Compression-release Cycles Research Articles

Pushing Pressure Detection Sensitivity to New Limits by Modulus-Tunable Mechanism.

Only microstructures are used to improve the sensitivity of iontronic pressure sensors. By modulating the compressive modulus, a breakthrough in the sensitivity of the iontronic pressure sensor is achieved. Furthermore, it allows for programmatic tailoring of sensor performance according to the requirements of different applications. Such a new strategy pushes the sensitivity up to a record-high of 25548.24kPa-1 and expands the linear pressure range from 15 to 127kPa. Additionally, the sensor demonstrates excellent mechanical stability over 10000 compression-release cycles. Based on this, a well-controlled robotic hand that precisely tracks the pressure behavior inside a balloon to autonomously regulate the gripping angle is developed. This paves the way for the application of iontronic pressure sensors in precise sensing scenarios.

Low-cost, reliable and flexible piezoresistive pressure sensors coated with single layer graphene and silver nanowires on three-dimensional polyurethane sponge

Flexible wearable sensors perform a significant role in practical applications such as medical monitoring, rehabilitation exercises, and artificial skin, as they can quantify and visualize external stress and strain. Polyurethane (PU) sponge stands out as a highly elastic and reproducible material frequently employed as a substrate in flexible electronic devices. Sponge materials with different pore sizes and densities exhibit varying degrees of deformation resistance under long-term cyclic pressure, thereby affecting the performance of sensors. Herein, piezoresistive pressure sensors based on PU with different pore diameters were successfully fabricated by dip-coating technology. By incorporating silver nanowires (AgNWs) and single layer graphene (SLG) into the sponge, both the low-density and high-density composite sponges can withstand over 70 % compressive strain, with stresses of 62.57 kPa and 31.79 kPa, respectively. Moreover, high-density composite sponge demonstrates a more stable conductive structure, more consistent relative resistance and better flame-retardant performance. The response and recovery time between 10 % and 40 % strains of the resulting pressure sensor based on high-density composite sponge (PU@AgNWs@SLG) are 110 ms and 106 ms and its gauge factor reaches 2.39. The pressure sensor exhibits a satisfactory reproducibility after over 3000 compression-release cycles, and performs during various applications, such as controlling brightness of LEDs, monitoring human activities, sensing finger movements.

Gradient pore structured Ppy/PDMS conductive sponge for flexible pressure sensor

Flexible pressure sensors based on porous conductive composites have potential applications in electronic skin, healthcare due to their pressure-responsive electrical conductivity. However, simultaneously enhancing sensitivity and expanding the working range remains a formidable challenge. In this work, we propose a bioinspired polypyrrole (Ppy)/polydimethylsiloxane (PDMS) sponge, which was fabricated using a one-step emulsion ice template process. This approach allows for the creation of a gradient pore structure along the thickness direction by adjusting the solid content of the latex solution and incorporating renewable carboxymethyl cellulose (CMC) additive. The technique effectively eliminates the issues associated with template residues, chemical pollutants, and multiple processes, thereby meeting the requirements for the sustainable development of flexible electronics in the future. The as-prepared Ppy/PDMS sponge exhibits a gradient modulus from top to bottom due to its gradient pore structure, thereby resulting in a combination of enhanced compressibility arising from larger pores and excellent stress adaptation characteristics originating from smaller pores. The Ppy/PDMS sponge based sensor exhibits a remarkable strain sensitivity of 31.4, a broad strain range from 0 % to 70 % and exceptional durability even after undergoing ∼ 10000 compression-release cycles. Additionally, the sensor can be utilized for health monitoring, human motion detection, and human–machine interfaces. Furthermore, an array of our sensors possesses the capability to collect spatiotemporally resolved signals for the purpose of monitoring and analyzing intricate actions. These advantages make our Ppy/PDMS sponge a highly promising candidate for flexible pressure sensor.

