A technique that reproduces the conditions of the Earth’s mantle at a depth of more than 2000 km could help researchers simulate our planet’s earliest days, when magma covered its surface. The technique, which combines laser-driven shock experiments with X-ray free-electron laser measurements, provides nanosecond-resolution information on the transformations that occur in silicate materials at ultrahigh pressures and temperatures. The work adds to our understanding of the present-day core-mantle boundary and may even shed light on conditions inside “super-Earths” – rocky exoplanets similar to Earth but larger in size.
Terrestrial planets like Earth have silicate-based mantles and iron-rich cores. This structure is thought to be the result of various material-differentiation processes that took place at an early stage of the planet’s development, including radiogenic decay of short-lived nuclides and numerous shock events. Together, these processes created temperatures high enough to sustain a planet-wide ocean of magma.
To better understand what happened during this epoch in the history of Earth and other rocky planets, researchers need to analyse the physical properties of liquid silicates under similar conditions. Such studies could help determine the composition and origin of molten or partially molten domains of liquid silicates that exist in the Earth’s upper mantle today, and possibly also at the boundary between the mantle and magmatic core, which is a vestige of those primordial days.
Doing away with extreme-condition apparatus
Temperatures as high as 6000 K and pressures of more than 100 GPa are, however, difficult to create in the laboratory. For this reason, researchers at Sorbonne University and the University of Grenoble-Alpes in France, together with colleagues at the US Department of Energy’s SLAC National Accelerator Laboratory, developed an alternative method that eliminates the need for ultrahigh-pressure/extreme temperature apparatus. The new technique involves first sending a shockwave through an amorphous magnesium silicate sample using an optical laser. This step, performed at SLAC’s Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL), compresses the sample to pressures of up to 130 GPa and heats it to temperatures of 6000 K. The silicate glass thus transforms into a liquid.
Next, they bombarded the sample with ultrafast femtosecond X-ray pulses from the LCLS at the precise moment when the shockwave reached the desired pressure and temperature. These X-rays produced two precise diffraction peaks as they scattered off the sample, enabling the researchers to monitor how the atoms in the sample rearranged themselves at such high pressure and temperatures. The resulting spectral fingerprint is related to the transition from four-fold to six-fold coordination of oxygen atoms around the silicon atoms.
Aside from this atomic rearrangement, the researchers saw no other major structural changes in the silicate melts at pressures of up to 130 GPa – a finding that should be important for better modelling these materials under the conditions present deep inside the Earth, they say.
The team backed up their results with measurements previously obtained in conventional diamond anvil analyses – in which a solid silicate sample is literally crushed to high pressures at room temperature – and molecular dynamics simulations.
Recreating Earth’s early days
“Through our experiments, we have been able to probe geophysical materials at the extremely high temperatures and pressures found deep inside the Earth, to characterize their liquid structure and learn how they behave,” explains study lead author Guillaume Morard. “These studies will allow us to recreate Earth’s early days and understand the processes that shaped our planet.”
The researchers now plan to repeat their experiments at higher X-ray energies. This should enable them to more precisely measure the way in which the atoms rearrange in the liquid silicates. They also hope to try out higher pressures and temperatures. “These latter studies will be important for better understanding how silicate liquids and glasses behave in super-Earth planets,” says Morard.
A new imaging technique has allowed researchers in the UK to create a 3D map that charts the flow of blood through a living zebrafish. Andrew Harvey and colleagues at the University of Glasgow used an optical setup that produces pairs of “Airy beams” corresponding to individual microscopic beads flowing in a fish’s blood. Their approach could lead to new and better ways to explore the characteristics of microscopic biological systems.
The spatial resolution of a conventional optical microscope is about 300 nm, which is on par with half wavelength of visible light. While smaller structures can be observed, their spatial features are blurred. In simple 2D systems, the blurring can be compensated for by locating the centres of blurred objects and constructing “point spread functions” (PSFs) on top of them. This can reduce blurred objects to single points of light to within about 10 nm precision – which is important when tracking single, fluorescently-labelled molecules in biological systems.
PSFs can also be used for simple 3D systems, since their shapes can indicate their depths, or axial distances relative to the imaging apparatus. However, the technique becomes far less effective when imaging 3D groups of objects, which often results in overlapping PSFs that are far more difficult to analyse.
Curved light
Harvey and colleagues have been developing a better 3D method that uses imaging optics to transform a PSFs into an “Airy beam”, a waveform that does not spread out over time and appears to curve as it travels. The shape of the Airy beam depends on the axial distance to the object, allowing the depth of the object to be determined. While this technique is effective, it can be difficult to implement.
