Although you might expect quantum phenomena and nanotechnology to walk hand in hand, a lot of superconductivity research has focused on material chemistry rather than material structures. However of late there seems to be an evident coupling between fundamental advances in superconductivity research and nanoscale science and technology – in particular 2D materials, as highlighted by the coinciding release of papers reporting superconductivity in graphene and 2D topologies in both Nature and Science this week.
The Nature paper focuses on graphene, which has previously been coerced into exhibiting superconducting properties through doping or a proximity effect when it is next to a superconducting material. As nanotechweb.org‘s freelance contributor Belle Dumé describes in her tech update, and Susan Curtis reports in Physics World, the superconducting properties in slightly twisted bilayer graphene reported this week arise in the absence of these approaches, and at record low carrier densities with the relatively high transition temperature of 1.7 K. (Superconductivity is one of the few fields where 1.7 degrees above absolute zero can be described as “high temperature”.)
The paper in Science reports on an iron-based superconductor – a type of material that can have really high transition temperatures (i.e. only a couple of a hundred degrees below freezing). Hong Ding, Shik Shin and colleagues in Japan, the US and China report 2D topologically superconducting surface states in FeTe1-xSex with a transition temperature of 14.5 K for x = 0.45. The significance of this observation is that it could host Majorana states for applications in topological quantum computing.
Of course certain nanostructures have been familiar features in superconductivity papers already, particularly when it comes to efforts towards applications using superconductivity. For example, a lot of these endeavours hinge in some way around some type of Josephson junction, often built from nanowires. However, big headlines marrying nanotechnology and superconductivity research have been arguably less common, and the recent rise in frequency of significant reports of superconductivity in nanostructures – from twisted bilayer graphene and 2D topological surface states in in FeTe1-xSex to stanene, superconductor-wrapped nanowires and nanorod powders to name a few from the past few weeks alone – hints at the two fields simultaneously reaching points where their symbiosis can be really fruitful.
On Wednesday Sarah Tesh and I took some time out of the APS March Meeting to visit the California Institute of Technology, just a 20-minute ride away from downtown Los Angeles. The focus of our visit was the newly opened Centre for Autonomous Systems Technology (CAST), which brings together Caltech’s expertise in such fields as robotics, space and Earth exploration to build futuristic systems that can adapt and respond to their surroundings.
Mory Gharib, director of the CAST facility, told us that the centre is working towards five different “moonshots” – ambitious visions that help to inspire and direct the research effort. As an example, the “explorers” challenge seeks to build integrated systems – most likely a combination of flying, walking and swimming devices – that can navigate unfamiliar environments and react to emerging situations.
An important step towards that goal is to build bipedal robots that can walk on different terrains. Robotics expert Aaron Ames showed us the latest iteration, dubbed Cassie, which can control its own gait and balance to walk unaided around the Caltech campus. But Ames also showed us amusing videos that show even the newest R&D devices falling over when walking on compressible materials such as sand.
Gharib, meanwhile, is focusing his efforts on the “transporters” moonshot, which is in part is attempting to build flying ambulances that could act as a first response for injured people in hard-to-reach locations. “The battery technology is already good enough for us to fly a machine at 75 miles per hour for about 15 to 20 minutes,” he says. “That would be enough to get a person to hospital, or to remove people from dangerous situations, such as a fire.”
Gharib and his team have built a working model one-fifth the size of the final design, which would eventually be about the size of a Toyota Prius, and have tested it in a specially constructed wind tunnel that has an array of almost 1300 fans to generate real-world weather conditions with wind speeds of up to 44 mph. Vertical take-off remains an engineering challenge, but the biggest hurdle will be to make such flying systems truly autonomous. “They will need to survive and operate without human intervention,” Gharib explains. “We want them to interact with their surroundings and react accordingly.”
