Bio-Inspired Design and Fabrication of Micro/Nano-Brush Dual Structural Surfaces for Switchable Oil Adhesion and Antifouling

Antifouling applications and fabrications of biomimetic micro-structured surfaces: A review

BACKGROUND

Since the inception of the term "Biomimetics" in 1991, the concept of utilizing natural solutions or deriving inspiration from nature to address contemporary engineering challenges has gained significant attention within the scientific community. Organisms, in order to thrive in harsh environments, have evolved a wide range of micro/nanostructured surfaces, which serve as a rich source of inspiration for the development of artificial micro/nano-structured surfaces. These natural adaptations provide valuable insights and novel pathways for fabricating such surfaces.

AIM

To conclude recent advances in micro/nano-structured surfaces from four aspects: biomimetic micro-structured surfaces of plants and animals, properties and applications of biomimetic surfaces, methods of preparations, and their limitation.

KEY SCIENTIFIC CONCEPTS

Artificial micro/nano-structured surfaces inspired by animals and plants are classified and demonstrated according to their living environment. The performances, principles and preparation techniques of natural superhydrophobic surfaces, slippery liquid-infused porous surfaces (SLIPS), anisotropic surfaces, etc. are described in detail. Moreover, the pros and cons of each preparation measures are compared and the challenges developing large-scale, cost-effective surface microstructure preparation processes are pointed out. In the end, the development trends of artificial micro/nano-structured surface are forecasted.

Preparation and Performance Study of HTPB-g-(PNIPAM/PEG) Thermoresponsive Polymer Brush

In recent years, a great deal of work has been devoted to the development of thermoresponsive polymers that can be made into new types of smart materials. In this paper, a branched polymer, HTPB-g-(PNIPAM/PEG), with polyolefin chain segments as the backbone and having polyethylene glycol (PEG) and poly(N-isopropylacrylamide) (PNIPAM) as side chains was synthesized by ATRP and click reactions using N3-HTPB-Br as the macroinitiator. This initiator was designed and synthesized using hydroxyl-terminated polybutadiene (HTPB) as the substrate. The temperature-responsive behavior of the branched polymer was investigated. The lower critical solution temperature (LCST) of the branched polymer was determined by ultraviolet and visible spectrophotometry (UV-vis) and was found to be 35.2 °C. The relationship between the diameter size of micelles and temperature was determined by dynamic light scattering (DLS). It was found that the diameter size changed at 36 °C, which was nearly consistent with the result obtained by UV-vis. The results of the study indicate that HTPB-g-(PNIPAM/PEG) is a temperature-responsive polymer. At room temperature, the polymer can self-assemble into composite micelles, with the main chain as the core and the branched chain as the shell. When the temperature was increased beyond LCST, the polyolefin main chain along with the PNIPAM branched chain assembled to form the nucleus, and the PEG branched chain constituted the shell.

Nature-Inspired Micro/Nano-Structured Antibacterial Surfaces

The problem of bacterial resistance has become more and more common with improvements in health care. Worryingly, the misuse of antibiotics leads to an increase in bacterial multidrug resistance and the development of new antibiotics has virtually stalled. These challenges have prompted the need to combat bacterial infections with the use of radically different approaches. Taking lessons from the exciting properties of micro-/nano-natural-patterned surfaces, which can destroy cellular integrity, the construction of artificial surfaces to mimic natural functions provides new opportunities for the innovation and development of biomedicine. Due to the diversity of natural surfaces, functional surfaces inspired by natural surfaces have a wide range of applications in healthcare. Nature-inspired surface structures have emerged as an effective and durable strategy to prevent bacterial infection, opening a new way to alleviate the problem of bacterial drug resistance. The present situation of bactericidal and antifouling surfaces with natural and biomimetic micro-/nano-structures is briefly reviewed. In addition, these innovative nature-inspired methods are used to manufacture a variety of artificial surfaces to achieve extraordinary antibacterial properties. In particular, the physical antibacterial effect of nature-inspired surfaces and the functional mechanisms of chemical groups, small molecules, and ions are discussed, as well as the wide current and future applications of artificial biomimetic micro-/nano-surfaces. Current challenges and future development directions are also discussed at the end. In the future, controlling the use of micro-/nano-structures and their subsequent functions will lead to biomimetic surfaces offering great potential applications in biomedicine.

A comprehensive review on the inherent and enhanced antifouling mechanisms of hydrogels and their applications

Biofouling remains a persistent challenge within the domains of biomedicine, tissue engineering, marine industry, and membrane separation processes. Multifunctional hydrogels have garnered substantial attention due to their complex three-dimensional architecture, hydrophilicity, biocompatibility, and flexibility. These hydrogels have shown notable advances across various engineering disciplines. The antifouling efficacy of hydrogels typically covers a range of strategies to mitigate or inhibit the adhesion of particulate matter, biological entities, or extraneous pollutants onto their external or internal surfaces. This review provides a comprehensive review of the antifouling properties and applications of hydrogels. We first focus on elucidating the fundamental principles for the inherent resistance of hydrogels to fouling. This is followed by a comprehensive investigation of the methods employed to enhance the antifouling properties enabled by the hydrogels' composition, network structure, conductivity, photothermal properties, release of reactive oxygen species (ROS), and incorporation of silicon and fluorine compounds. Additionally, we explore the emerging prospects of antifouling hydrogels to alleviate the severe challenges posed by surface contamination, membrane separation and wound dressings. The inclusion of detailed mechanistic insights and the judicious selection of antifouling hydrogels are geared toward identifying extant gaps that must be bridged to meet practical requisites while concurrently addressing long-term antifouling applications.

