Nanoliter droplet viscometer with additive-free operation.

Measurement of a solution's viscosity is an important analytic technique for a variety of applications including medical diagnosis, pharmaceutical development, and industrial processing. The use of droplet-based (e.g., water-in-oil) microfluidics for viscosity measurements allows nanoliter-scale sample volumes to be used, much smaller than those either in standard macro-scale rheometers or in single-phase microfluidic viscometers. By observing the flow rate of a sample plug driven by a controlled pressure through an abrupt constriction, we achieve accurate and precise measurement of the plug viscosity without addition of labels or tracer particles. Sample plugs in our device geometry had a volume of ~30 nL, and measurements had an average error of 6.6% with an average relative standard deviation of 2.8%. We tested glycerol-based samples with viscosities as high as 101 mPa s, with the only limitation on samples being that their viscosity should be higher than that of the continuous oil phase.

Flexible casting of modular self-aligning microfluidic assembly blocks.

The recent shift among developers of microfluidic technologies toward modularized "plug and play" construction reflects the steadily increasing realization that, for many would-be users of microfluidic tools, traditional clean-room microfabrication is prohibitively complex and/or expensive. In this work, we present an advanced modular microfluidic construction scheme in which pre-fabricated microfluidic assembly blocks (MABs) can be quickly fashioned, without expertise or specialized facilities, into sophisticated microfluidic devices for a wide range of applications. Specifically, we describe three major advances to the MAB concept: (1) rapid production and extraction of MABs using flexible casting trays, (2) use of pre-coated substrates for simultaneous assembly and bonding, and (3) modification of block design to include automatic alignment and sealing structures. Finally, several exemplary applications of these MABs are demonstrated in chemical gradient synthesis, droplet generation, and total internal reflection fluorescence microscopy.

Microfluidic chemical analysis systems.

The field of microfluidics has exploded in the past decade, particularly in the area of chemical and biochemical analysis systems. Borrowing technology from the solid-state electronics industry and the production of microprocessor chips, researchers working with glass, silicon, and polymer substrates have fabricated macroscale laboratory components in miniaturized formats. These devices pump nanoliter volumes of liquid through micrometer-scale channels and perform complex chemical reactions and separations. The detection of reaction products is typically done fluorescently with off-chip optical components, and the analysis time from start to finish can be significantly shorter than that of conventional techniques. In this review we describe these microfluidic analysis systems, from the original continuous flow systems relying on electroosmotic pumping for liquid motion to the large diversity of microarray chips currently in use to the newer droplet-based devices and segmented flow systems. Although not currently widespread, microfluidic systems have the potential to become ubiquitous.