Steric and electrostatic surface forces on sulfonated PEG graft surfaces with selective albumin adsorption

Addition of ionized terminal groups to PEG graft layers may cause additional interfacial forces to modulate the net interfacial interactions between PEG graft layers and proteins. In this study we investigated the effect of terminal sulfonate groups, characterizing PEG-aldehyde (PEG-CHO) and sulfonated PEG (PEG-SO3) graft layers by XPS and colloid probe AFM interaction force measurements as a function of ionic strength, in order to determine surface forces relevant to protein resistance and models of bio-interfacial interaction of such graft coatings. On the PEG-CHO surface the measured interaction force does not alter with ionic strength, typical of a repulsive steric barrier coating. An analogous repulsive interaction force of steric origin was also observed on the PEG-SO3 graft coating; however, the net interaction force changed with ionic strength. Interaction forces were modelled by steric and electrical double layer interaction theories, with fitting to a scaling theory model enabling determination of the spacing and stretching of the grafted chains. Albumin, fibrinogen, and lysozyme did not adsorb on the PEG-CHO coating, whereas the PEG graft with terminal sulfonate groups showed substantial adsorption of albumin but not fibrinogen or lysozyme from 0.15 M salt solutions. Under lower ionic strength conditions albumin adsorption was again minimized as a result of the increased electrical double-layer interaction observed with the PEG-SO3 modified surface. This unique and unexpected adsorption behaviour of albumin provides an alternative explanation to the "negative cilia" model used by others to rationalize observed thromboresistance on PEG-sulfonate coatings.

Biologically active dibenzofurans from Pilidiostigma glabrum, an endemic Australian Myrtaceae

In an effort to identify new anti-inflammatory and antibacterial agents with potential application in wound healing, five new dibenzofurans, 1,3,7,9-tetrahydroxy-2,8-dimethyl-4,6-di(2-methylbutanoyl)dibenzofuran (1), 1,3,7,9-tetrahydroxy-2,8-dimethyl-4-(2-methylbutanoyl)-6-(2-methylpropionyl)dibenzofuran (2), 1,3,7,9-tetrahydroxy-2,8-dimethyl-4,6-di(2-methylpropionyl)dibenzofuran (3), 1,3,7,9-tetrahydroxy-4,6-dimethyl-2-(2-methylbutanoyl)-8-(2-methylpropionyl)dibenzofuran (4), and 1,3,7,9-tetrahydroxy-4,6-dimethyl-2,8-di(2-methylpropionyl)dibenzofuran (5), were isolated from the leaves of Pilidiostigma glabrum together with one previously described dibenzofuran. Structure elucidation was achieved by way of spectroscopic measurements including 2D-NMR spectroscopy. Compounds with 2,8-acyl substitutions had potent antibacterial activity against several Gram-positive strains (MIC in the low micromolar range), while compounds with 4,6-acyl substitutions were less active. All compounds except 3 inhibited the synthesis of nitric oxide in RAW264 macrophages with IC(50) values in the low micromolar range. Compounds with 2,8-acyl substitutions also inhibited the synthesis of PGE(2) in 3T3 cells, whereas 4,6-acyl-substituted compounds were inactive. None of the compounds inhibited the synthesis of TNF-α in RAW264 cells. The compounds showed variable but modest antioxidant activity in the oxygen radical absorbance capacity assay. These findings highlight that much of the Australian flora remains unexplored and may yet yield many new compounds of interest. Initial clues are provided on structure/activity relationships for this class of bioactives, which may enable the design and synthesis of compounds with higher activity and/or selectivity.

Functionality of proteins bound to plasma polymer surfaces

The deposition of a thin film layer by plasma polymerization enables the surface functionalization of a wide range of substrate materials for biointerfacial interactions. Plasma polymers can surface-bind proteins specifically via covalent linkages or nonspecifically through other irreversible adsorption mechanisms; key questions are whether covalent chemisorption has indeed occurred, and whether the protein retains functionality. Here the mode of surface binding of streptavidin and the biotin binding functionality of the bound streptavidin layer are studied on plasma polymer (pp) surfaces deposited using propionaldehyde and ethanol that were plasma polymerized at different powers (P) to investigate possible mechanisms for protein binding to a range of different surface chemistries. As expected, with pp surfaces composed principally of aldehyde groups, protein conjugation appears to be specific (chemisorption) allowing the immobilization of streptavidin (SAV) molecules retaining the ability to bind biotinylated probes. To contrast with this, we present the first study of protein adsorption to ethanol pp surfaces prepared at different P. This provides an investigation into retention of the hydroxyl functionality in the pp at low P and its effect on protein adsorption. Adsorption of human serum albumin (HSA) to ethanol pp was similar to that on propionaldehyde pp except at low P (5 W) where hydroxyl group retention and hydration presumably has a role in reducing protein adsorption. Although we observed SAV adsorption to ethanol pp surfaces at all P, interestingly, the protein lost its ability to bind biotinylated probes. Thus we suggest that irreversible, nonspecific adsorption of protein on ethanol pp surfaces results in apparent protein denaturation despite the hydrophilic nature of the ethanol pp surface. We conclude by making inferences between the pp structure as measured by X-ray photoelectron spectroscopy (XPS) and the related protein adsorption mechanisms.

