A team of researchers at the Massachusetts Institute of Technology (MIT) has devised a 3D printing technique that uses a novel kind of hydrogel ink made from genetically programmed bacterial cells. The team’s results appear in the journal Advanced Materials.
New technique 3D prints genetically programmed bacterial cells into living devices. Image credit: Liu et al / Massachusetts Institute of Technology.
“Our technique can be used to fabricate ‘active’ materials for wearable sensors and interactive displays,” said co-lead authors Professor Xuanhe Zhao and Dr. Timothy Lu.
“Such materials can be patterned with live cells engineered to sense environmental chemicals and pollutants as well as changes in pH and temperature.”
The team are not the first to consider 3D printing genetically engineered cells — others have attempted to do so using live mammalian cells, but with little success.
“It turns out those cells were dying during the printing process, because mammalian cells are basically lipid bilayer balloons. They are too weak, and they easily rupture,” said MIT graduate student Hyunwoo Yuk, co-author of the study.
Instead, the scientists identified a hardier cell type in bacteria.
“Bacterial cells have tough cell walls that are able to survive relatively harsh conditions, such as the forces applied to ink as it is pushed through a printer’s nozzle,” they explained.
“Furthermore, bacteria are compatible with most hydrogels — gel-like materials that are made from a mix of mostly water and a bit of polymer.”
“We found that hydrogels can provide an aqueous environment that can support living bacteria.”
The researchers then carried out a screening test to identify the type of hydrogel that would best host bacterial cells.
After an extensive search, a hydrogel with pluronic acid was found to be the most compatible material.
“This hydrogel has ideal flow characteristics for printing through a nozzle,” Professor Zhao said.
“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 team then came up with a recipe for their 3D ink, using a combination of hydrogel, bacterial cells engineered to light up in response to a variety of chemical stimuli, 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. That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimeters,” Professor Zhao said.
The authors printed the ink using a custom 3D printer that they built using standard elements combined with fixtures they machined themselves.
To demonstrate the technique, they 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 UV radiation. They then adhere the transparent elastomer layer with the living patterns on it, to skin.
To test the patch, they smeared several chemical compounds onto the back of a test subject’s hand, 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.
The researchers also engineered bacteria to communicate with each other; for instance 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 lit up only when they overlapped and received input signals from corresponding bacteria.
“In the future, researchers may use our technique to print ‘living’ computers — structures with multiple types of cells that communicate with each other, passing signals back and forth, much like transistors on a microchip,” Yuk said.
“This is very future work, but we expect to be able to print living computational platforms that could be wearable.”
_____
Xinyue Liu et al. 3D Printing of Living Responsive Materials and Devices. Advanced Materials, published online December 5, 2017; doi: 10.1002/adma.201704821
