Multifunctional conductive hydrogel-based flexible wearable sensors

Facile Preparation of a Self-Adhesive Conductive Hydrogel with Long-Term Usability

Although conductive hydrogels (CHs) have been investigated as the wearable sensor in recent years, how to prepare the multifunctional CHs with long-term usability is still a big challenge. In this paper, we successfully prepared a kind of conductive and self-adhesive hydrogel with a simple method, and its excellent ductility makes it possible as a flexible strain sensor for intelligent monitoring. The CHs are constructed by poly(vinyl alcohol) (PVA), polydopamine (PDA), and phytic acid (PA) through the freeze-thaw cycle method. The introduction of PA enhanced the intermolecular force with PVA and provided much H+ for augmented conductivity, while the catechol group on PDA endows the hydrogel with self-adhesion ability. The PVA/PA/PDA hydrogel can directly contact with the skin and adhere to it stably, which makes the hydrogel potentially a wearable strain sensor. The PVA/PA/PDA hydrogel can monitor human motion signals (including fingers, elbows, knees, etc.) in real-time and can accurately monitor tiny electrical signals for smile and handwriting recognition. Notably, the composite CHs can be used in a normal environment even after 4 months. Because of its excellent ductility, self-adhesiveness, and conductivity, the PVA/PA/PDA hydrogel provides a new idea for wearable bioelectronic sensors.

Multifunctional acetylated distarch phosphate based conducting hydrogel with high stretchability, ultralow hysteresis and fast response for wearable strain sensors

The rapid development of flexible sensors has greatly increased the demand for high-performance hydrogels. However, it remains a challenge to fabricate flexible hydrogel sensors with high stretching, low hysteresis, excellent adhesion, good conductivity, sensing characteristics and bacteriostatic function in a simple way. Herein, a highly conducting double network hydrogel is presented by incorporating lithium chloride (LiCl) into the hydrogel consisting of poly (2-acrylamide-2-methylpropanesulfonic acid/acrylamide/acrylic acid) (3A) network and acetylated distarch phosphate (ADSP). The addition of ADSP not only formed hydrogen bonds with 3A to improve the toughness of the hydrogel but also plays the role of "physical cross-linking" in 3A by "anchoring" the polymer molecular chains together. Tuning the composition of the hydrogel allows the attainment of the best functions, such as high stretchability (∼770 %), ultralow hysteresis (2.2 %, ε = 100 %), excellent electrical conductivity (2.9 S/m), strain sensitivity (GF = 3.0 at 200-500 % strain) and fast response (96 ms). Based on the above performance, the 3A/ADSP/LiCl hydrogel strain sensor can repeatedly and stably detect and monitor large-scale human movements and subtle sensing signals. In addition, the 3A/ADSP/LiCl hydrogel shows a good biocompatibility and bacteriostatic ability. This work provides an effective strategy for constructing the conductive hydrogels for wearable devices and flexible sensors.

Preparation and properties of lignin-based dual network hydrogel and its application in sensing

Ionic conductive hydrogels prepared from various biological macromolecules are ideal materials for the manufacture of human motion sensors from the perspective of resource regeneration and environmental sustainability. However, it is now difficult to develop conductive hydrogels including excellent self-healing and mechanical properties, mainly due to their inherent trade-off between dynamic cross-linked healing and stable cross-linked mechanical strength. In this work, alkali lignin-Polyvinyl alcohol-polyacrylic acid double network conductive hydrogels with high mechanical strength and good self-healing properties were prepared. We formed the primary network structure by hydrogen bonding interaction between polyvinyl alcohol, alkali lignin and polyacrylic acid, and the secondary network structure by coordination interaction with polyacrylic acid through the addition of Fe3+. The added lignin acts as a dynamic linkage bridge in a porous network mediated by multiple ligand bonds, imparting superior mechanical properties to the hydrogels. The relationships between the alkali lignin and iron ion dosage and the comprehensive properties of hydrogels (adhesion, antibacterial, self-healing, electrical conductivity and mechanical properties) were studied in detail. On this basis, the hydrogels explored the role of lignin in the regulation of hydrogels properties and revealed the self-healing and conductive mechanism.

Multifunctional Motion Sensing Enabled by Laser-Induced Graphene

The development of flexible sensors based on laser-induced graphene (LIG) has recently attracted much attention. It was commonly generated by laser-ablating commercial polyimide (PI). However, the weak mechanical extensibility of PI limits the development and diversified applications of LIG-based sensors. In this work, we adopted medical polyurethane (PU) tapes to peel off the LIG generated on PI and developed flexible and wearable sensors based on the proposed LIG/PU composite structure. Compared with other methods for LIG transfer, PU tape has many advantages, including a simplified process and being less time-consuming. We characterized the LIG samples generated under different laser powers and analyzed the property differences introduced by the transfer operation. We then studied the impact of fabrication mode on the strain sensitivity of the LIG/PU and optimized the design of a LIG/PU-based strain sensor, which possessed a gauge factor (GF) of up to 263.6 in the strain range of 75-90%. In addition, we designed a capacitive pressure sensor for tactile sensing, which is composed of two LIG/PU composite structures and a PI space layer. These LIG flexible devices can be used for human motion monitoring and tactile perception in sports events. This work provides a simple, fast, and low-cost way for the preparation of multifunctional sensor systems with good performance, which has a broad application prospect in human motion monitoring.

