Nov 2017 IEEE Spectrum

 lasersTwo of the vertical cavity surface-emitting lasers used in a new optical cochlear implant are shown here next to a matchstick. Each laser rests within a sapphire box.

Blinking lights could soon serve a whole new purpose. Recent findings have led German, Swiss, and Austrian researchers to develop a prototype hearing implant based on the concept that a series of laser pulses can trigger auditory signals from hair cells located within the inner ear.

An array of near-infrared lasers can produce a soundwave using what’s called the optoacoustic effect, the researchers believe. In their device, tiny vertical cavity surface-emitting lasers, which pulsate light at a spectrum of 1.4 to 1.9 microns, act upon the fluid within the nautilus-shaped cochlear canals in the inner ear. Basically, the infrared light is absorbed by the liquid inside the cochlea. A small fraction of the liquid will expand due to heat. If that happens rapidly enough, it generates a soundwave inside the duct of the cochlea. This stimulates or moves tiny hair cells located there, which in turn sends a signal along the auditory nerve which the brain understands as sound.

Over the last three years, the researchers have built tiny laser arrays and completed tests on guinea pigs, finding they could generate action potentials, the signals carried by auditory nerves, using vertical laser light and the optoacoustic effect. They compared stimuli in the guinea pigs from the laser array with an acoustic click. Both generated nerve signals matching in form and amplitude.

It is still early days but the hope is that this technology can be used to replace or improve hearing devices and cochlear implants, says Mark Fretz, a physicist and project manager at the Centre Suisse d’Electronique et Microtechnique (CSEM), an applied research and technology nonprofit based in Alpnach, Switzerland. The next steps would be to improve the energy efficiency of the device and make it smaller. Individual components developed for the prototype—including a tiny sapphire case for hermetically sealing implanted body sensors and an improved laser lens design—may also find other uses, such as allowing laser light to shine within the ear to improve balance.

Today’s cochlear implants rely upon sets of electrodes threaded through the skull to the inner ear. The electrodes create an electrical field which stimulates the cochlear nerve, converting ambient sound into electrical signals that the nerve carries to the brain. It’s difficult to focus an electrical field, however, so it tends to flow into other tissues, generating noise. There are still design challenges for the prototype, including how to solve issues with power consumption and how to shrink the components. A body implant cannot generate too much heat or it will damage cells and tissue around it. The researchers found that creating many, many pulses of 50 nanoseconds each essentially replicates a single burst of 50 microseconds and reduces the heat that would be generated by sustained shining. That burst is what’s needed to create an acoustic compound action potential—a signal that travels down an auditory nerve.

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