Biomaterials shear rates

The in vitro study of the hemocompatibility of biomaterials requires the consideration of many parameters, static or dynamic contact, flow rate, wall shear rate, form of biomaterial to be tested, pathway to consider (platelet adhesion, platelet activation, complement activation, contact phase activation etc..) and duration of contact(39). It has previously been demonstrated t t hemodynamic circumstances play a significant role in determining localization, growth and fiagmentation of thrombi and platelet adhesion in vivo, and that flow rate controls platelet transport to a surface and their adhesion (40). This evidence is siqtpoited by observed differences in platelet activity predominance in venous and arterial flow (41). Qearly, defining the blood compatibility of a material is a conqrromise between a number of these factors. 

Rhodes. N.P, Kumary. T.V and Williams. D.F, Influence of wall shear rate on parameters of blood compatibility ofintravascular catheters. Biomaterials, 17,1995-2002(1996). 

In most studies in which attempts have been made to investigate the relationship between hydrodynamics and thrombus formation, the flow rate of blood through the test device was changed to alter the hydrodynamics. Unfortunately, for many configurations [for example, a tube (9)] a change in flow rate also changed the residence time of blood in the device, the mass transfer coefficients, as well as shear rates at the biomaterial-blood interface. This simultaneous variation in hemodynamic parameters makes it difficult to assign specific cause and effect relationships to the results obtained with these methods. 

Surface Tension. Interfacial surface tension between fluid and filter media is considered to play a role in the adhesion of blood cells to synthetic fibers. Interfacial tension is a result of the interaction between the surface tension of the fluid and the filter media. Direct experimental evidence has shown that varying this interfacial tension influences the adhesion of blood cells to biomaterials. The viscosity of the blood product is important in the shear forces of the fluid to the attached cells viscosity of a red cell concentrate is at least 500 times that of a platelet concentrate. This has a considerable effect on the shear and flow rates through the filter. The surface stickiness plays a role in the critical shear force for detachment of adhered blood cells. 

Epoxy-modified Mesua ferrea L. seed oil-based polyurethane/clay nanocomposites at different loadings (1-5 wt%) have been studied as biocompatible biomaterials. The partially exfoliated nanocomposites were prepared by an ex situ solution technique under high mechanical shearing and adequate ultrasonication at room temperature. The nanocomposites exhibited enhanced mechanical properties such as tensile strength (twofold) and scratch hardness (five-fold), thermostability (to about 40°C), and an increase in the rate of biodegradation between five- and ten-fold. All the nanocomposites showed RBC haemolysis inhibition observed by anti-hemolytic assay carried out on the sterilised films. 

Fig. 15.2 Schematic diagram of an in vitro perfusion system for laminar shear stress conditions. Flow chamber contains the biomaterial with cultivated cells (e.g. HUVECs). Perfusion with full blood, isolated leukocytes, platelets or cell culture media. Rolling pump applies various volume rates. After perfusion, the perfused cells on the biomaterial (e.g. proliferation, vitality, PCR, Western blotting) and the supernatant (e.g. flow cytometry, ELISA) should be analyzed...

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