A marriage of convenience

Individual particles are manipulated optically inside electrically controlled droplets by actuating OETs and EWOD on the same device. (Adapted with permission. Copyright 2009 Royal Society of Chemistry.)
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Incompatibility often leads to divorce. But a group led by Chang-Jin “CJ” Kim at the University of California Los Angeles (UCLA) has managed to marry two seemingly incompatible technologies by giving each its own space. The group’s device moves droplets around by electrowetting-on-dielectric (EWOD) and sorts particles inside the droplets with optoelectronic tweezers (OETs) (Lab Chip 2009, DOI 10.1039/b821508a). “We would like to manipulate individual cells inside a single droplet that can be moved around,” says UCLA coauthor Eric Chiou.

In EWOD, voltage is applied between a droplet and a conducting surface that is insulated with a dielectric layer and coated with hydrophobic material. If the voltage is applied to one end of the droplet, the droplet is attracted toward that end. The droplet can be moved again by applying voltage to another spot on the surface.

OETs, developed by Chiou and coauthor Ming Wu at the University of California Berkeley, use locally applied voltage to move particles, such as cells. However, the voltage is generated by light. Particles are placed on a photosensitive surface, such as amorphous silicon. When a pattern of light is shined on the surface, the illuminated areas behave like conducting electrodes and the dark areas like insulators. Therefore, particles move toward or away from the illuminated areas depending on their charge. In this respect, OETs resemble optical tweezers, but they require several orders of magnitude less light.

Both EWOD and OETs techniques require that the material to be manipulated be placed between two plates. But their requirements conflict, so it has been impossible to apply them in the same location. “EWOD needs the voltage to drop across the dielectric layer on the conductive surface,” says Chiou. “But OETs don’t allow such a dielectric layer on the photoconductive surface they use.”

Gaurav Shah, one of Kim’s graduate students, engineered the merger. The solution was to use just one plate in the sandwich for EWOD and the other for OETs and to make each serve as a reference plate for the other. The OETs plate has two arrays of electrodes interdigitated in the metal under its photoconductive layer (and therefore is a lateral-field OETs [LOETs] plate). Both technologies can manipulate the contents of the sandwich as long as they are not activated at the same time. “There might be better ways to combine EWOD and OETs in the future,” says Kim. “But this is the first report of how it can actually be done.”

“The engineering is very innovative—they generated appropriate electrode geometry to apply both fields sequentially,” says Kishan Dholakia at the University of St. Andrews (U.K.). “Also, this fits nicely into the current climate of people wanting to combine modalities.”

To test the device, Kim’s group sandwiched a droplet containing 19 live HeLa cells between the plates and observed it through a light microscope. After stretching the droplet by EWOD, the researchers moved a rectangular optical pattern across it, sweeping the cells forward like a sheepdog herding sheep. By repeating the sweep, they collected 13 cells at one end of the droplet, leaving 6 at the other. Then they used EWOD to split the droplet in two, isolating the 13 cells.

To separate one cell type from others, one could tag the former with a fluorescent dye or take advantage of inherent differential properties such as size, shape, or dielectric signature, says Chiou. The device also could remove cells from spent medium and unite them with droplets of fresh medium. Dholakia says it would be interesting to scale up the technology so it can be used with more particles and droplets.

“Independent control of analytical samples and their components is a holy grail of analytical technology,” says Brian Kirby at Cornell University. “This system provides a new step in that direction.” Because electric fields are involved, Kirby cautions that it will be necessary “to monitor the viability of cells processed with this technique, as well as to monitor the effects of processing on gene expression.”

The investigators say that the parent technologies have improved since the device was first conceived and are hopeful that new configurations of the setup can be developed. “We are looking forward to resuming this work,” Kim says, “because OETs have become more powerful.”