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

A short linear glucan nanocomposite hydrogel formed by in situ self-assembly with highly elastic, fatigue-resistant and self-recovery

Polyacrylamide (PAM) hydrogels are widely used in wide-ranging applications in biology, medicine, pharmaceuticals and environmental sectors. However, achieving the requisite mechanical properties, fatigue resistance, self-recovery, biocompatibility, and biodegradability remains a challenge. Herein, we present a facile method to construct a nanocomposite hydrogel by integrating short linear glucan (SLG), obtained by debranching waxy corn starch, into a PAM network through self-assembly. The resulting composite hydrogel with 10 % SLG content exhibited satisfactory stretchability (withstanding over 1200 % strain), along with maximum compressive and shear strengths of about 490 kPa and 39 kPa at 90 % deformation, respectively. The hydrogel demonstrated remarkable resilience and could endure repeated compression and stretching. Notably, the nanocomposite hydrogel with 10 % SLG content exhibited full stress recovery at 90 % compression deformation after 20 s, without requiring specific environmental conditions, achieving an energy dissipation recovery rate of 98 %. Meanwhile, these hydrogels exhibited strong adhesion to various soft and hard substrates, including skin, glasses and metals. Furthermore, they maintain solid integrity at both 37 °C and 50 °C after swelling equilibrium, unlike traditional PAM hydrogels, which exhibited softening under similar conditions. We hope that this PAM-SLG hydrogel will open up new avenues for the development of multifunctional electronic devices, offering enhanced performance and versatility.

High-strength, anti-fatigue, cellulose nanofiber reinforced polyvinyl alcohol based ionic conductive hydrogels for flexible strain/pressure sensors and triboelectric nanogenerators

Ionic conductive hydrogels (ICHs) have attracted great attention because of their excellent biocompatibility and structural similarity with biological tissues. However, it is still a huge challenge to prepare a high strength, conductivity and durability hydrogel-based flexible sensor with dual network structure through a simple and environmentally friendly method. In this work, a simple one-pot cycle freezing thawing method was proposed to prepare ICHs by dissolving polyvinyl alcohol (PVA) and ferric chloride (FeCl3) in cellulose nanofiber (CNF) aqueous dispersion. A dual cross-linked network was established in hydrogel through the hydrogen bonds and coordination bonds among PVA, CNF, and FeCl3. This structure endows the as-prepared hydrogel with high sensitivity (pressure sensitivity coefficient (S) = 5.326 in the pressure range of 0-5 kPa), wide response range (4511 kPa), excellent durability (over 3000 cycles), short response time (83 ms) and recovery time (117 ms), which can accurately detect various human activities in real time. Furthermore, the triboelectric nano-generator (TENG) made from PVA@CNF-FeCl3 hydrogel can not only supply power for commercial capacitors and LED lamps, but also be used as a self-powered sensor to detect human motion. This work provides a new approach for the development of the next generation of flexible wearable electronic devices.

Induction of polymer-grafted cellulose nanocrystals in hydrogel nanocomposites to increase anti-swelling, mechanical properties and conductive self-recovery for underwater strain sensing

Anti-swelling conductive hydrogels with simultaneous high tensile strength (>1 MPa) and fast self-recovery are promising candidates for underwater strain sensing, but their preparation remains challenging. Herein, novel anti-swelling conductive nanocomposite hydrogels were fabricated based on poly(acrylamide-co-acrylic acid) (P(AM-co-AA)), polymer-grafted cellulose nanocrystals (CNCs) and Fe3+ ions through a strategy combining nano-reinforcing and multiple physical crosslinking. Due to the presence of interfacial H-bonds, polymer-grafted cellulose nanocrystals played important role in endowing hydrogels with anti-swelling capacity and enhanced mechanical performance. The obtained nanocomposite hydrogels exhibited relatively low swelling ratio (2.9-3.3 g/g), high tensile strength (>1.5 MPa), fast self-recovery (86 % recovery of hysteresis within 5 min) and conductivities of 0.0534-0.0593 S/m. The combination of excellent tensile properties and conductivity endowed the hydrogel-based strain sensors with good sensitivity (GF ≈ 0.8) and reliable cycling repeatability in 0-100 % strain range. Notably, the nanocomposite hydrogels can maintain their mechanical and sensing performance after soaking in water for 14 days, making them applicable for human motion detection both in air and underwater. Hence, this work provided a facile method to construct highly robust and anti-swelling CNC-reinforced conductive hydrogels, which have potential applications in underwater strain sensing and beyond.

