Tag Archives: KTH Royal Institute of Technology

Stronger than steel and spider silk: artificial, biodegradable, cellulose nanofibres

This is an artificial and biodegradable are two adjectives you don’t usually see united by the conjunction, and. However, it is worth noting that the artificial material is initially derived from a natural material, cellulose. Here’s more from a May 16, 2018 news item on ScienceDaily,

At DESY’s [Deutsches Elektronen-Synchrotron] X-ray light source PETRA III, a team led by Swedish researchers has produced the strongest bio-material that has ever been made. The artifical, but bio-degradable cellulose fibres are stronger than steel and even than dragline spider silk, which is usually considered the strongest bio-based material. The team headed by Daniel Söderberg from the KTH Royal Institute of Technology in Stockholm reports the work in the journal ACS Nano of the American Chemical Society.

A May 16, 2018 DESY press release (also on EurekAlert), which originated the news item, provides more detail,

The ultrastrong material is made of cellulose nanofibres (CNF), the essential building blocks of wood and other plant life. Using a novel production method, the researchers have successfully transferred the unique mechanical properties of these nanofibres to a macroscopic, lightweight material that could be used as an eco-friendly alternative for plastic in airplanes, cars, furniture and other products. “Our new material even has potential for biomedicine since cellulose is not rejected by your body”, explains Söderberg.

The scientists started with commercially available cellulose nanofibres that are just 2 to 5 nanometres in diameter and up to 700 nanometres long. A nanometre (nm) is a millionth of a millimetre. The nanofibres were suspended in water and fed into a small channel, just one millimetre wide and milled in steel. Through two pairs of perpendicular inflows additional deionized water and water with a low pH-value entered the channel from the sides, squeezing the stream of nanofibres together and accelerating it.

This process, called hydrodynamic focussing, helped to align the nanofibres in the right direction as well as their self-organisation into a well-packed macroscopic thread. No glue or any other component is needed, the nanofibres assemble into a tight thread held together by supramolecular forces between the nanofibres, for example electrostatic and Van der Waals forces.

With the bright X-rays from PETRA III the scientists could follow and optimise the process. “The X-rays allow us to analyse the detailed structure of the thread as it forms as well as the material structure and hierarchical order in the super strong fibres,” explains co-author Stephan Roth from DESY, head of the Micro- and Nanofocus X-ray Scattering Beamline P03 where the threads were spun. “We made threads up to 15 micrometres thick and several metres in length.”

Measurements showed a tensile stiffness of 86 gigapascals (GPa) for the material and a tensile strength of 1.57 GPa. “The bio-based nanocellulose fibres fabricated here are 8 times stiffer and have strengths higher than natural dragline spider silk fibres,” says Söderberg. “If you are looking for a bio-based material, there is nothing quite like it. And it is also stronger than steel and any other metal or alloy as well as glass fibres and most other synthetic materials.” The artificial cellulose fibres can be woven into a fabric to create materials for various applications. The researchers estimate that the production costs of the new material can compete with those of strong synthetic fabrics. “The new material can in principle be used to create bio-degradable components,” adds Roth.

The study describes a new method that mimics nature’s ability to accumulate cellulose nanofibres into almost perfect macroscale arrangements, like in wood. It opens the way for developing nanofibre material that can be used for larger structures while retaining the nanofibres’ tensile strength and ability to withstand mechanical load. “We can now transform the super performance from the nanoscale to the macroscale,” Söderberg underlines. “This discovery is made possible by understanding and controlling the key fundamental parameters essential for perfect nanostructuring, such as particle size, interactions, alignment, diffusion, network formation and assembly.” The process can also be used to control nanoscale assembly of carbon tubes and other nano-sized fibres.

(There are some terminology and spelling issues, which are described at the end of this post.)

Let’s get back to a material that rivals spider silk and steel for strength (for some reason that reminded me of an old carnival game where you’d test your strength by swinging a mallet down on a ‘teeter-totter-like’ board and sending a metal piece up a post to make a bell ring). From a May 16, 2018 DESY press release (also on EurekAlert), which originated the news item,

The ultrastrong material is made of cellulose nanofibres (CNF), the essential building blocks of wood and other plant life. Using a novel production method, the researchers have successfully transferred the unique mechanical properties of these nanofibres to a macroscopic, lightweight material that could be used as an eco-friendly alternative for plastic in airplanes, cars, furniture and other products. “Our new material even has potential for biomedicine since cellulose is not rejected by your body”, explains Söderberg.