Gharib believes that the wind-tunnel tests will help to ensure that the flying devices can negotiate different weather conditions. “We are using human reactions as a leapfrog for our learning,” he says. “We use the wind tunnel to create different types of turbulence, and we monitor the reaction of a human operator to see how the machine can be kept on course. The output from the remote control, along with positioning data from a series of cameras, can then be used to train a machine learning system.”
Gharib admits that the vision of fully autonomous systems may not be realized for many years, but he believes that the moonshoot approach provides a powerful impetus for such interdisciplinary research. And despite having such an ambitious destination, he is confident that many useful technologies will emerge during the journey. “The moonshots are really a teaser to inspire and excite the scientists and students,” he says. “But the best fruit will be the technologies we develop along the way, just as the journey to the moon produced many of the technologies we use today.”
And if all this talk of thinking machines sounds a bit scary, Gharib stresses that Caltech’s intention is to create machines that work alongside us to achieve new things. “We need machines that help us discover new things,” he says. “Here at Caltech we want to create autonomous tools that will work as partners to help us do better science and engineering, and to do our jobs better.”
Academics are one of the occupational groups with the highest incidence of mental illness, according to a recent report on mental health in education by RAND Europe for the Royal Society and Wellcome Trust. According to the report, the risk of postgraduates and university staff having or developing mental-health problems is generally higher than many other working populations. In addition, the levels of work-related stress are comparable to groups classified as “high risk”, such as health-care workers.
The cover feature in the March issue of Physics World magazine shines a light on this problem through the eyes of one physicist in the UK. The story of their seemingly endless struggles highlights how the academic community doesn’t neccessarily handle mental-health problems well, both in terms of attitudes at a personal level and the support offered at an institutional scale. Indeed, the physicist in question chose to write the article anonymously to avoid their problems being publicly known.
In the video above, I outline our rationale for publishing the feature and explain why it’s an important topic to cover. You may also wish to look at the “Resilience toolkit”, created by the Institute of Physics (IOP), which publishes Physics World.
Remember that, as always, selected articles from Physics World magazine will appear on this website over the course of the month, but if you’re a member of the IOP you can read the entire March issue right now in digital format.
China is planning to build a next-generation X-ray observatory that will study some of the most violent objects in the universe such as black holes, neutron stars and quark stars. The enhanced X-ray Timing and Polarimetry mission (eXTP), which is estimated to cost about three billion yuan (£340m), will be launched by 2025 and involve collaboration with European scientists.
At a kick-off meeting on 2 March held in Beijing at the National Space Science Center, the Chinese Academy of Sciences (CAS), CAS vice president Bin Xiangli put his support behind the mission noting that it should become “China’s flagship science satellite”. The mission team will now spend the next couple of years finalizing the design before building a prototype by 2022. “As we only have seven years to go it sounds like mission impossible,” says Xiangli. “But we will coordinate international efforts and deliver it without delay.”
Dedicated to space research
X-rays are the perfect tool to study objects under extreme conditions. By measuring the electromagnetic fields in and around these objects over time, scientists expect to investigate how black holes spin and determine the “equation of state” for neutron stars. Using both focusing and collimating technologies, eXTP will study the details of these X-ray sources in the energy range 0.5-30 keV. It will be carry four instruments: the Spectroscopic Focusing Array; the Polarimetry Focusing Array; the Large Area Detector (LAD); and the Wide Field Monitor (WFM).
As a next-generation observatory, eXTP will have a total collecting area of 4.5 m2, which is crucial for precision measurements. Its focusing array, for example, will have a collecting area three times that of Europe’s XMM-Newton probe. eXTP will also be able to measure the polarimetry of the sources to collect information about the various asymmetries at, or near, the surface of black holes and neutron stars.
eXTP will be an example of large technical and engineering collaboration mission between Europe and China
Andrea Santangelo, University of Tübingen
China is a newcomer in X-ray astronomy and space science. The Hard X-ray Modulation Telescope (HXMT) – the country’s first and only X-ray satellite – was sent into orbit last June and is now taking data. Before 2015, China did not have a single satellite in orbit that was dedicated to space-based fundamental research. However, with the support from the Chinese government, the country is making big leaps forward having recently launched a number of small missions such as the Dark Matter Particle Explorer or Quantum Experiments at Space Scale.
