Living hydrogels with catalytic function grown from engineered yeast co-cultures

Imagine a world where the materials we use to build medical devices, soft robots, or even food are not manufactured in factories with heavy machinery and toxic chemicals, but are instead grown, much like a garden. This is the promise of engineered living materials, a field where scientists program tiny organisms to build structures for them. At the heart of this story are hydrogels, soft, jelly-like networks made mostly of water that can hold living cells and support life. These materials are essential for things like artificial skin, wound dressings, and tissue scaffolds. However, making them has traditionally been a difficult and often harmful process. To turn liquid proteins into a solid gel, manufacturers usually have to mix in harsh chemical crosslinkers or expensive, purified enzymes. These chemicals can be toxic to the very cells the gel is meant to protect, and the purification steps are energy-intensive and tedious. Nature, however, has been solving this problem for billions of years. Living cells in our own bodies, and in the natural world, constantly build and repair gel-like structures by linking proteins together without any toxic additives. The question researchers have been asking is whether we can teach microbes to do the same thing, creating these useful materials simply by growing them in a liquid culture.

A team of scientists at Aalto University and Imperial College London has now answered that question with a resounding yes. They have developed a system where engineered yeast cells grow a strong, functional hydrogel directly from a liquid broth, completely bypassing the need for external chemicals or complex manufacturing steps. The researchers started by looking at a specific type of enzyme called transglutaminase, which acts like a molecular glue, capable of stitching protein chains together to form a solid network. While some versions of this enzyme exist in nature, the team chose to use bacterial versions because they are robust and do not require extra helper molecules to work. They took four different bacterial enzymes and programmed yeast cells to produce them. To find the best candidate, they displayed these enzymes on the surface of the yeast cells and used a sorting process to identify which one the yeast could produce most effectively. They found that an enzyme from a bacterium called Streptomyces mobaraensis was the clear winner.

The next challenge was getting the yeast to secrete this enzyme into the liquid so it could work on the proteins in the surrounding environment. The researchers engineered the yeast to release the enzyme freely into the broth. They added a specific protein called casein, which is rich in the building blocks the enzyme needs to work, to the liquid culture. When the yeast were left to grow in a quiet, undisturbed container, they began to secrete the enzyme. This enzyme started linking the casein proteins together. However, a problem emerged: the yeast cells are too heavy to float, so they sank to the bottom of the container. As a result, the enzyme was only produced at the bottom, and the gel only formed in a thick layer there, leaving the rest of the liquid unchanged. The material was not uniform, and the process was inefficient.

To solve this, the researchers looked to nature for a clue. Yeast cells in the wild often cling to plant matter, such as decaying fruit or tree bark, which are rich in cellulose. The team wondered if they could use tiny fibers of cellulose, known as nanofibrils, to keep the yeast cells suspended in the liquid. They added a small amount of these nanofibrils to the culture. These fibers acted like a gentle, invisible scaffold, holding the yeast cells in place throughout the entire volume of the liquid. With the cells now evenly distributed, the enzyme they produced could reach the casein proteins everywhere in the container. The result was a transformation: the entire liquid turned into a uniform, self-standing hydrogel within a few days. This gel was not just a simple mixture; it had a unique dual structure. One part of the network was made of the cellulose fibers physically tangled together, and the other part was made of the proteins chemically glued together by the yeast's enzyme. This combination made the material remarkably strong and elastic, capable of holding its shape even when squeezed, while still being 94 percent water.

The true power of this living system, however, lies in its ability to do more than just build a gel. The researchers demonstrated that they could grow a gel that also performed a specific chemical task. They co-cultivated the gel-making yeast with a second type of yeast engineered to produce an enzyme that breaks down antibiotics. Because both types of yeast were growing together in the same liquid, the resulting hydrogel contained both the structural proteins and the antibiotic-breaking enzyme. When they tested the gel, they found it could successfully change the color of a chemical indicator, proving that the living material was actively performing a catalytic function. This means that in a single step, they grew a material that was both a structural gel and a functional tool.

This work represents a fundamental shift in how we might think about making materials. Instead of forcing chemicals to react in a vat, the scientists let the material grow itself, guided by the genetic instructions inside living cells. The process is simple, requiring only the cultivation of yeast in a liquid medium, and it avoids the use of toxic crosslinkers entirely. The resulting hydrogels are strong, flexible, and can be programmed to perform specific tasks, such as breaking down harmful substances. Because the yeast used are generally recognized as safe and the enzymes are already used in the food industry, this approach opens the door to creating new types of biodegradable materials for medicine, food, and advanced technology. The study shows that by working with the natural capabilities of living cells, we can grow complex, high-performance materials that were previously impossible to make without harsh industrial processes.