Songfang Zhao

Highly Stretchable and Sensitive Strain Sensor Based on Facilely Prepared Three-Dimensional Graphene Foam Composite

Wearable strain sensors with excellent stretchability and sensitivity have emerged as a very promising field which could be used for human motion detection and biomechanical systems, etc. Three-dimensional (3D) graphene foam (GF) has been reported before for high-performance strain sensors, however, some problems such as high cost preparation, low sensitivity, and stretchability still remain. In this paper, we report a highly stretchable and sensitive strain sensor based on 3D GF and polydimethylsiloxane (PDMS) composite. The GF is prepared by assembly process from graphene oxide via a facile and scalable method and possesses excellent mechanical property which facilitates the infiltration of PDMS prepolymer into the graphene framework. The as-prepared strain sensor can be stretched as high as 30% of its original length and the gauge factor of this sensor is as high as 98.66 under 5% of applied strain. Moreover, the strain sensor shows long-term stability in 200 cycles of stretching-relaxing. Implementation of the device for monitoring the bending of elbow and finger results in reproducibility and various responses in the form of resistance change. Thus, the developed strain sensors exhibit great application potential in fields of biomechanical systems and human-interactive applications.

In situ polymerization of mechanically reinforced, thermally healable graphene oxide/polyurethane composites based on Diels–Alder chemistry

Covalently bonded graphene oxide/polyurethane (GO/PU) composites with significant reinforcement and thermally healable properties were developed via in situ polymerization based on Diels–Alder (DA) chemistry. The PU prepolymer was prepared with GO, 4,4-diphenylmethane diisocyanate, and poly(tetramethylene glycol) and blocked by using furfuryl alcohol firstly. Then the prepolymer was cross-linked by using bifunctional maleimide via DA chemistry. SEM shows that the GO was dispersed uniformly in the PU matrix. The DA and retro-DA reactions were characterized by Fourier transform infrared spectroscopy and differential scanning calorimetry separately. Tensile tests showed that with the incorporation of 0.1 wt% of GO, the tensile modulus of GO/PU composites increased from 9.80 MPa to 21.95 MPa, and the tensile strength and elongation at break of the GO/PU composites increased by more than 367% and 210%, respectively. Furthermore, the composites had thermally healable ability which was inspected by using an atomic force microscope and the strain–stress test. The healing efficiency of 78% on average was achieved which was determined by the recovery of breaking stress and a healing mechanism was tentatively proposed. Therefore, the covalently bonded self-healing GO/PU composites could be used as smart materials and structural materials.

Recent Advancements in Flexible and Stretchable Electrodes for Electromechanical Sensors: Strategies, Materials, and Features

Stretchable and flexible sensors attached onto the surface of the human body can perceive external stimuli, thus attracting extensive attention due to their lightweight, low modulus, low cost, high flexibility, and stretchability. Recently, a myriad of efforts have been devoted to improving the performance and functionality of wearable sensors. Herein, this review focuses on recent remarkable advancements in the development of flexible and stretchable sensors. Multifunction of these wearable sensors is realized by incorporating some desired features (e.g., self-healing, self-powering, linearity, and printing). Next, focusing on the characteristics of carbon nanomaterials, nanostructured metal, conductive polymer, or their hybrid composites, two major strategies (e.g., materials that stretch and structures that stretch) and diverse design approaches have been developed to achieve highly flexible and stretchable electrodes. Strain sensing performances of recently reported sensors indicate that the appropriate choice of geometric engineering as well as intrinsically stretchable materials is essential for high-performance strain sensing. Finally, some important directions and challenges of a fully sensor-integrated wearable platform are proposed to realize their potential applications for human motion monitoring and human-machine interfaces.

