In this page, you will find a list of recent publications of the Celullar Biomechanics and Biotransport Laboratory, including research papers, book chapters and reviews, conference presentations, technical reports, and theses. This list is accompanied by abstracts of our published work, with a link to full text, that can be accessed by clicking the reference to a paper. Certain publications are available in PDF format for downloading.


Research papers:
  1. H. Lan and D.B. Khismatullin, "Numerical simulation of the pairwise interaction of deformable cells during migration in a microchannel," Phys. Rev. E (2014, submitted).

    When the blood flows in vessels and channels, circulating cells deform and move relatively to the blood flow in the lateral and translational directions. This migratory property plays a key role in immune response, hemostasis, cancer progression, delivery of nutrients, and microfluidic technologies such cell separation/enrichment and flow cytometry. Using our three-dimensional computational algorithm for multiphase viscoelastic flow, we have investigated the effect of pairwise interaction on the lateral and translational migration of deformable cells in a microchannel. The numerical simulation data show that when two cells with same size and close distance interact, they repel each other until they reach a same lateral equilibrium position where their equilibrium separation distance depends on their location relative to the centerline. When a series of closely spaced cells with same size are considered, they generally undergo damped oscillation in both lateral and translational directions until they reach equilibrium positions where they become evenly distributed in the flow direction (self-assembly phenomenon). For a series of cells with different sizes, bigger cells could collide repeatedly with smaller ones and enter the other side of the channel (above or below the centerline). For a series of cells with different deformability, more deformable cells upon impact with less deformable cells move to an equilibrium position closer to the centerline. The results of our study provide better understanding of cell margination in bloodstream and cell separation/enrichment in microfluidic devices.

  2. N.H. Hoang, H.Y. Murad, S.H. Ratnayaka, C. Chen, and D.B. Khismatullin, "Synergistic ablation of liver tissue and liver cancer cells with high-intensity focused ultrasound and ethanol," Ultrasound Med. Biol. (2014, in press).

    We have investigated the combined effect of ethanol and high-intensity focused ultrasound (HIFU) firstly on heating and cavitation bubble activity in tissue-mimicking phantoms and porcine liver tissues and secondly on the viability of HepG2 liver cancer cells. Phantoms or porcine tissues were injected with ethanol and then subjected to HIFU at acoustic power ranged from 1.17 to 20.5 W (HIFU levels 1 to 7). Cavitation events and the temperature around the focal zone were measured by a passive cavitation detector and embedded type K thermocouples, respectively. HepG2 cells were subjected to 4% ethanol solution in growth medium (v/v) just before the cells were exposed to HIFU at 2.73, 8.72, or 12.0 W for 30 seconds. The cell viability was measured 2, 24, and 72 hours post-treatment. The results indicate that ethanol and HIFU have a synergistic effect on liver cancer ablation as manifested by greater temperature rise and lesion volume in liver tissues and in reduced viability of liver cancer cells. This effect is likely caused by reduction of the cavitation threshold in the presence of ethanol and the increased rate of ethanol diffusion through the cell membrane by HIFU-induced streaming, sonoporation, and heating.

  3. C. Chen and D.B. Khismatullin, "Lipopolysaccharide induces the interactions of breast cancer and endothelial cells via activated monocytes," Cancer Lett., 345, 75-84 (2014).

    The adhesion of circulating cancer cells to vascular endothelium is a key step in hematogenous metastasis. Cancer cell-endothelium interactions are mediated by cell adhesion molecules that can also be involved in the arrest of circulating leukocytes on endothelium in inflammation. Static and microfluidic flow adhesion assays as well as flow cytometry were conducted in this study to elucidate the role of circulating monocytes, bacterial lipopolysaccharide (LPS), and histamine in breast cancer cell adhesion to vascular endothelial cells. Tumor necrosis factor-α (TNF-α) released from LPS-treated monocytes triggered the expression of intercellular cell adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) on endothelial cells. Histamine augmented the TNF-α effect, leading to a high number of arrested breast cancer cells under both static and shear flow conditions. LPS-treated monocytes were shown to enhance the arrest of breast cancer cells by anchoring the cancer cells to activated endothelial cells. This anchorage was achieved by binding cancer cell ICAM-1 to monocyte β2 integrins and binding endothelial ICAM-1 and VCAM-1 to monocyte β1 and β2 integrins. The results of this study imply that LPS is an important risk factor for cancer metastasis and that the elevated serum level of histamine further increases the risk of LPS-induced cancer metastasis. Preventing bacterial infections is essential in cancer treatment, and it is particularly vital for cancer patients affected by allergy.

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  4. C. Chen and D.B. Khismatullin, "Oxidized low-density lipoprotein contributes to atherogenesis via co-activation of macrophages and mast cells," PLOS One (2013, submitted).

