Topic of research
Development of a telemetric retina implant

Epi-Ret

Introduction

About 3 million people worldwide suffer from retinitis pigmentosa, making this one of the leading causes for blindness. In retinitis pigmentosa patients a slow and progressive degeneration of photoreceptors is observed, while about 30% of the retina’s ganglion cells remain intact. These ganglion cells are connected to the visual cortex of the brain via the optic nerve. Hence electrical stimulation of the remaining intact ganglion cells by placing micro electrodes onto the retina can in principal lead to visual sensation. The system concept for a visual prosthesis proposed by the EPI-RET group can be seen in Fig. 1.

Concept for a telemetric vision aid

Fig. 1: Concept for a telemetric vision aid

The idea is that the patient wears a pair of glasses which have an integrated CMOS camera. An image of the environment is taken and processed by a digital signal processor (the so-called retina encoder), which calculates a stimulation pattern for the electrodes placed onto the retina that reproduces the original image. Data and energy are then transferred via RF coupling to the implant inside the patient’s eye. Here a silicon chip generates bipolar current pulses that stimulate the intact ganglion cells of the retina via three-dimensional micro electrodes.

Fig. 2 shows an image of a second generation prototype of the implant with its main components mounted onto a flexible polyimide tape (from left to right): receiver coil, capacitor, diode, receiver chip, stimulation chip, micro cable and a 5x5 array of three-dimensional iridium oxide covered gold electrodes.

prototype

Fig. 2: Second generation prototype of a telemetric eye prosthesis

During surgery, the implant is pushed into the eye through a tiny cut in the cornea. Flexibility and foldability of the implant allow for a smaller cut and hence less invasive surgery. With these requirements a production process of the implant using thin polyimide tape and a flexible electroplated coil seemed to be the most promising choice.

Fabrication

The implants are produced using standard and non-standard wafer-level processes. 4” silicon wafers coated with a sacrificial layer are used as substrates for the production process. Polyimide is spin-coated onto the sacrificial layer and patterned. In an electroplating step, gold wiring is grown before a second polyimide film is spin-coated and patterned. A second gold electroplating step is performed to add 26 µm to the height of the electrodes. At this point the implants could in principal be assembled to fully functional telemetric eye prostheses.

However, gold as the electrode material has some disadvantages with regards to electrical stimulation of nerve cells. The charge delivery capacity Qcdc, which is a measure of the ability to transfer electrical charge to the tissue during stimulation, and the safe potential range, that is the voltage that can be applied without electrolysis taking place, are comparatively small for gold. Therefore, a suitable coating material like iridium oxide (IrOx) is added to the electrodes. Iridium oxide is magnetron DC-sputtered in a reactive argon/oxygen gas phase. Parylene C is next added from a gas phase to form a biocompatible protective coating. On the electrodes, holes are etched into the Parylene C film, so that the electrodes can electrically contact the ganglion cells of the retina. A schematic cross section through an electrode of an implant at this stage can be seen in Fig. 3.

Schmeatic view

Fig. 3: Schematic view of wafer-level cross section of an electrode and implant

Finally, the implants are removed from the wafer by etching away the sacrificial layer.

Performance

In-vitro tests have shown that the implant is fully functional. Fig. 4 shows a bipolar pulse (bottom line) measured at one of the electrodes after telemetric energy and data transmission. In this experiment the implant was placed 2 cm away from the transmitter coil and an RF field with a frequency of 13.56 MHz was applied while measurements were taken on pads and electrodes.

Measurement

Fig. 4: Bipolar pulse (bottom line) measured on an electrode after telemetric data and energy transmission (upper line)

Conclusions

The production process described in the previous section leads to a very small, flexible and light implant which can easily be implanted into the eye causing minor strain during and after surgery.

The iridium oxide films used as a coating material for functional electrical stimulation (FES) have a charge delivery capacity of 95 mC/cm² and hence show substantially larger values than other coating materials such as platinum or titanium nitride.

Outlook

Currently, implants of the 3rd are being developed. They will be used for temporary (i.e. 4 weeks duration) implantation in humans to test and prove the feasibility of a fully telemetric retina implant.

Christian Koch

Institute of Materials in Electrical Engineering, Chair 1
http://www.iwe1.rwth-aachen.de

IWE 1 logo

This work was funded by the Federal Ministry of Education and Research (EPI-RET-II).

The following institutes were involved in the development of the EPI-RET-III implant:


Last update: 4/23/2008

Imprint

Disclaimer