The hydrogel becomes as hard as glass

Hydrogels are a true masterpiece of all materials – they stick under water, become superabsorbents and are the main ingredient of soft contact lenses. Now, scientists have added one more property to the diverse properties of these cross-linked polymer scaffolds: they have developed a hydrogel that becomes hard as glass under pressure but immediately reverts to a soft gel state when the pressure is removed. The gel is so stable when compressed that it can even be run over by a car. The scientists explain that the principle of construction of these innovative hydrogels may open up new possibilities for applications.

It is produced by snails, some crabs use it, and even jellyfish are almost entirely hydrogel. This mass, slippery in a humid environment, is more than 80 percent water, but does not lose its structure even when submerged. At the same time, a hydrogel can become incredibly strong, withstand tremendous forces, or stick to almost any surface. All this is possible thanks to the special composition of hydrogels. Their backbone consists of cross-linked polymers that can bind water, but are themselves insoluble in water. As a result, the three-dimensional network of physically or chemically linked polymer chains swells in a humid environment and becomes soft and gel-like. On the other hand, when dry, depending on its composition and structure, the hydrogel may remain rubbery and elastic or may solidify and grind into a powder.

Cage particles as stabilizers

Scientists have now added another range of hydrogel properties: they have developed a gel that becomes rock-hard and stable as safety glass under pressure. “Humans have been making mostly rubber hydrogels for years, but that’s only half the picture,” says senior author Oren Scherman of the University of Cambridge. “We have now developed a new class of materials that can cover the entire range from soft rubber to as hard as glass.” The starting point for this new type of hydrogel was the observation that the crosslinks known as crosslinkers in most soft hydrogels yield little when applied force. They often rely on hydrogen bonds and other non-covalent bonds that quickly dissociate. “We hypothesize that extending the lifetime of these cross-linkers will allow them to form supramolecular polymer networks that behave like glassy materials,” explains Scherman, first author Zehuan Huang of the University of Cambridge, and colleagues.

To this end, scientists have developed a hydrogel whose crossbars consist of so-called squashes – cage-like organic molecules that enclose and hold the two ends of opposite cross-linking agents as ‘guest molecules’. The cages ensure that the crosslinkers respond very slowly to the pulling forces. “By changing the chemical structure of these guest molecules, we can control how well the material can withstand compressive forces,” explains Huang’s colleague Jade McCune.

It can also withstand the weight of the car

In the laboratory, scientists checked how stable the different variants of this hydrogel they produced were. Some gels have been found to withstand pressures greater than one gigapascal – almost 10,000 times the atmospheric pressure. Instead of yielding and crushing, they became as hard as glass under pressure. “The way the hydrogel withstood compression was surprising – we’ve never seen anything like this in hydrogels before,” says McCune. This was also confirmed in a test in which scientists drove a 1.2-tonne car over a hydrogel plate measuring approximately seven by five centimeters. “Even after repeating it 16 times, there were no visible cracks or irreversible deformation,” the research team said. Instead, the hydrogel compressed and hardened under pressure, but returned to its soft, original shape upon release.

“To our knowledge, this is the first time that glassy hydrogels have been made,” says Huang. “We are opening a new chapter in the field of high-performance gels.” According to the team, these modified hydrogels greatly expand the possible applications of such materials.

Source: Zehuan Huang (University of Cambridge, UK) et al., Nature Materials, doi: 10.1038 / s41563-021-01124-x

Leave a Comment