We present a new optical biosensor concept based on hydrogel waveguides with integrated fluorescent proteins developed as a platform for implantable sensors. In this prototype the sensor detects the presence or absence of calcium through changes in fluorescence resonance energy transfer (FRET) induced by conformational changes of a FRET protein. The protein is immobilized in a synthetic hydrogel matrix with cylindrical shape, which serves simultaneously as an optical waveguide for the excitation light. The specificity of the protein determines which molecule can be detected.