Biorheology

Determination with a Couette viscometer of the shear stress-shear rate relation for a non-Newtonian fluid is not simple because of the variation of shear rate within the viscometer gap. Exact methods for calculation of the shear rate have been presented by Krieger and Elrod, but they require extensive manipulation of the viscometric data. The present study shows that use of average values of shear stress and shear rate allows determination of the viscometric curve to within a few per cent of the exact relation. The error is found to remain finite even for extreme deviation from Newtonian behavior. For a pseudo plastic power-law fluid the maximum per cent error in calculated viscosity at a given shear rate is given by [ ( s − 1 ) / 2 s ] × 100 , where s is the ratio of the outer to inner cylinder diameters. The method has the advantage that the average shear rates used do not depend on the fluid rheology and thus can be selected in advance.

Measurements were taken of the viscoelastic properties of saliva and blood serum, by slightly modifying the capillary apparatus constructed earlier by two of the authors. The elastic property measured was the recoverable shear strain as a function of shear stress, which enables one to calculate the elastic shear modulus. The materials tested were collected from hospital patients suffering from a variety of diseases, such as diabetes, cystic fibrosis, malignant neoplastic disease and Guillian–Barré syndrome. Measured values of the viscoelastic properties of the patients are found to be quite different from those of healthy persons, suggesting that the method developed by the authors has a potential use for possible clinical application in testing saliva or serum of patients.

Rheogoniometric measurements were made of the viscosity of human plasma and serum systems, and of solutions of fibrinogen and other plasma proteins with and without guard ring in combined Couette and cone and plate geometry at shear rates from 10³ to less than 10⁻¹ sec⁻¹. The exclusion of measurements of surface layers by the use of the guard ring always resulted in Newtonian flow characteristics, while the “overall viscosity”, measured without the guard ring, exhibited non-Newtonian behavior. The addition of 0.4% fibrinogen to 5% plasma or to 0.25% albumin concentration showed a marked increase of overall viscosity, but such addition to 90% plasma or to 5% albumin exhibited no change in non-Newtonian behavior. These and other findings support the introduction of a new concept for the initiation of thrombosis which is based on the formation of polymolecular layers of fibrinogen and other plasma proteins, leading to obstruction of the affected blood vessel and impairment of the circulation. The aggregation of the proteins is considered to occur in two steps, viz. a first adsorption process on the surface of the vessel wall, followed by a growth process in which additional protein molecules adsorb on the first or previously formed adsorption layers.

Eighty-seven per cent fresh red cells suspended in fresh homologous (heparinized) plasma exhibited decreasing viscosity with continued shear at flow rates of 0.02–0.2 tube diameters per second in a horizontal glass capillary 0.55 mm wide. There was some correlation with cigarette smoking. There also appeared to be a tentative connection between flow properties and early cancer.

A variable shear rate, non-pulsatile capillary viscometer for measurements of blood circulating in chronic exteriorized A–V shunts in dogs has been constructed. Nominal values of reduced average velocity (Ū) obtainable with this instrument range from 28 to 560 sec⁻¹ in 200μ i.d. tubes. Experimental data are reported for measurements obtained from both native and heparinized blood. Conclusions are: (1) acute heparinization has no effect on the rheological properties of dog blood and (2) dog blood can be described as a pseudoplastic, non-Newtonian suspension, with apparent viscosity decreasing as either shear rate increases or hematocrit decreases. Both conclusions are limited to the range of hematocrit (20–50 per cent) and Ū studied.