Microfluidic flow-over designs

Micro fuel cell designs without polymeric membranes can overcome some PEM-related issues such as fuel crossover, anode dry-out or cathode flooding. In these membraneless laminar flow-based fuel cells (LF-EC) two or more liquid streams merge into a single microfluidic channel. The stream flows over the anode and the cathode electrodes placed on opposing side walls within the channel. The reaction of fuel and oxidant takes place at the electrodes while the two liquid streams and their liquid-liquid interface provide the necessary ionic transport [122,123]. 

Microfluidics is a concept that describes the science and technology of design, fabrication and operation of systems of microchannels that conduct liquids and gases. T q)ically, the channels have widths of tens to hundreds of micrometers and the speed of flow of the fluids is such that the viscous forces dominate over inertial ones. The resulting - linear - equations of flow and its laminar character provide for extensive control the speed of flow obeys the simple Hagen-Poiseuille equation that relates the speed linearly to the pressure drop through the particular channel and to its inverse hydraulic resistance, which in term is a function of the dimensions of the channel and the viscosity of the fluid. This property, when combined with t5q)ically large values of the Peclet number [1] that reflect the fact that diffiisional transport is t5q)ically slow in comparison to the flow, it is possible to control the profiles of concentration [2] of chemicals and... 

The first immunoassay performed in a capillary driven system was reported in 1978 [67]. Based on this technique, the commonly known over-the-counter pregnancy test was introduced into the market in the middle of the 80 s. Today, this microfluidic platform is commonly designated as a lateral flow test (LAT) [14]. Other terms are test strip , immunochromatographic strip , immunocapillary tests or sol particle immunoassay (SPIA) [68]. Astonishingly, hardly any publications from a microfluidic point of view or in terns of material classification exist, and apparently many company secrets are kept unpublished [69]. 

Another unique silica-based approach to microscale DNA extraction currently underdevelopment utilizes a serpentine channel design, combined with an immobilized silica-bead solid phase and fluidic oscillation. This method, developed by Chung et al., relies on silica beads immobilized on the plasma-oxidized surface of the polymethylmethacrylate (PMMA) channels, instead of a packed-silica solid phase, as depicted in Figure 43.1c. Following bead immobilization, the solutions required for DNA binding, purification, and release are flowed back-and-forth through the device. This fluidic oscillation over the immobilized phase results in marked improvement of recovery and extraction efficiency over the same extraction methods with free beads. This method represents yet another variation of silica-based purifications that has been accomplished in microfluidic systems, exploiting previously optimized chemistries. In summary, the development of macroscale, commercial, silica SPE protocols has enabled the facile translation of DNA, and now RNA, extraction into microfluidic systems for a variety of applications. 

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