Photothermally driven fast responding photo-actuators fabricated with comb-type hydrogels and magnetite nanoparticles

Facile Photoresponsive Actuators Based on Ferrocene-Doped Poly(butyl methacrylate)

This paper presents facile photoresponsive actuators comprising ferrocene as a guest chromophore and poly(butyl methacrylate) (PBMA) as a host matrix. The ferrocene-doped PBMA film exhibits mechanical expansion and contraction when a 445 nm laser is turned on and off, respectively. The photoresponsive film is attached by a commercially available acetylcellulose adhesive tape, which exhibits a bending motion that is controlled by turning the laser on and off. Thereafter, the double-layer film is employed to fabricate a table-shaped lifting machine (0.7 mg) that lifts a 10.5 mg object up and down by turning the laser on and off, respectively, and the mechanical force offered by the double-layer film is recorded. Additionally, the film is coated with gold and applied to an electric circuit that serves as a reversible photoresponsive switch. This film preparation technique is applied to other chromophores (e.g., Coumarin 343, Rhodamine 6G, Sudan Blue II, and Solvent Green 3) to independently control the motions of the films with 445, 520, and 655 nm lasers. The ferrocene-containing films also exhibit photoinduced healing from mechanical damage. Finally, the photoirradiation-accompanied morphological changes in the film are observed via small-angle X-ray scattering.

Artificial Cytoskeleton Assembly for Synthetic Cell Motility

Imitation of cellular processes in cell-like compartments is a current research focus in synthetic biology. Here, a method is introduced for assembling an artificial cytoskeleton in a synthetic cell model system based on a poly(N-isopropyl acrylamide) (PNIPAM) composite material. Toward this end, a PNIPAM-based composite material inside water-in-oil droplets that are stabilized with PNIPAM-functionalized and commercial fluorosurfactants is introduced. The temperature-mediated contraction/release behavior of the PNIPAM-based cytoskeleton is investigated. The reversibility of the PNIPAM transition is further examined in bulk and in droplets and it could be shown that hydrogel induced deformation could be used to controllably manipulate droplet-based synthetic cell motility upon temperature changes. It is envisioned that a combination of the presented artificial cytoskeleton with naturally occurring components might expand the bandwidth of the bottom-up synthetic biology.

Photoresponsive hydrogel-based soft robot: A review

Soft robots have received a lot of attention because of their great human-robot interaction and environmental adaptability. Most soft robots are currently limited in their applications due to wired drives. Photoresponsive soft robotics is one of the most effective ways to promote wireless soft drives. Among the many soft robotics materials, photoresponsive hydrogels have received a lot of attention due to their good biocompatibility, ductility, and excellent photoresponse properties. This paper visualizes and analyzes the research hotspots in the field of hydrogels using the literature analysis tool Citespace, demonstrating that photoresponsive hydrogel technology is currently a key research direction. Therefore, this paper summarizes the current state of research on photoresponsive hydrogels in terms of photochemical and photothermal response mechanisms. The progress of the application of photoresponsive hydrogels in soft robots is highlighted based on bilayer, gradient, orientation, and patterned structures. Finally, the main factors influencing its application at this stage are discussed, including the development directions and insights. Advancement in photoresponsive hydrogel technology is crucial for its application in the field of soft robotics. The advantages and disadvantages of different preparation methods and structures should be considered in different application scenarios to select the best design scheme.

Thermally and Magnetically Programmable Hydrogel Microactuators

The rapid development in micro-machinery enabled the investigation of smart materials that can embody fast response, programmable actuation, and flexibility to perform mechanical work. Soft magnetic actuators represent an interesting platform toward combining those properties. This study focuses on the synthesis of micro-actuators that respond to thermal and magnetic stimuli using micro-molding with a soft template as a fabrication technique. These microsystems consist of a hydrogel matrix loaded with anisotropic magnetic nanospindles. When a homogeneous magnetic field is applied, the nanospindles initially dispersed in monomer solution, align and assemble into dipolar chains. The ensuing UV-polymerization creates a network and conveniently arrests these nanostructures. Consequently, the magnetic dipole moment is coplanar with the microgel. Varying the shape, volume, and composition of the micro-actuators during synthesis provides a temperature-dependent control over the magnetic response and the polarizability. Beyond isotropic swelling, shaping the hydrogel as long thin ribbons with a passive layer on one side allows for differential swelling leading to bending and twisting deformations, for example, 2D- or 3D-spiral. These deformations involve a reversible amplification of the magnetic response and orientation of the hydrogels under magnetic field. Temperature control herewith determines the conformation and simultaneously the magnetic response of the micro-actuators.

