Fluid-Structure Interaction In Brains May Produce Net Flow From Pulsations

The flow of cerebrospinal fluid (CSF) through perivascular spaces (PVSs) plays an essential role in metabolic waste clearance in the brain. The driving mechanism of the CSF flow is poorly understood and may, in part, be explained by a valve mechanism. PVSs surrounding penetrating arteries in the brain cortex are bounded by an outer wall formed by tiled astrocyte endfeet, with gaps between. Increased pressure in the PVS presumably causes the outer wall to expand, enlarging the gaps. On the other hand, decreased pressure presumably shrinks the gaps. If fluid passes through the gaps as it moves between the PVS and the surrounding parenchyma, this expansion and shrinking would act like a rectifying valve, since fluid would flow more easily when the gaps are larger. Here, we study how much shrinking and expansion we can expect in realistic conditions and how effective the rectification would be with a prescribed pressure oscillation in the PVS. In our fluid-structure interaction simulation, we model the outer wall as a thin shell, solve for gap size change based on the hoop stress-strain relation, and calculate the flow transport across the gap. Our model provides a theoretical quantification of the impact of the gap size change on flow rectification, and insights into the potential role of endfeet in CSF flow.