Chinese leadership
eXTP will be the most expensive space-science satellite China has ever approved costing three times as much as an average Chinese mission. According to physicist Shuangnan Zhang, principal investigator of eXTP from the Institute of High Energy Physics in Beijing, about two-thirds of the cost will come from China with the remainder made up of “in-kind” contributions from the European members and the European Space Agency. The two focusing arrays will be mainly developed in China, while LAD will be built in Italy and WFM in Spain and Denmark.
“Our goal is to fly a truly large, flagship mission for astrophysics in the next decade,” says Andrea Santangelo from the University of Tübingen in Germany, who is eXTP’s international coordinator. “eXTP will be an example of large technical and engineering collaboration mission between Europe and China under the leadership of China.” Indeed, he sees no major technical problem for the eXTP to overcome. “The mission’s technical readiness is really high,” he says. “And I’m not really worried about the timeframe. China has shown its ability to keep the schedule,” he added.
Paul Ray, an astrophysicist at the US Naval Research Laboratory, notes that recent advances in solid-state X-ray detector technologies have facilitated new mission concepts. He is principal investigator of STROBE-X – a similar proposed X-ray mission that will feature a large collecting area and wide-sky coverage and that could be launched in the late 2020s, if approved. “These missions will be critical in the era of time-domain astronomy and will be an essential complement to optical, radio, and multi-messenger studies of the most dynamic and energetic processes in the cosmos,” says Ray.
Today is International Women’s Day and to celebrate we are highlighting some of our favourite content from the past 12 months that is by or about women.
“Society expected young women to get married, not make major astronomical discoveries!” says Bell Burnell about the attitude she faced as an early-career scientist. Fortunately, things have changed since the late 1960s and today we are also highlighting videos that look at the aspirations of two early-career scientists today. In “Faces of Physics: human organs on a chip“, Samira Musah talks about her multidisciplinary work as a post doc at Harvard University’s Wyss Institute. Meanwhile at the University of Bristol, PhD student Kate Wyness talks about her work on nuclear waste in “Faces of Physics: a nuclear-powered PhD“.
Estuarine wetlands along the US Pacific coast may be at risk of destruction if sea levels rise as much as feared, warns a study in Science Advances. Many such habitats can’t spread inland as they are hemmed in by steep geography or human coastal developments. This makes the region particularly vulnerable to sea-level rise, threatening the endangered species that call the wetlands home.
To make the assessment, Karen Thorne of the US Geological Survey and colleagues from the US and Canada modelled the effect of rising sea-level on 14 estuarine wetlands along the Pacific coast of the continental US.
If the upper estimates of sea-level rise come to pass and levels rise by around 1.5 m, 83% of tidal wetlands are predicted to become unvegetated, the team found. High and middle-height marsh habitats would disappear and only low marshes would survive past 2110. Even with less extreme predictions of sea-level rise, 95% of high and 60% of middle-height marshes look set to disappear.
This pattern differs from the Atlantic coast and Gulf of Mexico, where wetlands can often migrate inland, escaping much of the rise in sea-level. Many Pacific wetlands already occupy the maximum area available, hemmed in as they are by human developments or steep terrain, so rising sea levels will simply inundate the habitat.
The researchers simulated the effect of sea-level rise on 14 individual wetland sites in river estuaries, accounting for current wetland elevation and projections for how much sediment each wetland is set to accumulate in the next century. They simulated low, moderate and high sea-level rise scenarios for each individual site ranging from a rise of 12 cm to a 166 cm increase.
