Protein adsorption to poly(ethylene oxide) surfaces

Orientation of cell adhesion and growth on patterned heterogeneous polystyrene surface

Studies of neurite outgrowth or cell migration, two important processes in neuronal networks formation, are facilitated by cell culture models capable of orientating cellular growth and of designing a well-defined cellular pattern. Heterogeneous polystyrene surfaces composed of oxygen plasma-treated stripes (PSox) with a low hydrophobicity separated by non-treated areas (PS) have these properties. In this study, to guide cell growth, we developed a cell culture model using these supports and we identified the molecular factors involved in cellular orientation. When the heterogeneous supports were not coated, proteins from a serum culture medium were required for cells to line up on PSox. On the other hand, cell orientation on coated surfaces was clearly influenced by competitive adsorption of adhesive proteins such as fibronectin or collagen and anti-adhesive molecules as pluronic F68 or albumin. Attachment factors were adsorbed on PSox stripes while adsorption of anti-adhesive molecules on the most hydrophobic PS areas prevented cell adhesion or growth. Thus, we describe the preparation of a cell culture substrate that succeeded in orientating cell growth and that led to a line of cells on adhesive PSox stripes ranging from 2 to 100 microns width.

Patterned glass surfaces direct cell adhesion and process outgrowth of primary neurons of the central nervous system

Glass surfaces were patterned with cell-adhesive regions of laminin adhesive peptides YIGSR, RGD, and IKVAV, and cell-repulsive regions of poly(ethylene glycol) (PEG). The patterns were created by sputter-coating titanium and then gold onto glass coverslips through electron microscope grids. Gold surfaces were modified with cysteine-terminated peptides to have approximately 450 fmol/ cm2 of peptide incorporated on the glass coverslips as determined with radiolabeled CGYIGSR. Amine-functionalized glass coverslips were prepared using an amine-functionalized silane and then further modified with PEG-aldehyde by a Schiff base reduction. All surfaces were characterized by X-ray photoelectron spectroscopy and water contact angles. Hippocampal neurons, plated from a serum-free medium, adhered preferentially to peptide-functionalized surfaces over PEG-modified surfaces. Cell adhesion and neurite outgrowth were limited to the peptide region, demonstrating that neurite outgrowth could be directed by a combination of cell-adhesive and cell-repulsive cues.

Poly(ethylene oxide) Grafted to Silicon Surfaces: Grafting Density and Protein Adsorption

Poly(ethylene oxide) (PEO) polymer, in linear and star form, was covalently grafted to silicon surfaces, and the surfaces were tested for their ability to adsorb proteins. Linear PEG of molecular weight 3400, 10 000, and 20 000 g/mol and star PEO molecules were coupled via their terminal hydroxyl groups activated by tresyl chloride to aminosilane-treated silicon wafers. The amount of PEO coupled to the surface was varied by changing the concentration of the tresyl-PEO solution. The dry PEO thickness on the surface was measured using X-ray photoelectron spectroscopy (XPS) and ellipsometry, from which the grafting density was calculated. The PEO surfaces were exposed to solutions of each of three proteins: cytochrome-c, albumin, and fibronectin. The degree of adsorption of each protein was determined by XPS and ellipsometry and recorded as a function of PEO grafting density. All three proteins were found to reach zero adsorption at the highest grafting densities on all three PEG surfaces, which for all three PEG surfaces was a PEO content of 100 +/- 10 ng/cm2. On both star PEO surfaces, albumin and fibronectin decreased to zero adsorption at intermediate values of grafting density, whereas cytochrome-c continued to adsorb at all grafting densities, although with a decreasing trend. A physical model of the surface helped explain these protein adsorption results in terms of the spacing and degree of overlap of grafted PEO chains.