Pressure-Activatable Liquid Metal Composites Flexible Sensor with Antifouling and Drag Reduction Functional Surface

Flexible sensors produced through three-dimensional (3D) printing have exhibited promising results in the context of underwater sensing detection (for applications in navigational vehicles and human activities). However, underwater vehicles and activities such as swimming and diving are highly susceptible to drag, which can cause negative impacts such as reduced speed and increased energy consumption. Additionally, microbial adhesion can shorten the service life of these vehicles. However, natural organisms are able to circumvent such problems, with shark skin offering excellent barrier properties and ruffled papillae providing effective protection against fouling. Here, we show that a sandwich system consisting of a spraying layer, conductive elastomer composite, and encapsulation layer can be printed for multifunctional integrated underwater sensors. The modulated viscoelastic properties of liquid metal form the foundation for printing features, while its pressure-activated properties offer the potential for switchable sensors. An integrated drag reduction and antifouling layer were created by combining the shark skin surface shield scale structure with the lotus leaf surface papillae structure. A 3D-printed flexible sensor was designed using our approach to monitor attitude changes and strain in underwater environments, showcasing its capabilities. Our printed sensors can reduce biological attachment density by more than 50% and reduce underwater drag by 8.6-10.3%.

Research Progress on Low-Surface-Energy Antifouling Coatings for Ship Hulls: A Review

The adhesion of marine-fouling organisms to ships significantly increases the hull surface resistance and expedites hull material corrosion. This review delves into the marine biofouling mechanism on marine material surfaces, analyzing the fouling organism adhesion process on hull surfaces and common desorption methods. It highlights the crucial role played by surface energy in antifouling and drag reduction on hulls. The paper primarily concentrates on low-surface-energy antifouling coatings, such as organic silicon and organic fluorine, for ship hull antifouling and drag reduction. Furthermore, it explores the antifouling mechanisms of silicon-based and fluorine-based low-surface-energy antifouling coatings, elucidating their respective advantages and limitations in real-world applications. This review also investigates the antifouling effectiveness of bionic microstructures based on the self-cleaning abilities of natural organisms. It provides a thorough analysis of antifouling and drag reduction theories and preparation methods linked to marine organism surface microstructures, while also clarifying the relationship between microstructure surface antifouling and surface hydrophobicity. Furthermore, it reviews the impact of antibacterial agents, especially antibacterial peptides, on fouling organisms' adhesion to substrate surfaces and compares the differing effects of surface structure and substances on ship surface antifouling. The paper outlines the potential applications and future directions for low-surface-energy antifouling coating technology.

Progress in Studies of Surface Nanotextures and Coatings with Nanomaterials on Glass for Anti-Dust Functionality

Dust pollution presents a wide range of adverse effects to product functionalities and the quality of human life. For instance, when dust particles deposit on solar photovoltaic panels, sunlight absorption is significantly reduced, and solar-to-electrical energy conversion yield may be lowered by 51%- Conventional (manual) dust removal methods are costly, consume significant material resources, and cause irreparable damage to the solar glass surface. Therefore, it is critical to develop glass surfaces that can clean themselves or are easily cleaned by natural forces. Many approaches have been attempted to reduce dust deposition, such as developing superhydrophobic surfaces and preparing anti-static surfaces. This paper reviews the recent progress in studies of anti-dust and cleaning mechanisms or methodologies, which include investigation into micro- and nano-sized dust properties, dust deposition processes and adhesion mechanisms to surfaces, and the state-of-the-art approaches to anti-dust and easy-cleaning functions that tailor surface micro-/nanotextures, lowering surface energy via nanocoatings, and enhancing anti-static properties with nanomaterials. We compare the advantages and disadvantages of various approaches and discuss the research prospects. We envision that future research will be focused on developing transparent surfaces with multiple dust-proof functions to cope with dust-burdening operating environments.

Role of chemistry in bio-inspired liquid wettability

Chemistry and topography are the two distinct available tools for customizing different bio-inspired liquid wettability including superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superoleophilicity, and liquid infused slippery property. In nature, various living species possessing super and special liquid wettability inherently comprises of distinctly patterned surface topography decorated with low/high surface energy. Inspired from the topographically diverse natural species, the variation in surface topography has been the dominant approach for constructing bio-inspired antiwetting interfaces. However, recently, the modulation of chemistry has emerged as a facile route for the controlled tailoring of a wide range of bio-inspired liquid wettability. This review article aims to summarize the various reports published over the years that has elaborated the distinctive importance of both chemistry and topography in imparting and modulating various bio-inspired wettability. Moreover, this article outlines some obvious advantages of chemical modulation approach over topographical variation. For example, the strategic use of the chemical approach has allowed the facile, simultaneous, and independent tailoring of both liquid wettability and other relevant physical properties. We have also discussed the design of different antiwetting patterned and stimuli-responsive interfaces following the strategic and precise alteration of chemistry for various prospective applications.

