Madrid, 25 (European Press)
A new gelatinous design material that is able to withstand the equivalent of a passing car and fully restore its original shape, even though it is 80% water.
The soft and strong material, developed by a team at Cambridge University, looks like a soft gel, but acts like extremely tough and unbreakable glass when pressed, despite its high water content.
The non-aqueous part of the material is a network of polymers bound together by reversible on-off reactions that control the mechanical properties of the material. This is the first time that such a large compressive force has been incorporated into a soft material.
“Super gelatin” could be used for a wide range of potential applications, including soft robotics, bioelectronics, or even as a replacement for cartilage for biomedical use. The results were published in the journal Nature Materials.
The way materials behave, whether they are soft or hard, brittle or strong, depends on their molecular structure. Flexible, rubber-like hydrogels have many interesting properties that make them a popular research topic, such as their durability and ability to self-heal, but making hydrogels that can withstand pressure without cracking is a challenge.
“To make materials with the mechanical properties we want, we use cross-linkers, where two molecules are joined through a chemical bond,” said Dr. Zihuan Huang of Yusuf Hamid’s Department of Chemistry, first author of the study. “We use reversible cross-linkers to make hydrogels soft and pliable, but making the hydrogel rigid and compressible is difficult and designing a material with these properties is quite a contradiction.”
Working in the lab of Professor Oren Sherman, who led the research, the team used barrel-shaped molecules called cucurbits to make a hydrogel that could resist compression. Cucurbituril is a crosslinked molecule that has two guest molecules in its lumen, like a molecular bound. The researchers engineered guest molecules that prefer to stay inside the cavity longer than usual, keeping the polymer network tightly together, allowing it to resist pressure.
“At 80% water content, you would think it would break like a water balloon, but it doesn’t: it remains intact and withstands tremendous compressive forces,” said Sherman, director of the university’s Melville Polymer Synthesis Laboratory. “The properties of the hydrogel appear to differ.”
“The way the hydrogel could resist compression was amazing, it wasn’t like anything we’d seen in hydrogels,” said co-author Dr. Jade McKeown, from the Department of Chemistry. “We also found that compressive resistance can be easily controlled simply by changing the chemical structure of the host molecule within the shackles.”
To make the hydrogels resemble glass, the team selected specific molecules for the shackles. Altering the molecular structure of the guest molecules within the shackles allowed the material’s dynamics to “slow down” considerably, with the final mechanical performance of the hydrogel in a range of rubber-to-glass-like states.
“People have spent years making rubber-like hydrogels, but that’s only half of the picture,” Sherman said. “We have revised the traditional physics of polymers and created a new class of materials that covers the full range of material properties, from rubber-like to glass-like, to complement the whole picture.”
The researchers used the material to make a hydrogel pressure sensor to monitor human movements in real time, such as standing, walking and jumping.
“To our knowledge, this is the first time that glass-like hydrogels have been made. We write nothing new in textbooks, which is really exciting, but we are opening a new chapter in the field of high-performance soft materials,” Huang said.