Photodiodes may replace human photoreceptors
06/01/1997
Photodiodes may replace human photoreceptors
Over the last several decades, artificial hips and pacemakers have become commonplace. Cochlear implants restore partial hearing by directly stimulating the acoustic nerve. Now, a German research group is investigating a new approach to visual prostheses for blind people.
Most research on visual prostheses, in Germany and the US, has used an epiretinal approach. Images are collected and processed outside the body, then transmitted to the brain by direct stimulation of the optic nerve. Some patients, however, suffer from retinal lesions but have otherwise intact visual systems. According to M.B. Schubert, of the Institute for Physical Electronics at the University of Stuttgart, in a presentation at the Materials Research Society`s spring meeting in San Francisco, these patients may benefit from a subretinal approach instead. In its simplest form, the subretinal approach merely aims to replace degenerated photoreceptors, while taking advantage of whatever retinal cells and capabilities remain (Fig. 1).
A successful subretinal implant must:
be thin enough (10-20 ?m) and flexible enough to fit in the retina, yet be able to withstand rough handling during surgery,
allow nourishment to reach underlying tissue, and
be compatible with the subretinal environment.
Amorphous silicon can be deposited at temperatures as low as 100?C, which allows use of flexible plastic substrates like polyimide, polycarbonate, or biodegradable polylactate. Unfortunately, these films are mechanically unstable and difficult to process at the desired thickness. Likewise, many common transparent electrodes, like ITO (indium tin oxide) and ZnO, either are not biocompatible or have not been tested for biocompatibility.
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Figure 1. Schematic illustrating a photovoltaic pixel-array as a subretinal implant to the human eye.
Due to these additional restrictions, amorphous silicon appears to be the best choice for a substrate. The researchers used a glass substrate, coated with spin-on photoresist as a sacrificial layer, as the base for a Ti/Au/Ti electrode. Then, a Nd: YAG laser was used to drill holes to allow nutrients to pass through the array. Individual pixels were formed from an array of amorphous silicon pin diodes by CF4 etching. Next, appropriate contacts were patterned and created by liftoff. Finally, a passivating silicon nitride layer completed the microdiode array (Fig. 2). Workers at the University Eye Hospital and the Natural and Medical Science Institute at the University of Tuebingen are gathering fundamental biocompatibility data for these devices.
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Figure 2. Laser holes provide access for retina cell nourishment. The photoresist carrier can be seen through the holes prior to removal.
Once biocompatibility is achieved, subretinal implants will succeed or fail based on their ability to interface with retinal cells. The stimulation threshold for nerve cell activity ranges from 1-100 ?C/cm2, depending on the electrode-cell distance and the contact geometry. If incoming light does not induce sufficient stimulation, an additional energy supply will need to be incorporated into the devices. - K.D.