A Biomimetic Bilayer Hydrogel Actuator Based on Thermoresponsive Gelatin Methacryloyl-Poly(N-isopropylacrylamide) Hydrogel with Three-Dimensional Printability

Soft Actuators and Actuation: Design, Synthesis, and Applications

Soft actuators are one of the most promising technological advancements with potential solutions to diverse fields' day-to-day challenges. Soft actuators derived from hydrogel materials possess unique features such as flexibility, responsiveness to stimuli, and intricate deformations, making them ideal for soft robotics, artificial muscles, and biomedical applications. This review provides an overview of material composition and design techniques for hydrogel actuators, exploring 3D printing, photopolymerization, cross-linking, and microfabrication methods for improved actuation. It examines applications of hydrogel actuators in biomedical, soft robotics, bioinspired systems, microfluidics, lab-on-a-chip devices, and environmental, and energy systems. Finally, it discusses challenges, opportunities, advancements, and regulatory aspects related to hydrogel actuators.

Solvent-Triggered, Ultra-Adhesive, Conductive, and Biocompatible Transition Gels for Wearable Devices

The development of robust adhesive, conductive, and flexible materials has garnered significant attention in the realm of human-machine interface and electronic devices. Conventional preparation methods to achieve these exceptional properties rely on incorporating highly polar raw materials, multiple components, or solvents. However, the overexposure of functional groups and the inherent toxicity of organic solvents often render gels non-stick or potentially biocompatible making them unsuitable for human-contact devices. In this study, a straightforward three-step strategy is devised for preparing responsive adhesive gels without complex components. Structurally conductive poly(N-(2-hydroxyethyl)-acrylamide-co-p-styrene sulfonate hydrate) (PHEAA-NaSS) gels are synthesized by integrating ionic and hydrophilic networks with distinct solvent effects. Initially, the in-suit formed PHEAA-NaSS networks are activated by dimethyl sulfoxide, which substantially increases intramolecular hydrogen bonding and enhances the matrix stretchability and interfacial adhesion. Subsequently, ethanol exchange reduced solvent impact and led to a compact network that limited surface exposure of ionic and hydrophilic groups, resulting in nonstick, robust for convenient storage. Finally, upon contacting with water, the network demonstrates rehydration, resulting in favorable adhesion, biocompatibility, and conductivity. The proposed PHEAA-NaSS/W gels can stably and reliably capture joint motion and electrophysiological signals. Furthermore, this uncomplicated gel preparation method is also applicable to other electrolyte monomers.

Preparation of patterned hydrogels for anti-counterfeiting and directional actuation by shear-induced orientation of cellulose nanocrystals

Hydrogels with anisotropic structures are of great interest in the fields of bionic actuators, sensing and anti-counterfeiting due to their unique optical and stimulus response properties. Here we report an anisotropic cellulose nanocrystals/polyacrylamide (CNC/PAM) hydrogel with a patterned structure obtained by shear-induced orientation of CNC in precursor solution. Due to the difference in affinity between different slider surfaces and the precursor, patterned structures with different interference colors were realized by adhering the polypropylene (PP) film with a specific pattern to the bottom glass slider, which leads to differences in CNC orientation in different areas. These interfere color patterns can only be observed between crossed polarization, allowing the hydrogel to be used in applications of anti-counterfeiting and information encryption. Moreover, a complex and controllable 3D deformation is realized by introducing "zebra crossing" structure in the hydrogel. The opening and closing processes of flowers are vividly mimicked using the reversible swelling and shrinking properties of hydrogels in water and salt solutions, making the hydrogel promising for soft actuators.

