Biorheology - Volume 40, issue 5

Authors: El‐Khatib, F.H. | Damiano, E.R.

Article Type: Research Article

Abstract: A non‐Newtonian shear‐thinning constitutive relation is proposed to study pulsatile flow of whole blood in a cylindrical tube. The constitutive relation, which satisfies the principle of material frame indifference, is derived from viscometric data obtained from whole blood over a range of hematocrits. Assuming axisymmetric flow in a rigid cylindrical tube of constant diameter, a second‐order, nonlinear partial differential equation governing the axial velocity component is obtained. Imposing a periodic pressure gradient, the governing equation was solved numerically using finite difference methods over a range of Stokes values and hematocrits. For a forcing frequency of 1 Hz, results are presented …over tube diameters ranging between 0.1 and 2 cm and over hematocrits ranging between 10 and 80%. For a given hematocrit, velocity profiles predicted for the non‐Newtonian model under sinusoidal forcing reveal attenuated volume flow rate and enhanced vorticity transport over the tube cross‐section relative to a Newtonian fluid having a viscosity corresponding to the high shear‐rate limit. For moderate to high Stokes numbers, consistent with flow in large arteries, our results revealed a viscosity distribution that was nearly time invariant. An analytic solution was obtained for a fluid having arbitrarily prescribed radially varying, temporally invariant viscosity and density distributions under arbitrary periodic pressure forcing. Close agreement was observed between our numerical and analytical results when the imposed viscosity distribution was chosen to approximate the time‐averaged viscosity distribution predicted by the shear‐thinning non‐Newtonian model. For $\mathit{St}\gtrsim 100$ , the disparity between our results and those of a Newtonian fluid of constant viscosity grows with a decreasing ratio of the DC to AC components of the pressure‐gradient amplitude below 50%. In particular, for any purely oscillatory pressure‐gradient (vanishing DC component), the Womersley solution is a particularly poor predictor of the amplitude and phase of wall shear rate for over half of the flow cycle. Under such circumstances, the analytical models presented here provide a simple and accurate means of estimating instantaneous wall shear rate, knowing only the pressure gradient and hematocrit. Show more

Keywords: Viscosity, non‐Newtonian, shear thinning, hematocrit, Womersley, eigenfunction expansion

Citation: Biorheology, vol. 40, no. 5, pp. 503-522, 2003

Authors: Lawrence, A.R. | Gusic, R.J. | Gooch, K.J.

Article Type: Research Article

Abstract: Based on the decisive effects of the hemodynamic and mechanical environments on the development and remodeling of arteries in vivo, several groups have cultured tissue‐engineered vessels and excised vessels in various mechanically active perfusion systems. To facilitate the interpretation and design of such studies, accurate estimates of the applied forces and resulting stresses are required, which in turn require an accurate estimate of vessel dimensions. The measured pressure drop along the length of the vessel could be used to calculate the average inner diameter, but practical considerations, including the modest accuracy of many pressure transducers, limit this approach. Using nine …porcine arteries harvested from pigs weighing between 25 and 100 kg, we show that when real‐time measurements of the pressure drop and the outer diameter during a vasoactive event are fit to a theoretical model, offset errors in the pressure measurement can be compensated for and estimates of vessel wall transverse area with an average error of 4.1% (not exceeding 8.3%) are achieved. Show more

Keywords: Vascular biology, biomechanics, vessel geometry, noninvasive measurement

