NC State University has developed a Flexible Carbon Nanotube Film with a unique combination of thermal, electrical and physical properties that make it an an Excellent Candidate for Next-Generation of Smart Fabrics

Researchers reported in a new study that a material made of carbon nanotubes may be key in developing clothing that can heat or cool the wearer on demand. The film is twisted into a filament yarn and wound around a tube to show its flexibility. (Image: Kony Chatterjee) A film made of carbon nanotubes (CNT) may be a key material in developing clothing that can heat or cool the wearer on demand. A new North Carolina State University study finds that the CNT film has a combination of thermal, electrical and physical properties that make it an appealing candidate for next-generation smart fabrics.

The researchers were also able to optimize the thermal and electrical properties of the material, allowing the material to retain its desirable properties even when exposed to air for many weeks. Moreover, these properties were achieved using processes that were relatively simple and did not need excessively high temperatures.

“Many researchers are trying to develop a material that is non-toxic and inexpensive, but at the same time is efficient at heating and cooling,” said Tushar Ghosh, co-corresponding author of the study (ACS Applied Energy Materials, “In-plane Thermoelectric Properties of Flexible and Room Temperature Processable Doped Carbon Nanotube Films”). “Carbon nanotubes, if used appropriately, are safe, and we are using a form that happens to be inexpensive, relatively speaking. So it’s potentially a more affordable thermoelectric material that could be used next to the skin.” Ghosh is the William A. Klopman Distinguished Professor of Textiles in NC State’s Wilson College of Textiles.

“We want to integrate this material into the fabric itself,” said Kony Chatterjee, first author of the study and a Ph.D. student at NC State. “Right now, the research into clothing that can regulate temperature focuses heavily on integrating rigid materials into fabrics, and commercial wearable thermoelectric devices on the market aren’t flexible either.”

To cool the wearer, Chatterjee said, CNTs have properties that would allow heat to be drawn away from the body when an external source of current is applied.

“Think of it like a film, with cooling properties on one side of it and heating on the other,” Ghosh said.

The researchers measured the material’s ability to conduct electricity, as well as its thermal conductivity, or how easily heat passes through the material.

One of the biggest findings was that the material has relatively low thermal conductivity – meaning heat would not travel back to the wearer easily after leaving the body in order to cool it. That also means that if the material were used to warm the wearer, the heat would travel with a current toward the body, and not pass back out to the atmosphere.

The researchers were able to accurately measure the material’s thermal conductivity through a collaboration with the lab of Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State. The researchers used a special experimental design to more accurately measure the material’s thermal conductivity in the direction that the electric current is moving within the material.

“You have to measure each property in the same direction to give you a reasonable estimate of the material’s capabilities,” said Liu, co-corresponding author of the study. “This was not an easy task; it was very challenging, but we developed a method to measure this, especially for thin flexible films.”

The research team also measured the ability of the material to generate electricity using a difference in temperature, or thermal gradient, between two environments. Researchers said that they could take advantage of this for heating, cooling, or to power small electronics.

Liu said that while these thermoelectric properties were important, it was also key that they found a material that was also flexible, stable in air, and relatively simple to make.

“The point of this paper isn’t that we achieved the best thermoelectric performance,” Liu said. “We achieved something that can be used as a flexible, electronic, soft material that’s easy to fabricate. It’s easy to prepare this material, and easy to achieve these properties.”

Ultimately, their vision for the project is to design a smart fabric that can heat and cool the wearer, along with energy harvesting. They believe that a smart garment could help reduce energy consumption.

“Instead of heating or cooling a whole dwelling or space, you would heat or cool the personal space around the body,” Ghosh said. “If we could get the thermostat down a degree or two, that could save a tremendous amount of energy.”

Source: North Carolina State University

Breathable’ Electronics Pave the Way for More Functional Wearable Tech

This sleeve incorporates the new electronic material, allowing it to function as a video game controller.

Engineering researchers have created ultrathin, stretchable electronic material that is gas permeable, allowing the material to “breathe.” The material was designed specifically for use in biomedical or wearable technologies, since the gas permeability allows sweat and volatile organic compounds to evaporate away from the skin, making it more comfortable for users – especially for long-term wear.

