sweat would cause cross-interferences, and even short circuits, between adjacent electrodes, especially in the moving scenarios, or a hot and humid environment. To resolve these problems, a novel printable flexible Ag/AgCl dry electrode array was developed for EEG acquisition at forehead sites, mainly consisting of screen printing the Ag/AgCl coating, conductive sweat-absorbable sponges and flexible tines. Approach. A systematic method was also established to evaluate the flexible dry electrode array. Main results. The experimental results show the flexible dry electrode array has reproducible electrode potential, relatively low electrode–skin impedance, and good stability. Moreover, the EEG signals can be effectively captured with a high quality that is comparable to that of wet electrodes. Significance. All the results confirmed the feasibility of forehead EEG recording in real-world scenarios using the proposed flexible dry electrode array, with a rapid and facile operation as well as the advantages of self-application, user-friendliness and wearer comfort.

Self-Derived Superhydrophobic and Multifunctional Polymer Sponge Composite with Excellent Joule Heating and Photothermal Performance for Strain/Pressure Sensors.

Flexible strain or pressure sensors have potential applications in electronic skin, healthcare, etc. It remains a challenge to explore multifunctional strain or pressure sensors that possess excellent water repellent and heating performance and hence can be used in harsh environments such as high moisture and low-temperature conditions. Here, a self-derived superhydrophobic and multifunctional polymer composite foam is prepared by adsorption of the Ag precursor in tetrahydrofuran (THF) onto the rubber sponge followed by reduction of Ag+ to Ag nanoparticles (AgNPs). During the Ag+ reduction in hydrazine solution, the swollen rubber sponge by THF is partially precipitated based on the nonsolvent-induced phase separation (NIPS). The NIPS creates a porous structure on the sponge surface and thus a high surface roughness, contributing to the material superhydrophobicity. The precipitated polymer wrapping the AgNPs could enhance the interaction between the individual AgNPs. The obtained conductive sponge composite possesses excellent Joule heating and photothermal performance and can be used as both a strain and pressure sensor. The conductive sponge composite sensor possesses good reliability and durability and can be applied to real-time monitoring of human body movements.

A Highly Sensitive Piezoresistive Pressure Sensor Based on Graphene Oxide/Polypyrrole@Polyurethane Sponge.

In this work, polyurethane sponge is employed as the structural substrate of the sensor. Graphene oxide (GO) and polypyrrole (PPy) are alternately coated on the sponge fiber skeleton by charge layer-by-layer assembly (LBL) to form a multilayer composite conductive layer to prepare the piezoresistive sensors. The 2D GO sheet is helpful for the formation of the GO layers, and separating the PPy layer. The prepared GO/PPy@PU (polyurethane) conductive sponges still had high compressibility. The unique fragmental microstructure and synergistic effect made the sensor reach a high sensitivity of 0.79 kPa−1. The sensor could detect as low as 75 Pa, exhibited response time less than 70 ms and reproducibility over 10,000 cycles, and could be used for different types of motion detection. This work opens up new opportunities for high-performance piezoresistive sensors and other electronic devices for GO/PPy composites.

Sugar-templated conductive polyurethane-polypyrrole sponges for wide-range force sensing

Porous conductive polyurethane (PU) sponges were prepared by combining a sugar-templated fabrication of porous polyurethane elastomer and the in-situ polymerization of pyrrole inside the sponges. The conductive sponges had a uniform porous structure similar to the sugar cube templates and a unique micro-wrinkled structure of polypyrrole (PPy). Moreover, the conductive sponges exhibited good mechanical properties and electrical conductivity by the combination of elastomeric PU and conductive PPy, which make the conductive PPy-PU sponges as good candidates for pressure and strain sensitive materials. It was shown that a single PPy-PU conductive sponge was not only sensitive to large forces, but also sensitive to the tiny deformation by small forces such as 0.1 N, thus realizing a wide-range detection of pressure forces from 0.1 N to 200 N. Furthermore, the detection range could be further enlarged to more than 800 N by assembling a sponge array. A PPy-PU sponge matrix was also designed and showed potential application in multi-site position detection. These sugar-templated conductive sponges may find application for sensors in human physiological movement and intelligent monitoring.

