Liquid metal-hydrogel composites for flexible electronics.

As an emerging functional material, liquid metal-hydrogel composites exhibit excellent biosafety, high electrical conductivity, tunable mechanical properties and good adhesion, thus providing a unique platform for a wide range of flexible electronics applications such as wearable devices, medical devices, actuators, and energy conversion devices. Through different composite methods, liquid metals can be integrated into hydrogel matrices to form multifunctional composite material systems, which further expands the application range of hydrogels. In this paper, we provide a brief overview of the two materials: hydrogels and liquid metals, and discuss the synthesis method of liquid metal-hydrogel composites, focusing on the improvement of the performance of hydrogel materials by liquid metals. In addition, we summarize the research progress of liquid metal-hydrogel composites in the field of flexible electronics, pointing out the current challenges and future prospects of this material.

Wide Range Strain Distributions on the Electrode for Highly Sensitive Flexible Tactile Sensor with Low Hysteresis.

Flexible piezoresistive tactile sensors are widely used in wearable electronic devices because of their ability to detect mechanical stimuli. However, achieving high sensitivity and low hysteresis over a broad detection range remains a challenge with current piezoresistive tactile sensors. To address these obstacles, we designed elastomeric micropyramid arrays with different heights to redistribute the strain on the electrode. Furthermore, we mixed single-walled carbon nanotubes in the elastomeric micropyramids to compensate for the conductivity loss caused by random cracks in the gold film and increase the adhesion strength between the gold film (deposited on the pyramid surface) and the elastomer. Thus, the energy loss of the sensor during deformation and hysteresis (∼2.52%) was effectively reduced. Therefore, under the synactic effects of the percolation effect, tunnel effect, and multistage strain distribution, the as-prepared sensor exhibited a high sensitivity (1.28 × 106 kPa-1) and a broad detection range (4.51-54837.06 Pa). The sensitivity was considerably higher than those of most flexible pressure sensors with a microstructure design. As a proof of concept, the sensors were successfully applied in the fields of health monitoring and human-machine interaction.

Highly Sensitive and Omnidirectionally Stretchable Bioelectrode Arrays for In Vivo Neural Interfacing.

Flexible electrode array, a new-generation neural microelectrode, is a crucial tool for information exchange between living tissues and external electronics. Till date, advances in flexible neural microelectrodes are limited because of their high impedance and poor mechanical consistency at tissue interfaces. Herein, a highly sensitive and omnidirectionally stretchable polymeric electrode array (PEA) is introduced. Micropyramid-nanowire composite structures are constructed to increase the effective surface area of PEA, achieving an exponential reduction in impedance compared with gold (Au) and flat polypyrrole electrodes. Moreover, for the first time, a suspended umbrella structure to enable PEA with omnidirectional stretchability of up to ≈20% is designed. The PEA can withstand 1000 cycles of mechanical loads without decrease in performance. As a proof of concept, PEA is conformally attached to a rat heart and tibialis anterior muscle, and electrophysiological signals (electrocardiogram and electromyogram) of the rat are successfully recorded. This strategy provides a new perspective toward highly sensitive and omnidirectionally stretchable PEA that can facilitate the practical application of neural electrodes.

A Nonswelling Hydrogel with Regenerable High Wet Tissue Adhesion for Bioelectronics.

Reducing the swelling of tissue-adhesive hydrogels is crucial for maintaining stable tissue adhesion and inhibiting tissue inflammation. However, reported strategies for reducing swelling always result in a simultaneous decrease in the tissue adhesive strength of the hydrogel. Furthermore, once the covalent bonds break in the currently reported hydrogels, they cannot be rebuilt, and the hydrogel loses its tissue adhesive ability. In this work, a nonswelling hydrogel (named as "PAACP") possessing regenerable high tissue adhesion is synthesized by copolymerizing and crosslinking poly(vinyl butyral) with acrylic acid, gelatin, and chitosan-grafted N-acetyl-l-cysteine. The tissue adhesive strength of the obtained PAACP reaches 211.4 kPa, which is approximately ten times higher than that of the reported nonswelling hydrogels, and the hydrogel can be reused for multiple cycles. The as-prepared hydrogel shows great potential in soft bioelectronics, as muscle fatigue is successfully monitored via the electrode array and strain sensor integrated on PAACP substrates. The success of these bioelectronics offers potential applicability in the long-term diagnosis of muscle-related health conditions and prosthetic manipulations.