In their latest research, the team introduce an even more effective optical setup that is easier to calibrate. Their system converts a PSF into a twin Airy beam that appears as two spots on either side of the object. The separation between the spots increases with axial distance increases and therefore measuring the separation gives the depth of the object. This approach enabled the researchers to locate fluorescent nanocrystal beads to within 30 nm, across an axial range of over 7 micron.
As a proof of concept, they used the technique to observe the motions of 1 micron fluorescent beads when injected into the blood of living zebrafish –tracking their twin Airy beam PSFs at 26 frames-per-second. With high enough bead densities, Harvey and colleagues could clearly map out the 3D shapes of the fish arteries over a depth range of 0.1 mm.
The technique could soon offer significant new opportunities for optically imaging microscopic structures, including living systems on a cellular level. Harvey’s team describes how nanobeads could be made to fluoresce in the presence of oxygen or acidic conditions; or loaded into soft gels that are deformed by growing cells, potentially yielding new insights into the mechanical forces they exert.
Single-layer cell cultures are widely used as an alternative to animal models for investigating the effects of drugs on the brain. But the advantages that such 2D models have in terms of simplicity and accessibility are balanced by some significant shortcomings. Neural networks in 2D cultures respond differently to stimuli compared with those in the 3D physiological structures that they aim to emulate. Cells in 2D cultures also remain viable for just a short time, as the nutrient medium cannot penetrate to the culture’s interior.
To address these problems, researchers in China and the US have demonstrated a 3D tissue construct that sustains a population of neuronal cells for four weeks. The team used a cell-laden bioink to print a stack of three grid-shaped layers on an array of electrodes. Spaces between and within each layer allowed nutrients to reach the cells, while the electrode array measured the cells’ electrophysiological signals. Such constructs reproduce in vivo neural circuits more accurately and over much longer periods than 2D cell cultures, making them a better model for drug testing and studying the dynamics of neural networks.
To fabricate the 3D model, Yu Song, Ting Zhang and colleagues, at the Biomanufacturing and Rapid Forming Technology Key Laboratory of Beijing at Tsinghua University, suspended rat neuronal cells in a solution of gelatin, alginate and fibrinogen. With the right combination of nozzle diameter and flow rate, they found that 85% of cells in the bioink survived the printing process. The researchers also varied the proportion of bioink components so that the structure that they built – a stack of three square grids, each 0.5 mm thick and 8 mm across – had mechanical properties similar to those of living brain tissue.
In 3D models, primary cells (collected from rats) are mixed with biocompatible materials to form bioink, and then printed on a petri dish for imaging or a 4×4 electrode array for electrophysiological recording. 2D samples are used as controls. (Courtesy: Biofabrication 10.1088/1758-5090/ab7d76)
A week after printing, nearly all of the cells in the structure were still alive and had started to extend neurites to form a network. A 2D control sample at the same point in time had already lost more than a quarter of its cells. The rate of cell death in the 2D culture continued steadily until the end of the experiment four weeks after printing, at which point less than 25% of its cells were still alive. In the 3D structure, in contrast, more than three-quarters of cells survived to the final measurement.
After cultivating the cells for four weeks, Song and colleagues quantified the neural activity in the two cultures by measuring excitatory postsynaptic potentials (EPSPs). They triggered and detected these electrical signals using a 4 × 4 array of electrodes that lay under each culture. The amplitude of the EPSPs in the 3D structure was larger than in the 2D culture, which the researchers attribute to the printed construct’s greater proportion of surviving cells and their better connectivity.
These EPSPs dropped to zero when the team perfused the 3D construct with tetrodotoxin (TTX), a neurotoxin that inhibits the transmission of sodium ions across cell membranes. The speed with which TTX spread through the culture and shut down the cells’ activity showed that the construct is highly sensitive to the effects of such substances, indicating its suitability as a model for drug-screening applications.
“Our ongoing work includes studying how neuro drugs with different molecular structures diffuse at different speeds in our printed models, and administering electrophysiological stimulation and/or pentylenetetrazole to our models to study epilepsy,” says Wei Sun, director of the Biomanufacturing Center at Tsinghua University. “We are also interested in printing stem cells to build a brain-like model to study neurodevelopment, and integrating the printed brain-like models with microfludics to build a ‘brain-on-a-chip’ device.”
Full details of the work are reported in Biofabrication.
I got some strong reaction to my recent column about how to reduce carbon emissions from air travel. Don’t be so naïve, I was told, people will never stop flying for business – they don’t want to go on a “flight diet”. What a difference a few months makes. The entire global aviation industry has almost ground to a halt due to a virus (SARS-CoV-2) that emerged in China and isn’t especially deadly (in terms of the overall percentage death rate).
