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Best in physics: multi-target tracking and carotid plaque evaluation

Virtual conference: Speaker Paul Lui watches his own Best-in-Physics presentation on the online meeting platform. (Courtesy: Paul Liu)

The “Best-in-Physics” poster session at the American Association of Physicists in Medicine (AAPM) Annual Meeting highlights the top five abstracts in the imaging, therapy and multi-disciplinary science tracks. In 2020, however, as with many events this year, things were different.

This year’s annual meeting, held jointly with the Canadian Organization of Medical Physicists (COMP), went virtual. Instead of crowds gathering around the posters, presenters shared their research via talks presented on the meeting’s online portal. Here is part one of my selection from this year’s top scoring studies at the 2020 Joint AAPM|COMP Virtual Meeting. Look out for a second report later this week.

MRI-Linac enables simultaneous MLC tracking of two moving targets

Paul Liu, from the ACRF Image X Institute at the University of Sydney, described the use of an MRI-Linac to simultaneously track the motion of two treatment targets. “There are many radiotherapy cases that involve simultaneous treatment of multiple targets,” he said, citing examples such as locally advanced prostate or lung tumours, and oligometastases.

The challenge here is that both targets can move independently of each other. Currently, this is addressed by using large treatment margins, but this then confers extra dose to healthy tissue. Liu explained that the improved imaging capabilities of an MRI-Linac can be used to localize multiple targets simultaneously.

Liu and collaborators used the Australian MRI-Linac, which has a 1T magnet and a 120-leaf multileaf collimator (MLC), to track two spherical targets irradiated with a 6 MV conformal field. They used a motion platform to test three sinusoidal motion traces, plus lung and prostate motion traces recorded from patients.

Multi-target tracking is achieved via an extra pre-treatment aperture segmentation step, Liu explained. An MLC tracking algorithm uses the target volumes, geometries and MLC positions from the treatment plan to divide the MLC aperture into two segments specific to each target.

During treatment, the team recorded 4 Hz cine MR images and used a template matching algorithm to calculate the motion of each target. When motion was seen, rather than shift the entire MLC aperture, the leaf positions of each segment were calculated independently and then recombined into a single MLC aperture encompassing the motion of both targets.

For sinusoidal motion, the team recorded a tracking latency between the targets and their corresponding apertures of 328 ms, comparable to single-target MLC tracking systems. Liu noted that two-thirds of this time is related to the MRI step, potentially enabling latency reduction by using faster imaging and reconstruction techniques

For lung motion traces, “multi-target tracking does a good job overall in tracking both targets simultaneously,” said Liu. The geometric uncertainty – defined as the RMS error between the target and aperture positions during treatment – was reduced from 5.5 mm without tracking to 2.7 mm. After correcting for latency, this error reduced further to 1.2 mm. For prostate traces, the RMS error was reduced from 4.2 mm with no tracking to 1.4 mm with multi-target tracking.

Putting these findings into clinical context, simulations showed that in both cases, large margins of up to 7 mm were needed to maintain target coverage without tracking. With multi-target tracking, 3 mm margins provided over 99% coverage.

“This is the first experimental demonstration of tracking two independent moving targets on an MRI-Linac,” Liu concluded. “We showed that this technology can help reduce margin sizes in cases of differential motion.”

Quantitative ultrasound evaluates plaque vulnerability

Carotid plaques within arteries are at risk of rupture, which can cause adverse health effects such as cognitive deficits or stroke. But assessing plaque vulnerability – how likely it is to break off, travel to the brain and cause hypoxia – is difficult, as the mechanisms of plaque rupture are unknown and each plaque is unique in size, shape and composition.

“Our research tries to improve the ways that we can noninvasively and inexpensively see the carotid plaque that can cause stroke or other harmful heath events,” explained Catherine Steffel from the University of Wisconsin-Madison. “We want to give clinicians an inside look at which plaques are vulnerable.”

Steffel and colleagues are investigating the use of quantitative ultrasound imaging to noninvasively identify a plaque prone to rupture via its scattering structures. Their goal was to define quantitative ultrasound parameters that can identify plaque composition and help assess its risk.

