It is virtually impossible to burst a water balloon by throwing it at a wall unless the balloon has been pre-stretched. This is the finding of researchers at Princeton University in the US who have developed and experimentally tested a model that predicts how elastic capsules deform and rupture upon impact. The work could have practical implications in many areas, they say, including in the development of safer riot-control projectiles, better fire-extinguishing techniques and improved safety guidelines that minimize the risk of internal organ damage during car crashes.
From red blood cells to organs like the stomach and bladder, elastic-membraned capsules are ubiquitous in our bodies. Artificially engineered capsules that mimic their biological counterparts are also common, employed in a host of industrial applications. Whether organic or synthetic, these containers exist to transport and deliver liquids that would otherwise be prone to contamination. However, despite the prevalence of such capsules, little is known about the physics of what happens to them on impact.
Throwing water balloons at a wall
To fill this knowledge gap (and strictly in the interests of science), researchers led by Pierre-Thomas Brun of the Department of Chemical and Biological Engineering at Princeton performed experiments in which they threw custom-made elastomeric capsules and shop-bought water balloons filled with different liquids against a flat wall. They took a sequence of images showing how the capsules and balloons deformed as the impact velocity increased from roughly 6 m/s to nearly 40 m/s.
The researchers liken the impact of a capsule to that of a droplet falling onto a surface, but with two alterations. The first is that the capsule’s surface modulus (a parameter that depends on the capsule material’s shear modulus and thickness) replaces the drop’s surface tension. The second is that while the impacts of capsules may produce large deformations similar to those seen in drop impacts, capsules – unlike drops – can burst open and release their contents. A capsule also deforms as a function of its Reynolds number, which describes the flow of a fluid and expresses the ratio of the fluid’s inertial and viscous forces, and an elastic-shell equivalent of its Weber number – a measure of a fluid’s inertia compared to its surface tension.
Pre-stretching required
Based on their modified liquid-droplet analogy, Brun and colleagues created a model that quantitatively predicts the maximal deformation of a capsule or water balloon, and its possible rupture, upon impact.
“We found that to burst an elastic capsule, we need to pre-stretch it by filling it with a volume of liquid that exceeds the volume of the capsule at rest,” Brun tells Physics World. “We can now predict how much inflation is needed to break a capsule in a given impact speed range.”
According to the Princeton researchers, their model could make it easier to design capsules that burst only when intended. Possible applications include safer riot-control projectiles that would burst at lower impact speeds, thus avoiding serious injury to the people targeted, and fire-extinguishing elastomer capsules that could be thrown onto small domestic fires or released from aircraft to fight large-scale conflagrations. The model could even help devise safety guidelines that decrease the risk of internal organs bursting in a car crash, they say.
Although the present work, which is detailed in Nature Physics, focused on macroscopic objects, the researchers believe their model could be extended to the microscale. “The simple analogy between drops and elastic capsules that we have demonstrated could indeed come in useful for understanding other complex fluid-structure interactions involving capsules – for instance the deformation of microcapsules recently observed in microfluid channels,” Brun says. “Here the source of the deformations would come from viscous effects imparted by the surrounding fluid.”
Who can forget the Blue Planet II documentary broadcast by the BBC in 2017? It showed a turtle tangled in plastic netting, while albatross chicks in South Georgia could be seen ingesting large and potentially deadly pieces of plastic litter. Graphic evidence, if any was needed, of the damaging impact of plastics pollution – it was the TV show that underlined why we have to kick our plastics habit.
That year also saw a landmark study by US academics, which revealed that 8.3 billion tonnes of plastic had been produced since the 1950s. Almost 80% ended up in landfill or the natural environment, where it is set to last for hundreds or thousands of years (Sci. Adv.3 e1700782). The UK alone generates around 2.4 million tonnes of plastic packaging each year, barely a quarter of which gets collected for recycling. What’s worse, up to 27,000 tonnes of plastic waste washes into the UK’s seas and oceans each year (2015 Science347 768).
So why, when your print copy of Physics World magazine lands on your doormat, is it still wrapped in polythene? Like me, you probably rip the wrapper off and bin it, adding to the mountain of single-use plastic destined for landfill where it will persist for decades, possibly even releasing methane when it finally degrades. Surely there’s a better option for the planet, particularly when other magazines arrive in compostable wrappers or good old-fashioned paper? Come on Physics World, sort it out!
