The nondestructive characterization of small liquid volumes, as well as the detection of single biological cells, can lead to new insights for biomedical and biotechnological applications. The dielectric permittivity of the sample, which depends on the chemical composition, is detected using electromagnetic sensors in the microwave frequency range. The sensing system proposed here is based on resonant near-field sensors, which are combined with a microfluidic chip. The sample volume of a few picoliters enables single-cell measurements. A novel process featuring a biocompatible polymer material is used for fabricating the microfluidic channel for liquid samples. Two opposed sensing tips are integrated at the bottom of the channel to improve the sensitivity to small dielectric contrasts. They are connected to two resonators, which allows to simultaneously characterize the sample permittivity at two different frequencies. In a proof-of-principle experiment, single cells of the Chinese hamster ovary cell line are detected. This shows that the proposed sensing system is suitable for single-cell characterization due to its high sensitivity and spatial resolution. A potential biomedical application is the systematic investigation of various cell types and states to elucidate potential distinguishing characteristics.
