Tough, Repeatedly Adhesive, Cyclic Compression-Stable, and Conductive Dual-Network Hydrogel Sensors for Human Health Monitoring

Recent advances in flexible hydrogel sensors: Enhancing data processing and machine learning for intelligent perception

With the advent of flexible electronics and sensing technology, hydrogel-based flexible sensors have exhibited considerable potential across a diverse range of applications, including wearable electronics and soft robotics. Recently, advanced machine learning (ML) algorithms have been integrated into flexible hydrogel sensing technology to enhance their data processing capabilities and to achieve intelligent perception. However, there are no reviews specifically focusing on the data processing steps and analysis based on the raw sensing data obtained by flexible hydrogel sensors. Here we provide a comprehensive review of the latest advancements and breakthroughs in intelligent perception achieved through the fusion of ML algorithms with flexible hydrogel sensors, across various applications. Moreover, this review thoroughly examines the data processing techniques employed in flexible hydrogel sensors, offering valuable perspectives expected to drive future data-driven applications in this field.

Dynamic Metal-Coordinated Adhesive and Self-Healable Antifreezing Hydrogels for Strain Sensing, Flexible Supercapacitors, and EMI Shielding Applications

Dynamic metal-coordinated adhesive and self-healable hydrogel materials have garnered significant attention in recent years due to their potential applications in various fields. These hydrogels can form reversible metal-ligand bonds, resulting in a network structure that can be easily broken and reformed, leading to self-healing capabilities. In addition, these hydrogels possess excellent mechanical strength and flexibility, making them suitable for strain-sensing applications. In this work, we have developed a mechanically robust, highly stretchable, self-healing, and adhesive hydrogel by incorporating Ca2+-dicarboxylate dynamic metal-ligand cross-links in combination with low density chemical cross-links into a poly(acrylamide-co-maleic acid) copolymer structure. Utilizing the reversible nature of the Ca2+-dicarboxylate bond, the hydrogel exhibited a tensile strength of up to ∼250 kPa and was able to stretch to 15-16 times its original length. The hydrogel exhibited a high fracture energy of ∼1500 J m-2, similar to that of cartilage. Furthermore, the hydrogel showed good recovery, fatigue resistance, and fast self-healing properties due to the reversible Ca2+-dicarboxylate cross-links. The presence of Ca2+ resulted in a highly conductive hydrogel, which was utilized to design a flexible resistive strain sensor. This hydrogel can strongly adhere to different substrates, making it advantageous for applications in flexible electronic devices. When adhered to human body parts, the hydrogel can efficiently detect limb movements. The hydrogel also exhibited excellent performance as a solid electrolyte for flexible supercapacitors, with a capacitance of ∼260 F/g at 0.5 A/g current density. Due to its antifreezing and antidehydration properties, this hydrogel retains its flexibility at subzero temperatures for an extended period. Additionally, the porous network and high water content of the hydrogel impart remarkable electromagnetic attenuation properties, with a value of ∼38 dB in the 14.5-20.5 GHz frequency range, which is higher than any other hydrogel without conducting fillers. Overall, the hydrogel reported in this study exhibits diverse applications as a strain sensor, solid electrolyte for flexible supercapacitors, and efficient material for electromagnetic attenuation. Its multifunctional properties make it a promising candidate for use in various fields as a state-of-the-art material.

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.

Amphibious Multifunctional Hydrogel Flexible Haptic Sensor with Self-Compensation Mechanism

In recent years, hydrogel-based wearable flexible electronic devices have attracted much attention. However, hydrogel-based sensors are affected by structural fatigue, material aging, and water absorption and swelling, making stability and accuracy a major challenge. In this study, we present a DN-SPEZ dual-network hydrogel prepared using polyvinyl alcohol (PVA), sodium alginate (SA), ethylene glycol (EG), and ZnSO4 and propose a self-calibration compensation strategy. The strategy utilizes a metal salt solution to adjust the carrier concentration of the hydrogel to mitigate the resistance drift phenomenon to improve the stability and accuracy of hydrogel sensors in amphibious scenarios, such as land and water. The ExpGrow model was used to characterize the trend of the ∆R/R0 dynamic response curves of the hydrogels in the stress tests, and the average deviation of the fitted curves ϵ¯ was calculated to quantify the stability differences of different groups. The results showed that the stability of the uncompensated group was much lower than that of the compensated group utilizing LiCl, NaCl, KCl, MgCl2, and AlCl3 solutions (ϵ¯ in the uncompensated group in air was 276.158, 1.888, 2.971, 30.586, and 13.561 times higher than that of the compensated group in LiCl, NaCl, KCl, MgCl2, and AlCl3, respectively; ϵ¯ in the uncompensated group in seawater was 10.287 times, 1.008 times, 1.161 times, 4.986 times, 1.281 times, respectively, higher than that of the compensated group in LiCl, NaCl, KCl, MgCl2 and AlCl3). In addition, for the ranking of the compensation effect of different compensation solutions, the concentration of the compensation solution and the ionic radius and charge of the cation were found to be important factors in determining the compensation effect. Detection of events in amphibious environments such as swallowing, robotic arm grasping, Morse code, and finger-wrist bending was also performed in this study. This work provides a viable method for stability and accuracy enhancement of dual-network hydrogel sensors with strain and pressure sensing capabilities and offers solutions for sensor applications in both airborne and underwater amphibious environments.

