Changes in fibrinogen adsorbed to segmented polyurethanes and hydroxyethylmethacrylate-ethylmethacrylate copolymers

How Crystallographic Orientation-Induced Fibrinogen Conformation Affects Platelet Adhesion and Activation on TiO2

Control of protein adsorption is essential for successful integration of healthcare materials into the body. Human plasma fibrinogen (HPF), especially its conformation is a key upstream regulator for platelet behavior and thus pathological clot formation at the blood-biomaterial interface. A previous study by the authors revealed that the conformation of adsorbed HPF can be controlled by rutile surface crystallographic orientation. Therefore, it is hypothesized that pre-adsorbed HPF on specific rutile orientation can regulate platelets adhesion and activation. Here, it is shown that platelets exposed to the four low index (110), (100), (101), (001) facets of TiO2 (rutile) exhibit surface-specific behavior. Scanning electron microscopy (SEM) observations of platelets morphology and P-selectin expression measurement revealed that on (110) facets, platelets adhesion and activation are suppressed. In contrast, extensive surface coverage by fully activated platelets is observed on (001) facets. Platelets' behavior has been linked to the HPF conformation and thereby availability of platelet-binding sequences. Atomic force microscopy (AFM) imaging supported by immunochemical analysis shows that on (110) facets, HPF is adsorbed in trinodular conformation rendering the γ400-411 platelet-binding sequence inaccessible. This research has potential implications on the bioactivity of different materials crystal facets, reducing the risk of pathological clot formation and thromboembolic complications.

Fibrinogen adsorption to biomaterials

Fibrinogen (Fg) adsorption is an important mechanism underlying cell adhesion to biomaterials and was the major focus of the author's research career. This article summarizes our work on Fg adsorption, with citations of related work as appropriate. The molecular properties of Fg that promote adsorption and cell adhesion will be described. In addition, the adsorption behavior of Fg from buffer, binary solutions with other proteins, and blood plasma will be discussed, including the Vroman effect. Studies of platelet adhesion to surfaces preadsorbed with blood plasmas selectively deficient in Fg, vitronectin (Vn), fibronectin (Fn), or von Willebrand's factor (vWf) will be reviewed. These studies clearly showed a major role for Fg in platelet adhesion under static conditions and both Fg and vWf for adhesion from flowing suspensions, but no significant role for Vn or Fn. However, it was also shown that platelet adhesion was poorly correlated with the total amount of adsorbed Fg, but very well correlated with the binding of antibodies specific to the cell binding domains of Fg. A brief overview of nonfouling surfaces for prevention of Fg adsorption will be given. A more extensive discussion of structural changes in Fg after its adsorption is included, including changes detected with both physicochemical and biological methods. A short discussion of the state of the art of structural determination of adsorbed proteins with computational methods is also given. A final section identifies Fg adsorption as the single most important event determining the biocompatibility of implants in soft tissue and in blood. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2777-2788, 2018.

Proteins, platelets, and blood coagulation at biomaterial interfaces

Blood coagulation and platelet adhesion remain major impediments to the use of biomaterials in implantable medical devices. There is still significant controversy and question in the field regarding the role that surfaces play in this process. This manuscript addresses this topic area and reports on state of the art in the field. Particular emphasis is placed on the subject of surface engineering and surface measurements that allow for control and observation of surface-mediated biological responses in blood and test solutions. Appropriate use of surface texturing and chemical patterning methodologies allow for reduction of both blood coagulation and platelet adhesion, and new methods of surface interrogation at high resolution allow for measurement of the relevant biological factors.

Surface engineering of synthetic polymer materials for tissue engineering and regenerative medicine applications

When using polymer materials as scaffolds for tissue engineering or regenerative medicine applications the initial, and often lasting, interaction between cells and the material areviasurfaces.

