Highly stretchable and tough thermo-responsive double network (DN) hydrogels: Composed of PVA-borax and poly (AM-co-NIPAM) polymer networks

Anti-freezing, long-term-usability conductive organo-hydrogels containing lignin for the manufacture of high-performance flexible strain and temperature sensors

The development of an all-in-one hydrogel that possesses the desired characteristics of multiple sensing modalities, exceptional electrical conductivity, and high strain sensing performance remains a significant challenge. Here, we employed calcium ions to crosslink and chelate sodium alginate and sodium lignosulfonate within a polyacrylic acid hydrogel network. Additionally, we introduced a conductive polyaniline network through in situ polymerization and performed a solvent exchange with glycerol to produce a multifunctional conductive organo-hydrogel. The resulting hydrogels exhibited remarkable mechanical properties, with a strength of 268 kPa at a tensile strain of 400 %. They also demonstrated resistance to drying and freezing, remaining unfrozen at low temperatures of -60 °C, as well as temperature sensitivity. The incorporation of polyaniline and Ca2+ ions contributed to enhanced electrical conductivity and sensing capabilities, evidenced by gauge factors of 1.39 and 2.13 in the strain ranges of 0-210 % and 210-400 %, respectively. Furthermore, the temperature sensing properties were characterized by a temperature coefficient of resistance of -4.49 % °C-1 and -0.92 % °C-1 in the temperature ranges of -25 to 25 °C and 25 to 60 °C, respectively. Leveraging these advantageous properties, this hydrogel is anticipated to serve as a real-time motion detection device for human activities in extreme conditions.

A Facile Strategy to Construct Stretchable and Thermoreversible Double Network Hydrogels with Low Hysteresis and High Toughness Based on Entanglement and Hydrogen Bond Networks

High toughness and low hysteresis are of great significance for stretchable hydrogels to strengthen their reliability and practicability for cycle-loaded applications. Whereas, it is still challenging to simultaneously gain mutually repulsive properties due to the existence of dissipation structure. Here, stretchable and recoverable double network (DN) hydrogels comprising highly entangled network structure and temperature-induced dense hydrogen bond (HB)-associated network structure are synthesized, in which slidable entanglements and dense HBs act as the effective crosslinking in first and second networks, respectively. Moveable entanglements and stable HB structures endow hydrogels with high stretchability (999%), high tensile strength (0.82 MPa) and high fracture toughness (4.51 MJ·m-3) through stress transmission along long chains and HB clusters energy dissipation. Moreover, the imposed energy on hydrogels can saved in oriented polymer chains by disentanglement as an entropy loss, thus ensuring the low hysteresis (6.2% at 100% tensile strain) of hydrogels. In addition, deformed hydrogel can be well remodeled and recovered (stress recovery of 95.5%) by temperature-triggered sol-gel transition of HB network. By means of entanglement and hydrogen bond strategies, stretchable DN hydrogels not only equilibrate the conflict of low hysteresis and high toughness, but also provides a new perspective to design high-property hydrogels for cycle-loaded applications.

A Super-Robust and Ultra-Tough Hydrogel Prepared from Flower-Like Submicron Carbon Clusters Exhibited Excellent Resistance to Crack Propagation

Hydrogels are widely used in flexible sensing, drug delivery, and tissue engineering due to their outstanding flexibility and biocompatibility, etc. However, the development of conductive hydrogels with high strength, toughness, and fatigue resistance still exists significant challenges. This study introduced a novel toughening strategy based on the "pinning effect", utilizing submicron carbon cluster (CCs) with a unique π-conjugated core prepared with self-assembly and acrylamide to fabricate high strength and toughness hydrogels. The resulting CCs, coupled with stress dissipation, chain entanglement, and interfacial interactions with polyacrylamide (PAM), effectively arrested crack propagation during stretching, thereby enhancing mechanical performance. The mechanical properties of the PAM-CCs hydrogels are significantly improved compared to PAM hydrogel, showing a fracture strength of 2.33 MPa (2850% increase), an elongation of ≈2400% (700% increase), a fracture energy of 126.4 kJ m-2 (3461% increase), and toughness of 14.94 MJ m-3 (10571% increase). Besides, PAM-CCs hydrogel also revealed good adhesion, compression, and conductivity properties. This strategy do not require complex design or processing, using a simple and fast approach that showed immense potential for applications of hydrogels requiring high mechanical performance.

