T. Timmel

Tri‐Leaflet Valve Design With a Purge Flow for Heart‐Assist Devices: An In Vitro Optimization Study

The objective of this study is to assess the effect of a purge flow on valves of pulsatile heart-assist devices. Clinical applications of these devices are still limited because of frequent thromboembolic complications. These complications often originate at the valves and the unavoidable flow separation regions that are found behind the leaflets. The flow separations cause a long residence time of blood that is considered particularly detrimental. To solve this problem, a valve with a purge flow is proposed. A purge flow is a jet, which is separated from the main blood flow and directed behind the leaflets into the sinus to flush it. Even though the purge flow does not prevent a flow separation, it shortens the residence time of the blood in the sinus. Thus, the purge flow improves the periodic washout of the blood in the region of flow separation. The complex purge flow was investigated in a tri-leaflet valve. The geometrical parameters of the valve were varied systematically. A statistical technique--the Taguchi method--was used to reduce the number of investigated models to 12. The flows through the resulting valve models were numerically simulated and analyzed. The evaluated models with the best results were subsequently investigated experimentally using different methods: hemodynamic tests in a valve tester and flow visualization using the dye washout method. It was shown that the purge flow can effectively wash out the sinus. Therefore, the purge flow valve reduces the potential of a thrombus formation normally associated with the valve.

Defined Mass Transport in Monolayer Cell Cultures of Hepatocytes

Introduction: Monolayer culture systems are a frequently used form of cell culture for hepatocytes. In such a culture mass transport of gases and nutrients is driven only by diffusion. However, Fick’s Law reveals that only for very low oxygen consumption rates sufficient oxygen reaches the cells. At higher rates the state of the hepatocytes is not defined and hypoxia may occur.

Quantification of the Time Course of Proliferation and Scattering Activity of Cells in Time Lapse Videos

Introduction: Cell cultures frequently are observed using time lapse microscopic recording. Without the use of fluorescence markers the automatic quantification of morphological changes often fails. We established a semi automated technique allowing the quantification of the following parameters: number of cells and nuclei, cell death and division events and the extent of cell scattering. Errors caused by changing illumination conditions and scattering of frame positions have to be corrected.