While the sense of hearing is an entirely mechanical process, it's an extraordinarily complex process.
Hearing works something like this: the pinna or outer part of the ear collects nearby sound waves, and depending on the direction from which a sound is coming toward you, the brain decodes a variety of different positional markers by comparing the information collected by both ears.
The process of hearing is also controlled by the brain's determination of whether a sound has arrived at your left ear or right ear first, and differentiates between subtle changes in volume collected by both ears.
As sounds travel into the ear canal, they cause the tympanic membrane – a thin, cone-shaped flap of skin less than half an inch wide – to vibrate. The eardrum is rigid, and exceptionally sensitive to minute fluctuations in air-pressure and its movement is analogous to the diaphragm in a microphone or speaker. Sound waves and their attendant pressure changes cause the membrane to move back and forth. Higher-pitch sound waves move the drum faster, lower-pitch sound waves move it slower, and louder noise causes it to move a greater distance when compared to more quiet sounds.
The cochlea is by a good measure the most complicated structure in the ear. It takes the physical vibrations generated by a sound wave and translates them into electrical information the brain can process.
Now a German firm, Laser Zentrum Hannover (LZH), is using 3D printing technology to recreate those minute and complex components for use in cochlear repair surgery. As part of a collaboration with the Hannover Medical School, LZH uses laser sintering to construct the small bones encased deep within the inner ear.
Scientist say that some 95% of those who are highly hearing impaired do have an adequately intact auditory nerve, at least functional enough to provide partial hearing. To return their hearing to full functionality, doctors use cochlear implants and electronic acoustic aids or prosthesis to take over the work of damaged sensory cells in the inner ear. Consisting of an electrode placed directly in the cochlea and a microphone and speech processor behind the ear, the implants work by registering sound waves through the microphone and then translating the input into electrical impulses which are transferred to an electrode on the auditory nerve of the inner ear itself.
Working to repair damaged hearing is a dicey proposition. Surgeons must operate in an environment which can be permanently damaged by the slightest misstep in placement. A structure like the basilar membrane, covered as it is by minute, hair-like sensory cells, can't be damaged or disturbed without disrupting optimal hearing. If the membrane is damaged, a complete loss of residual hearing could result.
What it comes down to is this; the cochlea electrode has to be inserted with extreme care to prevent damage to the membrane, and LZH say they have a process to simplify the operation and improve the insertion technique of the electrode into the cochlea.
Part of the process involves the special properties of nickel-titanium shape memory alloys, or NiTi-SMA, used to make the CI electrodes. By heating the electrode, or through electrical impulses, the material essentially 'remembers' a shape it held when manufactured. That allows specific movement and fitting of the electrode. Laser sintering is used to form the NiTi-SMA into a customized, highly individual implant based on the patient's body structure.
"The surface of conventional cochlea implants is not subject to special treatment," says Elena Fadeeva of LZH. "We've learned from mother nature that the biological surfaces of, for example, lotus leaves or shark skin, have defined structures for special functions."
Fadeeva says that through the use of a femtosecond laser, platinum electrodes can be manufactured which, though they look very rough when magnified, reduce attachment of connective tissue and improve interaction with the nerve cells. Through nanostructuring, the implants can include essential fine details on a surface only 300 µm in diameter which is also curved.
The researchers add that cochlear implants could benefit some 200,000 people worldwide and that the success rate for children needing the implants should improve as well.