Microchannels system

Figure 11.1 Scale-up versus numbering up for microchannels. All critical channel dimensions remain constant in a microchannel system independent of the overall process capacity.

Only very few studies with alternative materials and fabrication methods have been published. Ekstrom et al. [35] demonstrated the feasibility of structuring inexpensive polymeric materials by means of a microfabricated master for the production of microchannel systems. The structured polymer film was mechanically clamped between two glass plates to form a closed channel system. Recently, a similar route for the fabrication of microchannel chips that relies on casting of an elastomeric polymer material against a microfabricated master has been presented by Effenhauser et al. [36] (see Sect. 3.4). 

FIGURE 2.15 A microchannel system in abasketweave pattern fabricated by three PDMS layers, (a) Optical micrograph of the middle PDMS layer, which contains the entire 8 x 8 channel system. The channels are 100 pm wide and 70 pm high, (b) Optical micrograph of a portion of the enclosed, fluid-filled channels. Channels in the up-down direction are filled with a solution of fluorescein and channels in the left-right direction are filled with a solution of Meldola s Blue dye [180]. Reprinted with permission from the American Chemical Society. 

FIGURE 2.17 A five-layer stacked PDMS microchannel system, (a) Schematic of a channel path structure, (b) High-magnification optical photograph of a vertical channel path [175]. Reprinted with permission from the Institute of Physics Publishing. 

Hofmann, O., Niedermann, P., Manz, A., Modular approach to fabrication of three-dimensional microchannel systems in PDMS — application to sheath flow microchips. Labchip 2001, 1, 108-114. 

Griffiths, S.K., Nilson, R.H., Modeling electrokinetic transport for the design and optimization of microchannel systems. Micro Total Analysis Systems, Proceedings 5th [lTAS Symposium, Monterey, CA, Oct. 21-25, 2001, 456 158. 

Figure 1.13 Diffusion bonding process chain. Starting top left stacking, diffusion bonding furnace with mechanical pressure force, diffusion bonding and a cut through a microchannel system after diffusion bonding.

In order to understand the fluid mechanics in microchannel systems one has to calcrdate the Reynolds numbers in a given system. The dimensionless Rejmolds number (Re) is the ratio of the inertial forces to the viscous forces acting on a small element of fluid, and can be seen as the ratio of shear stress due to turbulence to shear stress due to viscosity. 

Finally, microfilters are described as part of the microchannel system in order to assist sample preparation. Channels with diameters less than 30 /rm can easily be blocked by particulates in the sample, or crystals formed in solutions held in a micro-reservoir. While prefiltration outside of the microchannel system can separate most particles out of the sample solution prior to addition to the biosensor system, volumes required for macrofiltration are much larger than the volume finally applied to the biosensor. In addition, crystal or aggregate formation inside the channel cannot be avoided. Thus, the use of microfilters inside the microchannel system will be an important element in miniaturized biosensors. Filters have been described for trapping cells from blood [53], percolation filters for filtering solvents containing particulate materials ranging from dust to cells [54], and nanofilters that can separate particles as small as 44 nm [55]. 

Regarding optical transducers, a small prism can, for example, be coated with a thin gold layer in order to construct a SPR transducer, which subsequently is connected to a microchannel system. Also, optical fibers are... 

A different approach of site-specific polymerization and thus immobilization of a biorecognition element was shown by several research groups by utilizing electropolymerization [74-76]. In combination with microelectrodes in microchannel systems, a site directed simple immobilization of the biorecognition element could be achieved. Patterns can also be created by... 

As a flnal example, it should be pointed out that it is also possible to circumvent the need of immobilization of the biorecognition element in the microchannel system. Instead, it can be immobilized on superparamagnetic beads, silica beads, latex particles, etc. [79-81]. These beads are applied together with the sample into the microchannel system and can be collected on or near the transducer via magnetic or membrane separation. 

Due to the small volumes and feature sizes, reaction rates are found to be quite different in the microbiosensor system in comparison to their macro counter part. Most of this is due to the fact that diffusion is not the limiting factor in a reaction any longer. For example, the diffusion time of a particle with a diffusion coefficient = 10 m s is 15 min to travel a distance of 1 mm, but only 10 s to travel 100 /rm and only 0.1 s to travel 10/rm [37]. Therefore, DNA hybridization reactions, antibody-antigen binding events, and enzyme—substrate catalytic reactions take place in a fraction of the time required earlier. DNA hybridization can be accomplished in a matter of seconds in a microchannel system, while it takes in the order of an hour when employing standard Northern or Southern Blotting techniques with a piece of nylon membrane soaking in several milliliters of hybridization solution. 

Griffiths SK, NUson RH. Low-dispersion turns and junctions for microchannel systems. Anal Chem 2001 73 272-8. 

Pertinent references are given throughout, and the reader is referred to the primary works for more details. Where applicable, previous reviews of the subject matter are cited. As with reactive microchannel systems, the centers of development for the technology and applications discussed here tend to be in Europe (mainly Germany) [15], the United States [16], and Asia (mainly Japan) [17]. 

Fig. 7.10 Geometry of the microchannel systems employed by Nakajima and coworkers [137],...

In addition to the enhanced speed of synthesis, a microchannel system also provides a potential separation column and a non-turbulent environment for partition between solvents. Integration of a microreactor device, via purification, to one of the many highly sensitive microchannel-based biological assay systems would enable the compounds to be screened. Apart from the greatly reduced reaction times demonstrated for the microreactors, handling times to assay and chemical reagent costs would be virtually eliminated as shown diagrammatically in Fig. 14.5. 

While this project is relatively new, significant progress has been made regarding the component design and modeling, sorbent and catalyst development and microchannel system development tasks. We will continue to focus on these tasks and microcombustor/vaporizer development. In addition, methods will be demonstrated to integrate catalysts into the micro-reactors. 

Frame, M. D., Chapman, G B., Makino, Y., and Sarelius, I. H., Shear stress gradient over endothelial cells in a curved microchannel system. Biorheology, 35, 245,1998. 

Cell Culture in Microchannels MicroChannel systems can be used to generate continuous gradient of some growth factors in order to study the optimal proliferation and differentiation conditions of some cells. They can also be used to micropattem the cell culture surface area. Cells cultivated in microchannels are used to study cellular properties within fluidic environments, especially the impact from shear stress. For example, the geometry of the microchannel affects cell growth, orientation, and phenotype control of vascular smooth muscle cells. Smooth muscle cells cultured in wider... 

Unless the coalescence of the interfaces is the key operation for droplet merging, additional helper functionality is required to bring the target droplets into pairwise proximity for merging at a distinct position of microchannel system. 

As mentioned, nanofluids exhibit unusual thermal and fluid properties, which in conjunction with microchannel systems provide enhanced heat transfer performance in heat transfer and fluid flow. For example. Wen and Ding [23] reported a considerable convective heat transfer augmentation when employing 7-AI2O3 nanoparticles in water flow in a copper tube based on their experimental results. The test Y-AI2O3 nanoparticles had a size range of 27-56 nm. Figure 9 depicts the local heat transfer coefficient vs. axial distance from the entrance of the test section, which clearly shows that the enhancement of the local heat transfer coefficient... 

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