Accurate dispensing of volatile reagents on demand for EWOD chips

We demonstrate an accurate on-demand reagent dispensing system with impedance-based threshold sensing that is suitable for use with volatile solvents in air-filled electrowetting-on-dielectric (EWOD) chips for applications in chemical synthesis. Because volatile liquids cannot easily be stored on-chip, they are delivered from off-chip reservoirs via a syringe pump. Impedance sensing enables compensation for evaporative loss from the dispensing interface to ensure high repeatability of desired volumes. As a demonstration, we dispensed acetonitrile (MeCN) with varying volume (100 to 1000 nL) and varying time intervals (10 sec to 10 min) between subsequent droplets. Repeatability was found to be better than 3%. Copyright © (2011) by the Chemical and Biological Microsystems Society.