Design Characteristics of a Neoteric, Superhydrophilic, Mechanically Robust Hydrogel Engineered To Limit Fouling in the Ocular Environment

Current challenges with ocular drug delivery and the chronic nature of many ocular ailments render the use of traditional ocular devices for additional drug delivery purposes very attractive. To achieve this feat, there is the need to develop biomaterials that are biocompatible, mechanically robust for ocular applications, highly transparent (depending on the targeted ocular device), and with ultralow protein adhesion potential (the primary step in processes that lead to fouling and potential device failure). Herein is reported the facile synthesis of a novel, highly transparent, mechanically robust, nontoxic, bulk functionalized hydrogel with characteristics suited to scalable fabrication of ocular implantable and nonimplantable devices. Synergistic superhydrophilicity between methacrylated poly(vinyl alcohol) (PVAGMA) and zwitterionic sulfobetaine methacrylate was exploited to obtain a superhydrophilic polymer conjugate through facile photoinitiated cross-linking polymerization. Proton nuclear magnetic resonance (1H NMR), attenuated total reflectance-Fourier transform infrared spectroscopy (ATF-FTIR), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) were used to confirm the synthesis and establish the physicochemical parameters for both the starting materials, the conjugated polymer, and the hydrogels. Cytotoxicity and cell adhesion potential evaluated in primary human retinal epithelial cells showed no toxicity or adhesion of the ocular cells. Biofilm adhesion studies in Escherichia coli and Staphylococcus aureus showed over 85% reduction in biofilm adhesion for the best-modified polymer compared to the unconjugated PVAGMA, highlighting its antifouling potential.

Nano-FTIR spectroscopy of surface confluent polydopamine films - What is the role of deposition time and substrate material?

Polydopamine (PDA) is a widely used anchoring layer for multiple purposes. While simple to prepare, PDA is characterized by high chemical and topological diversity, which can limit its versatility. Unraveling the formation mechanism and physicochemical properties of continuous confluent layer and adherent nanoparticles on the nanoscale is crucial to further extend the prospective applications of PDA. Utilizing nano-FTIR spectroscopy, we investigate layers of PDA on three different substrates (silicon/silicon dioxide, nitrogen-doped titanium oxide, and gold substrates) at varying times of deposition (ToD). We observed a good correlation between the nano-FTIR and macroscopic FTIR spectra that reflected the changes in the relative abundance of PDA and polymerization intermediates as ToD increased. To gain analytical power, we utilized the principal component analysis (PCA) and extracted additional information from the resulting loadings spectral curves and data distribution in the score plots. We revealed a higher variability of the spectra of ultrathin surface confluent layers compared to the adherent nanoparticles. While the spectra of nanoparticles showed no apparent dependency on either ToD or the substrate material, the spectra of layers were highly affected by the increasing ToD and exhibited a rise in the absorption of PDA. Concomitantly, the spectra of layers grouped according to the substrate material at the lowest ToD point to the fact that the substrate material affects the PDA's initial physicochemical structure. The observed separation gradually diminished with the increasing ToD as the PDA physicochemical structure became less influenced by the substrate material.

Tackling the Root Cause of Surface-Induced Coagulation: Inhibition of FXII Activation to Mitigate Coagulation Propagation and Prevent Clotting

Factor XII (FXII) is a zymogen present in blood that tends to adsorb onto the surfaces of blood-contacting medical devices. Once adsorbed, it becomes activated, initiating a cascade of enzymatic reactions that lead to surface-induced coagulation. This process is characterized by multiple redundancies, making it extremely challenging to prevent clot formation and preserve the properties of the surface. In this study, a novel modulatory coating system based on C1-esterase inhibitor (C1INH) functionalized polymer brushes, which effectively regulates the activation of FXII is proposed. Using surface plasmon resonance it is demonstrated that this coating system effectively repels blood plasma proteins, including FXII, while exhibiting high activity against activated FXII and plasma kallikrein under physiological conditions. This unique property enables the modulation of FXII activation without interfering with the overall hemostasis process. Furthermore, through dynamic Chandler loop studies, it is shown that this coating significantly improves the hemocompatibility of polymeric surfaces commonly used in medical devices. By addressing the root cause of contact activation, the synergistic interplay between the antifouling polymer brushes and the modulatory C1INH is expected to lay the foundation to enhance the hemocompatibility of medical device surfaces.

