PVA/gelatin/β-CD-based rapid self-healing supramolecular dual-network conductive hydrogel as bidirectional strain sensor

Rapid Preparation Triggered by Visible Light for Tough Hydrogel Sensors with Low Hysteresis and High Elasticity: Mechanism, Use and Recycle-by-Design

Hydrogels have emerged as promising candidates for flexible devices and water resource management. However, further applications of conventional hydrogels are restricted due to their limited performance and lack of a recycling strategy. Herein, a tough, flexible, and recyclable hydrogel sensor via a visible-light-triggered polymerization is rapidly created. The Zn2+ crosslinked terpolymer is in situ polymerized using g-C3N4 as the sole initiator to form in situ chain entanglements, endowing the hydrogels with low hysteresis and high elasticity. In the use phase, the hydrogel sensor exhibited high ion conductivity, excellent mechanical properties, fast responsiveness, high sensitivity, and remarkable anti-fatigue ability, making it exceptionally effective in accurately monitoring complex human movements. At the end-of-life (EOL), leveraging the synergy between the photodegradation capacity of g-C3N4 and the adsorption function of the hydrogel matrix, the post-consumer hydrogel is converted into water remediation materials, which not only promoted the rapid degradation of organic pollutants, but also facilitated collection and reuse. This innovative strategy combined in situ entangling reinforcement and tailored recycle-by-design that employed g-C3N4 as key blocks in the hydrogel to achieve high performance in the use phase and close the loop through the reutilization at EOL, highlighting the cost-effective synthesis, specialized structure, and life cycle management.

A quadruple-stimuli responsive supramolecular hydrogel constructed from a poly(acrylic acid) derivative and β-cyclodextrin dimer

The fabrication of stimulus-responsive supramolecular hydrogels as smart materials has attracted much attention in recent years. However, the multi-stimuli responsiveness often requires complicated chemical synthesis and rational molecular design. Herein, a quadruple-stimuli responsive supramolecular hydrogel was designed through the host-guest interaction between a β-CD dimer and a methoxy-azobenzene (mAzo) and ferrocene (Fc) grafted poly(acrylic acid) derivative, as well as through the electrostatic interaction of negatively charged carboxyl side groups. Owing to the dynamic properties of the host-guest and electrostatic interactions, reversible sol-gel transition can be triggered by various stimuli, including temperature, light irradiations, pH changes and chemical redox reagents. As a result, the release of rhodamine B loaded in the hydrogel can be accelerated by green light irradiation, oxidizing agents and low pH, demonstrating potential applications in biomedical materials.

Antibacterial, Fatigue-Resistant, and Self-Healing Dressing from Natural-Based Composite Hydrogels for Infected Wound Healing

The treatment of infected wounds faces substantial challenges due to the high incidence and serious infection-related complications. Natural-based hydrogel dressings with favorable antibacterial properties and strong applicability are urgently needed. Herein, we developed a composite hydrogel by constructing multiple networks and loading ciprofloxacin for infected wound healing. The hydrogel was synthesized via a Schiff base reaction between carboxymethyl chitosan and oxidized sodium alginate, followed by the polymerization of the acrylamide monomer. The resultant hydrogel dressing possessed a good self-healing ability, considerable compression strength, and reliable compression fatigue resistance. In vitro assessment showed that the composite hydrogel effectively eliminated bacteria and exhibited an excellent biocompatibility. In a model of Staphylococcus aureus-infected full-thickness wounds, wound healing was significantly accelerated without scars through the composite hydrogel by reducing wound inflammation. Overall, this study opens up a new way for developing multifunctional hydrogel wound dressings to treat wound infections.

Development of polyvinyl alcohol/carrageenan hydrogels and fibers via KOH treatment for Morse code information transmission

To solve the inherent problems of conductive hydrogels, such as relatively low mechanical properties and fatigue resistance, inability to work after water loss, and difficulty weaving. In this study, the borax-crosslinked polyvinyl alcohol/k-carrageenan (kC) conducting hydrogels (BPKKOH) were prepared by a simple one-pot method, and KOH treatment was used instead of the cumbersome and time-consuming freeze-thaw cycle to improve the comprehensive properties. The KOH treatment increased the hydrogel hydrogen bonding content by 7.72 % and synergized with the induction of kC by K+ to enhance the tensile and compressive strengths by 8.12 and 34.6 times, respectively. Meanwhile, the BPKKOH hydrogel's fatigue resistance and shape recovery after water loss were improved. Additionally, the BPKKOH hydrogels can be monitored for finger bending, showing clear and stable differences in electrical signals. BPKKOH hydrogels combined with Morse code realize applications in information transmission and encryption/decryption. Notably, introducing KOH ensures the molding and preparation of BPKKOH hydrogel fibers while having good signal responsiveness and monitoring ability. More importantly, it can be woven into fabrics that can be loaded with heavy weights, which has the potential to be directly applied in smart wearables. This work provides new ideas for applying flexible sensors and wearable smart textiles.

