How Self - Powered Nanotech Machines WorkNanosize machines need still tinier power plantsBy Zhong Lin Wang Editor ' s Note : This story was originally printed in the January 2008 issue of Scientific American . We ' re reposting it because of new research out today by author Zhong Lin Wang . The watchmaker in the 1920 s who devised the self - winding wristwatch was on to a great idea : mechanically harvesting energy from the wearer ’ s moving arm and putting it to work rewinding the watch spring . Today we are beginning to create extremely small energy harvesters that can supply electrical power to the tiny world of nanoscale devices , where things are measured in billionths of a meter . We call these power plants nanogenerators . The ability to make power on a minuscule scale allows us to think of implantable biosensors that can continuously monitor a patient ’ s blood glucose level , or autonomous strain sensors for structures such as bridges , or environmental sensors for detecting toxins — all running without the need for replacement batteries . Energy sources are desperately needed for nanorobotics , microelectromechanical systems ( MEMS ) , homeland security and even portable personal electronics . It is hard to imagine all the uses such infinitesimal generators may eventually find . Researchers are pursuing several different routes toward power generation on a miniature scale . Options include exploiting random vibrations or motions ( such as those near a roadway ) , temperature gradients ( for example , ground temperature is fairly constant several meters below the surface ) , biochemistry , and external energy sources such as ultrasonic waves or even audible noises . A key advantage of nanodevices and nano - systems is that they usually operate at a very low power level , in the range of nanowatts to microwatts , bringing nanogenerators for powering them into the realm of the possible . Just think of the potential power sources a human body provides : mechanical energy , heat energy , vibration energy , chemical energy ( in the form of glucose ) and the hydraulic energy of the circulatory system . Converted into electricity , just a small fraction of this energy could be sufficient to power many types of small devices . Power to the Tiny Work on power generation for small devices has moved quickly since the late 1990 s , when the proliferation of portable electronic gadgets attracted researchers to the problem of finding new ways to power them . Experimenters at the Massachusetts Institute of Technology ’ s Media Lab , for example , devised an energy - scavenging shoe using the piezoelectric effect , whereby certain crystalline materials produce a voltage when mechanically stressed . But the difficulty of producing useful amounts of power soon drove scientists to explore generators that could meet the much smaller electrical power needs of MEMS . These silicon - based devices , whose dimensions are measured in microns ( millionths of a meter ) to millimeters ( thousandths of a meter ) , have found many uses , including as accelerometers for automobile air - bag systems and as ink - jet printer nozzles . Biology and chemistry also offer opportunities for producing power . In recent years , scientists have built small vibration - based generators using both piezoelectric and electromagnetic transducers . The electro - magnetic microgenerator utilizes a moving magnet or coil for inducing an alternating electric current in a circuit . Although some microgenerators have been fabricated at the scale of MEMS , the technology tends to require structures ranging from one to 75 cubic centimeters , which work in vibration ranges from 50 hertz ( cycles per second ) to five kilohertz . A typical piezoelectric vibration - based generator uses a two - layered beam of lead zirconium titanate , with a mass located at its unsupported end , somewhat like a swimmer poised at the end of a diving board . When gravity causes the beam to bend downward , the upper piezoelectric layer is under tensile strain and the lower layer is under compressive strain . The result is a positive and negative voltage across the beam . As the mass oscillates back and forth , an alternating voltage is created . But because this energy generator is relatively large , gravity is important in driving its oscillating mass . Now my research group at the Georgia Institute of Technology is working on piezoelectric power generation at the nanoscale . And at the nanoscale , things change . Gravity , which plays such a critical role in the larger world , is a very minor actor in the nanoworld as compared with the forces of chemical bonding and intermolecular attraction . Where Gravity Doesn ’ t Matter The force of gravity is not available to us on a useful scale in the nanoworld . If one attempted to construct a piezoelectric generator with a nanometer - scale beam , gravity would make almost no contribution to sustaining the beam ’ s motion , and the device would not work . Therefore , we need another method to build a nano - size generator for powering autonomous devices . Our team has been exploring innovative nano - technologies for converting mechanical energy ( such as body movement and muscle stretching ) , vibration energy ( such as acoustic and ultrasonic waves ) and hydraulic energy ( such as the flow of blood and other bodily fluids ) into electric energy to power nanodevices . My research focused on carbon nanotubes in the late 1990 s . We invented a few techniques for measuring the mechanical , electrical and field - emission properties of individual carbon nano - tubes using in situ microscopy . But we could not control a nanotube ’ s electrical properties . I immediately realized that metal oxides are a new world — and why not explore those nanostruc - tures ? In 2000 I started with nanobelts , white woollike products made by baking a metal oxide such as zinc in the presence of argon gas at 900 to 1 , 200 degrees Celsius , and with nanowires . Our research has become focused on aligned zinc oxide nanowires , each of them a perfect six - sided , columnlike crystal that is grown on a solid conductive substrate using a standard vapor - liquid - solid process in a small tube furnace . We deposit gold nanoparticles , which serve as catalysts , on a sapphire substrate . An argon gas carrier flows through the furnace as a zinc oxide powder is heated . The nanowires then grow underneath the gold particles . The typical diameter of the nanowires is 30 to 100 nanometers , and they measure one to three microns in length . The idea of converting mechanical energy into electricity came to my mind around August 2005 , when we were measuring the electromechanical coupled properties of the wires . Using an atomic force microscope ( AFM ) , we observed some voltage output peaks , but we were not sure what they were . We did systematic work through November of that year and learned that the voltage was from the piezoelectric effect of the zinc oxide ; our results excluded the contribution from friction , contact or other confounding artifacts . The next step was determining what the process