Steffel examined the effective scatterer diameter (ESD) – a parameter that describes the frequency dependence of acoustic backscatter and has been used previously to examine other tissues. She notes that her team’s previous studies on excised plaque showed that small ESD corresponded to calcified regions, while larger ESD was seen from lipid particles in plaque.

ESD map — Parametric map showing the variation in ESD throughout plaque. This example shows a 70% stenosis in the right internal carotid artery of an asymptomatic patient. (Courtesy: Catherine Steffel)

The researchers performed in vivo ultrasound imaging of carotid arteries in 52 patients scheduled for clinical removal of plaque. Just over half of the participants displayed symptoms of stroke or transient ischemic attack and they had a median stenosis (artery narrowing) of about 74%. After the plaque was surgically removed, it was scored by a surgeon and assessed by a pathologist. The team then compared the in vivo images with the surgical scores and histopathology assessment.

Analysis revealed weak correlations between ESD and surgical calcification rating, and between ESD and histopathology cholesterol/fibrinoid level. Stronger correlations were seen between ESD and histopathology hemosiderin score. Steffel explained that the hemosiderin scores are important for evaluating plaque composition as hemosiderin deposits are key markers of plaque vulnerability.

Best in physics: clinical prompt gamma measurements and FLASH dosimetry

“Our results for calcium and cholesterol/fibrinoid do not agree with previous literature, perhaps because our parameters were computed for the entire plaque rather than a region-of-interest,” said Steffel. “The hemosiderin findings intrigued us, as to our knowledge, this has not been studied previously.”

In particular, the team found that two parameters – the average ESD and standard deviation ESD – could differentiate between hemosiderin scores of 0 and 2. “Overall, this work demonstrates that ESD parameters may help identify plaque composition and microstructure, providing a future clinical tool for assessing plaque vulnerability with noninvasive, relatively inexpensive ultrasound imaging,” Steffel concluded.

Human hair soaks up oil spills, shortlist for astronomy photography contest, what if the Trinity test had failed?

Have you ever thought about how much human hair is cut and thrown away every day around the world, and wondered if something useful could be done with this renewable resource? Beyond making wigs – which is a major industry in itself – Megan Murray and colleagues at the University of Technology Sydney have found that human hair and dog fur are extremely good at soaking up oil spills on roads and other hard surfaces.

“Dog fur in particular was surprisingly good at oil-spill clean-up, and felted mats from human hair and fur were very easy to apply and remove from the spills,” says Murray. Although synthetic materials are readily available for soaking up oil, they are not biodegradable. As a result, the Australian researchers have tested several natural materials including peat moss – which was not as good as human hair. They report their results in Environments.

The shortlist for the Insight Investment Astronomy Photographer of the Year 2020 competition has been announced and includes some fantastic images of the sky. My favourite is a photo of polar stratospheric clouds above a snow landscape in Finland that was taken by Thomas Kast. He says the clouds “are something I have been searching for for many years and had seen only in photographs until that day”.

The winners will be announced on 10 September and the shortlisted works will be exhibited at the National Maritime Museum in London starting in October.

This week marks the 75th anniversary of the Trinity test and on his blog Restricted Data, Alex Wellerstein asks what would have happened if that first test of a nuclear weapon had failed? He considers three different types of failure and looks at some of the military and political implications.

Planet Nine: is it a planet, a primordial black hole or something else entirely?

Planet Nine is a hypothetical world in the far reaches of our solar system. Proposed in 2016 by Caltech astronomers Mike Brown and Konstantin Batygin, its existence would explain the unusual orbits of certain Kuiper belt objects (KBOs). But are we completely sure that Planet Nine in fact a planet?

A paper in September 2019 suggested the gravitational effects could instead be explained by the presence of a primordial black hole smaller than your fist. To get to the bottom of this mystery, there have been recent proposals to send fleets of tiny probes to the general region of this mysterious object.

In the July episode of Physics World Stories Andrew Glester gets the latest on the mystery of Planet Nine. Appearing in the podcast are Mike Brown and the University of Maryland’s Zeeve Rogoszinski, co-author of one of the mission proposals.

