Earthworm-Inspired Rough Polymer Coatings with Self-Replenishing Lubrication for Adaptive Friction-Reduction and Antifouling Surfaces

Earthworm-Inspired Ultra-Durable Sliding Triboelectric Nanogenerator with Bionic Self-Replenishing Lubricating Property for Wind Energy Harvesting and Self-Powered Intelligent Sports Monitoring

Triboelectric nanogenerators (TENGs), a promising strategy for harvesting distributed low-quality power sources, face inevitable bottlenecks regarding long-term abrasion and poor durability. Herein, both issues are addressed by selecting an earthworm-inspired self-replenishing bionic film (ERB) as the tribo-material of sliding-freestanding TENGs (SF-TENGs), it consists of an interconnected 3D porous network structure capable of storing and releasing lubricant under cyclic mechanical stimuli. Thanks to the superiority of self-replenishing property, there is no need for periodic replenishment and accurate content control of lubricant over the interfacial-lubricating SF-TENGs based on dense tribo-layers. Additionally, an SF-TENG based on ERB film (ERB-TENG) demonstrates remarkable output stability with only a slight attenuation of 1% after continuous operation for 100 000 cycles. Moreover, the ERB-TENG displays a distinguished anti-wear property, exhibiting no distinct abrasion with an ultra-low coefficient of friction (0.077) and maintaining output stability over a prolonged period of 35 days. Furthermore, integration with an energy management circuit enables the ERB-TENG to achieve a 39-fold boost in charging speed. This work proposes a creative approach to enhance the durability and extend the lifespan of TENG devices, which is also successfully applied to wind energy harvesting and intelligent sports monitoring.

Sea Anemone-Inspired Slippery Liquid-Infused Porous Surface (SLIPS) with Bionic Cilia for Responsive 4D Antifouling

The formation process of biofouling is actually a 4D process with both spatial and temporal dimensions. However, most traditional antifouling coatings, including slippery liquid-infused porous surface (SLIPS), are limited to performing antifouling process in the 2D coating plane. Herein, inspired by the defensive behavior of sea anemones' wielding toxic tentacles, a "4D SLIPS" (FSLIPS) is constructed with biomimetic cilia via a magnetic field self-assembly method for antifouling. The bionic cilia move in 3D space driven by an external magnetic field, thereby preventing the attachment of microorganisms. The FSLIPS releases the gaseous antifoulant (nitric oxide) at 1D time in response to light, thereby achieving a controllable biocide effect on microorganisms. The FSLIPS regulates the movement of cilia via the external magnetic field, and controls the release of NO overtime via the light response, so as to adjust the antifouling modes on demand during the day or night. The light/magnetic response mechanism endow the FSLIPS with the ability to adjust the antifouling effect in the 4D dimension of 1D time and 3D space, effectively realizing the intelligence, multi-dimensionality and precision of the antifouling process.

Bioinspired Interfacial Friction Control: From Chemistry to Structures to Mechanics

Organisms in nature have evolved a variety of surfaces with different tribological properties to adapt to the environment. By studying, understanding, and summarizing the friction and lubrication regulation phenomena of typical surfaces in nature, researchers have proposed various biomimetic friction regulation theories and methods to guide the development of new lubrication materials and lubrication systems. The design strategies for biomimetic friction/lubrication materials and systems mainly include the chemistry, surface structure, and mechanics. With the deepening understanding of the mechanism of biomimetic lubrication and the increasing application requirements, the design strategy of multi-strategy coupling has gradually become the center of attention for researchers. This paper focuses on the interfacial chemistry, surface structure, and surface mechanics of a single regulatory strategy and multi-strategy coupling approach. Based on the common biological friction regulation mechanism in nature, this paper reviews the research progress on biomimetic friction/lubrication materials in recent years, discusses and analyzes the single and coupled design strategies as well as their advantages and disadvantages, and describes the design concepts, working mechanisms, application prospects, and current problems of such materials. Finally, the development direction of biomimetic friction lubrication materials is prospected.

Droplet Manipulation on Lubricant Self-Mediating Slippery PDMS Films

Further exploration is needed for sustainable and precise droplet manipulation on intelligent surfaces, especially the problem of SLIPS failure caused by lubricant loss. In this work, a self-mediating photothermal lubrication surface was designed. Through a simple preparation method, it was possible to generate a new lubrication layer through near-infrared light (NIL) and perform sustainable and precise droplet manipulation even after the surface lubricant was consumed. The thermal expansion film obtained from polydimethylsiloxane (PDMS) and nano ferric oxide, combined with the connected structure obtained through laser etching technology, effectively preserve lubricating oil. After the surface lubricating oil is consumed, under the action of NIL, the lubricating oil inside the film is squeezed out, forming a new lubricating layer. At the same time, programmable droplet transport can be achieved by inducing the direction of NIL. After turning off NIL, the lubricating oil is absorbed into the network structure, achieving good circulation. This not only reduces the loss of lubricating oil, but also facilitates the manipulation of droplets. In addition, the movement (plane and antigravity) and splitting behavior of droplets are also discussed. This sustainable and precise manipulation of liquid droplets on the LSSPF (lubricant self-mediating slippery PDMS films) surface can be widely applied in various micro reaction devices.

