Heterogeneous Hydrogel Structures with Spatiotemporal Reconfigurability using Addressable and Tunable Voxels

Release and Transport of Nanomaterials from Hydrogels Controlled by Temperature

Understanding the transport of nanoparticles from and within hydrogels is a key issue for the design of nanocomposite hydrogels for drug delivery systems and tissue engineering. To investigate the translocation of nanocarriers from and within hydrogel networks triggered by changes of temperature, ultrasmall (8 nm) and small (80 nm) silica nanocapsules are embedded in temperature-responsive hydrogels and non-responsive hydrogels. The ultrasmall silica nanocapsules are released from temperature-responsive hydrogels to water or transported to other hydrogels upon direct activation by heating or indirect activation by Joule heating; while, they are not released from non-responsive hydrogel. Programmable transport of nanocarriers from and in hydrogels provides insights for the development of complex biomedical devices and soft robotics.

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.

Environment-Interactive Programmable Deformation of Electronically Innervated Synergistic Fluorescence-Color/Shape Changeable Hydrogel Actuators

Utilization of life-like hydrogels to replicate synergistic shape/color changeable behaviors of living organisms has been long envisaged to produce robust functional integrated soft actuators/robots. However, it remains challenging to construct such hydrogel systems with integrated functionality of remote, localized and environment-interactive control over synergistic discoloration/actuation. Herein, inspired by the evolution-optimized bioelectricity stimulus and multilayer structure of natural reptile skins, electronically innervated fluorescence-color switchable hydrogel actuating systems with bio-inspired multilayer structure comprising of responsive fluorescent hydrogel sheet and conductive Graphene/PDMS film with electrothermal effect is presented. Such rational structure enables remote control over synergistic fluorescence-color and shape changes of the systems via the cascading "electrical trigger-Joule heat generation-hydrogel shrinkage" mechanism. Consequently, local/sequential control of discoloration/actuation are achieved due to the highly controllable electrical stimulus in terms of amplitude and circuit design. Furthermore, by joint use with acoustic sensors, soft chameleon robots with unprecedented environment-interactive adaptation are demonstrated, which can intelligently sense environment signals to adjust their color/shape-changeable behaviors. This work opens previously unidentified avenues for functional integrated soft actuators/robots and will inspire life-like intelligent systems for versatile uses.

Blue-ringed octopus-inspired microneedle patch for robust tissue surface adhesion and active injection drug delivery

Intratissue topical medication is important for the treatment of cutaneous, mucosal or splanchnic diseases. However, penetrating surface barriers to providing adequate and controllable drug delivery while guaranteeing adhesion in bodily fluids remains challenging. Here, the predatory behavior of the blue-ringed octopus inspired us with a strategy to improve topical medication. For effective intratissue drug delivery, the active injection microneedles were prepared in a manner inspired by the teeth and venom secretion of blue-ringed octopus. With on demand release function guided by temperature-sensitive hydrophobic and shrinkage variations, these microneedles can supply adequate drug delivery at an early stage and then achieve the long-term release stage. Meanwhile, the bionic suction cups were developed to facilitate microneedles to stay firmly in place (>10 kilopascal) when wet. With wet bonding ability and multiple delivery mode, this microneedle patch achieved satisfactory efficacy, such as accelerating the ulcers' healing speed or halting early tumor progression.

Phase-separation facilitated one-step fabrication of multiscale heterogeneous two-aqueous-phase gel

Engineering heterogeneous hydrogels with distinct phases at various lengths, which resemble biological tissues with high complexity, remains challenging by existing fabricating techniques that require complicated procedures and are often only applicable at bulk scales. Here, inspired by ubiquitous phase separation phenomena in biology, we present a one-step fabrication method based on aqueous phase separation to construct two-aqueous-phase gels that comprise multiple phases with distinct physicochemical properties. The gels fabricated by this approach exhibit enhanced interfacial mechanics compared with their counterparts obtained from conventional layer-by-layer methods. Moreover, two-aqueous-phase gels with programmable structures and tunable physicochemical properties can be conveniently constructed by adjusting the polymer constituents, gelation conditions, and combining different fabrication techniques, such as 3D-printing. The versatility of our approach is demonstrated by mimicking the key features of several biological architectures at different lengths: macroscale muscle-tendon connections; mesoscale cell patterning; microscale molecular compartmentalization. The present work advances the fabrication approach for designing heterogeneous multifunctional materials for various technological and biomedical applications.