“The gas permeability is the big advance over earlier stretchable electronics,” says Yong Zhu, co-corresponding author of a paper on the work and a professor of mechanical and aerospace engineering at North Carolina State University. “But the method we used for creating the material is also important because it’s a simple process that would be easy to scale up.”

Specifically, the researchers used a technique called the breath figure method to create a stretchable polymer film featuring an even distribution of holes. The film is coated by dipping it in a solution that contains silver nanowires. The researchers then heat-press the material to seal the nanowires in place.

“The resulting film shows an excellent combination of electric conductivity, optical transmittance and water-vapor permeability,” Zhu says. “And because the silver nanowires are embedded just below the surface of the polymer, the material also exhibits excellent stability in the presence of sweat and after long-term wear.”

“The end result is extremely thin – only a few micrometers thick,” says Shanshan Yao, co-author of the paper and a former postdoctoral researcher at NC State who is now on faculty at Stony Brook University. “This allows for better contact with the skin, giving the electronics a better signal-to-noise ratio.

“And gas permeability of wearable electronics is important for more than just comfort,” Yao says. “If a wearable device is not gas permeable, it can also cause skin irritation.”

To demonstrate the material’s potential for use in wearable electronics, the researchers developed and tested prototypes for two representative applications.

The first prototype consisted of skin-mountable, dry electrodes for use as electrophysiologic sensors. These have multiple potential applications, such as measuring electrocardiography (ECG) and electromyography (EMG) signals.

“These sensors were able to record signals with excellent quality, on par with commercially available electrodes,” Zhu says.

The second prototype demonstrated textile-integrated touch sensing for human-machine interfaces. The authors used a wearable textile sleeve integrated with the porous electrodes to play computer games such as Tetris. Related video can be seen at https://youtu.be/7AO_cq8A_BE.

“If we want to develop wearable sensors or user interfaces that can be worn for a significant period of time, we need gas-permeable electronic materials,” Zhu says. “So this is a significant step forward.”

The paper, “Gas-Permeable, Ultrathin, Stretchable Epidermal Electronics with Porous Electrodes,” is published in the journal ACS Nano. First author of the paper is Weixin Zhou, a Ph.D. student at Nanjing University of Posts and Telecommunications (NUPT) who worked on the project while a visiting scholar at NC State.

The paper was co-authored by Hongyu Wang, a Ph.D. student at NC State, and by Qingchuan Du of NUPT. Co-corresponding author of the paper is Yanwen Ma, a professor at NUPT.

The work was done with support from the National Science Foundation, under grant number CMMI-1728370.

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Note to Editors: The study abstract follows.

“Gas-Permeable, Ultrathin, Stretchable Epidermal Electronics with Porous Electrodes”

Authors: Weixin Zhou, Qingchuan Du and Yanwen Ma, Nanjing University of Posts and Telecommunications; Shanshan Yao, North Carolina State University and Stony Brook University; and Hongyu Wang and Yong Zhu, North Carolina State University

Published: April 29, ACS Nano

DOI: 10.1021/acsnano.0c00906

Abstract: We present gas-permeable, ultrathin, and stretchable electrodes enabled by self-assembled porous substrates and conductive nanostructures. Efficient and scalable breath figure method is employed to introduce the porous skeleton and then silver nanowires (AgNWs) are dip-coated and heat-pressed to offer electric conductivity.

The resulting film has a transmittance of 61%, sheet resistance of 7.3 Ω/sq, and water vapor permeability of 23 mg cm^-2 h^-1. With AgNWs embedded below the surface of the polymer, the electrode exhibits excellent stability with the presence of sweat and after long-term wear.

We demonstrate the promising potential of the electrode for wearable electronics in two representative applications – skin-mountable biopotential sensing for healthcare and textile-integrated touch sensing for human-machine interfaces.

The electrode can form conformal contact with human skin, leading to low skin-electrode impedance and high quality biopotential signals. In addition, the textile electrode can be used in a self-capacitance wireless touch sensing system.

New materials Powering the battery Revolution

There are more mobile phones in the world than there are people. Nearly all of them are powered by rechargeable lithium-ion batteries, which are the single most important component enabling the portable electronics revolution of the past few decades. 

