Versatile Surface Modification of Hydrogels by Surface-Initiated, Cu0-Mediated Controlled Radical Polymerization

Tribology in Nature: Inspirations for Advanced Lubrication Materials

Friction-induced energy consumption is a significant global concern, driving researchers to explore advanced lubrication materials. In nature, lubrication is vital for the life cycle of animals, plants, and humans, playing key roles in movement, predation, and decomposition. After billions of years of evolution, natural lubrication exhibits remarkable professionalism, high efficiency, durability, and intelligence, offering valuable insights for designing advanced lubrication materials. This review focuses on the lubrication mechanisms of natural organisms and significant advancements in biomimetic soft matter lubrication materials. It begins by summarizing common biological lubrication behaviors and their underlying mechanisms, followed by current design strategies for biomimetic soft matter lubrication materials. The review then outlines the development and performance of these materials based on different mechanisms and strategies. Finally, it discusses potential research directions and prospects for soft matter lubrication materials. This review will be a valuable resource for advancing research in biomimetic lubrication materials.

Interfacial Nanoengineering of Hydrogel Surfaces via Block Copolymer Self-Assembly

Synthetic polymer hydrogels are valuable matrices for biotransformations, drug delivery, and soft implants. While the bulk properties of hydrogels depend on chemical composition and network structure, the critical role of interfacial features is often underestimated. This work presents a nanoscale modification of the gel-water interface using polymer brushes via a straightforward "grafting-to" strategy, offering an alternative to more cumbersome "grafting-from" approaches. Functional block copolymers with photoreactive anchor blocks are successfully self-assembled and UV-immobilized on hydrogel substrates despite their low solid content (<30 wt %). This versatile technique works on both bulk- and surface-immobilized hydrogels, demonstrated on poly(hydroxypropyl acrylate), poly(N-isopropylacrylamide), and alginate gels, allowing precise control over grafting density. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry revealed a homogeneous bilayered architecture. By "brushing-up", the hydrogels' interface can be tailored to enhance protein adsorption, improve cell adhesion, or impair the diffusive uptake of small molecules into the bulk gels. This effective interfacial nanoengineering method is broadly applicable for enhancing hydrogel performance across a wide range of applications.

Physical Aging of Poly(methyl methacrylate) Brushes and Spin-Coated Films

While there is significant attention aimed at understanding how one-dimensional confinement and chain confirmations can impact the glass transition temperature (Tg) of polymer films, there remains a limited focus on similar effects on sub-Tg processes, notably, structural relaxation. Using spectroscopic ellipsometry, we investigated the combined influence of confinement and molecular packing on Tg and physical aging, i.e., the property changes that accompany structural relaxation, at select film thicknesses and aging temperatures (Ta). We used poly(methyl methacrylate) (PMMA) films in the brush and spin-coated morphologies as model systems. We found that whether a PMMA film exhibited a decrease or increase in physical aging rate with confinement was dependent on the morphology. Notably, PMMA brushes exhibited higher physical aging rates compared to similarly thick spin-coated films at all values of Ta. These intriguing findings reveal the strong effects of confinement and molecular packing on the structural relaxation of polymer films. Results from this study have the potential to aid in the design of thin-film materials with controllable long-term glassy-state properties.

Fabrication of PHFPO Surface-Modified Conductive AgNWs/PNAGA Hydrogels with Enhanced Water Retention Capacity toward Highly Sensitive Strain Sensors