Bioinspired Superspreading Surface: From Essential Mechanism to Application

ConspectusThe dynamic behavior of liquids on surfaces is ubiquitous in nature and has aroused wide attention from researchers. Among them, the superspreading surface has been extensively investigated and applied in areas ranging from film fabrication to antibiofouling, separation, etc. However, the traditional equilibrium contact angle (CA) at the thermodynamic steady-state cannot completely depict the dynamic spreading process of liquids, because the performance of these surfaces is controlled not only by the final steady superhydrophilicity (CA < 5°) but also by the superspreading speed of liquids with a CA of ∼0°. Moreover, as the most basic prerequisite for superspreading, the long-held intrinsic wetting threshold (IWT) of 90°, which divides hydrophobic and hydrophilic surfaces, is also controversial.In this Account, we summarize and condense the commonalities of our related research, further formally propose the concept of "superspreading", and recommend using "superspreading time (ST)" and "curve of superspreading radius versus spreading time (SRST)" to quantify its performance. Learning from nature is the most effective way to artificially fabricate superspreading surfaces. To begin, we first review some typical superspreading surfaces we found in nature and introduce the strategies adopted by the surfaces for surviving or realizing special functions. Then, we systematically review our recent understanding of the essential mechanism of superspreading surfaces across multiple length scales─from the molecular origin of the newly found IWT of ∼65° for water to the macroscopic respective functions of nanostructure and microstructure in superspreading. Armed with the in-depth fundamental mechanism, we propose the designing principle of high-performance superspreading surfaces. Following that, we summarize the commonly utilized methods, including modifying surface composition to give the surface intrinsic hydrophilicity and changing surface structure to improve the superspreading performance. Subsequently, we introduce the recently developed practical applications by virtue of the outstanding property of the superspreading surface, including the fabrication of a self-assembled film on the solid-gas surface and solid-liquid interface, a self-assembled water barrier for antibiofouling and oil repellency, high-efficiency separation and heat dissipation, etc. Finally, we discuss the remaining major challenges and the future development trends in the superspreading field. This Account serves to arouse wide attention and efforts in the superspreading field to strengthen mechanism research and promote practical large-area applications.

WET-Induced Layered Organohydrogel as Bioinspired "Sticky-Slippy Skin" for Robust Underwater Oil-Repellency

Underwater superoleophobic surfaces featuring anti-oil-fouling properties are of great significance in widespread fields. However, their complicated engineering process and weak interfacial adhesion strength with underlying substrates severely hamper these ideal surfaces toward practical applications. Here, a moss-inspired sticky-slippy skin composed of layered organohydrogel is reported through a one-step wetting-enabled-transfer (WET) strategy, which unprecedentedly integrates robust inherent adhesion with durable anti-oil-fouling properties. The sticky organogel layer can be simply attached to various substrates, from metals and plastics to glass, independent of any surface pretreatment. The slippy hydrogel layer enables stable underwater superoleophobicity and ultralow oil adhesion for diverse kinds of oils. The sticky-slippy skin features outstanding comprehensive properties including easy-pasting, anti-swelling/anti-bending, compatibility with commercial adhesives, acid/alkali resistance, environmental friendliness, and substrate universality. The design strategy with integrated functions provides a clue to accelerate the development of bioinspired multifunctional interfacial materials toward real-world applications.

Dually reactive multilayer coatings enable orthogonal manipulation of underwater superoleophobicity and oil adhesion via post-functionalization

Fish scale-inspired underwater superoleophobic coatings with low oil adhesion can be achieved through the creation of hierarchical surface topography on water-compatible materials (including polymeric hydrogels, metal oxides, and electrostatic multilayers). While promising, these method do not allow for the underwater superoleophobicity and oil adhesion to be independently tuned, limiting their potential applications. Here we report the design of a conceptually novel class of coatings, dually reactive multilayer coatings, whose underwater superoleophobicity and oil adhesion can be independently tuned through the orthogonal functionalization of two types of reactive moieties at ambient conditions. Moreover, the cooperative assembly of amphiphiles on the modified underwater superoleophobic coating gives rise to a switchable oil adhesion while retaining the extreme oil-repellency (advancing oil contact angle >165°). Interestingly, the reversible change in the oil adhesion of the underwater superoleophobic coatings depends on the interplay between the molecular structure and concentration of the amphiphiles and the pH of the aqueous solution. Building on these findings, we developed superoleophobic sensors that enable the real-time and naked eye identification of (1) the charge of synthetic ionic surfactants and (2) the concentration of bile acids. Overall, the results reported in this work provide design principles by which molecular self-assembly and oil adhesion can be coupled at underwater superoleophobic surfaces, and hint at principles by which physiologically important amphiphiles and metabolites can be rapidly sensed with the naked eye using our novel class of superoleophobic surfaces.

Smart surfaces with reversibly switchable wettability: Concepts, synthesis and applications

As a growing hot research topic, manufacturing smart switchable surfaces has attracted much attention in the past a few years. The state-of-the-art study on reversibly switchable wettability of smart surfaces has been presented in this systematic review. External stimuli are brought about to render the alteration in chemical conformation and surface morphology to drive the wettability switch. Here, starting from the fundamental theories related to the surfaces wetting principles, highlights on different triggers for switchable wettability, such as pH, light, ions, temperature, electric field, gas, mechanical force, and multi-stimuli are discussed. Different applications that have various wettability requirement are targeted, including oil-water separation, droplets manipulation, patterning, liquid transport, and so on. This review aims to provide a deep insight into responsive interfacial science and offer guidance for smart surface engineering. It ends with a summary of current challenges, future opportunities, and potential solutions on smart switch of wettability on superwetting surfaces.

