Engineers at Massachusetts Institute of Technology (MIT) have found a way to 3-D print a "living tattoo" using a new kind of ink made from genetically programmed living bacteria cells.

The "living tattoo" -- a thin, transparent patch patterned with live bacteria cells in the shape of a tree -- could have implications for future wearable sensors and in the manufacturing of drug capsules and surgical implants.The cells were engineered to light up in response to a variety of stimuli, showed the study published in the journal Advanced Materials.The researchers came up with a recipe for their 3-D ink, using a combination of bacteria, hydrogel, and nutrients to sustain the cells and maintain their functionality.

"We found this new ink formula works very well and can print at a high resolution of about 30 micrometres per feature," said Xuanhe Zhao, Professor in MIT's Department of Mechanical Engineering."That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimetres," Zhao added.They printed the ink using a custom 3-D printer that they built using standard elements combined with fixtures they machined themselves. 

To test the patch, the researchers smeared several chemical compounds onto the back of a hand, then pressed the hydrogel patch over the exposed skin. Over several hours, branches of the patch's tree lit up when bacteria sensed their corresponding chemical stimuli.The researchers also engineered bacteria to communicate with each other.For instance they programmed some cells to light up only when they receive a certain signal from another cell. 

The researchers believe that the technique can be used to fabricate "active" materials for wearable sensors and interactive displays.
Such materials could be patterned with live cells engineered to sense environmental chemicals and pollutants as well as changes in temperature.In the future, researchers may also use the technique to print "living computers" -- structures with multiple types of cells that communicate with each other, passing signals back and forth, much like transistors on a microchip."This is very future work, but we expect to be able to print living computational platforms that could be wearable," said graduate student Hyunwoo Yuk.The researchers also envision their technique may be used to manufacture drug capsules and surgical implants, containing cells engineered to produce compounds such as glucose, to be released therapeutically over time.