A building material that lives and stores carbon

The idea seems futuristic: At ETH Zurich, various disciplines are working together to combine conventional materials with bacteria, algae and fungi. The common goal: to create living materials that acquire useful properties thanks to the metabolism of microorganisms – "such as the ability to bind CO₂ from the air by means of photosynthesis," says Mark Tibbitt, Professor of Macromolecular Engineering at ETH Zurich.

An interdisciplinary research team led by Tibbitt has now turned this vision into reality: it has stably incorporated photosynthetic bacteria – known as cyanobacteria – into a printable gel and developed a material that is alive, grows and actively removes carbon from the air. The researchers recently presented their "photosynthetic living material" in a study in the journal Nature Communications.

Key characteristic: Dual carbon sequestration

The material can be shaped using 3D printing and only requires sunlight and artificial seawater with readily available nutrients in addition to CO₂ to grow. "As a building material, it could help to store CO₂ directly in buildings in the future," says Tibbitt, who co-initiated the research into living materials at ETH Zurich.

The special thing about it: the living material absorbs much more CO₂ than it binds through organic growth. "This is because the material can store carbon not only in biomass, but also in the form of minerals – a special property of these cyanobacteria," reveals Tibbitt.

Yifan Cui, one of the two lead authors of the study, explains: "Cyanobacteria are among the oldest life forms in the world. They are highly efficient at photosynthesis and can utilise even the weakest light to produce biomass from CO₂ and water".

At the same time, the bacteria change their chemical environment outside the cell as a result of photosynthesis, so that solid carbonates (such as lime) precipitate. These minerals represent an additional carbon sink and – in contrast to biomass – store CO₂ in a more stable form.

Cyanobacteria as master builders

"We utilise this ability specifically in our material," says Cui, who is a doctoral student in Tibbitt's research group. A practical side effect: the minerals are deposited inside the material and reinforce it mechanically. In this way, the cyanobacteria slowly harden the initially soft structures.

Laboratory tests showed that the material continuously binds CO₂ over a period of 400 days, most of it in mineral form – around 26 milligrams of CO₂ per gram of material. This is significantly more than many biological approaches and comparable to the chemical mineralisation of recycled concrete (around 7 mg CO₂ per gram).

Hydrogel as a habitat

The carrier material that harbours the living cells is a hydrogel – a gel made of cross-linked polymers with a high water content. Tibbitt's team selected the polymer network so that it can transport light, CO₂, water and nutrients and allows the cells to spread evenly inside without leaving the material.

To ensure that the cyanobacteria live as long as possible and remain efficient, the researchers have also optimised the geometry of the structures using 3D printing processes to increase the surface area, increase light penetration and promote the flow of nutrients.

Co-first author Dalia Dranseike: "In this way, we created structures that enable light penetration and passively distribute nutrient fluid throughout the body by capillary forces." Thanks to this design, the encapsulated cyanobacteria lived productively for more than a year, the materials researcher in Tibbitt's team is pleased to report.

Infrastructure as a carbon sink

The researchers see their living material as a low-energy and environmentally friendly approach that can bind CO₂ from the atmosphere and supplement existing chemical processes for carbon sequestration. "In the future, we want to investigate how the material can be used as a coating for building façades to bind CO₂ throughout the entire life cycle of a building," Tibbitt looks ahead.

There is still a long way to go – but colleagues from the field of architecture have already taken up the concept and realised initial interpretations in an experimental way.

Two installations in Venice and Milan

Thanks to ETH doctoral student Andrea Shin Ling, basic research from the ETH laboratories has made it onto the big stage at the Architecture Biennale in Venice. "It was particularly challenging to scale up the production process from laboratory format to room dimensions," says the architect and bio-designer, who is also involved in this study.

Ling is doing her doctorate at ETH Professor Benjamin Dillenburger's Chair of Digital Building Technologies. In her dissertation, she developed a platform for biofabrication that can print living structures containing functional cyanobacteria on an architectural scale.

For the Picoplanktonics installation in the Canada Pavilion, the project team used the printed structures as living building blocks to construct two tree-trunk-like objects, the largest around three metres high. Thanks to the cyanobacteria, these can each bind up to 18 kg of CO₂ per year – about as much as a 20-year-old pine tree in the temperate zone.

"The installation is an experiment – we have adapted the Canada Pavilion so that it provides enough light, humidity and warmth for the cyanobacteria to thrive and then we watch how they behave," says Ling. This is a commitment: The team monitors and maintains the installation on site – daily. Until 23 November.

At the 24th Triennale di Milano, Dafne's Skin is investigating the potential of living materials for future building envelopes. On a structure covered with wooden shingles, microorganisms form a deep green patina that changes the wood over time: A sign of decay becomes an active design element that binds CO₂ and emphasises the aesthetics of microbial processes. Dafne's Skin is a collaboration between MAEID Studio and Dalia Dranseike. It is part of the exhibition "We the Bacteria: Notes Toward Biotic Architecture" and runs until 9 November.

The photosynthetic living material was created thanks to an interdisciplinary collaboration within the framework of ALIVE (Advanced Engineering with Living Materials). The ETH Zurich initiative promotes collaboration between researchers from different disciplines in order to develop new living materials for a wide range of applications.