Accurate Characterization by Dielectric Spectroscopy up to 25 GHz of Nano-Liter Range Liquid Volume Within a Microfluidic Channel

This paper reports the design, simulation and experimental validation of an accurate liquid sensing technique in the nano-liter range. It is suitable for the detection and quantification of very small contents of liquid samples containing biological cells. The biosensor is based on an open-ended interdigitated capacitor (IDC) within a microfluidic channel. The microwave structure including the IDC is patterned in coplanar waveguide technology. The microfluidic channel is $100 \mu \text{m}$ thick and the IDC covers an area of $150 \mu \text{m}$ width and $90 \mu \text{m}$ length. This leads to a volume under analysis much lower than a nano-liter which allows the noninvasive and contactless microwave investigation of biological cells in their culture medium. This micro-device is used to extract the complex permittivity of liquid media from the measured scattering parameters within the frequency band ranging from 0.2 to 25 GHz. For ease of use, a SMA-connectorized version of the device with spring contacts probes to the biosensor is also proposed. Results concerning microparticle detection within the media are also presented, extending the use of the sensors to cellular environments.