Albumin-binding surfaces: synthesis and characterization

Functional monolayers for improved resistance to protein adsorption: oligo(ethylene glycol)-modified silicon and diamond surfaces

The interaction of proteins with semiconductors such as silicon and diamond is of great interest for applications such as electronic biosensing. We have investigated the use of covalently bound oligo(ethylene glycol), EG, monolayers on diamond and silicon to minimize nonspecific protein adsorption. Protein adsorption was monitored by fluorescence scanning as a function of the length of the ethylene glycol chain (EG3 through EG6) and the terminal functional group (methyl- versus hydroxyl-terminated EG3 monolayer). More quantitative measurements were made by eluting adsorbed avidin from the surface and measuring the intensity of fluorescence in the solution. The attachment chemistry of the tri(ethylene glycol) molecules and monolayer orientation was studied by X-ray photoelectron spectroscopy. Improvement in the selectivity of surfaces modified with EG functionality was demonstrated in two model biosensing assays. We find that high-quality EG monolayers are formed on silicon and diamond and that these EG3 monolayers are as effective as EG3 self-assembled monolayers on gold at resisting nonspecific avidin adsorption. These results show promise for use of silicon and diamond materials in many potential applications such as biosensing and medical implants.

Protein adsorption on mixtures of hydroxyl- and methyl-terminated alkanethiols self-assembled monolayers

The effect of surface composition and wettability on the adsorption of human serum albumin (HSA) was studied. Self-assembled monolayers (SAMs) containing mixtures of longer chain methyl- and shorter chain hydroxyl-terminated alkanethiols on gold were used to produce a range of surfaces with different wettabilities and exposed functional groups. Different SAMs were characterized by X-ray photoelectron spectroscopy, water contact angles, and Fourier transform infrared reflection absorption spectroscopy (IRAS). HSA adsorption onto the different SAMs was evaluated by contact angle measurements (wetting tension determinations), radiolabeling of proteins, and IRAS. Concerning HSA adsorption, all the techniques demonstrated higher HSA adsorption on more hydrophobic surfaces. The wetting tension measurements and IRAS suggested a gradual decrease of the HSA adsorption with increases of surface hydrophilicity. Radiolabeled albumin measurements also demonstrated a significant decrease of HSA adsorption on the pure hydroxyl-terminated SAMs. However, no significant differences were detected between mixed and pure methyl-terminated SAMs. Studies of HSA exchangeability with human fibrinogen have suggested that an ideal percentage of hydroxyl groups on the surface may increase albumin affinity without fibrinogen adsorption.

Effects of base material, plasma proteins and FGF2 on endothelial cell adhesion and growth

Blood-contacting materials rapidly acquire a coating of plasma proteins which can lead to local platelet activation and thrombus formation. This phenomenon seriously limits the usefulness of small diameter synthetic vascular grafts. One solution to this problem is to pre-seed or encourage in situ colonisation of the material with endothelial cells to maintain a non-thrombogenic surface. We have investigated the effect of contact with plasma and serum on the subsequent ability of human endothelial cells to adhere to model hydrophobic and hydrophylic plastic surfaces, and the effect of surface bound fibroblast growth factor 2 (FGF2) on endothelial cell proliferation. Cell adhesion was mainly dependent on adsorbed fibrinogen or vitronectin, depending on the polymer surface, and correlated with antibody binding to these molecules rather than quantitative surface concentrations. Cell proliferation was directly correlated with surface bound FGF2. Surface binding of the latter was controlled both by the chemical nature of the polymer surface and by the presence of FGF-binding molecules adsorbed on the surface. FGF2 bound specifically to surface-adsorbed fibrinogen, fibronectin and vitronectin as well as to pre-coated heparan sulphate proteoglycan, perlecan. Binding was significantly inhibited by plasma and serum which contained high levels of FGF2 binding proteins. To be effective in supporting endothelialisation of vascular grafts in vivo, surface-bound FGF2 would need to be protected from surface dissociation into the circulating blood.

