A new color-changing ink could aid in health and environment monitoring—for example, allowing clothing that switches hues when exposed to sweat or a tapestry that shifts colors if a dangerous gas enters the room. The formulation could be printed on anything from a T-shirt to a tent.

Wearable sensing devices such as smart watches and patches use electronics to monitor heart rate, blood glucose, and more. Now researchers at Tufts University's Silklab say their new silk-based inks can respond to, and quantify, the presence of chemicals on or around the body. Silk's ability to “act like a protective ‘cocoon’ for biological materials” means the necessary sensing and color-changing compounds can be added to the ink without losing their function, says Fiorenzo Omenetto, a biomedical engineer at Silklab and senior author of a new paper on the technology published in July in Advanced Materials.

The researchers improved on an earlier iteration that worked with inkjet printers, thickening the ink with the chemical sodium alginate to make it viable for screen printing, and then added various reactive substances. With the new ink, they can now “easily print a large number of reactive elements onto large surfaces,” Omenetto says.

The team made the ink by breaking down raw silk fibers into constituent proteins, which the researchers suspended in water. Next they mixed in various reactive molecules and analyzed how the resulting products changed color when exposed to alterations in their environment. When printed on fabric and worn, pH indicators, for example, could convey information about skin health or dehydration; lactate oxidase could measure a wearer's fatigue levels. The changes are visible to the naked eye, but the researchers also used a camera-imaging analysis to continuously monitor the color variations and create a database of values.

“In the case of a T-shirt, the wearer ‘paints’ the shirt [through] exercise—with colors correlating to the acidity distribution of their sweat,” Omenetto says. The ink could also be adapted to track environmental changes in a room, he says, or to respond to bacteria and follow disease progression.

Mechanical engineer Tyler Ray of the University of Hawaii at Mānoa, who was not involved with the study, notes that most of today's wearable monitors are rigid and fairly bulky. The new ink technology has “the potential to transform consumer wearables from recreational novelty devices into body-worn, clinical-grade physiological measurement tools that yield physician-actionable information,” he says. But “one of the challenges with any colorimetric approach is the effect various environmental conditions have on accuracy, such as lighting ... or the camera used.” Future work needs to address these issues.