Smart Materials

30 September 2007
Presented by Chris Smith, Helen Scales.

This week, we're exploring the science of Smart Materials - we discover a Super-Non-Stick coating that even honey wont stick to and flexible plastic paper with E-Ink that we-writes itself on demand. We learn how potatoes could form the basis of future plastics and a new way to think about 'bone china', as ceramics and polymers could replace your broken bones.& Also, we discover where sea turtles spend their childhood, how a microRNA gene switch could put the brakes on the spread of cancer and how thousands of cases of breast cancer could be avoided without medication.& Plus, in Kitchen Science, we show you how a simple cotton handkerchief can hold back a torrent of water!

In this episode

01:51 - Eco-Idol: Hindu festivals threaten Indian Rivers

Eco-Idol: Hindu festivals threaten Indian Rivers

As festival season approaches for Indian Hindus, environmentalists are trying to avoid a catastrophe caused by the faithful tossing millions of decorated statues of gods into rivers and waterways. The usually elaborately decorated effigies often contain toxic levels of heavy metals including lead, mercury and chromium as well as cancer-causing dyes and plaster compounds that can deplete water oxygen levels. This leads to severe water pollution, which can kill fish and other aquatic organisms and threaten human health.

"The commercialization of holy festivals like Ganesh Chaturthi and Durga Puja has meant people want bigger and brighter idols and are no longer happy with the ones made from eco-friendly materials," said Ramapati Kumar, a toxics campaigner for Greenpeace India. "Traditionally, the idols were made from mud and clay and vegetable-based dyes were used to paint them, but now it's more like a competition between households and between corporates who sponsor the idols to gain publicity".

About 80% of India's 1.1 billion population are Hindus, but in recent years their religious activities have been subject to increased scrutiny due to growing public awareness of environmental issues.

"No one is saying the immersion of idols should not happen," says the Centre for Science and Environment's Suresh Babu, "but the government should impose guidelines to craftsmen who make the idols to use eco-friendly materials and organic paints so that we give the environment as much respect as we give god."

03:21 - Unravelling a turtle mystery

Unravelling a turtle mystery

For decades, there has been a wonderful mystery that has baffled marine biologists.  The question is, where do baby turtles go?

Sea turtles, as their name suggests, live their lives at sea, with females returning to land to lay their eggs on beaches.  And after the tiny baby turtles have hatched, they crawl down the beach, fight their way through the pounding surf and make their way back out to sea where they completely disappear into the wide blue ocean.

Until now we've had absolutely no idea where young turtles go or what they do until they turn up five years later closer to shore and much, much bigger.

Now at least part of that puzzle has been solved, thanks to a team of scientists from the Archie Carr Center for Sea Turtle Research in Florida who have been studying Green turtles from the Bahamas in the Caribbean.  

The researchers took small samples of the hard shells from Green turtles and analysed them for the presence of particular isotopes of carbon and nitrogen - most elements have several different isotopes that occur naturally with each isotope having a different number of neutrons, giving each them a slightly different atomic mass.

Different types of plants and animals naturally take up different amounts of the various isotopes in the environment, for example of the three main carbon isotopes, carbon 12, 13 and 14.  And by measuring the ratio of different carbon and nitrogen isotopes in sea turtle shells, the team found out what types of food the turtles feed on.

These isotope studies have shed a much-needed beam of light into the mysterious world of Green turtles, and shown that these creatures that we normally think of as vegetarian - feeding on seagrasses and seaweeds - are in fact carnivores when they swim out to sea where they adopt a diet of jellyfish and other marine animals.

This latest study also indicates that the green turtles stay far out at sea for the first three to five years of their life, before swimming back into shallower coastal waters.

The really neat thing about this type of study is that it can by applied to all sorts of other animals, by sampling feathers, skins, fish scales and bones, allowing us to learn more about what animals get up to when we can't see them.

In fact this is the same sort of technique that we talked about a few weeks ago, with a study that showed how penguins in the Antarctic have shifted their diets from mostly eating fish to taking advantage of the large populations of krill in the Southern Ocean.

06:23 - Scientists uncover key to cancer spread

Scientists uncover key to cancer spread

Scientists have discovered a molecular switch that turns on a cancer cell's ability to spread to other parts of the body.