None of those devices would be attractive to users if they didn’t have enough power to last at least several hours, without being particularly heavy.

Lithium-ion batteries are also useful in larger applications, like electric vehicles and smart-grid energy storage systems. And researchers’ innovations in materials science, seeking to improve lithium-ion batteries, are paving the way for even more batteries with even better performance. There is already demand forming for high-capacity batteries that won’t catch fire or explode. And many people have dreamed of smaller, lighter batteries that charge in minutes – or even seconds – yet store enough energy to power a device for days.

Research is finding better ways to make batteries both big and small. 

Researchers like me, though, are thinking even more adventurously. Cars and grid-storage systems would be even better if they could be discharged and recharged tens of thousands of times over many years, or even decades. Maintenance crews and customers would love batteries that could monitor themselves and send alerts if they were damaged or no longer functioning at peak performance – or even were able to fix themselves. And it can’t be too much to dream of dual-purpose batteries integrated into the structure of an item, helping to shape the form of a smartphone, car or building while also powering its functions.

All that may become possible as my research and others’ help scientists and engineers become ever more adept at controlling and handling matter at the scale of individual atoms.

3d Illustration of twist sodium ion battery technology

Emerging materials

For the most part, advances in energy storage will rely on the continuing development of materials science, pushing the limits of performance of existing battery materials and developing entirely new battery structures and compositions.

The battery industry is already working to reduce the cost of lithium-ion batteries, including by removing expensive cobalt from their positive electrodes, called cathodes. This would also reduce the human cost of these batteries, because many mines in Congo, the world’s leading source of cobalt, use children to do difficult manual labor.

Workers at a cobalt-copper mine in the Democratic Republic of Congo. Kenny-Katombe Butunka/Reuters

Researchers are finding ways to replace the cobalt-containing materials with cathodes made mostly of nickel. Eventually they may be able to replace the nickel with manganese. Each of those metals is cheaper, more abundant and safer to work with than its predecessor. But they come with a trade-off, because they have chemical properties that shorten their batteries’ lifetimes.

Researchers are also looking at replacing the lithium ions that shuttle between the two electrodes with ions and electrolytes that may be cheaper and potentially safer, like those based on sodium, magnesium, zinc or aluminum.

My research group looks at the possibilities of using two-dimensional materials, essentially extremely thin sheets of substances with useful electronic properties. Graphene is perhaps the best-known of these – a sheet of carbon just one atom thick. We want to see whether stacking up layers of various two-dimensional materials and then infiltrating the stack with water or other conductive liquids could be key components of batteries that recharge very quickly.

Looking inside the battery

It’s not just new materials expanding the world of battery innovation: New equipment and methods also let researchers see what’s happening inside batteries much more easily than was once possible.

In the past, researchers ran a battery through a particular charge-discharge process or number of cycles, and then removed the material from the battery and examined it after the fact. Only then could scholars learn what chemical changes had happened during the process and infer how the battery actually worked and what affected its performance.

X-rays generated by a synchotron can illuminate the inner workings of a battery. CLS Research Office/flickr, CC BY-SA

But now, researchers can watch battery materials as they undergo the energy storage process, analyzing even their atomic structure and composition in real time. We can use sophisticated spectroscopy techniques, such as X-ray techniques available with a type of particle accelerator called a synchrotron – as well as electron microscopes and scanning probes – to watch ions move and physical structures change as energy is stored in and released from materials in a battery.

Those methods let researchers like me imagine new battery structures and materials, make them and see how well – or not – they work. That way, we’ll be able to keep the battery materials revolution going.

Re-Posted from Veronica Augustyn  An Assistant Professor of Materials Science and Engineering, North Carolina State University

NV-doped Nanodiamonds may “serve as the basic building blocks” for quantum computing – N.Carolina State University

Researchers at North Carolina State University have developed a new technique for creating NV-doped single-crystal nanodiamonds, only four to eight nanometers wide, which could serve as components in room-temperature quantum computing technologies. These doped nanodiamonds also hold promise for use in single-photon sensors and nontoxic, fluorescent biomarkers.

