An implantable sensor includes a hydrogel, a glucose-binding protein and a reference molecule. The binding affinity of the reference molecule for glucose differs by at least a factor of ten from the binding affinity for glucose of the glucose-binding protein. At least one of the electromagnetic behavior and the fluorescent behavior of the glucose-binding protein and the reference molecule change when glucose is bound. An electrical functional adhesive bandage includes a measurement element for measuring at least one of electromagnetic properties and fluorescent properties. The bandage also includes a first communication element for wireless communication. Together, the implantable sensor, the bandage, and an evaluation device, which includes a computation unit, a display and a second communication element for wireless communication, form a kit for determining the blood sugar level in a patient.
