Development of implantable wireless biomicrosystem for measuring electrode-tissue impedance

The increase of impedance between electrode and tissue is the most relevant factor to cause the power loss in the implantable electrical stimulation application. This study aims to develop an implantable wireless biomicrosystem for impedance measurement between microelectrode and tissue through inductive link. A transcutaneous wireless transmission scheme was adopted in this study from which the power and command were transmitted into implantable module by using high efficient power transmitter and amplitude-shift keying (ASK) modulation technique. A sinusoidal wave approach with peak detector was applied for estimating impedance. The measured impedance were sampled and converted into digital signal which can be transmitted outwards through the same radio frequency (RF) link by using the load shift keying (LSK) modulation method. The developed wireless impedance measurement module was first validated during in-vitro test in which the increase in impedance was mainly due to the gradual adhesion of protein of blood plasma to microelectrode surface. The impedance measurements of our designed module were comparable to those obtained from the commercial LCR meter. For in-vivo monitoring the electrode-tissue impedance, the microelectrode was implanted between the skin and muscle of Wistar rat's dorsum. Those results indicated that total impedance decreased or maintained at the first three days and gradually increased around the fourth day after implantation.