The loss of wetland areas has far-reaching effects for both human and animal communities. Vegetated wetlands in large river estuaries provide protection from storm surges for upriver towns and cities. Wetlands are also important carbon sinks, their plants absorbing greenhouse gases such as carbon dioxide and methane, so their loss threatens to worsen climate change. The habitat is home to many endangered species found only on the Pacific coast, such as the salt marsh harvest mouse and Belding’s savannah sparrow. Many bird species rely on stopovers in wetland areas during their annual migrations, and various fish forage there during at least part of their lives.
While Pacific coast wetlands are clearly under serious threat, the relatively slow sea-level rise projected for the near future provides a chance to save them. Various control measures, such as wetland restoration and “managed retreat” through geoengineering, may help save this valuable habitat. For example, the wetlands in San Francisco Bay already have a management plan. In the face of climate change, this study reveals the potential benefits of such a plan for the whole US Pacific coast.
Body size has an influence on the dose conversion factors of a conventional chest posteroanterior (PA) examination for the organs in the field-of-view, except for the thyroid, according to an award-winning Belgian study presented last week at ECR 2018.
Previous studies have found similar results on the influence of the body mass index (BMI) or weight, and there is a growing consensus that using the automatic exposure control during a radiological examination will increase the exposure of overweight patients to maintain sufficient image quality.
“However, our study showed that the organ dose conversion factors decrease with increasing patient size,” noted lead author An Dedulle, a doctoral student in the department of imaging and pathology at the University of Leuven and also from the medical imaging software developer Qaelum, a spin-off company of the university. “This will result in a decreasing general conversion factor with increasing patient size: The radiation detriment of obese patients is lower than initially expected if only one, size independent, conversion factor would be used.”
This finding stimulates interest for patient-specific (organ) dose calculations in general radiography and should be considered in patient-specific reports of medical radiological procedures, the researchers explained in an e-poster that has received a prestigious magna cum laude award at ECR 2018.
The obesity epidemic
Nowadays, the radiation burden of the population is mostly an estimation based on general conversion factors (e.g., from dose area product to effective dose) for a reference normal-sized patient, but the prevalence of obesity has nearly tripled since 1975, and in 2008 more than 50% of the European population was overweight, according to Dedulle and colleagues.
The aim of their study was to examine the effect of BMI and water equivalent diameter (WED) on organ dose conversion factors in the case of conventional chest PA radiology examinations. They calculated dose conversion factors from voxel models made from total-body CT scans. All organs of importance could theoretically be included in the study, even though the current focus was chest PA imaging.
The study included 40 patients (20 female, 20 male) of different BMIs who underwent a CT exam from head to thighs. The researchers calculated the water equivalent diameter of every patient as the average WED over the central 20 cm of the axial CT slices in the lung region (from top to bottom of lungs).
They segmented the patients’ major radiosensitive organs: bones (for bone marrow and bone surface dose), thyroid, lungs, breasts, heart, liver, stomach, colon, kidneys, bladder, gonads, uterus (females)/prostate (males), skin, muscle, air inside the body, and the remainder. The team adjusted and configured an in-house developed Monte Carlo framework built for cone-beam CT for 2D radiography.
The researchers determined commonly used chest PA examination parameters from the dose monitoring system (Dose, Qaelum) in routine use at Leuven University Hospital, and they generated organ dose conversion factors for the typical, clinically used X-ray spectra and standard radiographic parameters.
The group used a statistical program (GraphPad Prism, GraphPad Software) to analyse and subsequently visualize the data and also analysed the female and male patients separately. The researchers tested normality and evaluated the influence of the BMI and water equivalent diameter on the organ dose conversion factors inside the field-of-view (red bone marrow, thyroid, lungs, heart and breasts) with two-tailed p-values with a confidence interval of 95% (p < 0.05).