Chemical modification and photograft polymerization upon expanded poly(tetrafluoroethylene)

Poly(tetrafluoroethylene) (PTFE) films were surface-modified by employing a reaction solution of benzophenone and sodium hydride in anhydrous dimethylformamide at a temperature of 150 degrees C for 12 h. Electron spectroscopy for chemical analysis (ESCA) showed defluorination, oxygen incorporation, and extensive unsaturation within the treated PTFE surfaces. The suitably of these reduced PTFE films as substrates for graft polymerization was initially assessed via photograft polymerization of the sodium salt of styrenesulfonic acid (SS-Na), which permitted unequivocal surface analysis by the introduction of a new atom, as well as poly(ethylene glycol) monoacrylate (PEG-Ac). All photograpt polymerization was performed employing ultraviolet irradiation with 2,2-dimethoxy-2-phenylacetophenone as an initiator. Photograft polymerization of SS-Na was verified by further reduction of fluorine atomic content and the appearance of new sulfur and sodium atomic peaks on ESCA survey spectra, and that of PEG-Ac was verified by further reduction of fluorine atomic content and increase of atomic percent ratio of O/C from ESCA survey spectra as well as appearance of a new ester peak on high resolution ESCA C 1s spectra. Dynamic water contact angles on reduced and PEG-Ac photograft polymerized films were measured and showed that the PTFE film surface became more hydrophilic after reduction (from 120 to 89 deg) and the reduced film became more hydrophilic after photograft polymerization with PEG-Ac (from 89 to 36 deg). Modification of the complete surface of expanded PTFE (ePTFE), i.e. of the lumenal, outside and pore surfaces, was performed by employing the reaction described above, except at 105 degrees C for 1 day, followed by photograft polymerization of PEG-Ac. ESCA was performed on the superficial surfaces (i.e. the lumen and exterior) as well as on cross-sections of the ePTFE to permit analysis of the pore surfaces. This analysis showed that both the initial surface reduction and subsequent photograft polymerization were successful as indicated from F/C and O/C atomic percent ratios from ESCA survey spectra, from overall peaks shapes of high resolution ESCA C 1s spectra and from generation of new ester peaks on high resolution ESCA C 1s spectra of ePTFE graft polymerized with PEG-Ac, which demonstrated an O/C atomic percent ratio close to that of PEG-Ac homopolymer. Low voltage scanning electron microscopy confirmed minimal morphological damage to the ePTFE microstructure after reduction and graft polymerization. The approach explored thus provides a means for modulation of biological interactions at ePTFE surfaces with only minimal modification of material morphology, with some surface texture appearing on a length scale of 50-100 nm.

Polymer-protein interactions in aqueous two phase systems: fluorescent studies of the partition behaviour of human serum albumin

This study describes the partitioning of fluorescent macromolecules in aqueous two-phase systems (ATPS) comprising phosphate salt and poly(ethylene glycol) of three different molecular masses (i.e. 1000, 1450 and 2000 Da). The impact of system assembly was studied with fluorescent macromolecules introduced in contact with either (i) first salt, then polymer or (ii) first polymer, then salt, or (iii) with both salt and polymer simultaneously. Native human serum albumin (HSA) and derivatives labelled with N-(iodoacetylaminoethyl)-5-naphthylamine-1-sulphonic acid (1,5-IAEDANS) were partitioned using selected ATPS. Partitioning behaviour was characterised by molecular rotational studies of recovered proteins based upon changes of depolarisation. Measurements were undertaken by steady-state fluorescence or time-decay fluorescence using a single-photon counting system. In addition, circular dichroism was used as a tool for the study of macromolecular secondary structure. Two discrete categories of stable molecular structure have been identified that exist irrespective of the phase environment. The findings form the basis for a discussion of polymer protein interactions and the molecular micro-environment of proteins in ATPS.

A novel method for surface modification to promote cell attachment to hydrophobic substrates

The ability to study and regulate cell behavior at a biomaterial interface requires strict control over material surface chemistry. Perhaps the greatest challenge to researchers working in this area is preventing the fouling of a given surface due to uncontrolled protein adsorption. This work describes a method for coupling peptides to hydrophobic materials for the purpose of simultaneously preventing nonspecific protein adsorption and controlling cell adhesion. A hexapeptide containing the ubiquitous RGD cell-adhesion motif was coupled to polystyrene (PS) via a polyethylene oxide (PEO) tether in the form of a modified PEO/PPO/PEO triblock copolymer. Triblocks were adsorbed onto PS at a density of 3.3 +/- (5.14 x 10(-4)) mg/m2 (1.4 x 10(5) +/- 2.12 x 10(1) molecules/microm2), which was determined by isotope 125I labeling. The peptide, GRGDSY, was activated at the N terminus with N-Succinimidyl 3-(2-pyridyldithio) propionate and coupled to immobilized triblocks where the terminal hydroxyls had been converted to sulfhydryl groups. Surface peptide density was measured by amino acid analysis and found to be 1.4 x 10(4) +/- 0.47 x 10(4) molecules/microm2. PS modified with PEO/PPO/PEO copolymers alone was found to be inert to cell adhesion both in the presence of serum proteins and when exposed to activated RGD peptide. In contrast, PS conjugated with RGD via endgroup-activated PEO/PPO/PEO copolymers supported cell adhesion and spreading. The surface coupling scheme reported here should prove valuable for studying cell-ligand interactions under simplified and highly controlled conditions.

