Synthesis of hydrogels with extremely low surface friction

Mechanically Robust Lubricating Hydrogels Beyond the Natural Cartilage as Compliant Artificial Joint Coating

Natural cartilage exhibits superior lubricity as well as an ultra-long service lifetime, which is related to its surface hydration, load-bearing, and deformation recovery feature. Until now, it is of great challenge to develop reliable cartilage lubricating materials or coatings with persistent robustness. Inspired by the unique biochemical structure and mechanics of natural cartilage, the study reports a novel cartilage-hydrogel composed of top composite lubrication layer and bottom mechanical load-bearing layer, by covalently manufacturing thick polyelectrolyte brush phase through sub-surface of tough hydrogel matrix with multi-level crystallization phase. Due to multiple network dissipation mechanisms of matrix, this hydrogel can achieve a high compression modulus of 11.8 MPa, a reversible creep recovery (creep strain: ≈2%), along with excellent anti-swelling feature in physiological medium (v/v0 < 5%). Using low-viscosity PBS as lubricant, this hydrogel demonstrates persistent lubricity (average COF: ≈0.027) under a high contact pressure of 2.06 MPa with encountering 100k reciprocating sliding cycles, negligible wear and a deformation recovery of collapse pit in testing area. The extraordinary lubrication performance of this hydrogel is comparable to but beyond the natural animal cartilage, and can be used as compliant coating for implantable articular material of UHMWPE to present, offering more robust lubricity than current commercial system.

Cartilage-Inspired, High-Strength, and Heat-Tolerant Lubricating Hydrogels by Macrophase Separation

Hydrogels are considered as a potential cartilage replacement material based on their structure being similar to natural cartilage, which are of great significance in repairing cartilage defects. However, it is difficult for the existing hydrogels to combine the high load bearing and low friction properties (37 °C) of cartilage through sample methods. Herein, we report a facile and new fabrication strategy to construct the PNIPAm/EYL hydrogel by using the macrophase separation of supersaturated N-isopropylacrylamide (NIPAm) monomer solution to promote the formation of liposomes from egg yolk lecithin (EYL) and asymmetric template method. The PNIPAm/EYL hydrogels possess a relatively high compressive strength (more than 12 MPa), fracture energy (9820 J/m2), good fatigue resistance, lubricating properties, and excellent biocompatibility. Compared with the PNIPAm hydrogel, the friction coefficient (COF 0.046) of PNIPAm/EYL hydrogel is reduced by 50%. More importantly, the COF (0.056) of PNIPAm/EYL hydrogel above lower critical solution temperature (LCST) does not increase significantly, exhibiting heat-tolerant lubricity. The finite element analysis further proves that PNIPAm/EYL hydrogel can effectively disperse the applied stress and dissipate energy under load conditions. This work not only provides new insights for the design of high-strength lubricating hydrogels but also lays a foundation for the treatment of cartilage injury as a substitute material.

Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing

Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.

A grape seed protein-tannic acid powder to transform various non-adhesive hydrogels into adhesive gels

Realizing adhesion between wet materials remains challenging because of the interfacial water. Current strategies depend on complicated surface modifications, resulting in limited functions. Herein, a facile strategy based on the powder of grape seed protein and tannic acid (GSP-TA) was reported to endow various non-adhesive hydrogels adhesion without chemical modifications for both hydrogels and adherents. The GSP-TA powder has the capability to absorb interfacial water, form an adhesive layer on the hydrogel surface, diffusion into the underneath hydrogel matrix, and establish the initial adhesion within 5 s. By forming multiple non-covalent interactions between powders and substrates, the GSP-TA powder served as an efficient surface treating agent, enabling robust adhesion to solid substrates (wood, cardboard, glass, iron, and rubber) and wet tissues (pigskin, muscle, liver and heart). The adhesive strength for wood, cardboard, glass, iron, and rubber was 145.92 ± 5.93, 123.93 ± 15.98, 66.24 ± 7.67, 98.22 ± 4.13, and 80.83 ± 7.48 kPa, respectively. Because of reversible interactions, the adhesion was also repeatable. Due to the merits of grape seed protein and plant polyphenol, it could be completely degraded within 11 days. Bearing several merits, this strategy has promising applications in wound patches, tissue repair, and sensors.

