Living cells used in construction to create concrete, bricks or tiles could also soak up CO2.
At a former Dutch royal family seat, Het Loo Palace in Apeldoorn, the Netherlands, ongoing renovations are using self-healing concrete that relies on living bacteria to repair any cracks that might develop in the structure. The bacteria in the concrete, which is currently being poured at the Dutch Baroque palace’s underground extension, were supplied by Green Basilisk, a biotech startup based in Delft, the Netherlands. It’s just the latest example of biology being harnessed by the construction industry. Aside from self-healing concrete, other companies are using bacteria to create bricks and tiles from scratch, or cultivating fungi to produce materials for wall panels and building blocks. “The field is quite small, but it’s growing rapidly,” says Wil Srubar III, a civil engineer at the University of Colorado Boulder.
Some of these biological building materials (Table 1) may offer an opportunity to reduce the construction industry’s environmental impact. The world produces over 4 billion tonnes of cement every year, which is responsible for about 8% of the world’s CO2 emissions. In contrast, bio-based materials could actually sequester CO2 from the atmosphere.
Timber is already experiencing a huge renaissance as an environmentally friendly building material, but it has a big drawback in comparison with bacteria and fungi. “Timber is slow in terms of absorbing carbon,” says Jan Wurm, director of research and innovation in Europe for Arup, the multinational engineering company. “Having these very fast-growing materials could be a step change in sustainability.”
Green Basilisk’s bacteria aim to fix another kind of sustainability problem with concrete. Although concrete is strong in compression, steel bars are often embedded in the material to improve its tensile strength. But it is common for tiny cracks to appear in concrete, allowing water to seep inside and corrode the steel, which weakens the whole structure and raises the risk of a catastrophic collapse. Countries spend billions of dollars every year to fix these cracks in vital infrastructure. If they are beyond repair, such derelict structures generate huge amounts of waste, while replacement buildings increase the global demand for concrete.
To tackle this, Green Basilisk seeds concrete with bacterial spores and calcium lactate. The spores can lay dormant for years before water ingress awakens them, at which point they begin to digest lactate and produce CO2. In concrete’s highly alkaline environment, this CO2 combines with calcium ions to form solid calcium carbonate, which can seal cracks up to one millimeter wide in a few weeks and prevent further water damage. The company now offers three products that either imbue virgin concrete with self-healing properties or repair cracks in existing structures.
Henk Jonkers at Delft University of Technology, who cofounded Green Basilisk in 2014, says his team spent years prospecting for wild-type bacteria that could thrive in such a harsh environment. The researchers found suitable strains of Bacillus pseudofirmus, Bacillus cohnii and Bacillus alkalinitrilicus in a desert region of northern Spain and at an alkaline lake in Russia, and confirmed that they could generate fresh calcium carbonate within concrete. “We only want to work with natural bacteria,” explains Jonkers. “There’s a lot of legislation that makes it very difficult if you’re going to work with genetically engineered bacteria.”
Green Basilisk now aims to develop nutrients for the bacteria that are cheaper than calcium lactate. At the moment, one cubic meter of fresh concrete can cost $68–91, and the self-healing bacteria and nutrients add another $46, which can put some customers off. But Jonkers reckons that after factoring in maintenance, repairs and other expenses over the lifetime of a building, the self-healing ingredients may account for just 1% of overall costs. “If you make that calculation, self-healing is a very cheap solution,” he says.
Major construction companies are beginning to take an interest. In 2015, UK construction firm Costain trialed four different self-healing concrete technologies, including one seeded with bacterial spores, at a road improvement project in Wales. Costain is now one of the industrial partners of a $6-million research project in the UK called Resilient Materials 4 Life (RM4L), largely aimed at developing self-healing building materials and embedded sensors that can identify damage. “What we’re trying to do is make concrete a bit like a biological system, so it can detect damage and then heal itself,” says Susanne Gebhard, a microbiologist at the University of Bath, UK, who leads part of the project.
“What we’re trying to do is make concrete a bit like a biological system, so it can detect damage and then heal itself.”
Most of RM4L’s work is focused on chemistry and materials science, but Gebhard is probing the interactions between bacteria, nutrients and concrete to find combinations that can outshine existing self-healing systems. “We’re taking a step back to find out how it works, and where it could work better,” she says. For example, her team has encapsulated B. cohnii spores in aerated concrete granules and covered them in a waterproof shell of polyvinyl alcohol. This encapsulation ensures the bacteria are not activated until a growing crack breaks through the shell, allowing water to reach the spore inside. When the researchers added a growth medium containing yeast extract and calcium nitrate into the granules, it enhanced the crack-healing effect of the bacteria.
RM4L researchers are also engineering the spore-forming bacterium Bacillus subtilis to better understand the biochemical pathways involved in making calcium carbonate. These pathways involve, for example, cell surface structures, ureolysis and the production of extracellular materials. Although European Union regulations on genetically modified organisms mean that these bacteria would not be used in construction, Gebhard says they can offer clues about which genes and properties researchers should look for in wild-type bacteria when seeking better strains.