Grafting of PEO to polymer surfaces using electron beam irradiation

A new method was developed for binding poly-(ethylene oxide) (PEO) to polymer surfaces that involves the use of electron beam irradiation in two steps. In the first, methacrylic acid was grafted and polymerized to a polymer surface, changing it from hydrophobic to hydrophilic. Exposure of this surface to aqueous PEO solutions resulted in strong hydrogen bonding of the PEO, which was covalently grafted in a second radiation step. The PEO grafts were stable; they could not be removed with extensive washing with water, soaking in basic solution, or gentle mechanical scraping. Both monolayers and multilayers of PEO were formed. The density of the monolayers were found to have little dependence on the molecular weight or concentration of the PEO solution; multilayers could be controlled by varying the viscosity of the PEO solution and the method of application. The PEO-grafted monolayers were tested for their ability to prevent protein adsorption of cytochrome-c, albumin, and fibronectin. Monolayers of star PEO were the most effective, at best showing a 60% decrease in adsorption from untreated controls. One million molecular wight linear PEO monolayers were almost as effective as star monolayers, and 35,000 g/mol linear PEO was bound too closely to the surface, owing to its small size, to have much impact in preventing protein adsorption. The reason for the continued protein adsorption was believed to be due to a close grafting of the PEO to the surface, as well as the grafted methacrylic acid chains being long enough to extend through the PEO monolayer, thus being accessible on the surface.

On the chemical modification of pacemaker electrodes and patterned surface functionalization of planar substrates

We report on experiments towards the chemical modification of metal electrodes in order to enhance biocompatibility or improve cell adhesion properties. In the first example pacemaker electrodes were modified with a thin polysiloxane network which allowed for further derivatization with a poly(ethylene glycol) layer. The primary goal was to suppress inflammatory response of tissue after implantation of electrodes. FTIR, ESCA and a.c.-impedance spectroscopy show the integrity of the ultrathin membrane. No significant reduction of the electrode capacitance was observed, providing further proof for the deposition of a homogeneously thin membrane. The second example deals with the patterned chemical modification of planar surfaces. The goal was to eventually effect selective adhesion of electrosensitive cells above microelectrodes for stimulation and/or recording. First results demonstrate the compatibility of monolayer deposition techniques with common photolithography. It is thus possible to create surfaces with patterned chemical functionality. A gas-phase silylation process was developed in order to control more precisely surface hydration and reaction parameters than is possible with common solution-based silylation procedures.

Bipolar pacemaker leads: new materials, new technology

This commentary is in response to a review published earlier in this journal. It is intended to provide additional information and supplement the original paper. A short review of the failure mechanisms of polyurethane pacing lead materials is provided. Two specific degradation mechanisms, environmental stress cracking and metal ion oxidation, are discussed. Environmental stress cracking has been extensively studied and is a well understood failure mechanism. Methods for reducing the problem have been developed and tested in vivo. As a result, stress cracking failures can be virtually eliminated. Metal ion oxidation failures now dominate pacing lead recalls. Two new materials, polycarbonate urethanes and ethylenetetrafluoroethylene, have been introduced as insulators for pacing leads. These materials do not fail by stress cracking and preliminary test results are very positive.

Mechanism of adsorption of human albumin to titanium in vitro

Our previous studies have shown that human albumin is one of the main salivary proteins that adsorb to titanium (Ti). The goal of the present study was to investigate the role of electrostatic interactions in the adsorption of human albumin to Ti-oxide (TiO2) in vitro. The binding profile of human albumin to Ti was analyzed according to an adsorption isotherm. Purified human serum albumin (HSA) was suspended with native, calcium-, magnesium-, or potassium-treated commercially pure Ti powders, at pH 3.0 and 7.0. The amount of unadsorbed protein in the supernatant fluid was measured. The maximum amount of adsorbed albumin was 0.13 mg/1.0 g Ti. The albumin-Ti association constant was 2.77 mL/mg. Pretreatment of Ti with calcium, or magnesium alone, or combined with increasing pH values (3.0-7.0) resulted in augmented adsorption of HSA to Ti. No increase in adsorption was observed following pretreatment of Ti with potassium. These results point to the involvement of electrostatic interactions in the adsorption of HSA to TiO2.