Trouble is, even well-intentioned decisions can have undesirable outcomes. Millions of people were once persuaded to buy diesel cars on account of their low carbon emissions – only to find out later, as I did, that their supposedly eco-friendly vehicle spews particulates that endanger human health. In a similar vein, plastic carrier bags have a lower overall environmental impact than paper bags or fully reusable cotton totes, according to a full “life-cycle” analysis carried out by the Danish Environmental Protection Agency in 2018.
Litter in the ocean Sea turtles spend much of their time in the open ocean, and are at risk of getting entangled in waste plastic, or ingesting it. (Courtesy: Shutterstock/Rich Carey)
“When we make decisions about materials, including what to use for bags, we have to consider all the environmental impacts,” says Sophie Hadden of WRAP UK, a not-for-profit organization that offers guidance on sustainable packaging. “It is never straightforward and there are always trade-offs.” Relevant factors include the amount of greenhouse gases emitted in producing the bag, whether the material is derived from renewable sources or fossil fuels, and if it can be recycled and incorporated into new packaging.
With many Physics World readers having written in to ask why it’s still encased in plastic, I’ve been exploring what exactly is the most eco-friendly magazine wrapper. Given the resources available to me, I couldn’t do a proper life-cycle analysis, so I chose three criteria to investigate. First, can the wrapper be reused or recycled? Second, what are the material and energy costs of producing and disposing it? And third, what are the practical pros and cons of the different options when it comes to sending thousands of copies of Physics World to readers all around the world?
Polymer problems
Let’s start with the current plastic wrapper, known in the publishing trade as “polywrap”. It is made from low-density polyethylene (LDPE) – the same material that’s used for supermarket carrier bags. Consisting of repeated units of ethylene (C2H2)n, LDPE is so ubiquitous because it is cheap to make, adds little weight to a package, and is strong enough to withstand wind, rain and rough handling. Polywrap is also transparent, which is vital to enable postal services to scan any barcodes printed inside and to let readers see what’s been delivered.
Best of all, it can be recycled, and the polywrap used for Physics World is made from 100% recycled material, according to our supplier. These factors might surprise you, given the perception that LDPE is hard to recycle and can only be reprocessed into low-grade products such as refuse sacks. But some reprocessing facilities can grind waste LDPE into pellets that can then be used for new plastic products. What’s more, several companies are examining promising new ways to recycle plastic wrappers (see box “New solutions to recycling plastic film”). WRAP UK suggests that recycling a single tonne of plastic film, rather than creating virgin polymer, could prevent 0.83 tonnes of carbon dioxide from being released into the atmosphere.
So far so good. But polywrap’s environmental credentials are far from perfect. The ethylene units in LDPE are obtained by cracking fossil fuels. The ethylene produced in this process is then piped into a processing plant where it is polymerized at pressures of about 300 MPa. According to the trade body Plastics Europe, this consumes about 77 GJ per tonne of plastic product, around two-thirds of which is needed to extract the raw ethylene.
Another problem is that very few local authorities (in the UK at least) collect any type of plastic film for recycling. Strong but flyaway, it gets easily tangled in the machines that sort and separate curbside collections. Around 4500 supermarkets across the UK do provide storefront collection points for carrier bags and anything else made from LDPE, but who bothers with that? And while some supermarkets’ home-delivery services take plastic bags and film back for recycling, coverage is patchy and working out which stores offer the service isn’t simple, even with the help of websites like Recycle Now.
More worrying still is that few people are aware that polywrap can be recycled at all. Some labelling schemes provide clear information to consumers, but fragmented and conflicting waste-management regimes make it hard for householders to know which plastics can be reprocessed in their area. According to WRAP UK, less than 5% of LDPE products are currently recycled in the UK, despite almost 400,000 tonnes of the stuff having been released into UK consumer markets in 2017.
