Gerard Cote, biomedical engineering professor in the Dwight Look College of Engineering, is testing an experimental system using fluorescent polymer microbeads implanted just under a patient's skin. Glucose levels affect how much light the beads emit, which could be measured with a wristwatch-like monitor.

The experimental microgel beads, injected just under the skin, are too big to enter cells - unlike tattooing, in which cells absorb the pigment. Instead, the beads remain in the spaces between the cells, called the interstitial spaces. Fluid in those spaces contains water and glucose molecules small enough to pass through the PEG and reach the fluorophore-tagged polymers. Cote said the level of glucose in interstitial fluid is related to the blood glucose level that's measured by the traditional needle-stick method.

The dextran molecules bind to the conA molecules. Together, under light from a laser or light-emitting diode, they emit a certain color under fluorescence. However, when glucose enters the picture, it competes with dextran, displacing the dextran molecules and binding to the conA. The fluorescent color changes according to the amount of glucose present.

In preliminary studies, the researchers injected the microbeads under a rat's skin and found that the rat tolerated the implant. The beads did fluoresce under the rat's skin and indicated a change in glucose level.

Active tattoos can be found in various sf works; consider the subdermal microchannels from the 1985 cyberpunk classic Stone Lives by Paul Di Filippo: June's body is a tracery of lambent lines, like some arcane capillary circuitry in the core of Mao/K'ung Fu-Tzu. Following the current craze, she has had a subdermal pattern of micro-channels implanted. The channels are filled with synthetic luciferase, the biochemical responsible for the glow of fireflies.
(Read more about Di Filippo's Subdermal Microchannels)

A similar idea is used in Nova Swing by M. John Harrison.

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