Tag Archives: bulletproof clothing

Comfortable, bulletproof clothing for Canada’s Department of National Defence

h/t to Miriam Halpenny’s October 14, 2019 Castanet article as seen on the Vancouverisawesome website for this news about bulletproof clothing being developed for Canada’s National Department of Defence. I found a September 4, 2019 University of British Columbia Okanagan news release describing the research and the funds awarded to it,

The age-old technique of dressing in layers is a tried and tested way to protect from the elements. Now thanks to $1.5 million in new funding for UBC’s Okanagan campus, researchers are pushing the practice to new limits by creating a high-tech body armour solution with multiple layers of protection against diverse threats.

“Layers are great for regulating body heat, protecting us from inclement weather and helping us to survive in extreme conditions,” says Keith Culver, director of UBC’s Survive and Thrive Applied Research (STAR) initiative, which is supporting the network of researchers who will be working together over the next three years. “The idea is to design and integrate some of the most advanced fabrics and materials into garments that are comfortable, practical and can even stop a bullet.”

The research network working to develop these new Comfort-Optimized Materials For Operational Resilience, Thermal-transport and Survivability (COMFORTS) aims to create a futuristic new body armour solution by combining an intelligent, moisture-wicking base layer that has insulating properties with a layer of lightweight, ballistic-resistant material using cross-linker technology. It will also integrate a water, dust and gas repellent outer layer and will be equipped with comfort sensors to monitor the wearer’s response to extreme conditions.

“Although the basic idea seems simple, binding all these different materials and technologies together into a smart armour solution that is durable, reliable and comfortable is incredibly complex,” says Kevin Golovin, assistant professor of mechanical engineering at UBCO and principal investigator of the COMFORTS research network. “We’re putting into practice years of research and expertise in materials science to turn the concept into reality.”

The COMFORTS network is a collaboration between the University of British Columbia, the University of Alberta and the University of Victoria and is supported by a number of industrial partners. The network has received a $1.5M contribution agreement from the Department of National Defence through its Innovation for Defence Excellence and Security (IDEaS) program, designed to support innovation in defence and security.

“The safety and security threats faced by our military are ever-changing,” says Culver. “Hazards extend beyond security threats from foreign forces to natural disasters now occurring more frequently than ever before. Almost every year we’re seeing natural disasters, forest fires and floods that put not just ordinary Canadians at risk but also the personnel that respond directly to those threats. Our goal is to better protect those who put their lives on the line to protect the rest of us.”

While the initial COMFORTS technologies developed will be for defence and security applications, Culver says the potential extends well beyond the military.

“Imagine a garment that could keep its users comfortable and safe as they explore the tundra of the Canadian arctic, fight a raging forest fire or respond to a corrosive chemical spill,” says Culver. “I imagine everyone from first responders to soldiers to extreme athletes being impacted by this kind of innovation in protective clothing.”

The research will be ongoing with eight projects planned over the next three years. Some of the protective materials testing will take place at UBC’s STAR Impact Research Facility (SIRF), located just north of UBC’s Okanagan campus. The ballistic and blast simulation facility is the only one of its kind in Canada—it supports research and testing of ballistic and blast-resistant armour, ceramic and other composite materials, as well as helmets and other protective gear.

“I anticipate we will see some exciting new, field-tested technologies developed within the next few years,” says Culver. “I look forward to seeing where this collaboration will lead us.”

To learn more about the COMFORTS project, visit: ok.ubc.ca/okanagan-stories/textile-tech

UBC Expert Q&A

Western Canada primed to be defense and security research hotspot

World-class vineyards and sunny lakeside resorts have long been the reputation for BC’s Okanagan Valley. That reputation has expanded with Kelowna’s growth as a tech hub, according to Professor Keith Culver, director of UBC’s Survive and Thrive Research (STAR) initiative, but core expertise in defense and security research has also been rapidly expanding since UBC launched the STAR initiative five years ago.

Culver is a professor, legal theorist, self-described convener and coach with proven expertise assembling multi-disciplinary research teams working at the vanguard of innovation. One of these teams, led by Assistant Professor of Mechanical Engineering Kevin Golovin, was recently awarded a $1.5 million contract by the Department of National Defense to develop next-generation, high-performance body armour that increases the safety and comfort of Canadian soldiers.