Publishing in this week's Nature, MIT researcher Robert Weinberg and his colleagues were examining microRNAs, small pieces of single-stranded genetic material produced in the cell nucleus. The actions of these RNA sequences are still poorly understood, but they are known to alter the activity of other genes. So, to find out how they might affect the behaviour of certain cancers, the team set out to compare the levels of 29 different microRNAs in tumour cells and healthy tissue. The team also looked at how the levels varied between cancers that had already begun to spread (metastasise), and those that hadn't. Intriguingly, one of the microRNAs - called microRNA-10b - was present at much higher levels in the metastatic tissue, suggesting that it might be involved in triggering the process.

To find out the researchers increased the levels of microRNA-10b in some human breast cancer cells and implanted them into mice. Compared with animals injected with the unmodified cells, animals that received cells containing higher levels of microRNA-10b rapidly developed spreading cancers. Next, to find out how microRNA-10b was triggering this process the team used a computer programme to screen for "targets" - other genes - that might be affected by the microRNA. They were able to home in on a gene called HoxD10, which works like a cellular handbrake, preventing cells from going AWOL. It's switched off by microRNA-10b during embryonic development so that cells can migrate to their correct future locations in the developing body, but in adulthood it is strongly expressed and helps to keep cells stationary.

"During normal development, this microRNA probably enables cells to move from one part of the embryo to another," points out Weinberg. "Its original function has been co-opted by carcinoma [cancer] cells". Excitingly, when the researchers increased the levels of HoxD10 in experimental cancers the cells lost their ability to migrate and invade. This means that it may be possible to exploit the discovery as a powerful anti-cancer target: "I was able to fully reverse microRNA-10b induced migration and invasion, suggesting that HoxD10 is indeed a functional target", says Li Ma, lead author in the study.

08:50 - New Species Found in Vietnam

New Species Found in Vietnam

In these days of globalization, international travel and exploration, it's hard to imagine that there could still creatures hiding out there that have never been seen by man.

So, it comes as a wonderful surprise that scientists have uncovered a treasure trove of brand new species in a mysterious and remote part of central Vietnam known as the Green Corridor.  

These are the latest findings of an expedition run by the conservation organisation WWF.  Among the eleven new species found are a snake called the white-lipped keel back - a black snake that grows up to 80cm long, and has a striking white stripe running along it's neck and belly, and there are also four species of orchid and two butterflies.

Another new species was a type of aspidistra - a group of plants well know as house plants - this new one has an almost completely black flower.

The region was already known as an area very high and unique biodiversity.  This is where most of the world's white-cheeked gibbons live - one of the rarest and most endangered species of primate in the world, and this is also the place to go if you want to see a soala, a rare species of wild cattle that was only discovered in 1992.

One reason why this part of the world is thought to be home so many different species is because the mountain range, known as the Ammanites, crosses the boundary between the cooler, temperate zone in the north and the warm, tropical zone in the south leading to a rich mix of different creatures.

But sadly, the Green Corridor and all the animals and plants that live there, are threatened by the actions of man, including illegal logging, so it's really encouraging that local authorities in the area, in collaboration with WWF, are pledging their commitment to conservation of these important forests and who knows what other amazing creatures might be discovered there.

Website:The Green Corridor Project:

10:35 - Anorexia leaves bad taste in the mouth

Anorexia leaves bad taste in the mouth

Scientists have discovered that the brains of anorexics respond differently to certain tastes than the brains of control subjects, possibly explaining why sufferers eschew tasty foods.

Writing in this month's Neuropsychopharmacology, University of Pittsburgh researcher Walter Kaye, Angela Wagner and their colleagues brain scanned sixteen recovered anorexics as they were fed a sweet drink containing 10% sucrose, or just plain water, and compared their patterns neural activity with those detected in sixteen healthy women. Amongst the normal subjects, whenever the sugary stimulus was presented, a region of the brain's grey matter known as the insula cortex lit up. The increase in activity in this region also tallied with the subjects' reports of how pleasant they found the sweet liquid to be. But amongst the recovered anorexics the levels of activity detected in the insula were much lower in response to both the plain water and sugar solutions.

The finding fits in with previous studies on this part of the brain which have shown that the insula seems to be involved in processing how the "value" of certain foods might affect the body. For instance animals with damage to this brain region lose the ability to avoid foods that have previously made them sick, and fail to stop eating high-calorie foods when they are full. This suggests that the insula may help to translate the experience of eating foods into pleasurable sensations, but this function seems to be abnormal amonst people with anorexia, possibly explaining why they avoid "pleasurable" foods, fail to respond appropriately to hunger and lose so much weight.

"We know that the insula and the connected regions are thought to play an important role in interoceptive information, which determines how the individual senses the condition of the entire body," says Kaye.

