Voltage field generated by a single photodiode in a volume conductor: simulation and measurements

One of the major challenges of neural stimulation is the mechanical stress and resulting neural trauma induced by the movement of the electrode interconnects. A potential way of eliminating interconnects is to use micro-stimulators that can be activated through optical means. To test this method, the voltage field generated by a single photodiode in a volume conductor was analyzed using finite element (FE) analysis. The voltage field was also experimentally tested using diodes with uncoated gold and electrodeposited iridium oxide film (EIROF) coated contacts pulsed with a near-infrared laser (NIR) beam. The EIROF coated microphotodiodes proved to have much larger current injection capabilities. Comparison of experimental results with the finite element model shows that the photodiode voltage is saturating at a few microamperes due to the high impedance of the contacts. However, the voltage generated is larger than 50 mV in peak at the cathodic contact. The voltage waveforms recorded directly from the contacts show that the interface needs a long time interval to discharge. This places a limit on the maximum frequency of stimulation.