by Neil Bauman, Ph.D.
Researchers are working on a cool new middle ear implantable hearing aid. It will consist of three parts: a case with a microphone and battery—probably to be worn in the ear; an electro-acoustic transducer implant attached to the round window in the middle ear; and a wireless optical signal and energy pathway between the external and internal parts.
The tiny electro-acoustic transducer is only about 1.2 mm in diameter, yet it can output volumes of up to a whopping 120 decibels. (1)
Two of the advantages of this implantable hearing aid, apart from its anticipated better sound quality, are its ease in implanting, and the minimal trauma it causes to the ear. The surgeon simply makes a slit in the eardrum, attaches the implant to the round window, then stitches up the eardrum again. That’s it.
How does it work? The transducer works on the same principle as a piezo-electric crystal. A voltage applied to the transducer causes it it bend, thus pushing on the round window. This causes a corresponding signal to be generated in the inner ear due to the resulting movement of the fluid in the inner ear.
Your inner ear is a marvelous organ and very ingeniously designed. There are two membrane-covered windows—the oval window and the round window.
The third of the three tiny bones in the middle ear, the stirrup (stapes) pushes on (vibrates against) the oval window. This causes movement of the fluid in the cochlea, resulting in a sound signal being produced and sent to the brain.
Since the cochlea is encased in unmovable bone and is filled with fluid, pushing on the oval window by itself would not cause any fluid movement as the fluid (perilymph) has nowhere to go so can’t move. Here’s where God’s ingenious solution comes in. He created the round window to allow for the necessary fluid movement.
When the stirrup (stapes) pushes the oval window in, the pressure exerted on the fluid (perilymph) pushes the round window out by the same amount. This allows the fluid to move (and thus generate a sound signal). (2)
Conceptually, think of a short glass tube (the cochlea) with balloons attached to each end. If you squeeze the balloon at the left end (oval window balloon), the fluid in the oval window balloon will flow down the tube and into the balloon at the other end (round window balloon). This makes the round window balloon bulge out to the same degree that the oval window balloon was pushed in.
Therefore when the stirrup (stapes) pushes the oval window in, the round window bulges out by the same amount. This means the two windows operate 180 degrees out of phase.
What the press release didn’t explain is exactly how this new implant will work. I assume they will feed the sound signal into the round window 180 degrees out of phase with the sound signal reaching the oval window via the ossicular chain. If they do this, the round window transducer will produce amplified mechanical vibrations working in unison with the sound signals reaching the oval window via the middle ear, thereby enhancing the residual hearing capacity of people wearing these implanted hearing aids.
Therefore, if the implant should ever fail, you would still have your residual hearing intact. Sounds like a win-win situation to me.
If all goes well, researchers hope to begin testing this new hearing aid in 2014. (1)
(1) Regaining proper hearing at last. April 2, 2013. Research News. Fraunhofer-Gesellschaft.
(2) Round window. Wikipedia.