Hydrodynamic evaluation of aortic cardiopulmonary bypass cannulae using particle image velocimetry.

The high velocity jet from aortic arterial cannulae used during cardiopulmonary bypass potentially causes a "sandblasting" injury to the aorta, increasing the possibility of embolisation of atheromatous plaque. We investigated a range of commonly available dispersion and non-dispersion cannulae, using particle image velocimetry. The maximum velocity of the exit jet was assessed 20 and 40 mm from the cannula tip at flow rates of 3 and 5 L/min. The dispersion cannulae had lower maximum velocities compared to the non-dispersion cannulae. Dispersion cannulae had fan-shaped exit profiles and maximum velocities ranged from 0.63 to 1.52 m/s when measured at 20 mm and 5 L/min. Non-dispersion cannulae had maximum velocities ranging from 1.52 to 3.06 m/s at 20 mm and 5 L/min, with corresponding narrow velocity profiles. This study highlights the importance of understanding the hydrodynamic performance of these cannulae as it may help in selecting the most appropriate cannula to minimize the risk of thromboembolic events or aortic injury.

Sensorless flow and head estimation in the VentrAssist rotary blood pump.

Flow rate and pressure difference (or head) are key variables needed in the control of implantable rotary blood pumps. However, use of flow and/or pressure probes can decrease reliability and increase system power consumption and expense. For a given fluid viscosity, the flow state is determined by any 2 of the 4 pump variables: Flow, pressure difference, speed, and motor input power can be used. Thus, if viscosity is known or if its influence is sufficiently small, flow rate and pressure difference can be estimated from the motor speed and motor input power. For the VentrAssist centrifugal blood pump, which uses a hydrodynamic bearing, sensorless flow and pressure head estimation accuracy of 2 of our impeller designs were compared for a viscosity range of 1.2 to 4.5 mPas. This showed impeller design optimization can improve estimation accuracy. We also compared estimation accuracy using 2 blood analogues used in vitro, aqueous glycerol and red blood cells suspended in Haemaccel. The nature of the blood analogue and not only the viscosity of the fluid seems to influence estimation accuracy in our pump.

Wear of human enamel: a quantitative in vitro assessment.

Many factors influence the extent and rate at which enamel wears. Clinical studies in humans are limited by difficulties in the accurate quantification of intra-oral wear and by a lack of control over the oral environment. The purpose of this study was to determine the wear characteristics of human dental enamel under controlled experimental conditions. An electro-mechanical tooth wear machine, in which opposing enamel surfaces of sectioned, extracted teeth were worn under various conditions, was used to simulate tooth grinding or bruxism. Enamel surface wear was quantified by weight to an accuracy of 0.1 mg, with water uptake and loss controlled. The variables considered included the structure and hardness of enamel, facet area, duration of tooth contact, relative speed of opposing surfaces, temperature, load, pH, and the nature of the lubricant. Enamel wear under non-lubricated conditions increased with increasing load over the range of 1.7 to 16.2 kg. The addition of a liquid lubricant (pH = 7) reduced enamel wear up to 6.7 kg, but when the load increased above this threshold, the rate of wear increased dramatically. With the viscosity of the lubricant constant and pH = 3, the rate of wear was further reduced to less than 10% of the non-lubricated rate at 9.95 kg, after which the rate again increased substantially. Under more extreme conditions (pH = 1.2, simulating gastric acids), the wear was excessive under all experimental loads. When saliva was used as a lubricant, the amount of wear was relatively low at 9.95 kg, but rapid wear occurred at 14.2 kg and above. These results indicate that under non-lubricated conditions, enamel wear remains low at high loads due to the dry-lubricating capabilities of fine enamel powder. Under lubricated conditions, low loads with an acidic lubricant lead to little enamel wear, whereas very low pH results in a high rate of wear under all loads.

Development of a ceramic conduit valve prosthesis for corrective cardiovascular surgery.

This paper reports some of the ways in which biomaterial considerations have influenced the design, development and construction methods for a prototype conduit valve prosthesis in which the valve body is made from alumina. This material is used principally for its ability to grow and support a thin (< 0.1 mm) tissue covering on the surfaces in contact with the blood. This non-vascular covering does not interfere with the operation of the valve, but is thick enough to camouflage the underlying surface from any further interaction with the blood. This is important for any conduit valve because of the large internal surface area, but would be especially beneficial for children if it could obviate the need for chronic anticoagulation.

Asymmetrical loads and lateral bending of the human spine.

The human spine is modelled as a cantilever-type beam column. Under the influence of static asymmetrical loads, muscle and low-back forces are predicted from a hypothetical but revealing model. Such forces produced by asymmetrical loads are much larger than for a corresponding symmetrical load. Asymmetrical loads can encourage, especially in young schoolchildren, lateral bending of the spine by alleviating muscle and low-back forces. This could possibly be a factor contributing to the surprisingly high percentage of schoolchildren with measurable scoliotic curves. The wearing of knapsack-type bags is advocated.

Computational fluid mechanical evaluation of steady-flow energy losses in cardiac valve prostheses.

A form of the general integral energy conservation equation is applied to the case of steady flow through a rigid control volume bounded by stationary walls and fixed inflow and outflow control surfaces. The total energy loss due to viscous dissipation in the control volume is then defined as the change in mean (pressure + kinetic energy + shear stress energy) quantities between the inflow and outflow surfaces. Computational fluid mechanical modelling can then be used to provide the extensive flow field data necessary to calculate the integral means of the total energy components--not only at the inflow/outflow surfaces, but throughout the control volume via a series of "cutting surfaces" progressing from the inlet to the outlet. This allows interchanges between the various forms of energy to be examined and provides an insight into the mechanisms responsible for these interchanges. Two illustrative examples are presented: laminar flow through a specially designed ball-occluded conduit valve in a long conduit run, and through a caged-ball annulus valve mounted in a ventricle-aorta model. Analysis of the results suggests that a new energy loss parameter be defined--a "valve only" loss, calculated by subtracting the total energy loss for the system with the valve absent, from the total energy loss for the same system with the valve in place. This developed concept of "valve only" energy loss has the potential of standardising the findings of different research groups by removing the arbitrary selection of measurement points from reported results.

Assessment of haemolytic and thromboembolic potentials--from CFD studies of Starr-Edwards cardiac valve prostheses.

Quantitative assessments of haemolysis, thromboembolism and endothelial damage are established using computational fluid dynamics and documented threshold values. The régime studied was steady forward (systolic) flow through a fully open Starr-Edwards model 1260 ball valve, in an aortic shaped chamber. Modelling was carried out using finite difference techniques for turbulent, Newtonian flow. Haemolysis was not predicted "in-bulk", but a haemolytic potential as high as 30% was suggested at prosthetic surfaces. Also a 10% thromboembolic potential and a 6% potential for endothelium damage were noted.