They observed significant linear correlations between the dose conversion factors and BMI, respectively WED, for both genders for the red bone marrow, lung, heart, and breast. The conversion factors fell with increasing BMI and WED, which can be explained by the extra shielding of the adipose tissue in patients with high BMI and WED. The researchers found nonsignificant results when correlating the BMI or WED with the thyroid dose conversion factors for both males and females (p > 0.05).
The results are summarized in the table below.
Two-tailed p-values to test the significance of the BMI and WED of the phantoms and the organ dose conversion factors inside the field-of-view
Study disclosures
The doctoral research of An Dedulle is supported by the Flanders Innovation & Entrepreneurship agency (grant number HBC.2016.0233). The work was conducted in cooperation with Qaelum and the University of Leuven.
A new experimental platform based on two misaligned graphene layers could be used to investigate strongly correlated physics – that is, the physics of systems in which the interactions between electrons lead to novel phenomena. The platform, which can be tuned by simply applying an electric field, could help shed important light on the underlying mechanisms at play in superconductors, in particular high-temperature ones based on cuprates, for which a fundamental understanding is still lacking.
A team of researchers led by Pablo Jarillo-Herrero of the Massachusetts Institute of Technology (MIT) in the US made the platform by stacking two sheets of atomic-thick carbon (graphene) on top of each other. They then twisted the sheets so that the angle between them, known as the (theoretically predicted) “magic angle”, was 1.1°. They found that the material became a superconductor (that is, it conducted electricity without resistance) at 1.7 K.
“We were not looking for superconductivity when we began our experiments,” explains Jarillo-Herrero. “We chose to study these structures because there were some theoretical predictions that interesting electronic properties would occur in the graphene moiré superlattice if the two layers were stacked at this angle. Our intuition also told us that there would be some interesting physics, but what we discovered went far beyond what we had anticipated.”
The researchers studied the conductivity of the graphene sheets by applying a voltage to them and then measuring the current that circulated through them. They also measured the density of the particles that carry electronic charge inside the sheets.
Two breakthrough results
“We found two things: first that we can electrically tune the graphene system so that it becomes a correlated insulator, which can happen thanks to electrons localized in the moiré superlattice. This ‘Mott’ insulator is a material that should be a metal but which, because of strong repulsion between electrons, does not conduct. We reported this result in the first of our two Nature papers published this week.
“Secondly, we found that by adding a few extra charge carriers to this insulator state (by applying a small electric field), we could tune the graphene superlattice so that it became a superconductor. This result is detailed in our second Nature paper.”
The researchers say that graphene superlattices containing a record-low 2D charge carrier density of about just 1011 per cm2 can become superconducting. This means that they can superconduct electricity with just 10-4 of the electron density of conventional superconductors (that work at temperatures near absolute zero, and which can be described by the well-established Bardeen–Cooper–Schreiffer theory of superconductivity).
This behaviour (the presence of an insulating state so close to the superconducting one) is characteristic of so-called unconventional, high-temperature superconductors, known as cuprates. These complex copper oxides can conduct electricity without resistance at the relatively “high” temperature of 133 K. Although physicists have been studying these materials for decades now, in their quest to make superconductors that work at even higher temperatures, and ideally at room-temperature, they are still unable to explain the fundamental mechanisms at play in them.
Magic-angle graphene is magic
“The technique to make our new misaligned graphene sounds simple, but it took years to perfect,” says Jarillo-Herrero. “The good thing, however, is that there are several groups around the world that can carry it out. There are also many other groups that will now be able to replicate it too and so use the platform to study unconventional superconductivity in a simple system.”
Normally, when researchers study high-temperature superconductors, they need to subject the materials to extremely high magnetic fields, he explains. With graphene, they might be able to do this by simply applying a modest magnetic field.
“First and foremost, our discovery represents an advance in terms of fundamental science, and we hope that it will allow us to gain insight into the properties of strongly correlated systems, such as high-temperature superconductors and quantum spin liquids,” he tells nanotechweb.org. “What is more, our platform is a general one and could be applied to any 2D material, not just graphene.”
