Sound travels through the ear drum, the middle ear, and is then processed by the inner ear.

FORBES: Deaf People Will Hear

It's not quite implantable, but Insound Medical's new Lyric hearing aid is about as close as you can get, with it designed to squeeze in deep in your ear canal and sit just a few millimeters from your ear drum.

ENGADGET: Insound's Lyric hearing aid cozies up right next to your ear drum

When nearly 30 years ago Dr. Jacob Fraden invented an ear thermometer working on this second principle, the infrared light sensor was so large that he needed a narrow gold-plated reflective tube to channel the light from the ear drum to the sensor.

FORBES: Body temperature.