In this work, we present first tests of a new implantable sensor concept based on acoustic metamaterials and ultrasound interrogation tested with animal tissue. The metamaterial sensor is passive and measures temperature with high sensitivity. The sensing performance is characterized in the [36], [42] °C physiological temperature range and it is also compared in presence of tissue mimicking materials (phantoms), as well as animal tissue. The high temperature sensitivity, observed without tissue (S= -3.3.10-³K-¹), is almost unaffected by the presence of tissue (S= -3.5·10-³K-¹). The temperature resolution degrades as expected in comparison to the experiment without tissue, but still reaches a value of 0.18 K. We also investigate two approaches to address challenges associated with sensor spatial localization under scattering tissue. A simple time-windowing approach is found to be lower in performances compared to a spectral approach in both ratio of positives (30.2% vs 65.80/0) and true positives (19.3%) detected values. This metamaterial-based sensing approach opens the way to a new class of passive temperature sensors for biomedical applications.