Protein adsorption and materials biocompatibility: A tutorial review and suggested hypotheses

Polyimides as biomaterials: preliminary biocompatibility testing

A number of commercially available polyimide materials were evaluated in vitro using a selected battery of levels I and II testing protocols prescribed by the National Institutes of Health Guidelines for Blood-Material Interactions. These procedures consisted of electron spectroscopy for chemical analysis and contact angle characterization surface studies, and protein adsorption, cell culture cytotoxicity, clotting time and haemolysis biocompatibility testing. The polyimide surfaces were invariant from the bulk composition with 60-80% C, 10-20% O and 2-5% N, producing advancing contact angles in the hydrophobic range (80-100 degrees). Consequently, they adsorbed significant amounts of albumin (2-3 micrograms/cm2) and fibrinogen (0.5-0.8 microgram/cm2). The polyimides also displayed an insignificant level of cytotoxicity and haemolysis, and clotting times ranged from 63 to 98% of normal. These clotting times and haemolytic index values were intermediate between the values observed for Teflon and Silastic controls. These factors, along with the strong adherence of polyimides to metal oxide substrates, indicate that polyimide materials are good candidates for further testing as encapsulants for implantable biosensors.

Integrin receptors and platelet adhesion to synthetic surfaces

The activation-independent and -dependent integrin receptors--glycoproteins GPIc-IIa (alpha 5-beta 1) and GPIIb-IIIa (alpha IIb-beta 3)--are involved in platelet adhesion and thrombus growth on damaged subendothelium through interactions with fibrinogen, fibronectin, von Willebrand factor, and other adhesive proteins. Because these receptors are used in normal in vivo hemostatic adhesion, they may also have a role for adhesion onto synthetic surfaces in the vasculature. Platelet adhesion in vitro was examined onto Formvar, glass, and four polyurethaneureas with various soft segment chemistries and surface properties. Platelets were pretreated with RGD peptides before and after adhesion. RGD peptide pretreatment inhibited spreading and close contact formation compared to treatment with saline or control RGE peptides, with no observable effect on the number of adherent platelets per area. High-voltage electron microscopy showed abnormally sparse and short microfilament structures with RGD peptide treatment, suggesting an indirect inhibition of actin filament formation. Video-enhanced light microscopy showed a cessation of spreading and a partial reversal of close contacts following RGD peptide application to adherent platelets. Because minimal amounts of plasma proteins are present in column-washed platelet suspensions, and as platelet secretion appeared to be minimal in these experiments, these observations suggest that RGD binding integrin receptors may function in platelet spreading even in the absence of exogenous ligand. As RGD peptides did not affect the numbers of adherent platelets, while producing substantial decreases in the extent of spreading, we suggest that platelet integrins, possibly GPIIb-IIIa, are involved in spreading on synthetic surfaces but not for initial adhesion.

Human umbilical vein endothelial cell culture on heparin-like microcarriers

Biospecific functional polymers, i.e., polymers randomly substituted with specific chemical functional groups, were designed to interact with living systems. Interactions between polystyrene sodium sulfonate (PSSO3Na) and insulin secreting RINm5F cells have been previously described. For the sake of comparison, interactions of PSSO3Na with human umbilical vein endothelial cells (HUVEC) were studied. In this case, the interaction is indirect, i.e., mediated by a binding protein, fibronectin (Fn). This was evidenced by HUVEC culture on Fn precoated PSSO3Na microcarriers. The interactions between PSSO3Na and HUVEC result in a biologically normal proliferation of cells and synthesis and secretion of Von Willebrand Factor (VWF). These results show that different biospecific interactions may occur between cells in culture, binding proteins and polymers randomly substituted with suitable functional groups. HUVEC, when cultured on heparin-like microcarriers, behave differently from other cells like RINm5F, whose interaction with the same polymers is not mediated by binding proteins.

Glucagon adsorption on polymer surfaces with alpha-helical and extended beta-strand conformations: a computational approach

The adsorption of glucagon with alpha-helical and extended beta-strand conformations on polymer surfaces was investigated using a computer simulation approach. An X-ray crystallographic structure of glucagon was employed in the study of the alpha-helical glucagon adsorption. The X-ray structure was then modified to simulate an extended beta-strand structure in the study of the beta-strand glucagon adsorption. Totally, 1632 different adsorption orientations of glucagon with each conformation were examined on polystyrene, polypropylene, polyethylene, poly(hydroxyethyl methacrylate) and poly(vinyl alcohol) surfaces. The calculation of the average adsorption energies among different orientations did not reveal any marked difference between the two conformations. However, when the lowest-energy orientations were compared, the adsorption energy of the extended beta-strand glucagon was always much lower than that of the alpha-helical glucagon, indicating that the adsorption of extended beta-strand glucagon was more energy favorable. The lower adsorption energy for the extended beta-strand glucagon than for the alpha-helical glucagon appeared to be contributed by the lower interaction potential energy of the former conformation. A greater number of the surface atoms in the extended beta-strand glucagon than in the alpha-helical glucagon was also observed. Fourier transform infrared spectroscopic (FTIR) studies in the literature showed conformational changes in adsorption of various larger proteins. Our study appears to provide a theoretical insight for the results from the FTIR studies.