Now I don’t wish to make light of the tragic deaths of loved ones or the job losses and economic damage incurred by global efforts to contain this exponentially spreading virus. COVID-19 is indeed a global tragedy on many levels. But could there be a lesson from it in terms of how we deal with climate change? Could the virus be the event that shifts our habits on driving and flying forever?
Could the virus be the event that shifts our habits on driving and flying forever?
The last major killer pandemic – Spanish flu – led to the death of about 50 million people, or just under 3% of the population. However, it didn’t greatly affect the world economy, which was already on its knees after the First World War. The same was true of the 2009 swine flu pandemic. About 284,000 people died (compared to 250–500,000 deaths from seasonal flu annually) but apart from a few travel advisories it was mostly “business as usual”.
So why is COVID-19 so different and such a threat? The trouble is, it’s a totally new virus and we have no immunity to it. What’s worse is that you can be contagious before you show any symptoms. Critically ill people, meanwhile, need lots of doctors, nurses and equipment to be treated. No health service could cope if the virus were allowed to “let rip”, which is why most governments have decided to stop everyone from moving about and interacting.
But the economic impact of that decision has been huge. Many businesses are starved of sales and cannot last long without cash coming in. In many countries, companies are being supported by the state, but no government can hold back the tide for ever. Hardly surprising then that, by 29 April, the US S&P stock-market index of 500 top firms was 13.2% below its February peak.US unemployment was last month nearing 15%, while oil prices fell to a 20-year low and global production began to be cut as demand is expected to slump further.
Until the world finds a vaccine or we all build up enough “herd immunity”, things won’t return to how they were for years, if ever
No-one knows how long this situation will last so we can’t yet say what its ultimate impact will be. But until the world finds a vaccine (and we’re all inoculated) or we all build up enough “herd immunity” not to spread this virus to the vulnerable, things won’t return to how they were for years, if ever. So we’d better adapt and embrace the amazing power of communication and connectivity that we have at our disposal.
E-mail, video conferences, virtual meetings, Internet speeds – wow, with this tech and a bit of planning you can be really productive. Plus, they remove all the “dead time” that you’d waste travelling. We all knew that these tools existed, but was it habit or the expectations of others that stopped us from using them more? And now that lots of other people aren’t stuck in traffic jams or on trains either, they’re more available too.
Since the lockdown began, I’ve discovered that this communication technology works really well – the UK’s investment in broadband and mobile data has really paid off. My kids are managing school and lectures online too, which would have been impossible 10 years ago. And even if I paid the annual subscription costs for the “premium” versions, it would still cost less than one train or car trip into London.
Sure, I miss going out with colleagues and new acquaintances, but social mixing will come back in some form
Sure, I miss going out with colleagues and new acquaintances, but social mixing will come back in some form. I can, however, feel a new set of habits developing especially as the lack of travel means that I have more time to do the things I want. And what a joy not to have to wear all those boring old work shoes, shirts and coats.
According to the UK Office for National Statistics, only 5% of the UK labour force worked mainly from home in 2019. I predict that this figure will rise as employers and employees drool over the prospect of reduced office costs, improved staff retention, a better work–life balance for staff and a wider talent pool to recruit from. Not all work can be done online but a surprising amount can be.
What’s more, the lockdown has made the air cleaner. Pollution levels in New York have almost halved since this time last year. Emissions in China fell by 25% at the start of the year, while coal use dropped by 40% at its six biggest power plants. The slow down is sure to benefit transport emissions, which made up 24% of global carbon emissions in 2016. With fewer vehicles on the road, I can now even hear the birds singing in my garden.
Yes, there will be winners and losers. In doing more online, businesses will save money on offices and travel costs. The big losers will be aircraft manufacturers, airlines, hotels, conference venues, commercial office-space providers and high-street shops. Pickpockets will also struggle, though I can live with that.
Will COVID-19 merely accelerate existing trends that would – and should – have happened anyway?
But for the survivors, will COVID-19 merely accelerate existing trends that would – and should – have happened anyway? Governments that are propping up their national economies will now find that they can now consider much stronger action on climate change. After all, it’s easier to push change if you’re paying for it. And if they’re smart enough, most of those governments will not just consider those changes, they’ll implement them too.
The only question is whether, once the economy has recovered, will we return to our bad old habits.