Quantum computers and photodetectors might benefit
Although there might be many potential applications most of these are likely to be a way off, realistically speaking, he adds. “For example, the most advanced technology to make prototype quantum computers today are based on superconducting devices. Magic-angle graphene superlattices could offer us a new type of electrically tunable superconductor, and who knows, they might one day be exploited in quantum computation and information technologies.
“Superconductors are also used in many other applications, such as ultrasensitive detectors of light, so our result may perhaps have an impact there too.”
There is no doubt that graphene is an exceptional material in so many ways. Its unique properties, such as extremely high mechanical strength (it is stronger than steel) and extremely high electrical conductivity, with electrons zipping through it at near-ballistic speeds, have been known for a while now. Although researchers had already shown that it could behave like a superconductor before too, the superconductivity was only observed when it was in contact with other superconducting materials. What is more, this could mostly be explained by the Bardeen–Cooper–Schreiffer theory, so it was considered to be conventional.
“The relatively high superconducting temperature of 1.7 K of twisted bilayer graphene that we observed, with its charge carrier density of just 1011 per cm2, now also makes this material among the strongest coupling superconductors known,” adds Jarillo-Herrero. He says that the material might be working in a regime close to the crossover between the Bardeen–Cooper–Schrieffer regime and a Bose–Einstein condensate (a state of matter in which all the particles in a system condense into a single state), but confirming or refuting this will be the subject of future research.
How many people study physics and then go on to forge a career in environmental sciences? Perhaps not a huge number, but those who have a “physics mindset” often bring a fresh perspective to environmental research. Today an increasing number of physicists are helping to tackle some of the world’s most pressing environmental challenges. For Daniel Kammen, a self-confessed Star Trek fan and director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley, US, the migration from physics to environmental science was serendipitous.
“My path was very random, driven by a love of physics and way too many interests,” he says. Initially, Kammen’s dream was to be an astronaut. “I learned to fly planes, took acrobatic and sea-plane landing lessons, but I was ultimately screened out of the NASA astronaut qualification on the basis of vision,” he explains. However, Kammen’s infectious enthusiasm for understanding the world around him soon opened many other doors.
While studying physics at Cornell University, Kammen learned about astronomy and cosmology, worked in the low-temperature physics laboratories and in solid-state physics, where he published his first papers on solid-state masers, and eagerly absorbed courses on electrodynamics, quantum mechanics and quantum field theory. Then at graduate school, first at Stanford University and then at Harvard University, he was drawn towards cosmology, computational physics and neural networks.
But it was while doing a postdoc in neural computing at Caltech that Kammen realized he could apply his talents to environmental problems. “During my summers I volunteered on an energy project, introducing solar ovens to communities in Nicaragua (the US was blockading the country at the time), and as a result I published my first paper on energy in Nature,” he says.
Accessing all energy
That chance volunteer work set Kammen’s career on the path of both academic and activist. For the last 25 years his focus has been finding solutions to the energy needs of developing countries. Today his passion is “energy access” and he works largely with communities in East Africa, Central America – including the country that originally inspired him, Nicaragua – and on Native American lands in the US. “Physics has provided me with the most amazing training, and I consistently use it today in work on solar cells, network studies of energy grids, and in dynamical systems methods applied to all sorts of things,” he says.
Physics has provided me with the most amazing training
Dan Kammen
Stepping sideways from physics into environmental sciences has required a flexible and open-minded approach, but Kammen relishes the challenge of learning new things. “I am keen to keep working in analytical methods and I always want to learn more in the humanities and social sciences, where I am just a baby,” says Kammen, who is editor-in-chief of the open-access journal Environmental Research Letters (produced by IOP Publishing, which also publishes Physics World).