Acousto-Photolithography for Programmable Shape Deformation of Composite Hydrogel Sheets

Stimuli-responsive hydrogels with programmable shapes produced by defined patterns of particles are of great interest for the fabrication of small-scale soft actuators and robots. Patterning the particles in the hydrogels during fabrication generally requires external magnetic or electric fields, thus limiting the material choice for the particles. Acoustically driven particle manipulation, however, solely depends on the acoustic impedance difference between the particles and the surrounding fluid, making it a more versatile method to spatially control particles. Here, an approach is reported by combining direct acoustic force to align photothermal particles and photolithography to spatially immobilize these alignments within a temperature-responsive poly(N-isopropylacrylamide) hydrogel to trigger shape deformation under temperature change and light exposure. The spatial distribution of particles can be tuned by the power and frequency of the acoustic waves. Specifically, changing the spacing between the particle patterns and position alters the bending curvature and direction of this composite hydrogel sheet, respectively. Moreover, the orientation (i.e., relative angle) of the particle alignments with respect to the long axis of laser-cut hydrogel strips governs the bending behaviors and the subsequent shape deformation by external stimuli. This acousto-photolithography provides a means of spatiotemporal programming of the internal heterogeneity of composite polymeric systems.

Poly(N-isopropylacrylamide)-Based Hydrogels for Biomedical Applications: A Review of the State-of-the-Art

A prominent research topic in contemporary advanced functional materials science is the production of smart materials based on polymers that may independently adjust their physical and/or chemical characteristics when subjected to external stimuli. Smart hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) demonstrate distinct thermoresponsive features close to a lower critical solution temperature (LCST) that enhance their capability in various biomedical applications such as drug delivery, tissue engineering, and wound dressings. Nevertheless, they have intrinsic shortcomings such as poor mechanical properties, limited loading capacity of actives, and poor biodegradability. Formulation of PNIPAM with diverse functional constituents to develop hydrogel composites is an efficient scheme to overcome these defects, which can significantly help for practicable application. This review reports on the latest developments in functional PNIPAM-based smart hydrogels for various biomedical applications. The first section describes the properties of PNIPAM-based hydrogels, followed by potential applications in diverse fields. Ultimately, this review summarizes the challenges and opportunities in this emerging area of research and development concerning this fascinating polymer-based system deep-rooted in chemistry and material science.

Fabrication of a tunable photothermal actuator via in situ oxidative polymerization of polydopamine nanoparticles in hydrogel bilayers

Photothermally triggered actuation enables the remote and local control of a material. The complex actuation can be achieved by controlling the photothermal efficiency of the material, which is crucial for the development of soft actuators. In this study, the photothermal efficiency of a hydrogel bilayer actuator consisting of a passive agarose/alginate double-network hydrogel layer and an active poly(N-isopropylacrylamide) (PNIPAm) layer was controlled via in situ oxidative polymerization of polydopamine nanoparticles (PDA NPs). Highly concentrated PDA NPs were successfully incorporated into the hydrogel bilayer without interrupting or weakening the polymer network during polymerization. The photothermal efficiency of the actuator was controlled using the number of polymerization cycles. Upon light irradiation, the heat generated by the photothermal effect of PDA NPs caused the shrinkage of the PNIPAm layer, resulting in the shape-morphing of the bilayer. The broad light absorption properties of PDA NPs allowed the bilayer to actuate under sunlight or visible light. Finally, we demonstrated controlled photothermal actuation using a pinwheel-shaped actuator consisting of four panels with different photothermal efficiencies.

Nanomaterials and their applications on bio-inspired wearable electronics

Wearable electronics featuring conformal attachment, sensitive perception and intellectual signal processing have made significant progress in recent years. However, when compared with living organisms, artificial sensory devices showed undeniable bulky shape, poor adaptability, and large energy consumption. To make up for the deficiencies, biological examples provide inspirations of novel designs and practical applications. In the field of biomimetics, nanomaterials from nanoparticles to layered two-dimensional materials are actively involved due to their outstanding physicochemical properties and nanoscale configurability. This review focuses on nanomaterials related to wearable electronics through bioinspired approaches on three different levels, interfacial packaging, sensory structure, and signal processing, which comprehensively guided recent progress of wearable devices in leveraging both nanomaterial superiorities and biorealistic functionalities. In addition, opinions on potential development trend are proposed aiming at implementing bioinspired electronics in multifunctional portable sensors, health monitoring, and intelligent prosthetics.

Superabsorbent polymers: A state-of-art review on their classification, synthesis, physicochemical properties, and applications

Superabsorbent polymers (SAP) and modified natural polymer hydrogels are widely and increasingly used in agriculture, health care textiles, effluent treatment, drug delivery, tissue engineering, civil concrete structure, etc. However, not many comprehensive reviews are available on this class of novel polymers. A review covering all the viable applications of SAP will be highly useful for researchers, industry persons, and medical, healthcare, and agricultural purposes. Hence, an attempt has been made to review SAPs with reference to their classifications, synthesis, modification by crosslinking, and physicochemical characterization such as morphology, swellability, thermal and mechanical properties, lifetime prediction, thermodynamics of swelling, absorption, release and transport kinetics, quantification of hydrophilic groups, etc. Besides, the possible methods of fine-tuning their structures for improving their absorption capacity, fast absorption kinetics, mechanical strength, controlled release features, etc. were also addressed to widen their uses. This review has also highlighted the biodegradability, commercial viability and market potential of SAPs, SAP composites, the feasibility of using biomass as raw materials for SAP production, etc. The challenges and future prospects of SAP, their safety, and environmental issues are also discussed.

