Biorheology - Volume 48, issue 2

Authors: Chatelin, Simon | Oudry, Jennifer | Périchon, Nicolas | Sandrin, Laurent | Allemann, Pierre | Soler, Luc | Willinger, Rémy

Article Type: Research Article

Abstract: Understanding the mechanical properties of human liver is one of the most critical aspects of its numerical modeling for medical applications or impact biomechanics. Generally, model constitutive laws come from in vitro data. However, the elastic properties of liver may change significantly after death and with time. Furthermore, in vitro liver elastic properties reported in the literature have often not been compared quantitatively with in vivo liver mechanical properties on the same organ. In this study, both steps are investigated on porcine liver. The elastic property of the porcine liver, given by the shear modulus G, was measured by both …Transient Elastography (TE) and Dynamic Mechanical Analysis (DMA). Shear modulus measurements were realized on in vivo and in vitro liver to compare the TE and DMA methods and to study the influence of testing conditions on the liver viscoelastic properties. In vitro results show that elastic properties obtained by TE and DMA are in agreement. Liver tissue in the frequency range from 0.1 to 4 Hz can be modeled by a two-mode relaxation model. Furthermore, results show that the liver is homogeneous, isotropic and more elastic than viscous. Finally, it is shown in this study that viscoelastic properties obtained by TE and DMA change significantly with post mortem time and with the boundary conditions. Show more

Keywords: Soft tissues, liver, viscoelasticity, transient elastography, dynamic mechanical analysis

DOI: 10.3233/BIR-2011-0584

Citation: Biorheology, vol. 48, no. 2, pp. 75-88, 2011

Authors: Zhong, Ziwei | Akkus, Ozan

Article Type: Research Article

Abstract: Bone marrow is the niche for stem cells and is within close proximity to bone lining cells. Forces experienced by these cells guide their differentiation and proliferation. As these forces are dependent on the viscosity of the medium, the knowledge about the viscosity of marrow is essential to modeling the mechanical environment of bone. This study sought to examine the effects of age on the rheological properties of human yellow bone marrow. Samples were harvested from the femurs of male donors ranging from 22 to 82 years of age (N=19) and subjected to stress and frequency sweeps to determine viscosity …and dynamic moduli, respectively. The viscosity of bone marrow at physiologically plausible shear rates ranged from 44 to 142 mPa · s. The coefficients of variation ranged up to 0.40 within subjects and 0.14 between subjects. Regressions of viscosity values against age did not generate a strong level of significance; therefore, earlier reported changes in the composition of marrow with age did not translate into variation in viscosity of marrow. Since age does not seem to offer a governing effect, the observed variation within and between donors may stem from other factors (genetic, nutrition, etc.). The wide range of variation in the viscosity of marrow within subject, between subjects and with age implies that the fluid shear experienced by cells resident in marrow may also vary substantially. Show more

Keywords: Osteoporosis, stem cells, shear stress, viscosity, loss modulus, shear modulus, fat

DOI: 10.3233/BIR-2011-0587

Citation: Biorheology, vol. 48, no. 2, pp. 89-97, 2011

Authors: Kamada, Hiroki | Tsubota, Ken-ichi | Nakamura, Masanori | Wada, Shigeo | Ishikawa, Takuji | Yamaguchi, Takami

Article Type: Research Article

Abstract: The purpose of this study was to evaluate the effects of stenosis geometry on primary thrombogenesis with respect to the dynamics of the blood flow. A two-dimensional computer simulation was carried out to simulate the formation of a primary thrombus under blood flow in two geometrically different blood vessels: one straight and the other stenosed. In the simulation, blood was modeled by particles that have characteristics of plasma and of platelets. Plasma and platelet flow was analyzed using the Moving Particle Semi-implicit (MPS) method, while the motion of adhered and aggregated platelets was expressed by mechanical spring forces. With these …models, platelet motion in the flowing blood and platelet aggregation and adhesion were successfully coupled with viscous blood flow. The results of the simulation demonstrated that the presence of a stenosis induced changes in blood flow and thereby altered the formation, growth, and destruction of a thrombus. In particular, whereas in the absence of stenosis, the thrombus evenly covered the injured site, in the presence of a stenosis, thrombus formation was skewed to the downstream side. The number of platelets that adhered to the injured site increased earlier as the stenosis became more severe. These results suggest that dynamic changes in blood flow due to the presence of a stenosis affect primary thrombogenesis. Show more

Keywords: Thrombus, stenosis, blood flow, platelet, computational fluid dynamics (CFD)

DOI: 10.3233/BIR-2011-0585

Citation: Biorheology, vol. 48, no. 2, pp. 99-114, 2011

Authors: Hightower, C. Makena | Yalcin, Ozlem | Salazar Vázquez, Beatriz Y. | Johnson, Paul C. | Intaglietta, Marcos

Article Type: Research Article

Abstract: The effect of low and high viscosity hemodilution with plasma expanders on the extent of the cell free layer (CFL) width was analyzed in the microcirculation of the exteriorized cremaster muscle preparation of Sprague–Dawley male rats. Anesthetized animals were subjected to 40% hemodilution by blood volume, using 5% human serum albumin (HSA) or 6% Hetastarch (hydroxyethyl starch 670 kDa). Arterioles (n=5 for each treatment) were investigated. Mean arterial pressure, heart rate, vessel flow velocity and CFL width were measured at baseline and 5, 20 and 40 min post-exchange transfusion. Blood and plasma viscosity was determined from terminal blood collections. CFL …width and pseudoshear rate, diameter and flow, normalized to baseline, were significantly elevated at all post-exchange assessments. Peripheral vascular resistance decreased. The increase of the CFL width was greater with HSA by comparison with Hetastarch hemodilution (p<0.05). Hetastarch blood and plasma viscosities increased significantly compared to those of HSA (p<0.05). This study shows that CFL widths are influenced by plasma expander viscosity, a phenomenon proportional to the increase in molecular weight of the colloids in solution. Show more

Keywords: Hemodilution, plasma viscosity, microcirculation, shear stress

DOI: 10.3233/BIR-2011-0586

Citation: Biorheology, vol. 48, no. 2, pp. 115-125, 2011

Authors: Kaliviotis, Efstathios | Yianneskis, Michael

Article Type: Research Article

Abstract: This paper reports on a theoretical examination of the hypothesis that red blood cell network characteristics influence the mechanical properties of the fluid. For this purpose a newly developed energy-rate based blood viscosity model, which incorporates network dynamics, was used to predict the transient behaviour of blood viscosity (steady-state results of this model have been reported in Biorheology 46 (2009), 487–508). The main network characteristic examined in the present work was the inter-aggregate branch size and its relationship to the evolving aggregates. Branch size was used to define a network integrity index that accounted for the strength of the developed …network. For the development and validation of the model, experiments performed with an optical shearing microscope, with different step-changes in shear rate, were utilised, as well as viscosity measurements under similar flow conditions performed in a double wall Couette instrument. The experimental data were compared with the response of the model, which incorporated the network integrity index. The results suggest that network characteristics may influence the viscosity of blood at low shear rates and exhibit good agreement with experimental observations. Show more

Keywords: Branch size, network formation, bi-phasic viscoelastic constant, red blood cell aggregation

DOI: 10.3233/BIR-2011-0588

Citation: Biorheology, vol. 48, no. 2, pp. 127-147, 2011