Researchers made hydrogels that are 90 percent water but extremely adhesive.
Pulling hydrogel away from a substrate shows the toughness of its bonding. Image: Hyunwoo Yuk
What's 90 percent water but really sticky? Hey, get your mind out of the gutter. I'm talking about new hydrogels that emulate some of nature's own sticky materials to create a new, super-tough adhesive or coating.
Researchers at MIT have detailed a new method to create synthetic hydrogels—polymer networks infiltrated with water—that stick to non-porous surfaces such as glass or silicon with a strength comparable to the bond between cartilage and bone.
"If you look at our body, except our teeth and bone, our other parts are hydrogels," explained lead author Xuanhe Zhao in a phone call. Natural hydrogels are also used as an adhesive by marine creatures such as mussels and barnacles to bond to rocks and ships. This kind of tough, water-heavy bonds inspired the researchers to pursue a synthetic version.
Their resulting hydrogels cling to glass even with a 25kg weight hanging off them, and held strong when applied to silicon even when that silicon was shattered with a hammer.
The most obvious applications for such a strong bonding substance are glues and coatings, such as protective coatings for boats. But given the hydrogels are mostly water, they could also be well-suited to coating medical devices or making bioelectronics.
Hydrogels could also be a perfect material for soft robots. It could be used in robotics to make more flexible joints—much like the tendons in human joints.
"If you think about it, the human body is a hydrogel soft robot."
But first the researchers needed to understand how natural hydrogels are so good at staying stuck. "The one question that challenged us is: What is the fundamental mechanism for this very robust bonding between hydrogels and diverse rigid materials in nature?" said Zhao.
First up, the hydrogel needs a "chemical anchor," meaning it needs to covalently bond to the other surface. The researchers achieved this by "silanating" the surfaces, a process which involves coating the surface with a compound that bonds better with the hydrogel's polymer network.
Secondly, the hydrogel needs to dissipate—disperse—energy so it doesn't retain it all in one place. "Basically, the hydrogel by itself needs to be dissipative, so not all the work is down to the material; it's stored as elastic energy in the material," explained Zhao. "So a significant amount of energy is dissipated during the deformation." This deformation could refer to trying to peel the hydrogel away from the surface it's stuck to.
The researchers created many different hydrogels and applied them to different non-porous materials. As well as exploring those fundamental materials science questions, they demonstrated some potential use cases. Zhao is particularly interested in further investigating applications in bioelectronics and soft robotics.
"If you think about it, the human body is a hydrogel soft robot," said Zhao. "Tissue in the human body contains around 70 percent water; it's basically many pieces of hydrogel together with bones, with skeletons, to form a soft robot."