(416b) A Microfluidic Model System for Investigating Transport Phenomena in a Focal Hemostatic Injury Model

Background: Small-scale flows are of critical importance for the effective transport of dissolved gasses, ions, and biomolecules throughout living tissues. In high-pressure closed circulatory systems, a compromised vessel wall induces a series of surface-catalyzed deposition and enzymatic reactions that stop blood loss while maintaining the bulk fluidity. The subsequent reaction-convection-diffusion system coupled to a small-scale flow provides a rich field for the investigation of transport processes of significant fundamental and clinical importance.

Methods: By coupling micropatterning techniques with a microfluidic vessel analog, it is possible to create focal injury analogs with precise spatial control in both the axial and transverse flow directions. The initial surface and inlet bulk conditions can be precisely manipulated, providing for a high degree of control while maintaining the physiologically relevant small-scale flow that provides both convective transport and shear stresses which are crucial to hemostatic processes. Real-time and post-hoc analysis of platelet adhesion, secondary signaling, and fibrin formation provides a suite of pertinent output signals that can be monitored under the wide range of flow conditions observed throughout the microvasculature in healthy and diseased states.

Results: The size and composition of molecules exposed at the site of vascular injury have implications on the reactive potential and subsequent hemostatic response under the small-flow conditions found throughout the vascular system. The complex behavior of aggregation and fragmentation of platelets and their subsequent signaling on a reactive surface are dependent on both the flow conditions and the surface composition. The competing phenomena of increased surface flux with increased displacing force and decreased interaction time causes and apparent maximal shear rate for platelet adhesion to a collagen surface of finite length. The polymerization of fibrin fibers from soluble precursors via a series of surface-catalyzed enzymatic reactions under a wide range of flows shows both the complexity and robustness of the system, with significant formation at wall shear rates up to 500sec^-1. At higher velocities, the local residence time of the active enzyme thrombin is diminished, limiting its role to that of a soluble secondary platelet activator until the accumulated mass is large enough to form small deformations in the flow field, allowing for localized polymerization in spite of unfavorable bulk flow conditions.