MicroChannel architecture

Micro heat exchangers based on microchannel architecture. Top left and bottom Reprinted with permission from W. Ehrfeld et al., Microreactors New Technology for Modern Chemistry. Wiley-VCH Weinheim, Germany, 2000. Top right Courtesy of the Institut fuer Mikrotechnik Mainz, GmbH. 

The microchannel emulsion technique has been extended to the formation of multiple emulsions [158-163], encapsulation [123, 158, 164—166], polymer bead formation [123, 125, 167-169], demulsification [116, 158, 170], and even microbubble formation [171]. New methods of stabilizing emulsions have also been investigated in this realm, including particle-stabilized [172] and protein-stabilized emulsions [173], with some work in emulsification without surfactants [135,146]. In the case of multiple emulsions, microchannel architecture can enable the formation of W/O/W emulsions in which two water droplets of different compositions can be encased in the same oil droplet [163]. 

Fig. 7.12 MicroChannel architecture for membrane emulsification. (Courtesy of Velocys, Inc.)...

Pervaporation can also be used as a micropump. Isopropanol was placed inside the polyimide membrane microchannels described above, and water was deposited on top of the permeation area. The large selectivity for water transport over alcohol resulted in water permeation at a rate of 70 pm s [263]. In a similar study, a micropump was developed using pervaporation to transport a volume of 300 pL of Ringer s solution out of a membrane over a 6-day period. Capillary forces then induced additional flow into the membrane device to produce a very constant flow of 35 nL min [265]. Although this device did not utilize microchannel architectures, the low fabrication costs and high reliability of such a system make pervaporation an attractive approach to pumping small flow rates for microfluidic devices. 

Researchers at Oregon State University have demonstrated the advantages of microchannel architecture in improving the hemodialysis process. Using microchannel architectare, they were able to show 70-80% reductions in the necessary transfer area relative to commercial hollow fiber systems for the clearance of creatinin (Fig. 7.23) and urea (Fig. 7.24) from a simulated blood stream [285]. The microchannel advantage, as has been seen in other applications, comes in the form of well-defined and narrow channels that facilitate rapid mass transfer into and out of the fluid media. This approach is expected to change the current paradigm in hemodialysis from clinical treatment to at-home use, and may allow for the creation of a wearable hemodialyzer [286]. 

Chapter 2, by Ariga, Ji and Hill, presents recent developments on the application of the layer-by-layer technique for encapsulating enzymes. Encapsulation strategies are demonstrated for enzymes in both thin film and particle formats to generate complex enzyme architectures for microreactions. The integration of such systems into advanced biodevices such as microchannels, field effect transistors and flow injection amperometric sensors is also presented. 

Given the recent emergence of microfluidic fuel ceU architectures, only a few microfluidic biofuel cells with biological catalysts have been reported to date. The first microfluidic biofuel cell with enzymes immobilized in a microchannel was introduced by Moore et al. [8]. Alcohol dehydrogenase (ADH) enzymes were immobilized onto a carbon microelectrode placed in a 100 pm deep microchannel. A thin film of methylene green, which is an... 

Similarly to partially overlapping channels, microchannels with mesh contactors (Figure 7.2h) are used to create the partial contact of fluids. The advantage of these contactors is that both modes of operation, cocurrent and countercurrent, can be apphed. Besides, the flow is stabilized because of the solid support between two fluids. The solid contactors are porous membrane [9, 10] and metal sheets with sieve-like structure [11]. Similarly to parallel flow, the mass transfer in both cases is only by diffusion and the flow is under laminar flow regime dominated by capillary forces. The membrane contactor has the advantage of being flexible with respect to the ratio of two fluids. In addition to flow velocities, the mass transfer is a function of membrane porosity and thickness. In another type of microextractor, two microchaimels are separated by a sieve-like wall architecture to achieve the separation of two continuous phases. However, the hydrodynamics in both types of contactors is more complex because of interfadal support and bursting of fluid... 

D. Qiu, T. Dritz, Superior emulsions in microchannel architecture, Chem. Eng. 2007, March, 38-40. 

Researchers at the Korean Institute of Energy Research developed a methanol fuel reformer [36]. The system included a vaporizer and reformer based on sheet architecture with etched microchannels. The microreactor consisted of a vaporizer and a reformer connected serially. The prototype generated sufficient hydrogen for a power output of 15 W. 

Lerou JJ, Tonkovich AL, Silva L, Perry S, McDaniel J. MicroChannel reactor architecture enables greener processes. Chemical Engineering Science 2010 65 380-385. 

Electrowetting, Applications, Figure 7 Experimental setup of the microchannel-like flow type electrowetting device of Satoh et al. [19]. Panels (a) and (b) are cross-sectional views of the electrode and flow channel geometry, and, panel (c) shows the architecture of the device (from [19])... 

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