9UNP0669SD_Xudong_Wang_WordPress

- Cassie Immel: Good evening. Thank you for watching Wednesday Nite at the Lab. I'm Cassie Immel from here at the Biotechnology Center. This evening, we have Dr. Xudong Wang. He is an assistant professor at the Department of Material Sciences and Engineering at the University of Wisconsin-Madison. He received his PhD from Georgia Tech in 2005. Currently, his research includes cellular growth and assembly of nanostructures. He is the recipient of the DARPA Young Faculty Award, the Non-Tenured Faculty Award, and the Ross Coffin Purdy Award. 

Without further ado, I present to you, Dr. Wang. 
[applause]

- Xudong Wang: Thank you. Can everyone hear me? Good. Thank you and good evening. It's my pleasure to be here, to give a talk today. Happy New Year, by the way. So, my talk will be some of my research I've been doing for the last five or six years, which is about nanoscale and mechanical energy harvesting. 

We have named it nanogenerators. So this is basically energy harvesting. Most of the folks are on nanotechnology and mechanics side. So as we know, contrary to -- stage of technology explosion, and you can see small electronic devices getting smaller and smaller and faster and faster. That's faster developments for nanotechnology and electronics. So, you will notice that all this nice technology can be achieved with these very small, tiny chips. But the one thing that is lagging behind is the battery. So you can see the heaviest part of our computer, heaviest part of our cell phone is a battery. It's the majority of the body of the device. So, there's many research that's going on right now to try to reduce the size and improve the capacity of the battery to match the development of the electronics. 

So that's why our research is going on here. And in the biomedical device field, it's more important. So right now, as we know, there's many devices for the implantable devices used for the human bodies. For example there's brain sensors, as we see right here, and pacemakers. There are some sensors that people need to put inside their body to help their health, monitor their health, and to help our body improvement. So, all these devices are very nice and very small. But they also need batteries, so that's a key challenge for those devices. Some people, for example the pacemakers, people need to replace the battery for every eight or ten years. For other sensors, we also need to charge the batteries for those devices. So the best way that people think about it for those devices is to use the energy from the surroundings. 

For example, for the power source for those devices, what we need, is we need a small size, and they'll need to be biocompatible, and then need to be very, they need to last very long. They need to have a long lifetime. So people think about, instead of uses more battery, the best way is to harvest energy from the environment, basically from our body itself. Now, we'll talk about how we can harvest energy from outside. Right now, there are many, many techniques for us to harvest energy from the surroundings. A lot of things we know about is the biofuels, the hydro power, wind energy, and solar energy. So, all those energies are larger scale. They may be able to apply to a small scale, but there's many technical challenges. What you see here is energy storage devices. What do we know right now, is lithium batteries. 

They can hold a significant amount of charge in a small size, and power devices for computers, for laptops, and for cell phones, we all use lithium batteries. I think batteries are supposed to have much higher power, but once of the size of this battery goes to micrometer scale, the energy stored in those batteries becomes significantly small, so they're not powerful enough to power small devices that we need to use in a micrometer scale. So right now, some people have brought up some concept about a three-dimensional micro battery, as the picture you see right here. So this device can really improve the capacity, but we're facing significant challenge of fabrication. It's a very complicated structure. So even though the scientists and technologists themselves think in the micrometer scale, maybe it's better for us to harvest energy rather than storing energy in such complicated structures. So, now let's take a look at the energy out there that allows us to harvest. The first one I see is solar energy, right here. Of course, outside at this size, a significant amount of energy are there. But if it's inside, you can see the indoor, the energy drops down to six, which is five thousand times smaller than outside. 

But the other types of energy, for example, mechanical energy, vibration, acoustic, and even the shoe insert, you can see the energy density is about ten times higher than solar energy that's inside. Okay, so this means we have sufficient amount of source for us to harvest as energy. Look at the battery capacitors down here, 47, which means the engine source is sufficient for us to get sufficient energy, comparable to those batteries that are being used right now. Now the question is, how are we going to harvest it, and where does the energy come from? Actually, more than 20 year ago, IBM brought up this concept called a human powered computer. They did this calculation and compared the energy density to the electronic devices. The picture on this chart shows the amount of energy from the human body. Here we see the blood flow, breath, exhalation, and movement, arms and fingers. The energy density is around the one watt range, and the last one is walking. When you run you can reach about 100 watts. 

