A Universal Coating Strategy for Controllable Functionalized Polymer Surfaces

Development of a universal and stable surface coating, irrespective of surface chemistry or material characteristics, is highly desirable but has proved to be extremely challenging. Conventional coating strategies including the commonly used catechol surface coating are limited to either a certain type of substrates or weak and unreliable surface bonding. Here, a simple, robust, and universal surface coating method capable for attaching any stimuli‐responsive glycidyl methacrylate (GMA)‐based copolymer, consisting of one surface‐adhesive moiety of epoxy groups and one stimuli‐responsive moiety, to any type of hydrophobic and hydrophilic surfaces via a one‐step ring‐opening reaction is proposed and demonstrated. The resultant GMA‐based copolymers are not only strongly adhered on different substrates (e.g., silicon, polypropylene, polyvinyl chloride, indium tin oxide, polyethylene terephthalate, aluminum, glass, polydimethylsiloxane, and even polyvinylidene fluoride with low surface energy), but also are possessed distinct thermal‐, pH‐, and salt‐responsive functions of bacterial killing, bacterial releasing, tunable multicolor fluorescence emission, and heavy metal detection. This coating method is also compatible with the directional quaternization of GMA‐based copolymers for further improving surface adhesion and functionality. This study provides a simple yet universal coating method to solve the long‐standing challenge of robust integration of stimuli‐responsive polymers with strong adhesion between various polymers and substrates.     

Photoresponsive Sponge‐Like Coating for On‐Demand Liquid Release

Many publications report on stimuli responsive coatings, but only a few on the controlled release of species in order to change the coating surface properties. A sponge‐like coating that is able to release and absorb a liquid upon exposure to light has been developed. The morphology of the porous coating is controlled by the smectic liquid crystal properties of the monomer mixture prior to its polymerization, and homeotropic order is found to give the largest contraction. The fast release of the liquid can be induced by a macroscopic contraction of the coating caused by a trans to cis conversion of a copolymerized azobenzene moiety. The liquid secretion can be localized by local light exposure or by creating a surface relief. The uptake of liquid proceeds by stimulating the back reaction of the azo compound by exposure at higher wavelength or by thermal relaxation. The surface forces of the sponge‐like coating in contact with an opposing surface can be controlled by light‐induced capillary bridging revealing that the controlled release of liquid gives access to tunable adhesion.     

Tunable Photonic Materials via Monitoring Step‐Growth Polymerization Kinetics by Structural Colors

The functional and responsive properties of elastomeric materials highly depend on crosslink density and molecular weight between crosslinks. However, tedious analytical steps are needed to obtain polymer network structure–property relationships. In this article, an in situ structure–property characterization method is reported by monitoring the structural color change in a photonic elastomeric material. The photonic materials are prepared in a two‐step polymerization process. First, linear chain extension occurs via Michael addition. Second, photopolymerization ensures crosslinking, resulting in the formation of an elastomeric photonic network. During the first step, the step‐growth polymer process can be monitored by following the photonic reflection band redshift, allowing to program the molecular weight between the crosslinks. During network formation, the crosslink density, chain length between crosslinks, and the colors are “frozen in.” These processes can be locally controlled creating both single‐layered multicolor patterned and broadband reflective coatings at room temperature. The scalability of the coating process is further demonstrated by using a gravure printing technique. Additionally, the final coatings are made responsive toward specific solvents and temperature. Here the modulus, response, and color of the coating are controlled by tuning the crosslink density and molecular weight between crosslinks of the elastomeric material.     

Self‐Healing Polyurethanes with Shape Recovery

Two new thermoresponsive self‐healing polyurethanes (1DA1T and 1.5DA1T) based on the Diels–Alder (DA) reaction between furan and maleimide moieties are developed that use the shape‐memory effect to bring crack faces into intimate contact such that healing can take place. Unlike other self‐healing polymers, these polymers do not require external forces to close cracks but rather they use the shape‐memory effect to autonomously close the crack. Both polyurethanes have a stable polymer structure and comparable mechanical properties to commercial epoxies. A differential scanning calorimeter is employed to check the glass transition temperature of the polymers as well as the DA and retro‐DA (rDA) reaction temperatures. These DA and rDA reactions are confirmed with variable‐temperature proton nuclear magnetic resonance. Healing efficiency is calculated using a measurement of the failure load from compact tension testing. The results show that the shape‐memory effect can replace external forces to close two crack surfaces and the DA reaction can be repeatedly employed to heal the cracks.     

