But scientists have recently melded the virtues of the infuriating tool and of the toxic microbe to produce an ink that is alive, made entirely from microbes. The microbial ink flows like toothpaste under pressure and can be 3D printed into various tiny shapes — a circle, a square and a cone — all of which hold their form and glisten like Jell-O.
The researchers describe their recipe for their programmable, microbial ink in a study published Tuesday in the journal Nature Communications. The material is still being developed, but the authors suggest that the ink could be a crucial renewable building material, able to grow and heal itself and ideal for constructing sustainable homes on Earth and in space.
This new substance is not the first living ink. Scientists have previously created printable gels that were cocktails of bacteria and polymers that helped provide structure when printed. One such ink contained hyaluronic acid, a seaweed extract and fumed silica — all agents that made the material thicker and more viscous.
But the new substance contains no additional polymers; it is produced entirely from genetically engineered E coli bacteria. The researchers induce bacterial cultures to grow the ink, which is also made of living bacteria cells. When the ink is harvested from the liquid culture, it becomes firm like gelatine and can be plugged into 3D printers and printed into living structures, which do not grow further and remain in their printed forms.
“They developed this really nice engineered platform where the microbes secrete their own ink,” said Sujit Datta, a chemical and biological engineer at Princeton University who was not involved with the research. “The microbes are creating the material themselves — you just have to feed them and keep them happy.”
Bacteria may seem an unconventional building block. But microbes are a crucial component of products such as perfumes and vitamins, and scientists have already engineered microbes to produce biodegradable plastics.
A material like a microbial ink has more grandiose ambitions, according to Neel Joshi, a synthetic biologist at Northeastern University and an author on the new paper. Such inks are an expanding focus of the field of engineered living materials. Unlike structures cast from concrete or plastic, living systems would be autonomous, adaptive to environmental cues and able to regenerate — at least, that is the aspirational goal, Joshi said.
“Imagine creating buildings that heal themselves,” Datta said.
To Joshi, the best analogy may be a seed’s transformation into a tree. A seed has all the information it needs to harvest the energy of the sun and organise its growth and development into something as complex and grand as a tree. In an engineered living system, a single engineered cell could function like a seed.
Microbes, on their own, aren’t great at making clearly defined shapes in three dimensions. “Think of pond scum,” Joshi said. “That’s kind of the level of complexity that bacteria are comfortable with, in terms of making shapes.”
Typically, microbial inks rely on a scaffold of polymers to stiffen their scummy forms. But polymers have their own limitations and can alter the mechanical properties of the ink in unwanted ways, Datta said. Also, the polymers must be biocompatible, so the microbes do not die. And synthetic polymers, such as polyethylene, are derived from oil and are not renewable.
Forgoing polymers and using only microbes “provides a lot more tunability in what you can print,” said R Kōnane Bay, a soft-matter physicist and an incoming assistant professor at the University of Colorado Boulder, who was not involved with the research.
Many engineered living materials take the form of hydrogels, structures that can absorb large quantities of water, like gelatine. In 2018, Joshi and Anna Duraj-Thatte, an engineer at Virginia Tech and an author on the new paper, successfully created a hydrogel entirely from E. coli that could grow and regenerate.
Although the hydrogel could be squeezed through a syringe, it was not stiff enough to stand on its own. “You could not make any structures,” Duraj-Thatte said.
The researchers needed to firm up the substance. “We came up with this strategy where we use fibrin, which is a polymer used in blood-clotting in humans and many other animals,” said team member Avinash Manjula-Basavanna, who completed the work while he was a researcher at Harvard University.
The researchers genetically engineered the E. coli to produce a protein polymer from fibrin designed to link into a meshlike network — imagine a heavy duty cargo net. This makes the material stiff enough to print while still able to flow from the nozzle of the 3D printer.
The authors took their microbial ink to 3D printers at the lab of Yu Shrike Zhang, a bioengineer at Harvard Medical School, which often uses the printers for tissue-engineering mammalian cells. The lab was one of the few brave enough to invite bacteria into its sanitary printing space.
“A lab that their bread and butter is just doing tissue engineering with mammalian cells would be kind of gun-shy about bringing bacteria anywhere near there,” Joshi said.
“It’s a lab that will try many different things,” Duraj-Thatte said.
The researchers printed the microbial ink into a number of shapes and patterns to test its ability to hold its shape: a lattice grid, a box, a ring and a cone that looked almost like an icicle. The ink was squeezed like toothpaste from the printer but did not ooze or melt once printed, passing all the tests.
They also put the ink to a fidelity test to see how far a strand of the ink would stretch without breaking. In the test, the printer’s nozzle extruded a half-millimetre-thick strand of ink across a line of successive pillars, each one a greater distance from the last — intended to show how far between pillars the ink strand could hold without breaking.
The strand could support its own weight between pillars that were 16 millimetres apart: a success. When Duraj-Thatte and Manjula-Basavanna recorded the test in real time in the lab, they said, they began screaming with excitement about the proof that the ink worked. (The video that accompanied the published study did not include audio.)
To test whether the printed structures could perform functions, Duraj-Thatte and Manjula-Basavanna also remixed the ink with other microbes that had been engineered to perform specific tasks. In one therapeutic test, the printed ink released the anti-cancer drug azurin when exposed to a chemical. In another test, the printed ink successfully trapped the toxic chemical BPA, suggesting that the material could potentially remove harmful contaminants from its surroundings.
The ink still needs a lot of work. It can’t withstand drying out, and is not currently stable enough to be the sole basis of larger constructions, such as a house fit for a human; the researchers are working on ways to make more robust printed structures. But researchers see few limits to its possible future applications.
Duraj-Thatte hopes to see the ink combined with tissue engineering, as it can be customised for medical applications. Joshi suggested the ink could eventually offer a greener, renewable way to construct buildings. Bay wondered if the ink could be created from other bacteria such as Pseudomonas putida, which can clean up the toxin phenol. “We can think about making them into biosensors,” Bay suggested.
Manjula-Basavanna is shooting for the moon, Earth’s satellite, where there are no forests to harvest for wood and no easy way to send bulk building materials. There, he said, the ink might be used as a self-regenerating substance to help build habitats on other planets, as well as places on Earth.
“There is a lot of work to be done to make it scalable and economic,” Datta conceded. But, he noted, just five years ago creating robust structures out of microbes was unimaginable; conceivably, self-healing buildings could be a reality in our lifetime.
“It’s hard to project into the future,” Datta said. “But given the pace in this area, the future looks very bright.”
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