Dual pH-Responsive Hydrogel Actuator for Lipophilic Drug Delivery

Ionic Polymerization-Based Synthesis of Bioinspired Adhesive Hydrogel Microparticles with Tunable Morphologies from Microfluidics

Shape-anisotropic hydrogel microparticles have attracted considerable attention for drug-delivery applications. Particularly, nonspherical hydrogel microcarriers with enhanced adhesive and circulatory abilities have demonstrated value in gastrointestinal drug administration. Herein, inspired by the structures of natural suckers, we demonstrate an ionic polymerization-based production of calcium (Ca)-alginate microparticles with tunable shapes from Janus emulsion for the first time. Monodispersed Janus droplets composed of sodium alginate and nongelable segments were generated using a coflow droplet generator. The interfacial curvatures, sizes, and production frequencies of Janus droplets can be flexibly controlled by varying the flow conditions and surfactant concentrations in the multiphase system. Janus droplets were ionically solidified on a chip, and hydrogel beads of different shapes were obtained. The in vitro and in vivo adhesion abilities of the hydrogel beads to the mouse colon were investigated. The anisotropic beads showed prominent adhesive properties compared with the spherical particles owing to their sticky hydrogel components and unique shapes. Finally, a novel computational fluid dynamics and discrete element method (CFD-DEM) coupling simulation was used to evaluate particle migration and contact forces theoretically. This review presents a simple strategy to synthesize Ca-alginate particles with tunable structures that could be ideal materials for constructing gastrointestinal drug delivery systems.

Remotely Controlled Surface Charge Modulation of Magnetoelectric Nanogenerators for Swift and Efficient Drug Delivery

We have developed a highly efficient technique of magnetically controlled swift loading and release of doxorubicin (DOX) drug using a magnetoelectric nanogenerator (MENG). Core-shell nanostructured MENG with a magnetostrictive core and piezoelectric shell act as field-responsive nanocarriers and possess the capability of field-triggered drug release in a cancerous environment. MENGs generate a surface electric dipole when subjected to a magnetic field due to the strain-mediated magnetoelectric effect. The capability of directional magnetic field-assisted modulation of the surface electrical dipole of MENG provides a mechanism to create/break ionic bonds with DOX molecules, which facilitates efficient drug attachment and on-demand swift detachment of the drug at a targeted site. The magnetic field-assisted drug-loading mechanism was minutely analyzed using spectrophotometry and Raman spectroscopy. The detailed time-dependent analysis of controlled drug release by the MENG under unidirectional and rotating magnetic field excitation was conducted using field-emission scanning electron microscopy, energy-dispersive X-ray, and atomic force microscopic measurements. In vitro, experiments validate the cytocompatibility and magnetically assisted on-demand and swift DOX drug delivery by the MENG near MCF-7 breast cancer cells, which results in a significant enhancement of cancer cell killing efficiency. A state-of-the-art experiment was performed to visualize the nanoscale magnetoelectric effect of MENG using off-axis electron holography under Lorentz conditions.

Recent advancements of hydrogels in immunotherapy: Breast cancer treatment

Breast cancer is the most prevalent cancer among women and the leading cause of cancer-related deaths in this population. Recent advances in Immunotherapy, or combined immunotherapy, offering a more targeted and less toxic approach, expand the survival rate of patients more than conventional treatment. Notably, hydrogels, a versatile platform provided promising avenues to combat breast cancer in preclinical studies and extended to clinical practices. With advantages such as the alternation of tumor microenvironment, immunomodulation, targeted delivery of therapeutic agents, and their sustained release at specific sites of interest, hydrogels can potentially be used for the treatment of breast cancer. This review highlights the advantages, mechanisms of action, stimuli-responsiveness properties, and recent advancements of hydrogels for treating breast cancer immunotherapy. Moreover, post-treatment and its clinical translations are discussed in this review. The integration of hydrogels in immunotherapy strategies may pave the way for more effective, personalized, and patient-friendly approaches to combat breast cancer, ultimately contributing to a brighter future for breast cancer patients.

Progress in the Preparation and Application of Inulin-Based Hydrogels

Inulin, a natural polysaccharide, has emerged as a promising precursor for the preparation of hydrogels due to its biocompatibility, biodegradability, and structural versatility. This review provides a comprehensive overview of the recent progress in the preparation, characterization, and diverse applications of inulin-based hydrogels. Different synthesis strategies, including physical methods (thermal induction and non-thermal induction), chemical methods (free-radical polymerization and chemical crosslinking), and enzymatic approaches, are discussed in detail. The unique properties of inulin-based hydrogels, such as stimuli-responsiveness, antibacterial activity, and their potential as fat replacers, are highlighted. Special emphasis is given to their promising applications in drug delivery systems, especially for colon-targeted delivery, due to the selective degradation of inulin via colonic microflora. The ability to incorporate both hydrophilic and hydrophobic drugs further expands their therapeutic potential. In addition, the applications of inulin-based hydrogels in responsive materials, the food industry, wound dressings, and tissue engineering are discussed. While significant progress has been achieved, challenges and prospects in optimizing synthesis, improving mechanical properties, and exploring new functionalities are discussed. Overall, this review highlights the remarkable properties of inulin-based hydrogels as a promising class of biomaterials with immense potential in the biomedical, pharmaceutical, and materials science fields.

Hydrogel microrobots for biomedical applications

Recent years have witnessed a surge in the application of microrobots within the medical sector, with hydrogel microrobots standing out due to their distinctive advantages. These microrobots, characterized by their exceptional biocompatibility, adjustable physico-mechanical attributes, and acute sensitivity to biological environments, have emerged as pivotal tools in advancing medical applications such as targeted drug delivery, wound healing enhancement, bio-imaging, and precise surgical interventions. The capability of hydrogel microrobots to navigate and perform tasks within complex biological systems significantly enhances the precision, efficiency, and safety of therapeutic procedures. Firstly, this paper delves into the material classification and properties of hydrogel microrobots and compares the advantages of different hydrogel materials. Furthermore, it offers a comprehensive review of the principal categories and recent innovations in the synthesis, actuation mechanisms, and biomedical application of hydrogel-based microrobots. Finally, the manuscript identifies prevailing obstacles and future directions in hydrogel microrobot research, aiming to furnish insights that could propel advancements in this field.

Porosity Engineering of Dried Smart Poly(N-isopropylacrylamide) Hydrogels for Gas Sensing

A recent study unveiled the potential of acrylamide-based stimulus-responsive hydrogels for volatile organic compound detection in gaseous environments. However, for gas sensing, a large surface area, that is, a highly porous material, offering many adsorption sites is crucial. The large humidity variation in the gaseous environment constitutes a significant challenge for preserving an initially porous structure, as the pores tend to be unstable and irreversibly collapse. Therefore, the present investigation focuses on enhancing the porosity of smart PNiPAAm hydrogels under the conditions of a gaseous environment and the preservation of the structural integrity for long-term use. We have studied the influence of polyethylene glycol (PEG) as a porogen and the application of different drying methods and posttreatment. The investigations lead to the conclusion that only the combination of PEG addition, freeze-drying, and subsequent conditioning in high relative humidity enables a long-term stable formation of a porous surface and inner structure of the material. The significantly enhanced swelling response in a gaseous environment and in the test gas acetone is confirmed by gravimetric experiments of bulk samples and continuous measurements of thin films on piezoresistive pressure sensor chips. These measurements are furthermore complemented by an in-depth analysis of the morphology and microstructure. While the study was conducted for PNiPAAm, the insights and developed processes are general in nature and can be applied for porosity engineering of other smart hydrogel materials for VOC detection in gaseous environments.

