A team of scientists has developed a transparent, self-healing, highly stretchable conductive material that can be electrically activated to power artificial muscles or used to improve batteries, electronic devices, and robots.
The findings, published Dec. 23 in the journal Advanced Materials, combine the fields of self-healing materials and ionic conductors (a material that ions can flow through). Ionic conductors are a class of materials with key roles in energy storage, solar energy conversion, sensors, and electronic devices.
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, the scientists say.
Inspired by wound healing in nature, the idea for self-healing materials is to repair damage caused by wear, lower the cost of materials and devices, and extend their lifetime.
“Creating a material with all these properties has been a puzzle for years,” said Chao Wang*, a University of California, Riverside adjunct assistant professor of chemistry who is one of the authors of a paper published in December in Advanced Materials.
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).
Limitations of current self-healing polymers
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 stable under electrochemical conditions. He combined a polar, stretchable polymer with a mobile, high-ionic-strength salt to create the material with the desired properties.
The soft rubber-like material they created is low-cost, easy to produce, and can stretch 50 times its original length. After being cut, it can completely re-attach, or heal, in 24 hours at room temperature. (After only five minutes of healing, the material can be stretched two times its original length.)
The scientists demonstrated that the material could be used to power an artificial muscle, or 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.
* 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.Abstract of A Transparent, Self-Healing, Highly Stretchable Ionic Conductor
Self-healing materials can repair damage caused by mechanical wear, thereby extending lifetime of devices. Here, a transparent, self-healing, highly stretchable ionic conductor is presented that autonomously heals after experiencing severe mechanical damage. The design of this self-healing polymer uses ion–dipole interactions as the dynamic motif. The unique properties of this material when used to electrically activate transparent artificial muscles are demonstrated.
By suspending tiny metal nanoparticles in liquids, Duke University scientists can use conductive ink-jet-printed conductive “inks” to print inexpensive, customizable RFID and other electronic circuit patterns on just about any surface — even on paper and plastics.
Printed electronics, which are already being used widely in devices such as the anti-theft radio frequency identification (RFID) tags you might find on the back of new DVDs, currently have one major drawback: for the circuits to work, they first have to be heated to 200° C (392°F) to melt all the nanoparticles together into a single conductive wire.
But Duke researchers have now found that electrons are conducted through films made of silver nanowires much more easily than with films made from other shapes (like nanospheres or microflakes). And the nanowire films can now function in printed circuits without the need to melt them first — heating at only 70° C (158°F) is required — and that means the circuits can be printed on cheaper plastics or paper.
“The nanowires [in their research] had a 4,000 times higher conductivity than the more commonly used silver nanoparticles that you would find in printed antennas for RFID tags,” said Benjamin Wiley, assistant professor of chemistry at Duke.
The technology could also be used to make lower-cost solar cells, printed displays, LEDs, touchscreens, amplifiers, batteries, and even some implantable bio-electronic devices. The results appeared online Dec. 16 in ACS Applied Materials and Interfaces.
The team is now experimenting with using aerosol jets to print silver nanowire inks in usable circuits. Wiley says they also want to explore whether silver-coated copper nanowires, which are significantly cheaper to produce than pure silver nanowires, will give the same effect.
This research was supported by funding from the National Science Foundation and a GAANN Fellowship through the Duke Chemistry Department.
Abstract of Effect of Morphology on the Electrical Resistivity of Silver Nanostructure Films
The relatively high temperatures (>200 °C) required to sinter silver nanoparticle inks have limited the development of printed electronic devices on low-cost, heat-sensitive paper and plastic substrates. This article explores the change in morphology and resistivity that occurs upon heating thick films of silver nanowires (of two different lengths; Ag NWs), nanoparticles (Ag NPs), and microflakes (Ag MFs) at temperatures between 70 and 400 °C. After heating at 70 °C, films of long Ag NWs exhibited a resistivity of 1.8 × 10–5 Ω cm, 4000 times more conductive than films made from Ag NPs. This result indicates the resistivity of thick films of silver nanostructures is dominated by the contact resistance between particles before sintering. After sintering at 300 °C, the resistivity of short Ag NWs, long Ag NWs, and Ag NPs converge to a value of (2–3) × 10–5 Ω cm, while films of Ag MFs remain ∼10× less conductive (4.06 × 10–4 Ω cm). Thus, films of long Ag NW films heated at 70 °C are more conductive than Ag NP films sintered at 300 °C. Adding 10 wt % nanowires to a film of nanoparticles results in a 400-fold improvement in resistivity.