One-Pot Construction of Articular Cartilage-Like Hydrogel Coating for Durable Aqueous Lubrication

Articular cartilage has an appropriate multilayer structure and superior tribological properties and provides a structural paradigm for design of lubricating materials. However, mimicking articular cartilage traits on prosthetic materials with durable lubrication remains a huge challenge. Herein, an ingenious three-in-one strategy is developed for constructing an articular cartilage-like bilayer hydrogel coating on the surface of ultra-high molecular weight polyethylene (BH-UPE), which makes full use of conceptions of interfacial interlinking, high-entanglement crosslinking, and interface-modulated polymerization. The hydrogel coating is tightly interlinked with UPE substrate through hydrogel-UPE interchain entanglement and bonding. The hydrogel chains are highly entangled with each other to form a dense tough layer with negligible hysteresis for load-bearing by reducing the amounts of crosslinker and hydrophilic initiator to p.p.m. levels. Meanwhile, the polymerization of monomers in the top surface region is suppressed via interface-modulated polymerization, thus introducing a porous surface for effective aqueous lubrication. As a result, BH-UPE exhibits an ultralow friction coefficient of 0.0048 during 10 000 cycles under a load of 0.9 MPa, demonstrating great potential as an advanced bearing material for disc prosthesis. This work may provide a new way to build stable bilayer coatings and have important implications for development of biological lubricating materials.

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.

Enhancing the mechanical strength of 3D printed GelMA for soft tissue engineering applications

Gelatin methacrylate (GelMA) hydrogels have gained significant traction in diverse tissue engineering applications through the utilization of 3D printing technology. As an artificial hydrogel possessing remarkable processability, GelMA has emerged as a pioneering material in the advancement of tissue engineering due to its exceptional biocompatibility and degradability. The integration of 3D printing technology facilitates the precise arrangement of cells and hydrogel materials, thereby enabling the creation of in vitro models that simulate artificial tissues suitable for transplantation. Consequently, the potential applications of GelMA in tissue engineering are further expanded. In tissue engineering applications, the mechanical properties of GelMA are often modified to overcome the hydrogel material's inherent mechanical strength limitations. This review provides a comprehensive overview of recent advancements in enhancing the mechanical properties of GelMA at the monomer, micron, and nano scales. Additionally, the diverse applications of GelMA in soft tissue engineering via 3D printing are emphasized. Furthermore, the potential opportunities and obstacles that GelMA may encounter in the field of tissue engineering are discussed. It is our contention that through ongoing technological progress, GelMA hydrogels with enhanced mechanical strength can be successfully fabricated, leading to the production of superior biological scaffolds with increased efficacy for tissue engineering purposes.

Durable hydrogel-based lubricated composite coating with remarkable underwater performances

HYPOTHESIS

Hydrogel coatings have received great attention in the field of such as medical devices, water treatment membranes, flexible electronics, and marine antifouling. However, when it comes to lubrication of hydrogel materials, though it has great potential applications in the field of industrial and medical drag reduction, some restrained properties are urgently needed to overcome for releasing the practical potential.

EXPERIMENTS

Durability of high lubrication was revealed from the sliding test during the long-term storage, as well as the long-distance sliding. Some variables which possibly affect the lubrication performance were examined to demonstrate that excellent lubricity of the coating would not be easily influenced by load, frequency, friction pair and temperature. The microstructure and mechanical characterization of the lubricative coating indicate that the resistance to harsh running conditions is premised on enough hydration extent and robustness. The formulae of Possion ratio and ball-on-disk contact stress which apply to soft matter were used for calculating contact stress values in tribology tests. Anti-swelling and bio-compatibility are also verified.

FINDINGS

This work found a route of achieving superior lubrication and coexisting with stability in lubrication, which can be used for drag reduction in medical devices and shipbuilding industry.

The role of poroelastic diffusion in the transient wetting behavior of hydrogels

Wetting and water absorption of hydrogels is often encountered in many applications. We seek to understand how wetting behavior can be affected by the time-dependent swelling of hydrogels. We measured the advancing contact angles of water droplets on hydrogels of varying thicknesses where thicker gels absorbed water more slowly. We also observed that, above a threshold advancing speed, water droplets would collapse into a lower contact angle state on the surface. We hypothesized that this collapse threshold speed is a result of competition between the poroelastic diffusion of water into the gel and the advance of the spreading droplet, the thickness of the surface, and the diffusion of water into the gel. Taking the ratio of the diffusion and advancing timescales results in a Peclet number with gel thickness as a characteristic length scale. Our results show that above a Peclet number of around 40, droplets will collapse on the surface across all gel thicknesses, confirming our hypothesis. This work provides simple insight to understand a complex time-dependent wetting phenomenon for a widely used hydrogel.

Pumping and sliding of droplets steered by a hydrogel pattern for atmospheric water harvesting

Atmospheric water harvesting is an emerging strategy for decentralized and potable water supplies. However, water nucleation and microdroplet coalescence on condensing surfaces often result in surface flooding owing to the lack of a sufficient directional driving force for shedding. Herein, inspired by the fascinating properties of lizards and catfish, we present a condensing surface with engineered hydrogel patterns that enable rapid and sustainable water harvesting through the directional pumping and drag-reduced sliding of water droplets. The movement of microscale condensed droplets is synergistically driven by the surface energy gradient and difference in Laplace pressure induced by the arch hydrogel patterns. Meanwhile, the superhydrophilic hydrogel surface can strongly bond inner-layer water molecules to form a lubricant film that reduces drag and facilitates the sliding of droplets off the condensing surface. Thus, this strategy is promising for various water purification techniques based on liquid-vapor phase-change processes.

