Measurement of outflow facility using iPerfusion

Elevated intraocular pressure (IOP) is the predominant risk factor for glaucoma, and reducing IOP is the only successful strategy to prevent further glaucomatous vision loss. IOP is determined by the balance between the rates of aqueous humour secretion and outflow, and a pathological reduction in the hydraulic conductance of outflow, known as outflow facility, is responsible for IOP elevation in glaucoma. Mouse models are often used to investigate the mechanisms controlling outflow facility, but the diminutive size of the mouse eye makes measurement of outflow technically challenging. In this study, we present a new approach to measure and analyse outflow facility using iPerfusion™, which incorporates an actuated pressure reservoir, thermal flow sensor, differential pressure measurement and an automated computerised interface. In enucleated eyes from C57BL/6J mice, the flow-pressure relationship is highly non-linear and is well represented by an empirical power law model that describes the pressure dependence of outflow facility. At zero pressure, the measured flow is indistinguishable from zero, confirming the absence of any significant pressure independent flow in enucleated eyes. Comparison with the commonly used 2-parameter linear outflow model reveals that inappropriate application of a linear fit to a non-linear flow-pressure relationship introduces considerable errors in the estimation of outflow facility and leads to the false impression of pressure-independent outflow. Data from a population of enucleated eyes from C57BL/6J mice show that outflow facility is best described by a lognormal distribution, with 6-fold variability between individuals, but with relatively tight correlation of facility between fellow eyes. iPerfusion represents a platform technology to accurately and robustly characterise the flow-pressure relationship in enucleated mouse eyes for the purpose of glaucoma research and with minor modifications, may be applied in vivo to mice, as well as to eyes from other species or different biofluidic systems

Multiphase measurement of blood flow in a microchannel

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Blood is a complex fluid comprising red blood cells (RBCs) suspended in a continuous medium. Recent studies have shown that the spatial concentration distributions of the RBCs have a considerable impact on their velocity distributions. By extending this analysis, we present the first multiphase experimental analysis of microscale blood flow to include local velocity and concentration distributions of both phases of the blood. Human blood is perfused though a PDMS microchannel comprising a sequentially bifurcating geometry with a 50×50μm cross-section. The flow rate and the proportion of flow entering the branches of the bifurcation are varied, and the effects on the velocity and concentration distributions of the RBCs and suspending medium are analysed. In addition, the influence of RBC aggregation is investigated. The relative velocity between the two phases of the blood is shown to be dependent to varying degrees on all of the independent parameters examined in this study. A mechanism for the observed trends based on collisions of RBCs with the channel walls in the bifurcation is proposed

Flow field characterisation of aggregating human blood in bifurcating microchannels

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Erythrocyte aggregation is a shear dependent physiological phenomenon that modifies local properties of blood flow. Blood flow characteristics in microvascular bifurcations are dependent on many parameters; however the influence of erythrocyte aggregation has not been investigated previously in vitro. In the present study, micro-PIV is used to provide high spatial resolution velocity data for both erythrocytes and suspending medium for aggregating and non-aggregating human blood samples in a microchannel with a T-bifurcation geometry on the scale of the microcirculation. Simultaneous hematocrit distributions are inferred from brightfield images. Full field shear distributions are described for an evenly split flow and single flow rate. Velocity profiles of cells upstream of the bifurcation are found to be less blunt than those of the suspended particles. Daughter branch velocity profiles downstream of the bifurcation are skewed towards the wall closest to the parent branch, and non-aggregating cell velocities are significantly less blunted than those of the aggregating case. The local hematocrit is increased at the channel wall opposite the parent branch and a cell-depleted layer is observed near the channel wall closest to the parent branch. Thus, it is shown that aggregation influences both hematocrit and velocity distributions around and downstream of a bifurcation

Flow pattern in inner cores of double emulsion droplets

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The efficacy of applications of water-in-oil-in-water (w/o/w) double emulsions moving in microchannels is significantly impacted by the flow conditions in the inner aqueous cores. For example in the case of shear sensitive cells transported in the cores, high shear conditions may be deleterious. This study reports on the flow topology inside w/o/w cores determined by means of micro-particle image velocimetry (μPIV) and compares it to the flow in single water-in-oil (w/o) microdroplets with equal sizes moving in a rectangular microchannel. The multiphase flow system employed in the study had a viscosity ratio, λ, between oil and aqueous phase of the order of unity (λ = 0.8) and both single and compound droplets filled the channels. This configuration resulted in a weak recirculating flow inside the w/o single droplet: the measured flow field exhibited a uniform low velocity flow field in the central region surrounded by small regions of reversed flow near the channel walls. This flow topology was maintained in the inner cores of w/o/w double emulsions for intermediate capillary numbers (Ca) ranging from 10-3 to 10-2, and core morphologies varying from large plug to pancake cores. The core morphology affected the magnitude and distribution of the velocity in the droplets. The similarity in the flow pattern results from the fact that inner cores were located at the back of the outer droplet in such a way that inner and outer interfaces were in contact for half of core surface area and separated by a thin lubricating film

