Rigid and Strong Thermoresponsive Shape Memory Hydrogels Transformed from Poly(vinylpyrrolidone- co-acryloxy acetophenone) Organogels

Silver and Copper Nanoparticle-Loaded Self-Assembled Pseudo-Peptide Thiourea-Based Organic-Inorganic Hybrid Gel with Antibacterial and Superhydrophobic Properties for Antifouling Surfaces

The escalating threat of antimicrobial resistance has become a global health crisis. Therefore, there is a rising momentum in developing biomaterials with self-sanitizing capabilities and inherent antibacterial properties. Despite their promising antimicrobial properties, metal nanoparticles (MNPs) have several disadvantages, including increased toxicity as the particle size decreases, leading to oxidative stress and DNA damage that need consideration. One solution is surface functionalization with biocompatible organic ligands, which can improve nanoparticle dispersibility, reduce aggregation, and enable targeted delivery to microbial cells. The existing research predominantly concentrates on the advancement of peptide-based hydrogels for coating materials to prevent bacterial infection, with limited exploration of developing surface coatings using organogels. Herein, we have synthesized organogel-based coatings doped with MNPs that can offer superior hydrophobicity, oleophobicity, and high stability that are not easily achievable with hydrogels. The self-assembled gels displayed distinct morphologies, as revealed by scanning electron microscopy and atomic force microscopy. The cross-linked matrix helps in the controlled and sustained release of MNPs at the site of bacterial infection. The synthesized self-assembled gel@MNPs exhibited excellent antibacterial properties against harmful bacteria such as Escherichia coli and Staphylococcus aureus and reduced bacterial viability up to 95% within 4 h. Cytotoxicity testing against metazoan cells demonstrated that the gels doped with MNPs were nontoxic (IC50 > 100 μM) to mammalian cells. Furthermore, in this study, we coated the organogel@MNPs on cotton fabric and tested it against Gram +ve and Gram -ve bacteria. Additionally, the developed cotton fabric exhibited superhydrophobic properties and developed a barrier that limits the interaction between bacteria and the surface, making it difficult for bacteria to adhere and colonize, which holds potential as a valuable resource for self-cleaning coatings.

Shape Memory Hydrogels for Biomedical Applications

The ability of shape memory polymers to change shape upon external stimulation makes them exceedingly useful in various areas, from biomedical engineering to soft robotics. Especially, shape memory hydrogels (SMHs) are well-suited for biomedical applications due to their inherent biocompatibility, excellent shape morphing performance, tunable physiochemical properties, and responsiveness to a wide range of stimuli (e.g., thermal, chemical, electrical, light). This review provides an overview of the unique features of smart SMHs from their fundamental working mechanisms to types of SMHs classified on the basis of applied stimuli and highlights notable clinical applications. Moreover, the potential of SMHs for surgical, biomedical, and tissue engineering applications is discussed. Finally, this review summarizes the current challenges in synthesizing and fabricating reconfigurable hydrogel-based interfaces and outlines future directions for their potential in personalized medicine and clinical applications.

Natural Oleanolic Acid-Tailored Eutectogels Featuring Multienvironment Shape Memory Performance

Shape memory gels, one of the primary modern smart materials, hold great promise in a myriad of applications spanning from soft robotics to medical devices. Nevertheless, most shape memory gels rely on water, organic solvents, and ionic liquids as dispersion mediums, posing the risks of freezing, dehydration, and toxicity to humans or environment. Herein, we have developed a thermoresponsive shape memory eutectogel by introducing an oleanolic acid-modified polyacrylamide network into a deep eutectic solvent (DES). The resulting eutectogel shows a fracture strength of 4.46 MPa along with elongation of 345%, Young's modulus of 14.83 MPa, and toughness of 9.51 MJ m-3. Thanks to the low freezing point and low volatility inherited from DES, this eutectogel possesses good antifreezing and long-term storage stability, which facilitate the shape memory behavior both in silicone oil and in air. The shape fixity and shape recovery ratios of this eutectogel maintain almost 90% during 10 cycles in silicone oil and more than 70% during four cycles in air that cannot be realized in hydrogels. By virtue of shape memory effect and conductivity, the eutectogel can be further used as a thermoswitch. This work presents a simple approach to fabricating shape memory eutectogels and imparts exciting prospects to smart eutectogels.