    Objective: Oxidized low density lipoprotein (OxLDL) is a risk factor for atherosclerosis due to its role in endothelial dysfunction and foam cell formation. Tissue-resident cells including macrophages and mast cells can release inflammatory mediators upon exposure to OxLDL that can in turn activate vascular endothelium and increase monocyte recruitment. In this study, we examined whether OxLDL can co-activate macrophages and mast cells and whether this co-activation leads to a synergistic effect on monocyte-endothelium adhesion.
    Approach and Results: Static adhesion assays and flow cytometry were conducted to measure the number of adherent THP-1 monocytes and the expression of adhesion molecules on endothelium activated by OxLDL-treated THP-1 macrophages and LUVA mast cells and directly activated by OxLDL. The low dose of OxLDL (8 μg/ml), when patients typically have no clinical evidence of cardiovascular disease, was sufficient to activate both macrophages and mast cells and synergistically increase monocyte-endothelium adhesion via released TNF-α and histamine, respectively. The direct exposure of endothelial cells to a much higher dose of OxLDL (80 μg/ml) had less effect on monocyte adhesion than the indirect activation through macrophages and mast cells.
    Conclusions: The results of this work indicate that endothelial dysfunction induced by OxLDL-activated macrophages and mast cells is an important mechanism for the development of atherosclerosis. The synergy between inflammatory mediators released from activated macrophages and mast cells indicates a key role of mast cells in atherogenesis and points out that allergic patients with a lipid-rich diet are at high risk for atherosclerosis and cardiovascular disease.

  5. S.H. Ratnayaka, T. Hillburn, O. Forouzan, S.S. Shevkoplyas, and D.B. Khismatullin, "PDMS well platform for culturing millimeter-size tumor spheroids," Biotechnol. Prog., 29, 1265-1269 (2013).

    Multicellular tumor spheroids are widely used as in vitro models for testing of anticancer drugs. The advantage of this approach is that it can predict the outcome of a drug treatment on human cancer cells in their natural three-dimensional environment without putting actual patients at risk. Several methods were utilized in the past to grow submillimeter-size tumor spheroids. However, these small models are not very useful for preclinical studies of tumor ablation where the goal is the complete destruction of tumors that can reach several centimeters in diameter in the human body. Here, we propose a PDMS well method for large tumor spheroid culture. Our experiments with HepG2 hepatic cancer cells show that three-dimensional aggregates of tumor cells with a volume as large as 40 mm3 can be grown in cylindrical PDMS wells after the initial culture of tumor cells by the hanging drop method. This is a 320 times more than the maximum volume of tumor spheroids formed inside hanging drops (0.125 mm3).

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  6. W. Wang, F. Graziano, V. Russo, A. J. Ulm, D. De Kee, and D.B. Khismatullin, "Giant intracranial aneurysm embolization with a yield stress fluid material: Insights from CFD analysis," Biorheology, 50, 99-114 (2013).

    The endovascular treatment of intracranial aneurysms remains a challenge, especially when the aneurysm is large in size and has irregular, non-spherical geometry. In this paper, we use computational fluid dynamics to simulate blood flow in a vertebro-basilar junction giant aneurysm for the following three cases: 1) an empty aneurysm, 2) an aneurysm filled with platinum coils, and 3) an aneurysm filled with a yield stress fluid material. In the computational model, blood and the coil-filled region are treated as a non-Newtonian fluid and an isotropic porous medium, respectively. The results show that yield stress fluids can be used for aneurysm embolization provided the yield stress value is 20 Pa or higher. Specifically, flow recirculation in the aneurysm and the size of the inflow jet impingement zone on the aneurysm wall are substantially reduced by yield stress fluid treatment. Overall, this study opens up the possibility of using yield stress fluids for effective embolization of large-volume intracranial aneurysms.

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  7. H. Lan, SJ Clair Hur, D. Di Carlo, and D.B. Khismatullin, "Lateral migration of living cells in inertial microfluidic systems explored by fully three-dimensional numerical simulation," Biorheology (2012, under revision).

    The effects of cell size and deformability on the lateral migration and deformation of living cells flowing through a rectangular microchannel has been numerically investigated and compared with the experimental data on the inertial microfluidics-based approach for detection and separation of cells. The results of this work indicate that the cells move closer to the centerline if they are bigger and/or more deformable and that their equilibrium position is largely determined by the solvent (cytosol) viscosity, which is much less than the polymer (cytoskeleton) viscosity measured in most rheological systems. Simulations also suggest that decreasing channel dimensions leads to larger differences in equilibrium position for particles of different viscoelastic properties, giving design guidance for the next generation of microfluidic cellseparation chips.

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  8. F. Graziano, V. Russo, W. Wang, D. Khismatullin, and A. J. Ulm, "3-D Computational Fluid Dynamics of a Treated Vertebro-Basilar Giant Aneurysm: A Multistage Analysis," Am. J. Neuroradiol. 34, 1387-1394 (2013).

