The team of scientists have created a material that looks and feels like a very soft gelatin, but which acts like an ultra-hard and unbreakable glass when strongly compressed. The 20% of the material that is not water is made up of a network of polymers that are held together by reversible on / off interactions that control the mechanical properties of the material. That is, the properties of this material change as interactions are turned on or off. The way materials behave depends on their molecular structure. The new super gelatin is a hydrogel, which are nothing more than hydrophilic polymers with specific properties. A hydrogel is made up of a three-dimensional network of flexible chains, made up of elements connected in a specific way and swollen by a liquid. Its interesting properties have made it a fairly popular research topic in recent years. These smart materials are often tough and self-healing, but until now it was a real challenge to come up with variants capable of withstanding high compression without being crushed. The macroscopic properties of any substance come from its microscopic properties: its molecular structure and the way its molecules interact. Due to the internal structure of hydrogels, it is extremely rare to obtain materials of this type that show both flexibility and strength. Hydrogel applications span a wide spectrum, from contact lenses to robotics.
The team worked with barrel-shaped molecules called cucurbiturils. They are macrocyclic molecules composed of glycoluril monomers linked by methylene bridges. There are oxygen atoms that run along the edges of the band and slope inward, forming a partially closed cavity. The name comes from its resemblance to a pumpkin of the Cucurbit family. There are other types of molecules that also behave like capsules: cyclodextrins, calixarenes, and pillararenos. It was the German organic chemist Robert Behrend who first synthesized cucurbiturils in 1905, from the condensation of glycoluril with formaldehyde. Despite this, its structure was not revealed until 1981. Already in 2000 Kim Kimoon managed to synthesize CB5, B7 and CB8. The number indicates the number of glycoluril units.
The study scientists used cucurbit [8] uril, that is, CB8. They took advantage of the shape of the cucurbiturils to house two guest molecules inside. They somehow do the work of handcuffs, to hold two separate molecules together in the network. They succeeded in designing a CB8 with slow dissociation kinetics. That is, the guest molecules designed by the team of scientists remain inside the cavity for longer than usual, which keeps the polymer network tightly together and thus resists compression. In general, researchers describe it as “slowing down” the dynamics of the material, so its performance can range from a rubber-like state to a glass-like state.
The article Highly compressible glass-like supramolecular polymer networks was published on November 25, 2021 in Nature Materials, by various authors. The result: a material that supports a weight of 4000 kg, a pressure of one hundred million pascals and that recovers its shape in less than two minutes. The article is accompanied by photographs showing stressful situations to which the material has been subjected. A super jelly sample was passed up to 16 times and then recovered its shape without fractures.
“To make materials with the mechanical properties we want, we use crosslinkers, in which two molecules are joined through a chemical bond,” said Dr. Zehuan Huang from the Department of Chemistry at Yusuf Hamied, first author of the study. “We use reversible crosslinkers to make soft hydrogels and elastic hydrogels, but making a hard, compressible hydrogel is difficult and designing a material with these properties is completely counterintuitive.” Huang says they are opening up a new field of high-performance soft materials.
“At 80% water content, you might think it would break like a water balloon, but it didn’t: it remains intact and withstands enormous compressive forces,” says Oren A. Scherman, Director of the Melville Laboratory for Polymer Synthesis. from the University of Cambridge.
“The way the hydrogel can resist compression was amazing, it was unlike anything we’ve seen in hydrogels,” said Dr. Jade McCune, a co-author of the paper. “We also found that compressive strength could be easily controlled just by changing the chemical structure of the host molecule inside.”
This super gelatin developed at Cambridge University could be used in a wide range of applications: bioelectronics, biomedical cartilage replacement for biomedical use, etc. It would also be useful for real-time monitoring of body movements, such as being still, moving, or jumping. It is possible that in a few years it will become part of our daily life. Perhaps one of the applications of this material with the most future is in the field of flexible robots, an entire area that is growing faster than it is possible to follow. They call it soft robotics and it is much more than just entertainment, science fiction or child’s play.