Anti-Biofouling Polymers with Special Surface Wettability for Biomedical Applications

The use of anti-biofouling polymers has widespread potential for counteracting marine, medical, and industrial biofouling. The anti-biofouling action is usually related to the degree of surface wettability. This review is focusing on anti-biofouling polymers with special surface wettability, and it will provide a new perspective to promote the development of anti-biofouling polymers for biomedical applications. Firstly, current anti-biofouling strategies are discussed followed by a comprehensive review of anti-biofouling polymers with specific types of surface wettability, including superhydrophilicity, hydrophilicity, and hydrophobicity. We then summarize the applications of anti-biofouling polymers with specific surface wettability in typical biomedical fields both in vivo and in vitro, such as cardiology, ophthalmology, and nephrology. Finally, the challenges and directions of the development of anti-biofouling polymers with special surface wettability are discussed. It is helpful for future researchers to choose suitable anti-biofouling polymers with special surface wettability for specific biomedical applications.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

Antibiotic Zwitterionic Nanogel Membrane: from Molecular Dynamics Simulation to Structure Manipulation

Membrane separation has been considered as one of the most revolutionary technologies for the removal of oils, dyes, or other pollutants from wastewater. However, most membranes still face great challenges in water permeability, antifouling property, and even antibiotic ability. Possessing a pathogen-repellent surface is of great significance as it can enable membranes to minimize the presence of active viral pathogens. Herein, we demonstrate a distinct design with a molecular dynamics simulation-guided experiment for the surface domination of antibiotic zwitterionic nanogel membranes. The zwitterionic nanoparticle gel (ZNG)/Cu²⁺/glutaraldehyde (GA) synergy system is first simulated by introducing a ZNG into a preset CuCl₂ brine solution and into a GA ethanol solution, in which the nanogel is observed to initially swell and subsequently shrink with the increase of GA concentration, leading to the membrane surface structure transition. Then, the corresponding experiments are performed under strict conditions, and the results suggest the surface structure transition from nanoparticles to network nanoflowers, which are consistent with the simulated results. The obtained network structure membrane with superhydrophilic and underwater superoleophobic abilities can significantly enhance the water permeability as high as almost 40% with its original rejection rate in comparison with unoptimizable ZNG-PVDF (polyvinylidene difluoride) membranes. Moreover, the obtained membrane achieves additional excellent antibiofouling capacity with the antibiotic efficiency exceeding 99.3%, manifesting remarkable potential for disinfection applications. By comparison, the conventional antibiotic methods generally improve the membrane's antibiotic property solely but can hardly improve the other properties of the membrane. That is to say, our simulation combined with the experimental strategy significantly improved the zwitterionic membrane property in this work, which provides a new perspective on the design of high-performance functional materials.

Marine antifouling coatings with surface topographies triggered by phase segregation

Marine biofouling is a ubiquitous and longstanding challenge that causes both economic and environmental problems. To address this, several antifouling strategies have been proposed, such as the release of biocidal compounds or surface chemical/physical design. Here we report a coating with surface structures (chemical heterogeneity) triggered by phase segregation, which endues the good antifouling properties, alongside robust mechanical properties, low underwater oil adhesion, and excellent optical transparency. This is achieved by arranging the hydrophobic and hydrophilic components to control the assembly and phase separation under the cross-linking and localized swelling process. The structure designs are based on the poly(ethylene glycols) (PEG), zwitterions, and hydrophobic components, which may lower the entropic and enthalpic driving forces for the adsorption of the marine organisms. Our approach could provide an effective way of manufacturing novel coating with amphiphilic micro/nanodomains structure, particularly for the marine industry. And we also showed that the coatings were stable under different temperatures and shear environments. To illustrate the applicability of such a robust coating in marine biofouling, we demonstrated significantly reduced algal adhesion and barnacle attachment in the sea (p < 0.01). We envision that this work will provide great potential for the application in antifouling marine coatings.

One-Step Versatile Fabrication of Superhydrophilic Filters for the Efficient Purification of Oily Water

As industrial oily wastewater can seriously damage ecosystems, the use of filtration technology with functional filters has emerged as an effective approach for purifying oily wastewater and protecting the environment. Although several methods for preparing functional filters with specific wettability have been reported, most methods are complicated, expensive, and time-consuming. Furthermore, these methods are only applicable to specific substrates, which hinder their practical applications. Here, a simple and versatile method for the fabrication of a superhydrophilic filter on any substrate using a one-step dipping process is reported. The method is easily scaled-up to fabricate large-area superhydrophilic filters; moreover, mass production is possible using a roll-to-roll process. The resulting filter is durable, stable, and, due to its stable hydrophilic layer, shows no deterioration in wetting behavior; it also exhibits self-cleaning properties. Based on its selective wetting characteristics, oil/water mixtures and oil-in-water emulsions stabilized by surfactants can be purified in a highly efficient manner. Importantly, owing to its self-cleaning properties, the filter can be reused after simply immersing and washing in water. This easy, cost-effective, fast, and versatile method for fabricating superhydrophilic filters can be practically applied in industries that need to purify oily water.

