Archive for the ‘Droplets’ Category

DMF Flip-Chips: an easy route to digital microfluidics

Steve C.C. Shih1, 2 and  Aaron R. Wheeler1, 2, 3
1 Institute for Biomaterials and Biomedical Engineering, University of Toronto, 164 College St., Toronto, ON, M5S 3G9
2 Donnelly Centre for Cellular and Biomolecular Research, 160 College St., Toronto, ON, M5S 3E1
3 Department of Chemistry, University of Toronto, 80 St George St., Toronto, ON, M5S 3H6

Why is this useful?


Digital microfluidics (DMF) is a technique in which discrete droplets are manipulated by applying electrical fields to an array of electrodes.1 An advantage of DMF is that droplets serve as discrete microvessels in which reactions can be carried out without cross-talk between samples or reagents. In contrast to the more conventional geometry of enclosed microchannels, each sample on a DMF device can be addressed individually, and reagents can be dispensed from reservoirs, moved, merged and split.2
A key component in a functioning DMF device is the insulating layer (i.e., a dielectric material) which is deposited on top of the actuation electrodes to facilitate the build-up of charge which drives droplet actuation.3 Fabrication of dielectric layers can be time-consuming and the costs for depositing them can be very expensive (e.g., a common method is chemical vapour deposition of parylene C).  Alternative methods have been described,4 but in all previous work, a separate dielectric layer must be positioned onto a device.

Here, we report a new method which we have called “DMF Flip-Chips,” in which the device substrate itself serves as the dielectric layer. In this method, actuation electrodes are patterned on one side of a glass coverslip (or other thin substrate), which is then flipped over such that the substrate serves as the insulating layer for droplet manipulation. This method is faster than conventional fabrication, and we speculate it will be useful for laboratories that do not have a dielectric coater but would like to use digital microfluidics.

What do I need?


  • Glass coverslips with thickness of 160 mu.gifm or less
  • Indium-tin-oxide coated glass (Delta Technologies Ltd, Stillwater, MN) to serve as top substrate
  • Scissors
  • Double-sided tape
  • Teflon-AF

How do I do it?


1. Pattern a glass coverslip with an array of electrodes. This can be accomplished using conventional cleanroom techniques2,3 or by rapid prototyping techniques such as microcontact printing,5 laser toner printing,6 or marker masking.7

Figure 1

2. Flip the substrate over and coat the “bottom” (now top) with Teflon-AF as described previously. Likewise, coat an ITO-glass substrate with Teflon-AF.2,3

Figure 2

3. Assemble the device with the patterned glass substrate on the bottom, double-sided tape spacers in the middle, and ITO-glass substrate on the top. Sandwich droplets between the two substrates.
4. Apply driving potentials (~500 VRMS and 15 kHz) between electrodes on the bottom and top plates, which causes droplets to a) dispense, b) move, c) merge, and d) mix as shown.

Figure 3

What else should I know?


  • Potentials of ~500 VRMS  are appropriate for bottom plates formed from a 160 mu.gifm coverslip.  Lower potentials can be used with thinner coverslips.
  • Additionally, lower voltages can be used for substrates formed from materials with higher dielectric constants than glass.
  • Clean surfaces with methanol or water after every use.
  • Thinner cover slips are difficult to use because they are very brittle and fragile.

Acknowledgements


We thank Irena Barbulovic-Nad for helpful discussions.

References


[1] A. R. Wheeler, Science, 2008, 322, 539-540.
[2] S. K. Cho, H. J. Moon and C. J. Kim, Journal of Microelectromechanical Systems, 2003, 12, 70-80.
[3] M. G. Pollack, A. D. Shenderov and R. B. Fair, Lab Chip, 2002, 2, 96-101.
[4] M. J. Jebrail, N. Lafreniere, H. Yang and A. R. Wheeler,  A two-for-one dielectric and hydrophobic layer for digital microfluidics, Chips & Tips (Lab on a Chip), 6 July 2010.
[5] M. W. Watson, M. Abdelgawad, G. Ye, N. Yonson, J. Trottier and A. R. Wheeler, Anal. Chem., 2006, 78, 7877-7885.
[6] M. Abdelgawad and A. R. Wheeler, Adv. Mat., 2007, 19, 133-137.
[7] M. Abdelgawad and A. R. Wheeler, Microfluid. Nanofluid., 2008, 4, 349-355.

