What are the possibilities and limitations of engineered living materials? Interview with biomedicine and smart materials expert Prof. Dr Andrés Díaz Lantada (Technical University of Madrid, Spain). He has served as principal investigator in several European projects, including TOMAX, UBORA, INKplant, and BIOMET4D.

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Can you explain, in simple terms, what engineered living materials (ELMs) are, and what biological systems - bacteria, fungi, etc. - are most promising as building blocks of ELMs?

Andrés Díaz Lantada: ELMs are materials in which living (biotic) entities, such as bacteria or archaea, interact with abiotic components - the extracellular matrix. We can use animal cells, bacteria, fungi, archaea and even a combination of these entities. While ELMs are linked to tissue engineering as precursor and facilitating field, ELMs transcend tissue engineering because they have many different industrial applications. They can be applied in medicine, architecture, robotics, engineering, construction, electronics, fashion and more. This is because we can embed a variety of cells with a great diversity of functionalities which can be engineered and fine-tuned. In construction, civil engineering and architecture, there has already been a lot of promising results with the use of fungi because they grow their own extracellular matrix called mycelium. The use of therapeutic bacteria is very promising in healthcare and there are also some bacteria that can create self-healing concrete or cement.

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How are these materials developed and produced?

Andrés Díaz Lantada: We can design the matrix, which is then 3D printed, and the cells are cultured in it. With the use of additive manufacturing, we can print the porous structures where the cells can grow and create hybrid living materials. Complementarily, cells in culture can also grow their extracellular matrices leading to biological living materials.

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How do you keep cells alive and functional once they're embedded in a material matrix? How long can these materials remain "alive" and functional under real-world conditions?

Andrés Díaz Lantada: Ensuring that the cells stay alive is a pressing challenge for many ELMs. The lifespan of cells is one of the holy grails of ELMs. While cells inside a human body can receive nutrients directly from the body, it is difficult to determine how to keep the cells alive in an environment outside the body. You can load the matrix, where the cells are grown, with nutrients, but these nutrients do not last long, and the cells will eventually die.

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Which applications do you focus on in your own research?

Andrés Díaz Lantada: I focus on medical devices and materials that respond to environmental stimuli for medical applications. These include sensors for diagnosis and actuators for therapeutic purposes. For example, we started looking into implants that would contain cells directly from the patient, which could prevent rejection of the implant by the body and make the implant more successful.

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How far along is the development of these materials?

Andrés Díaz Lantada: In some cases, we are already reaching patients. There are tissue engineering solutions that are already being applied, for example for skin implants, bone tissue engineering and articular joint repair. Some of these solutions are already deployed and some are in clinical trials.

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What do the living materials look like? What do they feel like if you touch them?

Andrés Díaz Lantada: Depending on the cells, the scaffold might look like real human tissue. If you use fungi, it can look like leather or cardboard.

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What are the possible limitations of living materials compared to traditional materials?

Andrés Díaz Lantada: Currently, we are limited by scalability. It is already challenging to create tiny structures in the lab. It can take weeks to grow a structure of 1 cm2. Scaling up the production is definitely one of the main challenges. Moreover, to reach the end users in a safe way, we need new regulations.

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Are you worried about societal acceptance of living materials?

Andrés Díaz Lantada: It can be challenging to ensure that people accept it. Some medical devices fail simply because of their colour. There was a well-known case of a polymer that worked perfectly but it was bright orange and surgeons would not use it because they were afraid of it being toxic. Now imagine that a device contains bacteria; for instance, if we use ELMs in construction, it might be difficult to explain to society how a building incorporates bacteria that also heal the building. It is important to remember that these are non-pathogenic bacteria, not dissimilar to the ones you can find in yoghurts, but the communication with society is important. That might be a challenge. There isn’t much research into the ethical implications of ELMs, but I’d like to assure people that all labs are safe and that all bacteria we use are non-pathogenic. Nothing will be deployed without a proper safety evaluation.

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Where do you think ELMs will be in ten years - niche research curiosity or mainstream material?

Andrés Díaz Lantada: In ten years, I believe there will be tissue engineering solutions that are popular and fully incorporated into clinical practice. I also think the biological cement will be used in civil engineering and for architectural purposes. Furthermore, I expect a variety of clothing and furniture made of ELMs because this approach is already popular with some artists.

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