    Background and Purpose: The treatment of vertebro-basilar junction (VBJ) giant aneurysms (GA) remains a challenging task in the neurosurgical practice and the gold standard therapy is still under debate. Through a detailed post-mortem study, the authors analyze the hemodynamic factors underlying the formation and recanalization of an aneurysm located at this particular site and its anatomic configuration.
    Methods: An adult fixed cadaveric specimen with a known VBJ GA, characterized radiographically and treated with endovascular embolization, was studied. 3-D computational fluid dynamic (CFD) models were built based on the specific angio-architecture of the specimen and each step of the endovascular treatment was simulated.
    Results: The 3-D CFD study showed at the neck region of the aneurysm, an area of hemodynamic stress (high wall shear stress, high static pressure, high flow velocity), matching the site of recanalization seen during the treatment period of the patient.
    Conclusions: Aneurysm morphology, location and patient specific angio-architecture are the principal factors to be considered in the management of the VBJ giant aneurysms. The 3-D CFD study is valuable tool that, coupled with the neuro-radiological work-up, may add valuable insights in the treatment planning of complex cerebrovascular diseases.

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  9. T.P. Brown, O. Forouzan, S.S. Shevkoplyas, and D.B. Khismatullin, "Histamine reduces GPIbα-mediated adhesion of platelets to TNF-α-activated vascular endothelium," Thromb. Res. 131, 150-157 (2013).

    Histamine and tumor necrosis factor-α (TNF-α) are critical mediators of acute and chronic inflammation that are generated by mast cells and macrophages in atherosclerotic lesions or systemically during allergic attacks. Both of them induce activation of vascular endothelium and thus may play a role in thrombosis. Here we studied the interplay between histamine and TNF-α in glycoprotein (GP) Ibα-mediated platelet adhesion to cultured human vascular endothelial cells under static and shear flow conditions. The stimulation of endothelial cells with histamine or TNF-α increased the number of adherent or slow rolling GP Ibα-coated microbeads or washed human platelets. However, the application of histamine to endothelium pre-activated by TNF-α inhibited GP Ibα-mediated platelet adhesion. These effects were found to be associated with changes in the concentration of ultra large von Willebrand factor (ULVWF) strings anchored to endothelium. The results of this study indicate that histamine released during mast cell degranulation may cause or inhibit thrombosis, depending on whether it acts on resting endothelial cells or on cells pre-activated by other inflammatory stimuli.

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  10. C. Chen and D.B. Khismatullin, "Synergistic effect of histamine and TNF-α on monocyte adhesion to vascular endothelial cells," Inflammation 36, 309-319 (2013).

    The histamine level is high during allergic attacks, and patients with allergy may have chronic inflammatory conditions at which tumor necrosis factor (TNF)-α is extensively released by macrophages. Here, in vitro static and microfluidic flow assays were conducted to investigate the combined influence of histamine and TNF-α on adhesion of monocytic THP-1 cells to human umbilical vein endothelial cells (HUVEC). In a static assay, histamine stimulation of TNF-α-activated HUVEC elevated the number of attached THP-1 cells. In a flow assay, the number of crawling and firmly adherent THP-1 cells was higher on TNF-α + histamine activated HUVEC than on HUVEC activated by TNF-α alone. This synergistic effect of histamine and TNF-α is caused by the increased endothelial surface expression of intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin. Since the exposure of TNF-α-activated endothelium to histamine favors monocyte recruitment, it may be a serious risk factor for atherosclerosis and other chronic inflammatory disorders.

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  11. P.A. Coghill, E. Kesselhuth, E. Shimp, D.B. Khismatullin, and D.W. Schmidtke, "Effects of microfluidic channel geometry on leukocyte rolling assays," Biomed. Microdev. 15, 183-193 (2013).

    Microfluidic cell adhesion assays have emerged as a means to increase throughput as well as reduce the amount of costly reagents. However as dimensions of the flow chamber are reduced and approach the diameter of a cell (Dc), theoretical models have predicted that mechanical stress, force, and torque on a cell will be amplified. We fabricated a series of microfluidic devices that have a constant width:height ratio (10:1) but with varying heights. The smallest microfluidic device (200µm x 20µm) requires perfusion rates as low as 40 nL/min to generate wall shear stresses of 0.5 dynes/cm2. When neutrophils were perfused through P-selectin coated chambers at equivalent wall shear stress, rolling velocities decreased by approximately 70% as the ratio of cell diameter to chamber height (Dc/H) increased from 0.08 (H=100µm) to 0.40 (H=20µm). Three-dimensional numerical simulations of neutrophil rolling in channels of different heights showed a similar trend. Complementary studies with PSGL-1 coated microspheres and paraformaldehyde-fixed neutrophils suggested that changes in rolling velocity were related to cell deformability. Using interference reflection microscopy, we observed increases in neutrophil contact area with increasing chamber height (9-33%) and increasing wall shear stress (28-56%). Our results suggest that rolling velocity is dependent not only on wall shear stress but also on the shear stress gradient experienced by the rolling cell. These results point to the Dc/H ratio as an important design parameter of leukocyte microfluidic assays, and should be applicable to rolling assays that involve other cell types such as platelets or cancer cells.

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  12. H. Lan and D.B. Khismatullin, "A Numerical Study of the Lateral Migration and Deformation of Drops and Leukocytes in a Rectangular Microchannel," Int. J. Multiphase Flow 47, 73-84 (2012).