Highly elastic conductive sponges by joule heat-driven selective polymer reinforcement at reduced graphene oxide junctions

Polymer reinforcement of reduced graphene oxide (rGO) sponges is widely employed to enhance mechanical strength and elasticity. However, the surplus polymer decreases electrical conductivity by passivating the conductive surface of rGO flakes. Here we firstly report the selective polydimethylsiloxane (PDMS) reinforcement at the flake junction of rGO sponge by the Joule heating process utilizing the high electrical contact resistance. The preferential Joule heating of the junction is theoretically simulated by finite element modeling and experimentally confirmed by micro-thermal infrared imaging. The local temperature increase results in the further reduction of rGO and preferential PDMS curing at the flake junction only. The PDMS/rGO mass ratio was carefully optimized at 3.96. The electrical conductivity (0.087 S m−1 at 0% strain) is more than an order of magnitude higher than that (0.00251 S m−1) of the conventional oven-heated sponge with a similar PDMS/rGO mass ratio. The mechanical strength is equivalent (210.3 kPa at 70% strain), in spite of the preferential polymer coating at the rGO flake junction only, with excellent elasticity. The Joule heating method is an excellent curing strategy to selectively reinforce flake junctions for conductive elastic rGO-polymer sponges.

Self-supporting three-dimensional carboxymethyl cellulose conductive sponges used as electrodes for lithium-ion batteries

In this study, two kinds of self-supporting three-dimensional (3D) carboxymethyl cellulose (CMC) conductive sponges were fabricated and used as electrodes for lithium-ion batteries (LIBs). One was used nano-silicon graphite (Si–graphite) and the other was used nano-ferriferous oxide (Fe–graphite) as active materials. The fabrication and microstructures of two kinds of self-supporting 3D CMC conductive sponges were investigated via scanning electron microscopy and their electrochemical performances were tested when they are used as electrodes. The results show that the hybrid particles are deposited on fibrous CMC layers to form conductive sponges during the freeze-drying. Combined with those of half-battery experiments, two kinds of self-supporting 3D CMC conductive sponges used as electrodes exhibit good electrochemical performances. Moreover, most of the active materials remain encased in the CMC fiber after recycling. These novel Si–graphite and Fe–graphite CMC conductive sponged electrodes can maintain reversible capacities of 1350 and 345.8 mAh/g after 200 and 1000 charge–discharge cycles, respectively, indicating that continuous fibrous CMC conductive networks can provide novel insight into electrode materials for LIBs with enhanced electrochemical performance.

A Triboelectric Nanogenerator as a Self‐Powered Sensor for a Soft–Rigid Hybrid Actuator

AbstractTriboelectric nanogenerators are a promising technology for use in self‐powered sensors, which take advantage of multiple choices of materials and high response to low‐frequency triggering. Here, a conductive sponge/porous silicone‐based triboelectric nanogenerator (SS‐TENG) is demonstrated as a tactile sensor for a hybrid actuator. To fabricate the SS‐TENGs, liquid silicone and ethanol are mixed well and then infiltrated inside a conductive sponge. When heated, the ethanol evaporates from the silicone, leaving plenty of empty pores. Every pore supplies the contact and separation space for the conductive sponge and silicone to form a micro TENG, and the whole bulk makes up the SS‐TENG. When deformed, the SS‐TENG outputs electric signals. The open‐circuit voltage linearly increases from 3.9 to 17 V as the force increases from 2.9 to 9.9 N. In addition, when touched, the electric signals generated by SS‐TENGs can differentiate the corresponding objects. Finally, the SS‐TENGs are integrated with a soft–rigid hybrid actuator to form a bionic skeleton–muscle–skin hybrid gripper. The gripper can catch different objects and feedback electric signals, which show great potential for use in bionic robots.