Tunable Sponge-Like Hierarchically Porous Hydrogels with Simultaneously Enhanced Diffusivity and Mechanical Properties

Crosslinked polymers and gels are important in soft robotics, solar vapor generation, energy storage, drug delivery, catalysis, and biosensing. However, their attractive mass transport and volume-changing abilities are diffusion-limited, requiring miniaturization to avoid slow response. Typical approaches to improving diffusion in hydrogels sacrifice mechanical properties by increasing porosity or limit the total volumetric flux by directionally confining the pores. Despite tremendous efforts, simultaneous enhancement of diffusion and mechanical properties remains a long-standing challenge hindering broader practical applications of hydrogels. In this work, cononsolvency photopolymerization is developed as a universal approach to overcome this swelling-mechanical property trade-off. The as-synthesized poly(N-isopropylacrylamide) hydrogel, as an exemplary system, presents a unique open porous network with continuous microchannels, leading to record-high volumetric (de)swelling speeds, almost an order of magnitude higher than reported previously. This swelling enhancement comes with a simultaneous improvement in Young's modulus and toughness over conventional hydrogels fabricated in pure solvents. The resulting fast mass transport enables in-air operation of the hydrogel via rapid water replenishment and ultrafast actuation. The method is compatible with 3D printing. The generalizability is demonstrated by extending the technique to poly(N-tertbutylacrylamide-co-polyacrylamide) and polyacrylamide hydrogels, non-temperature-responsive polymer systems, validating the present hypothesis that cononsolvency is a generic phenomenon driven by competitive adsorption.

Light-driven bimorph soft actuators: design, fabrication, and properties

Soft robots that can move like living organisms and adapt to their surroundings are currently in the limelight from fundamental studies to technological applications, due to their advances in material flexibility, human-friendly interaction, and biological adaptation that surpass conventional rigid machines. Light-fueled smart actuators based on responsive soft materials are considered to be one of the most promising candidates to promote the field of untethered soft robotics, thereby attracting considerable attention amongst materials scientists and microroboticists to investigate photomechanics, photoswitch, bioinspired design, and actuation realization. In this review, we discuss the recent state-of-the-art advances in light-driven bimorph soft actuators, with the focus on bilayer strategy, i.e., integration between photoactive and passive layers within a single material system. Bilayer structures can endow soft actuators with unprecedented features such as ultrasensitivity, programmability, superior compatibility, robustness, and sophistication in controllability. We begin with an explanation about the working principle of bimorph soft actuators and introduction of a synthesis pathway toward light-responsive materials for soft robotics. Then, photothermal and photochemical bimorph soft actuators are sequentially introduced, with an emphasis on the design strategy, actuation performance, underlying mechanism, and emerging applications. Finally, this review is concluded with a perspective on the existing challenges and future opportunities in this nascent research Frontier.

Fabrication of a Dual Stimuli-Responsive Assorted Film Comprising Magnetic- and Gold-Nanoparticles with a Self-Assembly Protein of α-Synuclein

Development of sensing elements for controllable soft materials is crucial to improve their responsiveness toward remotely provided external stimuli. Magnetic nanoparticles (MNPs) and gold nanoparticles (AuNPs) have been coassembled into a flexible free-floating 2D film to produce a shape deformable mobile structure in the presence of magnetic field and light irradiation by employing a self-assembly protein of α-synuclein (αS). αS was demonstrated to be essential for the preparation of a multisensory system because the intrinsically disordered protein led to a complete dispersion of MNPs to an average size of 10 nm in aqueous solution, pH-dependent closely packed single layer adsorption of αS-MNPs, and α-helix-mediated free-floating MNP monolayer film formation upon dissolving the underlying polycarbonate substrate with chloroform. As AuNPs were incorporated into the assorted hybrid film in the presence of MNPs, however, the β-sheet component became prominent. By placing the assorted film between a spin-coated thin layer of thermoresponsive P(AAc-co-NIPAAm) hydrogel comprising acrylic acid and N-isopropylacrylamide and a passive layer of silicone elastomer, the resulting triply structure exhibited not only magnet-induced locomotion but also shape deformation due to asymmetric contraction of the sandwiching two layers caused by the heat generated by AuNPs upon near IR irradiation. In fact, two adjoining planar layers of another triply structure were shown to form a three-dimensional lotus flower with the light. This multisensory system is suggested to be further functionalized by modifying the αS molecules and incorporating additional nanoparticles to react to diverse stimuli, which would make the system be utilized in the areas of not only soft robotics but also foldable electronics, high-performance sensors/actuators, and medical/wearable applications.