It's pretty significant energy. Compare that energy to the small electronics that we showed down here. All those devices require energy in the micro-watt range. Compare the two values and we can see, if we can make a device that is %10 efficient or %5 efficient, we can still provide a sufficient amount of energy to power small devices. It might be used for these devices and put inside our bodies. After this concept, progress has been made in this field. First, I'd like to talk about the current technology for harvesting energy from the human body. Basically, these are on a larger scale. The first example I want to show is energy harvesting from walking. This group made this device, a sort of large device, you can see right here. 

This is the size of the device. They can put it on the knee. This shows what is inside the device. There are many gears. As we are walking around we move the knee back and forth, and this drives the rotation of the gears inside. There is a little rotor right here. This electro-magnetic circuit will generate electricity, an electro-magnetic effect. This is the same concept as the windmill or as hydro-power. It's just in a smaller size. It's really working and gives about five watts when people are walking at a normal pace. 

It's quite a lot of energy and can be used to power an MP3 or cell phones. You can see, though that the size is quite big and I don't think it's that comfortable to wear, right on the knee, just for 5 watts of energy. This is just some progress that has been made in this type of energy harvesting. This is another technique, made by MIT, Medialine. The same mechanism, the electro-magnetic effect. They have this thing put on the heel, inside there are many gears to transfer the pressing force into the rotation of the gears rotor and generate electricity. This one is dense and a little bit smaller, so it can produce 16 watts of energy. From these two examples you can see the magnetic effect really can produce a lot of energy, but it requires a large size to realize this kind of complex structure. Piezoelectric is another effect that many people have used to harvest mechanical energy. Piezoelectric effect is basically a crystal response to an external force. 

We know that many crystals are formed by positive and negative ions distributed inside. Once those crystals experience some force outside, the negatively charge ions are moved away from the positively charged ions. The charge is displaced due to the change of the dimensions in the crystals. Based on the different directions in the change, we will build up a charge on either side. This is called the piezoelectric effect. The electricity is a response to the external pressure. Many crystals have this effect. Here we show zinc oxide. It's is one typical structure, it's a hexagonal structure. Here is the blue sphere to show zinc, which is positively charged and oxygen which is negatively charged ions. 

Here is just one zinc ion sitting inside of four oxygen tetragonal ions. This is the equilibrium state. If we apply force on top of this, we can shift this positive center away and that's how we create the polarization, a charge from this material. Based on this concept there are many materials that have the same characteristics. For example, the PZT, a contagnate, is a widely used piezoelectric material, and there are many other types of material that have these same types of properties. But all those materials have lead inside, basically. So in terms of bio-compatibility, this is not good material even though there is a good response. What we are looking at is this zinc oxide, which is a bio-safe material that is one essential element in our human bodies. This is good material that we can safely put inside our bodies, and also shows a good response. The typical technique we use with this material is called a cantilever type. 

Basically, we can use the material and make a cantilever and put a big mass on top of that. Once the material experiences some external driving force the cantilever will vibrate like this and create a bending of the material, generating electricity. The concept is kind of simple and therefore the set-up is simple, the devices are simple. Here are some devices with the piezoelectric cantilever. This is a quarter, so you can make it smaller than a quarter. But you can see, the majority of it is to produce the mass to drive the cantilever. The cantilever is right here, it takes up a small volume. The big volume is from this big mass to create the vibration. We can use this device to generate 100 micro watts of energy, to power some transistors. Another type of material in addition to the ceramic material is called a polymer, is a polymer-type material. 

PVDF is one typical material. It has a very simple structure. It has a carbon chain, right here, and hydrogen and fluorine right here. The hydrogen is positive and the fluorine is negative. They arrange themselves like this, and create a dipole, negative towards positive. Strain on this material will create a dipole on two sides of this material. Same result, just different crystal structure. This polymer is much more flexible compared to the ceramic materials. A lot of people have used this material to replace ceramic material in the energy harvesting area. This is one example. 

This is one, I think it's an Army based project, to make a backpack. Once you are walking and the bag is moving up and down, it generates electricity. This devices is made with an electromagnetic effect. You have a rotor back here, and it moves the rotor up and down to generate the electricity. This device is very heavy. Carrying the frame takes about 19 watts but the maximum power is 7 watts. Doesn't make sense, right? People who use the PVDF strive to repress this, by putting it in the backpack. That makes the weight much, much less and it can still produce 60 milliwatts of power based on how much load is put in the bag. The higher the load, the more power is produced. 