Designing Superlubricious Hydrogels from Spontaneous Peroxidation Gradients

Hydrogels are hydrated three-dimensional networks of hydrophilic polymers that are commonly used in the biomedical industry due to their mechanical and structural tunability, biocompatibility, and similar water content to biological tissues. The surface structure of hydrogels polymerized through free-radical polymerization can be modified by controlling environmental oxygen concentrations, leading to the formation of a polymer concentration gradient. In this work, 17.5 wt % polyacrylamide hydrogels are polymerized in low (0.01 mol % O2) and high (20 mol % O2) oxygen environments, and their mechanical and tribological properties are characterized through microindentation, nanoindentation, and tribological sliding experiments. Without significantly reducing the elastic modulus of the hydrogel (E* ≈ 200 kPa), we demonstrate an order of magnitude reduction in friction coefficient (from μ = 0.021 ± 0.006 to μ = 0.002 ± 0.001) by adjusting polymerization conditions (e.g., oxygen concentration). A quantitative analytical model based on polyacrylamide chemistry and kinetics was developed to estimate the thickness and structure of the monomer conversion gradient, termed the "surface gel layer". We find that polymerizing hydrogels at high oxygen concentrations leads to the formation of a preswollen surface gel layer that is approximately five times thicker (t ≈ 50 μm) and four times less concentrated (≈ 6% monomer conversion) at the surface prior to swelling compared to low oxygen environments (t ≈ 10 μm, ≈ 20% monomer conversion). Our model could be readily modified to predict the preswollen concentration profile of the polyacrylamide gel surface layer for any reaction conditions─monomer and initiator concentration, oxygen concentration, reaction time, and reaction media depth─or used to select conditions that correspond to a certain desired surface gel layer profile.

Force-triggered rapid microstructure growth on hydrogel surface for on-demand functions

Living organisms share the ability to grow various microstructures on their surface to achieve functions. Here we present a force stamp method to grow microstructures on the surface of hydrogels based on a force-triggered polymerisation mechanism of double-network hydrogels. This method allows fast spatial modulation of the morphology and chemistry of the hydrogel surface within seconds for on-demand functions. We demonstrate the oriented growth of cells and directional transportation of water droplets on the engineered hydrogel surfaces. This force-triggered method to chemically engineer the hydrogel surfaces provides a new tool in addition to the conventional methods using light or heat, and will promote the wide application of hydrogels in various fields.

Janus Hydrogel to Mimic the Structure and Property of Articular Cartilage

Designing hydrogels with adequate strength, remarkable swelling resistance, low friction coefficient, excellent biocompatibility, and osseointegration potential is essential for replacing articular cartilage. However, it remains challenging to integrate all these properties into one material. In this work, a Janus hydrogel was prepared from polyvinyl alcohol, chitosan, and sodium hyaluronate, followed by a one-sided dipping in situ precipitation mineralization to form a layer of hybridized hydroxyapatite (HAp), wherein the two surfaces had distinct compositions and functions. Because of the negative carboxyl groups from sodium hyaluronate, the top surface possessed a friction coefficient as low as 0.024. On account of the HAp mineralized layer, the bottom side had osteogenesis potential. Owing to the synergy of physical linkages, the hydrogel displayed compressive strength as high as 78 MPa. Furthermore, it demonstrated remarkable swelling resistance with strength retention near 100% even after soaking in PBS solution at 37 °C for 7 days. The absence of toxic chemicals maintained the merits of starting polymers and resulted in excellent biocompatibility (cell viability ≈ 100%), making it an ideal substitute for articular cartilage.

Functional tissue engineering of articular cartilage for biological joint resurfacing-The 2021 Elizabeth Winston Lanier Kappa Delta Award

Biological resurfacing of entire articular surfaces represents a challenging strategy for the treatment of cartilage degeneration that occurs in osteoarthritis. Not only does this approach require anatomically sized and functional engineered cartilage, but the inflammatory environment within an arthritic joint may also inhibit chondrogenesis and induce degradation of native and engineered cartilage. Here, we present the culmination of multiple avenues of interdisciplinary research leading to the development and testing of bioartificial cartilage for tissue-engineered resurfacing of the hip joint. The work is based on a novel three-dimensional weaving technology that is infiltrated with specific bioinductive materials and/or genetically-engineered stem cells. A variety of design approaches have been tested in vitro, showing biomimetic cartilage-like properties as well as the capability for long-term tunable and inducible drug delivery. Importantly, these cartilage constructs have the potential to provide mechanical functionality immediately upon implantation, as they will need to replace a majority, if not the entire joint surface to restore function. To date, these approaches have shown excellent preclinical success in a variety of animal studies, including the resurfacing of a large osteochondral defect in the canine hip, and are now well-poised for clinical translation.