What is UBC’s STAR initiative?

UBC STAR is a group of researchers and partners working together to solve human performance challenges. We know that solving complex problems requires a multi-disciplinary approach, so we build teams with specialized expertise from across both our campuses and other Western Canadian universities. Then we blend that expertise with the know-how and production capabilities of private and public sector partners to put solutions into practice. Above all, STAR helps university researchers and partners to work together in new, more productive ways.

You recently received considerable new funding from the Department of National Defence. Can you tell us about that research

A team of researchers from UBC, the University of Alberta and the University of Victoria have established a research network to invent and test new materials for the protection of humans operating in extreme environments – in this case, soldiers doing their jobs on foot. Assistant Professor Kevin Golovin of UBC Okanagan’s School of Engineering is leading the network with support from UBC STAR. The network brings together three leading Western Canadian universities to work together with industry to develop new technologies for the defence and security sector.

The network is developing several kinds of protective materials and hazard sensors for use in protective armour for soldiers and first responders. The name of the network captures its focus nicely: Comfort-Optimized Materials For Operational Resilience, Thermal-transport and Survivabilty (COMFORTS). Researchers in engineering, chemistry and other disciplines are developing new textile technologies and smart armour solutions that will be rigorously tested for thermal resistance to increase soldier comfort. We’re fortunate to be working with a great group of companies ready to turn our research into solutions ready for use. We’ll help to solve the challenges facing Canadian first responders and soldiers while enabling Canadian companies to sell those solutions to international markets.

What does the safety and security landscape look like in Western Canada?

I think there’s a perception out there that this kind of research is only happening in places like Halifax, Toronto or Waterloo. Western Canadian expertise is sometimes overlooked by Ottawa and Toronto, but there’s incredible expertise and cutting-edge research happening here in the west, and we are fortunate to have a strong private sector partner community that understands safety and security problems in military contexts, and in forestry, mining and wildfire and flood response. Our understanding of hazardous environments gives us a head start in putting technologies and strategies to work safely in extreme conditions, and we’re coming to realize that our creative solutions can both help Canadians and others around the world.

Why do companies want to work with UBC STAR and its Western Canadian partners?

We have great researchers and great facilities – our blast simulator and ballistics range are second to none – but we offer much more than expertise and equipment. UBC STAR is fundamentally about making the most of collaboration. We work together with our partners to understand the nature of problems and what could contribute to a solution. We readily draw on expertise from multiple universities and firms to assemble the right team. And we know that we are in the middle of a great living lab for testing solutions –with rural and urban areas of varying sizes, climates and terrains. We’re situated in an ideal place to work through technology development, while identifying the strategies and standards needed to put innovative technology to good use.

How do you expect this sector to develop over the next decade?

I see a boom coming in this sector. In Canada, and around the world, we are witnessing a rise in natural disasters that put first responders and others at risk, and our research can help improve their safety. At the same time, we are seeing a rise in global political tensions calling for Canadian military deployment in peacekeeping and other support roles. Our military needs help protecting its members so they can do their jobs in dangerous places. And, of course, when we develop protective materials for first responders and soldiers, the same solutions can be easily adapted for use in sport and health – such as protecting children playing contact sports or our aging population from slip and fall injuries. I think I speak for everyone involved in this research when I say that it’s incredibly rewarding to see how solutions found addressing one question often have far broader benefits for Canadians in every walk of life.

To learn more about STAR, visit: star.ubc.ca

About UBC’s Okanagan campus

UBC’s Okanagan campus is an innovative hub for research and learning in the heart of British Columbia’s stunning Okanagan Valley. Ranked among the top 20 public universities in the world, UBC is home to bold thinking and discoveries that make a difference. Established in 2005, the Okanagan campus combines a globally recognized UBC education with a tight-knit and entrepreneurial community that welcomes students and faculty from around the world.

To find out more, visit: ok.ubc.ca

Courtesy: UBC Okanagan

I have featured bulletproof clothing here in a November 4, 2013 posting featuring a business suit that included carbon nanotubes providing protection from bullets. Here’s where you can order one.

It’s a very ‘carbony’ time: graphene jacket, graphene-skinned airplane, and schwarzite

In August 2018, I been stumbled across several stories about graphene-based products and a new form of carbon.

