Liquid Metal-Doped Conductive Hydrogel for Construction of Multifunctional Sensors

Low hysteresis, water retention, anti-freeze multifunctional hydrogel strain sensor for human-machine interfacing and real-time sign language translation

Hydrogel strain sensors have received increasing attention due to their potential applications in human-machine interfaces and flexible electronics. However, they usually suffer from both mechanical and electrical hysteresis and poor water retention, which limit their practical applications. To address this challenge, a poly(acrylic acid-co-acrylamide) hydrogel crosslinked by silica nanoparticles is fabricated via photo polymerization and salting-out of hydrophilic ions for the strain sensor. The resulting hydrogel strain sensor possessed low electrical hysteresis (1.6%), low mechanical hysteresis (<7%), high cycle stability (>10 000 cycles), high durability, water retention and anti-freezing ability. Moreover, this strain sensor can be used as a wearable sensor for real-time control of robotic hands and hand gesture recognition. Finally, a sign language translation system has been demonstrated with the aid of machine learning, achieving recognition rates of over 98% for 15 different sign languages. This work offers a promising prospect for human-machine interfaces, smart wearable devices, and the Internet of Things.

Overview of Dynamic Bond Based Hydrogels for Reversible Adhesion Processes

Polymeric hydrogels are soft materials with a three-dimensional (3D) hydrophilic network capable of retaining and absorbing large amounts of water or biological fluids. Due to their customizable properties, these materials are extensively studied for developing matrices for 3D cell culture scaffolds, drug delivery systems, and tissue engineering. However, conventional hydrogels still exhibit many drawbacks; thus, significant efforts have been directed towards developing dynamic hydrogels that draw inspiration from organisms' natural self-repair abilities after injury. The self-healing properties of these hydrogels are closely associated with their ability to form, break, and heal dynamic bonds in response to various stimuli. The primary objective of this review is to provide a comprehensive overview of dynamic hydrogels by examining the types of chemical bonds associated with them and the biopolymers utilized, and to elucidate the chemical nature of dynamic bonds that enable the modulation of hydrogels' properties. While dynamic bonds ensure the self-healing behavior of hydrogels, they do not inherently confer adhesive properties. Therefore, we also highlight emerging approaches that enable dynamic hydrogels to acquire adhesive properties.

Robust and Lubricating Interface Semi-Interpenetrating Network on Inert Polymer Substrates Enabled by Subsurface-Initiated Polymerization

Lubricating hydrogel coatings on inert rubber and plastic surfaces significantly reduce friction and wear, thus enhancing material durability and lifespan. However, achieving optimal hydration lubrication typically requires a porous polymer network, which unfortunately reduces their mechanical strength and limits their applicability where robust durability and wear-resistance are essential. In the research, a hydrogel coating with remarkable wear resistance and surface stability is developed by forming a semi-interpenetrating polymer network with polymer substrate at the interface. By employing a good solvent swelling method, monomers, and photoinitiators are embedded within the substrates' subsurface, followed by in situ polymerization under ultraviolet light, creating a robust semi-interpenetrating and entangled network structure. This approach, offering a thicker energy-dissipating layer, outperforms traditional surface modifications in wear resistance while preserving anti-fatigue, hydrophilicity, oleophobicity, and other properties. Adaptable to various rubber and plastic substrates by using suitable solvents, this method provides an efficient solution for creating durable, lubricating surfaces, broadening the potential applications in multiple industries.

A Review of Conductive Hydrogel-Based Wearable Temperature Sensors

Conductive hydrogel has garnered significant attention as an emergent candidate for diverse wearable sensors, owing to its remarkable and tailorable properties such as flexibility, biocompatibility, and strong electrical conductivity. These attributes make it highly suitable for various wearable sensor applications (e.g., biophysical, bioelectrical, and biochemical sensors) that can monitor human health conditions and provide timely interventions. Among these applications, conductive hydrogel-based wearable temperature sensors are especially important for healthcare and disease surveillance. This review aims to provide a comprehensive overview of conductive hydrogel-based wearable temperature sensors. First, this work summarizes different types of conductive fillers-based hydrogel, highlighting their recent developments and advantages as wearable temperature sensors. Next, this work discusses the sensing characteristics of conductive hydrogel-based wearable temperature sensors, focusing on sensitivity, dynamic stability, stretchability, and signal output. Then, state-of-the-art applications are introduced, ranging from body temperature detection and wound temperature detection to disease monitoring. Finally, this work identifies the remaining challenges and prospects facing this field. By addressing these challenges with potential solutions, this review hopes to shed some light on future research and innovations in this promising field.