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.

Highly stretchable anti-freeze hydrogel based on aloe polysaccharides with high ionic conductivity for multifunctional wearable sensors

Conductive hydrogels have limitations such as non-degradability, loss of electrical conductivity at sub-zero temperatures, and single functionality, which limit their applicability as materials for wearable sensors. To overcome these limitations, this study proposes a bio-based hydrogel using aloe polysaccharides as the matrix and degradable polyvinyl alcohol as a reinforcing material. The hydrogel was crosslinked with borax in a glycerol-water binary solvent system, producing good toughness and compressive strength. Furthermore, the hydrogel was developed as a sensor that could detect both small and large deformations with a low detection limit of 1 % and high stretchability of up to 300 %. Moreover, the sensor exhibited excellent frost resistance at temperatures above -50 °C, and the gauge factor of the hydrogel was 2.86 at 20 °C and 2.12 at -20 °C. The Aloe-polysaccharide-based conductive hydrogels also functioned effectively as a wearable sensor; it detected a wide range of humidities (0-98 % relative humidity) and exhibited fast response and recovery times (1.1 and 0.9 s) while detecting normal human breathing. The polysaccharide hydrogel was also temperature sensitive (1.737 % °C-1) and allowed for information sensing during handwriting.

Optimization of rapid self-healing and self-adhesive gluten/guar gum crosslinked gel for strain sensors and electronic devices

In this study, a smart strain sensor based on gluten/guar gum (GG) copolymer containing a combination of additives was developed. The mix proportions of strain sensors were designed using Taguchi method coupled with Grey relational analysis. L16 orthogonal array with three factors, viz. tannic acid (TA), glycerol and sodium chloride (NaCl) at four-levels each was optimized. The addition of TA substantially enhanced tensile strength, self-adhesion ability and conductivity. The self-adhesion ability could also be improved by adding NaCl in range of 0-5 wt%. The presence of glycerol in strain sensors could reduce the self-healing time which was found in the range of 28.75-150 s. In addition, the incorporation of glycerol into gel also improved stretchability of strain sensors. The best mix proportion of strain sensor was found to be 3.75 wt% TA, 30 vol% glycerol and 5 wt% NaCl. The best mixture of stain sensor showed the highest gauge factor (GF) of 0.61 % at a stretchability of 665 % and rapid self-healing at 70 s. This strain sensor could be applied to monitor human limb movements in a wide temperature range from -20 °C to 50 °C. Furthermore, the obtained gel was successfully used as electronic devices and self-powered sensors.

β-Cyclodextrin functionalized agarose-based hydrogels for multiple controlled drug delivery of ibuprofen

Agarose hydrogels are three-dimensional hydrophilic polymeric frameworks characterised by high water content, viscoelastic properties, and excellent ability as cell and drug delivery systems. However, their hydrophilicity as gel systems makes loading of hydrophobic drugs difficult and often ineffective. The incorporation of amphiphilic molecules (e.g. cyclodextrins) into hydrogels as hosts able to form inclusion complexes with hydrophobic drugs could be a possible solution. However, if not properly confined, the host compounds can get out of the network resulting in uncontrolled release. Therefore, in this work, β-cyclodextrins-based host-guest supramolecular hydrogel systems were synthesised, with β-cyclodextrins (β-CD) covalently bound to the polymeric network, preventing leakage of the host molecules. Hydrogels were prepared at two different β-CD-functionalized polyvinyl alcohol (PVA)/agarose ratios, and characterised chemically and physically. Then ibuprofen, a drug often used as a gold standard in studies involving β-CD both in its hydrophilic and hydrophobic forms, was selected to investigate the release behavior of the synthesised hydrogels and the influence of β-CD on the release. The presence of β-CD linked to the polymeric 3D network ensured a higher and prolonged release profile for the hydrophobic drug and also seemed to have some influence on the hydrophilic one.