13:12 - Waterproof Hanky

Waterproof Hanky

15:26 - Beating Breast Cancer - News from the Cancer Conference

Kat Arney reports in with the latest news from the National Cancer Research Institute conference in Birmingham, UK.

Beating Breast Cancer - News from the Cancer Conference
with Dr Kat Arney

Each year, the National Cancer Research Institute (NCRI) holds a conference which "aims to provide the major international forum in the UK for the dissemination of research advances in cancer, across all disciplines."  Naked Scientist Kat Arney reports in with the latest news

Chris - So what's this conference all about?

Kat - Well this is the biggest cancer conference in the UK and this year they're hoping to get nearly 2000 people along; that's scientists, doctors and patients from all around the world, basically trying to get to grips with the latest in cancer research. So we've got people talking about new advances in drugs and drug development. I just spoke to a really fascinating guy called Greg Verdine, who's giving a talk tomorrow and he's basically developing an entirely new type of drug which could revolutionise cancer treatment. It's very exciting stuff.

Chris - What's the news on how breast cancer spreads around the body?

Kat - It's not so much that - it's more about the story on stats that has come out. It's breast cancer awareness month in October and the key thing is we're learning all about the genes that are involved in cancer and the new treatments. But there are so many things that women can actually do to prevent breast cancer themselves. One of Cancer Research UK's epidemiologists (stats guy), Professor Max Parkin, has actually calculated that potentially 6000 cases of breast cancer could be prevented every year by women changing their lifestyles.

Chris - What sorts of lifestyle changes are they advocating, because that's a lot of people!

Kat - It's a lot of women. For example, we know that prolonged use of HRT over a number of years does increase your risk of breast cancer. If the number of women taking HRT without clinical need drops then you could save around 2000 cases per year. By reducing obesity you could prevent another 1800 cases per year. By doing a bit of exercise, your half an hour a day, five days a week you can actually prevent more than 1000 cases per year. So it's pretty significant stuff.

Chris - Would dealing with those factors have an impact on other cancers as well? Could the prevention numbers be even higher?

Kat - Exactly, this is just to do with breast cancer but we know that, for example, being overweight can increase your risk of contracting other types of cancer like womb cancer and bowel cancer. Getting a bit physical - that can cut your risk of contracting many types of cancer. Keeping a healthy bodyweight is really good for that. Also cutting down on the amount of alcohol you drink too, that can be pretty handy. It's really a double edged thing here. We're finding out about the latest in cancer treatments and really getting to the nuts and bolts of what's wrong in cancer. And then thinking about prevention: there's going to be an interesting debate tonight with people from Breast Cancer Care. They'll be talking about the use of pills for the treatment of breast cancer or whether we should look to change our lifestyles.

Chris - And looking forward to the rest of the week, what else will you be focusing in on?

Kat - Well there's a really exciting lecture coming up on Tuesday on pancreatic cancer with a guy called David Tuveson, who's from Cambridge. He's probably one of the best people in the world at the moment working on pancreatic cancer: a cancer which has a really terrible survival rate. It's really very low so it's going to be interesting to see if people feel they're making progress in pancreatic cancer treatment and how we can go forward. Also there're some interesting seminars and symposia involving patients. They'll be looking at the impact of more and more people who are surviving cancer and the issues they have to deal with. They'll also talk about how can you go forward living with cancer as a more chronic disease, rather than something that doesn't do you any good at all.

Chris - Thanks Kat, see you soon.

Kat - See you soon!

19:50 - Can I water my houseplants with sea water?

Can I water my houseplants with sea water?

Unless your houseplants are a particular type of plant that love to be in the sea or salty water (halophytic plants), then no. Because what you're doing is adding too much salt to the plant. The roots of plants are only adapted to a certain level of salts or any kind of ions that are in the water. They're only able to take up water in a certain concentration. If that becomes more concentrated the osmotic pressures go crazy and the plant is unable to take up any more water. So unless you are growing seaweeds, sea grasses or salt marsh plants then I think, stick to fresh water!

20:32 - Super Non Stick Surfaces

Non-stick pans are great, they make it so much easier to clean cooked on scrambled egg! But the Thin Films and Interfaces group at Cambridge University have found a way to make a super non...

Super Non Stick Surfaces
with Professor Ullrich Steiner, Cambridge University

Chris - With us today we've got Ulrich Steiner: he's from the Thin Films and Interfaces group at Cambridge University: sounds like the reverse of widescreen TV, Ulrich. You're actually here to talk about some exciting new materials you're working on or a fancy new form of Teflon® that's ultra slippery.

