Lecture 4: The Plaster that Stretches Life

The humble plaster is one of the simplest steps on the road to repairing the damaged body, yet it is a marvel of chemical engineering - a miniature hospital, dispensing everything from antibiotics to aftercare. But how does the plaster stick, and how does it allow the wound to breathe, while at the same time keep it dry? Our knowledge of the chemistry of our bodies now extends far beyond plasters. Now we have soap, toothpaste & shampoo that make us smell nice, our parents look younger and our teeth last as long as we do. Check out the lecture to discover more!

Professor Tony Ryan
[Tony enters lecture theatre on mountain bike. He points to the overhead screen displaying a still picture of Tony's healed knee.] Only eight weeks ago, I fell off a bike like this, and this was the result, and now it looks like this. So, what happened to my knee? Why doesn't it look like something out of a horror movie any more, and what are the ways we can help our bodies repair themselves?
Today we're going to explore the chemistry that allows us to keep fit and healthy.

[Tony holds up a plaster.] Most accidents leads to cuts and grazes that are a lot less severe than my knee, and almost instinctively, we reach for a plaster. You see, the injury breaks the skin and that's a natural barrier. It prevents bacteria and fungi coming in, and its first job is to close the gap. But to compare a plaster to a piece of sticky tape would be unfair. Covering the gap is only the start of what this pocket hospital can do.

The first bandages were used in Egypt, we think, because they wrapped up their dead kings and queens as mummies, and if you got a cut in the 1850s, things would look very strange compared to the way they are now. [Music.]

[Black-and-white reconstruction sequence: Tony visits an 1850s pharmacist.] So you'd visit a pharmacist. I've run in from the present century to two centuries ago, and here he is - a rather dodgy-looking pharmacist, and he's preparing a bandage.
It's made from a piece of leather, so what he's doing is, he's mixing, putting glue on and then some sort of herbal remedy. Something that'll make it sting gets put into the middle of the piece of leather.
Ooh! What a horrible one that was. Ah, such pain!

Yeah, this pharmacist is called Hammy, I think, from the way he's acting. Yeah?
They were metal tongs that squeezed the metal on and there wasn't a National Health Service then, so I had to pay him. [Applause.]

[Tony holds up a modern-day plaster.] This is skin-coloured. It's tough and this one breathes. Our beautiful model over here of a big plaster also breathes, but it breathes through perforations, and most of the early bandages that were made out of plastic had those perforations, but these modern bandages can breathe on their own without the need for these obvious holes. But why is breathing so important in a bandage? Well, I'll show you.

[Demonstration to show breathable fabric.] So here is a membrane over some hot water that's not breathable and you can see the condensation.
We can use condensation to develop patterns that I've got on these gold wafers. I'll just breathe on one to show you how it works.
So if there's moisture around, it says "Ri", OK?
So that's our moisture detector, and I'll put one over this obviously breathable bandage, and we just need a little while for them to develop.

So here is the moisture detector from the membrane that doesn't transmit vapour, and here is the moisture detector from one that does. So this breathing is very important.

The transport of moisture across the membrane helps the healing. You see, the bandage not only covers you, but it covers - oops - a big, undamaged area, and your skin's covered in sweat glands. There are three million of them.

You can produce three litres of sweat in an hour. I'm going to weigh myself at the end of this lecture, see how I've done. And it has an important role.
You see, water evaporates and cools you down, and if you cover the skin with something like this, you get no evaporation.

The wounds become soggy and they're very, very slow to heal. So our breathable bandages are made from my favourite molecules.

[Close-up shot of block copolymers.] These are block copolymers and they're designed to be permeable, so the purple chains - these ones here - and the blue chains - these ones here - don't like each other, so they separate, and the colours separate in such a way that you get a structure on a tiny, tiny scale, so this instrument is called an atomic force microscope and we can read the structure, so over there, on the computer screen, is the structure that these molecules have formed.

Those blobs of blue and purple are a thousand times finer than a human hair. You see, the purple chains really like water, but the blue chains hate it, so in our breathable bandage, the water hops from purple chain to purple chain to purple chain, avoiding the bits of blue chain that have all clumped together, and comes out of the plaster.
Now those of you who came here in a waterproof coat or a cagoule had the same types of polymers inside your coats and they did exactly the same job as the membrane does in this bandage. They allowed the sweaty vapour to escape, but in the time between you cutting yourself and putting the bandage on, thousands of bugs can find their way into your body.

Here comes a bug now. Oh, there he is. [Tony chases Ri helper 'Bipin the Bacteria' round lecture theatre/demonstrates power of immune system.]
Here's Bipin the Bacterium and I'm a white cell and I'm gonna catch him. I am gonna catch him!
I missed him! There! I've caught him now, and what these do are, they protect you from invasion.
Your immune system has special white cells and the cells eat the invaders, and both cells die. [Aaah].
I tell you, Bipin the Bacterium isn't the worst thing we've called him this week. [Laughter.] And these dead cells are exuded as pus, and re-infect the cut.

You see, you need some way of sucking this evil liquid away, so here's the pus and away it goes, and this stuff can really, really do you some damage, so the gauze pad that collects the pus uses exactly the same technology as we find in a nappy, so nappies contain a special gel, and the gel can suck up vast amounts of water, so how much water do you think a nappy can absorb?
Its own weight? Twice its own weight? Well, let's have a look.

I need a volunteer to help me fill a nappy. [Laughter] This young man seems very keen! Down you come to help me fill the nappy.
OK, I want you to stand on this end, right, and you need to peep round the corner, yeah, when I tell you to, and read the scale. What's your name?

[Jaygo] Jaygo? Pleased to meet you. You all right? Good.
So here's a nappy. See how thin it is. Now put it on.
It weighs 22 grams, OK. I weighed it earlier. Now this - feel it - it's a warm liquid, isn't it. We're gonna pour it in here and you need to be careful, OK.
I tell you what, I'll hold it down here and you pour it in for me.
Whoa, whoa, whoa, whoa, whoa, whoa, whoa, whoa, whoa, whoa!
I bet you've never filled a nappy that fast before, have you! [Laughter]

OK.
See, we've poured the liquid in, but the membrane on the top is still white.
It appears dry, and we've already put - let's have a read - we've put [200 grams] 200 grams - wow! - so we've already put 10 times its own mass in there.
Let's see if it takes some more. See, it goes in really, really quickly, yeah?
What I'd like you to do for me is have a feel. Does the surface feel dry? [Hmm, no].
No? Really? You don't think so? Are you sure?
You see, I think it feels dry. I'll even wear it as a hat! [Laughter/applause.] Thank you very much. [Applause.]