Lutz Lab

Department of Bioengineering, University of Washington

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Oscillating flow microfluidics (“AC microfluidics”) for point-of-care diagnostics

The field of microfluidics was preceded by the Cold-War-era “fluidics” movement, which developed flow-based analogues of electrical circuits to create radiation-resistant control systems. In this analogy, current corresponds to flowrate; voltage corresponds pressure; and resistors (R), inductors (L), capacitors (C), and diodes can be related to device geometry and fluid properties. However, in the transition to microfluidic scales, much of this functionality was abandoned. Today, nearly all microfluidic devices are based on fluidic “resistors” driven by “DC” flow. In electrical circuits, sophisticated functions can be created using simple combinations of resistors, inductors, capacitors, and diodes. For example, classic radio tuners are simply band-pass filters based on resonant RLC circuits, where the station is tuned by changing the capacitance or other component using a dial. By using oscillating (“AC”) flow in microfluidic devices, the rich behavior of RLC circuits can be resurrected.

The Landers group recently exploited this analogy to create a 2-channel frequency-tunable RC pump [Begley, et al; Leslie, et al]. As suggested by those authors, we introduce inductive elements (L) to create complete RLC fluidic circuits with the high selectivity needed to turn pumps on and off. We are also working to use RLC fluidic circuits as an audio-based detector that measures changes in circuit elements via the shift in peak circuit resonance. Thus, simple plastic cards connected to a cell phone could be used to carry out multi-step assays and detect the assay results via audio signals. Smart phone cameras are being used as diagnostic instruments, but here we exploit the inherent audio capabilities found on every cell phone.

By choosing resistive (R), inductive (L), and capacitive (C) contributions that are approximately balanced in the range of audible frequency, we have created microfluidic networks with sharp resonant peaks (high Q-factor). The resonant frequency of each channel was set during fabrication by using a different fluidic capacitor (a diaphragm of different size) in otherwise identical fluidic circuits, and the entire system was driven by a single piezo disk oscillator attached to a diaphragm. The AC flowrate magnitude was measured in each of the three channels across a range of audible driving frequencies (measured at the location of colored R and L components). The figure below shows that each channel resonates at a distinct frequency, and the model accurately predicts the resonant behavior (no adjustable parameters were used in the model).

Classic diffuser valves [Morris, et al] were added to each leg of the circuit above. Resonance within a given leg produces large oscillating flow across the diffuser valve, and partial rectification of the oscillating flow produces a net flow (a pump). The figure below shows a 3-step fluid delivery using audio tones; each fluid (blue, red, colorless) was driven in sequence by selecting the corresponding audio tone for resonance in each channel. The high selectivity in this RLC design allowed each fluid to be turned on and off by simply changing the driving frequency. These pumps can be driven from the audio jack of a phone without amplification, although the magnitude of flow is significantly smaller.

Relevant papers, from us and others (new papers from our group coming soon – check back):

· Olsson, A., Stemme, G., and Stemme, E. Diffuser-element design investigation for valve-less pumps. Sensors and Actuators A, 57, 137-143 (1996).

· Morris, C. J. and Forster, F. K. The correct treatment of harmonic pressure-flow behavior in microchannels. Micro-Electro-Mechanical Systems (MEMS), 2, 473-479 (2000).

· Begley, M.R., Utz, M., Leslie, D.C., Haj-Hariri, H., Landers, J.P. Periodic response of fluidic networks with passive deformable features. Applied Physics Letters, 95(20), 203501/1-3 (2009).

· Leslie, C. D., Easley, C. J., Seker, E., Karlinsey, J. M., Utz, M., Begley, M. R., and Landers, J. P. Frequency specific flow control in microfluidic circuits with passive elastomeric features. Nature Physics, 5, 231-235 (2009).

· R. Phillips, R. Shah, Y. Browning, P. Yager, and B. Lutz. Resonant fluidic circuits for sound-controlled point-of-care diagnostics. Proceedings of MicroTAS (2011).

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