High-strength and highly electrically conductive hydrogels for wearable strain sensor

Recent Advances in the 3D Printing of Conductive Hydrogels for Sensor Applications: A Review

Conductive hydrogels, known for their flexibility, biocompatibility, and conductivity, have found extensive applications in fields such as healthcare, environmental monitoring, and soft robotics. Recent advancements in 3D printing technologies have transformed the fabrication of conductive hydrogels, creating new opportunities for sensing applications. This review provides a comprehensive overview of the advancements in the fabrication and application of 3D-printed conductive hydrogel sensors. First, the basic principles and fabrication techniques of conductive hydrogels are briefly reviewed. We then explore various 3D printing methods for conductive hydrogels, discussing their respective strengths and limitations. The review also summarizes the applications of 3D-printed conductive hydrogel-based sensors. In addition, perspectives on 3D-printed conductive hydrogel sensors are highlighted. This review aims to equip researchers and engineers with insights into the current landscape of 3D-printed conductive hydrogel sensors and to inspire future innovations in this promising field.

Hydrogel Wound Dressings Accelerating Healing Process of Wounds in Movable Parts

Skin is the largest organ in the human body and requires proper dressing to facilitate healing after an injury. Wounds on movable parts, such as the elbow, knee, wrist, and neck, usually undergo delayed and inefficient healing due to frequent movements. To better accommodate movable wounds, a variety of functional hydrogels have been successfully developed and used as flexible wound dressings. On the one hand, the mechanical properties, such as adhesion, stretchability, and self-healing, make these hydrogels suitable for mobile wounds and promote the healing process; on the other hand, the bioactivities, such as antibacterial and antioxidant performance, could further accelerate the wound healing process. In this review, we focus on the recent advances in hydrogel-based movable wound dressings and propose the challenges and perspectives of such dressings.

Low hysteresis zwitterionic supramolecular polymer ion-conductive elastomers with anti-freezing properties, high stretchability, and self-adhesion for flexible electronic devices

The fabrication of stretchable ionic conductors with low hysteresis and anti-freezing properties to enhance the durability and reliability of flexible electronics even at low temperatures remains an unmet challenge. Here, we report a facile strategy to fabricate low hysteresis, high stretchability, self-adhesion and anti-freezing zwitterionic supramolecular polymer ion-conductive elastomers (ICEs) by photoinitiated polymerization of aqueous precursor solutions containing a newly designed zwitterionic monomer carboxybetaine ureido acrylate (CBUIA) followed by solvent evaporation. The resultant poly(carboxybetaine ureido acrylate) (PCBUIA) ICEs are highly stretchable and self-adhesive owing to the presence of strong hydrogen bonds between ureido groups and dipole-dipole interactions of zwitterions. The zwitterion groups on the polymer side chains and loaded-lithium chloride endow PCBUIA ICEs with excellent anti-freezing properties, demonstrating mechanical flexibility and ionic transport properties even at a low temperature (-20 °C). Remarkably, the PCBUIA ICEs demonstrate a low hysteresis (≈10%) during cyclic mechanical loading-unloading (≤500%), and are successfully applied as wearable strain sensors and triboelectric nanogenerators (TENGs) for energy harvesting and human motion monitoring. In addition, the PCBUIA ICE-based TENG was used as a wireless sensing terminal for Internet of Things smart devices to enable wireless sensing of finger motion state detection.

