Do as want is ultimate judgment for any designed device. We can do very well for the devices with larger dimensions, but it's difficult for those of nano. Especially in the case where fine manipulation needs to do, the fabrication as well as device's behaving are all at loose ends. That's why the preferred designs for most of current nanodevices happen to coincide with self-sustaining. Roughly said, this preference is partly because small size really hinders us to access effective manipulation. More challenging come in biological cases where the biosafety is the first concern. Obviously, let it be is not a good answer for application, while fundamental science also needs to continues to strengthen why it should be to be, but where is the alternative route to this inextricable preference? 

For example, how can we power an implantable biosensor embedded in live body? One direct solution is using chemical cell but of much smaller size. However, possibly leaking of heavy metals or electrolyte used in the cell really cast great hesitations, let alone the durability of the cell.

There is a solution. As WANG Zhonglin and SONG Jinhui suggested last year, ZnO nanowires with strong piezoelectricity can be a substitute [ 1 ]. They found that releasing a ZnO nanowire bended with a conductive atomic force microscopy (AFM) tip can produce electricity with an energy-transfer-efficiency of 17 to 30%, which means a nanogenerator can be made to harvest mechanical energy from body environment. That's just a prototype of the piezoelectric ZnO nanogenerator because an AFM tip was used as the mechanical driver and the power collector. 

After one year's work, Wang's group finally got a better design of zigzag electrode right beside truly application, reported in the 6 April 2007 issue of Science [ 2 ]. The new nanogenerator can be switched on and powered by an ultrasonic wave (operation frequency: ~ 41 kHz).

As said as the authors', "an array of aligned ZnO nanowires was covered by a zigzag, (001) orientated Si electrode coated with 200 nm thick Pt film. The Pt coating not only enhanced the conductivity of the electrode, but also created a Schottky contact at the interface with ZnO. The ZnO nanowires were grown on either GaN substrates or sapphire substrates that were covered by a thin layer of ZnO film, which served as a common electrode for directly connecting the nanowires with an external circuit. The density of the nanowires was 10/μm^2, and the height and diameter were 1.0 µm and 40 nm, respectively. The top electrode was composed of parallel zigzag trenches fabricated on a Si wafer and coated with a thin layer of Pt (200 nm in thickness) ."

With the ultrasonic wave being turned on and off regularly, The current jump is about 0.15 nA. The resistance of the entire nanogenerator was very stable during these two periods (R = 3.560 ± 0.005 kΩ). This stability indicates that the current jump could not be due to the variation in resistance, as caused by the vibration of the nanowires, suggesting that the obtained current signal was created by the nanogenerator.

At present case, the power output of single nanowire is far below than that of the wire driven by an AFM tip, due to the difference of the bended amplitude. However, there is a group army of ZnO nanowires now, the power output simultaneously harvested from the group nanowires is much intense.

At first glance, it is surely a good mechanical nanocell. However, I would like to emphasize that, the most important potential for this kind of technology is its great possibility of remote controlling of the body-implanted biosensor or nanomachine, by the ultrasonic wave that is used commonly in today's therapeutics. What the fascinating meanings of the "controlling" here include, 1) activating or powering a resting sensor or machine directly once you want; 2) recharging an exhausted battery within a heart pacemaker!

So please note to use Zhonglin's nanogenerator as a nano-charger or trigger for therapeutic nanomachine. It's a splendid direction really deserved your attention. □ 

* Lin Pu is in the Physics Department of Nanjing University, Nanjing 210093, CHINA.