Application of Liquid Dielectrophoresis (L-DEP) for Microfluidic Devices

Karan V. I. S. Kaler* and Ravi Prakash

Schulich School of Engineering, University of Calgary
2500 University Drive NW Calgary, AB – T2N 1N4, CANADA
*Email: kaler@ucalgary.ca

In Liquid Dielectrophoresis (L-DEP), a pondermotive DEP [1] 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 [6].

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 [9]. 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 [9].

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 [7], micro/nano-suspensions [7] and functionalized lipid vesicles [10] 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].


References
  1. Pohl H. A., Dielectrophoresis, Cambridge University Press, Cambridge, 1978.
  2. Jones T. B., Liquid dielectrophoresis on the microscale, J. Electrostatics, vol. 51–52, pp. 290–299, 2001.
  3. Jones T. B., Gunji M., Washizu M., Feldman M. J., Dielectrophoretic liquid actuation and nanoliter droplet formation, J. Appl. Phys., vol. 89(3), pp. 1–8, 2001.
  4. Prakash R., Paul R., Kaler K. V. I. S., Liquid DEP Actuation and Precision Dispensing of Variable Volume Droplets, Lab Chip, vol. 10, pp. 3094-3102, 2010.
  5. D. Chatterjee, H. Shepherd, R.L. Garrell, Electromechanical model for actuating liquids in a two-plate droplet microfluidic device, Lab Chip, vol. 9, pp. 1219-1229, 2009.
  6. Lord Rayleigh, On capillary phenomena of jets, Proc. Roy. Soc. (London), vol. 27, pp. 71–97, 1879.
  7. Kaler K. V. I. S., Prakash R., Dielectrophoresis Based Droplet Microfluidic Devices for On-Chip Bioassays, Book Title: Microfluidics: Control, Manipulation and Behavioral Applications; pp. 1-50, Nova Science Publishers, New York, 2012.
  8. Renaudot R., Agache V., Daunay B., Lambert P., Kumemura M., Fouillet Y., Collard D., Fujita H., Optimization of Liquid DiElectroPhoresis (LDEP) Digital Microfluidic Transduction for Biomedical Applications, Micromachines, vol. 2, pp. 258-273, 2011.
  9. Prakash R., Papageorgiou D. P., Papathanasiou A. G., Kaler K. V. I. S., Dielectrophoretic Liquid Actuation on Nano-Textured Super Hydrophobic Surfaces, Sensors and Actuators B: Chemical, vol. 182, pp. 351-361, 2013.
  10. Prakash R., Kaler K. V. I. S., DEP actuation of emulsion jets and dispensing of sub-nanoliter emulsion droplets, Lab Chip., vol. 9, pp. 2836–2844, 2009.
Figures
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.