Hydrophobic association-improved multi-functional hydrogels with liquid metal droplets stabilized by xanthan gum and PEDOT:PSS for strain sensors

The synthesis of liquid metal-infused hydrogels, typically constituted by polyacrylamide networks crosslinked through covalent bonds, often encounters a conundrum: they exhibit restricted extensibility and a diminished capacity for self-repair, owing to the inherently irreversible nature of the covalent linkages. This study introduces a hydrophobically associated hydrogel embedding gallium (Ga)-droplets, realized through the in situ free radical copolymerization of hydrophobic hexadecyl methacrylate (HMA) and hydrophilic acrylamide (AM) in a milieu containing xanthan gum (XG) and PEDOT:PSS, which co-stabilizes the Ga-droplets. The Ga-droplets, synergistically functioning as conductive agents alongside PEDOT:PSS, also expedite the hydrogel's formation. The resultant XG/PEDOT:PSS-Ga-P(AM-HMA) hydrogel is distinguished by its remarkable extensibility (2950 %), exceptional toughness (3.28 MJ/m3), superior adherence to hydrophobic, smooth substrates, and an innate ability for hydrophobic-driven self-healing. As a strain sensing medium, this hydrogel-based sensor exhibits heightened sensitivity (gauge factor = 12.66), low detection threshold (0.1 %), and robust durability (>500 cycles). Furthermore, the inclusion of glycerol endows the XG/PEDOT:PSS-Ga-P(AM-HMA) hydrogel with anti-freezing properties without compromising its mechanical integrity and sensing acumen. This sensor adeptly captures a spectrum of human movements, from the nuanced radial pulse to extensive joint articulations. This research heralds a novel approach for fabricating multifaceted PAM-based hydrogels with toughness and superior sensing capabilities.

Liquid Metal Nanoparticles Physically Hybridized with Cellulose Nanocrystals Initiate and Toughen Hydrogels with Piezoionic Properties

Liquid metal (LM) particles can serve as initiators, functional fillers, and cross-linkers for hydrogels. Herein, we show that cellulose nanocrystals (CNCs) stabilize LM particles in aqueous solutions, such as those used to produce hydrogels. The CNC-coated LM particles initiate free-radical polymerization to form poly(acrylic acid) (PAA) hydrogel with exceptional properties─stretchability ∼2000%, excellent toughness ∼1.8 MJ/m3, mechanical resilience, and efficient self-healing─relative to cross-linked PAA networks polymerized using conventional molecular initiators. FTIR spectroscopy, rheology, and mechanical measurements suggest that physical bonds between PAA and both Ga3+ and LM-CNC particles contribute to the excellent mechanical properties. The gels are used to sense a wide range of strains, such as those associated with human motion, via changes in resistance through the gel. The sensitivity at low strains enables monitoring subtle physiological signals, such as pulse. Without significantly compromising the toughness, soaking the gels in salt solution brings about high ionic conductivity (3.8 S/m), enabling them to detect touch via piezoionic principles; the anions in the gel have higher mobility than cations, resulting in significant charge separation (current ∼30 μA, ∼10 μA/cm2) through the gel in response to touch. These attractive properties are promising for wearable sensors, energy harvesters, and self-powered ionic touch panels.

Using chitosan nanofibers to synergistically construct a highly stretchable multi-functional liquid mental-based hydrogel for assembling strain sensor with high sensitivity and broad working range

Liquid metal (LM) microdroplets have garnered significant interest as conductive materials for initiating free radical polymerization in the development of conductive hydrogels suited for strain sensors. However, crafting multi-functional conductive hydrogels that boast both high stretchability and superior sensing capabilities remains as a challenge. In this study, we have successfully synthesized LM-based conductive hydrogels characterized by remarkable stretchability and sensing performance employing acrylic acid (AA) to evenly distribute chitosan nanofibers (CSFs) and to subsequently catalyze the free radical polymerization of AA. The resultant polymer network was crosslinked within situ polyacrylic acid (PAA), facilitated by Ga3+ in conjunction with guar gum (GG)-stabilized Ga droplets. The strategic interplay between the rigid, and protonated CSFs and the pliable PAA matrix, coupled with the ionic crosslinking of Ga3+, endows the resulting GG-Ga-CSF-PAA hydrogel with high stretchability (3700 %), ultrafast self-healing, robust moldability, and strong adhesiveness. When deployed as a strain sensing material, this hydrogel exhibits a high gauge factor (38.8), a minimal detection threshold, enduring durability, and a broad operational range. This versatility enables the hydrogel-based strain sensor to monitor a wide spectrum of human motions. Remarkably, the hydrogel maintains its stretchability and sensing efficacy under extreme temperatures after a simple glycerol solution treatment.