Fibrinogen organization at the cell-material interface directs endothelial cell behavior

Fibrinogen (FG) adsorption on surfaces with controlled fraction of —OH groups was investigated with AFM and correlated to the initial interaction of primary endothelial cells (HUVEC). The —OH content was tailored making use of a family of copolymers consisting of ethyl acrylate (EA) and hydroxyl ethyl acrylate (HEA) in different ratios. The supramolecular distribution of FG changed from an organized network-like structure on the most hydrophobic surface (—OH 0) to dispersed molecular aggregate one as the fraction of —OH groups increases, indicating a different conformation by the adsorbed protein. The best cellular interaction was observed on the most hydrophobic (—OH 0) surface where FG assembled in a fibrin-like appearance in the absence of any thrombin. Likewise, focal adhesion formation and actin cytoskeleton development was poorer as the fraction of hydroxy groups on the surface was increased. The biological activity of the surface-induced FG network to provide 3D cues in a potential tissue engineered scaffold, making use of electrospun PEA fibers (—OH0), seeded with human umbilical vein endothelial cells was investigated. The FG assembled on the polymer fibers gave rise to a biologically active network able to direct cell orientation along the fibers (random or aligned), promote cytoskeleton organization and focal adhesion formation.

Measuring the time-dependent functional activity of adsorbed fibrinogen by atomic force microscopy

In this work, we measured time-dependent functional changes in adsorbed fibrinogen by measuring antigen-antibody debonding forces with atomic force microscopy (AFM). AFM probes were functionalized with monoclonal antibodies recognizing fibrinogen gamma 392-411, which includes the platelet binding dodecapeptide region. These probes were used to collect force measurements between the antibody and fibrinogen on mica substrates and the probability of antigen recognition was calculated. Statistical analysis showed that the probability of antibody-antigen recognition peaked at approximately 45 min postadsorption and decreased with increasing residence time. Macroscale platelet adhesion measurements on these mica substrates were determined to be greatest at fibrinogen residence times of approximately 45 min, which correlated well with the functional activity of adsorbed fibrinogen as measured by the modified AFM probes. These results demonstrate the utility of this approach for measuring protein function at or near the molecular scale and offers new opportunities for improved insights into the molecular basis for the biological response to biomaterials.

Anti-adhesive properties of fish tropomyosins

AIMS

We have recently found that preconditioning of stainless steel surfaces with an aqueous fish muscle extract can significantly impede bacterial adhesion. The purpose of this study was to identify and characterize the primary components associated with this bacteria-repelling effect.

METHODS AND RESULTS

The anti-adhesive activity was assayed against Escherchia coli K-12, and bacterial adhesion was quantified by crystal violet staining and sonication methods. Proteolytic digestion, elution and fractionation experiments revealed that the anti-adhesive activity of the extract was linked to the formation of a proteinaceous conditioning film composed primarily of fish tropomyosins. These fibrous proteins formed a considerable anti-adhesive conditioning layer on and reduced bacterial adhesion to several different materials including polystyrene, vinyl plastic, stainless steel and glass. The protein adsorption profiles obtained from the various materials did not differ significantly, but elution was often incomplete making minor qualitative/quantitative differences indiscernible.

CONCLUSIONS

The data highlights the significance of protein conditioning films on bacterial adhesion and emphasizes the importance of substratum's physiochemical properties and exposure time with regards to protein adsorption/elution efficiency and subsequent bacterial adhesion.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fish tropomyosin-coatings could potentially offer a nontoxic and relatively inexpensive measure of reducing bacterial colonization of inert surfaces.

Passivating protein coatings for implantable glucose sensors: Evaluation of protein retention

The long-term function of implantable biosensors is limited by the foreign-body reaction (FBR). Since the acute phase of the FBR involves macrophage attachment mediated by adsorbed fibrinogen, preadsorption, and retention of other proteins might reduce the FBR. The retention of preadsorbed albumin, hemoglobin, von Willebrand's factor, and high-molecular-weight kininogen was therefore measured after exposure to plasma. The retention of preadsorbed proteins after incubation with monocyte cultures and implantation in rats was also measured. Fibrinogen adsorption from plasma to the preadsorbed surfaces was also measured. Hemoglobin adsorption was higher than that for other proteins, and it also had the greatest retention after exposure to blood plasma. When surfaces preadsorbed with hemoglobin were incubated with monocytes, more of the hemoglobin was displaced than that after incubation in plasma, while still more hemoglobin was displaced when the surfaces were implanted in vivo. Protein preadsorption on polystyrene greatly reduced fibrinogen adsorption. However, polyurethane surfaces used for glucose sensors had low fibrinogen adsorption compared with polystyrene, and this low level was not further reduced by preadsorption with other proteins. Preadsorbed proteins on polymers appear to be removed by passive exchange and/or displacement by plasma proteins and by proteases released by monocytes.

