50th Anniversary Perspective: Polymer Brushes: Novel Surfaces for Future Materials

Ultrafast Tailoring of Carbon Surfaces via Electrochemically Attached Triazolinediones

The straightforward coupling between a triazolinedione (TAD) unit and citronellyl derivatives via an Alder-ene reaction has been exploited to tailor the physicochemical surface properties of glassy carbon (GC) surfaces in an ultrafast and additive-free manner. For this purpose, we first covalently grafted a TAD precursor onto GC via electrochemical reduction of an in situ generated diazonium salt, which was then electrochemically oxidized into the desired GC-bonded TAD unit. A kinetic study of the modification of this reactive layer with an electroactive ferrocene probe proved that a complete functionalization was obtained in merely 1 minute. Further modification experiments with a fluorinated probe demonstrated that the surface properties can be swiftly tailored on demand. The different modification steps, as well as the efficiency of this strategy, were investigated by electrochemistry, contact angle goniometry, and X-ray photoelectron spectroscopy analysis.

Laccase-Mediated Grafting on Biopolymers and Synthetic Polymers: A Critical Review

Laccase-mediated grafting on lignocelluloses has gained considerable attention as an environmentally benign method to covalently modify wood, paper and cork. In recent decades this technique has also been employed to modify fibres with a polysaccharide backbone, such as cellulose or chitosan, to infer colouration, antimicrobial activity or antioxidant activity to the material. The scope of this approach has been further widened by researchers, who apply mediators or high redox potential laccases and those that modify synthetic polymers and proteins. In all cases, the methodology relies on one- or two-electron oxidation of the surface functional groups or of the graftable molecule in solution. However, similar results can very often be achieved through simple deposition, even after extensive washing. This unintended adsorption of the active substance could have an adverse effect on the durability of the applied coating. Differentiating between actual covalent binding and adsorption is therefore essential, but proves to be challenging. This review not only covers excellent research on the topic of laccase-mediated grafting over the last five to ten years, but also provides a critical comparison to highlight either the lack or presence of compelling evidence for covalent grafting.

Biologically Complex Planar Cell Plasma Membranes Supported on Polyelectrolyte Cushions Enhance Transmembrane Protein Mobility and Retain Native Orientation

Reconstituted supported lipid bilayers (SLB) are widely used as in vitro cell-surface models because they are compatible with a variety of surface-based analytical techniques. However, one of the challenges of using SLBs as a model of the cell surface is the limited complexity in membrane composition, including the incorporation of transmembrane proteins and lipid diversity that may impact the activity of those proteins. Additionally, it is challenging to preserve the transmembrane protein native orientation, function, and mobility in SLBs. Here, we leverage the interaction between cell plasma membrane vesicles and polyelectrolyte brushes to create planar bilayers from cell plasma membrane vesicles that have budded from the cell surface. This approach promotes the direct incorporation of membrane proteins and other species into the planar bilayer without using detergent or reconstitution and preserves membrane constituents. Furthermore, the structure of the polyelectrolyte brush serves as a cushion between the planar bilayer and rigid supporting surface, limiting the interaction of the cytosolic domains of membrane proteins with this surface. Single particle tracking was used to analyze the motion of GPI-linked yellow fluorescent proteins (GPI-YFP) and neon-green fused transmembrane P2X2 receptors (P2X2-neon) and shows that this platform retains over 75% mobility of multipass transmembrane proteins in its native membrane environment. An enzyme accessibility assay confirmed that the protein orientation is preserved and results in the extracellular domain facing toward the bulk phase and the cytosolic side facing the support. Because the platform presented here retains the complexity of the cell plasma membrane and preserves protein orientation and mobility, it is a better representative mimic of native cell surfaces, which may find many applications in biological assays aimed at understanding cell membrane phenomena.

Substrate-Independent Approach to Dense Cleavable Polymer Brushes by Nitroxide-Mediated Polymerization

High grafting density polymer brushes are grown on an inimer coating bearing nitroxide-mediated polymerization (NMP) inimers and glycidyl methacrylate (GMA). The inimer coating is cross-linked on the substrate to provide an initiator layer with needed stability during long exposure to organic solvents at moderate to high temperatures. Surface-initiated nitroxide-mediated polymerization (SI-NMP) is conducted to grow polystyrene (PS) brushes on the coating with a sacrificial layer designed to cleave the brushes. The cleaved brushes have larger molecular weights than the corresponding free polymers. The grafting density of the brushes is as high as 1.12 chains/nm² throughout the brush growth, which is among the densest PS brushes reported so far. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) depth profiling are used to reveal the surface morphology and kinetics of the growth.

Lewis Pair-Mediated Surface-Initiated Polymerization

We present the first example of surface-initiated polymerization mediated by Lewis pairs for the synthesis of polymer brushes on planar substrates. The method enables the rapid grafting polymerization from the self-assembled monolayer or surface-attached macroinitiators, furnishing linear polymer brushes and bottle-brush brushes. Both homopolyester and block copolyester brushes can be synthesized via this versatile approach. This work not only opens up new opportunities for the application of Lewis pair-mediated polymerization but also enriches the surface-initiated polymerization on different surfaces.