Citation: Biorheology, vol. 40, no. 5, pp. 523-529, 2003

Authors: Antonova, Nadia | Todorov, Roumen | Exerowa, Dotchi

Article Type: Research Article

Abstract: The proposed in vitro model for studying the alveolar surface layer of the lungs enables one to investigate the surface intermolecular forces which influence the stability of the alveolus. The general role for the stability of the alveolus belongs to the phospholipids in the alveolar surfactant and predominantly to their main component dipalmitoylphosphatidylcholine (DPPC). The aim of the study was to investigate the rheological behavior of DPPC and exogenous surfactant preparations used in neonatal clinical practice. Data for the rheological behavior of the solutions of the commercially available surfactants, Infasurf, Exosurf and Survanta, as well as of DPPC (their main …phospholipid component) at shear rates from 0.024 to 94.5 s−1 under steady and transient flow conditions at 23°C were obtained. Infasurf and Exosurf showed Newtonian rheological behavior, while Survanta revealed the shear‐thinning behavior of a non‐Newtonian pseudoplastic fluid. The rheological properties of aqueous solutions of DPPC containing 0.14 M NaCl at concentrations from 100 and 630 μg/ml of phospholipid (chosen from the dependence of the probability for bilayer film formation) were studied. Differences observed in the rheological properties of the exogenous surfactants were interpreted on the basis of their composition, the presence of other phospholipid components, certain additives and surfactant proteins, as well as the bulk structures formed from them. The relevance of the results for the delivery of exogenous surfactants and their spreading in replacement therapy is discussed. Show more

Keywords: Phospholipids, lung surfactant, viscosity, bilayer films

Citation: Biorheology, vol. 40, no. 5, pp. 531-543, 2003

Authors: Nasseri, Simin | Bilston, Lynne | Tanner, Roger

Article Type: Research Article

Abstract: Conducting experiments on very soft biological tissues can be difficult. Traditionally, unconfined compression and shear have been used. Here, an improved method of compression testing, lubricated squeezing flow is described. This gives a uniform compression along the squeezing axis and almost uniform equi‐biaxial elongation at right angles to the squeezing axis, with minimal shear deformation due to the constant lubrication of the sample surfaces during testing. Sample results for porcine liver obtained using this method are described here.

Keywords: Viscoelastic, compression, liver, rheology

Citation: Biorheology, vol. 40, no. 5, pp. 545-551, 2003

Authors: Farges, E. | Grebe, R. | Baumann, M.

Article Type: Research Article

Abstract: The red cell deformation under the conditions of oscillating centrifugal fields was studied. Experiments were carried out with a modified Cell‐Elastometer operating in oscillating mode (0.02 to 0.30 Hz). Gravitational acceleration was sinusoidally modulated between 620g and 2250g. At low frequencies (below 0.08 Hz), native red cells followed the applied stress without delay. At 0.09 Hz and up, the cellular deformation was still periodical and included an additional perturbation due to intracellular movements. This perturbation was analysed and quantified. The influence of alterations on the erythrocyte membrane by diamide was analysed to verify the sensitivity of this method. On increasing …the membrane stiffness with low concentrations of diamide, the response to oscillatory centrifugal stress was impaired characteristically in terms of amplitude deformation. Based on tangential and centrifugal accelerations, a physical model was developed that describes the basic observable changes on varying the oscillation frequency. From the data it can be concluded that viscoelastic properties of red cells can be analysed and quantified using oscillatory centrifugal accelerations. The described method can become a valid tool to differentiate between membrane alterations or intracellular viscous modifications. Show more

Keywords: Centrifugal acceleration, diamide, laser, diffractometry, sinusoidal

Citation: Biorheology, vol. 40, no. 5, pp. 553-565, 2003

Authors: Jetha, Karim A. | Egginton, Stuart | Nash, Gerard B.

Article Type: Research Article

Abstract: Increase in the resistance to deformation of neutrophils upon exposure to the cold may impair their passage through microvessels. However, the potential for such rheological changes to cause prolonged microvascular obstruction in cooled tissue will depend on whether and at what rate the neutrophils recover on rewarming. We tested the ability of neutrophils to pass through micropore filters, and found that neutrophils cooled to 10°C for 10–20 minutes could block either 5 μm or 8 μm pore filters. On return to 37°C, flow resistance remained impaired briefly but recovered over about 5 minutes. The kinetics of changes in flow resistance …in the cold and on rewarming were linked to kinetics of actin polymerisation during these periods. However, they were not closely linked to distortion of cell shape in the cold, which recovered only slowly with rewarming. The results suggest that while rigid neutrophils might occlude capillaries in cold tissue, mechanical obstruction should not be long‐lived on rewarming. Moreover, rigid neutrophils washed out of cold tissue should experience only temporary mechanical trapping in remote tissues. Show more

Keywords: Neutrophil filterability, temperature, hypothermia

Citation: Biorheology, vol. 40, no. 5, pp. 567-576, 2003