Mixing together components of baby nappies and contact lenses in a vat of liquid, shining a light on it and hoping it will result in a magical new substance sounds like the mischievous adventures of an unsupervised toddler rather than real science, but this is pretty much what’s led researchers to remarkable new materials that are extraordinarily tough and flexible.
Describing these new materials in Nature Materials, the researchers say they are tougher than cartilage, incredibly stretchy, and have the ability to self-heal.
Falling under the broader category of ionogels – polymer networks that contain ionic liquids – the new materials retain mechanical properties despite being nearly 70% liquid.
They’re related to hydrogels, which are similar polymer networks that contain water instead of ionic liquids. Hydrogels are soft, translucent materials that can hold large volumes of liquid without losing their structural integrity – a property that makes them very useful in products like baby nappies. They’re reasonably commonplace, used in things like contact lenses and wound dressings.
By replacing water with liquid salts, ionogels offer a suite of advantages over their hydrogel counterparts, explains Dr Michael Dickey, co-author and professor of Chemical and Biomolecular Engineering at the North Carolina State University.
“Ionic liquids don’t evaporate like water, so you don’t have to worry about the ionogels drying out,” he says. “Ionogels are also electrically and thermally stable and conduct electricity well, raising some interesting opportunities for future applications.”
To make the new materials, the researchers started with polyacrylic acid (the hydrogel used in nappies) and polyacrylamide (the hydrogelused in contact lenses), placed them in a solution of ionic liquid, and exposed them to ultraviolet light. The resulting copolymer amalgamated not just the two monomers, but the ionic liquid they were submerged in.
“The end result is significantly better than an average of the two materials,” Dickey says. “It’s like adding 1+1 and getting 10. The resulting gel has the stretchability of polyacrylic acid and is even stronger than the polyacrylamide. In terms of toughness, it’s better than cartilage.”
The gel has some other wondrous properties, too. Two pieces of the ionogel can be bonded together into a single, seamless piece when exposed to heat, giving it self-healing potential. It also has shape memory – it can be deformed into new shapes, then returned to its original shape when heated. The rate at which the material heals or returns to an initial shape can be controlled through temperature, needing only tens of seconds to perform these impressive feats when heated to 60°C.
Despite the unusual process involved in making them, the researchers say ionogels are incredibly simple to create, and can even be 3D printed – another desirable property.
“We’re excited that we’ve made something with truly remarkable properties that can be made very easily – you just shine light on it – using widely available polymers,” Dickey says. “And you can tailor the properties of the ionogels by controlling the ratio of ingredients during the copolymerisation process.”
Creating materials that can be deformed and stretched without tearing is a common goal in materials science, but they’re tricky to achieve in the lab.
“Nature is good at this; think of cartilage as an example. But engineering synthetic materials with these properties has been difficult, which makes our work here exciting.”