Radio frequency – RF waves do not respond to the dead layer of skin on the outside of your finger – the part that might be chapped or too dry to be read with much accuracy – and instead reads only the living tissue underneath. This produces an extremely precise image of your print, and ensures that a severed finger is completely useless… This means that the Touch ID sensor should be remarkably accurate for living creatures, but it also means that only a finger attached to a beating heart will be able to unlock it. source