Continuous Surface Polymerization via Fe(II)-Mediated Redox Reaction for Thick Hydrogel Coatings on Versatile Substrates

PEG-Based Hydrogel Coatings: Design Tools for Biomedical Applications

Device failure due to undesired biological responses remains a substantial roadblock in the development and translation of new devices into clinical care. Polyethylene glycol (PEG)-based hydrogel coatings can be used to confer antifouling properties to medical devices-enabling minimization of biological responses such as bacterial infection, thrombosis, and foreign body reactions. Application of hydrogel coatings to diverse substrates requires careful consideration of multiple material factors. Herein, we report a systematic investigation of two coating methods: (1) traditional photoinitiated hydrogel coatings; (2) diffusion-mediated, redox-initiated hydrogel coatings. The effects of method, substrate, and compositional variables on the resulting hydrogel coating thickness are presented. To expand the redox-based method to include high molecular weight macromers, a mechanistic investigation of the role of cure rate and macromer viscosity was necessary to balance solution infiltration and gelation. Overall, these structure-property relationships provide users with a toolbox for hydrogel coating design for a broad range of medical devices.

In Situ Structural Densification of Hydrogel Network and Its Interface with Electrodes for High-Performance Multimodal Artificial Skin

The multisensory responsiveness of hydrogels positions them as promising candidates for artificial skin, whereas the mismatch of modulus between soft hydrogels and hard electrodes as well as the poor adhesion and conductance at the interface greatly impairs the stability of electronics devices. Herein, we propose an in situ postprocessing approach utilizing electrochemical reactions between metals (Zn, etc.) and hydrogels to synergistically achieve strong adhesion of the hydrogel-electrode interface, low interfacial impedance, and local strain isolation due to the structural densification of the hydrogel network. The mechanism is that Zn electrochemically oxidizes to Zn2+ and injects into the hydrogel, gradually forming a mechanically interlocked structure, Zn2+-polymer dual-helix structural nodes, and a high-modulus ZnO from the surface to the interior. Compared to untreated samples, the treated sample displays 8.7 times increased interfacial adhesion energy between the hydrogel and electrode (87 J/m2), 95% decreased interfacial impedance (218.8 Ω), and a high-strain isolation efficiency (εtotal/εisolation > 400). Akin to human skin, the prepared sensor demonstrates multimodal sensing capabilities, encompassing highly sensitive strain perception and simultaneous perception of temperature, humidity, and oxygen content unaffected by strain interference. This easy on-chip preparation of hydrogel-based multimodal sensor array shows great potential for health and environment monitoring as artificial skin.

Rapid, Tough, and Trigger-Detachable Hydrogel Adhesion Enabled by Formation of Nanoparticles In Situ

Integrating hydrogel with other materials is always challenging due to the low mass content of hydrogels and the abundance of water at the interfaces. Adhesion through nanoparticles offers characteristics such as ease of use, reversibility, and universality, but still grapples with challenges like weak bonding. Here, a simple yet powerful strategy using the formation of nanoparticles in situ is reported, establishing strong interfacial adhesion between various hydrogels and substrates including elastomers, plastics, and biological tissue, even under wet conditions. The strong interfacial bonding can be formed in a short time (60 s), and gradually strengthened to 902 J m-2 adhesion energy within an hour. The interfacial layer's construction involves chain entanglement and other non-covalent interactions like coordination and hydrogen bonding. Unlike the permanent bonding seen in most synthetic adhesives, these nanoparticle adhesives can be efficiently triggered for removal by acidic solutions. The simplicity of the precursor diffusion and precipitation process in creating the interfacial layer ensures broad applicability to different substrates and nanoparticle adhesives without compromising robustness. The tough adhesion provided by nanoparticles allows the hydrogel-elastomer hybrid to function as a triboelectric nanogenerator (TENG), facilitating reliable electrical signal generation and output performance due to the robust interface.

Growth of Double-Network Tough Hydrogel Coatings by Surface-Initiated Polymerization

Hydrogel coatings exhibit versatile applications in biomedicine, flexible electronics, and environmental science. However, current coating methods encounter challenges in simultaneously achieving strong interfacial bonding, robust hydrogel coatings, and the ability to coat substrates with controlled thickness. This paper introduces a novel approach to grow a double-network (DN) tough hydrogel coating on various substrates. The process involves initial substrate modification using a silane coupling agent, followed by the deposition of an initiator layer on its surface. Subsequently, the substrate is immersed in a DN hydrogel precursor, where the coating grows under ultraviolet (UV) illumination. Precise control over the coating thickness is achieved by adjusting the UV illumination duration and the initiator quantity. The experimental measurement of adhesion reveals strong bonding between the DN hydrogel coating and diverse substrates, reaching up to 1012.9 J/m2 between the DN hydrogel coating and a glass substrate. The lubricity performance of the DN hydrogel coating is experimentally characterized, which is dependent on the coating thickness, applied pressure, and sliding velocity. The incorporation of 3D printing technology into the current coating method enables the creation of intricate hydrogel coating patterns on a flat substrate. Moreover, the hydrogel coating's versatility is demonstrated through its effective applications in oil-water separation and antifogging glasses, underscoring its wide-ranging potential. The robust DN hydrogel coating method presented here holds promise for advancing hydrogel applications across diverse fields.