Elastic Cu@PPy sponge for hybrid device with energy conversion and storage

Elastic and electrical conductive sponges are attracting materials for energy storage and energy harvest devices. In this study, we have demonstrated that a flexible and durable Cu doped PDMS sponge (Cu) can be adopted as electrodes for triboelectric nanogenerators (TENG) and flexible supercapacitors (SC). The Cu sponge loading with polypyrrole (PPy) was newly applied as single-electrode TENG for the first time. The outputs of the TENG were tuned by varying the PPy content or by combining with PDMS sponge. Due to the feasibility of changing the PPy content in the Cu@PPy sponges, this study offers a facile way to enhance the performance of TENG from the material itself with a tunable manner. Moreover, this study potentiates a better understanding of the triboelectricity produced by the TENG from the view of materials. The Cu@PPy sponges were further utilized as electrodes for all solid-state symmetrical SCs, in which the Cu sponge and PPy showed a synergistic effect on promoting the stability and performance of the SC. The SC exhibits lightweight, good capacitance, high flexibility, excellent mechanical and long-term stability, which is suitable for wearable energy storage devices. For the application of the TENG and the SC, a self-powered hybrid device was assembled, which provides a prospect for integrated devices. As a summary, we reported a new metal sponge based TENG and self-powered hybrid device, which demonstrated our study provides new opportunities for elastic multifunctional power sources and potential applications in wearable electronics.

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.

Multifunctional WS2&M-AgNPs superhydrophobic conductive sponges for application in various sensors

Superhydrophobic conductive sponge is prepared by an easy method based on WS2 nanosheets and modified Ag nanoparticles in this work, which is promising to apply in various sensors derived from superior liquid repellence, thermo stability, conductive property, mechanical and chemical durability.

Electroless Ag-plated sponges by tunable deposition onto cellulose-derived templates for ultra-high electromagnetic interference shielding

Environmental-friendly cellulose-derived sponges were employed as porous templates to achieve uniform Ag deposition via a facile and economical electroless plating method. SEM and BET results confirmed an Ag layer was deposited on cellulose composite sponges without damaging the porous structure. XPS and XRD analysis demonstrated substantial interactions between porous substrates and Ag particles. When utilized as electromagnetic interference (EMI) shielding materials, a 3.2 mm thick silver-plated sponge exhibited a total shielding effectiveness (SEtotal) value of as high as 120.85 dB in the frequency range of 10–1500 MHz when the plating time was 120 min. Moreover, the conductive sponges displayed absorption-dominant mechanism to alleviate secondary radiation, making them promising candidates as high EMI shielding materials.

Soft Tubular Triboelectric Nanogenerator for Biomechanical Energy Harvesting

AbstractA triboelectric nanogenerator (TENG), fabricated by soft materials with advantages of flexibility, stretchability, and water resistance, is a potential means for biomechanical energy harvesting. Herein, a soft tubular TENG (ST‐TENG) is proposed, which is composed of a flexible conductive sponge as the inner electrode and a soft silicone tube as the dielectric layer with the helix outer electrode. The ST‐TENG can be built in outsoles and harvest various biomechanical energy including pressing, bending, and twisting. The output performance of ST‐TENG was thoroughly investigated and the surface charge density can reach to 175.4 µC m−2. The ST‐TENG has stable and durable characteristics for 1.2 × 106 working cycles, 100 h soaking, and 5000 times bending. By walking or jogging with the ST‐TENG tubes, several portable electronics such as thermometer, digital watch, and calculator can be continuously powered. The soft energy harvester in this work has made a significant progress in self‐powered devices.

