Engineers at the University of Colorado Boulder recently conducted an experiment that sounds almost like a kids' science project: They added colonies of green bacteria to a mix of sand and grocery-store gelatin.

The result, it turns out, is a novel building material that's a living, breathing organism. It can spawn its own babies.

The microbes in the brick are cyanobacteria, which perform photosynthesis to grow, taking in carbon dioxide. They produce a powdery substance called calcium carbonate — the main ingredient in cement — which toughens the material. 

"It's a lot like making rice crispy treats, where you toughen the marshmallow by adding little bits of hard particles," Wil Srubar, a structural engineer who led the project, told CU Boulder Today. "You can step on it, and it won't break."

Adding live bacteria also makes the dead sand and gelatin particles seem to spring to life. The bacteria colonies grow at an exponential rate, so bricks produce their own offspring after they're cut in half. Each half yields two spawn for up to three generations, meaning a single brick can turn into eight. 

The gelatin in the mixture helps glue the sand particles together, creating a stable structure for the brick. Adding the bacteria to that combination makes the material as tough as the mortar used hold brick and stone together in modern construction.

However, the atmosphere needs to be humid in order for the bacteria to grow.

To test the lifespan of the living brick, the researchers stored the mixture inside a shoe-box-sized mold under dry conditions at room temperature. After a few days, the bacteria began to die. After a month, only 9% to 14% of the colonies were still alive. But when the researchers made the atmosphere hotter and more humid, the colonies started growing again.

The process reminded the researchers of Frankenstein.

"Cyanobacteria are actually green, so it really does look like a Frankenstein material," Srubar told CU Engineering. 

The building and construction sector is responsible for nearly 40% of the world's carbon emissions.

A living brick could help offset some of this pollution, since cyanobacteria captures carbon dioxide.

Plus, since the bricks can produce offspring, builders could also grow the material at a construction site, reducing emissions from the manufacturing and transportation processes.

If researchers can develop a version of the mixture that can withstand dry temperatures, the bricks could even offer a way to build future structures on the moon or Mars, since less building material would need to be launched and carried on a spacecraft.

But first, the researchers are considering more practical applications. For example, the regenerative nature of the bacteria means the bricks could potentially heal their own cracks after a natural disaster like an earthquake. 

"We already use biological materials in our buildings, like wood, but those materials are no longer alive," Srubar told CU Boulder Today. "We're asking: Why can't we keep them alive and have that biology do something beneficial, too?"