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Fingernail storage is a method of writing data onto a human fingernail using a pulsed laser. The fluorescence of the nail, when exposed to ultraviolet (UV) light, is increased at points where data is written. Data can be read from the fingernail using a microscope while irradiating the nail with UV energy.
Recent experiments with fingernail storage, conducted by Yoshio Hayasaki of Tokushima University (Japan), have involved small regions of a single nail, measuring approximately two millimeters (mm) square. A pulsed laser is used, at a wavelength of 800 nanometers (nm), to write the data onto the nail. Each data bit measures approximately 0.003 millimeters (mm) in diameter. Individual data bits are spaced 0.005 mm apart, in three layers at depths of 0.04, 0.06, and 0.08 mm within the nail.
Fingernail storage has a limited life because human nails grow out. The average human fingernail is completely replaced by the body every six months, assuming the nail is clipped short at regular intervals. Fingernail storage has been suggested as a biometric means of identification, and also for storing critical medical information for use in emergencies.