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.
Fretz and CSEM are part of a group that includes auditory and laser researchers at the Medical Hochschule Hannover, which supports an experienced team of experts, as well as the Bavarian laser maker Vertilas, the precision lens and array manufacturer SUSS MicroOptics and others. Innsbruck’s MED-EL, one of the largest makers of cochlear implants, is also involved, as is STMicroelectronics, who developed hardware, and the VTT Technical Research Centre of Finland, which supplied an anti-fouling coating to protect the implant lead from excessive fibrous growth.
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.
The group used sapphire boxes to seal the laser and components from bodily fluids and corrosion, connecting the assembly to platinum ribbons and molding it all in silicon rubber. For the guinea pig tests, researchers used a plug-in version of their device, but in an updated prototype they were able to get the laser casings down to a length of two millimeters and a width of one millimeter.
The whole device is now the shape of a slimmed-down nine-volt battery and can fit in the palm of a hand. “It would need to be miniaturized further than that," Fretz says, for a commercial implant. “This is possible.”
Using light to transmit auditory signals is a topic that has seen a lot of debate, experimentation, and development in recent years, says Claus-Peter Richter, research vice-chair at Northwestern University’s department of otolaryngology, who’s been involved with this research from the beginning.
One early approach tried to directly stimulate auditory nerves using light; another aimed to genetically alter cells so they react to light—a concept pioneered by molecule-maker Ed Boyden at the Massachusetts Institute of Technology and Tobias Moser’s team at the University of Göttingen in Germany.
A decade ago, Richter and his team were using lasers to directly stimulate auditory nerves in gerbils for hours at a time. By 2013, they were able to use the same technology to perform tests in deaf and hearing-impaired cats for up to six weeks, while the cats wore modified backpacks.
Today, several other projects aim to use optics to improve hearing devices: Gentiana Wenzel, at the University of Saarland in Homburg, Germany, is experimenting with using a green laser coded for sound frequencies to activate the inner ear. The U.S. military, concerned about hearing injuries over the last decade of warfare, has funded laser-related implant research, as has the U.S. National Institute on Deafness and Other Communication Disorders.
It’s a delicate and difficult task to catch all the waveforms of a noisy world and turn them into signals. And Richter is quick to point out the animal studies done so far to evaluate the technology have limitations. “There’s only so much you can do with cats,” he says. “You can measure signals, but the cat is not telling you that I hear this, or don’t hear that.”