Conductive hydrogels, characterized by their unique features of flexibility, biocompatibility, electrical conductivity, and responsiveness to environmental stimuli, have emerged as promising materials for sensitive strain sensors. In this study, a facile strategy to prepare highly conductive hydrogels is reported. Through rational structural and synthetic design, silver nanowires (AgNWs) are incorporated into poly(N-acryloyl glycinamide) (PNAGA) hydrogels, achieving high electrical conductivity (up to 0.88 S m-1), significantly enhanced mechanical properties, and elevated deformative sensitivity. Furthermore, surface modification with polyhexafluoropropylene oxide (PHFPO) has substantially improved the water retention capacity and dressing comfort of this hydrogel material. Based on the above merits, these hydrogels are employed to fabricate highly sensitive wearable strain sensors which can detect and interpret subtle hand and finger movements and enable precise control of machine interfaces. The AgNWs/PNAGA based strain sensors can effectively sense finger motion, enabling the control of robotic fingers to replicate the human hand's gestures. In addition, the high deformative sensitivity and elevated water retention performance of the hydrogels makes them suitable for flow sensing. These conceptual applications demonstrate the potential of this conductive hydrogel in high-performance strain sensors in the future.

Hydrogel Coatings of Implants for Pathological Bone Repair

Hydrogels are well-suited for biomedical applications due to their numerous advantages, such as excellent bioactivity, versatile physical and chemical properties, and effective drug delivery capabilities. Recently, hydrogel coatings have developed to functionalize bone implants which are biologically inert and cannot withstand the complex bone tissue repair microenvironment. These coatings have shown promise in addressing unique and pressing medical needs. This review begins with the major functionalized performance and interfacial bonding strategy of hydrogel coatings, with a focus on the novel external field response properties of the hydrogel. Recent advances in the fabrication strategies of hydrogel coatings and their use in the treatment of pathologic bone regeneration are highlighted. Finally, challenges and emerging trends in the evolution and application of physiological environment-responsive and external electric field-responsive hydrogel coatings for bone implants are discussed.

Roughness-Mediated SI-Fe0CRP for Polymer Brush Engineering toward Superior Drag Reduction

Surface-initiated iron(0)-mediated controlled radical polymerization (SI-Fe0CRP) with low toxicity and excellent biocompatibility is promising for the fabrication of biofunctional polymer coatings. However, the development of Fe(0)-based catalysts remains limited by the lower dissociation activity of the Fe(0) surface in comparison to Cu(0). Here, we found that, by simply polishing the Fe(0) plate surface with sandpaper, the poly(methacryloyloxy)ethyl trimethylammonium chloride brush growth rate has been increased significantly to 3.3 from 0.14 nm min-1 of the pristine Fe(0) plate. The excellent controllability of roughness-mediated SI-Fe0CRP can be demonstrated by customizing multicompartment brushes and triblock brushes. Furthermore, we found that the resulting polymer brush coatings exhibit remarkably low water adhesion (0.097 mN) and an outstanding drag reduction rate of 52% in water. This work provides a promising strategy for regulating the grafting rate of polymer brushes via SI-Fe0CRP for biocompatible marine drag reduction coatings.

Investigating the Effect of the Crosslinking Factor on the Properties of Hydrogel Materials Containing Tilia platyphyllos Hydrolate

The use of natural ingredients in recent years has been of great importance in many industries and medicine. In biomedical applications, hydrogel materials also play a significant role. In view of this, the aim of this study was to synthesize and characterize hydrogel materials enriched with broadleaf linden hydrolate. An important aspect was to carry out a series of syntheses with varying types and amounts of crosslinking agents so as to test the possibility of synthesizing materials with controlled properties. The obtained hydrogels were subjected to detailed physicochemical analysis. The results of the tests confirmed the relationship between the selected properties and the type of crosslinking agent used. A crosslinking agent with a lower molar mass (575 g/mol) results in a material with a compact and strongly crosslinked structure, which is characterized by high surface roughness. The use of a crosslinking agent with a molecular weight of 700 g/mol resulted in a material with a looser-packed polymer network capable of absorbing larger amounts of liquids. The work also proved that regardless of the type of crosslinking agent used, the addition of linden hydrolate provides antioxidant properties, which is particularly important in view of the target biomedical application of such materials.

