Foreign body responses and bio-fouling caused by the physiological environment impair sensor performance due to alteration of the sensor/tissue interface. For in vivo applications longterm stability is a critical prerequisite and often affected due to host response towards the implant. In order to assess tissue response towards implants, we propose continuous measurements at the implant/tissue interface employing a microsensor device placed in contact with the chorioallantoic membrane (CAM) of the avian embryo. We introduce a biostable microsensor implant (MSI) to measure oxygen, pH and electrical impedance in situ. These parameters were chosen for their sensitivity with respect to the composition and properties of biological tissue. Micro fabrication technology in combination with electrochemical electrode functionalization was used to combine all sensors in a small planar array. The chorioallantoic membrane assay (CAM-assay) of avian ex ovo cultures served as a quasi-in vivo environment. Here we established an immune active and -deficient in vivo model, enabling comparison between weak and strong immune responses in the same organism. A miniaturized potentiostat unit (“MiniPot”) was developed for controlling the MSI in humid culture environments. Here we performed continuous measurements of all sensor parameters at the implant/tissue interface with the microsensor device placed in contact with the CAM of the avian embryo.