An effective DLP 3D printing strategy of high strength and toughness cellulose hydrogel towards strain sensing

Photocurable 3D printing technology has outperformed extrusion-based 3D printing technology in material adaptability, resolution, and printing rate, yet is still limited by the insecure preparation and selection of photoinitiators and thus less reported. In this work, we developed a printable hydrogel that can effectively facilitate various solid or hollow structures and even lattice structures. The chemical and physical dual-crosslinking strategy combined with cellulose nanofibers (CNF) significantly improved the strength and toughness of photocurable 3D printed hydrogels. In this study, the tensile breaking strength, Young's modulus, and toughness of poly(acrylamide-co-acrylic acid)_D/cellulose nanofiber (PAM-co-PAA)_D/CNF hydrogels were 375 %, 203 % and 544 % higher than those of the traditional single chemical crosslinked (PAM-co-PAA)_S hydrogels, respectively. Notably, its outstanding compressive elasticity enabled it to recover under 90 % strain compression (about 4.12 MPa). Resultantly, the proposed hydrogel can be utilized as a flexible strain sensor to monitor the motions of human movements, such as the bending of fingers, wrists, and arms, and even the vibration of a speaking throat. The output of electrical signals can still be collected through strain even under the condition of energy shortage. In addition, photocurable 3D printing technology can provide customized services for hydrogel-based e-skin, such as hydrogel-based bracelets, fingerstall, and finger joint sleeves.

3D printable, injectable amyloid-based composite hydrogel of bovine serum albumin and aloe vera for rapid diabetic wound healing

Protein-based biomaterials, particularly amyloids, have sparked considerable scientific interest in recent years due to their exceptional mechanical strength, excellent biocompatibility and bioactivity. In this work, we have synthesized a novel amyloid-based composite hydrogel consisting of bovine serum albumin (BSA) and aloe vera (AV) gel to utilize the medicinal properties of the AV gel and circumvent its mechanical frangibility. The synthesized composite hydrogel demonstrated an excellent porous structure, self-fluorescence, non-toxicity, and controlled rheological properties. Moreover, this hydrogel possesses inherent antioxidant and antibacterial properties, which accelerate the rapid healing of wounds. The in vitro wound healing capabilities of the synthesized composite hydrogel were evaluated using 3T3 fibroblast cells. Moreover, the efficacy of the hydrogel in accelerating chronic wound healing via collagen crosslinking was investigated through in vivo experiments using a diabetic mouse skin model. The findings indicate that the composite hydrogel, when applied, promotes wound healing by inducing collagen deposition and upregulating the expression of vascular endothelial growth factor (VEGF) and its receptors. We also demonstrate the feasibility of the 3D printing of the BSA-AV hydrogel, which can be tailored to treat various types of wound. The 3D printed hydrogel exhibits excellent shape fidelity and mechanical properties that can be utilized for personalized treatment and rapid chronic wound healing. Taken together, the BSA-AV hydrogel has great potential as a bio-ink in tissue engineering as a dermal substitute for customizable skin regeneration.

The design of highly conductive and stretchable polymer conductors with low-load nanoparticles

Highly conductive and stretchable polymer conductors fabricated from conductive fillers and stretchable polymers are urgently needed in flexible electronics, implants, soft robotics, etc. However, polymer conductors encounter the conductivity-stretchability dilemma, in which high-load fillers needed for high conductivity always result in the stiffness of materials. Herein, we propose a new design of highly conductive and stretchable polymer conductors with low-load nanoparticles (NPs). The design is achieved by the self-assembly of surface-modified NPs to efficiently form robust conductive pathways. We employ computer simulations to elucidate the self-assembly of the NPs in the polymer matrices under equilibrium and tensile states. The conductive pathways retain 100% percolation probability even though the loading of the NPs is lowered to ∼2% volume. When the tensile strain reaches 400%, the percolation probability of the ∼2% NP system is still greater than 25%. The theoretical prediction suggests a way for advancing flexible conductive materials.

Guar gum reinforced conductive hydrogel for strain sensing and electronic devices

Hydrophobically associated conductive hydrogels got great attention due to their excellent properties like stretchability, energy dissipation mechanism, and strain sensor. But hydrophobically associated hydrogels have poor mechanical properties and time response to external stimuli. To enhance the mechanical properties and response to stimuli, Acrylamide- co-Butyl acrylate/Gum based conductive hydrogels were prepared. SDS works as a cross-linker and micelle-forming agent while NaCl makes hydrogel as conductive. The results show that our % strain sensing reached up to 400 %, and fracture stress and fracture strain reached to 0.5 MPa and 401 % respectively. Besides this, it's having an excellent response to continuous stretching and unstretching multiple cycles without any fracture up to 180 s and an excellent time response of 190 s. The conductivity of the hydrogel was 0.20 Sm-1. The hydrophobic hydrogels showed a clear and quick response to human motions like finger, wresting, writing, speaking, etc. Interestingly, our prepared hydrogels can detect the mood of the human face. Similarly, the hydrogels were found efficient in bridging the surface of electronic devices with human skin. This indicates that our prepared hydrogels can monitor human body motion and will replace the existing materials used in strain sensors in the near future.

Mussel-inspired cellulose nanofiber/poly(vinyl alcohol) hydrogels with robustness, self-adhesion and antimicrobial activity for strain sensors

Flexible strain sensors have attracted substantial attention given their application in human-computer interaction and personal health monitoring. Due to the inherent disadvantages of conventional hydrogels, the manufacture of hydrogel strain sensors with high tensile strength, excellent adhesion, self-healing and antimicrobial properties in vitro, and conductive stability is still a challenge. Herein, a conductive hydrogel consisting of polydopamine-coated cellulose nanofibers (CNF@PDA), carbon nanotubes (CNT), and polyvinyl alcohol (PVA) was developed. The CNTs in PVA/CNF@PDA/CNT hydrogels were uniformly dispersed in the presence of CNF@PDA by hydrogen bonding, resulting in a nearly threefold increase in conductivity (0.4 S/m) over hydrogels without PDA. The hydrogel exhibited satisfactory tensile properties (tensile stress up to 0.79 MPa), good fatigue resistance, self-recovery and excellent antimicrobial activity in vitro. It showed excellent adhesion, especially the adhesion strength of pigskin was increased to 27 kPa. In addition, the hydrogel was used as a strain sensor, exhibiting excellent strain sensitivity (strain coefficient = 2.29), fast response (150 ms), and great durability (over 1000 cycles). The fabricated strain sensors can detect both large and subtle human movements (e.g., wrist bending and vocalization) with stable and repeatable electrical signals, indicating potential applications in personal health monitoring.

