Lasers can improve hearing

Ashley N. Paddock, ashley.paddock@photonics.com

New technologies to improve the insertion techniques and exact fitting of cochlear implants in the inner ear could soon provide better residual hearing for more than 200,000 people worldwide.

Approximately 95 percent of those who are highly hearing impaired have an auditory nerve intact enough to provide at least partial hearing. A cochlear implant – an electrical acoustic aid or prosthesis – takes over the function of damaged sensory cells in the inner ear. The device consists of an implant placed in the bone under the skin behind the ear, an electrode that is placed directly in the cochlea, and a microphone and speech processor placed behind the ear.

When sound waves are registered by the microphone, the device translates the waves into a series of electrical impulses, which are directed to the electrode on the auditory nerve in the inner ear.

Optimal hearing can take place only if the basilar membrane, which is covered by tiny sensory cells or hairs, is not damaged. If it is, complete loss of residual hearing occurs. It is essential to use extreme caution when inserting the cochlear implant so as not to damage these membranes.

Cochlear implant with an external microphone and microspeech processor. Courtesy of ©Cochlear.

To address the need for meticulously precise implantation, scientists in the Laser Zentrum Hannover eV (LZH) Surface Technology group are working to simplify the operation and improve the insertion technique of electrodes into the complex shape of the cochlea. The researchers used nickel-titanium shape memory alloys (NiTi-SMAs) in the manufacturing process for the implant electrodes. Heating the electrode enables the material to remember the form or shape in which it was manufactured, allowing specific movement and fitting of the electrode.

The team used a laser to melt the NiTi-SMAs into a highly individual implant. The special characteristics of the material are used to insert the implant into the cochlea without damaging the basilar membrane.

The deeper the material is inserted into the cochlea and the better the fit, the better the hearing will be.

To optimize the characteristics of cochlear implants, another group at LZH, Laser Micromachining, set out to improve the surface of implants by using laser structuring.

“The surface of conventional cochlear implants is not subject to special treatment, and a great potential is lost,” researcher Elena Fadeeva said. “We have learned from Mother Nature that biological surfaces – for example, of lotus leaves or shark skin – have defined structures for special functions.”

The group used a femtosecond laser to structure platinum electrodes in a special way. Special nanostructures look rough when magnified, but when manufactured, they can reduce attachment of connective tissue and improve interaction with the nerve cells. Nanostructuring also decreases frequency-dependent electrical resistance, so less energy is required.

The challenge is that the structure must be made on an implant with a curved surface and only 300 µm in diameter. The team is working to develop such a complex structure on a very small scale.

These newly developed “artificial inner ears” could help the hearing impaired to use the telephone again after sufficient training. For those who have recently lost hearing, or for children, the success rate should be even higher.

The developments were presented April 23-27 at Hannover Messe 2012.