Amine functionalised graphene embedded polyvinyl alcohol (PVA) and PVA-chitosan hydrogel composites

A novel amine-functionalized graphene oxide (AFG) doped polyvinyl alcohol (PVA)/chitosan (PVA-Ch) composite film was developed using an eco-synthesis approach, eliminating the need for halogenated compounds. The resulting AFG-doped PVA/Chitosan (PVA-Ch/AFG) polymer film exhibited promising properties for controlled delivery and biosensing applications. The investigation included assessing the swelling behaviour, dissolution percent, gel fraction, and mechanical properties of the polymer film. The swelling characteristics of PVA-Ch and PVA-Ch/AFG were found to be pH and temperature-dependent across various pH ranges (3, 5, 7, and 9). Interestingly, PVA-Ch/AFG demonstrated a stable swelling pattern at pH 5 and 7, unaffected by changes in chitosan concentration, indicating enhanced stability compared to PVA-Ch. The study also explored the use of PVA-Ch/AFG in a drug delivery system, revealing controlled release of the model antibiotic amphicillin, emphasizing its potential in medical applications. Furthermore, the eco-friendly synthesis route underscored the safety of PVA-Ch/AFG for use in food and medical applications. Biocompatibility assessments, including biodegradability studies and cytotoxicity tests on fibroblasts (3T3 cells), confirmed the safety profile of PVA-Ch/AFG. In conclusion, the study suggests that PVA-Ch/AFG holds promise for bio-sensing applications, offering a flexible and colorimetric platform capable of encapsulating, adsorbing, and desorbing biomolecules such as drugs and sensing compounds.

Multifunctional Conductive Double-Network Hydrogel Sensors for Multiscale Motion Detection and Temperature Monitoring

As typical soft materials, hydrogels have demonstrated great potential for the fabrication of flexible sensors due to their highly compatible elastic modulus with human skin, prominent flexibility, and biocompatible three-dimensional network structure. However, the practical application of wearable hydrogel sensors is significantly constrained because of weak adhesion, limited stretchability, and poor self-healing properties of traditional hydrogels. Herein, a multifunctional sodium hyaluronate (SH)/borax (B)/gelatin (G) double-cross-linked conductive hydrogel (SBG) was designed and constructed through a simple one-pot blending strategy with SH and gelatin as the gel matrix and borax as the dynamic cross-linker. The obtained SBG hydrogels exhibited a moderate tensile strength of 25.3 kPa at a large elongation of 760%, high interfacial toughness (106.5 kJ m-3), strong adhesion (28 kPa to paper), and satisfactory conductivity (224.5 mS/m). In particular, the dynamic cross-linking between SH, gelatin, and borax via borate ester bonds and hydrogen bonds between SH and gelatin chain endowed the SBG hydrogels with good fatigue resistance (>300 cycles), rapid self-healing performance (HE (healing efficiency) ∼97.03%), and excellent repeatable adhesion. The flexible wearable sensor assembled with SBG hydrogels demonstrated desirable strain sensing performance with a competitive gauge factor and exceptional stability, which enabled it to detect and distinguish various multiscale human motions and physiological signals. Furthermore, the flexible sensor is capable of precisely perceiving temperature variation with a high thermal sensitivity (1.685% °C-1). As a result, the wearable sensor displayed dual sensory performance for temperature and strain deformation. It is envisioned that the integration of strain sensors and thermal sensors provide a novel and convenient strategy for the next generation of multisensory wearable electronics and lay a solid foundation for their application in electronic skin and soft actuators.

Natural Polymer-Based Hydrogels: From Polymer to Biomedical Applications

Hydrogels prepared from natural polymer have attracted extensive attention in biomedical fields such as drug delivery, wound healing, and regenerative medicine due to their good biocompatibility, degradability, and flexibility. This review outlines the commonly used natural polymer in hydrogel preparation, including cellulose, chitosan, collagen/gelatin, alginate, hyaluronic acid, starch, guar gum, agarose, and dextran. The polymeric structure and process/synthesis of natural polymers are illustrated, and natural polymer-based hydrogels including the hydrogel formation and properties are elaborated. Subsequently, the biomedical applications of hydrogels based on natural polymer in drug delivery, tissue regeneration, wound healing, and other biomedical fields are summarized. Finally, the future perspectives of natural polymers and hydrogels based on them are discussed. For natural polymers, novel technologies such as enzymatic and biological methods have been developed to improve their structural properties, and the development of new natural-based polymers or natural polymer derivatives with high performance is still very important and challenging. For natural polymer-based hydrogels, novel hydrogel materials, like double-network hydrogel, multifunctional composite hydrogels, and hydrogel microrobots have been designed to meet the advanced requirements in biomedical applications, and new strategies such as dual-cross-linking, microfluidic chip, micropatterning, and 3D/4D bioprinting have been explored to fabricate advanced hydrogel materials with designed properties for biomedical applications. Overall, natural polymeric hydrogels have attracted increasing interest in biomedical applications, and the development of novel natural polymer-based materials and new strategies/methods for hydrogel fabrication are highly desirable and still challenging.