Biomimetic dextran coatings on silicon wafers : thin film properties and wetting

There has been much recent interest in polysaccharide coatings for biotechnology applications. We obtained highly wettable dextran coatings applied to flat silicon wafer surfaces through a two-step process: in the first step, the silicon is aminated by the deposition of a selfassembled monolayer of 3-aminopropyltriethoxysilane (APTES); in the second step, polydisperse and low dispersity dextrans with molecular weights ranging from 1 kDa to 100 kDa are covalently grafted along the backbone to the surface amino groups to achieve strong interfacial anchoring. The effect of dextran concentration on film thickness and contact angle is investigated. Atomic force microscopy (AFM) has been employed to characterize surface roughness and coverage of the dextrans as well as the APTES monolayers. The synthetic surfaces were also tested for gas bubble adhesion properties

Finite-sized gas bubble motion in a blood vessel: Non-Newtonian effects

We have numerically investigated the axisymmetric motion of a finite-sized nearly occluding air bubble through a shear-thinning Casson fluid flowing in blood vessels of circular cross section. The numerical solution entails solving a two-layer fluid model - a cell-free layer and a non-Newtonian core together with the gas bubble. This problem is of interest to the field of rheology and for gas embolism studies in health sciences. The numerical method is based on a modified front-tracking method. The viscosity expression in the Casson model for blood (bulk fluid) includes the hematocrit [the volume fraction of red blood cells (RBCs)] as an explicit parameter. Three different flow Reynolds numbers, Reapp=ΡlUmaxd/µapp, in the neighborhood of 0.2, 2, and 200 are investigated. Here, Ρl is the density of blood, Umax is the centerline velocity of the inlet Casson profile, d is the diameter of the vessel, and µapp is the apparent viscosity of whole blood. Three different hematocrits have also been considered: 0.45, 0.4, and 0.335. The vessel sizes considered correspond to small arteries, and small and large arterioles in normal humans. The degree of bubble occlusion is characterized by the ratio of bubble to vessel radius (aspect ratio), λ, in the range 0.9 ≤ λ≤1.05. For arteriolar flow, where relevant, the Fahraeus-Lindqvist effects are taken into account. Both horizontal and vertical vessel geometries have been investigated. Many significant insights are revealed by our study: (i) bubble motion causes large temporal and spatial gradients of shear stress at the endothelial cell (EC) surface lining the blood vessel wall as the bubble approaches the cell, moves over it, and passes it by; (ii) rapid reversals occur in the sign of the shear stress (+ → - → +) imparted to the cell surface during bubble motion; (iii) large shear stress gradients together with sign reversals are ascribable to the development of a recirculation vortex at the rear of the bubble; (iv) computed magnitudes of shear stress gradients coupled with their sign reversals may correspond to levels that cause injury to the cell by membrane disruption through impulsive compression and stretching; and (v) for the vessel sizes and flow rates investigated, gravitational effects are negligible

Dextran grafted silicon substrates : preparation, characterization and biomedical applications

Biodevices used in the cardiovascular system suffer from well-known problems associated with surface-induced gas embolism and thrombosis. In order to improve the biocompatibility of these devices, biomimetic coatings show good promise. We recently synthesized a coating layer of dextran, a relatively simple and well characterized neutral polysaccharide, with the purpose of mimicking the cells\u27 glycocalyx layer, that prevents non-specific cells-protein interactions. Systematic physical chemical characterization was performed on coatings obtained both from commonly used polydisperse dextrans and low-dispersity dextrans in the 1-100 kDalton molecular weight range. We have combined standard surface analysis techniques, such as ellipsometry, contact angle measurements and AFM, with less traditional vibrational spectroscopy techniques in the characterization of our biomimetic coatings. FTIR, micro-FTIR and micro-Raman spectroscopies were utilized to correlate the conformational and molecular aspects of the grafted poly- and monodisperse dextran chains to their attractive biological properties