Designing strong, fast, high-performance hydrogel actuators

Hydrogel actuators displaying programmable shape transformations are particularly attractive for integration into future soft robotics with safe human-machine interactions. However, these materials are still in their infancy, and many significant challenges remain presenting impediments to their practical implementation, including poor mechanical properties, slow actuation speed and limited actuation performance. In this review, we discuss the recent advances in hydrogel designs to address these critical limitations. First, the material design concepts to improve mechanical properties of hydrogel actuators will be introduced. Examples are also included to highlight strategies to realize fast actuation speed. In addition, recent progress about creating strong and fast hydrogel actuators are sumarized. Finally, a discussion of different methods to realize high values in several aspects of actuation performance metrics for this class of materials is provided. The advances and challenges discussed in this highlight could provide useful guidelines for rational design to manipulate the properties of hydrogel actuators toward widespread real-world applications.

A fucoidan-gelatin wound dressing accelerates wound healing by enhancing antibacterial and anti-inflammatory activities

Microbial infections and the slow regression of inflammation are major impediments to wound healing. Herein, a tilapia fish skin gelatin-fucose gum-tannic acid (Gel&Fuc-TA) hydrogel wound dressing (Gel&Fuc-TA) was designed to promote wound healing by mixing and reacting tannic acid (TA) with tilapia fish skin gelatin (Gel) and fucoidan (Fuc). Gel&Fuc-TA hydrogel has a good network structure as well as swelling and release properties, and shows excellent antibacterial, antioxidant, cell compatibility, and hemostatic properties. Gel&Fuc-TA hydrogel can promote the expression of vascular endothelial growth factor (VEGF), platelet endothelial cell adhesion molecule-1 (CD-31), and alpha-smooth muscle actin (α-SMA), enhance collagen deposition, and accelerate wound repair. Gel&Fuc-TA hydrogel can change the wound microbiome, reduce wound microbiome colonization, and decrease the expression of microbiome-related proinflammatory factors, such as lipopolysaccharide (LPS), Toll-like receptor 2 (TLR2), and Toll-like receptor 4 (TLR4). Gel&Fuc-TA hydrogel effectively regulates the conversion of wound macrophages to the M2 (anti-inflammatory phenotype) phenotype, decreases the expression of interleukin-6 (IL-6), interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α), and increases the expression of arginase-1 (Arg-1), interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β), thereby reducing the inflammatory response. In summary, Gel&Fuc-TA hydrogel prepared using a rational green cross-linking reaction can effectively accelerate wound healing.

Research Progress of Shape Memory Polymer and 4D Printing in Biomedical Application

As a kind of smart material, shape memory polymer (SMP) shows great application potential in the biomedical field. Compared with traditional metal-based medical devices, SMP-based devices have the following characteristics: 1) The adaptive ability allows the biomedical device to better match the surrounding tissue after being implanted into the body by minimally invasive implantation; 2) it has better biocompatibility and adjustable biodegradability; 3) mechanical properties can be regulated in a large range to better match with the surrounding tissue. 4D printing technology is a comprehensive technology based on smart materials and 3D printing, which has great application value in the biomedical field. 4D printing technology breaks through the technical bottleneck of personalized customization and provides a new opportunity for the further development of the biomedical field. This paper summarizes the application of SMP and 4D printing technology in the field of bone tissue scaffolds, tracheal scaffolds, and drug release, etc. Moreover, this paper analyzes the existing problems and prospects, hoping to provide a preliminary discussion and useful reference for the application of SMP in biomedical engineering.

