A device for measuring impedance in a subject, the device being configured to be implanted within the body of the subject and being configured to measure impedance within a body tissue of the subject resulting from an electrical current flowing through the body tissue, wherein the body tissue is sub-dermal or subcutaneous tissue of the subject, comprising: one pair of injection electrodes configured for injection of electrical current into the body tissue, wherein the electrical current is passed from one of the injection electrodes to the other of the injection electrodes through the body; one pair of sensing electrodes configured to detect the resulting voltage caused by the current flowing between the pair of injection electrodes and through the body tissue; a detector operatively connected to the sensing electrodes and configured to receive the voltage detected by the sensing electrodes, wherein the detector is configured to measure the impedance of the body tissue based on the voltage detected by the pair of sensing electrodes; a microcontroller operatively connected to the detector and being configured to receive impedance signals from the detector and to provide control signals to the current signal output circuit; a current signal output circuit operatively connected to the microcontroller and the injection electrodes and being configured to provide electrical current at predetermined frequencies to the injection electrodes; and a powering and communication circuit including a coil configured to be powered by an electromagnetic field produced by an external coil the powering circuit being operatively connected to the microcontroller and configured to power the microcontroller, the current signal output circuit and the detector; wherein the microcontroller is programmed to determine a glucose level in the subject by correlating the measured impedance with a predetermined relationship between impedance and blood glucose levels, and wherein the device for measuring impedance in a subject is a 2 kHz to 2-MHz bio-impedance sensor application-specific integrated circuit (ASIC), which is designed in 150nm CMOS technology and consumes 165μA at 1.8V when powered by an external reader. one pair of injection electrodes configured for injection of electrical current into the body tissue, wherein the electrical current is passed from one of the injection electrodes to the other of the injection electrodes through the body; one pair of sensing electrodes configured to detect the resulting voltage caused by the current flowing between the pair of injection electrodes and through the body tissue; a detector operatively connected to the sensing electrodes and configured to receive the voltage detected by the sensing electrodes, wherein the detector is configured to measure the impedance of the body tissue based on the voltage detected by the pair of sensing electrodes; a microcontroller operatively connected to the detector and being configured to receive impedance signals from the detector and to provide control signals to the current signal output circuit; a current signal output circuit operatively connected to the microcontroller and the injection electrodes and being configured to provide electrical current at predetermined frequencies to the injection electrodes; and a powering and communication circuit including a coil configured to be powered by an electromagnetic field produced by an external coil the powering circuit being operatively connected to the microcontroller and configured to power the microcontroller, the current signal output circuit and the detector; wherein the microcontroller is programmed to determine a glucose level in the subject by correlating the measured impedance with a predetermined relationship between impedance and blood glucose levels, and wherein the device for measuring impedance in a subject is a 2 kHz to 2-MHz bio-impedance sensor application-specific integrated circuit (ASIC), which is designed in 150nm CMOS technology and consumes 165μA at 1.8V when powered by an external reader.