The scientists started with commercially available cellulose nanofibres that are just 2 to 5 nanometres in diameter and up to 700 nanometres long. A nanometre (nm) is a millionth of a millimetre. The nanofibres were suspended in water and fed into a small channel, just one millimetre wide and milled in steel. Through two pairs of perpendicular inflows additional deionized water and water with a low pH-value entered the channel from the sides, squeezing the stream of nanofibres together and accelerating it.

This process, called hydrodynamic focussing, helped to align the nanofibres in the right direction as well as their self-organisation into a well-packed macroscopic thread. No glue or any other component is needed, the nanofibres assemble into a tight thread held together by supramolecular forces between the nanofibres, for example electrostatic and Van der Waals forces.

With the bright X-rays from PETRA III the scientists could follow and optimise the process. “The X-rays allow us to analyse the detailed structure of the thread as it forms as well as the material structure and hierarchical order in the super strong fibres,” explains co-author Stephan Roth from DESY, head of the Micro- and Nanofocus X-ray Scattering Beamline P03 where the threads were spun. “We made threads up to 15 micrometres thick and several metres in length.”

Measurements showed a tensile stiffness of 86 gigapascals (GPa) for the material and a tensile strength of 1.57 GPa. “The bio-based nanocellulose fibres fabricated here are 8 times stiffer and have strengths higher than natural dragline spider silk fibres,” says Söderberg. “If you are looking for a bio-based material, there is nothing quite like it. And it is also stronger than steel and any other metal or alloy as well as glass fibres and most other synthetic materials.” The artificial cellulose fibres can be woven into a fabric to create materials for various applications. The researchers estimate that the production costs of the new material can compete with those of strong synthetic fabrics. “The new material can in principle be used to create bio-degradable components,” adds Roth.

The study describes a new method that mimics nature’s ability to accumulate cellulose nanofibres into almost perfect macroscale arrangements, like in wood. It opens the way for developing nanofibre material that can be used for larger structures while retaining the nanofibres’ tensile strength and ability to withstand mechanical load. “We can now transform the super performance from the nanoscale to the macroscale,” Söderberg underlines. “This discovery is made possible by understanding and controlling the key fundamental parameters essential for perfect nanostructuring, such as particle size, interactions, alignment, diffusion, network formation and assembly.” The process can also be used to control nanoscale assembly of carbon tubes and other nano-sized fibres.

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

Multiscale Control of Nanocellulose Assembly: Transferring Remarkable Nanoscale Fibril Mechanics to Macroscale Fibers by Nitesh Mittal, Farhan Ansari, Krishne Gowda V, Christophe Brouzet, Pan Chen, Per Tomas Larsson, Stephan V. Roth, Fredrik Lundell, Lars Wågberg, Nicholas A. Kotov, and L. Daniel Söderberg. ACS Nano, Article ASAP DOI: 10.1021/acsnano.8b01084 Publication Date (Web): May 9, 2018

Copyright © 2018 American Chemical Society

This paper is open access and accompanied by this image illustrating the work,

Courtesy: American Chemical Society and the researchers [Note: The bottom two images of cellulose nanofibres, which are constittuents of an artificial cellulose fibre, appear to be from a scanning tunneling microsscope. Credit: Nitesh Mittal, KTH Stockholm

This news has excited interest at General Electric (GE) (its Wikipedia entry), which has highlighted the work in a May 25, 2018 posting (The 5 Coolest Things On Earth This Week) by Tomas Kellner on the GE Reports blog.

Terminology and spelling

I’ll start with spelling since that’s the easier of the two. In some parts of the world it’s spelled ‘fibres’ and in other parts of the world it’s spelled ‘fibers’. When I write the text in my post, it tends to reflect the spelling used in the news/press releases. In other words, I swing in whichever direction the wind is blowing.