Design, fabrication, and applications of bioinspired slippery surfaces

Bioinspired slippery surfaces (BSSs) have attracted considerable attention owing to their antifouling, drag reduction, and self-cleaning properties. Accordingly, various technical terms have been proposed for describing BSSs based on specific surface characteristics. However, the terminology can often be confusing, with similar-sounding terms having different meanings. Additionally, some terms fail to fully or accurately describe BSS characteristics, such as the surface wettability of lubricants (hydrophilic or hydrophobic), surface wettability anisotropy (anisotropic or isotropic), and substrate morphology (porous or smooth). Therefore, a timely and thorough review is required to clarify and distinguish the various terms used in BSS literature. This review initially categorizes BSSs into four types: slippery solid surfaces (SSSs), slippery liquid-infused surfaces (SLISs), slippery liquid-like surfaces (SLLSs), and slippery liquid-solid surfaces (SLSSs). Because SLISs have been the primary research focus in this field, we thoroughly review their design and fabrication principles, which can also be applied to the other three types of BSS. Furthermore, we discuss the existing BSS fabrication methods, smart BSS systems, antifouling applications, limitations of BSS, and future research directions. By providing comprehensive and accurate definitions of various BSS types, this review aims to assist researchers in conveying their results more clearly and gaining a better understanding of the literature.

Dependence of Slippery and Elastic Properties of Thin Polymer Films on the Grafted Flexible Sidechain Amount

In modern life, people face a wide number of sticky problems when adhesion is highly undesirable: water and dirt stick to clothes, useful materials stick to the walls of their containers and cannot be fully used, water sticking and freezing on airplane wings affects handling and can be dangerous, biological liquids can stick and form clots inside medical devices threatening patients' lives, etc. Slippery liquid-infused porous surfaces (SLIPSs) with pressure stable omniphobicity could help to solve these issues. Lubricant depletion from porous surface and subsequent degradation of omniphobic properties is the major problem for SLIPS. It could be resolved by attaching flexible, liquid-like sidechains to the polymer matrix. Understanding the relationship between the structure of such polymer films and wetting effects is therefore of great importance. The present work is devoted to the study of droplet pinning on crosslinked polydimethylsiloxane (PDMS) polymer films with varied amounts of attached flexible PDMS sidechains and clarification of the relationship between slippery and viscoelastic properties of the films. An one-stage approach to the synthesis of such slippery coatings on smooth and porous substrates in "eco-friendly" pressurized CO₂ solutions is proposed. Pinning force and Young's modulus (E) of the films on silicon substrates with variation of the grafted sidechains amount (x) are measured. The non-monotonic dependence of the pinning force on the amount of sidechains is obtained: the pinning force decreases at small x values (region I) and starts to increase at higher x (region II). The effects of the grafted sidechains amount, as well as matrix softening, are discussed for each case. It is demonstrated that the proposed method of film synthesis allows one to obtain thin, uniform coatings on fabrics without gluing the fibers. Such coatings with an optimal amount of PDMS sidechains demonstrate decreased sliding angles for droplets of water and aqueous alcohol solutions, as compared to PDMS coatings without grafted sidechains. The proposed technique may be of interest for deposition of coatings on porous surfaces having a complex morphology, such as textiles, aerogels, porous electrodes, etc.

Pruney Finger-Inspired Switchable Surface with Water-Actuated Dynamic Textures

Switchable surfaces play an important role in the development of functional materials. However, the construction of dynamic surface textures remains challenging due to the complicated structural design and surface patterning. Herein, a pruney finger-inspired switchable surface (PFISS) is developed by constructing water-sensitive surface textures on a polydimethylsiloxane substrate by taking advantage of the hygroscopicity of the inorganic salt filler and the 3D printing technology. Like human fingertips, the PFISS shows high water sensitivity with obvious surface variation in wet and dry states, which is actuated by water absorption-desorption of the hydrotropic inorganic salt filler. Besides, when the fluorescent dye is optionally added into the matrix of the surface texture, water-responsive fluorescent emitting is observed, providing a feasible surface-tracing strategy. The PFISS shows effective regulation of the surface friction and performs a good antislip effect. The reported synthetic strategy for the PFISS offers a facile way for building a wide range of switchable surfaces.

Fluid manipulation via multifunctional lubricant infused slippery surfaces: principle, design and applications

Water-repellent interfaces with high performance have emerged as an indispensable platform for developing advanced materials and devices. Inspired by the pitcher plant, slippery liquid-infused porous surfaces (SLIPSs) with reliable hydrophobicity have proven to possess great potential for various applications in droplet and bubble manipulation, droplet energy harvesting, condensation, fog collection, anti-icing, and anti-biofouling due to their excellent properties such as persistent surface hydrophobicity, molecular smoothness, and fluidity. This review aims to introduce the development history of interaction between SLIPSs and fluids as well as the design principles, preparation methods, and various applications of some of the more typical SLIPSs. The fluid manipulation strategies of the slippery surfaces have been proposed including the wettability pattern, oriented micro-structure, and geometric gradient. At last, the application prospects of SLIPSs in various fields and the challenges in the design and fabrication of slippery surfaces are analyzed. We envision that this review can provide an overview of the fluid manipulating processes on slippery surfaces for researchers in both academic and industrial fields.

