If humans are to live on Mars or the moon one day, we’ll need to be able to construct buildings to live, sleep, eat, and work in space. The way to do that, space agencies have said, is to 3D-print habitats or their components. But hauling enough of the Earth-derived materials used for most 3D printing from our planet to another celestial body isn’t a feasible option.
Biology could solve that problem, says Neel Joshi, associate professor of chemistry and chemical biology at Northeastern. And Joshi’s team may have devised just the technology for the job: a 3D-printable material that is alive.
“Like a tree has cells embedded within it and it goes from a seed to a tree by assimilating resources from its surroundings in order to enact these structure-building programs, what we want to do is a similar thing, but where we provide those programs in the form of DNA that we write and genetic engineering,” Joshi says.
The researchers have figured out how to program the bacterium Escherichia coli, also known as E. coli, to produce an entirely biological ink which can be used to 3D-print solid structures. That microbial ink, which is described in a paper published Tuesday in the journal Nature Communications, has yet to be tested on a cosmic scale, but the scientists have used the gelatinous material to print small shapes, such as a circle, a square, and a cone. They have also successfully programmed it to build materials with specified attributes with other applications that could be useful in medicine.
“We want to use living cells, microbes, as factories to make useful materials,” says Avinash Manjula-Basavanna, a postdoctoral fellow in Joshi’s laboratory and co-lead author on the new paper. The idea, he says, is to harness the properties that are unique to the materials that make up living things for a spectrum of purposes, ranging from therapeutic to industrial.
“Think about it as a platform for building many different things, not just bricks for building buildings or construction,” Joshi says. He explains the work by comparing it to the way a polymer chemist considers how to devise plastic materials that can serve distinct purposes. Some plastics are hard and retain their shape, while others are stretchy and soft.
“Biology is able to do similar things,” Joshi says. “Think about the difference between hair, which is flexible, and horns on a deer or a rhino or something. They’re made of similar materials, but they have very different functions. Biology has figured out how to tune those mechanical properties using a limited set of building blocks.”
The particular natural building block the scientists are taking advantage of is a protein produced by the bacterium E. coli. The material, called Curli fibers, is produced by the bacterial cells as they attach to a surface and to one another to form a community. The same properties that make the Curli fibers a sort of glue for the bacteria also make it an attractive material for microbial engineers like Joshi and his colleagues.
To make the microbial link, the scientists started by culturing genetically engineered E. coli in a flask. They fed the bacteria nutrients so that they would multiply, and as they divided they would produce the desired polymers, the Curli fibers. Then, the researchers filtered out the gelatinous polymers and fed that material into a 3D printing apparatus as the microbial ink.
Microbes have been used to make the ink for 3D printing before, but, Joshi and Manjula-Basavanna say, what sets this microbial ink apart is that it is not blended with anything else. Their gel is entirely biological.
One of the perks of a truly living material is that it is, in fact, alive, Manjula-Basavanna says. And that means that it can do what living things can do, such as heal itself, the way skin does. In the right conditions, the cells in the microbial gel could simply make more of itself.
It’s not necessarily always growing, Joshi says. For example, if the cells were left alive in the small cone that the team made from the microbial gel, “if you were to take that whole cone and dunk it into some glucose solution, the cells would eat that glucose and they would make more of that fiber and grow the cone into something bigger,” he says. “There is the option to leverage the fact that there are living cells there. But you can also just kill the cells and use it as an inert material.”
While the initial gel is made entirely from genetically engineered E. coli, the researchers also tried mixing the ink with other genetically engineered microbes with the goal of using the 3D-printed materials for specific purposes. That’s how they made a material that could deliver an anticancer drug, which it released when it encountered a specific chemical stimulus. In another experiment, they also programmed another material to trap the toxic chemical Bisphenol A (BPA) when it encountered BPA in the environment.
“You could think about taking a bottle cap and printing our material on the inside of it so that if there was BPA around, it would be sucked up by that and not be in your drink,” Joshi says.
This study was simply a proof-of-concept endeavor, but Joshi sees this microbial ink as opening a door to all kinds of possibilities for building things with biology.
“If there is a way to manufacture in a more sustainable manner, it’s going to involve using living cells,” he says. “This is advancing more towards that type of paradigm of building things with living cells.”