Highly efficient recognition of similar objects based on ionic robotic tactile sensors

Tactile sensing provides robots the ability of object recognition, fine operation, natural interaction, etc. However, in the actual scenario, robotic tactile recognition of similar objects still faces difficulties such as low efficiency and accuracy, resulting from a lack of high-performance sensors and intelligent recognition algorithms. In this paper, a flexible sensor combining a pyramidal microstructure with a gradient conformal ionic gel coating was demonstrated, exhibiting excellent signal-to-noise ratio (48 dB), low detection limit (1 Pa), high sensitivity (92.96 kPa−1), fast response time (55 ms), and outstanding stability over 15,000 compression-release cycles. Furthermore, a Pressure-Slip Dual-Branch Convolutional Neural Network (PSNet) architecture was proposed to separately extract hardness and texture features and perform feature fusion. In tactile experiments on different kinds of leaves, a recognition rate of 97.16% was achieved, and surpassed that of human hands recognition (72.5%). These researches showed the great potential in a broad application in bionic robots, intelligent prostheses, and precise human–computer interaction.

RGO-BCNT/PANI Three-Dimensional Flexible Aerogel Sponge Electrodes and Electrochemical Performance

Self-supported flexible supercapacitors have promising applications in wearable electronics. The electrode materials, as a crucial component of supercapacitors, have a decisive impact on the energy storage performance of the entire device. Herein, reduced graphene oxide-boron atom doped-carbon nanotubes/polyaniline (rGO-BCNT/PANI) (rBP) three-dimensional (3D) aerogel sponge electrode materials were prepared by a simple ultrasonic self-assembly followed by reduction-induced self-assembly reaction. The rBP aerogel sponge structure not only provided a channel for electrolyte exchange, but also provided enough space for PANI nanoparticles to withstand the volume change during charging and discharging, and inhibited the decomposition of PANI nanoparticles. As a result, the 3D rBP aerogel sponge with 60 mg PANI addition amount (rBP60) exhibited high specific capacitance (695 F·g-1), high power density (675 W·kg-1), and high energy density (60.95 Wh·kg-1) at 0.5 A·g-1 in a three-electrode system. The 3D rBP60 aerogel sponge electrode material can reach 610 F·g-1 at 2 A·g-1, with a retention rate of up to 88% after 2000 cycles. The Coulombic efficiency of the rBP60 aerogel sponge electrode material was close to or equal to 85.5% at different current densities. The 3D rBP aerogel sponge was exceptionally flexible, maintaining its morphology without damage after 100 compression-release cycles.

Superelastic chitin nanofibril/chitosan aerogel for effective circulating and continuous oil-water separation

Elastic and hydrophobic aerogels have received a lot of attention in dealing with the increasing oil pollution due to their recyclable properties. Herein, we present an ultralight and superelastic aerogel with highly oriented polygon structure based on chitin nanofibril (ChNF) and chitosan (CS) by directional freezing. The chemical cross-linking enables good mechanical strength at low aerogel density. After 500 compression-release cycles, the aerogel can retain the deformation recovery rate of 88 % in air, demonstrating the excellent resilience. The bio-based aerogel has high absorption capacity (52-114 g/g) for various oils and organic solvents, and it is able to achieve the absorption retention of 90 % even after 20 absorption-extrusion cycles. Moreover, owing to the good elasticity, the pore size of the aerogel can be adjusted by compression to selectively separate water-in-oil emulsions of different particle sizes with separation efficiencies higher than 99.5 %. The bio-based aerogel with good cycle performance has broad application prospects in the field of oil-water separation.

Mechano-Electrochemically Promoting Lithium Atom Diffusion and Relieving Accumulative Stress for Deep-Cycling Lithium Metal Anodes.

