Microfluidics definition

Segmented Flow Microfluidics Definition of segmented flow microfluidics ... 

The definition of secondary and follow-up assays is not straightforward. It really depends on the quality and economics of the assays. For example, functional ion channel assays are mostly used as a second line during the early phase of drug discovery. However, technological advances, such as the introduction of microfluidics and improved detection technologies make them more and more suitable for first-line profiling. 

M 14] [P 14] Splitting of droplets can only occur at aspect ratios <0.2 (for definition see Effect of aspect ratio) [29]. Since this is an important microfluidic action, it was worth finding suitable novel mixing strategies to overcome this limitation. 

Chemical reaction and mass transfer are two unique phenomena that help define chemical engineering. Chapter 8 described problems involving chemical reaction and mass transfer in a porous catalyst, and how to model chemical reactors when the flow was well defined, as in a plug-flow reactor. Those models, however, did not account for the complicated flow situations sometimes seen in practice, where flow equations must be solved along with the transport equation. Microfluidics is the chemical analog to microelectro-mechanical systems (MEMS), which are small devices with tiny gears, valves, and pumps. The generally accepted definition of microfluidics is flow in channels of size 1 mm or less, and it is essential to include both distributed flow and mass transfer in such devices. 

This review contains examples of microfluidic platforms for lab-on-a-chip applications which were selected as fitting to our platform definition and no comprehensiveness is claimed. The review should, however, provide the reader with some orientation in the field and the ability to select platforms with appropriate characteristics on the basis of application-specific requirements. 

THE NEED FOR THE MICROFLUIDIC PLATFORM APPROACH Definition of a Microfluidic Platform ... 

In this chapter, solutions for highly parallel assay processing are presented. These are not per se microfluidic platforms by our definition, since they do not offer a set of easily combined unit operations and are quite inflexible in terms of assay layout. They are nevertheless presented here, since the small reaction volumes per assay and partly the liquid control systems are based on microfluidic platforms. The significant market for repetitive analyses, which allows high development costs for proprietary, optimized systems, does not necessarily require a platform approach, but can benefit from microfluidic production technologies and liquid handling systems. 

Microfluidics is also a cross-disciplinary subject that uses the methods and principles of microelectronics to construct very small analogs or models of such macroscopic fluidic elements as wind tunnels, valves, or fluidic amplifiers. The natural question that comes to mind is at what dimensional scale does fluid motion depart from the extremely well understood and well established laws of fluid dynamics There is no definitive answer to that question yet since the study of fluid motion in microscale and nanoscale structures is still at an early stage. 

Between the definite regimes of laminar and turbulent flow there is a transitional Re range. The exact values of this number range are a function of many parameters, such as channel shape, surface roughness, and aspect ratio. The transition Re is generally expected to be in the range of 1,500 and 2,500 for most situations [39]. For microfluidic systems. Re are typically smaller than 100 and the flow is considered essentially laminar. This characteristic has a direct consequence on mixing within microfluidic devices. 

Laminar flow is the definitive characteristic of microfluidics. Fluids flowing in channels with dimensions on the order of 50mm and at readily achievable flow speeds are characterized by low Reynolds number. Re, defined as... 

Li and Harrison carried out the first cell assay in microchannels [2]. This seminal work made use of electrokinetically driven flow (electroosmosis and electrophoresis) to transport bacteria, yeast, and mammalian cells in channels and to implement low-volume chemical lysis (cell death). This theme of microfluidics-based cell transport, sorting, and lysis has continued to be a popular application, as well as related work in using microfluidics to culture cells and to pattern them into structures. The utility of these methods is acknowledged (and that they are featured in several good reviews [1] and other entries in the encyclopedia) but focuses here on describing microfluidics-based cell assays that fit the definition described above - application of a stimulus and measurement of a response. 

Extension to 2D and 3D Systems In the majority of microfluidic cases where 1/k is much smaller than the channel height, the Helmholtz-Smoluchowski equation provides a reasonable estimate of the flow velocity at the edge of the double layer field. As such when modeling two- and three-dimensional flow systems, it is common to apply this equation as a slip boundary condition on the bulk flow field. Since beyond the double layer by definition... 

Enclosed microchannels are by definition not accessible to laser desorption/ionization, which requires an open surface from which analytes can be sampled into the spectrometer. Several strategies have been adopted to circumvent this challenge, including the elution of bands of analytes from microfluidic devices onto an open substrate, where they are dried and analyzed. Alternatively, Musyimi et al. [11] employed a rotating ball to transfer analytes from polymer microchannels to a MALDl-MS system without compromising the vacuum required for mass spectrometry. 

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