Science - Latest Development

Highly Stretchable and Bio-compatible Material to Replace Damaged Cartilage in Joints Developed

Experts from various fields at Harvard School of Engineering and Applied Sciences (SEAS) have researched and created highly stretchable and tough material that can replace damaged cartilage in human joints.

The new material is a hydrogel, which is a strong hybrid of two weak gels.  It possesses a high degree of flexibility that can stretch to 21 times its original length. It’s extremely tough, self-healing, and bio-compatible attributes pave way to new opportunities being investigated in medicine and tissue engineering.

Conventional hydrogels (network of polymer chains) are very weak and brittle. But these hydrogels are likely to be used 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 a new hydrogel, researchers combined two common polymers. The primary component is polyacrylamide, commonly used in soft contact lenses and as the electrophoresis gel that separates mixed population of DNA and RNA fragments by length in biology labs; the second component is alginate, a seaweed extract that is commonly 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 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 permitting the gel to crack.

The portion of gel that is alginate consists of polymer chains that make weak ionic bonds with one another, capturing calcium ions (added to the water) in the process. When the gel is stretched, some of the bonds between chains break or unzip, releasing the calcium. As a result, the gel expands slightly, but the polymer chains remain intact. At the same time, the polyacrylamide chains form a grid like structure that bonds tightly with the alginate chains.

So, if the gel forms a tiny crack as it stretches, the polyacrylamide grid spreads over a large area, pulling on the alginate’s ionic bonds and unzipping them. Even with a huge crack, the hybrid gel can still able to stretch 17 times its initial length.

Important thing to note is that the new hydrogel is able to maintain its elasticity and toughness even after stretched multiple times. Only if the gel has given time to relax between stretches, the ionic bonds between the alginate and the calcium can re-zip. In this research, it was demonstrated that the process of healing can be accelerated by increase in ambient temperature.

The researchers suggest using the new hydrogel in places where it may need hydrogels of high stretchability and toughness like in soft robotics, in optics, in artificial muscle, and as a protective covering for wounds.

The research made on the material, its properties, and a simple method of synthesizing it is described in the September 6th issue of Nature.