Karan V. I. S. Kaler* and Ravi Prakash
Schulich School of Engineering, University of Calgary
2500 University Drive NW Calgary, AB – T2N 1N4, CANADA
In Liquid Dielectrophoresis (L-DEP), a pondermotive DEP  force is induced under the influence of spatially non-uniform electric fields that can act to convey a fluidic sample and collect it in regions of strong (positive DEP) or, weak (negative DEP) electric field intensity. Recent advances in L-DEP technology have given rise to a rapid and automated, precision liquid actuation, droplet dispensing and manipulation capability that can be potentially leveraged for a variety of miniaturized, lab-on-a-chip (LOC) bio-diagnostic assays.
The utility of L-DEP at microscopic scales to actuate fluidic media was first demonstrated by Jones [2, 3] using an insulated coplanar set of metal micro-electrodes, excited by AC voltage source. Figure 1 shows a typical L-DEP microfluidic electrode structure and its cross-sectional diagram. The placement of a parent liquid droplet (~ 1μL) on one end of the coplanar electrode arrangement and excitation of the electrodes by an AC voltage (typically between 200-500 Vpp at 100 kHz; depending on the electrode geometry and electrical conductivity of fluidic sample), resulted in ejection of a liquid jet emanating from the parent droplet and rapidly conveyed over the L-DEP electrodes. Upon removal of the A.C. excitation voltage, the liquid jet breaks up into multiple and in this case, identical microscopic sized droplets (volume ~ 1 nL). The dispensed droplet volume, location and the number of droplets are controlled by the electrode geometry [4, 5] and governed by the Rayleigh's instability criterion of the actuated liquid jet .
L-DEP micro-devices can be fabricated on passivated silicon, glass or polymeric substrates, utilizing micro-fabrication techniques, to realize the required dielectrically isolated micro-electrode structure [7, 8]. A key requirement of L-DEP microfluidic devices is the production of a suitable top surface to facilitate formation and retention of ultrafine droplets of complex fluidic samples. Hydrophobic surfaces (liquid contact angle > 90o) such as: spin coated and composite fluoropolymers (Teflon™ and plasma deposited fluorocarbon), cytop™ etc. are often suitable for L-DEP actuation of simple fluidic samples [7, 9]. However, suitably tailored nano-textured superhydrophobic (SH) surfaces (contact angle > 150o) are found to be more effective for application of L-DEP towards LOC bio-diagnostics assays due to reduction in bio-sample adsorption and prevention of droplet collapsing during assays . L-DEP actuation of both simple and complex fluidic sample, on hydrophobic and nano-textured SH surface is compared in Figure 2. L-DEP actuation of both homogeneous and complex fluidic samples is found to be superior (faster and more reliable dispensing) over SH surfaces .
L-DEP droplet dispensing methods have been utilized to dispense not only homogenous fluidic media, but also structured, multi-layered droplets, such as: aqueous-in-oil emulsions , micro/nano-suspensions  and functionalized lipid vesicles  in the nL to pL volume range. Figure 3 shows examples illustrations of the versatile dispensing and complex sample handling capabilities of L-DEP. The vesicles when functionalized with bio-probes such as: target nucleic acid, functionalized micro-particles and quantum dots® etc. [7, 10] can be utilized for on chip nucleic acid bio-sensing applications. In order to further leverage this L-DEP droplet dispensing capability for LOC based assays requires additional means of transporting the droplets to mixing, dilution, incubation and detection sites [7, 10].
|Figure 1: (a) Cross-sectional view of a L-DEP microfluidic device; (b) salient features of planar L-DEP electrode structure; (c) micrographs showing L-DEP actuation and droplet dispensing; (d) effect of size and geometry on the dispensed droplet array; (e) variable volume (1.5 nL to 100 pL) droplet formation with dispensed droplet density controlled by Rayleigh's instability; (f) variable volume droplet formation with dispensed droplet density controlled using pinches. De-ionized (DI) water used as liquid sample and L-DEP actuations conducted under a 5 cSt silicone oil bath.|
|Figure 2: (a) AFM image of composite fluorocarbon, hydrophobic surface; (b) dispensing of DI water droplets on composite hydrophobic surface; (c) poor dispensing due to adsorption of TAQ™ DNA polymerase (conc. 5 U/mL) during L-DEP actuation; (d) a 5 μL droplet on composite hydrophobic surface; (e) comparison of enzyme adsorption during L-DEP actuation over hydrophobic and superhydrophobic (SH) surfaces; (f) SEM image of nano-textured SH surface; (g) a 5 μL DI water droplet on SH surface; (h) superior dispensing of DI water droplets on SH surface and (i) superior handling of TAQ™ DNA polymerase (conc. 5 U/mL) during L-DEP actuation.|
|Figure 3: (a, b) Formation of single emulsion (SE) liquid jet and SE droplet array during L-DEP actuation; (c) dispensed 50 μm glycerol-DI-in-oil SE droplet; (d) procedure for formation of bilayer lipid vesicles using L-DEP; (e) quantum dots® functionalized supported lipid bilayers formed on micro-beads; (f, g) L-DEP actuation and handling of various micro-particle suspensions in a 5 cSt silicone oil bath and (h) plot illustrating the uniform dispensing of different polystyrene (PS) micro-particle suspensions, in the range of 1 μm – 16 μm.|