Researchers from Tokushima University in Japan demonstrated that data can be stored in a human fingernail. The data is "written" onto the nail with femtosecond laser pulses and read using a fluorescence microscope. In experiments on a clipped piece of nail, each "bit" of data is 3.1 microns in diameter and 5 microns apart from its closest neighboring bit. The system records in three layers at depths of 40, 60, and 80 microns. (A human hair is around 100 microns thick.) From Optics.org:
Capacities are said to be up to 5 mega bits and the stored data lasts for 6 months – the length of time it takes a fingernail to be completely replaced.
"I don't like carrying around a large number of cards, money and papers," (Yoshio) Hayasaki from Tokushima University told Optics.org. "I think that a key application will be personal authentication. Data stored in a fingernail can be used with biometrics, such as fingerprint authentication and intravenous authentication of the finger…"
Although the initial experiments have concentrated on small pieces of nail, the team is now developing a system that can write data to a fingernail which is still attached to a finger. "We will develop a femtosecond laser processing system that can record the data at the desired points with compensation for the movement of a finger," said Hayasaki.