Lithium metal batteries (LMBs) can double the energy density of lithium-ion batteries. However, the notorious lithium dendrite growth and large volume change are not well addressed, especially under deep cycling. Here, an in-situ mechanical-electrochemical coupling system is built, and it is found that tensile stress can induce smooth lithium deposition. Density functional theory (DFT) calculation and finite element method (FEM) simulation confirm that the lithium atom diffusion energy barrier can be reduced when the lithium foils are under tensile strain. Then tensile stress is incorporated into lithium metal anodes by designing an adhesive copolymer layer attached to lithium in which the copolymer thinning can yield tensile stress to the lithium foil. Elastic lithium metal anode (ELMA) is further prepared via introducing a 3D elastic conductive polyurethane (CPU) host for the copolymer-lithium bilayer to release accumulated internal stresses and resist volume variation. The ELMA can withstand hundreds of compression-release cycles under 10% strain. LMBs paired with ELMA and LiNi0.8 Co0.1 Mn0.1 O2 (NCM811) cathode can operate beyond 250 cycles with 80% capacity retention under practical condition of 4 mAh cm-2 cathode capacity, 2.86g Ah-1 electrolyte-to-capacity ratio (E/C) and 1.8 negative-to-cathode capacity ratio (N/P), five times of the lifetime using lithium foils.

Compression-tolerant supercapacitor based on NiCo2O4/Ti3C2Tx MXene/reduced graphene oxide composite aerogel with insights from density functional theory simulations

Compression-tolerant electrodes are critical for developing next-generation wearable energy storage devices. However, most of previous studies on compressible electrodes focus on carbon-based materials, whereas metal-based materials such as spinel metal oxide with faradaic nature have been rarely studied due to their lack of compressibility. Herein, NiCo2O4 (NCO) microtubes assembled by ultrathin and mesoporous nanosheets, are deposited on/into Ti3C2Tx MXene/reduced graphene oxide aerogel (MGA), an intrinsically compressible host template with high conductivity and specific surface areas. The optimized NCO/MGA-300 sample shows a reversible compressive strain of 60% and a superior durability. Density functional theory (DFT) calculations reveal that the NCO/MGA-300 heterojunction has high electronic conductivity, fast electron transfer ability, and low adsorption energy for OH– ions. As a result, the NCO/MGA-300 electrode exhibits superb electrochemical performance in terms of its high gravimetric capacitance (1633F g−1 at 1 A g−1), rate performance (1492F g−1 at 10 A g−1), and remarkable cycling stability of 86.6% after 10,000 charge-discharging cycles. Moreover, an assembled asymmetric supercapacitor based on compressible NCO/MGA-300 shows stable electrochemical performances under different compressive strains (20%. 40% and 60%), or after 100 compression-release cycles. This research finding demonstrates the possibility of metal-based electrode for wearable devices with high energy storage capability and good compressibility.

Multifunctional conductive sponge with excellent superhydrophobicity, piezoresistivity, electro/light to heat conversion, and oil/water separation performance

Flexible porous conductive composites have been widely applied for wearable sensing devices and there is continuous exploration towards multi-functionalization of porous conductive composites. In this work, through sugar template method and following immersion assisted reduction, polydimethylsiloxane (PDMS) sponge embedded with carbonyl iron (CI) was successfully anchored with silver nanoparticles (AgNPs) binded with block copolymer polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene (SEBS). A superhydrophobic multifunctional sponge type conductive composites (SCC) was prepared (CI-PDMS/AgNPs-SEBS Sponge, abbreviated as CP/AgE Sponge). CP/AgE sponge had excellent superhydrophobic properties (water contact angle≈153°), efficient Joule heating (1.6 V, 101 °C within 180 s) and photothermal conversion capabilities (250 mW/cm2, 102 °C within 40 s). The CP/AgE sponge pressure sensor had excellent piezoresistive properties, including high sensitivity (3.8 kPa−1), wide detection range (160 kPa) and robust stability when subjected to 1000 compression-release cycles, which could be further utilized to accurately and reliably monitor the changing biophysical signals. Besides, CP/AgE sponge had high oil/water separation efficiency as well as magnetically guided waste oil adsorption ability. This work provides a new strategy for preparing a conductive sponge with superhydrophobicity and enriched functionality, which has broad applications in the fields of next generation multifunctional flexible electronics, effective oil/water separation and smart protective skins.