A hydrogel system for drug loading toward the synergistic application of reductive/heat-sensitive drugs

The preparation of hydrogels as drug carriers via radical-mediated polymerization has significant prospects, but the strong oxidizing ability of radicals and the high temperatures generated by the vigorous reactions limits the loading for reducing/heat-sensitive drugs. Herein, an applicable hydrogel synthesized by radical-mediated polymerization is reported for the loading and synergistic application of specific drugs. First, the desired sol is obtained by polymerizing functional monomers using a radical initiator, and then tannic-acid-assisted specific drug mediates sol-branched phenylboric acid group to form the required functional hydrogel (New-gel). Compared with the conventional single-step radical-mediated drug-loading hydrogel, the New-gel not only has better chemical/physical properties but also efficiently loads and releases drugs and maintains drug activity. Particularly, the New-gel has excellent loading capacity for oxygen, and exhibits significant practical therapeutic effects for diabetic wound repair. Furthermore, owing to its high light transmittance, the New-gel synergistically promotes the antibacterial effect of photosensitive drugs. This gelation strategy for loading drugs has further promising biomedical applications.

Carbon Nitride Grafting Modification of Poly(lactic acid) to Maximize UV Protection and Mechanical Properties for Packaging Applications

Due to their biobased nature and biodegradability, poly(lactic acid) (PLA) rich blends are promising for processing in the packaging industry. However, pure PLA is brittle and UV transparent, which limits its application, so the exploration of nanocomposites with improved interfacial interactions and UV absorbing properties is worthwhile. We therefore developed and optimized synthesis routes for well-designed nanocomposites based on a PLA matrix and graphitic carbon nitride (g-C3N4; CN) nanofillers. To enhance the interfacial interaction with the PLA matrix, a silane-coupling agent (γ-methacryloxypropyl trimethoxysilane, KH570) is chemically grafted onto the CN surface after controlled oxidation with nitric acid and hydrogen peroxide. Interestingly, only 1 wt % of CNO-KH570, as synthesized under mild conditions, is needed to significantly improve the UV absorption, blocking even a large part of both UV-C, UV-B, and UV-A outperforming the UV absorption performance of PLA and, for instance, polyethylene terephthalate (PET). The low nanofiller loading of 1 wt % also results in a higher ductility with an increase in elongation at break (+73%), maintaining the tensile modulus. The results on a joint optimization of UV protection and mechanical properties are supported by a broad range of experimental characterizations, including FTIR, XRD, DSC, DSEM, FETEM, XPS, FTIR, TGA, and BET N2 adsorption-desorption analysis.

Surface-Initiated Zerovalent Metal-Mediated Controlled Radical Polymerization (SI-Mt0CRP) for Brush Engineering

ConspectusThe surface-tethered polymer brush has become a powerful approach to tailoring the chemical and physical properties of surfaces and interfaces and revealed broad application prospects in widespread fields such as self-cleaning, surface lubrication, and antibiofouling. Access to these diverse functional polymer brushes is highly dependent on versatile and powerful surface-initiated controlled radical polymerization (SI-CRP) strategies. However, conventional SI-CRP typically requires oxygen exclusion, large amounts of catalysts and monomer solution, and a long reaction time, making it time-consuming and sophisticated. When using a two-plate system consisting of an initiator-bearing substrate and a metal plate, we and our collaborators introduced surface-initiated zerovalent metal-mediated controlled radical polymerization (SI-Mt0CRP). In the SI-Mt0CRP setup, a metal(0) plate (Cu, Fe, Zn, or Sn) is placed proximately to an initiator-functionalized substrate and forms a confined polymerization system which considerably simplifies the synthesis of a wide range of polymer brushes with high grafting densities over large areas (up to the meter scale).In comparison to classical SI-ATRP (catalyzed by metal salts), SI-Mt0CRP demonstrates oxygen tolerance, high controllability, good retention of chain-end functionality, and facile recyclability of the metal catalysts (i.e., metal foil/plate). Taking advantage of the confined geometry of the SI-Mt0CRP setup, polymer brushes with various conformations and architectures are easily accessible while consuming only microliter volumes of monomer solution and without complicated operations under ambient conditions. Owing to these attractive characteristics, SI-Mt0CRP has become a versatile technique for functionalizing materials for targeted applications, ranging from the areas of surface science to materials science and nanotechnology.In this Account, we summarize the recent advances of SI-Mt0CRP catalyzed by zerovalent metals (e.g., Cu, Fe, Zn, and Sn) and highlight the intrinsic advantages of the featured experimental setup, compared with the "classical" SI-CRP in which metal salt, powder, or wire is applied. We further discuss the synthetic features and proposed mechanism of SI-Mt0CRP while emphasizing the various external technologies' (including "on water" reaction, galvanic replacement, lithography, and capillary microfluidic) integrated polymerization systems. We also describe structural polymer brushes, including block copolymers, patterned and gradient structures, and arrayed and binary polymer brushes. Finally, we introduce the diverse polymer brushes that have been prepared using these techniques, with a focus on targeted and emerging applications. We anticipate that the discussion presented in this Account will promote a better understanding of the SI-Mt0CRP technique and advance the future development of practical surface brushing.