Preparation of albumin preferential surfaces on poly(vinyl chloride) membranes via surface self-segregation

Poly(vinyl chlorides)-graft-[omega-stearyl-poly(ethylene oxide)] (PVC-g-SPEO), which has a poly(vinyl chloride) (PVC) backbone, poly(ethylene oxide) (PEO) side chain, and stearyl end groups, has been synthesized. Self-organizing blends of the amphiphilic comb polymer in poly(vinyl chlorides) have been examined as a means to create albumin preferential surfaces on polymer films. X-ray photoelectron spectroscopy (XPS) analysis indicates substantial surface segregation of the PVC-g-SPEO. A surface concentration of 59.9 EO wt % is achieved by the solution casting and heat treatment of a film with a bulk concentration of only 3.78 EO wt %. In the aqueous environment, the surface rearrangement of PVC-g-SPEO/PVC blend film is limited and presents a high interfacial energy and high depolar component of interfacial energy due to the "tail-like" SPEO side chain. Protein adsorption tests confirm that PVC-g-SPEO/PVC blend films absorb high levels of albumin and dramatically resist fibrinogen adsorption. Surfaces to attract and reversibly bind albumin, which might diminish the occurrence of thrombosis, inflammation, and infection, are developed by self-organizing blends of the amphiphilic comb polymer in poly(vinyl chlorides).

Effect of pre-adsorbed proteins on attachment, proliferation, and function of endothelial cells

As certain proteins control cell adhesion, it has been hoped that cell transplantation and tissue engineering could be augmented by pre-adsorption of specific proteins to biological or synthetic surfaces. The questions that remain, however, are whether such proteins can affect cell production as well as adhesion, and if so, whether in a protein-specific manner. We examined the adhesion and the biochemical secretion of bovine aortic endothelial cells (BAEC) on tissue culture polystyrene (TCPS) discs coated with fibronectin (Fn), laminin (Ln), or gelatin. The three coating proteins nonspecifically promote sub-confluent and post-confluent endothelial cell production of total protein up to 2.5-fold of the reference value. Total soluble glycosaminoglycan (GAG) production slightly increased with the different coatings only at low cell density. In contrast, Ln and Fn, not gelatin, drastically enhanced post-confluent BAEC production of prostaglandin (PGI2). However, antibody-blockage of the alpha5 integrin, constituent of the Fn receptor in BAEC, appeared to inhibit the upregulation of PGI2 production observed on Fn-coated surfaces. The results indicate that the cell adhesion mediators used as coating agents dictate cell biological production as well as adhesion and proliferation.

Reduced protein adsorption and platelet adhesion by controlled variation of oligomaltose surfactant polymer coatings

A series of oligomaltose surfactant polymers were prepared by the simultaneous coupling of hydrophilic maltolactone [of 2(M2), 7(M7), or 15(M15) glucose units] and hydrophobic N-(hexanoyloxy)succinimide (Hex) groups to the amino groups of a poly(vinyl amine) backbone. The surfactants were characterized by FTIR and 1H-NMR spectroscopies for purity and composition. Contact-angle and AFM measurements confirmed full monolayer adsorption for all surfactants on a model surface, highly oriented pyrolitic graphite, while full coverage was observed on polyethylene only for PVAm (M7:Hex) due to the optimal M7:Hex ratio and Hex chain density. On graphite, protein resistance increased with increasing coating thickness from 81.4 to 85.8 to 95.8% for the M2, M7, and M15 surfactants, respectively. Additionally, static platelet adhesion on all three surfactants dropped substantially to 15% (M2), 17% (M7), and 16% (M15)compared to glass (adhesion normalized to 100%) and a polyurethane (24%) surface. Protein- and platelet-resistant properties of the controlled oligomaltose layers are discussed by analysis of molecular modeling, oligomaltose and hexanoyl chain densities, and surfactant stability.

Albumin-binding surfaces: in vitro activity

Immobilized monoclonal antibodies (Mabs) have been used to attract specific molecules to a solid surface from complex mixtures such as blood, plasma or serum, thereby directing the response to the modified substrate, a key goal in rational biomaterial design. The nature of the Mab dictated the nature of the response: anti-albumin antibodies were used to prevent cell and platelet adhesion in vitro, whilst anti-fibronectin Mabs promoted attachment. Patterned surfaces could be formed, bearing Mabs that generated adhesive and non-adhesive regions. Fibrinogen adsorption from plasma showed a Vroman peak on unmodified control polymer, which was reduced by 64% in the presence of surface-bound anti-albumin Mab. Immobilization of a control Mab reduced fibrinogen adsorption only slightly, implying an albumin-mediated effect. In static tests, platelet adhesion from human platelet rich plasma was significantly reduced by the immobilization of anti-HSA Mab when compared to the untreated FEP surface (p < 0.0001). This effect was also seen with citrated blood flowing through Mab-treated polyurethane tubing at a shear rate of 132 s(-1) (p=0.034). Since platelets and proteins (as blood, plasma or serum) were introduced to the surface simultaneously, the generation of a defined protein film must have been sufficiently rapid as to shape the platelet or cell response.