Fibrinogen adsorption, platelet adhesion and activation on mixed hydroxyl-/methyl-terminated self-assembled monolayers

The effect of surface wettability on fibrinogen adsorption, platelet adhesion and platelet activation was investigated using self-assembled monolayers (SAMs) containing different ratios of longer chain methyl- and shorter chain hydroxyl-terminated alkanethiols (C15CH3 vs. C11OH) on gold. Protein adsorption studies were performed using radiolabeled human fibrinogen (HFG). Platelet adhesion and activation studies with and without pre-adsorbed fibrinogen, albumin and plasma were assessed using scanning electron microscopy (SEM) and a glutaraldehyde-induced fluorescence technique (GIFT). Results demonstrated a linear decrease of HFG adsorption with the increase of OH groups on the monolayer (increase of the hydrophilicity). Platelet adhesion and activation also decrease with increase of hydrophilicity of surface. Concerning SAMs pre-immersed in proteins, fibrinogen adsorption was related with high platelet adhesion and activation. The passivant effect of albumin on platelet adhesion and activation was only demonstrated on SAMs contained C11OH. When all the blood proteins are present (plasma) platelet adhesion was almost absent on SAMs with 65% and 100% C11OH. This could be explained by the higher albumin affinity of the SAMs with 65% C11OH and the lower total protein adsorption associated with SAMs with 100% C11OH.

Hemodialysis Membranes Based on Functionalized High-density Polyethylene

Grafting vinyl acetate (VAc) and maleic anhydride (MAn) comonomer onto high-density polyethylene (HDPE) was performed by means of gamma rays. Conditions for the minimum homopolymer formation, maximum grafting yield and alternate copolymer grafts were carried out. Further chemical treatment with sodium hydroxide, hydrochloric acid, ammonium hydroxide, sulfamic acid and amino pyridine were made on the grafted membranes to increase functionality. The swelling, mechanical, solute permeability and biocompatibility properties of these copolymers were evaluated for possible application as dialysis membranes. The introduction of functional groups on HDPE membranes enhanced their hydration and transport flux. The treated grafting membranes showed improved permeability towards urea, creatinine and uric acid over the ungrafted HDPE. The permeability rate of the solutes through the membranes depended on the molecular weight and the size of the solutes. The presence of hydrophilic groups on the membranes reduced protein adsorption and enhanced membrane transport. The swelling, solute dialysis permeability and protein low affinity properties of HDPE-g-(VAc-alt-MAn) treated with sulfamic acid or 2-aminopyridine indicate potential use as hemodialysis membranes.

Albumin and fibrinogen adsorption on PU-PHEMA surfaces

Materials that adsorb specific proteins may find a variety of applications in the biomedical field. The aim of this study was the preparation of a hydrophilic surface, with low protein adsorption, to be used in the future as a support for the immobilisation of several species, e.g. Cibacron Blue F3G-A, which has been described to induce specific albumin adsorption. Poly(hydroxyethylmethacrylate) (PHEMA) and poly(hydroxyethylacrylate) (PHEA) were chosen as the hydrophilic surface because they can be easily polymerised and possess hydroxyl groups that can be used for the immobilisation of different compounds. Thin films of PHEMA and PHEA were successfully graft polymerised onto the surface of a commercial poly(etherurethane) (PU) using ceric ion as initiator. Grafting polymerisations were followed by mass gain and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Since stability tests demonstrated that only PU-PHEMA was stable in alkaline solutions, a necessary condition to future immobilisations, the investigation was focused on the coating of PU with PHEMA. PU-PHEMA films were characterised in detail using several techniques as mass gain, ATR-FTIR, contact angle measurements, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Protein adsorption was evaluated using radiolabelled albumin and fibrinogen from pure solutions and from mixtures of both proteins. PU surfaces modified with PHEMA have demonstrated low protein adsorption, showing their potential use as substrates. This opens the possibly of exploring the advantages of selective adsorption by appropriate immobilisation of specific molecules.