Ibuprofen-Loaded Heparin Modified Thermosensitive Hydrogel for Inhibiting Excessive Inflammation and Promoting Wound Healing

Hydrogels have been investigated as ideal biomaterials for wound treatment owing to their ability to form a highly moist environment which accelerates cell migration and tissue regeneration for prompt wound healing. They can also be used as a drug carrier for local delivery, and are able to activate immune cells to enhance wound healing. Here, we developed heparin-conjugated poly(N-isopropylacrylamide), an injectable, in situ gel-forming polymer, and evaluated its use in wound healing. Ibuprofen was encapsulated into the hydrogel to help reduce pain and excessive inflammation during healing. In addition to in vitro studies, a BALB/c mice model was used to evaluate its effect on would healing and the secretion of inflammatory mediators. The in vitro assay confirmed that the ibuprofen released from the hydrogel dramatically reduced lipopolysaccharide-induced inflammation by suppressing the production of NO, PGE2 and TNF-α in RAW264.7 macrophages. Moreover, an in vivo wound healing assay was conducted by applying hydrogels to wounds on the backs of mice. The results showed that the ibuprofen-loaded hydrogel improved healing relative to the phosphate buffered saline group. This study indicates that ibuprofen loaded in an injectable hydrogel is a promising candidate for wound healing therapy.

Solid-phase colorimetric sensing probe for bromide based on a tough hydrogel embedded with silver nanoprisms

Sharp-tipped anisotropic silver (Ag) nanostructures are attracting increasing attention because of their unusual optical properties. However, the sharp tips make such nanostructures thermodynamically unstable; thus, they have been considered unsuitable for use in colorimetric sensing because of their tendency to aggregate or transform in a solution state. In the present study, a colorimetric sensing platform for detecting bromide (Br^-) in an aqueous medium was developed. The platform is based on the localized surface plasmon resonance (LSPR) properties of Ag nanoprisms with sharp tips. The key to using such Ag nanocrystals with extreme anisotropic structures is to adopt a solid-phase sensing platform. A Ag-nanoprism-embedded tough hydrogel with interpenetrating polymer networks was synthesized via aqueous-phase polymerization and crosslinking processes. The Ag nanoprisms immobilized inside the hydrogel were stable and did not exhibit aggregation or degradation over time; specifically, when the hydrogel was dried, the nanoprisms retained their inherent LSPR properties for an extended period. By taking advantage of the rapid and spontaneous morphological transformation of Ag nanoprisms inside the hybrid hydrogel exposed to Br^- and the corresponding changes in their LSPR properties, we designed a plasmonic sensing platform for the sensitive and selective detection of Br^- in an aqueous medium. The proposed colorimetric sensing platform was found to exhibit a wide sensing range and high selectivity, with a low limit of detection (LOD) of 10 μM, and offers substantial advantages over previously developed systems; specifically, it is portable, eco-friendly, safe to use and handle, stable for extended periods, and enables naked-eye detection. We believe that the as-proposed sensing platform can be used as a point-of-care analytical tool for detecting Br^- in a broad range of samples.

Ti3C2Tx MXene-Based Light-Responsive Hydrogel Composite for Bendable Bilayer Photoactuator

Soft actuators based on hydrogel materials, which can convert light energy directly into mechanical energy, are of the utmost importance, especially with enhancements in device development. However, the hunt for specific photothermal nanomaterials with distinct performance remains challenging. In this study, we successfully fabricated a bilayer hydrogel actuator consisting of an active photothermal layer from incorporated Ti₃C₂T_x MXene in poly(N-isopropylacrylamide) p(NIPAm)hydrogel structure and a passive layer from the N-(2-hydroxylethylpropyl)acrylamide (HEAA) hydrogel structure. The uniform and effective incorporation of MXene into the NIPAm hydrogel structures were characterized by a battery of techniques. The light responsive swelling properties of the MXene-embedded NIPAm-based hydrogel demonstrated fully reversible and repeatable behavior in the light on–off regime for up to ten consecutive cycles. The effect of MXene loading, the shape of the actuator, and the light source effects on the bilayer NIPAm-HEAA hydrogel structure were investigated. The bilayer hydrogel with MXene loading of 0.3% in the NIPAm hydrogel exhibited a 200% change of the bending angle in terms of its bidirectional shape/volume after 100 s exposure to white light at an intensity of 70 mW cm⁻². Additionally, the bending behavior under real sunlight was evaluated, showing the material’s potential applicability in practical environments.

Environmentally responsive hydrogels for repair of cardiovascular tissue

Cardiovascular diseases (CVDs) pose a serious threat to human health, which are characterized by high disability and mortality rate globally such as myocardial infarction (MI), atherosclerosis, and heart failure. Although stem cells transplantation and growth factors therapy are promising, their low survival rate and loss at the site of injury are major obstacles to this therapy. Recently, the development of hydrogel scaffold materials provides a new way to solve this problem, which have shown the potential to treat CVD. Among these scaffold materials, environmentally responsive hydrogels have great prospects in repairing the microenvironment of cardiovascular tissues and vascular regeneration. They provide a new method for the treatment of cardiovascular tissue repair and space-time control for the release of various therapeutic drugs, including small-molecule drugs, growth factors, and stem cells. Herein, this article reviews the occurrence and current treatment of CVD, as well as the repair of cardiovascular injury by several environmental responsive hydrogels systems currently used, mainly focusing on the delivery of growth factors or the application of cell therapy to revascularization. In addition, we will also discuss the enormous potential and personal perspectives of environmentally responsive hydrogels in cardiovascular repair.

