SCIENTISTS CREATE LIVING CONCRETE

Source: https://www.nytimes.com/2020/01/15/science/construction-concrete-bacteria-photosynthesis.html?fbclid=IwAR0m7LZK76O_mhR4VKSNcXm2SIb4A4L9Y4vZWs8CDLWbhBEoAyx0o85V48I

"A Frankenstein material"

...teeming with photosynthetic microbes, and ultimately made by them. And it can reproduce.

Wil Srubar (left), a structural engineer at the University of Colorado Boulder, and Sarah Williams, a materials engineering student, holding bricks of a building material made from cyanobacteria and other ingredients.

For centuries, builders have made concrete in much the same way: mixing hard materials such as sand with various binders and hoping it stays strong and rigid for a long time.

Now an interdisciplinary team of researchers at the University of Colorado Boulder has created a very different kind of concrete: one that is alive and can even reproduce.

The minerals in the new material are not laid down chemically but by cyanobacteria, a common group of microbes that draw their energy from photosynthesis. That photosynthetic process absorbs carbon dioxide, unlike the production of ordinary concrete, which releases vast amounts of greenhouse gases.

The photosynthetic bacteria also give the concrete another unusual property: a green colour. "It really does look like a Frankenstein material," said Wil Srubar, a structural engineer and the head of the research project. (The green fades as the material dries out.)

Other researchers have already looked at building biology into concrete, particularly concrete that can heal its own cracks. According to its creators, the main advantage of the new material is that, instead of adding bacteria to ordinary concrete, an inhospitable environment, their approach focuses on the bacteria themselves: getting them to produce the concrete and keeping them alive for later use.

The new living 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 Strathclyde University in Scotland.

To create the living concrete, the researchers first tried putting cyanobacteria into a mix of warm water, sand and nutrients. The microbes readily absorbed light and began producing calcium carbonate, which gradually bound the grains of sand together. The process was slow, however, and Darpa, the speculative research arm of the Department of Defense and the project's funder, wanted construction to happen very quickly. Necessity and a happy accident then gave rise to the invention.

An arch made of living building materials in Dr Srubar's laboratory

Dr Srubar had previously worked with gelatin, a food ingredient that forms distinctive bonds between its molecules once dissolved in water and cooled. Crucially, it can be used at mild temperatures that are kind to bacteria. He suggested adding gelatin to strengthen the matrix formed by the cyanobacteria, and the team was intrigued.

The researchers bought Knox gelatin at a local supermarket and dissolved it in a solution with the bacteria. When they poured the mixture into moulds and cooled it in the fridge, the gelatin formed its bonds. "Exactly the same as when you make Jell-O," Dr Srubar said. The gelatin provided a stronger structure and worked with the bacteria, helping the living concrete grow stronger and faster.

In about a day, the mixture formed concrete blocks in the shape of whatever mould the team used, including two-inch cubes, shoebox-sized blocks and pieces of roofing with struts and cutouts. The individual two-inch cubes were strong enough for a person to stand on, although the material is weaker than ordinary concrete. The shoebox-sized blocks showed the potential for real construction.

The first experiments

"When we first made a large structure with this system, we didn't know whether it would work; we went from this small size to this big brick," said Chelsea Heveran, a former postdoctoral researcher on the team, now an engineer at Montana State University and the lead author of the study. "We pulled it out of the mould and held it in our hands. It was beautiful, bright green, with the word 'Darpa' written on the side." (The mould bore the name of the project's funder.) "It was the first time we had reached the scale we had imagined, and it was really exciting."

When the team brought small samples to a regular meeting with Darpa representatives, they were impressed, Dr Srubar said: "Everyone wanted one to put on their desk."

Stored in relatively dry air at room temperature, the blocks reach their maximum strength within days and the bacteria gradually begin to die off. Even after several weeks, though, the blocks are still alive; when exposed again to high heat and humidity, many of the bacterial cells revive.

The team can take a single block, cut it with a diamond tip, put one half back into a warm tank with more raw materials, pour it into a mould and start making concrete again. Each block can thus yield three new generations, producing eight offspring.

The Department of Defense is interested in using the reproductive capacity of these "LBMs", or living building materials, to support construction in remote or barren areas. "You don't want to haul large amounts of material out into the desert," Dr Srubar said.

The advantage of the blocks is

...that they are made from a range of readily available materials. Most concrete requires virgin sand from rivers, lakes and oceans, which is in short supply worldwide, largely because of the enormous demand for concrete. The new living material is not so fussy. "We're not tied to using one particular kind of sand," Dr Srubar said. "We could also use waste materials such as ground glass or recycled concrete."

The research team is working to make the material more practical: strengthening the concrete, making the bacteria more resistant to drying out, rearranging the materials so they can be stored flat and assembled as easily as plasterboard, and looking for a different kind of cyanobacteria that does not need added gelatin.

Dr Srubar said the tools of synthetic biology could greatly widen the possibilities: building materials that can detect toxic chemicals and respond to them, for example, or that light up to flag structural damage. Living concrete could help in environments harsher than the driest deserts, such as other planets, including Mars.

"There is no way of transporting building materials into space," said Dr Srubar. "We are taking biology with us."