Scientist has been
exploring 3D printing in order to build many different structural items. 3D
printing allows structures to be created in layers, meaning that there are
multiple thin layers throughout the structure. At MIT scientist for  configuring a way to make a living tattoo using
“live ink” building the structure from 3D printing. Scientist wanted to know if
live cells could serve as responsive materials in 3D inks. These live
structures or “tattoos” can help with alerting the person wearing it when the
skin is exposed to environmental chemicals, pollutants, changes in PH and
temperatures.  

There are many
variables to have to figure out in order to answer the question “If living
cells can serve as responsive materials to 3d inks.” Scientists at MIT had to
find a cell that would be able to stay active through the printing process. As
MIT was not the first to try out the idea of using 3D printing using live
cells, so they learned from other scientists that mammalian cells were too
fragile to go through the printing process.  
MIT found that using bacteria had a tough enough wall to go through the
pressure of the nozzle that the printing process endures and be able to survive
once put on the skin. Before the cells can reach the printing process, they
have to be programmed for what they are detecting. The tree tattoo that MIT
produced has three differently programmed cells. According to MIT News “The
researchers also engineered bacteria to communicate with each other; for
instance they programmed some cells to only light up when they receive a
certain signal from another cell” (2017). Different areas of the tattoo will
light up at different times when a person encounters what the bacteria was
designed to detect. The tree tattoo is split up into thirds, the left side to
detect rham, the middle sensors IPTG and the right senses AHL.  

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MIT’s research found
that mixing hydrogel with pluronic acid and nutrients was the accurate mixture
to hold and support the bacteria for the ink to go through the 3D printer and
survive on the skin. The ink has to be the constancy of toothpaste to allow the
printer to be able to build layer upon layer. If the ink is, too loose, it will
flow out of the nozzle and if the ink is, too think, it will clog the nozzle.
MIT reported that “Ink can be printed into 3D structures when the concentration
of the hydrogel is between 18 and 36 wt% and the pressure is above a certain
threshold” (Advance Materials 2017). After the image is completed, the structure
is cured with UV radiation.

This live tattoo
is printed on a bilayer elastomeric sheet, that sheet containing the image that
will adhere to the skin. In the thin layer, it contains living sensors that are
embedded in the ink. These sensors will be activated when detecting certain
chemicals, the logic gates use green fluorescent protein gates to be able to
light up each area of the tree. When the tattoo is adhering to the skin, it
does not constrain movement to the person or cause tension to the skin. This
tattoo stays on the skin in three ways; the image can be stretched, compressed,
or twisted and not detach from the skin.  

Scientists at MIT
tested their results by first applying chemicals to the skin. Then they
attached the bilayer elastomeric sheet to the skin. Over several hours after
attaching the “tattoo”, the tree branches started to light up, depending on
which chemicals they were sensing. To test the bacteria that need a signal from
another bacteria in the tattoo, scientists had to create two overlays. When
they only put the input overlay on the tattoo, nothing happened. When they put
the output overlay on top of the input overlay, the tree branches in the tattoo
started to light up.

“The prediction of
the spatiotemporial patterns of 3D printing living materials relies on the
understanding of two processes: the diffusion of signaling chemicals in
hydrogel matrices and induction of encapsulated bacteria cells by signaling
chemicals” (Advance Materials pg 5). With this information, it enables
scientists at MIT to be able to explore other possibilities of living devices. “This
is future work, but we expect to be able to print living computational
platforms that could be wearable” (Chu, 2017). This technology opens up the
opportunity to be able to manufacture drug capsules, surgical implants that
will automatically be able to release drugs therapeutically on their own.