Heat microfluidic systems

A fifth reason for using microfluidics in electrochemistry would be the possibility to combine flow chemistry with an ultrafast mixer, which allows the generation and subsequent use of short-lived reactive ions or radicals, for example, in a "cation flow" process (Suga et al., 2001 Yoshida, 2008). Finally, a sixth reason for performing electrochemistry in a microfluidic system may be the desire to efficiently remove reaction heat (or joule heat due to high currents in combination with a high ohmic resistance) in fast electrochemical reactions (Yoshida, 2008). 

GlaxoSmithKline Pharmaceuticals in Harlow, UK, performed the Hantzsch synthesis of 2-bromo-4 -methylacetophenone and l-acetyl-2-thiourea in NMP (N-methyl-2-pyrrolidone) using a microchip reador under EOF conditions [10] (for EOF see [11]) [10] This is claimed to be the first example of a heated organic reaction performed on a glass chip reactor under electroosmotic flow control, whereas only room temperature reactions were made earlier. In a wider scope, the Hantzsch synthesis is a further example to evaluate the potential of microfluidic systems for high-throughput... 

Imprinting into plastic materials can help to overcome two main disadvantages of silicon-based microfluidic systems expense of fabrication and brittleness of the material. Imprinting can be carried out at elevated temperatures [6,7] or at room temperature [8]. While heating of the plastic material can result in better feature aspect ratios, it is limited by the breaking of silicon templates during the cooling process due to the different thermal... 

As pointed out earlier, microfluidic systems have a wide range of applications, e.g. heat exchange systems for electronic devices [28-31], medical diagnostic and analytical chemical applications [69, 70], and precision dilution systems with minimal dead volume for gas chromatography [71]. More may be anticipated as the technology matures. Current research at many laboratories has shown the need to provide flow control at extremely low levels for sensor-controlled implanted drug delivery systems [72] and portable diagnostic cards for polymerase chain reaction analysis [73]. 

Gottschlich et al. [134] developed a microfluidic system that integrated enzymatic reactions, electrophoretic separation of the reactants from the products, and postseparation labeling of the proteins and peptides prior to fluorescence detection (see Fig. 12). Tryptic digestion of oxidized insulin p-chain was performed in 15 min under stopped flow conditions in a heated channel serving as the reactor, and the separation was completed in 60 s. Localized thermal control of the reaction channel was achieved using a resistive heating element. The separated reaction products were then labeled with naphthalene-2,3-dicarboxaldehyde (NDA) and detected by fluorescence detection. 

The highly ordered mass and heat transfer processes within microreactors often produce very selective reactions. As will be seen later this has implications for nanomaterial synthesis, but even within well-known reactions this selectivity can be important. Burns and Ramshaw [31] showed that nitration processes within microfluidic systems produce cleaner products than bulk scale systems. Similarly high yields and low by-product formation has been reported in on-chip peptide formation [32]. This is almost certainly attributable to the thermal flatness found within microfluidic channels and the ordered and predictable mixing within the system. It appears that reactions within the microfluidic regime are cleaner and often quicker, than their bulk equivalents. 

There is a clear trend today within the bioanalytical and biomedical fields toward more frequent use of cell-based studies. The dimensions of microfluidic systems are well matched to meet the demand on cell-based systems. Still, new methods are needed that can efficiently handle and manipulate cells in those formats. Examples have already been given in this chapter where acoustic forces are used to trap and manipulate cells. The device in Figure 44.22 has been forther developed for use in cell-based bioassays. The temperature characteristics of the device have been examined to be able to control the temperature during the cell experiments. The major source of heat in the acoustic resonance systems presented here is the power dissipation in the transducer itself. The power dissipation follows a... 

The final step in PCSL fabrication of microfluidic systems is removal of the sacrificial material to yield clean, smooth microchannel inner surfaces. Paraffin wax PCSL is removed by heating the microdevice to 85°C and applying vacuum to a reservoir in the PMMA cover piece to aspirate the liquid PCSL (Figure 51.2h). Nonpolar organic solvents such as hexane or cyclohexane can be used subsequently to dissolve and remove residual sacrificial material from inside the channel. With... 

In microfluidic systems where the EDL is thin compared to the characteristic microfluidic channel dimension, it can be approximated that the net charge density is zero and the ionic concentration is uniform throughout the channel domain. Furthermore, if the Joule heating effect is not concerned, we have /conv 0. diff and CT const. In addition, as discussed earlier the... 

The process flow for the fabrication of the microfluidic system includes a single or double metallization layer, a polymer layer for the fluidic system, and a glass sealing cap. There have been some efforts during fabrication to minimize the thermal-dissipation loss. The temperature difference between the two points where the sensors are located is measured with a differential current amplifier, and the flow rate is calibrated. At low flow rates, the temperature difference is a linear function of the flow rate as in Fig. 6. Measurements without heat insulation decrease the sensitivity of the flow sensor and increase the lower limit of flow rate detection. The distance between the heater and the sensors is optimized for the maximum differential temperature. 

Erickson D, Sinton D, Li D (2003) Joule heating and heat transfer in poly(dimethylsiloxane) microfluidic systems. Lab Chip 2 141-149... 

Joule Heating and Chip Materials, Fig. 3 Computational geometry for a PDMS/PDMS or PDMS/glass hybrid microfluidic system... 

Greater control over reactions. The diffusion paths for heat and mass transfer in microfluidic systems are very short, making such systems ideal candidates for heat- or mass-transfer-limited reactions. The surface-to-volume ratio of microscopic structures is very high. Thus, surface effects are likely to dominate over volumetric effects, increasing selectivity and yield. 

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