Tunable swelling and deswelling of temperature- and light-responsive graphene oxide-poly(N-isopropylacrylamide) composite hydrogels

Swelling and deswelling behaviors of graphene oxide-poly(N-isopropylacrylamide) composite hydrogels can be tuned by the concentrations of a chemical crosslinker and graphene oxide, and the rate of the external temperature change.

An ultra-large deformation bidirectional actuator based on a carbon nanotube/PDMS composite and a chitosan film

Actuating materials can convert external stimuli (humidity, light, electricity, etc.) into mechanical energy and realize multiple forms of movements. However, a majority of current actuating materials are driven by a single stimulus with a small degree of actuation and rough control which is unfavorable for practical applications. Here, a new type of bidirectional actuating material based on carbon nanotube/PDMS composites and chitosan films is proposed. Thanks to the robust mechanical support by PDMS, due to the ultra-large water capacity in between chitosan chains and strong near-infrared light absorption by carbon nanotube layers, the actuator can be driven by humidity and light for an ultra-large actuation curvature (3.91 cm^-1 in humidity actuation, 3.84 cm^-1 in light actuation). The well-established light power-curvature, relative humidity-curvature profiles and a fine mechanic modelling of the actuator show the possibility of controlling the actuator's bending. A lab application as a cargo-moving device preliminarily demonstrates a robust mechanical functionality of this actuator with a low body weight.

Statistical Modeling of Photo-Bending Actuation of Hybrid Silicones Mixed with Azobenzene Powder

Mechanically responsive materials are promising as next-generation actuators for soft robotics, but have scarce reports on the statistical modeling of the actuation behavior. This research reports on the development and modeling of the photomechanical bending behavior of hybrid silicones mixed with azobenzene powder. The photo-responsive hybrid silicone bends away from the light source upon light irradiation when a thin paper is attached on the hybrid silicone. The time courses of bending behaviors were fitted well with exponential models with a time variable, affording fitting constants at each experimental condition. These fitted parameters were further modeled using the analysis of variance (ANOVA). Cubic models were proposed for both the photo-bending and unbending processes, which were parameterized by the powder ratio and the light intensity. This modeling process allows such photo-responsive materials to be controlled as actuators, and will possibly be effective for engineering mechanically responsive materials.

Synergy of Single-ion Conductive and Thermo-responsive Copolymer Hydrogels Achieving Anti-Arrhenius Ionic Conductivity

Artificial intelligence sensations have aroused scientific interest from electronic conductors to bio-inspired ionic conductors. The conductivity of electrons decreases with increasing temperature, while the ionic conductivity agrees with an Arrhenius equation or a modified Vogel-Tammann-Fulcher (VTF) equation. Herein, thermo-responsive poly(N-isopropyl amide) (PNIPAm) and single-ion-conducting poly(2-acrylamido-2-methyl-1-propanesulfonic lithium salt) (PAMPSLi) were copolymerized via a facile radical polymerization to demonstrate a very intriguing anti-Arrhenius ionic conductivity behaviour during thermally induced volume-phase transition. These smart hydrogels presented very promising scaffolds for architecting flexible, wearable or advanced functional ionic devices.

Dual-Gel 4D Printing of Bioinspired Tubes

The distribution of periodic patterns of materials with radial or bilateral symmetry is a universal natural design principle. Among the many biological forms, tubular shapes are a common motif in many organisms, and they are also important for bioimplants and soft robots. However, the simple design principle of strategic placement of 3D printed segments of swelling and nonswelling materials to achieve widely different functionalities is yet to be demonstrated. Here, we report the design, fabrication, and characterization of segmented 3D printed gel tubes composed of an active thermally responsive swelling gel (poly N-isopropylacrylamide) and a passive thermally nonresponsive gel (polyacrylamide). Using finite element simulations and experiments, we report a variety of shape changes including uniaxial elongation, radial expansion, bending, and gripping based on two gels. Actualization and characterization of thermally induced shape changes are of key importance to robotics and biomedical engineering. Our studies present rational approaches to engineer complex parameters with a high level of customization and tunability for additive manufacturing of dynamic gel structures.

Modeling of a fast-response magnetic-sensitive hydrogel for dynamic control of microfluidic flow

A magnetic-sensitive hydrogel-based microfluidic system is designed via a magneto-chemo-hydro-mechanical model for replicating various physiological and pathological conditions in the human body, by which the desired flow patterns can be generated in real time due to the fast-response deformation of the magnetic hydrogel. In the model, the fluid-structure interaction is characterized between the deformable magnetic hydrogel and surrounding fluid flow through the fully coupled arbitrary Lagrangian-Eulerian (ALE) method. Moreover, the physicochemical mechanisms including hydrogel magnetization, fluid diffusion, fluid flow, and hydrogel large deformation are characterized. After validation of the present model with both the finite difference and experimental results in the open literature, the transient behavior of the magnetic hydrogel is investigated, and the results show that the response time for the magnetic hydrogel is improved significantly in a uniform magnetic field compared with that of a hydrogel without the magnetic effect. Furthermore, various patterns of pulsatile flow are generated for mimicking the cell physiological microenvironment experienced by bone marrow stromal cells, and also for the pathological condition at the femoral artery during diastole and systole, respectively. Therefore, the present magnetic-sensitive hydrogel-based microfluidic system via the multiphysics model may provide a relevant humanized manipulation platform to investigate cell behavior and function through microfluidic chips.

Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures

Development of smart soft actuators is highly important for fundamental research and industrial applications but has proved to be extremely challenging. In this work, we present a facile, one-pot, one-step method to prepare dual-responsive bilayer hydrogels, consisting of a thermoresponsive poly( N-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly(3-(1-(4-vinylbenzyl)-1 H-imidazol-3-ium-3-yl)propane-1-sulfonate) (polyVBIPS) layer. Both polyNIPAM and polyVBIPS layers exhibit a completely opposite swelling/shrinking behavior, where polyNIPAM shrinks (swells) but polyVBIPS swells (shrinks) in salt solution (water) or at high (low) temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional bending in response to temperatures, salt concentrations, and salt types. Such bidirectional bending, bending orientation, and degree can be reversibly, repeatedly, and precisely controlled by salt- or temperature-induced cooperative swelling-shrinking properties from both layers. Based on their fast, reversible, and bidirectional bending behavior, we further design two conceptual hybrid hydrogel actuators, serving as a six-arm gripper to capture, transport, and release an object and an electrical circuit switch to turn on-and-off a lamp. Different from the conventional two- or multistep methods for preparation of bilayer hydrogels, our simple, one-pot, one-step method and a new bilayer hydrogel system provide an innovative concept to explore new hydrogel-based actuators through combining different responsive materials that allow us to program different stimuli for soft and intelligent materials applications.

Photothermally triggered disassembly of a visible dual fluorescent poly(ethylene glycol)/α-cyclodextrin hydrogel

The real-time tracking and adjustment of the disassembly and status of hydrogels in vivo are important challenges to accurate and precise assessment. In this article, a photothermally controllable, visible, dual fluorescent thermosensitive hydrogel was designed and developed based on a porphyrin-poly(ethylene glycol)/IR-820-α-cyclodextrin hydrogel. Due to the photothermal effect and fluorescence emission of IR-820, it can exert the dual functions of photothermal control and fluorescence imaging tracking. The IR-820 conjugated hydrogel can regulate the hydrogel disassembly by the photothermal effect of IR-820. Furthermore, each component of the hydrogel can be tracked by the fluorescence of IR-820 and porphyrin. Fluorescence imaging tracking and remote photothermal control were merged into the visible and controlled hydrogel disassembly after subcutaneous injection using mice as models. The dual fluorescence imaging visualization of cyclodextrin/poly(ethylene glycol) hydrogels revealed the disassembly process by tracking each component, and the hydrogel disassembly can be efficiently accelerated under laser irradiation with the photothermal effect of IR-820. This affords an important basis for understanding the disassembly process of the poly(ethylene glycol)/α-cyclodextrin hydrogel.

Polydopamine-Coated Main-Chain Liquid Crystal Elastomer as Optically Driven Artificial Muscle

Optically driven active materials have received much attention because their deformation and motion can be controlled remotely, instantly, and precisely in a contactless way. In this study, we investigated an optically actuated elastomer with rapid response: polydopamine (PDA)-coated liquid crystal elastomer (LCE). Because of the photothermal effect of PDA coating and thermal responsiveness of LCE, the elastomer film contracted significantly with near-infrared (NIR) irradiation. With a fixed strain, light-induced actuating stress in the film could be as large as 1.5 MPa, significantly higher than the maximum stress generated by most mammalian skeletal muscle (0.35 MPa). The PDA-coated LCE films could also bend or roll up by surface scanning of an NIR laser. The response time of the film to light exposure could be as short as 1/10 of a second, comparable to or even faster than that of mammalian skeletal muscle. Using the PDA-coated LCE film, we designed and fabricated a prototype of robotic swimmer that was able to swim near the water-air interface by performing "swimming strokes" through reversible bending and unbending motions induced and controlled by an NIR laser. The results presented in this study clearly demonstrated that PDA-coated LCE is a promising optically driven artificial muscle, which may have great potential for applications of soft robotics and optomechanical coupling devices.

Micro 3D Printing of a Temperature-Responsive Hydrogel Using Projection Micro-Stereolithography

Stimuli-responsive hydrogels exhibiting physical or chemical changes in response to environmental conditions have attracted growing attention for the past few decades. Poly(N-isopropylacrylamide) (PNIPAAm), a temperature responsive hydrogel, has been extensively studied in various fields of science and engineering. However, manufacturing of PNIPAAm has been heavily relying on conventional methods such as molding and lithography techniques that are inherently limited to a two-dimensional (2D) space. Here we report the three-dimensional (3D) printing of PNIPAAm using a high-resolution digital additive manufacturing technique, projection micro-stereolithography (PμSL). Control of the temperature dependent deformation of 3D printed PNIPAAm is achieved by controlling manufacturing process parameters as well as polymer resin composition. Also demonstrated is a sequential deformation of a 3D printed PNIPAAm structure by selective incorporation of ionic monomer that shifts the swelling transition temperature of PNIPAAm. This fast, high resolution, and scalable 3D printing method for stimuli-responsive hydrogels may enable many new applications in diverse areas, including flexible sensors and actuators, bio-medical devices, and tissue engineering.