Amphiphilic Marine Antifouling Coatings Based on a Hydrophilic Polyvinylpyrrolidone and Hydrophobic Fluorine-Silicon-Containing Block Copolymer

The development of environmentally friendly and highly efficient antifouling coatings is vastly desirable in the marine industry. Herein, we prepared a novel amphiphilic block copolymer that combined hydrophilic polyvinylpyrrolidone (PVP) with hydrophobic poly(1-(1H,1H,2H,2H-perfluorodecyloxy)-3-(3,6,9-trioxadecyloxy)-propan-2-yl acrylate) (PFA) and polydimethylsiloxane (PDMS). The amphiphilic copolymer (PVP-PFA-PDMS) was blended into a cross-linked PDMS matrix to form a set of controlled surface composition and surface-renewal coatings with efficient antifouling and fouling-release properties. These coatings incorporated the biofouling settlement resistance ability attributed to the hydrophilic PVP segments and the reduced adhesion strength attributed to the low surface energy of fluorine-silicon-containing segments. As expected, the coatings showed an excellent antifouling performance against bacteria and marine unicellular Navicula parva diatoms (98.1 and 98.5% of reduction, respectively) and fouling-release performance against pseudobarnacle adhesion (84.3% of reduction) compared to the pristine PDMS coating. Moreover, a higher-content PVP-based coatings presented higher ability to resist biofouling adhesion. The nontoxic antifouling coating developed in this paper hold the potential to be applied in a variety of marine industrial facilities.

Recent developments in stimuli responsive nanomaterials and their bionanotechnology applications

Bionanotechnology is an ever-expanding field as innovations in nanotechnology continue to be developed based on biological systems or to be applied to address unmet needs in biology, biomedicine, etc., including various sensor and drug delivery solutions. Amidst the wide range of bionanomaterials that have been developed, stimuli responsive bionanomaterials are of particular interest and are thus emphasized within this review. Here, we have highlighted the most recent advances for stimuli responsive bionanomaterials with focus on those possessing responses based on activation, expansion/contraction and self-assembly/disassembly. The aim of this review is to bring attention to some of the most current bionanotechnology research and the interesting applications within this field.

Biosurfaces Fabricated by Polymerization-Induced Surface Self-Assembly

Surface biofunctionalization provides an approach to the fabrication of surfaces with improved biological and clinical performances. Biosurfaces have found increasing applications in many areas such as sensing, cell growth, and disease detection. Efficient synthesis of biosurfaces without damages to the structures and functionalities of biomolecules is a great challenge. Polymerization-induced surface self-assembly (PISSA) provides an effective approach to the synthesis of surface nanostructures with different compositions, morphologies, and properties. In this research, application of PISSA in the fabrication of biosurfaces is investigated. Two different reversible addition-fragmentation chain transfer (RAFT) agents, RAFT chain transfer agent (CTA) on silica particles (SiO₂-CTA) and CTA on bovine serum albumin (BSA-CTA), were employed in RAFT dispersion polymerization of N-isopropylacrylamide (NIPAM) in water at a temperature above the lower critical solution temperature (LCST) of poly-(isopropylacrylamide) (PNIPAM). After polymerization, PNIPAM layers with BSA on the top surfaces are fabricated on the surfaces of silica particles. Transmission electron microscopy results show that the average PNIPAM layer thickness increases with monomer conversion. Kinetics study indicates that there is a turn point on a plot of ln([M]₀/[M]_t) versus polymerization time. After the critical point, surface coassembly of PNIPAM brushes and BSA-PNIPAM bioconjugates is performed on the silica particles. The secondary structure and the activity of BSA immobilized on top of the PNIPAM layers are basically kept unchanged in the PISSA process. To prepare permanently immobilized protein surfaces, PNIPAM layers on silica particles are cross-linked. BSA on the top surfaces presents a reversible "on-off" switching property. At a temperature below the LCST of PNIPAM, the activity of the immobilized BSA is retained; however, the BSA activity decreases significantly at a temperature above the LCST because of the hydrophobic interaction between PNIPAM and BSA. Based on this approach, many different biosurfaces can be fabricated and the materials will find applications in many fields, such as enzyme immobilization, drug delivery, and tissue engineering.

The toolbox of porous anodic aluminum oxide–based nanocomposites: from preparation to application

Anodic aluminum oxide (AAO) templates have been intensively investigated during the past decades and have meanwhile been widely applied through both sacrificial and non-sacrificial pathways. In numerous non-sacrificial applications, the AAO membrane is maintained as part of the obtained composite materials; hence, the template structure and topography determine to a great extent the potential applications. Through-hole isotropic AAO features nanochannels that promote transfer of matter, while anisotropic AAO with barrier layer exhibits nanocavities suitable as independent and homogenous containers. By combining the two kinds of AAO membranes with diverse organic and inorganic materials through physical interactions or chemical bonds, AAO composites are designed and applied in versatile fields such as catalysis, drug release platform, separation membrane, optical appliances, sensors, cell culture, energy, and electronic devices. Therefore, within this review, a perspective on exhilarating prospect for complementary advancement on AAO composites both in preparation and application is provided.

