A novel sensor for in situ monitoring of the body reaction to implants has been developed. A piezoelectric wafer active sensor was adapted for biomedical applications (bio-PWAS). A number of bio-PWAS sensors have been implanted in rats and left in place up to 64 days. The bio-PWAS were able to oscillate in several resonance modes, radial-wise (in-plane) and thickness-wise (out-of-plane). The electromechanical impedance was measured over a wide frequency band, covering several radial vibration modes and the first thickness mode. The recorded data was processed with impedance spectroscopy methods. Preliminary results indicate a correlation between the electromechanical impedance spectrum of the bio-PWAS and the state of implantation. Quantitative studies have shown that the first radial mode amplitude seems to correlate with the short-time inflammatory and immune response, while the thickness mode amplitude seems to correlate with both the short-term inflammatory response and long-term encapsulation and fibrosis response. Since radial vibrations generate shear waves in the surrounding tissue, while thickness vibrations generate pressure waves, it seems that the shear and pressure wave interactions have specificity in detecting the different stages of the body's reaction response to implants. These observations were supported by histological examinations of the tissue surrounding the bio-PWAS. Though these initial results are encouraging, further experiments need to be conducted and more data needs to be collected in correlation with histological determinations. In-depth impedance spectroscopy studies should be conducted on this extensive data and closely correlated with extensive histological studies.