Elsevier

Sensors and Actuators A: Physical

Volumes 123–124, 23 September 2005, Pages 172-178
Sensors and Actuators A: Physical

Production processes for a flexible retina implant (Eurosensors XVIII, Session C6.6)

https://doi.org/10.1016/j.sna.2005.03.061Get rights and content

Abstract

In this paper, an overview on different approaches for visual prostheses is presented and the different strategies are compared. A flexible implant is discussed which was developed for electrical stimulation of the retina of people suffering from retinitis pigmentosa.

Wafer-level production processes and assembly and packaging of a second-generation prototype are discussed with an emphasis on the production of three-dimensional electrodes. Iridium oxide thin-film coating used as stimulation material for the electrodes is described. Finally, the in vitro performance of the implants is discussed.

Introduction

Worldwide, about 3 million people suffer from retinitis pigmentosa, making this one of the leading causes for blindness. Macula degeneration, another degenerative retinal disease, is even more common, affecting more than 11% of the elderly [1].

In both groups of patients, a slow and progressive degeneration of photoreceptors is observed, while some ganglion cells of the retina remain intact, making the retina suitable for direct electrical stimulation [2].

Fig. 1 shows the detailed structure of the retina with the outer retina on the left comprised of photoreceptors and their cell bodies (rods and cones) and the inner retina on the right with its different layers (horizontal, bipolar, amacrine and ganglion cells). In a healthy eye, incoming light causes a photochemical reaction, resulting in the excitation of nerve potentials, that are then forwarded via the inner retina and the optic nerve to the visual cortex of the brain. In retinitis pigmentosa patients, the outer retina degenerates over several years starting in the mid periphery, then spreading to the periphery and finally to the central visual field [3], [4]. About 30% of ganglion cells however stay intact [5], [2].

By placing electrodes onto the retina, it is possible to electrically stimulate these ganglion cells, leading to a visual perception. The principal feasibility of an external stimulation of ganglion cells has been demonstrated in numerous animal trials [5], [2], [6], [7] and also in human trials [8]. The patients perceived simple forms in response to electrical stimulation of a simple geometric pattern on the retina [9]. Simulations have shown that by increasing the number of electrodes to 300 electrodes, even reading exercises might be performed [10].

In general, there are four approaches for developing visual prostheses that are pursued by research groups worldwide.

The system concept, proposed by our group, is shown in Fig. 2.

The patient wears a pair of glasses with an integrated CMOS camera. An image of the environment is processed by a digital signal processor (the so-called retina encoder), which calculates a stimulation pattern for the stimulation electrodes. Data and energy are then transferred via rf coupling to the implant inside the patient's eye. Here, data and energy signals are separated by a receiver chip. The signals are used to feed a stimulation chip which generates bipolar current pulses that stimulate the intact ganglion cells of the retina via three-dimensional micro stimulation electrodes.

The epiretinal concept, developed by IIP-Technologies (Germany), is comparable to the work presented in this paper. IIP-Technologies have recently done first human trials with electrodes placed onto the retina and connected to stimulation equipment outside the body via a cable [8]. Also, two groups in the USA are working on epiretinal implants: M. Humayun from John Hopkins University [2], [12] and J. Rizzo from Harvard University [5]. Both groups have tested non-telemetric implants on humans.

The DOE-funded Artificial Retina Project (USA) is based on a modified cochlear implant that was first introduced by Second Sight Inc., USA [13].

The implantation of a silicon chip with a photodiode array, called subretinal approach, is favoured by two companies: Retina Implant AG (Germany) [14], [15] and Optobionics Inc. (USA) [16], [17]. Both groups use silicon chips with micro photodiodes that are implanted underneath the retina and receive data and energy input via an optic channel (light or laser). While Retina Implant AG uses 1500 photodiodes with integrated amplifiers and has tested the implants on animals, Optobionics Inc. has increased the number of photodiodes to 3500 and done first human trials in June 2000.

The stimulation of the nervus opticus is pursued by the University of Louvain (Belgium) and funded through the European Project OPTIVIP [18], [19], [20]. The aim is to place an electrode around the optic nerve, so that this area can be electrically stimulated.

Researchers from the Dobelle Institute (USA) implanted electrodes onto the surface of the cortex, while R. Normann from the University of Utah (USA) implanted penetrating electrodes into the cortex [21], [22]. Both groups used cables to connect the electrodes to stimulation equipment outside the body. In Europe, hope is placed on the project CORTIVIS which recently started developing cortical implants [24].

This paper discusses the second-generation prototype of the epiretinal implant shown in Fig. 3 with its main components mounted onto a flexible polyimide tape [23].

