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Living Ink

Tattoos made from genetically programmed cells could serve as wearable sensors.
February 21, 2018
courtesy of the researchers

MIT engineers have devised a 3-D-printing technique that uses a new kind of ink made from genetically programmed living cells.

The cells are engineered to light up in response to certain stimuli. When mixed with a slurry of hydrogel and nutrients, they can be printed to form responsive three-dimensional structures and devices.

As a demonstration, the team has printed a “living tattoo”—a transparent patch patterned with bacterial cells in the shape of a tree. Each branch of the tree is lined with cells sensitive to a different chemical. When the patch is stuck to skin exposed to those compounds, corresponding regions of the tree light up in response.

The researchers, led by Xuanhe Zhao, an associate professor of mechanical engineering, and Timothy Lu, an associate professor of biological engineering and of electrical engineering and computer science, say their technique can be used to fabricate “active” materials for wearable sensors and interactive displays, patterned with live cells engineered to sense environmental chemicals as well as changes in pH and temperature.

Zhao and Lu realized that live cells might serve as responsive materials for 3-D-printed inks, particularly because they can be genetically engineered to respond to a variety of stimuli. They chose to work with bacterial cells, whose tough walls can survive relatively harsh conditions, such as the forces applied to ink as it is pushed through a printer’s nozzle. Bacteria are also compatible with most hydrogels—gel-like materials made from water and a bit of polymer. The group found that a hydrogel containing pluronic acid can sustain bacteria. It is also well suited to the printing process.

“This hydrogel has ideal flow characteristics for printing through a nozzle,” Zhao says. “It’s like squeezing out toothpaste. You need [the ink] to flow out of a nozzle like toothpaste, and it can maintain its shape after it’s printed.”

Lu provided the team with bacterial cells engineered to light up in response to a variety of chemical stimuli. The researchers then came up with a recipe for their 3-D ink, including nutrients to sustain the cells and maintain their functionality.

They printed the ink using a custom 3-D printer that they built using standard elements combined with fixtures they machined themselves. To demonstrate the technique, the team made the tree tattoo by printing a pattern on an elastomer layer. They then cured, or solidified, the patch by exposing it to ultraviolet radiation.

The researchers smeared several chemical compounds onto the back of a test subject’s hand and 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.

For near-term applications, the researchers envision flexible patches and stickers that could be engineered to detect a variety of chemical and molecular compounds. 

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