Quality assessment of heparin coatings by their binding capacities of coagulation and complement enzymes

In vitro testing of blood contacting materials before clinical application is generally advisable. Four heparin coatings from different manufacturers were tested for adsorbed proteins and soluble activation markers. The surface with the highest antithrombin, thrombin, high-molecular-weight-kininogen (HMWK) and the lowest fibrinogen binding capacity (Carmeda, Medtronic) showed significantly lower levels of granulocytes and platelet activation (beta-TG, PMN-elastase release). No statistically significant differences in soluble markers of the coagulation system could be detected (F1 + 2, TAT). Interestingly, complement activation (TCC) was significantly reduced within the group of the lowest adsorption of the complement factor C3. Our data demonstrate that there is a relation between the binding affinity of proteins (C1-inhibitor, C3-complement) and the consecutive changes in complement activation (TCC). Therefore, measuring adsorbed proteins on artificial surfaces is a suitable, sensitive and very reproducible method for assessing the thrombogenicity of biomaterials.

Residence-time dependent changes in fibrinogen adsorbed to polymeric biomaterials

It has generally been accepted that biomaterials adsorbing the least amount of the plasma protein fibrinogen following exposure to blood will support less platelet adhesion and therefore exhibit less thrombogenicity. Several studies suggest, however, that the conformation or orientation of immobilized fibrinogen rather than the total amount adsorbed plays an important role in determining the blood compatibility of biomaterials. The purpose of this study was to investigate time-dependent functional changes in fibrinogen adsorbed to polytetrafluoroethylene (PTFE), polyethylene (PE), and silicone rubber (SR). Fibrinogen was adsorbed to these materials for 1 min and then allowed to 'reside" on the surfaces for up to 2 h prior to assessing its biological activity. Changes in fibrinogen reactivity were determined by measuring the adhesion of 51Cr-labeled platelets, the binding of a monoclonal antibody (mAb) directed against an important functional region of the fibrinogen molecule (the gamma-chain dodecapeptide sequence 400-411), and the ability of blood plasma to displace previously adsorbed fibrinogen. Platelet adhesion differed among the polymeric materials studied, and PTFE and PE samples exhibited a small decrease in adhesion with increasing fibrinogen residence time. Platelet adhesion to SR was the least among all materials studied and showed no variation with residence time. When using PTFE and SR as substrates, mAb recognition of adsorbed fibrinogen did not change with residence time whereas that on PE decreased slightly. The mAb binding was least to fibrinogen adsorbed to SR, which is in agreement with the platelet adhesion results. Finally, the ability of plasma to displace previously adsorbed fibrinogen decreased dramatically with increasing residence time on all materials. These in vitro studies support the hypothesis that fibrinogen undergoes biologically significant conformational changes upon adsorption to polymeric biomaterials, a phenomenon that may contribute to the hemocompatibility of the materials following implantation in the body.

Rapid postadsorptive changes in fibrinogen adsorbed from plasma to segmented polyurethanes

Fibrinogen adsorbed to biomaterials plays a key role in mediating platelet interactions that can lead to blood clotting so its behavior on surfaces is of fundamental interest. In previous work showing that fibrinogen adsorbed to surfaces quickly becomes non-displaceable upon exposure to blood plasma, the fibrinogen was adsorbed from buffer, so we performed new studies in which the displaceability of fibrinogen adsorbed from plasma was characterized. Fibrinogen was adsorbed from 1% plasma to seven different surfaces for 1-64 min and then transferred to 100% plasma lacking radiolabeled fibrinogen and the amount adsorbed before and after transfer measured. The surfaces were glass, Silicone rubber, and five different polyurethanes. As adsorption time increased, the fibrinogen became increasingly resistant to displacement during the 100% plasma step, but the rate of increase in resistance varied greatly with surface type. Fibrinogen adsorbed from 1% plasma evidently undergoes rapid, surface dependent transitions. This work shows that the transitions that occur when the fibrinogen is adsorbed from blood plasma are similar to what we have previously observed for fibrinogen adsorbed from buffer.