A grape seed protein-tannic acid powder to transform various non-adhesive hydrogels into adhesive gels

Realizing adhesion between wet materials remains challenging because of the interfacial water. Current strategies depend on complicated surface modifications, resulting in limited functions. Herein, a facile strategy based on the powder of grape seed protein and tannic acid (GSP-TA) was reported to endow various non-adhesive hydrogels adhesion without chemical modifications for both hydrogels and adherents. The GSP-TA powder has the capability to absorb interfacial water, form an adhesive layer on the hydrogel surface, diffusion into the underneath hydrogel matrix, and establish the initial adhesion within 5 s. By forming multiple non-covalent interactions between powders and substrates, the GSP-TA powder served as an efficient surface treating agent, enabling robust adhesion to solid substrates (wood, cardboard, glass, iron, and rubber) and wet tissues (pigskin, muscle, liver and heart). The adhesive strength for wood, cardboard, glass, iron, and rubber was 145.92 ± 5.93, 123.93 ± 15.98, 66.24 ± 7.67, 98.22 ± 4.13, and 80.83 ± 7.48 kPa, respectively. Because of reversible interactions, the adhesion was also repeatable. Due to the merits of grape seed protein and plant polyphenol, it could be completely degraded within 11 days. Bearing several merits, this strategy has promising applications in wound patches, tissue repair, and sensors.

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.

Sodium carboxymethyl cellulose and MXene reinforced multifunctional conductive hydrogels for multimodal sensors and flexible supercapacitors

With the growing demand for eco-friendly materials in wearable smart electronic devices, renewable, biocompatible, and low-cost hydrogels based on natural polymers have attracted much attention. Cellulose, as one of the renewable and degradable natural polymers, shows great potential in wearable smart electronic devices. Multifunctional conductive cellulose-based hydrogels are designed for flexible electronic devices by adding sodium carboxymethyl cellulose and MXene into polyacrylic acid networks. The multifunctional hydrogels possess excellent mechanical property (stress: 310 kPa; strain: 1127 %), toughness (206.67 KJ m-3), conductivity (1.09 ± 0.12 S m-1) and adhesion (82.19 ± 3.65 kPa). The multifunctional conductive hydrogels serve as strain sensors (Gauge Factor (GF) = 5.79, 0-700 % strain; GF = 14.0, 700-900 % strain; GF = 40.36, 900-1000 % strain; response time: 300 ms; recovery time: 200 ms) and temperature sensors (Temperature coefficient of resistance (TCR) = 2.5755 °C-1 at 35 °C- 60 °C). The sensor detects human activities with clear and steady signals. A distributed array of flexible sensors is created to measure the magnitude and distribution of pressure and a hydrogel-based flexible touch keyboard is also fabricated to recognize writing trajectories, pressures and speeds. Furthermore, a flexible hydrogel-based supercapacitor powers the LED and exhibits good cyclic stability over 15,000 charge-discharge cycles.