Smart Hydrogel Swelling State Detection Based on a Power-Transfer Transduction Principle

Stimulus-responsive (smart) hydrogels are a promising sensing material for biomedical contexts due to their reversible swelling change in response to target analytes. The design of application-specific sensors that utilize this behavior requires the development of suitable transduction concepts. The presented study investigates a power-transfer-based readout approach that is sensitive to small volumetric changes of the smart hydrogel. The concept employs two thin film polyimide substrates with embedded conductive strip lines, which are shielded from each other except at the tip region, where the smart hydrogel is sandwiched in between. The hydrogel's volume change in response to a target analyte alters the distance and orientation of the thin films, affecting the amount of transferred power between the two transducer parts and, consequently, the measured sensor output voltage. With proper calibration, the output signal can be used to determine the swelling change of the hydrogel and, consequently, to quantify the stimulus. In proof-of-principle experiments with glucose- and pH-sensitive smart hydrogels, high sensitivity to small analyte concentration changes was found along with very good reproducibility and stability. The concept was tested with two exemplary hydrogels, but the transduction principle in general is independent of the specific hydrogel material, as long as it exhibits a stimulus-dependent volume change. The application vision of the presented research is to integrate in situ blood analyte monitoring capabilities into standard (micro)catheters. The developed sensor is designed to fit into a catheter without obstructing its normal use and, therefore, offers great potential for providing a universally applicable transducer platform for smart catheter-based sensing.

4D Multimaterial Printing of Soft Actuators with Spatial and Temporal Control

Soft actuators (SAs) are devices which can interact with delicate objects in a manner not achievable with traditional robotics. While it is possible to design a SA whose actuation is triggered via an external stimulus, the use of a single stimulus creates challenges in the spatial and temporal control of the actuation. Herein, a 4D printed multimaterial soft actuator design (MMSA) whose actuation is only initiated by a combination of triggers (i.e., pH and temperature) is presented. Using 3D printing, a multilayered soft actuator with a hydrophilic pH-sensitive layer, and a hydrophobic magnetic and temperature-responsive shape-memory polymer layer, is designed. The hydrogel responds to environmental pH conditions by swelling or shrinking, while the shape-memory polymer can resist the shape deformation of the hydrogel until triggered by temperature or light. The combination of these stimuli-responsive layers allows for a high level of spatiotemporal control of the actuation. The utility of the 4D MMSA is demonstrated via a series of cargo capture and release experiments, validating its ability to demonstrate active spatiotemporal control. The MMSA concept provides a promising research direction to develop multifunctional soft devices with potential applications in biomedical engineering and environmental engineering.

Self-Healing Hydrogel Bioelectronics

Hydrogels have emerged as powerful building blocks to develop various soft bioelectronics because of their tissue-like mechanical properties, superior bio-compatibility, the ability to conduct both electrons and ions, and multiple stimuli-responsiveness. However, hydrogels are vulnerable to mechanical damage, which limits their usage in developing durable hydrogel-based bioelectronics. Self-healing hydrogels aim to endow bioelectronics with the property of repairing specific functions after mechanical failure, thus improving their durability, reliability, and longevity. This review discusses recent advances in self-healing hydrogels, from the self-healing mechanisms, material chemistry, and strategies for multiple properties improvement of hydrogel materials, to the design, fabrication, and applications of various hydrogel-based bioelectronics, including wearable physical and biochemical sensors, supercapacitors, flexible display devices, triboelectric nanogenerators (TENGs), implantable bioelectronics, etc. Furthermore, the persisting challenges hampering the development of self-healing hydrogel bioelectronics and their prospects are proposed. This review is expected to expedite the research and applications of self-healing hydrogels for various self-healing bioelectronics.

Multimodal Soft Robotic Actuation and Locomotion

Diverse and adaptable modes of complex motion observed at different scales in living creatures are challenging to reproduce in robotic systems. Achieving dexterous movement in conventional robots can be difficult due to the many limitations of applying rigid materials. Robots based on soft materials are inherently deformable, compliant, adaptable, and adjustable, making soft robotics conducive to creating machines with complicated actuation and motion gaits. This review examines the mechanisms and modalities of actuation deformation in materials that respond to various stimuli. Then, strategies based on composite materials are considered to build toward actuators that combine multiple actuation modes for sophisticated movements. Examples across literature illustrate the development of soft actuators as free-moving, entirely soft-bodied robots with multiple locomotion gaits via careful manipulation of external stimuli. The review further highlights how the application of soft functional materials into robots with rigid components further enhances their locomotive abilities. Finally, taking advantage of the shape-morphing properties of soft materials, reconfigurable soft robots have shown the capacity for adaptive gaits that enable transition across environments with different locomotive modes for optimal efficiency. Overall, soft materials enable varied multimodal motion in actuators and robots, positioning soft robotics to make real-world applications for intricate and challenging tasks.

Advances in Hydrogel-Based Drug Delivery Systems

Hydrogels, with their distinctive three-dimensional networks of hydrophilic polymers, drive innovations across various biomedical applications. The ability of hydrogels to absorb and retain significant volumes of water, coupled with their structural integrity and responsiveness to environmental stimuli, renders them ideal for drug delivery, tissue engineering, and wound healing. This review delves into the classification of hydrogels based on cross-linking methods, providing insights into their synthesis, properties, and applications. We further discuss the recent advancements in hydrogel-based drug delivery systems, including oral, injectable, topical, and ocular approaches, highlighting their significance in enhancing therapeutic outcomes. Additionally, we address the challenges faced in the clinical translation of hydrogels and propose future directions for leveraging their potential in personalized medicine and regenerative healthcare solutions.

Magnetic multilayer hydrogel oral microrobots for digestive tract treatment

Oral administration is a convenient drug delivery method in our daily lives. However, it remains a challenge to achieve precise target delivery and ensure the efficacy of medications in extreme environments within the digestive system with complex environments. This paper proposes an oral multilayer magnetic hydrogel microrobot for targeted delivery and on-demand release driven by a gradient magnetic field. The inner hydrogel shells enclose designated drugs and magnetic microparticles. The outer hydrogel shells enclose the inner hydrogel shells, magnetic microparticles, and pH neutralizers. The drug release procedure is remotely implemented layer-by-layer. When the required gradient magnetic field is applied, the outer hydrogel shells are destroyed to release their inclusions. The enclosed pH neutralizers scour the surrounding environment to avoid damaging drugs by the pH environment. Subsequently, the inner hydrogel shells are destroyed to release the drugs. A set of experiments are conducted to demonstrate the wirelessly controllable target delivery and release in a Petri dish and biological tissues. The results demonstrated attractive advantages of the reported microrobot in microcargo delivery with almost no loss, remote controllable release, and drug protection by the pH neutralizers. It is a promising approach to advance next-generation precision oral therapies in the digestive system.

