Hydrogels are the only materials that have the potential to be used as a replacement material for functional tissues like cartilage, sinews or muscles. However, while the biological wet and soft materials have impressive mechanical properties and are generally very tough, conventional hydrogels are rather brittle and tend to disintegrate under duress. With one exception, though: Double Network hydrogels can take a lot more force, even exceeding biological tissues or rubber. I just read a paper in Soft Matter that discusses why this is so. The mechanism is rather intriguing.
Double-Network hydrogels are prepared by synthesizing a highly crosslinked hydrogel and afterwards treating it with an aqueous solution of another monomer and a low concentration of cross-linker. What happens is that the hydrogel swells in the solution, and within the swollen hydrogel the second polymer is synthesized and cross-linked with the hydrogel. The resulting jelly block contains about 90 percent water and is stronger than each of the polymers alone.
The strength of the gel increases with the molar ratio of the second polymer to the first one, which is about 20 to 30. Mechanical properties generally depend on the cross-linker added to the second monomer solution. As this movie from the researchers homepage demonstrates the stuff is a lot tougher than a normal hydrogel.
The reason for this is of great interest to researchers who want to create wet and soft materials of high strength. When a sheet of the material is stretched along one axis until breaking, it does so by forming necks, areas where the material narrows and changes behavior. In those necking places the firm hydrogel becomes soft and is easily pulled apart. What happens on the molecular level is this: The first Polymer is brittle and, upon stretching, breaks at one point. When this happens, the other polymer takes all the strain and expands, until all its polymer chains are stretched out to the maximum.
Researchers think that this behavior is responsible for the strength of such materials: Experiments show that the material changes its properties and even its shape under stress, even when not breaking. The reason for this appears to be that due to the brittle polymers microfractures open within the material. But as soon as this happens the other polymer becomes ductile and prevent further damage to the structure.
In the bigger picture, this strategy of stopping fracture propagation is one of two mechanisms that are important to strengthening materials. The other, of course, is to stop fractures from happening at all. This is usually achieved by making the microstructure more homogenous, so that there are no shortest cross-links which crack first. That strategy has been used for example in slide-ring gels, where the polymer chains aren’t linked by chemical bonds, but by macrocycles slipped over other polymer chains. This increases considerably. The next thing to do, of course, is to use both tricks sin the same hydrogel, and see what comes out of it.
Gong, J. (2010). Why are double network hydrogels so tough? Soft Matter DOI: 10.1039/b924290b