Thermosensitive hydrogel loaded with chitosan-carbon nanotubes for near infrared light triggered drug delivery

Controlled drug release with on demand is an important challenge for drug delivery. Near-infrared (NIR) light triggered drug delivery reflected the development of a significant strategy to control drug release based on photothermal effects. Herein, a sustained and controlled drug delivery system was developed based on a PCL-PEG-PCL thermosensitive hydrogel combined with chitosan-multiwalled carbon nanotubes for a near infrared light triggered drug delivery. Carbon nanotubes that incorporate hydrogel can enhance the sustained effect of drug delivery by a dual-stage release and allow drug delivery by controlling light irradiation. This in situ photothermal process was monitored by thermal imaging and the controlled drug delivery of doxorubicin was tracked in real-time by fluorescence imaging in vivo based on the fluorescence ability of the drug using nude mice as models. The results suggest that the photothermal effect of the carbon nanotubes can disrupt the structure of the hydrogel with a gel-sol transition, triggering the release of the drug from the sustained drug delivery system by NIR irradiation while responding on demand. The sustained and controlled drug delivery has the potential to implement the accurate administration of hydrogel-based drug delivery systems.

Shape-Morphing Materials from Stimuli-Responsive Hydrogel Hybrids

The formation of well-defined and functional three-dimensional (3D) structures by buckling of thin sheets subjected to spatially nonuniform stresses is common in biological morphogenesis and has become a subject of great interest in synthetic systems, as such programmable shape-morphing materials hold promise in areas including drug delivery, biomedical devices, soft robotics, and biomimetic systems. Given their ability to undergo large changes in swelling in response to a wide variety of stimuli, hydrogels have naturally emerged as a key type of material in this field. Of particular interest are hybrid systems containing rigid inclusions that can define both the anisotropy and spatial nonuniformity of swelling as well as nanoparticulate additives that can enhance the responsiveness and functionality of the material. In this Account, we discuss recent progress in approaches to achieve well-defined shape morphing in hydrogel hybrids. First, we provide an overview of materials and methods that facilitate fabrication of such systems and outline the geometry and mechanics behind shape morphing of thin sheets. We then discuss how patterning of stiff inclusions within soft responsive hydrogels can be used to program both bending and swelling, thereby providing access to a wide array of complex 3D forms. The use of discretely patterned stiff regions to provide an effective composite response offers distinct advantages in terms of scalability and ease of fabrication compared with approaches based on smooth gradients within a single layer of responsive material. We discuss a number of recent advances wherein control of the mechanical properties and geometric characteristics of patterned stiff elements enables the formation of 3D shapes, including origami-inspired structures, concatenated helical frameworks, and surfaces with nonzero Gaussian curvature. Next, we outline how the inclusion of functional elements such as nanoparticles can enable unique pathways to programmable and even reprogrammable shape-morphing materials. We focus to a large extent on photothermally reprogrammable systems that include one of a variety of additives that serve to efficiently absorb light and convert it into heat, thereby driving the response of a temperature-sensitive hydrogel. Such systems are advantageous in that patterns of light can be defined with very high spatial and temporal resolution in addition to offering the potential for wavelength-selective addressability of multiple different inclusions. We highlight recent advances in the preparation of light-responsive hybrid systems capable of undergoing reprogrammable bending and buckling into well-defined 3D shapes. In addition, we describe several examples where shape tuning of hybrid systems enables control over the motion of responsive hydrogel-based materials. Finally, we offer our perspective on open challenges and future areas of interest for the field.

Programmable Shape Recovery Process of Water-Responsive Shape-Memory Poly(vinyl alcohol) by Wettability Contrast Strategy

Water-responsive shape-memory polymers (SMPs) are desirable for biomedical applications, but their limited shape recovery process is problematic. Herein, we demonstrate a shape-memory poly(vinyl alcohol) (SM-PVA) with programmable multistep shape recovery processes in water via a wettability contrast strategy. A hexamethyldisilazane (HMDS)-treated SiO₂ nanoparticle layer with varying loading weights was rationally deposited onto the surface of SM-PVA, aiming to create surface-wettability contrast. The varying wettability led to different water adsorption behaviors of SM-PVA that could be well-described by the pseudo-first-order kinetic model. The results calculated from the kinetic model showed that both the pseudo-first order-adsorption rate constant and the saturated water absorption of SM-PVA demonstrated a declining trend as the loading weight of SiO₂ increased, which laid the foundation for the local regulation of the water-responsive rate of SM-PVA. Finally, two proof-of-concept drug-delivery devices with diverse three-dimensional structures and actuations are presented based on the water-responsive SM-PVA with preprogrammed multistep shape recovery processes. We believe the programmable shape-memory behavior of water-responsive SM-PVA could highly extend its use in drug delivery, tissue engineering scaffolds, and smart implantable devices, etc.

Photo-crosslinkable comb-type copolymers bearing a benzophenone moiety for the enhanced swelling kinetics of hydrogels

The fast responding 3D hydrogel object was fabricated using developed photo-crosslinkable copolymers bearing grafted PNIPAm and a benzophenone moiety.