Unravelling Surface Modification Strategies for Preventing Medical Device-Induced Thrombosis

The use of biomaterials in implanted medical devices remains hampered by platelet adhesion and blood coagulation. Thrombus formation is a prevalent cause of failure of these blood-contacting devices. Although systemic anticoagulant can be used to support materials and devices with poor blood compatibility, its negative effects such as an increased chance of bleeding, make materials with superior hemocompatibility extremely attractive, especially for long-term applications. This review examines blood-surface interactions, the pathogenesis of clotting on blood-contacting medical devices, popular surface modification techniques, mechanisms of action of anticoagulant coatings, and discusses future directions in biomaterial research for preventing thrombosis. In addition, this paper comprehensively reviews several novel methods that either entirely prevent interaction between material surfaces and blood components or regulate the reaction of the coagulation cascade, thrombocytes, and leukocytes.

Liquid-in-liquid printing of 3D and mechanically tunable conductive hydrogels

Conductive hydrogels require tunable mechanical properties, high conductivity and complicated 3D structures for advanced functionality in (bio)applications. Here, we report a straightforward strategy to construct 3D conductive hydrogels by programable printing of aqueous inks rich in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) inside of oil. In this liquid-in-liquid printing method, assemblies of PEDOT:PSS colloidal particles originating from the aqueous phase and polydimethylsiloxane surfactants from the other form an elastic film at the liquid-liquid interface, allowing trapping of the hydrogel precursor inks in the designed 3D nonequilibrium shapes for subsequent gelation and/or chemical cross-linking. Conductivities up to 301 S m^-1 are achieved for a low PEDOT:PSS content of 9 mg mL^-1 in two interpenetrating hydrogel networks. The effortless printability enables us to tune the hydrogels' components and mechanical properties, thus facilitating the use of these conductive hydrogels as electromicrofluidic devices and to customize near-field communication (NFC) implantable biochips in the future.

Construction of hexagonal spindle-shaped Fe-MOFs induced by cationic copolymer and its application for effective wastewater treatment

The environment and human health are in danger due to the long-term enrichment and buildup of organic pesticides, dyes, and harmful microbes in wastewater. The development of functional materials that are efficient for treating wastewater remains a significant problem. Eco-friendly hexagonal spindle-shaped Fe-MOFs (Hs-FeMOFs) were created in this study under the influence of cationic copolymer (PMSt). The mechanism of crystal growth and development of its unique morphology were described after looking into impact factors for the ideal circumstances and being characterized by XRD, TEM, XPS, and other techniques. It revealed that Hs-FeMOFs possess an enormous supply of adsorption active sites, a strong electropositivity, and the nanometer tip. Then, typical organic pollutants, such as herbicides and mixed dyes, as well as biological pollutants bacteria, were chosen to assess its efficacy in wastewater treatment. It was discovered that the pendimethalin could be quickly removed in wastewater and the removal rate reached 100% within 10 min. In separation of mixed dyes, the retention rate of malachite green (MG) reached 92.3% in 5 min and with a minimum inhibitory concentration of 0.8 mg/mL and demonstrated strong activity due to the presence of cationic copolymers. In actual water matrix, Hs-FeMOF could also play excellent adsorption and antibacterial activity. In summary, a novel, environmentally friendly MOF material with good activity was successfully created by cationic copolymer induction. It offers a fresh approach to develop functional materials in wastewater treatment.

UV-Initiated Frontal Polymerization for the Fast Synthesis of Bubble-Free, Self-Propagating Hydrogel Anticorrosive Coatings

Hydrogels are formed through the cross-linking of hydrophilic polymer chains within an aqueous microenvironment. A novel synthesis strategy, UV-initiated frontal polymerization to quickly synthesize bubble-free, self-propagating hydrogel anticorrosive coatings in aqueous conditions, was presented, in which the self-propagating polymerization rate reached 16.7 mm min^-1. SEM, AFM, FTIR, EIS, IC, XPS, and ultra-depth-of-field microscopy were used to characterize the surface morphology and anti-corrosion properties of the hydrogel coating. The addition of nano-SiO₂ formed dense hydrogen bonds, and the stability of the three-dimensional network was enhanced. Underwater, the hydrogel coating could physically block corrosive media from reaching steel surfaces, and it was 75% effective at blocking chloride ions after 7 days under osmotic pressure. In addition, 1,2,4-triazole accumulated on the steel surface, and then the active anti-corrosion function took effect. This paper provides the basis for the in situ synthesis of hydrogel anticorrosive coatings on the surface of mild steel in a wet or underwater state.

Small-Scale Robotics with Tailored Wettability

Small-scale robots (SSRs) have emerged as promising and versatile tools in various biomedical, sensing, decontamination, and manipulation applications, as they are uniquely capable of performing tasks at small length scales. With the miniaturization of robots from the macroscale to millimeter-, micrometer-, and nanometer-scales, the viscous and surface forces, namely adhesive forces and surface tension have become dominant. These forces significantly impact motion efficiency. Surface engineering of robots with both hydrophilic and hydrophobic functionalization presents a brand-new pathway to overcome motion resistance and enhance the ability to target and regulate robots for various tasks. This review focuses on the current progress and future perspectives of SSRs with hydrophilic and hydrophobic modifications (including both tethered and untethered robots). The study emphasizes the distinct advantages of SSRs, such as improved maneuverability and reduced drag forces, and outlines their potential applications. With continued innovation, rational surface engineering is expected to endow SSRs with exceptional mobility and functionality, which can broaden their applications, enhance their penetration depth, reduce surface fouling, and inhibit bacterial adhesion.