One-step of ionic liquid-assisted stabilization and dispersion: Exfoliated graphene and its applications in stimuli-responsive conductive hydrogels based on chitosan

Conductive hydrogels, as novel flexible biosensors, have demonstrated significant potential in areas such as soft robotics, electronic devices, and wearable technology. Graphene is a promising conductive material, but its dispersibility in aqueous solutions exists difficulties. Here, we discover that untreated graphene, after exfoliation by different ionic liquids, can disperse well in aqueous solutions. We investigate the impact of four ionic liquids with varying alkyl chain lengths ([Bmim]Cl, [Omim]Cl, [Dmim]Cl, [Hmim]Cl) on the dispersibility of grapheme, and a dual physically cross-linked network hydrogel structure is designed using acrylamide (AM), acrylic acid (AA), methyl methacrylate octadecyl ester (SMA), ionic liquid@graphene (ILs@GN), and chitosan (CS). Notably, SMA, CS, AA and AM act as dynamic cross-linking points through hydrophobic interactions and hydrogen bonding, playing a crucial role in energy dissipation. The resulting hydrogel exhibits outstanding stretchability (2250 %), remarkable toughness (1.53 MJ/m3) in tensile deformation performance, high compressive strength (1.13 MPa), rapid electrical responsiveness (response time ∼ 50 ms), high electrical conductivity (12.11 mS/cm), and excellent strain sensing capability (GF = 12.31, strain = 1000 %). These advantages make our composite hydrogel demonstrate high stability in extensive deformations, offering repeatability in pressure and strain and making it a promising candidate for multifunctional sensors and flexible electrodes.

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.

Tough and Strain-Sensitive Organohydrogels Based on MXene and PEDOT/PSS and Their Effects on Mechanical Properties and Strain-Sensing Performance

Conductive hydrogels have shown promising application prospects in the field of flexible sensors, but they often suffer from poor mechanical properties, low sensitivity, and lack of frost resistance. Herein, we report a tough, highly sensitive, and antifreeze strain sensor assembled from a conductive organohydrogel composed of a dual-cross-linked polyacrylamide and poly(vinyl alcohol) (PVA) network, as well as MXene nanosheets as nanofillers and poly(3,4-ethylenedioxythiophene)-doped poly(styrenesulfonate) (PEDOT/PSS) as the main conducting component (PPMP-OH organohydrogel). The tensile strength and toughness of PPMP-OH had been greatly enhanced by MXene nanosheets due to the mechanical reinforcement of MXene nanosheets, as well as various strong noncovalent interactions formed in the organohydrogels. The PPM1P-OH organohydrogels showed a tensile strength of 1.48 MPa at 772% and a toughness of 5.59 MJ/m3. Moreover, the conductivity and strain-sensing performance of PPMP-OH were significantly improved by PEDOT/PSS, which can form hydrogen bonds with PVA and electrostatic interactions with MXene. This was greatly beneficial for constructing a uniformly distributed and stable 3D conductive network and helped to obtain strain-dependent resistance of PPMP-OH. The strain sensors assembled from PPMP1-OH exhibited a high sensitivity of 5.16, a wide range of detectable strains up to 500%, and a short response time of 122 ms, which can effectively detect various physiological activities of the human body with high stability. In addition, the corresponding pressure sensor array also showed high sensitivity in identifying pressure magnitude and position.

Multifunctioning of carboxylic-cellulose nanocrystals on the reinforcement of compressive strength and conductivity for acrylic-based hydrogel

Simultaneously having competitive compressive properties, fatigue-resistant stability, excellent conductivity and sensitivity has still remained a challenge for acrylic-based conductive hydrogels, which is critical in their use in the sensor areas where pressure is performed. In this work, an integrated strategy was proposed for preparing a conductive hydrogel based on acrylic acid (AA) and sodium alginate (SA) by addition of carboxylic-cellulose nanocrystals (CNC-COOH) followed by metal ion interaction to reinforce its compressive strength and conductivity simultaneously. The CNC-COOH played a multifunctional role in the hydrogel by well-dispersing SA and AA in the hydrogel precursor solution for forming a uniform semi-interpenetrating network, providing more hydrogen bonds with SA and AA, more -COOH for metal ion interactions to form uniform multi-network, and also offering high modulus to the final hydrogel. Accordingly, the as-prepared hydrogels showed simultaneous excellent compressive strength (up to 3.02 MPa at a strain of 70 %) and electrical conductivity (6.25 S m-1), good compressive fatigue-resistant (93.2 % strength retention after 1000 compressive cycles under 50 % strain) and high sensitivity (gauge factor up to 14.75). The hydrogel strain sensor designed in this work is capable of detecting human body movement of pressing, stretching and bending with highly sensitive conductive signals, which endows it great potential for multi-scenario strain sensing applications.