Ullrich - Well the material itself is actually not so special. It's just Teflon®. What's different is the way we make the coating. Normal Teflon® is a rather smooth coating and we know that materials don't stick to it. If one changes how the coating is put down, it becomes even more slippery - more hydrophobic.

Chris - Why is Teflon® slippery?

Ullrich - Materials don't stick to it and that is to do with the fact that it has very low 'surface energy'. Water and what ever else you put on will stick to other materials that have high surface energy. Teflon® doesn't like to be covered by any other material and that's why it doesn't stick to it.

Chris - One of these questions that go around on email lists is that: if Teflon® is so unsticky, how do you stick it to the pan?

Ullrich - That's a good one. You need to go through a number of different steps. The company in the US (DuPont(TM)) that makes Teflon® has a detailed protocol. You first need to make the material rough so you sandblast it. Then you put a primer down to which the Teflon® will stick. I can't tell you what the primer is because DuPont(TM) will not tell me what it is. They have worked out how this goes.

Chris - So it's not just a flippant question, there is actually quite a lot of thought that has to go into solving that problem.

Ullrich - Oh yes absolutely, because Teflon® doesn't stick to things it also doesn't want to stick to the surface you want to put it down on. It's a technological challenge that people have worked out.

Chris - What's the step forward that you're exploring?

Ullrich - This is something that has been known for a long time from plants and animals. There are surfaces that are called superhydrophobic. These are surfaces that water doesn't stick to at all. So if you drop water on these surfaces it forms an almost perfect pearl-like sphere. For example, the leaves of the lotus plant have that property and that's why this effect is also called 'lotus effect'. You can also observe it in the garden. If you go out in the morning you'll see some of the drops are just sliding off the surface of these plants. This comes from the fact that these have a rough surface. They have a certain surface texture.

Chris - So they've got a sort of anatomic Teflon®. So what is the leaf doing if you zoom in on the surface? What is it doing to repel water in that way and why should a rough surface, paradoxically, repel water like that?

Ullrich - The surface of the leaf has small wax structures, wax needles. The water sits on top of these needles and doesn't really touch the surface of the plant leaf itself. The plants and sometimes even butterflies, for example, have these surfaces to keep clean. It's thought that they do that in order to prevent being infected from spores and so on. So the step forward that we made was to try and copy nature. We thought to make these structures, not from natural materials, but from something that we are more used to and we chose to use Teflon®.

Chris - So how do you apply your stuff? You can apply it to anything, can you, and make it really slippery?

Ullrich - We essentially do exactly the same thing as others do when the coat a Teflon® frying pan. So we follow all the steps that the manufacturer of Teflon® tells us we have to do. There are two little tricks that we play which make the top coating porous. So we add small particles in and these particles burn away during the treatment of the surface: making the surface rough. The second thing is a certain spray technology. We spray it on in a way that makes the top surface rough.

Chris - So can we see it working? I saw you playing around with a spoon earlier so I know you have brought something to show us.

Ullrich - Yes, I have a spoon here but unfortunately you cannot see it through the radio, which is a pity! It's a spoon that I covered and I've dropped some water on it. You can see when I move it around the water just moves with it, it doesn't stick to it at all.

Chris - And if you tip that water off, it'll just fall?

Chris - Oh my god! I'll just describe this to people at home. You know when you tip water off a surface it grips and adopts some of the shape of the surface it's falling from? Well, I've just seen this come off as a perfect droplet from the spoon.

Helen - It's quite beautiful actually. It's like a glass bead, almost just sitting on the spoon, moving independently

Chris - So if you were to dip that in something really sticky, e.g. honey, what would happen?

Ullrich - Well, the honey would also just bead-up: make a droplet. And if you turn the spoon it will also just roll off.

Chris - Like water?

Ullrich - Like water, well, slower than water because honey is slower. But eventually it will completely leave the spoon. Nothing will stay behind on the spoon. Also if you put a powder on it and you then put some water onto it the water will just move the powder/dirt off the surface. I think I just demonstrated that before the show to one of the team.  

Helen - Yeah we had chocolate cake! We cleaned the spoon of all the evidence!

Chris - Speaking of which, my wife bakes a cake each week and I was thinking we could have a Cake of the Week as well as Question of the Week. Cake of the Week this week is a rather wonderful fridge cake (chocolate and digestive biscuits). Very agreeable and it's got raisins in it too...just in case anyone's feeling peckish! What would be the applications of this technology, Ulrich?