Mechanically Tough and Regenerable Antibacterial Nanofibrillated Cellulose-Based Aerogels for Oil/Water Separation.

Nanofibrillated cellulose (NFC)-based aerogels have been widely used for various applications. However, the disadvantages of poor structural stability, low mechanical toughness, and easy contamination by bacteria hinder their large-scale application. In this work, 3-(3'-acrylicacidpropylester)-5,5-dimethyl hydantoin (APDMH) was grafted on oxidized NFC (ONC) to prepare antibacterial poly(APDMH)-g-ONC (PAC). PAC and poly(ethyleneimine) (PEI) were chemically cross-linked using 3-glycidoxypropyltrimethox (GPTMS), aiming at constructing a PAC-g-PEI aerogel with multiple network structures. The mechanical behaviors of composite aerogel and oil/water separation performances under different conditions were investigated. PAC-g-PEI aerogel exhibits outstanding fatigue resistance (>50 cycles of compression) and superior elasticity (96.76% height recovery after five compression-release cycles at 50% strain). The obtained superhydrophilic and underwater-oleophobic properties endow the aerogel with excellent oil/water separation performances, achieving a satisfactory separation efficiency of over 99% and flux of over 9500 L·m-2·h-1. Furthermore, the chlorinated aerogel of PAC-g-PEI-Cl shows highly efficient and rechargeable antibacterial properties, can inactivate 6.72-log Escherichia coli and 6.60-log Staphylococcus aureus within 10 min, and can still kill all inoculated bacteria after 50 cycles. In addition, PAC-g-PEI-Cl aerogel can inhibit biofilm formation, making it a promising candidate for highly efficient oil/water separation applications in diverse harsh conditions.

Highly compressible binder-free sponge supercapacitor electrode based on flower-like NiO/MnO2/CNT

The increasing demand for flexible electronics encourages the innovative and functional designs of electrode materials with high performance and compressibility. In this work, we report a compressible supercapacitor electrode which is prepared by coating electrically active NiO/MnO2/carbon nanotube (CNT) composite onto a sponge. A cube of sugar was used as the template to obtain the sponge through infiltration and cross-linking of polydimethylsiloxane (PDMS). NiO/MnO2/CNT was deposited on the PDMS sponge to generate substantial amount of interface, resulting in a specific capacitance of 23 F/g at 0.1 A /g in a three-electrode system and 1.32 F/g at 0.5 mA in a symmetric supercapacitor. The proposed system exhibits excellent cycling stability with capacitance retention over 10.000 cycles. The strong adhesion of the binary metal oxides and carbon material onto the porous nonconductive sponge enables mechanical stability under compression-release cycles. Our study indicates that this electrode is a promising candidate for applications in flexible electronics. Furthermore, this research might guide the development of flexible, high-performance, and low-cost electrodes, which will be useful in wearable energy storage systems.

A simple and green strategy for preparing flexible thermoplastic polyimide foams with exceptional mechanical, thermal-insulating properties, and temperature resistance for high-temperature lightweight composite sandwich structures

Polyimide (PI) foams possess excellent mechanical properties, high-temperature resistance, and other unique properties, which facilitate impact in aerospace, automotive, and microelectronics industries. However, when they are used as lightweight insulation structural layers for high-temperature carbon fiber reinforced composites, improvement in thermal-insulating properties and temperature resistance of PI foams without compromising the flexibility in extreme environments is a challenging task. In this study, the flexible blocks were linked into rigid polymeric structures to fabricate a thermoplastic structure through a simple aqueous strategy to construct hydrogen-bonding networks. Thus, with a low shrinkage of 8.42%, an ultrahigh compressive modulus of 11.17 MPa, and a specific modulus of 81.03 MPa cm3 g−1 was successfully achieved. Benefitting from the formation of ultra-strong network backbones, foams showed excellent thermal-insulating properties and temperature resistance. The robust porous material with thermal-insulating properties (0.0481 W m−1 K−1) exhibited a compression modulus of 4.21 MPa after heat treatment at 300 °C and maintained 91% strength retention, thus it can be used as lightweight heat-insulation composite sandwich structures up to 300 °C. The copolymer foam obtained by using 30 mol% flexible blocks reserves its high flexibility undergoing only 10% dimensional changes after 40,000 compression-release cycles. The lightweight thermoplastic PI foams with flexibility, thermal-insulating properties, and temperature resistance can be used for high-temperature lightweight composite sandwich structures in aeronautics and space exploration.

