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Graphene and silly putty make for a strain sensor

Graphene and Silly Putty Creates a Super-Sensitive Strain Sensor

For all the talk and research that has gone into exploiting graphene’s pliant properties for use in wearable and flexible electronics, most of the polymer composites it has been mixed with to date have been on the hard and inflexible side.

It took a team of researchers in Ireland to combine graphene with the children’s toy Silly Putty to set the nanomaterial community ablaze with excitement. The combination makes a new composite that promises to make a super-sensitive strain sensor with potential medical diagnostic applications.

In research described in the journal Science, scientists at AMBER, the Science Foundation Ireland-funded materials science research center based at Trinity College Dublin, discovered that if you added nanosheets into a low-viscosity material like silly putty, its electromechanical properties dramatically changed. You suddenly have an extremely sensitive strain sensor.

When you apply a voltage to the graphene-infused silly putty, the slightest touch results in a very large change in the current. Voila a strain sensor. 

“If you take the silly putty and stretch it just by one percent, then the current would change by a factor of five: that’s a very small mechanical change with a very big electrical change,” says Jonathan Coleman, the professor at Trinity College Dublin, who led the research.

In tests with the graphene-enabled putty, the researchers placed the composite onto the people’s chests and necks to measure breathing, pulse, and blood pressure. The results demonstrated an unprecedented sensitivity for a strain and pressure sensor, hundreds of times more sensitive than other sensors.

“What we found is that when you put graphene in extremely soft polymers like this, they act as strain sensors that are light years ahead of anything that had been created before,” says Coleman. “And this is intimately linked with the fact that these polymers are so soft.”

What is surprising about this line of research is that there has been so few investigations previously looking at combining graphene with a low-viscosity polymer. Graphene has been put in many different polymers to make composites,  but the vast majority of those polymers are at the harder end of the spectrum. This practice is typically driven by the aim of adding graphene to a strong, hard polymer to make it even stronger and harder.

While there has been some work in putting graphene into soft polymers, no one has put graphene into polymers anywhere near as soft as silly putty. The softest polymer that people had put graphene in previously would be something like rubber, according to Coleman.

Just as there has been a lot of work into adding graphene to polymers, the use of graphene to make a polymer a strain sensor has some history as well. But no one was quite expecting that these softer materials would enhance the effect so profoundly.

“We knew that there had been very little work done in this area,” said Coleman.  “But I have to be completely honest, we didn’t know that the material would have the interesting properties that we saw in the end result.”

While Coleman believes that there is a wide range of medical sensing applications for the material, by far the most clear-cut and important example would be the continuous measurement of pulse and blood pressure.

“We can measure pulse in a relatively straightforward way,” he says. “There are a number of ways to do it. But we can measure blood pressure simply, cheaply and continuously.”

You can see a demonstration of this blood pressure monitoring in the video below:

Coleman is pretty confident that the path to commercialization for this technology is pretty clear with no major obstacles in sight.

“First of all you would have to have a commercial supply of the material; I don’t see that as challenging,” said Coleman. “There are many commercial suppliers of graphene that can produce graphene in large quantities. And, of course, silly putty is commercially available. The procedure of mixing the two components together is fairly straightforward and that could certainly be industrialized. So making the material is not a problem.”

Engineering the sensing device would be a bit more of a challenge. Coleman explains that you would have to make some kind of housing that could be worn on the wrist and would have the sensing material inside of it. Then you would need the electronics to generate the current and measure the changes in it. You would also need some kind of communication system that would send the signal to a mobile phone where you would need an app to collect and analyze the data.

“To be completely honest, all of that stuff is not that far from being off-the-shelf,” he says. “I don’t think there is a great engineering challenge in this work. So really we are actually quite close to the ability to commercialize this material.”

While the strain sensor that Coleman and his colleagues have developed is sensitive, he believes that this research opens up an entirely new way of making composites that will lead to far more sensitive sensing technologies in the future.

“What we really want to do is to go on to the next generation of sensing,” he says. “We have extremely sensitive materials here, but we see an opportunity to make composites in a different way using different polymers that are up to a factor of ten more sensitive than the ones we have created here.”

he researchers’ machine uses a “nanoporous” stamp through which a solution of nanoparticles, or “ink,” can flow uniformly through the stamp and onto whatever surface is to be printed.

Forest of Carbon Nanotubes Stamps Electronic Ink Onto a Surface

About a dozen years ago, I wrote a report on the future of nanomaterials in printing and packaging.  I think copies of it may exist today only on my bookshelf, but it has served over that time to inform my opinion on how nanomaterials can be applied to these areas—most recently contributing to piece on these pages a few years back.  The basic question: How do you get a nanomaterial onto some packaging cheaply enough that it makes sense for what is essentially a throwaway item?

It has not been easy. But MIT researchers believe they have hit upon a low-cost, robust stamping method that manages to get carbon nanotubes onto a flexible surface so that they can serve as a transistor for controlling individual pixels in high-resolution displays.

