N. Dommel

Retinal Neurostimulator for a Multifocal Vision Prosthesis

A neurostimulator application-specific integrated circuit (ASIC) with scalable circuitry that can stimulate 14 channels, has been developed for an epi-retinal vision prosthesis. This ASIC was designed to allow seven identical units to be connected to control up to 98 channels, with the ability to stimulate 14 electrodes simultaneously. The neurostimulator forms part of a vision prosthesis, designed to restore vision to patients who have lost their sight due to retinal diseases such as retinitis pigmentosa and macular degeneration. For charge balance, the neurostimulator was designed to stimulate with current sources and sinks operating together, and with the ability to drive a hexagonal mosaic of electrodes to reduce the electrical crosstalk that occurs when multiple bipolar stimulation sites are active simultaneously. A hexagonal mosaic of electrodes surrounds each stimulation site and has been shown to effectively isolate each site, increasing the ability to inject localized independent charge into multiple regions simultaneously.

A CMOS retinal neurostimulator capable of focussed, simultaneous stimulation

Restoring vision to the blind by way of medical device technology has been an objective of several research teams for a number of years. It is known that spots of light-phosphenes-can be elicited by way of electrical stimulation of surviving retinal neurons. Beyond this our understanding of prosthetic vision remains rudimentary. We have designed and manufactured an integrated circuit neurostimulator with substantial versatility, able to provide focussed, simultaneous stimulation using current sources and sinks, steering the current to the intended site of stimulation. The ASIC utilizes high-voltage CMOS transistors in key circuits, to manage voltage compliance issues (due to an unknown or changing electrode/tissue interface impedance) given the relatively high stimulation thresholds necessary to elicit physiological excitation of retinal neurons. In addition, a unique multiplexing system comprised of electrodes arranged in a hexagonal mosaic is used, wherein each electrode can be addressed to be a stimulating electrode and all adjacent electrodes serve as the return path. This allows for simultaneous stimulation to be delivered while appropriately managing cross-talk between the stimulating electrodes. Test results indicate highly linear current sources and sinks (differential nonlinearity error of 0.13 least significant bits -2.6 microA), with the ASIC clearly able to provide focussed stimulation using electrodes immersed in a saline solution.

In-Vitro Testing of Simultaneous Charge Injection and Recovery in a Retinal Neuroprosthesis

In order to deliver sufficient phosphene quantities to convey effective vision in a prosthesis device, simultaneous stimuli is necessary. We present in vitro experimental results of the current distribution between stimulation sites during simultaneous stimulation of platinum electrodes immersed in physiological saline. Stimuli were delivered using circuitry that utilizes (a) current source only, (b) current sink only, and (c) the combination of a balanced current source and current sink, to deliver and recover balanced charge at each stimulation site. The results from these experiments support our decision to implement balanced combined current source and current sink circuitry in an application specific integrated circuit (ASIC)

A retinal neuroprosthesis design based on simultaneous current injection

We present an epiretinal neuroprosthesis design based on a hexagonally-latticed 98 electrode array and the capacity to multiplex up to 14 simultaneous current sources. The digital and analogue electronics required to perform this function and how this would be incorporated into an application specific integrated circuit (ASIC) are described. Simulation data and data from saline bath testing of a platinum/silicone electrode array (and associated driving electronics) are presented. Simulations were performed using a 2D computational model solved using a custom collocation method. The guarding affect of the hexagonal array is investigated and shown in simple simulations to be an approach worthy of further investigation.

The Design and Testing of an Epi-Retinal Vision Prosthesis Neurostimulator Capable of Concurrent Parallel Stimulation

An application specific integrated circuit (ASIC) neurostimulator capable of stimulating multiple electrodes in unison has been designed and tested. The ASIC utilizes multiple matched current sinks and sources to provide localized stimulation and is designed to drive electrodes organized in a hexagonal mosaic. This organization allows each stimulating electrode to be surrounded by up to six return electrodes, effectively isolating each stimulation site. The ASIC was manufactured using a high-voltage complementary metal-oxide-semiconductor process, which allows up to 20 V to be applied across the circuitry. This provides the greatest versatility for testing with electrodes and tissues of varying impedances in-situ and allows the device to be used in other neurostimulation applications such as functional electrical stimulation. The design has been thoroughly tested and meets all the design specifications

