Artificial symbiosis enables three-dimensional living materials to be produced with properties normally associated with organisms rather than industry – potentially changing how materials can be manufactured. The work could open new possibilities in areas ranging from wound healing and biological reactors to responsive materials that can repair themselves.
Most people know cellulose from plants, in which the biological material is extracted to make paper, textiles and other things.
What far fewer people realise is that certain bacteria can also produce cellulose. While in plants cellulose occurs as one of the components of the cell wall, to isolate it, many chemical treatments need to be applied; in bacteria, cellulose is produced as a pure nanoscale filament forming interwoven networks with exceptional mechanical properties.
The dense, fibrous structure of bacterial cellulose gives the material exceptional mechanical properties, including the ability to hold large quantities of water. This makes it attractive for applications such as wound healing and cosmetics.
The challenge is that bacteria that produce cellulose require oxygen to form such a network, limiting the amount of material that can be produced in traditional fermenters. In practice, this has confined production to the surface of the fermenter rather than to the volume. The researchers have now overcome this constraint, without the need for stirring or other advanced production methods typically used to increase oxygen supply in liquids.
“This is exactly the same process seen in kombucha cultures. Cellulose forms at the interface between water and air, where the bacteria access oxygen – and that is precisely what limits large-scale production. We have now removed that limitation,” says Silvia Vignolini, Professor at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany.
In a new study, her group show that cellulose-producing bacteria (Komagataeibacter hansenii) can be brought into an artificially created symbiosis with green microalgae (Chlamydomonas reinhardtii). The result is bacterial cellulose produced in large quantities, throughout the volume of the liquid, and in virtually any shape.
The study, carried out by Sing-Teng Chua and Kui Yu and others, has been published in Advanced Materials.
Algae let bacteria grow beyond the surface
The work grew out of a combination of two seemingly unrelated projects in Silvia Vignolini’s research group, in which two PhD students were working independently on algae and cellulose nanoparticles. They observed that algal culture could be “immobilised” by adding charged cellulose nanoparticles.
This led to the idea of letting the algae produce oxygen locally in the water container and using them as an oxygen source, which the bacteria producing cellulose could use directly. Because the algae remain distributed throughout the container, they generate oxygen in the deeper layer, enabling the bacteria to produce cellulose throughout the liquid, not just at the surface, where production is normally limited to a thin film. In addition, it avoids approaches that would otherwise require stirring the system or forcing oxygen into it.
Laboratory experiments showed that exactly this kind of collaboration can be created between the two organisms, which do not naturally occur together.
Changing the oxygen source also led the bacteria to produce cellulose with a different structure, forming a three-dimensional network of fibres.
Further, the interaction not only benefits the bacteria. In exchange for oxygen, the algae gain a porous network in which to grow, keeping them in place and offering protection.
The two organisms thus benefit each other – even as the environment gradually becomes more acidic during growth.
“We call it artificial symbiosis because we have linked two species that would not normally come together. But there are many other examples in nature in which algae and bacteria live symbiotically,” says Silvia Vignolini.
Nanoparticles keep algae suspended throughout the material
For the artificial symbiosis to work, the researchers first had to solve another problem: algae sink to the bottom when left undisturbed.
As a result, they would otherwise clump together at the bottom instead of being evenly distributed throughout the medium.
The solution was to add microscopic cellulose nanocrystals to the mixture. These interact with the surface of the algal cells and form a loose network around them, keeping them apart and preventing them from sinking – a problem that otherwise quickly arises when the liquid is left to stand.
The result is a living material that grows directly into its final shape throughout the entire volume of the liquid – from plates to cylinders and spheres – guided by the geometry of the container.
The researchers showed that the composite material can reach a thickness of up to four millimetres in just four days, with the vast majority of the material forming within the first three days.
As a living material in constant growth, it can do something conventional materials cannot: it repairs itself – while being far stronger than most living hydrogels, which are otherwise very soft and easily break.
“Because it is a living material that is constantly growing, it can actually regenerate itself if you cut into it or damage it,” says Silvia Vignolini.
From cosmetics to living reactors and shape-changing materials
In addition to wound-healing materials and cosmetics, Silvia Vignolini points to biological reactors as a particularly promising application of the symbiotic bacteria and microalgae.
Microalgae thrive within the cellulose network, which effectively becomes a living production platform for compounds such as omega-3 fatty acids, pigments and dietary supplements – unlike many existing systems, in which cells and materials are kept separate.
The research group is also exploring another possibility opened up by the symbiotic system.
Because the algae act as local oxygen sources, their position determines exactly where in the material cellulose is produced. This enables gradient materials to be created with varying densities and mechanical properties, depending on where the oxygen is – which would otherwise require far more advanced techniques.
“We can now control which parts of the material are dense and which are more open. And when you can precisely control the mechanical properties, you can create materials that respond to external stimuli – for example, by changing shape or moving. This positions bacterial cellulose as an important future material within responsive material design and soft robotics – a field that could transform how we build materials in industry and medicine,” says Silvia Vignolini.
