Chipping in to aid hearing

MIT INNOVATION

A team of chip designers and surgeons has devised a tool to ascertain the underlying causes behind auditory loss. The tool uses the ear’s own biological battery as the power source, reports Kalyan Ray

Whispers in the ear will have a whole new meaning if a bunch of chip designers and surgeons at Massachusetts succeed in realising their dream. They crossed the first hurdle when they harvested a tiny source of electricity inside the ear to activate a specially-designed chip, which sent out radio signals to an external receiver.

The whispering in the radio waves conveyed the conditions of the inner ear, which, in turn will inform a doctor if there is chance of deafness. This is a step forward in understanding deafness at the molecular level, which may lead to development of an accurate diagnostic tool for a disorder, which covers more than half of India’s population.

Delhi doctors showed that 63 million Indians suffer ‘significant auditory loss’. Nationwide disability surveys have estimated hearing loss to be the second most common cause of disability, said researchers at Maulana Azad Medical College in Delhi in a paper published in the National Medical Journal of India in 2009.

Biological battery

What the MIT team now offers is a tool to find out the causes behind this  disorder using the ear’s own biological battery as the power source. The chip-in-the-ear may also lead to the development of an infection-free cochlear implant and eventually a drug delivery tool in the vicinity of the ear.

Deep in the inner ear of mammals lies a natural battery — a chamber filled with ions that produces an electrical potential to drive neural signals, discovered in 1952. The power it generates varies between 0.07V and 0.1V. In contrast, an AA battery has a capacity of around 1.5V, higher voltage and power capability.

Although the inner ear voltage is the highest in the body (outside of individual cells, at least), it’s still very low. The amount of current that can be drawn without hearing loss was too tiny to deserve scientific attention till now. With advances in low power electronics, an inter-disciplinary team of surgeons and chip designers at MIT, however,
decided to explore the uncharted territory.

The most crucial hurdle was to capture the EP energy as the voltage and extractable power are very low — at least ten times lower than what can be captured using the most efficient existing circuits.  

Anantha Chandrakasan, Konstantina Stankovic and colleagues overcame this challenge using a specially designed electronics chip. With the chip placed on the surface of an anesthetised guinea pig and connected to tiny electrodes embedded in the cochlea, the authors succeeded in extracting about one nano watt of power for five hours, which was enough to run a wireless radio that transmitted EP measurements to a receiver.

“The immediate application is to sense the inner ear environment to discover molecular biomarkers of deafness to make emerging therapies for the inner ear safe (by preventing undesirable side-effects), and to prevent serious side-effects of cochlear implants,” Stankovic told Deccan Herald. The results of the experiment were reported in the November issue of Nature Biotechnology.

Though for the experiment, the chip was placed outside the ear of guinea pigs, keeping only the electrodes inside, scientists said implanting the 2.4mm x 2.4mm chip inside the ear by a routine surgery was not a problem. The chip does not cause any hearing impairment and there is no question of any damage due to ear wax because wax is made in the external ear canal while the chip is implanted in the middle ear. “We tested our devices on five guinea pigs.

EP is well known in mammals. We are confident the system will work but we will need to do a lot more testing before this can be used in humans,” said Anantha P Chandrakasan, department head at MIT Electrical Engineering and Computer Science division and one of the lead authors of the paper. Though the chip is small enough for implant, emerging technologies can make both chip and electrodes further miniaturised.

Lest it disrupt hearing, a device powered by the biological battery can harvest only a small fraction of its power. To circumvent the problem, the MIT team equipped their chip with an ultralow-power radio transmitter. An implantable medical monitor wouldn’t be of much use if there was no way to retrieve its measurements.

While the radio is much more efficient than those found in cell phones, it still couldn’t run directly on the biological battery. So the chip also includes power-conversion circuitry — like that in the boxy converters at the ends of power cables in many electronic devices— that gradually builds up charge in a capacitor.

The voltage of the biological battery fluctuates, but it would take the control circuit somewhere between 40 seconds and four minutes to amass enough charge to power the radio. The frequency of the signal was itself an indication of the electrochemical properties of the inner ear. “The capacitor in the system, explained Chandrakasan, was being trickle charged  from the EP. When the capacitor fills up, it starts the radio transmitter. To reduce its power consumption, the control circuit had to be simplified, but like the radio, it still needed a higher voltage than the biological battery could provide. Once the control circuit was up and running, it could drive itself. The problem was getting it up and running.
A one-time burst of radio waves solved the problem. Improvements in electrodes are required for long-term operation. The challenge is to make the tip of the electrode as small as possible to minimise trauma to the tissue that it penetrates, yet as big as possible to decrease electrode impedance, and thereby allow energy harvesting. 
Ear specialist Cliff Megerian at Case Western Reserve University said there could be three possible applications in cochlear implants, diagnostics and implantable hearing aids.
“Generation of power for a low voltage from the cochlea itself raises the possibility of using that as a power source to drive a cochlear implant,” said Megerian, who is not connected to the MIT research. Chandrakasan, however, pointed out that the biologic battery didn’t produce enough power for a cochlear implant.