PNIPAAm-based temperature responsive ionic conductive hydrogels for flexible strain and temperature sensing

Conductive hydrogels have received much attention in the field of flexible wearable sensors due to their outstanding flexibility, conductivity, sensitivity and excellent compatibility. However, most conductive hydrogels mainly focus on strain sensors to detect human motion and lack other features such as temperature response. Herein, we prepared a strain and temperature dual responsive ionic conductive hydrogel (PPPNV) with an interpenetrating network structure by introducing a covalent crosslinked network of N-isopropylacrylamide (NIPAAm) and 1-vinyl-3-butylimidazolium bromide (VBIMBr) into the skeleton of the hydrogel composed of polyvinylalcohol (PVA) and polyvinylpyrrolidone (PVP). The PPPNV hydrogel exhibited excellent anti-freezing properties (-37.34 °C) and water retention with high stretchability (∼930 %) and excellent adhesion. As a wearable strain sensor, the PPPNV hydrogel has good responsiveness and stability to a wide range of deformations and exhibits high strain sensitivity (GF=2.6) as well as fast response time. It can detect large and subtle body movements with good signal stability. As wearable temperature sensors, PPPNV hydrogels can detect human physiological signals and respond to temperature changes, and the volumetric phase transition temperature (VPTT) can be easily controlled by adjusting the molar ratio of NIPAAm to VBIMBr. In addition, a bilayer temperature-sensitive hydrogel was prepared with the temperature responsive hydrogel by two-step synthesis, which shows great promising applications in temperature actuators.

Designing fast-response porous hydrogel actuators with improved toughness

A design concept for porous hydrogel actuators is demonstrated, combining lower critical solution temperature (LCST)-type phase separation with crystallinity formation. In contrast to the existing methods of producing porous hydrogels, our concept could not only generate fast actuation speed, but also greatly enhance the hydrogel tensile strength and toughness.

Tunable thermoresponsive hydrogels for temperature regulation and warning in fruit and vegetables preservation

Fresh fruit and vegetables usually suffer from quality deterioration when exposed to inappropriate temperatures. Common energy-input temperature regulation is widely applied but there remain challenges of increasing energy consumption. Passive temperature management regulates the heat transfer without energy consumption, showing a sustainable strategy for food preservation. Here, thermoresponsive hydrogels were constructed by incorporating NaCl and sodium dodecyl sulfate (SDS) micelles into a poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-co-AM)) network. Due to the excellent mechanical properties and reversible thermochromism at 14 °C and 37 °C, Gel-8 wt%-NaCl could inhibit temperature rise and avoid sunburn damage to peppers under direct sunlight by blocking the input of solar energy and accelerating moisture evaporation. Additionally, hydrogels could act as a feasible sensor by providing real-time visual warnings for inappropriate temperatures during banana storage. Based on the self-adaptive thermoresponsive behaviour, the prepared hydrogels showed effective performance of temperature regulation and quality preservation of fruit and vegetables.

Responsive Acrylamide-Based Hydrogels: Advances in Interpenetrating Polymer Structures

Hydrogels, composed of hydrophilic homopolymer or copolymer networks, have structures similar to natural living tissues, making them ideal for applications in drug delivery, tissue engineering, and biosensors. Since Wichterle and Lim first synthesized hydrogels in 1960, extensive research has led to various types with unique features. Responsive hydrogels, which undergo reversible structural changes when exposed to stimuli like temperature, pH, or specific molecules, are particularly promising. Temperature-sensitive hydrogels, which mimic biological processes, are the most studied, with poly(N-isopropylacrylamide) (PNIPAm) being prominent due to its lower critical solution temperature of around 32 °C. Additionally, pH-responsive hydrogels, composed of polyelectrolytes, change their structure in response to pH variations. Despite their potential, conventional hydrogels often lack mechanical strength. The double-network (DN) hydrogel approach, introduced by Gong in 2003, significantly enhanced mechanical properties, leading to innovations like shape-deformable DN hydrogels, organic/inorganic composites, and flexible display devices. These advancements highlight the potential of hydrogels in diverse fields requiring precise and adaptable material performance. In this review, we focus on advancements in the field of responsive acrylamide-based hydrogels with IPN structures, emphasizing the recent research on DN hydrogels.