Antibacterial-Based Hydrogel Coatings and Their Application in the Biomedical Field-A Review

Hydrogels exhibit excellent moldability, biodegradability, biocompatibility, and extracellular matrix-like properties, which make them widely used in biomedical fields. Because of their unique three-dimensional crosslinked hydrophilic networks, hydrogels can encapsulate various materials, such as small molecules, polymers, and particles; this has become a hot research topic in the antibacterial field. The surface modification of biomaterials by using antibacterial hydrogels as coatings contributes to the biomaterial activity and offers wide prospects for development. A variety of surface chemical strategies have been developed to bind hydrogels to the substrate surface stably. We first introduce the preparation method for antibacterial coatings in this review, which includes surface-initiated graft crosslinking polymerization, anchoring the hydrogel coating to the substrate surface, and the LbL self-assembly technique to coat crosslinked hydrogels. Then, we summarize the applications of hydrogel coating in the biomedical antibacterial field. Hydrogel itself has certain antibacterial properties, but the antibacterial effect is not sufficient. In recent research, in order to optimize its antibacterial performance, the following three antibacterial strategies are mainly adopted: bacterial repellent and inhibition, contact surface killing of bacteria, and release of antibacterial agents. We systematically introduce the antibacterial mechanism of each strategy. The review aims to provide reference for the further development and application of hydrogel coatings.

Modular Metal-Quinone Networks with Tunable Architecture and Functionality

Nanostructured materials with tunable structures and functionality are of interest in diverse areas. Herein, metal ions are coordinated with quinones through metal-acetylacetone coordination bonds to generate a class of structurally tunable, universally adhesive, hydrophilic, and pH-degradable materials. A library of metal-quinone networks (MQNs) is produced from five model quinone ligands paired with nine metal ions, leading to the assembly of particles, tubes, capsules, and films. Importantly, MQNs show bidirectional pH-responsive disassembly in acidic and alkaline solutions, where the quinone ligands mediate the disassembly kinetics, enabling temporal and spatial control over the release of multiple components using multilayered MQNs. Leveraging this tunable release and the inherent medicinal properties of quinones, MQN prodrugs with a high drug loading (>89 wt %) are engineered using doxorubicin for anti-cancer therapy and shikonin for the inhibition of the main protease in the SARS-CoV-2 virus.

Room Temperature Ca2+-Initiated Free Radical Polymerization for the Preparation of Conductive, Adhesive, Anti-freezing and UV-Blocking Hydrogels for Monitoring Human Movement

In recent years, conductive hydrogels have received increasing attention as wearable electronics due to the electrochemical properties of conductive polymers combined with the softness of hydrogels. However, conventional hydrogels are complicated to prepare, require high temperature or UV radiation to trigger monomer polymerization, and are frozen at low temperatures, which seriously hinder the application of flexible wearable devices. In this paper, a conductive sensor integrating mechanical properties, adhesion, UV shielding, anti-dehydration, and anti-freeze was prepared based on Ca²⁺-initiated radical polymerization at room temperature using the synergy of sodium lignosulfonate, acrylamide (AM), and calcium chloride (CaCl₂). Metal ions can activate ammonium persulfate to generate free radicals that allow rapid gelation of AM monomers at room temperature without external stimuli. Due to ionic cross-linking and non-covalent interaction, the hydrogels have good tensile properties (1153% elongation and 168 kPa tensile strength), high toughness (758 KJ·m^-3), excellent adhesive properties (48.5 kPa), high ionic conductivity (7.2 mS·cm^-1), and UV resistance (94.4%). CaCl₂ can inhibit ice nucleation, so that the hydrogels have anti-dehydration and frost resistance properties and even at -80 °C can maintain flexibility, high conductivity, and adhesion. Assembled into a flexible sensor, it can sense various large and small movements such as compression, bending, and talking, which is a flexible sensing material with wide application prospects.

Programmable Thermo-Responsive Actuation of Hydrogels via Light-Guided Surface Growth of Active Layers on Shape Memory Substrates

Hydrogel shape memory and actuating functionalities are heavily pursued and have found great potential in various application fields. However, their combination for more flexible and complicated morphing behaviors is still challenging. Herein, it is reported that by controlling the light-initiated polymerization of active hydrogel layers on shape memory hydrogel substrates, advanced morphing behaviors based on programmable hydrogel shapes and actuating trajectories are realized. The formation and photo-reduction-induced dissociation of Fe³⁺ -carboxylate coordination endow the hydrogel substrates with the shape memory functionality. The photo-reduced Fe²⁺ ions can diffuse from the substrates into the monomer solutions to initiate the polymerization of the thermally responsive active layers, whose actuating temperatures and amplitudes can be facially tuned by controlling their thicknesses and compositions. One potential application, a shape-programmable 3D hook that can lift an object with a specific shape, is also unveiled. The demonstrated strategy is extendable to other hydrogel systems to realize more versatile and complicated actuating behaviors.

Strong Anchoring of Hydrogels through Superwetting-Assisted High-Density Interfacial Grafting

Strong adhesion of hydrogels on solids plays an important role in stable working for various practical applications. However, current hydrogel adhesion suffers from poor interfacial bonding with solid surfaces. Here, we propose a general superwetting-assisted interfacial polymerization (SAIP) strategy to robustly anchor hydrogels onto solids by forming high-density interfacial covalent bonds. The key of our strategy is to make the initiator fully contact solid surfaces via a superwetting way for enhancing the interfacial grafting efficiency. The designed anchored hydrogels show strong bulk failure with a high breaking strength of ≈1.37 MPa, different from weak interfacial failure that occurs in traditional strategies. The strong interfacial adhesion greatly enhances the stability of hydrogels against swelling destruction. This work opens up new inspirations for designing strongly anchored hydrogels from an interfacial chemistry perspective.