Toward the Effective Exploitation of Topological Phases of Matter in Catalysis: Chemical Reactions at the Surfaces of NbAs and TaAs Weyl Semimetals

By means of density functional theory and experiments, surface chemical reactivity of single crystals of NbAs and TaAs Weyl semimetals is studied. Weyl semimetals exhibit outstanding reactivity toward simple molecules (oxygen, carbon monoxide, and water), with several active sites available for surface chemical reactions (adsorption, decomposition, formation of reaction products, recombination of decomposition fragments). When different chemical species are adsorbed on Weyl semimetals, strong lateral interactions between coadsorbed species occur, evidenced by CO‐promoted water decomposition at room temperature. The resulting ? OH groups react with CO to form HCOO, which is an intermediate species in water–gas shift reaction. These findings unambiguously demonstrate that Weyl semimetals could be effectively used in catalysis, whereas their employment in nanoelectronics or plasmonics is complicated by the poor ambient stability, due to the rapid surface oxidation, inevitably occurring unless protective capping layers are used.     

Stimuli‐Responsive Materials Based on Interpenetrating Polymer Liquid Crystal Hydrogels

Stimuli‐responsive materials based on interpenetrating liquid crystal‐hydrogel polymer networks are fabricated. These materials consist of a cholesteric liquid crystalline network that reflects color and an interwoven poly(acrylic acid) network that provides a humidity and pH response. The volume change in the cross‐linked hydrogel polymer results in a dimensional alteration in the cholesteric network as well, which, in turn, leads to a color change yielding a dual‐responsive photonic material. Furthermore a patterned coating having responsive and static interpenetrating polymer network areas is produced that changes both its surface topography and color.     

Easily Processable and Programmable Responsive Semi‐Interpenetrating Liquid Crystalline Polymer Network Coatings with Changing Reflectivities and Surface Topographies

The fabrication of stimulus‐responsive coatings that change both reflectivity and topography is hampered by the lack of easy processable, patternable, and programmable elastomers. Here, an easily applied reflective coating based on a semi‐interpenetrating polymer network composed of a liquid crystal elastomer and a liquid crystal network (>15 wt%) is reported. The reflective wavelength of these polysiloxane elastomer photonic coatings can be readily programed by the concentration of chiral reactive mesogen dopant that forms the network. The coatings show a fast and reversible decrease in reflection band intensity with increasing temperature, which can be tuned by the polymer network density. In addition, hierarchical surface relief structures are prepared, which can be reversibly changed with temperature.     

Fabrication of All‐Water‐Based Self‐Repairing Superhydrophobic Coatings Based on UV‐Responsive Microcapsules

Superhydrophobic coatings that are also self‐healing have drawn much attention in recent years for improved durability in practical applications. Typically, the release of the self‐healing agents is triggered by temperature and moisture change. In this study, UV‐responsive microcapsules are successfully synthesized by Pickering emulsion polymerization using titania (TiO₂) and silica (SiO₂) nanoparticles as the Pickering agents to fabricate all‐water‐based self‐repairing, superhydrophobic coatings. These coatings are environmentally friendly and can be readily coated on various substrates. Compared to conventional superhydrophobic coatings, these coatings can regenerate superhydrophobicity and self‐cleaning ability under UV light, mimicking the outdoor environment, after they are mechanically damaged or contaminated with organics. They can maintain the superhydrophobicity after multiple cycles of accelerated weathering tests.     

Layer‐by‐Layer Assembly of Light‐Responsive Polymeric Multilayer Systems

Layer‐by‐Layer (LbL) assembly is a simple and highly versatile method to modify surfaces and fabricate robust and highly‐ordered nanostructured coatings over almost any type of substrate. Such versatility enables the incorporation of a plethora of building blocks, including materials exhibiting switchable properties, in a single device through a multitude of complementary intermolecular interactions. Switchable materials may undergo reversible physicochemical changes in response to a variety of external triggers. Although most of the works in the literature have been focusing on stimuli‐responsive materials that are sensitive to common triggers such as pH, ionic strength, or temperature, much less has been discussed on LbL systems which are sensitive to non‐invasive and easily controlled light stimulus, despite its unique potential. This review provides a deep overview of the recent progresses achieved in the design and fabrication of light‐responsive LbL polymeric multilayer systems, their potential future challenges and opportunities, and possible applications. Many examples are given on light‐responsive polymeric multilayer assemblies built from metal nanoparticles, functional dyes, and metal oxides. Such stimuli‐responsive functional materials, and combinations among them, may lead to novel and highly promising nanostructured smart functional systems well‐suited for a wide range of research fields, including biomedicine and biotechnology.     