X-rays unearth novel nanofeatures in bone

A new X-ray technique has revealed hitherto unknown structures in human bone. The technique, which uses a synchrotron beam to map the 3D orientation of nanocrystals and nanostructures within a material, advances our understanding of bone structure and how it relates to bone mechanics and bone disease. According to its developers, the method could also shed light on other natural and synthetic structures in which crystal orientation plays a role in determining the material’s properties.

Bone is an impressive natural material. It is both stiff, able to endure load without deforming much, and tough, meaning that it doesn’t break easily when bent or placed under load. Stiffness and toughness are rarely found together in a single material, and indeed bone gets its unusual properties from two main constituents: protein-based collagen fibrils, reinforced by nanocrystals of a calcium phosphate mineral called hydroxylapatite. These building blocks appear in various twisted patterns, and incorporate thin layers known as lamella that stack together like plywood. However, the precise way they fit together has proved difficult to understand because their organization is complex at every length scale.

Researchers from Aarhus University in Denmark, the European Synchrotron (ESRF) in France, Chalmers University in Sweden and the Paul Scherrer Institute in Switzerland have now used X-ray scattering data to show that the small-scale structure of bone is not uniform, as previously assumed, but contains deviations in the orientation of the hydroxylapatite nanocrystals with respect to the bone’s nanostructure. “To the best of our knowledge, this is the first time that that the ‘twisted plywood’ pattern of human lamellar bone has been mapped in 3D at such high resolution,” says Marianne Liebi, a materials physicist at Chalmers and the study’s co-first author.

Tensor tomography

The researchers focused on a type of human bone known as trabecular bone, which is porous and found at the ends of long bones like the femur. The pores in the bone are connected by thin rods and plates of bone tissue.

In their experiments, members of the team scanned a 40x40x40 micron cross-section of trabecular bone across a beam of synchrotron radiation while rotating the sample around its x and y axes. By combining small-angle X-ray scattering and wide-angle X-ray scattering, they were able to probe the bone’s nanoscale structures and the atomic structure of its biomineral crystal lattice at the same time.

To extract information on the bone’s 3D structure from their x-y scan, they used a technique called tensor tomography to analyse the intensity distribution of the scattered radiation in reciprocal space (an imaginary space in which planes of atoms are represented by reciprocal points and all lengths are the inverse of their length in real space). If, for example, the crystals in the material are not randomly oriented, the intensity of the X-ray scattering signals will be large for some orientations and small for others, explains co-team leader Henrik Birkedal, who is in the Department of Chemistry and iNANO at Aarhus. “This can be likened to the presence of landmasses on a globe,” he says. “By mapping the variation of the scattering signals with sample orientation (corresponding to making a topographic map in the globe analogy), we can determine how the crystals or nanostructures are oriented in 3D.”

The scattered intensity “projection” of the x-y scan revealed a clear lamellar structure of oriented material, interlaced with disordered regions. According to Tilman Grunewald, the study’s other first author and a former ESRF scientist, “it’s a bit too early” to explain why the orientation of the hydroxylapatite nanocrystals is sometimes disordered, but the data nevertheless provides a focus for later research. “Essentially, our approach provides a new way of looking into the underlying structure of bone,” he says.

Synchrotron advances

Manfred Burghammer, the study’s co-leader and the scientist in charge of the ESRF’s ID13 beamline where the measurements were made, says that this new approach was made possible by “drastic progress” that is currently being made in X-ray synchrotrons. “We are expecting that the current upgrades of synchrotron sources will increase our power immensely in the next years,” he adds.

The team now plan to use their method to analyse bone material from patients with diseases such as osteoporosis and other disorders associated with age-related changes in bone structure, which can impair the bone’s mechanical properties. They also hope to study types of bone microstructure other than the one analysed in this work.

“Finally, we wish to understand the origin of the localized differences in orientation between nanostructure and crystals that we have observed,” Birkedal tells Physics World. “This will likely require improving the resolution of our technique by using smaller X-ray beams.”

Full details of the technique are reported in Science Advances.

The virtual conference: thoughts from a first time ‘attendee’

This time last year I had just returned from the AAPM Annual Meeting in San Antonio, Texas, full of enthusiasm for the great talks I’d heard, old friends I’d caught up with, and all the new medical physics I’d learned. I was also somewhat exhausted by the scorching temperatures in Texas and the long-haul flight home. In 2020, however, things are inevitably very different.

