Watery, but powerful: Researchers have developed a hydrogel that becomes as firm as hardened glass under pressure. You can even roll a car over it without it being crushed or melted. The gel is soft and pliable under normal conditions, but then reversibly changes its consistency. This is made possible because the polymer structure of the hydrogel has special cross struts that give way only very slowly, as the team reports in the specialist journal “Nature Materials”.
Hydrogels consist of cross-linked polymers that are not water-soluble, but can bind large amounts of water to themselves. The characteristics and behavior of these gels can be varied in a variety of ways by specifically adapting the cross-linking of these frameworks. Some are used, for example, as underwater glue, others as scaffolding for organ growing or as seam glue in surgery. Because the gels can also absorb large amounts of water when dry, they are also used as superabsorbents in diapers and sanitary napkins.
It all depends on the connections
A hydrogel variant developed by researchers working with Zehuan Huang at the University of Cambridge has now proven that the mostly slimy gels can also become rock-hard and stable like safety glass. The starting point was the observation that the crosslinks known as crosslinkers give slightly in the case of most soft hydrogels. They are mostly based on hydrogen bonds and other non-covalent bonds that dissociate quickly.
“We suspect that extending the shelf life of these crosslinkers will make it possible to create supramolecular polymer networks that behave like glass-like materials,” the researchers explain. To do this, they developed a hydrogel whose cross struts consist of so-called cucurbiturils – cage-like organic molecules that hold the two ends of the opposing crosslinkers in place. These cages ensure that the crosslinkers only react very slowly to forces that pull apart.
Run over by the car – and still safe
The team then tested how stable these hydrogels react to pressure in laboratory tests and by being driven over with a 1.2-ton car. And indeed: the modified gels withstood pressures of more than one gigapascal – this is ten thousand times more than atmospheric pressure. Instead of yielding and being crushed, they became hard as glass under the compression. “The way the hydrogel withstood the compression was surprising – we had never seen anything like this in hydrogels before,” says Huang’s colleague Jade McCune.
This was also shown in the car test: The SUV rolled onto the approximately seven by five centimeter large hydrogel plate and stood on it for a minute. “Even after repeating it 16 times, no cracks or irreversible deformations were visible,” reports the research team. Instead, the hydrogel compressed and hardened under pressure, but regained its soft, original shape when the pressure was removed.
“New chapter in the textbooks”
“To the best of our knowledge, this is the first time that vitreous hydrogels have been made,” says Huang. “This opens a new chapter in the field of high-performance gels.” In the opinion of the team, these modified hydro gels expand the possible uses of such materials considerably.
“People spent years making rubbery hydrogels, but that was 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 rubber-soft to glass-hard.” Because how hard the gel becomes when compressed can be adjusted in a controlled manner by chemically modifying the crosslinker ends. (Nature Materials, 2021; doi: 10.1038/s41563-021-01124-x)
Quelle: University of Cambridge