Protein adsorption on surfaces with grafted polymers: a theoretical approach

A general theoretical framework for studying the adsorption of protein molecules on surfaces with grafted polymers is presented. The approach is a generalization of the single-chain mean-field theory, in which the grafted polymer-protein-solvent layer is assumed to be inhomogeneous in the direction perpendicular to the grafting surface. The theory enables the calculation of the adsorption isotherms of the protein as a function of the surface coverage of grafted polymers, concentration of protein in bulk, and type of solvent molecules. The potentials of mean force of the protein with the surface are calculated as a function of polymer surface coverage and amount of protein adsorbed. The theory is applied to model lysozyme on surfaces with grafted polyethylene oxide. The protein is modeled as spherical in solution, and it is assumed that the protein-polymer, protein-solvent, and polymer-solvent attractive interactions are all equal. Therefore, the interactions determining the structure of the layer (beyond the bare polymer-surface and protein-surface interactions) are purely repulsive. The bare surface-protein interaction is taken from atomistic calculations by Lee and Park. For surfaces that do not have preferential attractions with the grafted polymer segments, the adsorption isotherms of lysozyme are independent of the polymer length for chains with more than 50 ethylene oxide units. However, the potentials of mean force show strong variations with grafted polymer molecular weight. The competition between different conformations of the adsorbed protein is studied in detail. The adsorption isotherms change qualitatively for surfaces with attractive interactions with ethylene oxide monomers. The protein adsorption is a function of chain length--the longer the polymer the more effective it is in preventing protein adsorption. The structure of the layer and its deformation upon protein adsorption are very important in determining the adsorption isotherms and the potentials of mean force.

Adsorption behaviour of amphiphilic polymers at hydrophobic surfaces: effects on protein adsorption

The adsorption of four different amphiphilic polymers to a model surface has been studied, and the effects of the adsorbed amphiphiles on the subsequent adsorption of fibrinogen (Fg) and human serum albumin (HSA) at the surfaces were investigated. The amphiphilic polymers were one commercially available ABA block copolymer, Pluronic PE9400 (PE94), composed of poly(ethylene oxide) (A-blocks) and poly(propylene oxide) (B-block), and three graft copolymers, two with backbones of poly(styrene-co-acrylamide) (STY) and one with a backbone of poly(methyl methacrylate-co-ethylhexyl methacrylate) (ACRY). The backbones carried poly(ethylene oxide) (PEO) grafts. The model surface was a hydrophobic methylated silica surface (HMS). The amphiphilic polymers were adsorbed at the HMS surface from an ethanol/water solution. The adsorption process was monitored by ellipsometry. After rinsing with phosphate buffered saline (PBS), protein was added and the continued adsorption measured by ellipsometry. Surfaces modified by adsorption of the amphiphilic polymers were also characterized by contact angle measurements and X-ray photoelectron spectroscopy (XPS). According to these measurements the amphiphilic polymers adsorbed in significant amounts at the HMS surface. A limited study by atomic force microscopy (AFM), as well as the XPS measurements, suggests that both single molecules and micellar aggregates adsorb at the surface. ACRY and PE94 gave the highest levels of adsorption. As compared to the Pluronic block copolymer the graft copolymers were more strongly attached to the HMS surface, as shown by less desorption on rinsing with solvent. The ellipsometric results show that the adsorption of HSA and Fg at HMS surfaces containing preadsorbed amphiphilic polymer was significantly reduced as compared to the bare HMS surface. ACRY and PE94 showed the largest effects. Both polymers gave more than a 20-fold reduction of the Fg adsorption and a 10-fold reduction of the HSA adsorption. The STY polymers reduced the protein adsorption by a factor of 2-3.

