You've probably heard about pens with conductive ink, that allow users to draw circuits onto materials such as paper. Now, researchers at the University of California, San Diego have gone a step or two farther – they've created "bio-inks" that could be used to draw sensors onto a variety of surfaces, using an ordinary ballpoint pen.

You've probably heard about pens with conductive ink, that allow users to draw circuits onto materials such as paper. Now, researchers at the University of California, San Diego have gone a step or two farther – they've created "bio-inks" that could be used to draw sensors onto a variety of surfaces, using an ordinary ballpoint pen.

The inks are simply loaded into store-bought pens, and were initially designed as a means of measuring diabetics' glucose levels by being applied to their skin.

Specific enzymes are used in each type of ink, depending on what chemical it's designed to detect. Among the other ingredients are polyethylene glycol, which serves as a binder; graphite powder, for electrical conductivity; chitosan, which helps the ink adhere to surfaces; and xylitol, which stabilizes the enzymes. The mixture is reportedly safe for application to humans, and remains viable over long periods in storage prior to use.

So far, the bio-inks have been successfully used to measure both glucose beneath the skin, and pollutants on leaves. It was estimated that the ink in just one pen would be sufficient for about 500 individual glucose tests.

Down the road, the researchers believe that the inks could also be used for applications such as detecting explosives on the battlefield, measuring toxic gases on building walls, or to add health-monitoring functionality to smartphones.

Currently, although the drawn-on ink serves as the sensor itself, a separate device known as a potentiostat must be brought into contact with it in order to actually read it. The UC Diego team hopes to change that, however, by developing bio-ink sensors that communicate wirelessly with monitoring devices.

The research was led by Joseph Wang, whose lab has previously developed temporary-tattoo-like sensors that measure lactate levels and detect metabolic problems. A paper on the bio-ink was recently published in the journal Advanced Healthcare Materials.

As a side note, scientists at MIT have previously used pencil lead containing carbon nanotubes to draw sensors onto paper.