SI-PET-RAFT in flow: improved control over polymer brush growth

Surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain transfer (SI-PET-RAFT) provides a light-dependent tool to synthesize polymer brushes on different surfaces that tolerates oxygen and water, and does not require a metal catalyst. Here we introduce improved control over SI-PET-RAFT polymerizations via continuous flow conditions. We confirm the composition and topological structure of the brushes by X-ray photoelectron spectroscopy, ellipsometry, and AFM. The improved control compared to no-flow conditions provides prolonged linear growth of the polymer brush (up to 250 nm, where no-flow polymerization maxed out <50 nm), and improved polymerization control of the polymer brush that allows the construction of diblock polymer brushes. We further show the linear correlation between the molecular weight of the polymer brush and its dry thickness by combining ellipsometry and single-molecule force spectroscopy.

Formation of Antifouling Brushes on Various Substrates Using a Melanin-Inspired Initiator Film

In this study, we developed a substrate-independent initiator film that can undergo surface-initiated polymerization to form an antifouling brush. Inspired by the melanogenesis found in nature, we synthesized a tyrosine-conjugated bromide initiator (Tyr-Br) that contains phenolic amine groups as the dormant coating precursor and α-bromoisobutyryl groups as the initiator. The resultant Tyr-Br was stable under ambient air conditions and underwent melanin-like oxidation only in the presence of tyrosinase to form an initiator film on various substrates. Subsequently, an antifouling polymer brush was formed using air-tolerant activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP) of zwitterionic carboxybetaine. The entire surface coating procedure, including the initiator layer formation and ARGET ATRP, occurred under aqueous conditions and did not require organic solvents or chemical oxidants. Therefore, antifouling polymer brushes can be feasibly formed not only on experimentally preferred substrates (e.g., Au, SiO₂, and TiO₂) but also on polymeric substrates such as poly(ethylene terephthalate) (PET), cyclic olefin copolymer (COC), and nylon.

Antiviral Polymer Brushes by Visible-Light-Induced, Oxygen-Tolerant Covalent Surface Coating

This work presents a novel route for creating metal-free antiviral coatings based on polymer brushes synthesized by surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain transfer (SI-PET-RAFT) polymerization, applying eosin Y as a photocatalyst, water as a solvent, and visible light as a driving force. The polymer brushes were synthesized using N-[3-(decyldimethyl)-aminopropyl] methacrylamide bromide and carboxybetaine methacrylamide monomers. The chemical composition, thickness, roughness, and wettability of the resulting polymer brush coatings were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), water contact angle measurements, and ellipsometry. The antiviral properties of coatings were investigated by exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and avian influenza viruses, with further measurement of residual viable viral particles. The best performance was obtained with Cu surfaces, with a ca. 20-fold reduction of SARS-Cov-2 and a 50-fold reduction in avian influenza. On the polymer brush-modified surfaces, the number of viable virus particles decreased by about 5-6 times faster for avian flu and about 2-3 times faster for SARS-CoV-2, all compared to unmodified silicon surfaces. Interestingly, no significant differences were obtained between quaternary ammonium brushes and zwitterionic brushes.

The Relation Between Protein Adsorption and Hemocompatibility of Antifouling Polymer Brushes

Whenever an artificial surface comes into contact with blood, proteins are rapidly adsorbed onto its surface. This phenomenon, termed fouling, is then followed by a series of undesired reactions involving activation of complement or the coagulation cascade and adhesion of leukocytes and platelets leading to thrombus formation. Thus, considerable efforts are directed towards the preparation of fouling-resistant surfaces with the best possible hemocompatibility. Herein, a comprehensive hemocompatibility study after heparinized blood contact with seven polymer brushes prepared by surface-initiated atom transfer radical polymerization is reported. The resistance to fouling is quantified and thrombus formation and deposition of blood cellular components on the coatings are analyzed. Moreover, identification of the remaining adsorbed proteins is performed via mass spectroscopy to elucidate their influence on the surface hemocompatibility. Compared with an unmodified glass surface, the grafting of polymer brushes minimizes the adhesion of platelets and leukocytes and prevents the thrombus formation. The fouling from undiluted blood plasma is reduced by up to 99%. Most of the identified proteins are connected with the initial events of foreign body reaction towards biomaterial (coagulation cascade proteins, complement component, and inflammatory proteins). In addition, several proteins that are not previously linked with blood-biomaterial interaction are presented and discussed.

