Development of an implantable wireless biomicrosystem with impedance spectroscopy measurement and nerve stimulation functions

In neural prosthetic applications, cuff electrodes have been utilized for providing peripheral nerve electrical stimulation and signal sensation. It is imperative to monitor the impedance of implantable cuff electrodes for effective sensing and stimulating schemes. This study aimed to implement an implantable wireless biomicrosystem for providing constant stimulation currents as well as for measuring cuff electrode impedance via a magnetic-inductive link. For continuous in vivo impedance monitoring, a transcutaneous magnetic coupling technique was adopted for transmitting power and commands into the internal module of the biomicrosystem and transmitting outwards the impedance measurement. For impedance measurement, the two-terminal and four-terminal methods were adopted for measuring the electrode-tissue interfacing impedance and tissue impedance. To avoid the high sampling rate required, a gain-phase detector was utilized for direct output of the magnitude and phase shift of a sinusoidal current input for impedance measurement. The same voltage-controlled current source can be converted for the nerve stimulation function with a mono-phasic pulse input. The adjustable stimulation parameters, including the stimulation intensity and frequency, can be controlled via the external module. Validation tests of impedance measurement and microstimulation function were first performed in an impedance model of resistor in parallel with capacitor and later by immersing the entire implantable wireless biomicrosystem with cuff electrode in saline solution. The measured impedance differences were less than 10 % in comparison with those measured by precision LCR meter. After the test of the in vitro saline solution, the implantable biomicrosystem is now ready for in vivo animal experiments.