ETH Zurich develops living building material that absorbs CO₂

A team at ETH Zurich is developing an innovative building material that is literally alive. Tiny microorganisms within it remove carbon dioxide from the air, incorporate it chemically and gradually harden the material. The vision is for façades that absorb CO₂ like trees, become more stable over the years and, in the best case, can even compensate for minor damage themselves.

How algae bring ancient techniques to modern construction

The project centres on cyanobacteria, commonly referred to as blue-green algae. These living organisms are among the oldest on Earth and have spent billions of years converting sunlight into energy.

The basic process is well known: through photosynthesis, the bacteria take up CO₂, use water and light, and produce oxygen and energy-rich compounds. What makes them particularly interesting is what else they are capable of doing.

The new building material stores part of the absorbed carbon dioxide not only in biomass, but converts it directly into solid, lime-like minerals.

It is precisely this second ability that makes the material so compelling. Inside it, the microorganisms create a kind of mineral “skeleton”. This process, known as mineralisation, strengthens the material over the long term and locks away carbon very securely.

Unlike pure biomass, which can grow only to a limited extent and eventually breaks down, these mineral structures remain in place. The algae therefore provide a natural version of what cement achieves chemically – but in a far more climate-friendly way.

Hydrogel habitat: the “flowerpot” for microbial building materials

To prevent the organisms from simply drying out or dying, the researchers embed them in a specialised hydrogel. This water-rich, sponge-like substance has numerous pores, serving both as a habitat and as a working surface.

• high water content for the microorganisms
• fine pores to transport CO₂ and nutrients
• permeable to light so photosynthesis can take place
• mechanically shapeable and suitable for 3D printing

The gel can be processed in a 3D printer, allowing freely shaped structures such as columns, panels or ornamental façade elements to be produced. It enables light to penetrate deep into the material while water and gas circulate through minute channels. For the algae, this creates ideal living conditions.

The system was tested over 400 days. During that period, the material remained active and changed both in appearance and stability: it became stiffer, denser and visibly greener, before appearing somewhat lighter again as mineralisation progressed. At the same time, it fixed around 26 milligrams of CO₂ per gram of material – substantially more than other bio-based CO₂-storage concepts achieve.

Buildings as active CO₂ absorbers

The researchers are already looking well beyond laboratory samples. Their aim is to apply the material to façades and other building components directly exposed to outdoor air. This would turn houses into active parts of a city’s climate system.

At an architecture exhibition in Venice, the team presented prototypes shaped like artificial tree trunks. According to calculations, each of these “trunks” could absorb around 18 kilograms of CO₂ per year – roughly as much as a pine tree about 20 years old.

The vision: house walls that become stronger over the years while continuously drawing carbon dioxide from the surrounding air.

The key point is that the structure becomes more stable as more minerals form. The microorganisms gradually deposit a kind of lime layer inside it. Over time, this creates a more load-bearing, robust system that could ideally seal small cracks or reinforce weak points.

How much climate protection can these materials realistically provide?

Naturally, this concept is not a replacement for large industrial CO₂-capture facilities. However, the cumulative effect could be considerable if millions of square metres of façades were actively involved. It offers particularly exciting possibilities in densely built-up cities with extensive concrete and limited green space.

There are also synergies with architecture and urban planning: façades could provide shade, cool their surroundings and bind carbon dioxide at the same time. Combined with green roofs and other bio-based elements, this creates a very different view of buildings – not as part of the problem, but as part of the solution.

Biotechnology as an accelerator for “living” building materials

For the approach to be scaled up substantially, the natural performance of cyanobacteria may not be sufficient. The team is therefore working on genetically optimised variants. The aim is to increase photosynthetic performance and speed up mineral formation.

This raises ethical and regulatory questions, but it also creates opportunities. Such bacterial strains could be adapted to extreme conditions, including intense heat on sun-exposed façades or high salt levels near the sea.

Nutrient supply is another issue. In the trials so far, the researchers used artificially produced seawater solutions. Practical use will require concepts in which nutrients are integrated into the material or supplied through rainwater and airborne deposition without overloading it.

How sustainable is a living building material really?

Compared with energy-intensive CO₂-capture plants, the algae-based building material operates very frugally. Its most important energy source is sunlight. Pumps, compressors and high pressure are unnecessary as long as the structures remain openly exposed to the air.

This significantly reduces its environmental footprint. The production and disposal of the hydrogel matrix must still be examined critically, but it is already clear that the CO₂ yield per unit of energy used is considerably more favourable than with conventional technical methods.

Approach	Energy requirement	CO₂ binding
Industrial CO₂ capture	high (electricity, pressure, heat)	very high, but expensive
Reforestation with plants	low to medium	high, but slow and land-intensive
Algae-based building material	low (sunlight, minimal technology)	medium, can be integrated into buildings

Opportunities, risks and what this could mean for cities

A “living” building material brings its own set of questions. It must be safe, must not release unwanted substances and should be capable of being deactivated in an emergency. It also needs a defined life cycle: how long should the microorganisms remain active, and what happens when a building is demolished?

For urban planning, the concept opens up new options. Possible applications include:

• active façades in heavily polluted city centres
• building elements alongside busy roads
• temporary structures for major projects or exhibitions that absorb CO₂ for as long as they remain standing
• combinations with solar panels that generate electricity while also providing shade for the algae

Several technical issues also remain unresolved: how would these materials perform during freezing winters? How would they respond to persistent rain or extended dry periods? How much maintenance would be required, and what would large-scale production cost?

For non-specialists, a simple comparison may help: the material behaves somewhat like a thin coral shell on the outside of a house. Microorganisms deposit minerals, reinforce their surroundings and gradually build up a solid structure. The difference is that these “corals” are designed in the laboratory and deliberately controlled.

There are also prospects for combining it with conventional concrete or bricks. The algae material does not have to replace everything; instead, it can act as an additional layer – an active, breathing external system in front of a traditional load-bearing structure. This would make it possible to complement established construction methods with biotechnological elements without having to reinvent the entire system.