Researchers have discovered an enzymatic ink that can sense chemicals on walls, leaves, or skin. Image credit: Jacobs School of Engineering/UC San DiegoNanoengineers at the University of California, San Diego have developed a bio-ink that reacts to chemicals and can be delivered with an ordinary ballpoint pen.

The team said the ink can be used to draw on walls to detect pollution, or on smartphones to monitor vital signs. It might have military or industrial use, too – a line drawn on a wall could warn of explosives or hazardous chemicals in an area.

"Our new biocatalytic pen technology, based on novel enzymatic inks, holds considerable promise for a broad range of applications on site and in the field," team leader Joseph Wang said. Wang is the chairman of the Department of NanoEngineering at the University of California, San Diego, and also directs the Center for Wearable Sensors at the university. 

At its core, the sensor ink is made out of enzymes. Researchers already know how the enzymes react to different chemicals, so the real challenge was making sure the ink stayed together and could last for long periods of time. It was also critical to make sure the ink wasn’t harmful to humans or plants.

They used biocompatible polyethylene glycol to bind the various elements together. Chitosan, an antibacterial agent which is used in bandages to reduce bleeding, was also added to help the ink adhere to any surface on which it might be used. Xylitol, a sugar substitute, helps stabilize enzymes that react with the chemicals the sensors are designed to monitor.

 The team of researchers were originally trying to find a way to make glucose testing easier for people with diabetes. They came up with glucose sensors in the form of temporary tattoos: ink drawn on a flexible, transparent film that included an electrode.

The sensor was reusable; the researchers used the bio-ink to draw patterns on the film, then added a blood sample from the patient. The enzymes in the ink reacted to the glucose and the electrode recorded the measurement, which could then be transmitted to an external device. From this, the idea of more uses for the enzymatic ink was born.

The scientists have yet to test how the sensors function in extreme conditions, such as high or low temperature or extended exposure to harsh sunlight. As of now, the sensors still require a wired connection to a detection device, although the team is working on having the ink function wirelessly.

The team has demonstrated that the sensors can communicate with a Bluetooth-enabled pentiostat, used to control data-collecting electrodes. The ink could also be drawn directly on a patient’s skin to take various chemical readings. More applications may not yet have been discovered.

The team’s findings were published on Feb. 26 in Advanced Healthcare Materials.