Progress Reported by Researchers on Retinal Implant

Hoping to restore vision to those suffering from age-related macular degeneration and retinitis pigmentosa, Massachusetts Institute of Technology (MIT) and medical researchers are working on a chronic retinal implant.

Researcher Luke Theogarajan reported at the International Solid-State Circuits Conference that, “The goal is to stimulate the remaining healthy layers of retinal neurons using brief biphasic current pulses.” Progress in the miniaturization of retinal and cochlear prosthesis was also reported at the conference.

This is heartening news for more than 14 million people worldwide suffering from these vision problems or from hearing difficulties. Unfortunately, its application in humans is rare. It is the loss of photoreceptors that cause blindness, but other retinal neurons maintain an active connection to the brain.

Researchers from MIT and Massachusetts Eye and Ear Infirmary at Harvard University noted that the design requirements for the implant include an external power source, wireless communication of external commands to the implant, and the ability to provide wireless tuning of pulse amplitude, duration and inter-pulse timing.

An inductively coupled power and data link met the source and wireless requirements. Seamless scaling of the number of electrodes that can be physically implanted in a minimally invasive approach was made possible by using flexible stimulator chip architecture developed for this purpose.

Only the electrode array is placed in the eye beneath the retina, while the secondary coils and stimulator chip are surgically attached to the eyeball. According to researcher Theogarajan, surgical trauma to the eye is greatly minimized by placing the bulky electronics in the more compliant eye socket, rather than the delicate retina.

The implant was made from a paralyene-encapsulated flexible polyimide substrate onto which the chip and electrode array are bonded. Frequency-independent operation is provided by the stimulator chip architecture and it compensates for non-idealities due to process, temperature and voltage.

Power from an inductive link is rectified and filtered using off-chip diodes and capacitors housed on the flexible substrate, with 2.5 V supply voltage regulated by an off-chip zener diode.

Nominal carrier frequencies for power and data are 125 kHz and 13.56 MHz, respectively. The chip can drive 15 electrodes, contains 30,000 transistors in a 0.5-micron technology, occupies 2.3 by 2.3 square millimeters, and consumes 1.3 mW at a data rate of 100 kbits/second (excluding the current sources).

Researchers from two German companies, design house Sci-Worx and medical electronics firm IIP Technologies, are working on an epiretinal prosthesis, which restores basic vision through electrical nerve stimulation within the eyeballs of blind people with retinal degeneration.

Eyes and Ears

The German design presents the concept of an array of fully digitally interfaced and programmable stimulation pad cells for a retinal implant in 0.35-micron HVCMOS, which has a maximum voltage swing of 15 V. It includes full-custom ESD protection and an innovative active charge balancer.

Researchers at IIP Technologies state that a complete retinal stimulator chip is being fabricated, which includes all global functions and stimulation pad cells. The clinical trials of the chip have started.

Hearing-impaired cochlear prostheses have evolved significantly during the past 20 years. A cochlear implant bypasses the failed hair cells of the inner ear to electrically stimulate the auditory nerve using 16 to 22 wire electrodes.

The development of electrode arrays that increase the number of stimulating sites perception is seen as a potential solution to these problems by the Researchers at the University of Michigan.

Researcher Pamela Bhatti reported on a thin-film cochlear electrode array that achieves high site density and incorporates on-board circuitry for stimulus generation and position sensing.

Source: EE Times Online