Microsystems microfluidics

Gas flows are encountered in many microsystems like micro-motors, micro-turbines, micro-sensors, and microfluidic systems in the presence of air or gas environment. Since the ratio of surface area to volume increases in such microsystems, surface forces become dominant over the body forces, and gas flows have great affects on the performance and reliability of many microdevices. 

Mapping of transport parameters in complex pore spaces is of interest for many respects. Apart from classical porous materials such as rock, brick, paper and tissue, one can think of objects used in microsystem technology. Recent developments such as lab-on-a-chip devices require detailed knowledge of transport properties. More detailed information can be found in new journals such as Lab on a Chip [1] and Microfluidics and Nanofluidics [2], for example, devoted especially to this subject. Electrokinetic effects in microscopic pore spaces are discussed in Ref. [3]. 

I.A. Ges, B.L. Ivanov, D.K. Schaffer, F.A. Lima, A.A. Werdich, and FJ. Baudenbacher, Thin-film IrOx pH microelectrode for microfluidic-based microsystems. Biosens. Bioelectron. 21, 248-256 (2005). 

There are three types of mass transport processes within a microfluidic system convection, diffusion, and immigration. Much more common are mixtures of three types of mass transport. It is essential to design a well-controlled transport scheme for the microsystem. Convection can be generated by different forces, such as capillary effect, thermal difference, gravity, a pressurized air bladder, the centripetal forces in a spinning disk, mechanical and electroosmotic pumps, in the microsystem. The mechanical and electroosmotic pumps are often used for transport in a microfluidic system due to their convenience, and will be further discussed in section 11.5.2. The migration is a direct transport of molecules in response to an electric field. In most cases, the moving... 

Microsystems are also expected to be introduced in the near future, including for example artificial noses, fingerprint sensing systems, bar code readers, rf-tag-ging systems, microfluidic pumps and dosing systems, gas flow control systems, new flexible and low cost displays or electronic paper. 

Some of these functions could be monitored with improved sensors, instruments and microsystems, like microspectrometers and color sensors, thermopiles, artificial noses, etc. Also some dosing and mixing functions (e. g. of herbs and spices) could be controlled by microfluidic systems. 

Electrochemical detection offers also great promise for CZE microchips, and for other chip-based analytical microsystems (e.g., Lab-on-a-Chip) discussed in Section 6.3 (77-83). Particularly attractive for such microfluidic devices are the high sensitivity of electrochemical detection, its inherent miniaturization of both the detector and control instrumentation, low cost, low power demands, and compatibility with micromachining technologies. Various detector configurations, based on different capillary/working-electrode... 

Electrophoretic microfluidic chips feature a number of microreactor characteristics and have been used for conducting chemical and biochemical reactions in channels and microfabricated chambers, mixing reagents, microextraction and microdialysis, post- and preseparation derivatizations, etc. The most recent achievements are reviewed in Ref. 63 and other similar publications. These integrated microdevices perform PCR amplification, cell sorting, enzymatic assays, protein digestion, affinity-based assays, etc. In this section we describe such integrated microsystems and the most recent advances in this field. 

More sophisticated in vitro systems exist, such as human on chip microsystems, in which various cell types are cultivated in different miniaturized chambers linked by a microfluidic network in which a cell culture medium is circulated to mimic blood flow through organs [5, 6]. However, even in that case, we need extrapolation modelling to scale up the data obtained to the target animal species (typically humans). A purely experimental approach would also be quite expensive if PK were to be predicted for various dosing schedules and levels. We will see in the following section how PBPK computational models can help. 

Chung AJ, Kim D, Erikson D. Electrokinetic microfluidic devices for rapid low power delivery in autonomous microsystems. Lab Chip 2008 8 330-338. 

The characteristic features of microsystems stem from the small size of the space in the microstructures. Therefore, microsystems are not necessarily small systems in total size. They can be large in total size as long as they contain microstructures that can be used for chemical reactions. This sharply contrasts with the concept of a lab-on-a-chip, which should be small in total size. It is also important to note that microsystems are normally set up as flow-type reactors with a constant flow of solutions through a microstructured reaction chamber or channel. Although the reactor s capacity at any one time is small, total production capacity over time is much greater than may be imagined. Therefore, microflow systems are not necessarily used solely to produce small quantities of chemical substances. In fact, a microfluidic device has been developed that fits in the palm of the hand but can produce several tons of a product per year (see Chapter 10). 

S. Kaka9 et al. (eds.), Microfluidics Based Microsystems Fundamentals and Applications, DOI 10.1007/978-90-481-9029-4 1, Springer Science + Business Media B.V. 2010... 

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