stretchable e-skin changes color when touched

The chameleon-inspired e-skin’s two main components are a pressure-sensitive polymer (“pyramid layer”) and an electrochromic polymer. At bottom left, the two chemical structures of the electrochromic polymer are shown that emit red and blue light, respectively. (Ambanja panther chameleon and hand images from 123rf.com) Credit: Chou, et al. ©2015 Macmillan Publishers Limited.

Scientia — Researchers at Stanford University have fabricated a stretchable, color-changing, pressure-sensitive material–basically the closest thing yet to an artificial chameleon skin. Touching the new electronic skin (e-skin) with varying amounts of pressure causes it to change colors, as the pressure indirectly alters the chemical structure, and subsequently the optical properties, of the “electrochromic” material. The e-skin could have applications in interactive wearable devices, artificial prosthetics, and smart robots.

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Previously, similar materials have been fabricated that can change color, and a few of these are even touch-sensitive, but so far none has also been stretchable. The new e-skin combines all three of these properties for the first time.

“We show an all-solution processed chameleon-inspired stretchable electronic skin (e-skin), in which the e-skin color can easily be controlled through varying the applied pressure along with the applied pressure duration,” Ho-Hsiu Chou of Stanford University, who is first author of the study, told Phys.org. “As such, the e-skin’s color change can also in turn be utilized to distinguish the pressure applied.”

The e-skin consists of two main components: a stretchable microstructured polymer that can modify its voltage upon an applied pressure, and a stretchable electrochromic polymer that can be either red or blue, depending on the applied voltage.

The researchers demonstrated how the e-skin works by using an item not commonly found in most engineering labs: a teddy bear. They attached the pressure-sensitive polymer to the bear’s paw, and connected it to the electrochromic polymer which they mounted on the bear’s abdomen. The electrochromic polymer first appears red, but after giving the bear a weak handshake (about 50 kilopascals [kPa] of pressure) it turns blue-gray. Once the handshake is removed, the polymer again turns red, but a stronger handshake (about 200 kPa) causes it to turn pale blue.

What’s happening, as the researchers explain, is a multi-step process that ultimately changes the chemical structure of the electrochromic polymer into a different chemical structure that emits light at a different wavelength, or color. Pressure from the handshake causes a drop in the electrical resistance of the pressure-sensitive polymer (which is connected to a low-voltage power supply) by up to several orders of magnitude. The drop in resistance increases the voltage to the electrochromic polymer and oxidizes the material, slightly altering its chemical structure. Although the structural change is small, it causes a large change in the material’s light absorption spectrum, which can be quickly reversed by releasing the pressure.

When attached to a teddy bear, the e-skin changes color upon a handshake, since the pressure-sensitive polymer is attached to the paw. Credit: Chou, et al. ©2015 Macmillan Publishers Limited.

While the electrochromic polymer used here can only switch between shades of red and blue, the researchers expect that other electrochromic polymers can be designed to exhibit a wide range of colors that can be modulated by various pressures. This could lead to a wide variety of applications.

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“The e-skin can potentially be integrated into the things that we wear and carry, i.e., clothes, smart phones, smart watches, and any other kind of wearable devices,” Chou said. “By integrating with this color-changeable e-skin, you can imagine that all the colors can be integrated into one device, and the user can change it interactively for decoration or to express emotion.

“Because the e-skin’s color change can also be in turn utilized to distinguish and quantify the magnitude of pressure we applied, the other potential application is that we can integrate the system into any surface where we want to know the magnitude of pressure applied on it. Also, the e-skin can provide the camouflage function for prosthetics and smart robots. In addition, the stretchable system allows it to attach on curvilinear or dynamic surfaces well, while conventional rigid devices cannot. This advantage can reduce the interface between the device and human body.”

As the e-skin has the potential to find use in a wide variety of consumer applications, the researchers addressed the fact that the material contains carbon nanotubes, which have raised concerns about toxicity and carcinogenicity because they have similar shapes as asbestos. Here, the carbon nanotubes are sprayed onto the pressure-sensitive polymer and play an important role in controlling the resistance response to pressure. Fortunately, the carbon nanotubes used here are relatively short and small in diameter, whereas studies have shown that longer and thicker carbon nanotubes induce more DNA damage and inflammation. Still, the researchers recommend that the system be encapsulated in stretchable materials, such as silicone, as a safety precaution.

“Another important target is to make the whole system biodegradable since the device is applied on the human,” Chou said.