Fully recyclable multifunctional adhesive with high durability, transparency, flame retardancy, and harsh-environment resistance

Recyclable/reversible adhesives have attracted growing attention for sustainability and intelligence but suffer from low adhesion strength and poor durability in complex conditions. Here, we demonstrate an aromatic siloxane adhesive that exploits stimuli-responsive reversible assembly driven by π-π stacking, allowing for elimination and activation of interfacial interactions via infiltration-volatilization of ethanol. The robust cohesive energy from water-insensitive siloxane assembly enables durable strong adhesion (3.5 MPa shear strength on glasses) on diverse surfaces. Long-term adhesion performances are realized in underwater, salt, and acid/alkali solutions (pH 1-14) and at low/high temperatures (-10-90°C). With reversible assembly/disassembly, the adhesive is closed-loop recycled (~100%) and reused over 100 times without adhesion loss. Furthermore, the adhesive has unique combinations of high transparency (~98% in the visible light region of 400-800 nm) and flame retardancy. The experiments and theoretical calculations reveal the corresponding mechanism at the molecular level. This π-π stacking-driven siloxane assembly strategy opens up an avenue for high-performance adhesives with circular life and multifunctional integration.

Biology and nature: Bionic superhydrophobic surface and principle

Nature is the source of human design inspiration. In order to adapt to the environment better, creatures in nature have formed various morphological structures during billions of years of evolution, among which the superhydrophobic characteristics of some animal and plant surface structures have attracted wide attention. At present, the preparation methods of bionic superhydrophobic surface based on the microstructure of animal and plant body surface include vapor deposition, etching modification, sol-gel method, template method, electrostatic spinning method and electrostatic spraying method, etc., which have been used in medical care, military industry, shipping, textile and other fields. Based on nature, this paper expounds the development history of superhydrophobic principle, summarizes the structure and wettability of superhydrophobic surfaces in nature, and introduces the characteristics differences and applications of different superhydrophobic surfaces in detail. Finally, the challenge of bionic superhydrophobic surface is discussed, and the future development direction of this field is prospected.

Biomimetic superhydrophobic metal/nonmetal surface manufactured by etching methods: A mini review

As an emerging fringe science, bionics integrates the understanding of nature, imitation of nature, and surpassing nature in one aspect, and it organically combines the synergistic complementarity of function and structure–function integrated materials which is of great scientific interest. By imitating the microstructure of a natural biological surface, the bionic superhydrophobic surface prepared by human beings has the properties of self-cleaning, anti-icing, water collection, anti-corrosion and oil–water separation, and the preparation research methods are increasing. The preparation methods of superhydrophobic surface include vapor deposition, etching modification, sol–gel, template, electrostatic spinning, and electrostatic spraying, which can be applied to fields such as medical care, military industry, ship industry, and textile. The etching modification method can directly modify the substrate, so there is no need to worry about the adhesion between the coating and the substrate. The most obvious advantage of this method is that the obtained superhydrophobic surface is integrated with the substrate and has good stability and corrosion resistance. In this article, the different preparation methods of bionic superhydrophobic materials were summarized, especially the etching modification methods, we discussed the detailed classification, advantages, and disadvantages of these methods, and the future development direction of the field was prospected.

Liquid-Repellent Surfaces

Surfaces are vibrant sites for various activities with environments, especially as the transfer station for mass and energy exchange. In nature, natural creatures exhibit special wetting and interfacial properties such as water repellency and water affinity to adapt to various environmental challenges by taking advantage of air or liquid infusion media. Inspired by natural surfaces, various engineered liquid-repellent surfaces have been developed with a wide range of applications in both open and closed underwater environments. In particular, underwater conditions are characterized by high viscosity, high pressure, and complex compositions, which pose more challenges for the design of robust and functional repellent surfaces. In this Perspective, we take a parallel approach to introduce two classical liquid-repellent surfaces: an air-infused repellent surface and a lubricated liquid-repellent surface. Then we highlight fundamental challenges and design configurations of robust liquid-repellent surfaces both in air and underwater. We summarize the advantages and drawbacks of two kinds of repellent surfaces and list several applications of liquid-repellent surfaces for use in the ocean, medical care, and energy harvesting. Finally, we provide an outlook of research directions for robust liquid-repellent surfaces.

Design Methodology and Application of Surface Texture: A Review

Surface texture is regarded as a promising solution for enhancing the tribological features of industrial materials due to its outstanding benefits, such as minimization of the contact area, enhancement of the load bearing capacity, storage of the lubricant, and management of the transition between lubrication regimes. Surface texture can be processed under either liquid or gas conditions. As compared to laser ablation in air, employing liquids or other gases as ablation media provides high accuracy and uniformity by limiting the heat-affected zone (HAZ) and other undesired defects to a large extent, as well as high crater structural features. In addition, the synergistic use of different liquid, solid, and additive lubricants with surface roughness recently demonstrated excellent performance. Therefore, surface texture helps to improve the tribological characteristics of a material. This paper reviews the design methodologies and applications of surface texture, emphasizing the proper selection of the appropriate laser parameters and ambient conditions for the best texture quality and functionality. Recent texture geometric design features to improve the film thickness and the self-lubricating system are presented. The ablation environment is explored using various media. The interaction between the lubricants’ types and surface textures is explored based on the operating conditions. Furthermore, surface texture applications using superhydrophobic surfaces, anti-drag, and vibration and noise friction are discussed. We hope that this review plays an enlightening role in follow-up research on laser surface texture.

