Biomaterial-based inks that respond to and quantify chemicals released from the body or the environment by changing colour have been developed for use in clothing and footwear.

The development, made by researchers at Tufts University's School of Engineering in Massachusetts, means inks can be screen printed onto textiles in complex patterns and at high resolution, providing a detailed map of human response or exposure.

The advance in wearable sensing could simultaneously detect and quantify a wide range of biological conditions, molecules and, possibly, pathogens over the surface of the body using conventional garments and uniforms, the researchers say.

"The use of novel bioactive inks with the very common method of screen printing opens up promising opportunities for the mass-production of soft, wearable fabrics with large numbers of sensors that could be applied to detect a range of conditions," says Fiorenzo Omenetto, co-author and the Frank C. Doble Professor of Engineering at Tufts' School of Engineering. "The fabrics can end up in uniforms for the workplace, sports clothing, or even on furniture and architectural structures."

Wearable sensing devices have attracted considerable interest in monitoring human performance and health. Many such devices have been invented incorporating electronics in wearable patches, wristbands, and other configurations that monitor either localised or overall physiological information such as heart rate or blood glucose. The research presented by the Tufts team takes a different, complementary approach – non-electronic, colorimetric detection of a theoretically very large number of analytes using sensing garments that can be distributed to cover very large areas, from a patch to the entire body.

The components that make the sensing garments possible are biologically activated silk-based inks. The soluble silk substrate in these ink formulations can be modified by embedding various "reporter" molecules – such as pH sensitive indicators, or enzymes like lactate oxidase to indicate levels of lactate in sweat. The former could be an indicator of skin health or dehydration, while the latter could indicate levels of fatigue of the wearer. Many other derivatives of the inks can be created due to the versatility of the silk fibroin protein by modifying it with active molecules such as chemically sensitive dyes, enzymes, antibodies and more. While the reporter molecules could be unstable on their own, they can become shelf-stable when embedded within the silk fibroin in the ink formulation.

The inks are formulated for screen printing applications by combining with a thickener (sodium alginate) and a plasticiser (glycerol). While the changes in colour presented by the inks can provide a visual cue to the presence or absence of an analyte, use of camera imaging analysis scanning the garments or other material can gather more precise information on both quantity and high resolution, sub-millimetre mapping.

"The screen printing approach provides the equivalent of having a large, multiplexed arrangement of sensors covering extensive areas of the body, if worn as a garment, or even on large surfaces such as room interiors," adds Giusy Matzeu, research assistant professor of biomedical engineering at Tufts School of Engineering and first author of the paper. "Coupled with image analysis, we can obtain a high-resolution map of colour reactions over a large area and gain more insight on overall physiological or environmental state. In theory, we could extend this method to track air quality, or support environmental monitoring for epidemiology."