• Engineers have invented a self-replicating brick that pulls CO2 from the air.
  • The brick is produced by harnessing photosynthesis and bacteria.
  • While refinements are needed, researchers say the new technique is an advance in the search for lower-energy, lower-carbon building materials.

What do you get if you mix sand, bacteria and sunlight? A self-replicating building material that pulls CO2 straight out of the atmosphere, according to research engineers at one university in the United States.

At the heart of this new building material is Synechococcus, a type of bacteria which is found in plankton and uses photosynthesis to generate energy.

The bacteria is combined with sand and gelatin, then soaked in warm nutrient-rich saltwater. Photosynthesis does the rest, producing calcium carbonate along with oxygen and glucose.

Common forms of calcium carbonate include marble, limestone and chalk. That compound makes up around 4% of the Earth’s crust, and is a major component of building materials – in particular, cement.

A new approach

Cement and brick production are very energy-intensive processes that also call for a lot of extracted raw materials. Quarries are dug for clay and other aggregate material, such as limestone, and temperatures of more than 1,000ºC are needed to form some bricks for construction. This process consumes great amounts of energy, directly and indirectly. Some estimates suggest the production of cement and bricks could be responsible for 7-8% of global CO2 emissions.

The search for lower-energy, lower-carbon building materials is gaining momentum. In 2010, German researchers created a new binding agent that reduces the energy consumption and CO2 emissions of cement production. In the US, researchers at the Massachusetts Institute of Technology (MIT) have proposed a method for producing cement using electrolysis rather than furnaces to cause the chemical reactions needed. But the researchers working with the Synechococcus at the University of Colorado, Boulder, claim they are breaking new ground.

“I would say that our approach is fundamentally different because we are using photosynthetic bacteria and CO2 and sunlight to make the material,” says Wil Srubar, a materials scientist and architectural engineer on the project.

“We enabled the bacteria ... to help in the manufacturing process of the actual material.”

The work is part of a sharpened focus on living building materials, which are produced using biochemistry, and which can be fully recycled after use.

The Life Cycle of Living Building Materials
Living building materials considerably reduce carbon emissions.
Image: Matter

A new framework

The combination of sand and gelatin create a rigid framework or scaffold for the bacteria to inhabit and multiply across. The original parent generation of Synechococcus was able to produce three subsequent generations.

In one experiment, a brick formed from the new material was broken in two and resubmerged in the saline solution, where it repaired itself, growing a fully matching new half where the old one had been.

Formation of the LBM Mineralized Scaffold
How to make a living brick.
Image: Matter

The material has its limitations. The bacteria, for example, requires the right conditions and moisture to thrive.

Additionally, there are some questions regarding the material’s toughness, according to the journal Scientific American, which writes: “Compared with a similar material that contained no cyanobacteria, the living version was 15% tougher in terms of resisting fractures. But it fell short of the resilience of standard bricks or cement, performing more like low-strength cement or hardened mortar.”

As a result, the research team behind the living brick does not see the material as a new mass-market building product. Instead, they say: “We were motivated by building infrastructure in really resource-limited environments.”

Despite the need for ongoing improvements, the innovation could have potential for use in remote locations where materials, energy and money are not always freely available.