Graphene jacket

The company producing this jacket has as its goal “… creating bionic clothing that is both bulletproof and intelligent.” Well, ‘bionic‘ means biologically-inspired engineering and ‘intelligent‘ usually means there’s some kind of computing capability in the product. This jacket, which is the first step towards the company’s goal, is not bionic, bulletproof, or intelligent. Nonetheless, it represents a very interesting science experiment in which you, the consumer, are part of step two in the company’s R&D (research and development).

Onto Vollebak’s graphene jacket,

Courtesy: Vollebak

From an August 14, 2018 article by Jesus Diaz for Fast Company,

Graphene is the thinnest possible form of graphite, which you can find in your everyday pencil. It’s purely bi-dimensional, a single layer of carbon atoms that has unbelievable properties that have long threatened to revolutionize everything from aerospace engineering to medicine. …

Despite its immense promise, graphene still hasn’t found much use in consumer products, thanks to the fact that it’s hard to manipulate and manufacture in industrial quantities. The process of developing Vollebak’s jacket, according to the company’s cofounders, brothers Steve and Nick Tidball, took years of intensive research, during which the company worked with the same material scientists who built Michael Phelps’ 2008 Olympic Speedo swimsuit (which was famously banned for shattering records at the event).

The jacket is made out of a two-sided material, which the company invented during the extensive R&D process. The graphene side looks gunmetal gray, while the flipside appears matte black. To create it, the scientists turned raw graphite into something called graphene “nanoplatelets,” which are stacks of graphene that were then blended with polyurethane to create a membrane. That, in turn, is bonded to nylon to form the other side of the material, which Vollebak says alters the properties of the nylon itself. “Adding graphene to the nylon fundamentally changes its mechanical and chemical properties–a nylon fabric that couldn’t naturally conduct heat or energy, for instance, now can,” the company claims.

The company says that it’s reversible so you can enjoy graphene’s properties in different ways as the material interacts with either your skin or the world around you. “As physicists at the Max Planck Institute revealed, graphene challenges the fundamental laws of heat conduction, which means your jacket will not only conduct the heat from your body around itself to equalize your skin temperature and increase it, but the jacket can also theoretically store an unlimited amount of heat, which means it can work like a radiator,” Tidball explains.

He means it literally. You can leave the jacket out in the sun, or on another source of warmth, as it absorbs heat. Then, the company explains on its website, “If you then turn it inside out and wear the graphene next to your skin, it acts like a radiator, retaining its heat and spreading it around your body. The effect can be visibly demonstrated by placing your hand on the fabric, taking it away and then shooting the jacket with a thermal imaging camera. The heat of the handprint stays long after the hand has left.”

There’s a lot more to the article although it does feature some hype and I’m not sure I believe Diaz’s claim (August 14, 2018 article) that ‘graphene-based’ hair dye is perfectly safe ( Note: A link has been removed),

Graphene is the thinnest possible form of graphite, which you can find in your everyday pencil. It’s purely bi-dimensional, a single layer of carbon atoms that has unbelievable properties that will one day revolutionize everything from aerospace engineering to medicine. Its diverse uses are seemingly endless: It can stop a bullet if you add enough layers. It can change the color of your hair with no adverse effects. [emphasis mine] It can turn the walls of your home into a giant fire detector. “It’s so strong and so stretchy that the fibers of a spider web coated in graphene could catch a falling plane,” as Vollebak puts it in its marketing materials.

Not unless things have changed greatly since March 2018. My August 2, 2018 posting featured the graphene-based hair dye announcement from March 2018 and a cautionary note from Dr. Andrew Maynard (scroll down ab out 50% of the way for a longer excerpt of Maynard’s comments),

Northwestern University’s press release proudly announced, “Graphene finds new application as nontoxic, anti-static hair dye.” The announcement spawned headlines like “Enough with the toxic hair dyes. We could use graphene instead,” and “’Miracle material’ graphene used to create the ultimate hair dye.”

From these headlines, you might be forgiven for getting the idea that the safety of graphene-based hair dyes is a done deal. Yet having studied the potential health and environmental impacts of engineered nanomaterials for more years than I care to remember, I find such overly optimistic pronouncements worrying – especially when they’re not backed up by clear evidence.