    When deformable particles (e.g., drops or living cells) are perfused through a flow channel, they drift into a specific lateral position that depends on their size and mechanical properties. This characteristic can be used for deformability-based particle sorting. Using a fully three-dimensional algorithm for viscoelastic drop dynamics, we study numerically the effects of particle size, bulk shear viscosity and elasticity, interfacial (or cortical) tension, and fluid inertia on lateral migration and deformation of small liquid drops and leukocytes (white blood cells) in a rectangular microfluidic flow chamber. Our numerical data show that there is an almost linear increase in the lateral equilibrium position of liquid drops or leukocytes with the particle diameter-to-channel height ratio increased from 0.1 to 0.5. Excluding the case of drops with high interfacial tension, an increase in bulk viscosity of these particles leads to a substantial decrease in their equilibrium position. Overall, the results of this work indicate that 1) drops with different bulk viscosities can be efficiently separated in a rectangular microchannel if their interfacial tension is low or the flow rate is sufficiently high; and 2) the microfluidic technology is well suited for the separation of leukocytes with different cytoplasmic viscosities and relaxation times, but it is much less sensitive to cortical tension. This investigation opens up the possibility of using microfluidic systems for deformability-based flow cytometry.

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  13. D.B. Khismatullin and G.A. Truskey, "Leukocyte rolling on P-selectin: A three-dimensional numerical study of the effect of cytoplasmic viscosity," Biophys. J. 102, 1757-1766 (2012).

    Rolling leukocytes deform and show a large area of contact with endothelium under physiological flow conditions. We studied the effect of cytoplasmic viscosity on leukocyte rolling using our novel three-dimensional numerical algorithm that treats leukocyte as a compound droplet in which the core phase (nucleus) and the shell phase (cytoplasm) are viscoelastic fluids. The algorithm includes the mechanical properties of the cell cortex by cortical tension and considers leukocyte microvilli that deform viscoelastically and form viscous tethers at supercritical force. Stochastic binding kinetics describes binding of adhesion molecules. The leukocyte cytoplasmic viscosity plays a critical role in leukocyte rolling on an adhesive substrate. High-viscosity cells are characterized by high mean rolling velocities, increased temporal fluctuations in the instantaneous velocity, and a high probability for detachment from the substrate. A decrease in the rolling velocity, drag and torque with the formation of a large, flat contact area in low-viscosity cells leads to a dramatic decrease in the bond force and stable rolling. Using values of viscosity consistent with step aspiration studies of human neutrophils (5 to 30 Pa·s), our computational model predicts the velocities and shape changes of rolling leukocytes as observed in vitro and in vivo.

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  14. C. Chen, Y. Liu, S. Maruvada, M. Myers, and D.B. Khismatullin, "Effect of ethanol injection on cavitation and heating of tissues exposed to high intensity focused ultrasound," Phys. Med. Biol. 57, 937-961 (2012).

    Cavitation activity and temperature rise have been investigated in a tissue-mimicking material and excised bovine liver treated with ethanol and insonated with a 0.825 MHz focused acoustic transducer. The acoustic power was varied from 1.3 W to 26.8 W to find the threshold leading to the onset of inertial cavitation. Cavitation events were quantified by three independent techniques: B-mode ultrasound imaging, needle hydrophone measurements, and passive cavitation detection. Temperature in or near the focal zone was measured by thermocouples embedded in the samples. The results of this study indicate that the treatment of tissue phantoms and bovine liver samples with ethanol reduces their threshold power for inertial cavitation. This in turn leads to a sudden rise in temperature in ethanol-treated samples at a lower acoustic power than that in untreated ones. The analysis of passive cavitation detection data shows that once the threshold acoustic power is reached, inertial cavitation becomes a major contributor to acoustic scattering in ethanol-treated phantoms and bovine liver samples as compared to control. This study opens up the possibility of improved tumour ablation therapy via a combination of percutaneous ethanol injection and high-intensity focused ultrasound.

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  15. W. Wang, B. Meng, D. De Kee, and D.B. Khismatullin, "Numerical investigation of plate edge and slot size effects in low yield stress measurements with a slotted plate device," Rheol. Acta 51, 151-162 (2012).

    There is a need for accurate yield stress measurements, especially in the case of low yield stress complex materials such as biological samples. This task cannot be accomplished with conventional rotational rheometers due to significant wall slip effects and the necessity to operate the device at very low shear rates, often beyond the limit that such rheometers can achieve. In this paper, we focus on the slotted plate method proposed recently for low yield stress measurements. Using computational fluid dynamics, we study the effects of plate geometry on the measurement accuracy of the slotted plate method. Results of this study indicate that both wall slip effects and pressure drag force can be substantially reduced by adopting a thin plate with sharp front and rear edges, high slot area ratio, and large number of slots. If the plate has 30 degree triangular edges, a slot area ratio of 80%, and 12 slots, the slotted plate method overpredicts the yield stress of a 0.09 wt.% Carbopol dispersion (yield stress of 9.17 Pa) by only 8.4% under no slip conditions and underpredicts the yield stress by 12.3% under free slip conditions. Similar results were obtained for human saliva characterized by a very low yield stress (0.073 Pa).