Currently, computers use binary logic, in which each binary unit – or bit – is in one of two states: 1 or 0. Quantum computing makes use of superposition and entanglement, allowing the creation of quantum bits – or qubits – which can have a vast number of possible states. Quantum computing has the potential to significantly increase computing power and speed.

A number of options have been explored for creating quantum computing systems, including the use of diamonds that have “nitrogen-vacancy” centers. That’s where this research comes in.

Normally, diamond has a very specific crystalline structure, consisting of repeated diamond tetrahedrons, or cubes. Each cube contains five carbon atoms. The NC State research team has developed a new technique for creating diamond tetrahedrons that have two carbon atoms; one vacancy, where an atom is missing; one carbon-13 atom (a stable carbon isotope that has six protons and seven neutrons); and one nitrogen atom. This is called the NV center. Each NV-doped nanodiamond contains thousands of atoms, but has only one NV center; the remainder of the tetrahedrons in the nanodiamond are made solely of carbon.

It’s an atomically small distinction, but it makes a big difference.

“That little dot, the NV center, turns the nanodiamond into a qubit,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and lead author of a paper describing the work. “Each NV center has two transitions: NV0 and NV-. We can go back and forth between these two states using electric current or laser. These nanodiamonds could serve as the basic building blocks of a quantum computer.”

To create these NV-doped nanodiamonds, the researchers start with a substrate, such as such as sapphire, glass or a plastic polymer. The substrate is then coated with amorphous carbon – elemental carbon that, unlike graphite or diamond, does not have a regular, well-defined crystalline structure. While depositing the film of amorphous carbon, the researchers bombard it with nitrogen ions and carbon-13 ions. The carbon is then hit with a laser pulse that raises the temperature of the carbon to approximately 4,000 Kelvin (or around 3,727 degrees Celsius) and is then rapidly quenched. The operation is completed within a millionth of a second and takes place at one atmosphere – the same pressure as the surrounding air. By using different substrates and changing the duration of the laser pulse, the researchers can control how quickly the carbon cools, which allows them to create the nanodiamond structures.

“Our approach reduces impurities; controls the size of the NV-doped nanodiamond; allows us to place the nanodiamonds with a fair amount of precision; and directly incorporates carbon-13 into the material, which is necessary for creating the entanglement required in quantum computing,” Narayan says. “All of the nanodiamonds are exactly aligned through the paradigm of domain matching epitaxy, which is a significant advance over existing techniques for creating NV-doped nanodiamonds.”

“The new technique not only offers unprecedented control and uniformity in the NV-doped nanodiamonds, it is also less expensive than existing techniques,” Narayan says. “Hopefully, this will enable significant advances in the field of quantum computing.”

The researchers are currently talking with government and private sector groups about how to move forward. One area of interest is to develop a means of creating self-assembling systems that incorporate entangled NV-doped nanodiamonds for quantum computing.

The paper, “Novel synthesis and properties of pure and NV-doped nanodiamonds and other nanostructures,” is published in the journal Materials Research Letters.

 

Also Read: Electron ‘spin control’ of levitated nanodiamonds could bring advances in sensors, quantum information processing Read more at: http://phys.org/news/2016-07-electron-levitated-nanodiamonds-advances-sensors.html#jCp

More information: Jagdish Narayan et al, Novel synthesis and properties of pure and NV-doped nanodiamonds and other nanostructures, Materials Research Letters (2016). DOI: 10.1080/21663831.2016.1249805

Provided by: North Carolina State University

 

Wearable sensor clears path to long-term EKG, EMG monitoring

January 20, 2015

Source: North Carolina State University

Summary:

A new, wearable sensor that uses silver nanowires to monitor electrophysiological signals, such as electrocardiographyor electromyography, has been developed by researchers. The new sensor is as accurate as the ‘wet electrode’ sensors used in hospitals, but can be used for long-term monitoring and is more accurate than existing sensors when a patient is moving.

Researchers from North Carolina State University have developed a new, wearable sensor that uses silver nanowires to monitor electrophysiological signals, such as electrocardiography (EKG) or electromyography (EMG). The new sensor is as accurate as the “wet electrode” sensors used in hospitals, but can be used for long-term monitoring and is more accurate than existing sensors when a patient is moving.