Preparation of tough and stiff ionogels via phase separation

Ionogels have the advantages of thermal stability, non-volatility, ionic conductivity and environmental friendliness, and they can be used in the field of flexible electronics and soft robotics. However, their poor mechanical strength and complex preparation methods limit their practical application. Herein, we propose a simple strategy to improve the performance of ionogels by adjusting their phase separation behavior. In a polymer-ionic liquid (IL) binary system with an upper critical solution temperature (UCST) and Berghmans' point, the phase separation behavior will be frozen below the temperature corresponding to the Berghmans' point, and thus, the degree of phase separation can be adjusted by controlling the cooling rate. We found that a polyacrylamide (PAM)-IL binary system possessed a UCST and Berghmans' point and the resulting ionogels had excellent mechanical properties. Their tensile strength, tensile modulus, compressive strength and compressive modulus reached 31.1 MPa, 319.8 MPa, 122 MPa and 1.7 GPa, respectively, while these properties of the other ionogels were generally less than 10 MPa. Furthermore, they were highly transparent, stretchable, stable and multifunctional.

Ultrastretchable and highly conductive hydrogels based on Fe3+- lignin nanoparticles for subzero wearable strain sensor

Conductive hydrogels have attracted considerable interest for potential applications in soft robotics, electronic skin and human monitoring. However, insufficient mechanical characteristics, low adhesion and unsatisfactory electrical conductivity severely restrict future application possibilities of hydrogels. Herein, lignin nanoparticles (LNPs)-Fe3+-ammonium persulfate (APS) catalytic system was introduced to assemble Poly(2-hydroxyethyl methacrylate)/LNPs/Ca2+ (PHEMA/LNPs/Ca) hydrogels. Due to the abundant metal coordination and hydrogen bonds, the composite hydrogel displayed ultrahigh stretchable capacity (3769 %), adhesion properties (248 kPa for skin) and self-healing performance. Importantly, hydrogel sensors possess with high durability, strain sensitivity (GF = 8.75), fast response time and freeze resistance (-20 °C) that could be employed to monitor motion signals in low-temperature regime. Therefore, the LNPs-Fe3+ catalytic system has great potential in preparing hydrogel for various applications such as human-computer interaction, artificial intelligence, personal healthcare and subzero wearable devices. At the same time, incorporation of natural macromolecules into polymer hydrogels is tremendous research significance for investigating high-value utilization of lignin.

High-Performing Conductive Hydrogels for Wearable Applications

Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human-machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity.

Lignin-containing hydrogels with anti-freezing, excellent water retention and super-flexibility for sensor and supercapacitor applications

We developed a highly conductive and flexible, anti-freezing sulfonated lignin (SL)-containing polyacrylic acid (PAA) (SL-g-PAA-Ni) hydrogel, with a high concentration of NiCl₂. Ni²⁺ contributes multi-functions to the preparation of the hydrogel and its final properties, such as fast polymerization reaction as a result of the presence of redox pairs of Ni³⁺/Ni²⁺ and hydroquinone/quinone, and anti-freezing properties of the hydrogel due to the salt effects of NiCl₂ so that at -20 °C the hydrogel shows similar properties to those at the room temperature. Thanks to the effective coordinations of Ni²⁺ with catecholic groups and carboxylic groups, as well as the rich hydrogen bonding capacity, the resultant hydrogel possesses excellent mechanical properties. High ionic conductivity (6.85 S·m^-1) of the hydrogel is obtained due to the supply of high concentration of Ni²⁺. Moreover, the ionic solvation effect of NiCl₂ in the hydrogel imparts excellent water retention ability, with water retention of ~93 % after 21-day storage. The SL-g-PAA-Ni hydrogel can accurately detect various human motions at -20 °C. The supercapacitor assembled from SL-g-PAA-Al hydrogel at -20 °C manifests a high specific capacitance of 252 F·g^-1, with maximum energy density of 26.97 Wh·kg^-1, power density of 2667 W·kg^-1, and capacitance retention of 96.7 % after 3000 consecutive charge-discharge cycles.