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  16. W. Wang, D. De Kee, and D. Khismatullin, "Numerical simulation of power law and yield stress fluid flows in double concentric cylinder with slotted rotor and vane geometries," J. Non-Newtonian Fluid Mech. 166, 734-744 (2011).

    In our previous report, we showed that a rheometer equipped with a double concentric cylinder geometry with slotted rotor could effectively reduce wall slip effects and thus it could be used as an alternative to a rheometer with a vane geometry in yield stress measurements. Here, we use three-dimensional CFD simulation to compare these two geometries for rheological measurements of power law and yield stress fluids. Our results indicate that the double concentric cylinder rheometer with slotted rotor (DCCR/SR) is able to accurately measure rheological properties of a wider spectrum of test fluids than a vane rheometer because of significant reduction of the end and secondary flow effects.

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  17. P. C. Stapor, W. Wang, W. L. Murfee, and D. B. Khismatullin, "The distribution of fluid shear stresses in capillary sprouts," Cardiovas. Eng. Tech. 2, 124-136 (2011).

    Fluid shear stress has been implicated as a regulator of sprouting angiogenesis. However, whether endothelial cells within capillary sprouts in vivo experience physiologically relevant shear stresses remains unclear. The objective of our study is to estimate the shear stress distribution along the length of a capillary sprout through computational modeling of blood flow in a blind ended channel branching off a host vessel. In this model, we use sprout geometries typical for rat mesenteric microvasculature and consider three types of boundary conditions: 1) non-permeable vessel wall, 2) uniformly permeable vessel wall, and 3) a non-permeable vessel wall with open slots (representative of endothelial clefts). Our numerical simulation predicts that for each boundary condition a local maximum shear stress (13.9, 8.9, and 13.3 dyne/cm2 respectively) occurs at the entrance of a 50 um long, 6 um diameter sprout branching at 90 degrees off of a 11 um diameter host vessel. The shear stress drops below 0.2 dyne/cm2, a threshold for endothelial cell activation, within 4.1 um of the entrance for the non-permeable wall case and 4.2 um for the uniformly permeable wall case. Shear stress magnitudes within the sprout are above 0.2 dyne/cm2 for longer sprout scenarios and peaked at 5.9 dyne/cm2 at endothelial cell clefts. These results provide a first estimate of relative fluid shear stress magnitudes along a capillary sprout and highlight the importance of investigating endothelial cell responses to flow conditions during angiogenesis in tumors and other altered microenvironments.

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  18. J.C. Chrispell, R. Cortez, D.B. Khismatullin, and L.J. Fauci, "Shape oscillations of a droplet in an Oldroyd-B fluid," Physica D 240, 1593-1601 (2011).

    We present a Navier-Stokes/Oldroyd-B immersed boundary algorithm that captures the interaction of a flexible structure with a viscoelastic fluid. In particular, we study the effects of bulk viscoelasticity on freely decaying shape oscillations of an Oldroyd-B fluid droplet suspended in an Oldroyd-B matrix. Our numerical data indicate that if the fluid viscosity is low, viscoelasticity plays a modulating role in the drop shape relaxation; specifically, it increases the oscillation frequency and decreases the decay rate when the fluid relaxation time is above a critical value. In the high-viscosity limit, i.e., when a Newtonian droplet is expected to return to a spherical shape with an aperiodic decay, an increase in the relaxation time eventually results in the reappearance of the oscillations. Both these results are in line with the prediction of small deformation theory for viscoelastic droplet oscillations. The algorithm was also validated by direct comparison with linear asymptotics.

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  19. W. Wang, H. Zhu, D. De Kee, and D. Khismatullin, "Numerical investigation of the reduction of wall slip effects for yield stress fluids in a double concentric cylinder rheometer with slotted rotor," J. Rheol. 54, 1267-1283 (2010).

    Wall slip effects in a rheometer with double concentric cylinder geometry may lead to significant errors in measurement of the apparent viscosity. Previously, we proposed to use a slotted rotor design to reduce these effects. In this paper, we conduct two- as well as three-dimensional computational fluid dynamics (CFD) simulations to determine the differences in rheological measurements of yield stress fluids between the slotted and non-slotted rotor designs. The test fluid and the slip wall boundary of a rotor are characterized in our computational model by the constitutive equation of Zhu et al. (2005) and the wall-slip length method, respectively. The model has been validated against the existing rheological data measured using a vane rheometer. The results of this study indicate that the rheometer equipped with a slotted rotor can measure the fluid properties with enhanced accuracy and less sensitivity to the wall slip velocity than a rheometer with a non-slotted rotor. We also show that the wall slip effects can be further reduced by either increasing the slot ratio or adding more slots to the rotor. This work illustrates that CFD analysis can be a powerful tool in rheometer design.

