An implantable bioimpedance monitor using 2.45 GHz band for telemetry

This paper describes a multi-frequency single-channel electrical implantable bioimpedance monitor (35 mm x 35 mm x 10 mm, weight 52 g) powered by a NiMH battery. By using the tetrapolar method and injecting 10 mu A(peak), the monitor is capable of measuring at 14 different frequencies, from 100 Hz to 200 kHz. It contains a ZigBee transceiver to monitor the measurements performed, and has an embedded memory for backing up the data. RC networks and in-situ heart excised tissues were used to test the system. When measuring a full spectrum every 5 min, 35 days of autonomy are possible due to the low power consumption of the monitor. Temperature drift was estimated by short-term and long-term measurements. Temperature cycling was used to measure modulus and phase angle stability. The result was a very low effect on a modulus decrease of 2.34 Omega, with respect to an impedance of 322 Omega, at 100 Hz and a phase angle increase of 1.1 degrees, at 200 kHz. In addition, measurement errors were bigger at low frequencies because of the high impedance of the electrodes used, which was higher than 10 k Omega at frequencies below 1 kHz.