Evaluation of an enhanced electrical charge controlled stimulation method for retinal bipolar cells
Objective: To present an improvement of the charge-controlled stimulation method (ChCS) for achieving a high charge control, which is independent of the electrode area. Concept: The decreasing electrode area results in a higher electrical tissue impedance (ETZ). Therefore, the voltage-controlled stimulation offers no charge control, because the stimulus current changes with ETZ. In the case of current-controlled method (CCS), operating voltages above 16 V are necessary in order to achieve a constant charge injection. It decreases the stimulation safety because the electrode potentials are above the water window. To compensate the disadvantages of both methods, a high pulse density modulated ChCS has been developed by Marzouk. Within a switched-capacitor circuit, small defined charge packets will be transferred into the tissue and accumulate to the total injected charge quantity. His implementation has the effect, that the stimulator efficiency depends on ETZ. The capacitance of ETZ saturates the stimulator and occurs a clockwise decreasing of the charge packet. Within a feedback, the influence of ETZ to the charge packet will be suppressed per each transfer. The charge control efficiency has been verified with tissue emulators. The improved ChCS has been integrated in a CMOS-Chip, which includes two recording amplifiers. A local bidirectional communication between tissue and the stimulator is possible, which allows a simultaneous stimulation of neuron cells and recording of created action potentials. Results and Discussion: It is shown that the improved ChCS has huge charge control which is independent of increasing ETZ. In compare to CCS, ChCS has a higher stimulation safety because of low operating voltage of 5 V. Each recording amplifier can achieve a signal-to-noise ratio of 20.6 dB with a rd10 action potential of 0.24 mV, which allows an undisturbed detection of action potentials. Acknowledgment: German Research Foundation (DFG), contract No. GR3328/4-3.