Laboratoire Matière et Systèmes Complexes

The Dynamics of Two Biological Interfaces

Gerald G. Fuller Chemical Engineering Stanford University

Abstract : Biological systems are normally high-interface systems and these surfaces are laden with biological molecules and cells that render them mechanically complex. The resulting nonlinearities with response to surface stresses and strain are often essential to their proper function and these are explored using recently developed methods that reveal an intricate interplay between applied stress and dynamic response. Two applications are discussed.

1. Vascular endothelial cells are nature’s "rheologists" and line the interior walls of our blood vessels and are sensitive to surface shear stresses. These stresses are known to affect the shape and orientation of endothelial cells. It is evident that the spatial homogeneity of flow can affect vascular health and it is well-documented that lesions form in regions of high curvature, bifurcations, and asperities in blood vessels. Experiments are described where stagnation point flows are used to create regions of well controlled flow stagnation and spatial variation of wall shear stresses. Live-cell imaging is used to monitor the fate of cells attached to surfaces experiencing flow impingement and it is revealed that endothelial cells migrate and orient in such flows to create remarkable patterns of orientation and cell densification. This response, termed "rheotaxis", is used to explore mechano-transduction pathways within these cells.

Cell‐cell junctions of lymphatic endothelial cells in impinging jet flow

2. The tear film of the eye is a composite structure of an aqueous solution of protein and biomacromolecules. This thin layer is further covered by a film comprised of meibomian lipids excreted during each blink. The purpose of the meibum has been largely unexplained although one prevailing suggestion is that it suppresses evaporation. Recent measurements in our laboratory demonstrate that this layer is strongly viscoelastic and this property has dramatic effects on the dynamics of the moving contact line and stability against dewetting.

Stabilization of the tear film by viscoelastic lipids