Bioinspired Polyacrylamide/(polyvinyl alcohol-copper acetate) Hydrogel with Cooling-triggered Shape Memory, Color Changing, and Self-healing Behavior

Inspired by many living creatures with adjustment of shape and color in ever-changing environment, color changeable shape memory hydrogels are designed and expected to be potential candidates in the fields spanning from anti-counterfeiting to biomedical devices. However, they normally require complex synthesis, and more importantly, the cooling-induced shape recovery hydrogel is still rare and in its infancy so far. Herein, we have developed a unique color changeable shape memory hydrogel by simply incorporating polyvinyl alcohol and copper acetate into covalent polyacrylamide network. As core functional element, copper ions serve as reversible crosslinks after heating to achieve excellent cooling-triggered shape memory effect, color shifting and self-healing behavior, showing significant potential in diverse applications like grabbing, information encryption, and biomimetic designs. This work may guide the development of cooling-triggered smart hydrogels for practical applications. This article is protected by copyright. All rights reserved.

Universal Strategy for Designing Shape Memory Hydrogels

Smart polymeric biomaterials have been the focus of many recent biomedical studies, especially those with adaptability to defects and potential to be implanted in the human body. Herein we report a versatile and straightforward method to convert non-thermoresponsive hydrogels into thermoresponsive systems with shape memory ability. As a proof of concept, a thermoresponsive polyurethane mesh was embedded within a methacrylated chitosan (CHTMA), gelatin (GELMA), laminarin (LAMMA) or hyaluronic acid (HAMA) hydrogel network, which afforded hydrogel composites with shape memory ability. With this system, we achieved good to excellent shape fixity ratios (50-90%) and excellent shape recovery ratios (∼100%, almost instantaneously) at body temperature (37 °C). Cytocompatibility tests demonstrated good viability either with cells on top or encapsulated during all shape memory processes. This straightforward approach opens a broad range of possibilities to convey shape memory properties to virtually any synthetic or natural-based hydrogel for several biological and nonbiological applications.

Freeze-thaw and solvent-exchange strategy to generate physically cross-linked organogels and hydrogels of curdlan with tunable mechanical properties

Physical gels from natural polysaccharides present the advantage of no toxic cross-linking agents and no chemical modification during preparation. Herein, novel physical gels, transparent organogels and opaque hydrogels from the microorganism-derived (1,3)-β-D-glucan of curdlan were prepared in dimethyl sulfoxide (DMSO) using the freeze-thaw technique, followed by a solvent-exchange strategy with water. The mechanical and structural properties of these gels were investigated by rheology, scanning electron microscopy, attenuated total reflection infrared spectroscopy, wide-angle X-ray diffraction and small-angle X-ray scattering. Gelation mechanisms and intermolecular interaction models have also been proposed. The good solvent DMSO serves as both a crosslinker and a pore-foaming agent in organogels. The reversible macromolecular conformation changes and phase separation of curdlan endow the gels with reversible transparency, volume change and tunable mechanical strength. The new design strategy of facile preparation and performance tuning provides a platform for developing new organogels and sterile hydrogels of curdlan.

Reversible Protein Capture and Release by Redox-Responsive Hydrogel in Microfluidics

Stimuli-responsive hydrogels have a wide range of potential applications in microfluidics, which has drawn great attention. Double cross-linked hydrogels are very well suited for this application as they offer both stability and the required responsive behavior. Here, we report the integration of poly(N-isopropylacrylamide) (PNiPAAm) hydrogel with a permanent cross-linker (N,N'-methylenebisacrylamide, BIS) and a redox responsive reversible cross-linker (N,N'-bis(acryloyl)cystamine, BAC) into a microfluidic device through photopolymerization. Cleavage and re-formation of disulfide bonds introduced by BAC changed the cross-linking densities of the hydrogel dots, making them swell or shrink. Rheological measurements allowed for selecting hydrogels that withstand long-term shear forces present in microfluidic devices under continuous flow. Once implemented, the thiol-disulfide exchange allowed the hydrogel dots to successfully capture and release the protein bovine serum albumin (BSA). BSA was labeled with rhodamine B and functionalized with 2-(2-pyridyldithio)-ethylamine (PDA) to introduce disulfide bonds. The reversible capture and release of the protein reached an efficiency of 83.6% in release rate and could be repeated over 3 cycles within the microfluidic device. These results demonstrate that our redox-responsive hydrogel dots enable the dynamic capture and release of various different functionalized (macro)molecules (e.g., proteins and drugs) and have a great potential to be integrated into a lab-on-a-chip device for detection and/or delivery.

