An electromagnetic wave sensor for determining an interstitial fluid parameter in vivo comprises an implantable housing, and a sensor component hermetically encapsulated within the implantable housing. The sensor component comprises an electromagnetic wave transmitter unit configured to emit an electromagnetic wave signal in a frequency range between 300 MHz and 3 THz penetrating the implantable housing and penetrating an interstitial fluid probe volume, an electromagnetic wave receiver unit configured to receive the electromagnetic wave signal modified by the interstitial fluid probe volume, and a transceiver unit configured to transmit radio frequency signals related to the electromagnetic wave signal modified by the interstitial fluid probe volume. A system for determining an interstitial fluid parameter in vivo comprises the electromagnetic wave sensor and an external reader.