Long-term monitoring of electrophysiological signals can be used to track patient health or assist in medical research, and may also be used in the development of new powered prosthetics that respond to a patient’s muscular signals.

The silver nanowire sensors conform to a patient’s skin, creating close contact. Image credit: Yong Zhu. Click to enlarge.

Electrophysiological sensors used in hospitals, such as EKGs, use wet electrodes that rely on an electrolytic gel between the sensor and the patient’s skin to improve the sensor’s ability to pick up the body’s electrical signals. However, this technology poses problems for long-term monitoring, because the gel dries up — irritating the patient’s skin and making the sensor less accurate.

The new nanowire sensor is comparable to the wet sensors in terms of signal quality, but is a “dry” electrode — it doesn’t use a gel layer, so doesn’t pose the same problems that wet sensors do.

“People have developed other dry electrodes in the past few years, and some have demonstrated the potential to rival the wet electrodes, but our new electrode has better signal quality than most — if not all — of the existing dry electrodes. It is more accurate,” says Dr. Yong Zhu, an associate professor of mechanical and aerospace engineering at NC State and senior author of a paper describing the work. “In addition, our electrode is mechanically robust, because the nanowires are inlaid in the polymer.”

The sensors stem from Zhu’s earlier work to create highly conductive and elastic conductors made from silver nanowires, and consist of one layer of nanowires in a stretchable polymer.

The new sensor is also more accurate than existing technologies at monitoring electrophysiological signals when a patient is in motion.

“The silver nanowire sensors conform to a patient’s skin, creating close contact,” Zhu says. “And, because the nanowires are so flexible, the sensor maintains that close contact even when the patient moves. The nanowires are also highly conductive, which is key to the high signal quality.”

The new sensors are also compatible with standard EKG- and EMG-reading devices.

“I think these sensors are essentially ready for use,” Zhu says “The raw materials of the sensor are comparable in cost to existing wet sensors, but we are still exploring ways of improving the manufacturing process to reduce the overall cost.”

An uncorrected proof of the paper, “Wearable Silver Nanowire Dry Electrodes for Electrophysiological Sensing,” was published online Jan. 14 in RSC Advances, immediately after acceptance. Lead author of the paper is Amanda Myers, a Ph.D. student at NC State. The paper was co-authored by Dr. Helen Huang, an associate professor in the joint biomedical engineering program at NC State and the University of North Carolina at Chapel Hill.

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Story Source:

The above story is based on materials provided by North Carolina State University. Note: Materials may be edited for content and length.

Direct printing of liquid metal 3D microstructures

By Michael Berger. Copyright © Nanowerk

(Nanowerk Spotlight) The ability to pattern materials  into arbitrary three-dimensional (3D) microstructures is important for  electronics, microfluidic networks, tissue engineering scaffolds, photonic band  gap structures, and chemical synthesis.

However, existing commercial processes  to 3D print metals usually require expensive equipment and large temperatures.

In contrast, a novel, relatively simple method developed by researchers at North  Carolina State University can print metal structures at room temperature. This  makes the technique it compatible with many other materials including plastics.  Also, the resulting structures are liquid and are therefore soft and  stretchable.

“The key concept is that the liquid metal forms spontaneously a  thin oxide layer on its surface,” Michael Dickey, an Associate Professor of chemical and  biomolecular engineering at NC State, tells Nanowerk. This oxide layer is solid  and allows the metal to be printed into 3D shapes despite being a liquid.  When  two droplets of water come together, they form a larger droplet.  However, this  does not happen with the liquid metal due to the oxide ‘skin’.”   As the team reports in a recent issue of Advanced  Materials (“3D Printing of Free Standing Liquid Metal  Microstructures”), they have demonstrated that it is possible to direct  write structures composed of a low-viscosity liquid with metallic conductivity  at room temperature. The liquid metal is useful for soft, stretchable, or shape  reconfigurable electronics.

