3D tattoos for chemical sensing

Engineers at Massachusetts Institute of Technology have devised a 3D printing technique that uses a novel kind of ink made from genetically programmed living cells. The cells are engineered to light up in response to a variety of stimuli. When mixed with a slurry of hydrogel and nutrients, the ink can be printed, layer by layer, to form a near-flat but three-dimensional, interactive structure for wearable technologies and other applications.

Xuanhe Zhao and Timothy Lu say that their approach to wearables could be used to make "active" materials for sensors and interactive displays. Patterning with living cells means such a sensor could detect environmental chemicals, changes in pH, and temperature and perhaps even display the results.

The team has demonstrated proof of principle by printing a "living tattoo" - a thin, transparent patch patterned with live bacteria cells in the shape of a tree. Each branch of the tree is lined with cells sensitive to a different chemical. A person with the patch stuck to their skin could get an indicator of exposure to those chemicals as the cellular lights on the tree switch on. Zhao et al., Adv. Mater (2017); DOI: 10.1002/adma.201704821]

To test the tattoo, the team smeared different chemicals on the back of a test subject's hand. They then applied the fabricated hydrogel patch on this area of skin. Over the course of several hours, parts of the tattoo tree lit up as the bacteria detected the chemical stimuli.

The team adds that their system could be adapted to a wide range of applications. They can predict what interactions occur between the cells within a given printed structure, under different conditions, the mode thus derived could be used to design other responsive living materials.

3D-printed inks have been tested previously, with temperature-sensitive polymers for heat-responsive shape-shifting objects, just one success story. Other researchers have printed photoactive polymer structures that shrink and stretch in response to light. Attempts have been made to print mammalian cells but these tend to die in the process. The teams of Zhao and Lu recognized that living bacterial cells might be more suitable and certainly more adaptable than polymeric materials. Bacterial cells have tough cell walls that are able to survive relatively harsh conditions, such as the forces applied to ink as it is pushed through a printer's nozzle, the researchers explain. Bacteria are also compatible with hydrogels, and these can provide an aqueous, supportive environment for the bacteria.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.