Strain-Driven and Ultrasensitive Resistive Sensor/Switch Based on Conductive Alginate/Nitrogen-Doped Carbon-Nanotube-Supported Ag Hybrid Aerogels with Pyramid Design

Flexible strain-driven sensor is an essential component in the flexible electronics. Especially, high durability and sensitivity to strain are required. Here, we present an efficient and low-cost fabrication strategy to construct a highly sensitive and flexible pressure sensor based on a conductive, elastic aerogel with pyramid design. When pressure is loaded, the contact area between the interfaces of the conductive aerogel and the copper electrode as well as among the building blocks of the nitrogen-doped carbon-nanotube-supported Ag (N-CNTs/Ag) aerogel monoliths, changes in reversible and directional manners. This contact resistance mechanism enables the hybrid aerogels to act as strain-driven sensors with high sensitivity and excellent on/off swithching behavior, and the gauge factor (GF) is ∼15 under strain of 3%, which is superior to those reported for other aerogels. In addition, robust, elastomeric and conductive nanocomposites can be fabricated by injecting polydimethylsiloxane (PDMS) into alginate/N-CNTs/Ag aerogels. Importantly, the building blocks forming the aerogels retain their initial contact and percolation after undergoing large-strain deformation, PDMS infiltration, and cross-linking of PDMS, suggesting their potential applications as strain sensors.

A facile method to prepare highly compressible three-dimensional graphene-only sponge

Endowing graphene sponge with compressibility and conductivity offers the possibility to regenerate piezoresistivity and is therefore of great interest in the field of sensors. In this work, highly compressible three-dimensional graphene-only sponge (CGS) was prepared through a facile method by using ammonium sulfide and ammonia solutions under mild conditions. The morphologies and microstructures of the as-prepared CGSs can be controlled by adjusting the mass ratio of graphene oxide (GO) to ammonium sulfide which changed from a metallic sheen bulk with a leaf-shaped structure to a black sponge with a porous structure. Besides, by simply changing the concentrations of GO, CGSs with different porosity, conductivity as well as mechanical strength were obtained. Moreover, the resultant CGSs show ultralow density (as low as 4.9 mg cm−3), high porosity (as much as 99.8%), great compressibility (as much as the strain of 80%), and excellent stability (100 cycles) during compression. Furthermore, the sensitive variation of electrical resistance and cycle stability was validated under the compressive strain of 50% which make CGSs great candidates for pressure-responsive sensors, elastic conductors and other applications.

Percolation threshold-inspired design of hierarchical multiscale hybrid architectures based on carbon nanotubes and silver nanoparticles for stretchable and printable electronics

Conductive elastomers, an irreplaceable component of stretchable electronics, have recently gained significant attention. Herein, we report highly conductive, sensitive, stretchable, and fully printed hybrid composites comprising carbon nanotubes (CNTs), silver nanoparticles (Ag NPs) and hydroxyl-poly(styrene-block-butadiene-block-styrene) (OH-SBS) polymers. The electrically conductive composites are fabricated via direct evaporation of CNT-dispersed OH-SBS suspension under mild heating conditions, followed via an iterative process of silver precursor absorption and reduction, generating large amounts of Ag NPs on both the surface and inner regions of the CNT-embedded composites. The obtained CNT–Ag NP embedded composites possess a superior electrical conductivity of 1228 S cm−1, a high break elongation of 540%, and a high gauge factor of 26 500. The unique hierarchical multiscale hybrid architecture of CNT–Ag NPs and the utilization of OH-SBS enable the as-prepared composites to exhibit huge piezoresistive behavior with a broad range of tensile strains. Moreover, handwritten electric circuits with diverse geometries are designed, and the printed strain gauge sensor could successfully detect sign language via its strain-sensing behavior. We believe that our hierarchical multiscale hybrid design could pave the way for the simple fabrication of stretchable circuits for wearable electronics.

Binary Synergistic Sensitivity Strengthening of Bioinspired Hierarchical Architectures based on Fragmentized Reduced Graphene Oxide Sponge and Silver Nanoparticles for Strain Sensors and Beyond