Stimuli‐Responsive Hydrogels for Antibacterial Applications

Hydrogels have emerged as promising candidates for biomedical applications, especially in the field of antibacterial therapeutics, due to their unique structural properties, highly tunable physicochemical properties, and excellent biocompatibility. The integration of stimuli-responsive functions into antibacterial hydrogels holds the potential to enhance their antibacterial properties and therapeutic efficacy, dynamically responding to different external or internal stimuli, such as pH, temperature, enzymes, and light. Therefore, this review describes the applications of hydrogel dressings responsive to different stimuli in antibacterial therapy. The collaborative interaction between stimuli-responsive hydrogels and antibacterial materials is discussed. This synergistic approach, in contrast to conventional antibacterial materials, not only amplifies the antibacterial effect but also alleviates adverse side effects and diminishes the incidence of multiple infections and drug resistance. The review provides a comprehensive overview of the current challenges and outlines future research directions for stimuli-responsive antibacterial hydrogels. It underscores the imperative for ongoing interdisciplinary research aimed at unraveling the mechanisms of wound healing. This understanding is crucial for optimizing the design and implementation of stimuli-responsive antibacterial hydrogels. Ultimately, this review aims to offer scientific guidance for the development and practical clinical application of stimuli-responsive antibacterial hydrogel dressings.

Thermochromic Hydrogels with Adjustable Transition Behavior for Smart Windows

With the fast economic development and accelerating urbanization, more and more skyscrapers made entirely of concrete and glass are being constructed. To keep a comfortable indoor environment, massive energy for air conditioning or heating appliances is consumed. A huge amount of heat (>30%) is gained or released through glass windows. Using smart windows with the capability to modulate light is an effective way to reduce building energy consumption. Thermochromic hydrogel is one of the potential smart window materials due to its excellent thermal response, high radiation-blocking efficiency, cost-effectiveness, biocompatibility, and good uniformity. In this work, polyhydroxypropyl acrylate (PHPA) hydrogels with controllable lower critical solution temperature (LCST) were prepared by photopolymerization. The transition temperature and transition rate under "static transition" conditions were investigated. Unlike "static" conditions in which the transition temperature was not affected by the initial and final temperature and heating/cooling ramp, the transition temperature varied with the rate of temperature change under dynamic conditions. The "dynamic" transition temperature of the PHPA hydrogel gradually increased with the increase of the heating rate. It was the result of the movement of the molecular chains lagging behind the temperature change when the temperature change was too fast. The results of the solar irradiation experiment by filling PHPA hydrogels into double glazing windows showed that the indoor temperature was about 15 °C lower than that of ordinary glass windows, indicating that it can significantly reduce the energy consumption of air conditioning. In addition, a wide range of adjustable transition temperatures and fast optical response make PHPA hydrogels potentially applicable to smart windows.

Mechanically Tough and Conductive Hydrogels Based on Gelatin and Z-Gln-Gly Generated by Microbial Transglutaminase

Gelatin-based hydrogels with excellent mechanical properties and conductivities are desirable, but their fabrication is challenging. In this work, an innovative approach for the preparation of gelatin-based conductive hydrogels is presented that improves the mechanical and conductive properties of hydrogels by integrating Z-Gln-Gly into gelatin polymers via enzymatic crosslinking. In these hydrogels (Gel-TG-ZQG), dynamic π-π stacking interactions are created by the introduction of carbobenzoxy groups, which can increase the elasticity and toughness of the hydrogel and improve the conductivity sensitivity by forming effective electronic pathways. Moreover, the mechanical properties and conductivity of the obtained hydrogel can be controlled by tuning the molar ratio of Z-Gln-Gly to the primary amino groups in gelatin. The hydrogel with the optimal mechanical properties (Gel-TG-ZQG (0.25)) exhibits a high storage modulus, compressive strength, tensile strength, and elongation at break of 7.8 MPa at 10 °C, 0.15 MPa at 80% strain, 0.343 MPa, and 218.30%, respectively. The obtained Gel-TG-ZQG (0.25) strain sensor exhibits a short response/recovery time (260.37 ms/130.02 ms) and high sensitivity (0.138 kPa-1) in small pressure ranges (0-2.3 kPa). The Gel-TG-ZQG (0.25) hydrogel-based sensors can detect full-range human activities, such as swallowing, fist clenching, knee bending and finger pressing, with high sensitivity and stability, yielding highly reproducible and repeatable sensor responses. Additionally, the Gel-TG-ZQG hydrogels are noncytotoxic. All the results demonstrate that the Gel-TG-ZQG hydrogel has potential as a biosensor for wearable devices and health-monitoring systems.