Even though it produces less power, the total weight is less. It is a good improvement in using this polymer based piezoelectric material for the mechanical energy harvesting. All right, so this is another example to show the use of PVDF. It's a shoe insert. You can see, the shoe looks much nicer compared to the previous shoe we saw in the previous slides. So it just has a PVDF film put inside the shoe, and the moving create the electricity to power the small devices outside. You can get in the milliwatt range of energy from this kind of device. At this point all we see is still on a larger scale. You can see, on a millimeter or centimeter scale. If we want to use this for biomedical devices, of course, we want it much smaller. 

How can we do that? The next topic that we're going to talk about is the development of nanogenerators. The basic concept is to use nano-sized material to replace the bulk sized material and still realize this kind of harvesting functionality. Of course we know the advantage of mechanical energy harvesting. It's everywhere and can be harvested easily from everywhere. It's accessible. We can find it inside rooms, outside, in the body our most energy source is mechanical. This is a perfect solution and provides a lot of answers for a lot of biomedical implantable devices. So this is great. But the challenge right now, is that the larger devices we saw previously, is the size. 

As we see, the size has to be very large. It creates a fatigue issue. The material has to vibrate many times and it can become very fatiguing. Because of the larger size and the need to integrate this large mass with the material, there is some issue with the sensitivity to small level mechanical defamation. It also has to challenge, to be adaptable to the micro-meter size of implantable devices. Our solution is to use nanowires to replace those large size materials. Nanowires, this is one, the structure that we see. This one is a 15 nanometers wide and about 2 micrometers long. This size is 10 times smaller than the macro meter size devices. This one we can really use inside biological systems without changing the environment. 

The thing is, if we can use the piezoelectric material to make this wire shape and create defamation, we can also create polarization on two sides. If I can think about a way to connect both sides, just like we did for the larger scale device, then we can create power from this small devices. It is easy to talk, but very hard to realize. This is such a small size, about ten times smaller than the --. The principle development was done in 2006, using a device called the atomic force microscopy. Using this very small tip that is coated with a platinum and used as a metal tip, a metal contact, and you'll see the nanowatts. It's a very small size and it's growing on the side of the substrate. To use this small tip we can create the bending of these small wires, and once we create the bending the conductive atomic force microscopy tip will make contact with the nanowire and release a charge of nanowatts. What you see on the chart down here is the charge released from the nanowires you see right here. Once the tip moves over those nanowires the charge is released towards the tip. 

This is a very exciting result. We can really generate the electricity from these small wires. So the next question is how to connect all the devices that are used in such expensive equipment, and use it in an applicable device, right? That's the second question to ask. The intuitive thought for this one is, okay, there's one tip bending one nanowire at one time. How about if we can make an array of tips and move an array of nanowires at one time. Or we can make a two dimensional tip to bend an array of two dimensional nanowires to create more power. Here is the concept and here is the device. This is the first device that we built in 2007. It still uses the nanowires and a zig-zag electrode on top, sealed with a flexible spacer. 

Okay? Then we create a vibration over this device and the zig-zag electrode will create the bending of the nanowires sitting beneath it and then create electricity in an outside circuit. This is the concept and this is how we make it. Based on the concept of the bending of the nanowires to create electricity, we use the vertical nanowires with less density and then we integrate it with a silicon trench, coated with platinum as an electrode. We simply kept these two together, as we see right here. So put a zig-zag electrode on top of nanowires. Or you can see the nanowires are sitting beneath it. Most of them are in contact with the electrodes. Then we can apply external vibrations towards this device, so the nanowires inside will be bent from the vibrating, and will create electricity toward the outside circuit based on the same concept. This is the first generation of nanogenerators. 

Another aspect that I want to talk about here is encoding on the nanowires. It's called the Schottky type of contact. That means the electrons can come from only one way, instead of from the other. The electrons can only grow from the electrode towards the nanowires based on the piezoelectric effect. This will generate DC output from this device. Once we've applied a sonic wave toward device, we can see the current output is a DC type of output generated from this device. Here is an IV curve that has been measured based on this device and you can see that the maximum voltage we can produce is about 10 millivolts. The maximum current we can produce is one microamp per square centimeter. This is the point at which we can run the device with the maximum output of power. Based on this first generation device, we can produce the maximum output energy, about 10 nanowatt per square centimeter. 

It's still very small, okay? The operational power is about 2.5 nanowatt per square centimeter. So this is a very small amount of energy and much less than our expectation. All the nanowires, seemingly, are not really doing their jobs. If everyone is doing their job, we could get a microwatt output. Fabrication is still the challenge on such a small scale. The researcher on this work did many improvements for this kind of device. He used nice -- techniques and put nanowires laterally in between the array of electrodes. He made it in larger scale devices you see right here. On top of this is an array of nanowire devices you see here inside. 