Smart and programmed thermo-wetting yarns for scalable and customizable moisture/heat conditioning textiles

Programmable smart textiles with adaptive moisture/heat conditioning (MHC) capabilities are globally being sought to meet the requirements of comfort, energy efficiency, and health protection. However, a universal strategy for fabricating truly scalable and customizable MHC textiles is lacking. In this study, we introduce a scalable in situ grafting approach for the continuous fabrication of two series of smart textile yarns with opposite thermoresponsive wetting behaviors. In particular, the wetting transition temperature can be precisely programmed by adjusting the grafting formula, making the yarns highly customizable. The smart yarns demonstrated excellent mechanical strength, whiteness, weavability, biocompatibility, and washability (with more than 60 home washes), comparable to those of regular textile yarns. They can serve as building blocks independently or in combination to create smart textiles with adaptive sweat wicking and intelligent moisture/heat regulation capabilities. A proposed hybrid textile integrating both the two series of smart yarns can offer dry-contact and cooling/keep-warming effects of approximately 1.6/2.8 °C, respectively, in response to changes in ambient temperature. Our method provides a rich array of design options for nonpowered MHC textiles while maintaining a balance between traditional wearing conventions and large-scale production.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

Hydrogels with tailor-made swelling-shrinkable properties have aroused considerable interest in numerous biomedical domains. For example, as swelling is a key issue for blood and wound extrudates absorption, the transference of nutrients and metabolites, as well as drug diffusion and release, hydrogels with high swelling capacity have been widely applicated in full-thickness skin wound healing and tissue regeneration, and drug delivery. Nevertheless, in the fields of tissue adhesives and internal soft-tissue wound healing, and bioelectronics, non-swelling hydrogels play very important functions owing to their stable macroscopic dimension and physical performance in physiological environment. Moreover, the negative swelling behavior (i.e., shrinkage) of hydrogels can be exploited to drive noninvasive wound closure, and achieve resolution enhancement of hydrogel scaffolds. In addition, it can help push out the entrapped drugs, thus promote drug release. However, there still has not been a general review of the constructions and biomedical applications of hydrogels from the viewpoint of swelling-shrinkable properties. Therefore, this review summarizes the tactics employed so far in tailoring the swelling-shrinkable properties of hydrogels and their biomedical applications. And a relatively comprehensive understanding of the current progress and future challenge of the hydrogels with different swelling-shrinkable features is provided for potential clinical translations.

External Stimuli-Responsive Characteristics of Poly(N,N'-diethylacrylamide) Hydrogels: Effect of Double Network Structure

Swelling experiments and NMR spectroscopy were combined to study effect of various stimuli on the behavior of hydrogels with a single- and double-network (DN) structure composed of poly(N,N'-diethylacrylamide) and polyacrylamide (PAAm). The sensitivity to stimuli in the DN hydrogel was found to be significantly affected by the introduction of the second component and the formation of the double network. The interpenetrating structure in the DN hydrogel causes the units of the component, which is insensitive to the given stimulus in the form of the single network (SN) hydrogel, to be partially formed as globular structures in DN hydrogel. Due to the hydrophilic PAAm groups, temperature- and salt-induced changes in the deswelling of the DN hydrogel are less intensive and gradual compared to those of the SN hydrogel. The swelling ratio of the DN hydrogel shows a significant decrease in the dependence on the acetone content in acetone-water mixtures. A certain portion of the solvent molecules bound in the globular structures was established from the measurements of the 1H NMR spin-spin relaxation times T2 for the studied DN hydrogel. The time-dependent deswelling and reswelling kinetics showed a two-step profile, corresponding to the solvent molecules being released and absorbed during two processes with different characteristic times.