Advanced injectable hydrogels for bone tissue regeneration

Diseases or defects of the skeleton are hazardous because of their specificity and intricacy. Bone tissue engineering has become an important area of research that offers promising new tools for making biomimetic hydrogels that can be used to treat bone diseases. New hydrogels with a distinctive 3D network structure, high water content, and functional capabilities are ranked among the most promising candidates for bone tissue engineering. This makes them helpful in treating cartilage injury, skull deformity, and arthritis. This review will briefly introduce the variety of biocompatible functional hydrogels used in cell culture and bone tissue regeneration. Many gel design concepts, such as crosslinking procedures, controlled release properties, and alternative bionic methodology, were stressed regarding injectable hydrogels to form bone tissue. Hydrogels manufactured from biocompatible materials are a promising option for minimally invasive surgery because of their adaptable physicochemical qualities, ability to fill irregularly shaped defect sites, and ability to grow hormones or release drugs in response to external stimuli. Also included in this overview is a quick rundown of the more practical designs employed in treating bone disorders. Essential details on injectable hydrogel scaffolds for bone tissue regeneration are described in this article.

Fabrication of submillimeter-sized spherical self-oscillating gels and control of their isotropic volumetric oscillatory behaviors

In this study, we established a fabrication method and analyzed the volumetric self-oscillatory behaviors of submillimeter-sized spherical self-oscillating gels. We validated that the manufactured submillimeter-sized spherical self-oscillating gels exhibited isotropic volumetric oscillations during the Belousov-Zhabotinsky (BZ) reaction. In addition, we experimentally elucidated that the volumetric self-oscillatory behaviors (i.e., period and amplitude) and the oscillatory profiles depended on the following parameters: (1) the molar composition of N-(3-aminopropyl)methacrylamide hydrochloride (NAPMAm) in the gels and (2) the concentration of Ru(bpy)₃-NHS solution containing an active ester group on conjugation. These clarified relationships imply that controlling the amount of Ru(bpy)₃ in the gel network could influence the gel volumetric oscillation during the BZ reaction. These results of submillimeter-sized and spherical self-oscillating gels bridge knowledge gaps in the current field because the gels with corresponding sizes and shapes have not been systematically explored yet. Therefore, our study could be a cornerstone for diverse applications of (self-powered) gels in various scales and shapes, including soft actuators exhibiting life-like functions.

Oxygen Tolerance in Surface-Initiated Reversible Deactivation Radical Polymerizations: Are Polymer Brushes Turning into Technology?

