Broadband permittivity measurements of liquid and biological samples using microfluidic channels

We have developed an integrated microfluidic-microelectronic measurement platform in order to accurately measure the dielectric properties of sub-mu L volumes of liquids at frequencies up to 40 GHz. We use on-wafer calibration techniques to obtain accurate S-parameter measurements of fluids and biological samples in solution contained within microfluidic structures directly integrated with patterned coplanar waveguide transmission lines. Our analysis yields the capacitance and conductance per unit length of our fluid-loaded transmission lines, and finite element simulations yield the relative permittivity as a continuous function of frequency up to 40 GHz. We demonstrate the technique with measurements of the complex permittivity of de-ionized water and methanol. Results for the frequency-dependent permittivity obtained in our microfluidic test structures show good agreement with bulk fluid measurements over a comparable frequency range. We expect this technique to be useful for rapid electromagnetic characterization of sub-microliter volumes of both liquids and biological samples in solution.