Advancements in Soft Robotics: A Comprehensive Review on Actuation Methods, Materials, and Applications

The flexibility and adaptability of soft robots enable them to perform various tasks in changing environments, such as flower picking, fruit harvesting, in vivo targeted treatment, and information feedback. However, these fulfilled functions are discrepant, based on the varied working environments, driving methods, and materials. To further understand the working principle and research emphasis of soft robots, this paper summarized the current research status of soft robots from the aspects of actuating methods (e.g., humidity, temperature, PH, electricity, pressure, magnetic field, light, biological, and hybrid drive), materials (like hydrogels, shape-memory materials, and other flexible materials) and application areas (camouflage, medical devices, electrical equipment, and grippers, etc.). Finally, we provided some opinions on the technical difficulties and challenges of soft robots to comprehensively comprehend soft robots, lucubrate their applications, and improve the quality of our lives.

Robust Hydrogel Actuators Functioning in Multi-Environments Enabled by Thermo-Responsive Polymer Nanoparticle Coatings on Hydrogel Surfaces

Hydrogel actuators with anisotropic structures exhibit reversible responsiveness upon the trigger of various external stimuli, rendering them promising for applications in many fields including artificial muscles and soft robotics. However, their effective operation across multiple environments remains a persistent challenge, even for widely studied thermo-responsive polymers like poly(N-isopropyl acrylamide) (PNIPAm). Current attempts to address this issue are hindered by complex synthetic procedures or specific substrates. This study introduces a straightforward methodology to grow a thin, dense PNIPAm nanoparticle layer on diverse hydrogel surfaces, creating a highly temperature-sensitive hydrogel actuator. This actuator demonstrates adaptability across various environments, including water, oil, and open air, owing to its distinct structure facilitating self-water circulation during actuation. The thin PNIPAm layer consists of interconnected PNIPAm nanoparticles synthesized via in situ interfacial precipitation polymerization, seamlessly bonded to the hydrogel substrate through an interfacial layer containing hybrid hydrogel/PNIPAm nanoparticles. This unique anisotropic structure ensures exceptional structural stability without interfacial delamination, even enduring harsh treatments such as freezing, ultrasonic irradiation, and prolonged water immersion. Remarkably, PNIPAm films on hydrogel surfaces which enable programmable 3D actuation can also be precisely patterned. This synthetic approach opens a novel pathway for fabricating advanced hydrogel actuators with broad-ranging applications.

Injectable and Multifunctional Hydrogels Based on Poly(N-acryloyl glycinamide) and Alginate Derivatives for Antitumor Drug Delivery

Chemotherapy is a conventional treatment that uses drugs to kill cancer cells; however, it may induce side effects and may be incompletely effective, leading to the risk of tumor recurrence. To address this issue, we developed novel injectable thermal/near-infrared (NIR)-responsive hydrogels to control drug release. The injectable hydrogel formulation was composed of biocompatible alginates, poly(N-acryloyl glycinamide) (PNAGA) copolymers with an upper critical solution temperature, and NIR-responsive cross-linkers containing coumarin groups, which were gelated through bioorthogonal inverse electron demand Diels-Alder reactions. The hydrogels exhibited quick gelation times (120-800 s) and high drug loading efficiencies (>90%). The hydrogels demonstrated a higher percentage of drug release at 37 °C than that at 25 °C due to the enhanced swelling behavior of temperature-responsive PNAGA moieties. Upon NIR irradiation, the hydrogels released most of the entrapped doxorubicin (DOX) (97%) owing to the cleavage of NIR-sensitive coumarin ester groups. The hydrogels displayed biocompatibility with normal cells, while induced antitumor activity toward cancer cells. DOX/hydrogels treated with NIR light inhibited tumor growth in nude mice bearing tumors. In addition, the injected hydrogels emitted red fluorescence upon excitation at a green wavelength, so that the drug delivery and hydrogel degradation in vivo could be tracked in the xenograft model.

Dual Cross-Linked Stimuli-Responsive Alginate-Based Hydrogels

Sodium alginate with different molecular weights (55, 170, and 320 kg mol-1) were chemically modified by grafting methacrylic moieties onto the hydroxyl groups of the alginate backbone. The methacrylation was optimized to obtain different degrees of modification. Chemically cross-linked hydrogels were obtained following UV-light irradiation in the presence of a photoinitiator. The swelling behavior and the mechanical properties were observed to depend on both the degree of methacrylation and the alginate molecular weight. Due to the chain entanglement present in high-viscosity sodium alginate, lower degrees of modification were required to tune the hydrogel properties. Moreover, in the presence of Ca2+, secondary cross-linking was introduced by the coordination of the alginate guluronate moieties with the Ca2+ ions. The addition of this secondary cross-linking caused fast volume shrinkage and a reinforcement of the mechanical properties. The secondary cross-linking was reversible, and the hydrogels regained their original shape for at least three cycles. Additionally, the dual cross-linked network can be used to induce adhesion between hydrogels and serve as a building block for self-folding actuators.

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.

Injectable and Photocurable Gene Scaffold Facilitates Efficient Repair of Spinal Cord Injury

RNA interference-based gene therapy has led to a strategy for spinal cord injury (SCI) therapy. However, there have been high requirements regarding the optimal gene delivery vector for siRNA-based SCI gene therapy. Here, we developed an injectable and photocurable lipid nanoparticle GelMA (PLNG) hydrogel scaffold for controlled dual siRNA delivery at the SCI wound site. The prepared PLNG scaffold could efficiently protect and retain the bioactivity of the siRNA nanocomplex. It facilitated sustainable siRNA release along with degradation in 7 days. After loading dual siRNA targeting phosphatase and tensin homologue (PTEN) and macrophage migration inhibitory factor (MIF) simultaneously, the locally administered siRNAs/PLNG scaffold efficiently improved the Basso mouse scale (BMS) score and recovered ankle joint movement and plantar stepping after treatment with only three doses. We further proved that the siRNAs/PLNG scaffold successfully regulated the activities of neurons, microglia, and macrophages, thus promoting neuron axon regeneration and remyelination. The protein array results suggested that the siRNAs/PLNG scaffold could increase the expression of growth factors and decrease the expression of inflammatory factors to regulate neuroinflammation in SCI and create a neural repair environment. Our results suggested that the PLNG scaffold siRNA delivery system is a potential candidate for siRNA-based SCI therapy.