Advances in Ultrasound-Responsive Hydrogels for Biomedical Applications

Various intelligent hydrogels have been developed for biomedical applications because they can achieve multiple, variable, controllable and reversible changes in their shape and properties in a spatial and temporal manner,...

Engineering Tough Metallosupramolecular Hydrogel Films with Kirigami Structures for Compliant Soft Electronics

A simple and effective approach is demonstrated to fabricate tough metallosupramolecular hydrogel films of poly(acrylic acid) by one-pot photopolymerization of the precursor solution in the presence of Zr⁴⁺ ions that form coordination complexes with the carboxyl groups and serve as the physical crosslinks of the matrix. Both as-prepared and equilibrated hydrogel films are transparent, tough, and stable over a wide range of temperature, ionic strength, and pH. The thickness of the films can be easily tailored with minimum value of ≈7 μm. Owing to the fast polymerization and gelation process, kirigami structures can be facilely encoded to the gel films by photolithographic polymerization, affording versatile functions such as additional stretchability and better compliance of the planar films to encapsulate objects with sophisticated geometries that are important for the design of soft electronics. By stencil printing of liquid metal on the hydrogel film with a kirigami structure, the integrated soft electronics shows good compliance to cover curved surfaces and high sensitivity to monitor human motions. Furthermore, this strategy is applied to diverse natural and synthetic macromolecules containing carboxyl groups to develop tough hydrogel films, which will open opportunities for the applications of hydrogel films in biomedical and engineering fields.

One-step mild preparation of tough and thermo-reversible poly(vinyl alcohol) hydrogels induced by small molecules

To overcome shortcomings of the traditional freeze-thaw method for PVA hydrogel preparation, we develop a one-step mild method, which induces PVA crystallization to form hydrogels through small molecules containing hydroxyl and carboxyl groups. The obtained hydrogels showed high mechanical properties, untypical plasticity with short gelation time and repeatable sol-gel transformation.

Photoregulated Gradient Structure and Programmable Mechanical Performances of Tough Hydrogels with a Hydrogen-Bond Network

Gradient materials exist widely in natural living organisms, affording fascinating biological and mechanical properties. However, the synthetic gradient hydrogels are usually mechanically weak or only have relatively simple gradient structures. Here, we report on tough nanocomposite hydrogels with designable gradient network structure and mechanical properties by a facile post-photoregulation strategy. Poly(1-vinylimidazole-co-methacrylic acid) hydrogels containing gold nanorods (AuNRs) are in a glassy state and show typical yielding and forced elastic deformation at room temperature. The gel slightly contracts its volume when the temperature is above the glass-transition temperature that results in a collapse of the chain segments and formation of denser intra- and interchain hydrogen bonds. Consequently, the mechanical properties of the gels are enhanced, when the temperature returns to room temperature. The mechanical performances of hydrogels can also be locally tuned by near-infrared light irradiation due to the photothermal effect of AuNRs. Hydrogels with arbitrary two-dimensional gradients can be facilely developed by site-specific photoirradiation. The treated and untreated regions with different stiffness and yielding stress possess construct behaviors in stretching or twisting deformations. A locally reinforced hydrogel with the kirigami structure becomes notch-insensitive and exhibits improved strength and stretchability because the treated regions ahead the cuts have better resistance to crack advancement. These tough hydrogels with programmable gradient structure and mechanics should find applications as structural elements, biological devices, etc.