Effects of Snake-Bioinspired Surface Texture on the Finger-Sealing Performance under Varied Working Conditions

The tribological performance of the friction pair between the rotor and finger feet is a crucial index affecting the service life of finger seals. In recent years, the surface texture has attracted a considerable number of researchers owing to its extraordinary potential in improving antifriction and wear resistance. This paper, inspired by snakeskins, introduces three texture forms (e.g., diamond, ellipse, and hexagon) into the rotor. The effects on finger-sealing performance are analyzed by considering finger seals’ varied working conditions. First, a numerical model of textured finger seals under hydrodynamic lubrication is established based on the Reynolds equation. Then, the sealing performance analysis of textured finger seals is performed considering varied working conditions given rotation speed, pressure difference, seal clearance, and working temperature. The numerical results show that: (1) the textured domain produces a noticeable hydrodynamic pressure effect and cavitation, which effectively improves the bearing capacity of the fluid film; (2) the higher the rotation speed or the lower the inlet/outlet pressure difference, the stronger the dynamic pressure effect of textured finger seals and the better the antifriction and wear resistance; (3) for good antifriction and wear resistance of a textured finger seal, the seal clearance should be as shallow as possible (≤10 μm), and the working temperature should be as low as possible (≤120 °C); and (4) the ellipse texture has a higher average dimensionless pressure and a lower friction coefficient, which is superior to diamond and hexagon ones in terms of friction and wear performance.

Mechanical Durability of Low Ice Adhesion Polydimethylsiloxane Surfaces

Elastomeric surfaces and oil-infused elastic surfaces reveal low ice adhesion, in part because of their deformability. However, these soft surfaces might jeopardize their mechanical durability. In this work, we analyzed the mechanical durability of elastic polydimethylsiloxane (PDMS) surfaces with different balances between elasticity and deicing performances. The durability was studied in terms of shear/tensile ice adhesion strength before and after different wear tests. These tests consisted of abrasion/erosion cycles using standard procedures aimed to reproduce different environmental wearing agents. The main objective is to evaluate if our PDMS surfaces can become long-lasting solutions for ice removal in real conditions. We found that our elastic surfaces show excellent durability. After the wear tests, the ice adhesion strength values remained low or even unaltered. Although the oil-infused PDMS surface was the softest one, it presented considerable durability and excellent low ice adhesion, being a promising solution.

Molecular weight tuning optimizes poly(2-methoxyethyl acrylate) dispersion to enhance the aging resistance and anti-fouling behavior of denture base resin

Poly(methyl methacrylate) (PMMA)-based denture base resins easily develop oral bacterial and fungal biofilms, which may constitute a significant health risk. Conventional bacterial-resistant additives and coatings often cause undesirable changes in the resin. Reduced bacterial resistance over time in the harsh oral environment is a major challenge in resin development. Poly(2-methoxyethyl acrylate) (PMEA) has anti-fouling properties; however, due to the oily/rubbery state of this polymer, and its surface aggregation tendency in a resin mixture, its direct use as a resin additive is limited. This study aimed to optimize the use of PMEA in dental resins. Acrylic resins containing a series of PMEA polymers with various molecular weights (MWs) at different concentrations were prepared, and the mechanical properties, surface gloss, direct transmittance, and cytotoxicity were evaluated, along with the distribution of PMEA in the resin. Resins with low-MW PMEA (2000 g mol^-1) (PMEA-1) at low concentrations satisfied the clinical requirements for denture resins, and the PMEA was homogeneously distributed. The anti-fouling performance of the resin was evaluated for protein adsorption, bacterial and fungal attachment, and saliva-derived biofilm formation. The PMEA-1 resin most effectively inhibited biofilm formation (∼50% reduction in biofilm mass and thickness compared to those of the control). Post-aged resins maintained their mechanical properties and anti-fouling activity, and polished surfaces had the same anti-biofilm behavior. Based on wettability and tribological results, we propose that the PMEA additive creates a non-stick surface to inhibit biofilm formation. This study demonstrated that PMEA additives can provide a stable and biocompatible anti-fouling surface, without sacrificing the mechanical properties and aesthetics of denture resins.

Supramolecular Oleogel-Impregnated Macroporous Polyimide for High Capacity of Oil Storage and Recyclable Smart Lubrication

Developing smart lubrication materials to achieve recyclable and durable lubrication and excellent wear resistance under various running conditions has great significance in fields ranging from aerospace to advanced engineering machinery but has proven challenging. Herein, a supramolecular oleogel with reversible gel-to-liquid transition was impregnated into macroporous polyimide (MPPI-gel) to obtain a smart lubrication material, which exhibited recyclable smart lubrication with an enhanced oil content and oil retention. The self-assembly of the gelator in polyalphaolefin10 (PAO10) formed three-dimensional networks that encapsulated the PAO10 during the service process, and the MPPI-gel could exhibit a high oil retention (approximately 99%). The gel-to-liquid transition allows the lubricant to be extruded and transferred to the surface of the macroporous matrix (MPPI) under thermal-mechano-stimuli and vice versa. The extruded lubricant can be sucked back into the MPPI pores through the capillary force and recovered to the oleogel when removing the external stimuli. Due to the high oil content, high oil retention, and recyclable lubricant releasing/reabsorbing, MPPI-gel exhibited recyclable smart lubrication (at least 1852 cycles; each cycle lasted for 1 h), a stable coefficient of friction (∼0.06) under alternating conditions (the frequency varied from 1 to 20 Hz, and the load varied from 10 to 46 N), and long-term conditions (at least 10 days). Therefore, MPPI-gel holds the promise of realizing smart lubrication according to the external stimuli with both high oil storage and recyclable lubricant releasing/reabsorbing with the porous matrix.