Biomedical applications of supramolecular hydrogels with enhanced mechanical properties

Supramolecular hydrogels bound by hydrogen bonding, host-guest, hydrophobic, and other non-covalent interactions are among the most attractive biomaterials available. Supramolecular hydrogels have attracted extensive attention due to their inherent dynamic reversibility, self-healing, stimuli-response, excellent biocompatibility, and near-physiological environment. However, the inherent contradiction between non-covalent interactions and mechanical strength makes the practical application of supramolecular hydrogels a great challenge. This review describes the mechanical strength of hydrogels mediated by supramolecular interactions, and focuses on the potential strategies for enhancing the mechanical strength of supramolecular hydrogels and illustrates their applications in related fields, such as flexible electronic sensors, wound dressings, and three-dimensional (3D) scaffolds. Finally, the current problems and future research prospects of supramolecular hydrogels are discussed. This review is expected to provide insights that will motivate more advanced research on supramolecular hydrogels.

Mussel-inspired self-healing hydrogel based on gelatin and oxidized tannic acid for pH-responsive controlled drug release

A series of mussel-inspired pH-responsive self-healing hydrogels based on gelatin crosslinked by oxidized tannic acid (GLT-OTAs) were prepared and used as controlled drug delivery systems (CDDS). An antiphlogistic drug, indomethacin (IDMC) was used as model drug to be immobilized into the hydrogels. The obtained hydrogel samples were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mechanical stability, biocompatibility and self-healing property of the hydrogels were estimated, respectively. Swelling and drug release behaviors of these hydrogels were measured in phosphate buffered saline (PBS) with pH 7.4 (simulated intestinal fluid) and HCl solution with pH 1.2 (simulated gastric fluid) at 37 °C. Effect of the OTA content on the structures and characteristics of all samples was discussed. FTIR spectra revealed the covalent cross-linking between gelatin and OTA induced by Michael addition and Schiff base reaction. Both XRD and FTIR all showed the drug (IDMC) was loaded successfully and existed steadily. GLT-OTA hydrogels had satisfactory biocompatibility, superior self-healing. Results also illustrated that the mechanical strength, internal structure, the swelling and drug release behaviors of the GLT-OTAs hydrogel were greatly affected by the OTA content. With increasing the OTA content, the mechanical stability of GLT-OTAs hydrogel became better and better, and their internal structure became more and more compact. The swelling degree (SD) and cumulative drug release of all hydrogel samples tended to decrease with increasing OTA content and both had obvious pH responsiveness. For each hydrogel sample, the cumulative drug release in PBS at pH 7.4 was greater than that in HCl solution at pH 1.2. These results indicated that the obtained GLT-OTAs hydrogel had promising potential to be used as effective pH-responsive and self-healing drug delivery materials.

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.

Bone-inspired (GNEC/HAPAAm) hydrogel with fatigue-resistance for use in underwater robots and highly piezoresistive sensors

A novel bone-inspired fatigue-resistant hydrogel with excellent mechanical and piezoresistive properties was developed, and it exhibited great potential as a load and strain sensor for underwater robotics and daily monitoring. The hydrogel was created by using the high edge density and aspect ratio of graphene nanosheet-embedded carbon (GNEC) nanomaterials to form a three-dimensional conductive network and prevent the expansion of microcracks in the hydrogel system. Multiscale progressive enhancement of the organic hydrogels (micrometer scale) was realized with inorganic graphene nanosheets (nanometer scale). The graphene nanocrystals inside the GNEC film exhibited good electron transport properties, and the increased distances between the graphene nanocrystals inside the GNEC film caused by external forces increased the resistance, so the hydrogel was highly sensitive and suitable for connection to a loop for sensing applications. The hydrogels obtained in this work exhibited excellent mechanical properties, such as tensile properties (strain up to 1685%) and strengths (stresses up to 171 kPa), that make them suitable for use as elastic retraction devices in robotics and provide high sensitivities (150 ms) for daily human monitoring.

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.

Current Progress in Conductive Hydrogels and Their Applications in Wearable Bioelectronics and Therapeutics

Wearable bioelectronics and therapeutics are a rapidly evolving area of research, with researchers exploring new materials that offer greater flexibility and sophistication. Conductive hydrogels have emerged as a promising material due to their tunable electrical properties, flexible mechanical properties, high elasticity, stretchability, excellent biocompatibility, and responsiveness to stimuli. This review presents an overview of recent breakthroughs in conductive hydrogels, including their materials, classification, and applications. By providing a comprehensive review of current research, this paper aims to equip researchers with a deeper understanding of conductive hydrogels and inspire new design approaches for various healthcare applications.