Tough, high conductivity pectin polysaccharide-based hydrogel for strain sensing and real-time information transmission

Hydrogels from natural polymers are eco-friendly, biocompatible and adjustable for manufacturing wearable sensors. However, it is still challenging to prepare natural polymer hydrogel sensors with excellent properties (e.g., high conductivity). Here, we developed a physically cross-linked, highly conductive and multifunctional hydrogel (named PPTP) to address this challenge. The natural renewable pectin-based PPTP hydrogel is synthesized by introducing tannic acid (TA), calcium chloride (CaCl2), and sodium chloride (NaCl) into the pectin/polyvinyl alcohol (PVA) dual network structure. The hydrogel exhibits excellent characteristics, including unique tensile strength (2.6155 MPa), high electrical conductivity (7 S m-1), and high sensitivity (GF = 3.75). It is also recyclable, further enhancing its eco-friendly nature. The PPTP hydrogel can be used for monitoring human joint activities, as flexible electrodes for monitoring electrocardiogram (ECG) signals, and touchable screen pen for electronic skin. Moreover, when combined with Morse code and wireless Bluetooth technology, PPTP hydrogels can be used for underwater and land information encryption, and decryption. Our unique PPTP hydrogel offers promising opportunities for medical monitoring, information transfer, and human-computer interaction.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Fabrication of versatile polyvinyl alcohol and carboxymethyl cellulose-based hydrogels for information hiding and flexible sensors: Heat-induced adjustable stiffness and transparency

With the growing demand for wearable electronics, designing biocompatible hydrogels that combine self-repairability, wide operating temperature and precise sensing ability offers a promising scheme. Herein, by interpenetrating naturally derived carboxymethyl cellulose (CMC) into a polyvinyl alcohol (PVA) gel matrix, a novel hydrogel is successfully developed via simple coordination with calcium chloride (CaCl2). The chelation of CMC and Ca2+ is applied as a second crosslinking mechanism to stabilize the hydrogel at relatively high temperature (95 °C). In particular, it has unique heat-induced healing behavior and unexpected tunable stiffness & transparency. Like the sea cucumber, the gel can transform between a stiffened state and a relaxed state (nearly 23 times modulated stiffness from 453 to 20 kPa) which originates from the reconstruction of the crystallites. The adjustable transparency enables the hydrogel to become an excellent information hiding material. Due to the presence of Ca2+, the hydrogels show favorable conductivity, anti-freezing and long-term stability. Based on the advantages, a self-powered sensor, where chemical energy is converted to electrical energy, is assembled for human motion detection. The low-cost, environmentally friendly strategy, at the same time, complies to the "green" chemistry concept with the full employment of the biopolymers. Therefore, the proposed hydrogel is deemed to find potential use in wearable sensors.

Low-temperature strain-sensitive sensor based on cellulose-based ionic conductive hydrogels with moldable and self-healing properties

Bioelectronics based on high-performance conductive ionic hydrogels, which can create novel technological interfaces with the human body, have attracted significant interest from both academia and industry. However, it is still a challenge to fabricate hydrogel sensor with integration of good mechanical properties, fast self-healing ability and flexible strain sensitivity below 0 °C. In this paper, we present a moldable, self-healing and adhesive cellulose-based ionic conductive hydrogel with strain-sensitivity, which was prepared by forming dual-crosslinked networks using poly(vinyl alcohol) (PVA) with borax, calcium chloride (CaCl2), zinc chloride (ZnCl2) and 2,2,6,6-tetramethylpiperidine-1-oxyl oxidized cellulose nanofibril (TCNF). The hydrogel exhibited fast self-healing within 10 s, moderate modulus of 5.13 kPa, high elongation rate of 1500 % and excellent adhesion behavior on various substrates. Due to multiple hydrogen bonding and the presence of CaCl2 and ZnCl2, the hydrogel presented a reduced freezing point as low as -41.1 °C, which enabled its application as a low-temperature strain sensor. The proposed hydrogel provides a simple and facile method for fabricating multi-functional hydrogels that can be used as suitable strain sensors for applications such as wearable electronic sensor, soft robotics and electronic skins in a wide temperature range.

Copolymer-grafted cellulose nanocrystal induced nanocomposite hydrogels with enhanced strength, high elasticity and adhesiveness for flexible strain and pressure sensors

