Polyimide Coplanar Waveguides for Broadband Dielectric Spectroscopy to Monitor Effects of Long-Term Fluid Exposure

Polyimide thin films are commonly used as encapsulation and insulation material in implantable medical devices. Fluid transport can result in unwanted electrical or ionic pathways, adversely affecting device function. However, it is unknown how fluid transport in or under polyimide films contributes to the composite broadband dielectric properties. Notably at microwave frequencies, dielectric loss contributed by the fluid is large due to the characteristic relaxation frequency of water (similar to 20 GHz). We fabricated polyimide coplanar waveguides and immersed them in deionized water or phosphate-buffered saline and measured the frequency-dependent, broadband dielectric properties of the films subject to fluid exposure across 5 months. We observed changes in broadband permittivity (similar to 3.1 to 3.25) and peak loss tangent (0.006 to 0.011), tracking the dispersive dielectric properties of water. We observed no film delamination in the span of the study. Ultimately, these results suggest that fluid transport has a considerable effect on film dielectric properties and integrity. Quantification of such effects are crucial for evaluating the long-term use of polyimide in wireless, implantable, biomedical micromechanical systems (MEMS) that may operate at microwave frequency ranges (e.g., 2.45 GHz).