Bio-inspired hemocompatible surface modifications for biomedical applications

When blood first encounters the artificial surface of a medical device, a complex series of biochemical reactions is triggered, potentially resulting in clinical complications such as embolism/occlusion, inflammation, or device failure. Preventing thrombus formation on the surface of blood-contacting devices is crucial for maintaining device functionality and patient safety. As the number of patients reliant on blood-contacting devices continues to grow, minimizing the risk associated with these devices is vital towards lowering healthcare-associated morbidity and mortality. The current standard clinical practice primarily requires the systemic administration of anticoagulants such as heparin, which can result in serious complications such as post-operative bleeding and heparin-induced thrombocytopenia (HIT). Due to these complications, the administration of antithrombotic agents remains one of the leading causes of clinical drug-related deaths. To reduce the side effects spurred by systemic anticoagulation, researchers have been inspired by the hemocompatibility exhibited by natural phenomena, and thus have begun developing medical-grade surfaces which aim to exhibit total hemocompatibility via biomimicry. This review paper aims to address different bio-inspired surface modifications that increase hemocompatibility, discuss the limitations of each method, and explore the future direction for hemocompatible surface research.

Complement Activation Dramatically Accelerates Blood Plasma Fouling On Antifouling Poly(2-hydroxyethyl methacrylate) Brush Surfaces

Non-specific protein adsorption (fouling) triggers a number of deleterious events in the application of biomaterials. Antifouling polymer brushes successfully suppress fouling, however for some coatings an extremely high variability of fouling for different donors remains unexplained. The authors report that in the case of poly(2-hydroxyethyl methacrylate) (poly(HEMA)) this variability is due to the complement system activation that causes massive acceleration in the fouling kinetics of blood plasma. Using plasma from various donors, the fouling kinetics on poly(HEMA) is analyzed and correlated with proteins identified in the deposits on the surface and with the biochemical compositions of the plasma. The presence of complement components in fouling deposits and concentrations of C3a in different plasmas indicate that the alternative complement pathway plays a significant role in the fouling on poly(HEMA) through the "tick-over" mechanism of spontaneous C3 activation. The generated C3b binds to the poly(HEMA) surface and amplifies complement activation locally. Heat-inactivated plasma prevents accelerated fouling kinetics, confirming the central role of complement activation. The results highlight the need to take into account the variability between individuals when assessing interactions between biomaterials and blood plasma, as well as the importance of the mechanistic insight that can be gained from protein identification.

Coupled Monte Carlo and Molecular Dynamics Simulations on Interfacial Properties of Antifouling Polymer Membranes

We use molecular simulation to study the wetting behavior of antifouling polymer-tethered membranes. We obtain the interfacial properties (e.g., contact angle) of water at various temperatures for five polymer membranes, including a base polysulfone (PSF) membrane and four other PSF membranes grafted with antifouling polymers (two poly(ethylene glycol) (PEG) tethers and two zwitterionic tethers). We implement a coupled Monte Carlo (MC)/molecular dynamics (MD) approach to determine the interface potentials of water on the membrane surfaces in an efficient manner. Within this method, short MC and MD simulations are performed in cycles to collect the surface excess free energy of a thin water film on polymer membrane surfaces. Simulation results show that the grafting of zwitterionic tethers provides a more significant enhancement in the hydrophilicity of the PSF membrane than that of the PEG tethers. Water completely wets the surface of zwitterionic polymer membranes.

Unraveling the Mechanism and Kinetics of Binding of an LCI-eGFP-Polymer for Antifouling Coatings

The ability of proteins to adsorb irreversibly onto surfaces opens new possibilities to functionalize biological interfaces. Herein, the mechanism and kinetics of adsorption of protein-polymer macromolecules with the ability to equip surfaces with antifouling properties are investigated. These macromolecules consist of the liquid chromatography peak I peptide from which antifouling polymer brushes are grafted using single electron transfer-living radical polymerization. Surface plasmon resonance spectroscopy reveals an adsorption mechanism that follows a Langmuir-type of binding with a strong binding affinity to gold. X-ray reflectivity supports this by proving that the binding occurs exclusively by the peptide. However, the lateral organization at the surface is directed by the cylindrical eGFP. The antifouling functionality of the unimolecular coatings is confirmed by contact with blood plasma. All coatings reduce the fouling from blood plasma by 8894% with only minor effect of the degree of polymerization for the studied range (DP between 101 and 932). The excellent antifouling properties, combined with the ease of polymerization and the straightforward coating procedure make this a very promising antifouling concept for a multiplicity of applications.