Ullrich - We are still trying to work this out but evident things are, of course, surfaces that are easy to clean: in the household. It could also be for industrial applications, like machine parts that are not easily accessible. Then you could clean them by just spraying water over it.

Chris - What about clothing? Could you decorate clothing with a similar technique so that furniture, shoes etc need less care and washing?

Ullrich - That's not one of the things we're thinking of because I think fibres could be made more easily self-cleaning. Another thing to develop would be in biomedical technology, surfaces where dirt doesn't stick, are easy to clean and are more hygienic could be useful.

Chris - Does that include bacteria, because you're saying dirt can't stick? I'm thinking if bacteria can't stick this could be good for prostheses.

Ullrich - I know too little about this to say yes or no. I would think for implants it's probably not that good because they have rather complicated surface requirements. I was thinking more in terms of syringe needles and materials that you use in a medical environment and that would be easily cleanable.

Chris - There I was thinking they'd be so slippery they'd just slide in and they wouldn't hurt. But thank you very much: that's Ulrich Steiner, he's from the Thin Film and Interfaces group at Cambridge University with an amazing new application of Teflon® which is so slippery that water just flies off it like a beautiful pearly drop!

28:16 - Plastic Logic - Plastic Paper and E-Ink

Now, if you listen carefully you will notice that we, in the Naked Scientists are surrounded by paper. We usually try not to make you hear but similarly in offices all around the country...

Plastic Logic - Plastic Paper and E-Ink
with Professor Henning Sirringhaus and Simon Jones, Plastic Logic

Now, if you listen carefully you will notice that we, in the Naked Scientists are surrounded by paper. We usually try not to make you hear but similarly in offices all around the country incredible amounts of paper are thrown away on a daily basis. I'd just like to point out that I take all my paper home to recycle. Maybe a better solution would be to remove the need to use all this paper in the first place and do away with all those hours spent photocopying and all those pages that come out of printers. We sent Azi to find out about the design and uses of plastic electronics and e-ink.

Azi - I've come to Cambridge University's Cavendish Labs and I'm joined by Professor Henning Sirringhaus, who is a founder and chief scientists for a company called Plastic Logic. Now, Professor Sirringhaus what exactly is Plastic Logic?

Henning - Plastic Logic is a little start-up company in Cambridge that make plastic electronics. So if you imagine your plastic bag that you use to do your shopping that's a very uninteresting thing. It doesn't do much. But you can make electronic devices such as transistors, light emitting diodes, solar cells using these plastic materials in the same way that you would use silicon for conventional electronics. Plastic Logic is a company that's exploiting this materials research for displays.

Azi - But how do you make plastic do the things that silicon does?

Henning - If you imagine a normal plastic material is made out of long-chain organic molecules, and depending on the way the atoms are arranged with respect to each other, that affects the electronic properties the material has. You can make it so that the material is completely insulating, conducting (like gold or copper) or a semiconductor.

Azi - So how do these flexible plastic displays actually work?

Henning - The core of the technology is to make plastic transistors using a plastic semiconductor. So what you need for a flexible display are lots of transistors on a flexible substrate. Plastic Logic uses PET, which is the material from which coke bottle are made as a substrate. It then deposits an array of transistors onto that substrate. It can be up to a million transistors. You then put that together with a display medium which consists of solid capsules that are filled with a liquid. Inside the liquid are coloured particles: some are white, some are black. The white ones are negatively charged and the black ones are positively charged. So when you apply a voltage the white ones might go to the top or the black ones might go to the top. Whether you get white or black is determined by the signal that the transistor beneath applies to the display medium. This is how you build up an image. Then you can make this flexible display: if you talk to my colleague, Simon Jones, he can tell you all of the wonderful things that you can do with it.

Simon - I'm Simon Jones and I'm responsible for product development at Plastic Logic.

Azi - Can I just ask you to explain what you've got here?

Simon - I've got a couple of examples to show you. One is a thin sheet of plastic, which I'm bending in my hands. But it's a real display. It's nothing like the display on your laptop screen. You could throw this to the other side of the room and it would not break. If you integrate a display like this with some electronics you can make a reading device and this is the other demonstrator I've got here. This screen is very unusual because it looks very much like paper. You can look at it from any angle and read it, which you can't do with a laptop screen. It only uses battery power when you change what's on the screen. It's incredibly power efficient.

Azi - So are you competing with pre-existing displays that are already incorporated into mobile phones and laptops?