Thermal-Sinterable EGaIn Nanoparticle Inks for Highly Deformable Bioelectrode Arrays.

Liquid metal (especially eutectic gallium indium, EGaIn) nanoparticle inks overcome the poor wettability of high surface tension EGaIn to elastomer substrates and show great potential in soft electronics. Normally, a sintering strategy is required to break the oxide shells of the EGaIn nanoparticles (EGaIn NPs) to achieve conductive paths. Herein, for the first time, thermal-sinterable EGaIn NP inks are prepared by introducing thermal expansion microspheres (TEMs) into EGaIn NP solution. Through the mechanical pressure induced by the expansion of the heated TEMs, the printed EGaIn NPs can be sintered into electrically conductive paths to achieve highly stretchable bioelectrode arrays, which exhibit giant electromechanical performance (up to 680% strain), good cyclic stability (over 2 × 104  cycles), and stable conductivity after high-speed rotation (6000 rpm). Simultaneously, the recording sites are hermetically sealed by ionic elastomer layers, ensuring the complete leakage-free property of EGaIn and reducing the electrochemical impedance of the electrodes (891.16 Ω at 1 kHz). The bioelectrode is successfully applied to monitor dynamic electromyographic signals. The sintering strategy overcomes the disadvantages of the traditional sintering strategies, such as leakage of EGaIn, reformation of large EGaIn droplets, and low throughput, which promotes the application of EGaIn in soft electronics.

Strategies for interface issues and challenges of neural electrodes.

Neural electrodes, as a bridge for bidirectional communication between the body and external devices, are crucial means for detecting and controlling nerve activity. The electrodes play a vital role in monitoring the state of neural systems or influencing it to treat disease or restore functions. To achieve high-resolution, safe and long-term stable nerve recording and stimulation, a neural electrode with excellent electrochemical performance (e.g., impedance, charge storage capacity, charge injection limit), and good biocompatibility and stability is required. Here, the charge transfer process in the tissues, the electrode-tissue interfaces and the electrode materials are discussed respectively. Subsequently, the latest research methods and strategies for improving the electrochemical performance and biocompatibility of neural electrodes are reviewed. Finally, the challenges in the development of neural electrodes are proposed. It is expected that the development of neural electrodes will offer new opportunities for the evolution of neural prosthesis, bioelectronic medicine, brain science, and so on.

Electrostatic Interaction-Based High Tissue Adhesive, Stretchable Microelectrode Arrays for the Electrophysiological Interface.

The drift or fall of stretchable neural microelectrodes from the surface of wet and dynamic tissues severely hampers the adoption of microelectrodes for electrophysiological signal monitoring. Endowing the stretchable electrodes with adhesive ability is an effective way to overcome these problems. Current adhesives form tough adhesion to tissues by covalent interaction, which decreases the biocompatibility of the adhesives. Here, we fabricate a strong electrostatic adhesive (noncovalent interaction), highly conformal, stretchable microelectrode arrays (MEAs) for the electrophysiological interface. This MEA was composed of polypyrrole (PPy) as the electrode material and hydrogel as the stretchable substrate [the cross-linked and copolymerized hydrogel of 2-acrylamido-2-methylpropane sulfonic acid (AMPS), gelatin, chitosan, 2-methoxyethyl acrylate, and acrylic acid is named PAGMA]. Strong and stable electrostatic adhesion (85 kPa) and high stretchability (100%) allow for the integration of PPy MEAs based on the PAGMA hydrogel substrate (PPy-PAGMA MEAs) on diverse wet dynamic tissues. Additionally, by adjusting the concentration of AMPS in PAGMA, the hydrogel (PAGMA-1) can produce tough adhesion to many inorganic and elastomer materials. Finally, the PPy-PAGMA MEAs were toughly and conformally adhered on the rat's subcutaneous muscle and beating heart, and the rat's electrophysiological signals were successfully recorded. The development of these adhesive MEAs offers a promising strategy to establish stable and compliant electrode-tissue interfaces.

Amperometric sensor based on ZIF/g-C3N4/RGO heterojunction nanocomposite for hydrazine detection.

