There are a lot of creative things you can do with tattoos, particularly the temporary stick-on kind. You can get temporary tattoos made from metallic ink, glitter, and even crystals – but engineers at MIT may be the first to have come up with a “living tattoo” that lights up. The engineers used a 3D printing method to create the tattoo, which is made from live bacteria cells patterned in the shape of a tree on a thin, transparent patch. The cells were printed in the same way that cells are 3D printed in many bioprinting applications – they were mixed with hydrogel and nutrients and then extruded layer by layer to form the shape.
Each of the tree’s branches is composed of cells that are sensitive to different chemical or molecular compounds; these branches light up when the skin the patch is adhered to has been exposed to those compounds. According to the researchers, the technology could be used in the creation of wearable sensors and interactive displays. The devices can be patterned with live cells engineered to sense things like chemicals and pollutants as well as changes in pH and temperature.
The team also came up with a model to predict how cells will act within a 3D printed structure, under a variety of conditions. The research was published in a paper entitled “3D Printing of Living Responsive Materials and Devices,” which you can access here.
Other responsive 3D printed materials have been created from temperature-sensitive polymers, or polymers that shrink or expand in response to light. To create a responsive material from live cells, the MIT researchers looked at ways to avoid some of the problems that had plagued other scientists 3D printing cells – particularly mammalian cells.
“It turns out those cells were dying during the printing process, because mammalian cells are basically lipid bilayer balloons,” said graduate student Hyunwoo Yuk. “They are too weak, and they easily rupture.”
Bacteria cells, on the other hand, were hardier. Their tough cell walls can survive being pushed through a 3D printer’s extruder, and they’re compatible with most hydrogels. After a screening test, the MIT researchers found that a hydrogel with pluronic acid was the most compatible material, and it also had an ideal consistency for 3D printing.
“This hydrogel has ideal flow characteristics for printing through a nozzle,” said Xuanhe Zhao, the Noyce Career Development Professor in MIT’s Department of Mechanical Engineering. “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.”
The researchers then combined the hydrogel with bacteria engineered to light up in response to different chemical stimuli, as well as nutrients to sustain the cells.
“We found this new ink formula works very well and can print at a high resolution of about 30 micrometers per feature,” Zhao said. “That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimeters.”
They used a custom built 3D printer to print the shape of a tree on an elastomer patch, which was then cured using UV light. Then it was time to test the patch. A test subject’s hand was smeared with several different chemical compounds, and the “tattoo” was placed on top. After several hours, the branches of the tree began to light up as the bacteria sensed their corresponding chemical stimuli.
The researchers also engineered some of the bacteria to communicate with each other. Some cells were programmed to light up only when they received a signal from another cell. To test this communication method, the scientists printed a thin sheet of hydrogel with “input,” or signal-producing bacteria and chemicals, overlaid with a layer of “output,” or signal-receiving bacteria. The output bacteria lit up only when they overlapped and received signals from the input bacteria.
According to Yuk, scientists in the future could use these techniques to 3D print living computers, or devices that have multiple types of cells communicating with each other, passing signals back and forth 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 Yuk.
In the more near future, the researchers want to create customized sensors in the form of flexible patches and stickers – or tattoos – that could be engineered to detect different chemical or molecular compounds. They’re also looking into creating drug capsules and surgical implants containing cells engineered to produce compounds such as glucose, which could be released over time.
“We can use bacterial cells like workers in a 3-D factory,” said graduate student Xinyue Liu. “They can be engineered to produce drugs within a 3-D scaffold, and applications should not be confined to epidermal devices. As long as the fabrication method and approach are viable, applications such as implants and ingestibles should be possible.”
Authors of the study include Xinyue Liu, Hyunwoo Yuk, Shaoting Lin, German Alberto Parada, Tzu-Chieh Tang, Eléonore Tham, Cesar de la Fuente-Nunez, Timothy K. Lu and Xuanhe Zhao.
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