This year’s American Association for Physicists in Medicine (AAPM) Annual Meeting, held jointly with the Canadian Organization of Medical Physicists (COMP), was meant to take place this week in Vancouver. But due to the coronavirus pandemic, the organizers decided to turn the entire event virtual. Despite missing out on a trip to Canada, I nevertheless “attended” this year’s meeting. As well as hearing about some of the latest scientific developments, I was also keen to find out how a virtual conference could actually work.

The 2020 Joint AAPM|COMP Virtual Meeting incorporated the presentations that would have been given in Vancouver into five days of online sessions. Speakers had recorded their scientific presentations in advance, and these were then played during the scheduled session over a live Zoom call. The presenters were also present at their respective sessions, to answer any questions from the audience after their recordings finished. The first talk that I listened to had a few teething troubles, with some erroneous ghostly chat playing out over the recorded presentation. But overall, and considering the immense technical task involved, all ran pretty smoothly on the technology side.

One major advantage of a virtual conference is that there’s no tie to any particular time zone. After their scheduled slots, all of the scientific sessions are available to watch on-demand for six weeks after the meeting. Indeed, being in Central European Summer Time, some six hours ahead of the conference’s Eastern Time-scheduled agenda, I watched all of the talks on catch-up the next day. While this felt a little disconnected from the live event, it was great to be able to choose exactly which sessions I wanted to hear, and when to listen – with none of those disappointing timetable clashes that are inevitable at this type of large event.

Alongside, this virtual format opens up meeting attendance to many attendees who, for geographical, financial or other constraints, may not otherwise been able to attend. Looking at the map of delegates that the organizers had created revealed that this truly was a global event. And also perhaps more of a family-friendly event than in previous years. Judging by the photos that I saw on Twitter, of delegates watching the conference talks accompanied by their children, babies, flatmates and cats (lots of cats), the ability to take part whenever, from wherever, is certainly a big bonus to many. As one attendee Tweeted: “I miss seeing colleagues in person, but as a new parent I definitely appreciate the benefits of a virtual #AAPMCOMP2020!”

Of course, online attendance does come with issues of its own. “Overall, I’ve found it harder to focus during the virtual meeting relative to an in-person meeting,” Catherine Steffel, a PhD student at the University of Wisconsin-Madison, told me. “Someone on Twitter mentioned that the amount of screen time you’re exposed to during a virtual conference makes it a challenge, while another person said that what makes a virtual conference tiring is sitting in the same place for long periods. I definitely agree with both comments.”

Another obvious downside to the meeting going virtual is the lack of social contact. Whether reconnecting with old friends and colleagues, or random meetings with strangers who may turn into new collaborators or contacts for future research – without the evening social events, the crowded coffee breaks, and the opportunity to simply bump into people wandering around the show, such interactions are beyond reach in this new regime.

The organizers definitely tried to address these shortfalls, for example by including “social hour discussions” in the agenda, running various Twitter-based competitions and introducing an app-based fitness challenge to replace the traditional 5K race that’s run each year. A personal favourite of mine was the “Medical Physics Family Storytime” held on the first evening, in which “fellow medical physicists share STEM-centric books for children of all ages”.

Overall, as a delegate, I experienced both benefits and downsides to the virtual conference. As a journalist, there are pros and cons too. While it’s great to be able to pause a recorded presentation to take notes, it’s also impossible to catch a speaker after their talk to ask a quick question or take an on-site photo to use in a future article.

While the conference has now finished, there are still many presentations that I’d like to listen to. And with six weeks to catch up on the content, my main problem may be knowing when to stop.

Hear more of my thoughts on the 2020 Joint AAPM|COMP Virtual Meeting in this week’s Physics World Weekly podcast.

New metasurface lens banishes chromatic aberration

A new type of metasurface lens developed by US researchers is far less susceptible to optical dispersion than previous designs – which means that it can focus a wider range of wavelengths more efficiently. Boubacar Kanté of the University of California, Berkeley and colleagues believe their new lens could find a range applications including hi-tech medical imaging, energy harvesting and virtual reality.