Immobilized streptavidin gradients as bioconjugation platforms

Surface density gradients of streptavidin (SAV) were created on solid surfaces and demonstrated functionality as a bioconjugation platform. The surface density of immobilized streptavidin steadily increased in one dimension from 0 to 235 ng cm(-2) over a distance of 10 mm. The density of coupled protein was controlled by its immobilization onto a polymer surface bearing a gradient of aldehyde group density, onto which SAV was covalently linked using spontaneous imine bond formation between surface aldehyde functional groups and primary amine groups on the protein. As a control, human serum albumin was immobilized in the same manner. The gradient density of aldehyde groups was created using a method of simultaneous plasma copolymerization of ethanol and propionaldehyde. Control over the surface density of aldehyde groups was achieved by manipulating the flow rates of these vapors while moving a mask across substrates during plasma discharge. Immobilized SAV was able to bind biotinylated probes, indicating that the protein retained its functionality after being immobilized. This plasma polymerization technique conveniently allows virtually any substrate to be equipped with tunable protein gradients and provides a widely applicable method for bioconjugation to study effects arising from controllable surface densities of proteins.

Instability of Antibacterial Serrulatane Compounds from the Australian Plant Species Eremophila duttonii

Hydrophilically substituted diterpenes of the structural class of serrulatanes have attracted attention as novel antibacterial compounds that are effective even against multidrug-resistant Staphylococcus aureus, a key bacterium involved in human infections. The mechanism of action has, however, not been established yet. Available data on structure–activity relationships suggest that the aromatic hydroxy group is essential for activity, and the strongest activity has been found for naphthyl compounds. In this context, it is reported that two highly active serrulatanes isolated from leaf resin of the Australian plant species Eremophila duttonii showed instability upon separation. Acetylation of hydroxy groups generated stable compounds that could be isolated and identified by NMR spectroscopy. The acetylated compounds showed little antibacterial activity, but such activity, as well as oxidative instability, was restored after hydrolysis of the acetate groups. Thus, phenolic hydroxy groups are essential for the mechanism of action of these compounds. The reaction products were not purifiable in sufficient quantities, but indications point to oxidation to quinones. Such oxidation may be a key aspect of the antibacterial activity of this class of compounds.

Individual and Population Quantitative Analyses of Calcium Flux in T-Cells Activated on Functionalized Material Surfaces

We have developed a novel method for activating T-cells on material surfaces that enable individual and population-based analyses of intracellular calcium flux, as a quantitative measure of T-cell receptor engagement. Functionalized material surfaces were created using a plasma-polymerized foundation layer to immobilize stimulatory T-cell ligands, which could induce T-cell receptor-dependent calcium flux in naive T-cells. Real-time confocal microscopic detection and quantification of calcium flux using paired fluorescent ratiometric probes facilitated the tracking and analysis of response profiles of individual T-cells, as well as population analyses using a combination of individual T-cell events. This type of combined analysis cannot be achieved using traditional population-based flow cytometric approaches, and thus provides a logical step towards developing the capacity to assess the magnitude and quality of inherently heterogeneous effector T-cell responses to antigenic challenge.

Controlled release of levofloxacin sandwiched between two plasma polymerized layers on a solid carrier

Targeted delivery and controlled local release of drugs has a number of advantages over conventional systemic drug delivery approaches. Novel platforms for local delivery from solid drug carriers are needed to satisfy the requirements of various medical applications, in particular for the incorporation and release of hydrophilic drugs from a solid carrier material. We have utilized the plasma polymerization of n-heptylamine for the generation of two thin coated layers that serve two distinct purposes. First, an n-heptylamine plasma polymer layer is applied onto the surface of the solid carrier material in order to facilitate spreading of the drug, which is applied by solvent casting; levofloxacin in ethanol was used for this study. A second n-heptylamine plasma polymer coating then serves as a thin barrier coating to control the release. We show that the rate of release can be adjusted via the thickness of the plasma polymer overlayer. We also show that this modality of controlled release of levofloxacin completely inhibits Methicillin-resistant Staphylococcus aureus (MRSA) colonization and biofilm formation on and near the coated biomaterial surface.

