We present a novel force sensor for the injection of both suspended and adherent cells. Unlike most configurations, this force sensor is independent of the tool interacting with the cells. It is a planar structure that provides a surface sensitive to out-of-plane forces where living cells can be placed for manipulation. It also integrates two beam resonators. Forces perpendicular to the sensor's plane 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 report the fabrication and characterization of a first prototype designed for the injection of spherical cells with a diameter of ~100-600 μm. In air, our prototype presently offers a quality factor of 700, and a linear force sensitivity of ~2.6 Hz/mN. The measurement of forces applied upon lobster eggs is also experimentally demonstrated.