 

Direct writing of liquid metal 3D structures of varying sizes.  (Image: Dickey Research Group, North Carolina State University) (click image to  enlarge)  

Metals have unique electrical, optical, and thermal properties.  With this novel technique, it is now possible to print metal microstructures  directly to creates various parts including electronics. The resulting parts, if  designed correctly, can be stretchable.    The general approach for printing liquid metal structures  involves applying modest gauge pressure to a syringe needle that then extrudes  the liquid metal – for this work they used the binary eutectic alloy of gallium  and indium but they say that any alloy of gallium will also work – onto a  substrate controlled by a motorized translation stage.   Upon exposure to air, the metal forms a thin (∼1 nanometer)  passivating ‘skin’ composed of gallium oxide. This oxide skin on the surface of  the metal stabilizes the liquid metal wire against gravity and surface tension  of the liquid. Once detached from the syringe, the wires maintain their shape.

3D printing of liquid metals at room temperature.  

“The formation of the wires is remarkable and unexpected” says  Dickey. “The process of forming the wires begins by forming a bead of the metal  on the tip of the syringe.

Although the metal is under pressure the entire time,  it does not flow out of the syringe due to the stabilizing influence of the  oxide skin. Without increasing or decreasing the pressure in the syringe, wires  form when the metal contacts the substrate and the tip of the syringe withdraws  away from the substrate. Because the oxide skin spans from the nozzle of the  syringe to the substrate, increasing the distance between the nozzle and  substrate generates a tensile force along the axis of the wire that yields the  skin and allows the wire to elongate.

The pressure of the liquid metal retards  any destabilizing capillary forces long enough for new skin to form and thereby  mechanically stabilizes the wire.”   Altogether, the researchers describe four different methods to  direct write 3D, free standing, liquid metal microstructures by extruding the  liquid metal through a capillary: “In addition to extruding wires, it is  possible to form free standing liquid metal microstructures using at least three  additional methods,” Dickey explains: “1) Expelling rapidly the metal to form a  stable liquid metal filament; 2) stacking droplets; and 3) injecting the metal  into microchannels and subsequently removing the channels chemically.”

The smallest components that the team fabricated were about 10  µm, but they note that there may be opportunities to create smaller structures  through, for example, the use of smaller nozzles.   Dickey’s team is currently exploring how to further develop  these techniques, as well as how to use them in various electronics applications  and in conjunction with established 3-D printing technologies.

Dickey notes that the work by an undergraduate student, Collin  Ladd, also the paper’s first author, was indispensable to this project. “He  helped develop the concept, and literally created some of this technology out of  spare parts he found himself.”

Read more: http://www.nanowerk.com/spotlight/spotid=32574.php#ixzz2hLXHYA7S

Self-Healing Solar Cells

To understand how solar cells heal themselves, look no further than the nearest tree leaf or the back of your hand.

The “branching” vascular channels that circulate life-sustaining nutrients throughout leaves and hands serve as the inspiration for solar cells that can restore themselves efficiently and inexpensively.

 

 

In a new paper, North Carolina State University researchers Orlin Velev and Hyung-Jun Koo show that creating solar cell devices with channels that mimic organic vascular systems can effectively reinvigorate solar cells whose performance deteriorates due to degradation by the sun’s ultraviolet rays. Solar cells that are based on organic systems hold the potential to be less expensive and more environmentally friendly than silicon-based solar cells, the current industry standard.

 

The nature-mimicking devices are a type of dye-sensitized solar cells (DSSCs), composed of a water-based gel core, electrodes, and inexpensive, light-sensitive, organic dye molecules that capture light and generate electric current. However, the dye molecules that get “excited” by the sun’s rays to produce electricity eventually degrade and lose efficiency, Velev says, and thus need to be replenished to reboot the device’s effectiveness in harnessing the power of the sun.

“Organic material in DSSCs tends to degrade, so we looked to nature to solve the problem,” Velev said. “We considered how the branched network in a leaf maintains water and nutrient levels throughout the leaf. Our microchannel solar cell design works in a similar way. Photovoltaic cells rendered ineffective by high intensities of ultraviolet rays were regenerated by pumping fresh dye into the channels while cycling the exhausted dye out of the cell. This process restores the device’s effectiveness in producing electricity over multiple cycles.”