Bilayer Hydrogels with Low Friction and High Load-Bearing Capacity by Mimicking the Oriented Hierarchical Structure of Cartilage

Natural articular cartilages exhibit extraordinary lubricating properties and excellent load-bearing capacity based on their penetrated surface lubricated biomacromolecules and gradient-oriented hierarchical structure. Hydrogels are considered as the most promising cartilage replacement materials due to their excellent flexibility, good biocompatibility, and low friction coefficient. However, the construction of high-strength, low-friction hydrogels to mimic cartilage is still a great challenge. Here, inspired by the structure and functions of natural articular cartilage, anisotropic hydrogels with horizontal and vertical orientation structure were constructed layer by layer and bonded with each other, successfully developing a bilayer oriented heterogeneous hydrogel with a high load-bearing capacity, low friction, and excellent fatigue resistance. The bilayer hydrogel exhibited a high compressive strength of 5.21 ± 0.45 MPa and a compressive modulus of 4.06 ± 0.31 MPa due to the enhancement mechanism of the anisotropic structure within the bottom anisotropic hydrogel. Moreover, based on the synergistic effect of the high load-bearing capacity of the bottom layer and the lubrication of the surface layer, the bilayer hydrogel possesses excellent biotribological properties in hard/soft (0.032) and soft/soft (0.028) contact, which is close to that of natural cartilage. It is worth noting that the bilayer oriented heterogeneous hydrogel is able to withstand repeated loading without fatigue crack. Therefore, this work could open up a new avenue for constructing cartilage-like materials with both high strength and low friction.

Light-Guided Growth of Gradient Hydrogels with Programmable Geometries and Thermally Responsive Actuations

Hydrogel actuators have gained considerable interest and experienced significant advancements in recent years. However, the programming of their actuating behaviors is still challenging. Herein, we report the development and regulation of gradient structures of hydrogels for programmable thermally responsive actuating behaviors. The hydrogel actuators are developed by controlling the photoreduction of Fe³⁺ ions coordinated with carboxylate groups from the substrates and their limited diffusion into the precursor solutions to act as both initiators and crosslinkers. The developed hydrogels show well-defined external geometries and controllable thicknesses under spatiotemporal control of ultraviolet irradiation. The shapes and the actuation amplitudes of the hydrogel actuators can be independently regulated by controlling the formation and photodissociation of Fe³⁺-carboxylate coordination in the formed gradient networks. Some interesting applications such as the lifting of an object with a specific shape and directional walking are realized. The proposed method can be extended to other hydrogel actuators with different compositions and stimuli-responsive behaviors.

Fluid-driven Hydrogel Actuators with an Origami Structure

Owing to the innate good biocompatibility, tissue-like softness and other unique properties, hydrogels are of particular interest as promising compliant materials for biomimetic soft actuators. However, the actuation diversity of hydrogel actuators is always restricted by their structure design and fabrication methods. Herein, origami structures were introduced to the design of fluid-driven hydrogel actuators to achieve diverse actuation movements, and a facile fabrication strategy based on removable templates and inside-out diffusion-induced in situ hydrogel crosslinking was adopted. As a result, three types of modular cuboid actuator units (CAUs) achieved linear motion, bending, and twisting. Moreover, combinations of multiple CAUs achieved different actuation modes, including actuation decoupling, superposition, and reprogramming. The diverse actuation functionality would enable new possibilities in application fields for hydrogel soft actuators. Several simple application demos, such as grippers for grasping tasks and a multi-way circuit switch, demonstrated their potential for further applications.

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Origami structures were introduced to fluid-driven hydrogel actuators

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Three types of cuboid actuator units (CAUs) achieved linear motion, bending, and twisting

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A fabrication strategy was based on removable templates and in situ formation

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Combinations of multiple CAUs achieved different actuation modes

Colorimetric Nanoparticle-Embedded Hydrogels for a Biosensing Platform

Hydrogels containing colorimetric nanoparticles have been used for ion sensing, glucose detection, and microbial metabolite analyses. In particular, the rapid chemical reaction owing to both the hydrogel form of water retention and the sensitive color change of nanoparticles enables the rapid detection of target substances. Despite this advantage, the poor dispersibility of nanoparticles and the mechanical strength of nanoparticle-hydrogel complexes have limited their application. In this study, we demonstrate a milliliter agarose gel containing homogeneously synthesized polyaniline nanoparticles (PAni-NPs), referred to as PAni-NP-hydrogel complexes (PNHCs). To fabricate the optimal PNHC, we tested various pH solvents based on distilled water and phosphate-buffered saline and studied the colorimetric response of the PNHC with thickness. The colorimetric response of the prepared PNHC to the changes in the pH of the solution demonstrated excellent linearity, suggesting the possibility of using PNHC as a pH sensor. In addition, it was verified that the PNHC could detect minute pH changes caused by the cancer cell metabolites without cytotoxicity. Furthermore, the PNHC can be stably maintained outside water for approximately 12 h without deformation, indicating that it can be used as a disposable patch-type wearable biosensing platform.

A Universal Strategy for Growing a Tenacious Hydrogel Coating from a Sticky Initiation Layer

Controllably coating the surfaces of substrates/medical devices with hydrogels exhibits great application potential, but lacks universal techniques. Herein, a new method, namely ultraviolet-triggered surface catalytically initiated radical polymerization (UV-SCIRP) from a sticky initiation layer (SIL) (SIL@UV-SCIRP), is proposed for growing hydrogel coatings. The method involves three key steps: 1) depositing a sticky polydopamine/Fe³⁺ coating on the surface of the substrates-SIL, 2) reducing Fe³⁺ ions to Fe²⁺ ions as active catalysts by UV illumination with the assistance of citric acid, and 3) conducting SCIRP in a monomer solution at room temperature for growing hydrogel coatings. In this manner, practically any substrate's surface (natural or artificial materials) can be modified by hydrogel coatings with controllable thickness and diverse compositions. The hydrogel coatings exhibit good interface bonding with the substrates and enable easy changes in their wettability and lubrication performances. Importantly, this novel method facilitates the smooth growth of uniform hydrogel lubrication coatings on the surface of a range of medical devices with complex geometries. Finally, as a proof-of-concept, the slippery balls coated with hydrogel exhibited smooth movement within the catheter and esophagus. Hence, this method can prove to be a pioneering universal modification tool, especially in surface/interface science and engineering.

