A team of Harvard scientists has managed to create an extremely stretchy and water-based tough gel that may one day be capable of replacing damaged cartilage in human joints.
Not only can this new gel stretch to 21 times its original length, but it is also exceptionally tough, self-healing and biocompatible.
"Conventional hydrogels are very weak and brittle. [Just] imagine a spoon breaking through jelly," explained Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS).
"But because they are water-based and biocompatible, people would like to use them for some very challenging applications like artificial cartilage or spinal disks. For a gel to work in those settings, it has to be able to stretch and expand under compression and tension without breaking."
To create the tough new hydrogel, Sun and his team combined two common polymers. The primary component is polyacrylamide, known for its use in soft contact lenses and as the electrophoresis gel that separates DNA fragments in biology labs, while the second component is alginate, a seaweed extract frequently used to thicken food.
Separately, these gels are both quite weak - alginate, for example, can stretch to only 1.2 times its length before it breaks. Combined in an 8:1 ratio, however, the two polymers form a complex network of crosslinked chains that reinforce one another. The chemical structure of this network allows the molecules to pull apart very slightly over a large area instead of permitting the gel to crack.
The alginate portion of the gel consists of polymer chains that form weak ionic bonds with one another, capturing calcium ions (added to the water) in the process. When the gel is stretched, some of these bonds between chains break - or "unzip" - releasing the calcium.
As a result, the gel expands slightly, but the polymer chains themselves remain intact. Meanwhile, the polyacrylamide chains form a grid-like structure that bonds covalently (very tightly) with the alginate chains.
"So even if the gel acquires a tiny crack as it stretches, the polyacrylamide grid helps to spread the pulling force over a large area, tugging on the alginate's ionic bonds and unzipping them here and there," said Jeong-Yun Sun. "Even with a huge crack, a critically large hole, the hybrid gel can still stretch to 17 times its initial length."
Perhaps most importantly, the new hydrogel is capable of maintaining its elasticity and toughness over multiple stretches. Provided the gel has some time to relax between stretches, the ionic bonds between the alginate and the calcium can "re-zip," a process that can be accelerated by simply raising the ambient temperature.
"The unusually high stretchability and toughness of this gel, along with recovery, are exciting," said researcher Zhigang Suo. "Now that we've demonstrated that this is possible, we can use it as a model system for studying the mechanics of hydrogels further, and explore various applications."
Indeed, beyond artificial cartilage, the researchers believe the new hydrogel could be used in soft robotics, optics, artificial muscle, as a tough protective covering for wounds, or "any other place where hydrogels of high stretchability and high toughness" are required.