Waterproof and Breathable Wound Dressing Composited By Expanded Polytetrafluoroethylene Backing and Hydrogel

Wound dressings with waterproof, breathable and bacterial-resistant properties are still rarely realized. In this work, a newly hydrogel-based dressing is designed with a backing of expanded polytetrafluoroethylene (ePTFE) film. The ePTFE grafting with polyvinylpyrrolidone (PVP) brush is composited with hydrogel successfully with an adhesion energy of ∼80 kJ/m² . In this resultant composite, the ePTFE backing contributes excellent breathability, water resistance and bacterial barrier property. The water vapor transmission rate of the composite is 4.83 × 10³  g/m² ·24 h, which can maintain the mosit evironment of wound and relive pain by evaporating water. Notably, it can withstand 500 mm water column for over 300 s, which is obviously better than the commonly used nonwoven fabric backing materials. It can also prevent the invasion of bacteria, because the pores of ePTFE backing are smaller than those of most common bacterial. As a result, the composite with an ePTFE film backing has a positive effect in accelerating wound healing, promoting the reconstruction of intact epidermis and reducing inflammation. This article is protected by copyright. All rights reserved.

Hydration Lubrication in Biomedical Applications: From Cartilage to Hydrogels

In the course of evolution, nature has achieved remarkably lubricated surfaces, with healthy articular cartilage in the major (synovial) joints being the prime example, that can last a lifetime as they slide past each other with ultralow friction (friction coefficient μ = the force to slide surfaces past each other/load compressing the surfaces < 0.01) under physiological pressures (up to 10 MPa or more)). Such properties are unmatched by any man-made materials. The precise mechanism of low friction between such sliding cartilage tissues, which is closely related to osteoarthritis (OA), the most widespread joint disease, affecting hundreds of millions worldwide, has been studied for nearly a century, but is still not fully understood. Traditionally, the roles of load bearing by interstitial fluid within the cartilage bulk and that of thin exuded fluid films at the interface between the sliding cartilage surfaces have been proposed as the main lubrication mechanism. More recent work, however, suggests that molecular boundary layers at the surfaces of articular cartilage and other tissues play a major role in their lubrication. In particular, in recent years hydration lubrication has emerged as a new paradigm for boundary lubrication in aqueous media based on subnanometer hydration shells which massively reduce frictional dissipation. The vectors of hydration lubrication include trapped hydrated ions, hydrated surfactants, biological macromolecules, biomimetic polymers, polyelectrolytes and polyzwitterionic brushes, and close-packed layers of phosphatidylcholine (PC) vesicles, all having in common the exposure of highly hydrated groups at the slip plane. Among them, vesicles (or bilayers) of PC lipids, which are the most widespread lipid class in mammals, are exceptionally efficient lubricating elements as a result of the high hydration of the phosphocholine headgroups they expose. Such lipids are ubiquitous in joints, leading to the proposal that macromolecular surface complexes exposing PC bilayers are responsible for the remarkable lubrication of cartilage. Cartilage, comprising ∼70% water, may be considered to be a complex biological hydrogel, and studying the frictional properties of hydrogels may thus provide new insights into its lubrication mechanisms, leading in turn to novel, highly lubricious hydrogels that may be used in a variety of biomedical and other applications. A better understanding of cartilage lubrication could moreover lead to better treatments for OA, for example, through intra-articular injections of appropriate lubricants or through the creation of low-friction hydrogels that may be used as tissue engineering scaffolds for diseased cartilage. In this Account, we begin by introducing the concept and origin of hydration lubrication, extending from the seminal study of lubrication by hydrated simple ions to more complex systems. We then briefly review different modes of lubrication in synovial joints, focusing primarily on boundary lubrication. We consider modes of hydrogel lubrication and different kinds of such low-friction synthetic gels and then focus on cartilage-inspired, boundary-lubricated hydrogels. We conclude by discussing challenges and opportunities.

Bioinspired Design of a Cartilage-like Lubricated Composite with Mechanical Robustness

Natural articular cartilages show extraordinary tribological performance based on their penetrated surface lubricated biomacromolecules and good mechanical tolerance. Hydrogels are considered to be potential alternatives to cartilages due to their low surface friction and good biocompatibility, although the poor mechanical properties limited their applications. Inspired by the excellent mechanical properties and the remarkable surface lubrication mechanism of natural articular cartilages, one kind of cartilage-like composite material with a lubrication phase (Composite-LP) was developed by chemically grafting a thick hydrophilic polyelectrolyte brush layer onto the subsurface of a three-dimensional manufactured elastomer scaffold-hydrogel composite architecture. The Composite-LP exhibited good load-bearing capacities because of the nondissipation strategy and the stress dispersion mechanism resulting from the elastomer scaffold enhancement. In the presence of the top lubrication layer, the Composite-LP showed superior friction reduction functionality and wear resistance under a dynamic shearing process. This design concept of coupling the non-dissipative mechanism and interface lubrication provides a new avenue for developing cartilage-like hydrogels and soft robots.

