Discovering Surprisingly-Strong Hydrogels From Everyday Ingredients

A few years ago we had an amazing postdoc at UH (Bin Zhang, now at Brunel) working on a commercial project. One of the challenges we needed to address for the client was how to make super-strong hydrogels in a setting without normal laboratory facilities like fume cupboards.

Hydrogels are very useful. They’re jelly-like materials full of water – usually over 90% water by weight. The presence of water in this solid-like material means that you can do handy things with them – for example make tissue-like synthetic materials, absorb crud from wounds, and soothe burns. Usually we make hydrogels using polymers, which molecules that are really long – you can think of them like a piece of string. There are two common ways to make hydrogels:

1. Use a water-soluble polymer that interacts with itself strongly enough to form a kind of knotted-up network that holds water (physical hydrogels).
2. Make the polymer from scratch using chemical synthesis, but include a chemical cross-linker that ties the chains together (we call them chemical hydrogels)

Physical hydrogels are pretty easy to make – it’s exactly what goes on when you make a jelly at home. Heating up gelatin (or agar if you’re veggie) makes the polymer dissolve, then cooling down again makes the chains knot-together to form the hydrogel. Chemical hydrogels are much stronger but you need a proper lab to make them.

Tough “Double-Network” Hydrogels from Glue and Jelly

Bin found a very safe and easy way to make really strong physical hydrogels – no more dangerous than cooking your dinner. If you mix together agar (vegan jelly, vide supra) and PVA (yes, the thing in glue) when they’re hot you get a viscous liquid, a bit like wallpaper paste. When you cool the liquid down in a mold you can form a hydrogel that’s pretty soft by the agar knotting up. The magic is that we can then activate the physical entanglement of the PVA by freezing the hydrogel and thawing it out again. This gives us an amazing material. A hydrogel that’s so tough it’s almost impossible to break by hand, but that’s 88% water. You can stretch it out like an elastic band (see below) without it breaking. We have a secret application for this, but there are loads of things that could be done with it.