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A two-for-one dielectric and hydrophobic layer for digital microfluidics

Mais J. Jebrail, Nelson Lafreniere, Hao Yang and Aaron R. Wheeler
Department of Chemistry, University of Toronto, Ontario, Canada

Why is this useful?


Digital microfluidics (DMF) is a technique in which droplets of reagents in micro- to nano-liter volumes are manipulated by applying a series of electrical potentials to an array of electrodes[1]. In DMF devices, the actuation electrodes are coated with an insulating layer. Upon application of electrical potentials, charges accumulate on either side of the insulator, a phenomenon that can be exploited to make droplets move, merge, mix, split, and dispense from reservoirs. The insulating layer is covered by an additional hydrophobic coating, which reduces droplet sticking to the surface[2]. The instruments and materials required for forming these layers are expensive (tens-to-hundreds of thousands of dollars) and the deposition methods are time-consuming (many hours). We recently [3] demonstrated a new strategy for reusing DMF devices by fitting them with insulating polymer coverings (e.g., food wrap) that are spin-coated with Teflon. Here, we share an even simpler method that is cheap (tens of dollars) and fast (minutes)  featuring a two-for-one insulating and hydrophobic layer formed from laboratory wrap (Parafilm®, Alcan Packaging, Neenah, WI). No Teflon is required for fabricating these devices, and we speculate that this will be useful for laboratories interested in rapid prototyping for various applications.

What do I need?


  • Bottom substrate patterned with working electrodes (typically chromium or gold on glass); electrodes can be formed using conventional cleanroom techniques [4,5] or by rapid prototyping techniques such as microcontact printing [6], laser toner printing [7], or marker masking [8]
  • Indium-tin-oxide coated glass (can be purchased from Delta Technologies Ltd, Stillwater, MN) to serve as top substrate
  • Parafilm® and wax paper backing
  • Scissors
  • Scalpel
  • Hot plate

How do I do it?


1. With scissors, cut a piece of Parafilm® and stretch the film horizontally and vertically to its limits (a), and place over the bottom plate of the DMF device (with patterned electrodes) (b)

2. With a wax paper apply pressure with your finger on the film to release any air trapped between the electrode(s) and film (a). Then, score film with a scalpel and peel off excess Parafilm® (b,c).

3. Repeat steps 1 and 2 to apply parafilm layer to the top substrate (indium-tin oxide coated glass)
4. Place substrates on hotplate for 30 seconds at 80-85°C.
5. Allow substrates to cool down to room temperature (a), and then assemble the device with a top plate4,5 to dispense, merge and split droplets as shown in Figure (b-e).

What else should I know?


  • A ~4.5 x 4.5 cm piece of Parafilm® stretched to its limits will give a thickness of 6 – 9 mu.gifm.
  • A voltage of 300 – 500V is appropriate for actuation for above thickness.
  • Use of wax paper when applying pressure is important as it avoids contamination of Parafilm® surface.
  • Device can be recycled by simply peeling off old Parafilm® and replacing it with a new film.

References


[1] A. R. Wheeler, Science, 2008, 322, 539.
[2] J. Lee, H. Moon, J. Fowler, T. Schoellhammer, C. J. Kim, Sens. Actuator A-Phys., 2002, 95, 259.
[3] H. Yang, V. N. Luk, M. Abdelgawad, I. Barbulovic-Nad and A. R. Wheeer, Anal. Chem., 2009, 81, 1061.
[4] M. G. Pollack, A. D. Shenderov and R. B. Fair, Lab Chip, 2002, 2, 96.
[5] S. K. Cho, H. J. Moon and C. J. Kim, J. Microelectromech. Syst., 2003, 12, 70.
[6] M. W. L. Watson, M. Abdelgawad, G. Ye, N. Yonson, J. Trottier and A. R. Wheeler, Anal. Chem.,  2006, 78, 7877.
[7] M. Abdelgawad and A. R. Wheeler, Adv. Mater., 2007, 19, 133.
[8] M. Abdelgawad and A. R. Wheeler, Microfluid. Nanofluid., 2008, 4, 349.

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