Cardiogenic Airflow in the Lung Revealed Using Synchrotron-Based Dynamic Lung Imaging.

The beating heart is known to produce pressure and airflow oscillations in the lungs of mammals. This phenomenon is often disregarded as detailed measurement of its effects in the lung have hitherto not been possible. Previous studies have attempted to measure the effect of these oscillations on gas mixing. However, the results have proven inconclusive, due to the lack of a direct measurement tool capable of flow measurement throughout the entire bronchial tree. Here we present the first detailed measurement of cardiogenic oscillations, using synchrotron-based dynamic lung imaging of live mechanically ventilated mice. The results demonstrate large flow oscillations and pendelluft in the airways due to the mechanical action of the beating heart. Using a virtual tracer modelling analysis we show that cardiogenic oscillations produced up to 4 times increased gas mixing, but only in the absence of tidal ventilation. The results highlight the importance of considering this often-disregarded phenomenon when investigating lung function, particularly in situations where tidal ventilation is reduced or absent.

Haemodynamic forces on in vitro thrombi: a numerical analysis.

The flow of blood past an axisymmetric thrombus analogue, within an in vitro geometry, is computed via solution of the discrete three-dimensional (3D) Navier-Stokes equations. Particle tracking is used to model the behaviour of thrombocytes (platelets) moving throughout the domain and to investigate behaviour with respect to the platelets. The system is explored using shear rate to quantify the effects an idealised thrombus has with respect to an undisturbed in vitro geometry over 'Poiseuille flow' shear rate conditions applicable to in vivo and in vitro experiments (1,200-10,000 s⁻¹). Local shear rate variations show peaks in shear rate greater than double that of Poiseuille flow conditions. These local shear rate variations are observed to be non-linear, despite the low Reynolds number (5.2-43.4) within the system. Topological transitions of shear rate are observed, limiting the height of peak shear rate within the system, suggesting a thrombus growth limiting behaviour. Temporal gradients of shear rate, measured with respect to individual platelets, were calculated. Multiple regions of peak shear rate gradient were observed throughout the flow, suggesting that platelet-platelet interaction may not be limited to regions near to the surface of the thrombus.

Feedback control system simulator for the control of biological cells in microfluidic cross slots and integrated microfluidic systems.

Control systems for lab on chip devices require careful characterisation and design for optimal performance. Traditionally, this involves either extremely computationally expensive simulations or lengthy iteration of laboratory experiments, prototype design, and manufacture. In this paper, an efficient control simulation technique, valid for typical microchannels, Computed Interpolated Flow Hydrodynamics (CIFH), is described that is over 500 times faster than conventional time integration techniques. CIFH is a hybrid approach, utilising a combination of pre-computed flows and hydrodynamic equations and allows the efficient simulation of dynamic control systems for the transport of cells through micro-fluidic devices. The speed-ups achieved by using pre-computed CFD solutions mapped to an n-dimensional control parameter space, significantly accelerate the evaluation and improvement of control strategies and chip design. Here, control strategies for a naturally unstable device geometry, the microfluidic cross-slot, have been simulated and optimal parameters have been found for proposed devices capable of trapping and sorting cells.

Effect of hemodynamic forces on platelet aggregation geometry.

The shear rate dependence of platelet aggregation geometries is investigated using a combination of in vitro and numerical experiments. Changes in upstream shear rate, γ(Pw), are found to cause systematic changes in mature platelet aggregation geometries. However, γ(Pw) is not the only factor determining the shear rate experienced by a platelet moving over, and adhering to, a platelet aggregation: flow simulations demonstrate that naturally occurring variations in platelet aggregation geometry cause the local shear rate on the surface of a mature platelet aggregation to vary between zero and up to eight times γ(Pw). Additionally, as a platelet aggregation grows, systematic changes in geometry are found, indicating that the local shear field over a growing platelet aggregation will differ from that over mature platelet aggregations.