Last week I was enthusing about how lidar has been used to discover a huge Mayan structure in Mexico – and this week, ground-penetrating radar (GPR) takes the spotlight in the Red Folder. The technique has been used by archaeologists at the University of Cambridge and Ghent University to map a complete Roman city that is still buried underground. Located near Rome, Falerii Novi was first occupied in 241 BC and was populated for over 900 years.
The extensive measurements were taken by Ghent’s Lieven Verdonck as part of an ongoing project to improve GPR technology. The city stretches over 30.5 ha and Verdonck took a GPR reading every 12.5 cm across the entire site. Vast amounts of data were collected, and it could be some time before it is all analysed. Writing in the journal Antiquity, the team says the improved technique “has the potential to revolutionize archaeological studies of urban sites”.
If Verdonck and colleagues ever want to excavate Falerii Novi, they might consider using a “mole-bot” that has been optimized for underground and space exploration. Created by researchers at the Korea Advanced Institute of Science and Technology, the mole-bot is a drilling biomimetic robot inspired by the African mole-rat and European mole. The mole-bot is 25 cm wide, 84 cm long and weighs 26 kg. The team says the robot is three times faster – and has six times higher directional accuracy – than conventional boring machines. You can watch it in action in the video above.
And if you prefer your robots to remain above ground, researchers in France have created a cable-driven robot to track and film flying insects.
The solar system may have been formed in a long-ago collision between the Milky Way and its orbiting companion the Sagittarius dwarf galaxy. That is the conclusion of astrophysicists in Spain, who have analysed data from the Gaia space observatory. This cosmic “fender bender” – which occurred as Sagittarius’ orbit plunged it through the plane of our galaxy – helped to concentrate cosmic dust in and usher in a period of heightened star formation.
First identified as a satellite galaxy of the Milky Way in 1994, the Sagittarius Dwarf Spheroidal Galaxy is around one tenth of the diameter of the Milky Way. Made up of four main globular clusters of stars, our elliptical-shaped neighbour is spiralling around our galaxy on a polar orbit about 50,000 light-years from the galactic core. This brought the galaxy through the plane of the Milky Way several times in the past. Some researchers have even suggested that these collisions over the past six billion years helped to create the Milky Way’s trademark spiral structure.
“It is known from existing models that Sagittarius fell into the Milky Way three times – first about five or six billion years ago, then about two billion years ago, and finally one billion years ago,” says team member and astrophysicist Tomás Ruiz-Lara of the Instituto de Astrofísica de Canarias (IAC) in Tenerife.
Luminosities and colours
In their study, Ruiz-Lara and colleagues used data from the European Space Agency’s Gaia telescope to examine the luminosities and colours of the stars that lie within around 6500 light-years of the Sun, as to determine the star formation history of our stellar neighbourhood. They then compared this with existing models of stellar evolution.
The researchers identified three periods in galactic history where star formation appeared to occur at an increased rate — peaking at around 5.7, 1.9 and 1 billion years ago. This corresponds broadly to the times at which the Sagittarius galaxy is believed to have been passing directly through the Milky Way’s disc.
“At the beginning you have a galaxy, the Milky Way, which is relatively quiet. After an initial violent epoch of star formation, partly triggered by an earlier merger […] the Milky Way had reached a balanced state in which stars were forming steadily,” explains Ruiz-Lara. The effect of Sagittarius falling into the Milky Way, he added, was to “disrupt the equilibrium, causing all the previously still gas and dust inside the larger galaxy to slosh around like ripples on water.”
Concentrating dust and gas
The effect of these so-called ripples would have been to concentrate dust and gas in certain areas of the galaxy — promoting the more rapid formation of new stars as gravity pulled the material together. At the same time, the collisions also act to strip Sagittarius of some of its gas and dust.
Team member and IAC astrophysicist Carme Gallart observes, “It seems that not only did Sagittarius shape the structure and influenced the dynamics of how stars are moving in the Milky Way, it has also led to [its] build-up”. Without these recurring collisions with the dwarf galaxy, she adds, part of the Milky Way’s stellar mass may not have come to exist – at least, not in the form with which we are familiar – and such may have even included our very own solar system.
The Sagittarius effect
“The Sun formed at the time when stars were forming in the Milky Way because of the first passage of Sagittarius,” explains Gallart. “We don’t know if the particular cloud of gas and dust that turned into the Sun collapsed because of the effects of Sagittarius or not. But it is a possible scenario because the age of the Sun is consistent with a star formed as a result of the Sagittarius effect.”
According to ESA Gaia project scientist Timo Prusti – who was not directly involved in the analysis – such detailed insights into the Milky Way’s history would not have been possible before the Gaia observatory’s first data release in 2016.