Slippery Passive Radiative Cooling Supramolecular Siloxane Coatings

Polymer coatings with comprehensive properties including passive radiative cooling, anti-fouling, and self-healing constitute a promising energy-saving strategy but have not been well documented yet. Herein, we reported a class of novel multifunctional supramolecular polysiloxane composite coatings showing the combination of these features. The coatings have a hybrid structure with a slippery liquid-infused porous surface and a gradient polymer-Al₂O₃ composite matrix constructed by reversible hydrogen bonding. The gradient matrix consists of a polymer-rich top and a particle-rich bottom favoring coating attachment on rigid substrates. Such a complex structure can be obtained by simply casting the suspending solutions of the polydimethylsiloxane (PDMS)-urea copolymer and Al₂O₃ on substrates followed by swelling silicone oil. Obtained coatings display good passive daytime radiative cooling (a temperature drop of ∼2 °C), self-healing ability, and anti-fouling properties. Since the comprehensive performances and the facile fabrication, the coatings should have application potential for various thermal management purposes.

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.

Study of the Droplet Pinning Force in the Transition from Dry to Liquid-Infused Thin Polymer Films

The change in the pinning force during the transition from dry to oil-impregnated thin polymer films is studied for droplets of water and hexadecane. A careful variation of the oil amount in the films is performed by means of supercritical impregnation. The film thickness dependence on the oil content is measured using ellipsometry and compared to gel swelling theory estimates. Depending on the oil content, two cases of pinning force behavior have been identified. For each case, the factors that determine the pinning force are discussed. The pinning force in the transition from dry to equilibrium swollen gel films is well approximated by the Joanny and de Gennes hysteresis model of dilute defects.

Green Superlubricity Enabled by Only One Water Droplet on Plant Oil-Infused Surfaces

The increase in energy loss due to friction and the use of large amounts of lubricants to improve it are major challenges we face from a global environmental perspective. Pitcher-plant-inspired liquid-infused surfaces (LISs) are emerging super-repellent surfaces against liquids. However, their coefficient of friction (CoF) against solids is higher than that of conventional lubricant surfaces. Herein, we demonstrate superlubricity with a single water droplet placed on a LIS holding oleic acid, a component of plant oil. When a water droplet is placed on the fluid layer, the CoF under reciprocating and rotating friction is 0.012 and 0.0098, respectively. A force in the direction opposite to the loading due to the Laplace pressure on the droplet and an autonomous positional movement of the water accompanied by the optimization of surface energy maintain the fluid-lubrication state and prevent direct contact between the surface and the friction material, resulting in a decrease of the dependence of the CoF on the friction velocity. The key technology here will not only present a novel strategy for preparing LISs against solids but also serve as a step toward a sustainable green strategy for friction reduction and lubrication, which would greatly reduce energy loss and environmental degradation.

Dynamic Anti-Icing Surfaces (DAIS)

Remarkable progress has been made in surface icephobicity in the recent years. The mainstream standpoint of the reported antiicing surfaces yet only considers the ice-substrate interface and its adjacent regions being of static nature. In reality, the local structures and the overall properties of ice-substrate interfaces evolve with time, temperature and various external stimuli. Understanding the dynamic properties of the icing interface is crucial for shedding new light on the design of new anti-icing surfaces to meet challenges of harsh conditions including extremely low temperature and/or long working time. This article surveys the state-of-the-art anti-icing surfaces and dissects their dynamic changes of the chemical/physical states at icing interface. According to the focused critical ice-substrate contacting locations, namely the most important ice-substrate interface and the adjacent regions in the substrate and in the ice, the available anti-icing surfaces are for the first time re-assessed by taking the dynamic evolution into account. Subsequently, the recent works in the preparation of dynamic anti-icing surfaces (DAIS) that consider time-evolving properties, with their potentials in practical applications, and the challenges confronted are summarized and discussed, aiming for providing a thorough review of the promising concept of DAIS for guiding the future icephobic materials designs.

Recent Progress of Bioinspired Scalephobic Surfaces with Specific Barrier Layers

Bioinspired superwettable surfaces have been widely harnessed in diverse applications such as self-cleaning, oil/water separation, and liquid transport. So far, only a little work is focused on scalephobic capability of those superwettable surfaces. However, the troublesome scale deposition will inevitably be observed in our daily production and life, greatly reducing heat transfer efficiency and inhibiting the liquid transport. To address this annoying problem, as the emerging strategy, specific barrier layers are introduced onto superwettable surfaces to reduce or even avoid the direct contact between scale and the surfaces. In this feature article, we first provide the basic concept of bioinspired scalephobic surfaces with specific barrier layers. Then, we briefly introduce the typical fabrication methods of scalephobic surfaces. Later, we summarize recent progress of bioinspired scalephobic surfaces with specific barrier layers. Furthermore, we point out the guiding theory and criteria for the stability of barrier layers. Finally, we put forward the forecast on the existing problems and future direction in bioinspired scalephobic surfaces.