Blood velocity and viscosity in bifurcating microchannels

Blood is a complex fluid comprised of predominantly red blood cells (RBCs) suspended in a continuous, Newtonian phase, the plasma. Blood viscosity is highly dependent on the RBC concentration (haematocrit) and also displays shear thinning properties, as a result of RBC deformation and aggregation at high and low shear rates, respectively. However, these two phenomena also lead to uneven haematocrit distributions, which are exacerbated in microvascular bifurcations. In the present study, multifaceted experiments of human blood, perfused through bifurcating microchannels, are used to further elucidate the relationship between haematocrit, velocity and viscosity. A custom pressure based perfusion system was developed and was coupled with image acquisition for velocity measurement with μPIV and further processing. The acquired data was analysed in order to investigate the flow characteristics of human blood in two different idealised bifurcation geometries. The `cell-depleted layer' (CDL), a region of reduced haematocrit which occurs near the walls of the channel, and the continuous haematocrit distribution were experimentally measured. Analytical and numerical approaches were used to extract further information on the effect of flow rate, flow ratio and the presence of aggregation on microhaemodynamics. In the parent branch of the bifurcation, RBC aggregation was observed to increase the radial migration of RBCs away from the vessel wall. This enhanced the non-uniformity of the haematocrit downstream of the bifurcation and altered the relative velocity between the RBCs and the suspending medium. A skewed distribution of cells was observed downstream of the bifurcation, which resulted in skewed velocity profiles, which were also captured in the analytical and computational approaches. The geometry of the bifurcation was observed to influence the results and RBC aggregation quite significantly modified the haemodynamic characteristics even at high flow rates

Quantifying local characteristics of velocity, aggregation and hematocrit of human erythrocytes in a microchannel flow

The effect of erythrocyte aggregation on blood viscosity and microcirculatory flow is a poorly understood area of haemodynamics, especially with relevance to serious pathological conditions. Advances in microfluidics have made it possible to study the details of blood flow in the microscale, however, important issues such as the relationship between the local microstructure and local flow characteristics have not been investigated extensively. In the present study an experimental system involving simple brightfield microscopy has been successfully developed for simultaneous, time-resolved quantification of velocity fields and local aggregation of human red blood cells (RBC) in microchannels. RBCs were suspended in Dextran and phosphate buffer saline solutions for the control of aggregation. Local aggregation characteristics were investigated at bulk and local levels using statistical and edge-detection image processing techniques. A special case of aggregating flow in a microchannel, in which hematocrit gradients were present, was studied as a function of flowrate and time. The level of aggregation was found to strongly correlate with local variations in velocity in both the bulk flow and wall regions. The edge detection based analysis showed that near the side wall large aggregates are associated with regions corresponding to high local velocities and low local shear. On the contrary, in the bulk flow region large aggregates occurred in regions of low velocity and high erythrocyte concentration suggesting a combined effect of hematocrit and velocity distributions on local aggregation characteristics. The results of this study showed that using multiple methods for aggregation quantification, albeit empirical, could help towards a robust characterisation of the structural properties of the fluid

Quantifying local characteristics of velocity, aggregation and hematocrit of human erythrocytes in a microchannel flow

The effect of erythrocyte aggregation on blood viscosity and microcirculatory flow is a poorly understood area of haemodynamics, especially with relevance to serious pathological conditions. Advances in microfluidics have made it possible to study the details of blood flow in the microscale, however, important issues such as the relationship between the local microstructure and local flow characteristics have not been investigated extensively. In the present study an experimental system involving simple brightfield microscopy has been successfully developed for simultaneous, time-resolved quantification of velocity fields and local aggregation of human red blood cells (RBC) in microchannels. RBCs were suspended in Dextran and phosphate buffer saline solutions for the control of aggregation. Local aggregation characteristics were investigated at bulk and local levels using statistical and edge-detection image processing techniques. A special case of aggregating flow in a microchannel, in which hematocrit gradients were present, was studied as a function of flowrate and time. The level of aggregation was found to strongly correlate with local variations in velocity in both the bulk flow and wall regions. The edge detection based analysis showed that near the side wall large aggregates are associated with regions corresponding to high local velocities and low local shear. On the contrary, in the bulk flow region large aggregates occurred in regions of low velocity and high erythrocyte concentration suggesting a combined effect of haematocrit and velocity distributions on local aggregation characteristics. The results of this study showed that using multiple methods for aggregation quantification, albeit empirical, could help towards a robust characterisation of the structural properties of the fluid

Localized and controlled delivery of nitric oxide to the conventional outflow pathway via enzyme biocatalysis: towards therapy for Glaucoma

Nitric oxide (NO) has been shown to lower intraocular pressure (IOP), however its therapeutic effects on outflow physiology are location- and dose-dependent. Here, a NO delivery platform that directly targets the resistance-generating region of the conventional outflow pathway and locally liberates a controlled dose of NO is reported. An increase in outflow facility (decrease in IOP) is demonstrated in mouse model