Multivapor-Responsive Controlled Actuation of Starch-Based Soft Actuators

Multivapor-responsive biocompatible soft actuators have immense potential for applications in soft robotics and medical technology. We report fast, fully reversible, and multivapor-responsive controlled actuation of a pure cassava-starch-based film. Notably, this starch-based actuator sustains its actuated state for over 60 min with a continuous supply of water vapor. The durability of the film and repeatability of the actuation performance have been established upon subjecting the film to more than 1400 actuation cycles in the presence of water vapor. The starch-based actuators exhibit intriguing antagonistic actuation characteristics when exposed to different solvent vapors. In particular, they bend upward in response to water vapor and downward when exposed to ethanol vapor. This fascinating behavior opens up new possibilities for controlling the magnitude and direction of actuation by manipulating the ratio of water to ethanol in the binary solution. Additionally, the control of the bending axis of the starch-based actuator, when exposed to water vapor, is achieved by imprinting-orientated patterns on the surface of the starch film. The effect of microstructure, postsynthesis annealing, and pH of the starch solution on the actuation performance of the starch film is studied in detail. Our starch-based actuator can lift 10 times its own weight upon exposure to ethanol vapor. It can generate force ∼4.2 mN upon exposure to water vapor. To illustrate the vast potential of our cassava-starch-based actuators, we have showcased various proof-of-concept applications, ranging from biomimicry to crawling robots, locomotion near perspiring human skin, bidirectional electric switches, ventilation in the presence of toxic vapors, and smart lifting systems. These applications significantly broaden the practical uses of these starch-based actuators in the field of soft robotics.

Synthesis and Structural Studies of Nucleobase Functionalized Hydrogels for Controlled Release of Vitamins

The development of biomolecule-derived biocompatible scaffolds for drug delivery applications is an emerging research area. Herein, we have synthesized a series of nucleobase guanine (G) functionalized amino acid conjugates having different chain lengths to study their molecular self-assembly in the hydrogel state. The gelation properties have been induced by the correct choice of chain lengths of fatty acids present in nucleobase functionalized molecules. The effect of alkali metal cations, pH, and the concentration of nucleobase functionalized amino acid conjugates in the molecular self-assembly process has been explored. The presence of Hoogsteen hydrogen bonding interaction drives the formation of a G-quadruplex functionalized hydrogel. The DOSY nuclear magnetic resonance is also performed to evaluate the self-assembling behavior of the newly formed nucleobase functionalized hydrogel. The nanofibrillar morphology is responsible for the formation of a hydrogel, which has been confirmed by various microscopic experiments. The mechanical behaviors of the hydrogel were evaluated by rheological experiments. The in vitro biostability of the synthesized nucleobase amino acid conjugate is also investigated in the presence of hydrolytic enzymes proteinase K and chymotrypsin. Finally, the nucleobase functionalized hydrogel has been used as a drug delivery platform for the control and sustained pH-responsive release of vitamins B2 and B12. This synthesized nucleobase functionalized hydrogel also exhibits noncytotoxic behavior, which has been evaluated by their in vitro cell viability experiment using HEK 293 and MCF-7 cell lines.

Chemically Driven Oscillating Soft Pneumatic Actuation

Pneumatic actuators are widely studied in soft robotics as they are facile, low cost, scalable, and robust and exhibit compliance similar to many systems found in nature. The challenge is to harness high energy density chemical and biochemical reactions that can generate sufficient pneumatic pressure to actuate soft systems in a controlled and ecologically compatible manner. This investigation evaluates the potential of chemical reactions as both positive and negative pressure sources for use in soft robotic pneumatic actuators. Considering the pneumatic actuation demands, the chemical mechanisms of the pressure sources, and the safety of the system, several gas evolution/consumption reactions are evaluated and compared. Furthermore, the novel coupling of both gas evolution and gas consumption reactions is discussed and evaluated for the design of oscillating systems, driven by the complementary evolution and consumption of carbon dioxide. Control over the speed of gas generation and consumption is achieved by adjusting the initial ratios of feed materials. Coupling the appropriate reactions with pneumatic soft-matter actuators has delivered autonomous cyclic actuation. The reversibility of these systems is demonstrated in a range of displacement experiments, and practical application is shown through a soft gripper that can move, pick up, and let go of objects. Our approach presents a significant step toward more autonomous, versatile soft robots driven by chemo-pneumatic actuators.

Inchworm-like Soft Robot with Multi-Responsive Bilayer Films

As an important branch of robotics, soft robots have the advantages of strong flexibility, a simple structure, and high safety. These characteristics enable soft robots to be widely used in various fields such as biomedicine, military reconnaissance, and micro space exploration. However, contemporary soft crawling robots still face problems such as the single drive mode and complex external equipment. In this study, we propose an innovative design of an inchworm-like soft crawling robot utilizing the synergistic interaction of electricity and moisture for its hybrid dual-drive locomotion. The legs of the soft robot are mainly made of GO-CNT/PE composite film, which can convert its own volume expansion into a corresponding bending motion after being stimulated by electricity or moisture. Unlike other drive methods, it requires less power and precision from external devices. The combination of the two driving methods greatly improves the environmental adaptability of the soft robot, and we developed visible light as the driving method on the basis of the dual drive. Finally, we also verified the robot's excellent load capacity, climbing ability, and optical drive effect, which laid the foundation for the application of soft robots in the future.

Cellulose Nanocrystal-Based Gradient Hydrogel Actuators with Controllable Bending Properties

Stimuli-responsive hydrogel actuators are being increasingly used in microtechnology, but typical bilayer hydrogel actuators have significant drawbacks due to weak adhesive interface between the two layers. In this study, thermoresponsive single-layer hydrogel actuators are produced by generating a gradient distribution of cellulose nanocrystals (CNCs) in a poly(N-isopropylacrylamide) (PNIPAAm) hydrogel network by electrophoresis. Tunable bending properties of the composite hydrogels, such as the thermoresponsive bending speed and angle, are realized by varying the electrophoresis time, applied voltage, and CNC concentration. By varying these conditions, the gradient distribution of the CNCs can be optimized, leading to fast bending and large bending angles of the hydrogels. Bending properties are attributed to the gradient distribution of CNCs causing different deswelling rates across the hydrogel network owing to reinforcing effects. Bending ability is also influenced by differences in the CNC dimensions based on the sources of cellulose, which determine the rigidity of the CNC-rich layer of the polymer composite. It is thus shown that thermoresponsive single-layer gradient hydrogels with tunable bending properties can be realized.

A dual stimuli-responsive smart soft carrier using multi-material 4D printing

This paper proposes a 4D printed smart soft carrier with a hemispherical hollow and openable lid. The soft carrier is composed of a lid with a slot (with a shape of 4 legs), a border, and a hemisphere. The soft carrier is fabricated by 4D printing using smart hydrogels. Specifically, the lid, border, and hemisphere are fabricated using a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel, a non-responsive polyethylene glycol (PEG) hydrogel with superparamagnetic iron oxide nanoparticles (SPIONs), and a PEG hydrogel, respectively. Since the SPIONs are included in the border, the slot in the center of the lid is opened and closed according to the temperature change caused by near-infrared (NIR) irradiation, and the proposed soft carrier is magnetically driven by an external magnetic field. The hemisphere enables the storage and transport of cargo. The proposed soft carrier can control the opening and closing of the slot and movement to a desired position in water. Several cargo delivery experiments were conducted using various shapes and numbers of cargo. In addition, the proposed soft carrier can successfully handle small living marine organisms. This soft carrier can be manufactured by 4D printing and operated by dual stimuli (NIR and magnetic field) and can safely deliver various types of cargo and delicate organisms without leakage or damage. The flexibility of 4D printing enables the size of the soft carrier to be tailored to the specific physical attributes of various objects, making it an adaptable and versatile delivery approach.