Others are moving beyond concrete repair, employing bacteria to craft entirely new building materials. Biotech company bioMASON, based in Durham, North Carolina, uses a wild-type Bacillus strain that can stick grains of rock and sand together into sturdy tiles. The bacteria hydrolyze a urea feedstock to form CO2, which combines with calcium ions to form calcium carbonate. This binds the surrounding aggregate particles into a solid matrix within a tile mold. After three days, these bioLITH tiles can hold their shape like wet sandcastles, and only need ambient drying before they are ready to use on walls or floors. The company can already produce about 10,000 square meters of the tiles per year at its pilot plant.
Urea is mostly used as a fertilizer in agriculture, and it is produced at industrial scale by combining ammonia and CO2. Ammonia is made in the Haber–Bosch process, which reacts nitrogen and hydrogen over an iron catalyst at a high temperature and pressure. The hydrogen that feeds this reaction is made in a process called steam methane reforming, which generates about 3% of our global CO2 emissions and also supplies the CO2 for urea synthesis.
Turning urea’s carbon into calcium carbonate within bioLITH tiles means that “we are effectively sequestering the carbon from urea,” says Michael Dosier, the company’s chief technology officer. As a result, the tiles have a carbon footprint that is less than 1% that of conventional cement-based materials. The tiles look like natural stone, and have a cost similar to that of conventional stone and high-end concrete products, he says. The company’s first big commission was to supply pavers for the San Francisco headquarters of technology company Dropbox in 2016.
The company has also received an undisclosed amount of funding from the $47-million Engineered Living Materials (ELM) program, run by the US Defense Advanced Research Projects Agency (DARPA). As part of this program, bioMASON developed a form of marine cement that uses a community of different wild-type bacteria to lay down calcium carbonate, as a way to slowly maintain underwater concrete structures. Some of the bacteria produce urea, while others feed on the urea and absorb calcium from seawater to produce the required calcium carbonate.
In another demonstration for DARPA, dubbed Project Medusa, the company showed that its industrious bacteria could build a 230-square-meter helicopter landing pad using local soil. Delivering construction materials into conflict zones can be dangerous, complicated and expensive, so “if we can start to utilize biology to build materials on site, then we may have the opportunity to reduce that logistical burden,” explains Blake Bextine, ELM program manager.
DARPA has also funded University of Colorado Srubar’s research into cyanobacteria that can form living building bricks. The team adds cyanobacteria to a hydrogel matrix, where they absorb CO2 through photosynthesis, which increases the pH at the cells’ surface. This triggers calcium carbonate precipitation, which binds the hydrogel into solid blocks that are about as strong as mortar. Crucially, each brick still contains living cyanobacteria, so it can act as a ‘starting culture’ to generate more bricks. Srubar’s team hopes to produce stronger blocks and is testing different strains of cyanobacteria that could perform the same feat without needing a hydrogel. Next year, he plans to launch a new company, called Minus Materials, to commercialize the technology.
Bacteria are not the only biological builders on site. Fungi, such as mushrooms, absorb nutrients through underground tendrils called the mycelium, and this fast-growing material is now being used to produce a range of products, including textiles, packaging and building materials.
Biotech company Ecovative Design, based in Green Island, New York, has pioneered mycelium production, and in 2017 it won $9.1 million in funding through DARPA’s ELM program. Ecovative packs agricultural byproducts, such as maize husks or hemp fiber, into a mold, and then seeds it with mycelium. Over seven to ten days in an incubation chamber, the mycelium feeds on the plant waste and breaks down long-chain carbohydrate molecules such as cellulose. The resulting mass of mycelium and residual plant matter forms a sturdy composite material. “In nature, mycelium is a recycling system in and of itself,” says Andy Bass, Ecovative’s director of marketing.
The company initially aimed to find a biodegradable alternative to Styrofoam packaging, but in 2014 it partnered with Arup and design studio The Living to create the Hy-Fi tower, a temporary, 13-meter-high structure made of mycelium bricks that was installed at New York’s Museum of Modern Art. The bricks have a strength similar to that of Styrofoam, and at the end of their life they were composted and added to the soil of local community gardens.
Ecovative now licenses this technology to other companies, including New Frameworks in Burlington, Vermont, which uses mycelium to make door cores, and Mogu in Inarzo, Italy, which has collaborated with Arup to produce prefabricated mycelium acoustic panels for offices and conference spaces. “We’re already working on reducing the cost and making mycelium more available,” Wurm says. For now, there are still barriers to overcome. “The challenges in the field right now are scale and cost,” says Srubar. Traditional construction materials are used on a vast scale and tend to be very cheap, so the fledgling companies making biological construction materials are still fighting to break into the market.
Another hurdle is the preconceived notion that the built environment should be sterile and entirely inorganic. “In our earlier days,” Dosier says, “the word ‘bacteria’ came with a negative connotation, that bacteria are somehow ‘bad’.”
But Bextine hopes that as more demonstration projects emerge, such as those supported by ELM, it should allay these fears and attract investment to the companies that are building with biology. And Wurm believes that construction companies will turn to biological materials as increasingly stringent regulations on recycling and carbon emissions come into force. “I think these materials will become a more mainstream choice,” he says. “They hold great potential.”
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Peplow, M. Bioconcrete presages new wave in environmentally friendly construction. Nat Biotechnol 38, 776–778 (2020). https://doi.org/10.1038/s41587-020-0595-z