Microelectronic Retinal Prosthesis: II. Use of High-Voltage CMOS in Retinal Neurostimulators

For part 1, see ibid., p. Z004336-9 (2006). This paper presents the design, implementation, and simulated and measured results of a complementary metal-oxide-semiconductor neurostimulator implemented in a 0.35 mum high-voltage process. To allow for a high stimulation voltage, and hence the greatest versatility of the neurostimulator in situ, a high-voltage CMOS process was used. The neurostimulator utilized current sources and sinks to simultaneously deliver and recover charge. It has the ability to deliver stimulus in three output current ranges using a current sink only, current source only, or both a current source and sink combined to provide focused stimulation. The worst case integral non-linearity and differential non-linearity errors were 0.2 LSB and 0.1 LSB respectively, and the current source and sink turn-on times were under 500 ns, providing fast switching time in response to stimuli instructions. The total die area was under 13 mm2, well within the area constraints of our implantable vision prosthesis device

Implant electronics for intraocular epiretinal neuro-stimulators

In this paper, we discuss system architectures, design challenges and circuit implementation principles for intraocular epiretinal neuro-stimulators to be used as part of vision prostheses in partially restoring vision to the blind. Our unique hexagonal electrode placement allows focused simultaneous stimulation which allow the electrode count to scale. A new dual-channel transcutaneous inductive link is used to transfer power and data efficiently to the implant, and a new dual-voltage power rectifier provides two implant supplies without the need for a DC-DC converter. Low-power designs of current stimulators and ECAP amplifiers are also outlined in the paper.

Optical Imaging of Electrically Evoked Visual Signals in Cats: I. Responses to Corneal and Intravitreal Electrical Stimulation

Microelectronic retinal prostheses have been shown to restore the perception of light to the blind through electrical stimulation. Conventional recording techniques such as recording electrode arrays on the visual cortex can give a basic understanding of the events that occur during such stimulation events, but their finite size and number limits the spatial resolution achievable with them. Optical imaging of intrinsic signals (OIS imaging) allows for greater resolution (approximately 50 μm) of the activity in the cortex. This can be used to facilitate a greater understanding of the complex neurophysiological events that allow prosthetic vision. This paper shows responses to visual and electrical stimulation of the retina, and demonstrates that OIS imaging may be an effective technique in further refining stimulation techniques and implant designs for retinal prostheses.

An FPGA-Based Vision Prosthesis Prototype: Implementing an Efficient Multiplexing Method for Addressing Electrodes

A prototype of an epi-retinal vision prosthesis based upon an efficient electrode addressing schema has been developed. This system has the ability to stimulate multiple electrode regions simultaneously, hence greatly improving the maximum rate of stimulation compared to many currently available neural stimulation devices based on serial stimulation protocols. To minimize the problem of cross talk between stimulating electrodes, a hexagon layout of electrodes was implemented. Basic tests were completed using a field programmable gate array logic system driving analogue circuitry to inject current into physiological saline via electrodes in hexagon arrangements and in a simple paired arrangement. The hexagon layout of electrodes was shown to clearly reduce the interaction between multiple current sources and hence cross talk

Charge recovery during concurrent stimulation for a vision prosthesis

Parallel or concurrent stimulation in an epiretinal neuroprosthesis is likely necessary in order to deliver sufficient phosphenes for effective vision. Important issues with concurrent stimulation are the effect of current distribution which introduces current leakage or ‘cross talk’ between adjacent electrodes and charge recovery which determines balanced charge being delivered/recovered at each electrode from the previous phase. In this paper, we present the effect of concurrent stimulation of two hexagonally arranged platinum electrode arrays on charge recovery. Balanced and imbalanced (unequal) currents were delivered to the hexagonal arrays when they were immersed in physiological saline. Both simulation and experimental results revealed that charge was not recovered at individual electrodes, particularly when imbalanced currents were delivered. However, total charge injected to both hexagonal arrays was recovered.