Unraveling the influence of substrate on the growth rate, morphology and covalent structure of surface adherent polydopamine films

Polydopamine (PDA), also known as synthetic melanin, is widely used as a biomimetic anchoring layer for the modification of various solid substrates. PDA is utilized for a wide range of biomedical, sensing and tribological applications, even though the polymer's precise covalent structure has not been completely revealed yet. Even more, it is not evident to which extent the chemical nature of the substrate, on which the layer is formed, influences and predetermines the covalent structure of resulting PDA. In this contribution, we have studied the growth of PDA using various surface-sensitive techniques such as spectroscopic ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy. We supplemented grazing angle attenuated total reflection FTIR spectroscopy with multivariate statistical analysis to further gain analytical power. We have particularly focused on the effects of polymerization time and substrate on the PDA structure. We found notable differences in the chemical composition of PDA formed on gold and on surfaces terminated with oxides/reactive hydroxides such as silicon and N-dopped-TiO₂ in the early stages of the layer formation. At the later stages of layer formation, a merely unified chemical structure was observed independently on the type of substrate.

Fabrication and Impact of Fouling-Reducing Temperature-Responsive POEGMA Coatings with Embedded CaCO3 Nanoparticles on Different Cell Lines

In the present work, we have successfully prepared and characterized novel nanocomposite material exhibiting temperature-dependent surface wettability changes, based on grafted brush coatings of non-fouling poly(di(ethylene glycol)methyl ether methacrylate) (POEGMA) with the embedded CaCO3 nanoparticles. Grafted polymer brushes attached to the glass surface were prepared in a three-step process using atom transfer radical polymerization (ATRP). Subsequently, uniform CaCO3 nanoparticles (NPs) embedded in POEGMA-grafted brush coatings were synthesized using biomineralized precipitation from solutions of CaCl2 and Na2CO3. An impact of the low concentration of the embedded CaCO3 NPs on cell adhesion and growth depends strongly on the type of studied cell line: keratinocytes (HaCaT), melanoma (WM35) and osteoblastic (MC3T3-e1). Based on the temperature-responsive properties of grafted brush coatings and CaCO3 NPs acting as biologically active substrate, we hope that our research will lead to a new platform for tissue engineering with modified growth of the cells due to the release of biologically active substances from CaCO3 NPs and the ability to detach the cells in a controlled manner using temperature-induced changes of the brush.

Fouling in ocular devices: implications for drug delivery, bioactive surface immobilization, and biomaterial design

The last 30 years has seen a proliferation of research on protein-resistant biomaterials targeted at designing bio-inert surfaces, which are prerequisite for optimal performance of implantable devices that contact biological fluids and tissues. These efforts have only been able to yield minimal results, and hence, the ideal anti-fouling biomaterial has remained elusive. Some studies have yielded biomaterials with a reduced fouling index among which high molecular weight polyethylene glycols have remained dominant. Interestingly, the field of implantable ocular devices has not experienced an outflow of research in this area, possibly due to the assumption that biomaterials tested in other body fluids can be translated for application in the ocular space. Unfortunately, progression in the molecular understanding of many ocular conditions has brought to the fore the need for treatment options that necessitates the use of anti-fouling biomaterials. From the earliest implanted horsehair and silk seton for glaucoma drainage to the recent mini telescopes for sight recovery, this review provides a concise incursion into the gradual evolution of biomaterials for the design of implantable ocular devices as well as approaches used to overcome the challenges with fouling. The implication of fouling for drug delivery, the design of immune-responsive biomaterials, as well as advanced surface immobilization approaches to support the overall performance of implantable ocular devices are also reviewed.

Surface Design of Antifouling Vascular Constructs Bearing Biofunctional Peptides for Tissue Regeneration Applications

Antifouling polymer layers containing extracellular matrix-derived peptide motifs offer promising new options for biomimetic surface engineering. In this contribution, we report the design of antifouling vascular grafts bearing biofunctional peptide motifs for tissue regeneration applications based on hierarchical polymer brushes. Hierarchical diblock poly(methyl ether oligo(ethylene glycol) methacrylate-block-glycidyl methacrylate) brushes bearing azide groups (poly(MeOEGMA-block-GMA-N₃)) were grown by surface-initiated atom transfer radical polymerization (SI-ATRP) and functionalized with biomimetic RGD peptide sequences. Varying the conditions of copper-catalyzed alkyne-azide “click” reaction allowed for the immobilization of RGD peptides in a wide surface concentration range. The synthesized hierarchical polymer brushes bearing peptide motifs were characterized in detail using various surface sensitive physicochemical methods. The hierarchical brushes presenting the RGD sequences provided excellent cell adhesion properties and at the same time remained resistant to fouling from blood plasma. The synthesis of anti-fouling hierarchical brushes bearing 1.2 × 10³ nmol/cm² RGD biomimetic sequences has been adapted for the surface modification of commercially available grafts of woven polyethylene terephthalate (PET) fibers. The fiber mesh was endowed with polymerization initiator groups via aminolysis and acylation reactions optimized for the material. The obtained bioactive antifouling vascular grafts promoted the specific adhesion and growth of endothelial cells, thus providing a potential avenue for endothelialization of artificial conduits.