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.

Voltage-dependent capacitance as a probe for albumin adsorption onto a solid surface

The process of adsorption of bovine serum albumin onto a platinum electrode was monitored through the measurement of a nonlinear electrochemical property. The principle of the new method is that a sinusoidal voltage source is applied to a test solution and the waveform of the output current is analyzed by Fourier transformation. It was found that the intensities of the higher harmonics in the Fourier transformation change depending on the concentration of albumin and with time. From the higher harmonics, voltage dependence of the capacitance was quantitatively evaluated. The change of the state of albumin adsorbed onto the platinum plate was also monitored from the pattern of 'crack' of adsorbed albumin by using scanning electron microscopy. These results were discussed in relation to the mechanism of bimodal adsorption of albumin.

Study on the prevention of surface-induced platelet activation by albumin coating

To understand how albumin on the surface inhibits surface-induced platelet activation, we adsorbed albumin on dimethyldichlorosilane-coated glass (DDS-glass) and modified the adsorbed albumin by three different methods. The adsorbed albumin was crosslinked with glutaraldehyde, dried and rehydrated, or digested with trypsin. Surface albumin concentration did not change by crosslinking; however, it decreased by about 15% by a simple dry-and-rehydration process. Trypsin digestion reduced the surface albumin concentration by 50%. Platelets were found to adhere and activate on albumin coated DDS-glass, if the adsorbed albumin was modified. The extent of platelet activation was quantified with two numeric parameters, the spread area and circularity. Fibrinogen adsorption to the dried or digested albumin layer resulted in enhancement of platelet activation, while adsorption of more albumin inhibited platelet activation. The results suggest that albumin can inhibit platelet activation as long as it covers the surface completely and remains flexible on the surface. This study indicates that steric repulsion is one of the mechanisms of surface passivation by albumin.

Diffusion properties of an artificial membrane used for Langerhans islets encapsulation: an in vitro test

Glucose and insulin permeability of an artificial membrane (AN69, HOSPAL, Sweden) used for Langerhans islets encapsulation were investigated. In vivo, a 1 and 7 d intraperitoneal implantation of the AN69 membrane in rats induced a loss of permeability towards glucose and insulin probably due to a protein-coating performed after implantation. In vitro, a protein-coating of the AN69 membrane with fetal calf serum solution reproduced similar results. Thus this in vitro test which mimicks in vivo conditions should be proposed to evaluate rapidly the physicochemical properties of a membrane suitable for pancreatic islets encapsulation.

Influence of a preadsorbed terpolymer on human platelet accumulation, fibrinogen adsorption, and ex vivo blood activation in hemodialysis hollow fibers

Results are presented on kinetics of platelet accumulation in charged polyacrylonitrile (AN69) hollow fibers by continuous data recording under flow conditions (wall shear rate 108-1050 s-1), using suspensions of washed 111In-labeled human platelets in Tyrode's-albumin buffer, containing washed red blood cells (0-40%). Preadsorption of a terpolymer of acrylonitrile, poly(ethyleneoxide) methacrylate and trimethylaminoethyl chloride methacrylate leads to very efficient passivation with respect to platelet accumulation and fibrinogen adsorption. In human ex vivo tests, evaluation of complement peptide C3a, platelet beta-thromboglobulin, leucocyte-polymorphonuclear neutrophile elastase and fibrinopeptide A shows no detectable activation. Furthermore, preadsorption appears to result in simultaneous improvement in hemocompatibility of the blood lines leading to and from the dialysis module. This single pretreatment of dialysis membranes should allow injection of lower doses of anticoagulant to patients submitted to hemodialysis.