Based on this device you can generate, you can see each nanowire contacts with an electrode, and each nanowire is doing its job. There is a significant amount of power out of this device. The output is about 40 millivolts and the power is nearly a milliwatt, at least 10 to 100 microwatts. This chart you see here, they use this device to power a PH sensor. You can see this different response to show different PH value. This is the first development to show that this nanogenerator device can be used for some electronic devices. So this is pretty recent progress done by the Georgia Tech Group, where I graduated from. Now probably many people have the question, how much power can really be provided by this kind of device? I've been asked this question many times. Here is the answer. 

We did this calculation about two years ago. Of course we know, based on, this equation that I developed, I won't go through that- Intuitively we can see, the larger the size of the nanowatt, the more power we can produce. And the smaller the wire, the less mechanical energy that we need to bend that nanowire. So there is a trade-off between these two. The optimal voltage can reach a few volts with this nanowire, but in terms of power, this is what one nanowire can produce in terms of power and efficiency. This is the diameter, and this the length. You can see, basically, the larger the size of nanowire, the more power that it can produce but we need much more mechanical energy input. If the power is high, the efficiency is very low. If we have very small wire, efficiency can be very high, but the power can be very low. So there is an optimal point, right here at this point, where we can achieve reasonable good power and reasonable good efficiency. 

This is at the size of about three micrometers long and 50 to 100 nanometers in diameter for those nanowires. We can reach about .1 nanowatt per nanowire, and the efficiency is about 4%. Based on this calculation we can see, okay, this one can really give us a sufficient amount of energy if we can integrate many, many nanowatts together. This is still progress to be made in the next devices. So what is the application potential for this device? In 2007, maybe it's too small to see right here. In 2007, after the first development of this concept, -- Think of this as research that benefits the nation. They put this report into congress. A year later there was Scientific America, they published this article to project this application of this nanogenerator. This is a conceptual device, made by nanowires. 

This is actually a blood vessel. These are red cells inside. The change of blood pressure can generate the power to charge this capacitor and then charge this remote sensor to sense the blood sugar level or blood pressure, or all sorts of things inside of the blood vessels. All right, so this is a conceptual article published in Scientific American. So there are some real developments. Recently, that also has been done by the Georgia Tech Group, There are several developments. This is a nano, small generator, you'll see, put on top of the finger. Remember in the first, previous slides we showed, moving the finger can generate microwatt energy. It's put on top of the finger and it will produce mini volt voltage output. Okay? 

So this is a pretty significant output compared to the vibrational nanogenerator. And they also put this nanogenerator on top of this little hamster. So the hamster running inside its little cage can generate electricity. 
[inaudible]

 This one? 

- Yes. 

- This is a National Scientific Foundation, financial year 2008. The budget request to congress.

The hamster is running and producing voltage and once it stops, there's not output. It's basically muscle movement producing electricity. 

This bottom one here is kind of an extreme example. They put this nanogenerator on top of a heart, I think it's rabbit. Then the heart beating produces electricity as well. That's why I kind of brought it up, sorry. It does show some application of this nanogenerator and integrating with biological systems. Another development is called Power Fiber, done back in 2008. This is another concept for harvesting energy from human motion. The basic concept is, we integrate those nanogenerators on the fiber that we use to make our cloths. The concept is that we have this fiber and we will grow the nanowire on those fibers. Some fibers are coated with gold to act as an electrode. 

Some fibers are just pure nanowire. We wire these two types of fibers together. Then if there's any relative movement between those two fibers, those nanowires can create defamation of those nanowires, and then we can produce electricity. This is what we call Power Fibers and it also attracted a lot of attention once the paper published, and is also recognized by the National Nanotechnology Initiative, the second report to congress. So they highlighted this development on the cover to show this nano scale energy harvesting. Of course the development is still way far from the closest developments. The first proof of concept is just two fibers, one with gold and one without, just entangle them together. Then we create movement between those two fibers. The other concept is still the same. We have this gold coated wires as electrodes. 

They are moving relevant to this zinc-oxide nanowire. Once they move, the nanowire beneath it will be deflected and it will create electricity and it will be harvested by the nanowire with metal coating. The concept is the same, but it does take a lot of effort to make this device. This is the Kevlar fiber that the Army uses to make bulletproof jackets. It's one fiber with micro size, what we see on top, is zinc-oxide nanowires growing around it. It's a very uniform coating as you can see. The growth technique is a solution based technique. You put the fibers in a liquid and after about a day, take it out and you'll see something like this. It's basically a larger scale manufacturing technique just like dying fiber with color. It's a similar process. 