Recently, the application of cellulose nanocrystals (CNCs) in the field of hydrogel sensors has attracted much attention. However, it remains challenging to construct CNC-reinforced conductive hydrogels with a combination of enhanced strength, low hysteresis, high elasticity and remarkable adhesiveness. Herein, we present a facile method to prepare conductive nanocomposite hydrogels with the above-mentioned properties by reinforcing chemically crosslinked poly(acrylic acid) (PAA) hydrogel with rational-designed copolymer-grafted CNCs. The copolymer-grafted CNCs interact with PAA matrix to form carboxyl-amide conventional hydrogen bonds and carboxyl-amino ionic hydrogen bonds, among which the ionic hydrogen bonds with rapid recovery capability are critical to the low hysteresis and high elasticity of hydrogel. The introduction of copolymer-grafted CNCs endowed the hydrogels with enhanced tensile/compressive strength, high resilience (>95 %) during tensile cyclic loading, rapid self-recovery during compressive cyclic loading and improved adhesiveness. Thanks to the high elasticity and durability of hydrogel, the assembled hydrogel sensors exhibited good cycling repeatability and durability in detecting various strains, pressures and human motions. The hydrogel sensors also showed satisfying sensitivity. Hence, the proposed preparation method and the obtained CNC-reinforced conductive hydrogels would open new avenues in flexible strain and pressure sensors for human motion detection and beyond.

Tremella polysaccharide-based conductive hydrogel with anti-freezing and self-healing ability for motion monitoring and intelligent interaction

The application of conductive hydrogels in flexible wearable devices has garnered significant attention. In this study, a self-healing, anti-freezing, and fire-resistant hydrogel strain sensor is successfully synthesized by incorporating sustainable natural biological materials, viz. Tremella polysaccharide and silk fiber, into a polyvinyl alcohol matrix with borax cross-linking. The resulting hydrogel exhibits excellent transparency, thermoplasticity, and remarkable mechanical properties, including a notable elongation (1107.3 %) and high self-healing rate (91.11 %) within 5 min, attributed to the dynamic cross-linking effect of hydrogen bonds and borax. A strain sensor based on the prepared hydrogel sensor can be used to accurately monitor diverse human movements, while maintaining exceptional sensing stability and durability under repeated strain cycles. Additionally, a hydrogel touch component is designed that can successfully interact with intelligent electronic devices, encompassing functions like clicking, writing, and drawing. These inherent advantages make the prepared hydrogel a promising candidate for applications in human health monitoring and intelligent electronic device interaction.

Self-Healable, Adhesive, Anti-Drying, Freezing-Tolerant, and Transparent Conductive Organohydrogel as Flexible Strain Sensor, Triboelectric Nanogenerator, and Skin Barrier

Conductive hydrogels have attracted tremendous interest in the construction of flexible strain sensors and triboelectric nanogenerators (TENGs) owing to their good stretchability and adjustable properties. Nevertheless, how to simultaneously achieve high transparency, self-healing, adhesion, antibacterial, anti-freezing, anti-drying, and biocompatibility properties through a simple method remains a challenge. Herein, a transparent, freezing-tolerant, and multifunctional organohydrogel (PAOAM-PDO) as electrode for strain sensors and TENGs was constructed through a free radical polymerization in the 1,3-propanediol (PDO)/water binary solvent system, in which oxide sodium alginate, aminated gelatin, acrylic acid, and AlCl3 were used as raw materials. The obtained PAOAM-PDO exhibited good transparency (>90%), self-healing, adhesiveness, antibacterial property, good conductivity (1.13 S/m), and long-term environmental stability. The introduction of PDO endowed PAOAM-PDO with freezing resistance with a low freezing point of -60 °C, and PAOAM-PDO could serve as a protective skin barrier to prevent frostbite at low temperature. PAOAM-PDO could be assembled as strain sensors to monitor heterogeneous human movements with high strain sensitivity (gauge factor of 7.05, strain = 233%). Meanwhile, PAOAM-PDO could be further fabricated as a TENG with a "sandwich" structure in single electrode mode. Moreover, the resulting TENG achieved electrical outputs with simple hand tapping and served as a self-powered device to light light-emitting diodes. This work displays a feasible strategy to build environment-tolerant and multifunctional organohydrogels, which possess potential applications in the wearable electronics and self-powered devices.

Wearable Sensors for Respiration Monitoring: A Review

This paper provides an overview of flexible and wearable respiration sensors with emphasis on their significance in healthcare applications. The paper classifies these sensors based on their operating frequency distinguishing between high-frequency sensors, which operate above 10 MHz, and low-frequency sensors, which operate below this level. The operating principles of breathing sensors as well as the materials and fabrication techniques employed in their design are addressed. The existing research highlights the need for robust and flexible materials to enable the development of reliable and comfortable sensors. Finally, the paper presents potential research directions and proposes research challenges in the field of flexible and wearable respiration sensors. By identifying emerging trends and gaps in knowledge, this review can encourage further advancements and innovation in the rapidly evolving domain of flexible and wearable sensors.

