Viola Rieger

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.

On-chip spatial filter for subretinal implants

Retinal implants are raising hope for blind people to restore part of their visual capabilities. Enormous advances have taken place in this field while several groups worldwide are striving to achieve this long-yearned-for accomplishment. Current studies about visual perception parameters from patients using subretinal implants showed that effects like electric field interference limit the achievable visual acuity. According to our previous work, discussing a model of the implant-plus-retina system, this effect could be moderated by the implementation of a spatial high-pass filter. This work introduces an on-chip realization of a contrast enhancement circuit using a four-neighbor high-pass filter, extending the functionality of an existing retinal stimulator. For the evaluation of different filter kernels a new approach, combining the methods of image processing with the techniques of analog circuit design, is proposed. The circuits discussed in this work are already designed and they will be fabricated in a 180nm CMOS technology in near future.

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.

Low-latency system for evaluation of image-enhancement-algorithms on patients using subretinal implants

In this paper, a real-time system consisting of a camera device, computational unit and head mounted display, adjusted to the needs of patients using subretinal implants, is presented. Retinal implants demonstrated to partially restore useful vision to patients suffering from hereditary retinal degeneration diseases. Even though various implant-mediated visual perceptions in daily-life were reported, perceived vision could be enhanced using algorithms well known from image-processing. Due to strict area limitations subretinal implants can only cover well-chosen and carefully examined functionality within the silicon device. To gain flexibility in testing different kinds of image enhancement algorithms, a software solution allowing quick changes is desired. The system presented here, allows recording and displaying reality on a head mounted display with low latency, while maintaining true to scale representation. Additionally different types of pixel-based image-enhancement-algorithms can be applied on the captured content to modify the perceived image.

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.