Freedawn Scientia –

More information: Ho-Hsiu Chou, et al. “A chameleon-inspired stretchable electronic skin with interactive colour changing controlled by tactile sensing.” Nature Communications. DOI: 10.1038/ncomms9011

Journal reference: Nature Communications

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stretchable e-skin changes color when touched

Unlocking nanofibers’ potential

Prototype boosts production of versatile fibers fourfold, while cutting energy consumption by 92 percent.

Scientia — Nanofibers, polymer filaments only a couple of hundred nanometers in diameter — have a huge range of potential applications, from solar cells to water filtration to fuel cells. But so far, their high cost of manufacture has relegated them to just a few niche industries.

A scanning electron micrograph of the new microfiber emitters, showing the arrays of rectangular columns etched into their sides.
Courtesy of the researchers

In the latest issue of the journal Nanotechnology, MIT researchers describe a new technique for producing nanofibers that increases the rate of production fourfold while reducing energy consumption by more than 90 percent, holding out the prospect of cheap, efficient nanofiber production.

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“We have demonstrated a systematic way to produce nanofibers through electrospinning that surpasses the state of the art,” says Luis Fernando Velásquez-García, a principal research scientist in MIT’s Microsystems Technology Laboratories, who led the new work. “But the way that it’s done opens a very interesting possibility. Our group and many other groups are working to push 3-D printing further, to make it possible to print components that transduce, that actuate, that exchange energy between different domains, like solar to electrical or mechanical. We have something that naturally fits into that picture. We have an array of emitters that can be thought of as a dot-matrix printer, where you would be able to individually control each emitter to print deposits of nanofibers.”

Tangled tale

Nanofibers are useful for any application that benefits from a high ratio of surface area to volume — solar cells, for instance, which try to maximize exposure to sunlight, or fuel cell electrodes, which catalyze reactions at their surfaces. Nanofibers can also yield materials that are permeable only at very small scales, like water filters, or that are remarkably tough for their weight, like body armor.

The standard technique for manufacturing nanofibers is called electrospinning, and it comes in two varieties. In the first, a polymer solution is pumped through a small nozzle, and then a strong electric field stretches it out. The process is slow, however, and the number of nozzles per unit area is limited by the size of the pump hydraulics.

The other approach is to apply a voltage between a rotating drum covered by metal cones and a collector electrode. The cones are dipped in a polymer solution, and the electric field causes the solution to travel to the top of the cones, where it’s emitted toward the electrode as a fiber. That approach is erratic, however, and produces fibers of uneven lengths; it also requires voltages as high as 100,000 volts.

Thinking small

Velásquez-García and his co-authors — Philip Ponce de Leon, a former master’s student in mechanical engineering; Frances Hill, a former postdoc in Velásquez-García’s group who’s now at KLA-Tencor; and Eric Heubel, a current postdoc — adapt the second approach, but on a much smaller scale, using techniques common in the manufacture of microelectromechanical systems to produce dense arrays of tiny emitters. The emitters’ small size reduces the voltage necessary to drive them and allows more of them to be packed together, increasing production rate.

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At the same time, a nubbly texture etched into the emitters’ sides regulates the rate at which fluid flows toward their tips, yielding uniform fibers even at high manufacturing rates. “We did all kinds of experiments, and all of them show that the emission is uniform,” Velásquez-García says.

To build their emitters, Velásquez-García and his colleagues use a technique called deep reactive-ion etching. On either face of a silicon wafer, they etch dense arrays of tiny rectangular columns — tens of micrometers across — which will regulate the flow of fluid up the sides of the emitters. Then they cut sawtooth patterns out of the wafer. The sawteeth are mounted vertically, and their bases are immersed in a solution of deionized water, ethanol, and a dissolved polymer.

When an electrode is mounted opposite the sawteeth and a voltage applied between them, the water-ethanol mixture streams upward, dragging chains of polymer with it. The water and ethanol quickly dissolve, leaving a tangle of polymer filaments opposite each emitter, on the electrode.

The researchers were able to pack 225 emitters, several millimeters long, on a square chip about 35 millimeters on a side. At the relatively low voltage of 8,000 volts, that device yielded four times as much fiber per unit area as the best commercial electrospinning devices.

The work is “an elegant and creative way of demonstrating the strong capability of traditional MEMS [microelectromechanical-systems] fabrication processes toward parallel nanomanufacturing,” says Reza Ghodssi, a professor of electrical engineering at the University of Maryland. Relative to other approaches, he adds, there is “an increased potential to scale it up while maintaining the integrity and accuracy by which the processing method is applied.”

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– Credit and Resource –

Larry Hardesty | MIT News Office

Unlocking nanofibers’ potential