(113a) In Vitro Electrical Impedance Characterization of Huvecs Undergoing Hydrodynamic Shear Stress

The pathophysiological processes of atherosclerosis [1], [2], thrombosis [3], [4], and their related complications like cardiovascular diseases have all been linked to increased endothelium permeability. Though studies have strived to understand endothelial biology and kinetics through in vitro models, the majority of endothelium studies have been primarily with static cell culture models or macro-scale models that rely on end-point analysis, as well as, do not properly represent the physiological sizes, structures, and environmental conditions of human blood vessels. This work presents a microfluidic platform that replicates physiological shear stress/fluid flow conditions and the length scale of the blood vessels with the ability to electrically characterize the endothelium morphology and permeability. An electrical impedance biosensor provides a real time evaluation of cell-cell interaction and cell migration [5]. The microfluidic chip consists of two arrays of independently addressable gold electrodes of various diameters (50 µm, 100 µm, and 200 µm), and a common large counter electrode. Different electrodes sizes were used to test and compare the behavior of different subpopulations sizes within the monolayer. The microfluidic chip also includes a two-channel design, such that one may contain cells, while the other remains without cells. This design feature helps to identify environmental variables affecting the endothelial cell data. Human umbilical vein endothelial cells (HUVECs) were used as an initial endothelium model and were cultured on top of micro-patterned gold electrodes. A peristaltic pump was connected to the chip subjecting the endothelial cells to a constant shear stress conditions (15 dyne/cm²), a stepped shear condition of (20-30-50 dyne/cm²), and a control static (0 dyne/cm²) condition. An in-house designed printed circuit board was programmed to automatically record impedance spectra from a high-precision impedance analyzer (HP4294a) at specified time intervals. Impedance spectra (100 Hz- 1 MHz) collected were fitted to an equivalent circuit model in order to extract the cell behavior parameters such as cell monolayer resistance (R_TER). Experimental results show there is a shear-dependent increase in cell monolayer resistance at the onset of shear and it is greater for higher shear stress values. The cell monolayer resistance jump is only observed for acute flow as sheared cells do not generate a substantial increase in cell monolayer resistance when subjected to a higher shear stress. For both the constant shear condition and stepped shear condition, this increase is temporary and proceeded by a decline before it begins to stabilize below the baseline value (measurement at the onset of shear). Although the work presented here, focuses on general endothelium studies under fluid flow, this platform is very versatile. Other possible applications include drug testing, brain barrier characterization, role of mechanical stimuli in tumor metastasis and wound healing.