Power/Performance Bits: Feb. 11

Researchers at MIT have created a cochlear implant that can be wirelessly recharged using the natural microphone of the middle ear rather than a skull-mounted sensor; meanwhile, scientists at the University of Cambridge have developed a new method to probe silicon batteries and determined what causes the expansion to take place in order to pave the way for high-capacity batteries.

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Low-power chip for cochlear implants without external hardware
Existing versions of cochlear implants require that a disk-shaped transmitter about an inch in diameter be affixed to the skull, with a wire snaking down to a joint microphone and power source that looks like an oversized hearing aid around the patient’s ear…until now. Researchers at MIT’s Microsystems Technology Laboratory alongside physicians from Harvard Medical School and the Massachusetts Eye and Ear Infirmary have created a new, low-power signal-processing chip that could lead to a cochlear implant that requires no external hardware.

The implant would be wirelessly recharged and would run for about eight hours on each charge. The researchers have also demonstrated a prototype charger that plugs into an ordinary cell phone and can recharge the signal-processing chip in roughly two minutes.

Existing cochlear implants use an external microphone to gather sound, but the new implant would instead use the natural microphone of the middle ear, which is almost always intact in cochlear-implant patients. However, this design exploits the mechanism of a different type of medical device, known as a middle-ear implant. Delicate bones in the middle ear, known as ossicles, convey the vibrations of the eardrum to the cochlea, the small, spiral chamber in the inner ear that converts acoustic signals to electrical. In patients with middle-ear implants, the cochlea is functional, but one of the ossicles — the stapes — doesn’t vibrate with enough force to stimulate the auditory nerve. A middle-ear implant consists of a tiny sensor that detects the ossicles’ vibrations and an actuator that helps drive the stapes accordingly.

The new device would use the same type of sensor, but the signal it generates would travel to a microchip implanted in the ear, which would convert it to an electrical signal and pass it on to an electrode in the cochlea. Lowering the power requirements of the converter chip was the key to dispensing with the skull-mounted hardware.

Silicon, meet lithium
Researchers at the University of Cambridge believe that resolving the mystery of what happens inside batteries when silicon comes into contact with lithium could accelerate the commercialization of next-generation high capacity batteries for use in electric vehicles, mobile phones and other applications.

Using a combination of nanotechnology and nuclear magnetic resonance (NMR) techniques, the team developed a new probing system that gives a view into what is happening inside the batteries at the atomic level, enabling greater control over the properties of the materials.

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Silicon has been proposed as a replacement for carbon in battery anodes (negative electrodes) for the past 20 years, as it has roughly ten times more storage capacity than carbon. However, difficulty in managing silicon’s properties has prevented the technology from being applied at scale, the researchers said.

The primary problem with using silicon in a lithium-ion battery is that silicon atoms absorb lithium atoms, and the silicon expands up to three times in volume, degrading the battery. Although controlling this expansion has become easier over the past decade, a lack of understanding about what is happening inside the batteries and what governs the reactions have continued to hold silicon batteries back but the researchers have developed a new method to probe silicon batteries and determined what causes the expansion to take place.

Using nanoscale wires made of silicon and NMR techniques, the researchers developed a robust model system able to accommodate the expansion of the silicon over multiple cycles, and integrated it with short-range probing techniques that reveal what is happening inside the battery at the atomic level. The team found that the reactions proceed with interactions of various sizes of silicon networks and clusters, energetics of which partly govern the path of the reaction.

Using these combined techniques, the researchers were able to develop a ‘map’ of how silicon transforms when it is put into contact with lithium in a battery. The insights opened up by the technology will boost further developments of silicon batteries, as it will be easier for engineers to control their properties.

They believe using this technique will help make battery design much more systematic, with less trial and error.