Hydrogel-Based Biosensors for Effective Therapeutics

Nanotechnology and polymer engineering are navigating toward new developments to control and overcome complex problems. In the last few decades, polymer engineering has received researchers' attention and similarly, polymeric network-engineered structures have been vastly studied. Prior to therapeutic application, early and rapid detection analyses are critical. Therefore, developing hydrogel-based sensors to manage the acute expression of diseases and malignancies to devise therapeutic approaches demands advanced nanoengineering. However, nano-therapeutics have emerged as an alternative approach to tackling strenuous diseases. Similarly, sensing applications for multiple kinds of analytes in water-based environments and other media are gaining wide interest. It has also been observed that these functional roles can be used as alternative approaches to the detection of a wide range of biomolecules and pathogenic proteins. Moreover, hydrogels have emerged as a three-dimensional (3D) polymeric network that consists of hydrophilic natural or synthetic polymers with multidimensional dynamics. The resemblance of hydrogels to tissue structure makes them more unique to study inquisitively. Preceding studies have shown a vast spectrum of synthetic and natural polymer applications in the field of biotechnology and molecular diagnostics. This review explores recent studies on synthetic and natural polymers engineered hydrogel-based biosensors and their applications in multipurpose diagnostics and therapeutics. We review the latest studies on hydrogel-engineered biosensors, exclusively DNA-based and DNA hydrogel-fabricated biosensors.

Wearable Sensor: An Emerging Data Collection Tool for Plant Phenotyping

The advancement of plant phenomics by using optical imaging-based phenotyping techniques has markedly improved breeding and crop management. However, there remains a challenge in increasing the spatial resolution and accuracy due to their noncontact measurement mode. Wearable sensors, an emerging data collection tool, present a promising solution to address these challenges. By using a contact measurement mode, wearable sensors enable in-situ monitoring of plant phenotypes and their surrounding environments. Although a few pioneering works have been reported in monitoring plant growth and microclimate, the utilization of wearable sensors in plant phenotyping has yet reach its full potential. This review aims to systematically examine the progress of wearable sensors in monitoring plant phenotypes and the environment from an interdisciplinary perspective, including materials science, signal communication, manufacturing technology, and plant physiology. Additionally, this review discusses the challenges and future directions of wearable sensors in the field of plant phenotyping.

Highly water dispersible collagen/polyaniline nanocomposites with strong adhesion for electrochromic films with enhanced cycling stability

Electrochromic materials have attracted extensive attention recently due to their versatile applications in smart windows, displays, antiglare rearview mirrors, and so on. Herein we report a new electrochromic composite prepared from collagen and polyaniline (PANI) through a self-assembly assisted co-precipitation method. The introduction of hydrophilic collagen macromolecules into PANI nanoparticles makes the collagen/PANI (C/PANI) nanocomposite obtain excellent dispersibility in water, which provides good environmental-friendly solution processability. Furthermore, the C/PANI nanocomposite exhibits excellent film-forming properties and adhesion to the ITO glass matrix. The resulting electrochromic film of the C/PANI nanocomposite displays significantly improved cycling stability compared with the pure PANI film after 500 coloring-bleaching cycles. On the other hand, the composite films also exhibit yellow, green and blue polychromatic properties at different applied voltages and high average transmittance at the bleaching state. The C/PANI electrochromic material illustrates scaling potential for the application of electrochromic devices.

A liquid metal/carbon nanotubes complex enabling ultra-fast polymerization of super-robust, stretchable adhesive hydrogels for highly sensitive sensor

Carbon nanotubes (CNTs) usually served as conductive and reinforcing nanofillers for making nanocomposites have never been reported to play a role in accelerating fabrication of hydrogels. Herein, we report an important discovery that by involving CNTs and liquid metal (LM) to form a complex (LM@CNTs), multifunctional hydrogels are rapidly prepared from vinyl monomers without heating or adding any initiators and crosslinkers. Study results demonstrate that LM@CNTs not only performs as both initiator and crosslinker for synthesizing hydrogels, but also dramatically reduces the polymerization duration from 3 days to minute levels, compared with that of only LM involved in hydrogel fabrication. Specifically, the complex initiates (<60 s) and crosslinks (<8min) monomers to form the high-performance hydrogels, which significantly reduces energy consumptions. The resulting polyacrylic acid (PAA) hydrogel possesses super stretchability (∼1200 %), high tensile strength (0.96 MPa), outstanding strain sensitivity (Gauge factor = 15.40 at 300-500 % strain), and excellent adhesion to various substrate surfaces. Additionally, the injectable molding performance will benefit the mass production of the hydrogels, which exhibits great potential for applications of wearable flexible sensors. This study provides an environmentally friendly, rapid polymerization, and energy-saving strategy by effectively applying nano-fillers for viable fabrication and application of multifunctional hydrogels.

Biomolecular sensors for advanced physiological monitoring

Body-based biomolecular sensing systems, including wearable, implantable and consumable sensors allow comprehensive health-related monitoring. Glucose sensors have long dominated wearable bioanalysis applications owing to their robust continuous detection of glucose, which has not yet been achieved for other biomarkers. However, access to diverse biological fluids and the development of reagentless sensing approaches may enable the design of body-based sensing systems for various analytes. Importantly, enhancing the selectivity and sensitivity of biomolecular sensors is essential for biomarker detection in complex physiological conditions. In this Review, we discuss approaches for the signal amplification of biomolecular sensors, including techniques to overcome Debye and mass transport limitations, and selectivity improvement, such as the integration of artificial affinity recognition elements. We highlight reagentless sensing approaches that can enable sequential real-time measurements, for example, the implementation of thin-film transistors in wearable devices. In addition to sensor construction, careful consideration of physical, psychological and security concerns related to body-based sensor integration is required to ensure that the transition from the laboratory to the human body is as seamless as possible.

