Electrowetting on Dielectric
Electrowetting on Dielectric (EWOD) is a method for microfluidic control of small droplets of fluid using a voltage differential to change surface tension. SSI Biology is currently prototyping an EWOD liquid handling system for the DNA Synthesizer project.
The wettability of a surface is determined by changes in the electric potential across that surface. When a high enough voltage (~300V) is applied across a surface, it becomes more hydrophilic and wettable, while a lack of voltage causes the surface to become more hydrophobic. Digital electrowetting devices are composed of paths or grids of individually controlled electrode surfaces coated with insulating dielectric layers, on top of which droplets are placed. A cover surface made of insulating material (such as parafilm) and coated in silicone oil can be placed over the droplets. Applying voltage to individual electrodes results in a change in surface tension and contact angle of the liquid droplets on top of those dielectric surfaces, which leads to droplet motion. Droplets containing arbitrary reagents can be moved between adjacent electrodes, combined, and split apart using this principle.
The primary benefit of EWOD is its programmability and flexibility, as any reagent can be programmed to move to any electrode location. EWOD also allows for reagents (nucleotides, enzymes, buffers, and verification reagents) to be stored in reservoirs on electrodes and minimizes reagent waste. EWOD is used in the volTRAX liquid handling system produced by Oxford Nanopore, as well as by OpenDrop. An EWOD system has been shown to work with Pyrosequencing.
SSI Biology will use an EWOD-based system in their DNA Synthesizer. The growing DNA strands will be immobilized on magnetic beads in a droplet on one electrode. Droplets containing DNA synthesis reagents like Terminal Deoxynucleotidyl Transferase can be merged with the droplet containing the DNA to start reactions. When a given reaction step is complete, the droplet can then be moved off the electrode containing the DNA to a waste chamber, without the magnetic bead-bound DNA being washed away. After the desired DNA sequence is synthesized, it can be cleaved off the beads and moved to a desired location for downstream applications.