Slippery Antifouling Polysiloxane-Polyurea Surfaces with Matrix Self-Healing and Lubricant Self-Replenishing

The inferior mechanical properties and the difficulty in repairing damaged substrates and lubricant films of slippery liquid-infused porous surfaces significantly hampered their practical applications. To solve this problem, we fabricated a polysiloxane-polyurea slippery elastomer with lubricant self-replenishing and matrix self-healing properties by encapsulating silicone oil into the thermoplastic elastomers. By optimizing the chemical compositions and molecular interactions, the obtained slippery elastomer exhibits unique mechanical properties with a maximum breaking strength of 0.12 MPa, elongation of 1600%, and self-healing efficiency of 98%. Moreover, the lubricant stored in the capsule of the slippery elastomer can be controlled released under mechanical stimulation, further realizing surfaces' self-lubricating and liquid manipulation switching between slippery and pinning states. Furthermore, the textile-reinforced slippery elastomer with superior mechanical strength also exhibited liquid repellency, anti-biofouling, and drag reduction properties. Therefore, this textile-reinforced omniphobic surface with high mechanical property, matrix self-healing, and lubricant self-replenishing property shows a broad application prospect in surface protection, underwater antifouling, and drag reduction.

Polymeric Microparticles Generated via Confinement-Free Fluid Instability

In-fiber fluid instability can be harnessed to realize scalable microparticles fabrication with tunable sizes and multifunctional characteristics making it competitive in comparison to conventional microparticles fabrication methods. However, since in-fiber fluid instability has to be induced via thermal annealing and the resulting microparticles can only be collected after dissolving the fiber cladding, obtaining contamination-free particles for high-temperature incompatible materials remains great challenge. Herein, confinement-free fluid instability is demonstrated to fabricate polymeric microparticles in a facile manner induced by the ultralow surface energy of the superamphiphobic surface. The polymer solution columns break up into uniform droplets then form spherical particles spontaneously in seconds at ambient temperature. This method can be applied to a variety of polymers spanning an exceptionally wide range of sizes: from 1 mm down to 1 µm. With the aid of microfluidic spinning instrument, a large quantity of microparticles can be obtained, making this method promising for scaling up production. Notably, through simple modification of the feed solution configuration, composite/structured micromaterials can also be produced, including quantum-dots-labeled fluorescent particles, magnetic particles, core-shell particles, microcapsules, and necklace-like microfibers. This method, with general applicability and facile control, is envisioned to have great prospects in the field of polymer microprocessing.

Adhesion behaviors on four special wettable surfaces: natural sources, mechanisms, fabrications and applications

The study of adhesion behaviors on solid-liquid surfaces plays an important role in scientific research and development in various fields, such as medicine, biology and agriculture. The contact angle and sliding angle of the liquid on the solid surface are commonly used to characterize and measure the wettability of a particular surface. They have a wide range of values, which results in different wettability. It boils down to the adhesion of solid surfaces to liquids. This feature article is aimed at revealing the essence of the adhesion behavior from the aspects of controlling the chemical composition or changing the geometrical microstructure of the surface, and reviewing the natural sources, wetting models, preparation methods and applications of four kinds of typical solid-liquid surfaces (low-adhesion superhydrophobic surfaces, high-adhesion superhydrophobic surfaces, slippery liquid-infused porous surfaces (SLIPS) and hydrophilic/superhydrophilic surfaces). Last, a summary and outlook on this field are given to point out the current challenges and the potential research directions of surface adhesion in the coming future.

Slippery liquid-infused porous surfaces with inclined microstructures to enhance durable anti-biofouling performances

In the development of biocompatible materials for biomedical applications, infections and their resulting inflammation responses are important issues caused typically by the adhesion of micro-organisms on medical devices. Recently slippery liquid-infused porous surfaces (SLIPS) has provided a new strategy for anti-biofouling and low-adhesion surfaces, however, there are still some bottlenecks in practical uses, particularly the loss of lubricant significantly restricts the durability and stability of SLIPS. In this paper, we micro-fabricated well-controlled micro-cavities with different profiles (vertical or inclined walls) to investigate the long-term anti-biofouling effect of SLIPS. We explored microstructure geometries in two aspects: the aspect ratio and the slope angle relevant with the Laplace pressure and the oil contact area which lead to different oil-locking abilities. High aspect ratio and inclined slope were demonstrated with better oil-locking ability as well as significantly increased anti-fouling performances. Under the same experimental setup, the Escherichia coli and Staphylococcus aureus bacteria coverage on SLIPS with 80 μm-depth 20° inclined micro-cavities was only ∼30 % of that with vertical micro-cavities, while increasing aspect ratio by 4 times induced ∼3 times enhanced anti-fouling effect. On basis of these findings, we propose the enhanced SLIPS with inclined microstructures to achieve better oil-locking ability and long-term anti-biofouling performance, which may broaden many practical applications of SLIPS.