Optical dispersion occurs when the refractive index of a medium varies with the light’s wavelength. One of its most visible manifestations is chromatic aberration in a lens: the lens has a shorter focal length for wavelengths for which its refractive index is higher, resulting in a blurred image. Since dispersion was first explained by Isaac Newton in 1660, people have developed ways to mitigate it. One involves achromatic doublet lenses, comprising two lenses with different refractive indices stuck together such that the dispersion of one roughly compensates the dispersion of the other.

Today, however, optical technology is moving away from traditional lenses and towards flat optical metasurfaces. These are thinner and lighter and offer functionalities such as reconfigurability and variation of refractive index with polarization that are impossible to achieve with traditional lenses. However, there are difficulties associated with metasurfaces – and among the most prominent are increased chromatic aberration and other performance variations with wavelength.

Wavelength independence needed

The principal cause of these problems is that, whereas bulk lenses control light through cumulative effects on the waves passing through, metasurfaces use optical elements to interact with the wavefronts directly and tailor their shape and phase. To achieve this interaction, these optical elements comprise structures on the scale of the wavelength of the light they control. Variations in the wavelength therefore significantly affect this interaction. Moreover, many of the most efficient metasurface designs are based on resonators, which have a sharp peak at one wavelength: “If you want to make a laser, you want one colour, so a resonant cavity is perfect,” explains Kanté, “But if you want to make a lens, one colour is not good enough!”

In the new work, Kanté and colleagues took a different approach. Instead of using resonators, they used a “fishnet” structure – a lattice of interconnected 350 nm titanium dioxide grids spaced 370 nm apart. Each element behaves as a waveguide: “You need to make those little waveguides as broadband as possible,” explains Kanté. Waveguide-based metasurfaces have been designed before, but they have invariably had efficiencies well below those of the current design. However, the researchers had another, subtler trick up their sleeves.

The phase of each waveguide must be carefully selected to produce a surface with maximum possible efficiency. Previously, researchers have selected the phase of each waveguide independently, by assuming it to be part of a lattice of identical waveguides. “When I make my lens, the waveguide next to it is different,” explains Kanté, “but I assume that it’s not that different, so the error that I get by assuming that it’s the same is not that big.” In 2017, however, Kanté and colleagues showed that this is simply inadequate. They also presented a method for calculating the correct phase. In their latest research, they used this to select the orientation of each pillar in the lattice. The resulting lens focused at least 70% of incident light in the wavelength range 640-1200 nm to the same point.

Fluorescence imaging

The researchers believe that, if a scalable production process can be developed, their design could perform a range of tasks, such as producing virtual reality images and focusing light in solar cells – a process that helps harvest energy more efficiently. “If you are now comparable in efficiency [with traditional lenses] the next question becomes ‘how large scale can you make these things and how much do they cost?’” says Kanté. Even without large-scale production, however, Kanté believes the lenses could contribute to techniques such as fluorescence imaging, which is a Nobel-prize winning super-resolution microscopy technique now widely used in research and medicine: “You pump the molecules with one colour and they emit at another,” explains Kanté, “Now that our lenses are broadband, you could actually pump and collect with the same ultrathin lens.”

Francesco Monticone of Cornell University is impressed “While several recent papers have proposed a large variety of design methods to realize broadband achromatic metasurfaces – with some impressive results…the experimental demonstration and measurements presented [by Kanté’s team] are outstanding.” Monticone’s group recently published fundamental limits on the bandwidth of optical metalenses: “The fabricated metalenses [made by Kanté’s team] represent an important step toward reaching these physical bounds, extending the current state of the art of metalenses and their potential for many applications requiring broad bandwidth and high efficiency,” he says.

The metalenses are described in Nature Communications.

How a longhorn beetle could help keep you cool, the pros and cons of online conferences

Nature in all its glorious diversity has come up with some very clever solutions to difficult problems. In this episode of the Physics World Weekly podcast the science writer Michael Allen talks about a longhorn beetle from south-east Asia, whose ability to survive sizzling temperatures has inspired the creation of a highly reflective material that could soon be keeping you cool.

The COVID-19 pandemic has caused profound changes in the ways that physicists meet to exchange ideas about their research. This week, Physics World’s Tami Freeman attended her first virtual conference – the joint meeting of the American Association of Physicists in Medicine and the Canadian Organization of Medical Physicists (AAPM/COMP) – and chats about the pros and cons of online meetings.