Advances in biomolecule inspired polymeric material decorated interfaces for biological applications

With the development of surface modification technology, interface properties have great effects on the interaction between biomedical materials and cells and biomolecules, which significantly affects the biocompatibility and functionality of materials. As an orderly and perfect system, biological organisms in nature effectively integrate all kinds of bio-interfaces with physiological functions, which shed light on the importance of biomolecules in organisms. It gives birth to a bio-inspiration strategy to design and fabricate smart materials with specific functionalities, e.g. osteogenic and chondrocytic induced materials inspired by bone sialoprotein and chondroitin sulfate. Through this mimicking approach, various functional materials were utilized to decorate the interfaces and further optimize the performance of biomedical materials, which would widely expand their applications. In this review, followed by a summary and brief introduction of surface modification methods, we highlight recent advances in the fabrication of functional polymeric materials inspired by a range of biomolecules for decorating interfaces. Then, the other applications of biomolecule inspired materials including tissue engineering, diagnosis and treatment of diseases and physiological function regulation are presented and the future outlook is discussed as well.

A stimuli-responsive gel impregnated surface with switchable lipophilic/oleophobic properties

In this paper, we developed a novel morphing surface technique consisting of a 3D printed miniature groove structure and injected stimuli-responsive hydrogel pattern, which is capable of switching between lipophilicity and oleophobicity under certain stimuli. Under swelling, the geometrical change of the hydrogel will buckle the surface due to the structural confinement and create a continuous transition of surface topology. Thus, it will yield a change in the surface wetting property from oleophilic to super-oleophobic with a contact angle of oil of 85° to 165°. We quantitatively investigate this structure-property relationship using finite element analysis and analytical modeling, and the simulation results and the modeling are in good agreement with the experimental ones. This morphing surface also holds potential to be developed into an autonomous system for future sub-sea/off-shore engineering applications to separate oil and water.

Protein-resistant surface based on zwitterion-functionalized nanoparticles for marine antifouling applications

A functional nanocomposite (PSBMA@VTMO@silica/TA-PEG) coating was successfully developed through a facile deposition method, and was able to effectively block the adhesion of proteins and subsequent biofouling deposition.

Bioinspired surfaces with wettability for antifouling application

Wettability is a special character found in nature, including the superhydrophobicity of lotus leaves, the underwater superoleophobicity of fish scales and the slipperiness of pitcher plants. These surfaces exhibit unique properties such as resistance to icing, corrosion, and the like. The antifouling properties of the material surface have important applications in a variety of areas, such as in hulls, in medical equipment, in water pipes and underwater equipment. However, the traditional anti-fouling surface is usually combined with toxic substances or its manufacturing process is complicated and expensive, which cannot meet the current antifouling demand. These wettable surfaces have always exhibited good anti-biofouling and self-cleaning properties, and their use as antifouling surfaces can well solve the problems of the above-mentioned traditional antifouling surfaces. Here, we divided the wettable surfaces into superhydrophobic surfaces, underwater superoleophobic surfaces and slippery surfaces, respectively, summarizing their development in the field of antifouling. Their research progress in antibacterial, antibiotic flocculation and antiplatelet adhesion is highlighted. Furthermore, we provide our own insights into the shortcomings and development prospects of wettable surface applications in the field of antifouling.

NIR-Triggered Photothermal Responsive Coatings with Remote and Localized Tunable Underwater Oil Adhesion

Tunable underwater oil adhesion is a critical issue in interfacial science and industrial applications. Although much progress has been made to date, development of novel smart coating materials that can selectively change the wetting property at different areas is considerably scarce. Here, a simple strategy is proposed to fabricate photothermal responsive coatings, which can change the oil adhesion behavior from low-adhesive rolling state to high-adhesive pinning state for a variety of oily liquids in a remote, local, and reversible manner. Owing to this unique controllability, the adhesion and no-adhesion of oil droplets on the coated surfaces can be easily manipulated by remote and local near-infrared radiation.

Synthesis of poly(acrylic acid) coated magnetic nanospheres via a multiple polymerization route

Magnetic nanospheres are versatile candidates for both fundamental and practical applications. Before they are applied in more complicated fields, their surface must be modified by several functionalities. However, the surface modification can be affected by the magnetic nanoparticles (MNP) embedded in the polymer matrix. Herein, the synthesis of poly(acrylic acid) coated magnetic nanospheres via a multiple polymerization route is described. During the synthesis process, seed emulsion polymerization was applied to redistribute the MNP in the polymer matrix, and the relationship between the structure of magnetic nanospheres and the thickness of the grafted poly(acrylic acid) layer was investigated. The development of size, morphology and magnetic properties of the nanospheres were characterized by transmission electron microscopy, dynamic light scattering, thermogravimetric analysis, X-ray diffraction and vibrating sample magnetometry. This work would pave the way to design and preparation of new structure of functional magnetic nanospheres with precise surface modification.

