Researchers create a self-healing, transparent, highly stretchable material that can be electrically activated and used to improve batteries, electronic devices, and robots
Scientists, including several from the University of California, Riverside, have developed a transparent, self-healing, highly stretchable conductive material that can be electrically activated to power artificial muscles and could be used to improve batteries, electronic devices, and robots.
The findings, which were published today in the journal Advanced Material, represent the first time scientists have created an ionic conductor, meaning materials that ions can flow through, that is transparent, mechanically stretchable, and self-healing.
The material has potential applications in a wide range of fields. It could give robots the ability to self-heal after mechanical failure; extend the lifetime of lithium ion batteries used in electronics and electric cars; and improve biosensors used in the medical field and environmental monitoring.
“Creating a material with all these properties has been a puzzle for years,” said Chao Wang, an adjunct assistant professor of chemistry who is one of the authors of the paper. “We did that and now are just beginning to explore the applications.”
This project brings together the research areas of self-healing materials and ionic conductors.
Inspired by wound healing in nature, self-healing materials repair damage caused by wear and extend the lifetime, and lower the cost, of materials and devices. Wang developed an interest in self-healing materials because of his lifelong love of Wolverine, the comic book character who has the ability to self-heal.
Ionic conductors are a class of materials with key roles in energy storage, solar energy conversion, sensors, and electronic devices.
Another author of the paper, Christoph Keplinger, an assistant professor at the University of Colorado, Boulder, previously demonstrated that stretchable, transparent, ionic conductors can be used to power artificial muscles and to create transparent loudspeakers – devices that feature several of the key properties of the new material (transparency, high stretchability and ionic conductivity) – but none of these devices additionally had the ability to self-heal from mechanical damage.
The key difficulty is the identification of bonds that are stable and reversible under electrochemical conditions. Conventionally, self-healing polymers make use of non-covalent bonds, which creates a problem because those bonds are affected by electrochemical reactions that degrade the performance of the materials.
Wang helped solve that problem by using a mechanism called ion-dipole interactions, which are forces between charged ions and polar molecules that are highly stabile under electrochemical conditions. He combined a polar, stretchable polymer with a mobile, high-ionic-strength salt to create the material with the properties the researchers were seeking.
The low-cost, easy to produce soft rubber-like material can stretch 50 times its original length. After being cut, it can completely re-attach, or heal, in 24 hours at room temperature. In fact, after only five minutes of healing the material can be stretched two times its original length.
Timothy Morrissey and Eric Acome, two graduate students working with Keplinger, demonstrated that the material could be used to power a so-called artificial muscle, also called dielectric elastomer actuator. Artificial muscle is a generic term used for materials or devices that can reversibly contract, expand, or rotate due to an external stimulus such as voltage, current, pressure or temperature.
The dielectric elastomer actuator is actually three individual pieces of polymer that are stacked together. The top and bottom layers are the new material developed at UC Riverside, which is able to conduct electricity and is self-healable, and the middle layer is a transparent, non-conductive rubber-like membrane.
The researchers used electrical signals to get the artificial muscle to move. So, just like how a human muscle (such as a bicep) moves when the brain sends a signal to the arm, the artificial muscle also reacts when it receives a signal. Most importantly, the researchers were able to demonstrate that the ability of the new material to self-heal can be used to mimic a preeminent survival feature of nature: wound-healing. After parts of the artificial muscle were cut into two separate pieces, the material healed without relying on external stimuli, and the artificial muscle returned to the same level of performance as before being cut.
Learn more: A Wolverine Inspired Material
Charles McLaren, a Ph.D. candidate in materials science and engineering at Lehigh, arrived last fall for a semester of research at the University of Marburg in Germany with his language skills lagging significantly behind his scientific prowess.
“It was my first trip to Germany, and I barely spoke a word of German,” he confessed.
With the help of his new German colleagues, he got past the point-and-eat phase of the international experience in no time. “The group members there were very welcoming. They showed me around and helped me learn enough vocabulary to order some food, at least.”
The main purpose of McLaren’s exchange study in Marburg was far from culinary, however. He was there to learn more about a complex process involving transformations in glass that occur under intense electrical and thermal conditions. New understanding of these mechanisms could lead the way to more energy-efficient glass manufacturing, and even glass supercapacitors that leapfrog the performance of batteries now used for electric cars and solar energy.
“This technology is relevant to companies seeking the next wave of portable, reliable energy,” said Himanshu Jain, the T. L. Diamond Distinguished Chair in Materials Science and Engineering at Lehigh and director of its International Materials Institute for New Functionality in Glass.
“A breakthrough in the use of glass for power storage could unleash a torrent of innovation in the transportation and energy sectors, and even support efforts to curb global warming.”