Biofouling-resistant tubular fluidic devices with magneto-responsive dynamic walls

Biofouling of tubular fluidic devices limits the stability, accuracy, and long-term uses of lab-on-a-chip systems. Healthcare-associated infection by biofilm formations on body-indwelling and extracorporeal tubular medical devices is also a major cause of mortality and morbidity in patients. Although diverse antifouling techniques have been developed to prevent bacterial contamination of fluidic devices based on antimicrobial materials or nanoscale architectures, they still have limitations in biocompatibility, long-term activity, and durability. In this study, a new conceptual tubular fluidic device model that can effectively suppress bacterial contamination based on dynamic surface motions without using bactericidal materials or nanostructures is proposed. The fluidic device is composed of a magneto-responsive multilayered composite. The composite tube can generate dynamic surface deformation with controlled geometries along its inner wall in response to a remote magnetic field. The magnetic field-derived surface wave induces the generation of vortices near the inner wall surface of the tube, enabling sweeping of bacterial cells from the surface. As a result, the dynamic composite tube could effectively prevent biofilm formation for an extended time of 14 days without surface modification with chemical substances or nanostructures.

The challenge of lubricant-replenishment on lubricant-impregnated surfaces

Lubricant-impregnated surfaces are two-component surface coatings. One component, a fluid called the lubricant, is stabilized at a surface by the second component, the scaffold. The scaffold can either be a rough solid or a polymeric network. Drops immiscible with the lubricant, hardly pin on these surfaces. Lubricant-impregnated surfaces have been proposed as candidates for various applications, such as self-cleaning, anti-fouling, and anti-icing. The proposed applications rely on the presence of enough lubricant within the scaffold. Therefore, the quality and functionality of a surface coating are, to a large degree, given by the extent to which it prevents lubricant-depletion. This review summarizes the current findings on lubricant-depletion, lubricant-replenishment, and the resulting understanding of both processes. A multitude of different mechanisms can cause the depletion of lubricant. Lubricant can be taken along by single drops or be sheared off by liquid flowing across. Nano-interstices and scaffolds showing good chemical compatibility with the lubricant can greatly delay lubricant depletion. Often, depletion of lubricant cannot be avoided under dynamic conditions, which warrants lubricant-replenishment strategies. The strategies to replenish lubricant are presented and range from spraying or stimuli-responsive release to built-in reservoirs.

Towards programmable friction: control of lubrication with ionic liquid mixtures by automated electrical regulation

For mechanical systems in relative motion it would be fascinating if a non-mechanical stimulus could be used to directly control friction conditions. Therefore, different combinations of lubricants and external triggers for tribological influence have already been investigated. We show that when two metallic friction partners are lubricated with ionic liquid mixtures (ILM), consisting of long-chain cation and two different high charge/mass ratio anion containing ILs, the application of an electric impulse induces a permanent change of the frictional response. Such mixtures are able to alter the coefficient of friction (COF) to a greater extent, more accurately and faster than the respective single-component ILs. This change in the frictional properties is presumably due to changes in the externally induced electrical polarization at the surface, which influences the molecular adsorption, the exchange of adsorbed ions and their molecular orientation. The correlation between surface charges and friction can be used to control friction. This is achieved by implementing an electric tribo-controller which can adjust preset friction values over time. Programming friction in this way is a first step towards tribosystems that automatically adapt to changing conditions.

Fully Repairable Slippery Organogel Surfaces with Reconfigurable Paraffin-Based Framework for Universal Antiadhesion

Constructing a slippery lubricant-infused surface (SLIS) whose internal microstructure and surface properties can be fully repaired helps to improve its property stability and extend technological implications but has presented a huge challenge. A class of fully repairable slippery organogel surfaces (SOSs), which uses microstructured paraffin as reconfigurable supporting structure and silicone oil as lubricant dispersion medium, is reported here. Attributed to nearly 90 wt % of silicone oil stored in the slippery organogel system and good compatibility with the paraffin-based framework, SOSs combine continuous lubricity and reliable lubricant storage stability. Furthermore, the thermally sensitive paraffin-based framework can quickly switch between solid supporting structure and liquid solution according to the ambient temperature, thereby achieving rapid regeneration of microstructure. This unique system consisting of reconfigurable framework and flowable lubricant derives two types of repairs aimed at varying degrees of damage. Significantly, the easy-to-prepare SOS, on the other hand, allows the adoption of various substrate surfaces for different purposes to form an antiadhesion coating and exhibits excellent antistain, antialgae, and anti-icing performance, thus greatly improving the flexibility of such materials in practical applications.

Droplets Self-Born in the Dynamic Polymer for Generating Functional Coatings

Droplet-embedded structures are useful in functionalizing polymer composites but difficult to prepare. Herein, we report a facile self-born method for creating droplets in supramolecular gels to mediate the material's functions. This method is based on the amplification of the defects of polymer matrices generated in curing by swelling-driving reconfiguration of supramolecular polymer networks. The system of poly(urea-co-polydimethylsiloxane) that can cross-link via hydrogen-bond interaction is used to demonstrate our concept. The elastomer matrices are prepared via a casting method and exhibit a heterogeneous structure with both strong- and weak-cross-linking domains. When these materials are swelled in solvents, solvent molecules concentrate in the weak-cross-linking domains to nucleate. With the reconfiguration of the matrices, the nuclei grow into pure droplets, leading to the formation of droplet-embedded structures. This method is applicable to different material systems. We also show that obtained coatings with such droplet-embedded structures exhibit various interesting properties including self-replenishment of the surface liquid, mechanoresponsiveness, and self-healing ability. Moreover, after the droplets are consumed, this method can be used to regenerate the droplet-embedded structure for refunctionalizing the materials. Therefore, we envision its applications in preparation of many useful polymer composites.