Biosensor platform promises fast, low-cost detection of water contamination

With 80% of the world’s population at risk of water insecurity, there’s a clear need for a fast, simple and reliable way to test water for a wide range of contaminants. Julius Lucks and his team at Northwestern University believe they have the answer. Its name is ROSALIND – RNA output sensors activated by ligand induction.

In their recent study, published in Nature Biotechnology, Lucks and his team didn’t invent a sophisticated new technique. Instead, they took advantage of the elegant solutions that have evolved in nature to “taste” the water’s contents. This approach provides a straightforward positive or negative test for up to 17 different water contaminants, and it works in minutes.

Putting nature to a new use

The researchers’ approach is to take molecular machinery (including DNA, RNA and proteins) and reprogramme it for their own needs. In this case, the machinery comes from bacterial cells and uses the components that normally copy the DNA’s genetic code into a messenger form to be turned into the cell’s building blocks. Instead, the team re-engineered these molecular machines to work outside the cell for a new purpose.

Usually, DNA is copied when the code is needed by the cell. For this testing platform, that control is instead used to trigger DNA copying only when specific water contaminants are present. The water samples being tested are mixed in a tube pre-loaded with these molecular machines. When contaminants are present, the machines copy out a messenger form that lights up in the tube – giving off a signal that’s easily seen by the human eye. This allows ROSALIND to turn a contaminant we could never see into a light signal we can’t miss.

Importantly, the acronym ROSALIND also honours Rosalind Franklin, whose work was vital to the discovery of the DNA double-helix structure. According to Lucks, “her work essentially eventually enabled us to learn how to reprogram DNA to act in our technology”. Franklin’s 100th birthday would have been the same month that this research was published, he notes.

Taking technology where it is needed

Whilst current methods to test water require significant expertise and equipment – making them expensive – ROSALIND is different, according to Lucks. “We’re offering a technology that enables anyone to directly test their own water and know if they have contamination within minutes,” he explains. “It’s so simple to use that we can put it into the hands of the people who need it most.” The researchers have tested their method in the field, where it successfully matched the results of a gold standard laboratory test – flame atomic absorption spectroscopy – whilst being much quicker and more affordable.

Putting the technology into people’s hands doesn’t just mean making it cheaper, but also making it easier to use. Using the molecular machines outside of the cell allows them to be freeze dried, which keeps all of the components stable until they are needed. All the user needs to do is add a drop of water. Not only is this process simple, but the whole system is highly flexible. Currently, 17 different contaminants can be detected – including toxic metals, antibiotics and additives to personal care products – and ROSALIND could be easily updated to add more relevant contaminants in future.

Having previously tried to test water in his own home, Lucks was frustrated to find it a difficult or impossible task. “To ensure access to safe and clean drinking water, we need technologies that will allow easy monitoring of water quality,” he says. As the researchers move from creating their concept in the lab to developing this platform via a start-up company, Stemloop, Lucks is hoping that ROSALIND will soon bring water testing technology to the masses.

Mediator atoms help graphene self-heal

Graphene and other carbon materials are known to change their structure and even self-heal defects, but the processes involved in these atomic rearrangements often have high energy barriers and so shouldn’t occur under normal conditions. An international team of researchers in Korea, the UK, Japan, the US and France has now cleared up the mystery by showing that fast-moving carbon atoms catalyse many of the restructuring processes.

Graphene – a carbon sheet just one atomic layer thick – is an ideal system for studying defects because of its simple two-dimensional single-element structure. Until now, researchers typically explained the structural evolution of graphene defects via a mechanism known as a Stone-Thrower-Wales type bond rotation. This mechanism involves a change in the connectivity of atoms within the lattice, but it has a relatively large activation energy, making it “forbidden” without some form of assistance.

Using some of the best transmission electron microscopes available, researchers led by Alex Robertson of Oxford University and Kazu Suenaga of AIST Tsukuba found that so-called “mediator atoms” – carbon atoms that do not fit properly into the graphene lattice – act as catalysts to help bonds break and form. “The importance of these rapid, unseen ‘helpers’ has been previously underestimated because they move so fast and have been next-to-impossible to observe,” says co-team leader Christopher Ewels, a nanoscientist at the University of Nantes.