Adhesive, Antibacterial, Conductive, Anti-UV, Self-Healing, and Tough Collagen-Based Hydrogels from a Pyrogallol-Ag Self-Catalysis System

Recently, versatile hydrogels with multifunctionality have been widely developed with emerging applications as wearable and implantable devices. In this work, we reported novel versatile hydrogels by self-catalyzing the gelation of an interpenetrating polymer network consisting of acrylic acid (AA) monomers and GA-modified collagen (GCOL) in situ decorated silver nanoparticles (AgNPs). The resultant hydrogel, namely AgNP@GCOL/PAA, has many desirable features, including good mechanical properties (such as 123 kPa, 916%, and 1961 J m^-2 for the fracture stress, strain and tearing energy) that match with those of animal skin, excellent self-healing performance, favorable conductivity and strain sensitivity as a flexible biosensor, and excellent antibacterial and anti-UV properties, as well as the strong adhesiveness on skin. Moreover, AgNP@GCOL/PAA showed excellent biocompatibility via in vitro cell culture. Remarkably, AgNP@GCOL/PAA displayed superior hemostatic properties with sharply decreasing blood loss for a mouse liver incision, closely related to its strong self-adhesion which produced anchoring strength to the bleeding site and thus formed a network barrier with liver tissue. This study provides new opportunities for the facile preparation of widely used multifunctional collagen-based hydrogels based on a simple pyrogallol-Ag system.

Mucosa-Like Conformal Hydrogel Coating for Aqueous Lubrication

Mucosa is a protective and lubricating barrier in biological tissue, which has a great clinical inspiration because of its slippery, soft, and hydrophilic surface. However, mimicking mucosal traits on complex surface remains an enormous challenge. Herein, a novel approach to create mucosa-like conformal hydrogel coating is developed. A thin conformal hydrogel layer mimicking the epithelial layer is obtained by first absorbing micelles, followed by forming covalent interlinks with the polymer substrate via interface-initiated hydrogel polymerization. The resulting coating exhibits uniform thickness (≈15 µm), mucosa-matched compliance (Young's modulus = 1.1 ± 0.1 kPa) and lubrication (coefficients of friction = 0.018 ± 0.003), robust interfacial bonding against peeling (peeling strength = 1218.0 ± 187.9 J m^-2 ), as well as high water absorption capacity. It effectively resists adhesion of proteins and bacteria without compromising biocompatibility. As demonstrated by an in vivo cynomolgus monkey model and clinical trial, applications of the mucosa-like conformal hydrogel coating on the endotracheal tube significantly reduce intubation-related complications, such as invasive stimuli, mucosal lesions, laryngeal edema, inflammation, and postoperative pain. This work offers a promising prototype for surface decoration of biomedical devices and holds great prospects for clinical translation to enable interventional operations with minimally invasive impacts.

Esophagus-Inspired Actuator for Solid Transportation via the Synergy of Lubrication and Contractile Deformation

Directional transportation of objects has important applications from energy transfer and intelligent robots to biomedical devices. Although breakthroughs in liquid migration on 2D surfaces or 3D tubular devices have been achieved, realizing smooth/on-demand transportation of constrained solids within a 3D cavity environment under harsh pressurized environment still remains a daunting challenge, where strong interface friction force becomes the main obstacle restricting the movement of solids. Inspired by typical feeding mechanism in natural esophagus system which synergistically couples a lubricating mucosa surface with the peristaltic contraction deformation of the cavity, herein, this challenge is addressed by constructing an esophagus-inspired layered tubular actuator with a slippery inner surface and responsive hydrogel matrix to realize spherical solid propulsion by photo(thermo)-induced cavity deformation. The as-constructed tubular actuator containing Fe₃ O₄ nanoparticles exhibits local volumetric shrinkage upon NIR-irradiation, which can generate large hydrodynamic pressure and considerable mechanical extrusion force (F_{driving force} ≈ 0.18 N) to overcome low interface friction force (f_{friction force} ≈ 0.03 N), enabling on-demand transportation of constrained (pressure: 0.103 MPa) spherical solids over a long distance in an arbitrary direction. This actuator is anticipated to be used as bionic medicine transportation devices or artificial in vitro esophagus simulation systems, for example, to help formula eating-related physiotherapy plans for patients and astronauts.

Synthesis of charged chitosan nanoparticles as functional biolubricant

Osteoarthritis has been a major disease in recent years, which is mainly related to the breakdown of the lubrication function of the cartilage sliding interface, along with the inflammation of the joint capsule. In this paper, one kind of novel biomimetic nanoparticles (NPs) lubricant, named CS-PS, is synthesized through chemically grafting hydrophilic sulfonic acid (SO₃^-) groups onto the surface of biocompatible and biodegradable chitosan (CS) NPs. Compared with control CS NPs, the as-synthesized CS-PS NPs exhibits excellent hydration and stability because of negatively charged surface zeta potential, along with extraordinary lubrication performance in water for realizing a super-low friction coefficient (COF) as ∼0.01 at the sliding interface of PDMS elastomer-Ti₆Al₄V disk. Correspondingly, the CS-PS NPs can also be used as a drug carrier for aspirin, which presents very good drug loading and release behavior in PBS (pH = 7.4). MCS cells culture experiment proves that this kind of novel lubricant is nontoxic and biocompatible, for which may be expected to use as potential articular injective material for the treatment of osteoarthritis.

