Photovoltaic Pixels for Neural Stimulation: Circuit Models and Performance

Abstract

Photovoltaic conversion of pulsed light into pulsed electric current enables optically-activated neural stimulation with miniature wireless implants. In photovoltaic retinal prostheses, patterns of near-infrared light projected from video goggles onto subretinal arrays of photovoltaic pixels are converted into patterns of current to stimulate the inner retinal neurons. We describe a model of these devices and evaluate the performance of photovoltaic circuits, including the electrode-electrolyte interface. Characteristics of the electrodes measured in saline with various voltages, pulse durations, and polarities were modeled as voltage-dependent capacitances and Faradaic resistances. The resulting mathematical model of the circuit yielded dynamics of the electric current generated by the photovoltaic pixels illuminated by pulsed light. Voltages measured in saline with a pipette electrode above the pixel closely matched results of the model. Using the circuit model, our pixel design was optimized for maximum charge injection under various lighting conditions and for different stimulation thresholds. To speed discharge of the electrodes between the pulses of light, a shunt resistor was introduced and optimized for high frequency stimulation.

Fig. 1.

Photovoltaic pixel arrays with 140 μm pixels in (a) and (b) and 70 μm pixels in (c) and (d). 1 – central active electrode, 2 – return electrode, 3 – conductive bridges, 4 – filled trenches, 5 – open trenches. (e) Electric circuit of a 3-diode pixel.

Fig. 2.

Images of the 2- and 1-diode pixels. (a) 2-diode, 140 μm. (b) 1-diode, 140 μm. (c) 2-diode, 70 μm. (d) 1-diode, 70 μm.

Fig. 3.

Diagram of the experimental setup for measurement of the electric current in electrolyte above the illuminated pixel.

Fig. 4.

Bipolar and monopolar wired electrodes on a glass substrate. The disc electrodes are 10, 20, 40 and 80 ìm in diameter.

Fig. 5.

Electrical circuit model of a photovoltaic pixel in electrolyte. R_a is the access resistance, R_F is the Faradaic resistance, C is the capacitance of the electrode-electrolyte interface, R_e is the bulk resistance of the electrolyte medium, R_s is the shunt resistance.

Fig. 6.

(a), (c) Circuit diagrams and (b), (d) current/voltage waveforms at the SIROF electrode-electrolyte interface with wired electrodes.

Fig. 7.

(a) Voltage dependence of the capacitance, measured with 1 ms and 10 ms pulses on 3 different electrodes. Markers with dark fill correspond to 80 μm diameter electrodes, with light fill – to 40 μm, with white fill – to 20 μm electrodes. (b) Faradaic resistance of the 20 μm electrode as a function of voltage. (c) Electrolyte plus access resistance R_e + R_a for 80 μm electrode as a function of the inverse concentration of the electrolyte.

Fig. 8.

(a) A simplified circuit, consisting of a capacitor, a resistor and a photodiode, (b) illuminated by a rectangular pulse of light, produces (c) a current waveform. (d) I-V curves of the dark (blue) and illuminated (red) photodiode, and the resistor plus capacitor (discharged in black and charged in brown).

Fig. 9.

(a), (c) Calculated and (b), (d) measured current waveforms generated by small pixels in ACSF medium illuminated with 1 ms pulses of (a), (b) 0.1 mW/mm² and (c), (d) 2.7 mW/mm² irradiance. Scale bars are the same for the model and for experimental results.

Fig. 10.

Charge injected by 70 μm pixels in electrolyte of 1000 Ω · cm resistivity during 4 ms pulses as a function of light intensity. Lines depict the model calculations, and dots represent experimental data for s2 and s3 pixels. On the left is shown the optimum number of diodes per pixel corresponding to the minimum light intensity required to reach the target charge delivery.

Fig. 11.

Measured current generated by the 2-diode, 70 μm device (s2) under repetitive pulsed illumination of 18 mW/mm². The first pulse is shown in red, and the black waveforms represent 20 traces at 1 s intervals.

Fig. 12.

Model of an s2 pixel irradiated with 4 ms, 10 mW/mm² pulses at 33 Hz. (a) Current decreases over time. (b) Voltage across the active and return electrode capacitors. (c) The first (red) and steady-state (black) pulse shapes. (d) I-V curves illustrating the first pulse (OPQR loop) and the steady-state regime (EFGH loop).

Fig. 13.

Modeling the s2 pixel performance under the same conditions as in Fig. 12, but with a 2 MΩ shunt resistor.

Fig. 14.

Simulated efficiency of light-to-current conversion of small photodiode pixels.