Ultrafast and Programmable Shape Memory Hydrogel of Gelatin Soaked in Tannic Acid Solution

Shape memory hydrogels have been paid plenty of attention as a kind of intelligent soft material. However, complicated preparation and slow and uncontrollable shape change have hindered their applications in smart actuators. In this work, a temperature-responsive strong hydrogel was prepared by a facial soaking method without any chemical reactions, i.e., soaking gelatin hydrogel in aqueous tannic acid solution. The hydrogel was constructed by hydrogen bonding between gelatin and tannic acid beside the triple helix of gelatin chains without any chemical cross-linkers. The hydrogel showed ultrafast shape memory and body-temperature response. The hydrogel can be fixed in temporary shape in only 1 s at 25 °C and recover to the original shape in also 1 s at 37 °C, superior to the reported shape memory hydrogels. Furthermore, the hydrogel shape change can be programmed by fixing the temperature, and the designed shape is achieved stepwise by adjusting the recovery temperature. In addition, the hydrogel is stable in water without further swelling. These excellent features will initiate new prosperity of the shape memory hydrogel in biomedical technology, underwater actuators, and soft robots.

Mechanically Strong, Tough, and Shape Deformable Poly(acrylamide-co-vinylimidazole) Hydrogels Based on Cu2+ Complexation

Shape deformable hydrogels have drawn great attention due to their wide applications as soft actuators. Here we report a novel kind of mechanically strong, tough, and shape deformable poly(acrylamide-co-vinylimidazole) [poly(AAm-co-VI)] hydrogel prepared by photoinitiated copolymerization and the followed immersing in a Cu²⁺ aqueous solution. Strong Cu²⁺ complexation with imidazole groups dramatically enhances the mechanical properties of the hydrogels, whose tensile strength, elastic modulus, toughness, and fracture energy reach up to 7.7 ± 0.76 MPa, 15.4 ± 1.2 MPa, 23.2 ± 2.5 MJ m^-3, and 22.1 ± 2.3 kJ m^-2, respectively. More impressively, shape deformation (bending) can be easily achieved by coating Cu²⁺ solution on one side of hydrogel strips. Furthermore, precise control of the shape deformation from 1D to 2D and 2D to 3D can be achieved by adjusting Cu²⁺ concentration, coating time, region, and one or two side(s) of hydrogel samples. The Cu²⁺ complexation provides a simple way to simultaneously improve the mechanical properties of hydrogels and enable them with shape deformability. The mechanically strong, tough, and shape deformable hydrogels might be a promising candidate for soft actuators.

Cation-induced shape programming and morphing in protein-based hydrogels

Smart materials that are capable of memorizing a temporary shape, and morph in response to a stimulus, have the potential to revolutionize medicine and robotics. Here, we introduce an innovative method to program protein hydrogels and to induce shape changes in aqueous solutions at room temperature. We demonstrate our approach using hydrogels made from serum albumin, the most abundant protein in the blood plasma, which are synthesized in a cylindrical or flower shape. These gels are then programmed into a spring or a ring shape, respectively. The programming is performed through a marked change in stiffness (of up to 17-fold), induced by adsorption of Zn2+ or Cu2+ cations. We show that these programmed biomaterials can then morph back into their original shape, as the cations diffuse outside the hydrogel material. The approach demonstrated here represents an innovative strategy to program protein-based hydrogels to behave as actuators.

Comparative study of gelatin cryogels reinforced with hydroxyapatites with different morphologies and interfacial bonding

