Design and Fabrication of Highly Stretchable and Tough Hydrogels

Hydronium-Crosslinked Inorganic Hydrogel Comprised of 1D Lepidocrocite Titanate Nanofilaments

When a few drops of acid (hydrochloric, acrylic, propionic, acetic, or formic) are added to a colloid comprised of 1D lepidocrocite titanate nanofilaments (1DLs)-2 × 2 TiO6 octahedra in cross-section-a hydrogel forms, in many cases, within seconds. The 1DL synthesis process requires the reaction between titanium diboride with tetramethylammonium (TMA+), hydroxide. Using quantitative nuclear magnetic resonance (qNMR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), the mass percent of TMA+ after synthesis is determined to be ≈ 13.1 ± 0.1%. The TMA+ is completely removed from the gels after 2 water soak cycles, resulting in the first completely inorganic, TiO2-based hydrogels. Ion exchanging the TMA+ with hydronium results in gels with relatively strong hydrogen bonds. The hydrogels' compression strengths increased linearly with 1DL colloid concentration. At a 1DL concentration of 45 g L-1, the compressive strength, at 80% deformation when acrylic acid is used, is ≈325 kPa. The strengths are ≈ 50% greater after the TMA+ is removed. The removal of all residual organic components in the hydrogels, including TMA+, is confirmed by qNMR, Fourier-transformed infrared spectroscopy (FTIR), and TGA/DSC. The 1DL phase is retained after gelation, TMA+ removal, and 80% compression.

Rational Design of a Multifunctional Hydrogel Trap for Water and Fertilizer Capture: A Review

Water scarcity and land infertility pose significant challenges to agricultural development, particularly in arid and semiarid regions. Improving soil-water-retention capacity and fertilizer utilization efficiency through the application of soil additives has become a pivotal approach in agricultural practices. Hydrogels exhibit exceptional water absorption and fertilizer retention capabilities, making them extensively utilized in the fields of agriculture, forestry, and desert control. Currently, most reviews primarily focus on the raw materials, classification, synthesis methods, and application prospects of hydrogels, with limited attention given to strategies for enhancing water-retention performance, mechanisms underlying fertilizer absorption, and environmental risks. This review covers the commonly used cross-linking methods in hydrogel synthesis and the structure-activity relationship between hydrogels and water as well as fertilizer. Additionally, a thorough analysis of the ecological benefits and risks associated with hydrogels is presented. Finally, future prospects and challenges are delineated from the perspectives of material design and engineering applications.

Wireless Sensor System Based on Organohydrogel Ionic Skin for Physiological Activity Monitoring

Supermolecular hydrogel ionic skin (i-skin) linked with smartphones has attracted widespread attention in physiological activity detection due to its good stability in complex scenarios. However, the low ionic conductivity, inferior mechanical properties, poor contact adhesion, and insufficient freeze resistance of most used hydrogels limit their practical application in flexible electronics. Herein, a novel multifunctional poly(vinyl alcohol)-based conductive organohydrogel (PCEL5.0%) with a supermolecular structure was constructed by innovatively employing sodium carboxymethyl cellulose (CMC-Na) as reinforcement material, ethylene glycol as antifreeze, and lithium chloride as a water retaining agent. Thanks to the synergistic effect of these components, the PCEL5.0% organohydrogel shows excellent performance in terms of ionic conductivity (1.61 S m-1), mechanical properties (tensile strength of 70.38 kPa and elongation at break of 537.84%), interfacial adhesion (1.06 kPa to pig skin), frost resistance (-50.4 °C), water retention (67.1% at 22% relative humidity), and remoldability. The resultant PCEL5.0%-based i-skin delivers satisfactory sensitivity (GF = 1.38) with fast response (348 ms) and high precision under different deformations and low temperature (-25 °C). Significantly, the wireless sensor system based on the PCEL5.0% organohydrogel i-skin can transmit signals from physiological activities and sign language to a smartphone by Bluetooth technology and dynamically displays the status of these movements. The organohydrogel i-skin shows great potential in diverse fields of physiological activity detection, human-computer interaction, and rehabilitation medicine.

Effects of concentration of hydrophobic component and swelling in saline solutions on mechanical properties of a stretchable hydrogel

An elastic biopolymer, resilin possesses exceptional qualities such as high stretchability and resilience. Such attributes are utilized in nature by many species for mechanical energy storage to facilitate movement. The properties of resilin are attributed to the balanced combination of hydrophilic and hydrophobic segments. To mimic the properties of resilin, we developed a hydrogel system composed of hydrophilic acrylic acid (AAc) and methacrylamide (MAM) chains and hydrophobic poly(propylene glycol diacrylate) (PPGDA) chains. The gel was produced through free-radical polymerization in 0.8 M NaCl solutions using KPS as an initiator. In these gels, AAc and MAM can form hydrogen bonds, whereas the association between PPGDA chains can lead to hydrophobic domains. The PPGDA concentration affects the level of hydrogen bonding and gel mechanical properties. Tensile experiments revealed that the elastic modulus increased with a higher PPGDA concentration. Retraction experiments demonstrated increased velocity and acceleration when released from a stretched state with increasing PPGDA concentration. Swelling and deswelling of gels in saline solutions led to a change in mechanical properties and retraction behavior. This study shows that the stretchability and resilience of these hydrogels can be adjusted by changing the concentration of hydrophobic components.