Surface wettability effect on aqueous lubrication: Van der Waals and hydration force competition induced adhesive friction

HYPOTHESIS

Wettability effect has long been a concern in various aqueous lubrication systems including biological and industrial applications. The wettability may affect lubrication performance by changing interfacial viscosity or hydration force. The key point to reveal the mechanism is to design an ideal experimental system to exclude other bulk factors other than surface wettability.

EXPERIMENTS

In this work, silicon surfaces with different treatments were used to study the single factor effect of wettability on aqueous lubrication. The normal and friction forces of these surfaces were quantified by atomic force microscopy (AFM) in water environment. The interfacial viscosity was evaluated according to the probe dynamic approaching process. Macroscale and microscale lubrication experiments of other materials were also conducted as verification and supplement.

FINDINGS

A semi-quantitative relationship between friction and wettability was revealed and attributed to the competition between the attractive van der Waals interactions and wettability-dependent repulsive hydration interaction, which determined the strength of the adhesive interaction and dominated the sliding energy dissipation. The contribution of viscous effect of water was considered to be relatively minor. The findings provide an in-depth understanding of aqueous lubrication and outline important guidelines for tuning adhesion and friction.

Orthogonal photochemistry-assisted printing of 3D tough and stretchable conductive hydrogels

3D-printing tough conductive hydrogels (TCHs) with complex structures is still a challenging task in related fields due to their inherent contrasting multinetworks, uncontrollable and slow polymerization of conductive components. Here we report an orthogonal photochemistry-assisted printing (OPAP) strategy to make 3D TCHs in one-pot via the combination of rational visible-light-chemistry design and reliable extrusion printing technique. This orthogonal chemistry is rapid, controllable, and simultaneously achieve the photopolymerization of EDOT and phenol-coupling reaction, leading to the construction of tough hydrogels in a short time (tgel ~30 s). As-prepared TCHs are tough, conductive, stretchable, and anti-freezing. This template-free 3D printing can process TCHs to arbitrary structures during the fabrication process. To further demonstrate the merits of this simple OPAP strategy and TCHs, 3D-printed TCHs hydrogel arrays and helical lines, as proofs-of-concept, are made to assemble high-performance pressure sensors and a temperature-responsive actuator. It is anticipated that this one-pot rapid, controllable OPAP strategy opens new horizons to tough hydrogels.

Direct 2D-to-3D transformation of pen drawings

Pen drawing is a method that allows simple, inexpensive, and intuitive two-dimensional (2D) fabrication. To integrate such advantages of pen drawing in fabricating 3D objects, we developed a 3D fabrication technology that can directly transform pen-drawn 2D precursors into 3D geometries. 2D-to-3D transformation of pen drawings is facilitated by surface tension-driven capillary peeling and floating of dried ink film when the drawing is dipped into an aqueous monomer solution. Selective control of the floating and anchoring parts of a 2D precursor allowed the 2D drawing to transform into the designed 3D structure. The transformed 3D geometry can then be fixed by structural reinforcement using surface-initiated polymerization. By transforming simple pen-drawn 2D structures into complex 3D structures, our approach enables freestyle rapid prototyping via pen drawing, as well as mass production of 3D objects via roll-to-roll processing.

Infinite Approaching Superlubricity by Three-Dimensional Printed Structures

The rapid development of three-dimensional (3D) printing technology opens great opportunities for the design of various multiscale lubrication structures. 3D printing allows high customization of arbitrary complex structures and rapid prototyping of objects, which provides an avenue to achieve effective lubrication. Current experimental observations on superlubricity are limited to atomically smooth clean surfaces, extreme operating conditions, and nano- or microscales. With the in-depth exploration of 3D printed lubrication, construction of multifunctional 3D structures with refined dimensions spanning from micronanoscale to macroscale is increasingly regarded as an important means to approach superlubricity and has aroused great scientific interest. To document recent advances in 3D printing for structural lubrication, a detailed literature review is provided. Emphasis is given on the design and lubrication performance of geometric and bioinspired lubrication structures with characteristic dimensions. The material requirements, merits, drawbacks, and representative applications of various 3D printing techniques are summarized. Potential future research trends aiming at the design strategy and manufacturing process of 3D printed lubrication structures are also highlighted.

3D hollow-structured hydrogels with editable macrostructure, function, and mechanical properties induced by segmented adjustments

The segmental adjustment of the microstructure of the hydrogel by the combination of plasticizer and freeze–thaw enables the PVA–borax hydrogel to achieve plasticine-like plasticity and rubber-like strength successively.

