This story on a bone conduction hearing aid you wear in your mouth caught my attention far more than you might expect. My previous employer was a manufacturer of an implantable hearing aid. It was a pretty cool device that worked on people with middle ear hearing loss. A sensor was attached to the incus to sense the incoming vibrations and convert it to an electrical signal. The signal was amplified (the electronics were in a small case that is under the skin and just behind the ear) and sent to a transducer on the stapes which then used the natural operation of the inner ear to do the rest of the work. (My particular job was to protect pretty much everything with various polymeric materials so that nothing would corrode.)
A huge advantage of the technology over regular hearing aids was that it didn't fill the ear canal, a body part that is evolved to collect and isolate sounds, particularly conversations, in a noisy setting. Another advantage was that being completely implanted, you could even swim with it.
One of the challenges that we faced arose because hearing actually involves the whole head. All the bones in your head can be used as vibration collectors to produce signals in the inner ear, thus leading to bone conduction hearing aids. While bone conduction seems like nothing but a positive, in our case, it was a huge negative as it was a troubling source of feedback. If we overdrove the stapes, the signal could be tranmitted through the head back to the incus, which would then further amplify the signal, ultimately leading to feedback that even Jimmy Hendrix could have never imagined.
Furthering the problem was that we could not come up with a set of synthetic materials that would mimic the bone conduction pathways in the human head. That led us to our the only option available:
we worked with whole human heads.
(Now that's something I never thought I would encounter in my life as a polymer scientist!)