Infusing building materials with living microorganisms has already lent inanimate objects new powers. Self-healing concrete, for example, uses bacteria or fungi to fix its own cracks. Now researchers have developed a living substance that can transform from a gooey sand mixture into a solid brick—and then help build more copies of itself. Proponents say it could be used to make a building material that requires relatively few resources and absorbs greenhouse gases instead of releasing them.
“We enabled the bacteria that we chose to help in the manufacturing process of the actual material,” says Wil Srubar, a materials scientist and architectural engineer at the University of Colorado Boulder. His team used a type of cyanobacterium from the genus Synechococcus. Powered by photosynthesis, these microorganisms absorb sunlight, nutrients, and carbon dioxide and spit out calcium carbonate—the rigid compound found in seashells and cement. The researchers grew the cyanobacteria in a bath of artificial seawater and other nutrients, heated to a very warm summer-day temperature of 30 degrees Celsius, then combined this liquid with gelatin and sand. Poured into a mold, the mixture cooled, and the gelatin began to set, creating a gummy “scaffold” that supported more bacterial growth. The Synechococcus seeded calcium carbonate throughout this structure, turning soft goo into a harder mineralized substance that held the sand in place. The results were published Wednesday in Matter.
“I think the idea of using cyanobacteria for making this—specifically, the materials—it’s a really good idea,” says mechanical engineer Lina González, currently a visiting faculty lecturer at the University of Massachusetts Lowell, who was not involved in the research. She notes that cyanobacteria absorb carbon dioxide. The traditional process of making cement usually does the opposite: it requires significant heat, usually from burning fossil fuels that dump greenhouse gases into the atmosphere. Some estimates suggest cement accounts for about 7 percent of carbon emissions worldwide. “You can think of [greener] building materials as a way to deal with the current problem with a lot of CO2 in the environment,” González says.
This environmental advantage has also inspired other researchers, and there is already commercially available cement made with sand and bacteria in a process that emits less carbon dioxide than traditional manufacturing methods. But “I would say that our approach is fundamentally different, because we are using photosynthetic bacteria and CO2 and sunlight to make to make the material,” Srubar says. “And our viabilities are orders of magnitude higher than one would see in traditional cement-based materials.” By “viabilities,” he means the microorganisms in his team’s material remain alive longer than those currently used in self-healing cement: after 30 days had passed following the bricks’ solidification, 9 to 14 percent of the microorganisms within them had survived. In comparison, the bacteria in self-healing cement would have had a viability of a fraction of a percent over the same time period, according to the Matter paper.
Extended viability allows Srubar’s bricks to perform a unique trick: partial self-reproduction. Split a completed brick and put half of it back in its mold with a fresh batch of gelatin and sand, and bacteria from the original piece will grow into it and harden it to produce a whole new brick. “We showed here that up to three ‘child’ generations of living building materials can be grown from one ‘parent’ generation,” Srubar says. “So we effectively took one parent block; split it into two, which grew into a full two blocks; split that in half, which then resulted in four and then eight. And theoretically, this process can go on in perpetuity.”
Although this manufacturing method would be more environmentally-friendly than many conventional ones, Srubar does not propose that bacterial bricks would entirely supplant more traditional materials. Instead he suggests his work could help people construct buildings in areas with scant access to resources, such as military installations in the desert or human settlements on other planets. “We were motivated by building infrastructure in really resource-limited environments,” he explains. If “you have sunlight, which is free, and CO2—and perhaps some water and just a little bit of nutrients—you could have a way in which you can grow the physical materials that you can use to build infrastructure.”
But the experimental process has its limitations. For one thing, the bacteria thrive only when the air around them contains enough water: when the researchers tested how long the microorganisms in the bricks could survive, they kept the material at 50 percent relative humidity. González suggests that a bacterial species that forms protective spores at times of environmental stress might prove more resilient.
Second, the material would ideally be stronger. The researchers tested the living bricks, applying pressure to see how much they could take and measuring their resistance to cracking. Compared with a similar material that contained no cyanobacteria, the living version was 15 percent 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. Srubar’s team plans to experiment with materials other than sand, hoping to make the bricks stronger.
There is also the intriguing possibility that different types of bacteria could allow living bricks to interact with their surroundings. González and Srubar, who both work with living materials, agree that this prospect could potentially offer a broad range of applications. “The major thing is that organisms have the ability to sense the environment,” González says. “They can see if there’s light or if there’s a high pH in the environment and things like that. Furthermore, you can genetically engineer them” so they respond to the things they sense in specific ways in order “to do whatever you want them to do.”
“When you look at this approach and this material as a platform technology,” Srubar says, “you can start to envision multiple bacteria with different functionalities that could be used in the production [of building materials]: bacteria that could self-heal the material or that could sense and respond to airborne toxins to change color—and perhaps fluoresce under certain types of light.”