High-Sensitivity Wearable Sensor Based On a MXene Nanochannel Self-Adhesive Hydrogel

To address the shortcomings of traditional filler-based wearable hydrogels, a new type of nanochannel hydrogel sensor is fabricated in this work through a combination of the unique structure of electrospun fiber textile and the properties of a double network hydrogel. Unlike the traditional Ti₃C₂T_x MXene-based hydrogels, the continuously distributed Ti₃C₂T_x MXene in the nanochannels of the hydrogel forms a tightly interconnected structure similar to the neuron network. As a result, they have more free space to flip and perform micromovements, which allows one to significantly increase the electrical conductivity and sensitivity of the hydrogel. According to the findings, the Ti₃C₂T_x MXene nanochannel hydrogel has excellent mechanical properties as well as self-adhesion and antifreezing characteristics. The hydrogel sensor successfully detects different human motions and physiological signals (e.g., low pulse signals) with high stability and sensitivity. Therefore, the proposed Ti₃C₂T_x MXene-based hydrogel with a unique structure and properties is very promising in the field of flexible wearable devices.

Carboxymethylcellulose reinforced, double-network hydrogel-based strain sensor with superior sensing stability for long-term monitoring

Hydrogel-based strain sensors have garnered significant attention for their potential for human health monitoring. However, its practical application has been hindered by water loss, freezing, and structural impairment during long-term motion monitoring. Here, a strain sensor based on double-network (DN) hydrogel of polyacrylamide (PAAm)/carboxymethylcellulose (CMC) was developed in a ternary solvent system of lithium chloride (LiCl)/ethylene glycol (EG)/H₂O through a facile one-pot radical polymerization strategy. The incorporation of EG effectively mitigated the hydration of lithium salts by generating stable ion clusters with Li⁺ and stronger hydrogen bonds within the polymer matrix. The sensor demonstrated excellent mechanical properties, including a stretchability of 1858 %, toughness of 1.80 MJ/m³, and recoverability of 102 %. Furthermore, the LiCl/EG/H₂O ternary system resulted in high conductivity, excellent anti-freezing performance, and superior sensing stability. In addition, the sensor exhibited remarkable sensitivity, enabling the monitoring of human movements ranging from subtle to significant deformations, including throat motion and bending of the elbow, wrist, finger, and lower limb. This study presents a viable approach for constructing hydrogel-based strain sensors with exceptional sensing stability for long-term tracking of human motions.

Engineering Smart Composite Hydrogels for Wearable Disease Monitoring

Growing health awareness triggers the public's concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person's physiological data and analyzing it locally or remotely. During the health monitoring process, different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring.

Hydrogel-Based Flexible Electronics

Flexible electronics is an emerging field of research involving multiple disciplines, which include but not limited to physics, chemistry, materials science, electronic engineering, and biology. However, the broad applications of flexible electronics are still restricted due to several limitations, including high Young's modulus, poor biocompatibility, and poor responsiveness. Innovative materials aiming for overcoming these drawbacks and boost its practical application is highly desirable. Hydrogel is a class of 3D crosslinked hydrated polymer networks, and its exceptional material properties render it as a promising candidate for the next generation of flexible electronics. Here, the latest methods of synthesizing advanced functional hydrogels and the state-of-art applications of hydrogel-based flexible electronics in various fields are reviewed. More importantly, the correlation between properties of the hydrogel and device performance is discussed here, to have better understanding of the development of flexible electronics by using environmentally responsive hydrogels. Last, perspectives on the current challenges and future directions in the development of hydrogel-based multifunctional flexible electronics are provided.

Hydrogel with Robust Adhesion in Various Liquid Environments by Electrostatic-Induced Hydrophilic and Hydrophobic Polymer Chains Migration and Rearrangement

Hydrogels with wet adhesion are promising interfacial adhesive materials; however, their adhesion in water, oil, or organic solvents remains a major challenge. To address this, a pressure-sensitive P(AAm-co-C₁₈ )/PTA-Fe hydrogel is fabricated, which exhibits robust adhesion to various substrates in both aqueous solutions and oil environments. It is demonstrated that the key to wet adhesion under liquid conditions is the removal of the interfacial liquid, which can be achieved through rational molecular composition regulation. By complexing with hydrophilic polymer networks, phosphotungstic acid (PTA) is introduced into the hydrogel network as a physical cross-linker and anchor point to improve the cohesion strength and drive the migration of polymer chains. The migration and rearrangement of hydrophilic and hydrophobic polymer chains on the hydrogel surface are induced by the electrostatic interactions of Fe³⁺ , which create a surface with interfacial water- and oil-removing properties. By co-regulating the hydrophilic and hydrophobic polymer chains, the P(AAm-co-C₁₈ )/PTA-Fe hydrogel is able to act as a pressure-sensitive adhesive under water and oils with adhesion strength of 92.6 and 90.0 kPa, respectively. It is anticipated that this regulation strategy for polymer chains will promote the development of wet adhesion hydrogels, which can have a wide range of applications.

Dual-network polyvinyl alcohol/polyacrylamide/xanthan gum ionic conductive hydrogels for flexible electronic devices

Ionic conductive hydrogels (ICHs) have received widespread attention as an ideal candidate for flexible electronic devices. However, conventional ICHs failed in widespread applications due to their inability to simultaneously possess high toughness, high ionic conductivity, and anti-freezing properties. Here, polyvinyl alcohol (PVA) and polyacrylamide (PAAm) were first dissolved in the zinc chloride solution, in which zinc ions (Zn²⁺) act as ionic cross-linkers and conducting ions, followed by the introduction of xanthan gum (XG) with a unique structure of trisaccharide side chains into the PVA/PAAm semi-interpenetrating network to prepare a dual-network ICHs (refers as PPXZ). Enabled by the synergistic effect of intermolecular chemical covalent cross-linking and physical cross-linking, PPXZ hydrogels exhibit significantly improved mechanical properties without sacrificing electrical conductivity. Furthermore, PPXZ hydrogels are successfully applied to flexible electronic devices, such as strain sensors and zinc ion hybrid supercapacitors, exhibiting satisfactory sensing sensitivity and cycling stability at a wide temperature range, respectively. Even at a high current density (10 A g^-1), the capacity of the supercapacitor retains 88.24 % after 10,000 cycles. This strategy provides new insight for ICHs in wide temperature-applied flexible electronic devices.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Recent Development of Self-Powered Tactile Sensors Based on Ionic Hydrogels

Hydrogels are three-dimensional polymer networks with excellent flexibility. In recent years, ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications. In this paper, we introduce the basic properties of ionic hydrogels and highlight their application in self-powered sensors working in triboelectric, piezoionic, ionic diode, battery, and thermoelectric modes. We also summarize the current difficulty and prospect the future development of ionic hydrogel self-powered sensors.

