Jimenez Lab

Microfluidic devices are an important new tool for use in the study of dynamics of biologically significant molecules. For example, microfluidic mixers can initiate a chemical or biological process in microseconds and allow the dynamics to be monitored over many orders of magnitude in time – from microseconds to seconds and longer. Our group is using these devices to study protein folding dynamics and enzyme reactions. We construct devices using soft lithography techniques. The micron-sized channels are molded in a plastic called PDMS (poly-dimethylsiloxane) and bonded to a microscope slide. Because this material is transparent over a broad spectral range,(from infrared to ultraviolet) we can use these devices to perform many different types of spectroscopic studies on biological systems.

Steps to Constructing Microfluidic Devices

In the micron-sized channels, the fluid flow is completely laminar, and fast mixing is achieved by hydrodynamic focusing. Hydrodynamic focusing requires three inlet channels that merge together into one channel at a junction. The sample of interest is sent down the center channel. Upon reaching the junction, the sample is sandwiched between the fluid flowing from each of the two side channels which speeds up the flow in the outlet channel causing the center channel to be “squeezed” into a thin sheet. The thickness of this sheet can be adjusted by controlling the relative flow rates of the center and side channels so that sheets as thin as 1 micron or less can be produced. At these length scales, diffusion occurs rapidly and mixing can occur on microsecond timescales. The fastest mixing time demonstrated to date is 8 μs by Hertzog et al. (Analytical Chemistry, 2004).

Our group uses two-photon imaging to characterize our microfluidic mixers.  Using this imaging technique we can measure the mixing properties of our microfluidic device and compare these results to our models. The inherent three-dimensional capability of multiphoton imaging is important to fully characterize the flow and mixing throughout the entire volume of the device.  By placing a fluorescence tracer (Dextran-conjugated Rhodamine dye) in the center channel, we can image the hydrodynamic focusing. The mixing time can be quantitatively measured using the quenching of the fluorescence of the Rhodamine dye when 500 mM of KI is added to the side channels.

Two-photon absorption fluorescence images of the focused channel stream with no KI in side channels (left) and with 500 mM KI in side channels (right) showing fluorescence quenching

Measured fluorescence intensity compared to calculated KI concentration in the center stream