**6.2. Integrated microfluidic device for generating microwave-induced temperature gradients using a microstrip waveguide configuration**

Figure 7 shows a cross-sectional schematic and a picture of the microwave-heated microfluidic device for generating temperature gradients. The devices are fabricated using an adhesive copper tape on cyclic olefin copolymer (COC) using photolithographic procedures. A CNC milling machine is used to cut the substrate material to precise chip dimensions and to carve out the microchannel and the fluidic access ports. For the device in Figure 7 the microchannel is 340 *μ*m wide, 7 mm long and is machined all the way through a 300 *μ*m thick COC substrate. The channel is positioned 1 cm away from the front of the device. The signal line is 370 *μ*m wide and 5 cm long. The signal line and the ground plane are patterned on the copper tape after the tape is fixed on the top and bottom surfaces of the COC substrate. The tape has a 40 *μ*m thick acrylic adhesive on a 35 *μ*m thick copper foil. The acrylic adhesive serves as a cover plate for the microchannel and isolates the fluid from the copper electrodes. A slit, 100 *μ*m wide and 2 cm long, is formed in the ground plane to allow optical detection of the fluid for temperature measurement. The dimensions of the microchannel, the transmission-line and the slit were determined from electromagnetic simulations performed using Sonnet Software. The slit is designed to run parallel to the microchannel, and it is positioned in the middle of the microchannel (Figure 7(*b*)) with a photolithographic process. Such positioning allows for spatially resolved, optical fluidtemperature measurements throughout the length of the microchannel and laterally in the middle of the channel. Two 2.4 mm end-launch connectors are mounted on the device (Figure 7(*a*)) to convert from the microstrip geometry to the coaxial geometry of the test equipment.

**Figure 7.** (a) A picture of the integrated microfluidic device for generating microwave-induced temperature gradients. (b) A cross-sectional view of the microwave heating device.

The method described here for fabricating conductors is easily transferable to other microfluidic substrates such as glass and PDMS. Furthermore, the one-step method for conductor fabrication obviates the need for electroplating, which is typically required following thin-film deposition to achieve sufficient conductor thickness. This method also provides easy bonding of the top and bottom cover plates to create enclosed channel structures, which has proven challenging for thermoplastic materials (Shah et al., 2006). In contrast to previously published reports (Shah et al., 2007a; Booth et al., 2006; Facer et al., 2001), the transmission line structure isolates the fluid from the metal conductors making these devices suitable for a variety of biochemical applications in which reagent contamination due to electrolysis or corrosion is undesirable.
