**Author details**

206 The Development and Application of Microwave Heating

to temperature gradient generation.

the gap width and thickness of the CPW conductors.

**7. Conclusions** 

potentially allowing live organisms and microwave transducers to co-exist on a lab-on-achip device. The approach explained to establish temperature gradients appears to be especially well suited for thermal gradient focusing methods for analyte separations of cell metabolites in lab-on-a-chip devices. Other potential applications of integrated microwave heaters include cell lysis and PCR (Liu et al., 2002), as mentioned previously. The localized nature of on-chip microwave heating means that separate microwave heaters optimized for these different tasks could also co-exist on a single lab-on-a-chip device. Therefore, it is believed that the technique outlined here will facilitate the application of microfluidics to other biological and chemical applications requiring spatial temperature gradients as well as

The ability to rapidly and selectively control temperature within microchannel environment is crucial for many fluidic applications including high-efficiency PCR and temperature induced cell lysing. In this chapter, localized microwave dielectric heating of fluids at micrometer scale geometry using integrated planar microwave transmission line as a power source has been presented. The measured temperature increases with increasing frequency is in agreement with theory. The device offers several advantages. It is simple, easy to use and fabricate. The planar structure of the transmission line used as a power source lends itself to easy integration with the microchannel and allows for optical integration of the microchannel with widely used fluorescence microscopy techniques. The performance of the CPW for heating could be substantially improved by: applying high power pulses over shorter heating times, using a lower thermal diffusivity substrate than glass, and increasing

A microwave power absorption model to understand power distribution through the device and to differentiate microwave heating of the fluid from conductive heating of the fluid because of microwave power absorption in thin-film CPW conductors has been presented. Based on the fitting of the experimental data using the power absorption model, it can be stated that the temperature rise of fluid is predominantly due to the absorbed microwave power. It is believed that this model can be useful for evaluating the performance of various complex and lossy transmission line configurations, such as CPW or microstrip lines, for heating fluid in the microchannel environment. Tt is believed that the microwave dielectric heating approach will be of particular use in rapid thermocycling applications and will lead to new applications exploiting heating in microfluidic environment. Such heaters are potentially very useful for single use, disposable, and integrated microfluidic systems.

The integrated microwave microstrip transmission line is also very attractive to generate temperature gradients rapidly and selectively in a microfluidic device. The shape of the temperature profile can be adjusted by varying the microwave excitation frequency and the amplitude of the profile can be adjusted by varying the microwave power. With this technique, the temperature gradients can be established locally and selectively by positioning the transmission line in the region of interest over the fluidic network. This Mulpuri V.Rao *Department of Electrical and Computer Engineering, George Mason University,Fairfax, VA, USA* 

Jayna J. Shah *National Institute of Standards and Technology, Semiconductor Electronics Division, Gaithersburg, MD, USA Presently at: Raytheon Company, Dallas, TX, USA* 

Jon Geist and Michael Gaitan

*National Institute of Standards and Technology, Semiconductor Electronics Division, Gaithersburg, MD, USA* 