Construction of a nano-phase-separated structure on a hydrogel surface

A hydrogel surface with a nano-phase-separated structure was successfully fabricated by grafting a fluorine-containing polymer using activators regenerated by electron transfer atom transfer radical polymerisation (ARGET ATRP). The modified hydrogel surface exhibits water repellency and high elasticity with maintaining transparency.

Photocurable bioresorbable adhesives as functional interfaces between flexible bioelectronic devices and soft biological tissues

Flexible electronic/optoelectronic systems that can intimately integrate onto the surfaces of vital organ systems have the potential to offer revolutionary diagnostic and therapeutic capabilities relevant to a wide spectrum of diseases and disorders. The critical interfaces between such technologies and living tissues must provide soft mechanical coupling and efficient optical/electrical/chemical exchange. Here, we introduce a functional adhesive bioelectronic-tissue interface material, in the forms of mechanically compliant, electrically conductive, and optically transparent encapsulating coatings, interfacial layers or supporting matrices. These materials strongly bond both to the surfaces of the devices and to those of different internal organs, with stable adhesion for several days to months, in chemistries that can be tailored to bioresorb at controlled rates. Experimental demonstrations in live animal models include device applications that range from battery-free optoelectronic systems for deep-brain optogenetics and subdermal phototherapy to wireless millimetre-scale pacemakers and flexible multielectrode epicardial arrays. These advances have immediate applicability across nearly all types of bioelectronic/optoelectronic system currently used in animal model studies, and they also have the potential for future treatment of life-threatening diseases and disorders in humans.

Bioinspired, Recyclable, Stretchable Hydrogel with Boundary Ultralubrication

Although hydrogels exhibit excellent low frictional behavior, their friction coefficients cannot meet the requirements for biology, especially at low sliding velocities. Inspired by the natural lubrication mechanism from animals, plants, or even microorganisms, a nonionic surfactant, Tween 80, was introduced into a biofriendly poly(vinyl alcohol) (PVA) hydrogel to construct a composite hydrogel with ultralubrication. Such a combination endows PVA hydrogels with an ultralow coefficient of friction (10^-3 to 10^-4) under an extremely low sliding velocity (0.01 mm/s). Tween 80 micelles and aggregates, together with hydrophobic molds, induce rough surfaces and high carbon contents on the surface of the hydrogel, promoting excellent lubrication behavior of the composite hydrogel. In addition to the desirable lubrication, this environmentally friendly composite hydrogel also exhibited excellent flexibility at subzero temperatures, tensile properties, and good recyclability. Additionally, the method of introducing Tween 80 into hydrogels to reduce friction is also effective in chemically crosslinked double-network hydrogels.

Oxygen inhibition of free-radical polymerization is the dominant mechanism behind the "mold effect" on hydrogels

Hydrogel surfaces are of great importance in numerous applications ranging from cell-growth studies and hydrogel-patch adhesion to catheter coatings and contact lenses. A common method to control the structure and mechanical/tribological properties of hydrogel surfaces is by synthesizing them in various mold materials, whose influence has been widely ascribed to their hydrophobicity. In this work, we examine possible mechanisms for this "mold effect" on the surface of hydrogels during polymerization. Our results for polyacrylamide gels clearly rule out the effect of mold hydrophobicity as well as any thermal-gradient effects during synthesis. We show unequivocally that oxygen diffuses out of certain molding materials and into the reaction mixture, thereby inhibiting free-radical polymerization in the vicinity of the molding interface. Removal of oxygen from the system results in homogeneously cross-linked hydrogel surfaces, irrespective of the substrate material used. Moreover, by varying the amount of oxygen at the surface of the polymerizing solutions using a permeable membrane we are able to tailor the surface structures and mechanical properties of PAAm, PEGDA and HEMA hydrogels in a controlled manner.

A Presentation of Ionic Liquids as Lubricants: Some Critical Comments

Ionic liquids (ILs) are liquid materials at room temperature with an ionic intrinsic nature. The electrostatic interactions therefore play a pivotal role in dictating their inner structure, which is then expected to be far from the traditional pattern of classical simple liquids. Therefore, the strength of such interactions and their long-range effects are responsible for the ionic liquid high viscosity, a fact that itself suggests their possible use as lubricants. More interestingly, the possibility to establish a wide scenario of possible interactions with solid surfaces constitutes a specific added value in this use. In this framework, the ionic liquid complex molecular structure and the huge variety of possible interactions cause a complex aggregation pattern which can depend on the presence of the solid surface itself. Although there is plenty of literature focusing on the lubricant properties of ionic liquids and their applications, the aim of this contribution is, instead, to furnish to the reader a panoramic view of this exciting problematic, commenting on interesting and speculative aspects which are sometimes neglected in standard works and trying to furnish an enriched vision of the topic. The present work constitutes an easy-to-read critical point of view which tries to interact with the imagination of readers, hopefully leading to the discovery of novel aspects and interconnections and ultimately stimulating new ideas and research.

