Stretchy Biocompatible Hydrogel Holds Promise as Artificial Cartilage
By themselves, polyacrylamide gels and alginate gels are brittle and not very elastic. A new hydrogel, however, created by Harvard researchers, has a more complex molecular structure that helps to disperse stress across a wide area.
Known as a hydrogel, for the reason that its main ingredient is water, the new material is a hybrid of two weak gels that as a composite create something much stronger. Not only can this new gel stretch to 21 times its original length, but it is also extraordinarily tough, self-healing, and biocompatible. This is an important compilation of attributes which opens up new possibilities in medicine and tissue engineering.
Beyond applications in artificial cartilage, the researchers suggest that the new hydrogel could be used in actuators for optics and fluidics, in soft robotics, artificial muscle, as a tough protective covering for wounds, or “any other place where we need hydrogels of high stretchability and high toughness.”
The September 6 issue of Nature has a feature describing the new material, its properties, and a simple method of synthesis.
A Stretch of Imagination
“Conventional hydrogels are very weak and brittle—imagine a spoon breaking through jelly,” said Jeong-Yun Sun, lead author and postdoctoral fellow at the Harvard School of Engineering and Applied Sciences. “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 fabricate the tough new hydrogel, they combined two common polymers. The primary component is polyacrylamide, which is more known for its use in soft contact lenses and as the electrophoresis gel that separates DNA fragments in biology labs; the second component is alginate, a seaweed extract that is frequently used to thicken food.
Alone, both these gels are fairly weak. For instance, alginate can stretch to only 1.2 times its length before it breaks. Combined in an 8:1 ratio, though, the two polymers form a complex network of cross-linked 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 allowing the gel to crack.
Unzipping Ionic Bonds
The alginate part of the gel is made of polymer chains that form weak ionic bonds with one another, capturing calcium ions, which are added to the water, in the process. When the gel is stretched, some of these bonds between chains break—or “unzip,” as the researchers put it—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, i.e. very tightly, with the alginate chains.
So if the gel develops even 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. The research team showed that even with a huge crack, a critically large hole, the hybrid gel can still stretch to 17 times its initial length.
Note that the team’s hydrogel is capable of keeping its elasticity and toughness over multiple stretches. As long as the gel has some time to relax between stretches, the ionic bonds between the alginate and the calcium can “re-zip,” and the researchers have shown that this process can be accelerated by raising the ambient temperature.
The group was able to apply two concepts from mechanics, crack bridging and energy dissipation, to a new problem because of their combined expertise in mechanics, materials science, and bioengineering.
“The unusually high stretchability and toughness of this gel, along with recovery, are exciting,” says 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. It’s very promising.”