Over the past three decades, the development of reversible deactivation radical polymerizations (RDRP), and advancements toward more user-friendly and accessible experimental setups have opened the door for nonexperts to design complex macromolecules with well-defined properties. External mediation, improved tolerance to oxygen, and increased reaction volumes for higher synthetic output are some of the many noteworthy technical improvements. The development of RDRPs in solution was paralleled by their application on solid substrates to synthesize surface-grafted "polymer brushes" via surface-initiated RDRP (SI-RDRP). This Viewpoint paper provides a current perspective on recent developments in SI-RDRP methods that are tolerant to oxygen, especially highlighting those that could potentially enable scaling up of the synthesis of brushes for the functionalization of technologically relevant materials.

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.

Versatile, Oxygen-Insensitive Surface-Initiated Anionic Polymerization to Prepare Functional Polymer Brushes in Aqueous Solutions

Surface-initiated polymerization is an attractive approach to achieve desired interfacial compositions and properties on a wide range of substrates and surfaces. Due to mild reaction conditions, multiple surface-initiated polymerization methods, such as atom-transfer radical polymerization (ATRP), reversible addition-fragmentation chain-transfer polymerization, and so forth, have been developed and studied in academia and industry. However, the current methods require the combination of metal catalysts, special initiators, and oxygen removal. Herein, we developed a surface-initiated carbanion-mediated anionic polymerization (SI-CMAP), which can be conducted in aqueous solutions in the presence of oxygen without the need for metal catalysts. Zwitterionic 2-(N-3-sulfopropyl-N,N-dimethyl ammonium)ethyl methacrylate (SBMA) was selected as a model monomer to develop and demonstrate this strategy. The vinyl sulfone (VS) groups displayed on substrate surfaces reacted with N-methylimidazole (NMIM), which was used as the in situ initiator. The polymerization mechanism was extensively studied from many aspects at room temperature, including the changes in reaction conditions, factors affecting the polymerization extent, and substrate surfaces. We also demonstrated the compatibility of this method with a broad spectrum of monomers ranging from SBMA to other acrylates and acrylamides by using glycine betaine as a reaction additive. This method was also evaluated for the preparation of polymer-coated nanoparticles. For polymer-coated silica nanoparticles, their hydrodynamic diameter, copper contamination, and effects of salt and protein concentrations were compared with SI-ATRP in parallel. SI-CMAP in aqueous solutions in air and the absence of metal catalysts make this method sustainable and cost-effective. We believe that SI-CMAP can be readily adapted to the industrial surface coating and large-scale nanoparticle preparation under mild conditions.

Molecularly Tailored Interface for Long-Term Xenogeneic Cell Transplantation

Encapsulation of therapeutic cells in a semipermeable device can mitigate the need for systemic immune suppression following cell transplantation by providing local immunoprotection while being permeable to nutrients, oxygen, and different cell-secreted biomolecules. However, fibrotic tissue deposition around the device has been shown to compromise the long-term function of the transplanted cells. Herein, a macroencapsulation device design that improves long-term survival and function of the transplanted cells is reported. The device is comprised of a semipermeable chitosan pouch with a tunable reservoir and molecularly engineered interface. The chitosan pouch interface decorated with 1,12-dodecanedioic acid (DDA), limits the cell adhesion and vigorous foreign body response while maintaining the barrier properties amenable to cell encapsulation. The device provides long-term protection to the encapsulated human primary hepatocytes in the subcutaneous space of immunocompetent mice. The device supports the encapsulated cells for up to 6 months as evident from cell viability and presence of human specific albumin in circulation. Solutions that integrate biomaterials and interfacial engineering such as the one described here may advance development of easy-to manufacture and retrievable devices for the transplantation of therapeutic cells in the absence of immunosuppression.