Articular and Artificial Cartilage, Characteristics, Properties and Testing Approaches-A Review

The design and manufacture of artificial tissue for knee joints have been highlighted recently among researchers which necessitates an apt approach for its assessment. Even though most re-searches have focused on specific mechanical or tribological tests, other aspects have remained underexplored. In this review, elemental keys for design and testing artificial cartilage are dis-cussed and advanced methods addressed. Articular cartilage structure, its compositions in load-bearing and tribological properties of hydrogels, mechanical properties, test approaches and wear mechanisms are discussed. Bilayer hydrogels as a niche in tissue artificialization are presented, and recent gaps are assessed.

Cartilage Mimics Adaptive Lubrication

The natural cartilage layer exhibits excellent interface low friction and good load-bearing properties based on the mechanically controlled adaptive lubrication mechanism. Understanding and imitating such a mechanism is important for developing high-load-bearing water-lubrication materials. Here, we report the successful preparation of thermoresponsive layered materials by grafting a poly(3-sulfopropyl methacrylate potassium salt) (PSPMA) polyelectrolyte brush onto the subsurface of an initiator-embedded high strength hydrogel [poly(N-isopropylacrylamide-co-acrylic acid-co-initiator/Fe³⁺)] [P(NIPAAm-AA-iBr/Fe³⁺)]. The top soft hydrogel/brush composite layer provides aqueous lubrication, while the bottom thermoresponsive hydrogel layer exhibits adaptive load-bearing capacity that shows tunable stiff or modulus in response to the temperature above and below the lower critical solution temperature (LCST, 32.5 °C). An obvious friction-reduction feature is realized above the LCST, resulting from the dynamic increase of the bottom layer mechanical modulus. Furthermore, in situ lubrication-improvement behavior is achieved upon applying a near-infrared (NIR) laser onto the surface of Fe₃O₄ nanoparticle (NP)-integrated layered materials. Such a typical lubrication-regulated behavior can be attributed to the synergy effect of the improved load-bearing capacity of the bottom layer and the enhanced lubrication behavior of the top layer with an increase in the polyelectrolyte brush chain density, which is similar to the mechanically controlled adaptive lubrication mechanism of the natural cartilage layer. Current research results provide an inspiration for developing novel biomimetic lubrication materials with considerable load-bearing capacity and also propose a strategy for designing intelligent/stable friction-actuation devices.

Poly(N,N'-Diethylacrylamide)-Based Thermoresponsive Hydrogels with Double Network Structure

Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N,N'-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) or poly(N,N`-dimethylacrylamide) (PDMAAm) was studied by a combination of swelling measurements, differential scanning calorimetry (DSC) and 1H NMR and UV-Vis spectroscopies. Presence of the second hydrophilic network in DN hydrogels influenced their thermal sensitivity significantly. DN hydrogels show less intensive changes in deswelling, smaller enthalpy, and entropy changes connected with phase transition and broader temperature interval of the transition than the single network (SN) hydrogels. Above the transition, the DN hydrogels contain significantly more permanently bound water in comparison with SN hydrogels due to interaction of water with the hydrophilic component. Unlike swelling and DSC experiments, a rather abrupt transition was revealed from temperature-dependent NMR spectra. Release study showed that model methylene blue molecules are released from SN and DN hydrogels within different time scale. New thermodynamical model of deswelling behaviour based on the approach of the van't Hoff analysis was developed. The model allows to determine thermodynamic parameters connected with temperature-induced volume transition, such as the standard change of enthalpy and entropy and critical temperatures and characterize the structurally different states of water.

Functional hydrogel coatings

Hydrogels—natural or synthetic polymer networks that swell in water—can be made mechanically, chemically and electrically compatible with living tissues. There has been intense research and development of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960. More recently, functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity, biocompatibility and anti-biofouling properties, with the advantages of substrates, such as stiffness, toughness and strength. In this review, we focus on three aspects of functional hydrogel coatings: (i) applications and functions enabled by hydrogel coatings, (ii) methods of coating various substrates with different functional hydrogels with tough adhesion, and (iii) tests to evaluate the adhesion between functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review.

