Retinal prosthesis on track for further testing

Fort Lauderdale, FL-The 256+ channel Boston sub retinal prosthesis is on track for in vivo validation of function and an FDA investigational device exemption application, according to Joseph F. Rizzo III, MD, a neuro-ophthalmologist at Massachusetts Eye & Ear Infirmary.

Dr. Rizzo, who is leading the retinal prosthesis team, said that development of the high-density wireless neurostimulator is nearly complete. Milestones expected by late 2012 or early 2013 will include completion of bench testing and initial preclinical testing in Yucatan mini-pigs.

Dr. Rizzo and two other members of the development team, engineers John L. Wyatt, PhD, and Douglas B. Shire, PhD, described progress on the implanted component of the prosthesis in a presentation at the annual meeting of the Association for Research in Vision and Ophthalmology. Well-designed packaging technology for the large number of stimulating electrodes necessary to get high-quality vision is a critical step in creating an implantable prosthesis, noted Dr. Rizzo, who initiated the retinal implant project in 1988.

This prosthesis and others being developed by groups worldwide are intended to restore vision lost to degenerative retinal diseases such as age-related macular degeneration and retinitis pigmentosa.

Providing details on the Boston group’s work, Dr. Shire said that three progressively better versions of the stimulator’s custom-designed, application-specific IC computer chip have been fabricated and tested, and although one more round of modification may take place soon, the current model should be suitable for animal trials. Dr. Shire is a research health scientist at the Boston VA Medical Center and engineering manager for the VA Center for Innovative Visual Rehabilitation.

The prosthesis consists of a camera mounted on an eyeglass frame that wirelessly supplies images to an implant containing an array of tiny stimulating electrodes. The 5 mm by 5 mm, low-power chip decodes this information and stimulates the retinal ganglion cells, which induces precepts that correspond to the strength and location of the stimuli. The integrated circuit drives the electrodes with biphasic current pulses in response to an external controller via a frequency-shift keyed radio transmitter.

The development team recently modified the electrode array with a coiled cable design that reduces surgical implantation time and allows flexibility in selecting the entry point to the subretinal space.

The wireless communication protocol is extremely important and includes an error-detecting code, Dr. Shire said. The chip does not respond if a command is invalid and will wait for another, valid command.

The implant also has several other built-in safety precautions. Hardware limits prevent the total charge from exceeding safe values and damaging the retina. Independent circuits monitor electrode voltages between each positive and negative pulse and can determine if the voltage is too high or too low. In addition, electrodes are shorted to ground between pulses.

The technology for creating paths for individual electrodes to pass through the hermetically sealed titanium case containing the electronics is extremely important since it protects these sensitive components from damage in the saline environment in which they are implanted, said John L. Wyatt, PhD, professor of electrical engineering and computer science at Massachusetts Institute of Technology. He is the head of the MIT engineering team for the implant project.

The package contains circuitry mounted on the internal surface of the ceramic feedthrough that contains one conductive path for each electrode. The package is sealed by welding, and connections to the outside are protected by a plastic header that is dimpled to allow the package to rest against the sclera. A completed package is approximately 11 mm in diameter and 2 mm thick.

The internal assembly requires attachment of a circuit board containing the stimulator chip to all the signal feedthrough pads simultaneously. On the external side, the flexible electrode array and coil are attached by first electroplating gold onto the electrode array and onto the ceramic disk, and then bonding the two together using a thermosonic process.

Drs. Rizzo, Shire, and Wyatt are co-inventors of intellectual property related to the prosthesis and own equity in Bionic Eye Technologies, a medical device company developing novel solutions to assist the blind. Dr. Rizzo and Dr. Wyatt serve on the company’s board, and Dr. Rizzo is an ex-officio member.

For more articles in this issue of Ophthalmology Times eReport, click here.

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