Conductive and self-healing hydrogel for flexible electrochemiluminescence sensor

A flexible electrochemiluminescence (ECL) hydrogel sensor exhibiting good self-healing was constructed. A transparent self-healing oxidized sodium alginate/hydrazide polyethylene glycol (OSA/PEG-DH) hydrogel was prepared by crosslinking dynamic covalent acylhydrazone bond. The introduction of 4-amino-DL-phenylalanine, a catalyst with good biocompatibility, allows rapid gelation and self-healing of hydrogel under mild conditions. Using the hydrogel as the sensing substrate, the ionic liquid (IL) 2-hydroxy-N,N,N-trimethylethanaminium chloride and the luminescent reagent N-(aminobutyl)-N-(ethylisoluminol) (ABEI) were simultaneously immobilized in the OSA/PEG-DH hydrogel to obtain the ABEI/IL/OSA/PEG-DH hydrogel. The ABEI/IL/OSA/PEG-DH hydrogel can be directly used as a semi-solid electrolyte for constructing a flexible ECL hydrogel sensor for the detection of H₂O₂, which acted as a coreactant of ABEI. The prepared flexible ECL sensor showed good self-healing performance, can restore ECL signal intensity within 20 min after physical damage, and showed high accuracy in the analysis of complex serum samples. This research shed new light on the development of flexible ECL sensor for bioanalytical applications.

Liquid Metal-Doped Conductive Hydrogel for Construction of Multifunctional Sensors

Interest in wearable and stretchable multifunctional sensors has grown rapidly in recent years. The sensing elements must accurately detect external stimuli to expand their applicability as sensors. However, the sensor's self-healing and adhesion to a target object have been major challenges in developing such practical and versatile devices. In this study, we prepared a hydrogel (LM-SA-PAA) composed of liquid metal (LM), sodium alginate (SA), and poly(acrylic acid) (PAA) with ultrastretchable, excellent self-healing, self-adhesive, and high-sensitivity sensing capabilities that enable the conformal contact between the sensor and skin even during dynamic movements. The excellent self-healing performance of the hydrogel stems from its double cross-linked networks, including physical and chemical cross-linked networks. The physical cross-link formed by the ionic interaction between the carboxyl groups of PAA and gallium ions provide the hydrogel with reversible autonomous repair properties, whereas the covalent bond provides the hydrogel with a stable and strong chemical network. Alginate forms a microgel shell around LM nanoparticles via the coordination of its carboxyl groups with Ga ions. In addition to offering exceptional colloidal stability, the alginate shell has sufficient polar groups, ensuring that the hydrogel adheres to diverse substrates. Based on the efficient electrical pathway provided by the LM, the hydrogel exhibited strain sensitivity and enabled the detection of various human motions and electrocardiographic monitoring. The preparation method is simple and versatile and can be used for the low-cost fabrication of multifunctional sensors, which have broad application prospects in human-machine interface compatibility and medical monitoring.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Self-Healable Elastomeric Network with Dynamic Disulfide, Imine, and Hydrogen Bonds for Flexible Strain Sensor

Self-healable and stretchable elastomeric material is essential for the development of flexible electronics devices to ensure their stable performance. In this study, a strain sensor (PIH₂ T₁ -tri/CNT-3) composed of self-repairable crosslinked elastomer substrate (PIH₂ T₁ -tri, containing multiple reversible repairing sites such as disulfide, imine, and hydrogen bonds) and conductive layer (carbon nanotube, CNT) was prepared. The PIH₂ T₁ -tri elastomer had excellent self-healing ability (healing efficiency=91 %). It exhibited good mechanical integrity in terms of elongation at break (672 %), tensile strength (1.41 MPa). The Young's modulus (0.39 MPa) was close to that of human skin. The PIH₂ T₁ -tri/CNT-3 sensor also demonstrated an effective self-healing function for electrical conduction and sensing property. Meanwhile, it had high sensitivity (gauge factor (GF)=24.1), short response time (120 ms), and long-term durability (4000 cycles). This study offers a novel self-healable elastomer platform with carbon based conductive components to develop flexible strain sensors towards high performance soft electronics.

Novel Wearable Optical Sensors for Vital Health Monitoring Systems-A Review

Wearable sensors are pioneering devices to monitor health issues that allow the constant monitoring of physical and biological parameters. The immunity towards electromagnetic interference, miniaturization, detection of nano-volumes, integration with fiber, high sensitivity, low cost, usable in harsh environments and corrosion-resistant have made optical wearable sensor an emerging sensing technology in the recent year. This review presents the progress made in the development of novel wearable optical sensors for vital health monitoring systems. The details of different substrates, sensing platforms, and biofluids used for the detection of target molecules are discussed in detail. Wearable technologies could increase the quality of health monitoring systems at a nominal cost and enable continuous and early disease diagnosis. Various optical sensing principles, including surface-enhanced Raman scattering, colorimetric, fluorescence, plasmonic, photoplethysmography, and interferometric-based sensors, are discussed in detail for health monitoring applications. The performance of optical wearable sensors utilizing two-dimensional materials is also discussed. Future challenges associated with the development of optical wearable sensors for point-of-care applications and clinical diagnosis have been thoroughly discussed.

