Hydrogels are normally pretty soft materials that tear easily. Jello is the example that most people of familiar with. Ever had chewy jello? I don't think so. If you've ever made "Slime" from white glue and borax, you've seen another soft hydrogel. There are countless more examples. If you've ever had an EKG or EEG done in the last 20 years, the electrodes that were stuck to you have a hydrogel. What makes a hydrogel a hydrogel is that they have a crosslinked polymer network made from a water-soluble polymer and lots of water. Because of the crosslinking, the water is not able to dissolve the polymer but instead, the polymer absorbs it much like a sponge.
Researchers at Harvard have now created a hydrogel that can be stretched a very large amount, but maybe more importantly, if a hole is put into the hydrogel, the hole doesn't form a runaway tear, but instead stays constrained. If you have a subscription to Nature or feel like paying $32, can read the report at their website. If you don't, you're still in luck. Harvard has recently required that all their publications be made available to the public (HOOP YEAH!), so you can still read the article (4 pages) and the supplementary materials (17 pages!).
For this new material, the authors used two inter-crosslinked networks of a synthetic polymer - polyacrylamide - and an algae-based biopolymer - alginate. The nature of the inter-crosslinking is important here, and there are 3 linkages of concern. The polyacrylamide is covalently crosslinked with itself and the alginate polymer, while the alginate is ionically-crosslinked with itself with divalent calcium ions. These are weak crosslinks which are critical to the overall performance of the hydrogel. More on that in a minute.
I mentioned above the covalent crosslinks of the alginate and their mobility. This is key in preventing tears from running across the entire hydrogel. When there is a hole in the stressed hydrogel, the stress is concentrated right at the edges of the hole. For a relatively brittle hydrogel such as polyacrylamide, this higher stress on the network continues to tear the gel. But when the alginate is added, the weak crosslinks allow the alginate chains to move and absorb the energy, releasing it as they move to a lower energy state. This prevents the stress from being being applied to the polyacrylamide network.
It is well known that viscous dissipation works very well at reducing energy applied to a material, so the mechanism employed here is not revolutionary by any means. That it works this well however, is.