“Some determinations of star formation history in the Milky Way existed before, based on data from ESA’s early 1990s Hipparcos mission,” he explained, adding: “But these observations were focused on the immediate neighbourhood of the Sun. It wasn’t really representative and so it couldn’t uncover those bursts in star formation that we see now.”
“This is really the first time that we see a detailed star formation history of the Milky Way. It’s a testament to the scientific power of Gaia that we have seen manifest again and again in countless ground-breaking studies in a period of only a couple of years.”
Moisture harvested from the atmosphere at night by a hydrogel can be used to cool down solar panels during the day, boosting their efficiency. So say researchers at the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and the Hong Kong Polytechnic University (PolyU) who made the hydrogel from a mix of carbon nanotubes in polymers with a hygroscopic calcium chloride salt. The technology could be an environmentally friendly way to increase photovoltaic electricity generation and also cool down other devices.
Solar photovoltaic (PV) panels currently produce more than 600 GW of the world’s power, and this figure is expected to increase to 1500 GW by 2025 and 3000 GW by 2030. While solar energy is an abundant, inexhaustible and very clean energy resource, commercial silicon-based PV cells can only convert between 6–25% of absorbed sunlight into electric current. The rest is transformed into waste heat, which increases the temperature of a solar panel by up to 40 °C. This makes the cells less efficient, and it can also damage them – especially in hot climates, where the problem is even more serious than in more temperate areas.
Current technologies to cool PV panels include refrigeration or air conditioning, but these can be energy-hungry. Water-cooling systems also exist, but they require abundant water supplies, as well as storage tanks and a complicated network of pipes and pumps.
A gel-like material with a high affinity for water molecules
A team of researchers led by Peng Wang of KAUST’s Water Desalination and Reuse Center and the Department of Civil and Environmental Engineering at PolyU recently developed an alternative cooling method. Their technique is based on a gel-like material that comprises heat-absorbing carbon nanotubes (CNTs) embedded in a cross-linked polyacrylamide (PAM) and calcium chloride (CaCl2). This gel has a high affinity for water molecules and can therefore take up large quantities of water vapour from ambient air. According to lead author Renyuan Li, the gel also has the ability to self-adhere to numerous surfaces, including solar panels, through strong hydrogen bonding.
In their experiments, the researchers pressed a 1-cm-thick layer of the hydrogel against the underside of a standard silicon solar panel. When the temperature drops in the evening and overnight, the water absorbed by the material condenses to form liquid water, explains Wang. During the daytime, as the temperature increases, the heat from the PV panel causes the water to evaporate – a process that not only removes heat from the panel, but also regenerates the vapour sorbent so that the atmospheric water harvester (AWH) is ready for the next night-day cycle.
Experiments by the KAUST-PolyU team on PV panels in the laboratory showed that a fully-engorged gel could free enough water to produce a cooling power of 295 W/m2 under 1000 W/m2 solar radiation and reduce the temperature of the panel by 10 °C. This reduced temperature improves the panel’s efficiency enough to boost the amount of electric current produce by an average of 15%. When the team tested the system on an outdoor prototype during winter and summer months on the KAUST campus, this figure increased to 19% – probably because wind enhanced the cooling effect, the researchers say.
With global PV capacity expected to reach 1500 GW by 2025, the researchers calculate that cooling all these panels using their approach would generate more than 150 GW of additional power. This boost to the electricity supply would translate into a reduction of 8.52 × 107 metric tonnes of coal consumed per year, and a drop in CO2 emissions of more than 1.48× 108 metric tonnes per year (assuming 20% solar PV electricity generation efficiency).
Improving water vapour sorption-desorption kinetics
Wang and colleagues, who report their work in Nature Sustainability, are now working to improve the corrosion resistance of their atmospheric water harvester (AWH) and increase its water vapour sorption-desorption kinetics (and thus its capacity). They are also investigating better ways of dealing with dust on PV panels – a particular problem in arid and semi-arid regions, where particles deposited by frequent dust storms can reduce the power output of a solar panel by as much 20% if not removed. One design option for the AWH cooling here, Wang says, would be to use the system to trap and condense water after it has evaporated from the hydrogel. This water could not only be used to remove any dust build up on the PV panels, it might even be clean enough to drink.
On another positive note, the technology could easily be adapted to different scales, Li adds. “It could be made as small as several millimetres for cooling down electronic devices, hundreds of square metres for a building, or even larger for passive cooling of industrial-sized PV farms.”
The researchers say they will now be testing the long-term stability of their AWH. “Our long-term goal is to demonstrate its commercial value and make it market-competitive,” Wang tells Physics World.
