In this work, we show the realization of a fully-implantable device for monitoring free-moving small animals. The device integrates a microfabricated sensing platform, a coil for power and data transmission and two custom designed integrated circuits. The device is intended to be implanted in mice, free to move in a cage, to monitor the concentration of metabolites. We show the system level design of each block of the device, and we present the fabrication of the passive sensing platform and its employment for the electrochemical detection of endogenous and exogenous metabolites. Moreover, we describe the assembly of the device to test the biocompatibility of the materials used for the microfabrication. To ensure biocompatibility, an epoxy enhanced polyurethane membrane was used to cover the device. We proved through an in-vitro characterization that the membrane was capable to retain enzyme activity up to 35 days. After 30 days of implant in mice, in-vivo experiments proved that the membrane promotes the integration of the sensor with the surrounding tissue, as demonstrated by the low inflammation level at the implant site.