Numbers game Plastics can be recycled, but sorting them into different types can be a minefield. The polywrap traditionally used for Physics World, which is low-density polyethylene (LDPE), can be recycled along with any carrier bags carrying the triangular logo with a “4” inside. Compostable wrappers, in contrast, which are usually more opaque, should be added to your home composting system or included for collection with green and/or food waste. Always keep compostable wrappers separate from your plastic recycling as cross-contamination between waste streams can send a whole batch of recycling direct to landfill. (Courtesy: iStock/Norberthos)
Starchy option
So if polywrap is bad, what are the alternatives? In a high-profile move in 2018, the National Trust (a charity that manages places of historic interest or natural beauty in England, Wales and Northern Ireland) switched all 2.4 million copies of its membership magazine to a compostable wrapper containing up to 30% of potato starch, a waste product created when potatoes are made into crisps and other foodstuffs. Starch is composed of long-chain polysaccharide molecules (C6H10O5)n that come in both linear and branched varieties.
“The imperative for the trust was to eliminate single-use plastic as soon as possible,” says Graham Prichard, the organization’s former print manager who is now an independent publishing consultant. “We had already started to investigate various alternatives to plastic, but the starch wrap fitted the brief and was readily available from our mailing house. The executive board made an extraordinarily quick decision to make the switch.”
The wrapper used by the National Trust is called Bioplast 300, which is made by Alfaplas in Hereford, UK. “Bioplast 300 has been certified to European standards as fully compostable at home or in industrial composting facilities,” says the firm’s sales manager Martin Peacey. “It is made by mixing the starch with other biopolymers and oil-derived co-polyesters, and the resulting molecular structure ensures that it is fully metabolized by micro-organisms into carbon dioxide, water and organic humus. No undesirable particles are created that could find their way into the food chain.”
The appeal of using a decomposible wrapper with a more natural end-of-life has prompted other organizations to follow suit, with both the Guardian and the Times now using potato-starch wrappers to distribute their weekend supplements. At Advanced Direct Mail (ADM) – the mailing company used by the National Trust and other large membership organizations such as English Heritage and the RSPB – business development manager Jo Lucas says that nearly all of the firm’s clients have adopted the bioplastic wrapping. “We are now using 90 tonnes of the material every month, and as such a large buyer we have been able to negotiate a very competitive rate for our customers,” Lucas says.
One snag is that the material is opaque, which makes it difficult for postal services to read barcodes printed on the magazines for automated sorting – such as those used in Royal Mail’s popular Mailmark system. But ADM has been working with Alfaplas to make the film slightly more transparent. “We have replaced a single 24 μm layer of Bioplast with three much thinner layers, producing a film with a thickness of just 21 μm,” explains Peacey. After some successful trials, Lucas says that an entire mailing of National Trust magazines was sent via the Mailmark scheme without any problems.
The potato-starch wrapper is still not an ideal eco-solution, however. “Not all councils will accept it in their recycling collections, and consumers are confused about the best way to dispose of it,” says Prichard. This becomes a real problem if starch-based compostable wrappers are taken for recycling with plastic carrier bags: the two materials must be treated differently and the whole batch of recycling can become contaminated.
Another problem is that some bioplastics degrade too slowly for industrial composting facilities that use anaerobic digestion to treat food waste, which means they just float to the top and must be removed. Worse still, a compostable wrapper is designed to be used only once, which means that the energy used to create it – which Peacey says is comparable to the polymerization process for LDPE – can never be recovered. Bioplastics also continue to incorporate materials derived from fossil fuels.
Bottom line: biowrappers can reduce plastic pollution, but are not a perfectly sustainable solution.
New solutions to recycling plastic film
Back to basics Swindon, UK, start-up Recycling Technologies uses pyrolysis to break down waste plastic into a hydrocarbon oil called Plaxx. (Courtesy: Recycling Technologies)
Plastic film – such as the low-density polyethylene (LDPE) polywrap used for years to wrap print copies of Physics World magazine – can be recycled, although that often means having to take it to your local supermarket. One company keen to recycle plastic film from such stores is Polystar Plastics in the UK, which has recently invested £5m on new equipment to convert it into a material called PCWflex. According to the firm, the material can be made with anything between 30 and 100% recycled plastic, and has the same properties as “virgin” LDPE.