In his doctoral research, McLaren discovered that applying a direct current field across glass reduced its melting temperature. In lab experiments, he and Jain placed a block of glass between a cathode and anode, and then exerted steady pressure on the glass while gradually heating it. Together with colleagues at the University of Colorado, the Lehigh researchers reported their results last fall in Applied Physics Letters.
The implications for the finding were intriguing. In addition to making glass formulation possible at lower temperatures and reducing energy needs, designers using electrical current in glass manufacturing would have a tool to make precise manipulations not possible with heat alone.
“You could make a mask for the glass, for example, and apply an electrical field on a micron scale,” said Jain. “This would allow you to deform the glass with high precision, and soften it in a far more selective way than you could with heat, which gets distributed throughout the glass.”
Though McLaren and Jain had isolated the phenomenon and determined how to dial up the variables for optimal results, they did not yet fully understand the mechanisms behind it. McLaren and Jain had been following the work of Bernard Roling at the University of Marburg, who had discovered some remarkable characteristics of glass using electro-thermal poling, a technique that employs both temperature manipulation and electrical current to create a charge in normally inert glass. The process imparts useful optical and even bioactive qualities to glass.
Roling invited McLaren to spend a semester at Marburg to analyze the behavior of glass under electro-thermal poling, to see if it would reveal more about the fundamental science underlying what McLaren and Jain had observed in their Lehigh lab.
A high-speed avalanche
McLaren’s work in Marburg revealed a two-step process in which a thin sliver of the glass nearest the anode, called a depletion layer, becomes much more resistant to electrical current than the rest of the glass as alkali ions in the glass migrate away. This is followed by a catastrophic change in the layer, known as dielectric breakdown, which dramatically increases its conductivity. McLaren likens the process of dielectric breakdown to a high-speed avalanche, and uses spectroscopic analysis with electro-thermal poling as a way to see what is happening in slow motion.
“The results in Germany gave us a very good model for what is going on in the electric field-induced softening that we did here. It told us about the start conditions for where dielectric breakdown can begin,” said McLaren.
“Charlie’s work in Marburg has helped us see the kinetics of the process,” Jain said. “We could see it happening abruptly in our experiments here at Lehigh, but we now have a way to separate out what occurs specifically with the depletion layer.”
Learn more: NEW CURRENTS IN GLASS STUDIES
Visible lasers offer exquisite control of x-ray light in a tabletop apparatus, potentially providing access to new insights to chemical reactions, proteins, and even atoms’ inner workings.
By crossing two counter-rotating ultrafast laser beams in a gas target, scientists controlled the direction and polarization of laser-like beams in the extreme ultraviolet and soft x-ray portions of the spectrum. This represents a new ability to manipulate x-ray light using visible light, and obviates the need for inefficient and expensive optics that other approaches must use to filter and steer such beams.
This source enables, for example, tabletop measurements of dynamics in novel magnetic materials occurring on the fastest time scales. It also allows scientists to study chiral molecules, such as proteins or DNA, that come in left- and right-handed versions. Furthermore, the method used to generate the beams provides a path to generating isolated attosecond (one quintillionth of a second) pulses of light with circular polarization.
Researchers at JILA, a joint research institute of the University of Colorado and the National Institute of Standards and Technology, have developed a method to produce ultrafast pulses of circularly polarized extreme ultraviolet (EUV) light in a tabletop setup. The approach uses high-harmonic generation (HHG) driven by ultrafast laser pulses. In this process, laser pulses rip electrons from atoms, accelerate the electrons to high energy, and smash them back into the parent ion to generate pulses of extreme ultraviolet light at harmonics of the driving laser frequency. Specifically, the researchers developed a new experimental configuration in collaboration with the Colorado School of Mines, in which the HHG process is driven by two ultrafast laser beams of opposite circular polarization that are crossed in a gas sample. This novel HHG geometry simultaneously generates left- and right-circularly polarized EUV beams at each of the emitted harmonic wavelengths. This approach can be implemented on a laboratory tabletop, and the EUV beams of different helicity and harmonic order are physically separated from each other as well as from the driving lasers. The angular separation of the EUV beams eliminates the need for expensive filters, mirrors, or gratings that otherwise would attenuate and temporally broaden the pulse. This flexible arrangement allows researchers to make measurements at a particular wavelength and polarization by simply placing a sample into the isolated beam path. The researchers demonstrated the practical use of this new light source by measuring the magnetic circular dichroism of a 20-nm iron film. Furthermore, numerical simulations demonstrate that this phase-matching configuration makes possible the generation of isolated attosecond pulses with circular polarization. Before this discovery, there were no experimentally realized methods for generating isolated circularly polarized high harmonics.