The exoskeleton in your closet

We are delighted to announce that the Royal Society for Chemistry have awarded us a grant for a new user-led public science project about nappies: Apatite for Destruction.

I wanted to call it Nappy Science Sleuths or something, but one of our volunteers suggested Apatite for Destruction, because the thing we are investigating involves a mineral called hydroxyl apatite. I put it to the group vote, and obviously the terrible pun won. Hoist by my own democratic petard. But apparently the RSC like crap puns too, so it’s worked out OK.

The project is going to be a detective story, with analytical chemistry.

In the course of our investigations last year into reusable nappies, we found some nappies with an ‘exoskeleton’ of a mineral called hydroxyl apatite. This is the stuff that bones and teeth are partly made of.

We had got people to send us nappies they were having problems with and we got them tested in a forensic lab. We expected to find maybe detergent build-up, or a build-up of oils and fats from nappy creams, or maybe even limescale from the ones in hard water areas. We found hardly anything soluble in the nappies. But when the lab did an ‘ash test’ to find if there were any insoluble minerals in the nappies, they found two out of the five had about 20% w/w of hydroxyl apatite in the fibres. 20% of the weight of the dry nappy was made up of this mineral.

The technician who did the test said he had never seen or even heard of an ash value that high. 1-2% might be more normal. 3-4% would be a high value.

We took a sample to a microscopy lab and looked at them down an electron microscope. The hydroxyl apatite forms a kind of exoskeleton around the fibres. They are completely coated in it. Like the nappies are turning into bones.


The testing lab suggested that hydroxyl apatite is formed from phosphates in detergent reacting with calcium ions in hard water and they think that is the most likely cause. But the EU has banned phosphates in detergent since 2013. We know that phosphates are added to the water in some areas. We also know that phosphates are sometimes present in urine (depending on diet and various medical things). So it seems like there could be various possible causes. But we don’t really know why this is happening with some nappies and not others. We don’t know how widespread it is. It could be only these two nappies have got a hydroxyl apatite exoskeleton. Or it could be that it’s really common.

We also don’t know if there’s a connection between the exoskeleton and the smelliness. ALL the smelly nappies had a high ‘bioburden’ (they had lots of bacteria living in them). Does the mineral help bacteria hang around in the nappies by giving them something to cling on to? Does it encourage biofilms to form? Is the smell related to the exoskeleton at all (given that 3/5 smelled but had no exoskeleton), or is it just a coincidence?

What we do know is that no-one in the forensic fabric testing lab predicted this result. None of the nappy manufacturers predicted it. None of the nappy libraries. None of the people who know a lot about nappies knew that these mineral exoskeletons were forming, and so extensively. But our (mostly non-science trained) volunteers dug around, asked questions, talked about what they wanted to know about, and that is how we came to be doing scientific tests on smelly nappies and discovering what seems to be an unknown phenomenon. Yay for citizen science!

So we want to investigate it and find out what the hell is going on, what causes it, whether it happens anywhere else, and how to prevent it or remove it. Our volunteers plan to get more smelly nappies tested to see if we can find more with the exoskeleton and try to work out the common factors. Our volunteers will talk to various scientists to see what is already known about how hydroxyl apatite forms, the chemical composition of urine and so on. And we’ll do loads of analytical chemistry, with the help of the Cardiff Catalysis Institute.

If you are a scientist with an interest in hydroxyl apatite, the composition of wee, biofilms forming on fabric, or anything else that seems relevant, and you like engaging the public and having impact (and who doesn’t?) then please get in touch.

It’s going to be a joint project between Nappy Science Gang and the Cardiff Catalysis Institute, over the next eight months. And we’ll let you know what we find out.


4 thoughts on “The exoskeleton in your closet

  1. What are ‘nappy libraries’? I imagine them as a sort of fabric resource with pamphlets on the histories of nappies but suspect the reality might just be a way of checking out and returning nappies, which is a bit worse than books 😉 I’m not going to just google this myself you know oh OK then 🙂


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