But the lack of a reliable supply chain makes Polystar Plastics’ business model precarious. “These greener options are much more expensive than virgin LDPE, partly because it’s incredibly difficult for us to find material that’s suitable for us to reprocess,” says Polystar Plastics account manager Steven White-Taptiklis. “We have contracts to buy back plastic waste from certain retailers and resupply them with plastic products, but the government needs to help local authorities improve their roadside collections and prevent cross-contamination between different waste streams.”
Some of those problems could be addressed instead by using chemical techniques to break down the polymer chains. For example, Recycling Technologies, a start-up based in Swindon, UK, uses pyrolysis to transform plastic films and laminates into a hydrocarbon oil called Plaxx, which serves as a feedstock to make new plastics, including food and medical packaging. High temperatures are needed, but non-condensable gases generated during the process are captured to provide some of the thermal energy, making it up to 85% energy efficient.
Recycling Technologies has already built a pre-production machine for research, training and initial trials, and now plans to build a commercial system that can reprocess 7000 tonnes of plastic waste per year by the end of 2020. Its system is small, modular and cheap, meaning that units can easily be installed within existing recycling facilities. The firm has also teamed up with TOTAL, Mars and Nestlé to show that plastic waste in France can be turned first into Plaxx and then into virgin plastic. “This will be a fantastic demonstration of how plastic can be a truly circular economy material,” says Elena Parisi, Recycling Technologies’ sales and marketing director.
Other firms are developing similar chemical recycling approaches too, but a 2019 study by Brussels-based pressure group Zero Waste Europe warns they are unlikely to reach large-scale production before 2025. It also suggests that the broader environmental impact of chemical recycling has not been properly evaluated. That could, it says, leave “a risk of putting too much expectation on a solution whose potential is yet to be proven”.
Paper solution
So if there are problems with both polywrap and starchy wrappers, why not ditch polymers altogether? Paper seems a much more attractive option, since it is routinely recycled and comes from a sustainable source. Until recently, however, there wasn’t a viable paper-wrapping solution for magazines. Traditional envelopes aren’t practical for large mailings because each magazine would have to be inserted by hand rather than by machine. What’s more, the envelopes’ mass increases postage costs and overall carbon emissions.
Large publishers have therefore worked with their print suppliers to introduce lightweight paper wrappers that add negligible extra mass. National Geographic was the first high-profile magazine to switch as part of its “Planet or Plastic?” campaign, and now sends 2.5 million copies wrapped in paper to subscribers in the US, UK and India. Country Life magazine has also moved to paper for its 20,000 postal copies, while London-based publisher Immediate Media has switched around 50 titles – including 12 BBC-branded products – to paper wrapping for all posted copies.
“We first introduced paper wrapping in March 2019 for BBC Wildlife and Countryfile magazines, and we have been using it for all 520,000 subscriber copies since the beginning of this year,” says Louisa Molter, Immediate Media’s production manager. “We are committed to reducing our impact through the whole distribution chain, and we felt that paper was the easiest material for our readers to recycle.” Molter discovered, however, that the industry wasn’t initially geared up for paper and it wasn’t easy finding a supplier that could offer paper wrapping in large enough quantities. “But many of the big players have since converted and there is now enough capacity to meet our needs,” she adds.
One of the earliest adopters was Stephens and George, which in March 2019 was the first printer in the UK to install a paper-wrapping machine. “It was only the second installation for the manufacturer,” says the firm’s estimation manager Gareth Collins. “It was a real challenge to get the process right, and there were lots of hurdles to overcome that even the machine manufacturer hadn’t anticipated.” The effort paid off, with around a quarter of the company’s client base now sending their magazines in paper wrap.
Immediate Media has also worked with its distribution partner, Mailing and Marketing Solutions (MAMS), to refine the process and minimize the difference in cost, with MAMS having invested £1m over the last 12 months to install three dedicated paper-wrapping lines. “We have agreed prices that are sustainable for us and close to cost-neutral compared to polywrap,” says Molter. “But that took quite a bit of commitment from our suppliers, and in turn we have made a commitment to put a certain volume through them.”
That’s torn it
After weighing up its options, Physics World believes that paper is the right choice for this magazine. Indeed, it was all set to change this month before the COVID-19 pandemic put those plans on hold. But the switch to paper will happen – and the good news, according to Immediate Media, is that while some paper wrappers do get torn or damaged, the magazines themselves remain in good condition.
