Experts from a wide variety of fields have collaborated to research and create an extremely tough and stretchy biocompatible material that may be used in the future to replace damaged cartilage in human joints.
The material is a hydrogel, which means that its main component is water and that it is a hybrid of two weak gels combined to create a material much stronger than either was on its own.
This newly created gel is able to stretch to 21 times its original length, and is, more impressively, also extremely tough, biocompatible, and capable of self-healing. That’s an extremely valuable collection of attributes when taken together, opening up many new possibilities and opportunities in medical and tissue engineering fields.
The research on the new material, its properties, and an easy way to synthesize it are described in the September 6 issue of Nature.
“Conventional hydrogels are very weak and brittle — imagine a spoon breaking through jelly,” explains lead author 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.”
The very tough new hydrogel was created by combining two common polymers, the primary being polyacrylamide (used in soft contact lenses), and the secondary being alginate (a seaweed extract used to thicken food).
“Separately, these gels are both quite weak — alginate, for instance, 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 allowing the gel to crack.”
The portion of the gel that is alginate is made of polymer chains that make weak ionic bonds with one another, “capturing calcium ions (added to the water) in the process.” If the gel is stretched out it allows some of these bonds between the chains to break, releasing the calcium. When this happens the gel slightly expands, while still leaving the polymer chains themselves intact. At the same time as this, the polyacrylamide chains form into a “grid-like structure that bonds covalently (very tightly) with the alginate chains.”
So, if the gel forms even a tiny crack as it’s stretched, the polyacrylamide grid spreads out over a large area the force from the pulling, putting pressure on the alginate’s ionic bonds and breaking them in some spots. Even with a huge crack, the hybrid gel is still able to stretch to 17 times its beginning length.
An important thing to note is that the new hydrogel is able to maintain its toughness and elasticity even after being stretched many times. As long as some time is provided to relax in between the stretches, the ionic bonds between the alginate and the calcium can “un-break.” In experiments, it was demonstrated that this healing process can be intentionally accelerated by increasing the ambient temperature.
“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.”
Beyond artificial cartilage, the researchers suggest that the new hydrogel could be used in soft robotics, optics, artificial muscle, as a tough protective covering for wounds, or “any other place where we need hydrogels of high stretchability and high toughness.” Perhaps in some new cleantech?
Source: Harvard School of Engineering and Applied Sciences
Image Credit: Jeong-Yun Sun and Widusha R. K. Illeperuma
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