Antifouling Polymer Brushes via Oxygen-Tolerant Surface-Initiated PET-RAFT

This work presents a new method for the synthesis of antifouling polymer brushes using surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain-transfer polymerization with eosin Y and triethanolamine as catalysts. This method proceeds in an aqueous environment under atmospheric conditions without any prior degassing and without the use of heavy metal catalysts. The versatility of the method is shown by using three chemically different monomers: oligo(ethylene glycol) methacrylate, N-(2-hydroxypropyl)methacrylamide, and carboxybetaine methacrylamide. In addition, the light-triggered nature of the polymerization allows the creation of complex three-dimensional structures. The composition and topological structuring of the brushes are confirmed by X-ray photoelectron spectroscopy and atomic force microscopy. The kinetics of the polymerizations are followed by measuring the layer thickness with ellipsometry. The polymer brushes demonstrate excellent antifouling properties when exposed to single-protein solutions and complex biological matrices such as diluted bovine serum. This method thus presents a new simple approach for the manufacturing of antifouling coatings for biomedical and biotechnological applications.

Substrate-independent Cu(0)-mediated controlled radical polymerization: grafting of block copolymer brushes from poly(dopamine) modified surfaces

A method is presented combining poly(dopamine) chemistry and surface-initiated Cu(0)-mediated controlled radical polymerization (SI-CuCRP). Polymer brushes and block copolymer brushes are grafted independent of the original surface properties.

Surface-Initiated ARGET ATRP of Antifouling Zwitterionic Brushes Using Versatile and Uniform Initiator Film

In this study, we developed a uniform initiator layer that can be formed on various surfaces, and formed site-selectively, for the subsequent antifouling polymer brush formation. Initially, metal-organic films composed of tannic acid (TA) and Fe^III ions (TA-Fe^III) were formed on various surfaces, followed by functionalization with an aryl azide-based initiator (ABI) under photoreaction. In particular, combination with a photolithographic technique enabled the presentation of initiators only on the intended region within a single-surface platform. A resultant initiator film (TF-ABI) was formed under mild reaction conditions and meets the uniformity and transparency requirements concurrently. Subsequently, we showed that TF-ABI can be further utilized to form a polymer brush by proceeding with surface-initiated polymerization using a zwitterionic monomer, namely, sulfobetaine acrylamide (SBAA). Instead of applying a classical, yet air-sensitive atom transfer radical polymerization (ATRP) technique, we utilized an activator regenerated by electron transfer (ARGET) ATRP under air conditions without a cumbersome deoxygenation step. Overall, our initiator layer allowed the antifouling poly(SBAA) brush to be used on various surfaces, and enabled their pattern generation.

Impact of Bioactive Peptide Motifs on Molecular Structure, Charging, and Nonfouling Properties of Poly(ethylene oxide) Brushes

Polymer layers capable of suppressing protein adsorption from biological media while presenting extracellular matrix-derived peptide motifs offer valuable new options for biomimetic surface engineering. Herein, we provide detailed insights into physicochemical changes induced in a nonfouling poly(ethylene oxide) (PEO) brush/polydopamine (PDA) system by incorporation of adhesion ligand (RGD) peptides. Brushes with high surface chain densities (σ ≥ 0.5 chains·nm^-2) and pronounced hydrophilicity (water contact angles ≤ 10°) were prepared by end-tethering of heterobifunctional PEOs ( M_n ≈ 20 000 g·mol^-1) to PDA-modified surfaces from a reactive melt. Using alkyne distal end group on the PEO chains, azidopentanoic-bearing peptides were coupled through a copper-catalyzed Huisgen azide-alkyne "click" cycloaddition reaction. The surface concentration of RGD was tuned from complete saturation of the PEO surface with peptides (1.7 × 10⁵ fmol·cm^-2) to values which may induce distinct differences in cell adhesion (<6.0 × 10² fmol·cm^-2). Infrared reflection-absorption and X-ray photoelectron spectroscopies proved the PDA-PEO layers covalent structure and the immobilization of RGD peptides. The complete reconstruction of experimental electrohydrodynamics data utilizing mean-field theory predictions further verified the attained brush structure of the end-tethered PEO chains which provided hydrodynamic screening of the PDA anchor. Increasing the surface concentration of immobilized RGD peptides led to increased interfacial charging. Supported by simulations, this observation was attributed to the ionization of functional groups in the amino acid sequence and to the pH-dependent adsorption of water ions (OH^- > H₃O⁺) from the electrolyte. Despite the distinct differences observed in the electrokinetic analysis of the surfaces bearing different amounts of RGD, it was found that the peptide presence on PEO(20 000)-PDA layers does not have a significant effect on the nonfouling properties of the system. Notably, the presented PEO(20 000)-PDA layers bearing RGD peptides in the surface concentration range 5.9 to 1.7 × 10⁵ fmol·cm^-2 reduced the protein adsorption from fetal bovine serum to less than 30 ng·cm^-2, that is, values comparable to the ones obtained for pristine PEO(20 000)-PDA layers.