Fibrinogen adsorption to receptor-like biomaterials made by pre-adsorbing peptides to polystyrene substrates

Two peptides from the ligand-binding site of the platelet receptor GPIIb/IIIa, residues 296-306 of GPIIb, designated B12 by D'Souza et al. (1991), and 300-311 of GPIIb, designated G13 by Taylor et al., (1992), as well as two control peptides, designated C14 and C20, were adsorbed to treated polystyrene substrates. Fibrinogen adsorption to the peptide-coated substrates was characterized. The specificity of I-125 labeled fibrinogen binding to the peptide-coated substrates was investigated by measuring the amount of fibrinogen adsorbed to each substrate and the inhibition of fibrinogen binding by RGDS peptide, bovine serum albumin, a divalent ion chelator (ethylene diamine tetra-acetic acid disodium salt), unlabeled fibrinogen and the B12 peptide. The results show that non-specific binding of fibrinogen to the B12-coated substrate is predominant under most conditions. Binding of monoclonal antibodies to fibrinogen adsorbed to the peptide coated substrates was characterized. The failure of several antibodies to bind fibrinogen adsorbed to the B12 substrate suggested that adsorption of fibrinogen to the B12-coated substrate alters its conformation relative to fibrinogen adsorbed to the bare substrate.

Poly(ethylene oxide)-modified carboxylated polystyrene latices--immobilization chemistry and protein adsorption

alpha,omega-Diamino poly(ethylene oxides) (PEOs) with different molecular weights (148, 1000, and 3400) were covalently immobilized onto carboxylated polystyrene latices. The immobilization of PEO was carried out with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) in aqueous media. The reaction conditions were optimized to obtain a maximal coupling of PEO. The degree of coupling was determined by the surface concentration of amino groups. The maximal surface concentrations of amino groups were close to what is expected for a complete coverage of the surface with PEO. Adsorption of albumin from a buffer solution onto PEO-containing surfaces was about 85% less than the albumin uptake by unmodified polystyrene latices. Protein adsorption from plasma dilutions was lower on surfaces containing PEO molecules with a higher molecular weight. The reduction of the protein uptake from plasma by surfaces containing PEO-3400 molecules was only 40% compared to the adsorption to unmodified surfaces. These results indicate that plasma proteins have a low affinity for surfaces modified with PEO. However the PEO modified surfaces are by no means 'protein resistant' when exposed to plasma.

Platelet adhesion and cellular interaction with poly(ethylene oxide) immobilized onto silicone rubber membrane surfaces

Cellular interaction and platelet adsorption were investigated on poly(ethylene oxide) (PEO) immobilized silicone rubber membrane (SR) which has polyacrylic acid grafts on the surfaces. Polyacrylic acid (PAA) had been introduced to the SR surface after Ar plasma treatment of SR surfaces to introduce peroxide groups. Surface characterizations were made using ATR-FTIR, ESCA, SEM, and contact angle measurements. Experimental results obtained by ESCA high resolution curve fitting spectra indicated that the amount of bisamino PEO of different molecular weights immobilized onto SR surfaces were similar, which showed that the influence of the length of molecular chains (-C-C-O-) on the reactivity of terminal amino group is negligible. The wettability of modified SR surfaces increased with an increase in PEO molecular weight. Biological studies such as corneal epithelial cell culture and blood platelet adhesion were performed to understand the biocompatibility of modified SR surfaces. Biological studies using corneal epithelial cells showed that cell migration, attachment and proliferation onto PEO-20000 immobilized SR surface were suppressed, whereas these biological activities on PEO-600 were enhanced. Another study on platelet adhesion revealed that many platelets attached to PEO-600 immobilized SR, while platelet deposition was rarely observed on SR grafted with PEO-3350. The effects of different PEO molecular chains on biological response were discussed.

Contact activation of the plasma coagulation cascade. II. Protein adsorption to procoagulant surfaces

A study of blood protein adsorption to procoagulant surfaces utilizing a coagulation time assay, contact angles, Wilhelmy balance tensiometry, and electron spectroscopy (ESCA) is presented. Using a new contact angle method of measuring protein adsorption termed "adsorption mapping" it was demonstrated that protein-adsorbent surfaces were inefficient activators of the intrinsic pathway of the plasma coagulation cascade whereas water-wettable, protein-repellent surfaces were efficient procoagulants. Repeated use of fully water-wettable (spreading) glass procoagulants in the coagulation time assay demonstrated that putative "activating sites" were not consumed in the coagulation of platelet-poor porcine plasma. Furthermore, these procoagulant surfaces retained water-wettable surface properties after incubation with blood proteins and saline rinse. The interpretation of these observations was that plasma and serum proteins were not adsorbed to water-wettable surfaces. However, ESCA of these same surfaces revealed the presence of a thin protein layer. Wilhelmy balance tensiometry resolved these seemingly divergent observations by demonstrating that protein was "associated" with a bound hydration layer, but not formally adsorbed through a surface dehydration or ionic interaction mechanism.