Exfoliated graphite blocks with resilience prepared by room temperature exfoliation and their application for oil-water separation

Exfoliated graphite (EG) blocks are prepared from the ultra-large flakes of graphite by intercalation of H2SO4 using a large amount of H2O2 at 5 °C and following exfoliation at 30 °C. By the exfoliation in a closed container, EG blocks with the bulk densities of 0.008–0.024 g/cm3 are successfully prepared. The resultant EG blocks have high sorption capacities for a diesel oil, up to 45 g/g. The EG blocks after oil sorption can get certain resilience for compressive stress with high reproducibility by compression-release cycles, which allows us to apply the compression-releasing for the oil sorption-desorption of the EG blocks. The performance of cyclic oil sorption-desorption by compression-releasing of EG block is compared with those of filtration and distillation. Since the resultant EG blocks had sufficient mechanical strength, the continuous removal of oil floating on the water surface is possible, exporting oil through a catheter inserted into the block and connected to a peristaltic pump. By warming up by Joule heating, even a crude oil having high viscosity can be continuously removed from the water with sufficient rate. The high hydrophobicity and lipophilicity of EG make selective removal of oil from water possible.

In-situ metallized carbon nanotubes/poly(styrene-butadiene-styrene) (CNTs/SBS) foam for electromagnetic interference shielding

The porous conductive polymer composites have received great attention owing to their superiority of being utilized in electromagnetic interference (EMI) shielding applications. Herein, lightweight and highly compressible conductive composite foam composed of poly(styrene-block-butadiene-block-styrene) (SBS) polymer matrix, carbon nanotubes (CNTs) and silver nanoparticles (Ag NPs) is fabricated by freeze-drying technology, silver precursor absorption and in-situ reduction process. Three-dimensional CNTs conductive network and partially embedded Ag NPs synergistically endow the foam with excellent mechanical, electrical and EMI shielding performance. The flexible 8Ag/4CNTs/10SBS composite foam with a low density of 0.42 g/cm3 exhibits moderate compressive strength of 169.2 kPa at strain of 50%, remarkable conductivity of 753 S/m, and prominent EMI shielding effectiveness (SE) value of around 91 dB in the electromagnetic wave frequency range of 8.2–12.5 GHz. In addition, the 4Ag/4CNTs/10SBS composite foam exhibits robust mechanical–electrical property upon 1000 compression-release cycles at strain of 50%, and its SE value increased by about 33% compared with the initial state after the cyclic compression test. These results suggest that the Ag/CNTs/SBS foams possess promising potential to be used as lightweight, flexible, and high-performance EMI shielding materials in modern electronic devices.

PHYSIOLOGICAL RESPONSE OF PERIPHERAL NERVE TO REPETITIVE COMPRESSION: A RABBIT EXPERIMENTAL MODEL

Objective: The objective of this study is to clarify the differences in physiological responses following the different modality of repetitive compressions versus continuous compression (CC) of the peripheral nerve. Methods: Thirty-two rabbits were used. Under anesthesia, four conditions of compression; CC, low-frequency release compression (LFR, one second release time every 30 s), high-frequency release compression (HFR, one second release time every 10 s), and high-frequency compression (HFC, one second compression time every 10 s) were each applied with 80[Formula: see text]mmHg of pressure for 90[Formula: see text]min to the sciatic nerve. Compound nerve action potential (CNAP) and nerve blood flow were evaluated from the beginning of compression to 90[Formula: see text]min after compression release. Endoneurial microvascular permeability was evaluated histologically. Results: During compression, the nerve blood flow and the amplitude of CNAP decreased for all four groups. There were significant delays in the recovery of the amplitude and nerve blood flow in the LFR and HFR groups compared to CC group ([Formula: see text]). Endoneurial edema was observed in the repetitive compression groups. Conclusions: Even if the overall compression time was short, the accumulation of repetitive compression-release cycles causes to delay the recovery of nerve physiological function with endoneurial edema.