Polymer Brush-Functionalized Chitosan Hydrogels as Antifouling Implant Coatings

Implantable sensor devices require coatings that efficiently interface with the tissue environment to mediate biochemical analysis. In this regard, bioinspired polymer hydrogels offer an attractive and abundant source of coating materials. However, upon implantation these materials generally elicit inflammation and the foreign body reaction as a consequence of protein fouling on their surface and concomitant poor hemocompatibility. In this report we investigate a strategy to endow chitosan hydrogel coatings with antifouling properties by the grafting of polymer brushes in a "grafting-from" approach. Chitosan coatings were functionalized with polymer brushes of oligo(ethylene glycol) methyl ether methacrylate and 2-hydroxyethyl methacrylate using photoinduced single electron transfer living radical polymerization and the surfaces were thoroughly characterized by XPS, AFM, water contact angle goniometry, and in situ ellipsometry. The antifouling properties of these new bioinspired hydrogel-brush coatings were investigated by surface plasmon resonance. The influence of the modifications to the chitosan on hemocompatibility was assessed by contacting the surfaces with platelets and leukocytes. The coatings were hydrophilic and reached a thickness of up to 180 nm within 30 min of polymerization. The functionalization of the surface with polymer brushes significantly reduced the protein fouling and eliminated platelet activation and leukocyte adhesion. This methodology offers a facile route to functionalizing implantable sensor systems with antifouling coatings that improve hemocompatibility and pave the way for enhanced device integration in tissue.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Ultrathin Monomolecular Films and Robust Assemblies Based on Cyclic Catechols

We introduce a newly designed catechol-based compound and its application for the preparation of homogeneous monomolecular layers as well as for robust assemblies on various substrates. The precisely defined cyclic catechol material (CyCat) was prepared from ortho-dimethoxybenzene in a phenolic resin-like synthesis and subsequent deprotection, featuring molecules with up to 32 catechol units. The CyCat's chemical structure was carefully assessed via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF), proton nuclear magnetic resonance (¹H NMR), diffusion ordered spectroscopy (2D DOSY) and high resolution electrospray ionization mass spectrometry (ESI MS) experiments. The formation of colloidal aggregates of the CyCat material in alkaline solution was followed by dynamic light scattering (DLS) and further verified by dropcasting CyCat from solution on highly oriented pyrolytic graphite (HOPG), which was examined by Kelvin probe force microscopy (KPFM). The adsorption behavior of the CyCat to form monomolecular layers was investigated in real time by surface plasmon resonance (SPR). Formation of these thin CyCat layers (1.6-2.1 nm) on Au, SiO₂ and TiO₂ substrates was corroborated by spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). The prepared coating perfectly reflects the surface structure of the underlying substrate and does not exhibit CyCat colloidal aggregates as verified by atomic force microscopy (AFM). The functional nature of the prepared catechol monolayers was evidenced by reaction with 4-bromophenethylamine and bis(3-aminopropyl)-terminated poly(ethylene oxide) (PEO). Multilayer assemblies were prepared by a simple procedure of iterative immersion in solutions of CyCat and a multifunctional amine on Au, SiO₂ and TiO₂ substrates forming thicker coatings (up to 12 nm). Postmodification with small organic molecules was performed to covalently attach trifluoroacetyl, tetrazole and 2-bromo-2-methylpropanoyl moieties to the amine groups of the multilayer assembly coating. Furthermore, the versatility of the novel multilayer coating was underpinned by "grafting-to" of phenacyl sulfide-terminated PEO and "grafting-from" of poly(methyl methacrylate) via surface-initiated atom transfer radical polymerization (ATRP).

Polymer brushes based on PLLA-b-PEO colloids for the preparation of protein resistant PLA surfaces

Deposition of PLLA-b-PEO colloidal nanoparticles from selective solvents onto a polylactide surface resulting in an anti-fouling and cell repulsive surface.