Gelatin cryogels are good candidate scaffolds for tissue engineering because of their interconnected macroporous structure. For bone regeneration, inorganic components are chosen to reinforce gelatin cryogels: (i) to mimic the compositions of natural bone tissue and (ii) to meet the mechanical requirements of bone repairing. Cryogels were prepared from methacrylated gelatin (GelMA) in this study, and hydroxyapatite nanorods (HANRs) with surface-grafted acrylate groups (D-HANRs) were synthesized to reinforce the cryogels, in which, the crosslinking between GelMA and D-HANRs was expected. In parallel, HANRs and hydroxyapatite nanowires (HANWs) were also composited with the GelMA cryogels to investigate the effects of filler morphology and interfacial bonding on the overall properties of the resulting composite cryogels comparatively. All these composite cryogels demonstrated potential as bone repairing materials by displaying excellent performances such as high porosity, appropriate water retention, shape recovery, and fast resilience features, as well as good biocompatibility and cell affinity. In comparison with the HANR composited GelMA cryogel, the HANWs were able to ameliorate the compression and the rheology performances of the resulting composite cryogels more efficiently due to the fact that the one-dimensional HANWs played a bridging role in the gelatin matrix. Among all the preparations, however, it was the D-HANRs that achieved the strongest reinforcement efficiency in mechanical properties because the double bonds on their surface could be photo-crosslinked with GelMA to form interfacial bonding. With these findings, we concluded that it was preferable for inorganic fillers designed for cryogel-type bone repairing materials to be in a one-dimensional morphology with surface functional groups to strengthen their interfacial bonding with the polymeric matrix.

Engineering hydrogels by soaking: from mechanical strengthening to environmental adaptation

The soaking strategy could not only strengthen hydrogels with superior mechanical properties but also provide the hydrogels with environmentally adapting properties.

Chemical unfolding of protein domains induces shape change in programmed protein hydrogels

Programmable behavior combined with tailored stiffness and tunable biomechanical response are key requirements for developing successful materials. However, these properties are still an elusive goal for protein-based biomaterials. Here, we use protein-polymer interactions to manipulate the stiffness of protein-based hydrogels made from bovine serum albumin (BSA) by using polyelectrolytes such as polyethyleneimine (PEI) and poly-L-lysine (PLL) at various concentrations. This approach confers protein-hydrogels with tunable wide-range stiffness, from ~10-64 kPa, without affecting the protein mechanics and nanostructure. We use the 6-fold increase in stiffness induced by PEI to program BSA hydrogels in various shapes. By utilizing the characteristic protein unfolding we can induce reversible shape-memory behavior of these composite materials using chemical denaturing solutions. The approach demonstrated here, based on protein engineering and polymer reinforcing, may enable the development and investigation of smart biomaterials and extend protein hydrogel capabilities beyond their conventional applications.

Highly Tough Hydrogels with the Body Temperature-Responsive Shape Memory Effect

Shape memory hydrogels (SMHs), a promising class of smart materials for biomedical applications, have attracted increasing research attention owing to their tissue-like water-rich network structure. However, preparing SMHs with high mechanical strength and body temperature-responsiveness has proven to be an extreme challenge. This study presents a facile and scalable methodology to prepare highly tough hydrogels with a body temperature-responsive shape memory effect based on synergetic hydrophobic interactions and hydrogen bonding. 2-Phenoxyethyl acrylate (PEA) and acrylamide were chosen as the hydrophobic monomer and the hydrophilic hydrogen bonding monomer, respectively. The prepared hydrogels exhibited a maximum tensile strength of 5.1 ± 0.16 MPa with satisfactory stretchability, and the mechanical strength showed a strong dependence on temperature. Besides, the hydrogel with 60 mol % PEA shows an excellent body temperature-responsive shape memory behavior with almost 100% shape fixity and shape recovery. Furthermore, we applied the hydrogels as a shape memory embolization plug for simulating vascular occlusion, and the embolism performance was preliminarily explored in vitro.

Photodirected Morphing Structures of Nanocomposite Shape Memory Hydrogel with High Stiffness and Toughness

Shape memory hydrogels have drawn increasing attention in recent years. Practical applications require these hydrogels to have good mechanical properties as well as contactless stimulations to trigger the shape deformations. Here we report a stiff and tough shape memory hydrogel that can transform to various configurations sequentially by phototriggered site-specific deformations. Response of the shape memory hydrogel to near-infrared (NIR) light irradiation was achieved by incorporating gold nanorods (AuNRs) into the glassy gel matrix of poly(methacrylic acid-co-methacrylamide) without compromising the excellent mechanical properties. Owing to the photothermal effect of the AuNRs, the localized temperature rise led to a dramatic decrease in Young's modulus (from 200 to 2 MPa) of the prestretched hydrogel and bending deformation with a programmable direction and amplitude. More complex three-dimensional configurations can be obtained by multidirectional prestretching and shape memorizing the individual parts of the nanocomposite hydrogel. Furthermore, the AuNRs embedded in the gel were aligned along the prestretching direction, leading to anisotropic plasmon resonance. These photomediated programmable deformations of tough shape memory hydrogels should find applications in the biomedical and engineering fields.