Densely crosslinked polymer networks of poly(ethylene glycol) in trimethylolpropane triacrylate for cell-adhesion-resistant surfaces

Densely crosslinked semi-interpenetrating polymer networks (semi-IPNs) of poly(ethylene glycol) (PEG) were synthesized by photopolymerizing a melt of PEG of various molecular weights and end-group functionalities in neat trimethylolpropane triacrylate (TMPTA). Increasing the molecular weight of PEG in the matrix from 1000 to 100,000 g/mol reduced the advancing and receding contact angles, contact angle hysteresis, and adsorption of human fibrinogen and bovine serum albumin. Crosslinked TMPTA homonetworks supported human fibroblast adhesion in vitro, whereas the resistance to cell adhesion of the semi-IPNs depended upon PEG molecular weight: Lower molecular weight PEG reduced the number of adherent cells; higher molecular weight PEG further reduced and eliminated cell adhesion, as did networks containing acrylate-functionalized PEG. A polymer system incorporated with PEG throughout a hydrophobic, densely crosslinked matrix, rather than as a blend or surface treatment, may be particularly useful for limiting biologic interactions when bulk material properties must be independent of the solvent environment and where surface abrasion may occur.

Effects of branching and molecular weight of surface-bound poly(ethylene oxide) on protein rejection

To understand better the origin of protein rejection observed with surface-bound poly(ethylene oxide) (or PEO), we have measured fibrinogen adsorption for a series of linear and branched, low-molecular-weight PEOs bound to solid polystyrene surfaces. The results show that a dependence on molecular weight is found below 1500 g mol-1 for linear PEO. Branched PEOs are less effective at protein rejection than linear PEOs. The branched PEOs have smaller exclusion volumes (from GPC) than the corresponding linear PEOs, consistent with restriction in conformational freedom for the branched compounds. The protein rejection results are interpreted in terms of entropy changes that result upon protein adsorption. In addition, some practical problems in preparation of PEO glycidyl ethers have been clarified, thus making these PEO derivatives more useful for surface modification.

The surface density gradient of grafted poly (ethylene glycol): preparation, characterization and protein adsorption

A surface density gradient of grafted poly (ethylene glycol) (PEG) chains was prepared using two-phase silanization of a flat silica surface. The first step was to create the surface density gradient of isocyanatopropyldimethylsilyl groups and to hydrolyze the isocyanato moiety into an amine. These surface amines were reacted with an excess of aldehyde-terminated PEG. The PEG-silica surface was characterized by dynamic contact angle measurements, X-ray photoelectron spectroscopy and ellipsometry. The length of the PEG gradient region was approximately 7 mm and the thickness in air ranged from zero to 1.1 nm. The maximum surface density of the PEG layer, as calculated from ellipsometric data, amounted to an average 0.4 PEG (molecular weight Mw = 2000 Da) molecule nm-2, while the surface density average of the amine groups was 1.4 molecules nm-2, indicating that only a fraction of the surface amines reacted with aldehyde-terminated PEG. The PEG segment density profile in the gradient PEG region was computed by a self-consistent mean field theory. The PEG (Mw = 2000 Da) segments profile was not parabolic, but showed a thin depletion zone next to the surface. The influence of the surface density of the grafted PEG chains on protein repellence was tested by the adsorption of fibrinogen from solution and from a ternary protein solution mixture containing fibrinogen, albumin and immunoglobulin G. Fibrinogen adsorption onto the silica end of the gradient was extremely low, both in the presence of the other two proteins and in their absence. As the surface density of the grafted PEG chains increased, so did the fibrinogen adsorption (up to 0.024 μg cm-2). It is not clear whether this low fibrinogen adsorption resulted from the interactions of the protein with the grafted PEG chains or with residual surface amines that were available due to some imperfections in the grafted PEG layer.

