Balance is formed in the vestibular region of the inner ear that tracks the motion and position of the head employing three liquid filled semicircular canals, found at perpendicular angles to each other.
When the head moves quickly in a certain direction, the fluid in the corresponding canal pushes against a membrane, with sensitive “hair” cells attached to a nerve that sends the information to the brain that adjust heads movements.
“It’s the fastest reflex in the body. Without it, the world looks like you’re watching it through a hand-held video camera,” explained Charles Santina at Johns Hopkins School of Medicine in Boston, Massachusetts, US,
who is looking for an implant to restore the vestibular-ocular reflex, in humans. “People who have lost their sense of balance could one day be fitted with an inner ear implant modeled on the body’s own balance organs”.
Current models fit to animals, but there is a need for smaller devices with a longer battery life to be fitted for human use.
People reach this situation when the vestibular hair cells are killed by genetic disorders, infections or antibiotic poisoning. Their death also causes hearing loss and cochlear implants restored partial hearing. Cochlear (auditive) implants employ a microphone and processor to code sound and send it directly to the cochlear (auditive) nerve through electrodes implanted in the inner ear.
A similar vestibular implant employs tiny gyroscopic sensors to track head movement and sends that information straight to the vestibular nerve through electrodes. The first vestibular device was designed by Daniel Merfeld and his team at the Jenks Vestibular Physiology Lab at Harvard in Boston, Massachusetts and worked in one plane of movement in animals.
But Santina's team developed a device tracking motion in three dimensions, with a circuit with three gyros oriented perpendicularly, exactly like the semicircular canals, which worked in chinchillas. “We’ve been able to restore their vestibular-ocular reflex,” says Santina. “We turn their head and measure how their eyes move.”
But the technology must advance to be applied in humans. The current device is too thick, 6 mm, to be implanted under the skin behind the human ear. And it is also too power hungry, discharging the battery every 5 or 6 hours. “Battery life is the biggest challenge facing vestibular implants,” says Lawrence Lustig, director of the Douglas Grant Cochlear Implant Center at the University of California in San Francisco, US. “When the battery runs out in a cochlear implant the person simply cannot hear, but if the battery ran out on someone with a vestibular implant they’d be violently dizzy and vomiting”.
The team wants to replace gyroscopes with accelerometers, which are much smaller and more energy efficient: a 9-volt battery will last 24 hours. “It’s a way of creating a virtual inner ear to try out our designs,” says Santina. “It’s an exciting technology coming down the turnpike,” says Lustig. “There is a small but definite number of people who will greatly benefit from this.”
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