Powerful vibrations

Tiny sensors that harvest energy from passing cars, people or even the flow of your blood may one day be powering sensors inside you...
01 October 2014

Interview with 

Dr Sohini Kar-Narayan, Cambridge University


Piezo-electric materials produce an electric current when they change shape,Electricity usually by being squeezed or hit. Dr Sohini Kar-Narayan, from Cambridge University works on nano-scale piezo-electrics which are sensitive enough to respond to tiny vibrations harvested from the environment. The output is small, but nonetheless sufficient to power tiny sensors in clothes, the environment or even embedded in the body. Sohini told Chris Smith how this could power a blood sugar detector with your heart beat, or a temperature sensor in a room simply from the vibrations of cars driving past. 

Plus, Dave Ansell tests his very own piezo-electric crystal he made at home!

Sohini -   So, you've heard about producing big energy and looking for big energy solutions.  I work at the other end of the spectrum which is looking for small energy solutions.  In particular, I'm looking at harvesting energy from ambient sources in our environment to power devices which don't require a lot of power really.  You might wonder, small power should be a lot easier but it isn't because as the devices are shrinking in size, the batteries which you would normally use for example to power them, they aren't quite keeping up.  And so, to enable this technology to progress, we need to look at alternative energy sources.  And this is piezoelectric materials come in.  piezoelectric materials are a special class of materials which when you distort their shape, so basically, literally, when you squeeze them, they produce an electrical charge which you can then access via circuit.  The reverse is also true.  You can actually apply an electric fuel to a piezoelectric material and it will change shape.

Chris -   Is this how your crystal in your clock keeps time?

Sohini -   Absolutely and even the way your microphone works, it basically converts the vibrations from sound.

Chris -   Lighters, the barbecue lighter.

Sohini -   Absolutely, yes.

Chris -   So, squeezing a crystal.

Sohini -   Exactly, to produce a voltage essentially.

Chris -   Why does that happen?  If I squeeze a crystal, why should squeezing a crystal make some electricity come out?

Sohini -   So, certain crystals can be thought of as being made up of charges which are separated within them and we call these dipoles.  So imagine a crystal with a positive and a negative charge separated.  Essentially, what happens is when you squeeze this, you change the position of the positive and the negative charge.  So essentially, you can imagine that you're creating more of a charge across the surface and I think Ginny has a demonstration.

Ginny -   Yeah, so we were thinking about this this afternoon and we were thinking it's quite a difficult thing to imagine.  So, we came up with a little way that you might be able to imagine it at home.  Okay, so what we've got here is a pillow case and you can imagine that this is like one molecule inside the crystal.  So, I'm going to ask Dave to hold one end of the pillow case and so he needs to hold the other end.

Dave -   So, the actual crystal will be made up of millions and billions of these, all lined up next to each other and stacked in every possible direction.

Ginny -   So, if you stretch the pillowcase out nice and tight - now, what we've got here is a Ping-Pong ball with a plus sign on it.  So, that is a positive charge.  So, if you imagine our molecule has its positive charge at one side, like so, then if we change the shape of the crystal by moving where your hands are...oops!  It falls off.  But you can see that if you bring...

Dave -   So, the side of it squashed it, the ping pong ball is rolling towards it.  So, we squashed the side towards the audience the ping pong ball is moved towards that.  If we squashed the other side, the ping pong ball moves away.

Ginny -   So, you can imagine that being like, if every molecule in the crystal is being squashed in the same direction then your charge is going to move from one side of it to another.

Dave -   And so, if you imagine, each molecule has a little bit of charge that move from one side to the other then the next molecule, the same bit moves, the same bit moves.  So overall, a charge is moved effectively from one side of the crystal to the other side of the crystal.  And that piece is quite a large voltage and you can produce sparks with a gas lighter with it.

Chris -   If you take the voltage out of the crystal, so those charges flow around a circuit like they do in say, a barbecue lighter, doesn't that leave the crystal without some charge?

Dave -   And then if you let the crystal relax then the charge will want to flow back the other way and then you have the second spark because you get a spark when you crush the crystal and you get another spark when the current goes the other direction when you uncrush the crystal.