In research described in the journal Science Advances, the technique developed for getting the carbon nanotubes onto the surface eschews the use of inkjet printing techniques, which have been thought to be way forward in this application space. Instead they turned to a rather old printing technique: the stamp.

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Image of Mona Lisa that appears and disappears

Novel Nanomaterial Enables Rewritable Optical Circuits

Researchers at the University of Texas at Austin have developed a hybrid nanomaterial that enables the writing, erasing and rewriting of optical components. The researchers believe that this nanomaterial and the techniques used in exploiting it could create a new generation of optical chips and circuits.

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Droplets of ‘photonic water’ can change color by using chemical or magnetic forces to adjust the alignment of internal stacks of reflective nanosheet crystals

'Photonic Water' Could Be Boon for Optoelectronic Applications

A two-dimensional metal oxide material called titanate nanosheets has remained pretty much off the radar of flatland materials expected to transform the worlds of electronics and optoelectronics. Its biggest claim to fame has been that it is pretty effective at cleaning up contaminants.

However, it would seem that titanate nanosheets history of being overlooked  in the catalogue of 2D materials may have come to an end thanks to a serendipitous discovery by researchers in Japan.

Researchers at the RIKEN Center for Emergent Matter Science in Japan were experimenting with the material to see if they could get the nanosheets to break into more uniform pieces rather than the varied sizes they typically take. Unfortunately, they weren’t able to solve this problem. But they did discover that when the material was centrifuged in water, it changed from being transparent to taking on a deep purple color.

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TEM image of sulphur-filled PPy-MnO2 coaxial nanotubes.

Novel Electrode Structure Provides New Promise for Lithium-Sulfur Batteries

Lithium-sulfur batteries (Li-S) can hold as much as five times the energy per unit mass that lithium-ion (Li-ion) batteries can. However, Li-S batteries suffer from the propensity for polysulfides to pass through the cathode, foul the electrolyte, then pass through to the other electrode, depleting it of sulfur after just a few charge-discharge cycles. This phenomenon is known as the “shuttle effect.”

Now researchers at the University of Texas at Austin have developed an electrode structure for a Li-S battery that makes use of coaxial polypyrrole-manganese dioxide (PPy-MnO2) nanotubes. This novel electrode combats the shuttle effect by essentially encapsulating the electrodes with the nanotubes.

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Graphene-based NFC antenna is flexible

Graphene-based Antenna Still Looking for Path to Commercialization

Graphene is a pure conductor, just like a metal, which has led researchers to examine applications like antennas to see if graphene could serve as a replacement.

Along this line of inquiry, researchers at the National Research Council Institute for Organic Synthesis and Photoreactivity (CNR-ISOF) in Italy, which is part of Europe’s Graphene Flagship,  have developed a graphene-based near-field communication (NFC) antenna. Unlike today’s NFC antennas their devices are flexibile and have greater durability.

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DGIST's senior researcher Jeong Min-kyung

First Graphene Photodetector To Operate in the Microwave

While graphene may be losing its luster in the field of digital electronics because of its lack of an inherent band gap,  in the world of optoelectronics graphene’s gapless band structure seems to be winning a new set of acolytes. This is seen no more keenly than in photodetectors, where graphene is enabling near-terahertz-speed photodetectors that are more energy efficient.

Now, researchers at the Daegu Gyeongbuk Institute of Science and Technology (DGIST) in South Korea and the University of Basel in Switzerland have developed a new graphene-based photodetector that operates at microwave wavelengths—a departure from graphene photodetectors that detect only optical wavelengths from the near-infrared to ultraviolet light.

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Schematic of the mono- and bilayer crystal structures. Purple and red spheres correspond to indium and selenium atoms, respectively.

Indium Selenide Takes on the Mantle of the New Wonder Material

Is there a research institute with a more distinguished pedigree in graphene research than the University of Manchester? There certainly haven’t been any that have “gone all in” the way Manchester has with its construction of a $71 million graphene research facility near the campus, which is operated under the auspices of the newly established National Graphene Institute (NGI).

This dedication to graphene makes sense considering the fact that Andre Geim and Konstantin Novoselov were at Manchester when they became the first researchers to synthesize graphene—the advance for which they were awarded the 2010 Nobel Prize in Physics.

But now it appears that a new material developed at Manchester, based on indium selenide (InSe), has taken some of graphene’s spotlight at Manchester, at least in terms of meeting the demands of future super-fast electronics.

“Ultra-thin InSe seems to offer the golden middle between silicon and graphene,” said Geim in a press release. “Similar to graphene, InSe offers a naturally thin body, allowing scaling to the true nanometer dimensions. Similar to silicon, InSe is a very good semiconductor.”

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The optical image of a folded graphene oxide film looks like baked filo dough

Graphene Solar Absorber Could Enable Cheap Thermal Desalination

To those uninitiated to the costs of thermal desalination of water, the idea of simply vaporizing water to take out the impurities seems like it would offer a limitless supply of fresh water just by using it on the world’s oceans. However, the energy costs for thermal desalination has been estimated at around 80 megawatt-hours per megaliter of water produced, rendering it too costly for just about everyone except Gulf States rich in oil and desperate for fresh water.

