David M. Eckmann

Hematocrit, volume expander, temperature, and shear rate effects on blood viscosity.

Our goal was to determine and predict the effects of temperature, shear rate, hematocrit, and different volume expanders on blood viscosity in conditions mimicking deep hypothermia for cardiac operations. Blood was obtained from six healthy adults. Dilutions were prepared to hematocrits of 35%, 30%, 22.5%, and 15% using plasma, 0.9% NaCl, 5% human albumin, and 6% hydroxyethyl starch. Viscosity was measured over a range of shear rates (4.5–450 s−1) and temperature (0°–37°C). A parametric expression for predicting blood viscosity based on the study variables was developed, and its agreement with measured values tested. Viscosity was higher at low shear rates and low temperatures, especially at temperatures less than 15°C (P < 0.016 for all conditions in comparison with 37°C). Decreasing hematocrit, especially to less than 22.5%, decreased viscosity. Hemodilution with albumin or 0.9% NaCl decreased blood viscosity more than hemodilution with plasma or 6% hydroxyethyl starch (P < 0.01 for all cases). The derived mathematical model for viscosity as a function of temperature, hematocrit, shear rate, and diluent predicted viscosity values that correlated well with the measured values in experimental samples (r2 > 0.92, P < 0.001). Implications A theoretical model for blood viscosity predicted independent effects of temperature, shear rate, and hemodilution on viscosity over a wide range of physiologic conditions, including thermal extremes of deep hypothermia in an experimental setting. Moderate hemodilution to a hematocrit of 22% decreased blood viscosity by 30%–50% at a blood temperature of 15°C, suggesting the potential to improve microcirculatory perfusion during deep hypothermia.

Experiments on transition to turbulence in oscillatory pipe flow

A laser-Doppler velocimeter is used to analyse volume-cycled oscillatory flow of a Newtonian viscous fluid in a straight circular tube. The axial velocity is measured at radial positions across the diameter of the tube for a wide range of amplitude A = stroke distance/tube radius (2.4 [les ] A [les ] 21.6) and Womersley parameter (9 R δ R δ = α A √2 is the Reynolds number based on Stokes-layer thickness. The turbulence is confined to an annular region which is a few times the Stokes-layer thickness near the wall. Hot-film anemometer measurements indicate the core flow remains stable when the boundary layer becomes turbulent for R δ up to 1310.

Computational model for nanocarrier binding to endothelium validated using in vivo, in vitro, and atomic force microscopy experiments

A computational methodology based on Metropolis Monte Carlo (MC) and the weighted histogram analysis method (WHAM) has been developed to calculate the absolute binding free energy between functionalized nanocarriers (NC) and endothelial cell (EC) surfaces. The calculated NC binding free energy landscapes yield binding affinities that agree quantitatively when directly compared against analogous measurements of specific antibody-coated NCs (100 nm in diameter) to intracellular adhesion molecule-1 (ICAM-1) expressing EC surface in in vitro cell-culture experiments. The effect of antibody surface coverage (σs) of NC on binding simulations reveals a threshold σs value below which the NC binding affinities reduce drastically and drop lower than that of single anti-ICAM-1 molecule to ICAM-1. The model suggests that the dominant effect of changing σs around the threshold is through a change in multivalent interactions; however, the loss in translational and rotational entropies are also important. Consideration of shear flow and glycocalyx does not alter the computed threshold of antibody surface coverage. The computed trend describing the effect of σs on NC binding agrees remarkably well with experimental results of in vivo targeting of the anti-ICAM-1 coated NCs to pulmonary endothelium in mice. Model results are further validated through close agreement between computed NC rupture-force distribution and measured values in atomic force microscopy (AFM) experiments. The three-way quantitative agreement with AFM, in vitro (cell-culture), and in vivo experiments establishes the mechanical, thermodynamic, and physiological consistency of our model. Hence, our computational protocol represents a quantitative and predictive approach for model-driven design and optimization of functionalized nanocarriers in targeted vascular drug delivery.