Stable polymer brushes with effectively varied grafting density synthesized from highly crosslinked random copolymer thin films

Crosslinkable epoxy copolymers enable achieving highly stable P(S-b-MMA) brushes with controlled grafting density for close examination of phase separation behaviors.

Physical biology of the cancer cell glycocalyx

The glycocalyx coating the outside of most cells is a polymer meshwork comprising proteins and complex sugar chains called glycans. From a physical perspective, the glycocalyx has long been considered a simple 'slime' that protects cells from mechanical disruption or against pathogen interactions, but the great complexity of the structure argues for the evolution of more advanced functionality: the glycocalyx serves as the complex physical environment within which cell-surface receptors reside and operate. Recent studies have demonstrated that the glycocalyx can exert thermodynamic and kinetic control over cell signalling by serving as the local medium within which receptors diffuse, assemble and function. The composition and structure of the glycocalyx change markedly with changes in cell state, including transformation. Notably, cancer-specific changes fuel the synthesis of monomeric building blocks and machinery for production of long-chain polymers that alter the physical and chemical structure of the glycocalyx. In this Review, we discuss these changes and their physical consequences on receptor function and emergent cell behaviours.

Droplet Transport in a Nanochannel Coated by Hydrophobic Semiflexible Polymer Brushes: The Effect of Chain Stiffness

We study the influence of chain stiffness on droplet flow in a nanochannel, coated with semiflexible hydrophobic polymers by means of nonequilibrium molecular dynamics simulations. The studied system is then a moving droplet in the slit channel, coexisting with its vapor and subjected to periodic boundary conditions in the flow direction. The polymer chains, grafted by the terminal bead to the confining walls, are described by a coarse-grained model that accounts for chain connectivity, excluded volume interactions and local chain stiffness. The rheological, frictional and dynamical properties of the brush are explored over a wide range of persistence lengths. We find a rich behavior of polymer conformations and concomitant changes in the friction properties over the wide range of studied polymer stiffnesses. A rapid decrease in the droplet velocity was observed as the rigidity of the chains is increased for polymers whose persistence length is smaller than their contour length. We find a strong relation between the internal dynamics of the brush and the droplet transport properties, which could be used to tailor flow properties by surface functionalization. The monomers of the brush layer, under the droplet, present a collective "treadmill belt" like dynamics which can only be present due the existence of grafted chains. We describe its changes in spatial extension upon variations of polymer stiffness, with bidimensional velocity and density profiles. The deformation of the polymer brushes due to the presence of the droplet is analyzed in detail. Lastly, the droplet-gas interaction is studied by varying the liquid to gas ratio, observing a 16% speed increase for droplets that flow close to each other, compared to a train of droplets that present a large gap between consecutive droplets.

Homopolymer Nanolithography

Future progress in nanoscience and nanotechnology necessitates further development of versatile, labor-, and cost-efficient surface patterning strategies. A new approach to nanopatterning is reported, which utilizes surface segregation of a smooth layer of an end-grafted homopolymer in a poor solvent. The variation in polymer grafting density yields a range of surface nanostructures, including randomly organized pinned spherical micelles, worm-like structures, networks, and porous films. The capability to use the polymer patterns for site-specific deposition of small molecules, polymers, or nanoparticles is shown. This versatile strategy enables patterning of curved surfaces with direct access to the substrate and no need in changing polymer composition to realize different surface patterns.

Protein Resistant Polymeric Biomaterials

Toward improving implantable medical devices as well as diagnostic performance, the development of polymeric biomaterials having resistance to proteins remains a priority. Herein, we highlight key strategies reported in the recent literature that have relied upon improvement of surface hydrophilicity via direct surface modification methods or with bulk modification using surface modifying additives (SMAs). These approaches have utilized a variety of techniques to incorporate the surface hydrophilization agent, including physisorption, hydrogel network formation, surface grafting, layer-by-layer (LbL) assembly and blending base polymers with SMAs. While poly(ethylene glycol) (PEG) remains the gold standard, new alternatives have emerged such as polyglycidols, poly(2-oxazoline)s (POx), polyzwitterions, and amphiphilic block copolymers. While these new strategies provide encouraging results, the need for improved correlation between in vitro and in vivo protein resistance is critical. This may be achieved by employing complex protein solutions as well as strides to enhance the sensitivity of protein adsorption measurements.

Photoactive Surface-Grafted Polymer Brushes with Phthalocyanine Bridging Groups as an Advanced Architecture for Light-Harvesting

Surface-grafted polymer brushes of novel ladder-like architecture were proposed for inducing ordering of chromophores embedded therein. The brushes with acetylene side groups were obtained by surface-initiated photoiniferter-mediated polymerization. The acetylene moieties reacted then through a "click" process with an axially azide-bifunctionalized silicon phthalocyanine bridging the neighboring chains that inherently adopt extended conformations in dense brushes. FTIR, quartz crystal microbalance, and atomic force microscopy were used to study formation and structure of the photoactive brushes varying in grafting densities. Importantly, photophysical properties of the chromophores were virtually unaffected upon embedding them into the brushes, as evidenced by UV/Vis absorption and emission spectroscopy. Owing to the unique ordering of the chromophores, the proposed method may open new opportunities for the fabrication of light-harvesting systems suitable for photovoltaic or sensing applications.