Platelet adhesion onto wettability gradient surfaces in the absence and presence of plasma proteins

A wettability gradient was prepared on lowdensity polyethylene (PE) sheets by treating them in air with a corona from a knife-type electrode the power of which increased gradually along the sample length. The PE surfaces oxidized gradually with the increasing corona power and a wettability gradient was created on the surfaces, as evidenced by the measurement of water contact angles, Fourier transform infrared spectroscopy in the attenuated total reflectance mode, and electron spectroscopy for chemical analysis. The wettability gradient surfaces prepared were used to investigate the adhesion behavior of platelets in the absence and presence of plasma proteins in terms of the surface hydrophilicity/hydrophobicity of polymeric materials. The platelets adhered to the wettability gradient surfaces along the sample length were counted and examined by scanning electron microscopy (SEM). It was observed that the platelet adhesion in the absence of plasma proteins increased gradually as the surface wettability increased along the sample length. The platelets adhered to the hydrophilic positions of the gradient surface also were more activated (possessed more pseudo pods as examined by SEM) than on the more hydrophobic ones. However, platelet adhesion in the presence of plasma proteins decreased gradually with the increasing surface wettability; the platelets adhered to the surface also were more activated on the hydrophobic positions of the gradient surface. This result is closely related to plasma protein adsorption on the surface. Plasma protein adsorption on the wettability gradient surface increased with the increasing surface wettability. More plasma protein adsorption on the hydrophilic positions of the gradient surface caused less platelet adhesion, probably due to platelet adhesion inhibiting proteins, such as high-molecular-weight kininogen, which preferably adsorbs onto the surface by the so-called Vroman effect. It seems that both the presence of plasma proteins and surface wettability play important roles for platelet adhesion and activation.

Hemocompatibility studies of surface-treated polyurethane-based chronic indwelling catheters

The objectives of this research were to evaluate and compare the interactions of several polyurethane-based central venous catheter materials with blood. Specifically, measurements of fibrinogen adsorption, platelet adhesion, kallikrein generation, and fibrinopeptide A (FPA) release were performed. The catheter materials examined in this study included: platinum-cured, 50 shore A durometer, barium sulfate-filled, silicone (SI); Tecoflex EG85A-B20 polyurethane (PU); PU catheters whose outer surface had been impregnated with ion beam-deposited silver atoms (AgI and AgII); PU catheters coated with a hydrophilic, polyacrylic acid polymer (UC); PU catheters coated with an air-cured PTFE emulsion (CS); and PU catheters coated with an aminofunctional dimethylsiloxane copolymer (JG). The time course of fibrinogen adsorption from plasma to the SI, JG, PU, and CS materials was similar, with CS exhibiting the least amount of adsorbed fibrinogen after 1 h (65 +/- 4.7 ng cm-2) and PU the greatest (144 +/- 16.5 ng cm-2). After 90 min of contact, AgI and AgII exhibited the greatest number of adherent platelets, levels that were approximately two to three times higher than those on the other catheter materials. With the exception of UC and PU, which caused kallikrein generation levels approximately half that of the positive (glass) control, little kallikrein formation was observed for any of the materials relative to the negative control. Finally, FPA generation was greatest using the SI, CS, and PU materials, with the latter causing the production of almost four times the amount of FPA as the negative control. This preliminary assessment of the hemocompatibility of the various catheters suggests that the surface treatments did not adversely affect their interactions with blood components; further investigations of these materials are therefore warranted in order to completely characterize their behavior prior to use in clinical situations.

Initial hemocompatibility studies of titanium and zirconium alloys: prekallikrein activation, fibrinogen adsorption, and their correlation with surface electrochemical properties

Two novel metal alloys, Ti-13Nb-13Zr and Zr-2.5Nb, have been engineered for applications in orthopedic implants because of their favorable mechanical properties, corrosion resistance, and compatibility with bone and tissue. These alloys also have the ability to form a hard, abrasion-resistant, ceramic surface layer upon oxidative heat treatment (diffusion hardening, DH). Previous studies have indicated that these and other ceramics cause limited hemolysis and exhibit remarkable structural integrity after extended exposure to physiological environments. Such observations suggest that DH Ti-13Nb-13Zr and ZrO2/Zr-2.5Nb could be used successfully as components in blood-contacting devices. Materials intended for such applications must possess properties that do not elicit adverse physiological responses, such as the initiation of the coagulation cascade or thrombus formation. In the present study measurements of prekallikrein activation, fibrinogen adsorption from diluted human plasma, and the strength of fibrinogen attachment as judged by residence-time experiments were performed to evaluate the potential hemocompatibility of these materials. The results of the prekallikrein activation and fibrinogen-retention studies correlated well with two electrochemical properties of the alloys, the open circuit potential and reciprocal polarization resistance. The results indicate that both the original and treated Ti and Zr alloys activate prekallikrein and adsorb as well as retain fibrinogen in amounts similar to other materials used as components of blood-contacting devices. On the basis of these studies, these alloys appear to be promising candidates for cardiovascular applications and merit further investigation.

