Researchers at the University of Tokushima and at Hokkaido University in Sapporo, both in Japan, have used human fingernails as a medium for three-dimensional optical data storage. Employing a femtosecond laser to write and a fluorescence microscope to read, they stored data at a density of 2 Gb/cm3. That's concentrated enough to pack more than 0.5 Mb in a space 0.5 cm on a side and 0.1 mm deep, dimensions that can be easily accommodated by any fingernail.Using a femtosecond laser, data can be written in a fingernail (top). Data retrieval is performed using fluorescence microscopy (bottom). The multilayer technique offers a data storage density of 2 Gb/cm3. Courtesy of Yoshio Hayasaki.Yoshio Hayasaki, an associate professor of physics at the University of Tokushima, explained that the work is more than a novelty. "A key application is personal authentication," he said.He envisions fingernail optical storage being employed for biometrics and user identification, much as a fingerprint is used today. Unlike fixed fingerprints, however, nail-based imprinting of data will allow stored information to be tailored for a given situation and to be changed over time. It also may find use in the identification and authentication of pets and food animals.In a demonstration of the technique, the researchers used a Tsunami mode-locked Ti:sapphire laser and a Spitfire Ti:sapphire regenerative amplifier, both from Spectra-Physics, to generate pulses with a width of less than 100 fs and a center wavelength of 800 nm. An Olympus IX70 inverted microscope focused the pulses down to a spot within a fingernail sample whose surface had been polished with an abrasive. By adjusting the focal point and moving the motorized microscope stage, the investigators could irradiate locations in a three-dimensional volume within the fingernail.When the femtosecond laser pulses strike the sample, structural changes occur at the focal point. Spectrometer measurements performed in the demonstration indicated an increase in fluorescence. The researchers believe that multiphoton absorption and a microexplosion cause keratin protein in the fingernail to denature and the fluorescence to increase.To read the recorded data, they used a Nikon Corp. ME600 fluorescence microscope equipped with various filters. When they illuminated the fingernail with ultraviolet radiation from a xenon arc lamp, they could observe the laser-induced spots with a CCD image sensor. These imprinted points survived heat and lasted at least 172 days, which is comparable to the six months it takes a fingernail to grow out. Under the experimental conditions, the bit diameter was approximately 3.1 µm, with a spacing of about 5 µm horizontally and 20 µm vertically.It will be several years at least before a data-writing fingernail system makes it out of the lab. Until then, you won't chew up your ID when you bite your nails.