Air Bubble Growth in Water

1 describing experiments of air bubble growth in water during exposure to 100% nitrous oxide, 100% xenon, or 50% xenon-50% oxygen. Although the experiments were nicely conducted, they explore a physics of gas flux in an unconstrained bubble permitted to grow spherically. Importantly, this geometry has limited biologic relevance for bubbles occluding vessels in the size range they have studied. The authors have referenced our previous work on xenon transport, 2 but they have mistakenly interpreted the findings presented therein to indicate the growth of bubbles as spheres. Rather, that study presents some simulations for bubbles that are initially spherical and just fill the vessel lumen. Such bubbles cannot grow radially because they are constrained by the vessel wall and therefore elongate during growth while maintaining a fixed curvature on the interface. This results in a much different force balance across the gas-liquid interface and, hence, a different pressure condition on the interior of the bubble from that which occurs in the case of a time-varying interfacial shape, which the authors have studied. We have described these differences in our previous theoretical and experimental studies of intravascular gas embolism. [2][3][4] 2 Whereas others have studied growth of similarly unconstrained air bubbles during cardiopulmonary bypass, 6 our work has not provided any data for direct comparisons such as the authors have made, based on the different gas constituents and the governing physics dictated by the shape constraint. I find it fascinating, however, that they have couched their results in terms of bubble diameter growth. When transferred to the volume domain, one readily sees that the spherical bubbles exposed to 100% xenon or 100% nitrous oxide had grown to more than twice their initial volume in 25 min (figs. 2 and 3) and continued growing when the solutions were switched (downward arrow). The time required for this is surprisingly similar to the volume doubling times we reported for many of the cases we explored, despite the differences in our model and these experiments. The curve fitting by a double exponential suggests that there will be continuous exponential growth of bubble diameter. So although the physics and gas transport are different from what we studied, the indication of the studies are the same. In Reply:-Our article 1 on the expansion of gas bubbles by xenon and nitrous oxide investigated how air bubbles of various dimensions in aqueous solution would expand when suddenly exposed to solutions containing certain gas mixtures (particularly mixtures containing xenon). The motivation behind this work was simple: Would air bubbles that were entrained while on cardiopulmonary bypass during cardiac surgery expand to a worrying extent if xenon were used during the procedure, hence potentially exacerbating damage caused by air emboli? Xenon has been proposed for use as a neuroprotectant, 2 and it might be beneficial in reducing the cognitive deficits that are known to occur during cardiopulmonary bypass. 3 However, if entrained gas bubbles expanded greatly, xenon may do more harm than good. Indeed, Dr. Eckmann and his colleagues have suggested exactly that, 4 based on theoretical calculations that concluded that small gas bubbles would expand rapidly and indefinitely if they were trapped in fine blood vessels. (We fully understand that the model assumes that the bubbles are constrained by the size of the capillaries.) For example, their calculations suggest that a 50-nl bubble of oxygen exposed to 70% xenon-30% oxygen would grow to 250 nl in approximately 20 min with an ever-increasing rate of growth. Because we thought that these predictions were implausible, and because there were a large number of variables that had to be estimated, we conducted our experiments, which were designed to measure bubble growth directly under a well-defined set of conditions. We studied the expansion of both air and oxygen bubbles, and the results were similar; our data show bubble expansions of the order of 10% in diameter and 30% in volume under conditions likely to be encountered during cardiopulmonary bypass. We concluded that this is unlikely to represent a significant clinical problem. We disagree with Dr. Eckmann's claim that his calculations predict similar expansions to those we observed. Apart from the extent of the volume expansions that were predicted, 4 their most striking aspect was the ever-increasing rates of expansion that seemed to predict unlimited bubble growth. In contrast, we observed limited bubble growth with volumes tending toward finite equilibrium values. Even making allowances for the differences between the model and the gas compositions, we believe our experimental observations probably better reflects reality than the theoretical calculations that Dr. Eckmann has published. Furthermore, in our recently published feasibility and tolerability clinical study involving exposure of cardiac surgical patients to xenon while on cardiopulmonary bypass, there was no Drs. Franks and Maze have a financial interest in an Imperial College spin-out company (Protexeon Ltd., London, United Kingdom) that is interested in developing clinical applications for medical gases, particularly xenon, and both are paid consultants for this activity. Drs. Franks and Maze also sit on a Strategic Advisory Board that advises Air Products, Allentown, Pennsylvania, on possible medical applications for gases, including anesthetic gases. The funding for the current study was provided by Carburos Metálicos, a wholly owned subsidiary of Air Products