Chris -   And so, you're saying Sohini that this is a way that we could harness this to extract energy that we would otherwise throw away in the environment.

Sohini -   I mean, that's really the key thing.  this is energy that is available to us.  It's widely accessible.  It's pretty much everywhere.  You're probably always going to be situated near a source of vibration.  So, it seems like a good place to start.  What I need to stress is that the amount of energy that we're trying to harness or to harvest is actually quite small.  But this is important because if you think of the applications for the small energy, they're really limitless.  So, a big thing which you might have heard of is the internet of things which is essentially having everything really connected via sensors.

Chris -   Someone told us the other day that he bought a slow cooker which is on the internet and he said that he's discovered he can dial in from work to turn on the slow cooker.

Sohini -   Exactly.

Chris -   But then he discovered that it's the same log-in and password for every single one of those slow cookers that everyone owns.  So then he said, "I can ruin someone's beef stew if I just know where to find it."

Sohini -   Indeed.  I mean to be honest, there are lots of security concerns with the internet of things, but then that's a debate to be had possibly.

Chris -   But the question is, how do you power them?

Sohini -   Exactly and the point is, you can look at the energy crisis from two angles.  We are running out of fossil fuels and we need to look at renewable energy sources.  So, one way to do it is to look for new ways to produce energy, but the other way to do it is to try and save energy.  So, the energy which I'm trying to harvest is not necessarily going to light up this building, but it can light millions of sensors in this building, such that you can save up to 30% on your electricity which sounds like a good deal.

Chris -   So, would this be then say, air current sort of wafting past something or if you did it say, on a light bulb, you could get air currents near a light bulb because it's hot.  You could get those vibrations or something.

Sohini -   Sure, absolutely.  So like I said, these are ubiquitous really.  So, you could imagine sticking it on your washing machine.  That vibrates while it's on.

Chris -   So, anything that moves, you can get energy out of it.

Sohini -   Yes, including yourself.  I mean, I think this is still a bit far off but in principle, if you could make these devices on a large scale and if you could integrate it for example into your clothing which is very possible.  As you walk, as you move, you could generate enough electricity to charge your mobile phone.

Chris -   Will that make walking really difficult though?

Sohini -   As I said, piezoelectric materials have been on for a long time and usually, research has been focused on bulky ceramic crystals which as you said are quite hard to move around with.  My research focuses on nano piezoelectric material.  We're looking at really tiny amounts of these materials which the idea is that they should be able to blend into the environment, into your clothing and practically be invisible for all practical purposes.  So that you're not aware that they actually exist, but they are constantly harvesting energy.

Ginny -   So, we actually have an example of one of those super bulky piezoelectric crystals that Dave made yesterday.  That's quite impressive Dave.  How did you make one of these?

Dave -   I basically made them by using cream of tartar and if you heat it up and dissolve it, that's an acid and then I reacted it with some sodium carbonate.  I spent about 3 hours carefully adding the two together and mixing it up and eventually, the solution went clear.  I let it cool overnight and you get these really beautiful quite large crystals.

Ginny -   So, they look a bit like a really huge salt crystal.  You can actually see some beautiful geometric shapes on the side.

Dave -   Because a crystal is when you've got lots and lots of acids and molecules lining up in a very organised shape.  So, repeating again and again, and again.  The reason why you see the edges of that is that kind of zoomed out in huge scales if you've got billions and billions of them together.  You get these sharp shapes because that's the shape of the fundamental crystal underneath.

Ginny -   So, what are we going to do?  I'm not sure I believe that those are piezoelectric.  They just look like crystals to me.

Dave -   It's taken a while to persuade myself that they are, but now, I'm fairly sure they are.  What I've done is, it's a bit delicate, so I've got it sitting here.  I've put one in a vice and I've put two tinfoil electrodes connected to this crystal and I've wired it up to an audio amplifier and attached that to a speaker.

Ginny -   So, the vice is just to hold it still and to hold the electrodes that it's connected to onto it.

Dave -   Yeah and so, the idea is that any electrical signals that's produced will be amplified by the amplifier and then should be turning into sound which you should be able to hear.

