Department of Mechanical Engineering, Clemson University, Clemson, SC 29634, USA
Dielectrophoresis (DEP) is the translation of a particle either along (i.e., positive dielectrophoresis) or against (i.e., negative dielectrophoresis) an electric field gradient if the particle is more or less polarizable than the suspending medium.1 The polarizability of a particle is dependent on its electrical and mechanical properties as well as the frequency of the applied electric field.2 This makes DEP a versatile tool for particle and cell handling, especially in microdevices due to its favorable scaling.3-5 Traditional DEP is realized through patterning pair(s) of microelectrodes onto the surface of a micro channel.6,7 Recently DEP has also been implemented by the use of channel geometry, which can be the variation in channel cross-section or the curvature of the channel itself.8,9 Both methods, however, rely on in-channel electrical or mechanical parts to create electric field gradients, which complicates device fabrication and causes fouling trouble due to electrochemical reactions and electrothermal flow effects etc.10,11
Reservoir-based dielectrophoresis (rDEP) is a newly developed microfluidic method that exploits negative dielectrophoresis induced at the reservoir-microchannel junction to manipulate particles inside a reservoir.12 As seen from Figure 1(a), the size mismatch between the reservoir and the microchannel creates inherent electric field gradients at their junction, which causes a dielectrophoretic particle motion, UDEP, both parallel and normal to the electrokinetic flow, UEK. The latter, UDEP,n, focuses particles to the channel mid-plane while the former, UDEP,s, slows down particles and can even trap them within the reservoir. The streaming or trapping of a particle is determined by its electrokinetic to dielectrophoretic mobility ratio, which is a function of intrinsic particle properties.12 This dependence has been utilized to implement the selective concentration and continuous sorting of 1 μm and 3 μm polystyrene particles by size [Figure 1(b)],12 3 μm fluorescent and 3 μm non-fluorescent polystyrene particles by surface charge [Figure 1(c)],13 and live and dead yeast cells by viability [Figure 1(d)].14 As the rDEP focusing, concentration and separation of particles all take place inside a reservoir, the entire microchannel can be spared for pre- and post-analysis. This makes the rDEP method perfectly positioned for lab-on-a-chip applications.
|Figure 1: Illustration of the principle and applications of reservoir-based dielectrophoresis (rDEP): (a) particle velocity analysis at the reservoir-microchannel junction (the background shows the electric field contour), (b) sized-based separation of 1 μm and 3 μm polystyrene particles,12 (c) charge-based separation of 3 μm fluorescent and non-fluorescent polystyrene particles,13 and (d) viability-based separation of live and dead yeast cells.14 The particle moving direction is from left to right in all plots.|