Oxidative polymerization of catecholamines: structural access by high-resolution mass spectrometry

We introduce an avenue for the structural elucidation of bio-inspired functional poly(catecholamines) via high-resolution electrospray ionization mass spectrometry.

Hepatitis B plasmonic biosensor for the analysis of clinical serum samples

A plasmonic biosensor for rapid detection of protein biomarkers in complex media is reported. Clinical serum samples were analyzed by using a novel biointerface architecture based on poly[(N-(2-hydroxypropyl) methacrylamide)-co-(carboxybetaine methacrylamide)] brushes functionalized with bioreceptors. This biointerface provided an excellent resistance to fouling even after the functionalization and allowed for the first time the direct detection of antibodies against hepatitis B surface antigen (anti-HBs) in clinical serum samples using surface plasmon resonance (SPR). The fabricated SPR biosensor allowed discrimination of anti-HBs positive and negative clinical samples in 10min. Results are validated by enzyme-linked immunoassays of the sera in a certified laboratory. The sensor could be regenerated by simple treatment with glycine buffer.

Synthesis and Characterization of Aminopropyltriethoxysilane-Polydopamine Coatings

Polydopamine coatings are of interest due to the fact that they can promote adhesion to a broad range of materials and can enable a variety of applications. However, the polydopamine-substrate interaction is often noncovalent. To broaden the potential applications of polydopamine, we show the incorporation of 3-aminopropyltriethoxysilane (APTES), a traditional coupling agent capable of covalent bonding to a broad range of organic and inorganic surfaces, into polydopamine coatings. High energy X-ray photoelectron spectroscopy (HE-XPS), conventional XPS, near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), and ellipsometry measurements were used to investigate changes in coating chemistry and thickness, which suggest covalent incorporation of APTES into polydopamine. These coatings can be deposited either in Tris buffer or by using an aqueous APTES solution as a buffer without Tris. APTES-dopamine hydrochloride deposition from solutions with molar ratios between 0:1 and 10:1 allowed us to control the coating composition across a broad range.

Electrokinetic Properties of Lubricin Antiadhesive Coatings in Microfluidic Systems

Lubricin is a glycoprotein found in articular joints which has long been recognized as being an important biological boundary lubricant molecule and, more recently, an impressive antiadhesive that readily self-assembles into a well ordered, polymer brush layer on virtually any substrate. The lubricin molecule possesses an overabundance of anionic charge, a property that is atypical among antiadhesive molecules, that enables its use as a coating for applications involving electrokinetic processes such as electrophoresis and electroosmosis. Coating the surfaces of silica and polymeric microfluidic devices with self-assembled lubricin coatings affords a unique combination of excellent fouling resistance and high charge density that enables notoriously "sticky" biomolecules such as proteins to be used and controlled electrokinetically in the device without complications arising from nonspecific adsorption. Using capillary electrophoresis, we characterized the stability, uniformity, and electrokinetic properties of lubricin coatings applied to silica and PTFE capillaries over a range of run buffer pHs and when exposed to concentrated solutions of protein. In addition, we demonstrate the effectiveness of lubricin as a coating to minimize nonspecific protein adsorption in an electrokinetically controlled polydimethylsiloxane/silica microfluidic device.

Fast and Simple Preparation of Patterned Surfaces with Hydrophilic Polymer Brushes by Micromolding in Capillaries

Micropatterns of hydrophilic polymer brushes were prepared by micromolding in capillaries (MIMIC). The polymers are covalently bound to the surfaces by a rapid hetero Diels-Alder reaction, constituting the first example of polymers grafted to surfaces in a defined pattern by MIMIC. The polymers [poly(acrylic acid), poly(hydroxyethyl acrylate), and poly(tetraethylene glycol acrylate) ranging in molecular weight from 1500 to 6000 g mol(-1)] were prepared with narrow dispersities via the reversible addition-fragmentation chain transfer (RAFT) process using a highly electron deficient RAFT agent that can react with surface-anchored dienes such as cyclopentadiene. We demonstrate that the anchoring method is facile to perform and highly suitable for preparing patterned surfaces that are passivated against biological impact in well-defined areas.