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Book chapters and review papers:
  1. D.B. Khismatullin, "The cytoskeleton and deformability of white blood cells," in Klaus Ley (Ed.), “Current Topics in Membrane. Vol. 64. Leukocyte adhesion”, Burlington: Elsevier/Academic Press, pp. 47-111 (2009).

    White blood cells (WBCs), also known as leukocytes, migrate to sites of infection to destroy pathogenic microorganisms. The ability of WBCs to deform is essential for this function but it is also an important determinant of healthy vasculature. This chapter analyzes the effects of leukocyte deformability on leukocyte–endothelial interactions, presents the evidence for the critical role of the cytoskeleton in bulk mechanical properties of leukocytes, summarizes recent advances in rheological measurements of leukocytes, and discusses pathologies associated with leukocyte activation and reduced deformability of these cells.

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Conference presentations:
  1. Y. Teng, W. Wang, and D.B. Khismatullin, "Development of multiple-particle-tracking microrheology for fluids experiencing deterministic motion," 82nd Annual Meeting of the Society of Rheology, October 24-28, 2010 — Santa Fe, New Mexico.

    In Multiple-Particle-Tracking Microrheology (MPTM), rheological properties of fluids are determined from the Stokes-Einstein theory applied to Brownian motion of small suspended particles. As compared to conventional rheometers, this noncontact method does not have the problem of wall slip effects and requires a very small amount of a test fluid. MPTM measurements are typically performed in a quiescent fluid to ensure all particles are subject to random motion. Unfortunately, it is very difficult if possible to completely eliminate the deterministic motion of a test fluid during measurement because of thermal convection of the fluid, fluctuations and inclination of the experimental platform, and active transport of particles. In this work, we report our first results on development of MPTM that takes into account the deterministic velocity of a test fluid. In our approach, 0.1 or 0.9 um diameter Latex beads were suspended in a fluid located between a glass microscope slide and a glass coverslip. The movement of the particles was visualized through an inverted microscope with 40x and 60x objectives and recorded by a high-speed camera at 30 frames per second. The trajectories of particles were analyzed using a MATLAB code in which the viscosity of a test fluid was determined from the ensemble averaged MSD vs. lag time curves with the deterministic component of the fluid removed under the assumption that the ensemble-averaged velocity for randomly moving particles was equal to zero. We applied this approach to measure the viscosity of water and 5% dextran-water solution at different temperatures. With the elimination of the deterministic component, our results agree well with published viscosity data for these fluids.

  2. W. Wang, D.B. Khismatullin, H. Zhu, and D. De Kee, "Numerical analysis of double concentric cylinder rheometer with slotted rotor," 82nd Annual Meeting of the Society of Rheology, October 24-28, 2010 — Santa Fe, New Mexico.

    The apparent wall slip phenomenon is inevitably encountered in the standard rheological measurement for concentrated suspensions. It may significantly underestimate the apparent viscosity in the experiments. Previously we have proposed a slotted rotor design to reduce such effects. The objective of this study is to validate this design by conducting 3-D computational fluid dynamics (CFD) simulation and analyzing the velocity and shear stress fields in the double concentric cylinder rheometer with and without slotted rotor and the vane rheometer. Both shear thinning and yield stress fluids, modeled by a continuous viscosity constitutive equation, are considered in the simulations. The wall slip effects are taken into account using the wall slip length method. The results indicate that the double concentric cylinder rheometer equipped with a slotted rotor can measure the fluid properties with enhanced accuracy and less sensitivity to the wall slip velocity than a rheometer with a non-slotted rotor. The wall slip effects can be further reduced by either increasing the slot ratio or adding more slots to the rotor. Although the vane rheometer can reduce the wall slip effects too, it is only suitable for high shear thinning or high yield stress fluids due to the large end effects. As a conclusion, the use of a slotted rotor in the double concentric cylinder rheometer may significantly reduce both wall slip and end effects, making this design an excellent choice for rheological measurements.

  3. C. Chen and D.B. Khismatullin, "Histamine induces monocyte interactions with arterial endothelium in vitro," 2010 BMES Annual Fall Meeting, October 6-9, 2010 — Austin, Texas.

    Histamine plays an important role in both normal physiology and various pathophysiological conditions. It upregulates the expression of inflammatory cytokines, including TNF-?, and endothelial cell adhesion molecules such as P-selectin and integrin ligands. These factors may trigger monocyte adhesion to arterial endothelium and subsequent accumulation of monocytes/macrophages in the arterial intima, leading to development of an atherosclerotic plaque. In this work, we target to investigate the effect of histamine on interactions of THP-1 (human acute monocytic leukemia cell line) with HUVEC monolayer in vitro. This analysis is conducted under flow conditions using the Bioflux 200 microfluidic shear flow system. According to our data, activation of HUVEC with histamine increases the rolling flux and decreases the rolling velocity of THP-1. Flow cytometric analysis indicates that P-selectin is the primary cell adhesion molecule involved in histamine-induced monocyte rolling. The histamine effect on cell rolling becomes more pronounced when it is used in combination with TNF-?. Histamine+TNF-? also lead to a significant increase in the number of firmly adherent monocytes. Parallel studies with OxLDL-stimulated endothelium show a less effect of OxLDL on monocyte rolling and adhesion than that of histamine+TNF-?. Overall, this study suggests that histamine may be an important regulator of atherogenesis.

