Dr. Zhong Lin Wang, Georgia Institute of Technology
Diana - For Naked Engineering this week, Dave Ansell’s been finding out how to solve the problem of powering the tiny nanoscale devices like medical implants that are currently dwarfed by the batteries needed to supply them. Helping to solve this problem is Dr. Zhong Lin Wang from Georgia Institute of Technology.
Zhong - In the past 50 years, I have devoted my whole time to nanotechnology, making things smaller, better, higher performance, and hopefully cheaper. But when a thing becomes too small, having a battery drive it may not be the adequate choice. If you have a battery that's so large that it means the whole system is large, how do we miniaturise the size of the battery? That's one way, but the other way is, can we harvest energy from the environment because the power consumption of the device is small. The energy harvest may be enough to drive this.
Dave - Is the problem that as a device shrinks, the amount of power it uses doesn’t shrink as much as it's volume, so the equivalent battery wouldn’t be big enough to keep it running for a long time.
Zhong - That's correct.
Dave - How are you going to replace these batteries?
Zhong - Power consumption is in the range of microwatts to milliwatts. That's a thousandth to a millionth of a watt, that's the power needed to drive this kind of device. This working environment is exposed to light, heat, and mechanical vibration, to all kinds of disturbance we experience every day. So can we convert those kinds of small energies to drive this?
Dave - I guess you're going to have to use different techniques to absorb different types of energy, so how are you going to go about it?
Zhong - Solar and thermal, the most popular today, will go for larger scale applications. But when you have tiny nano devices, these devices are not always under the sun. It could be implanted inside a biological system or hidden in a room in the dark. But there's always mechanical vibration. When I talk to you, my voice is a vibration. Can we convert this into electricity?
Dave - So how much power actually is that?
Zhong - The measure is different for each source. For example, you have ultrasonic waves, you have mechanical x-rays. For example when you drive a car, inside of the tyre there's a lot of mechanical disturbance. When you walk in your shoes, you're tapping your shoes back and forth, those are generating mechanical energy. Just let me give you an example: how much energy do each of us have? You're tapping your finger, there's milliwatts. You’re breathing, one watt. You're walking, 67 watts. If you can harvest a fraction of those watts, that's enough to power your present electronics.
Dave - So you're just powering them from energy which is being wasted all the time, normally it just gets turned into heat.
Zhong - Exactly.
Dave - Practically, how are you looking at harvesting this energy?
Zhong - Using nanowires, tiny nanowires, the diameter is about a 500th to a 1,000th of a hair’s width, the length is about a 10th of a hair width, and then we put this on a rod of substrate to convert a tiny physical motion into electricity. And we can generate 3 to 5 volts output today. What is 3 volts? Two double A batteries. What is the power? The power can reach microwatts to milliwatts. You'll say, “That's too small.” But for a lot of industrial applications, they're looking for that range.
Dave - So how do these nanowires actually generate electricity?
Zhong - These nanowires are not just any wire. They're a material called zinc oxide. It has a part called a piezoelectric part. So what is this? Piezoelectricity involves applying a force or pressure to produce a voltage inside a crystal. That voltage drives electricity to flow. That's the way you convert mechanical energy to electricity.
Dave - This is working on the same principle as the gas lighters, where you deform a crystal that produces the thousands of volts you need to produce a spark and then that spark lights your gas. But in this case, you're using tiny nanofibres of a similar material. When these are bent, they'll add together to produce a useful voltage and a useful current. So when you’ve perfected these nanofibres, where are we actually going to see them?
Zhong - This is a platform technology, just like you need the batteries, you need these things. I’ll give you a number of areas – first, environmental detection. Are there any toxic gases present that we can’t smell? You can make a self-powered sensor system that you can distribute around. So environmental monitoring, how do you track an animal, how do you track people, how do you track the product you shipped?
Dave - So with all of these, the major limiting factor is the batteries rather than the actual technologies to do the detecting or the tracking.
Zhong - If you dispose of a lot of batteries on the ground, the environmental damage that you will face in years to come will cost you a lot more. The other application is for medical purposes- implantable medical devices. Today, any materials used for batteries are biologically fairly toxic. And national security, if you have a thousand miles of border, how are you going to check those borders?
Dave - So if you can make detectors so cheap that you can just throw them away and they will keep running for years on their own, it’s very easy to build a huge sensor network.
Zhong - In the future, we’re going to make the sensor unit bean sized. We can distribute millions of these. Let’s say 30% or 50% of them fail, it doesn’t matter because as long as half of them work, they're all independent. Nano sensors work independently and self-sufficiently so we call them self-powered nanosystems. With self-powered systems, you don't need a battery. These work independently and wirelessly by themselves.
Diana - Zhong Lin Wang from the Georgia Institute of Technology, exploring new ways of using kinetic energy from gusts of wind or wasted energy inside moving tyres to power a wide range of nanotechnology.
Chris - There’s a video which Meera Senthilingam has made of that interview. You can find that online at nakedscientists.com/engineering.