Combat biofouling with microscopic ridge-like surface morphology: a bioinspired study

Biofouling refers to the unfavourable attachment and accumulation of marine sessile organisms (e.g. barnacles, mussels and tubeworms) on the solid surfaces immerged in ocean. The enormous economic loss caused by biofouling in combination with the severe environmental impacts induced by the current antifouling approaches entails the development of novel antifouling strategies with least environmental impact. Inspired by the superior antifouling performance of the leaves of mangrove tree Sonneratia apetala, here we propose to combat biofouling by using a surface with microscopic ridge-like morphology. Settlement tests with tubeworm larvae on polymeric replicas of S. apetala leaves confirm that the microscopic ridge-like surface morphology can effectively prevent biofouling. A contact mechanics-based model is then established to quantify the dependence of tubeworm settlement on the structural features of the microscopic ridge-like morphology, giving rise to theoretical guidelines to optimize the morphology for better antifouling performance. Under the direction of the obtained guidelines, a synthetic surface with microscopic ridge-like morphology is developed, exhibiting antifouling performance comparable to that of the S. apetala replica. Our results not only reveal the underlying mechanism accounting for the superior antifouling property of the S. apetala leaves, but also provide applicable guidance for the development of synthetic antifouling surfaces.

Wetting Criteria of Intrinsic Contact Angle To Distinguish between Hydrophilic and Hydrophobic Micro-/Nanotextured Surfaces: Experimental and Theoretical Analysis with Synchrotron X-ray Imaging

In this study, the existing knowledge on the wetting criterion, that is, the intrinsic contact angle, for distinguishing between hydrophilic and hydrophobic textured surfaces is verified experimentally. A precise apparent contact angle is measured on micro-, nano-, and micro-/nanotextured surfaces to quantitatively define the surface-wetting conditions. In particular, X-ray tomography is introduced to measure precise geometric morphologies of nano- and micro-/nanotextured surfaces, and the wetting state of the textured surfaces is clearly visualized using synchrotron X-ray imaging. By comparing previous theoretical models and experimental results, it is verified that the intrinsic contact angle for distinguishing between hydrophilic and hydrophobic textured surfaces should be corrected from 90° to 43°. In addition, nonwetting phenomena in the region of the intrinsic contact angle between 43° and 90° are discussed.

A 64-pin Nanowire Surface Fastener Like a Ball Grid Array Applied for Room-temperature Electrical Bonding

Surface-mount techniques primarily depend on soldering. However, soldering techniques have encountered some challenges in recent years. These challenges include rare metal recycling, thermal problems, and Pb toxicity. We recently developed a metallic nanowire surface fastener (NSF) to resolve the abovementioned problems. This fastener can be used to connect electronic components on a substrate at room temperature using the van der Waals force between each nanowire. This study demonstrates a 64-pin NSF that behaves like a ball grid array (BGA) for application to actual electronic devices. The adhesion strength and electrical properties of the NSF were investigated by adjusting the nanowire parameters, such as diameter, length, density (number per area), preload, and shape. The shape control of the nanowires greatly contributed to the improvement of the properties. A maximum adhesion strength of 16.4 N/cm² was achieved using a bent, hook-like NSF. This strength was 4-5 times the value of the straight NSF. The contact resistivity was 2.98 × 10^-2 Ω∙cm². The NSF fabricated through the simple template method showed the room temperature bonding ability and adaptability to a highly ordered electrode like the BGA.

Smart, Piezo-Responsive Polyvinylidenefluoride/Polymethylmethacrylate Surface with Triggerable Water/Oil Wettability and Adhesion

The design of smart surfaces with externally triggerable water/oil wettability and adhesion represents one of the most up-to-date challenges in the field of material science. In this work, the intelligent surface with electrically triggerable wettability and water/oil adhesion is presented. As a basic material background exhibiting electric field (EF) sensitivity, the piezo-responsive polymethylmethacrylate/polyvinylidenefluoride polymer fibers were used. To expand the available range of water/oil contact angles (CAs) and adhesion, the fibers were grafted with hydrophilic or hydrophobic functional groups using diazonium chemistry. The fiber functionality was evaluated using the static CA and wettability hysteresis measurements (increasing/decreasing drop volume and tilting angles), drops adhesion/repellence and graphite self-cleaning test performed with and without the application of EF. It was found that the proposed method enables tuning the surface wettability in the superhydrophobic/superoleophobic-hydrophilic/oleophilic range and changing of surface properties from low adhesive to high adhesive for water and oil. More convincing results were achieved in the case of fiber surface modification by ADT-C₈F₁₇, which may result from a rearrangement of the grated -C₆H₄C₈F₁₇ functional group under the application of EF triggering. Moreover, the triggering which can be performed in the extremely fast way (the surface responds to the EF switching on/off in seconds) was found to be fully reversible. Finally, the additional tests indicate the satisfactory stability of created fiber-based coating against the mechanical treatment.

Nanoporous Anodic Alumina Photonic Crystals for Optical Chemo- and Biosensing: Fundamentals, Advances, and Perspectives

Optical sensors are a class of devices that enable the identification and/or quantification of analyte molecules across multiple fields and disciplines such as environmental protection, medical diagnosis, security, food technology, biotechnology, and animal welfare. Nanoporous photonic crystal (PC) structures provide excellent platforms to develop such systems for a plethora of applications since these engineered materials enable precise and versatile control of light⁻matter interactions at the nanoscale. Nanoporous PCs provide both high sensitivity to monitor in real-time molecular binding events and a nanoporous matrix for selective immobilization of molecules of interest over increased surface areas. Nanoporous anodic alumina (NAA), a nanomaterial long envisaged as a PC, is an outstanding platform material to develop optical sensing systems in combination with multiple photonic technologies. Nanoporous anodic alumina photonic crystals (NAA-PCs) provide a versatile nanoporous structure that can be engineered in a multidimensional fashion to create unique PC sensing platforms such as Fabry⁻Pérot interferometers, distributed Bragg reflectors, gradient-index filters, optical microcavities, and others. The effective medium of NAA-PCs undergoes changes upon interactions with analyte molecules. These changes modify the NAA-PCs' spectral fingerprints, which can be readily quantified to develop different sensing systems. This review introduces the fundamental development of NAA-PCs, compiling the most significant advances in the use of these optical materials for chemo- and biosensing applications, with a final prospective outlook about this exciting and dynamic field.

