Scientists at the University of Cambridge have developed a soft-yet-strong material that looks and feels like a squishy jelly that can survive being run over by a car. This super jelly also acts like an ultra-hard, shatterproof glass when compressed, despite its high water content.
The super jelly consists of the non-water portion of the material. This portion is a network of polymers held together by reversible on/off interactions that control the material’s mechanical properties. This is the first time that such significant resistance to compression has been incorporated into a soft material.
Material behavior is utterly dependent on its molecular structure. Stretchy, rubber-like hydrogels have properties like toughness and self-healing capabilities. However, developing such hydrogels that can withstand being compressed without getting crushed is a challenge.
Scientists used crosslinkers to make materials with the mechanical properties they wanted. In this crosslinker, two molecules are joined through a chemical bond.
Dr. Zehuan Huang from the Yusuf Hamied Department of Chemistry, the study’s first author, said, “We use reversible crosslinkers to make soft and stretchy hydrogels, but making a hard and compressible hydrogel is difficult and designing a material with these properties is completely counterintuitive.”
Using barrel-shaped molecules called cucurbiturils, scientists made a hydrogel that can withstand compression. The cucurbituril is the crosslinking molecule that holds two guest molecules in its cavity – like a molecular handcuff. They then designed guest molecules that prefer to stay inside the cavity for longer than normal, which keeps the polymer network tightly linked, allowing it to withstand compression.
Scherman, Director of the University’s Melville Laboratory for Polymer Synthesis, said, “At 80% water content, you’d think it would burst apart like a water balloon, but it doesn’t: it stays intact and withstands huge compressive forces. The properties of the hydrogel are seemingly at odds with each other.”
Co-author Dr. Jade McCune, also from the Department of Chemistry, said, “The way the hydrogel can withstand compression was surprising, it wasn’t like anything we’ve seen in hydrogels. We also found that the compressive strength could be easily controlled through simply changing the chemical structure of the guest molecule inside the handcuff.”
To develop their glass-like hydrogels, scientists chose specific guest molecules for the handcuff. They then altered the molecular structure of guest molecules to slow down the dynamics of the material with the mechanical performance of the final hydrogel ranging from rubber-like to glass-like states.
Scherman said, “People have spent years making rubber-like hydrogels, but that’s just half of the picture. We’ve revisited traditional polymer physics and created a new class of materials that span the whole range of material properties from rubber-like to glass-like, completing the full picture.”
Scientists also used the material to make a hydrogel pressure sensor for real-time monitoring of human motions, including standing, walking, and jumping.
Huang said, “To the best of our knowledge, this is the first time that glass-like hydrogels have been made. We’re not just writing something new into the textbooks, which is exciting, but we’re opening a new chapter in the area of high-performance soft materials.”
Scientists are working to develop further this super jelly for application in biomedical and bioelectronic fields.
- Zhuan Huang et al. ‘Highly compressible glass-like supramolecular polymer networks.’ Nature Materials (2021). DOI: 10.1038/s41563-021-01124-x