A front tracking method for a deformable intravascular bubble in a tube with soluble surfactant transport

Based on a front tracking scheme, we have presented a comprehensive algorithm for the study of a deformable bubble moving in a tube in the presence of a soluble or an insoluble surfactant. The emphasis here is on the dynamic adsorption of the soluble surfactant which non-linearly alters the surface tension, and this in turns affects the flow and transport in a complicated way. Furthermore, since a bubble-liquid interface is being examined, there is a need to accommodate a concentration jump across the interface in the evaluation of flow and transport. Standard numerical procedures need to be modified to accommodate this feature. Based on the physics governing the problem, an axisymmetric formulation is found to be adequate and is thus considered. The adsorption scheme for the soluble surfactant is carefully designed such that the total mass of the surfactant is well conserved, and the mass flux is accurately resolved by using an interface indicator function. This represents an advance in treating problems of this class. Tests on the efficacy of various aspects of the algorithm have been carried out. The algorithm has the flexibility of studying different models for adsorption/desorption and surfactant surface tension models, such as the Langmuir and the Frumkin models. These models have significant practical relevance. The numerical results obtained are qualitatively consistent with results where available. The results presented include an example of Marangoni flow which causes a bubble to propel out of its initial static location due to the development of a surface tension gradient. It is also shown that the bubble motion in Poiseuille flow may be significantly slowed down due to the presence of a soluble surfactant in the bulk medium. In that case, the Marangoni induced motion is in a direction opposite to that driven by the bulk pressure. Our study indicates that as the location of the adsorptive interface gets closer to the tube wall, the bulk fluid in the vicinity of the interface may become depleted of surfactant, an observation that has particular significance in understanding gas embolism and for developing therapeutic measures.

Nanogel Carrier Design for Targeted Drug Delivery.

Polymer-based nanogel formulations offer features attractive for drug delivery, including ease of synthesis, controllable swelling and viscoelasticity as well as drug loading and release characteristics, passive and active targeting, and the ability to formulate nanogel carriers that can respond to biological stimuli. These unique features and low toxicity make the nanogels a favorable option for vascular drug targeting. In this review, we address key chemical and biological aspects of nanogel drug carrier design. In particular, we highlight published studies of nanogel design, descriptions of nanogel functional characteristics and their behavior in biological models. These studies form a compendium of information that supports the scientific and clinical rationale for development of this carrier for targeted therapeutic interventions.

Accelerated arteriolar gas embolism reabsorption by an exogenous surfactant

Background Cerebrovascular gas embolism can cause profound neurologic dysfunction, and there are few treatments. The authors tested the hypothesis that an exogenous surfactant can be delivered into the bloodstream to alter the air–blood interfacial mechanics of an intravascular gas embolism and produce bubble conformations, which favor more rapid bubble absorption. Methods Microbubbles of air were injected into the rat cremaster microcirculation after intravascular administration of either saline (control, n = 5) or Dow Corning Antifoam 1510US (surfactant, n = 5). Embolism dimensions and dynamics were directly observed after entrapment using intravital microscopy. Results To achieve embolization, the surfactant group required twice as many injections as did controls (3.2 ± 1.3 vs. 1.6 ± 0.9;P < 0.05). There was no difference in the initial lodging configuration between groups. After bubble entrapment, there was significantly more local vasoconstriction in the surfactant group (24.2% average decrease in diameter) than in controls (3.4%;P < 0.05). This was accompanied by a 92.7% bubble elongation in the surfactant group versus 8.2% in controls (P < 0.05). Embolism shape change was coupled with surfactant-enhanced breakup into multiple smaller bubbles, which reabsorbed nearly 30% more rapidly than did parent bubbles in the control group (P < 0.05). Conclusions Intravascular exogenous surfactant did not affect initial bubble conformation but dramatically increased bubble breakup and rate of reabsorption. This was evidenced by both the large shape change after entrapment and enhancement of bubble breakup in the surfactant group. These dynamic surfactant-induced changes increase the total embolism surface area and markedly accelerate bubble reabsorption.

Correlating macrophage morphology and cytokine production resulting from biomaterial contact

The morphological and inflammatory responses of adherent macrophages are correlated to evaluate the biocompatibility of surfaces. Monocyte-derived macrophage (MDM), THP-1, and THP-1 cells expressing GFP-actin chimeric protein were seeded onto glass, polyurethane (PU), and glass surface modified with quaternary ammonium salt functionalized chitosan (CH-Q) and hyaluronic acid (HA). Using confocal microscopy, the surface area, volume and 3D shape factor of adherent macrophages was quantified. For comparison, functional consequences of cell-surface interactions that activate macrophages and thereby elicit secretion of a proinflammatory cytokine were evaluated. Using an enzyme linked immune sorbent assay, tumor necrosis factor-alpha (TNF-α) was measured. On glass, macrophages exhibited mainly an amoeboid shape, exhibited the largest surface area, volume, and 3D shape factor and produced the most TNF-α. On PU, macrophages displayed mainly a hemispherical shape, exhibited an intermediate volume, surface area and 3D shape factor, and produced moderate TNF-α. In contrast, on CH-Q and HA surfaces, macrophages were spherical, exhibited the smallest volume, surface area, and 3D shape factor, and produced the least TNF-α. These studies begin to validate the use of GFP-actin-modified MDM as a novel tool to correlate cell morphology with inflammatory cell response.

