“Imagine creating buildings that heal themselves,” Dr. Datta said.
To Dr. 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 organize 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,” Dr. 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, Dr. 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 gelatin. In 2018, Dr. 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,” Dr. 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.