For diehards only

As i understand the terminology situation, nanocellulose and cellulose nanomaterials are interchangeable generic terms. Further, cellulose nanofibres (CNF) seems to be another generic term and it encompasses both cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF). Yes, there appear to be two CNFs. Making matters more interesting is the fact that cellulose nanocrystals were originally christened nanocrystalline cellulose (NCC). For anyone who follows the science and technology scene, it becomes obvious that competing terminologies are the order of the day. Eventually the dust settles and naming conventions are resolved. More or less.

Ordinarily I would reference the Nanocellulose Wikipedia entry in my attempts to clarify the issues but it seems that the writers for the entry have not caught up to the current naming convention for cellulose nanocrystals, still referring to the material as nanocrystalline cellulose. This means, I can’t trust the rest of the entry, which has only one CNF (cellulose nanofibres).

I have paid more attention to the NCC/CNC situation and am not as familiar with the CNF situation. Using, NCC/CNC as an example of a terminology issue, I believe it was first developed in Canada and it was Canadian researchers who were pushing their NCC terminology while the international community pushed back with CNC.

In the end, NCC became a brand name, which was trademarked by CelluForce, a Canadian company in the CNC market. From the CelluForce Products page on Cellulose Nanocrystals,

CNC are not all made equal. The CNC produced by CelluForce is called CelluForce NCCTM and has specific properties and are especially easy to disperse. CelluForce NCCTM is the base material that CelluForce uses in all its products. This base material can be modified and tailored to suit the specific needs in various applications.

These, days CNC is almost universally used but NCC (not as a trademark) is a term still employed on occasion (and, oddly, the researchers are not necessarily Canadian).

Should anyone have better information about terminology issues, please feel free to comment.

Cellulose aerogels for new wood-based composites

‘Frozen smoke’ or ‘solid smoke’ as it’s sometimes described, aerogel fascinates scientists.The latest on cellulose aerogels derived from wood is the focus for a February 14, 2018 Nanowerk Sportlight article by Michael Berger (Note: Links have been removed),

Aerogels, sometimes called frozen smoke, are nanoscale foams: solid materials whose sponge-like structure is riddled by pores as small as nanometers across. They can be made from different materials, for instance silicon.

Aerogels are among the lightest solid substances in the world yet flexible, extremely strong and water repellent, which makes them very interesting materials for engineers.

Cellulose aerogels, made from nanofibrils found in plants, have several unique features, one of which is super high oil absorption capacity that is several times higher than commercial sorbents available in the market.

“Encouraged from our previous work on transparent wood (“Transparent wood for functional and structural applications”; “Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance”; “Nanostructured Wood Hybrids for Fire-Retardancy Prepared by Clay Impregnation into the Cell Wall”), we started to develop porous wood/epoxy biocomposite materials, which preserves the original hierarchical and porous structure of wood,” Qi Zhou, an associate professor in the Department of Chemistry at KTH Royal Institute of Technology, tells Nanowerk.

“Our strategy is different from traditional wood modification methods,” explains Zhou. “It involves two steps, a simple chemical treatment to remove the lignin (delignification) at first, then back infiltration of the wood cell wall with epoxy, leaving the lumen (a void space) open. In traditional wood polymer composites, both the cell wall and cell lumen are filled with polymer.”

The scientists don’t seem to have any particular applications in mind but they are hopeful that new materials will inspire new uses. Here’s a link to and a citation for Zhou’s latest paper,

Wood Nanotechnology for Strong, Mesoporous, and Hydrophobic Biocomposites for Selective Separation of Oil/Water Mixtures by Qiliang Fu, Farhan Ansari, Qi Zhou, and Lars A. Berglund. ACS Nano, Article ASAP DOI: 10.1021/acsnano.8b00005 Publication Date (Web): February 7, 2018

Copyright © 2018 American Chemical Society

This paper is behind a paywall.

Microneedle patch from Sweden

Strictly speaking this isn’t a ‘nano’ story but this work from Sweden provides a complement and contrast to the Australian nanopatch I mentioned in a post earlier today (Dec. 16, 2016). From a Dec. 12, 2016 news item on Nanowerk,

It’s only a matter of time before drugs are administered via patches with painless microneedles instead of unpleasant injections. But designers need to balance the need for flexible, comfortable-to-wear material with effective microneedle penetration of the skin. Swedish researchers say they may have cracked the problem.