Optimizing Endothelial Targeting by Modulating the Antibody Density and Particle Concentration of Anti-ICAM Coated Carriers

Targeting of drug carriers to cell adhesion molecules expressed on endothelial cells (ECs) may improve treatment of diseases involving the vascular endothelium. This is the case for carriers targeted to intercellular adhesion molecule 1 (ICAM-1), an endothelial surface protein overexpressed in many pathologies. In order to optimize our design of anti-ICAM carriers, we have explored in this study the influence of two carrier design parameters on specific and efficient endothelial targeting in vitro and in vivo: carrier dose and density of targeting molecules (antibodies-Ab) on the carrier surface. Using radioisotope tracing we assessed the role of these parameters on the biodistribution of model polymer carriers targeted to ICAM-1 ((125)I-anti-ICAM carriers) in mice. Increasing the carrier dose enhanced specific accumulation in the lung vasculature (a preferential endothelial target) and decreased non-specific hepatic and splenic uptake. Increasing the Ab density enhanced lung accumulation with minimally reduced liver and spleen uptake. These studies account for the influence of blood hydrodynamic forces on carrier binding to endothelium, relevant to arterioles, venules and larger vessels. Yet, carriers may rather bind to the extensive capillary bed where shear stress is minimal. We used fluorescence microscopy to determine binding kinetics of FITC-labeled anti-ICAM carriers in static conditions, at the threshold found in vivo and conditions mimicking low vs high ICAM-1 expression on quiescent vs activated ECs. Binding to activated ECs reached similar saturation with all tested Ab densities and carrier concentrations. In quiescent cells, carriers reached ~3-fold lower binding saturation, even at high carrier concentration and Ab density, and carriers with low Ab density did not reach saturation, reflecting avidity below threshold. Binding kinetics was positively regulated by anti-ICAM carrier concentration and Ab density. Counterintuitively, binding was faster in quiescent ECs (except for carriers with high Ab density and concentration), likely due to fast saturation of fewer binding sites on these cells. These results will guide optimization of ICAM-1-targeted carriers, e.g., in the context of targeting healthy vs diseased endothelium for prophylactic vs therapeutic interventions.

Flow dynamics, binding and detachment of spherical carriers targeted to ICAM-1 on endothelial cells

Vascular drug delivery by administration of carriers targeted to endothelial surface determinants, such as intercellular adhesion molecule (ICAM-1), holds considerable promise to improve disease treatment. As a model to define elusive factors controlling the interplay between carrier motion in the bloodstream and its interactions with molecular targets in the endothelial wall, we used 1 mum beads coated with ICAM-1 monoclonal antibody (Ab) at 370, 1100 or 4100 Ab/microm2. Carriers were perfused at two shear rates over resting or activated endothelial cells, expressing minimum vs. maximum ICAM-1 levels, to determine carrier rolling, binding and detachment. Even at 0.1 Pa and 4100 Ab/microm2, carriers attached only to activated cells (21 fold increase over resting cells), ideal for specific drug targeting to sites of pathology. Binding was increased by raising the Ab surface density on the carrier, e.g., 59.4+/-11.1% increase for carriers having 4100 vs. 1100 Ab/microm2, as a consequence of decreased rolling velocity. Carrier binding was stable even under a high shear stress: carriers with 1100 and 4100 Ab/microm2 withstand shear stress over 3 Pa without detaching from the cells. This is further supported by theoretical modeling. These results will guide vascular targeting of drug carriers via rational design of experimentally tunable parameters.

Oscillatory flow and mass transport in a curved tube

Transport of soluble material is analysed for volume-cycled oscillatory flow in a curved tube. The equations of motion are solved using a regular perturbation method for small ratio of tube radius to radius of curvature and order unity amplitude over a range of the Womersley parameter. The transport equation is similarly solved by a regular perturbation scheme where uniform steady end concentrations and no wall flux are assumed. The time-average axial transport of solute is calculated. There is substantial modification of transport compared to the straight-tube case and the results are interpreted with respect to pulmonary gas exchange.