Naser Pour Aryan

A CMOS Chip With Active Pixel Array and Specific Test Features for Subretinal Implantation

This paper presents a CMOS imager chip that is aimed at subretinal implantation for partially restoring human vision. It has low supply voltage (plusmn 2 V) and all DC free terminals for long life wired operation. Stimulation voltage is increased to approximately 4 Vpp by low voltage drop design. 40 x 40 pixel cells including light sensors, amplifiers, control logic and electrode drivers are addressed sequentially to improve power consumption and spatial resolution of perception. Pad count is limited to 6, which requires a specific test procedure. The 3 x 3 mm2 design is fabricated in a 0.35 mum CMOS technology optimized for optical performance.

In vitro study of titanium nitride electrodes for neural stimulation

For neural stimulation, reliable high density charge transfer into tissue is required. One electrode material for these applications is titanium nitride (TiN). In this paper, a method for lifetime analysis of TiN electrodes is discussed. Our method significantly differs from open literature. The tests were run for much longer durations. Special attention was paid to the optical appearance and electrode voltage response to different input current pulses. According to our investigations, TiN electrodes are able to deliver at most 0.2mC/cm2 charge density for square shaped electrodes with 50μm × 50μm dimensions in safe operation, which is less compared to previous reports. The safe operation window for TiN was confirmed to be ±1V in terms of electrode potential with the counter electrode considered as reference. We found that the shape of the waveform does not affect electrode lifetime. Our measurements show that rectangular voltage waveforms inject the most amount of charge into the electrodes compared to other shapes. This makes rectangular electrode voltage signals optimal for highest charge injection at a given lifetime. In our case with square electrodes, the absolute electrode potential is found to be the more important parameter in electrode lifetime, compared to Helmholtz capacitor voltage drop.

Electrode Materials: State-of-the-Art and Experiments

Platinum is the most commonly used electrode material. The charge injection limit for platinum electrodes was found to be 400 μC∕cm2 in [4]. Tim Boretius et al. have reported a value of only 75 μC∕cm2 [3]. Platinum electrodes have proven success in practice, for example in many cochlear implants. Because of their relatively low charge injection capacity, they are usually used where large electrodes are applicable as in intracortical implant [8]. The limit for neural stimulation regarding tissue safety has been determined to be 1 mC∕cm2. In order to increase the charge injection capacity of platinum, various approaches have been proposed in the past few decades, like the galvanization of platinum black or gray. Although platinum black possesses a highly porous layer and therefore high charge injection capacity, its deposition often requires a lead containing electrolyte which limits its application because of cytotoxicity concerns.

Power control by magnetic field diminishment in inductively powered biomedical implants

A power control circuit by magnetic field diminish-ment in inductively powered biomedical implants is presented. Due to large and fast coupling variations in inductively powered retinal implants caused by the eye movements, excessive power has to be controlled. This proposal attenuates the magnetic field in the secondary coil to reduce received power. Coil shorting is realized by applying short enough pulses to enable a parallel ASK data transmission. The system was fabricated and measured using a 350 nm BiCMOS High Voltage technology. Measurement results show a reduction of the thermal power generated by the implant.

An ambient light adaptive subretinal stimulator

This paper presents a subretinal stimulator which is able to automatically adapt to varying ambient light levels. Stimulation timing and intensity are controlled using an input amplifier with an offset current cancellation circuitry to eliminate offset current imposed on the pixel amplifier for accurate stimulation control and optimized power consumption. In order to enable the chip to operate over a wide range of luminance conditions without manual tuning, automatic ambient light adaptation is required. Comparing the local photosensor output with a global luminance reference signal forms the basis of the adaptation mechanism. Global brightness information is obtained by logarithmic conversion of the summed photocurrents to generate a global luminance reference voltage. The chip has been realized in Austria Micro Systems 0.35µm CMOS Opto technology.

Benefit of spatial filtering for visual perception with a subretinal implant

Subretinal implants have proven to be capable of restoring vision to patients suffering from hereditary retinal degeneration diseases like retinitis pigmentosa and cone-rod dystrophy. Although they already provide basic visual perception, there is still much room for improvement in this field. Effects like electric field interference limit the visual acuity and may be the cause of the perceived vision to be blurred. This influence could be reduced by means of highpass spatial filtering. In this paper, based on the available reports about the visual perception parameters from the patients using the alpha-IMS subretinal implant, a model for the blurring effect of the patients retina is proposed. On this basis, highpass filters are suggested which will compensate the obscuring effect of the stimulator device plus retina system to some extent.

In vitro study of iridium electrodes for neural stimulation

Iridium is one of the main electrode materials for applications like neural stimulation. Iridium has a higher charge injection capacity when activated and transformed into AIROF (activated iridium oxide film) using specific electrical signals [1]. Activation is not possible in stimulating devices, if they do not include the necessary circuitry for activation. We introduce a method for iridium electrode activation requiring minimum additional on-chip hardware. In the main part, the lifetime behavior of iridium electrodes is investigated. These results may be interesting for applications not including on-chip activation hardware, and also because activation has drawbacks such as worse mechanical properties and reproducibility of AIROF.

Experiments Hardware and Methods

Three materials were extensively experimented with throughout the research here: TiN, iridium and iridium oxide (IrOx). The solution used in the experiments to mimic the surrounding tissue in the application is phosphate buffered saline (PBS). Before continuing with the investigation of different electrode materials and their characteristics, the hardware and methods developed and used here are explained briefly.

Stimulation and Recording Electrodes: General Concepts

In prosthetic devices, electrodes are the interfaces between the implant system and the body. Electrodes may be used for neural stimulation or neural signal recording according to the application. The signals to be recorded are typically small, i.e. some tens of microvolts for single pulse activity to a maximum amplitude of around 80 mV for intracellular potentials measured for example by fine-tipped electrolyte-filled glass micropipettes in cognitive studies in brain machine interface. On the opposite, neural stimulation sometimes needs relatively high electrode voltages and current densities, sometimes as high as several volts, so it may lead to a high enough energy transfer triggering chemical reactions that involve corrosion and changes in electrode properties.

Wireless power delivery for a biomedical retinal prosthesis

A wireless power delivery system for a biomedical retinal prosthesis is presented and analyzed. Due to large and fast coupling variations in the inductively powered retinal implant caused by the eye movements, excessive power has to be controlled. This power delivery setup enables excessive power control measurements by featuring movements of an eye model. The turning of the eye model is controlled with a microcontroller board. Measurements and control of the model is automatized using Matlab. Measurement results show an enormous reduction of the transmitted power for large eye movements and therewith the need for power control in biomedical implants.