Recent Advances in Design Strategies of Tough Hydrogels

Hydrogels are a fascinating class of materials popular in numerous fields, including tissue engineering, drug delivery, soft robotics, and sensors, attributed to their 3D network porous structure containing a significant amount of water. However, traditional hydrogels exhibit poor mechanical strength, limiting their practical applications. Thus, many researchers have focused on the development of mechanically enhanced hydrogels. This review describes the design considerations for constructing tough hydrogels and some of the latest strategies in recent years. These tough hydrogels have an up-and-coming prospect and bring great hope to the fields of biomedicine and others. Nonetheless, it is still no small challenge to realize hydrogel materials that are tough, multifunctional, intelligent, and zero-defect. This article is protected by copyright. All rights reserved.

Graphene oxide-based composite organohydrogels with high strength and low temperature resistance for strain sensors

In recent years, a rapid development of polymeric hydrogel-based sensors has been witnessed. However, conventional hydrogels often exhibit poor mechanical properties. Additionally, the use of these sensors at temperatures <0 °C is limited due to the freezing of the water molecules in the hydrogel matrix. In this study, graphene oxide/poly(acrylamide-co-N-(3-amino propyl)methacrylamide) [poly(AAm-co-APMA)/GO] hydrogels have been synthesized by UV photo-initiation polymerization. Subsequently, the poly(AAm-co-APMA)/GO-Gly (PAAG-Gly) organohydrogels were obtained by glycerol replacement. GO and glycerol had multiple interactions with the polymer chains, which endowed the physically crosslinked organohydrogel with a high fracture stress of up to 782.9 ± 38.6 kPa. Also, the glycerol molecules formed hydrogen bonds with the water molecules, thus inhibiting the formation of ice crystals. After storage at -20 °C for 24 h, the PAAG-Gly organohydrogels retained their superior mechanical properties, adhesion strength, and electrical conductivity. Once the cut surfaces of the organohydrogel were contacted, the conductive path was rapidly self-healed. Moreover, the PAAG-Gly organohydrogels exhibited excellent cytocompatibility. At 100% strain, the gauge factor of the organohydrogel-based sensor reached 4.22. The organohydrogel-based sensor revealed the capability to monitor human motions, such as finger, wrist and knee movements.

Self-healable nanocellulose composite hydrogels combining multiple dynamic bonds for drug delivery

Herein, we developed two nanocomposite polysaccharide hydrogels TPP-CNC and TPP-CNF via simple mixing method, which were constructed with multiple dynamic bonds. The microstructural features, mechanical properties, rheological properties, healable ability and biocompatibility of the complex hydrogels were evaluated. The TPP-CNC and TPP-CNF complex hydrogels exhibited higher tensile strength than pure polysaccharide hydrogel, from ~259 KPa to ~890 KPa and ~910 KPa, respectively, that was attributed to the contribution of ionic crosslinked network and hydrogen bonds. In addition, the hydrogels indicated superior fatigue resistance and high energy dissipation ratio during loading-unloading tests because of the physical sacrifice bonds, which also decreased the self-healing time at room temperature (~15 min). More importantly, the drug loaded nanocomposite hydrogels showed sustained release, reduction burst release, increased release under acidic environment, and the drug release kinetics belonged to Fickian diffusion mechanism. Therefore, the nanocellulose polysaccharide hydrogels have the highly promising to explore as biomaterials for drug delivery.