Residence time effects on monoclonal antibody binding to adsorbed fibrinogen

Fibrinogen adsorbed to polymeric surfaces and then allowed to reside on the surface while it is kept in a buffer solution for a period of time (the 'residence time') undergoes postadsorptive changes that decrease its SDS elutability, displaceability by plasma, polyclonal antifibrinogen binding, and ability to support platelet adhesion (summarized in Chinn et al. J. Biomed. Mater. Res. 26, 757 (1992)). In order to better understand the nature of the changes in adsorbed fibrinogen, the binding of ten different monoclonal antifibrinogen molecules to fibrinogen adsorbed from plasma to Biomer and several other surfaces has been measured after increasing residence time in buffer. Three of the monoclonal antibodies used bind to sequences that have been implicated in platelet binding to fibrinogen. One of these (M1) binds to the C-terminal region of the gamma chain (402-411), another (R1) binds to the N-terminal region of the A alpha chain containing an RGDF sequence (95-98), and the third (R2) binds to the C-terminal region of the A alpha chain containing an RGDS sequence (572-575). Two other antibodies (P1 and K4) also bind to the C-terminal region of the gamma chain (373-385 and 392-406, respectively). Five other antibodies that bind to other regions in fibrinogen were also used. Two of the antibodies (K4 and P1) are also known to be sensitive to conformational changes in the fibrinogen molecule. The binding of the various antibodies changed with residence time in ways that were highly dependent on the particular antibody. The binding of some antibodies was very stable with respect to residence time, others rose with time, some declined with residence time and one appears to pass through a maximum. However, none of the changes in antibody binding were nearly as fast as has been observed for the changes in platelet binding reported previously. Binding to the platelet binding region near the gamma chain C-terminal region either did not change with residence time (M1), increased with residence time (K4), or else decreased more slowly than observed for platelets (P1). Binding of the antibodies to the RGD sequences near the N-terminus of the A alpha chain (95-98) was very low initially but increased with residence time, while the binding to the RGD sequence near the C-terminus of the A alpha chain (572-575) increased slightly at short residence times but then declined substantially after longer residence times. Thus, the changes in the expression of the putative platelet binding domains do not correlate with the declines in platelet binding to plasma preadsorbed Biomer.(ABSTRACT TRUNCATED AT 400 WORDS)

Adsorption of fibrinogen and some other proteins from blood plasma at a variety of solid surfaces

Enzyme linked immunosorbent assay (ELISA) was used for the estimation of protein adsorption from blood plasma at some model solid surfaces. The majority of those surfaces were made in the wells of microtiterplates of polystyrene commonly used for ELISA purposes. Three of the model surfaces were made by radio frequency plasma discharge polymerization (RFPD) of the microtiterplates of polystyrene. The monomers we used were diaminocyclohexane, hexamethylenedisiloxane, and acrylic acid. Other surfaces investigated were: unmodified polystyrene, oxidized polystyrene, hydrophilic silicon oxide, and methylized silicon oxide. Two substances, Tween and bovine serum albumin (BSA), for the prevention of unintended adsorption of ELISA conjugate were also tested and the BSA method was found to be superior for this kind of investigation. Human blood plasma at different dilutions was incubated in the surface-modified microtiterplates followed by incubation of rabbit antibodies against fibrinogen (FG), fibronectin (FN), human serum albumin (HSA), complement factor 3 (C3), and immunoglobulin G (IgG). Visualization of bound antibodies was then made by standard ELISA procedure. At low blood plasma concentrations (plasma dil 1/1000), anti-IgG and anti-HSA were detected at high levels at the majority of surfaces. At high blood plasma concentration (plasma dil 1/10), anti-FG dominated at most surfaces. ELISA activity of FN and C3 were low at most of the surfaces at both plasma concentrations. An 'optimum' plasma dilution for the detection of surface bound FG (the Vroman effect) was not found with the use of the ELISA on any of the surfaces except for the silicon oxide surface. This is in contrast to findings by others who had used isotope-labelled fibrinogen diluted in plasma. However, 'false' Vroman effects occurred if nonionic surfactant was used for the prevention of unspecific binding in the ELISA.