Recently, stretchable electronics have been highly desirable in the Internet of Things and electronic skins. Herein, an innovative and cost-efficient strategy is demonstrated to fabricate highly sensitive, stretchable, and conductive strain-sensing platforms inspired by the geometries of a spiders slit organ and a lobsters shell. The electrically conductive composites are fabricated via embedding the 3D percolation networks of fragmentized graphene sponges (FGS) in poly(styrene-block-butadiene-block-styrene) (SBS) matrix, followed by an iterative process of silver precursor absorption and reduction. The slit- and scale-like structures and hybrid conductive blocks of FGS and Ag nanoparticles (NPs) provide the obtained FGS-Ag-NP-embedded composites with superior electrical conductivity of 1521 S cm-1 , high break elongation of 680%, a wide sensing range of up to 120% strain, high sensitivity of ≈107 at a strain of 120%, fast response time of ≈20 ms, as well as excellent reliability and stability of 2000 cycles. This huge stretchability and sensitivity is attributed to the combination of high stretchability of SBS and the binary synergistic effects of designed FGS architectures and Ag NPs. Moreover, the FGS/SBS/Ag composites can be employed as wearable sensors to detect the modes of finger motions successfully, and patterned conductive interconnects for flexible arrays of light-emitting diodes.

Layer-by-Layer Assembly of Multifunctional Porous N-Doped Carbon Nanotube Hybrid Architectures for Flexible Conductors and Beyond

Coassemble diverse functional nanomaterials with carbon nanotubes (CNTs) to form three-dimensional (3D) porous CNTs hybrid architectures (CHAs) are potentially desirable for applications in energy storage, flexible conductors, and catalysis, because of diverse functionalities and synergistic effects in the CHAs. Herein, we report a scalable strategy to incorporate various functional nanomaterials with N-doped CNTs (N-CNTs) into such 3D porous CHAs on the polyurethane (PU) sponge skeletons via layer-by-layer (LbL) assembly. To investigate their properties and applications, the specific CHAs based on N-CNTs and Ag nanoparticles (NPs), denoted as PU-(N-CNTs/Ag NPs)n, are developed. The unique binary structure enables these specific CHAs conductors to possess reliable mechanical and electrical performance under various elastic deformations as well as excellent hydrophilicity. Moreover, they are employed as strain-gauge sensor and heterogeneous catalyst, respectively. The sensor could detect continuous signal, static signal, and pulse signal with superior sustainability and reversibility, indicating an important branch of electromechanical devices. Furthermore, the synergistic effects among N-CNTs, Ag NPs, and porous structure endow the CHAs with excellent performance in catalysis. We have a great expectation that LbL assembly can afford a universal route for incorporating diverse functional materials into one structure.

A crack-based nickel@graphene-wrapped polyurethane sponge ternary hybrid obtained by electrodeposition for highly sensitive wearable strain sensors

Stretchable strain sensors, as crucial components in wearable intelligent devices, have become one of the recent research hotspots with promising potential in human-interactive, personal health monitoring, and flexible smartphones. Graphene-based materials have been reported for high-performance strain sensors. However, there still remain some limitations such as their high production cost and low sensitivity and stretchability. Herein, a highly stretchable and ultra-sensitive strain sensor based on nickel nanoparticles and a graphene-coated polyurethane sponge (Ni@GPUS) ternary hybrid material has been reported. Herein, Ni@GPUS was fabricated via a series of techniques including preparation of a graphene-coated polyurethane sponge, electrodeposition of nickel nanoparticles, and encapsulation by polydimethylsiloxane. The obtained sensors can be stretched up to 65% and exhibit a remarkable gauge factor of up to 3360.09. Furthermore, a fast signal response (<100 ms) and 1000 cycles of stretching and bending prove the rapid steady state response and long-term durability of the sensor, respectively. In addition, the working mechanisms of the sensor have been proposed. Moreover, the strain sensor was used as a bodily motion sensor to monitor finger bending and facial muscle tension, showing great potential in the fields of flexible, stretchable, and wearable electronics.

Investigation on the properties and processability of polymeric insulation layers for through silicon via

3D packaging using through silicon via (TSV) technology is becoming important in IC packaging industry. In this paper, the TSV process uses a polymeric liner as insulation material and a buffer for thermo-mechanical stress relaxation. Fourier transform infrared spectroscopy (FTIR), thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), dielectric and contact angle tests are applied to select a suitable dielectric from two kinds of polymers. All the properties show that the linear o-crosel phenolic (LOPF) is suitable for acting as an insulation liner in the TSV process. Then the LOPF liquid is spun on the wafer, followed by soft baking at the temperature of 115 °C, the processed wafer is inspected using optical microscope, step profiler and scanning electron microscope (SEM). All the results indicate that LOPF has good potential to be the insulation layers for TSV.