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.

Radiolabelling diverse positron emission tomography (PET) tracers using a single digital microfluidic reactor chip

“…However, because [ 18 F]fallypride synthesized by EWOD has some of the highest reported specific activity (19 Ci/µmole), fluorine-19 contamination from Teflon® appears not likely to be a major issue [23].…”

“…Past reports have demonstrated a range of droplet functions (i.e., generating, moving, splitting, and merging) [17], precise control of droplet volumes [18], on chip liquid composition measurement [19], and suitability for multiple reagent chemical synthesis [20]. Our group has recently reported the use of EWOD for the synthesis of the most commonly used PET radiotracer, [ 18 F]FDG [21].…”

Advantages of Radiochemistry in Microliter Volumes

“…An in-depth investigation led to the observation that the dependence of SA on reaction parameters was very different for macroscale (100-5000 μL) and microscale (2-8 μL) syntheses. At the macroscale, the SA strongly varied with the reaction volume (amount of reagents) as well as the starting radioactivity (from 10s to 100 s of mCi), whereas the SA was much higher and nearly constant (20)(21)(22)(23) Ci/μmol), under all conditions when performed in microdroplets. These results suggest that in the macroscale synthesis, reagents and solvents are the dominant source of fluorine-19, while in microscale volumes, these sources have been nearly eliminated and the fluorine-19 contribution from the cyclotron dominates [44,45] (Fig.…”

“…In preliminary studies, our group observed that microfluidic synthesis on EWOD chips resulted in significantly higher (up to 25-50 times) SA compared to macroscale synthesis, starting with the same amount of radioactivity [23,24]. This suggested the potential for microfluidics to significantly reduce fluorine-19 contamination in the synthesis process.…”

Dimensionless model for impedimetric sensing of particle laden droplets in digital microfluidic devices

“…In order to manipulate droplets in DMF devices, the droplet must be grounded and at least a portion of the contact line of the interface must be above the active electrode (Mugele and Baret 2005;Nelson and Kim 2012;Choi et al 2012). In the most common configuration, each addressable location in the device can be modeled as a resistor and capacitor in parallel (Lederer et al 2012;Sadeghi et al 2012;Schertzer et al 2012;Shih et al 2013). As such, previous investigations have used capacitance (Su et al 2004;Ren et al 2004;Su et al 2005;Gong and Kim 2008;Shih et al 2011;Yafia and Najjaran 2013) and impedance (Lederer et al 2012;Sadeghi et al 2012;Schertzer et al 2012;Shih et al 2013) measurements to provide real-time feedback at every addressable position in the device without requiring optical access, or increasing fabrication complexity.…”