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.

Clock recovery PLL with gated PFD for NRZ ON-OFF Modulated Signals in a retinal implant system.

A Clock Recovery Phase Locked Loop with Gated Phase Frequency Detector (GPLL) for NRZ ON-OFF Modulated Signals with low data transmission rates for an inductively powered subretinal implant system is presented. Low data transmission rate leads to a long absence of inductive powering in the system when zeros are transmitted. Consequently there is no possibility to extract any clock in these pauses, thus the digital circuitry can not work any more. Compared to a commonly used PLL for clock extraction, no certain amount of data transitions is needed. This is achieved by having two operating modes. In one mode the GPLL tracks the HF input signal. In the other, the GPLL is an adjustable oscillator oscillating at the last used frequency. The proposed GPLL is fabricated and measured using a 350 nm High Voltage CMOS technology.

An economical and convenient experiment setup for electrode investigation.

Electrodes are among the critical components of neural stimulation devices. Investigating electrode properties like electrode impedance, charge injection capacity, and electrode corrosion limits plays an important role in electrode development. There are many commercial devices available for this purpose. Although useful, these devices are usually expensive and often offer more functions than required. We propose a versatile setup, composed of a LabVIEW program, a National Instruments multifunctional board, and a circuit built of discrete commercial elements. The system offers basic functions used in electrochemical investigation like current and voltage injection, cyclic voltammetry, and impedance spectroscopy. It offers the functionalities of both a potentiostat and an arbitrary waveform generator. It has already been applied elsewhere.

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. 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.

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 0.2 mC/cm(2) charge density for square shaped electrodes with 50 µm × 50 µm dimensions in safe operation, which is less compared to previous reports. The safe operation window for TiN was confirmed to be ± 1 V 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.