  4. W. Wang, F. Graziano, V. Russo, and D.B. Khismatullin, “Numerical study of blood flow after embolization of cerebral aneurysm with yield stress fluids,” 2010 BMES Annual Fall Meeting, October 6-9, 2010 — Austin, Texas.

    Coiling embolization in which platinum wires are inserted into the aneurysm through an artery is a common treatment in therapy of cerebral aneurysms. This procedure, however, may result in non-uniform and incomplete closure of the aneurysm, thereby reducing therapeutic effects. In this work, we propose to use a yield stress fluid material instead of platinum wires to close the aneurysm. This material behaves like solid when the applied shear stresses are less than the critical yield stress but it starts to flow when the shear stresses are larger than this critical value. Our objective is to investigate the viability of this approach using computational fluid dynamics. The giant aneurysm geometry used in our simulation was modeled from deceased patient’s CT images. The velocity and shear stress fields in the blood circulation surrounding the aneurysm were computed before and after embolization with a yield stress fluid using the incompressible and time-steady computational fluid dynamics solver. Our simulation results indicate that the shear stress distribution along the aneurysm surface becomes more uniform after filling it with the yield stress fluid. This reduces the risk of aneurysm rupture. We also determine the minimum yield stress at which the embolizing material will not leak out of the aneurysm. Overall, our analysis shows that yield stress fluids can be potentially used for treatment of cerebral aneurysms.

  5. W. Wang, P. C. Stapor, W.L. Murfee, and D.B. Khismatullin, “Influence of permeability on shear stress distribution along capillary sprouts,” 2010 BMES Annual Fall Meeting, October 6-9, 2010 — Austin, Texas.

    Shear stress has been implicated as a modulator of angiogenesis. However, a full understanding of how shear stresses influence endothelial cell phenotype and function during angiogenesis requires identification of local stress distribution along capillary sprouts. The objective of this study was to investigate the influence of vessel permeability on shear stress distribution along a capillary sprout. Using Fluent, the shear stress was computed for Newtonian flow (1.2 cP viscosity) through a 2-D blind-ended vessel (5 ?m diameter, 200 ?m length) originating at a 90° angle off a host vessel (10 ?m diameter). Vessel walls were modeled as a solid wall 1) without or 2) with holes of 0.5 ?m in diameter spaced 5 ?m apart or 3) as a 0.3 ?m thick porous layer. The average inflow velocity was equal to 1 mm/s. For the last two models the hydraulic conductivity was matched to 0.018 ?m/(s mmHg). Independently of the model used, the entrance to the sprout experiences a local maximum in wall shear stress. The stress is reduced from the upstream value of 7 dyne/cm2 by four orders of magnitude within 8 ?m of the entrance in the solid wall model and by 2.5 orders in permeable models. For the solid wall with pores, the stress locally peaks at pore sites. Simulations for 45° & 135° sprout angles indicate similar stress magnitudes but with asymmetric distribution along upstream & downstream walls of the sprout. The presented data give insights into the shear stress distribution in capillary sprouts that can be linked to endothelial cell phenotypic changes during angiogenesis.

  6. H. Lan and D.B. Khismatullin, “3-D numerical simulation of lateral migration of cells and deformable particles in shear flow,” 2010 BMES Annual Fall Meeting, October 6-9, 2010 — Austin, Texas.

    Cellular deformability may lead to lateral migration of cells toward the centerline during their perfusion in a microfluidic flow chamber. This property can be used for separation of cells of different deformabilities, such as, for example, red and white blood cells. Here, we study the effect of bulk viscoelasticity on lateral migration of cells and particles in shear flow using custom computational fluid dynamics code. The cells and particles are modeled as multiphase (a nucleus surrounded by a layer of cytoplasm) and single-phase viscoelastic drops, respectively. The numerical algorithm is based on the volume-of-fluid continuous-surface-force (VOF-CSF) method and the semi-implicit solvers for the Navier-Stokes equations and the Giesekus constitutive equation for bulk viscoelasticity. Our simulations show the cell/particle with larger deformability moves at a lower translational velocity than the fluid flow. At the same time, it migrates with a relatively constant velocity toward the channel centerline until reaching the equilibrium position. A more deformable cell is characterized by a higher lateral migration velocity, especially when its cytoplasmic viscosity drops to the value of less than 10 P.

  7. D.B. Khismatullin, M. Pospieszalska, and K. Ley, “Influence of cell deformation, tether formation and catch/slip bond behavior on leukocyte rolling,” 2010 BMES Annual Fall Meeting, October 6-9, 2010 — Austin, Texas.