Diving–Surfacing Smart Locomotion Driven by a CO2‐Forming Reaction,with Applications to Minigenerators

Harvesting energy from environment has attracted increasing attention for its potential applications in fabricating minigenerator. However, most studies in the fabrication of mini‐ or nanogenerators are based on the concept of piezoelectricity or triboelectrification while few of the reports paid attention to the classical theory of Faraday's law. Herein, a pH responsive smart surface is combined with the reaction between CaCO₃ and HCl to develop a new minigenerator, which can convert mechanical energy generated from the chemical reaction into electrical energy through cutting magnetic lines with moving conductive lines. The conductive lines are connected with a smart device consisting of a pH‐responsive cube, a hydrophobic cube, and a quartz cell window; the device can perform diving‐surfacing cycled motions with an intelligent initiation through the adjustment of the solution. The device can surface through gathering CO₂ bubbles from the reaction between CaCO₃ and HCl and dive by releasing the bubbles on the water/air interface. Moreover, the results demonstrate that the inert CO₂ was nonhazardous to the smart surfaces, which is meaningful for durable electricity generation.     

Recent Progress and Future Directions of Multifunctional (Super)Wetting Smooth/Structured Surfaces and Coatings

Research on superwetting surfaces/coatings that artificially mimic biological surfaces/systems has a long history, and still garners significant worldwide interest as it is expected to provide superior solutions to conventional engineering approaches that attempt to solve challenges facing mankind. To broaden the utility of these superwetting surfaces/coatings, there is a strong demand for these surfaces to exhibit multiple practical functionalities. Here, the progress being made in multifunctional surfaces with superwettability is explored. In each section, state‐of‐the‐art works are summarized and the concepts, materials, processes, and the effects of both physical (smooth or structured surfaces) and chemical (low or high surface energies) factors on the resulting surface are described. Finally, the outlook of this prospective research field is considered, and its future directions briefly discussed, with a focus on preserving longevity in both functionality and structural integrity to produce truly useful biomimetic surfaces/coatings.     

Stimuli‐Responsive Thin Coatings Using Elastin‐Like Polymers for Biomedical Applications

Smart thin coatings using a recombinant elastin‐like polymer (ELP) containing the cell attachment sequence arginine–glycine–(aspartic acid) (RGD) are fabricated for the first time through simple deposition of the ELP dissolved in aqueous‐based solutions. The biopolymer is produced and characterized using electrophoresis and mass spectroscopy. The temperature and pH responsiveness are assessed by aggregate size measurements and differential scanning calorimetry. The deposition of the studied ELP onto chitosan is followed in situ with a quartz‐crystal microbalance with dissipation monitoring (QCM‐D). Contact angle measurements are performed at room temperature and at 50 °C, showing reversible changes from a moderate hydrophobic behavior to an extremely wettable surface. AFM analysis performed at room temperature reveals a smooth surface and no organized structure. At 50 °C, the surface presents spherical nanometer‐sized structures of collapsed biopolymer chains. Such results suggest that the ELP chains, when collapsed, aggregate into micelle‐like structures at the surface of the substrate, increasing its water affinity. Cell adhesion tests on the developed coatings are conducted using a SaOS‐2 cell line. Enhanced cell adhesion could be observed in the H‐RGD6‐coated surfaces, as compared with the original chitosan monolayer. An intermediate behavior is found in chitosan coated with the corresponding ELP without the RGD sequence. Therefore, the developed films have great potential as biomimetic coatings of biomaterials for different biomedical applications, including tissue engineering and controlled delivery of bioactive agents. Their thermo‐responsive behavior can also be exploited for tunable cell adhesion and controlled protein adsorption.     

Metal‐Based Nanomaterials in Biomedical Applications: Antimicrobial Activity and Cytotoxicity Aspects

Microbial colonization on material surfaces is ubiquitous. Biofilms derived from surface‐colonized microbes pose serious problems to the society from both an economical perspective and a health concern. Incorporation of antimicrobial nanocompounds within or on the surface of materials, or by coatings, to prevent microbial adhesion or kill the microorganisms after their attachment to biofilms, represents an important strategy in an increasingly challenging field. Over the last decade, many studies have been devoted to preparing meta‐based nanomaterials that possess antibacterial, antiviral, and antifungal activities to combat pathogen‐related diseases. Herein, an overview on the state‐of‐the‐art antimicrobial nanosized metal‐based compounds is provided, including metal and metal oxide nanoparticles as well as transition metal nanosheets. The antimicrobial mechanism of these nanostructures and their biomedical applications such as catheters, implants, medical delivery systems, tissue engineering, and dentistry are discussed. Their properties as well as potential caveats such as cytotoxicity, diminishing efficacy, and induction of antimicrobial resistance of materials incorporating these nanostructures are reviewed to provide a backdrop for future research.     