Diffusion-Limited Processes in Hydrogels with Chosen Applications from Drug Delivery to Electronic Components

Diffusion is one of the key nature processes which plays an important role in respiration, digestion, and nutrient transport in cells. In this regard, the present article aims to review various diffusion approaches used to fabricate different functional materials based on hydrogels, unique examples of materials that control diffusion. They have found applications in fields such as drug encapsulation and delivery, nutrient delivery in agriculture, developing materials for regenerative medicine, and creating stimuli-responsive materials in soft robotics and microrobotics. In addition, mechanisms of release and drug diffusion kinetics as key tools for material design are discussed.

Dynamics of hydroxide-ion-driven reversible autocatalytic networks

In living systems adaptive regulation requires the presence of nonlinear responses in the underlying chemical networks. Positive feedbacks, for example, can lead to autocatalytic bursts that provide switches between two stable states or to oscillatory dynamics. The stereostructure stabilized by hydrogen bonds provides an enzyme its selectivity, rendering pH regulation essential for its functioning. For effective control, triggers by small concentration changes play roles where the strength of feedback is important. Here we show that the interaction of acid-base equilibria with simple reactions with pH-dependent rate can lead to the emergence of a positive feedback in hydroxide ion concentration during the hydrolysis of some Schiff bases in the physiological pH range. The underlying reaction network can also support bistability in an open system.

Fast-response photothermal bilayer actuator based on poly(N-isopropylacrylamide)-graphene oxide-hydroxyethyl methacrylate/polydimethylsiloxane

Demands for highly deformable and responsive intelligent actuators are increasing rapidly. Herein, a photothermal bilayer actuator consisting of a photothermal-responsive composite hydrogel layer and a polydimethylsiloxane (PDMS) layer is presented. The photothermal-responsive composite hydrogel is prepared by compositing hydroxyethyl methacrylate (HEMA) and the photothermal material graphene oxide (GO) with the thermal-responsive hydrogel poly(N-isopropylacrylamide) (PNIPAM). The HEMA improves the transport efficiency of water molecules inside the hydrogel network, eliciting a fast response and large deformation, facilitating greater bending behavior of the bilayer actuator, and improving the mechanical and tensile properties of the hydrogel. Moreover, GO enhances the mechanical properties and the photothermal conversion efficiency of the hydrogel in the thermal environment. This photothermal bilayer actuator can be driven under various conditions, such as hot solution, simulated sunlight, and laser, and can achieve large bending deformation with desirable tensile properties, broadening the application conditions for bilayer actuators, such as artificial muscles, bionic actuators, and soft robotics.

Current Principles, Challenges, and New Metrics in pH-Responsive Drug Delivery Systems for Systemic Cancer Therapy

The paradigm of drug delivery via particulate formulations is one of the leading ideas that enable overcoming limitations of traditional chemotherapeutic agents. The trend toward more complex multifunctional drug carriers is well-traced in the literature. Nowadays, the prospectiveness of stimuli-responsive systems capable of controlled cargo release in the lesion nidus is widely accepted. Both endogenous and exogenous stimuli are employed for this purpose; however, endogenous pH is the most common trigger. Unfortunately, scientists encounter multiple challenges on the way to the implementation of this idea related to the vehicles' accumulation in off-target tissues, their immunogenicity, the complexity of drug delivery to intracellular targets, and finally, the difficulties in the fabrication of carriers matching all imposed requirements. Here, we discuss fundamental strategies for pH-responsive drug delivery, as well as limitations related to such carriers' application, and reveal the main problems, weaknesses, and reasons for poor clinical results. Moreover, we attempted to formulate the profiles of an "ideal" drug carrier in the frame of different strategies drawing on the example of metal-comprising materials and considered recently published studies through the lens of these profiles. We believe that this approach will facilitate the formulation of the main challenges facing researchers and the identification of the most promising trends in technology development.

Evolution of nanostructured skin patches towards multifunctional wearable platforms for biomedical applications

Recent advances in the field of skin patches have promoted the development of wearable and implantable bioelectronics for long-term, continuous healthcare management and targeted therapy. However, the design of electronic skin (e-skin) patches with stretchable components is still challenging and requires an in-depth understanding of the skin-attachable substrate layer, functional biomaterials and advanced self-powered electronics. In this comprehensive review, we present the evolution of skin patches from functional nanostructured materials to multi-functional and stimuli-responsive patches towards flexible substrates and emerging biomaterials for e-skin patches, including the material selection, structure design and promising applications. Stretchable sensors and self-powered e-skin patches are also discussed, ranging from electrical stimulation for clinical procedures to continuous health monitoring and integrated systems for comprehensive healthcare management. Moreover, an integrated energy harvester with bioelectronics enables the fabrication of self-powered electronic skin patches, which can effectively solve the energy supply and overcome the drawbacks induced by bulky battery-driven devices. However, to realize the full potential offered by these advancements, several challenges must be addressed for next-generation e-skin patches. Finally, future opportunities and positive outlooks are presented on the future directions of bioelectronics. It is believed that innovative material design, structure engineering, and in-depth study of fundamental principles can foster the rapid evolution of electronic skin patches, and eventually enable self-powered close-looped bioelectronic systems to benefit mankind.

Advanced Temporally-Spatially Precise Technologies for On-Demand Neurological Disorder Intervention

Temporal-spatial precision has attracted increasing attention for the clinical intervention of neurological disorders (NDs) to mitigate adverse effects of traditional treatments and achieve point-of-care medicine. Inspiring steps forward in this field have been witnessed in recent years, giving the credit to multi-discipline efforts from neurobiology, bioengineering, chemical materials, artificial intelligence, and so on, exhibiting valuable clinical translation potential. In this review, the latest progress in advanced temporally-spatially precise clinical intervention is highlighted, including localized parenchyma drug delivery, precise neuromodulation, as well as biological signal detection to trigger closed-loop control. Their clinical potential in both central and peripheral nervous systems is illustrated meticulously related to typical diseases. The challenges relative to biosafety and scaled production as well as their future perspectives are also discussed in detail. Notably, these intelligent temporally-spatially precision intervention systems could lead the frontier in the near future, demonstrating significant clinical value to support billions of patients plagued with NDs.

Silane coupling agent in biomedical materials

Medical devices are becoming more and more significant in our daily life. For implantable medical devices, good biocompatibility is required for further use in vivo. Thus, surface modification of medical devices is really important, which gives a wide application scene for a silane coupling agent. The silane coupling agent is able to form a durable bond between organic and inorganic materials. The dehydration process provides linking sites to achieve condensation of two hydroxyl groups. The forming covalent bond brings excellent mechanical properties among different surfaces. Indeed, the silane coupling agent is a popular component in surface modification. Metals, proteins, and hydrogels are using silane coupling agent to link parts commonly. The mild reaction environment also brings advantages for the spread of the silane coupling agent. In this review, we summarize two main methods of using the silane coupling agent. One is acting as a crosslinker mixed in the whole system, and the other is to provide a bridge between different surfaces. Moreover, we introduce their applications in biomedical devices.