In today’s fast-changing world, we are continuously upgrading our existing technologies and developing new ones. Yet, the basis of many breakthrough technologies lies in fundamental principles of physics. One example is the Doppler effect, which states that the frequency of a wave changes when its source is in motion relative to an observer. A car engine, for example, sounds different depending upon whether it is approaching us or receding. Likewise, light from an astronomical object moving toward us appears blue-shifted, while light is red-shifted from an object moving away.
The Doppler effect also plays a key role in coherent ranging, a technique being developed for long-range three-dimensional detection of distance and speed in autonomous driving. Coherent ranging, also known as frequency-modulated continuous-wave (FMCW) laser-based light detection and ranging (lidar), works by measuring the Doppler shift of reflected laser light coherently, thus preventing interference from sunlight and other lidar systems.
Coherent detection enhances the distance resolution, which is a critical factor in autonomous driving. But improvements often come at a cost. Coherent ranging results in a low acquisition speed and requires a highly coherent, as well as precisely chirped, laser source. Another major barrier to overcome is the technical capability to precisely control the many narrow-linewidth frequency-agile lasers used in parallel FMCW lidar.
A massively parallel coherent lidar scheme
To address this obstacle, a group of researchers led by Tobias Kippenberg at EPFL has developed a novel way to implement parallel FMCW lidar. Their approach, published in Nature, multiplexes a single FMCW laser using a high-quality silicon-nitride microresonator on a photonic chip. This frequency-modulated pump laser generates a soliton microcomb on the photonic chip. Basically, the continuous-wave laser light is converted into a stable optical pulse train, due to the double balance of dispersion and nonlinearity effects.
EPFL researchers (left to right): Johann Riemensberger, Anton Lukashchuk and Tobias Kippenberg.
In a soliton microcomb, multiple comb teeth reconstitute the equidistant optical frequency components from a single pump laser. In other words, each individual comb tooth serves as a source of frequency-modulated laser light and, channel-by-channel, recovers and reconstructs the reflected laser signal. Frequency comb generation based on dispersive spreading, for example using an optical grating, results in illumination of multiple pixels on the photonic chip and consequently can map the distance and velocity of multiple target objects simultaneously.
The technique has potential to greatly enhance the frame rate of imaging in coherent lidar systems via parallelization of ranging and detection. More importantly, this parallel architecture retains the advantages of continuous-wave operation, such as the avoidance of high peak powers. It is also inherently more eye-safe than contemporary time-of-flight-based lidar systems.
“Parallel detection, using arrays of lasers and photodetectors, was a key step in the development from 2D laser scan sensors to the 3D imaging systems that support the autonomous driving revolution,” says first author Johann Riemensberger. “Our technology can extend this principle for superior coherent detection lidar systems, while avoiding the technical challenges of operating large arrays of complex frequency-agile laser systems.”
The Elekta Unity MR-Linac is in the vanguard of a new generation of MR-guided radiotherapy (MR/RT) systems that enable clinicians to visualize a tumour target, as well as its surrounding anatomy, with exceptional soft-tissue contrast both prior to and during treatment. Those capabilities are now poised to transform workflows in the radiation oncology clinic, delivering resource efficiencies and improving patient outcomes in the process.
It’s a compelling picture, one in which MR/RT points the way to personalized medicine tailored to the unique requirements of each patient – adjusting radiation delivery to address the daily variation in the tumour and surrounding healthy tissue, while enabling the clinician to adapt the plan for tumours that respond rapidly to treatment, as well as those that prove unresponsive to standard doses of radiation. Furthermore, that ability to capture the tumour and its environment “on the fly” will, in turn, make it possible to increase the radiation dose to diseased tissue in real-time without damaging adjacent organs at risk and other critical structures.
Among the early-adopting clinical customers for Elekta Unity is the US-based Allegheny Health Network (AHN), which is currently putting the finishing touches to a new $100 million academic cancer centre at Allegheny General Hospital (AGH) in Pittsburgh, Pennsylvania. This integrated cancer clinic will begin patient treatments later this summer using an array of cutting-edge radiotherapy systems, including the Elekta Unity MR-Linac and two additional Elekta machines (an Elekta Versa HD with onboard imaging and robotic table; also a Leksell Gamma Knife Icon with cone-beam CT and motion management).
Work in progress
Right now, the MR-Linac is still under construction at AGH, with the Covid disruption forcing the launch schedule back a couple of months. “Hopefully we’ll be moving into commissioning for the Unity over the next few weeks and are pushing for initial patient treatments in late September or early October,” explains Tom Colonias, a radiation oncologist at AGH and clinical lead on the hospital’s Elekta Unity project.
