**6. Microscale microwave applicators for microfluidic heating**

In recent years the interest in microwave heating lie in integrating miniaturized microwave heating elements, in the form of transmission lines, with microfluidic channels for on-chip heating applications. In one such case, microwave power can be delivered to an elastomeric microfluidic channel via an integrated thin-film coplanar waveguide (CPW) two-conductor transmission line. The rest of the chapter focuses on the work our group has done on this topic in the past 10 years.

### **6.1. Coplanar waveguide to deliver microwave power to the microfluidic channel**

A schematic of a prototype coplanar waveguide (CPW) coupled with the microfluidic channel is shown in Figure 5. Typical dimensions are given in Figure 6. In this system a defined poly(dimethylsiloxane) (PDMS) microfluidic channel is aligned in the gap between the signal plane and one of the ground planes. The reason for choosing the CPW for delivering the microwave power to the microfluidic channel is that it can be easily formed on a substrate using a lift-off lithography process and its planar nature allows for easy integration with the microfluidic channels. The planar geometry also allows fabrication of an array of these devices on a common substrate with high yield, which later can be separated into individual devices.

The dimensions of the center conductor, the gap between the center conductor and the ground plane, the thickness and effective dielectric constant of the substrate determine the characteristic impedance (ZO) and effective dielectric constant of the line. Since, the CPW is a two-conductor transmission line with different dielectrics above and below the device plane, it supports a quasi-TEM mode as well as surface wave modes (Riaziat et al., 1990). The surface wave modes lead to energy transfer from the guided wave into the substrate leading to the attenuation of the guided wave. The CPW design solution is optimized to avoid all potential problems associated with the surface waves in choosing the substrate thickness less than the dielectric substrate wavelength, λd (Riaziat et al., 1990). For the design features shown in Figure 6, the substrate thickness is one-sixth of the λd value. The conductor loss in CPW is inversely proportional to the skin depth and metallization thickness. This leads to increased conductor attenuation with increasing microwave frequency and decreasing conductor thickness, however, the effect of conductor thickness on the conductor attenuation is not as dramatic as that of frequency. For a glass substrate, the dielectric attenuation (Gupta et al., 1996) is very insignificant compared to the conductor attenuation and can be ignored.

**Figure 5.** Schematic of coplanar waveguide (in orange) coupled with the microchannel structure (in pink)

**Figure 6.** Schematic of a cross-section of a coplanar waveguide (CPW) transmission line integrated with a poly(dimethylsiloxane) (PDMS) microchannel for microwave dielectric heating of fluids. The CPW conductors are 1.5 cm long; the signal conductor is 140 μm wide and it is separated by a 25 μm gap on either side from 300 μm wide ground conductors. The microchannel consists of a trapezoidal cross section defined by a silicon template and it is 7 μm deep, 25 μm wide at the bottom, and 3.62 mm long.

The CPW conductors are formed of Cr/Au on the substrate using standard lift-off lithography metallization process. Thickness of the CPW conductors is increased by electroplating of Au. A trapezoidal cross-section PDMS microfluidic channel is defined by using a silicon template. The PDMS is poured into the silicon master and cured for 4 hours at 65 C for this purpose. The PDMS structure is then detached from the silicon master and holes are punched into the PDMS structure for creating fluid inlet and outlet for the channel. The PDMS microchannel is then carefully aligned and attached by contact adhesion in the lateral direction over the gap between the signal-ground metal lines of the CPW. For maximum electric field coupling, the width of the PDMS microchannel is made equal to the CPW signal-ground metal spacing. According to computer simulations about 50% of the electric field is contained within 10 microns from the CPW surface. Hence, the depth of the microchannel should be designed accordingly to couple as much of the electric field as possible to the microfluid in the channel.

188 The Development and Application of Microwave Heating

and can be ignored.

pink)

to the attenuation of the guided wave. The CPW design solution is optimized to avoid all potential problems associated with the surface waves in choosing the substrate thickness less than the dielectric substrate wavelength, λd (Riaziat et al., 1990). For the design features shown in Figure 6, the substrate thickness is one-sixth of the λd value. The conductor loss in CPW is inversely proportional to the skin depth and metallization thickness. This leads to increased conductor attenuation with increasing microwave frequency and decreasing conductor thickness, however, the effect of conductor thickness on the conductor attenuation is not as dramatic as that of frequency. For a glass substrate, the dielectric attenuation (Gupta et al., 1996) is very insignificant compared to the conductor attenuation

**Figure 5.** Schematic of coplanar waveguide (in orange) coupled with the microchannel structure (in

**Figure 6.** Schematic of a cross-section of a coplanar waveguide (CPW) transmission line integrated with a poly(dimethylsiloxane) (PDMS) microchannel for microwave dielectric heating of fluids. The CPW conductors are 1.5 cm long; the signal conductor is 140 μm wide and it is separated by a 25 μm gap on either side from 300 μm wide ground conductors. The microchannel consists of a trapezoidal cross section defined by a silicon template and it is 7 μm deep, 25 μm wide at the bottom, and 3.62 mm long.

Microwave power is applied at the input port of the CPW using a signal generator. The frequency of the microwaves can be varied for optimum coupling of the microwave power to the microfluid. S-parameters of the system are measured for empty channel case and also for liquids of varying ionic concentrations. The fluid temperature is obtained by measuring the temperature dependent fluorescence intensity of a dilute fluorophore dye added to the fluid and comparing it to the calibrated fluorescence intensity at a known temperature. Rhodamyne B is used as the dye solution for this purpose.