Anisotropic, Hydrogel Microparticles as pH-Responsive Drug Carriers for Oral Administration of 5-FU

In the last 20 years, the development of stimuli-responsive drug delivery systems (DDS) has received great attention. Hydrogel microparticles represent one of the candidates with the most potential. However, if the role of the cross-linking method, polymer composition, and concentration on their performance as DDS has been well-studied, still, a lot needs to be explained regarding the effect caused by the morphology. To investigate this, herein, we report the fabrication of PEGDA-ALMA-based microgels with spherical and asymmetric shapes for 5-fluorouracil (5-FU) on-demand loading and in vitro pH-triggered release. Due to anisotropic properties, the asymmetric particles showed an increased drug adsorption and higher pH responsiveness, which in turn led to a higher desorption efficacy at the target pH environment, making them an ideal candidate for oral administration of 5-FU in colorectal cancer. The cytotoxicity of empty spherical microgels was higher than the cytotoxicity of empty asymmetric microgels, suggesting that the gel network's mechanical proprieties of anisotropic particles were a better three-dimensional environment for the vital functions of cells. Upon treatment with drug-loaded microgels, the HeLa cells' viability was lower after incubation with asymmetric particles, confirming a minor release of 5-FU from spherical particles.

Hydrogel-Based Flexible Electronics

Flexible electronics is an emerging field of research involving multiple disciplines, which include but not limited to physics, chemistry, materials science, electronic engineering, and biology. However, the broad applications of flexible electronics are still restricted due to several limitations, including high Young's modulus, poor biocompatibility, and poor responsiveness. Innovative materials aiming for overcoming these drawbacks and boost its practical application is highly desirable. Hydrogel is a class of 3D crosslinked hydrated polymer networks, and its exceptional material properties render it as a promising candidate for the next generation of flexible electronics. Here, the latest methods of synthesizing advanced functional hydrogels and the state-of-art applications of hydrogel-based flexible electronics in various fields are reviewed. More importantly, the correlation between properties of the hydrogel and device performance is discussed here, to have better understanding of the development of flexible electronics by using environmentally responsive hydrogels. Last, perspectives on the current challenges and future directions in the development of hydrogel-based multifunctional flexible electronics are provided.

UV and IR dual light triggered cellulose-based invisible actuators with high sensitivity

Actuators are widely used in bionic devices and soft robots, among which invisible actuators have some unique applications, including performing secret missions. In this paper, highly visible transparent cellulose-based UV-absorbing films were prepared by dissolving cellulose raw materials using N-methylmorpholine-N-oxide (NMMO) and using ZnO nanoparticles as UV absorbers. Furthermore, transparent actuator was fabricated by growing highly transparent and hydrophobic polytetrafluoroethylene (PTFE) film on regenerated cellulose (RC)-ZnO composite film. In addition to its sensitive response to Infrared (IR) light, the as-prepared actuator also shows a highly sensitive response to UV light, which is attributed to the strong absorption of UV light by ZnO NPs. Thanks to the drastic differences in adsorption capacity between the RC-ZnO and PTFE materials for water molecules, the asymmetrically- assembled actuator demonstrates extremely high sensitivity and excellent actuation performance, with a force density of 60.5, a maximum bending curvature of 3.0 cm^-1, and a response time of below 8 s. Bionic bug, smart door and the arm of excavator made from the actuator all exhibit sensitive responses to UV and IR lights.

Instant formation of horizontally ordered nanofibrous hydrogel films and direct investigation of peculiar neuronal cell behaviors atop

BACKGROUND

Hydrogels have been widely used in many research fields owing to optical transparency, good biocompatibility, tunable mechanical properties, etc. Unlike typical hydrogels in the form of an unstructured bulk material, we developed aqueous dispersions of fiber-shaped hydrogel structures with high stability under ambient conditions and their application to various types of transparent soft cell culture interfaces with anisotropic nanoscale topography.

METHOD

Nanofibers based on the polyvinyl alcohol and polyacrylic acid mixture were prepared by electrospinning and hydrogelified to nano-fibrous hydrogels (nFHs) after thermal crosslinking and sulfuric acid treatment. By modifying various material surfaces with positively-charged polymers, negatively-charged superabsorbent nFHs could be selectively patterned by employing micro-contact printing or horizontally aligned by applying shear force with a wired bar coater.

RESULTS

The angular distribution of bar-coated nFHs was dramatically reduced to ± 20° along the applied shear direction unlike the drop-coated nFHs which exhibit random orientations. Next, various types of cells were cultured on top of transparent soft nFHs which showed good viability and attachment while their behaviors could be easily monitored by both upright and inverted optical microscopy. Particularly, neuronal lineage cells such as PC 12 cells and embryonic hippocampal neurons showed highly stretched morphology along the overall fiber orientation with aspect ratios ranging from 1 to 14. Furthermore, the resultant neurite outgrowth and migration behaviors could be effectively controlled by the horizontal orientation and the three-dimensional arrangement of underlying nFHs, respectively.

CONCLUSION

We expect that surface modifications with transparent soft nFHs will be beneficial for various biological/biomedical studies such as fundamental cellular studies, neuronal/stem cell and/or organoid cultures, implantable probe/device coatings, etc.

Fabrication of a new photoluminescent and pH-responsive nanocomposite based on a hyperbranched polymer prepared from amino acid for targeted drug delivery applications

In this study, the Fe₃O₄ nanoparticles were encapsulated in the hyperbranched poly _L-lysine citramid (HBPLC). The Fe₃O₄-HBPLC nanocomposite modified with _L-arginine and quantum dots (QDs) to obtain Fe₃O₄-HBPLC-Arg/QDs as a new photoluminescent and magnetic nanocarrier for the pH-responsive release and targeted delivery of Doxorubicin (DOX). The prepared magnetic nanocarrier was fully characterized using different techniques. Its various potential as a magnetic nanocarrier was evaluated. The in-vitro drug release studies exhibited that the prepared nanocomposite has pH-responsive behavior. The antioxidant study revealed good antioxidant properties of the nanocarrier. Also, the nanocomposite revealed excellent photoluminescence with a quantum yield of 48.5 %. Cellular uptake studies showed that Fe₃O₄-HBPLC-Arg/QD has high cell uptake in MCF-7 cells and can be used for bioimaging applications. In-vitro cytotoxicity, colloidal stability, and enzymatic degradability studies revealed that the prepared nanocarrier is non-toxic (with cell viability of 94%), stabile and biodegradable (about 37%). The nanocarrier was hemocompatible with 8% hemolysis. Also, according to the apoptosis and MTT assays, the Fe₃O₄-HBPLC-Arg/QD-DOX induced greater toxicity and cellular apoptosis against breast cancer cells about 47.0 %.

pH-Responsive Super-Porous Hybrid Hydrogels for Gastroretentive Controlled-Release Drug Delivery

Super-porous hydrogels are considered a potential drug delivery network for the sedation of gastric mechanisms with retention windows in the abdomen and upper part of the gastrointestinal tract (GIT). In this study, a novel pH-responsive super-porous hybrid hydrogels (SPHHs) was synthesized from pectin, poly 2-hydroxyethyl methacrylate (2HEMA), and N, N methylene-bis-acrylamide (BIS) via the gas-blowing technique, and then loaded with a selected drug (amoxicillin trihydrate, AT) at pH 5 via an aqueous loading method. The drug-loaded SPHHs-AT carrier demonstrated outstanding (in vitro) gastroretentive drug delivery capability. The study attributed excellent swelling and delayed drug release to acidic conditions at pH 1.2. Moreover, in vitro controlled-release drug delivery systems at different pH values, namely, 1.2 (97.99%) and 7.4 (88%), were studied. These exceptional features of SPHHs—improved elasticity, pH responsivity, and high swelling performance—should be investigated for broader drug delivery applications in the future.