Tough Hydrogel Electrolytes for Anti-Freezing Zinc-Ion Batteries

As the soaring demand for energy storage continues to grow, batteries that can cope with extreme conditions are highly desired. Yet, existing battery materials are limited by weak mechanical properties and freeze-vulnerability, prohibiting safe energy storage in devices that are exposed to low temperature and unusual mechanical impacts. Herein, a fabrication method harnessing the synergistic effect of co-nonsolvency and "salting-out" that can produce poly(vinyl alcohol) hydrogel electrolytes with unique open-cell porous structures, composed of strongly aggregated polymer chains, and containing disrupted hydrogen bonds among free water molecules, is introduced. The hydrogel electrolyte simultaneously combines high strength (tensile strength 15.6 MPa), freeze-tolerance (< -77 °C), high mass transport (10× lower overpotential), and dendrite and parasitic reactions suppression for stable performance (30 000 cycles). The high generality of this method is further demonstrated with poly(N-isopropylacrylamide) and poly(N-tertbutylacrylamide-co-acrylamide) hydrogels. This work takes a further step toward flexible battery development for harsh environments.

Bistable Joints Enable the Morphing of Hydrogel Sheets with Multistable Configurations

Joints, as a flexing element to connect different parts, are widespread in natural systems. Various joints exist in the body and play crucial roles to execute gestures and gaits. These scenarios have inspired the design of mechanical joints with passive, hard materials, which usually need an external power supply to drive the transformations. The incorporation of soft and active joints provides a modular strategy to devise soft actuators and robots. However, transformations of responsive joints under external stimuli are usually in uni-mode with a pre-determined direction. Here, hydrogel joints capable of folding and twisting transformation in bi-mode are reported, which enable the composite hydrogel to form multiple configurations under constant conditions. These joints have an in-plane gradient structure and comprise stiff, passive gel as the frame and soft, active gel as the actuating unit. Under external stimuli, the response mismatch between different gels leads to out-of-plane folding or twisting deformation with the feature of bistability. These joints can be modularly integrated with other gels to afford complex deformations and multistable configurations. This approach favors selective control of hydrogel's architectures and versatile design of hydrogel devices, as demonstrated by proof-of-concept examples. It shall also merit the development of metamaterials, soft actuators, and robots, etc.

α,ε-Hybrid Peptide-Stabilized Magnetic Nanoparticle-Coated Paper-Based Actuators

The development of α,ε-hybrid peptide-stabilized magnetic nanoparticles and their application to fabricate a paper-based actuator has been reported. From single-crystal diffraction analysis, the nitropeptide 2 has an extended structure with a trans geometry. The one-pot in situ multiple oxidation-reduction reaction of a synthetic nitropeptide solution in ammonium hydroxide and FeCl₂ leads to the formation of Fe₃O₄ nanoparticles. The reduction reaction replaces the nitro group with an amine group, which finally acts as capping agent for the stabilization of the Fe₃O₄ nanoparticles. Paper-based soft magneto machines with multivariant actuation modes such as contraction-expansion, bending, and uplifting locomotion have been studied. The device has potential as controllable paper-based soft robots.

Engineering viscoelastic mismatch for temporal morphing of tough supramolecular hydrogels

Viscoelasticity is a generic characteristic of soft biotissues and polymeric materials, endowing them with unique time- and rate-dependent properties. Here, by spatiotemporally tailoring the viscoelasticity in tough supramolecular hydrogels, we demonstrate reprogrammable morphing of the gels based on differential viscoelastic recovery processes that lead to internal strain mismatch. The spatial heterogeneity of viscoelasticity is encoded through integrating dissimilar hydrogels or by site-specific treatment of a singular hydrogel. The temporal morphing behavior of tough gels, including a fast deformation process and then a slow shape-recovery process, is related to the kinetics of associative interactions and the entropic elasticity of supramolecular networks after pre-stretching and release, which takes place spontaneously in the absence of external stimuli. Such a kinetically driven morphing mechanism resolves the trade-off between the mechanical robustness and shape-changing speed in tough hydrogels with dense entanglements and physical associations, and should be applicable to other viscoelastic materials. A numerical theory for the temporal morphing of tough supramolecular gels has been formulated by dynamic coupling of viscoelastic recovery and mechanics of deformations, which is further implemented to predict the sophisticated morphed structures. Furthermore, magnetic particles are incorporated into the morphed tough hydrogels to devise versatile soft actuators and robots for specific applications.

MSCs-laden injectable self-healing hydrogel for systemic sclerosis treatment

As a novel cellular therapy, the anti-inflammatory and immunomodulatory virtues of mesenchymal stem cells (MSCs) make them promising candidates for systemic sclerosis (SSc) treatment. However, the clinical efficacy of this stratagem is limited because of the short persistence time, poor survival, and engraftment of MSCs after injection in vivo. Herein, we develop a novel MSCs-laden injectable self-healing hydrogel for SSc treatment. The hydrogel is prepared using N, O-carboxymethyl chitosan (CS-CM) and 4-armed benzaldehyde-terminated polyethylene glycol (PEG-BA) as the main components, imparting with self-healing capacity via the reversible Schiff-base connection between the amino and benzaldehyde groups. We demonstrate that the hydrogel laden with MSCs not only promoted the proliferation of MSCs and increased the cellular half-life in vivo, but also improve their immune-modulating functions. The tube formation assay indicates that the MSCs could significantly promote angiopoiesis. Moreover, the MSCs-laden hydrogel could inhibit fibrosis by modulating the synthesis of collagen and ameliorate disease progression in SSc disease model mice after subcutaneous injection of bleomycin. All these results highlight this novel MSCs-laden hydrogel and its distinctive functions in treatment of chronic SSc, indicating the additional potential to be used widely in the clinic.