Combinational Hydrogel and Xerogel Actuators Showing NIR Manipulating Complex Actions

Near-infrared (NIR)-driven shape memory hydrogels are synthesized with a one-pot polymerization of N,N-dimethylacrylamide in the inorganic clay and graphene oxide (GO) suspension. The hydrogel consists of only a physically crosslinked network, which is partially thermoreversible. With the efficient photothermal energy transformation of GO in the hydrogels, the shape recovery from the temporal shape is achieved by NIR irradiation. The optimal shape fixing percentage and recovery rate are found at moderate monomer and crosslinker contents. Meanwhile, the xerogel dried from the hydrogel also shows a fast NIR response shape change. The NIR manipulating combinational hydrogel-xerogel actuators are prepared by combining the wet and soft hydrogel and its dry and rigid xerogel together. The actuators achieve complex actions of turning and lifting under sequential NIR irradiation to carry an object up- and downward and around obstacles, or to transfer an object to a target position. This work provides a new idea for designing combinational actuators to fulfil complex actions.

Modern Strategies To Achieve Tissue-Mimetic, Mechanically Robust Hydrogels

Hydrogels are frequently used biomaterials due to their similarity in hydration and structure to biological tissues. However, their utility is limited by poor mechanical properties, namely, a lack of strength and stiffness that mimic that of tissues, particularly load-bearing tissues. Thus, numerous recent strategies have sought to enhance and tune these properties in hydrogels, including interpenetrating networks (IPNs), macromolecular cross-linking, composites, thermal conditioning, polyampholytes, and dual cross-linking. Individually, these approaches have achieved hydrogels with either high strength (σ f > 10 MPa), high stiffness (E > 1 MPa), or, less commonly, both high strength and stiffness (σ f > 10 MPa and E > 1 MPa). However, only certain unique combinations of these approaches have been able to synergistically achieve retention of a high, tissuelike water content as well as high strength and stiffness. Applying such methods to stimuli-responsive hydrogels has also produced robust, smart biomaterials. Overall, methods to achieve hydrogels that simultaneously mimic the hydration, strength, and stiffness of soft and load-bearing tissues have the potential to be used in a much broader range of biomedical applications.

One-Step Synthesis of Healable Weak-Polyelectrolyte-Based Hydrogels with High Mechanical Strength, Toughness, and Excellent Self-Recovery

Excellent self-recovery is critically important for soft materials such as hydrogels and shape memory polymers. In this work, weak-polyelectrolyte-based hydrogels with high mechanical strength, toughness, healability, and excellent self-recovery are fabricated by one-step polymerization of acrylic acid and poly(ethylene glycol) methacrylate in the presence of oppositely charged branched polyethylenimine. The synergy of electrostatic and hydrogen-bonding interactions and the in situ formed polyelectrolyte complex nanoparticles endow the hydrogels with a tensile strength of ∼4.7 MPa, strain at break of ∼1200%, and toughness of ∼32.6 MJ m^-3. The hydrogels can recover from an ∼300% strain to their initial state within 10 min at room temperature without any external assistance. Moreover, the hydrogels can heal from physical cut at room temperature and exhibit a prominent shape-memory performance with rapid shape recovery speed and high shape-fixing and shape-recovery ratios.

Super-strong and tough poly(vinyl alcohol)/poly(acrylic acid) hydrogels reinforced by hydrogen bonding

Super-strong and tough poly(vinyl alcohol)/poly(acrylic acid) hydrogels based on hydrogen bonding are prepared by the strategy of immersing and cold-drawing.