In the recent volume of PLOS ONE (“Flexible and Stretchable Microneedle Patches with Integrated Rigid Stainless Steel Microneedles for Transdermal Biointerfacing”), a research team from KTH Royal Institute of Technology in Stockholm reports a successful test of its microneedle patch, which combines stainless steel needles embedded in a soft polymer base – the first such combination believed to be scientifically studied. The soft material makes it comfortable to wear, while the stiff needles ensure reliable skin penetration.

A Dec. 12, 2016 KTH Royal Institute of Technology press release, which originated the news item, describes exactly the limitation that the scientists are trying to surmount,

Unlike epidermal patches, microneedles penetrate the upper layer of the skin, just enough to avoid touching the nerves. This enables delivery of drugs, extraction of physiological signals for fitness monitoring devices, extracting body fluids for real-time monitoring of glucose, pH level and other diagnostic markers, as well as skin treatments in cosmetics and bioelectric treatments.

Frank Niklaus, professor of micro and nanofabrication at KTH, says that practically all microneedle arrays being tested today are “monoliths”, that is, the needles and their supporting base are made of the same – often hard and stiff – material. While that allows the microneedles to penetrate the skin, they are uncomfortable to wear. On the other hand, if the whole array is made from softer materials, they may fit more comfortably, but soft needles are less reliable for penetrating the skin.

“To the best of our knowledge, flexible and stretchable patches with arrays of sharp and stiff microneedles have not been demonstrated to date,” he says.

They actually tested two variations of their concept, one which was stretchable and slightly more flexible than the other. The more flexible patch, which has a base of molded thiol-ene-epoxy-based thermoset film, conformed well to deformations of the skin surface and each of the 50 needles penetrated the skin during a 30 minute test.

A successful microneedle product could have major implications for health care delivery. “The chronically ill would not have to take daily injections,” says co-author Niclas Roxhed, who is research leader at the Department of Micro and Nano Systems at KTH.

In addition to addressing people’s reluctance to take painful shots, microneedles also offer a hygiene benefit. The World Health Organization estimates that about 1.3 million people die worldwide each year due to improper handling of needles.

“Since the patch does not enter the bloodstream, there is less risk of spreading infections,” Roxhed says.

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

Flexible and Stretchable Microneedle Patches with Integrated Rigid Stainless Steel Microneedles for Transdermal Biointerfacing by Mina Rajabi, Niclas Roxhed, Reza Zandi Shafagh, Tommy Haraldson,  Andreas Christin Fischer, Wouter van der Wijngaart, Göran Stemme, Frank Niklaus. PLOS [Public Library of Science] http://dx.doi.org/10.1371/journal.pone.0166330 Published: December 9, 2016

This paper is open access.

Transparent wood instead of glass for window panes?

The transparent wood is made by removing the lignin in the wood veneer. (Photo: Peter Larsson

The transparent wood is made by removing the lignin in the wood veneer. (Photo: Peter Larsson

Not quite ready as a replacement for some types of glass window panes, nonetheless, transparent (more like translucent) wood is an impressive achievement. According to a March 30, 2016 news item on ScienceDaily size is what makes this piece of transparent wood newsworthy,

Windows and solar panels in the future could be made from one of the best — and cheapest — construction materials known: wood. Researchers at Stockholm’s KTH Royal Institute of Technology [Sweden] have developed a new transparent wood material that’s suitable for mass production.

Lars Berglund, a professor at Wallenberg Wood Science Center at KTH, says that while optically transparent wood has been developed for microscopic samples in the study of wood anatomy, the KTH project introduces a way to use the material on a large scale. …

A March 31 (?), 2016 KTH Institute of Technology press release, which originated the news item, provides more detail,

“Transparent wood is a good material for solar cells, since it’s a low-cost, readily available and renewable resource,” Berglund says. “This becomes particularly important in covering large surfaces with solar cells.”

Berglund says transparent wood panels can also be used for windows, and semitransparent facades, when the idea is to let light in but maintain privacy.

The optically transparent wood is a type of wood veneer in which the lignin, a component of the cell walls, is removed chemically.

“When the lignin is removed, the wood becomes beautifully white. But because wood isn’t not naturally transparent, we achieve that effect with some nanoscale tailoring,” he says.