Magnetic-responsive Covalent Adaptable Networks

Covalent adaptable networks (CANs) are reprocessable polymers whose structural arrangement is based on the recombination of dynamic covalent bonds. Composite materials prepared by incorporating magnetic particles into CANs attract much attention due to their remote and precise control, fast response speed, high biological safety and strong penetration of magnetic stimuli. These properties often involve magnetothermal effect and direct magnetic-field guidance. Besides, some of them can also respond to light, electricity or pH values. Thus, they are favorable for soft actuators since various functions are achieved such as magnetic-assisted self-healing (heating or at ambient temperature), welding (on land or under water), shape-morphing, and so on. Although magnetic CANs just start to be studied in recent two years, their advances are promised to expand the practical applications in both cutting-edge academic and engineering fields. This review aims to summarize recent progress in magnetic-responsive CANs, including their design, synthesis and application.

Soft robot-enabled controlled release of oral drug formulations

The creation of highly effective oral drug delivery systems (ODDSs) has long been the main objective of pharmaceutical research. Multidisciplinary efforts involving materials, electronics, control, and pharmaceutical sciences encourage the development of robot-enabled ODDSs. Compared with conventional rigid robots, soft robots potentially offer better mechanical compliance and biocompatibility with biological tissues, more versatile shape control and maneuverability, and multifunctionality. In this paper, we first describe and highlight the importance of manipulating drug release kinetics, i.e. pharmaceutical kinetics. We then introduce an overview of state-of-the-art soft robot-based ODDSs comprising resident, shape-programming, locomotive, and integrated soft robots. Finally, the challenges and outlook regarding future soft robot-based ODDS development are discussed.

Remotely Controlled Light/Electric/Magnetic Multiresponsive Hydrogel for Fast Actuations

As a kind of soft smart material, hydrogel actuators have extensive development prospects, but it is still difficult for these actuators to integrate multiresponsiveness, multiple remote actuation, high strength, fast responsiveness, and programmable complex deformation. Herein, we have explored an anisotropic bilayer hydrogel actuator with an Fe3O4/co-poly(isopropylacrylamide-4-benzoylphenyl acrylate) [Fe3O4/P(NIPAM-ABP)] active layer and an isotropic conductive adhesive (ICAs) passive layer based on the layer-by-layer method. Benefiting from the fibrosis and porosity of the Fe3O4/P(NIPAM-ABP) hydrogel, the ICAs-Fe3O4/P(NIPAM-ABP) hydrogel actuator has excellent mechanical strength (tensile strength of 3.1 ± 0.3 MPa) and response speed (temperature (45 °C): bending speed of 2400.3°/s; near-infrared (NIR) light: bending speed of 356.4°/s; electricity (2 V): bending speed of 180°/s; water (10 °C): recovery speed of 30.0°/s). In addition, the good photothermal properties and magnetic conductivity of Fe3O4 nanoparticles provide precise remotely controllable light- and magnetic-actuated properties for the hydrogel actuator. The Ag microsheets with excellent conductivity (1.4 × 104 S/cm) provide remotely controllable electrical-actuated property for the hydrogel actuator. Combined with the responsiveness of P(NIPAM-ABP), the actuator can achieve short-range actuation including temperature-, ethanol-, and salt-responses. More importantly, it can achieve remote actuation including light, electrical, and magnetic responses. Finally, the Fe3O4/P(NIPAM-ABP) fibers can provide excellent anisotropic structures for the actuator to achieve precise deformational programmability. Inspired by some phenomena in nature, several actuating devices with the above characteristics have been successfully developed. This study can provide a general method for multifunctional anisotropic hydrogel actuators and will provide a new strategy for exploring smart materials suitable for complex bioinspired systems.

Nanocarriers based novel and effective drug delivery system

Nanotechnology has ultimately come into the domain of drug delivery. Nanosystems for delivery of drugs are promptly emerging science utilizing different nanoparticles as carriers. Biocompatible and stable nanocarriers are novel diagnosis tools or therapy agents for explicitly targeting locates with controllable way. Nanocarriers propose numerous advantages to treat diseases via site-specific as well as targeted delivery of particular therapeutics. In recent times, there are number of outstanding nanocarriers use to deliver bio-, chemo-, or immuno- therapeutic agents to obtain effectual therapeutic reactions and to minimalize unwanted adverse-effects. Nanoparticles possess remarkable potential for active drug delivery. Moreover, conjugation of drugs with nanocarriers protects drugs from metabolic or chemical modifications, through their way to targeted cells and hence increased their bioavailability. In this review, various systems integrated with different types of nanocarriers (inorganic. organic, quantum dots, and carbon nanotubes) having different compositions, physical and chemical properties have been discussed for drug delivery applications.

Designing Multistimuli-Responsive Anisotropic Bilayer Hydrogel Actuators by Integrating LCST Phase Transition and Photochromic Isomerization

Stimuli-responsive hydrogel actuators have attracted tremendous interest in switches and microrobots. Based on N-isopropylacrylamide (NIPAM) monomers with LCST phase separation and photochromic molecule spiropyran which can respond to ultraviolet light and H+, we develop a novel multistimuli-responsive co-polymer anisotropic bilayer hydrogel, which can undergo complex deformation behavior under environmental stimuli. Diverse bending angles were achieved based on inhomogeneous swelling. By controlling the environmental temperature, the bilayer hydrogels achieved bending angles of 83.4° and -162.4° below and above the critical temperature of PNIPAM. Stimulated by ultraviolet light and H+, the bilayer hydrogels showed bending angles of -19.4° and -17.3°, respectively. In addition, we designed a strategy to enhance the mechanical properties of the hydrogel via double network (DN). The mechanical properties and microscopic Fourier transform infrared (micro-FTIR) spectrum showed that the bilayer hydrogel can be well bonded at the interfaces of such bilayers. This work will inspire the design and fabrication of novel soft actuators with synergistic functions.