Hydrogel dressing integrating FAK inhibition and ROS scavenging for mechano-chemical treatment of atopic dermatitis

Atopic dermatitis (AD) is a chronic skin disease caused by skin immune dyshomeostasis and accompanied by severe pruritus. Although oxidative stress and mechanical scratching can aggravate AD inflammation, treatment targeting scratching is often overlooked, and the efficiency of mechano-chemically synergistic therapy remains unclear. Here, we find that enhanced phosphorylation of focal adhesion kinase (FAK) is associated with scratch-exacerbated AD. We then develop a multifunctional hydrogel dressing that integrates oxidative stress modulation with FAK inhibition to synergistically treat AD. We show that the adhesive, self-healing and antimicrobial hydrogel is suitable for the unique scratching and bacterial environment of AD skin. We demonstrate that it can scavenge intracellular reactive oxygen species and reduce mechanically induced intercellular junction deficiency and inflammation. Furthermore, in mouse AD models with controlled scratching, we find that the hydrogel alleviates AD symptoms, rebuilds the skin barrier, and inhibits inflammation. These results suggest that the hydrogel integrating reactive oxygen species scavenging and FAK inhibition could serve as a promising skin dressing for synergistic AD treatment.

Skin-Inspired Ultra-Tough Supramolecular Multifunctional Hydrogel Electronic Skin for Human-Machine Interaction

Multifunctional supramolecular ultra-tough bionic e-skin with unique durability for human-machine interaction in complex scenarios still remains challenging. Herein, we develop a skin-inspired ultra-tough e-skin with tunable mechanical properties by a physical cross-linking salting-freezing-thawing method. The gelling agent (β-Glycerophosphate sodium: Gp) induces the aggregation and binding of PVA molecular chains and thereby toughens them (stress up to 5.79 MPa, toughness up to 13.96 MJ m-3). Notably, due to molecular self-assembly, hydrogels can be fully recycled and reprocessed by direct heating (100 °C for a few seconds), and the tensile strength can still be maintained at about 100% after six recoveries. The hydrogel integrates transparency (> 60%), super toughness (up to 13.96 MJ m-3, bearing 1500 times of its own tensile weight), good antibacterial properties (E. coli and S. aureus), UV protection (Filtration: 80%-90%), high electrical conductivity (4.72 S m-1), anti-swelling and recyclability. The hydrogel can not only monitor daily physiological activities, but also be used for complex activities underwater and message encryption/decryption. We also used it to create a complete finger joint rehabilitation system with an interactive interface that dynamically presents the user's health status. Our multifunctional electronic skin will have a profound impact on the future of new rehabilitation medical, human-machine interaction, VR/AR and the metaverse fields.

Highly stretchable, injectable hydrogels with cyclic endurance and shape-stability in dynamic mechanical environments, by microunit reformation

Traditional injectable hydrogels have so far found it difficult to accommodate resistance to large deformation and shape-stability under cyclic deformation. Polyampholyte (PA) hydrogels exhibit resistance to large deformation, good fatigue-resistance and rapid self-healing under dynamic forces. The limitations of the preparation process result in non-injectability of polyampholyte (PA) hydrogels. Electrostatic interactions as a medium for resistance to large deformation and shape-stability after cyclic deformation in reformed injectable hydrogels has been explored in this study. The prepared hydrogels (as-prepared PA-N) were dried and smashed into microunits and then mixed with 0.9% NaCl solution to transform them into reformed hydrogels (as-reformed PA-N) via a needle to achieve injectability. The as-reformed PA-N could exhibit 913.6% elongation at break and showed shape-stability under cyclic deformation due to the efficient self-healing abilities of the microunits and the inherited structure of the prepared hydrogels, which are superior to those of current tough injectable hydrogels. Potential applications in elbow cyclic bending and frequent movement of mobile wounds have been proved in this study. Overall, the results showed that the as-reformed PA-N achieved convenient injectability with resistance to large deformation and shape-stability under cyclic deformation at the same time.

Bio-macromolecular design roadmap towards tough bioadhesives

Emerging sutureless wound-closure techniques have led to paradigm shifts in wound management. State-of-the-art biomaterials offer biocompatible and biodegradable platforms enabling high cohesion (toughness) and adhesion for rapid bleeding control as well as robust attachment of implantable devices. Tough bioadhesion stems from the synergistic contributions of cohesive and adhesive interactions. This Review provides a biomacromolecular design roadmap for the development of tough adhesive surgical sealants. We discuss a library of materials and methods to introduce toughness and adhesion to biomaterials. Intrinsically tough and elastic polymers are leveraged primarily by introducing strong but dynamic inter- and intramolecular interactions either through polymer chain design or using crosslink regulating additives. In addition, many efforts have been made to promote underwater adhesion via covalent/noncovalent bonds, or through micro/macro-interlock mechanisms at the tissue interfaces. The materials settings and functional additives for this purpose and the related characterization methods are reviewed. Measurements and reporting needs for fair comparisons of different materials and their properties are discussed. Finally, future directions and further research opportunities for developing tough bioadhesive surgical sealants are highlighted.

