Take a moment to think about what your ears do. The human ear has a complex system for turning mere sound waves into electric messages for your brain, and it serves to keep you balanced. In order to turn sound waves (which are mechanical energy) into the electrochemical energy that sends messages to the brain, the waves have to go through the three parts of the ear.
The outer ear is what you generally think of, the fleshy outside and ear canal, and it funnels sounds into your ear and helps with identifying where the sound came from. The middle ear uses the eardrum and some very small bones to make the sound louder. The inner ear uses the vibrations from the middle ear to move liquid in the cochlea, which moves small, sensitive hairs that help turn the sound into the signals the brain can read. It’s a pretty cool process.
The battery is the result of a membrane in the cochlea which has unequal amounts of potassium and sodium molecules on either side. When the membrane lets them pass through it, electricity is created. Most of it goes toward making the ear work, but the researchers were able to redirect a little to a small radio transmitter. The transmitter is specially designed to be extremely energy efficient and able to store energy when not in use. It does need to be jump-started when first implanted though, which is done with sending radio waves to it.
So far the device has only been put in guinea pigs, but in the future could expand from simple medical measurements to providing therapy to people who have hearing disabilities.