Electrical Droplet Position Feedback System for a Digital Microfluidic Platform

Abstract:
Digital microfluidic systems employ the electrowetting-on-dielectric mechanism to actuate nanoliter droplets on a dielectric-coated electrode array. Due to their flexibility and their ability to perform lab-on-a-chip experiments in an automated manner, they have gained considerable interest especially for biochemical applications. However, full automation of experimental procedures requires sensing of the droplet position to detect actuation failures. A novel position sensing system is presented in this study, based on a relaxation oscillator circuit. The impedance between the electrodes of the digital microfluidic chip, which changes in the presence of a droplet, determines the frequency of the output signal. The sensing capability of the system is demonstrated for a range of liquids relevant for lab-on-a-chip applications: deionized water, potassium chloride solution, 1-octanol, and the ionic liquid [bmim]PF6. The system is highly sensitive and thus capable of detecting small droplet position changes. On a 2∙2 mm² square electrode, changes in electrode coverage between 0.29% for a droplet of DI water and 1.36% for 1-octanol can be resolved. Since many applications involve a reaction step during which the droplets remain stationary for several minutes up to about an hour, we demonstrate that the circuit is sufficiently frequency-stable to monitor the droplet position over dwell times of at least 90 min.