One way around these high-energy costs has been thought to be solar-powered thermal desalination, which can help produce clean water in remote areas and developing countries. However, the solar approach to water desalination is rather limited in the amount of fresh water it can produce and is further hampered by the need for optical concentrators and for thermal insulation, both of which have limited the large-scale use of this approach.

Now researchers at Nanjing University in China have developed a solar absorber material made from graphene oxide that enables a solar approach to desalinating water without the need for solar concentrators and thermal insulation. The result could be a low-cost, portable water desalination solution ideally suited for developing countries and remote areas.

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An artist’s rendering of nonlinear light scattering by a dimer of two silicon particles with a variable radiation pattern.

Nanoantenna Changes Direction of Light and the Prospects of Optical Computing

Russian and U.S. researchers have developed a technique whereby the direction of light can be manipulated using a novel optical nanoantenna. The researchers believe that this  nanoantenna could help lead to a new era in optical information processing in telecommunications systems.

Of course, replacing electrons with photons is the basis of optical computing. However, realizing this switch is fraught with difficulties—not the least of which is the fact that because a photon has neither mass nor an electric charge, it is far more difficult to steer than an electron, which can be pushed or pulled simply by applying an electric field. For optical computing to work, there needs to a control mechanism for photons that is just as simple. Waveguides are able to contain light and guide it in a certain direction over long distances. A nanoantenna works differently. Instead of guiding light, it bounces the photons that strike it in a specific direction. That directionality is determined by the materials and geometry of the nanoantenna, just like in classical antennas.

But what sets the nanoantenna developed by the research team from ITMO University in St. Petersburg, Russia, the Moscow Institute of Physics and Technology (MIPT), and the University of Texas in Austin, apart is that this photon-scattering proerty is tunable. The researchers say they can change the direction to which it scatters the incident light without changing its physical dimensions.

In the journal Laser & Photonics Reviews, the international research team described the development of a tiny (less than 200 by 200 by 500 nanometers) silicon-based nanoantenna that pushes photons in a particular direction depending on the intensity of the incoming wave of light.

“The new device will allow us to change the direction of light propagation at a much better rate compared to electronic analogues,” said Sergey Makarov, a senior researcher at ITMO University, in a press release.

As with previous research out of ITMO where researchers gained control over light scattering for optical computing, this proposed nanoantenna is built from silicon nanoparticles.

When silicon nanoparticles are subjected to laser light, they produce an electron plasma. This electron plasma in not the well-known surface plasmons we have become acquainted with in the field of plasmonics. This plasma is simply a bunch of conduction (free) electrons that are injected into the conduction band of a semiconductor when it absorbs light. They are free in a sense that they can move freely through the semiconductor (until they lose their energy and fall to the valence band from whence they originated).

“Surface plasmons are the special oscillations of these free electrons, but these oscillations may arise only when density of free electrons is quite large—and we dont have that many electrons in our situation,” explained Denis Baranov, a postgraduate student at MIPT, in an e-mail interview with IEEE Spectrum. “So, these are the same electrons that give rise to plasmons, but the density is not enough for them.”

It is this plasma excitation that serves as the mechanism for the nanoantenna to rotate the radiation pattern. Essentially, the more intense the incoming light, the greater the rotation becomes; it can change light’s direction by as much as 20-degrees.

“Once you choose specific diameters and positions of the particles and you gather them into a single antenna, then the direction of the light scattering is fixed,” says Baranov. “But, when plasma is generated within the particles (upon irradiation with a strong pulse), it changes their refractive index and, consequently, optical properties of the whole nanoantenna.”

In particular, it’s the plasma changes the direction of scattering. “Effectively, one may think of this as slightly changing the material the particles are made of: you change the material but leave the geometry unchanged—you have a different antenna with different light scattering,” added Baranov.

Also part of the nanoantenna’s design is the fact that one of the silicon nanoparticles needs to be resonant while the other is not. This is done to enhance the effect of the beam routing.

“Suppose that we have a nanoantenna composed of two identical particles. It cannot scatter light sideways; it always scatters it forward, due to symmetry,” explained Baranov.

However, when one of the particles is resonant, it experiences intense generation of plasma, while the other, non-resonant particle, does not. This provides the desired asymmetry in the behavior of the antenna.

“Now you see, that the same nanoantenna is capable of scattering light sideways or forward depending on the incident intensity,” said Baranov. “For weak pulses, there is not plasma generation, and since the antenna is not symmetric it scatters light sideways. When an intense pulse is applied, plasma is generated within the resonant particle, and the antenna becomes kind of "symmetric", so it scatters light forward.”

The optical antenna developed here can support data rates as fast as at 250 gigabits per second—a speed that could help it bridge the gulf between optical data transmission rates and electronic data processing speeds. Fiber optic cables are transmitting data at hundreds of gigabits per second but our electron-based computers can only process these signal at a fraction of those speeds.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
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Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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