Microfluidic thermal heating

R.M. Cotta, S. Kakag, M.D. Mikhailov, F.V. Castellos, and C.R. (Tardoso, Transient flow and thermal analysis in microfluidics, Microscale Heat Transfer-Fundamentals and Applications in Biological Systems and MEMS, edited by... 

The use of a focused infrared laser beam on a surface results in local heating, which has been applied to synthesize MIPs locally by thermal polymerization [72]. In this case, a common thermo-initiator, azobisisobutyronitrile, was used together with a standard MIP recipe, to generate MIP microdots in a microfluidic device. 

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... 

Sanchez, C., Pouloit, M., Renard, D., and Paquin, P. (1999). Uniaxial compression of thermal gels based on microfluidized blends of WPl and heat-denatured WPl. /. Agric. Food Chem. 47,1162-1167. 

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. 

The concept of polymer-based 3D microstructure utilizes replica molding. Therefore, the key fabrication processes include 3D mold fabrication and pattern transfer. The material property is a very important factor when choosing the polymer and mold materials for special applications. For easy handling and accurate shape transfer, the mold requires smooth surface and relatively low thermal expansion coefficient since the curing of the polymer often involves heat. Nowadays, several polymer materials such as PDMS, PMMA, and SU-8 are popular choices for microfluidic applications. 

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. 

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