MIT engineers have devised a 3D 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 a variety of stimuli. When mixed with a slurry of hydrogel and nutrients, the cells can be printed, layer by layer, to form 3D interactive structures and devices.
The team has demonstrated its technique by printing a “living tattoo,” a transparent, thin patch patterned with live bacteria cells in the shape of a tree. Each branch of the tree is lined with cells sensitive to a different chemical or molecular compound. When the patch adheres to skin that has been exposed to the same compounds, corresponding regions of the tree light up in response.
The researchers led by Xuanhe Zhao, the Noyce Career Development Professor in MIT’s Department of Mechanical Engineering, and Timothy Lu, associate professor of biological engineering and of electrical engineering and computer science, say that their technique can be used to fabricate “active” materials for wearable sensors and interactive displays. These materials can be patterned with live cells engineered to sense environmental chemicals and pollutants as well as changes in pH and temperature.
The team also developed a model to predict the interactions between cells within a given 3D-printed structure, under a variety of conditions. The team says researchers can use the model as a guide in designing responsive living materials.
In recent years scientists have explored a variety of responsive materials as the basis for 3D-printed inks. For instance, scientists have used inks made from temperature-sensitive polymers to print heat-responsive, shape-shifting objects. Others have printed photo-activated structures from polymers that shrink and stretch in response to light.
Zhao’s team realized, with bioengineers in Lu’s lab, that live cells might also serve as responsive materials for 3D-printed inks, particularly as they can be genetically engineered to respond to a variety of stimuli. The researchers are not the first to consider 3D printing genetically engineered cells. Others have attempted to do so using live mammalian cells, but they were not successful.
"It turns out those cells were dying during the printing process because mammalian cells are basically lipid bilayer balloons," Yuk says. "They are too weak, and they easily rupture."
Instead, the team identified a hardier cell type in bacteria. Bacterial cells have tough cell walls that are able to survive relatively harsh conditions, like the forces applied to ink as it is pushed through the printer’s nozzle. Bacteria are better than mammalian cells because they are compatible with most hydrogels. The group found that hydrogels can provide an aqueous environment that can support living bacteria.
The researchers carried out a screening test to identify the type of hydrogel that would best host bacterial cells. After a search, a hydrogel with pluronic acid was found to be the most compatible material. The hydrogel also exhibited an ideal consistency for 3D printing.
"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 his researchers 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 3D 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 micrometers per feature," Zhao says. "That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimeters."
The team printed the ink using a custom 3D printer they built using standard elements combined with fixtures they machined themselves. In order to demonstrate this technique, the team printed a pattern of hydrogel with cells in the shape of a tree on an elastomer layer. After printing, they solidified or cured, the patch by exposing it to ultraviolet radiation. They then adhere the transparent elastomer layer with the living patterns on it, to the skin.
In order to test the patch, 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 a few hours, branched of the patch’s tree lip up with bacteria sensed their corresponding stimuli.
The researchers engineered bacteria to communicate with each other. For example, they programmed some cells to light up only when they receive a certain signal from another cell. To test this type of communication in a 3D structure, they printed a thin sheet of hydrogel filaments with “input," or signal producing bacteria and chemicals, overlaid with another layer of filaments of an “output", or signal-receiving bacteria. They found the output filaments lip up only when they were overlapped and received input signals from corresponding bacteria.
Yuk says the future, researchers may use the team’s technique to print “living computers” which are structures with multiple types of cells that communicate with each other, passing signals back and for, 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," Yuk says.
For near-term applications, the researchers are aiming to fabricate customized sensors in the form of flexible patches and stickers that could be engineered to detect a variety of chemical and molecular compounds. They envision their technique may be used to manufacture drug capsules and surgical implants that contain engineered produce compounds like glucose, to be released therapeutically over time.
The paper on this research was published in Advanced Materials.