A dense  zeolitic imidazolate framework (ZIF) nanosheet is for the first time molded by reduced graphite oxide (RGO) and graphitic carbon nitride (g-C3N4) to fabricate an original 2D/2D/2D heterojunction (ZIF/g-C3N4/RGO nanohybrid), which is pipetted onto carbon cloth electrode (CCE) (ZIF/g-C3N4/RGO/CCE) as an electrochemical sensor. Profiting from the renowned synergistic and coupling effects, the resulting nanohybrid endows excellent electrocatalytic activity towards hydrazine. Amperometric detection reveals that the hybrid sensor possesses a low detection limit of 32 nM (S/N = 3) in a monitoring range of 0.0001 to 1.0386 mM, along with a high sensitivity 93.71 µA mM-1 cm-2. Importantly, the minimum detection concentration of hydrazine in the actual sample is lower than the maximum allowable limit of the World Health Organization (WHO) and has high reproducibility (RSD = 4.82%). As expected, the high sensing capability  of ZIF/g-C3N4/RGO combines the advantages of abundant surface-active sites and high conductivity along with 2D interfaces between ZIF, g-C3N4, and RGO nanosheets. This study provides a promising to expand 2D-based ternary nanojunction as a bridge for promoting sensing performance.Graphical abstract.

A redox-active covalent organic framework for the efficient detection and removal of hydrazine.

The removal and detection of soluble hydrazine is of importance due to its harm to soil and subterranean water, but challenging. Herein, we preferentially disposed a porous and redox active covalent-organic framework (DAAQ-TFP COF, denoted as DQ-COF) to simultaneously removal and detect hydrazine. Electroactive sites (anthraquinone units) can be intelligently incorporated into the channel walls/pores of COF. DQ-COF has high crystallinity and good thermal stability, and DQ-COF dropped onto nickel matrix (DQ-COF/Ni composite) still retains high surface area, characterized by PXRD, FT-IR, nitrogen adsorption and TGA. Subsequently, a detailed study of DQ-COF towards hydrazine uptake and detection potentials is explored. DQ-COF as adsorbent unfolds strong removal ability towards hydrazine, the maximum removal capacity of which is up to 1108 mg g-1, following Friedrich and pseudo-second-order kinetic models. Meanwhile, the DQ-COF supported on nickel renders attractive electrochemical properties, which is efficiently responsive to hydrazine at a part per billion (ppb) level, coupled with a wide linear range (0.5 ˜ 1223 µM), low detection limit (0.07 µM) and high anti-interference ability. There is no other COFs with such a favorable capability in synchronous removal and selective detection towards hydrazine, probably applying in superintending water quality and disposing wastewater.

The incorporation of bismuth(III) into metal-organic frameworks for electrochemical detection of trace cadmium(II) and lead(II).

The first example of metallic bismuth encapsulated into a mesoporous metal-organic framework of the type MIL-101(Cr) matrix is presented. Bi(III)-impregnated MIL-101(Cr) (Bi(III)/MIL-101(Cr)) was dropped onto a conductive carbon cloth electrode (CCE). Then, bismuth was generated by electrochemical reduction of the Bi(III)/MIL-101(Cr) supported on CCE (Bi/MIL-101(Cr)/CCE). The resulting Bi/MIL-101(Cr)/CCE display impressive performance in terms of peak currents for the ions Cd(II) and Pb(II) when compared to the single-component counterparts. Differential pulse anodic stripping voltammetry (DPASV) enabled sensing of the two ions over linear working range of 0.1 to 30 µg L-1 and 30 to 90 µg L-1. The parameters are refined before the detection of two metal ions, including the amount of bismuth in MIL-101(Cr), optimum pH (5.0), deposition potential (-1.2 V) and deposition time (600 s). The respective detection limits are 60 and 70 ng L-1 (at S/N = 3). This is strikingly lower than the guideline values of domestic water given by the WHO which are 3 µg L-1 for Cd(II) and 10 µg L-1 for Pb(II). The Bi/MIL-101(Cr) onto CCE is fairly specific for Cd(II) (at around -0.76 V) and Pb(II) (at around -0.54 V), well reproducible and has excellent recovery in real water analysis. Graphical abstract Schematic illustration of the preparation of a Bi(III)/MIL-101(Cr) metal-organic framework, its deposition on a carbon cloth electrode (CCE), and its application for detection of Cd(II) and Pb(II) by differential pulse adsorptive stripping voltammetry (DPASV).