The breakthrough, Ewels says, came when they realized that these usually fast-moving atoms slow down when bound to existing defects in the lattice. “Our technique can be likened to those nature programmes on television in which cameramen often have a hard time filming some of the animals, which can be shy,” he explains. “They therefore sometimes set themselves up in hides next to a watering hole where they know the animals are sure to go, and that’s how they get their film footage.

“In our case, the mediator atoms shoot around too fast for our cameras. By instead imaging and watching pre-existing defects (the watering holes) that occasionally trap mediator atoms (which can linger near the defects for seconds, minutes or even hours), we are able to image them and observe how they influence defect restructuring.”

Since the images were blurred because of the speed of the processes involved, detailed theoretical modelling calculations were required to interpret them. These calculations were done at Seoul National University by Gun-Do Lee and his team and Ewels and his colleagues at the CNRS in Nantes.

Active catalysts

The mediator atoms act as catalysts thanks to their reactive dangling bonds, Ewels explains. By bonding to defective sites and lowering the activation energy of reactions, they trigger a variety of bond-breaking and bond-forming processes. In some cases, the mediators become incorporated into the lattice, kicking out atoms that were already there and causing the ejected atoms to mediate further rearrangements (see image above). “These processes may occur in many other environments, from interstellar carbon chemistry to graphite moderators in nuclear reactors,” Ewels tells Physics World. The new work thus provides an important fundamental understanding of how graphene and related 2D materials structurally rearrange and repair themselves, he adds.

The researchers, who report their results in Science Advances, speculate that analogous mediator atom species may be present in other bulk materials. They now plan to explore this idea further. “We suspect similar process may underlie mechanical deformation processes of many 2D and 3D materials,” Ewels says.

Impurities toughen tooth enamel, but could make teeth vulnerable to decay

The fundamental building blocks of human tooth enamel contain characteristic impurities that could contribute to their toughness, but also make teeth more vulnerable to decay. US researchers led by Derk Joester at Northwestern University made the discovery using two atomic-scale imaging techniques, which revealed distinctive distributions of impurities within the crystal structure of enamel. Their findings could lead to new ways to improve the health of our enamel, and to repair the damage created by tooth decay.

Coating the entire crown of the human tooth, enamel is an extremely hard substance that is well adapted to withstanding the wear and mechanical forces associated with chewing. Yet in many people, the material is broken down through tooth decay – which is one of the most widespread chronic diseases. So far, techniques for repairing and synthesizing new enamel have had limited success, largely due to limitations in our understanding of the material’s deeply hierarchical structure.

At the microscopic level, enamel is made up of rods composed of thousands of long, thin crystals with rectangular cross-sections – called crystallites. The rods are arranged in complex meshess. No more than 170 nm wide, the crystallite building blocks are primarily composed of the mineral hydroxyl-apatite. This has an orderly lattice containing calcium, phosphate, and hydroxyl ions. However, the crystallites are also known to possess cross-sectional variations in chemical composition on atomic scales.

Atom-probe tomography

To explore these variations in more detail, Joester’s team first carried out scanning transmission electron microscopy (STEM) on thin, ultra-cold crystallite cross-sections. The resulting images revealed that within the periodic lattice of hydroxyl-apatite, crystallites have central cores with slightly different compositions. These cores are sandwiched between two distinctive layers (see figure). Since these structures were damaged by STEM electron beams, Joester’s team also employed the technique of atom-probe tomography (APT), which can locate individual atoms at sub-nanometre resolutions. Within these images, they saw that crystallite cores contain high concentrations of sodium, fluoride, and carbonate ion impurities, and are flanked by two magnesium-rich layers.

Through simulations, combined with experiments involving X-ray diffraction, Joester and colleagues revealed that these chemical gradients create stress patterns in enamel, which could be partly responsible for the materials’s high mechanical resilience. However, the impurities could also increase the solubility of the material in acidic conditions, making it more vulnerable to decay. The team’s findings could offer important new insights into the crystal growth that takes place during enamel formation, and how it could be controlled artificially. If achieved, such techniques could lead to new treatments for people suffering from tooth decay, and to new ways of maintaining healthy enamel.

The study is described in Nature.