Development of contactless emulsification method using electrostatic phenomena for biological application

Water-in-oil (W/O) emulsion has been used in various biological applications and combinatorial chemistries. In W/O emulsion, water droplets are served as micro-reactors, being separated from each other, and several reactions can be carried out simultaneously. Stirrers and homogenizers are generally used for emulsification. Since these devices are not disposable, their use implies the risk of contamination. In addition, these methods require certain amount of volume, at least 1 mL, because of their dimension. In this paper, a new emulsification method using AC electric field is described. The method can emulsify very small amount of water–oil mixtures such as 100 μl of oil and 5 μl of water. For emulsification, electrodes are placed outside of a vessel. Therefore, test solution is not in contact with emulsification devises; hence, under this configuration, risk of contamination is negligibly small. A plastic micro-tube (0.2 mL in volume) was used as a vessel. Aluminum tapes, as the electrodes, were placed outside around the micro-tube at the bottom and top level of the water–oil mixture. Rapeseed oil and aqueous solutions were loaded in the micro-tube. Then AC high voltage of 5 kV0–P at 17 kHz, for example, was applied. Although the electrode was not in contact with the mixture, emulsification was made due to the electric field formed by the exterior electrodes. Moreover, it was confirmed that the emulsification method did not affect polymerase chain reaction that multiplied DNA molecules using enzymatic reactions.