Cell adhesion and growth enabled by biomimetic oligopeptide modification of a polydopamine-poly(ethylene oxide) protein repulsive surface

Protein-repulsive surfaces modified with ligands for cell adhesion receptors have been widely developed for controlling the cell adhesion and growth in tissue engineering. However, the question of matrix production and deposition by cells on these surfaces has rarely been addressed. In this study, protein-repulsive polydopamine-poly(ethylene oxide) (PDA-PEO) surfaces were functionalized with an RGD-containing peptide (RGD), with a collagen-derived peptide binding fibronectin (Col), or by a combination of these peptides (RGD + Col, ratio 1:1) in concentrations of 90 fmol/cm(2) and 700 fmol/cm(2) for each peptide type. When seeded with vascular endothelial CPAE cells, the PDA-PEO surfaces proved to be completely non-adhesive for cells. On surfaces with lower peptide concentrations and from days 1 to 3 after seeding, cell adhesion and growth was restored practically only on the RGD-modified surface. However, from days 3 to 7, cell adhesion and growth was improved on surfaces modified with Col and with RGD + Col. At higher peptide concentrations, the cell adhesion and growth was markedly improved on all peptide-modified surfaces in both culture intervals. However, the collagen-derived peptide did not increase the expression of fibronectin in the cells. The deposition of fibronectin on the material surface was generally very low and similar on all peptide-modified surfaces. Nevertheless, the RGD + Col surfaces exhibited the highest cell adhesion stability under a dynamic load, which correlated with the highest expression of talin and vinculin in the cells on these surfaces. A combination of RGD + Col therefore seems to be the most promising for surface modification of biomaterials, e.g. vascular prostheses.

Phototriggered functionalization of hierarchically structured polymer brushes

The precise design of bioactive surfaces, essential for the advancement of many biomedical applications, depends on achieving control of the surface architecture as well as on the ability to attach bioreceptors to antifouling surfaces. Herein, we report a facile avenue toward hierarchically structured antifouling polymer brushes of oligo(ethylene glycol) methacrylates via surface-initiated atom transfer radical polymerization (SI-ATRP) presenting photoactive tetrazole moieties, which permitted their functionalization via nitrile imine-mediated tetrazole-ene cyclocloaddition (NITEC). A maleimide-functional ATRP initiator was photoclicked to the side chains of a brush enabling a subsequent polymerization of carboxybetaine acrylamide to generate a micropatterned graft-on-graft polymer architecture as evidenced by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Furthermore, the spatially resolved biofunctionalization of the tetrazole-presenting brushes was accessed by the photoligation of biotin-maleimide and subsequent binding of streptavidin. The functionalized brushes bearing streptavidin were able to resist the fouling from blood plasma (90% reduction with respect to bare gold). Moreover, they were employed to demonstrate a model biosensor by immobilization of a biotinylated antibody and subsequent capture of an antigen as monitored in real time by surface plasmon resonance.

Self-assembled anchor layers/polysaccharide coatings on titanium surfaces: a study of functionalization and stability

Composite materials based on a titanium support and a thin, alginate hydrogel could be used in bone tissue engineering as a scaffold material that provides biologically active molecules. The main objective of this contribution is to characterize the activation and the functionalization of titanium surfaces by the covalent immobilization of anchoring layers of self-assembled bisphosphonate neridronate monolayers and polymer films of 3-aminopropyltriethoxysilane and biomimetic poly(dopamine). These were further used to bind a bio-functional alginate coating. The success of the titanium surface activation, anchoring layer formation and alginate immobilization, as well as the stability upon immersion under physiological-like conditions, are demonstrated by different surface sensitive techniques such as spectroscopic ellipsometry, infrared reflection-absorption spectroscopy and X-ray photoelectron spectroscopy. The changes in morphology and the established continuity of the layers are examined by scanning electron microscopy, surface profilometry and atomic force microscopy. The changes in hydrophilicity after each modification step are further examined by contact angle goniometry.

A survey of state-of-the-art surface chemistries to minimize fouling from human and animal biofluids

Fouling of artificial surfaces by biofluids is a plague Biotechnology deeply suffers from. Herein, we inventory the state-of-the-art surface chemistries developed to minimize this effect from both human and animal biosamples.

Antifouling property of monothiol-terminated bottle-brush poly(methylacrylic acid)-graft-poly(2-methyl-2-oxazoline) copolymer on gold surfaces

A series of well-controlled bottle-brush poly(methylacrylic acid)-graft-poly(2-methyl-2-oxazoline) copolymers were grafted to gold surfaces through an in situ aminolysis reaction to reduce protein adsorption and platelet adhesion.

Quantifying bacterial adhesion on antifouling polymer brushes via single-cell force spectroscopy

The adhesion forces between a single bacterial cell and different polymer brushes were measured directly with an atomic force microscope and correlated with their resistance to fouling.

Polydopamine-assisted deposition of heparin for selective adsorption of low-density lipoprotein

Low-density lipoprotein (LDL) is the main carrier of blood cholesterol, with elevated levels of LDL increasing the risk of atherosclerosis.