(476g) Directional Oscillatory Shear Index in Orbiting Petri Dishes

Wall shear stress (WSS) is widely accepted as a primary influence affecting characteristics of anchored cells subjected to fluid flow. Common orbital shakers provide a means of simultaneously applying shear stress to cells for tens to hundreds of cases by loading the shaker with multiple dishes. Orbital shakers allow simultaneous cell culture experiments and are widely used throughout the cell culture industry because of their simplicity. The movement of fluid in a dish that derives its motion from an orbiting shaker platform will be oscillatory in nature, somewhat like the pulsing fluid movement in the human vasculature system, with a wave whose peak rotates around the dish at an angular velocity corresponding to the orbital velocity of the dish. However, the complex flow in orbiting dishes is generally not amenable to analytical solution for resolving shear created by the fluid motion except for limited simplified conditions. The only existing quantification of shear in this flow is an equation that estimates a constant scalar value of shear for the entire surface of the dish. In practice, WSS will be oscillatory in both magnitude and direction rather than steady due to the traveling waveform and will vary across the surface of the dish at any instant in time. Computational fluid dynamics (CFD) was, therefore, employed to simulate the motion of fluid in orbiting dishes. A model was created that provides complete spatial and temporal resolution of WSS over the bottom surface of the dish and validated using particle image velocimetry.

An oscillating shear index (OSI) was previously defined for use in the analysis of arterial disease to describe the cyclic departure of the WSS vector from its predominant axial alignment (Ku et al., Atherosclerosis, 5, 293-302,1985). We modified the index for the case of an orbiting petri dish where the fluid motion can be described in terms of radial and tangential components and termed it a directional oscillating shear index (DOSI). While the DOSI does not describe the frequency of oscillation, it does quantify the oscillatory nature between the radial and tangential contributions to the shear stress. The value ranges from 0 that indicates 180o cyclic variation or bidirectional flow to a maximum value of 1 that indicates no cyclic variation between the radial and tangential components or unidirectional flow. Examples are presented that show how endothelial cell proliferation and morphology are affected by shear magnitude and the DOSI, which both vary across the area of the dish, and help explain why cellular responses depend on location in the dish.