Kammen’s unusual career trajectory led him to contribute to the Intergovernmental Panel on Climate Change (IPCC) in its early days; work which was rewarded in 2007 when the IPCC shared the Nobel Peace Prize. These days his goal is to “de-carbonize” society. Last year he joined a list of eminent scientists, business leaders, economists, analysts, influencers and representatives of non-governmental organizations to set up Mission 2020 – a collaborative campaign that aims “to bend the greenhouse-gas emissions curve downwards” by 2020. Over time Kammen’s research interests have taken many twists and turns, but his enthusiasm for Star Trek is one thing that hasn’t changed. “I still own Spock ears and generally win the game ‘identify the Star Trek episode with the shortest quote’,” he laughs.
Down to Earth
For Anny Cazenave – director for earth sciences at the International Space Science Institute in Bern, Switzerland and senior scientist at the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales at the French space centre (CNES) in Toulouse, France – the journey to environmental science began with an interest in what lies beyond Earth. While doing her first degree in mathematics and physics, Cazenave, like Kammen, was fascinated by space, and had ambitions of becoming an astronomer. Gradually her interests evolved towards geophysics, and she did a PhD at the University of Toulouse on the rotation of the Earth. This led to a permanent position at CNES to develop satellite geodesy – the use of satellites to study the shape of the Earth, its gravity field and its rotation, solid Earth tides and so on.
Courtesy: istock\suprun
When Cazenave accepted the position, she had no inkling of how her work might transform environmental research. “At that time [the 1970s] environmental science was not at the forefront of space activities,” she explains.
It wasn’t until the mid-1990s, when satellite technology was far more advanced, that scientists began to fully explore the use of satellites for environmental applications. In particular altimeter satellites – which send a microwave pulse down to Earth and measure altitude from the time it takes the pulse to return – started employing two different wavelengths, massively increasing the resolution at which they could map the Earth’s surface.
Scientists, including Cazenave, spotted the potential of high-resolution satellites for mapping the peaks and troughs of the sea surface, and realized that they represented a new way of monitoring sea level changes and ocean circulation. “Although I was not an oceanographer, I learned about it while working,” says Cazenave. “At the beginning of the 2000s, I also started to develop hydrology from space – the study of terrestrial waters using space techniques.”
Interdisciplinary research needs hard work but it is highly motivating too, and I’m passionate about learning new things
Anny Cazenave
Today Cazenave’s focus is using satellite data to monitor climate change, for example, sea level rise, land ice melt, ocean thermal expansion and changes in the global water cycle. She feels that her original background in maths and physics has been a useful tool, but flexibility and willingness to learn have also been key to enabling her to move into a new field. “Interdisciplinary research needs hard work, to gain experience in the field in which we are a newcomer, but it is highly motivating too, and I’m passionate about learning new things,” she says.
Naturally outdoors
Unlike Kammen and Cazenave who came to environmental science via curiosity about space, Jennifer Burney of the University of California, San Diego, US, found her enthusiasm for the environment to be a consistent thread throughout her life. “I’ve always been an outdoorsy person, and growing up in New Mexico always had a strong interest in the natural world,” she explains.
Following a degree in history and science, Burney began a physics PhD at Stanford, developing a superconducting camera that captures images of cosmic bodies such as pulsars or exoplanets. Partway through her studies, Burney decided to defer for a year, so that she could volunteer with rebuilding efforts in Nicaragua after 1998’s Hurricane Mitch. “It was exciting to be in the field devising creative solutions,” she says.
After finishing her PhD, Burney’s desire to bring positive change to other people’s lives resurfaced and she followed a non-academic route, working for non-governmental organization the Solar Electric Light Fund on rural electrification around the world. “One project was solar-powered drip irrigation in West Africa,” she says. “They needed somebody to figure out how to evaluate the technology. That required assessing the design and how to make it cost-effective and sustainable.”
Over time Burney became intrigued by how energy and climate affect food security, water availability and agriculture, and in 2008 she transitioned back into academia via a postdoc at Stanford on food security and the environment. Her research has continued in this vein ever since. These days Burney investigates the couplings between human activity and the environment. However, her physics mindset is still at the forefront of everything she does.