Highly Flexible and Broad-Range Mechanically Tunable All-Wood Hydrogels with Nanoscale Channels via the Hofmeister Effect for Human Motion Monitoring

Wood-based hydrogel with a unique anisotropic structure is an attractive soft material, but the presence of rigid crystalline cellulose in natural wood makes the hydrogel less flexible. In this study, an all-wood hydrogel was constructed by cross-linking cellulose fibers, polyvinyl alcohol (PVA) chains, and lignin molecules through the Hofmeister effect. The all-wood hydrogel shows a high tensile strength of 36.5 MPa and a strain up to ~ 438% in the longitudinal direction, which is much higher than its tensile strength (~ 2.6 MPa) and strain (~ 198%) in the radial direction, respectively. The high mechanical strength of all-wood hydrogels is mainly attributed to the strong hydrogen bonding, physical entanglement, and van der Waals forces between lignin molecules, cellulose nanofibers, and PVA chains. Thanks to its excellent flexibility, good conductivity, and sensitivity, the all-wood hydrogel can accurately distinguish diverse macroscale or subtle human movements, including finger flexion, pulse, and swallowing behavior. In particular, when "An Qi" was called four times within 15 s, two variations of the pronunciation could be identified. With recyclable, biodegradable, and adjustable mechanical properties, the all-wood hydrogel is a multifunctional soft material with promising applications, such as human motion monitoring, tissue engineering, and robotics materials.

Fabrication of highly stretchable hydrogel based on crosslinking between alendronates functionalized poly-γ-glutamate and calcium cations

We report a highly stretchable hydrogel based on the crosslinking structure between calcium cations and alendronates (ALN) conjugated with poly-γ-glutamate (γ-PGA), a typical biodegradable polymer. γ-PGA with ALN (γ-PGA-ALN) forms the hydrogel in the aqueous solution containing CaCl2. The hydrogel shows 2000% of stretchability and reversible stretching without failure at a strain of 250%. The fracture strain and stress are tunable by varying the concentration of either γ-PGA-ALN or CaCl2, indicating the importance of fine-tuning of the density of the cross-linkage to control the mechanical properties of the hydrogel. We believe the biodegradable polymer based highly stretchable hydrogel has potential for use in various fields such as tissue engineering.

Elastic MXene Hydrogel Microfiber-Derived Electronic Skin for Joint Monitoring

Effective and timely joint monitoring has been a significantly vital research direction in human healthcare. As an emerging technology, flexible electronics provides more possibilities and applicabilities for practical sensing and signal transmission. Here, we provide novel elastic MXene microfibers of controllable morphologies at a microscale through microfluidic technology for actual joint motion monitoring. Double-network hydrogels including covalently cross-linking polyacrylamide and ionically cross-linking alginate were chosen for superelasticity. For the improvement of the electrical conductivity of superelastic hydrogel microfibers, MXene was selected to mix with them. By introducing the cross-linker to the outer channel, microfibers with controllable diameters along with high electrical conductivities and tensile properties could be fabricated successfully. The practical value of the synthesized microfibers in joint movement sensing has been demonstrated by acting as the element of new motion sensors. Based on these features, it is believed that these elastic MXene hydrogel microfibers have high potential for rapid sensing and diagnosis of joint diseases.

A highly sensitive strain sensor based on a silica@polyaniline core-shell particle reinforced hydrogel with excellent flexibility, stretchability, toughness and conductivity

Hydrogel-based flexible strain sensors for personal health monitoring and human-machine interaction have attracted wide interest among researchers. In this paper, hydrophobic association and nanocomposite conductive hydrogels were successfully prepared by introducing polyaniline coated silica (SiO2@PANI) core-shell particles into an acrylamide-lauryl methacrylate (P(AM/LMA)) copolymer matrix. The hydrophobic interaction between the SiO2@PANI core-shell particles and the hydrophobic LMA in the P(AM/LMA) chains induced the hydrogels with outstanding mechanical properties. Furthermore, the polyaniline on the SiO2 surface and the inorganic salt formed a conductive network, which synergistically enhanced the conductivity of the hydrogels. The obtained hydrogels integrate high tensile strength (1398 kPa), ultra-stretchability (>1000%), wonderful strain sensitivity (gauge factor = 10.407 at 100-1100% strain), quick response (300 ms), and excellent durability (>300 cycles) due to the hydrophobic association and nanocomposite effect. The prepared SiO2@PANI-P(AM/LMA) hydrogel shows high stress sensitivity to detect human movements and displays a broad application prospect in flexible strain-sensor field.

Applications of Highly Stretchable and Tough Hydrogels

Stretchable and tough hydrogels have drawn a lot of attention recently. Due to their unique properties, they have great potential in the application in areas such as mechanical sensing, wound healing, and drug delivery. In this review, we will summarize recent developments of stretchable and tough hydrogels in these areas.