Aqueous surface gels as low friction interfaces to mitigate implant-associated inflammation

Aqueous surface gels are fragile yet resilient biopolymer-based networks capable of sustaining extremely low friction coefficients despite tribologically-challenging environments. These superficial networks are ubiquitous in natural sliding interfaces and protect mechanosensitive cells from excessive contact pressures and frictional shear stresses from cell-fluid, cell-cell, or cell-solid interactions. Understanding these complex lubrication mechanisms may aid in the development of materials-based strategies for increasing biocompatibility in medical devices and implants. Equally as important is characterizing the interplay between soft and passive yet mobile implant materials and cellular reactions in response to direct contact and frictional shear stresses. Physically interrogating living biological systems without rupturing them in the process is nontrivial. To this end, custom biotribometers have been designed to precisely modulate contact pressures against living human telomerase-immortalized corneal epithelial (hTCEpi) cell layers using soft polyacrylamide membrane probes. Reverse-transcription quantitative polymerase chain-reaction (RT-qPCR) indicated that increased duration and, to a much greater extent, the magnitude of frictional shear stress lead to increased production of pro-inflammatory (IL-1β, IL-6, MMP9) and pro-apoptotic (DDIT3, FAS) genes, which in clinical studies are linked to pathological pain. The hierarchical structure often found in biological systems has also been investigated through the fabrication of high-water content (polyacrylamide) hydrogels through free-radical polymerization inhibition. Nanoindentation experiments and friction coefficient measurements indicate that these "gradient surface gels" reduce contact pressures and frictional shear stresses at the surface of the material while still maintaining stiffness within the bulk. Reducing frictional shear stresses through informed materials and surface design may concomitantly increase lubricity and quiet the immune response, and thus provide bio-inspired routes to improve patient outcomes and quality of life.

Uncovering friction dynamics using hydrogel particles as soft ball bearings

Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontrivial rate-dependent lubrication behavior. Unlike Newtonian lubrication or dry solid friction, hydrogel particles in suspension transition through four frictional regimes as a function of sliding velocity. We relate the different regimes to the deformation of the particles at different gap sizes, which changes the effective contact area between the sliding surfaces. By systematically varying the particle characteristics and the surface properties of the sliding surfaces, we assign potential mechanisms for each of the different lubricating regimes as a function of velocity: (I) relatively high friction due to particle flattening and direct contact between interacting bodies (II) decrease of friction owing to the presence of rolling particles (III) large inflow of particles in a confined space leading to compressed particles and (IV) the formation of a thick lubricating layer. Using these suspensions with soft, deformable particles as a ball bearing system, we provide new insights into soft material friction with applications in emulsions, powders, pastes or other granular materials.

A metalloprotein-inspired thermo-gene for thermogels

Coordinating hydrogels may act as a thermo-gene to endow heat-melting conventional hydrogels with superior thermal stability at temperatures over 80 °C.

Linking Friction and Surface Properties of Hydrogels Molded Against Materials of Different Surface Energies

Biological tissues subjected to rubbing, such as the cornea and eyelid or articular cartilage, are covered in brushy, hydrated mucous structures in order to reduce the shear stress on the tissue. To mimic such biological tissues, we have prepared polyacrylamide (PAAm) hydrogels with various concentrations of un-cross-linked chains on their surfaces by synthesizing them in molds of different surface energies. The selected molding materials included hydrophilic glass, polyoxymethylene (POM), polystyrene (PS), polyethylene (PE), polypropylene (PP), and polytetrafluoroethylene (PTFE). After synthesis, demolding, and equilibration in water, the elastic modulus at the hydrogel surface decreased with increasing water contact angle of the mold. The softer, brushier surfaces did not completely collapse under compressive pressures up to 10 kPa, remaining better hydrated compared to their denser, cross-linked analogs. The hydrogels with brushier surfaces displayed an order of magnitude lower coefficient of friction than the cross-linked ones, which is attributed to the ability of their near-surface regions to retain larger amounts of liquid at the interface. The characteristic speed-dependent friction of the denser, cross-linked hydrogel surface is compared to the speed-independent friction of the brushy hydrogels and discussed from the perspectives of (elasto)hydrodynamic lubrication, permeability, and shear-induced hydrodynamic penetration depth.

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.

Creating an Interface: Rendering a Double-Network Hydrogel Lubricious via Spontaneous Delamination

Hydrogels engineered with specific surface chemistries and architectures have found myriad applications in electronics, biofouling, biolubrication, and biomedical devices. Free-radical polymerization is frequently employed to construct covalently bonded networks in hydrogels, and any inhibition of the radical reactions by oxygen at the surface of the reaction mixture is generally undesirable. The internal stress caused by the resulting gradient in the cross-linking density during polymerization can give rise to a physical deformation of the surface, resulting in wrinkles, creases, or cracks. However, this oxygen-inhibition effect can be positively utilized to create finely controlled surface structures. We describe a two-step cross-linking strategy for the fabrication of a P(AAm-AMPS)/alginate double-network hydrogel in the presence of air, which enables greater independent control over surface chemistry and functionality than homogeneously processed conventional double-network hydrogels. An alginate-rich "skin" layer is spontaneously delaminated due to the mechanical instability and osmotic mismatch between the swollen double-network hydrogel matrix and the rigid "skin" layer. Removal of the "skin" layer results in a lubricious surface with coefficients of friction as low as 0.02 against glass in aqueous solutions. The proposed strategy can be generalized to develop soft functional materials with unique structures and properties and precise control over the surface chemistry.

