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The conformation adopted by the plasma protein fibrinogen upon its adsorption onto synthetic surfaces has been implicated to play an important role in determining the blood compatibility of biomaterials. It has recently been shown that adsorbed fibrinogen undergoes biologically significant conformational changes with increasing residence time on the surface of selected biomaterials. The purpose of this study was to examine the effects of co-adsorbed proteins and shear forces on such time-dependent functional changes in fibrinogen adsorbed onto polyethylene (PE), polytetrafluoroethylene (PTFE), and silicone rubber (SR). Fibrinogen was adsorbed onto these materials for 1 min and then allowed to 'reside' on these 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 and the ability of blood plasma to displace previously adsorbed fibrinogen. The magnitude of platelet adhesion to substrates adsorbed with pure fibrinogen increased in the presence of shear, compared with static conditions; at the lowest shear rate of 200 s(-1), samples exhibited a 20-fold increase in adhered platelet levels. In contrast, at a higher shear rate of 1000 s(-1), the three polymers supported minimal levels of platelet attachment. Surfaces pre-adsorbed with 10% plasma did not promote a significant increase in the number of adherent platelets with increasing shear when compared with the pure fibrinogen-coated substrates. The presence of shear also significantly altered the materials' ability to retain fibrinogen. Under static conditions, the amount of fibrinogen retained following incubation in blood plasma increased on all materials with increasing fibrinogen residence time. However, the materials varied distinctly in their ability to retain adsorbed fibrinogen with increasing fibrinogen residence time, shear rate, and nature of the co-adsorbed proteins. Thus, the results from this study indicate that fluid shear, residence time of the adsorbed protein, nature of the co-adsorbed proteins, and surface chemistry of the material all play important roles in influencing platelet-surface interactions and that they act in a complex manner to influence the biocompatibility of a material.

Fibrinogen readily adsorbs to the surface of biomaterials and, because of its demonstrated ability to support platelet adhesion and aggregation, plays a role in thrombotic events associated with the implantation of synthetic materials in the human body. Thus, understanding the factors influencing the interactions of fibrinogen with biomaterials, and how platelet responses are affected, is crucial for the development of synthetic materials exhibiting improved blood compatibility. In this study, the effects of fibrinogen residence time and shear rate on the procoagulant activity of adherent platelets, along with their morphologic status, as deduced from scanning electron microscopy, were investigated. To examine whether adherent platelets promoted the generation of thrombin, polymeric materials (polytetrafluoroethylene, polyethylene, and silicone rubber) preadsorbed with fibrinogen were exposed to platelet suspensions at different wall shear rates and then incubated with clotting factors for 5 minutes under static conditions. The amount of thrombin generated per platelet was calculated from the optical density of the color developed by adding substrate S-2238. Scanning electron microscopy images of the platelets revealed that the platelets exhibited different morphologies, depending on the shear rate and residence time of the adsorbed fibrinogen. Platelets ranged from their normal discoid shape observed primarily under static conditions, to that of fully spread platelets. Results from this study show that platelets, in the presence of shear forces, undergo activation on exposure to surfaces on which adsorbed fibrinogen has resided for short residence times rather than long residence times. Interestingly, studies examining the procoagulant responses of such adherent platelets demonstrated that the platelets attached to the fibrinogen coated materials did not promote significant thrombin generation. Such low prothrombinase activity of adherent platelets suggests that adsorbed fibrinogen, while capable of supporting platelet adhesion and spreading on biomaterials, does not necessarily enhance the procoagulant activity of adherent platelets.

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., (Copyright 1999 John Wiley & Sons, Inc.)

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.

Fibrinogen adsorption from blood to biomaterials may regulate platelet adhesion and thrombus formation because of fibrinogen's central role in the coagulation cascade and its ability to bind specifically to the platelet membrane glycoprotein (GP) IIb-IIIa. Adsorption of fibrinogen from blood plasma to many materials exhibits a maximum with respect to plasma dilution and exposure time (the Vroman effect). In this study fibrinogen adsorption to several polymers was examined to ascertain the influence of controlled changes in surface chemistry on the Vroman effect. The materials included hydroxyethylmethacrylate-ethylmethacrylate (HEMA/EMA) copolymers, Biomer, and a series of segmented polyurethanes (PEUs), two of which contained fluorinated chain extenders. Each material exhibited maximal adsorption of fibrinogen at intermediate plasma concentrations. Little effect of soft-segment type or molecular weight was observed and no significant differences in fibrinogen adsorption to the fluorinated PEUs were seen. Changes in the strength of fibrinogen attachment to these materials with time after adsorption were also assessed. Fibrinogen adsorbed for 1 min was displaced more readily by blood plasma than that adsorbed for 1 h, regardless of the material. The more hydrophobic polymers exhibited greater retention of adsorbed fibrinogen. In addition, the fraction of fibrinogen retained by polyethylene depended on the amount of fibrinogen adsorbed to the surface, being greatest when the surface loading was the least. These studies indicate that spreading or transition of adsorbed fibrinogen molecules from a weakly to tightly bound state is a general consequence of protein adsorption to solid surfaces.

