title: On chip droplet characterization: a practical, high-sensitivity measurement of droplet impedance in digital microfluidics.
abstract: We demonstrate a new approach to impedance measurement on digital microfluidics chips for the purpose of simple, sensitive, and accurate volume and liquid composition measurement. Adding only a single series resistor to existing AC droplet actuation circuits, the platform is simple to implement and has negligible effect on actuation voltage. To accurately measure the complex voltage across the resistor (and hence current through the device and droplet), the designed system is based on software-implemented lock-in amplification detection of the voltage drop across the resistor which filters out noise, enabling high-resolution and low-limit signal recovery. We observe picoliter sensitivity with linear correlation of voltage to volume extending to the microliter volumes that can be handled by digital microfluidic devices. Due to the minimal hardware, the system is robust and measurements are highly repeatable. The detection technique provides both phase and magnitude information of the real-time current flowing through the droplet for a full impedance measurement. The sensitivity and resolution of this platform enables it to distinguish between various liquids which, as demonstrated in this paper, could potentially be extended to quantify solute concentrations, liquid mixtures, and presence of analytes.
authors.affiliation: Department of Molecular and Medical Pharmacology, David Geffen School of Medicine, University of California, Los Angeles, California, USA.
authors.forename: Saman
authors.initials: S
authors.lastname: Sadeghi
authors.forename: Huijiang
authors.initials: H
authors.lastname: Ding
authors.forename: Gaurav J
authors.initials: GJ
authors.lastname: Shah
authors.forename: Supin
authors.initials: S
authors.lastname: Chen
authors.forename: Pei Yuin
authors.initials: PY
authors.lastname: Keng
authors.forename: Chang-Jin
authors.initials: CJ
authors.lastname: Kim
authors.forename: R Michael
authors.initials: RM
authors.lastname: van Dam
mesh_terms: D015374:Biosensing Techniques; D003198:Computer Simulation; D017097:Electric Impedance; D046210:Microfluidic Analytical Techniques / Q000295:instrumentation / Q000379:methods*; D044085:Microfluidics / Q000295:instrumentation / Q000379:methods*; D008962:Models, Theoretical; D012996:Solutions / Q000737:chemistry*
chemical_list: D012996:Solutions