Skin-adhesive lignin-grafted-polyacrylamide/hydroxypropyl cellulose hydrogel sensor for real-time cervical spine bending monitoring in human-machine Interface

Developing a straightforward method to produce conductive hydrogels with excellent mechanical properties, self-adhesion, and biocompatibility remains a significant challenge. While current approaches aim to enhance mechanical performance, they often require additional steps or external forces for fixation, leading to increased production time and limited practicality. A novel lignin-grafted polyacrylamide/hydroxypropyl cellulose hydrogel (L-g-PAM/HPC hydrogel) with a semi-interpenetrating polymer network structure had been developed in this research that boasted exceptional adhesion to the skin (∼68 kPa) and stretchability properties (∼1637 %) compared to PAM-based hydrogels. By incorporating conductive additives such as silver nanowires and carbon nanocages to construct a bridge-like structure within the hydrogel matrix, the resulting AgC@L-g-PAM/HPC hydrogel exhibited impressive electrical conductivity, surpassing that of other PAM-based hydrogels relying on MXene, with a maximum value of 0.76 S/m. Furthermore, the AgC@L-g-PAM/HPC hydrogel retained its efficient electrical signal transmission capability even under mechanical stress. These make it an ideal flexible strain sensor capable of detecting various human motions. In this study, a smart real-time monitoring system was successfully developed for tracking cervical spine bending, serving as an extension for monitoring human activities.

Simple and Rapid Way to a Multifunctionally Conductive Hydrogel for Wearable Strain Sensors

Conductive hydrogels have gained increasing attention in the field of wearable smart devices. However, it remains a big challenge to develop a multifunctionally conductive hydrogel in a rapid and facile way. Herein, a conductive tannic acid-iron/poly (acrylic acid) hydrogel was synthesized within 30 s at ambient temperature by the tannic acid-iron (TA@Fe3+)-mediated dynamic catalytic system. The TA@Fe3+ dynamic redox autocatalytic pair could efficiently activate the ammonium persulfate to initiate the free-radical polymerization, allowing the gelation to occur easily and rapidly. The resulting hydrogel exhibited enhanced stretchability (3560%), conductivity (33.58 S/m), and strain sensitivity (gauge factor = 2.11). When damaged, it could be self-healed through the dynamic and reversible coordination bonds between the Fe3+ and COO- groups in the hydrogel network. Interestingly, the resulting hydrogel could act as a strain sensor to monitor various human motions including the huge movement of deformations (knuckle, wrist) and subtle motions (smiling, breathing) in real time due to its enhanced self-adhesion, good conductivity, and improved strain sensitivity. Also, the obtained hydrogel exhibited efficient electromagnetic interference (EMI) shielding performance with an EMI shielding effectiveness value of 24.5 dB in the X-band (8.2-12.4 GHz). Additionally, it displayed antibacterial properties, with the help of the activity of TA.

Sodium alginate reinforced polyacrylamide/xanthan gum double network ionic hydrogels for stress sensing and self-powered wearable device applications

Strong and ductile sodium alginate (SA) reinforced polyacrylamide (PAM)/xanthan gum (XG) double network ionic hydrogels were constructed for stress sensing and self-powered wearable device applications. In the designed network of PXS-Mⁿ⁺/LiCl (short for PAM/XG/SA-Mⁿ⁺/LiCl, where Mⁿ⁺ stands for Fe³⁺, Cu²⁺ or Zn²⁺), PAM acts as a flexible hydrophilic skeleton, and XG functions as a ductile second network. The macromolecule SA interacts with metal ion Mⁿ⁺ to form a unique complex structure, significantly improving the mechanical strength of the hydrogel. The addition of inorganic salt LiCl endows the hydrogel with high electrical conductivity, and meanwhile reduces the freezing point and prevents water loss of the hydrogel. PXS-Mⁿ⁺/LiCl exhibits excellent mechanical properties and ultra-high ductility (a fracture tensile strength up to 0.65 MPa and a fracture strain up to 1800%), and high stress-sensing performance (a high GF up to 4.56 and pressure sensitivity of 0.122). Moreover, a self-powered device with a dual-power-supply mode, i.e., PXS-Mⁿ⁺/LiCl-based primary battery and TENG, and a capacitor as the energy storage component was constructed, which shows promising prospects for self-powered wearable electronics.

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.

