The diameter of circulating cells that may adhere to the vascular endothelium spans an order of magnitude from approximately 2 microm (e.g., platelets) to approximately 20 microm (e.g., a metastatic cell). Although mathematical models indicate that the adhesion exhibited by a cell will be a function of cell diameter, there have been few experimental investigations into the role of cell diameter in adhesion. Thus, in this study, we coated 5-, 10-, 15-, and 20-microm-diameter microspheres with the recombinant P-selectin glycoprotein ligand-1 construct 19.ek.Fc. We compared the adhesion of the 19.ek.Fc microspheres to P-selectin under in vitro flow conditions. We found that 1) at relatively high shear, the rate of attachment of the 19.ek.Fc microspheres decreased with increasing microsphere diameter whereas, at a lower shear, the rate of attachment was not affected by the microsphere diameter; 2) the shear stress required to set in motion a firmly adherent 19.ek.Fc microsphere decreased with increasing microsphere diameter; and 3) the rolling velocity of the 19.ek.Fc microspheres increased with increasing microsphere diameter. These results suggest that attachment, rolling, and firm adhesion are functions of particle diameter and provide experimental proof for theoretical models that indicate a role for cell diameter in adhesion.

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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.

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.

Both hemostasis and its pathological correlate. thrombosis. are dynamic events. That is. the various biochemical mechanisms that determine the extent of thrombosis at a particular site of injury depend on the local supply of reactive materials (cells and zymogens) and the senlor-a1 of the subsequent products (activated cells and enzymes). There is a rapidly growing literature indicating that the local fluid dynamic con-ditio~ls influence not only the quantity of blood elements which deposit but also the quality or type of deposit. For example, it has long been known that thrombi that form in arteries differ significantly from those that form 011 the venous side with respect to platelet, red cell and fibrinaceous content. However. the intravascular mechanisms underlying such differences have not been fully delineated. More recently, it has been realized that the phi-sical forces to which blood and vascular cells are exposed under physiological conditions may well alter the reactivity of these elements in flowing blood. For example. platelets exposed to physiologic shear forces in various types of viscometers (i.e.. devices that produce a uniform shear force throughout the test fluid) will aggregate without the addition of exogenous agonists. The mechanism by which shear force is recognized and translated across a cell membrane is not kvell understood: in endothelial cells. shear forces have been sho\vn to activate potassium channels in the cell wall. thereby leading to the release of the vasodilator nitric oxide (1). Despite the apparent recogliition of flow as a primary contributor to thrombotic events. the properties of flowing systems are not broadly appreciated. The intentio~ls of this article are to 1) introduce the basic concepts of fluid dynamics essential for an understanding of the multifaceted role of Ao\\ in surface-mediated thrombosis: 2) examine current hypotheses es regarding the mechanism(s) by which flow alters the activity ty of blood and vascular components: 3) briefly review the extensive evidence demonstrating that selected hemostatic defects are intrinsically dependent on flow; and 1) discuss how flow through regions of stenosed vessels. such as are present at sites of atherosclerotic lesions or during transient ischeluic attacks. may contribute to the severity of the disorder. Basic concepts of fluid dynamics Thrombosis at sites of vascular injury involves complex interactions between blood-borne elements. e.g., platelets and coagulation proteins. and constituents of the \ ascular wall. Blood flow constitutes an essential element of thrombotic