In-situ preparation of water chestnut-based carbon aerogel and its application in binder-less electric double layer electrode and stress sensing

The water chestnut-based carbon aerogel (WCA) that was directly in-situ prepared via a simple one-step hydrothermal method. The as-prepared WCA had excellent electrochemical properties as a binder-less electric double layer (EDL) electrode in SCs. Specifically, the WCA after annealing exhibited a superior mass specific capacitance of 343.4 F/g at 0.5 A/g in 3 M KOH. Also, the WCA displayed ultra-high capacitance retention rate, which significantly remained close to 100% after 20,000 cycles and still maintained 98.3% after 40,000 cycles. In addition, the green and sustainable WCA yet demonstrated ultra-high sensitivity (42.8 kPa−1) and recoverable performance, which had potential application value in flexible pressure sensors. After 3000 compression-release cycles, the recovery rate exceeded 90%.

Robust, Lightweight, Hydrophobic, and Fire-Retarded Polyimide/MXene Aerogels for Effective Oil/Water Separation.

Polyimide (PI) aerogels have attracted great attention owing to their low density and excellent thermal stability. However, hydrophobic surface modification is required for PI aerogels to improve their ability in oil/water separation due to their amphiphilic characteristic. Two-dimensional MXenes (transition metal carbides/nitrides) can be utilized as nanofillers to enhance the properties of polymers because of their unique layered structure and versatile interface chemistry. Herein, the robust, lightweight, and hydrophobic PI/MXene three-dimensional architectures were fabricated via freeze-drying of polyamide acid/MXene suspensions and thermal imidization. Polyamide acid was synthesized using N-N-dimethylacetamide and 4,4'-oxydianiline. MXene (Ti3C2Tx) dispersion was obtained via the etching of Ti3AlC2 and ultrasonic exfoliation. Taking advantage of the strong interaction between PI chains and MXene nanosheets, the interconnected, highly porous, and hydrophobic PI/MXene aerogels with low density were fabricated, resulting in the improved compressive performance, remarkable oil absorption capacity, and efficient separation of oil and water. For the PI/MXene-3 aerogel (weight ratio, 5.2:1) without any surface modification, the water contact angle was 119° with a density of 23 mg/cm3. This aerogel can completely recover to its original height after 50 compression-release cycles, exhibiting superelasticity and exceptional fatigue-resistant ability. It also showed high absorption capacities to various organic liquids ranging from approximately 18 to 58 times of their own weight. This hybrid aerogel can rapidly separate the chloroform, soybean oil, and liquid paraffin from the water-oil system. The thermally stable hybrid aerogel also exhibited excellent fire safety properties and outstanding reusability under an extreme environment.

Compressible, durable and conductive polydimethylsiloxane-coated MXene foams for high-performance electromagnetic interference shielding

Lightweight and compressible electromagnetic interference (EMI) shielding polymer nanocomposites are urgently required to solve increasingly serious electromagnetic pollutions. Two-dimensional transition metal carbides and nitrides (MXenes), especially Ti3C2Tx, are ideal candidates for constructing highly efficient conduction networks in polymer matrices due to their intriguing layered structure and high electrical conductivity. Herein, compressible and electrically conductive polydimethylsiloxane (PDMS)-coated MXene foams are fabricated by preforming three-dimensional (3D) MXene aerogel architectures assisted with sodium alginate (SA) followed by coating a thin layer of PDMS to enhance structural stability and durability of the porous architectures. Consequently, the lightweight MXene/SA hybrid aerogel achieves an outstanding conductivity of 2211 S m−1 and a high average EMI shielding efficiency of 70.5 dB. Furthermore, the PDMS coating effectively endows the 3D conductance network with excellent compressibility and durability. The PDMS-coated MXene foam with 6.1 wt% of MXene reserves its high EMI shielding efficiency of 48.2 dB after 500 compression-release cycles. The lightweight, compressible and conductive PDMS-coated MXene foam is thus promising for applications in EMI shielding gaskets, wearable electronics, sensors and other specific areas.