Impedance measurements on an engineered cementitious composite: A critical evaluation of testing protocols

An end-to-end (two-point) electrical measurement technique is traditionally employed in evaluating the bulk resistivity of hardened cementitious materials. This testing methodology is critically evaluated by studying the electrical impedance of an engineered cementitious composite (ECC), using cuboidal specimens with four different dimensions viz. 50 mm, 70 mm, 100 mm and 150 mm. In the present work, the impedance response (in the form of Nyquist plots) of the ECC specimens was measured over the frequency range 1 Hz–10 MHz via two external plate-electrodes placed at opposite faces of each specimen. A conductive gel, or saturated, synthetic sponges inserted between the plate/specimen interface were used as the electrode-specimen contacting medium. It is shown that the sponge contacting medium introduced a spurious response which was detectable across the entire frequency range and was particularly evident in specimens with smaller electrode contact area and in specimens with air-voids present on the contact surface. This effect was considerably reduced when either the conductive gel or sponges saturated with highly-conductive liquid was used. It is also shown that the true bulk resistivity of the material, identified from the Nyquist plot, can be evaluated by subtracting the resistivity of the sponge itself from the bulk resistivity of the ECC with the sponges present. Benchmarking studies were also undertaken on specimens with an embedded electrode arrangement.

Conductive Graphene-Melamine Sponge Prepared via Microwave Irradiation.

A conductive graphene-melamine sponge (MS) prepared via microwave irradiation is reported in this paper. Graphene oxide supported on the MS was prereduced first at 100 °C and then further reduced in a household microwave oven at over 1000 °C. It was surprising to find that graphene oxide on the MS was reduced perfectly while the three-dimensional structure of the MS was kept well after high-temperature reduction via microwave irradiation. Slight pyrolysis of MS was also found during 5 s microwave irradiation, resulting in nitrogen generation from the pyrolysis of the MS being doped into graphene, which could benefit the electric conductivity of the prepared graphene-MS. The electric conductivity of the prepared graphene-MS is about 0.12-1.0 S/m because of the high reduction degree of graphene oxide and nitrogen doping. On the other hand, different from the pure MS, the newly developed conductive graphene-MS possesses superhydrophobic and superoleophilic properties. Overall, the newly developed conductive graphene-MS contained 94.3 wt % MS and 5.7 wt % N-doped graphene and is a cost-effective material with good elasticity, high conductivity, superhydrophobicity, and superoleophilicity.

Low-Cost Carbon Fiber-Based Conductive Silicone Sponge EEG Electrodes.

We propose a novel carbon fiber-based conductive silicone sponge for low electrode-skin impedance EEG recordings. When this sponge is used with water or saline solution, no gel is required, lowering the setup time drastically compared to classical wet electrodes. Moreover, the wet conductive carbon fiber silicone sponges achieve an electrode-skin impedance as low as $2.5\mathrm {k} \Omega $ at 1kHz when wet, making them better than state of the art gel electrodes. Additionally, even as the sponge dries out, it continues to remain conductive and performs as a reliable dry electrode. We demonstrate through experiments that these conductive carbon fiber silicone sponge electrodes, wet or dry, are able to measure alpha wave activity. Our carbon fiber conductive sponge electrodes are low-cost and are highly suitable for designs of portable high density EEG measurement systems.

Resistive pressure sensor for high-sensitivity e-skin based on porous sponge dip-coated CB/MWCNTs/SR conductive composites

Three-dimensional (3D) porous conductive sponge with excellent electrical and mechanical properties has drawn great attention for developing wearable electronic skin (e-skin). Here, we reported a simple, practical and efficient ‘dip-coating’ technique to fabricate a resistive pressure sensor with excellent elasticity, high pressure sensitivity of 0.35 kPa−1, long-term reproducibility and durability (∼2000 cycles), fast response time (∼45 ms), based on homogenous 3D hybrid conductive network of carbon black (CB)/multi-walled carbon nanotube (MWCNTs)/silicon rubber (SR) nanocomposites layer-by-layer dip-coated on porous polyurethane (PU) sponge. The good electrical and mechanical performances are achieved using the three-dimensional porous microstructure of polyurethane sponge and the synergistic conductive network of CB/MWCNTs. The fabricated sensor can be used to detect a small-scale human motion, including swallowing, coughing, breathing, clenching, screwing and exercising. The CB/MWCNTs/SR/PU based resistive pressure sensor with high performance and a simple fabrication process will provide new insight into the application flexible wearable electronic skin for real-time health monitoring, human activity monitoring and human-machine interfaces.