"Across the world, people are increasingly looking to find new value in resources that would otherwise simply be incinerated or composted," said Hans Priks, a junior research fellow in materials science. He was particularly interested in biologically derived waste, which is generated in large quantities both in agriculture — for example as straw left over from harvesting — and in wastewater treatment.

Many such residues can be used as so-called food for microbes, transforming them into something new and useful. "Cotton clothing made of cellulose would also be suitable. If it is shredded and broken down, you can obtain glucose, which is one of microbes' favorite nutrients," he said by way of example.

In his recently defended doctoral dissertation, Priks sought to make production processes that use microorganisms more efficient than before. At present, production microbes are mostly cultivated in liquid: the cells are placed in a nutrient solution, the environment is kept uniform through stirring and aeration and at the end of the process the product and biomass are separated.

"That works well, but when you think about upcycling waste outside an ideal laboratory — for example at a wastewater treatment plant or in another real-world environment — two major concerns emerge," Priks explained. The first is safety and control: if genetically enhanced microbes are used to boost productivity, it is crucial that they do not escape into the environment. The second is stability: external environments contain their own microbes, fluctuating chemistry and mechanical stress, all of which require protection for the producers.

For this reason, Priks experimented with a new type of hydrogel cube as a workplace for microbes. "If you place them inside a material that can keep them contained, it creates room to deploy the technology more safely and on a broader scale," the newly minted Ph.D. said.

Factory, hotel or a Norwegian prison?

In his doctoral research, Hans Priks set out to create what he describes as a kind of factory for microorganisms where they could figuratively carry out their work in peace. "The goal was to create a structure that would allow nutrients to access the cells as efficiently as possible and where the cells could live, grow and function," he said. At the same time, such a factory must be sufficiently enclosed: if placed, for example, in a wastewater treatment plant, microbes living in that environment must not be able to invade and take over.

In his work, Priks produced these factories from micellar hydrogels. These are water-rich polymer materials composed of more than 70 percent water. "Once polymerized, the gel can resemble rubber or dried silicone: when you touch it, it feels moist but surprisingly tough," the newly minted Ph.D. explained. Although a hydrogel can resemble gelatin, Priks opted for a more rubbery composition. "The material has to withstand intensive stirring and maintain its shape even when there are hundreds or thousands of pieces in a reactor."

When chilled, the materials he used were liquid; at room temperature, they resembled toothpaste. Priks mixed various microorganisms into his materials, including yeasts, bacteria and algae. He also added an initiator — a substance that causes the material to harden after printing so the object retains its shape.

"I loaded the paste into a syringe and used a 3D printer to produce objects with different structures," he recalled. While the materials allow for highly complex designs, in laboratory tests he mostly confined himself to minimalist hydrogel cubes. "You could also imagine it as a kind of hotel for microbes. With printing, I can determine very precisely where each one resides — whether on one floor or another," Priks said.

At the same time, the microbes must have comfortable living conditions: figuratively speaking, the food is served and the room is cleaned, meaning byproducts are removed. According to Priks, the cube also resembles a comfortable Norwegian prison, since ideally no microbe should escape into the environment. "I came to understand how the materials need to be further developed in order to keep the cells contained for longer," he said.

Slower, but cleaner and reusable production

Hans Priks tested the reliability of the finished cubes using brewer's yeast. He fed the yeast glucose and measured the resulting production of ethanol. "It's a simplified system, but the idea is broader: in more complex applications, microbes could produce pharmaceutical compounds, enzymes and proteins or precursor chemicals needed in the chemical industry," the new Ph.D. said.

According to Priks, hydrogel cubes could make it possible to move toward continuous production. Instead of emptying and cleaning a reactor after each production cycle, the system could ideally be fed continuously from one end while the product is collected from the other. "That would reduce downtime and the need for sterilization, resulting in significant time and energy savings. In addition, the same cubes could be reused if necessary," he explained.

At the same time, hydrogel cubes in their current form have limitations. For example, bioprocesses proceed more slowly in them than in cells growing freely in solution. As a result, Priks said the cubes are not yet well suited for the pharmaceutical industry, where large molecules such as therapeutic proteins are produced — their movement through the gel would simply be too slow. "If the network is too dense, it hinders the movement of substances. We need to figure out how to make the network more porous without losing its mechanical properties," he said.

The cube in space and 'one size does not fit all'

Hydrogels are increasingly being studied worldwide as materials for biofactories because they make it possible to combine biology and engineering, Priks said. "Living cells provide the function, the material provides the control," he explained.

Looking ahead, Priks said the cubes could even be used on space missions. "If the cubes are carried in dried form, they could simply be placed in a growth medium in a crisis situation. The cube would rehydrate, the cells would, so to speak, wake up and produce the necessary compound, such as a medication," he said. For now, this is not yet a solution for large-scale production, but in special cases it could prove highly promising, in the view of the scientist.

The material studied by Priks was able to keep cells contained for up to seven days, which he said is a very good result at laboratory scale. The process itself, however, took place within the cube over the course of 24 hours. Going forward, Priks and his colleagues will study how to modify the cube's structure to expand its range of applications. He is also interested in whether more durable cubes could be made from other materials, such as cellulose.

"I gained an understanding of which parameters will need to be monitored in the future when we set out to solve a specific problem," he said. One size does not fit all. But if the right kind of gel "hotels" are built for microbes, waste could gain new value and biotechnology could become a safer, cleaner and more flexible tool, according to Priks.

Hans Priks defended his doctoral dissertation in environmental technology, titled "Life within 3D-printed engineered living materials based on micellar hydrogels," on Jan. 29 at the University of Tartu. The dissertation was supervised by University of Tartu Professor Tarmo Tamm. The opponent was Associate Professor Johan Ulrik Lind of the Technical University of Denmark.

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