Construction of strong and tough carboxymethyl cellulose-based oriented hydrogels by phase separation

Anisotropic hydrogels have attracted extensive attention because they are similar to natural hydrogel-like materials and exhibit superiority and new functions that isotropic hydrogels cannot. Here, we fabricated strong and tough carboxymethyl cellulose-based conductive hydrogels with oriented hierarchical structures through pre-stretching, solvent displacement induced phase separation, and subsequent ionic crosslinking immobilization. Solvent displacement made the pre-stretched carboxymethyl cellulose-based polymer network more dense and linear, while the toughness of the hydrogel was further improved under the effect of phase separation. Strong and tough hydrogels were prepared by combining pre-stretching and phase separation; the variation range (tensile strength of 2.24-6.19 MPa and toughness of 19.41-22.92 MJ/m³) can be adjusted by the stretching ratio. Compared with traditional carboxymethyl cellulose-based hydrogels, the tensile strength and toughness were increased by 49 times and 15 times, respectively. In addition, the hydrogels had good underwater stability, ion cross-linking made the hydrogels have good conductivity, and the directional stratification structure gave the hydrogels conductive anisotropy. These characteristics give hydrogel sensors broad application prospects in flexible wearable devices, anisotropic sensors, and intelligent underwater devices.

Flexible and Stretchable Carbon-Based Sensors and Actuators for Soft Robots

In recent years, the emergence of low-dimensional carbon-based materials, such as carbon dots, carbon nanotubes, and graphene, together with the advances in materials science, have greatly enriched the variety of flexible and stretchable electronic devices. Compared with conventional rigid devices, these soft robotic sensors and actuators exhibit remarkable advantages in terms of their biocompatibility, portability, power efficiency, and wearability, thus creating myriad possibilities of novel wearable and implantable tactile sensors, as well as micro-/nano-soft actuation systems. Interestingly, not only are carbon-based materials ideal constituents for photodetectors, gas, thermal, triboelectric sensors due to their geometry and extraordinary sensitivity to various external stimuli, but they also provide significantly more precise manipulation of the actuators than conventional centimeter-scale pneumatic and hydraulic robotic actuators, at a molecular level. In this review, we summarize recent progress on state-of-the-art flexible and stretchable carbon-based sensors and actuators that have creatively added to the development of biomedicine, nanoscience, materials science, as well as soft robotics. In the end, we propose the future potential of carbon-based materials for biomedical and soft robotic applications.

An Electroconductive Hydrogel Scaffold with Injectability and Biodegradability to Manipulate Neural Stem Cells for Enhancing Spinal Cord Injury Repair

Spinal cord injury (SCI) generally leads to long-term functional deficits and is difficult to repair spontaneously. Many biological scaffold materials and stem cell treatment strategies have been explored, but very little research focused on the method of combining exogenous neural stem cells (NSCs) with a biodegradable conductive hydrogel scaffold. Here, a NSC loaded conductive hydrogel scaffold (named ICH/NSCs) was assembled by amino-modified gelatin (NH₂-Gelatin) and aniline tetramer grafted oxidized hyaluronic acid (AT-OHA). Desirably, the well-conducting ICH/NSCs can be simply injected into the target site of SCI for establishing a good electrical signal pathway of cells, and the proper degradation cycle facilitates new nerve growth. In vitro experiments indicated that the inherent electroactive microenvironment of the hydrogel could better manipulate the differentiation of NSCs into neurons and inhibit the formation of glial cells and scars. Collectively, the ICH/NSC scaffold has successfully stimulated the recovery of SCI and may provide a promising treatment strategy for SCI repair.

Wearable artificial intelligence biosensor networks

The demand for high-quality healthcare and well-being services is remarkably increasing due to the ageing global population and modern lifestyles. Recently, the integration of wearables and artificial intelligence (AI) has attracted extensive academic and technological attention for its powerful high-dimensional data processing of wearable biosensing networks. This work reviews the recent developments in AI-assisted wearable biosensing devices in disease diagnostics and fatigue monitoring demonstrating the trend towards personalised medicine with highly efficient, cost-effective, and accurate point-of-care diagnosis by finding hidden patterns in biosensing data and detecting abnormalities. The reliability of adaptive learning and synthetic data and data privacy still need further investigation to realise personalised medicine in the next decade. Due to the worldwide popularity of smartphones, they have been utilised for sensor readout, wireless data transfer, data processing and storage, result display, and cloud server communication leading to the development of smartphone-based biosensing systems. The recent advances have demonstrated a promising future for the healthcare system because of the increasing data processing power, transfer efficiency and storage capacity and diversifying functionalities.

Wearable Hydrogel-Based Epidermal Sensor with Thermal Compatibility and Long Term Stability for Smart Colorimetric Multi-Signals Monitoring

Hydrogel-based wearable epidermal sensors (HWESs) have attracted widespread attention in health monitoring, especially considering their colorimetric readout capability. However, it remains challenging for HWESs to work at extreme temperatures with long term stability due to the existence of water. Herein, a wearable transparent epidermal sensor with thermal compatibility and long term stability for smart colorimetric multi-signals monitoring is developed, based on an anti-freezing and anti-drying hydrogel with high transparency (over 90% transmittance), high stretchability (up to 1500%) and desirable adhesiveness to various kinds of substrates. The hydrogel consists of polyacrylic acid, polyacrylamide, and tannic acid-coated cellulose nanocrystals in glycerin/water binary solvents. When glycerin readily forms strong hydrogen bonds with water, the hydrogel exhibits outstanding thermal compatibility. Furthermore, the hydrogel maintains excellent adhesion, stretchability, and transparency after long term storage (45 days) or at subzero temperatures (-20 °C). For smart colorimetric multi-signals monitoring, the freestanding smart colorimetric HWESs are utilized for simultaneously monitoring the pH, T and light, where colorimetric signals can be read and stored by artificial intelligence strategies in a real time manner. In summary, the developed wearable transparent epidermal sensor holds great potential for monitoring multi-signals with visible readouts in long term health monitoring.