Poly(ethylene oxide)-graft-poly(L-lysine) copolymers to enhance the biocompatibility of poly(L-lysine)-alginate microcapsule membranes

A graft copolymer having poly(L-lysine) (PLL) as the backbone and monomethoxy poly(ethylene glycol) (MPEG) as pendent chains was synthesized. This polycationic copolymer was used to form microcapsules with sodium alginate, a polyanion. Microcapsules and model surfaces formed with PLL-graft-MPEG demonstrated reduced protein adsorption, complement binding and cell adhesion in vitro compared to materials with unmodified PLL. Microcapsules with PLL-g-MPEG on the surface were seen to be much more biocompatible than the widely used alginate/PLL/alginate microcapsule in a mouse intraperitoneal implant model. The graft copolymers demonstrated a lower affinity for alginate and increased microcapsule permeability more than PLL. To correct this, pentalayered alginate/PLL/alginate/PLL-g-MPEG/alginate microcapsules were fabricated, and these demonstrated both appropriate permselectivity and enhanced biocompatibility.

Protein adsorption from human plasma is reduced on phospholipid polymers

Protein adsorption from human plasma was investigated on phospholipid polymers, poly (2-methacryloyloxyethyl phosphorylcholine (MPC)-co-n-butyl methacrylate (BMA) or glass by radioimmunoassay and immunogold labeling techniques. In the present studies the focus was to determine the composition and distribution of proteins at the surface of these materials after contact with human blood plasma. On all materials, protein adsorption was detected and included identification of albumin, IgG, fibrinogen, fibronectin, Hageman factor (factor XII), factor VIII/von Willebrand factor, high-molecular-weight kininogen (HMWK) and the complement protein C5. The amount of protein adsorbed decreased with an increase in the MPC composition and appeared to adsorb to the surfaces in a uniform and evenly distributed manner. Therefore, we suggest that MPC moieties play an important role in suppression of protein adsorption. From these findings, it is concluded that the reduction of protein adsorption at the blood contacting surface of phospholipid polymers may result in the inhibition of thrombus formation.

Endothelial cell-selective materials for tissue engineering in the vascular graft via a new receptor

We have found a novel adhesion receptor on the human endothelial cell for the peptide sequence Arg-Glu-Asp-Val (REDV), which is present in the III-CS domain of human plasma fibronectin, with a dissociation constant of 2.2 x 10(-6) M and 5.8 x 10(6) sites/cell. When a synthetic peptide containing this sequence was immobilized on otherwise cell nonadhesive substrates, endothelial cells attached and spread but fibroblasts, vascular smooth muscle cells, and platelets did not. Endothelial monolayers on REDV were nonthrombogenic: endothelial cells attached and spread upon other receptor-binding domains of fibronectin and laminin, but with lesser degrees of specificity or with a loss of nonthrombogenicity. This approach may provide a basis for a tissue engineered vascular graft where endothelial cell attachment is desired, but not the attachment of other blood vessel wall cells and blood platelets.

Immobilization of binding proteins on nonporous supports. Comparison of protein loading, activity, and stability

Four different nonporous particulate materials, nylon, polystyrene, soda-lime silicate glass, and fused silica glass, have been evaluated for their appropriateness as immobilization supports for immunoglobulins. A method of protein quantitation that is usually applied to solutions, the bicinchoninic acid (BCA) assay, was used successfully to directly measure ng amounts of protein immobilized on the supports. Two proteins, a monoclonal antibody to theophylline and the biotin binding protein avidin, were studied. Radioactive theophylline and radioactive biotin were used to measure the activity of the immobilized protein. Ligand binding capacity per mm2 of support was measured as a function of amount of protein immobilized. By measuring both the amount of protein immobilized and its ligand binding capacity, we have determined that antitheophylline antibody adsorbed on polystyrene balls loses almost 90% of its binding activity after 65 h, although little protein is lost from the balls over this time. Avidin retains nearly full activity for biotin on polystyrene. The binding activity of biotinyl-antibody conjugate immobilized on avidin-adsorbed polystyrene is stable, even when stored for over 22 wk. Antibody covalently immobilized on soda-lime silicate glass beads retains its binding activity over long-term storage, although only 0.1 mol of 3H-theophylline bind per mol of immobilized antibody. Using fused silica glass particles as the solid support, the same antibody binds approx 0.6 mol of ligand per mol of immobilized antibody protein. The structural "softness" of the immunoglobulin requires that interaction with the surface be prevented in order to maintain activity.

Self-assembled organic monolayers: model systems for studying adsorption of proteins at surfaces

Self-assembled monolayers (SAMs) of omega-functionalized long-chain alkanethiolates on gold films are excellent model systems with which to study the interactions of proteins with organic surfaces. Monolayers containing mixtures of hydrophobic (methyl-terminated) and hydrophilic [hydroxyl-, maltose-, and hexa(ethylene glycol)-terminated] alkanethiols can be tailored to select specific degrees of adsorption: the amount of protein adsorbed varies monotonically with the composition of the monolayer. The hexa(ethylene glycol)-terminated SAMs are the most effective in resisting protein adsorption. The ability to create interfaces with similar structures and well-defined compositions should make it possible to test hypotheses concerning protein adsorption.