Inhibition of platelet spreading from plasma onto glass by an adsorbed layer of a novel fluorescent-labeled poly(ethylene oxide)/poly(butylene oxide) block copolymer: characteristics of the exclusion zone probed by means of polystyrene beads and macromolecules

We have investigated the anti-adhesive properties of a newly synthesized fluorescent triblock copolymer containing poly(ethylene oxide). This adsorbs from aqueous solution onto glass that has been rendered hydrophobic. When the polymer-treated surface was exposed to human platelet-rich plasma (PRP) or whole blood at 37 degrees C, platelet adhesion and spreading were prevented. Avid adhesion and rapid platelet spreading occurred along tracks scraped in the adsorbed polymer coating, as seen by video-enhanced interference reflection microscopy. Leukocytes from whole blood are eventually able to adhere to the polymer-treated surface and were seen to remove labeled polymer from their vicinity and accumulate it at the cell body. Interferometry using polystyrene spheres showed that they do not adhere to polymer-coated glass and are unable to approach closer than 70-95 nm. On scraped tracks, beads make molecular contacts with the glass. Because the fully extended solvated (EO)400 arms may extend up to 100 nm from the glass, this suggests that the polymer forms a monolayer with the hydrophilic arms projecting into the water, whereas the hydrophobic (BO)55 segment binds the molecule to the hydrophobic surface. Another tri-bloc copolymer with shorter hydrophilic arms allows particles to approach more closely.

Adsorption of a novel fluorescent derivative of a poly(ethylene oxide)/poly(butylene oxide) block copolymer on octadecyl glass studied by total internal reflection fluorescence and interferometry

We have used total internal reflection fluorescence (TIRF) to measure the adsorption kinetics of a newly synthesized fluorescent derivative of a triblock copolymer comprising two poly(ethylene oxide) arms connected by a poly(butylene oxide) segment. The composition is (EO)400 (BO)55 (EO)400, in which EO represents ethylene oxide, BO represents butylene oxide, and one or both of the terminal OH groups of the two (EO)400 arms are labeled with tetramethylrhodamine. The poly(butylene oxide) segment binds to hydrophobic octadecyl glass, used as a substratum. The TIRF signal is shown to be derived almost entirely from surface-adsorbed polymer. This facilitates calculation of adsorption isotherms from 0.1-0.005% bulk polymer solution by means of diffusion kinetics. Information about the effective thickness of the adsorbed polymer, determined by optical interference microscopy, corresponds with what is known about the conformation of similar molecules at interfaces and indicates monolayer adsorption on the glass.

Protein adsorption and platelet adhesion onto polyurethane grafted with methoxy-poly(ethylene glycol) methacrylate by plasma technique

Graft polymerization of methoxy-poly(ethylene glycol) methacrylate, an ester of methacrylic acid and monomethoxy-poly(ethylene glycol) (PEO), was performed onto a polyetherurethane (PU) film and tube under different polymerization conditions by a plasma treatment technique. The surface of grafted PU film was characterized by staining with dye, x-ray photoelectron spectroscopy, contact angle, and zeta potential. All these measurements indicated that water-soluble chains were immobilized on the PU surface, their location being restricted to the film surface region. The PU surface showed reduced protein adsorption in vitro and reduced platelet adhesion in vitro and ex vivo. The optimum graft density suppressing the protein adsorption was as low as 5 micrograms cm(-2). When a small amount of dimethacrylate was added to the monomer solution for graft polymerization to introduce crosslinking in the grafted layer, protein adsorption was further slightly reduced. The extent of reduction in serum albumin adsorption was always less than that of gamma-globulin. Although platelet adhesion was largely reduced by the surface graft polymerization, a definite amount of protein was always adsorbed to the grafted surface.

X-ray photoelectron spectroscopy studies, surface tension measurements, immobilization of human serum albumin, human fibrinogen and human fibronectin onto ammonia plasma treated surfaces of biomaterials useful for cardiovascular implants and artificial cornea implants

XPS studies of untreated and ammonia plasma treated surfaces of PTFE, ePTFE, Dacron, P(HEMA), PMMA, Silastic and PS were carried out. Ammonia plasma treatment caused significant changes in the surface composition. The curve-fitting results confirmed the incorporation of nitrogen and oxygen in the form of functional groups such as C-N, C=O, C-O, Si-N, Si-OH etc. Increases in the values of surface tension occurred. The surface tension of plasma treated surfaces varied between 44-48 erg/cm2 with the exception of Dacron which became wettable. Enhanced immobilization of human albumin on plasma treated surfaces was achieved. When washed with 0.2% Tween in buffer, these albuminated surfaces were found to be stable compared to control samples. Increased immobilization of human fibrinogen was also observed. The ammonia plasma treated surfaces showed high binding properties and retention for human fibronectin. Ionic interaction between proteins solution and plasma treated surfaces may be cause of the increase attachment of these biological molecules.