Air Bubble Growth in Water

1 describing experiments of air bubble growth in water during exposure to 100% nitrous oxide, 100% xenon, or 50% xenon-50% oxygen. Although the experiments were nicely conducted, they explore a physics of gas flux in an unconstrained bubble permitted to grow spherically. Importantly, this geometry has limited biologic relevance for bubbles occluding vessels in the size range they have studied. The authors have referenced our previous work on xenon transport, 2 but they have mistakenly interpreted the findings presented therein to indicate the growth of bubbles as spheres. Rather, that study presents some simulations for bubbles that are initially spherical and just fill the vessel lumen. Such bubbles cannot grow radially because they are constrained by the vessel wall and therefore elongate during growth while maintaining a fixed curvature on the interface. This results in a much different force balance across the gas-liquid interface and, hence, a different pressure condition on the interior of the bubble from that which occurs in the case of a time-varying interfacial shape, which the authors have studied. We have described these differences in our previous theoretical and experimental studies of intravascular gas embolism. [2][3][4] 2 Whereas others have studied growth of similarly unconstrained air bubbles during cardiopulmonary bypass, 6 our work has not provided any data for direct comparisons such as the authors have made, based on the different gas constituents and the governing physics dictated by the shape constraint. I find it fascinating, however, that they have couched their results in terms of bubble diameter growth. When transferred to the volume domain, one readily sees that the spherical bubbles exposed to 100% xenon or 100% nitrous oxide had grown to more than twice their initial volume in 25 min (figs. 2 and 3) and continued growing when the solutions were switched (downward arrow). The time required for this is surprisingly similar to the volume doubling times we reported for many of the cases we explored, despite the differences in our model and these experiments. The curve fitting by a double exponential suggests that there will be continuous exponential growth of bubble diameter. So although the physics and gas transport are different from what we studied, the indication of the studies are the same. In Reply:-Our article 1 on the expansion of gas bubbles by xenon and nitrous oxide investigated how air bubbles of various dimensions in aqueous solution would expand when suddenly exposed to solutions containing certain gas mixtures (particularly mixtures containing xenon). The motivation behind this work was simple: Would air bubbles that were entrained while on cardiopulmonary bypass during cardiac surgery expand to a worrying extent if xenon were used during the procedure, hence potentially exacerbating damage caused by air emboli? Xenon has been proposed for use as a neuroprotectant, 2 and it might be beneficial in reducing the cognitive deficits that are known to occur during cardiopulmonary bypass. 3 However, if entrained gas bubbles expanded greatly, xenon may do more harm than good. Indeed, Dr. Eckmann and his colleagues have suggested exactly that, 4 based on theoretical calculations that concluded that small gas bubbles would expand rapidly and indefinitely if they were trapped in fine blood vessels. (We fully understand that the model assumes that the bubbles are constrained by the size of the capillaries.) For example, their calculations suggest that a 50-nl bubble of oxygen exposed to 70% xenon-30% oxygen would grow to 250 nl in approximately 20 min with an ever-increasing rate of growth. Because we thought that these predictions were implausible, and because there were a large number of variables that had to be estimated, we conducted our experiments, which were designed to measure bubble growth directly under a well-defined set of conditions. We studied the expansion of both air and oxygen bubbles, and the results were similar; our data show bubble expansions of the order of 10% in diameter and 30% in volume under conditions likely to be encountered during cardiopulmonary bypass. We concluded that this is unlikely to represent a significant clinical problem. We disagree with Dr. Eckmann's claim that his calculations predict similar expansions to those we observed. Apart from the extent of the volume expansions that were predicted, 4 their most striking aspect was the ever-increasing rates of expansion that seemed to predict unlimited bubble growth. In contrast, we observed limited bubble growth with volumes tending toward finite equilibrium values. Even making allowances for the differences between the model and the gas compositions, we believe our experimental observations probably better reflects reality than the theoretical calculations that Dr. Eckmann has published. Furthermore, in our recently published feasibility and tolerability clinical study involving exposure of cardiac surgical patients to xenon while on cardiopulmonary bypass, there was no Drs. Franks and Maze have a financial interest in an Imperial College spin-out company (Protexeon Ltd., London, United Kingdom) that is interested in developing clinical applications for medical gases, particularly xenon, and both are paid consultants for this activity. Drs. Franks and Maze also sit on a Strategic Advisory Board that advises Air Products, Allentown, Pennsylvania, on possible medical applications for gases, including anesthetic gases. The funding for the current study was provided by Carburos Metálicos, a wholly owned subsidiary of Air Products