In the meantime, there’s a comprehensive training and applications programme in place so that Colonias and his colleagues are ready, from day one, to exploit Unity to the full. A case in point is the system’s on-board MR imaging capability. “We use MR scans all the time in radiation oncology,” says Colonias, “but we don’t perform the scans – they’re done in radiology. As a result, the whole department has to be MR-trained regarding the health and safety aspects of working in a magnetic-field environment.” To provide a focal point for that expertise, AGH is putting three of its radiation therapists through MR certification so that they can run the MR scanner on a day-to-day basis (although such certification is not mandatory for Unity).
Elekta’s engineering and physics teams are also working closely with AGH medical physicists to implement a rigorous and standardized approach to system acceptance, commissioning and quality assurance (QA). “We’ll be using specialized MR-safe phantoms for the commissioning QA,” says Colonias, “as well as recruiting volunteers so that we can verify the performance of the MR scanner on real people.” What’s more, all of the QA techniques and equipment for the Unity are developed to be MR-safe – from end-to-end workflow tests utilizing both the linac and MRI components of the system to patient-specific QA for each adapted plan of treatment.
In the run-up to the Unity system going live, AGH’s multidisciplinary clinical team will make recommendations on initial patient selection. On a practical level, it’s likely they will kick things off with prostate treatments and pelvic malignancies (as in each case there’s no significant organ motion and there are plenty of patients available). “It’s going to be a learning curve at the outset, getting the Unity workflow defined and really efficient,” explains Colonias. “Within six months of being operational, though, we aim to be doing real-time adaptive planning and treating up to 10 patients a day.”
Longer term, the adaptive capabilities of Unity will enable AGH to address the inherent complexities of a wide range of disease indications (see “The AHN roadmap for MR/RT”, below). Between treatment sessions, for example, patients can gain or lose weight; their stomach, bladder and bowel contents change; their organs may shift, rotate or deform; and their tumours may shrink, move or rotate. With this in mind, AGH clinicians are already planning MR-Linac studies of liver stereotactic body radiotherapy (SBRT) and pancreas SBRT, alongside a specific interest in exploring stereotactic ablative radiotherapy (SABR) to treat ventricular tachycardia (a potentially fatal condition in which the heart beats at more than 100 beats/min).
Another priority is head-and-neck cancer. “Owing to potential weight loss and tumour response during therapy, head-and-neck cancer patients can be ideal candidates for the adaptive planning afforded by the Unity,” notes Colonias. “Such adaptive treatments may result in better functional outcomes and improved quality-of-life post-treatment.”
Collaborate and accumulate
More broadly, membership of Elekta’s MR-Linac Consortium has been fundamental to the MR/RT learning experience for Colonias and his colleagues. One aspect of the Consortium is a knowledge-transfer programme that brings together more than 35 clinical institutions – all of them Elekta Unity customers – as part of a collective conversation focused on driving improved patient outcomes through the application of MR-Linac technology.
Tom Colonias: “The MR-Linac Consortium is a great vehicle for us to talk, collaborate and learn along the way with other clinical institutions.”
AHN has been involved with the Consortium for a couple of years, taking advantage of the regular international meetings to fast-track the training of its radiation oncology team on all aspects of Unity best practice – from treatment planning and MR safety to QA protocols and online adaptive workflows. “We’re starting out-of-the-box here at AHN with a new way to treat patients,” says Colonias. “As such, the Consortium is a great vehicle for us to talk, collaborate and learn along the way with other clinical institutions.”
What’s notable, Colonias adds, is that the Consortium is set up as an independent entity, with Elekta “creating the conditions” for collaboration rather than opting for top-down micromanagement. “Elekta plays an integral role in the Consortium and acts as a facilitator, letting the clinical community define priorities and share their clinical and technical outcomes,” he explains. “Of course, the Elekta product development and engineering teams are also front-and-centre, providing specialist support and training as well as gathering feedback direct from the clinical end-users.”
Down the line, the AHN team plans to work with other Consortium members on multicentre MR-Linac clinical studies – including the application of Unity’s functional MR imaging capabilities. “We get scale when we collaborate, share and analyse our study data together,” Colonias concludes. “If we have multiple Unity clinics contributing MR data on, for example, 100+ patients with pancreatic cancer [versus fewer patients as individual institutions], we can compare post-treatment outcomes and hopefully draw robust conclusions on best practice and treatment efficacy.”