Self‐Replenishing Dual Structured Superhydrophobic Coatings Prepared by Drop‐Casting of an All‐In‐One Dispersion

Robust dual structured superhydrophobic coatings which replenish spontaneously their surface chemical composition on new multi‐scale structured surfaces, recreated upon damage, are described. The surface repair occurs at room temperature, via intrinsic elements of the coatings, all covalently bonded. These coatings can be prepared from all‐in‐one dispersions by a simple drop‐cast method, with different thicknesses and on various substrates. The critical factors to optimize the self‐replenishment are described and three main design principles are postulated. The superhydrofobicity of the coatings is maintained even after 500 abrasion cycles. The principles reported can be extended towards self‐healing other surface‐dependent functionalities, that is, anti‐bacteria, anti‐fouling, or drag‐reduction, which will maintain high performance levels all through their life‐cycle with low cost and energy demand for maintenance and surface repair.     

Biosensing Properties of TitanateNanotube Films: Selective Detection of Dopamine in the Presence of Ascorbate and Uric Acid

A novel strategy based on titanate nanotubes (TNTs) for developing an electrochemical biosensor is proposed. Stable TNT films are fabricated on glassy carbon (GC) electrodes by a casting technique. Cyclic voltammetry, electrochemical impedance spectrometry, and linear‐sweep voltammetry are used to characterize the TNT membrane‐covered GC electrodes (TNT/GCs). The TNT film is shown to demonstrate selectivity by charge exclusion. The TNT film is also shown to be capable of improving the mass transport to the electrode surface and electron transfer between dopamine (DA) and the electrode. Therefore, DA exhibits a quasireversible electrochemical reaction at the TNT/GC electrode. The voltammetric signal of DA is well resolved from those of ascorbate (AA) and uric acid (UA) at the TNT/GC electrode; therefore, DA can be selectively detected in the presence of a large excess of AA and UA at physiological pH. The linear calibration curve for DA is obtained over the concentration range 0.1–30 μM in a physiological solution that contains 0.1 mM AA and 0.3 mM UA.     

Self‐Supporting,Double Stimuli‐Responsive Porous Membranes From Polystyrene‐block‐poly(N,N‐dimethylaminoethyl methacrylate) Diblock Copolymers

Asymmetric membranes are prepared via the non‐solvent‐induced phase separation (NIPS) process from a polystyrene‐block‐poly(N,N‐dimethylaminoethyl methacrylate) (PS‐b‐PDMAEMA) block copolymer. The polymer is prepared via sequential living anionic polymerization. Membrane surface and volume structures are characterized by scanning electron microscopy. Due to their asymmetric character, resulting in a thin separation layer with pores below 100 nm on top and a macroporous volume structure, the membranes are self‐supporting. Furthermore, they exhibit a defect‐free surface over several 100 µm². Polystyrene serves as the membrane matrix, whereas the pH‐ and temperature‐sensitive minority block, PDMAEMA, renders the material double stimuli‐responsive. Therefore, in terms of water flux, the membranes are able to react on two independently applicable stimuli, pH and temperature. Compared to the conditions where the lowest water flux is obtained, low temperature and pH, activation of both triggers results in a seven‐fold permeability increase. The pore size distribution and the separation properties of the obtained membranes were tested through the pH‐dependent filtration of silica particles with sizes of 12–100 nm.     

Internally Referenced Remote Sensors for HF and Cl2 Using Reactive Porous Silicon Photonic Crystals

Remote detection of reactive analytes using optical films constructed from electrochemically prepared porous Si‐based photonic crystals is demonstrated. Porous Si samples are prepared to contain either surface oxide or surface Si‐H species, and analyte detection is based on irreversible reactions with HF_(aq) or Cl_2(g) analytes, respectively. HF dissolves silicon oxide from the porous matrix, causing an irreversible blue‐shift in the resonance peak of the photonic crystal. Cl₂ reacts with the native Si‐H species present on the surface of as‐etched porous Si to generate reactive silicon halides that evaporate from the surface and/or react with air to convert to silicon oxide. Either Cl₂‐related process reduces the net refractive index of the material that is detected as a blue shift in the spectrum. With sufficient analyte concentrations or exposure times, the spectral blue shifts are visible to the unaided eye. A portion of the porous nanostructure is filled with inert polystyrene, which acts as an internal spectral reference. The polymer fiducial protects that portion of the sensor from attack by the corrosive analytes. Reflectance spectra from both the polymer‐filled and the unfilled, reactive porous layers are acquired simultaneously. The fiducial marker also allows elimination of artifacts associated with shifts of the resonance peak upon changing the angle of incidence of the optical probe. Theoretical angle‐resolved spectra (transfer matrix method) show a good match with the experimental data. High‐temperature air or room‐temperature ozone oxidation reactions are used to prepare the HF‐reactive surface, and it is found that the ozone oxidation reaction produces a greater sensitivity to HF (LLOD of 0.1% HF in water).     

