To make a material comparable to a natural biomineral involves using known techniques and procedures that hope to capture the essential elements of the natural system. In this video clip, Professor Kate McGrath, Director of the MacDiarmid Institute, explains how a hydrogel based on chitin is used as the framework upon which calcium carbonate mineralisation can be induced.
PROFESSOR KATE MCGRATH
In order to get a material that is comparable to the natural biomineral doesn’t mean you have to do it the same way that a biological organism does it. You can utilise other techniques and other knowledge in science and apply those, and what you’ll generate won’t be identical but it will hopefully capture the essential elements of the natural system.
So one of the ways is through the formation of this chitin or carbohydrate-based three-dimensional network. We replicate that synthetically using a hydrogel, which is essentially jelly.
So if you take jelly, which is effectively a protein-based crystal system, and if you add hot water, the protein crystals will actually dissolve.
At that high temperature, the protein changes its three-dimensional structure, which is called unfolding, so it unfolds. And in doing that, it exposes all of these chemical functionalities that love water, and so those water molecules that you’ve now got will intimately associate with the protein and that will allow the protein to start taking up a much larger volume, and eventually, individual proteins will start to interact with each other and form a three-dimensional gel network. And what happens then is that, as the jelly cools, the water isn’t let go, so the protein wants the water to stay there, and we end up with this lovely wobbly jelly. That’s an example of a hydrogel, but you can make hydrogels from carbohydrates as well.
Because in nacre the major organic material is a carbohydrate, chitin, that’s what we start from. So everybody knows what a crab is or a lobster, and that shell is chitin. Pāua is predominantly calcium carbonate with a little bit of chitin. Lobster is predominantly chitin with a little bit of calcium carbonate. Crab shells, squid pens, all those kind of things are chitin. It’s cheap and readily available, but it has a really big problem – it’s not very soluble in water. Quite useful for a lobster or a crab, not so useful from a synthetic point of view.
So what we do is we slightly modify the chitin and we turn it into something called chitosan. There’s a chemical functionality in chitin that makes it not interact very well with water. You can chop it off and turn it into chitosan, and you have now a chemical functionality that likes water much more.
So we take the chitosan, dissolve it in water, and then if you get the concentration correct, it will form, just like jelly, a nice hydrogel system. If you were to look at the structure of the hydrogel more closely, then you can correlate it with a kitchen sponge. There’s lots of pores, lots of holes in it, but there’s this continuous solid system. And that’s exactly what you’ve got in a hydrogel, so the pores are filled with water in the case of a hydrogel, and then your protein or your carbohydrate forms the continuous matrix. So that’s where we start.
U.S. National Oceanic and Atmospheric Administration
Dr Natasha Munro