Ginny -   Let's give it a go then shall we?  

Dave -   So effectively, what I've built is a very, very rubbish microphone.  With rather better engineering, you can produce a perfectly good microphone and actually, quite a lot of the cheap microphones are made like this.

Chris -   But the point is, you're squashing the crystal, putting a force onto the crystal and that is, as you say, moving charges around and making them go on to the electrodes, flow to the amplifier and those little clicks we were hearing, they're the surges of current coming off the crystal.

Dave -   That's exactly right, yeah.

Ginny -   But we weren't making very much electricity there and it's quite a big crystal.

Dave -   So, the trick is to A. if you can bend the crystal a bit more, you'll get more voltage out of it.  Also, if you use much better materials, which is I think what you've been doing over there.

Chris -   Yours better?

Sohini -   I'd like to think so, yes.  In fact, a lot of research into piezoelectric materials concerns ceramics.  So what Dave just showed you was a ceramic material and the image that comes to mind when you think of ceramics is they're brittle, they're stiff, and that's exactly the problem.  We're talking about an energy harvester which can sustain repeated vibrations, or heatings as you may want to think about it.  And so, it's important that this material can sustain that level of impact.  The problem with ceramic materials is that they're stiff and hence, they're prone to mechanical failure.  And so, I work with piezoelectric polymers which are slightly less well-studied class of materials.  But they're very interesting because being polymers, they're flexible which means that they can take a lot more beating and bashing as it were.  They have several advantages over ceramics.  For example, they are actually relatively cheap and easy to fabricate which is important if you want to make commercial devices.

Chris -   How much electricity will they make?

Sohini -   So, with these nano generators, we're looking at anywhere between 10 nanowatts to a microwatt and I know that that doesn't sound like a lot, but a lot of wireless sensors these days, their power consumptions are also coming down.  So, it might just be enough to power these devices.  The other thing that you need to remember is, a lot of these devices don't need to be on all the time.  So, imagine a glucose monitor embedded within your body.  You don't need to read your glucose off every second.  I mean, you might want to but it might not be necessary.  But you might be able to generate enough energy by just your blood flowing across one of these nano generators to emit a signal every 6 hours maybe and that's all you need really.

Chris -   How far away are you?  Have you got this actually working?

Sohini -   So, I can show you a device which I've brought here and hopefully, it will work.  If I just hold it up, the little circle there that you see consists of about 10 billion piezoelectric polymer nanowires.

Chris -   So, just for the benefit of the people at home, what we've got is something, it's about 2 inches long by about an inch wide slide of glass.  And what's the circle in the middle of it?

Sohini -   The circle is the device.  So, the circle is basically about 2 cm in diameter.  It's about 60 microns thick which means it's about as thick as the width of your hair.  As I said, it's packed with these piezoelectric polymer nanowires.  If I can get this to work, it should respond to my touch and that just shows you that it creates a voltage.

Chris -   So, at the top of the box, we've got lots of little red LEDs and as soon as you touched it, they all lit up.

Sohini -   That's right.

Chris -   They're all powered by you, touching the device and pressing on it.

Sohini -   So, that's really an indicator that you're generating some voltage and that is sensitive to how much I touch.  So, just a light tap would give you that.  A big push would give you that.  So yes, they are quite sensitive.

Chris -   So, it's like world's strongest man competition where you have to hit the thing with a big hammer, but for microscopic people.

Sohini -   Something like that, but to be fair, with that particular device, you would need to tap on it, not very hard for about 20 minutes to generate enough electricity to power an LED.  So, that doesn't sound very exciting sitting around tapping for 20 minutes.  But the point is that if you can upscale the production, if you can make more of these and connect them in series then you can bring that time down to a lot less.  And that's really the goal - to be able to make lots of these in a cheap and reproducible and reliable manner.

Chris -   Any questions from the floor?

Malcolm -   My name is Malcolm and I'm from Longstanton.  Why can't you just put a weight on top of the button and leave it for about 20 minutes to power a LED?