Combined Experimental and Simulation Studies of Cross-Linked Polymer Brushes under Shear

We have studied the effect of cross-linking on the tribological behavior of polymer brushes using a combined experimental and theoretical approach. Tribological and indentation measurements on poly(glycidyl methacrylate) brushes and gels in the presence of dimethylformamide solvent were obtained by means of atomic force microscopy. To complement experiments, we have performed corresponding molecular dynamics (MD) simulations of a generic bead-spring model in the presence of explicit solvent and cross-linkers. Our study shows that cross-linking leads to an increase in friction between polymer brushes and a counter-surface. The coefficient of friction increases with increasing degree of cross-linking and decreases with increasing length of the cross-linker chains. We find that the brush-forming polymer chains in the outer layer play a significant role in reducing friction at the interface.

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

The development of versatile generalized strategies for easy surface modification is of immense scientific interest. Herein, a novel mechanism to form functional hydrogel coatings on a wide variety of substrate materials including polymers, polymeric resins, ceramics, and intermetallic compounds, enabling easy change of the surface wettability and lubrication property, is reported. In situ polymerization and hydrogel coating formation is initiated by free radicals generated through the redox reaction between Fe²⁺ and S₂ O₈ ^2- at the solid-liquid interface, which shows controllable growth kinetics. Hydrogel modification is fast, controllable, and performed in mild conditions at room temperature. The chemical components, thickness, and network structure of the hydrogel coating can be well controlled. The surface catalytically initiated radical polymerization method allows reinitiation of the polymerization when the grafted hydrogel coating is polished away, and allows continuous surface polymerization to form multi-interpenetrating network hydrogel coatings. Interestingly, it is fully compatible with 3D-printing technology, and by using 3D-printed composites as the catalytic template, it demonstrates an extreme advantage for engineering 3D hollow hydrogel objects with various complex structures. The versatility of this method makes it generate potential applications in the field of surface/interface and biological engineering.

Development and Characterization of a Poly (Vinyl Alcohol)/Graphene Oxide Composite Hydrogel as An Artificial Cartilage Material

Poly (vinyl alcohol) hydrogel (PVA-H) is expected to be a suitable artificial articular cartilage material because of its high biocompatibility. However, it is difficult to affix to the surface of a living joint because it is bioinert and its mechanical strength needs to be improved. In this study, graphene oxide (GO) subjected to two oxidation rounds was used to form a nanocomposite material and the composite hydrogel PVA-GO-H was prepared by low-temperature crystallization. Scanning electron microscope (SEM) images showed that the addition of GO can increase roughness of the hydrogel surface. Contact angle measurements showed that the surface of PVA-GO-H exhibited hydrophobicity that increased with GO concentration and not with that of PVA-H, indicating that the hydrophilic parts of PVA and GO form hydrogen bonds and the hydrophobic part of GO was exposed on the surface. Tensile tests demonstrated that Young’s modulus was enhanced on the addition of GO. Osteoblast cells showed more affinity for PVA-GO-H than PVA-H, which much more cells adhere to than to PVA-GO-H after a certain period of culturing, suggesting GO can improve the cell attachment of PVA-H. Further studies on the influence of the oxidation time of GO are still required.

Tribological Properties of Double-Network Gels Substituted by Ionic Liquids

Since human body joints have a gel-like structure with low friction that persists for several decades, hydrogels have attracted much interest for developing low-friction materials. However, such advantages can hardly be realized in industrial usage because water in the gel evaporates easily and the gel deswells. The substitution of water with an ionic liquid (IL) is one of the effective ways to overcome this problem. In this study, we substituted water in a double network (DN) hydrogel with 3-ethyl-1-methyl-imidazolium ethylsulfate (EMI-EtSulf), a hydrophilic IL, via a simple solvent exchange method to obtain a DN ion gel. A compressive test and thermogravimetric analysis showed that the DN ion gel has a high compression fracture stress and improved thermal properties, with the difference in 10% loss of temperature being ΔT10 = 234 °C. A friction test conducted using a reciprocating tribometer showed that the friction of a glass ball/DN ion gel was relatively higher than that of a glass ball/DN hydrogel. Because the minimum coefficient of friction (COF) value increased after substitution, the increase in polymer adhesion caused by the electrostatic shielding of the surface moieties of glass and poly 2-acrylamidomethylpropanesulfonic acid (PAMPS) was considered the main contributor to the high friction. As the COF value decreased with increasing temperature, the DN ion gel can achieve low friction via the restriction of polymer adhesion at high temperatures, which is difficult in the DN hydrogel owing to drying.