The AHN roadmap for MR/RT
Multidisciplinary AHN teams have been participating in the Elekta MR-Linac Consortium meetings since September 2018, with many AHN staff (physicians and physicists) taking a prominent role within the consortium’s specialist tumour-site groups (TSGs) and brainstorming initiatives. As a result of that engagement, AHN will be contributing to multicentre clinical studies and data/outcome analysis in the following areas.
Pancreatic SBRT
There’s emerging evidence that radiation dose escalation provides improved outcomes for locally advanced, unresectable pancreatic cancer
Project driver: targeting will be easier and more reliable using Unity’s on-board MR imaging, with potential for dose escalation across 5–15 fractions (depending on surrounding structures)
A treatment protocol is currently being developed by the MR-Linac Consortium’s pancreatic TSG
AHN clinical lead: Rodney Wegner MD
Liver SBRT
Project driver: application of adaptive stereotactic radiotherapy for primary and metastatic hepatic malignancies
The study will exploit superparamagnetic iron-oxide nanoparticles (SPION) and on-board MR imaging for avoidance of healthy liver tissue (hepatic parenchyma) during 3D conformal treatment planning
Subsequently, MR-Linac imaging will examine the dose–response of tumour sites and hepatic parenchyma to SBRT
AHN clinical lead: Alexander Kirichenko MD
SABR for refractory ventricular tachycardia
MR is a superior imaging modality (versus CT) for visualizing the heart in terms of image planning and guidance
This study will evaluate the Unity platform’s “marriage of MRI-based cardiac planning and treatment delivery via radioablation”
The goal is MR imaging, real-time planning and treatment delivery to address tachycardia in one session
Researchers at Princeton University in the US have become the first to observe a robust supercurrent at the edge of a superconductor that is very different to the supercurrent in the material’s bulk. This “topological superconductivity” could come in useful for a host of new applications, they say.
Topological materials are materials that have very different properties at the surface compared to those in their bulk thanks to special topologically protected “edge states”. Topological insulators – materials that act as insulators in their interior but conduct currents on their surface – have been a hot topic in condensed-matter research for several years, but their superconducting counterparts are less well-studied.
To find out what happens when the interior of a topological material is not an insulator but a superconductor, Nai Phuan Ong and colleagues turned their attention to molybdenum ditelluride (MoTe2). This material is a Weyl semimetal, a recently discovered class of topological material in which electrons (which are fermions, and thus have spin-1/2) behave as if they have no mass. These oddly-behaved particles were predicted in 1929 by theoretical physicist Herman Weyl as a solution to the Dirac equation (which describes the physics of normal fermions), and they travel faster and dissipate less energy than electrons in ordinary metals or semiconductors. They also show the “chiral magnetic effect” when placed in a magnetic field, which generates a current of positive and negative Weyl particles that move parallel and antiparallel to the field.
Saw-tooth pattern in critical current oscillations
The researchers began by preparing slivers of crystals of MoTe2 that were between 60 to 120 nm thick. They then cooled these crystalline samples down to below 100 millikelvin – the superconducting transition temperature of MoTe2. Next, they applied a weak magnetic field to the samples while measuring the current flow through them.
The Princeton team observed a quantity known as the critical current oscillating in a saw-tooth pattern as they increase the applied magnetic field. Both the height and frequency of these oscillations fit well with predictions of how these fluctuations arise from the quantum behaviour of electrons confined to the edges of the material, they say.
In superconducting materials, electrons overcome their mutual electrostatic repulsion to form Cooper pairs thanks to interactions between the electrons and vibrations of the material’s crystalline lattice. Once formed, these pairs behave as bosons, which have an integer spin. This means they can condense to form a “superfluid” state that behaves as a single entity, carrying electrical current through the material with no resistance at temperatures below the material’s transition temperature.
Superfluid velocity adjusts
In MoTe2 and other Weyl semimetals, this Cooper-pairing of electrons in the bulk also appears to induce a similar pairing on the edges, says Ong. The saw-tooth pattern in the critical current as the applied magnetic field increases would come from the fact that the superfluid velocity adjusts to maintain an integer number of overall twists in the phase of the superconductor’s wave function, he explains.
While he and his colleagues say they do not yet fully understand the reason for why the edge supercurrent remains independent of bulk supercurrent, they believe it could come from the topologically protected edge states in MoTe2. To find out whether this is true, they plan to repeat their experiment on other unconventional superconductors and search for similar edge supercurrents.
Although conventional superconductors are already widely employed in areas like magnetic resonance imaging (MRI) and long-distance transmission lines, new types of superconductivity like this one could help us move beyond the limitations of familiar superconducting technologies, Ong says.
The paper, entitled “Evidence for an edge supercurrent in the Weyl superconductor MoTe2” is published in Science.