Simon - No, we're not competing. We're enabling a brand new kind of consumer activity which is comfortable digital reading. If we're competing with anything, we're competing with paper. We're seeking to replace the amount of paper that people print and carry around with them. Independent research has shown that it generates about 3kg of CO2 to get an average paperback book into the hands of the consumer. Depending on how much people read on these devices there could be a fantastic environmental benefit because of the paper that hasn't got to be manufactured.

Azi - Professor Sirringhaus, do you thin that a lot of people will rush to the shops to buy a paper substitute product?

Henning - I think there are a number of interesting applications that might convince people to rush to the shops and buy one. If you imagine for example, that you travel a lot around the world but you would still like to read your Cambridge Evening News when you are in Singapore you could download your newspaper from the internet and display it on this flexible display. That is clearly an application that some people might find attractive.

Helen - That was professor Henning Sirringhaus talking to Azi at the Cavendish Labs in Cambridge about a revolutionary design for reading. I certainly think it would be lovely to take the newspaper around the world with you as I do a bit of travelling myself. It may be the end of the crossword...unless you have a wipe-off pen.

Chris - I'd be quite disappointed because I do like the crossword. I'm struggling with the Telegraph one: I've done half of it and I can't do the rest.

34:08 - Biocomposites - the Future for Plastics?

Biocomposites, materials similar to plastics but made from biological material, could be the ideal replacement for petrochemical based plastics. Paul Fowler explained more to Chris...

Biocomposites - the Future for Plastics?
with Paul Fowler, Univeristy of Wales, Bangor

Chris - Now joining us, we have Paul Fowler on the line. He's from the Biocomposites Centre of Bangor University. He specialises in the design and study of biocomposite materials: that's things like bioplastics and bioresins. He's trying to help companies across the globe practice greener technology. Paul, what exactly is a biocomposite material?

Paul - It's a material that consists of two components, a matrix and a fibre reinforcement. Now in a biocomposite we look for the matrix and the fibre composite to be derived from natural materials.

Chris - The stats are that one trillion plastic bags get made all around the world every year and most of them probably end up in landfills. Are bioplastics something that could replace these plastic bags and will therefore break down and be better for the environment?

Paul - Bioplastics certainly would go some way towards remedying some of the problems of plastics in the environment. The issue around plastic bags is a contentious one and there is a school of thought that perhaps biodegradability isn't necessarily the way to go with plastic bags.

Chris - Why not?

Paul - I'm thinking in terms of the biodegradability of plastic and its inclusion into landfill where it may not degrade in an aerobic fashion but in an anaerobic fashion.

Chris - And that makes methane?

Paul - In which case it would make methane, yes. So the whole issue of biodegradability may, in fact, be a red herring. Unless the application you're putting the material to actually demands biodegradability.

Helen - Paul, I've got an email here from Lizzie Jones and she points out a particular issue. We're getting more of this potato/starch based packaging and cups. She's wondering what to do with them. Now you mentioned that maybe putting them in to landfill isn't going to be the best thing. Can we compost them at home? Our kerb-side collections say clearly not to put these plastics into the compost for waste (they certainly do that in Cambridge). Can we do that at home?

Paul - Well certainly, I think home composting is one way that packaging materials is going. Certainly starch-based materials should compost quite well. If you manage a compost heap in quite an active way and make sure you turn it: keeping it reasonably well topped-up with green matter as well.

Helen - Do you have any idea why council collections say 'don't put them in your green bin'?

Paul - Well I guess it's a question of where the material's going and how it's segregated. It's a question of really being absolutely aware of what material is made from and its ultimate degradation fate.

Chris - Paul, if we could just cut to the chase on what bioplastics are: how can you make a plastic out of a potato?

Paul - The active component is the starch that is rendered typically by adding a 'plasticiser'. The starch is then processed to make a film. The starch is a key polymer which is then transformed to make a plastic film.

Chris - So what is going on with that 'plasticiser' to make the starch molecules, which are potato-like, into something plastic and stretchy and flexible?

Paul - What is happening is that the starch granules are being de-structured by applying mechanical and heat forces to them in the presence of a plasticiser. That plasticiser may be water. The individual molecules become sort-of 'flexibalised'. The plasticiser slides in between them to enable the material to become plastic and film-like.

Chris - What could we do with this material?

Paul - Biodegradable plastics, as you are aware, have seen utility in food packaging and carrier bag applications. There's no reason why a bioplastic couldn't be injection-molded or thermoformed into things like caps for products such as deodorants or hairspray. The thermoformed things could be used to package meat or vegetables.