Microstructural and micromechanical modeling of gum-gelatin-based soft tissue engineering scaffolds

Oral and dental diseases, including periodontal disease, are among the most common conditions in the field of dentistry. The best treatment for this complication is the use of different polymers and multi-component biological tissue prepared through the freeze-drying technique. In this study, biocompatible and biodegradable polymers, namely polyvinyl alcohol (PVA) and gelatin (GN), were used for this purpose, along with Arabian gum-hydroxyapatite (HA) for its antibacterial properties. Arabian gum, with weight percentages of 0, 2, 4, and 6 wt%, was added to the polyvinyl alcohol-gelatin composition at -55 °C for 28 h in the freezer and 48 h at -45 °C under a pressure of 0.01 mbar. The resulting porous biological tissue, with four different ratios, was tested for mechanical and biological analysis in a physiological solution. Then, the samples were analyzed using a scanning electron microscope (SEM) and X-ray diffraction (XRD) technique to study the morphology and structure of the compounds before and after placement in biological solutions. Additionally, a wettability and antibacterial test were performed on the nanocomposite specimen. The SEM observations revealed that this method can create a porous structure with a porosity of about 30-50 μm with a spherical and circular architecture, which was further improved by the addition of gum, reducing the percentage of porosity and improving the tissue's tensile strength and elastic modulus. The porosity changes showed a decrease from 72 % to 60 %, and the tensile strength increased from 53.5 KPa to 76 KPa, resulting in an elastic modulus of 510 KPa to 800 KPa. The addition of gum also reduced the rate of destruction of the biological tissue, making it more suitable for soft tissue applications. The obtained results of the pH test showed that the concentration changes were neutral. The contact angle of water droplets was measured to determine hydrophilicity, indicating an improvement in hydrophilicity after the addition of gum. The results showed that the use of PVA and gelatin, due to their ductility and suitable mechanical properties, along with Arabian gum-HA, could accelerate the healing process of dental periodontal problems.

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.

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.

An injectable and self-healing hydrogel with dual physical crosslinking for in-situ bone formation

Although hydrogels have been widely studied because of their satisfactory biocompatibility and plasticity, their application is limited in bone tissue engineering (BTE) owing to their inadequate mechanical properties and absence of osteogenic activity. To address this issue, we developed an updated alendronate (ALN)-Ca²⁺/Mg²⁺-doped supramolecular (CMS) hydrogel based on our previously developed mechanically resilient "host-guest macromer" (HGM) hydrogel to improve the hydrogel's mechanical properties and osteogenic activity. The CMS hydrogel was prepared by introducing a new physical crosslinking comprising the strong chelation of the comonomer acrylate alendronate (Ac-ALN) and Ca²⁺/Mg²⁺ in the HGM hydrogel. Compared with the previously developed HGM hydrogel, the upgraded CMS hydrogel presented better mechanical properties because of the additional physical crosslinking, while possessing injectable and self-healing properties like the HGM hydrogel. Moreover, the addition of Ac-ALN and Ca²⁺/Mg²⁺ also effectively promoted the in vitro proliferation, migration, and osteogenic differentiation of bone marrow-derived stem cells. The healing effect of a rat cranial defect further proved that the in vivo bone regeneration ability of CMS hydrogel was better than that of HGM hydrogel. The updated CMS hydrogel shows significant potential for BTE application.

Self-Healing Hydrogels: Development, Biomedical Applications, and Challenges

Polymeric hydrogels have drawn considerable attention as a biomedical material for their unique mechanical and chemical properties, which are very similar to natural tissues. Among the conventional hydrogel materials, self-healing hydrogels (SHH) are showing their promise in biomedical applications in tissue engineering, wound healing, and drug delivery. Additionally, their responses can be controlled via external stimuli (e.g., pH, temperature, pressure, or radiation). Identifying a suitable combination of viscous and elastic materials, lipophilicity and biocompatibility are crucial challenges in the development of SHH. Furthermore, the trade-off relation between the healing performance and the mechanical toughness also limits their real-time applications. Additionally, short-term and long-term effects of many SHH in the in vivo model are yet to be reported. This review will discuss the mechanism of various SHH, their recent advancements, and their challenges in tissue engineering, wound healing, and drug delivery.

A robust and sensitive flexible strain sensor based on polyurethane cross-linked composite hydrogels for the detection of human motion

Polyurethane cross-linked composite hydrogel and wireless Bluetooth module were assembled for the detection of human motion.