Infrared light can be used to replace or improve hearing aids and cochlear implants

The flashing lights will soon have a new effect. Researchers in Germany, Switzerland, and Austria have recently developed a concept to develop a prototype hearing implant that uses a series of laser pulses to trigger an auditory signal from a hair cell located in the inner ear.

Researchers believe that a series of near-infrared lasers can generate sound waves using so-called photoacoustic effects. In their device, a tiny vertical cavity surface emitting laser (which pulsates light in the 1.4 to 1.9 micron spectrum) acts on the liquid in the "artificial cochlear tube" of the inner ear.

Basically, infrared light is absorbed by the liquid in the cochlea. A small portion of the liquid will expand due to heat. If this happens quickly, it produces a sound wave in the cochlear duct. This stimulates or moves tiny hair cells located there, and the cells send signals along the auditory nerves that the brain will understand.

In the past three years, researchers have established tiny laser arrays and completed tests on guinea pigs, which have been found to produce action potentials, signals carried by auditory nerves, using vertical laser and photoacoustic effects. They compared the irritations of the laser array to the guinea pigs and the hum of the sound. Both produce neural signals that match form and amplitude.

"It's still in its early stages, but it is hoped that this technology can be used to replace or improve hearing aids and cochlear implants," said Mark Fretz, a physicist and project manager at the Center for Electronics and Microtechnology (CSEM). The Applied Research Center is a technology non-profit organization based in Alpnach, Switzerland.

The next step will be to increase the energy efficiency of the equipment and make it smaller. The various components developed for the prototype, including the tiny sapphire shell used to seal the implanted body sensor and the improved laser lens design, can also find other uses, such as allowing the laser to illuminate the ear to improve balance.

Today's cochlear implants rely on electrodes to pass through the skull to the inner ear. The electrodes generate an electric field that stimulates the cochlear nerves, converting the surrounding sound into electrical signals that the nerves carry to the brain. However, focusing the electric field is difficult, so it tends to flow into other tissues and produce noise.

Fretz and CSEM are members of the Hochschule Hearing and Laser Research team at Hannover Medical, as well as Bavarian laser manufacturer Vertilas, precision lens and array manufacturer SUSS MicroOptics. The team has a wealth of expertise. As one of the largest manufacturers of cochlear implants, Innsbruck's MED-EL is also involved in the development of hardware. The VTT Technology Research Center in Finland provides an anti-fouling coating to protect the implanted wires from excessive fiber growth.

Prototypes also face design challenges, including how to solve power consumption problems and how to shrink components. Implanted into the body can not produce too much heat, otherwise it will damage the surrounding cells and tissues. The researchers found that generating many 50 nanosecond pulses can essentially replicate a 50 microsecond pulse and reduce the amount of heat generated by the continuous flash. This burst is required to create an acoustic composite action potential - a signal that is transmitted along the auditory nerve.

The team used a sapphire box to seal the laser and components to isolate body fluids and corrosion, attach the components to a platinum strip, and shape them with silicone rubber. In the guinea pig test, the researchers used a plug-in version of their device, but in the newer prototype, they were able to reduce the length of the laser casing to two millimeters and a width of one millimeter.

The entire device is now in the shape of a slimming nine-volt battery that can be placed on the palm of your hand. “This needs to be further narrowed,” Fretz said. “For commercial implants, “this is possible.”

Claus-Peter Richter, vice president of otolaryngology at Northwestern University, participated in the study from the beginning. He said, "In recent years, the use of optical transmission of auditory signals has caused much controversy. On the topic, there are many experiments at the same time, and there are also great developments."

An early method attempts to directly stimulate the auditory nerve with light; another purpose is to genetically alter cells and make them react to light - by Ed Boyden, a molecular maker at the Massachusetts Institute of Technology, and Tobias Moser from the University of Göttingen, Germany. The team pioneered a concept.

Ten years ago, Richter and his team used lasers to directly stimulate the squirrels' auditory nerves for hours. By 2013, they were able to use the same technique for six weeks of testing on sputum and hearing impaired cats, while the cat was wearing a modified backpack.

Today, there are several other projects whose goal is to use optics to improve hearing aids: Gentiana Wenzel of the University of Saarland, Humboldt, Germany, is trying to activate the inner ear using a green laser coded as audio. The US military has been concerned about hearing impairment in the wars of the past decade, and it has jointly funded laser-related implant research with the National Deafness and Other Communication Disorders Institute.

Capturing all the waveforms of a noisy world and turning it into a signal is a tricky and arduous task. Richter pointed out that the animal research evaluation techniques made so far have limitations. He said: "There are only so many experiments you can do with cats. You can measure the signal, but the cat will not tell you that I heard it, or I can't hear it.