Tannic Acid-Silver Dual Catalysis Induced Rapid Polymerization of Conductive Hydrogel Sensors with Excellent Stretchability, Self-Adhesion, and Strain-Sensitivity Properties

The application of conductive hydrogels in intelligent biomimetic electronics is a hot topic in recent years, but it is still a great challenge to develop the conductive hydrogels through a rapid fabrication process at ambient temperature. In this work, a versatile poly(acrylamide) @cellulose nanocrystal/tannic acid-silver nanocomposite (NC) hydrogel integrated with excellent stretchability, repeatable self-adhesion, high strain sensitivity, and antibacterial property, was synthesized via radical polymerization within 30 s at ambient temperature. Notably, this rapid polymerization was realized through a tannic acid-silver (TA-Ag) mediated dynamic catalysis system that was capable of activating ammonium persulfate and then initiated the free-radical polymerization of the acrylamide monomer. Benefiting from the incorporation of TA-Ag metal ion nanocomplexes and cellulose nanocrystals, which acted as dynamic connecting bridges by hydrogen bonds to efficiently dissipate energy, the obtained NC hydrogels exhibited prominent tensile strain (up to 4000%), flexibility, self-recovery, and antifatigue properties. In addition, the hydrogels showed repeatable adhesiveness to different substrates (e.g., glass, wood, bone, metal, and skin) and significant antibacterial properties, which were merits for the hydrogels to be assembled into a flexible epidermal sensor for long-term human-machine interfacial contact without concerns about the use of external adhesive tapes and bacterial breeding. Moreover, the remarkable conductivity (σ ∼ 5.6 ms cm^-1) and strain sensitivity (gauge factor = 1.02) allowed the flexible epidermal sensors to monitor various human motions in real time, including huge movement of deformations (e.g., wrist, elbow, neck, shoulder) and subtle motions. It is envisioned that this work would provide a promising strategy for the rapid preparation of conductive hydrogels in the application of flexible electronic skin, biomedical devices, and soft robotics.

Freeform Three-Dimensionally Printed Microchannels via Surface-Initiated Photopolymerization Combined with Sacrificial Molding

Precise freeform microchannels within an aqueous environment have several biomedical applications but remain a challenge to fabricate. Carbohydrate glass materials have shown potential for three-dimensionally (3D) printing precise, microscale structures and are suitable as a sacrificial material to reconstruct complex channel architectures, but due to the rapid dissolution kinetics in hydrogels and the aqueous environment, protective coatings are required. Here, conformal coatings were applied to carbohydrate structures via surface-initiated photopolymerization (SIP) by incorporating a photoinitiator (PI) into freeform 3D printed isomalt structures using a custom 3D printer. Structures were then immersed into a photocurable prepolymer bath and exposed to light for reaction initiation. To achieve uniform distribution of photoinitiator molecules in 3D printed constructs, miscibility between commercial photoinitiators and isomalt was modeled using the group contribution method. A dye-based, type-two photoinitiator, Eosin Y disodium salt (EY), was selected for its miscibility with isomalt and stability under high temperature. A previously described Eosin Y (EY)/triethanolamine (TEA) radical polymerization system was used to polymerize poly(ethylene glycol) diacrylate (PEGDA). Attenuated total reflectance-Fourier transform infrared (ATR-FTIR), surface morphology, and swelling ratio characterizations via SIP were performed. Coatings around freeform structures and solid surfaces were presented to demonstrate the capability of coating complex architectures. This coating method should facilitate the application of 3D sacrificial molding in a variety of hydrogels toward building biomimetic vascular constructs.

Camacho-Cruz L, Bucio E, Kadlubowski SS. An Updated Review of Macro, Micro, and Nanostructured Hydrogels for Biomedical and Pharmaceutical Applications

Hydrogels are materials with wide applications in several fields, including the biomedical and pharmaceutical industries. Their properties such as the capacity of absorbing great amounts of aqueous solutions without losing shape and mechanical properties, as well as loading drugs of different nature, including hydrophobic ones and biomolecules, give an idea of their versatility and promising demand. As they have been explored in a great number of studies for years, many routes of synthesis have been developed, especially for chemical/permanent hydrogels. In the same way, stimuli-responsive hydrogels, also known as intelligent materials, have been explored too, enhancing the regulation of properties such as targeting and drug release. By controlling the particle size, hydrogel on the micro- and nanoscale have been studied likewise and have increased, even more, the possibilities for applications of the so-called XXI century materials. In this paper, we aimed to produce an overview of the recent studies concerning methods of synthesis, biomedical, and pharmaceutical applications of macro-, micro, and nanogels.

Recent progress in creating complex and multiplexed surface-grafted macromolecular architectures

Surface-grafted macromolecules, including polymers, DNA, peptides, etc., are versatile modifications to tailor the interfacial functions in a wide range of fields. In this review, we aim to provide an overview of the most recent progress in engineering surface-grafted chains for the creation of complex and multiplexed surface architectures over micro- to macro-scopic areas. A brief introduction to surface grafting is given first. Then the fabrication of complex surface architectures is summarized with a focus on controlled chain conformations, grafting densities and three-dimensional structures. Furthermore, recent advances are highlighted for the generation of multiplexed arrays with designed chemical composition in both horizontal and vertical dimensions. The applications of such complicated macromolecular architectures are then briefly discussed. Finally, some perspective outlooks for future studies and challenges are suggested. We hope that this review will be helpful to those just entering this field and those in the field requiring quick access to useful reference information about the progress in the properties, processing, performance, and applications of functional surface-grafted architectures.

Visible-light-assisted multimechanism design for one-step engineering tough hydrogels in seconds

Tough hydrogels that are capable of efficient mechanical energy dissipation and withstanding large strains have potential applications in diverse areas. However, most reported fabrication strategies are performed in multiple steps with long-time UV irradiation or heating at high temperatures, limiting their biological and industrial applications. Hydrogels formed with a single pair of mechanisms are unstable in harsh conditions. Here we report a one-step, biocompatible, straightforward and general strategy to prepare tough soft hydrogels in a few tens of seconds under mild conditions. With a multimechanism design, the network structures remarkably improve the mechanical properties of hydrogels and maintain their high toughness in various environments. The broad compatibility of the proposed method with a spectrum of printing technologies makes it suitable for potential applications requiring high-resolution patterns/structures. This strategy opens horizons to inspire the design and application of high-performance hydrogels in fields of material chemistry, tissue engineering, and flexible electronics.

