A hydrogel that transitions from solid to liquid and back the more water is added

Chemists know that you can turn a solid hydrogel into a liquid by diluting it in water. And if you want the reverse to happen, all you need to do is increase the concentration of the hydrogel’s components instead.

But in an accidental discovery, researchers have found a liquid solution that transitions into a solid hydrogel when it’s diluted – and goes back to a liquid again by adding even more water.

This phenomenon had never been described before and will have important applications in many areas of chemistry and biology.

“It is very valuable to show that what you learn in your first chemistry classes does not always hold,” says senior author Bert Meijer, a professor of organic chemistry at the Eindhoven University of Technology, Netherlands. “When diluting, not all gels and solutions become liquids by definition.”

The new study has been published in Science.

What are hydrogels?

Hydrogels are water-containing gels made from 3D networks of polymers – molecules made up of repeating units – physically entangled and crosslinked together by chemical bonds.

The most well-known and widely used hydrogel is the humble jelly, a favourite dessert amongst those young and old.

It’s made by dissolving gelatin – a collection of protein polymers produced from collagen that is extracted from the skin, bones and connective tissues of animals – in warm water.

As the mixture cools the polymers cross-link with each other into a network that holds water molecules, forming a solid hydrogel in a process that can be reversed by increasing the temperature again.

The natural and/or synthetic polymers in hydrogels can be crosslinked reversibly or irreversibly through a few different mechanisms. Temperature changes, enzymes, light, the presence of certain ions, and spontaneous reactions between chemical groups are used by scientists to form crosslinks.

By changing the polymers used and the types of crosslinks made, hydrogels can have different properties and numerous applications. Some are bio-compatible and can be used inside the body to deliver drugs, others can remove pollutants from water, some are even used as fabrication materials for soft robots, and much more.

Discovered by accident

This research focused on a hydrogel that self-assembles when its two components are at a specific ratio in water. These components are benzene-1,3,5-tricarboxamide (BTA-EG4) – which can join together to form into supramolecular polymers – and a positively charged ionic surfactant.

The team knew that when you decrease the concentration of the hydrogel by adding water (without changing the ratio between the two components) the gel dissolves and turns into a liquid.

But in a surprising accidental discovery back in 2019, the team found that if the solution is then diluted even further, a gel forms again.

“It was a special, accidental finding, and I immediately saw more possibilities,” explains lead author Dr Lu Su, a chemical engineer in the Meijer research group at Eindhoven. “What if we were able to demonstrate a double transition? So, from a gel to a liquid, back to a gel, and back to a liquid just by adding more water.”

Amazingly, they found that was exactly the case – by adding more water the gel would once again dissolve into a liquid.

Through additional experiments they determined the correct proportions the active substances had to be in and at which concentrations the transitions take place. These transitions are also fully reversible. So if the concentrations of the substances are increased the transitions (from liquid to gel to liquid to gel) occur in reverse at the same points.

“Such hydrogels may be good solutions for existing challenges, such as for the cultivation of stem cells,” adds Meijer. “In the gel, the cells can safely divide into three dimensions and, once there are enough, you dilute the solution and the cells can be used right away.

“The trick is, of course, to find the right substances that exhibit this behaviour but do not react to the cell walls or stick to them. In this way, our research leads to other research.”