Blood shear

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. 

Heat Exchangers Using Non-Newtonian Fluids. Most fluids used in the chemical, pharmaceutical, food, and biomedical industries can be classified as non-Newtonian, ie, the viscosity varies with shear rate at a given temperature. In contrast, Newtonian fluids such as water, air, and glycerin have constant viscosities at a given temperature. Examples of non-Newtonian fluids include molten polymer, aqueous polymer solutions, slurries, coal—water mixture, tomato ketchup, soup, mayonnaise, purees, suspension of small particles, blood, etc. Because non-Newtonian fluids ate nonlinear in nature, these ate seldom amenable to analysis by classical mathematical techniques. 

The release of NO from the endothelium is induced by various chemical substances, including acetylcholine polypeptides such as substance P, bradykinin, and arginine vasopressin histamine ATP/ADP a2-adrenoceptor agonists thrombin and Ca2+ iono-phores. NO formed in response to mechanical stimuli like shear stress or transmural pressure plays an important role in maintaining basal blood flow. Endothelial NO causes vasodilatation, decreased... 

Opitz F, Schenke-Layland K, Richter W, Martin DP, Degenkolbe I, Wahlers T, and Stock UA. Tissue engineering of ovine aortic blood vessel substitutes using applied shear stress and enzymatically derived vascular smooth muscle cells. Ann Biomed Eng, 2004, 32, 212-222. 

The ratio (p/G) has the units of time and is known as the elastic time constant, te, of the material. Little information exists in the published literature on the rheomechanical parameters, p, and G for biomaterials. An exception is red blood cells for which the shear modulus of elasticity and viscosity have been measured by using micro-pipette techniques 166,68,70,72]. The shear modulus of elasticity data is usually given in units of N m and is sometimes compared with the interfacial tension of liquids. However, these properties are not the same. Interfacial tension originates from an imbalance of surface forces whereas the shear modulus of elasticity is an interaction force closely related to the slope of the force-distance plot (Fig. 3). Typical reported values of the shear modulus of elasticity and viscosity of red blood cells are 6 x 10 N m and 10 Pa s respectively 1701. Red blood cells typically have a mean length scale of the order of 7 pm, thus G is of the order of 10 N m and the elastic time constant (p/G) is of the order of 10 s. 

Shear stress A risk factor for pressure ulcers that is generated when the head of a patient s bed is elevated, causing deeper blood vessels to crimp, which leads to ischemia. 

As is evident from Eq. (3-20) or (3-21), the Bingham plastic exhibits a shear thinning viscosity i.e., the larger the shear stress or shear rate, the lower the viscosity. This behavior is typical of many concentrated slurries and suspensions such as muds, paints, foams, emulsions (e.g., mayonnaise), ketchup, or blood. 

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

It has been claimed that the magnitude of this stress in some blood vessels is similar to the shear stress at the nozzle of the famous water fountain in Lake Geneva. The fountain spouts 500 litres of water per second with a mean velocity of 200 km/hr which reaches a height of about 140 m the shear stress at the nozzle is about 40 dynes/cm . 

Hypertension canses damage to endothelial cells dne to the shear stress as the blood flows throngh the arteries, especially at positions where arteries branch. 

It should be noted that the shear stress caused by the flow of blood through arteries and capillaries can be very high. 

Although the titanium oxide layer at the surface of the nitinol is highly biocompatible and protects the underlying substrate from electrochemical corrosion, the titanium oxide layer itself is mechanically very brittle. Under mechanical stress, such as the shear of blood flow in the aorta or under the bending moments of aortic pulsations, the titanium oxide surface layer can fracture, exposing the underlying metal to corrosion. Not only is corrosion undesirable in terms of biocompatibility (i.e., leaching of nickel and its... 

Almost all patients with TTP have a deficiency in protease activity. As a result, TTP is associated with the presence of unusually large vWF (uLvWF) multimers in the circulation. These uLvWF multimers can bind to GPIb/IX receptor complex on platelets under conditions of high shear stress in the microcirculation and can also bind to the GPIIb/ina receptor on activated platelets, causing aggregation of platelets and subsequent blood vessel occlusion (69). 

It has been shown that plasma protein adsorption, which occurs as soon as blood comes into contact with a given surface, is clearly influenced by the local shear stress [7]. 

Weiss et al. [10] have shown that platelet deposition is at a minimum at a shear rate of 50 s while fibrin deposition on subendothelium from non-anticoagulated blood is at a maximum at the same shear rate. At low shear rates (250 s ), fibrin... 

It will be later proposed that an ideal blood-contacting surface would be one on which endothelial cells could adhere and grow normally to create a complete endothelium endowed with the physiological functions of such tissue. But the correct expression of such physiological functions requires normal blood flow conditions. And the correlation of thrombosis with regions of disturbed flow suggests that shear stress may alter the production of endothelial cell-derived products and directly affect endothelial cell function. 

The role of nitric oxide in maintaining laminar blood flow. A sudden vasoconstriction in one branch of a vascular bed will cause an immediate increase in the upstream pressure that could cause turbulent flow. The myogenic constriction of the upstream branches responding to the turbulent flow will be counterbalanced by increased synthesis of nitric oxide, shown as a radial gradiant, due to shear stress on endothelium. 

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