Around the electrodes, the polyimide tape is patterned in the shape of cloverleafs each carrying four electrodes. This gives the implant additional flexibility, allowing it to be better adapted to the round shape of the retina and hence improving electrical contact between the electrodes and the retina.

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.

Section snippets

Wafer-level production processes

The implants are produced using standard and non-standard wafer-level processes. Four inches silicon wafers coated with a sacrificial layer are used as substrates for the production process. Polyimide is spin-coated onto the sacrificial layer and structured along the outlines of the implants and around the above-mentioned cloverleafs of the electrode array. This structuring is not only necessary to define separation lines between implants, but also to speed up the etching of the sacrificial

Three-dimensional electrodes

The hat-shaped electrode shown in Fig. 7 has several advantages over cylindrical electrodes. The large base area results in better adhesion of the electrode to the layer underneath, with no major decrease in flexibility. Moreover, the slopes on the electrode edges can more easily be covered with sputtered iridium oxide films, allowing for a larger tissue area to be contacted by the electrodes without increasing electrode dimensions.

The barb-shaped electrodes fabricated by placing several

Conclusions

The production process described in Section 2 leads to a very small, flexible and light implant which can easily be implanted into the eye causing minor strain during and after surgery.

Prototypes of three-dimensional barb-shaped electrodes were manufactured which show good mechanical stability in primary tests.

Stimulation electrodes coated with iridium oxide films were tested, showing excellent electrochemical properties.

Acknowledgements

We would like to thank the German Ministry for Education and Research (BMBF) for funding this project (Grant 01KP001, EPI-RET-II) and the FhG-IMS, Duisburg, for manufacturing the CMOS chips used in the implants.

Kaspar Hungar received his Master degree in electrical engineering from RWTH Aachen University, Germany, in 2003. He is currently working at the Institute of Materials in Electrical Engineering I at RWTH Aachen University. Apart from being responsible for the lithography laboratory and gold electroplating, he is supervising the production of flexible retina implants. Currently, he is also part-time studying for his PhD, the topic of which is the integration of flexible silicon chips in medical

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    Kaspar Hungar received his Master degree in electrical engineering from RWTH Aachen University, Germany, in 2003. He is currently working at the Institute of Materials in Electrical Engineering I at RWTH Aachen University. Apart from being responsible for the lithography laboratory and gold electroplating, he is supervising the production of flexible retina implants. Currently, he is also part-time studying for his PhD, the topic of which is the integration of flexible silicon chips in medical implants.

    Michael Görtz received his Diplom from the Niederrhein University of Applied Sciences in 2001 in electrical engineering with a focus on electrical/optic communication technology and production technologies for micro systems. From 2001 to 2004, he worked at the Fraunhofer Institute of Microelectronic Circuits and Systems in Duisburg, Germany, where his tasks included design and testing of neural vision prostheses with micro system technologies. Currently, he is working at the Institute of Materials in Electrical Engineering I at RWTH Aachen University and is responsible for active implantable medical devices.

    Evelina Slavcheva graduated from Sofia University of Chemical Technology and Metallurgy in 1981 where later on she received her PhD degree with a thesis on corrosion inhibition. Since 1994, Ms. Slavcheva has a permanent position as a research associate at the Institute of Electrochemistry and Energy Systems (IEES) at the Bulgarian Academy of Sciences. Her main topics of research are electrochemical characterisation of materials, electrocatalysis, fuel cells and corrosion of metals. Currently, she is working at the Institute of Materials in Electrical Engineering I at RWTH Aachen University, Germany.

    Gerd Spanier received his Diplom (German equivalent to Master degree) in electrical engineering with a focus on solid state electronics at RWTH Aachen University, Germany, in 1994. After working several years in industry, he is now working at the Institute of Materials in Electrical Engineering I at RWTH Aachen University, leading the Department for Advanced Assembling and Packaging for Medical Implants.

    Christopher Weidig studied electrical engineering at RWTH Aachen University. During his Master thesis, he analysed different techniques for fabricating multilayer gold electrodes with barb-shaped structures.

    Wilfried Mokwa received his Diplom (German equivalent to Master degree) in physics from RWTH Aachen University, Germany, in 1977 and 4 years later his PhD from 1981 onwards, he worked at the Department of Experimental Condensed Matter Physics at RWTH Aachen University, studying catalytic reactions on gas sensor surfaces. In 1985, he joined the Fraunhofer Institute of Microelectronic Circuits and Systems (IMS) in Duisburg, Germany. There, he managed a group working on the monolithic integration of silicon sensors. In 1996, he became a full professor in the Faculty for Electrical Engineering at RWTH Aachen University where he is Director of Chair 1 of the Institute of Materials in Electrical Engineering I (IWE-1) with special interests in the field of MEMS technologies.

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