Sohini -   That's a very good question and I'll take you back to the demonstration that Ginny and Dave just gave.  The point is that if you leave a weight on, yes, you will generate some charge, but then that's about it.  In order to make a current flow in a circuit, you need to be able to do this repeatedly over a prolonged period of time.  so, by moving back and forth on that material, you produce what is known as an alternating current.  And then you can rectify that and use that to power something.  But just leaving something on there would just produce a spike of current and that's about it.  But you want this to work repeatedly.

Jasmine -   I'm Jasmine from Cambridge.  What's the smallest touch that you can do?  As in, what device can you touch really lightly and it works very well?

Sohini -   So, these piezoelectric materials as you've just seen, they're very sensitive to the touch and nano piezoelectric materials are supersensitive to very little forces.  We're actually looking at applications where you can put these into say, biological samples so that you can actually detect cellular motion.  So, really very, very tiny forces which you would not even be aware of, these can pick them up.  So yes, the answer is, very tiny forces and if I should put a number to that, we're talking on the scale of piconewtons or less.

Chris -   So, pretty small.

Sohini -   Very small, yes.

Chris -   Any other questions for Sohini.

Lowen -   Lowen from Cambridge.  So, with this material, you can have it in potentially nearly any electrical device.  How easy would it be to recycle it from one device to the other when the device was finished like clothes, etc.?

Sohini -   Again, that's a very good question.  I think one of the drawbacks of these kind of generators is that it is dependent on the actual source of vibration and that can be very intermittent in nature.  So obviously, if you had one of these in your shoes for example, you wouldn't be walking around all day at the same pace for example.  So, to move that into a different application will probably not be trivial.  And so, I would say that these nano generators need to be designed with specific applications in mind.

Holly -   I'm Holly from Florida.  People use phone every day.  Do you think you guys could start putting the stuff into screens on your phone?

Sohini -   In fact, I think there's a similar prototype being developed where you have it at the back of your keyboards because as you're typing away furiously, you can use that energy.  The beauty really is that you can integrate it into just about anything.  So yes, I can see applications where you have them on touchscreens where you can harvest the energy of just swiping or clicking on things, absolutely.

Dave -   I guess the phones would have to get a lot more efficient before it becomes very useful because they're using watts at the moment, not microwatts.

Sohini -   Indeed.  Sorry, so yes, so I should say that the energy which you would get out of it is still limited just because there isn't that much energy to harvest.  So, this would power maybe one function on your phone, but not necessarily charge your phone.  Having said that, as you correctly pointed out, we're at a very unique stage with a power consumption of modern electronics is reduced to such an extent that it is now slowly becoming feasible to power them from vibrations in our environment.  So, who knows?  Someday maybe.

Chris -   So, if you could have an amazing device that you're going to power on your things, what would it be?

Sohini -   Gosh!  I can think of so many of them at the top of my head, but I'm really interested in biomedical applications really.  So, tiny sensors which are implantable and which can just run off just your say, blood flowing through your veins and then which can give out vital information about your blood pressure, temperature, what have you.  I think it will make an enormous impact on healthcare.

Ginny -   So, I've got a question that's coming from social media and (Steven Pates) asks, "How can the UK be self-sufficient for 100% of its energy needs?"  So, he wants us to not be importing oil, coal, nothing.  So, I think that's kind of a question for the whole panel.  Do you see that in the future and how far away is it?  Who wants to come in on that first, Richard?

Richard -   Well, I was saying to someone today, what is the price of not trusting the French?  And if we were not to rely on our imports and exports to France, that would cost us a lot of money in extra kits.  So actually, I think 100% energy sales efficiency is not a good goal.  I think we should share with our neighbours.

Ginny -   But do you think it would be possible if we did want to or have to?

Richard -   Of course, but it's probably not the cheapest solution.

Sohini -   I think that energy saving will become more important as we go into the future.  And also, as more cities are being built, the concept of having smart environments will reduce basically how much energy goes into pretty much everything that you can think about starting from resource management to waste pickup.  So, if you have lots of sensors in the environment, they can more effectively communicate with each other and also, make energy use more sustainable and then that will have an impact.


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