Collagen-Based Organohydrogel Strain Sensor with Self-Healing and Adhesive Properties for Detecting Human Motion

Conductive hydrogels are ideal for flexible sensors, but it is still a challenge to produce such hydrogels with combined toughness, self-adhesion, self-healing, anti-freezing, moisturizing, and biocompatibility properties. Herein, inspired by natural skin, a highly stretchable, strain-sensitive, and multi-environmental stable collagen-based conductive organohydrogel was constructed by using collagen (Col), acrylic acid, dialdehyde carboxymethyl cellulose, 1,3-propylene glycol, and AlCl₃. The resulting organohydrogel exhibited excellent tensile (strain >800%), repeatable adhesion (>10 times), self-healing [self-healing efficiency (SHE) ≈ 100%], anti-freezing (-60 °C), moisturizing (>20 d), and biocompatible properties. This organohydrogel also possessed good electrical conductivity (σ = 3.4 S/m) and strain-sensitive properties [GF (gauge factor) = 13.65 with the maximal strain of 400%]. Notably, the organohydrogel had a considerable low-temperature self-healing performance (SHE = 88% at -24 °C) and rapid underwater self-healing property (SHE = 92%, self-healing time <20 min). This type of strain sensor could not only accurately and continuously monitor the large-scale motions of the human body but also provide an accurate response to the human tiny motions. This work not only proposes a development strategy for a multifunctional conductive organohydrogel with multiple environmental stability but also provides potential research value for the construction of biomimetic electronic skin.

Self-healing, self-adhesive, and stretchable conductive hydrogel for multifunctional sensor prepared by catechol modified nanocellulose stabilized poly(α-thioctic acid)

Self-healing, self-adhesive, and stretchable bio-based conductive hydrogels exhibit properties similar to those of biological tissues, making them an urgent requirement for emerging wearable devices. The primary challenge lies in devising straightforward strategies to accomplish all the aforementioned performances and achieve equilibrium among them. This study used the natural compound thioctic acid (TA) and modified cellulose to prepare conductive hydrogels with stretchability, healing, and self-adhesion through a simple one-step strategy. Metastable poly(TA) was obtained through ring-opening polymerization of lithiated TA, followed by the introduction of dopamine-grafted cellulose nanofibers (DCNF) to stabilize poly(TA) and prepare PTALi/DCNF hydrogels with the aforementioned properties. The hydrogels demonstrated remarkable conductivity, attributed to the existence of Li + ions, with a maximum conductivity of 17.36 mS/cm. The self-healing capacity of the hydrogels was achieved owing to the presence of disulfide bond in TA. The introduction of DCNF can effectively stabilize poly(TA), endow the hydrogel with self-adhesion ability, improve the mechanical properties, and further enhance the formability of hydrogels. Generally, bio-based PTALi/DCNF hydrogels with stretchability, self-healing, self-adhesion, and conductivity are obtained through a simple strategy and used as a sensor with a wide response range and high sensitivity. Hydrogels have significant potential for application in wearable electronic devices, electronic skins, and soft robots.

Polydopamine-Reinforced Hemicellulose-Based Multifunctional Flexible Hydrogels for Human Movement Sensing and Self-Powered Transdermal Drug Delivery

The preparation of bio-based hydrogels with excellent mechanical properties, stable electrochemical properties, and self-adhesive properties remains a challenge. In this study, nano-polydopamine-reinforced hemicellulose-based hydrogels with typical multistage pore structures were prepared. The nanocomposite hydrogels exhibit stable mechanical properties and show no significant crushing phenomenon after 1000 cycles of cyclic compression. Its ultimate tensile strain was 101%, which is significantly higher than that of native skin. The shear adhesion strength of the hydrogel to skin tissue reaches 7.52 kPa, which is better than fibrin glue (Greenplast) (5 kPa), and the excellent adhesion property prolongs the service time of the hydrogel in biomedicine applications. The impedance of the hydrogel was reduced and the electrical conductivity was increased with the addition of nano-polydopamine. The prepared nanocomposite hydrogel can detect various body movements (even throat vibrations) in real time as a motion sensor while being able to rapidly load cationic drugs and facilitate transdermal introduction of electrically stimulated drug ions as a drug patch. It provides theoretical support for the fabrication of hemicellulose-based hydrogels with excellent properties through molecular design and nanoparticle reinforcement. This has important implications for the development of next-generation flexible materials suitable for health monitoring and self-administration.