Highly sensitive, reliable and flexible piezoresistive pressure sensors featuring polyurethane sponge coated with MXene sheets

Flexible pressure sensors still face a great challenge to combine fast frequency response, wide pressure range, multiple detection modes, satisfactory mechanical and environmental stability, and simple fabrication process into a sensor. Herein, flexible piezoresistive pressure sensors are fabricated by treating the backbone of polyurethane (PU) sponge with chitosan (CS) to obtain positively charged CS@PU sponge, followed by dip-coating of negatively charged Ti3C2Tx MXene sheets. The resulting MXene@CS@PU sponge-based sensor provides a versatile sensing platform with potentials for detecting both small and large pressure signals. Due to the highly compressive resilience of the PU sponge and its polar interaction with the MXene sheets, the MXene@CS@PU sensor has high compressibility and stable piezoresistive response for compressive strains of up to 85% with a stress of 245.7 kPa, and it also exhibits a satisfactory reproducibility for 5000 compression-release cycles. Even after washing in water for 1 h, the sensor still shows good performances. With a rapid response time of only 19 ms and a low detection limit of 30 μN corresponding to a pressure of 9 Pa, the MXene sponge sensor is promising for detecting human physiological signals and insect movements. In addition to the contact mode detection, the sponge sensor could detect voices and human breaths by a non-contact detection mode.

Flexible Polydimethylsiloxane Foams Decorated with Multiwalled Carbon Nanotubes Enable Unprecedented Detection of Ultralow Strain and Pressure Coupled with a Large Working Range.

Low-cost piezoresistive strain/pressure sensors with large working range, at the same time able to reliably detect ultralow strain (≤0.1%) and pressure (≤1 Pa), are one of the challenges that have still to be overcome for flexible piezoresistive materials toward personalized health-monitoring applications. In this work, we report on unprecedented, simultaneous detection of ultrasmall strain (0.1%, i.e., 10 μm displacement over 10 mm) and subtle pressure (20 Pa, i.e., a force of only 2 mN over an area of 1 cm2) in compression mode, coupled with a large working range (i.e., up to 60% for strain-6 mm in displacement-and 50 kPa for pressure) using piezoresistive, flexible three-dimensional (3D) macroporous polydimethylsiloxane (pPDMS) foams decorated with pristine multiwalled carbon nanotubes (CNTs). pPDMS/CNT foams with pore size up to 500 μm (i.e., twice the size of those of commonly used foams, at least) and porosity of 77%, decorated with a nanostructured surface network of CNTs at densities ranging from 7.5 to 37 mg/cm3 are prepared using a low-cost and scalable process, through replica molding of sacrificial sugar templates and subsequent drop-casting of CNT ink. A thorough characterization shows that piezoresistive properties of the foams can be finely tuned by controlling the CNT density and reach an optimum at a CNT density of 25 mg/cm3, for which a maximum change of the material resistivity (e.g., ρ0/ρ50 = 4 at 50% strain) is achieved under compression. Further static and dynamic characterization of the pPDMS/CNT foams with 25 mg/cm3 of CNTs highlights that detection limits for strain and pressure are 0.03% (3 μm displacement over 10 mm) and 6 Pa (0.6 mN over an area of 1 cm2), respectively; moreover, good stability and limited hysteresis are apparent by cycling the foams with 255 compression-release cycles over the strain range of 0-60%, at different strain rates up to 10 mm/min. Our results on piezoresistive, flexible pPDMS/CNT foams pave the way toward breakthrough applications for personalized health care, though not limited to these, which have not been fully addressed to date with flexible strain/stress sensors.