Stretching tough - hybrid hydrogel could replace damaged cartilage

Materials World magazine
29 Oct 2012

It is tougher than human cartilage, stretches to 21 times its original length and contains 90% water. A hybrid hydrogel developed at Harvard University, USA, could ease the pain of bandy knees and improve soft robotics.

Existing hydrogels are brittle, which is one reason why they cannot be used to replace damaged cartilage. Their consistency could be likened to jelly or tofu. ‘If you use these (materials) as cartilage between bone, they cannot stand much load,’ explains lead researcher Zhigang Suo. ‘They will break into pieces, but cartilage is tough.’  

To address this, Suo’s team has mixed two relatively weak gels – polyacrylamide (found in contact lenses) and alginate (seaweed extract). With their powers combined, they form crosslinked networks that reinforce each other and allow the material to stretch.  

The synthesis process reads like a simple cooking recipe. Polyacrylamide and alginate (without crosslinks) are mixed in a ratio of 8:1 in water. The solution is then cured under UV light at 50ºC for an hour.  

According to Suo, the resultant gel has a fracture energy of about 9,000 J/m-2. To put that into perspective, cartilage has a fracture energy of about 1,000 J/m-2 while tofu’s is only about 10 J/m-2. The researchers attribute the gel’s toughness to the synergy of the covalent crosslinks that bridge cracks and the ionic crosslinks that ‘unzip’ to dissipate energy.  

When tiny cracks appear in the stretching material, the polyacrylamide grid spreads the pulling force over a larger area, tugging on the alginate’s ionic bonds and unzipping them. Even if a crack yawns large, the researchers claim the hydrogel can stretch to 17 times its original length.  

While the material has a tough matrix, it is nowhere near as stiff as cartilage. To improve this, the team is burying fibres in the hydrogel to toughen it. They have experimented with steel wool and are working with another research group that is applying collagen fibres to hydrogel.  

While Suo emphasises that Harvard’s hydrogel is just one offering in a vibrant field, it still represents a significant advance. ‘What people are excited about is the fact that you can make a gel with 90% water that is so tough – as tough as rubber itself.’