Molter admits though that not everything has gone smoothly for Immediate Media. “It’s an evolving journey and we have not yet nailed the perfect specification – particularly for overseas mailings handled by multiple distribution and postal services,” she says. But most of their readers have responded positively to paper wrappers and the organization is sending out ever-fewer replacement copies.
The big advantage of paper is that it is easy to recycle, and previously used paper has become an important feedstock for the paper industry, where it is widely combined with fresh wood to reduce the energy needed to produce new paper with good mechanical properties. Indeed, according to the Confederation of European Paper Industries (CEPI), the energy used to make a tonne of paper fell from 15.7 GJ in 1991 to 12.9 GJ in 2017. That’s far less than the 77 GJ needed to make a tonne of LDPE.
Paper producers have also reduced their reliance on fossil fuels. More than half of their energy requirements are now supplied by renewable biomass – such as burning discarded bark – and most modern paper mills recover waste heat to reduce their use of primary energy.
But paper’s not perfect either. CEPI estimates that Europe’s paper and pulp industry still emits 32 million tonnes of carbon dioxide per year – that’s 1% of all EU emissions. You also need about 35,000 litres of water to make each tonne of product, since paper is usually produced by cooking wood chips and recycled material in a chemical bath containing sodium hydroxide and sodium sulphide. The resulting pulp is mixed with chalk, clay and other fillers, along with chemical additives like titanium oxide, before being squeezed through rollers to remove excess water and then dried through steam-heated rollers.
Much of the waste water is teeming with pollutants such as nitrogen, phosphorous, suspended solids, iron, manganese and organic compounds that don’t easily degrade. And although on-site treatment facilities have led to the chemicals in the waste water from papermills plummeting from 23 kg to 5 kg per tonne of paper over the last 30 years, reducing the impact of paper production on rivers and oceans is a work in progress. Paper manufacturers are also under pressure to cut emissions of sulphur compounds and nitrous oxides into the air.
A naked future?
So with no perfect wrapper available, why not send Physics World with no wrapper at all? Dubbed “naked mailing”, this solution has been adopted by the Chartered Institution of Wastes Management (CIWM), which in 2019 relaunched its own membership magazine as a bi-monthly title called Circular.
“We looked at all the different options available to us for making the magazine as sustainable as possible,” says Paul Sloggett, the CIWM’s head of member engagement. “We use 100% recycled paper and now avoid any sort of waste by sending it without a wrapper.” And for anyone worried that the magazines might have got torn in the post, only two of the first 5500 copies that the CIWM sent members were reported as being damaged; even then the recipients weren’t particularly bothered.
But could naked mailing work for Physics World, which often sends supplements, such as its series of subject-specific Briefings, as well as inserts from advertisers and the Institute of Physics (IOP)? And would naked mailing work given that lots of Physics World copies are sent overseas? In the case of Circular, Sloggett says it uses paper wrapping when supplements are delivered with the magazine, and inserts are only included for those issues. Binding in supplements to the main magazine is another option, although that might not go down well with advertisers.
You might even be wondering why Physics World sends print copies of its magazine at all. There’s been a digital app-based version of Physics World for many years, plus the website is more popular than ever. So why not just ditch print entirely? Problem is, lots of readers and advertisers still enjoy reading a physical product. A monthly print copy of Physics World magazine is also, for many readers, a tangible sign of their membership of the IOP and one they enjoy landing on their doormat.
So rest assured, the print issue of Physics World is still going strong. And while the disruption caused by the COVID-19 pandemic meant that Physics World was unable to switch to paper wrapping this month as planned, the change should happen soon.
Over 1600 researchers from more than 60 countries around the world have been recognized by IOP Publishing for their services to journal publishing. The Reviewer Awards for 2019, which have just been unveiled, sees researchers honoured for their peer-review contributions to 53 journals that are published by IOP Publishing, which publishes Physics World.
For each journal, one reviewer was selected as a “Reviewer of the Year” while several others were picked as an “outstanding reviewer”. Among those was Abhik Sanyal from Jangipur College, University of Kalyani in India, who was announced as reviewer of the year for the journal Classical and Quantum Gravity.
