Colorado researchers create living concrete
For centuries, builders have been making concrete roughly the same way: by mixing hard materials like sand with various binders, and hoping it stays fixed and rigid for a long time to come.
Now, an interdisciplinary team of researchers at the University of Colorado, Boulder, has created a rather different kind of concrete — one that is alive and can even reproduce.
Minerals in the new material are deposited not by chemistry but by cyanobacteria, a common class of microbes that capture energy through photosynthesis. The photosynthetic process absorbs carbon dioxide, in stark contrast to the production of regular concrete, which spews huge amounts of that greenhouse gas.
Photosynthetic bacteria also give the concrete another unusual feature: a green color. “It really does look like a Frankenstein material,” said Wil Srubar, a structural engineer and the head of the research project. (The green color fades as the material dries.)
Other researchers have worked on incorporating biology into concrete, especially concrete that can heal its own cracks. A major advantage of the new material, its creators say, is that instead of adding bacteria to regular concrete their process is oriented around bacteria: enlisting them to build the concrete, and keeping them alive so they make more later on.
The new concrete, described in the journal Matter, “represents a new and exciting class of low-carbon, designer construction materials,” said Andrea Hamilton, a concrete expert at the University of Strathclyde, in Scotland.
To build the living concrete, the researchers first tried putting cyanobacteria in a mixture of warm water, sand and nutrients. But the process was slow — and DARPA, the U.S. Defense Advanced Research Projects Agency and the project’s funder, wanted the construction to go very quickly. Necessity, happily, birthed invention.
Srubar had previously worked with gelatin, a food ingredient that, when dissolved in water and cooled, forms special bonds between its molecules. He suggested adding gelatin to strengthen the matrix being built by the cyanobacteria, and the team was intrigued.
The researchers bought Knox brand gelatin at a local supermarket and dissolved it in the solution with the bacteria. When they poured the mixture into molds and cooled it in a refrigerator, the gelatin formed its bonds — “just like when you make Jell-O,” Srubar said. The gelatin provided more structure, and worked with the bacteria to help the living concrete grow stronger and faster.
After about a day, the mixture formed concrete blocks in the shape of whatever molds the group used, including 2-inch cubes, shoe box-size blocks and truss pieces with struts and cutouts. Individual 2-inch cubes were strong enough for a person to stand on, although the material is weak compared to most conventional concretes.
“The first time we made a big structure using this system, we didn’t know if it was going to work, scaling up from this little-bitty thing to this big brick,” said Chelsea Heveran, a former postdoc with the group — now an engineer at Montana State University — and the lead author of the study. “We took it out of the mold and held it — it was a beautiful, bright green and said ‘DARPA’ on the side.” (The mold featured the name of the project’s funder.) “It was the first time we had the scale we were envisioning, and that was really exciting.”