Numerical study of wall effects on buoyant gas-bubble rise in a liquid-filled finite cylinder

The wall effects on the axisymmetric rise and deformation of an initially spherical gas bubble released from rest in a liquid-filled, finite circular cylinder are numerically investigated. The bulk and gas phases are considered incompressible and immiscible. The bubble motion and deformation are characterized by the Morton number Mo, Eötvös number Eo, Reynolds number Re, Weber number We, density ratio, viscosity ratio, the ratios of the cylinder height and the cylinder radius to the diameter of the initially spherical bubble (H* =H/d0, R*=R/d0). Bubble rise in liquids described by Eo and Mo combinations ranging from (1,0.01) to (277.5,0.092), as appropriate to various terminal state Reynolds numbers (ReT) and shapes have been studied. The range of terminal state Reynolds numbers includes 0.02T\u3c70. Bubble shapes at terminal states vary from spherical to intermediate spherical-cap–skirted. The numerical procedure employs a front tracking finite difference method coupled with a level contour reconstruction of the front. This procedure ensures a smooth distribution of the front points and conserves the bubble volume. For the wide range of Eo and Mo examined, bubble motion in cylinders of height H*=8 and R≥3, is noted to correspond to the rise in an infinite medium, both in terms of Reynolds number and shape at terminal state. In a thin cylindrical vessel (small R*) the motion of the bubble is retarded due to increased total drag and the bubble achieves terminal conditions within a short distance from release. The wake effects on bubble rise are reduced, and elongated bubbles may occur at appropriate conditions. For a fixed volume of the bubble, increasing the cylinder radius may result in the formation of well-defined rear recirculatory wakes that are associated with lateral bulging and skirt formation. The paper includes figures of bubble shape regimes for various values of R*, Eo, Mo, and ReT. Our predictions agree with existing results reported in the literature

Effect of a Soluble Surfactant on a Finite-Sized Bubble Motion in a Blood Vessel

We present detailed results for the motion of a finite-sized gas bubble in a blood vessel. The bubble (dispersed phase) size is taken to be such as to nearly occlude the vessel. The bulk medium is treated as a shear thinning Casson fluid and contains a soluble surfactant that adsorbs and desorbs from the interface. Three different vessel sizes, corresponding to a small artery, a large arteriole, and a small arteriole, in normal humans, are considered. The haematocrit (volume fraction of RBCs) has been taken to be 0.45. For arteriolar flow, where relevant, the Fahraeus–Lindqvist effect is taken into account. Bubble motion causes temporal and spatial gradients of shear stress at the cell surface lining the vessel wall as the bubble approaches the cell, moves over it and passes it by. Rapid reversals occur in the sign of the shear stress imparted to the cell surface during this motion. Shear stress gradients together with sign reversals are associated with a recirculation vortex at the rear of the moving bubble. The presence of the surfactant reduces the level of the shear stress gradients imparted to the cell surface as compared to an equivalent surfactant-free system. Our numerical results for bubble shapes and wall shear stresses may help explain phenomena observed in experimental studies related to gas embolism, a significant problem in cardiac surgery and decompression sickness

Mitochondrial Dynamics and Respiration Within Cells with Increased Open Pore Cytoskeletal Meshes