Design of Fe3+-Rich, High-Conductivity Lignin Hydrogels for Supercapacitor and Sensor Applications

Preparation of natural polymer-based highly conductive hydrogels with tunable mechanical properties for applications in flexible electronics is still challenging. Herein, we report a facile method to prepare lignin-based Fe³⁺-rich, high-conductivity hydrogels via the following two-step process: (1) lignin hydrogels are prepared by cross-linking sulfonated lignin with poly(ethylene glycol) diglycidyl ether (PEGDGE) and (2) Fe³⁺ ions are impregnated into the lignin hydrogel by simply soaking in FeCl₃. Benefiting from Fe³⁺ ion complexation with catechol groups and other functional groups in lignin, the resultant hydrogels exhibit unique properties, such as high conductivity (as high as 6.69 S·m^-1) and excellent mechanical and hydrophobic properties. As a strain sensor, the as-prepared lignin hydrogel shows high sensitivity when detecting various human motions. With the flow of moist air, the Fe³⁺-rich lignin hydrogel generates an output voltage of 162.8 mV. The assembled supercapacitor of the hydrogel electrolyte demonstrates a high specific capacitance of 301.8 F·g^-1, with a maximum energy density of 26.73 Wh·kg^-1, a power density of 2.38 kW·kg^-1, and a capacitance retention of 94.1% after 10 000 consecutive charge-discharge cycles. These results support the conclusion that lignin-based Fe³⁺-rich, high-conductivity hydrogels have promising applications in different fields, including sensors and supercapacitors, rendering a new platform for the value-added utilization of lignin.

Graphene oxide composite hydrogels for wearable devices

For graphene oxide (GO) composite hydrogels, a two-dimensional GO material is introduced into them, whose special structure is used to improve their properties. GO contains abundant oxygen-containing functional groups, which can improve the mechanical properties of hydrogels and support the application needs. Especially, the unique-conjugated structure of GO can endow or enhance the stimulation response of hydrogels. Therefore, GO composite hydrogels have a great potential in the field of wearable devices. We referred to the works published in recent years, and reviewed from these aspects: (a) structure of GO; (b) factors affecting the mechanical properties of the composite hydrogel, including hydrogen bond, ionic bond, coordination bond and physical crosslinking; (c) stimuli and signals; (d) challenges. Finally, we summarized the research progress of GO composite hydrogels in the field of wearable devices, and put forward some prospects.

Graphite-Based Bioinspired Piezoresistive Soft Strain Sensors with Performance Optimized for Low Strain Values

This paper presents the custom-made graphite-based piezoresistive strain sensor with gecko foot-inspired macroscopic features realized using a Velcro tape on Ecoflex substrate. The Velcro-based design provides an inexpensive and easy approach for the development of soft sensors with appreciable improvement in the performance even at low strain values. The sensor demonstrated excellent response (sensitivity of ∼16 500%, gauge factor of ∼3800) for 24% linear strain. The fabricated device showed a high gauge factor (>100) even for very low strain values. The sensor has been extensively characterized with a view to potentially use in soft robotics applications where high performance is needed at lower strain values. It is observed that the piezoresistive behavior of strain sensors is governed by several factors such as the supporting elastic medium, architecture of the strain sensor, material properties, strain rate and deformation sequence, and direction.

Recent progress in the fabrication of flexible materials for wearable sensors

Wearable sensors have been widely studied because of their small size, light weight, and potential for the noninvasive tracking and monitoring of human physiological information. Wearable flexible sensors generally consist of two parts: a flexible substrate in contact with the skin and a signal processing module. At present, wearable electronics cover many fields, such as machinery, physics, chemistry, materials science, and biomedicine. The design concept and selection of materials are very important to the function of a sensor. In this review, we summarize the latest developments in flexible materials for wearable sensors, including developments in flexible materials, electrode materials, and new flexible biodegradable materials, and describe the important role of innovation in material and sensor design in the development of wearable flexible sensors. Strategies and challenges related to the improvement of the performances of wearable flexible sensors, as well as the development prospects of wearable devices based on flexible materials, are also discussed.

