The First Invisible Hearing Aid
From: Popular Science - 12/2006 - page 98

Whereas standard hearing aids require external batteries or microphones, the
implantable Esteem is totally out of sight—and could boost hearing by 30
decibels. Surgically inserted under the skin behind the ear, the device
circumvents the faulty connections between the inner and outer ear that
create hearing loss. An internal sensor registers vibrations received by the
eardrum and transmits the signals to a sound processor. The processor, in
turn, sends vibrations to the cochlea, where the brain interprets them as
sound. 

Source:
http://www.popsci.com/popsci/flat/bown/2006/product_92.html

Links:
Envoy Medical
http://www.envoymedical.com/

How the Esteem works (video 1:01)
http://www.envoymedical.com/how-the-esteem-works