These studies need to be approached with care, as the precise risks of graphene exposure will depend on how the material is used, how exposure occurs and how much of it is encountered. Yet there’s sufficient evidence to suggest that this substance should be used with caution – especially where there’s a high chance of exposure or that it could be released into the environment.

The full text of Dr. Maynard’s comments about graphene hair dyes and risk can be found here.

Bearing in mind  that graphene-based hair dye is an entirely different class of product from the jacket, I wouldn’t necessarily dismiss risks; I would like to know what kind of risk assessment and safety testing has been done. Due to their understandable enthusiasm, the brothers Tidball have focused all their marketing on the benefits and the opportunity for the consumer to test their product (from graphene jacket product webpage),

While it’s completely invisible and only a single atom thick, graphene is the lightest, strongest, most conductive material ever discovered, and has the same potential to change life on Earth as stone, bronze and iron once did. But it remains difficult to work with, extremely expensive to produce at scale, and lives mostly in pioneering research labs. So following in the footsteps of the scientists who discovered it through their own highly speculative experiments, we’re releasing graphene-coated jackets into the world as experimental prototypes. Our aim is to open up our R&D and accelerate discovery by getting graphene out of the lab and into the field so that we can harness the collective power of early adopters as a test group. No-one yet knows the true limits of what graphene can do, so the first edition of the Graphene Jacket is fully reversible with one side coated in graphene and the other side not. If you’d like to take part in the next stage of this supermaterial’s history, the experiment is now open. You can now buy it, test it and tell us about it. [emphasis mine]

How maverick experiments won the Nobel Prize

While graphene’s existence was first theorised in the 1940s, it wasn’t until 2004 that two maverick scientists, Andre Geim and Konstantin Novoselov, were able to isolate and test it. Through highly speculative and unfunded experimentation known as their ‘Friday night experiments,’ they peeled layer after layer off a shaving of graphite using Scotch tape until they produced a sample of graphene just one atom thick. After similarly leftfield thinking won Geim the 2000 Ig Nobel prize for levitating frogs using magnets, the pair won the Nobel prize in 2010 for the isolation of graphene.

Should you be interested, in beta-testing the jacket, it will cost you $695 (presumably USD); order here. One last thing, Vollebak is based in the UK.

Graphene skinned plane

An August 14, 2018 news item (also published as an August 1, 2018 Haydale press release) by Sue Keighley on Azonano heralds a new technology for airplans,

Haydale, (AIM: HAYD), the global advanced materials group, notes the announcement made yesterday from the University of Central Lancashire (UCLAN) about the recent unveiling of the world’s first graphene skinned plane at the internationally renowned Farnborough air show.

The prepreg material, developed by Haydale, has potential value for fuselage and wing surfaces in larger scale aero and space applications especially for the rapidly expanding drone market and, in the longer term, the commercial aerospace sector. By incorporating functionalised nanoparticles into epoxy resins, the electrical conductivity of fibre-reinforced composites has been significantly improved for lightning-strike protection, thereby achieving substantial weight saving and removing some manufacturing complexities.

Before getting to the photo, here’s a definition for pre-preg from its Wikipedia entry (Note: Links have been removed),

Pre-preg is “pre-impregnated” composite fibers where a thermoset polymer matrix material, such as epoxy, or a thermoplastic resin is already present. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture.

Haydale has supplied graphene enhanced prepreg material for Juno, a three-metre wide graphene-enhanced composite skinned aircraft, that was revealed as part of the ‘Futures Day’ at Farnborough Air Show 2018. [downloaded from https://www.azonano.com/news.aspx?newsID=36298]

A July 31, 2018 University of Central Lancashire (UCLan) press release provides a tiny bit more (pun intended) detail,

The University of Central Lancashire (UCLan) has unveiled the world’s first graphene skinned plane at an internationally renowned air show.

Juno, a three-and-a-half-metre wide graphene skinned aircraft, was revealed on the North West Aerospace Alliance (NWAA) stand as part of the ‘Futures Day’ at Farnborough Air Show 2018.

The University’s aerospace engineering team has worked in partnership with the Sheffield Advanced Manufacturing Research Centre (AMRC), the University of Manchester’s National Graphene Institute (NGI), Haydale Graphene Industries (Haydale) and a range of other businesses to develop the unmanned aerial vehicle (UAV), which also includes graphene batteries and 3D printed parts.