Bactericidal Lubricating Synthetic Materials for Three-Dimensional Additive Assembly with Controlled Mechanical Properties

3D printable synthetic materials have been developed to realize desired surface and mechanical properties. Lubricating synthetic surfaces have broad technological impacts on many applications including food packaging, microfluidic systems, and biomedical devices. However, combining soft materials with lubricants leads to significant phase separation and swelling phenomena, together with lowered mechanical strength, impeding full utilization of lubricating synthetic surfaces with desired shapes in a highly controllable manner. Here, we report a new platform to create a 3D printable lubricant-polymer composite (3D-LUBRIC) for the seamless fabrication of multidimensional structures with diverse functionalities. The rationally designed lubricant-polymer mixtures including silica aerogel particles not only exhibit suitable rheological properties for direct ink writing without phase separation but also enable the deterministic additive assembly of heterogeneous materials, which have large mismatches of oil permeability, with no distinct shape distortion. While exhibiting excellent lubricating properties for a variety of liquids, 3D-LUBRIC shows tunable mechanical properties with desired functionalities, such as optical transparency, flexibility and stretchability, and anti-icing and antibacterial/bactericidal properties. We employ the proposed platform to fabricate self-cleanable containers and antibacterial/bactericidal medical tubes. Our platform can offer new opportunities for building low-adhesive, multifunctional synthetic materials with customized shapes for diverse applications.

Non-sticky and Free-forward Performances of Grubs against Soil

The intriguing non-sticky and free-forward performances of grubs against soil deeply attract our interests. In this study, the life cycle and body morphology of a kind of grubs, larvae of Japanese rhinoceros beetles, are introduced. The uniformly oriented hierarchical micro structures pattern on the back epidermis is firstly reported. The rotating and forwarding motion configuration of grubs in soil is unraveled. The friction and adhesion properties of grubs are evaluated and compared with typical materials. The biological electroosmosis induced adhesion reduction effect and the hierarchical structures pattern induced anisotropic friction feature are highlighted.

Supramolecular silicone coating capable of strong substrate bonding, readily damage healing, and easy oil sliding

Polymer coatings with a combined competence of strong bonding to diverse substrates, broad liquid repellency, and readily damage healing are in substantial demand in a range of applications. In this work, we develop damage-healable, oil-repellent supramolecular silicone (DOSS) coatings to harvest abovementioned properties by molecular engineering siloxane oligomers that can self-assemble onto coated substrates via multivalent hydrogen bonding. In addition to the readily damage-healing properties provided by reversible association/dissociation of hydrogen bonding motifs, the unique molecular configuration of the siloxane oligomers on coated substrates enables both robust repellency to organic liquids and strong bonding to various substrates including metals, plastics, and even Teflon. We envision that not only DOSS coatings can be applied in a range of energy, environmental, and biomedical applications that require long-term services in harsh environmental conditions but also the design strategy of the oligomers can be adopted in the development of supramolecular materials with desirable multifunctionality.

Photothermal Actuation of Diverse Liquids on an Fe3O4-Doped Slippery Surface for Electric Switching and Cell Culture

The photoinduced manipulation of liquids on a slippery lubricant-infused porous surface (SLIPS) has attracted a tremendous amount of attention because of its merits of contactless stimulation and excellent spatial and temporal control. However, tedious fabrication methods by a combination of template transfer and fluorination for a photothermal-material-doped SLIPS and the lack of deeper systematically quantitative analysis with respect to droplet hydrokinetics are greatly perplexing in both academic research and industrial applications. Here we demonstrate a kind of Fe₃O₄-doped SLIPS by one-step femtosecond laser cross-scanning, which can readily steer diverse liquids toward arbitrary directions with a fast velocity of up to 1.15 mm/s in the presence of a unilateral NIR stimulus. The underlying mechanism is that the wettability gradient force (F_wet-grad) induced by the temperature gradient arising from asymmetric near-infrared-irradiation (NIR) loading would be generated within 1 s to actuate a targeted droplet's sliding behavior. Through tuning the NIR irradiating sites, we can slide a targeted droplet with controllable directions and routes. On the basis of fundamental physics, we have quantitatively analyzed the relationship among Fe₃O₄-doped content, lubricant rheological performance, droplet wettability variations, F_wet-grad, and the sliding velocity for diverse liquid species. Accordingly, we can remotely steer liquid droplets to realize the on-off state of an electrical circuit on demand, the droplet fusion of a microfluidic reactor, and the culture/inhibition of biological cells.

Epidermal Gland Inspired Self-Repairing Slippery Lubricant-Infused Porous Coatings with Durable Low Ice Adhesion

The limited durability of slippery lubricant-infused porous surfaces (SLIPS) restricts their practical applications. Inspired by the epidermal glands of skins, we developed a facile approach to durable SLIPS with gland-like storage and release functions for icephobicity. By introducing a hybrid surfactant as a lubricant into the polydimethylsiloxane (PDMS) matrix, lubricant capsules were formed and mono-dispersed in the matrix, working as gland-like structures to release lubricant. The obtained SLIPS showed durable low ice adhesion strength and thermal durability simultaneously. In detail, the enhanced durability for icephobicity was demonstrated by 20 icing/deicing tests, in which the lubricant remains on the surface; the coatings showed negligible weight loss when stored at 100 °C for 60 h, displaying pronounced thermal durability of the slippery effect. Our current strategy sheds new light on a facile fabrication of mechanically and thermally durable SLIPS for icephobicity.

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.