A versatile and robust system for electrical droplet position sensing in a digital microfluidic platform

Digital microfluidic systems enable the actuation of micro- to nanoliter droplets on a hydrophobic surface using the electrowetting-on-dielectric mechanism. Due to their capability to dispense, move, merge, split, and mix droplets, digital microfluidic systems provide the capability to perform lab-on-a-chip experiments automatically. A prerequisite for automated experiments is the reliability of droplet actuation. A closed-loop feedback of the droplet position is required in order to determine whether the droplet actuation was successful, and adjust the droplet drive parameters accordingly. One common approach for droplet position sensing relies on measuring the electrical impedance change that occurs when a droplet is present on a device electrode. The position sensing feedback system presented in this work is based on a relaxation oscillator circuit and capable of determining the droplet position for a broad range of liquids relevant for digital microfluidic applications, such as deionized water, 1 M aqueous potassium chloride solution, organic solvents (1-octanol, ethanol, dimethyl sulfoxide), as well as the ionic liquid [bmim]PF 6 , extending its potential application to lab-on-a-chip platforms for reaction monitoring, analyte processing, and bioanalytical workflows. On a typical square electrode, changes in liquid coverage between for DI water and for 1-octanol can be detected. The system is stable to monitor the droplet position over at least and insensitive to interference. A novelty of the presented feedback system is its compatibility with all driver circuits currently in use for droplet actuation by electrowetting, including those based on integrated high-voltage serial-to-parallel converters.