I fundamentally see the world as a physicist, and ultimately most of my projects have that kind of ‘flavour’
Jennifer Burney
“I fundamentally see the world as a physicist, and ultimately most of my projects have that kind of ‘flavour’ – for example, in our projects trying to understand what role air pollutants play in impacting both climate and humans, I tend to think about how they change the radiative properties of the atmosphere and much less about the biological or chemical processes for example,” she says.
But Burney relishes the cross-disciplinary nature of her work. “You learn to see the world in a new way,” she says. And it is this willingness to see things from other people’s point of view, combined with a thirst for knowledge, that seems to have enabled Burney, Cazenave and Kammen to slide smoothly between physics and the environmental sciences. “Physics provides a fantastic toolkit, but environmental problems are the biggest challenge we have,” says Burney. “It will take all hands on deck.”
One of the challenges in using 3D printing to biofabricate cell-laden constructs for tissue repair and organ regeneration is ink optimization. Part of the puzzle is producing a printable blend, but that’s not the only consideration. It’s important to determine compositions that maximize the ability of cells to flourish and generate functional tissue.
“Here, factors such as the stiffness, composition and degradation rate of the ink play key roles in providing an appropriate niche to direct stem-cell fate,” explains Daniel Kelly, director of the Trinity Centre for Bioengineering, Ireland. “Highlights so far involve developing inks that can support vascular networks and engineer spatially complex tissues such as the osteochondral unit.”
Reporting their results in the journal Biofabrication, Kelly and his team have been comparing the performance of hydrogel bioinks by examining the printability of different blends and their capacity to support cartilage development.
Hydrogels (water-swollen polymers) have proven to be a useful addition to the mix. Softer versions of the material – with less cross-linking – favour the differentiation of cells, but construct developers also have to think about the mechanical integrity of their designs.
“This structural stability, often referred to as shape fidelity, is tightly related to the rheological properties of the bioink,” says Jos Malda, an expert in 3D bioprinting based at Utrecht University in the Netherlands. “An ideal bioink should exhibit shear-thinning behaviour – flowing as a low-viscosity fluid when extruded and behaving as a stable gel after printing.”
Malda points out that this transition should be as quick as possible to maintain the imposed shape, though the materials considerations don’t stop there. The target location of the biofabricated structure also adds to the list of design criteria and has led researchers to pursue a variety of solutions.
“The toughest applications revolve around implantation in mechanically challenging environments, which include bone, cartilage and tendons,” adds Malda. “To overcome such limitations we have been designing strategies to co-print bioinks with reinforcing thermoplastic polymers, hydrogels and microfibrous meshes.”
Fortunately, when it comes to screening potential bioinks there are some useful early indicators that can help to speed up the materials selection, particularly for characteristics such as printabilty and shape fidelity.
“Tests based on yield stress and viscosity measurements, coupled with observations of how printed bioink filaments deform due to gravity or surface-tension effects, have been proven as simple but effective approaches to evaluate bioink shape fidelity,” says Malda.
Writing up their findings in the journal Biofabrication, Malda and his colleagues highlight yield stress as a key factor in determining the bioprintability of hydrogels based on gelatin-methacryloyl and gellan gum for cartilage repair.
Adding to the appeal of 3D printed scaffolds are other developments such as being able to tune the release profile of the various active elements contained in the structure. “Increasingly, we are also designing bioinks to act as delivery systems to temporally control the release of genes, growth factors and other regulatory cues to cells within printed constructs,” Kelly reveals.
This article forms part of a series of reports reviewing progress on high-impact research originally published in the IOP Publishing journal Biofabrication.
Body size has an influence on the dose conversion factors of a conventional chest posteroanterior (PA) examination for the organs in the field-of-view, except for the thyroid, according to an award-winning Belgian study presented last week at 