Chris - And there's no danger that my car bumper might dissolve when I put it in the carwash?

Paul - Hehe. That's an interesting question - there's lots of scope for modifying starches to make them less biodegradable. Then you can actually extend the life of the product, if you will, with modifications of other biomaterials to prevent them from being so biodegradable as they first appear.

Skull 3d printed in polylactic acid (PLA)

39:25 - Medical Materials

Dr Ruth Cameron and Dr Serena Best from the Centre for Medical material at the University of Cambridge spoke to Chris about how they are using ceramics and polymers to help mend broken...

Medical Materials
with Dr Ruth Cameron and Dr Serena Best, Cambridge University

Dr Ruth Cameron and Dr Serena Best from the Centre for Medical material at the University of Cambridge spoke to Chris earlier about how they are using Ceramics and polymers to help mend broken bones.

Ruth - Well what we're doing is to develop materials that are designed to replace body tissue such as bone after certain medical situations, for example if you've had a tumour or if you've had an accident and you've broken a bone, you may need to have bone replacement material to reinforce the bone you've got left.

Chris - And to help you heal up better and faster I should think?

Ruth - Exactly.

Chris - Serena, what sort of materials are you developing to make that happen?

Serena - Well we're looking at materials that are available at the moment and most of the hip implants at the moment, are made of metals and what we find is those materials are actually too stiff, so they don't flex quite as well as the bone does, so what we're trying to do is to actually have a look at the structure and the properties of bone and try and find some materials that we could replace those metals with to make them behave in a more similar manner and more chemically similar so you can get a direct bond between the bone and the implants.

Chris -   And what sorts of materials are you exploring?

Serena - We're interested in ceramics, and strangely, although ceramics make people think about tea cups and flower pots and things, the mineral part of your bone is a calcium phosphate and if you synthesis that chemically and if you then heat treat it, that will turn into a ceramic. But what we're interested in is using ceramics that are as similar as possible to the mineral component of bone, but we're also interested in using polymers and Ruth knows a lot more about the polymer side than me...

Chris -   Ruth, what sorts of polymers do you sue then?

Ruth -   There are a range of options, but one of the strategies we use is to tak polymer that would be degradable within the body, so once you get it in the body it gets wet and breaks down into its component parts and you can choose that such that it was something the body would be happy dealing with.

Chris -   So it won't irritate the body having that in there?

Ruth -   Exactly.

Chris -   And what does the breaking down?

Ruth -   Simply by getting it wet with a lot of the polymers that we use. So there's a chemical reaction that happens and the polymers, which are long string-like molecules, actually break down into much smaller molecules with that reaction and you can get the body to start to take over the function of your original implants.

Chris -   And you can control how long it takes for that to happen presumably?

Ruth -   Yes, there are parameters you can adjust and there are different time scales that are appropriate for different applications.

Chris -   And what sorts of polymers are you using?

Ruth -   One example is polylactic acid, which is lots of repeating units of lactic acid, and that gets wet slowly over weeks and months and years and that breaks down into lactic acid. Lactic acid is something the body has already, when you run very hard and get a stitch-that's a build up of lactic acid.

Chris -   So it should be very bio-compatible, the body shouldn't argue with it being there then, how are you hoping it will help the body by putting this stuff in?

Ruth -   Well the idea is that it can give you very strong, mechanical support initially but then after a period of time you're trying to stimulate the body to create its own new, natural body tissue.

Chris -   Can you, for instance, imbed cells in there and put those in or could you imbed factor that's would make cells grow more and therefore heal better in the polymers ad well?

Ruth -   Well both of those are strategies you can take. So you can put in drugs that will release slowly as the polymer degrades and as the material breaks down you're getting them in exactly the right place to be stimulating new tissue. But, the idea of tissue engineering, where you create a scaffold of degradable polymer and you put cells into it is also an idea that we use as well.

Chris -   And what sorts of body bits could you make to do this?

Ruth -   Well, often we're just replacing spaces in the body or you're just trying to approach a particular site in the body, so we're not out to make entire new organs or entire new bones, but really kind of repair situations.

Chris -   And Serena, presumably your ceramics don't get eaten away in the same way as the polymers Ruth has been talking about?

Serena -   Some of the ceramics do actually, there's a whole family of calcium phosphates which are, well your bone mineral is basically calcium phosphate an the one that's most similar to it is one called hydroxyapatite, and around that hydroxyapatite formula there are other calcium phosphates and depending on the number of calcium atoms in relation to the number of phosphorus atoms, then as you get fewer ad fewer calcium atoms the material becomes much more biodegradable, so what people sometimes want to do, just as Ruth has been describing, is to put something into the body which will serve a function and then it will disappear with time.