But now we can achieve lots of nanowires growing like this. There are a lot of spaces left between those nanowires as well. So we coat those nanowires with gold on one side, and some wires are just bare. Then we also develop a technique to tightly seal those nanowires around those fibers to make sure that they will not fall off. On this image you can see tow fibers tangled around. The golden one is the one coated gold, and the whiter one is coated with zinc-oxide. So you move them around and you can create a very small amount of electricity. It's about five picoamps, a thousand times smaller than ultrasonic generators, and the voltage is small as well. The key problem is the resistivity of the fiber is very huge. There's a huge loss when the electrons transfer from each nanowire towards the end. 

Also the contact area is limited. We did improve both. Here we put six fibers together instead of two. Of course that improved the contact area, and you can see the current output increased nearly 100 times. We also improved the conductivity by coating the conduct layer beneath the nanowire. By doing this we increase the current by 1,000 times, from four picoamps to four nanoamps. We also showed the current to output relationship with the resistivity of the fiber. The thing is, if we just need to make the fiber conductive then we can get a majority charge out of the nanowires. This is a very exciting improvement. The last part I want to talk about is a recent development, that we did here at UW-Madison last year using a piezoelectric polymer microbelt to harvest energy from respiration, from breath. 

Basically this kind of development is to harvest energy from air flow, right? So, everyone knows, wind, harvesting energy from air flow and we have windmills. But for the human breath it's a much smaller scale. We can't put such a big device inside our nose. That sounds weird. It won't work. You want something different. And the larger scale wind energy harvesting will not work in a small scale. It basically has a so-called cut-off wind speed. The wind has to be above a certain level to drive this device. 

If it's smaller then it won't work. Once I had this idea, I saw another development from another company. They call it a windbelt. They make a belt like this and one end is attached with a magnetic bar. This magnetic bar sits inside a copper coil, just an electromagnetic effect. The vibrations of beam will create a vibration of this magnetic bar inside the coil and generate electricity. So they can make it in a small scale. This is a very small scale, and this is a larger scale that they make and put outside of the building. It can create, like, a milliwatt or watt level energy, to light the building. I think that's one device they put in Hong Kong, or Singapore. 

Yes. 
[inaudible]

Mostly because of friction inside of the rotor, the big rotor here. Based on this concept, we can make it even smaller. Because it uses this electromagnetic effect we can't reduce the device further. You have to have a significant amount of currant go through this circuit to generate power. So here our concept is to use piezoelectric to replace the belt and generate electricity just by vibrations and remove the rest of the parts. First of all, I'm sorry about this. I use PC and this is a MAC so we have some conversion problems. Anyway, this is not important. 

We did this calculation to show, in order to create the vibrations for the belt we have to have the right size. We have to have the right thickness and the right length, or the right ratio. You want to make the belt vibrate in a small wind. This is our calculation. You can see, as the thickness drops so we can create a higher amplitude of vibration and generate a high power, the more important than airflow speed. The smallest thickness, it will show some response at a very slow airflow speed. For the larger thickness there won't have any response, because the vibrations will not be driven by those kind of wind speeds. This is our output prediction, and we will see if we can produce about a microwatt level of energy based on the airflow that is less than one meter per second with the thickness in the micrometer size range. Okay, so based on this, we're using the PVDF as a polymer. That's the piezoelectric polymer material I showed you previously. 

It's used for this development. One key change for us is that we cannot make this device very small, very thin, because for the PVDF material has different phases. This is the phase that we want. We want all the hydrogen to stay one side and all the fluorine to stay on the other side. This is a high energy phase, and it will not happen until we apply a very high strain on this material. This means we cannot make it very thin. If we make it very thin it will not survive under high strain. It will break. We won't know while we are at this kind of phase. So we tried different ways and we found out that we can start with a thick material in the beta phase, which is a piezoelectric phase. 

We are using a top-down action technique. An ion bean action to shoot ions towards the material to blow away material and make it thinner. This way we can reduce the thickness from 25 to almost 10 micrometers. This is within the range that we think we can produce a significant amount of power under very slow wind flow. We also are very excited to find out that this kind of technique will keep the phase unchanged. We did this FTL measurement to show this peak right here, and this peak right here, is a characteristic peak of the beta phase. We are very glad to see that this beta phase exists over the long term action and we can make it very thin. We are very happy to see that. I apologize about this too, because of the MAC computer. So, you can look at this. 

