Harvard material scientists and bioengineers have developed an extremely tough and stretchy biocompatible material that could have applications in replacing damaged cartilage in human joints.
The biomaterial is dubbed a hydrogel because its main ingredient is water. It is a hybrid of two weak gels that combine to form a much stronger substance. The new gel can stretch up to 21 times its original length, in addition to be exceptionally tough, self-healing and biocompatible – valuable attributes for use in tissue engineering.
“Conventional hydrogels are very weak and brittle – imagine a spoon breaking through jelly,” lead author Jeong-Yun Sun explained. “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 tough new hydrogel was created by combining two common polymers. The primary component is polyacrylamide – the gel used in DNA electrophoresis – the second is alginate, a seaweed extract frequently used to thicken food.
Separately, the gels are both quite weak, but combined in an 8:1 ratio, the two polymers form a complex network of cross-linked chains that reinforce one another, allowing the molecules to pull apart very slightly over a large area instead of allowing the gel to crack.
The alginate component consists of polymer chains forming weak ionic bonds with each other capturing calcium ions (added to the water). Stretching the gel breaks some of the bonds between the chains, releasing the calcium. Consequently, the gel expands slightly but the chains themselves remain intact.
The polyacrylamide chains form a grid-like structure that forms covalent bonds with the alginate chains.
The new hydrogel is capable of maintaining its elasticity and toughness over multiple stretches. The material, its properties and a simple method of synthesis are described in Nature.
Beyond artificial cartilage, the researchers predict that the hydrogel could be used in soft robotics, optics, artificial muscle or as a tough protective covering for wounds.