An efficient multiplexing method for addressing large numbers of electrodes in a visual neuroprosthesis

Progress in artificial vision through suprachoroidal retinal implants

Retinal implants have proven their ability to restore visual sensation to people with degenerative retinopathy, characterized by photoreceptor cell death and the retina's inability to sense light. Retinal bionics operate by electrically stimulating the surviving neurons in the retina, thus triggering the transfer of visual sensory information to the brain. Suprachoroidal implants were first investigated in Australia in the 1950s. In this approach, the neuromodulation hardware is positioned between the sclera and the choroid, thus providing significant surgical and safety benefits for patients, with the potential to maintain residual vision combined with the artificial input from the device. Here we review the latest advances and state of the art devices for suprachoroidal prostheses, highlight future technologies and discuss challenges and perspectives towards improved rehabilitation of vision.

Visual Prosthesis: Interfacing Stimulating Electrodes with Retinal Neurons to Restore Vision

The bypassing of degenerated photoreceptors using retinal neurostimulators is helping the blind to recover functional vision. Researchers are investigating new ways to improve visual percepts elicited by these means as the vision produced by these early devices remain rudimentary. However, several factors are hampering the progression of bionic technologies: the charge injection limits of metallic electrodes, the mechanical mismatch between excitable tissue and the stimulating elements, neural and electric crosstalk, the physical size of the implanted devices, and the inability to selectively activate different types of retinal neurons. Electrochemical and mechanical limitations are being addressed by the application of electromaterials such as conducting polymers, carbon nanotubes and nanocrystalline diamonds, among other biomaterials, to electrical neuromodulation. In addition, the use of synthetic hydrogels and cell-laden biomaterials is promising better interfaces, as it opens a door to establishing synaptic connections between the electrode material and the excitable cells. Finally, new electrostimulation approaches relying on the use of high-frequency stimulation and field overlapping techniques are being developed to better replicate the neural code of the retina. All these elements combined will bring bionic vision beyond its present state and into the realm of a viable, mainstream therapy for vision loss.

Cortical responses following simultaneous and sequential retinal neurostimulation with different return configurations

Researchers continue to develop visual prostheses towards safer and more efficacious systems. However limitations still exist in the number of stimulating channels that can be integrated. Therefore there is a need for spatial and time multiplexing techniques to provide improved performance of the current technology. In particular, bright and high-contrast visual scenes may require simultaneous activation of several electrodes. In this research, a 24-electrode array was suprachoroidally implanted in three normally-sighted cats. Multi-unit activity was recorded from the primary visual cortex. Four stimulation strategies were contrasted to provide activation of seven electrodes arranged hexagonally: simultaneous monopolar, sequential monopolar, sequential bipolar and hexapolar. Both monopolar configurations showed similar cortical activation maps. Hexapolar and sequential bipolar configurations activated a lower number of cortical channels. Overall, the return configuration played a more relevant role in cortical activation than time multiplexing and thus, rapid sequential stimulation may assist in reducing the number of channels required to activate large retinal areas.

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).

Towards a chip scale neurostimulator: system architecture of a current-driven 98 channel neurostimulator via a two-wire interface

With more clinical trials proving viability of visual prosthesis follows the demand for higher resolution devices. As the number of electrodes increases, due to surgical difficulties, it is preferred to keep their length short by placing the implant close to the stimulation site, where there are considerable constraints on device size. On the contrary, the physical volume of the implant generally increases with increasing number of electrodes. Splitting the implant into two modules and placing only the essential circuits near the site of stimulation solves the aforementioned problem. However now the problem is redirected to the robustness and the safety of the interface joining these modules. A novel two-wire interface driving a 98 channel neurostimulator incorporating the split-architecture is presented. The stimulator is provided with both power and data by sending square current waveforms via the two-wire interface. The stimulator itself is fabricated using 0.35 μm HVCMOS technology and occupies 4.9 × 4.9 mm(2) and requires no external decoupling capacitor.

Fabrication of Pillar Shaped Electrode Arrays for Artificial Retinal Implants

Polyimide has been widely applied to neural prosthetic devices, such as the retinal implants, due to its well-known biocompatibility and ability to be micropatterned. However, planar films of polyimide that are typically employed show a limited ability in reducing the distance between electrodes and targeting cell layers, which limits site resolution for effective multi-channel stimulation. In this paper, we report a newly designed device with a pillar structure that more effectively interfaces with the target. Electrode arrays were successfully fabricated and safely implanted inside the rabbit eye in suprachoroidal space. Optical Coherence Tomography (OCT) showed well-preserved pillar structures of the electrode without damage. Bipolar stimulation was applied through paired sites (6:1) and the neural responses were successfully recorded from several regions in the visual cortex. Electrically evoked cortical potential by the pillar electrode array stimulation were compared to visual evoked potential under full-field light stimulation.

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.

Current distribution during parallel stimulation: implications for an epiretinal neuroprosthesis

A simplified mathematical model has been developed in order to better understand local current spread when multiple simultaneous current sources are used in an epiretinal neuroprosthesis. To test the model, pairs of platinum electrodes of 430 μm diameter and an intra-pair spacing of 1 mm between centers, were arranged either in-line or in parallel, in a bath of physiological saline. Each pair was separated by distances from 1 mm to 6 mm. The currents in each electrode in the bath were measured and compared with the computational model of the same arrangement. This approach allowed us to quantify return current interaction between parallel sources. As predicted, with parallel electrodes and matching currents in each electrode pair, there is no current cross-talk. However with imbalanced current sources, significant cross-talk is evident. The cross-talk decreases as a function of electrode pair separation. The implication of this work in the design of an epiretinal neuroprosthesis is discussed.

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.

Microelectronic retinal prosthesis: II. Use of high-voltage CMOS in retinal neurostimulators

This paper presents the design, implementation, and simulated and measured results of a complementary metal-oxide-semiconductor neurostimulator implemented in a 0.35 microm 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.