Mussel-inspired hydrogels as tough, self-adhesive and conductive bioelectronics: a review

To overcome the wearable sensor's defects and achieve the goal of robust mechanical properties, long-term adhesion, sensitive electrical conductivity, the multifunctional hydrogels were inspired by various mussels on the base of catechol and its analogues. In this review, we review the strategies for improving the mechanical strength, adhesion, conductivity and antibacterial properties of mussel-inspired hydrogels as bioelectronics. Double network structures, nanocomposites, supramolecular block polymers and other strategies were utilized for achieving tough hydrogels to prevent tensile fractures under high deformation. Many mussel-inspired chemistries were incorporated for constructing skin-attachable hydrogel strain sensors and some strategies for controlling the oxidation of catechol were employed to achieve long-term adhesion. In addition, electrolytes, conductive fillers, conductive polymers and their relevant hydrophilic modifications were introduced for fabricating the conductive hydrogel bioelectronics to enhance the conductivity properties. Finally, the challenges and outlooks in this promising field are featured from the perspective of materials chemistry.

Crosslinking Trends in Multicomponent Hydrogels for Biomedical Applications

Multicomponent-based hydrogels are well established candidates for biomedical applications. However, certain aspects of multicomponent systems, e.g., crosslinking, structural binding, network formation, proteins/drug incorporation, etc., are challenging aspects to modern biomedical research. The types of crosslinking and network formation are crucial for the effective combination of multiple component systems. The creation of a complex system in the overall structure and the crosslinking efficiency of different polymeric chains in an organized fashion are crucially important, especially when the materials are for biomedical applications. Therefore, the engineering of hydrogel has to be, succinctly understood, carefully formulated, and expertly designed. The different crosslinking methods in use, hydrogen bonding, electrostatic interaction, coordination bonding, and self-assembly. The formations of double, triple, and multiple networks, are well established. A systematic study of the crosslinking mechanisms in multicomponent systems, in terms of the crosslinking types, network formation, intramolecular bonds between different structural units, and their potentials for biomedical applications, is lacking and therefore, these aspects require investigations. To this end, the present review, focuses on the recent advances in areas of the physical, chemical, and enzymatic crosslinking methods that are often, employed for the designing of multicomponent hydrogels.

Alkyl Chain Length Effects of Imidazolium Ionic Liquids on Electrical and Mechanical Performances of Polyacrylamide/Alginate-Based Hydrogels

Conductive hydrogels with stretchable, flexible and wearable properties have made significant contributions in the area of modern electronics. The polyacrylamide/alginate hydrogels are one of the potential emerging materials for application in a diverse range of fields because of their high stretch and toughness. However, most researchers focus on the investigation of their mechanical and swelling behaviors, and the adhesion and effects of the ionic liquids on the conductivities of polyacrylamide/alginate hydrogels are much less explored. Herein, methacrylated lysine and different alkyl chain substituted imidazole-based monomers (IMCx, x = 2, 4, 6 and 8) were introduced to prepare a series of novel pAMAL-IMCx-Ca hydrogels. We systematically investigated their macroscopic and microscopic properties through tensile tests, electrochemical impedance spectra and scanning electron microscopy, as well as Fourier transform infrared spectroscopy, and demonstrated that an alkyl chain length of the IMCx plays an important role in the designing of hydrogel strain sensors. The experiment result shows that the hexyl chains of IMC6 can effectively entangle with LysMA through hydrophobic and electrostatic interactions, which significantly enhance the mechanical strength of the hydrogels. Furthermore, the different strain rates and the durability of the pAMAL-IMC6-Ca hydrogel were investigated and the relative resistance responses remain almost the same in both conditions, making it a potential candidate for wearable strain sensors.