William A. Braff and Cullen R. Buie*
Department of Mechanical Engineering, Massachusetts Institute of Technology
77 Massachusetts Avenue, Room 3-248, Cambridge, MA 02139
*Email: firstname.lastname@example.org; Tel: (617) 253-9379; Web: web.mit.edu/lemi/
The phenomenon known as dielectrophoresis (DEP) was first reported by Pohl more than sixty years ago.1 DEP occurs when a polarizable particle is subject to a non-uniform electric field. The resulting force is due to differences in polarizability between the particle and the surrounding media. Recently, insulator-based dielectrophoresis (iDEP) (or sometimes called electrodeless DEP)2 has been developed as an alternative to electrode-based DEP.2-4 Rather than patterned electrodes, iDEP devices use the channel geometry to impose large electric field gradients on particles (or cells) in a microchannel. iDEP devices simplify system design by eliminating the need for microelectrodes. Devices employing iDEP have been demonstrated for live-dead cell sorting,4 cell concentration,5 as well as to enhance the kinetics of DNA hybridization.6 One disadvantage of iDEP compared to electrode based DEP has been that large electric potentials are needed to generate non-negligible DEP forces. High applied potentials often result in deleterious Joule heating and electrothermal flows, which can injure living cells and disrupt transport in the microfluidic device.7 Increasing the constriction ratio, χ, between the open channel cross-sectional area and the area in the constricted region increases the electric field gradient at the constriction. The resulting increased electric field gradient reduces the potential required for iDEP immobilization. However, χ is limited in most systems by fabrication constraints. Recent work has demonstrated that micro-milling can be used to develop iDEP devices with constriction ratios of 100:1 by making them three-dimensional.8 A schematic illustrating the functional difference between conventional iDEP (2D-iDEP) and three-dimensional iDEP (3D-iDEP) is provided in Figure 1. Previous embodiments of iDEP have employed three-dimensional features,5,9-11 but these features were not used to achieve high constriction ratios. Compared to previous work on insulator-based dielectrophoresis, 3D-iDEP devices have an order of magnitude higher sensitivity.8 The increased sensitivity lowers the potential required for actuation, thereby also lowering Joule heating in the microchannel.7,8 Our experimental results demonstrate 3D insulator-based dielectrophoretic immobilization of bacteria at the lowest electric fields reported in the literature. Ultimately 3D-iDEP reduces the potential required for iDEP (to values closer to electrode based DEP) while retaining the fabrication simplicity of iDEP.
|Figure 1. Schematic depicting 2DiDEP (a) vs. 3DiDEP (b). Compared to (a), the large constriction ratio in (b) resulting from the three dimensional geometry results in a higher dielectrophoretic force at a given applied potential. (c) 3DiDEP immobilization of fluorescently labeled Escherichia coli at 5 V/mm.|