A high-pressure resistant ternary network hydrogel based flexible strain sensor with a uniaxially oriented porous structure toward gait detection

Gait abnormalities have been widely investigated in the diagnosis and treatment of neurodegenerative diseases. However, it is still a great challenge to achieve a comfortable, convenient, sensitive and high-pressure resistant flexible gait detection sensor for real-time health monitoring. In this work, a polyaniline (PANI)@(polyacrylic acid (PAA)-polyvinyl alcohol (PVA)) (PANI@(PVA-PAA)) ternary network hydrogel with a uniaxially oriented porous featured structure was successfully prepared using a simple freeze-thaw method and in situ polymerization. The PANI@(PVA-PAA) hydrogel shows excellent compressive mechanical properties (423.44 kPa), favorable conductivity (2.02 S m^-1) and remarkable durability (500 loading-unloading cycle), and can sensitively detect the effect of pressure with a fast response time (200 ms). The PANI@(PVA-PAA) hydrogel assembled into a flexible sensor can effectively identify the movement state of the shoulder, knee and even the sole of the plantar for gait detection. The uniaxially oriented porous structure enables the hydrogel-based sensor to have a high rate of change in the longitudinal direction and can effectively distinguish various gaits. The construction of a hydrogen bond between PANI and the PVA-PAA hydrogel ensures the uniform distribution of PANI in the hydrogel to form a ternary network structure, which improves the pressure resistance and conductivity of the PANI@(PVA-PAA) hydrogel. Thus, PANI@(PVA-PAA) hydrogel flexible sensor for gait detection can not only effectively monitor some serious diseases but also detect some unscientific exercise in people's daily life.

Multistimuli-Responsive PNIPAM-Based Double Cross-Linked Conductive Hydrogel with Self-Recovery Ability for Ionic Skin and Smart Sensor

Multistimuli-responsive conductive hydrogels have been appealing candidates for multifunctional ionic skin. However, the fabrication of the multistimuli-responsive conductive hydrogels with satisfactory mechanical property to meet the practical applications is still a great challenge. In this study, a novel poly(N-isopropylacrylamide-co-sodium acrylate)/alginate/hectorite clay Laponite XLS (PNIPAM-SA/ALG/XLS) double cross-linked hydrogel with excellent mechanical property, self-recovery ability, temperature/pH-responsive ability, and strain/temperature-sensitive conductivity was fabricated. The PNSAX hydrogel possessed a moderate tensile strength of 290 kPa at a large elongation rate of 1120% and an excellent compression strength of 2.72 MPa at 90%. The hydrogel also possessed excellent mechanical repeatability and self-recovery ability. Thus, the hydrogel could withstand repetitive deformations for long time periods. Additionally, the hydrogel could change its transparency and volume once at a temperature of 44 °C and change its volume at different pHs. Thus, the visual temperature/pH-responsive ability allowed the hydrogel to qualitatively harvest environmental information. Moreover, the hydrogel possessed an excellent conductivity of 0.43 S/m, and the hydrogel could transform large/subtle deformation and temperature information into electrical signal change. Thus, the ultrafast strain/temperature-sensitive conductivity allowed the hydrogel to quantitatively detect large/small-scale human motions as well as environmental temperature. A cytotoxicity test confirmed the good cytocompatibility. Taken together, the hydrogel was suitable for human motion detecting and environmental information harvesting for long time periods. Therefore, the hydrogel has a great application potential as a multifunctional ionic skin and smart sensor.

Peptide-enhanced tough, resilient and adhesive eutectogels for highly reliable strain/pressure sensing under extreme conditions

Natural gels and biomimetic hydrogel materials have been able to achieve outstanding integrated mechanical properties due to the gain of natural biological structures. However, nearly every natural biological structure relies on water as solvents or carriers, which limits the possibility in extreme conditions, such as sub-zero temperatures and long-term application. Here, peptide-enhanced eutectic gels were synthesized by introducing α-helical "molecular spring" structure into deep eutectic solvent. The gel takes full advantage of the α-helical structure, achieving high tensile/compression, good resilience, superior fracture toughness, excellent fatigue resistance and strong adhesion, while it also inherits the benefits of the deep eutectic solvent and solves the problems of solvent volatilization and freezing. This enables unprecedentedly long and stable sensing of human motion or mechanical movement. The electrical signal shows almost no drift even after 10,000 deformations for 29 hours or in the -20 °C to 80 °C temperature range.

Natural gum-based electronic ink with water-proofing self-healing and easy-cleaning properties for directly on-skin electronics

On-skin electronic systems, which can facilitate noninvasive continuous acquisition of low-artifact physiological signals, are a promising technique for future wearable devices in healthcare. Inspired by the nature of Arabic gum (AG), we developed a costless, easy-to-prepare, easy-to-use, and environment-friendly electronic ink (E-ink) that can be used to construct multiform on-skin electronic systems through simple painting or stamping. In addition to its competitive electrical properties, the E-ink has the following advantages: waterproof (0.5 m/s water flushing for 10 s), self-healing (1.5 mm wide wound), and easy-cleaning (can be easily removed using cotton ball with 5% surfactant), making it environmentally tolerant and highly reliable for practical use. We demonstrated that our E-ink can act as electric wires for epidermal circuits, sensors to handle a variety of physiological data measurements. This research provides an effective strategy for direct integration of electronics and skin, which can accelerate the realization of the next generation of imperceptible, scalable, cost-effective and customized wearable devices.

Nanoparticle–Hydrogel Based Sensors: Synthesis and Applications

Hydrogels are hydrophilic three-dimensional (3D) porous polymer networks that can easily stabilize various nanoparticles. Loading noble metal nanoparticles into a 3D network of hydrogels can enhance the synergy of the components. It can also be modified to prepare intelligent materials that can recognize external stimuli. The combination of noble metal nanoparticles and hydrogels to produce modified or new composite materials has attracted considerable attention as to the use of these materials in sensors. However, there is limited review literature on nanoparticle–hydrogel-based sensors. This paper presents the detailed strategies of synthesis and design of the composites, and the latest applications of nanoparticle–hydrogel materials in the sensing field. Finally, the current challenges and future development directions of nanoparticle–hydrogel-based sensors are proposed.