Naked-Eye Visual Thermometer Based on Glycerol─Nonclose-Packed Photonic Crystals for Real-Time Temperature Sensing and Monitoring

Real-time sensing and monitoring of temperature are of great significance for assessing human health. The sensitivity and stability are inevitable issues for thermometers. In this study, a thermometer with the cylindrical thermochromic hydrogel was prepared for real-time visual monitoring of temperature, which had excellent temperature sensitivity, angle-independence axially, and environmental stability. The customization of their initial optical properties depended on the PMMA concentrations and the content of the hydrogel monomer. The glycerol introduced with solvent displacement formed hydrogen bonds with the hydrogel network, which stabilized their mechanical properties, and the reflection peak blue-shifted from 653 to 499 nm when tensile strain was 57.85%. At the same time, the environmental stability originated from the moisturizing properties of the glycerol, which enabled the hydrogel to reliably transmit the information on temperature into the air without losing moisture. The reflection peak of the cylindrical thermochromic hydrogel shifted from 657 to 455 nm when the temperature increased from 22 to 45 °C, which realized temperature visual monitoring in the full-color range. The temperature sensitivity of the glycerol─nonclose-packed photonic crystals remained stable for 1 month, which provided an optimal option for continuous visual temperature monitoring.

Novel Acid-Driven Bioinspired Self-Resettable Bilayer Hydrogel Actuator Mimicking Natural Muscles

Soft robots have great potential applications in manufacturing, disaster rescue, medical treatment, etc. Artificial muscle is one of the most important components of a soft robot. In previous years, hydrogel actuators that can be controllably deformed by the stimuli of external signals have been developed as good candidates for muscle-like materials. In this article, we successfully prepared a chemical fuel-driven self-resettable bilayer hydrogel actuator mimicking natural muscles with the aid of a new negative feedback reaction network. The actuator can temporarily deform upon the addition of H+ (chemical fuel). Subsequently, H+ accelerated the reaction between BrO3- and Fe(CN)64-, which consume H+. It resulted in the spontaneous recovery of the pH as well as the shape of the actuator. Such an actuator exhibits a great similarity with natural muscles in actuation mechanisms and automaticity in the manipulation compared to the widely reported stimuli-responsive hydrogel actuators. This illustrates that fuel-driven self-resettable hydrogel is a promising dynamic material for mimicking the functions of living creatures.

4D-Printed Hydrogel Actuators through Diffusion-Path Architecture Design

Recently, smart hydrogels have garnered considerable attention as biomedical devices, and several approaches have been introduced for their fabrication, including the incorporation of stimulus-responsive additives, utilization of molecular imprinting techniques, and application of multilayered hydrogels. However, the nonuniform properties resulting from these approaches limit the practical applications of hydrogels by causing inconsistent performance and behavior. In this study, we propose a novel approach to manipulating the swelling kinetics of hydrogels by engineering their diffusion-path architecture. By simply adjusting the diffusion path length within the hydrogel, we achieved a significant change in swelling kinetics. This approach enables precise control over the diffusion and transport processes within the hydrogel, resulting in enhanced swelling kinetics when reducing the diffusion path length. Furthermore, by strategically designing the diffusion-path architecture of a 3D-printed hydrogel specimen, we can fabricate smart hydrogel actuators that exhibit reversible shape transformations during swelling and deswelling through a nonequilibrium differential swelling. The proposed approach eliminates the need to modify the spatial properties of hydrogel structures such as cross-linking density, polymer, or additive compositions, thereby achieving uniform properties throughout the hydrogel and creating new possibilities for the development of advanced 4D-printed biomedical devices.

