Christian Brendler

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.

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.

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.

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.

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 350nm High Voltage CMOS technology.

Robust demultiplexing for erroneous MPEG-2 Transport Streams

Many standards for digital TV use the MPEG-2 Transport Stream for multiplexing several programs into one stream. In the case of poor reception, data also get lost in the demultiplexing process due to bit errors in the Transport Stream packet headers. In this paper we propose an algorithm which adjusts disturbed information in the packet headers to deliver more audio data to the source decoder. We verified the amount of audio samples after decoding to be increased by up to 17%.

Capacitive coupled full-wave rectifier for rectangular power signals

In this paper, a fully integrated bridge rectifier is presented. It is specialized for fully differential rectangular power supply waveforms. The rectifier is implemented in AMS 0.18 μm 6M/1P high voltage CMOS process with metal insulator metal (MIM) capacitors. Combining self driven switches with capacitive coupled switching gates the proposed rectifier shows very little output ripple without the need for a big output capacitance (Cout). By introducing coupling capacitors (CC), switching time is significantly reduced and the losses during polarity change can almost be eliminated. Simulation results show that the output voltage ripple (AVout) becomes mostly a function of the load current. The output capacitance can be reduced at least by a factor of 2 and still achieve the same output ripple as compared to non-capacitive coupled architectures.

Closed loop inverse load modulation power control by magnetic field diminishment in inductively powered biomedical implants

A combination of an on-chip power control and a closed loop power control for inductively powered biomedical implants using inherent inverse load modulation as back channel is presented. Inverse load modulation is realized using an innovative internal on-chip power control. By shorting the secondary receiving coil to ground using varying switching pulse repetition rates the magnetic field is diminished and the load is nearly canceled. This reduces the energy absorbed by the implant and consequently the transmitted power decreases with a slight increase of the voltage amplitude in the primary transmitting coil. The increased amplitude peaks on the transmitter side are measured in order to decrease the on-chip power control state. Finally the transmitting power is adjusted in a way to transmit only sufficient power for chip operation. This reduces the transmitting power. Therefore the power control unit in the implant has to control less excessive power.

Resistorless BiCMOS voltage reference with isolated substrate potential for biomedical implants

This paper shows a resistorless BiCMOS voltage reference with isolated substrate potential for an inductively powered biomedical implant. The reference voltage source has been designed in a 350nm High Voltage BiCMOS process. The reference voltage is used as part of an biomedical implant system to generate the positive supply voltage VDD and the substrate potential VSS used as negative supply. Therefore the reference voltage has to be highly insensitive and independent on substrate variations. The proposed design shows 1.4% accuracy over process variations and mismatch. The line sensitivity was measured to be 4mV/V. Measurements have proven that the isolation of the output of the reference voltage from the substrate potential is working properly. The power supply rejection ratio without any filtering capacitor at 10Hz and 10MHz is lower than -60dB and -35 dB, respectively.