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We proposed novel MSCs-laden injectable self-healing hydrogel for SSc treatment.

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The hydrogel was constructed by PEG-BA and CS-CM.

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MSCs-laden hydrogel promotes angiopoiesis and inhibit fibrosis.

Multi-Jet Fusion 3D Voxel Printing of Conductive Elastomers

3D voxel printing enables the fabrication of parts with site-specific materials and properties at voxel-scale resolution, while the current research mainly focuses on the variations in mechanical properties and colors. In this work, the design and fabrication of voxelated conductive elastomers using Multi Jet Fusion 3D voxel printing actualized by a newly developed multifunctional agent (MA) are investigated. The MA, mainly consisting of carbon nanotubes and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, serves as an infrared-absorbing colorant, a reinforcement, and a conductive filler simultaneously. By controlling the drop-on-demand dispensing of the agents on thermoplastic polyurethane powder, the electrical conductivity across a single printed part can be tailored over a wide range from 10^-10 to 10^-1 S cm^-1 at a voxel resolution of ≈100 µm. Assembly-free strain sensors comprising conductive sensing layers and insulating frames are fabricated to demonstrate the capability of the technique in manufacturing all-printed wearable devices.

High-performance electrified hydrogel actuators based on wrinkled nanomembrane electrodes for untethered insect-scale soft aquabots

Hydrogels have diverse chemical properties and can exhibit reversibly large mechanical deformations in response to external stimuli; these characteristics suggest that hydrogels are promising materials for soft robots. However, reported actuators based on hydrogels generally suffer from slow response speed and/or poor controllability due to intrinsic material limitations and electrode fabrication technologies. Here, we report a hydrogel actuator that operates at low voltages (<3 volts) with high performance (strain > 50%, energy density > 7 × 10 5 joules per cubic meter, and power density > 3 × 10 4 watts per cubic meter), surpassing existing hydrogel actuators and other types of electroactive soft actuators. The enhanced performance of our actuator is due to the formation of wrinkled nanomembrane electrodes that exhibit high conductivity and excellent mechanical deformation through capillary-assisted assembly of metal nanoparticles and deswelling-induced wrinkled structures. By applying an electric potential through the wrinkled nanomembrane electrodes that sandwich the hydrogel, we were able to trigger a reversible and substantial electroosmotic water flow inside a hydrogel film, which drove the controlled swelling of the hydrogel. The high energy efficiency and power density of our wrinkled nanomembrane electrode–induced actuator enabled the fabrication of an untethered insect-scale aquabot integrated with an on-board control unit demonstrating maneuverability with fast locomotion speed (1.02 body length per second), which occupies only 2% of the total mass of the robot.

Self-sorting in macroscopic supramolecular self-assembly via additive effects of capillary and magnetic forces

Supramolecular self-assembly of μm-to-mm sized components is essential to construct complex supramolecular systems. However, the selective assembly to form designated structures at this length scale is challenging because the short-ranged molecular recognition could hardly direct the assembly of macroscopic components. Here we demonstrate a self-sorting mechanism to automatically identify the surface chemistry of μm-to-mm components (A: polycations; B: polyanions) based on the A-B attraction and the A-A repulsion, which is realized by the additivity and the competence between long-ranged magnetic/capillary forces, respectively. Mechanistic studies of the correlation between the magnetic/capillary forces and the interactive distance have revealed the energy landscape of each assembly pattern to support the self-sorting results. By applying this mechanism, the assembly yield of ABA trimers has been increased from 30%~40% under conventional conditions to 100% with self-sorting. Moreover, we have demonstrated rapid and spontaneous self-assembly of advanced chain-like structures with alternate surface chemistry.

Graphene-Lined Porous Gelatin Glycidyl Methacrylate Hydrogels: Implications for Tissue Engineering

Despite rigorous research, inferior mechanical properties and structural homogeneity are the main challenges constraining hydrogel's suturability to host tissue and limiting its clinical applications. To tackle those, we developed a reverse solvent interface trapping method, in which organized, graphene-coated microspherical cavities were introduced into a hydrogel to create heterogeneity and make it suturable. To generate those cavities, (i) graphite exfoliates to graphene sheets, which spread at the water/ heptane interfaces of the microemulsion, (ii) heptane fills the microspheres coated by graphene, and (iii) a cross-linkable hydrogel dissolved in water fills the voids. Cross-linking solidifies such microemulsion to a strong, suturable, permanent hybrid architecture, which has better mechanical properties, yet it is biocompatible and supports cell adhesion and proliferation. These properties along with the ease and biosafety of fabrication suggest the potential of this strategy to enhance tissue engineering outcomes by generating various suturable scaffolds for biomedical applications, such as donor cornea carriers for Boston keratoprosthesis (BK).