    Neutrophils roll by reversible binding and unbinding of P-selectin Glycoprotein Ligand-1 (PSGL-1) to P-selectin. The average lifetime of P-selectin—PSGL-1 bonds may increase with applied force until the force reaches a critical value of 11 pN. Here, we apply this "catch bond" behavior and the transition to slip bonds at supercritical forces to study their effect on neutrophil rolling. Leukocyte adhesion significantly depends on cellular deformability, viscoelastic extension of leukocyte microvilli, and membrane tether pulling. We use two custom computational models: ETMA in which the leukocyte is a rigid sphere and Visco-Elastic Cell Adhesion Model (VECAM) in which the leukocyte is a deformable particle. Both models incorporate microvillus viscoelasticity and tether pulling and describe receptor-ligand binding kinetics using a stochastic Monte Carlo approach, with on- and off-rates determined according to the spring model of Dembo. Bonds are characterized by bound state and transition state spring constants, with the latter being higher of the two in the case of catch bonds. Our simulations show that catch bonds lead to firm adhesion of a rigid cell at wall shear stress of 0.5 dyn/cm2, but the transition of these bonds to slip bonds induces stable rolling. Catch bonds favor tether pulling in a deformable cell that eventually results in cell detachment at this subthreshold shear stress. We conclude that catch bonds, tether dynamics and cell deformation all contribute to leukocyte rolling.

  8. J. Chrispell, R. Cortez, D.B. Khismatullin, and L. Fauci, “The dynamics of immersed boundaries in viscoelastic fluids,” 62nd Annual Meeting of the APS Division of Fluid Dynamics, November 22-24, 2009 — Minneapolis, Minnesota.

    Many biological fluids are viscoelastic and require a nonlinear constitutive equation to describe the evolution of the extra-stress tensor. We use an immersed boundary framework to model processes that involve the movement of immersed elastic boundaries interacting with a surrounding viscoelastic fluid. We present recent results on applications including dynamics of a closed membrane moving under surface tension, and phase-locking of swimming sheets.

  9. D.B. Khismatullin, C. Chen, and G.A. Truskey, “Quantitative models of monocyte-endothelial cell interactions in atherosclerosis,” 81st Annual Meeting of the Society of Rheology, October 18-22, 2009 — Madison, Wisconsin.

    Atherosclerosis is a progressive disorder of medium-to large-size arteries characterized by hardening and narrowing of the vessels due to formation and calcification of atheromatous plaques on the inside of the vessel walls. It is established that this disorder develops near vessel bifurcations and curvatures (where separation and reversal of blood flow occur) as a result of oxidative damage to vascular endothelium caused by oxidized low-density lipoproteins (oxLDL). Such endothelial dysfunction leads to increased adhesion of monocytes to endothelial cells and accumulation of monocytes/macrophages in the intimal layer of the arterial wall. In this talk, we present three-dimensional computational models of monocyte-endothelium interactions that take into account 1) monocyte viscoelasticity, 2) complex flow conditions existing at atherosclerosis-prone sites, and 3) chemokine-stimulated and multiple-receptor-mediated cell adhesion kinetics. We also discuss our in vitro experiments on oxLDL-induced adhesion of monocytic cell line THP-1 to HUVEC in a micro-fluidic flow chamber. Through comparison of in vitro and computational studies, we show that firm adhesion of monocytes to endothelial cells is very sensitive to monocyte rheological properties and flow conditions to which endothelial cells and monocytes are exposed.

  10. P. Stapor, W.L. Murfee, and D.B. Khismatullin, “Determination of Shear Stress Magnitudes Along Capillary Sprouts,” 2009 BMES Annual Fall Meeting, October 7-10, 2009 — Pittsburgh, Pennsylvania,”

  11. D.B. Khismatullin and G.A. Truskey, “Modeling the mechanical behaviour of white blood cells using a viscoelastic Volume-of-Fluid algorithm,” 10th US National Congress on Computational Mechanics, July 16-19, 2009 — Columbus, Ohio.

  12. D.B. Khismatullin, “Application of the Volume-of-Fluid algorithm to biological systems”, 2009 Spring Southeastern Meeting of the American Mathematical Society, April 4-5, 2009 — Raleigh, North Carolina.

    Biological systems are characterized by a signicant level of heterogeneity and, on the macro-scale, behave as viscoelastic materials. To study the mechanical behavior of biological systems, we have developed a novel parallel algorithm for fully three-dimensional numerical simulation of multiphase viscoelastic ow. The algorithm consists of the second order Volume-of-Fluid method for tracking uid-uid interfaces, the projection method for solving the Navier-Stokes equations, and the semi-implicit factorized scheme for the constitutive equation for the stress tensor (Giesekus, Oldroyd-B, or Upper-Convected Maxwell uid). We will talk about the application of the algorithm to the problems in microvascular hemodynamics, such as leukocyte-endothelial cell adhesion and blood ow in channels with complex geometry. We will show that the code we developed can accurately predict leukocyte rolling on vascular endothelium and blood ow in sprouting vessels. Proposals for extending the algorithm to other biological problems will also be discussed.

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