Bioactive hydrogel coatings of complex substrates using diffusion-mediated redox initiation

Hydrogels have long been established as materials with tunable stiffness and chemistry that enable controlled cellular interactions. When applied as coatings, hydrogels can be used to introduce biofunctionality to medical devices with minimal effect on bulk properties. However, it remains challenging to uniformly apply hydrogel coatings to three dimensional geometries without substantially changing the manufacturing process and potentially affecting device function. Herein, we report a new redox-based crosslinking method for applying conformable hydrogel coatings with tunable thickness and chemistry. This new diffusion-mediated strategy of redox initiation and hydrogel crosslinking enabled coating of a variety of three dimensional substrates without changing the primary fabrication process. Following adsorption of the reducing agent to the construct, hydrogel coating thickness was readily controlled by immersion time with desorption and diffusion of the reducing agent initiating hydrogel crosslinking from the surface. The process was used to generate a range of hydrogel properties by varying the macromer molecular weight and concentration. In addition, we demonstrated that these coatings can be applied sequentially to generate multilayered constructs with distinct features. Finally, incorporation of proteins into the bulk of the hydrogel coating or as a final surface layer permitted the controlled introduction of bioactivity that supported cell attachment. This work provides a versatile method for assembling bioactive coatings with a simple post-fabrication process that is amenable to diverse geometric substrates and chemistries.

Photoresponsive Shape Memory Hydrogels for Complex Deformation and Solvent-Driven Actuation

A new design for photoresponsive shape memory hydrogels and their possible applications are demonstrated in the present study. We show that the photodissociable Fe³⁺-carboxylate coordination can be utilized as a molecular switch to realize photocontrol of shape memory on both macroscopic and microscopic scales and enable a number of functions. Indeed, Fe³⁺-carboxylate coordination can fix a large tensile strain (up to 680%) of the sodium alginate/polyacrylamide hydrogel through cross-linking of sodium alginate chains, and subsequent UV irradiation allows strain energy release in spatially selected regions through reduction of Fe³⁺ to Fe²⁺. By manipulating light irradiation, complex 3D structures are obtained from 2D hydrogel sheets, and they exhibit complex solvent-driven actuation behaviors due to a light-changeable modulus and cross-linking density in the hydrogel. Based on the same approach, micropatterns can be inscribed on the hydrogel surface using mask-assisted irradiation, and they exhibit chain orientation-mediated anisotropic topography change upon solvent exchange. Moreover, light-controlled strain energy release also enables changing hydrogel surface wettability by solvent replacement. The demonstrated mechanism for photoresponsive hydrogels is highly efficient and applicable to many systems, which offers new perspectives in developing hydrogels with multiple photoresponsive functions.

Radical polymerization as a versatile tool for surface grafting of thin hydrogel films

The surface of solid substrates is the main part that interacts with the environment.

Boosting or moderating surface-initiated Cu(0)-mediated controlled radical polymerization with external additives

External additives regulate the copper disproportionation/comproportionation equilibrium to control polymer brush growth in surface-initiated Cu(0)-mediated controlled radical polymerization.

Supramolecular Fabrication of Complex 3D Hollow Polymeric Hydrogels with Shape and Function Diversity

Inspired by the high importance of hollow structures in nature such as blood vessels and bamboos in matter transportation, properties enhancement, or even survival of living creatures, the creation of hollow materials remains of considerable interest. However, constructing hollow unique living-like soft and wet polymeric hydrogels with desirable structures and functionalities is still a big challenge. Here, we reported a robust and effective strategy to fabricate complex three-dimensional (3D) hollow polymeric hydrogel with designed shape and function diversity on the basis of supramolecular interactions. By placing a Ca²⁺ included gelatin core into the solution of alginate, hydrogel shell could be formed along with the shape of the gelatin core via coordination between alginate chains and Ca²⁺ diffused from gelatin. The hollow hydrogel could finally be obtained by dissolving the gelatin core. Various complex 3D hollow structures could be achieved by designing/constructing assembled gelatin core as a building block with adjustable supramolecular metal coordination position and strength. Moreover, hollow hydrogels with function diversity could be developed by introducing functional polymers or nanoparticles into the hydrogel wall. This work has made important progress in developing hollow polymeric hydrogel with desirable structures, shapes, and various functional applications including soft actuators and chemical reaction containers.

Recent Advances in Bioinspired Gel Surfaces with Superwettability and Special Adhesion

Engineering surface wettability is of great importance in academic research and practical applications. The exploration of hydrogel-based natural surfaces with superior properties has revealed new design principles of surface superwettability. Gels are composed of a cross-linked polymer network that traps numerous solvents through weak interactions. The natural fluidity of the trapped solvents confers the liquid-like property to gel surfaces, making them significantly different from solid surfaces. Bioinspired gel surfaces have shown promising applications in diverse fields. This work aims to summarize the fundamental understanding and emerging applications of bioinspired gel surfaces with superwettability and special adhesion. First, several typical hydrogel-based natural surfaces with superwettability and special adhesion are briefly introduced, followed by highlighting the unique properties and design principles of gel-based surfaces. Then, the superwettability and emerging applications of bioinspired gel surfaces, including liquid/liquid separation, antiadhesion of organisms and solids, and fabrication of thin polymer films, are presented in detail. Finally, an outlook on the future development of these novel gel surfaces is also provided.