Tough Adhesive, Antifreezing, and Antidrying Natural Globulin-Based Organohydrogels for Strain Sensors

Hydrogels are often used to fabricate strain sensors; however, they also suffer from freezing at low temperatures and become dry during long-time storage. Encapsulation of hydrogels with elastomers is one of the methods to solve these problems although the adhesion between hydrogels and elastomers is usually low. In this work, using bovine serum protein (BSA) as the natural globulin model and glycerol/H₂O as the mixture solvent, BSA/polyacrylamide organohydrogels (BSA/PAAm OHGs) were prepared by a facile photopolymerization approach. At the optimal condition, BSA/PAAm OHG demonstrated not only high toughness but also tough adhesion properties, which could strongly adhere to various substrates, such as glass, metals, rigid polymeric materials (even poly(tetrafluoroethylene), i.e., PTFE), and soft elastomers. Moreover, BSA/PAAm OHG was flexible and showed tough adhesion at -20 °C. The toughening mechanism and the adhesive mechanism were proposed. On being encapsulated by poly(dimethylsiloxane) (PDMS), it illustrated good antidrying performance. After introducing a conductive filler, the encapsulated BSA/PAAm OHG could be used as a strain sensor to detect human motions. This work provides a better understanding of the adhesive mechanism of natural protein-based organohydrogels.

Ramie Fabric Treated with Carboxymethylcellulose and Laser Engraved for Strain and Humidity Sensing

Wearable fabric sensors have attracted enormous attention due to their huge potential in human health and activity monitoring, human-machine interaction and the Internet of Things (IoT). Among natural fabrics, bast fabric has the advantage of high strength, good resilience and excellent permeability. Laser engraving, as a high throughput, patternable and mask-free method, was demonstrated to fabricate fabric sensors. In this work, we developed a simplified, cost-effective and environmentally friendly method for engraving ramie fabric (a kind of bast fabric) directly by laser under an ambient atmosphere to prepare strain and humidity sensors. We used carboxymethylcellulose (CMC) to pretreat ramie fabric before laser engraving and gained laser-carbonized ramie fabrics (LCRF) with high conductivity (65 Ω sq-1) and good permeability. The strain and humidity sensors had high sensitivity and good flexibility, which can be used for human health and activity monitoring.

Highly Stretchable and Sensitive Flexible Strain Sensor Based on Fe NWs/Graphene/PEDOT:PSS with a Porous Structure

With the rapid development of wearable smart electronic products, high-performance wearable flexible strain sensors are urgently needed. In this paper, a flexible strain sensor device with Fe NWs/Graphene/PEDOT:PSS material added under a porous structure was designed and prepared. The effects of adding different sensing materials and a different number of dips with PEDOT:PSS on the device performance were investigated. The experiments show that the flexible strain sensor obtained by using Fe NWs, graphene, and PEDOT:PSS composite is dipped in polyurethane foam once and vacuum dried in turn with a local linearity of 98.8%, and the device was stable up to 3500 times at 80% strain. The high linearity and good stability are based on the three-dimensional network structure of polyurethane foam, combined with the excellent electrical conductivity of Fe NWs, the bridging and passivation effects of graphene, and the stabilization effect of PEDOT:PSS, which force the graphene-coated Fe NWs to adhere to the porous skeleton under the action of PEDOT:PSS to form a stable three-dimensional conductive network. Flexible strain sensor devices can be applied to smart robots and other fields and show broad application prospects in intelligent wearable devices.

Hydrogel Nanoarchitectonics: An Evolving Paradigm for Ultrasensitive Biosensing

The integration of nanoarchitectonics and hydrogel into conventional biosensing platforms offers the opportunities to design physically and chemically controlled and optimized soft structures with superior biocompatibility, better immobilization of biomolecules, and specific and sensitive biosensor design. The physical and chemical properties of 3D hydrogel structures can be modified by integrating with nanostructures. Such modifications can enhance their responsiveness to mechanical, optical, thermal, magnetic, and electric stimuli, which in turn can enhance the practicality of biosensors in clinical settings. This review describes the synthesis and kinetics of gel networks and exploitation of nanostructure-integrated hydrogels in biosensing. With an emphasis on different integration strategies of hydrogel with nanostructures, this review highlights the importance of hydrogel nanostructures as one of the most favorable candidates for developing ultrasensitive biosensors. Moreover, hydrogel nanoarchitectonics are also portrayed as a promising candidate for fabricating next-generation robust biosensors.

Fabrication of heterogeneous chemical patterns on stretchable hydrogels using single-photon lithography

Curved hydrogel surfaces bearing chemical patterns are highly desirable in various applications, including artificial blood vessels, wearable electronics, and soft robotics. However, previous studies on the fabrication of chemical patterns on hydrogels employed two-photon lithography, which is still not widely accessible to most laboratories. This work demonstrates a new patterning technique for fabricating curved hydrogels with chemical patterns on their surfaces without two-photon microscopy. In this work, we show that exposing hydrogels in fluorophore solutions to single photons via confocal microscopy enables the patterning of fluorophores on hydrogels. By applying this technique to highly stretchable hydrogels, we demonstrate three applications: (1) improving pattern resolution by fabricating patterns on stretched hydrogels and then returning the hydrogels to their initial, unstretched length; (2) modifying the local stretchability of hydrogels at a microscale resolution; and (3) fabricating perfusable microchannels with chemical patterns by winding chemically patterned hydrogels around a template, embedding the hydrogels in a second hydrogel, and then removing the template. The patterning method demonstrated in this work may facilitate a better mimicking of the physicochemical properties of organs in tissue engineering and may be used to make hydrogel robots with specific chemical functionalities.