77 Mass Ave
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
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 charac-
teristics for printing through a nozzle,”
Zhao says. “It’s like squeezing out tooth-
paste. You need [the ink] to flow out of
a nozzle like toothpaste, and it can main-
tain 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 research-
ers 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 molecu-
lar compounds. —Jennifer Chu C O
Tattoos made from genetically programmed cells could serve as wearable sensors.
The branches of this “living tattoo” are
3-D-printed with live bacterial cells that
sense different compounds on the skin.