Liquid dielectrophoresis and surface microfluidics

Abstract

Liquid dielectrophoresis (L-DEP), when deployed at microscopic scales on top of hydrophobic surfaces, offers novel ways of rapid and automated manipulation of very small amounts of polar aqueous samples for microfluidic applications and development of laboratory-on-a-chip devices. In this article we highlight some of the more recent developments and applications of L-DEP in handling and processing of various types of aqueous samples and reagents of biological relevance including emulsions using such microchip based surface microfluidic (SMF) devices. We highlighted the utility of these devices for on-chip bioassays including nucleic acid analysis. Furthermore, the parallel sample processing capabilities of these SMF devices together with suitable on- or off-chip detection capabilities suggest numerous applications and utility in conducting automated multiplexed assays, a capability much sought after in the high throughput diagnostic and screening assays.

Figure 1

Top and sectional views of various L-DEP based SMF devices.

Figure 2

Initial demonstration of the DEP effect and actuation of nonaqueous dielectric fluids.

Figure 3

(a) Typical electrode configuration and arrangement for DEP liquid actuation and droplet dispensing. (b) Video frames captured at different times: (i) t=0, (ii) t=18 ms—liquid finger ejected from parent sample, (iii) t=40 ms—liquid finger covers the entire electrode structure, and (iv) t=42 ms—voltage turned off and droplets dispensed at the electrode bump sites. Electrode dimensions: w=g=15 μm, length=6 mm.

Figure 4

L-DEP actuation recorded through a high-speed camera at 2000 fps illustrating the breakup of liquid jet. Scale bar in (a) is 100 μm. L-DEP electrode dimensions: w=40 μm; g=40 μm; λ=540 μm.

Figure 5

Experimental setup utilized for recording high-speed L-DEP actuations, fluorescence imaging, and volumetric analysis of dispensed droplets.

Figure 6

Microdroplets dispensed by L-DEP actuations on structures with different electrode architectures: (a) uniform width∕gap L-DEP electrode; (b) uniform width∕gap L-DEP electrode pinched at lambda; (c) continuous tapering width∕gap L-DEP electrode pinched at a fixed separation l; and (d) stepped tapering L-DEP electrode with uniform width∕gap electrode sections.

Figure 7

(a) Bright field and fluorescence images of variable volume Pico green labeled DNA sample droplets. (b) The PMT current confirms the uniform dispensing of DNA sample in equivolume (black) and variable volume dispensing arrangement (red).

Figure 8

Emulsion droplet dispensing schemes leveraging L-DEP.

Figure 9

Photographs of emulsion jets and subsequently dispensed daughter droplets of various samples: (a) sample2-5cSt oil jet and (b) daughter droplets; (c) emulsion jet of sample3-5cSt oil and (d) formed daughter droplets; (e) emulsion jet of sample4-5cSt oil; and (f) formed daughter droplets; (g) emulsion jet of DI water(sample1)-5cSt oil on w=g=30 μm structure; and (h) emulsion jet of DI-20cSt oil on w=g=15 μm structure. Reprinted with permission from R. Prakash and K. V. I. S. Kaler, Lab Chip, 9, 2836 (2009). Copyright 2009, The Royal Society of Chemistry.

Figure 10

Micrographs showing (a) bright field and (b) fluorescent images of an array of 30 pl sample2 (glycerol 12.5% by volume in DI) emulsion droplets dispensed using L-DEP actuation scheme on w=g=15 μm electrode structure; observed under 10x magnification; a typical single emulsion droplet observed under 50x magnification. Reprinted with permission from R. Prakash and K. V. I. S. Kaler, Lab Chip, 9, 2836 (2009). Copyright 2009, The Royal Society of Chemistry.

Figure 11

Micrographs comparing dispensed single and double emulsion droplets. (a) Section of an array of aqueous glycerol solution (25% by volume)-in-5cSt silicone oil single emulsion droplets. (b) A single emulsion droplet observed under 50x magnification. (c) Section of an array of aqueous glycerol solution (25% by volume)-in-5cSt silicone oil-in-aqueous glycerol solution (25% by volume) double emulsion droplets. (d) A double emulsion droplet observed under 50x magnification (water-in-oil-in-water-type double emulsion).

Figure 12

Integrated L-DEP and droplet actuation structures. Once an array of droplets is dispensed via L-DEP actuation, a low frequency (20 Hz) ac voltage is applied across fishbone-shaped electrodes for droplet transport.

Figure 13

Combinatorial biochemical assay on a 2×2 matrix.