The white porous veneer substrate is impregnated with a transparent polymer and the optical properties of the two are then matched, he says.

“No one has previously considered the possibility of creating larger transparent structures for use as solar cells and in buildings,” he says

Among the work to be done next is enhancing the transparency of the material and scaling up the manufacturing process, Berglund says.

“We also intend to work further with different types of wood,” he adds.

“Wood is by far the most used bio-based material in buildings. It’s attractive that the material comes from renewable sources. It also offers excellent mechanical properties, including strength, toughness, low density and low thermal conductivity.”

The American Chemical Society has a March 30, 2016 news release about the KTH achievement on EurekAlert  highlighting another potential use for transparent wood,

When it comes to indoor lighting, nothing beats the sun’s rays streaming in through windows. Soon, that natural light could be shining through walls, too. Scientists have developed transparent wood that could be used in building materials and could help home and building owners save money on their artificial lighting costs. …

Homeowners often search for ways to brighten up their living space. They opt for light-colored paints, mirrors and lots of lamps and ceiling lights. But if the walls themselves were transparent, this would reduce the need for artificial lighting — and the associated energy costs. Recent work on making transparent paper from wood has led to the potential for making similar but stronger materials. Lars Berglund and colleagues wanted to pursue this possibility.

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

Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance by Yuanyuan Li, Qiliang Fu, Shun Yu, Min Yan, and Lars Berglund. Biomacromolecules, Article ASAP DOI: 10.1021/acs.biomac.6b00145 Publication Date (Web): March 4, 2016

Copyright © 2016 American Chemical Society

This paper appears to be open access.

Paper as good at storing electrical energy as commercial supercapacitors

This is another potential nanocellulose application according to a Dec. 3, 2015 news item on ScienceDaily,

Researchers at Linköping University’s Laboratory of Organic Electronics, Sweden, have developed power paper — a new material with an outstanding ability to store energy. The material consists of nanocellulose and a conductive polymer. …

One sheet, 15 centimetres in diameter and a few tenths of a millimetre thick can store as much as 1 F, which is similar to the supercapacitors currently on the market. The material can be recharged hundreds of times and each charge only takes a few seconds.

A Dec. 3, 2015 Linköping University press release (also on EurekAlert), which originated the news item, provides more detail,

It’s a dream product in a world where the increased use of renewable energy requires new methods for energy storage — from summer to winter, from a windy day to a calm one, from a sunny day to one with heavy cloud cover.

“Thin films that function as capacitors have existed for some time. What we have done is to produce the material in three dimensions. We can produce thick sheets,” says Xavier Crispin, professor of organic electronics and co-author to the article just published in Advanced Science.

Other co-authors are researchers from KTH Royal Institute of Technology, Innventia, Technical University of Denmark and the University of Kentucky.

The material, power paper, looks and feels like a slightly plasticky paper and the researchers have amused themselves by using one piece to make an origami swan — which gives an indication of its strength.

The structural foundation of the material is nanocellulose, which is cellulose fibres which, using high-pressure water, are broken down into fibres as thin as 20 nm in diameter. With the cellulose fibres in a solution of water, an electrically charged polymer (PEDOT:PSS), also in a water solution, is added. The polymer then forms a thin coating around the fibres.

“The covered fibres are in tangles, where the liquid in the spaces between them functions as an electrolyte,” explains Jesper Edberg, doctoral student, who conducted the experiments together with Abdellah Malti, who recently completed his doctorate.

The new cellulose-polymer material has set a new world record in simultaneous conductivity for ions and electrons, which explains its exceptional capacity for energy storage. It also opens the door to continued development toward even higher capacity. Unlike the batteries and capacitors currently on the market, power paper is produced from simple materials – renewable cellulose and an easily available polymer. It is light in weight, it requires no dangerous chemicals or heavy metals and it is waterproof.

This press release also offers insight into funding and how scientists view requests for reports and oversight,

The Power Papers project has been financed by the Knut and Alice Wallenberg Foundation since 2012.

“They leave us to our research, without demanding lengthy reports, and they trust us. We have a lot of pressure on us to deliver, but it’s ok if it takes time, and we’re grateful for that,” says Professor Magnus Berggren, director of the Laboratory of Organic Electronics at Linköping University.