Self‐Healing and Antifouling Multifunctional Coatings Based on pH and Sulfide Ion Sensitive Nanocontainers

Application of mesoporous silica nanoparticles (MSNs) as delivery tools for self‐healing coatings is limited by spontaneous leakage and specifically responsive release of small molecular inhibitors. In this work, a pH/sulfide ion responsive release system based on MSNs using a Cu‐BTA complex forms at the openings of the mesopores into which BTA (corrosion inhibitor) and benzalkonium chloride (biocide) are loaded. The spontaneous leakage of active species is completely avoided and premature release of the loaded composition was lowered to 0.02. The responsive release begins when the pH is lower than 5 or [S^2?] is higher than 0.02 mM (about 0.6 ppm). The hybrid coating containing the responsive release system exhibits feedback self‐healing property sensitive to lowering of pH and sulfide ion concentration and, at the same time, provides a high barrier level for a long time. Due to incorporation of biocide in the release system, the coating is also provided with antifouling properties.     

Lab in a Tube: Purification,Amplification, and Detection of DNA Using Poly(2‐oxazoline) Multilayers

Fast and easy purification and amplification of DNA are prerequisites for the development of point‐of‐care diagnostics. For this reason covalent coatings of amine containing poly(2‐oxazoline)s (POx) on glass and poly(propylene) surfaces are prepared, to reversibly bind genetic material directly from biological samples. The polymer is deposited in a layer‐by‐layer process, whereas initial immobilization of macromolecules on the surface is accomplished by the use of an epoxy functionalized siloxane monolayer. Alternating treatment with polymer and cross‐linker leads to the construction of amine containing POx multilayers on the substrates. Successful deposition is investigated by confocal laser scanning microscopy (using labeled polymers), contact angle measurements, as well as reflectometric interference spectroscopy. The interaction of these layer systems with DNA regarding binding and temperature dependent release is studied using labeled genetic material. Finally, polymerase chain reaction (PCR) vessels are coated with POx layers on the inside, and used for quantitative real‐time PCR (qPCR) experiments. It is possible to bind genetic material directly from cell lysates to perform qPCR assays from surface adsorbed DNA within the same tube including amplification, as well as detection. The presented system displays an easy to use device for a point of care diagnostic.     

Enhanced Osteoblast Adhesion to Epoxide‐Functionalized Surfaces

As an alternative to expensive extracellular matrix (ECM) proteins generally applied as coatings in Petri dishes used for cell binding, an innovative system based on epoxide‐functionalized monolayers capable of protein binding is proposed. Since cells bind to material surfaces through proteins, protein‐binding surfaces should also promote cell binding. Here we investigate how the cell‐binding properties of an epoxide‐functionalized surface compares with ECM protein gel coated surfaces and tissue culture polystyrene control surfaces. Glass surfaces are functionalized with glycidoxypropyltriethoxysilane (GOPS), which results in an epoxide‐functionalized surface capable of binding proteins through an epoxide–amine reaction. Advancing contact angle measurements and atomic force microscopy measurements confirm the formation of a homogeneous GOPS monolayer. This monolayer is micropatterned with fluorescein‐labeled ECM protein gel by microcontact printing (µCP). Confocal laser scanning microscopy (CLSM) shows accurately transferred ECM protein gel micropatterns. Osteoblasts that are seeded on these micropatterned substrates show a clear preference for adhering to the epoxide‐functionalized areas. The morphology of these cultured osteoblasts is needle‐like with high aspect ratios. As controls, osteoblasts are cultured on GOPS‐functionalized surfaces, unstructured ECM protein gel surfaces, and tissue culture polystyrene (TCPS). The GOPS surfaces demonstrate a drastic increase in cell adhesion after 2 h, whilst the other tests show no adverse effects of this surface on the osteoblasts as compared to ECM and TCPS. CLSM shows healthy cell morphologies on each surface. It is demonstrated for the first time that epoxide groups outperform ECM protein gel in cell adhesion, thereby providing new routes for cost‐effective coatings that improve biocompatibility as well as exciting, new methodologies to control and direct cell adhesion.