Enhanced molecular chaperone activity of the small heat-shock protein alphaB-cystallin following covalent immobilization onto a solid-phase support

The well-characterized small heat-shock protein, alphaB-crystallin, acts as a molecular chaperone by interacting with unfolding proteins to prevent their aggregation and precipitation. Structural perturbation (e.g., partial unfolding) enhances the in vitro chaperone activity of alphaB-crystallin. Proteins often undergo structural perturbations at the surface of a synthetic material, which may alter their biological activity. This study investigated the activity of alphaB-crystallin when covalently bound to a support surface; alphaB-crystallin was immobilized onto a range of solid material surfaces, and its characteristics and chaperone activity were assessed. Immobilization was achieved via a plasma-deposited thin polymeric interlayer containing aldehyde surface groups and reductive amination, leading to the covalent binding of alphaB-crystallin lysine residues to the surface aldehyde groups via Schiff-base linkages. Immobilized alphaB-crystallin was characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and quartz crystal microgravimetry, which showed that 300 ng cm(-2) (dry mass) of oligomeric alphaB-crystallin was bound to the surface. Immobilized alphaB-crystallin exhibited a significant enhancement (up to 5000-fold, when compared with the equivalent activity of alphaB-crystallin in solution) of its chaperone activity against various proteins undergoing both amorphous and amyloid fibril forms of aggregation. The enhanced molecular chaperone activity of immobilized alphaB-crystallin has potential applications in preventing protein misfolding, including against amyloid disease processes, such as dialysis-related amyloidosis, and for biodiagnostic detection of misfolded proteins.

Surface Immobilization of Synthetic Proteins Via Plasma Polymer Interlayers

Coatings of biologically active molecules on synthetic ”bulk“materials are of much interest for biomedical applications since they can in principle elicit specific, predictable. controlled responses of the host environment to an implanted device. However, issues such as shelf life. storage conditions, biological safety, and enzymatic attack in the biological environment must be considered; synthetic proteins may offer advantages. In this study we investigated the covalent immobilization onto polymeric materials of synthetic proteins which possess some properties that mimic those of the natural protein collagen, particularly the ability to form triple helical structures, and thus may provide similar bio-responses while avoiding enzymatic degradation. In order to perform immobilization of these collagen-like molecules (CLMs) under mild reaction conditions, the bulk materials are first equipped with suitable surface groups using rf plasma methods. Plasma polymer interlayers offer advantages as versatile reactive platforms for the immobilization of proteins and other biologically active molecules. Application of a thin plasma polymer coating from an aldehyde monomer is particularly suitable as it enables direct immobilization of CLMs by reaction with their terminal amine groups, using reductive amination chemistry. An alternative route is via plasma polymer layers that contain carboxylic acid groups and using carbodiimnide chemistry. A third route makes use of alkylamme plasma polymer interlayers, which are less process sensitive than aldehyde and acid plasma coatings. A layer of poly-carboxylic acid compounds such as carboxylic acid terminated PAMAM-starburst dendrimers or carboxymethylated dextran is then attached by carbodiimide chemistry onto the amine plasma layer. Amine-terminated CLMs can then be immobilized onto the poly-carboxylic acid layer. Surface analytical methods have been used to characterize the immobilization steps and to assess the surface coverage. Initial cell attachment and growth assays indicate that the biological performance of the CLMs depends on their amino acid sequence.

Comprehensive characterization of grafted expanded poly(tetrafluoroethylene) for medical applications

Successful implantation of any biomaterial depends on its mechanical, architectural, and surface properties. Materials with good bulk properties seldom possess the appropriate surface characteristics required for good biointegration. The present study investigates the results of surface modification of a highly porous, fully fluorinated polymeric substrate, expanded poly(tetrafluoroethylene) (ePTFE), with a view to improving the surface bioactivity and hence ultimately its biointegration. Modification involved gamma irradiation-induced graft copolymerization with the monomers monoacryloxyethyl phosphate (MAEP) and methacryloxyethyl phosphate (MOEP) in various solvent systems (water, methanol, methyl ethyl ketone, and mixtures thereof). In order to determine the penetration depth of the graft copolymer into the pores and/or the bulk of the ePTFE membranes, angle-dependent X-ray photoelectron spectroscopy (XPS) and magnetic resonance imaging (MRI) were used. It was found that the penetration depth was critically affected by the choice of monomer and solvent as well as by the technique used to remove dissolved oxygen from the grafting mixture: nitrogen degassing versus vacuum. Difficulties due to the porous nature of the membranes in establishing the lateral position of the graft copolymers were largely overcome by combining data from microattenuated total reflectance Fourier transfer infrared (μ-ATR-FTIR) mapping and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging. Results show that the large variation in graft heterogeneity found between different samples is largely an effect of the underlying substrate and choice of monomer. The results from this study provide the necessary knowledge and experimental data to control both the graft copolymer lateral position and depth of penetration in these porous ePTFE membranes.

