We present a novel force sensor for cell indentation and cell injection. This force sensor is a monolithic structure that integrates two resonators. It provides a surface sensitive to out-of-plane forces where a living cell can be conveniently placed for manipulation. Normal forces applied upon the cell under study are estimated via frequency shifts of the resonators. In this paper, we develop a theoretical study for predicting and optimizing the structure's sensitivity. As a proof of concept, we also report the fabrication and experimental characterization of a first prototype. In ambient conditions, our prototype presently offers a quality factor of ~700, and a linear sensitivity of ~5.75 Hz/??m. In addition, we report the implementation of a compact and low-cost optical fiber setup to monitor the resonators' frequency. Potential applications are illustrated with the measurement of forces applied on lobster eggs.