MIT scientists said today they’ve just created one the strongest materials known (ten times stronger than steel, but also one of the lightest, with a density of just 5 percent of that of steel) by compressing and fusing flakes of graphene, a two-dimensional form of carbon.
In its two-dimensional form, graphene is thought to be the strongest of all known materials. But researchers until now have had a hard time translating that two-dimensional strength into useful three-dimensional materials.
It’s all about the geometrical configuration
But it’s not about the material itself; it’s about their unusual 3-D geometrical configuration, the researchers discovered. That suggests that similar strong, lightweight materials (in addition to graphene) could be made from a variety of materials by creating similar geometric features.
“You can replace the material itself with anything,” says Markus Buehler, the head of MIT’s Department of Civil and Environmental Engineering (CEE) and the McAfee Professor of Engineering. “The geometry is the dominant factor. It’s something that has the potential to transfer to many things.”
The findings were reported today an open-access paper in the journal Science Advances.
By analyzing the material’s behavior down to the level of individual atoms within the structure, the engineers were able to produce a mathematical framework that very closely matches experimental observations.
Two-dimensional materials — basically flat sheets that are just one atom in thickness but can be indefinitely large in the other dimensions — have exceptional strength as well as unique electrical properties. But because of their extraordinary thinness, “they are not very useful for making 3-D materials that could be used in vehicles, buildings, or devices,” says Buehler. “What we’ve done is to realize the wish of translating these 2-D materials into three-dimensional structures.”
The trick: heat + pressure
The solution for compressing small flakes of graphene turned out to be a combination of heat and pressure. This process produced a strong, stable structure whose form resembles that of some corals and microscopic creatures called diatoms. These new shapes, which have an enormous surface area in proportion to their volume, proved to be remarkably strong.
Buehler says the process resembles what would happen with sheets of paper. Paper has little strength along its length and width, and can be easily crumpled up. But when made into certain shapes, for example rolled into a tube, suddenly the strength along the length of the tube is much greater and can support substantial weight. Similarly, the geometric arrangement of the graphene flakes after treatment naturally forms a very strong configuration.
Melanie Gonick/MIT | One of the strongest, lightweight materials known
But many other possible applications of the material could eventually be feasible, the researchers say, for uses that require a combination of extreme strength and light weight. “You could either use the real graphene material or use the geometry we discovered with other materials, like polymers or metals,” Buehler says, to gain similar advantages of strength combined with advantages in cost, processing methods, or other material properties (such as transparency or electrical conductivity).
The unusual geometric shapes that graphene naturally forms under heat and pressure look something like a Nerf ball — round, but full of holes. These shapes, known as “gyroids,” are so complex that “actually making them using conventional manufacturing methods is probably impossible,” Buehler says.
The team used 3-D-printed models of the structure, enlarged to thousands of times their natural size, for testing purposes. For actual synthesis, the researchers say, one possibility is to use the polymer or metal particles as templates, coat them with graphene by chemical vapor deposit before heat and pressure treatments, and then chemically or physically remove the polymer or metal phases to leave 3-D graphene in the gyroid form.
The same geometry could even be applied to large-scale structural materials, they suggest. For example, concrete for a structure such a bridge might be made with this porous geometry, providing comparable strength with a fraction of the weight. This approach would have the additional benefit of providing good insulation because of the large amount of enclosed airspace within it.
Because the shape is riddled with very tiny pore spaces, the material might also find application in some filtration systems, for either water or chemical processing. The mathematical descriptions derived by this group could facilitate the development of a variety of applications, the researchers say.
The research was supported by the Office of Naval Research, the Department of Defense Multidisciplinary University Research Initiative, and BASF-North American Center for Research on Advanced Materials.