HIGH ASPECT RATIO INERTIAL MICROFLUIDIC FOCUSING FOR PASSIVE SIZE-SELECTIVE SORTING AND ENRICHMENT

Nathan Fletcher, Amy E. Reece, John S. Oakey*

University of Wyoming, Laramie, WY USA

ABSTRACTMicrofluidic inertial focusing has been used to passively enrich particles with minimal sample loss and high efficiency.

The function and general utility of inertial focusing devices is demonstrated by concentrating cell and particle suspensions in a single pass; effectively on-chip centrifugation. We investigate single and multiple pass concentration efficiency and results indicate that these devices are appropriate for routine cell handling operations, including media exchange. This approach is particularly useful for delicate samples that cannot tolerate centrifugation. The size selectivity of inertial focusing is demonstrated by collecting a monodisperse fraction from a polydisperse hydrogel particle suspension.

KEYWORDS: High-Throughput Microfluidics, Centrifuge, Inertial Focusing, Cell Sorting, Hydrogels

INTRODUCTIONInertial microfluidics has been recently introduced as a high-

throughput method for focusing particles and cells with applications in sample filtration, particle encapsulation[1] and flow cytometry[2]. Inertial particle focusing occurs as the result of fluidic inertial forces that arise at higher flow rates. These hydrodynamic forces arise from interactions between particles, fluids, surfaces and other particles, combining to produce stable, well-defined focusing behavior that can be quantified and predicted for well-defined samples. For relatively monodisperse samples, including suspensions of cells, the focusing behavior is uniform and tremendously stable. Figure 1 shows both a schematic, high speed camera photos and long-exposure particle streak images that illustrate typical focusing behavior within high-aspect ratio microchannels (in this case, h/w>1). Here, two distinct equilibrium positions are observed as particles are focused to long channel faces by competing shear gradient and wall-induced lift forces. Short channel faces are unpopulated as there exists no shear gradient lift force to oppose wall effects in the more narrow dimension. The resulting

focused stream, which consists of two symmetrical positions, depletes the fluid volume of particles in the remainder of the channel. To most effectively concentrate and conveniently collect particles, however, a single stream is desired. We accomplish this by staging asymmetrically curved inertial focusing channels with straight channels. As seen in Figure 2, the curved channel section biases particles to one half of the straight channel, where they are subsequently focused into a tight streakline.

There is much interest in utilizing microfluidic inertial focusing devices for the handling, concentrating and sorting of complex biological suspensions[3]. This is particularly desirable where more conventional instrumentation is impractical or cost-prohibitive. These settings may include field clinics, general practitioners’ offices or even biomedical laboratories that work with delicate samples that are not sufficiently robust to survive centrifugation. We show that inertial focusing can be applied very effectively to concentrate particle suspensions with very high throughput and single pass efficiency.

EXPERIMENTALSuspensions of 10 µm fluorescent microspheres were utilized exclusively to test inertial focusing device behavior. Hybrid PDMS-glass microfluidic devices were fabricated with standard soft-lithography protocols and flow was generated and controlled via syringe pump.

Figure 1: Inertial focusing in straight high aspect ratio channels. Particles focus to a single postion along the long channel face while populating equilibria positions at each face.

Figure 2. Overview of focusing behavior: inertial focusing in straight high aspect ratio channels and curved channels with varying modes of symmetry are shown. All streak images are taken from the top of the channel, as indicated by the cross-sectional schematics that show particle equilibrium focusing positions.

978-0-9798064-4-5/µTAS 2011/$20©11CBMS-0001 1642 15th International Conference onMiniaturized Systems for Chemistry and Life Sciences

October 2-6, 2011, Seattle, Washington, USA

Micrographs were taken with a high-speed camera using an exposure of ~1 µsec and a CCD camera under long-exposure conditions (100-500 msec). Bead solutions were run through devices at flow rates on the order of 100 µl/min, which is sufficient to produce a single tightly focused stream at the terminus of the straight channel. Here, the channels were designed to flare outwards in a wedge shape, slowing the fluid velocity by spreading the fluid across a much wider cross section. At the end of this expansion region, the channel divided into three outlets and sample fractions were collected from each.

RESULTS & DISCUSSIONWe demonstrate the application of inertial focusing to

concentrate dilute particle suspensions without the need for centrifugation and sort polydisperse particle suspensions on the basis of size. To obtain single particle streams for collection, we must design beyond the limits of inertial focusing channels. This is accomplished by introducing channel curvature, which, as Figure 2 shows, can reposition focused particles via Dean flow orthogonal to the primary flow direction. In channels with symmetrical curvature, Dean flow produces recirculating vortices that repositions focused particles from the channel center to the inside wall of the curved focusing region. Thus, when particles exit the curved section and enter the straight section, they are biased to one half of the channel. There exist no forces to push particles back across the centerline and so one of the two stable equilibrium positions remains unpopulated. Figure 3 shows a micrograph of this focused particle stream after the channel undergoes a linear expansion. The collected effluent is enriched by as much as 17x, as shown in Figure 3’s inset graph, with minimal sample loss to the other collection channels.

Figure 4 demonstrates the enrichment achieved by collecting focused particles from serial curved channels. Here, these channels were designed to select for a narrow particle size by collecting a focused fraction from a small channel. That fraction was subsequently passed through a large channel where the non-focused effluent is collected. By fabricating channels with very small overlap in their focusing range, a very narrow size distribution may be collected at the terminal outlet of the second channel.

CONCLUSIONSApplications for inertial focusing devices for concentrating particle

suspensions are diverse in biomedicine. Here we have broadly demonstrated particle enrichment as well as size-selective sorting. Single pass concentration efficiencies are sufficient for routine tasks, such as media exchange or more complex steps, such as removing background antibodies or fluorophores during assays. Because suspensions need not be pelletized and resuspended during concentration, inertial focusing should prove to be much gentler than centrifugation, with less sample loss or corruption. And, finally, given the inherent size-selectivity provided by inertial focusing, this approach has proven a versatile alternative to microchannel emulsification for producing monodisperse microgel particle suspensions[4].

ACKNOWLEDGEMENTSThe authors wish to acknowledge funding provided under   Award

Number P20RR16474 from the National Center For Research Resources.

REFERENCES[1] Edd, J.; Di Carlo, D.; Humphry, K.; Koster, S.; Irimia, D.; Weitz, D.; Toner, M. Lab on a Chip 2008, 8, 1262.[2] Oakey, J.; Applegate Jr, R. W.; Arellano, E.; Carlo, D. D.; Graves, S. W.; Toner, M. Anal Chem 2010, 82, 3862-3867.[3] Di Carlo, D.; Edd, J.; Irimia, D.; Tompkins, R. G.; Toner, M. Anal. Chem. 2008, 80, 2204-11.[4] Hwang, D. K.; Oakey, J.; Toner, M.; Arthur, J. A.; Anseth, K.; Lee, S.; Zeiger, A.; VanVliet, K.; Doyle, P. Journal of

the American Chemical Society 2009, 131, 4499-4504.

CONTACT*John S. Oakey, email: joakey@uwyo.edu

Figure 3. Particle concentration and collection at an expansion following a staged inertial focusing channel.

Figure 4. Particle enrichment from a polydisperse homogenized, photopolymerized emulsion (left) to a monodisperse particle suspension (right).

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