Other times we just want to put a material into the body, which will do its job but actually stay there with the bone having grown into it.

Chris -   So if I had a broken bone, for example, and I had  a bit of bone that was splintered and had to be removed, you could make a model bone with your ceramics, put that in, and it would act as a template for new bone growth?

Serena -   That would be an ideal situation. One of the problems that we have with these materials is that their mechanical properties are not great, so there are many ceramic materials that we know are very strong, but the calcium phosphates are a bit like chalk, they're a little bit brittle and not particularly strong in intention, and so what we're more likely to do with these materials is produce little granules and the granules are used to fill defects. For example, we use them to help people in spinal fusion (sticking two vertebrae together), and the little granules can be packed either side of the vertebrae and you might need to out some metal work in as well to hold everything in place. Or, you might use them at the end of the hole that's been created when somebody's had a hip operation, if they have to have that hip implant removed, then a big holes is left behind and sop we might pack the granules in too, to fill up those spaces.

Chris -   Is there any grounds Ruth, for combining a polymer with a ceramic and sort of mix the two technologies together?

Ruth -   Absolutely. This is where a lot of our research is going o at the moment so you can get the benefit of both worlds. As Serena has said, the ceramics tend to be very brittle on their own, it's like putting tea cup into your body, you can't expect it to be load bearing. So, if you can add the nice tough properties of a polymer to that, then you can get mechanical properties that are right, you can also get something that is resorbable over a period of time and you can get the bioactivity from choosing the right ceramic.

Chris -   So there's never been a better time to grow old then?

Ruth -   I guess so, yes.

46:32 - Sea Shell Sea Sounds?

This week, Diana was asked why the sounds in a sea shell are the sounds of the sea shore...

Sea Shell Sea Sounds?

This answer is from Matthew Mason, lecturer in physiology, development and neuroscience at the University of Cambridge.

The answer involves the shell acting as something called a helmholtz resonator. Now, a simple form of helmholtz resonator would be something like an empty wine bottle. You have a contained volume of air, which is connected, to the outside world through the neck of the bottle. We're familiar with the idea that if you blow across the neck of a wine bottle you hear a certain tone, and different sizes and shapes of wine bottle will produce different sounds. If you hold a shell to your ear, the shell is exposed to background noise from the environment around you. There's always noise wherever you are in the world and even your ear can produce background noise itself through the blood passing through the blood vessels within the ear. As a helmholtz resonator, what the shell is doing is it's selectively amplifying some o f the frequencies in that background noise, relative to the others. The larger the shell, the lower the frequencies you would tend to hear, so it would sound deeper with more bass. It could be any kind of solid, hollow body, for example, a coffee cup-you can try this yourself if you tip out all of the coffee first. Hold it next to your ear, but leave a little gap between the cup and your ear in order to hear something. If you try this and rattle your chair or wander around a little bit, what you might find is that some of the things that you do sound particularly loud to you in that ear, and those are the frequencies that are being particularly exaggerated by the helmholtz resonator. As to how long it would last, because the shell itself is not making the noise, the noise is coming from the background environment around you, it would carry on forever, or at least until your hearing went at those particular frequencies.

So the noise we're hearing is an amplified background noise that can be amplified by any partially enclosed vessel. There has to be a gap for the sound waves to enter, and once inside they bounce around the walls of the container, some reinforcing each other, before they're picked up by your ear. There's also the possibility that your ear actually generates the noise from blood vessel flow and that this is also amplified by the shell.

56:04 - Do you get wetter running or walking in the rain?

Do you get wetter running or walking in the rain?

Lots of people responded to this question with some very good points:

If you run, you're more likely to splash in puddles, and so your ankles and shoes will get wetter by running - BUT, this assumes it's been raining for a while, and there are already puddles around.

Also, running on wet ground is much more likely to result in you falling over, when you will get very wet indeed!

Obviously, the exact conditions of the rain means that it's very difficult to give the best speed to travel through rain, but Ronaldo Menezes sent us a link to Doug Craigen's calculator to work out exactly how much rain would hit you for given speed and rain conditions.

We tried the calculator with Chris' values, and found that if he jogs at 2 meters per second through 200m of vertical rain falling at 5m/s, he will be hit by 30.4ml of rain, but if he goes at 3m/s - only 27ml of water will hit him.

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