The cytoskeletal architecture directly affects the morphology, motility, and tensional homeostasis of the cell. In addition, the cytoskeleton is important for mitosis, intracellular traffic, organelle motility, and even cellular respiration. The organelle responsible for a majority of the energy conversion for the cell, the mitochondrion, has a dependence on the cytoskeleton for mobility and function. In previous studies, we established that cytoskeletal inhibitors altered the movement of the mitochondria, their morphology, and their respiration in human dermal fibroblasts. Here, we use this protocol to investigate applicability of power law diffusion to describe mitochondrial locomotion, assessment of rates of fission and fusion in healthy and diseased cells, and differences in mitochondria locomotion in more open networks either in response to cytoskeletal destabilizers or by cell line. We found that mitochondria within fibrosarcoma cells and within fibroblast cells treated with an actin-destabilizing toxin resulted in increased net travel, increased average velocity, and increased diffusion of mitochondria when compared to control fibroblasts. Although the mitochondria within the fibrosarcoma travel further than mitochondria within their healthy counterparts, fibroblasts, the dependence on mitochondria for respiration is much lower with higher rates ofhydrogen peroxide production and was confirmed using the OROBOROS O2K. We also found that rates of fission and fusion of the mitochondria equilibrate despite significant alteration of the cytoskeleton. Rates ranged from 15% to 25%, where the highest rates were observed within the fibrosarcoma cell line. This result is interesting because the fibrosarcoma cell line does not have increased respiration metrics including when compared to fibroblast. Mitochondria travel further, faster, and have an increase in percent mitochondria splitting or joining while not dependent on the mitochondria for a majority of its energy production. This study illustrates the complex interaction between mitochondrial movement and respiration through the disruption of the cytoskeleton

Biomimetic surfaces via dextran immobilization : grafting density and surface properties

Biomimetic surfaces were prepared by chemisorption of oxidized dextran (Mw = 110 kDa) onto SiO2 substrates that were previously modified with aminopropyl-tri-ethoxy silane (APTES). The kinetics of dextran oxidation by sodium metaperiodate (NaIO4) were quantified by 1H NMR and pH measurements. The extent of oxidation was then used to control the morphology of the biomimetic surface. Oxidation times of 0.5, 1, 2, 4, and 24 hours resulted in \u3c20, ~30, ~40, ~50 and 100% oxidation, respectively. The surfaces were characterized by contact angle analysis and atomic force microscopy (AFM). Surfaces prepared with low oxidation times revealed a more densely packed brushy layer when imaged by AFM than those prepared at low oxidation times. Finally, the contact angle data revealed, quite unexpectedly, that the surface with the greatest entropic freedom (0.5 h) wetted the fastest and to the greatest extent (THETAAPTES \u3e THETA1h \u3e THETA2,4h \u3e THETA0.5h)

Generalized Langevin dynamics of a nanoparticle using a finite element approach: Thermostating with correlated noise

A direct numerical simulation (DNS) procedure is employed to study the thermal motion of a nanoparticle in an incompressible Newtonian stationary fluid medium with the generalized Langevin approach. We consider both the Markovian (white noise) and non-Markovian (Ornstein-Uhlenbeck noise and Mittag-Leffler noise) processes. Initial locations of the particle are at various distances from the bounding wall to delineate wall effects. At thermal equilibrium, the numerical results are validated by comparing the calculated translational and rotational temperatures of the particle with those obtained from the equipartition theorem. The nature of the hydrodynamic interactions is verified by comparing the velocity autocorrelation functions and mean square displacements with analytical results. Numerical predictions of wall interactions with the particle in terms of mean square displacements are compared with analytical results. In the non-Markovian Langevin approach, an appropriate choice of colored noise is required to satisfy the power-law decay in the velocity autocorrelation function at long times. The results obtained by using non-Markovian Mittag-Leffler noise simultaneously satisfy the equipartition theorem and the long-time behavior of the hydrodynamic correlations for a range of memory correlation times. The Ornstein-Uhlenbeck process does not provide the appropriate hydrodynamic correlations. Comparing our DNS results to the solution of an one-dimensional generalized Langevin equation, it is observed that where the thermostat adheres to the equipartition theorem, the characteristic memory time in the noise is consistent with the inherent time scale of the memory kernel. The performance of the thermostat with respect to equilibrium and dynamic properties for various noise schemes is discussed