This contribution describes a sensor for GHz characterization of in vitro cell culture, that overcomes the limitation in high frequency of classical bio-impedance spectroscopy. The sensor we developed enables to monitor a thin layer of approximatively 30µm of cells in their liquid culture medium for a relative permittivity in the range of 5 to 55. Here, we demonstrate the measurement principle using a Finite Element model, which has been validated with experimental response. Our sensor has a minimal sensitivity of 0.3M Hz. We also used a multiphysics model to demonstrate that this technology does not affect tissue safety for input power level compatible with a sufficient measurement signal on noise ratio. This technology will be used in experiments aiming at the exploration of biomarkers for post-implantation inflammatory reactions of active devices.