Complex shape deformations of homogeneous poly(N-isopropylacrylamide)/graphene oxide hydrogels programmed by local NIR irradiation

The homogeneous PNIPAM/GO hydrogels could undergo complex shape deformations (e.g., imitating the postures of human) under local NIR irradiation.

Dual-responsive and Multi-functional Plasmonic Hydrogel Valves and Biomimetic Architectures Formed with Hydrogel and Gold Nanocolloids

We present a straightforward approach with high moldability for producing dual-responsive and multi-functional plasmonic hydrogel valves and biomimetic architectures that reversibly change volumes and colors in response to temperature and ion variations. Heating of a mixture of hybrid colloids (gold nanoparticles assembled on a hydrogel colloid) and hydrogel colloids rapidly induces (within 30 min) the formation of hydrogel architectures resembling mold shapes (cylinder, fish, butterfly). The biomimetic fish and butterfly display reversible changes in volumes and colors with variations of temperature and ionic conditions in aqueous solutions. The cylindrical plasmonic valves installed in flow tubes rapidly control water flow rate in on-off manner by responding to these stimuli. They also report these changes in terms of their colors. Therefore, the approach presented here might be helpful in developing new class of biomimetic and flow control systems where liquid conditions should be visually notified (e.g., glucose or ion concentration changes).

Electric Field Actuation of Tough Electroactive Hydrogels Cross-Linked by Functional Triblock Copolymer Micelles

Multiresponsive polyelectrolyte hydrogels with extraordinary toughness have great potential in soft device applications. Previously we have demonstrated a series of tough and multiresponsive hydrogels by using multifunctional triblock copolymer (Pluronic F127 diacrylate, F127DA) micelles to cross-link cationic polyelectrolyte chains into 3D network. Herein, we further synthesize negatively charged hydrogels comprising 2-acrylamido-2-methyl propylsulfonic acid (AMPS) monomers by using F127DA micelles as cross-linkers. Similar to the positive nanomicelle (NM) hydrogels, the negative NM hydrogels exhibited a compressive strength up to 59 MPa with a fracture strain up to 98%, and tensile fracture strain higher than 2000%. These charged hydrogels were actuated by electric field when immersed in salt solutions. The effects of electrolyte concentration, electric field strength, and ionic monomer content on the electric actuation behavior of these electroactive hydrogels (EAHs) have been systematically investigated. It is concluded that the electroactive hydrogels show a fast actuation rate with a bending angle up to 87° at 120 s and the bending angle was cyclically reversed upon changing bias direction without a large decrease. This study demonstrates that such tough and multiresponsive electroactive hydrogels may have great potential in sensors, actuators, switches, and artificial muscles.

Highly bendable bilayer-type photo-actuators comprising of reduced graphene oxide dispersed in hydrogels

To avoid the problem of reduced graphene oxide (rGO) restacking in aqueous solution, the preparation of light-responsive poly(N-isopropylacrylamide) incorporating rGO (PNIPAm/rGO) was achieved by the chemical reduction of GO dispersed in the hydrogel matrix. Due to the enhanced photothermal efficiency of the rGO, the prepared PNIPAm/rGO underwent large volume reductions in response to irradiation by visible light of modest intensity. With respect to potential applications, bilayer-type photo-actuators comprising a PNIPAm/rGO active layer and poly(acrylamide) passive layer were fabricated; these achieved a full bending motion upon visible-light exposure. Adjusting the swelling ratio of each layer in the initial state yielded bidirectional photo-actuators that showed the active motion of turning inside out. Furthermore, we demonstrated that the fabricated actuation system would exhibit controlled bending motion in response to solar radiation.

Comb-Type Grafted Hydrogels of PNIPAM and PDMAEMA with Reversed Network-Graft Architectures from Controlled Radical Polymerizations

Dual thermo- and pH-responsive comb-type grafted hydrogels of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and poly(N-isopropylacrylamide) (PNIPAM) with reversed network-graft architectures were synthesized by the combination of atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization and click chemistry. Two kinds of macro-cross-linkers with two azido groups at one chain-end and different chain length [PNIPAM⁻(N₃)₂ and PDMAEMA⁻(N₃)₂] were prepared with N,N-di(β-azidoethyl) 2-halocarboxylamide as the ATRP initiator. Through RAFT copolymerization of DMAEMA or NIPAM with propargyl acrylate (ProA) using dibenzyltrithiocarbonate as a chain transfer agent, two network precursors with different content of alkynyl side-groups [P(DMAEMA-co-ProA) and P(NIPAM-co-ProA)] were obtained. The subsequent azido-alkynyl click reaction of macro-cross-linkers and network precursors led to the formation of the network-graft hydrogels. These dual stimulus-sensitive hydrogels exhibited rapid response, high swelling ratio and reproducible swelling/de-swelling cycles under different temperatures and pH values. The influences of cross-linkage density and network-graft architecture on the properties of the hydrogels were investigated. The release of ceftriaxone sodium from these hydrogels showed both thermal- and pH-dependence, suggesting the feasibility of these hydrogels as thermo- and pH-dependent drug release devices.