Broadband Extraction of Sample Permittivity From Microwave Planar Transmission Lines

Electric fields and their interactions with living organisms and molecular components are central to the development of novel biomedical diagnostic and therapeutic techniques. The electric field interaction with biomaterials is dictated by their dielectric permittivity. However, progress in this field is often hindered by the need for large sample volumes to determine complex permittivity or by lengthy measurement times. Here, we introduce a universal method for extracting broadband complex dielectric permittivity, demonstrated using a conductor-backed coplanar waveguide (CBCPW). This method enables rapid, broadband characterization of microliter-scale (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula>L) biomolecular solutions in high-permittivity, water-based buffers across a broad frequency range. First, the technique is validated through <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i>-parameters obtained from full-wave electromagnetic simulations. It is then applied to experimentally measured <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i>-parameters of aqueous solutions containing specific biomolecules at varying concentrations. Comparisons with an independent reference method confirm the accuracy of the extracted permittivity values. Overall, this new method provides a fast, precise, and sample-efficient means of measuring broadband complex permittivity, demonstrating its potential as a powerful tool for biomedical research.