Surface modification of polymeric biomaterials with poly(ethylene oxide), albumin, and heparin for reduced thrombogenicity

Appropriate surface modification has significantly improved the blood compatibility of polymeric biomaterials. This article reviews methods of surface modification with water-soluble polymers, such as polyethylene oxide (PEO), albumin, and heparin. PEO is a synthetic, neutral, water-soluble polymer, while albumin and heparin are a natural globular protein and an anionic polysaccharide, respectively. When grafted onto the surface, all three macromolecules share a common feature to reduce thrombogenicity of biomaterials. The reduced thrombogenicity is due to the unique hydrodynamic properties of the grafted macromolecules. In aqueous medium, surface-bound water-soluble polymers are expected to be highly flexible and extend into the bulk solution. Biomaterials grafted with either PEO, albumin, or heparin are able to resist plasma protein adsorption and platelet adhesion predominantly by a steric repulsion mechanism.

Does polyethylene oxide possess a low thrombogenicity?

Because of the 'bland' nature of polyethylene oxide towards proteins and cells, considerable effort has been devoted to preparing surfaces rich in polyethylene oxide, using block copolymers, surface immobilization or other methods. It is clear that these modifications result in reduced levels of cell (including platelet) adhesion and protein adsorption, when compared to unmodified and typically hydrophobic substrates. It is far less clear whether the reduced adhesion or adsorption is due specifically to the thermodynamic effects of polyethylene oxide or to the increase in surface hydrophilicity after its immobilization. Even more so, it is unclear whether the reduction in such parameters is evidence of a reduced thrombogenicity.

Adsorption of baboon fibrinogen and the adhesion of platelets to a thin film polymer deposited by radio-frequency glow discharge of allylamine

Platelet adhesion under static and flow conditions from a washed platelet suspension containing albumin to a polymer deposited by radio-frequency glow discharge of allylamine vapour on a poly(ethylene terephthalate) substrate was measured. Electron spectroscopy for chemical analysis was used to characterize the surface. Fibrinogen adsorption from a series of dilute plasma solutions to radio-frequency glow discharge/allylamine, measured using 125I radiolabelled baboon fibrinogen, increased with decreasing plasma dilution to a level much higher than that previously observed on polyurethanes. Elutability by sodium dodecyl sulphate of fibrinogen adsorbed from dilute plasma also increased with increasing plasma concentration, but fibrinogen preadsorbed from plasma became non-elutable when surfaces were stored in buffer for 5 d before contact with sodium dodecyl sulphate. Platelet adhesion to substrates which had been pre-adsorbed with dilute plasma was measured using baboon platelets radiolabelled with 111In. Adhesion greatly decreased as the plasma concentration used for preadsorption increased, suggesting that non-specific platelet binding to the bare surface occurs when protein coverage is incomplete. Non-specific platelet binding was inhibited to varying degrees by preadsorption of different proteins to the surface. Platelet adhesion to surfaces preadsorbed with dilute (1.0%) baboon and human plasmas lacking fibrinogen (i.e. serum, heat-defibrinogenated plasma and congenitally afibrinogenemic plasma) was diminished compared with normal plasma. Addition of exogenous fibrinogen to the deficient plasma partially restored platelet adhesion to normal levels. Adhesion to surfaces preadsorbed with human plasma deficient in von Willebrand factor was comparable to that observed with normal plasma. The plasma preadsorption studies with fibrinogen deficient media suggested that adsorbed fibrinogen is necessary for platelet adhesion to the radio-frequency glow discharge/allylamine substrate at high protein coverage. However, since adhesion was greatly reduced when the plasma preadsorbed substrate was stored in buffer before platelet contact, the conformation of adsorbed fibrinogen is also important in mediating platelet adhesion to radio-frequency glow discharge.