Biorheology - Volume 37, issue 3

Authors: Liu, Zizhen | Bilston, Lynne

Article Type: Research Article

Abstract: This paper presents the results of recent investigations into the viscoelastic character of liver tissue, especially the linear response. The linear domain of liver tissue has been determined by a strain sweep oscillation test. The fluid‐like viscoelastic behaviour of liver tissue was confirmed by stress relaxation experiments. To demonstrate the consistency and quality of the experimental data, the relaxation spectrum was used to reconstruct the relaxation modulus, as well as the storage and loss moduli. The generalised Maxwell model was successfully employed to model the mechanical behaviour in the linear viscoelastic domain.

Keywords: Soft tissue mechanics, biomechanics, linear viscoelasticity, biorheology, liver tissue

Citation: Biorheology, vol. 37, no. 3, pp. 191-201, 2000

Authors: Gastaldi, Ada C. | Jardim, José R. | King, Malcolm

Article Type: Research Article

Abstract: Frozen, stored mucus has been extensively used for transport studies but there is no clear evidence of the influence that the temperature and length of time of storage may have on the results. We stored frog mucus samples at –20 and –80°C and analysed them on days zero, 2, 10, 30 and 90. At each temperature, a sample was thawed, studied and refrozen on each of the study days, at the same time that one sample was thawed only on the study day. Displacement in a simulated cough machine and on the frog palate, as well as contact angle measurements, …were determined for the mucus samples on each study day. Mucus cytologic analyses on each of the study days were done with special regard to neutrophil counts and cell integrity. Friedman analysis of variance did not show any difference between the different periods of storage and the two temperatures for any of the parameters studied. The medians for the relative transport velocity on the frog palate varied between 0.88 and 1.03, for the contact angle between 21 and 28°, and for the displacement in the simulated cough machine between 58 and 95 mm over the 90 days of the experiment. There were no cytologic alterations compatible with cell degeneration. We conclude that the storage of frog mucus either at –20 or -80°C for periods up to 90 days does not lead to any significant differences in mucus transportability. Show more

Citation: Biorheology, vol. 37, no. 3, pp. 203-211, 2000

Authors: Yamamoto, Ryoichi | Fujii, Shuhei | Tanimoto, Eiichi | Nevins, Donald J.

Article Type: Research Article

Abstract: Okra hypocotyl segments were incubated in solutions of 0.3 or 0.4 M sorbitol at various temperatures and their shrinkage was measured. The result yielded an apparent activation energy for shrinkage of 4.8 kcal/mol, which is close to that of the viscosity of water. This coincidence suggests that the viscosity of water, i.e., the reciprocal function of water conductivity, is a limiting factor for osmotic shrinkage. Abrasion of okra hypocotyl segments with Carborundum substantially increased the rate of their osmotic shrinkage, indicating that the cuticle is the major barrier to water uptake by segments. The apparent activation energy for osmotic shrinkage …was 4.5 kcal/mol in abraded segments. By introducing water conductivity into an algorithm, osmotic shrinkage and expansion of hypocotyl segments was successfully predicted by computation with this algorithm. Hence the extent of the contribution of water conductivity in osmotic shrinkage and expansion can be evaluated. Based on this simulation, water conductivity was identified as one of the major factors in governing the elongation growth rate of cells along with the osmotic pressure of the cell sap and the mechanical properties of the cell wall. Show more

Keywords: Activation energy, computer simulation, okra hypocotyl, the cell wall, water conductivity

Citation: Biorheology, vol. 37, no. 3, pp. 213-223, 2000

Authors: Janzen, Johan | Elliott, Thomas G. | Carter, Cedric J. | Brooks, Donald E.

Article Type: Research Article

Abstract: The viscosity of whole blood measured at low shear rates is determined partly by shear resistance of the red cell aggregates present, stronger aggregation increasing the viscosity in the absence of other changes. Effects of cell deformability can confound interpretation and comparison in terms of aggregation, however, particularly when the plasma viscosity is high. We illustrate the problem with a comparison of hematocrit‐adjusted blood from type 1 diabetes patients and controls in which it is found the apparent and relative viscosities at a true shear rate of 0.20 s^{-1} are lower in the patient samples than age matched …controls, in spite of reports that aggregation is increased in such populations. Because the plasma viscosities of the patients were higher on average than controls, we performed a series of experiments to examine the effect of plasma protein concentration and viscosity on normal blood viscosity. Dilution or concentration by ultrafiltration of autologous plasma and viscosity measurements at low shear on constant hematocrit red cell suspensions showed (a) suspension viscosity at 0.25 and 3 s^{-1} increased monotonically with plasma protein concentration and viscosity but (b) the relative viscosity increased, in concert with the microscopic aggregation grade, up to a viscosity of approximately 1.25 mPa‐s but above this the value the relative viscosity no longer increased as the degree of aggregation increased in concentrated plasmas. It is suggested that in order to reduce cell deformation effects in hyperviscous pathological plasmas, patient and control plasmas should be systematically diluted before hematocrit is adjusted and rheological measurements are made. True shear rates should be calculated. Comparison of relative viscosities at low true shear rates appears to allow the effects of red cell aggregation to be distinguished by variable shear rate viscometry in clinical blood samples. Show more

Citation: Biorheology, vol. 37, no. 3, pp. 225-237, 2000

Authors: Das, B. | Johnson, P.C. | Popel, A.S.

Article Type: Research Article

Abstract: The study of the effect of leukocyte adhesion on blood flow in small vessels is of primary interest to understand the resistance changes in venular microcirculation. Available computational fluid dynamic studies provide information on the effect of leukocyte adhesion when blood is considered as a homogeneous Newtonian fluid. In the present work we aim to understand the effect of leukocyte adhesion on the non‐Newtonian Casson fluid flow of blood in small venules; the Casson model represents the effect of red blood cell aggregation. In our model the blood vessel is considered as a circular cylinder and the leukocyte is considered …as a truncated spherical protrusion in the inner side of the blood vessel. The cases of single leukocyte adhesion and leukocyte pairs in positions aligned along the same side, and opposite sides of the vessel wall are considered. The Casson fluid parameters are chosen for cat blood and human blood and comparisons are made for the effects of leukocyte adhesion in both species. Numerical simulations demonstrated that for a Casson fluid with hematocrit of 0.4 and flow rate Q=0.072 nl/s, a single leukocyte increases flow resistance by 5 % in a 32 μm diameter and 100 μm long vessel. For a smaller vessel of 18 μm, the flow resistance increases by 15%. Show more

Keywords: Leukocyte adhesion, computational model, Casson model, microvessel resistance

Citation: Biorheology, vol. 37, no. 3, pp. 239-258, 2000

Authors: Barnes, H.A.

Article Type: Other

Citation: Biorheology, vol. 37, no. 3, pp. 259-264, 2000