[125I]-Fibrinogen was used to measure the adsorption of fibrinogen from baboon plasma to two types of glass (Pyrex and a borosilicate glass) at 25 and 37 degrees C using two different buffers to dilute the plasma, the first being citrate-phosphate buffered saline (CPBSz) and the second isotonic Tris-saline (TRIS), both pH 7.4. In addition, the effects of hydration conditions, rinsing techniques, and glass-cleaning treatments on fibrinogen adsorption were evaluated. The data reveal that lower temperatures and the use of TRIS to dilute the plasma significantly enhance fibrinogen adsorption to both types of glass. As has been observed in the past, fibrinogen adsorption peaked at intermediate plasma concentrations on both Pyrex and borosilicate glass (the so-called Vroman effect), but almost twice as much fibrinogen adsorbed to glass when TRIS was used to dilute the plasma instead of CPBSz. Moreover, up to five times as much fibrinogen adsorbed to both types of glass at 25 degrees C compared with 37 degrees C. No effects of the rinsing technique or glass-cleaning treatment were observed.

A direct enzyme-linked immunosorbent assay (ELISA), using a polyclonal anti-fibrinogen conjugated to horseradish peroxidase, was used to detect fibrinogen adsorption from blood plasma to ten different materials. Adsorption was also measured with [125I]-fibrinogen. The materials studied included glass, Biomer, Immulon I, and a series of hydroxyethylmethacrylate (HEMA) and ethylmethacrylate (EMA) co-polymers. For all the materials studied, the results from the ELISA technique closely paralleled those obtained using [125I]-fibrinogen. The cross-reactivity of the antibody with proteins other than fibrinogen was generally small. Both experimental methods detected the presence of a maximum in fibrinogen adsorption (as a function of the plasma dilution) to the more hydrophobic materials. For all but two HEMA/EMA co-polymers, a linear correlation between the ELISA and [125I]-fibronogen measurements was indicated by inspection of cross plots as well as by a statistical test.

Fibrinogen adsorption from plasma exhibits an unusual displacement phenomenon (the Vroman effect) in that decreases in adsorption occur after longer contact time or as the plasma concentration increases. Fibrinogen adsorption and the displacement effect were found to depend markedly on the nature of the surface, the time and temperature of adsorption, and to a lesser extent on certain contact activation factors. Displacement still occurred from plasmas lacking kininogens, factor IX, and prekallikrein, and from plasma that had been treated with BaSO4 to remove factors II, VII, IX, and X. Furthermore, displacement was also observed from fibrinogen solutions to which either albumin or hemoglobin had been added. In addition, competitive adsorption of binary protein mixtures was also shown to depend strongly on surface type. It therefore appears that fibrinogen adsorption from plasma is subject to similar if not identical competitive processes that occur in simpler protein mixtures. The final adsorption then reflects the influence of all the proteins in plasma, each competing for the limited number of adsorption sites according to the fundamental physical properties of surface activity and mass concentration.

The conformation adopted by the plasma protein fibrinogen upon its adsorption onto synthetic surfaces has been implicated to play an important role in determining the blood compatibility of biomaterials. It has recently been shown that adsorbed fibrinogen undergoes biologically significant conformational changes with increasing residence time on the surface of selected biomaterials. The purpose of this study was to examine the effects of co-adsorbed proteins and shear forces on such timedependent functional changes in fibrinogen adsorbed onto polyethylene (PE), polytetrafluoroethylene (PTFE), and silicone rubber (SR). Fibrinogen was adsorbed onto these materials for 1 min and then allowed to 'reside' on these surfaces for up to 2 h prior to assessing its biological activity. Changes in fibrinogen reactivity were determined by measuring the adhesion of [sup 51]Cr-labeled platelets and the ability of blood plasma to displace previously adsorbed fibrinogen. The magnitude of platelet adhesion to substrates adsorbed with pure fibrinogen increased in the presence of shear, compared with static conditions; at the lowest shear rate of 200 s[sup -1], samples exhibited a 20-fold increase in adhered platelet levels. In contrast, at a higher shear rate of 1000 s[sup -1], the three polymers supported minimal levels of platelet attachment. Surfaces pre-adsorbed with 10% plasma did not promote a significant increase in the number of adherent platelets with increasing shear when compared with the pure fibrinogencoated substrates. The presence of shear also significantly altered the materials' ability to retain fibrinogen. Under static conditions, the amount of fibrinogen retained following incubation in blood plasma increased on all materials with increasing fibrinogen residence time. However, the materials varied distinctly in their ability to retain adsorbed fibrinogen with increasing fibrinogen residence time, shear rate, and nature of the co-adsorbed proteins. Thus, the results from this study indicate that fluid shear, residence time of the adsorbed protein, nature of the co-adsorbed proteins, and surface chemistry of the material all play important roles in influencing platelet–surface interactions and that they act in a complex manner to influence the biocompatibility of a material. [ABSTRACT FROM AUTHOR]