Nanowire surface fastener fabrication on flexible substrate

The market for wearable devices has increased considerably in recent years. In response to this demand, flexible electronic circuit technology has become more important. The conventional bonding technology in electronic assembly depends on high-temperature processes such as reflow soldering, which result in undesired thermal damages and residual stress at a bonding interface. In addition, it exhibits poor compatibility with bendable or stretchable device applications. Therefore, there is an urgent requirement to attach electronic parts on printed circuit boards with good mechanical and electrical properties at room temperature. Nanowire surface fasteners (NSFs) are candidates for resolving these problems. This paper describes the fabrication of an NSF on a flexible substrate, which can be used for room temperature conductive bonding. The template method is used for preparing high-density nanowire arrays. A Cu thin film is layered on the template as the flexible substrate. After etching the template, a Cu NSF is obtained on the Cu film substrate. In addition, the electrical and mechanical properties of the Cu NSF are studied under various fabrication conditions. The Cu NSF exhibits high shear adhesion strength (∼234 N cm^-2) and low contact resistivity (2.2 × 10^-4 Ω cm²).

Thermoresponsive Arrays Patterned via Photoclick Chemistry: Smart MALDI Plate for Protein Digest Enrichment, Desalting, and Direct MS Analysis

Sample desalting and concentration are crucial steps before matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) analysis. Current sample pretreatment approaches require tedious fabrication and operation procedures, which are unamenable to high-throughput analysis and also result in sample loss. Here, we report the development of a smart MALDI substrate for on-plate desalting, enrichment, and direct MS analysis of protein digests based on thermoresponsive, hydrophilic/hydrophobic transition of surface-grafted poly(N-isopropylacrylamide) (PNIPAM) microarrays. Superhydrophilic 1-thioglycerol microwells are first constructed on alkyne-silane-functionalized rough indium tin oxide substrates based on two sequential thiol-yne photoclick reactions, whereas the surrounding regions are modified with hydrophobic 1H,1H,2H,2H-perfluorodecanethiol. Surface-initiated atom-transfer radical polymerization is then triggered in microwells to form PNIPAM arrays, which facilitate sample loading and enrichment of protein digests by concentrating large-volume samples into small dots and achieving on-plate desalting through PNIPAM configuration change at elevated temperature. The smart MALDI plate shows high performance for mass spectrometric analysis of cytochrome c and neurotensin in the presence of 1 M urea and 100 mM NaHCO₃, as well as improved detection sensitivity and high sequence coverage for α-casein and cytochrome c digests in femtomole range. The work presents a versatile sample pretreatment platform with great potential for proteomic research.

External-Field-Induced Gradient Wetting for Controllable Liquid Transport: From Movement on the Surface to Penetration into the Surface

External-field-responsive liquid transport has received extensive research interest owing to its important applications in microfluidic devices, biological medical, liquid printing, separation, and so forth. To realize different levels of liquid transport on surfaces, the balance of the dynamic competing processes of gradient wetting and dewetting should be controlled to achieve good directionality, confined range, and selectivity of liquid wetting. Here, the recent progress in external-field-induced gradient wetting is summarized for controllable liquid transport from movement on the surface to penetration into the surface, particularly for liquid motion on, patterned wetting into, and permeation through films on superwetting surfaces with external field cooperation (e.g., light, electric fields, magnetic fields, temperature, pH, gas, solvent, and their combinations). The selected topics of external-field-induced liquid transport on the different levels of surfaces include directional liquid motion on the surface based on the wettability gradient under an external field, partial entry of a liquid into the surface to achieve patterned surface wettability for printing, and liquid-selective permeation of the film for separation. The future prospects of external-field-responsive liquid transport are also discussed.

Underwater Transparent Miniature "Mechanical Hand" Based on Femtosecond Laser-Induced Controllable Oil-Adhesive Patterned Glass for Oil Droplet Manipulation

Development of underwater superoleophobic surfaces has captured the imagination of researchers because of their applications; especially, oil manipulation based on such surfaces has attracted much attention. Here, we show a simple and effective way to fabricate an underwater transparent miniature "mechanical hand" based on controllable oil-adhesive patterned glass using a femtosecond laser. The underwater oil-adhesive force of the patterned glasses that compose the "mechanical hand" device can be controlled from ultralow to ultrahigh by adjusting the ratio of the untreated flat glass area to the laser-ablated rough area. These surfaces also showed favorable transparency in water. Various oils such as chloroform, hexadecane, n-dodecane, decane, liquid paraffin, and petroleum ether were tested, and their repellency against the as-prepared surfaces in water medium was confirmed. Moreover, the "mechanical hand" was used to implement oil transportation, fusion, and rapid capture, which can be applied in the construction of microfluidic devices, in situ detectors, and bioreactors.