Piezo-Phototronic LEDs May Help Robots Feel

A new "piezo-phototronic" approach to the human-machine interface has been proposed by piezo-electric material expert Zhong Lin Wang, a professor at the Georgia Institute of Technology (Georgia Tech, Atlanta). First proposed by Wang as a new semiconductor-device category, his team has since produced prototypes that could act as artificial skin for robots to give them human-like touch sensitivity, for inkless fingerprint scanners, for biological imaging, and to enhance the performance of micro-electro-mechanical systems (MEMS).

The piezo-phototronic effect uses the charge polarization caused by deforming piezo-electric nanowires to enhance the charge transport and recombination of tiny light-emitting-diodes. The LEDs were formed by growing p-type zinc-oxide piezo-electric nanowires atop an n-type gallium nitride layer on an insulating sapphire substrate (see illustration). Every time the nanowire arrays feel strain from the touch of a finger -- human or robotic -- the p-n junction forming the LED glows with an intensity proportional to the applied pressure. That glowing image -- of an fingerprint, for instance -- can then be sensed with conventional optical techniques.

Imaging pressure with the piezo-phototronic effect arrays nanowire-LED sensors on a sapphire substrate (A) to enable a touch (B) to turn on zinc-oxide LEDs (bottom) in a character pattern.
(Source: Georgia Tech)

In his recent demonstration, Wang's team grew 2.7 micron diameter zinc-oxide nanowires into arrays that were able to sense touch with a resolution of 6,300 dots per inch and a switching time of 90 milliseconds. A low-temperature chemical growth technique patterned the nanowires atop a gallium nitride film with a thermoplastic providing support between nanowires. Patterned nickel-gold electrodes formed the ohmic contact with the gallium nitride film on the bottom while transparent indium-tin oxide (ITO) was deposited on top as the common electrode. For the future, the team plans to increase the resolution further with a higher-temperature nanowire growth process.

Professor Zhong Lin Wang of the Georgia Institute of Technology has developed a sensor that converts mechanical pressure directly into an optical image.
(Source: Georgia Tech)

Very interesting! Is there any indication of the degree of proportional response, i.e. the sensitivity of the measurement? The resolution and response time are certainly far beyond human touch capabilities. If the proportional response is in the same ballpark as the other specs there should be quite a few possibilities for this technology.

The authors claim that light output is directly proportional to the stress on the piezo-material--here zinc oxide, which is know to very sensitive. Also it will work with other piezo-electric materials, so you could probably design a material to meet most engineering needs.

The only thing that differentiate a human from robot is feeling and emotions. If robots can get that, we are making close to humans, but yes the the way robot would behave must be already programmed. Although humans may behave differently in same situations.

Looking at how robots are being visualized and used in many different applications, it is worthwhile to have them feel.

Not being a specialist in biology, I'm curious about how these specs stack up to human sensitivity: "Wang's team grew 2.7 micron diameter zinc-oxide nanowires into arrays that were able to sense touch with a resolution of 6,300 dots per inch and a switching time of 90 milliseconds." Anyone?

The authors claim their prototype offers sensitivity "comparable" to humans plus that next they are improving them by growing even smaller nanowires. However, the big boon for robotics will be their ability to assemble things they can't today. In particular, robots are not good at putting in screws, because their fingers cannot sense when the threads are properly lined up. Robotic skin with built-in touch sensors like these could solve that problem.

I'm no expert on the biology, but 6,300 dpi and 90 millisec are far beyond the specs that I would expect to see if they did the same tests on me. On the other hand, maybe I'm just insensitive and a little on the slow side... :-)

It seems like this could also go beyond robotics. There are other places where a touch-sensitive surface can be used for input. A touchpad for a laptop that could distinguish a user by fingerprints, for example.

Right on, Larry. Sensitivity already rivals humans and is only getting better. You are also spot-on regarding applications--robots is just a headline grabber, but any touch enabled application today is a candidate.

Very interesting reading. If this is a big success, robots could be in stores quicker than I imagined. I still believe that within the next few years a customer will be able to go to a store and buy a robot just like you purchase a computer or other devices. The cost of the robot will be based on the memory chip, the robots artificial real-like skin that can sense touch, and the robot features. My only recommendation is that it should be sold in a specialty store and only qualified workers who are trained and know the advantage and disadvantage of what a robot can do. Some type of certification and training should be required. Yes, a Robot Certification. OK engineering executives, I just gave you a wonderful idea.

Yes, there will definitely need to be user training classes included with general purpose robots, but we will probably see turnkey special purpose robots long before that (like a "cooking" robot whose arms extend from the top of your stove) where just pushing a few buttons will control its behaviors.

I don't think we'll be able to consider robots to truly be mainstream until they're easy enough to use without a training class. It really needs to be an appliance.

Of course, computers are mainstream and are still challenging enough to a lot of people that training classes would be a big help. But the number of people who operate them without training would indicate that most people would not bother with training for a robot. I think the only reason people get training in cars before getting a license is because the government requires it.

You may be right that training should not be necessary. In fact, one trend to take robots mainstream is called "co-robots"--which are supposed to accept instruction from an untrained human who just demonstrates the task while the robot watches. That is one goal, at least, of Obama's National Robotics Initiative:

All of the current robot makers have training programs for operators, although most don't call it certification (for instance, DaVinci surgical robots go to lengths to say its not certification--but I think that is to deflect lawsuits :)