Billy Beggs, UCLan’s Engineering Innovation Manager, said: “The industry reaction to Juno at Farnborough was superb with many positive comments about the work we’re doing. Having Juno at one the world’s biggest air shows demonstrates the great strides we’re making in leading a programme to accelerate the uptake of graphene and other nano-materials into industry.

“The programme supports the objectives of the UK Industrial Strategy and the University’s Engineering Innovation Centre (EIC) to increase industry relevant research and applications linked to key local specialisms. Given that Lancashire represents the fourth largest aerospace cluster in the world, there is perhaps no better place to be developing next generation technologies for the UK aerospace industry.”

Previous graphene developments at UCLan have included the world’s first flight of a graphene skinned wing and the launch of a specially designed graphene-enhanced capsule into near space using high altitude balloons.

UCLan engineering students have been involved in the hands-on project, helping build Juno on the Preston Campus.

Haydale supplied much of the material and all the graphene used in the aircraft. Ray Gibbs, Chief Executive Officer, said: “We are delighted to be part of the project team. Juno has highlighted the capability and benefit of using graphene to meet key issues faced by the market, such as reducing weight to increase range and payload, defeating lightning strike and protecting aircraft skins against ice build-up.”

David Bailey Chief Executive of the North West Aerospace Alliance added: “The North West aerospace cluster contributes over £7 billion to the UK economy, accounting for one quarter of the UK aerospace turnover. It is essential that the sector continues to develop next generation technologies so that it can help the UK retain its competitive advantage. It has been a pleasure to support the Engineering Innovation Centre team at the University in developing the world’s first full graphene skinned aircraft.”

The Juno project team represents the latest phase in a long-term strategic partnership between the University and a range of organisations. The partnership is expected to go from strength to strength following the opening of the £32m EIC facility in February 2019.

The next step is to fly Juno and conduct further tests over the next two months.

Next item, a new carbon material.

Schwarzite

I love watching this gif of a schwarzite,

The three-dimensional cage structure of a schwarzite that was formed inside the pores of a zeolite. (Graphics by Yongjin Lee and Efrem Braun)

An August 13, 2018 news item on Nanowerk announces the new carbon structure,

The discovery of buckyballs [also known as fullerenes, C60, or buckminsterfullerenes] surprised and delighted chemists in the 1980s, nanotubes jazzed physicists in the 1990s, and graphene charged up materials scientists in the 2000s, but one nanoscale carbon structure – a negatively curved surface called a schwarzite – has eluded everyone. Until now.

University of California, Berkeley [UC Berkeley], chemists have proved that three carbon structures recently created by scientists in South Korea and Japan are in fact the long-sought schwarzites, which researchers predict will have unique electrical and storage properties like those now being discovered in buckminsterfullerenes (buckyballs or fullerenes for short), nanotubes and graphene.

An August 13, 2018 UC Berkeley news release by Robert Sanders, which originated the news item, describes how the Berkeley scientists and the members of their international  collaboration from Germany, Switzerland, Russia, and Italy, have contributed to the current state of schwarzite research,

The new structures were built inside the pores of zeolites, crystalline forms of silicon dioxide – sand – more commonly used as water softeners in laundry detergents and to catalytically crack petroleum into gasoline. Called zeolite-templated carbons (ZTC), the structures were being investigated for possible interesting properties, though the creators were unaware of their identity as schwarzites, which theoretical chemists have worked on for decades.

Based on this theoretical work, chemists predict that schwarzites will have unique electronic, magnetic and optical properties that would make them useful as supercapacitors, battery electrodes and catalysts, and with large internal spaces ideal for gas storage and separation.

UC Berkeley postdoctoral fellow Efrem Braun and his colleagues identified these ZTC materials as schwarzites based of their negative curvature, and developed a way to predict which zeolites can be used to make schwarzites and which can’t.

“We now have the recipe for how to make these structures, which is important because, if we can make them, we can explore their behavior, which we are working hard to do now,” said Berend Smit, an adjunct professor of chemical and biomolecular engineering at UC Berkeley and an expert on porous materials such as zeolites and metal-organic frameworks.

Smit, the paper’s corresponding author, Braun and their colleagues in Switzerland, China, Germany, Italy and Russia will report their discovery this week in the journal Proceedings of the National Academy of Sciences. Smit is also a faculty scientist at Lawrence Berkeley National Laboratory.