Cellulose nanocrystals boosted hydrophobic association in dual network polymer hydrogels as advanced flexible strain sensor for human motion detection

Conductive hydrogels attract the attention of researchers worldwide, especially in the field of flexible sensors like strain and pressure. These flexible materials have potential applications in the field of electronic skin, soft robotics, energy storage, and human motion detection. However, its practical application is limited due to low stretchability, high hysteresis energy, low conductivity, long-range strain sensitivity, and high response time. It's still a challenging job to endow all these properties in a single hydrogel network. In the present work, cellulose nano crystals (CNCs) reinforced hydrophobically associated gels were developed using APS as a source of radical polymerization, acrylamide and lauryl methacrylate were used as a monomer. CNCs reinforced the hydrophobically associated hydrogels through hydrogen bonding to retain the hydrogel's network structure. Hydrogels consist of dual crosslinking, which demonstrate exceptional mechanical performance (fracture stress and strain, toughness, and Young's modulus). The low hysteresis energy (10.9 kJm^-3) and high conductivity (22.97 mS/cm) make the hydrogels a strong candidate for strain sensors with high sensitivity (GF = 19.25 at 700% strain) and a fast response time of 200 ms. Cyclic performance was also investigated up to 300 continuous cycles. After 300 cycles, the hydrogels were still stable and no considerable change was observed. These hydrogels are capable of sensing different human motions like wrist, finger bending, and neck (up-down and straight and right/left motion of neck). The hydrogels also demonstrate changes in current in response to swallowing, different speaking words, and writing different alphabets. These results suggest that our prepared materials can sense different small and large human motions, and also could be used in any electronic device where strain sensing is required.

Stretchable, compressible, and conductive hydrogel for sensitive wearable soft sensors

Conductive hydrogels hold great promises in wearable soft electronics. However, the weak mechanical properties, low sensitivity and the absence of multifunctionalities (e.g., self-healing, self-adhesive, etc.) of the conventional conductive hydrogels limit their applications. Thus, developing multifunctional hydrogels may address some of these technical issues. In this work, a multifunctional conductive hydrogel strain sensor is fabricated by incorporating a conductive polymer Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) into a mechanically robust poly (vinyl alcohol) (PVA)/ poly (acrylic acid) (PAA) double network (DN) hydrogel. The as-prepared hydrogel sensor could span a wide spectrum of mechanical properties by simply tuning the polymer composition and the number of freezing-thawing cycles. In addition, the dynamic hydrogen bonding interactions endow the hydrogel sensor with self-healing property and reversible adhesiveness on diverse substrates. Moreover, the hydrogel sensor shows high sensitivity (Gauge Factor from 2.21 to 3.82) and can precisely detect some subtle human motions (e.g., pulse and vocal cord vibration). This work provides useful insights into the development of conductive hydrogel-based wearable soft electronics.

Highly Transparent, Self-Healing, and Self-Adhesive Double Network Hydrogel for Wearable Sensors

Hydrogel-based flexible electronic devices are essential in future healthcare and biomedical applications, such as human motion monitoring, advanced diagnostics, physiotherapy, etc. As a satisfactory flexible electronic material, the hydrogel should be conductive, ductile, self-healing, and adhesive. Herein, we demonstrated a unique design of mechanically resilient and conductive hydrogel with double network structure. The Ca²⁺ crosslinked alginate as the first dense network and the ionic pair crosslinked polyzwitterion as the second loose network. With the synthetic effect of these two networks, this hydrogel showed excellent mechanical properties, such as superior stretchability (1,375%) and high toughness (0.57 MJ/m³). At the same time, the abundant ionic groups of the polyzwitterion network endowed our hydrogel with excellent conductivity (0.25 S/m). Moreover, due to the dynamic property of these two networks, our hydrogel also performed good self-healing performance. Besides, our experimental results indicated that this hydrogel also had high optical transmittance (92.2%) and adhesive characteristics. Based on these outstanding properties, we further explored the utilization of this hydrogel as a flexible wearable strain sensor. The data strongly proved its enduring accuracy and sensitivity to detect human motions, including large joint flexion (such as finger, elbow, and knee), foot planter pressure measurement, and local muscle movement (such as eyebrow and mouth). Therefore, we believed that this hydrogel had great potential applications in wearable health monitoring, intelligent robot, human-machine interface, and other related fields.

A review of recent advances in metal ion hydrogels: mechanism, properties and their biological applications

The mechanisms, common properties and biological applications of different types of metal ion hydrogels are summarized.