A prototype 64-electrode stimulator in 65 nm CMOS process towards a high density epi-retinal prosthesis

Face identity recognition in simulated prosthetic vision is poorer than previously reported and can be improved by caricaturing

The visual prosthesis (or "bionic eye") has become a reality but provides a low resolution view of the world. Simulating prosthetic vision in normal-vision observers, previous studies report good face recognition ability using tasks that allow recognition to be achieved on the basis of information that survives low resolution well, including basic category (sex, age) and extra-face information (hairstyle, glasses). Here, we test within-category individuation for face-only information (e.g., distinguishing between multiple Caucasian young men with hair covered). Under these conditions, recognition was poor (although above chance) even for a simulated 40×40 array with all phosphene elements assumed functional, a resolution above the upper end of current-generation prosthetic implants. This indicates that a significant challenge is to develop methods to improve face identity recognition. Inspired by "bionic ear" improvements achieved by altering signal input to match high-level perceptual (speech) requirements, we test a high-level perceptual enhancement of face images, namely face caricaturing (exaggerating identity information away from an average face). Results show caricaturing improved identity recognition in memory and/or perception (degree by which two faces look dissimilar) down to a resolution of 32×32 with 30% phosphene dropout. Findings imply caricaturing may offer benefits for patients at resolutions realistic for some current-generation or in-development implants.

Redundant safety features in a high-channel-count retinal neurostimulator

Safety features embedded in a 256-channel retinal prosthesis integrated circuit are presented. The biology of the retina and the electrochemistry of the electrode-tissue interface demand careful planning and design of the safety features of an implantable retinal stimulation device. We describe the internal limits and communication safety features of our ASIC, but we focus on monitoring and protection circuits for the electrode-tissue interface. Two independent voltage monitoring circuits for each channel measure the electrode polarization voltage at two different times in the biphasic stimulation cycle. The monitors ensure that the charged electrode stays within the electrochemical water window potentials, and that the discharged electrode is within a small window near the counter electrode potential. A switch to connect each electrode to the counter electrode between pulses protects against a wide range of device failures. Additionally, we describe work on an active feedback system to ensure that the electrode voltage is at zero.

A fully intraocular high-density self-calibrating epiretinal prosthesis

This paper presents a fully intraocular self-calibrating epiretinal prosthesis with 512 independent channels in 65 nm CMOS. A novel digital calibration technique matches the biphasic currents of each channel independently while the calibration circuitry is shared among every 4 channels. Dual-band telemetry for power and data with on-chip rectifier and clock recovery reduces the number of off-chip components. The rectifier utilizes unidirectional switches to prevent reverse conduction loss in the power transistors and achieves an efficiency > 80%. The data telemetry implements a phase-shift keying (PSK) modulation scheme and supports data rates up to 20 Mb/s. The system occupies an area of 4.5 ×3.1 mm². It features a pixel size of 0.0169 mm² and arbitrary waveform generation per channel. In vitro measurements performed on a Pt/Ir concentric bipolar electrode in phosphate buffered saline (PBS) are presented. A statistical measurement over 40 channels from 5 different chips shows a current mismatch with μ = 1.12 μA and σ = 0.53 μA. The chip is integrated with flexible MEMS origami coils and parylene substrate to provide a fully intraocular implant.

ASIC design and data communications for the Boston retinal prosthesis

We report on the design and testing of a custom application-specific integrated circuit (ASIC) that has been developed as a key component of the Boston retinal prosthesis. This device has been designed for patients who are blind due to age-related macular degeneration or retinitis pigmentosa. Key safety and communication features of the low-power ASIC are described, as are the highly configurable neural stimulation current waveforms that are delivered to its greater than 256 output electrodes. The ASIC was created using an 0.18 micron Si fabrication process utilizing standard 1.8 volt CMOS transistors as well as 20 volt lightly doped drain FETs. The communication system receives frequency-shift keyed inputs at 6.78 MHz from an implanted secondary coil, and transmits data back to the control unit through a lower-bandwidth channel that employs load-shift keying. The design's safety is ensured by on-board electrode voltage monitoring, stimulus charge limits, error checking of data transmitted to the implant, and comprehensive self-test and performance monitoring features. Each stimulus cycle is initiated by a transmitted word with a full 32-bit error check code. Taken together, these features allow researchers to safely and wirelessly tailor retinal stimulation and vision recovery for each patient.

A precise charge balancing and compliance voltage monitoring stimulator front-end for 1024-electrodes retinal prosthesis

In this paper, we present a precise charge balancing and compliance voltage monitoring stimulator front-end for 1024-electrode retinal prosthesis. Our stimulator is based on current mode stimulation. To generate a precisely matched biphasic current pulse, a dynamic current copying technique is applied at the stimulator front-end. A compliance voltage monitoring circuitry is included at the stimulator front-end to detect if a voltage across electrode-tissue interface goes beyond a predefined compliance voltage. Simulation results show the mismatch of a biphasic current pulse (at a maximum stimulation current of 476µA) is less than 0.1%. Also, the stimulator issues alarm signals, when a voltage compliance occurs during stimulation due to high tissue impedance. Our stimulator is implemented using a 65nm low voltage (LV) CMOS process, which helps reducing implementation area and power consumption.

Crosstalk current measurements using multi-electrode arrays in saline

This paper investigates how the configuration of return electrodes in an electrode array affects the amount of current crosstalk when electrodes are driven simultaneously in saline. Two pairs of electrodes in different return configurations were stimulated with different-amplitude biphasic currents. Stimulating electrodes were controlled by current sinks and current sources while return electrodes were connected to supply voltage or ground. Measurement results show that no matter what return configuration was used, the return current was almost equally distributed amongst the return electrodes, which is problematic in bipolar concurrent stimulation, at least in saline. This result is due to the fact that the spreading impedance of saline solution is small compared to the electrode-electrolyte impedance, which makes the saline solution have almost the same potential. This result suggests that monopolar stimulation using a common remote return electrode be used in simultaneous stimulation to avoid crosstalk.