Playing with carbon

Diamond and graphite are well-known three-dimensional crystalline arrangements of pure carbon, but carbon atoms can also form two-dimensional “crystals” — hexagonal arrangements patterned like chicken wire. Graphene is one such arrangement: a flat sheet of carbon atoms that is not only the strongest material on Earth, but also has a high electrical conductivity that makes it a promising component of electronic devices.

schwarzite carbon cage

The cage structure of a schwarzite that was formed inside the pores of a zeolite. The zeolite is subsequently dissolved to release the new material. (Graphics by Yongjin Lee and Efrem Braun)

Graphene sheets can be wadded up to form soccer ball-shaped fullerenes – spherical carbon cages that can store molecules and are being used today to deliver drugs and genes into the body. Rolling graphene into a cylinder yields fullerenes called nanotubes, which are being explored today as highly conductive wires in electronics and storage vessels for gases like hydrogen and carbon dioxide. All of these are submicroscopic, 10,000 times smaller than the width of a human hair.

To date, however, only positively curved fullerenes and graphene, which has zero curvature, have been synthesized, feats rewarded by Nobel Prizes in 1996 and 2010, respectively.

In the 1880s, German physicist Hermann Schwarz investigated negatively curved structures that resemble soap-bubble surfaces, and when theoretical work on carbon cage molecules ramped up in the 1990s, Schwarz’s name became attached to the hypothetical negatively curved carbon sheets.

“The experimental validation of schwarzites thus completes the triumvirate of possible curvatures to graphene; positively curved, flat, and now negatively curved,” Braun added.

Minimize me

Like soap bubbles on wire frames, schwarzites are topologically minimal surfaces. When made inside a zeolite, a vapor of carbon-containing molecules is injected, allowing the carbon to assemble into a two-dimensional graphene-like sheet lining the walls of the pores in the zeolite. The surface is stretched tautly to minimize its area, which makes all the surfaces curve negatively, like a saddle. The zeolite is then dissolved, leaving behind the schwarzite.

soap bubble schwarzite structure

A computer-rendered negatively curved soap bubble that exhibits the geometry of a carbon schwarzite. (Felix Knöppel image)

“These negatively-curved carbons have been very hard to synthesize on their own, but it turns out that you can grow the carbon film catalytically at the surface of a zeolite,” Braun said. “But the schwarzites synthesized to date have been made by choosing zeolite templates through trial and error. We provide very simple instructions you can follow to rationally make schwarzites and we show that, by choosing the right zeolite, you can tune schwarzites to optimize the properties you want.”

Researchers should be able to pack unusually large amounts of electrical charge into schwarzites, which would make them better capacitors than conventional ones used today in electronics. Their large interior volume would also allow storage of atoms and molecules, which is also being explored with fullerenes and nanotubes. And their large surface area, equivalent to the surface areas of the zeolites they’re grown in, could make them as versatile as zeolites for catalyzing reactions in the petroleum and natural gas industries.

Braun modeled ZTC structures computationally using the known structures of zeolites, and worked with topological mathematician Senja Barthel of the École Polytechnique Fédérale de Lausanne in Sion, Switzerland, to determine which of the minimal surfaces the structures resembled.

The team determined that, of the approximately 200 zeolites created to date, only 15 can be used as a template to make schwarzites, and only three of them have been used to date to produce schwarzite ZTCs. Over a million zeolite structures have been predicted, however, so there could be many more possible schwarzite carbon structures made using the zeolite-templating method.

Other co-authors of the paper are Yongjin Lee, Seyed Mohamad Moosavi and Barthel of the École Polytechnique Fédérale de Lausanne, Rocio Mercado of UC Berkeley, Igor Baburin of the Technische Universität Dresden in Germany and Davide Proserpio of the Università degli Studi di Milano in Italy and Samara State Technical University in Russia.

Here’s a link to and a citation for the paper,

Generating carbon schwarzites via zeolite-templating by Efrem Braun, Yongjin Lee, Seyed Mohamad Moosavi, Senja Barthel, Rocio Mercado, Igor A. Baburin, Davide M. Proserpio, and Berend Smit. PNAS August 14, 2018. 201805062; published ahead of print August 14, 2018. https://doi.org/10.1073/pnas.1805062115

This paper appears to be open access.