Polymer-mediated electrostatic interactions between charged lipid assemblies and electrolyte solutions: a tentative model of the polyethylene glycol-induced cell fusion

We developed a theoretical model to investigate the interaction between charged lipid aggregates and a water solution containing ions and uncharged polymers. The local concentration of ions and polymer chains around the lipid aggregate have been treated as variational parameters which can be found by minimizing the total energy of the system. We divided the energy into the following main contributions: (a) Solvation energy of the ions. This depends on the local polymer concentration through the variation of the solvent dielectric properties. (b) Ions-lipid aggregate interactions. These depend on the local concentrations both of the ion cloud and polymer chains. (c) Conformational energy of the polymer. This term is related to the inhomogeneous spatial density of the polymer segments. Any direct interaction between the charged lipid surface and the polymer coils has been intentionally neglected. The minimization procedure leads to a non-linear Poisson-Boltzmann equation coupled with a non-linear algebraic equation describing the polymer distribution. The solution of the above system allows one to calculate the ions and polymer spatial distribution around the lipid aggregate. The knowledge of such parameters is useful to predict the effect of non-ionic polymers on the structure and properties of lipid assemblies such as the mean area per lipid molecule, the aggregation number, the critical micellar concentration and the formation of immiscibility gaps in mixed lipid systems. A possible involvement of these parameters into the fusion process between lipid vesicles is discussed.

Surface exclusion and molecular mobility may explain Vroman effects in protein adsorption

Data on protein adsorption usually show that for increasing surface coverage the adsorption velocity decreases much faster than linearly. This contrasts to the classical Langmuir model with an adsorption velocity proportional to the number of unoccupied binding sites. It has been shown that this non-linearity may explain phenomena like transient adsorption of different proteins from a protein mixture or dilution-dependent changes in binding properties, collectively called Vroman effects. However, the molecular mechanisms explaining this non-linear behavior remain to be established. A Monte Carlo simulation model is presented that incorporates steric hindrance, lateral mobility and mutual interactions of adsorbed molecules. Experimental data on the adsorption kinetics of prothrombin and annexin V, a recently discovered anticoagulant protein, at phospholipid bilayers are analyzed with this model. A major conclusion is that the steep decline in adsorption rates for increasing surface coverage can be explained, without assuming repulsive forces between adsorbed molecules, as a surface exclusion effect combined with lateral mobility of adsorbed molecules. The fact that annexin V shows this effect to a much lesser degree than prothrombin is tentatively explained by clustering of adsorbed annexin V molecules. A qualitative effect of lateral mobility on the adsorption characteristics, predicted by the model, is confirmed in experiments in which the fluidity of the bilayers was manipulated.

Influence of surface charge on adsorption of fibrinogen and/or albumin on a rotating disc electrode of platinum and carbon

The adsorption of fibrinogen on to platinum and carbon and of albumin on to carbon was investigated for various changes of the surface by recording the variations of the double-layer capacitance of the electrochemical interface during adsorption, as a function of time. The rate of the second step of the adsorption decreased with increasing negative charge on to platinum but was charge-independent on to carbon. In contrast, on both surfaces, the rate of the first step and the area of the electrode surface in close contact with adsorbed proteins were both found to increase with increasing negative charge of the surface, although at pH 7.4 albumin and fibrinogen are negatively charged. The hypothesis of ion coadsorption inside the proteic layer has been proposed to account for this result.

Role of association on protein adsorption isotherms. Beta-lactoglobulin A adsorbed on a weakly hydrophobic surface

This paper explores the role of association on the adsorption isotherms of beta-lactoglobulin A on a weakly hydrophobic stationary phase at 4 degrees C and mobile phases of 0.85 M and 1 M ammonium sulfate, pH 4.5. The isotherms, obtained by frontal analysis, show an S-shape and the corresponding Scatchard plots indicate positive cooperativity. The slopes and intercepts of the Scatchard plots at low solute concentration are analyzed in terms of two species--a promoter and a higher order stronger adsorbing species. An explicit equation of the isotherm is developed based on this model, and this expression is shown to reproduce the isotherm shape using the appropriate derived parameters. It is further shown from this equation that a Langmuir-shaped adsorption isotherm can be obtained if the higher order associate or aggregate binds weaker to the support than the promoter. These results indicate that protein-protein interactions and the formation of associates can play a significant role on the shape of the isotherm and ultimately on the behavior of the species in preparative scale chromatography.