Antibacterial surfaces by adsorptive binding of polyvinyl-sulphonate-stabilized silver nanoparticles

This paper presents a novel and facile method for the generation of efficient antibacterial coatings which can be applied to practically any type of substrate. Silver nanoparticles were stabilized with an adsorbed surface layer of polyvinyl sulphonate (PVS). This steric layer provided excellent colloidal stability, preventing aggregation over periods of months. PVS-coated silver nanoparticles were bound onto amine-containing surfaces, here produced by deposition of an allylamine plasma polymer thin film onto various substrates. SEM imaging showed no aggregation upon surface binding of the nanoparticles; they were well dispersed on amine surfaces. Such nanoparticle-coated surfaces were found to be effective in preventing attachment of Staphylococcus epidermidis bacteria and also in preventing biofilm formation. Combined with the ability of plasma polymerization to apply the thin polymeric binding layer onto a wide range of materials, this method appears promising for the fabrication of a wide range of infection-resistant biomedical devices.

Platforms for controlled release of antibacterial agents facilitated by plasma polymerization

Bacterial infections present an enormous problem causing human suffering and cost burdens to the healthcare systems worldwide. Herein we present several versatile strategies for controlled release of antibacterial agents which include silver ions as well as traditional antibiotics. At the heart of these release platforms is a thin film deposited by plasma polymerization. The use of plasma polymerization makes these strategies applicable to the surface of many types of medical devices since the technique for deposition of a polymer film from plasma in practically substrate independent.

Tunable antibacterial coatings that support mammalian cell growth

Bacterial infections present an enormous problem causing human suffering and cost burdens to healthcare systems worldwide. Here we present novel tunable antibacterial coatings which completely inhibit bacterial colonization by Staphylococcus epidermidis but allow normal adhesion and spreading of osteoblastic cells. The coatings are based on amine plasma polymer films loaded with silver nanoparticles. The method of preparation allows flexible control over the amount of loaded silver nanoparticles and the rate of release of silver ions.

End terminal, poly(ethylene oxide) graft layers: surface forces and protein adsorption

Covalently grafted poly(ethylene oxide) coatings have been widely studied for use in biomedical applications, particularly for the reduction of protein and other biomolecule adsorption. However, many of these studies have not characterized the hydrated structure of the coatings. This new study uses a combination of silica colloid probe interaction force measurements using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) in order to determine the grafting density and hydrated layer structure of monomethoxy poly(ethylene oxide) aldehyde layers, covalently grafted onto amine plasma polymer surfaces, and their interactions with silica surfaces. For high grafting densities, purely repulsive interactions were measured as expected for densely grafted polymer brushes. These interactions could be described by theoretical expectations for compression of one polymer brush layer. However, at lower grafting densities, attractive interactions were observed at larger separation distances, originating from bridging interactions due to adsorption of the PEO chains on the surface of the silica colloid probe. This is a new finding indicating that the coupled PEO molecules have sufficient conformational freedom to interact strongly with an adjacent surface or, for example, protein molecules for which there is an affinity. The attractive interactions could be removed by grafting an additional PEO layer onto the silica colloid probe. Protein adsorption measurements confirmed that at high grafting densities, the amount of adsorbed protein on the PEO grafted surfaces was greatly reduced, to the order of the detection limit for the XPS technique.

Template-assisted generation of nanocavities within plasma polymer films

The generation of nanosized cavities within thin film layers is of interest for a number of fundamental and applied reasons. One challenge is to make such systems sufficiently robust mechanically. Plasma polymer (pp) films possess excellent mechanical stability if deposition conditions are selected such as to achieve a sufficient density of cross-linking and resistance to extraction of polymeric material by solvents. In this study, gold nanoparticles of 15 and 70 nm diameter were used as sacrificial templates to generate nanocavities in pp films of various thickness values in the tens of nanometers range. A first pp layer was deposited onto substrates using n-heptylamine (HA) to a thickness of 20 nm. Carboxy-thiolated gold nanoparticles were electrostatically bound onto the surface amine groups of the n-heptylamine plasma polymer (HApp) layer. A second HApp layer was then coated to various thicknesses onto the nanoparticle/HApp surface. The template particles embedded thus in-between the two HApp layers were then dissolved using aqueous KCN solution; monitoring of the plasmon resonance band of the gold nanoparticles enabled verification of template stripping and measurement of the kinetics of stripping. AFM topography images showed little change on extraction of the template nanoparticles, indicating that the plasma polymer layer maintained structural integrity upon template extraction and subsequent drying, and thereby prevented collapse of the empty nanocavities. The concept of template stripping to generate controlled size free volume in thin plasma polymer layers is thus shown to produce robust structures.