Velev, Invista Professor of Chemical and Biomolecular Engineering at NC State and the lead author of a paper in Scientific Reports describing the research, adds that the new gel-microfluidic cell design was tested against other designs, and that branched channel networks similar to the ones found in nature worked most effectively.

Study co-author Dr. Hyung-Jun Koo is a former NC State Ph.D. student who is now a postdoctoral researcher at the University of Illinois. The study was funded by the National Science Foundation and the U.S. Department of Energy.

Koo and Velev reported earlier a new type of biomimetic hydrogel solar cell.

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Note to editors: The abstract of the paper follows.

“Regenerable Photovoltaic Devices with a Hydrogel-Embedded Microvascular Network”

Authors: Hyung-Jun Koo and Orlin D. Velev, NC State University

Published: Aug. 5, 2013, in Scientific Reports

DOI: 10.1038/srep02357

Abstract: Light-driven degradation of photoactive molecules could be one of the major obstacles to stable long term operation of organic dye-based solar light harvesting devices. One solution to this problem may be mimicking the regeneration functionality of a plant leaf. We report an organic dye photovoltaic system that has been endowed with such microfluidic regeneration functionality. A hydrogel medium with embedded channels allows rapid and uniform supply of photoactive reagents by a convection-diffusion mechanism. A washing-activation cycle enables reliable replacement of the organic component in a dye-sensitized photovoltaic system.

 

Release Date: 08.07.13 Filed under Releases

 

 

Interface Properties of Graphene Paves Way for New Applications

Researchers from North Carolina State University and the University of Texas have revealed more about graphene’s mechanical properties and demonstrated a technique to improve the stretchability of graphene – developments that should help engineers and designers come up with new technologies that make use of the material.

Graphene is a promising material that is used in technologies such as transparent, flexible electrodes and nanocomposites. And while engineers think graphene holds promise for additional applications, they must first have a better understanding of its mechanical properties, including how it works with other materials.

“This research tells us how strong the interface is between graphene and a stretchable substrate,” says Dr. Yong Zhu, an associate professor of mechanical and aerospace engineering at NC State and co-author of a paper on the work. “Industry can use that to design new flexible or stretchable electronics and nanocomposites. For example, it tells us how much we can deform the material before the interface between graphene and other materials fails. Our research has also demonstrated a useful approach for making graphene-based, stretchable devices by ‘buckling’ the graphene.”

The researchers looked at how a graphene monolayer – a layer of graphene only one atom thick – interfaces with an elastic substrate. Specifically, they wanted to know how strong the bond is between the two materials because that tells engineers how much strain can be transferred from the substrate to the graphene, which determines how far the graphene can be stretched.

The researchers applied a monolayer of graphene to a polymer substrate, and then stretched the substrate. They used a spectroscopy technique to monitor the strain at various points in the graphene. Strain is a measure of how far a material has stretched.

Initially, the graphene stretched with substrate. However, while the substrate continued to stretch, the graphene eventually began to stretch more slowly and slide on the surface instead. Typically, the edges of the monolayer began to slide first, with the center of the monolayer stretching further than the edges.

“This tells us a lot about the interface properties of the graphene and substrate,” Zhu says. “For the substrate used in this study, polyethylene terephthalate, the edges of the graphene monolayer began sliding after being stretched 0.3 percent of its initial length. But the center continued stretching until the monolayer had been stretched by 1.2 to 1.6 percent.”

The researchers also found that the graphene monolayer buckled when the elastic substrate was returned to its original length. This created ridges in the graphene that made it more stretchable because the material could stretch out and back, like the bellows of an accordion. The technique for creating the buckled material is similar to one developed by Zhu’s lab for creating elastic conductors out of carbon nanotubes.

The paper, “Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate,” was published online Aug. 1 in Advanced Functional Materials. Lead author of the paper is Dr. Tao Jiang, a postdoctoral researcher at NC State. The paper was co-authored by Dr. Rui Huang of the University of Texas. The research was funded by the National Science Foundation (NSF) and the NSF’s ASSIST Engineering Research Center at NC State.

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Note to Editors: The study abstract follows.

“Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate”

Authors: Tao Jiang and Yong Zhu, North Carolina State University; Rui Huang, University of Texas at Austin

Published: Aug. 1 2013, Advanced Functional Materials

Elastic conductors for new sensing applications

Researchers from North Carolina State University have developed elastic conductors made from silver nanowires, as the basis of stretchable electronic devices.

Stretchable circuitry could be used, for example, to create tactile, strain and motion sensors in wearable or conformable applications.

Dr Yong Zhu, an assistant professor of mechanical and aerospace engineering at NC State, and Feng Xu, a PhD student in Zhu’s lab have developed elastic conductors using silver nanowires. Silver has very high electric conductivity. The technique developed at NC State embeds silver nanowires in a polymer that can withstand significant stretching without adversely affecting the material’s conductivity. This makes it attractive as a component for use in stretchable electronic devices.

Simple fabrication

Silver nanowires are placed on a silicon plate and a liquid polymer is poured over the silicon substrate, which flows around the silver nanowires. High heat turns the polymer from a liquid into an elastic solid, trapping the nanowires in the polymer. The polymer is peeled off the silicon plate.

Zhu says the elastic conductor technology could be commercially viable within five years. Fabrication is simple and is compatible with printing and patterning techniques, including screen and inkjet. Zhu’s team has made some prototypes, filed for patents and discussions about next steps towards commercialisation are taking place. When the polymer is stretched and relaxed, the surface containing nanowires buckles, creating a composite that is wavy on the side that contains silver nanowires and flat on the other.

After the nanowire-embedded surface has buckled, the material can be stretched up to 50% of its elongation, or tensile strain, without affecting the conductivity of the silver nanowires, because the buckled shape of the material allows the nanowires to stay in a fixed position in relation to each other, as the polymer is being stretched.

The research was supported by the National Science Foundation.

Nano-particles Release Insulin into Diabetics’ Bloodstream

Diabetics could cut their need for injections to less than once a week thanks  to new insulin-releasing “smart” particles.

Researchers in the US have developed a type of nanoparticle that  automatically releases insulin into the blood when glucose levels get too high,  and have demonstrated that its effects last for 10 days in mice.

Regular injections of the particles could mean type 1 diabetics  wouldn’t have to check their blood sugar levels several times a day, or inject  the exact right amount of insulin when needed, which can result in too high or  low doses being administered, with further health problems following.

‘We’ve created a ‘smart’ system that is injected into the body and  responds to changes in blood sugar by releasing insulin, effectively controlling  blood-sugar levels,’ said Dr Zhen Gu, an assistant professor in the joint  biomedical engineering program at North Carolina State University and the  University of North Carolina.

‘This technology effectively creates a ‘closed-loop’ system that mimics  the activity of the pancreas in a healthy person, releasing insulin in response  to glucose level changes. This has the potential to improve the health and  quality of life of diabetes patients.’

The nanoparticles have a solid core of insulin surrounded by a layer of  a modified glucose-based material known as dextran and another of glucose  oxidase enzymes.

When the enzymes are exposed to high glucose levels they effectively  convert the sugar into gluconic acid, which breaks down the modified dextran and  releases the insulin.

The insulin then brings the glucose levels under control. The gluconic  acid and dextran are biocompatible and dissolve in the body.

The nanoparticle cores are given a biocompatible coating that makes  them positively or negatively charged, causing them to form a network that  prevents them from dispersing throughout the body.

The positively charged coatings are made of chitosan (a material  normally found in shrimp shells), abnd the negatively charged coatings are made  of alginate (a material normally found in seaweed).

When the solution of coated nanoparticles is mixed together, the  positively and negatively charged coatings are attracted to each other to form a “nano-network.”

Once injected into the subcutaneous layer of the skin, the nano-network  holds the nanoparticles together. Both the nano-network and the coatings are  porous, allowing blood – and blood sugar – to reach the nanoparticle cores.

Gu’s research team is now in discussions to move the technology into  clinical trials for use in humans.

A paper on the research has been published in the scientific journal  ACS Nano.

Read more:  http://www.theengineer.co.uk/medical-and-healthcare/news/smart-particles-release-insulin-into-diabetics-bloodstream/1016213.article#ixzz2ScNHSXmx