Optofluidics

Our group has an interest in the integration of optics with microfluidics, a field sometimes known as “optofluidics”.  We describe below two recent examples.  In the first, we demonstrated microfluidic chips incorporating arrays of diffractive elements (zone plates) for fluorescence measurements.  In the second, we demonstrated microfluidic chips incorporating pressure sensor elements.

Fluorescence detection using an integrated zone-plate array

We have demonstrated a zone-plate array system integrated into a microfluidic device with both a large field of view and high collection efficiency, ideal for high-throughput fluorescence measurements and analysis. The design is shown in Fig. 1. A flow-focus structure generates drops, which are subsequently split and travel in 64 parallel channels. An 8-by-8 zone-plate array is aligned to the channels, with each zone plate focuses the excitation laser to a spot in one of the channelsThe fluorescence emission is collected also by the zone plates and imaged to a CCD camera. Several frames of the image stack are shown in Fig. 2. We demonstrate fluorescence measurements at up to ~184,000 drops per second. This work was published in Lab on a chip [1].

Fig. 1 Schematic of (a) the tilted view and (b) the cross section of the zone-plate array system for fluorescence detection.

Fig. 2 Frames of the collected image stack. Each zone plate observes fluorescence in drops from a different microfluidic channel.

A similar design using a zone-plate array for bead counting and sizing has also been demonstrated. The system is illustrated in Fig. 3a. The zone plates are aligned to a microfluidic channel containing fast-moving fluorescent beads. Each bead produces a peak in the fluorescent signals when it passes by the detection region of a zone plate. Fig. 3b shows a time trace of the fluorescent signals at two detection regions. The number of peaks thus represents the number of beads passing by, and the height of each peak indicates the size of its corresponding bead. This work was published in Optics Express [2].

Fig. 3 (a) Schematic of the zone-plate array system for bead counting and sizing. (b) Time trace of the signals at two detection regions.

Pressure measurements on-a-chip

We have developed an optical pressure measurement system that easily integrates into the design of microfluidic devices [3].  The sensor consists of an elastomeric membrane that is free to move up and down in response to the local fluid pressure.  These membranes are at the ends of taps leading from a main microchannel (Fig. 4a)).  The microfluidic device is placed in a microscope in transmission mode (Fig. 4b)).  In response to the pressure in the microchannel, the membrane takes on a convex shape. It thereby acts as a lens, focusing the light from the microscope’s LED. The microscope image of the sensor therefore contains a focal spot (Fig. 4b inset), whose brightness increases with pressure.  We calibrate the device by applying known pressures and recording the focal spot brightness.  It can then be used in an experiment to determine (unknown) pressures by comparing the measured brightness to that recorded in the calibration procedure.

The merits of this pressure sensing technique include 1) ease of material integration – the membrane and microfluidic chip are both made out of PDMS – alleviating bonding issues; 2) low equipment overhead – only a microscope is needed to make pressure measurements; 3) fast time response – the pressure sensing structure is extremely small (40 microns in diameter), minimizing hydraulic compliance; 4) spatial multiplexing – the small size of the sensor enables multiple sensors to be designed into the chip.  This last property enables novel visualization of pressure fields in microflows, as shown in Fig. 5.  We have also used the fast time response to detect the pressure change across a channel constriction when occupied by red blood cells – a physiologically relevant phenomenon that is understood to shed light on erythrocytic diseases such as malaria and sickle cell anemia.

Fig. 4: a) Expanded view of multilayered microfluidic device incorporating pressure sensors.  A PDMS membrane is sandwiched between two thick pieces of PDMS that define the microfluidic network.  b) Typical experimental setup showing the focusing property of the convex membrane.  Inset: CCD camera image.  Red circle denotes focal spot of a single pressure sensor.

Fig. 5: Demonstration of spatial multiplexing of pressure measurements in a microfluidic device. a)  Micrograph of a shallow flow cell with integrated pressure sensors.  b)  Pressure throughout the flow cell as reported by the membrane pressure sensors.  The device is comprised of 155 pressure sensors.

References:

[1] E. Schonbrun, A. R. Abate, P. E. Steinvurzel, D. A. Weitz, and K. B. Crozier, “High-throughput fluorescence detection using an integrated zone-plate array,” Lab Chip, 10, 852 (2010).

[2] E. Schonbrun, P. E. Steinvurzel, and K. B. Crozier, “A microfluidic fluorescence measurement system using an astigmatic diffractive microlens array,” Opt. Express, 19, 1385 (2011).

[3] Antony Orth, Ethan Schonbrun and Kenneth B. Crozier, “Multiplexed pressure sensing with elastomer membranes,” Lab on a Chip vol. 11, pp. 3810-3815 (2011)