Monitoring the composition of droplets in digital microfluidic chips
Electrowetting-on-dielectric (EWOD) digital microfluidics are well-suited for microscale chemical synthesis such as molecular imaging tracers. Since movement of droplets in digital microfliudic devices is achieved by sequentially applying AC signals (typically f~10 kHz, amplitude~100V) to electrodes, it is straightforward to integrate impedance measurements into the EWOD platform. The current drawn from the actuation circuit (which depends on the impedance of the EWOD chip and the droplet at the activated site) can be measured by placing a known resistor (of low impedance) in series and measuring the voltage drop across it.
We have developed a simple circuit and analysis software to provide impedance measurement in real-time. We have observed a linear dependence on droplet size as well as a dependence on the solute concentration and type of solvent in droplets (see Figure 1).
With careful analysis, the impedance signal can lead to accurate measurements of the capacitive or resistive properties of the droplet.
Such measurements could provide the basis for an error-checking mechanism during a complex, multi-step reaction on the EWOD chip, and could potentially provide information about the progress of reactions in situ.
Figure 1: (Left) Schematic of impedance sensing circuit in the EWOD chip. (Right) Cartoon of information that can be studied by monitoring the impedance signal (i.e. droplet volume, concentration of solutes in droplet, identify of solvent in droplet).
Current Status and Next Steps
We are exploring methods to improve the sensitivity and dynamic range of measurements.
- S. Sadeghi, H. Ding, G. J. Shah, S. Chen, P. Y. Keng, C.-J. “CJ” Kim, and R. M. van Dam, “On Chip Droplet Characterization: A Practical, High-Sensitivity Measurement of Droplet Impedance in Digital Microfluidics,” Anal. Chem., vol. 84, no. 4, pp. 1915–1923, Feb. 2012 (Journal Link)
- X. Ma, S. Chen, C.-J. “CJ” Kim, and van Dam, R. Michael, “High-sensitivity electrical conductivity measurement of droplets in digital microfluidics,” in Proceedings of the 18th International Conference on Miniaturized Systems for Chemistry and Life Sciences, San Antonio, TX, 2014
- Xiaoxiao "Helen" Ma (postdoctoral scholar)
- Saman Sadeghi (postdoctoral scholar)
- Huijiang "John" Ding (research staff)
- Gaurav J. Shah (postdoctoral scholar; later collaborator from Sofie Biosciences, Inc.)
- Prof. Pei Yuin Keg (postdoctoral scholar; later collaborator)
- Chang-Jin "CJ" Kim (collaborator)