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

Development of hydrogel-based actuators with programmable deformation is an important topic that arouses much attention in fundamental and applied research. Most of these actuators are nonbiodegradable or work under nonphysiological conditions. Herein, a temperature-responsive and biodegradable gelatin methacryloyl (GelMA)-poly(N-isopropylacrylamide) hydrogel (i.e., GN hydrogel) network was explored as the active layer of a bilayer actuator. Small-angle X-ray scattering (SAXS) revealed that the GN hydrogel formed a mesoglobular structure (∼230 Å) upon a thermally induced phase transition. Rheological data supported that the GN hydrogel possessed 3D printability and tunable mechanical properties. A bilayer hydrogel actuator composed of active GN and passive GelMA layers was optimized by varying the layer thickness and compositions to achieve large, reproducible, and anisotropic bending with a curvature of ∼5.5 cm^-1. Different patterns of the active layer were designed for actuation in programmable control. The 3D printed GN hydrogel constructs showed significant volume reduction (∼25-60% depending on construct design) at 37 °C with the resolution enhanced by the thermo-triggered actuation, while they were able to fully reswell at room temperature. A more intricate 3D printed butterfly actuator demonstrated the ability to mimic the wing movement through thermoresponsiveness. Furthermore, myoblasts laden in the GN hydrogel exhibited significant proliferation of ∼376% in 14 days. This study provides a new fabrication approach for developing biomimetic devices, artificial muscles, and soft robotics for biomedical applications.

Construction nanoenzymes with elaborately regulated multi-enzymatic activities for photothermal-enhanced catalytic therapy of tumor

In order to solve the limitation of tumor microenvironment on the anticancer effect of nanozymes, a multifunctional nanoenzyme Co/La-PB@MOF-199/GOx was designed in this work. By doping Co²⁺ and La³⁺ in different proportions, Co/La-PB with the optimal photothermal-enhanced catalytic performance was screened, which can catalyze H₂O₂ to generate more hydroxyl radicals (•OH) and oxygen, showing peroxidase (POD)-like and catalase(CAT)-like property. Through MOF-199 coating and loading glucose oxidase (GOx), a multifunctional nanoenzyme Co/La-PB@MOF-199/GOx was achieved. Due to the pH response of MOF-199, GOx can be accurately released into tumors to catalyze the reaction of glucose and oxygen to produce H₂O₂. In this process, the oxygen consumption can be compensated by the CAT-like property to realize continuous consumption of glucose and self-supply of H₂O₂ to continuously produce •OH. In the presence of high oxidation state metal ions (Co³⁺ and Fe³⁺), GSH consumption is accelerated to avoid weakening of •OH, showing the glutathione oxidase (GPx-like) activity. Besides, magnetic resonance imaging (MRI) experiments showed the potential application in imaging guided therapy. In vivo anti-tumor experiments showed a satisfactory anti-cancer effect through multi-enzymatic activities.

Biomembrane-Based Nanostructure- and Microstructure-Loaded Hydrogels for Promoting Chronic Wound Healing

Wound healing is a complex and dynamic process, and metabolic disturbances in the microenvironment of chronic wounds and the severe symptoms they cause remain major challenges to be addressed. The inherent properties of hydrogels make them promising wound dressings. In addition, biomembrane-based nanostructures and microstructures (such as liposomes, exosomes, membrane-coated nanostructures, bacteria and algae) have significant advantages in the promotion of wound healing, including special biological activities, flexible drug loading and targeting. Therefore, biomembrane-based nanostructure- and microstructure-loaded hydrogels can compensate for their respective disadvantages and combine the advantages of both to significantly promote chronic wound healing. In this review, we outline the loading strategies, mechanisms of action and applications of different types of biomembrane-based nanostructure- and microstructure-loaded hydrogels in chronic wound healing.

Engineering the Nonmorphing Point of Actuation for Controlled Drug Release by Hydrogel Bilayer across the pH Spectrum

Hydrogel-based pH-responsive bilayer actuators exhibit bidirectional actuation due to the differences in the concentration gradient developed across the thickness, the volume expansion due to swelling, and the mechanical stiffness of the layers involved. At a pH value (point), where the sum of these factors generates moments of equal magnitudes, the moments cancel each other and result in no net actuation. This pH point is termed here as a "nonmorphing point". In this work, we present a bilayer of chitosan (CS) and carboxymethyl cellulose (CMC) cross-linked with citric acid (CA) with tunable nonmorphing points across the pH spectrum by modulating the concentration and cross-linking density of the layers involved. The standard CS/CMC bilayer films took about 40 s to completely fold (clockwise) in 0.1 M HCl and 78 s to completely fold (anticlockwise) in 0.1 M NaOH. Generally, pH-responsive actuators are designed for targeted drug delivery to a specific site inside the body as they show bidirectional (clockwise/anticlockwise) actuation around a single nonmorphing point. The same pH-responsive system cannot be applied for drug release at another site with a different functioning pH. Thus, having a pH-responsive system with multiple nonmorphing points is highly desirable. Drug release experiments were performed with FITC and EtBr as model drugs loaded in CS and CMC layers. Moreover, the clockwise/anticlockwise actuation of the bilayer around the nonmorphing point can facilitate or inhibit the release of a drug. The clockwise actuation resulted in 55% FITC release and inhibited EtBr release to 4%; anticlockwise actuation resulted in 50% EtBr release and inhibited FITC release to 5%. We demonstrated morphing induced drug release by hydrogel bilayer films with tunable nonmorphing points across the pH spectrum.

Self-strengthening stimuli-responsive nanocomposite hydrogels

Stimuli-responsive hydrogels with self-strengthening properties are promising for the use of autonomous growth and adaptation systems to the surrounding environments by mimicking biological materials. However, conventional stimuli-responsive hydrogels require structural destruction to initiate mechanochemical reactions to grow new polymeric networks and strengthen themselves. Here we report continuous self-strengthening of a nanocomposite hydrogel composed of poly(N-isopropylacrylamide) (PNIPAM) and nanoclay (NC) by using external stimuli such as heat and ionic strength. The internal structures of the NC-PNIPAM hydrogel are rearranged through the swelling-deswelling cycles or immersing in a salt solution, thus its mechanical properties are significantly improved. The effects of concentration of NC in hydrogels, number of swelling-deswelling cycles, and presence of salt in the surrounding environment on the mechanical properties of hydrogels are characterized by nanoindentation and tensile tests. The self-strengthening mechanical performance of the hydrogels is demonstrated by the loading ability. This work may offer promise for applications such as artificial muscles and soft robotics.

Temporary Actuation of Bilayer Polymer Hydrogels Mediated by the Enzymatic Reaction

Most soft actuators have the ability of monotonic responsiveness. That is, there is only one response action after being stimulated once. In this work, a temporarily responsive bilayer hydrogel actuator is designed and fabricated by combining a tertiary amine-containing pH-responsive layer and a urease-containing non-responsive layer. The hydrogel actuator can achieve programed deformation and recovery driven by chemical fuels (i.e., acidic urea solutions), which is essentially regulated by rapid acidification and slow enzymatic production of ammonia for recovering the pH of the system. The actuation extent and duration can be simply controlled by the fuel levels, and the repeated actuations are also possible via refueling. Furthermore, we fabricate several hydrogel devices that can display specific patterns or lift an item. This enzymatic method shows new possibilities to control the temporary actuation of polymer hydrogels potentially useful in many fields such as soft robotics, biomimetic devices, and environmental sensing.