Long-range forces affecting equilibrium inertial focusing behavior in straight high aspect ratio microfluidic channels.

The controlled and directed focusing of particles within flowing fluids is a problem of fundamental and technological significance. Microfluidic inertial focusing provides passive and precise lateral and longitudinal alignment of small particles without the need for external actuation or sheath fluid. The benefits of inertial focusing have quickly enabled the development of miniaturized flow cytometers, size-selective sorting devices, and other high-throughput particle screening tools. Straight channel inertial focusing device design requires knowledge of fluid properties and particle-channel size ratio. Equilibrium behavior of inertially focused particles has been extensively characterized and the constitutive phenomena described by scaling relationships for straight channels of square and rectangular cross section. In concentrated particle suspensions, however, long-range hydrodynamic repulsions give rise to complex particle ordering that, while interesting and potentially useful, can also dramatically diminish the technique's effectiveness for high-throughput particle handling applications. We have empirically investigated particle focusing behavior within channels of increasing aspect ratio and have identified three scaling regimes that produce varying degrees of geometrical ordering between focused particles. To explore the limits of inertial particle focusing and identify the origins of these long-range interparticle forces, we have explored equilibrium focusing behavior as a function of channel geometry and particle concentration. Experimental results for highly concentrated particle solutions identify equilibrium thresholds for focusing that scale weakly with concentration and strongly with channel geometry. Balancing geometry mediated inertial forces with estimates for interparticle repulsive forces now provide a complete picture of pattern formation among concentrated inertially focused particles and enhance our understanding of the fundamental limits of inertial focusing for technological applications.

Microfluidic techniques for high throughput single cell analysis.

The microfabrication of microfluidic control systems and the development of increasingly sensitive molecular amplification tools have enabled the miniaturization of single cells analytical platforms. Only recently has the throughput of these platforms increased to a level at which populations can be screened at the single cell level. Techniques based upon both active and passive manipulation are now capable of discriminating between single cell phenotypes for sorting, diagnostic or prognostic applications in a variety of clinical scenarios. The introduction of multiphase microfluidics enables the segmentation of single cells into biochemically discrete picoliter environments. The combination of these techniques are enabling a class of single cell analytical platforms within great potential for data driven biomedicine, genomics and transcriptomics.

Staged Inertial Microfluidic Focusing for Complex Fluid Enrichment.

Microfluidic inertial focusing reliably and passively aligns small particles and cells through a combination of competing inertial fluid forces. The equilibrium behavior of inertially focused particles in straight channels has been extensively characterized and has been shown to be a strong function of channel size, geometry and particle size. We demonstrate that channels of varying geometry may be combined to produce a staged device capable of high throughput particle and cell concentration and efficient single pass complex fluid enrichment. Straight and asymmetrically curved microchannels were combined in series to accelerate focusing dynamics and improve concentration efficiency. We have investigated single and multiple pass concentration efficiency and results indicate that these devices are appropriate for routine cell handling operations, including buffer exchange. We demonstrate the utility of these devices by performing a ubiquitous fluorescence staining assay on-chip while sacrificing very little sample or processing time relative to centrifugation. Staged concentration is particularly desirable for point of care settings in which more conventional instrumentation is impractical or cost-prohibitive.

Unusual cold denaturation of a small protein domain.

A thermal unfolding study of the 45-residue α-helical domain UBA(2) using circular dichroism is presented. The protein is highly thermostable and exhibits a clear cold unfolding transition with the onset near 290 K without denaturant. Cold denaturation in proteins is rarely observed in general and is quite unique among small helical protein domains. The cold unfolding was further investigated in urea solutions, and a simple thermodynamic model was used to fit all thermal and urea unfolding data. The resulting thermodynamic parameters are compared to those of other small protein domains. Possible origins of the unusual cold unfolding of UBA(2) are discussed.