by Gordy Slack
Professor Ali Javey and his UC Berkeley colleagues work at the intersection of chemistry, electrical engineering, materials science, and physics. They are masters of the small, finding new ways of joining microdevices on flexible, durable or otherwise exceptional substrates and thus opening up new worlds of applications.
The sensors plus substrates are called “skins,” Javey explains, because, like human skin, they cover and protect what is under them. But just what kind of work these skins do is up for grabs. Javey’s shorthand for what he does is XoY. “The skin concept is generic,” says Javey. “Essentially, you could put any kind of semiconductor or sensor in there. We just put x on y, when x is a functional device/sensor network and y is a synthetic substrate.”
What are new—and what it took a chemist like Javey to devise—are the methods of wedding those tiny devices to materials that can be wrapped around, say, an airplane’s fuselage, a robot’s body, or a prosthetic human arm.
Javey and his collaborators, working in Cory Hall and in the Marvell Nanofab next door, have developed ways of rolling and printing nanowires onto sticky, bendable sheets of film interlaced with sensors that can, for example, detect temperature, stress, pressure, or analyze air quality.
For one application, Javey has recently demonstrated a device for skin embedded with pressure sensors that could potentially allow prosthetics to restore a sense of feeling to the artificial limbs of amputees. Sensors are distributed throughout the material so that the patient could ‘feel’ how much pressure was applied and potentially the temperature of an object being touched. Such information would be important for something as simple as picking up a hot cup of coffee or naturally shaking someone’s hand.
Such pressure sensitive skins could revolutionize the ways prosthetics compensate for lost human parts, says Javey, but they could also help engineers to create more “sensitive” robots. While industrial robots are pretty good at moving multi-ton metal units across a warehouse, they have more difficulty with delicate maneuvers that require varying amounts of pressure. Certainly any domestic humanoid helper-bot would need a sense of touch to just to empty the dishwasher without breaking wineglasses. Remote tele-surgery, another CITRIS-related project, would also be improved by tools that could sense resistance and transmit that information to a surgeon working elsewhere.
Javey’s skins could be stretched to cover vehicles or structures to monitor both the integrity of the thing itself and the environment around it. “In the future, these could be stretched over airplane wings to monitor their structural integrity in real time, for example” he says. Employing such skins, the airlines could detect cracks or stress points and initiate repairs long before the defects were visible to the human eye.
Javey’s skins could also be applied to bridges and buildings, providing streams of useful real-time information about the integrity of key structural parts. During storms, or right after natural disasters, such as earthquakes, this type of information would be particularly useful.
In another vein altogether, but still applying the XoY methodology, Javey’s group is working on fusing another kind of semiconductor—photovoltaic cells—onto flexible sheets of aluminum foil that could be rolled onto roofs or other structures.
“Installation is now about half of the cost of setting up a solar energy system,” says Javey. “If you can unroll solar collectors onto your roof the way you install carpet, then the cost of a system would go down drastically.”
Lightweight, efficient, and easy-to-install solar components could lower the threshold for many solar energy applications, opening new markets and reducing C02 emissions at the same time.
There are other flexible photovoltaics on the market. Made with a single flat layer of a light-absorbing semiconductor such as amorphous silicon, however, they are not efficient at converting sunlight into energy. These new collectors use a film of vertically grown nanoconductor wires that trap light and optimize light absorption that can theoretically achieve efficiencies of 20% or more. Currently, the best amorphous Si collector has an efficiency of 10%.
The key for Javey, though, is affixing these nanoconductors directly to aluminum foil, which is robust, inexpensive, flexible, and easy to process. Such a skin-like material could be used to collect solar energy on buildings, vehicles, tools, and phones. “Anything that has a surface exposed to light,” he says.