A systematic review of the role of 4D printing in sustainable civil engineering solutions

This systematic review, not financially supported by any funding body, aims to synthesize the current knowledge on the applications, potential benefits, and challenges of 4D printing in civil engineering, with a focus on its role in sustainable solutions. Comprehensive searches were conducted in Scopus, Web of Science, and Google Scholar using related keywords. Articles that discussed 4D printing within civil engineering and construction contexts, encompassing both conceptual and empirical studies, were included. The findings suggest that 4D printing, with its time-responsive transformation feature, can enhance design freedom, improve structural performance, and increase environmental efficiency in construction. However, challenges persist in material performance, scalability, and cost. Despite these, ongoing advancements signal potential future developments that could widen the opportunities for large-scale applications of 4D printing in civil engineering. The potential use of renewable, bio-based materials could also lead to more sustainable construction practices. This review highlights the transformative potential of 4D printing, underlining the need for further research to fully leverage its capabilities and address current limitations. 4D printing emerges as a promising avenue for sustainable civil engineering solutions, offering a transformative approach that calls for continued exploration and development.

Biomimetic Hydrogel Applications and Challenges in Bone, Cartilage, and Nerve Repair

Tissue engineering and regenerative medicine is a highly sought-after field for researchers aiming to compensate and repair defective tissues. However, the design and development of suitable scaffold materials with bioactivity for application in tissue repair and regeneration has been a great challenge. In recent years, biomimetic hydrogels have shown great possibilities for use in tissue engineering, where they can tune mechanical properties and biological properties through functional chemical modifications. Also, biomimetic hydrogels provide three-dimensional (3D) network spatial structures that can imitate normal tissue microenvironments and integrate cells, scaffolds, and bioactive substances for tissue repair and regeneration. Despite the growing interest in various hydrogels for biomedical use in previous decades, there are still many aspects of biomimetic hydrogels that need to be understood for biomedical and clinical trial applications. This review systematically describes the preparation of biomimetic hydrogels and their characteristics, and it details the use of biomimetic hydrogels in bone, cartilage, and nerve tissue repair. In addition, this review outlines the application of biomimetic hydrogels in bone, cartilage, and neural tissues regarding drug delivery. In particular, the advantages and shortcomings of biomimetic hydrogels in biomaterial tissue engineering are highlighted, and future research directions are proposed.

An Anisotropic Hydrogel by Programmable Ionic Crosslinking for Sequential Two-Stage Actuation under Single Stimulus

As one of the most important anisotropic intelligent materials, bi-layer stimuli-responsive actuating hydrogels have proven their wide potential in soft robots, artificial muscles, biosensors, and drug delivery. However, they can commonly provide a simple one-actuating process under one external stimulus, which severely limits their further application. Herein, we have developed a new anisotropic hydrogel actuator by local ionic crosslinking on the poly(acrylic acid) (PAA) hydrogel layer of the bi-layer hydrogel for sequential two-stage bending under a single stimulus. Under pH = 13, ionic-crosslinked PAA networks undergo shrinking (-COO−/Fe3+ complexation) and swelling (water absorption) processes. As a combination of Fe3+ crosslinked PAA hydrogel (PAA@Fe3+) with non-swelling poly(3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate) (PZ) hydrogel, the as-prepared PZ-PAA@Fe3+ bi-layer hydrogel exhibits distinct fast and large-amplitude bidirectional bending behavior. Such sequential two-stage actuation, including bending orientation, angle, and velocity, can be controlled by pH, temperature, hydrogel thickness, and Fe3+ concentration. Furthermore, hand-patterning Fe3+ to crosslink with PAA enables us to achieve various complex 2D and 3D shape transformations. Our work provides a new bi-layer hydrogel system that performs sequential two-stage bending without switching external stimuli, which will inspire the design of programmable and versatile hydrogel-based actuators.