Fabricating scalable, personalized wound dressings with customizable drug loadings via 3D printing

In recent times, 3D printing has been gaining traction as a fabrication platform for customizable drug dosages as a form of personalized medicine. While this has been recently demonstrated as oral dosages, there is potential to provide the same customizability and personalization as topical applications for wound healing. In this paper, the application of 3D printing to fabricate hydrogel wound dressings with customizable architectures and drug dosages was investigated. Chitosan methacrylate was synthesized and mixed with Lidocaine Hydrochloride and Levofloxacin respectively along with a photoinitiator before being used to print wound dressings of various designs. These designs were then investigated for their effect on drug release rates and profiles. Our results show the ability of 3D printing to customize drug dosages and drug release rates through co-loading different drugs at various positions and varying the thickness of drug-free layers over drug-loaded layers in the wound dressing respectively. Two scale-up approaches were also investigated for their effects on drug release rates from the wound dressing. The influence that each wound dressing design has on the release profile of drugs was also shown by fitting them with drug release kinetic models. This study thus shows the feasibility of utilizing 3D printing to fabricate wound dressings with customizable shapes, drug dosage and drug release rates that can be tuned according to the patient's requirements.

Fabrication of Three-Dimensional Polymer-Brush Gradients within Elastomeric Supports by Cu0-Mediated Surface-Initiated ATRP

Cu⁰-mediated surface-initiated ATRP (Cu⁰ SI-ATRP) emerges as a versatile, oxygen-tolerant process to functionalize three-dimensional (3D), microporous supports forming single and multiple polymer-brush gradients with a fully tunable composition. When polymerization mixtures are dispensed on a Cu⁰-coated plate, this acts as oxygen scavenger and source of active catalyst. In the presence of an ATRP initiator-bearing microporous elastomer placed in contact with the metallic plate, the reaction solution infiltrates by capillarity through the support, simultaneously triggering the controlled growth of polymer brushes. The polymer grafting process proceeds with kinetics that are determined by the progressive infiltration of the reaction solution within the microporous support and by the continuous diffusion of catalyst regenerated at the Cu⁰ surface. The combination of these effects enables the accessible generation of 3D polymer-brush gradients extending across the microporous scaffolds used as supports, finally providing materials with a continuous variation of interfacial composition and properties.

Silanization of a Metal-Polyphenol Coating onto Diverse Substrates as a Strategy for Controllable Wettability with Enhanced Performance to Resist Acid Corrosion

Wettability is a crucial characteristic of materials that plays a vital role in surface engineering. Surface modification is the key to changing the wettability of materials, and a simple and universal modification approach is being extensively pursued by researchers. Recently, metal-phenolic networks (MPNs) have been widely studied because they impart versatility and functionality in surface modification. However, an MPN is not stable for long periods, especially under acidic conditions, and is susceptible to pollution by invasive species. Spurred by the versatility of MPNs and various functionalities achieved by silanization, we introduce a general strategy to fabricate functionally stable coatings with controllable surface wettability by combining the two methods. The formation process of MPN and silane-MPN coatings was characterized by spectroscopic ellipsometry (SE), UV-visible-near-infrared (UV-vis-NIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle (WCA), etc. We found that the stability of the MPN was greatly enhanced after silanization, which is attributed to the cross-linking effect that occurs between silane and the MPN, namely, the cross-linking protection produced in this case. Additionally, the wettability of an MPN can be easily changed through our strategy. We trust that our strategy can further extend the applications of MPNs and points toward potential prospects in surface modification.

Controlling the Friction of Gels by Regulating Interfacial Oxygen During Polymerization

Hydrogel surfaces are of great interest in applications ranging from cell scaffolds and transdermal drug-delivery patches to catheter coatings and contact lenses. In this work, we propose a method to control the surface structure of hydrogels, thereby tailoring their frictional properties. The method is based on oxygen inhibition of the free-radical polymerization reaction during synthesis and enables (i) control of friction over more than an order in magnitude and (ii) spatial control of friction as either a continuous gradient or a distinct pattern. The presented method has successfully been applied to acrylamide-, diacrylate- and methacrylate-based gels, illustrating the universality of the presented method, and its potential use in the above-mentioned applications.

Mechanism and application of surface-initiated ATRP in the presence of a Zn0 plate

SI-Zn⁰-ATRP enables the synthesis of chemically different polymer brushes under full ambient conditions, using just microliter volumes of reaction solutions.

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.