was for the charge output from a single nanowire . After studying a book on semiconductor devices , I proposed the working mechanism of what would become the nanogenerator . Zinc oxide has the rare attribute of possessing both piezoelectric and semiconducting properties , which we put to use in creating and accumulating piezoelectric charges in the nanowires . We have shown that when the conductive tip of an AFM bends a straight , vertical nanowire , a strain field is established , with the stretched surface showing positive strain and the compressed surface showing negative strain . As the tip scans over the top of the zinc oxide nanowires , we observe many peaks in the corresponding voltage output image for each contact position . The piezoelectric effect creates an electric field inside the nanowire ’ s volume , with the stretched and compressed sides of the wire showing positive and negative voltages . The idea came first , but we needed experimental support . Just before Christmas 2005 , I designed an experiment to visualize directly the voltage output of a large wire under optical and AFM microscopy . My student and I did the experiments , and one evening in late December , we were rewarded with several videos that directly proved my model . The next day I worked with Jinhui Song in my office to edit the movie . Then we sent the paper to Science for publication . To be useful in practical applications , our nanogenerator needs to contain an array of nano - wires , all of them continuously generating electricity that can be collected and delivered to a device . And the energy to be converted into electricity has to come in the form of a wave or vibration from the environment so the nano - generator can operate independently and wirelessly . We have developed a novel design that addresses these requirements . The next challenge was to increase the power of the nanogenerator . Three objectives have to be achieved : eliminate the use of the AFM , make many nanowires generate electricity simultaneously and continuously , and excite the nanowires in an indirect wave , such as an ultrasonic wave . I came out with a new design using a ridged electrode to replace the AFM tips and presented the idea to my postdoctoral assistant , Xudong Wang . It took him about four months of experiments before compiling the first group of data . The signal was rather small . From May to October 2006 we focused on the optimum packaging of the nanogenerator to enhance its output . By the end of the year we realized that the nanogenerator could at last be reported to the scientific community . Our experimental setup provided the first demonstration of continuous direct current produced by a piezoelectric nanogenerator . It consists of an array of parallel zinc oxide nanowires and a platinum - coated silicon electrode with a ridged surface in place of the microscope ’ s tip . Coating the electrode with platinum both enhances its conductivity and causes it to act like a diode that allows current to flow in only one direction , from metal to semiconductor . The electrode is placed above the nanowire array at a controlled distance and can be moved laterally so that it bends the nanowires from side to side . Thanks to its surface ridges , the electrode acts like an array of aligned microscope tips . Flexible Future Since January 2007 we have been fully involved in improving our nanogenerator . The ceramic or semiconductor substrates that we initially used for growing zinc oxide nanowires are hard and brittle , for instance , making them unsuitable for applications that require a foldable or flexible power source , such as biosensors implanted in muscles or joints , or power generators built into shoes . Here is where conductive polymers can provide a substrate that is likely to be biocompatible . In experiments we discovered that many available flexible plastic substrates are suitable for growing the zinc oxide nanowire arrays , which ultimately could find applications in portable and flexible electronics . Because of the flexibility of the substrate , the nanowire surface profile was wavy , causing some missed contacts . We believe that providing suitable bonding strength between the nanowires and substrate as well as optimizing the wire spacing will be important in increasing discharge efficiency . Although our approach has demonstrated the principle of the nanogenerator , we must drastically improve its performance to make it practical . All the nanowires must generate electricity simultaneously and continuously , and all the electricity must be effectively collected and distributed . A large - scale method for growing zinc oxide nanowires can be cost - effective because it does not require expensive high - temperature manufacturing processes . Hurdles that lie ahead in our research include learning how to grow perfectly uniform arrays of nanowires that all produce electricity and how to extend their working life . The lifetime of the current nano - generator is about 50 hours . The main reason for the device ’ s failure is likely the packaging technology for assembling the top electrode and the nanowire arrays . If the electrode presses on the nanowires too firmly , for example , no current will be generated . We are working hard to improve the packaging . The process used to produce the arrays involves evaporating a thin layer of gold onto the substrate , where it acts as a catalyst for nano - wire growth . The zinc oxide crystals look something like a forest without branches . To improve the adhesion of the nanowires with the substrate , we have added a thin layer of polymer onto the substrate after growth so that the roots of the nanowires are partially embedded . We have achieved electrical output of about 10 milli - volts and 800 nanoamperes from a nano - generator that is about six square millimeters in size . We have also shown that nanogenerators can be arranged in series to improve the output voltage and in parallel to improve the output current , as is commonly done with power sources such as batteries or fuel cells . But to produce higher voltages , we need to make nanowires with identical height and diameter . Nanogenerators may never power our homes or even our flashlights ; the amount of power available from them will be quite small . But nanowire arrays can be ideal generators for devices that need to work only intermittently , such as sensors that collect and transmit data for one second of every minute . In the years to come , nanogenerators will be used to harvest and recycle the energy wasted in our daily life , such as that created by pressure changes in a car tire , a moving vehicle ’ s mechanical vibration or even the fluttering surface of a camper ’ s tent . Consider how many small energy resources are all around us . ABOUT THE AUTHOR ( S ) Zhong Lin Wang is director of the Center for Nanostructure Characterization at the Georgia Institute of Technology . He discovered the nanobelt in 2000 and the world  s smallest balance , the nanobalance , in 1998 . Among his many honors , he received the 1999 Burton Medal of the Microscopy Society of America .