This is the device itself. We can make this very thin belt right here. We coat it in gold, the electron on top, and the other coated electron bottom. We've got this change. The belt itself is about 15 micrometers thick. Then connect this end to the voltage meter. We can see, this is that output. Once blow air through this small belt, we can see output on the different wind speeds. Even at a very low wind speed we can still see some voltage generation from about 10 millivolts at a slow wind speed. So this is very good improvement that we see. 

We made different assemblies with different thicknesses and we can see the trend in voltage and this is the in power. Of course, with the reducing of airflow speed the voltage drops quickly and the power drops quickly as well. One thing we want to take a look at here is this. Once we reduce this thickness the voltage increases at a slow airflow rate. This is a good part that I really like to see. We reduced the thickness and increased the power output. That's why we want to use it to harvest energy from the respiration. Using a similar approach to calculate the power, this is around a microwatt range to a 10 nanowatt range using a single belt. Then we are starting to see how much energy can really be stored by this device. We then integrated the device with a capacitor, a simple capacitor as a battery. 

We can see, the charge of the battery is increasing in voltage measured on the battery. You can see, the higher the wind speed, of course, the more energy that can be stored on the capacitor. This is just because of the balance of the charging rate and the leaking rate of the capacitor. With a better capacitor I believe we can achieve better storage. But just based on this capacitor we can achieve energy storage around a 100 microjoules to 1,000 microjoules based on different airflow speed. This energy is sufficient for many small electronic devices. For example, a lab-based stopwatch. We hook up a stopwatch with this generator. Here's a video you can see. I hope it will work. 

In the video, you see right here, we took the battery out of this stopwatch and simply hook up these two wires with our microbelt. Then we have airflow through right here. Now you can see the belt starting to vibrate and the watch is starting to show. If we block it, it disappears. We can actually operate it normally instead of just showing the screen. It's a little longer.

So we can press a button and do the normal functions. Of course the response take a little bit longer than a normal battery, but it shows that it works very well based on this small belt. I apologize again for this. It was a good picture. 

I liked it. So we are starting to use this energy harvesting from respiration. Of course we didn't put it in the nose. We made it as a simulated device. We made a little tube, a very small tube. Then we put our belt inside the tube and the other end it is connected to a lung simulator. So expansion and compression, to pump airflow through to simulate the respiration. On this chart is a normal flow rate of normal human respiration. It's basically a sin curve. The maximum speed of airflow from normal respiration is about two meters per second. 

So of course, if we have the belt that has a slow cut off wind speed, then more energy will be harvested from this respiration. We used the belt that we made with about 15 micrometer thickness and then connected with this belt. Through this we can see the voltage generation from the belt based on the breath. Connect it with the capacitor, after about ten minutes of operation we can store the charge and the voltage can be about a 200 millivolts and the charge is about 100 microjoules. This is enough for most electronic devices. OF course, right now, if you asked me how to put this in a nose, I don't know. 
[laughter]

Here I just show that there is a material that is available to be able to make into a small size and harvest energy from such a small flow-rate from human breath. This is a prospect of this nanogenerator, the different kinds that we see. Nanogenerators can convert mechanical energy into electricity on a small scale. 

And around us there are many mechanical energy sources, from small scale to large scale. From breath, as in the one I just showed, and blood flow or heartbeat. On a large scale, from the wind, the waves and body movement, a car running or even some random vibration inside a building, such as air conditioning of from some acoustic waves on the floor. All of them can be used as a source for this nanogenerator to power this and generate electricity. Based on different sources that we use we can use it for implantable devices, such as a pacemaker, for this kind of energy source. And we can use it for some defense technology, such as lighting sensors. Like the ones put in enemy territories, where no one can be able to replace the batteries. Or robots that needs to run by itself. Also some names of nano-devices. Of course this one has already been realized. 

If we can produce a sufficient amount of energy, then I hope that someday we can use it to power small personal electronics based on this techniques. Then we will not need to carry the charger around with us. This basically concludes my talk. My last slide is acknowledgement. I thank my group, my young group. This is my third year here and my new group with five graduate students and a previous post-doc, and some funding support from NSF and -- and also significant funding from the UW graduate school and --. There was also some work that was done at my previous school, at Georgia Tech -- Group before I came here. All right. Thank you very much. 
[applause]