Naturally, commercialization efforts are already in the works. (Canadian nanocellulose community watch out! The Swedes are coming!),

The new power paper is just like regular pulp, which has to be dehydrated when making paper. The challenge is to develop an industrial-scale process for this.

“Together with KTH, Acreo and Innventia we just received SEK 34 million from the Swedish Foundation for Strategic Research to continue our efforts to develop a rational production method, a paper machine for power paper,” says Professor Berggren.

Here’s a link to and a citation for the team’s study,

An Organic Mixed Ion–Electron Conductor for Power Electronics by Abdellah Malti, Jesper Edberg, Hjalmar Granberg, Zia Ullah Khan, Jens W. Andreasen, Xianjie Liu, Dan Zhao, Hao Zhang, Yulong Yao, Joseph W. Brill, Isak Engquist, Mats Fahlman, Lars Wågberg, Xavier Crispin, and Magnus Berggren. Advanced Science DOI: 10.1002/advs.201500305 Article first published online: 2 DEC 2015

© 2015 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is open access.

Touchy feely breakthrough at the nano scale

This first posting back after a three week hiatus (I’m baaack) concerns a study in Sweden where scientists found that people can discern nano wrinkles with their fingertips. From the Sept. 16, 2013 news item on Nanowerk,

In a ground-breaking study, Swedish scientists have shown that people can detect nano-scale wrinkles while running their fingers upon a seemingly smooth surface. The findings could lead such advances as touch screens for the visually impaired and other products, says one of the researchers from KTH Royal Institute of Technology.

The study marks the first time that scientists have quantified how people feel, in terms of a physical property. One of the authors, Mark Rutland, Professor of Surface Chemistry, says that the human finger can discriminate between surfaces patterned with ridges as small as 13 nanometres in amplitude and non-patterned surfaces.

The KTH Sept. 16, 2013 news release by David Callahan, which originated the news item, describes the new understanding of touch and its possible applications,

The study highlights the importance of surface friction and wrinkle wavelength, or wrinkle width – in the tactile perception of fine textures.

When a finger is drawn over a surface, vibrations occur in the finger. People feel these vibrations differently on different structures. The friction properties of the surface control how hard we press on the surface as we explore it. A high friction surface requires us to press less to achieve the optimum friction force.

“This is the breakthrough that allows us to design how things feel and are perceived,” he says. “It allows, for example, for a certain portion of a touch screen on a smartphone to be designed to feel differently by vibration.”

The research could inform the development of the sense of touch in robotics and virtual reality. A plastic touch screen surface could be made to feel like another material, such as fabric or wood, for example. The findings also enable differentiation in product packaging, or in the products themselves. A shampoo, for example, can be designed to change the feel of one’s hair.

The news release goes on to describe how the research was conducted,

With the collaboration of National Institute of Standards and Technology (NIST) material science labs, Rutland and his colleagues produced 16 chemically-identical surfaces with wrinkle wavelengths (or wrinkle widths) ranging from 300 nanometres to 90 micrometres, and amplitudes (or wrinkle heights) of between seven nanometres and 4.5 micrometres, as well as two non-patterned surfaces. The participants were presented with random pairs of surfaces and asked to run their dominant index finger across each one in a designated direction, which was perpendicular to the groove, before rating the similarity of the two surfaces.

The smallest pattern that could be distinguished from the non-patterned surface had grooves with a wavelength of 760 nanometres and an amplitude of only 13 nanometres.

Rutland says that by bringing together professors and PhD students from two different disciplines – surface chemistry and psychology – the team succeeded in creating “a truly psycho-physical study.”

“The important thing is that touch was previously the unknown sense,” Rutland says. “To make the analogy with vision, it is as if we have just revealed how we perceive colour.

“Now we can start using this knowledge for tactile aesthetics in the same way that colours and intensity can be combined for visual aesthetics.”

Here’s a citation for and link to the researchers’ study,

Feeling Small: Exploring the Tactile Perception Limits by Lisa Skedung, Martin Arvidsson, Jun Young Chung, Christopher M. Stafford, Birgitta Berglund & Mark W. Rutland. Scientific Reports 3, Article number: 2617 doi: 10.1038/srep02617 Published 12 September 2013

This paper is open access.