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.

Hydrogel-based treatments for spinal cord injuries

Spinal cord injury (SCI) is characterized by damage resulting in dysfunction of the spinal cord. Hydrogels are common biomaterials that play an important role in the treatment of SCI. Hydrogels are biocompatible, and some have electrical conductivity that are compatible with spinal cord tissues. Hydrogels have a high drug-carrying capacity, allowing them to be used for SCI treatment through the loading of various types of active substances, drugs, or cells. We first discuss the basic anatomy and physiology of the human spinal cord and briefly discuss SCI and its treatment. Then, we describe different treatment strategies for SCI. We further discuss the crosslinking methods and classification of hydrogels and detail hydrogel biomaterials prepared using different processing methods for the treatment of SCI. Finally, we analyze the future applications and limitations of hydrogels for SCI. The development of biomaterials opens up new possibilities and options for the treatment of SCI. Thus, our findings will inspire scholars in related fields and promote the development of hydrogel therapy for SCI.

Ultra-robust, high-adhesive, self-healing, and photothermal zwitterionic hydrogels for multi-sensory applications and solar-driven evaporation

Zwitterionic hydrogels have received considerable attention owing to their characteristic structures and integrating multifunctionality. However, the superhydrophilicity-induced poor mechanical properties severely hinder their potential applications. Besides, from the perspective of wide applications, zwitterionic hydrogels with integrated high mechanical properties, conductivity and multifunctionalities including self-adhesive, self-healing, and photothermal properties are highly desirable yet challenging. Herein, a new class of high-performance and multifunctional zwitterionic hydrogels are designed based on the incorporation of polydopamine-coated liquid metal nanoparticles (LM@PDA). Due to the efficient energy dissipation endowed by the isotropically extensible deformation of LM@PDA and the multiple interactions within the hydrogel matrix, the resultant hydrogels exhibited ultrahigh robustness with tensile strength of up to 1.3 MPa, strain of up to 1555% and toughness of up to 7.3 MJ m-3, superior or comparable to those of most zwitterionic hydrogels. The introduced LM@PDA also endows the hydrogels with high conductivity, versatile adhesion, autonomous self-healing, excellent injectability, three-dimensional printability, degradability, and photothermal conversion performance. These preferable properties enable the hydrogels promising as wearable sensors with multiple sensory capabilities for a wide range of strain values (1-500%), pressures (0.5-200 kPa) and temperatures (20-80 °C) with an impressive temperature coefficient of resistance (up to 0.15 °C-1). Moreover, these hydrogels can be also applied as solar evaporators with a high water evaporation rate (up to 2.42 kg m-2 h-1) and solar-thermal conversion efficiency (up to 90.3%) for solar desalination and wastewater purification. The present work can pave the way for the future development of zwitterionic hydrogels and beyond.

Stretchable Organohydrogel with Adhesion, Self-Healing, and Environment-Tolerance for Wearable Strain Sensors

Stretchable hydrogels as landmark soft materials have been efficiently utilized in the field of wearable sensing devices. However, these soft hydrogels mostly cannot integrate transparency, stretchability, adhesiveness, self-healing, and environmental adaptability into one system. Herein, a fully physically cross-linked poly(hydroxyethyl acrylamide)-gelatin dual-network organohydrogel is prepared in a phytic acid-glycerol binary solvent via a rapid ultraviolet light initiation. The introduction of gelatin as the second network endows the organohydrogel with desirable mechanical performance (high stretchability up to 1240%). The presence of phytic acid not only synergizes with glycerol to impart environment-tolerance to the organohydrogel (from -20 to 60 °C) but also increases the conductivity. Moreover, the organohydrogel demonstrates a durable adhesive performance toward diverse substrates, a high self-healing efficiency through heat treatment, and favorable optical transparency (transmittance of 90%). Furthermore, the organohydrogel achieves high sensitivity (gauge factor of 2.18 at 100% strain) and rapid response time (80 ms) and could detect both tiny (a low detection limit of 0.25% strain) and large deformations. Therefore, the assembled organohydrogel-based wearable sensors are capable of monitoring human joint motions, facial expression, and voice signals. This work proposes a facile route for multifunctional organohydrogel transducers and promises the practical application of flexible wearable electronics in complex scenarios.