Sanyal says he enjoys peer review, particularly when it comes to learning about the latest research. He doesn’t receive funding for research from his institution as it is a teaching college, but following the lockdown in India due to the global COVID-19 health crisis, Sanyal has been still able to do a “huge amount of work” via online tools, especially to support his students. He usually peer reviews between 12 and 18 articles each year, but says that the number of requests to do so has reduced following the pandemic.
But for Yi Weng, a senior optical-systems engineer at NeoPhotonics Corporation in the US, there hasn’t been a huge change in the number of requests he has had to peer review articles. Weng reviews more than 100 manuscripts a year and was Reviewer of the Year for Journal of Optics. He says that despite COVID-19, communication technologies have helped to “overcome the tremendous obstacles” resulting from the COVID-19 pandemic. “I am having the feeling that our scientific community has become an integral part of the solution,” he says.
Weng says NeoPhotonics has been maintaining a skeleton workforce for only essential operations during the lockdown. But he adds that their research projects have not been hugely hit thanks to the ability to remotely control experiments. “We are all learning to communicate and do research and business in innovative ways that will be a big part of the new normal going forward,” says Weng, adding that he thinks the restrictions due to COVID-19 pandemic will be “fully overcome” over the coming year.
To see the complete list of IOP Reviewer awards, see here.
One of the challenges in storing gas is that, as molecules are so dispersed, it can be difficult to store a useful amount of the stuff. It’s not a problem if you just need a little canister of butane to power a camp stove for a couple of days. But it is a big problem if you’re hoping to drive a methane-powered car without stopping every couple of miles to top-up your fuel canister. Or if you are looking to capture a vast amount of carbon dioxide emitted from a coal plant.
A solution comes from a field known as reticular chemistry, which describes metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). These “crystalline sponges” are characterized by molecular building blocks connected by strong bonds, resulting in relatively large pores that can be filled with gas. Find out more about this class of nanomaterials in this video interview with Felipe Gándara from the Materials Science Institute of Madrid (ICMM).
You can learn more about the materials research taking place at the ICMM in this video profile of the lab. Also take a look at the Physics World Nanotechnology Briefing, published in April 2020. This free-to-read collection celebrates how nanotechnology is playing an increasingly important role in applications as diverse as medicine, fire safety and quantum information.
The way the eye moves in the moments after a head impact serves as a reliable proxy for the acceleration experienced by the brain, reports a research team in the US. The researchers observed the effect in a physical head phantom and a human volunteer, and say that the measurement could one day be made using “smart” contact lenses. Routine eye-motion measurements in athletes could allow sports-related traumatic brain injury (TBI) to be detected at the side-line (Physiol. Meas. 10.1088/1361-6579/ab78bd).
In American football, players selected in large part for their size and speed undergo multiple high-energy collisions per game. Perhaps unsurprisingly, mild TBI (concussion) is common, though players often do not realize it, or may deliberately downplay its symptoms to stay on the pitch. The most reliable ways to diagnose TBI are those that can only be administered away from the playing field: brain-imaging techniques like MRI or X-ray CT, or the detection of certain protein biomarkers in the blood.
Looking for a more accessible method that returns immediate results, some researchers have used helmet-based or head-mounted sensors, but with limited success. The reason that these whole-head measurements make a poor diagnostic tool is that the head kinematics do not necessarily reveal those of the brain inside. TBI occurs when the brain responds heterogeneously to linear and rotational accelerations of the skull. Shear forces that result from this heterogeneity are the cause of the brain trauma, but the details of the process are not fully understood.
“There are many known and unknown factors that may contribute to how the brain responds to a head impact,” says Yuan Meng of Auburn University. “One of the disappointing findings of the large-scale helmet-based telemetry system studies is that the same head kinematics can have a wide range of consequences, from no concussion at all to severe concussion.”
Meng, with colleagues at Auburn University and the University of Alabama at Birmingham, propose that the way the brain moves within the skull in response to a head impact might be reflected in the much more easily measured motion of the eye. The crucial interval is the 20 or so milliseconds immediately after the impact — before the ocular muscles impose stability, and when the eye moves purely according to the laws of dynamics.
The test apparatus with the 3D-printed human head phantom. (Courtesy: Physiol. Meas. 10.1088/1361-6579/ab78bd)
To test their proposal, the researchers constructed a human head phantom and subjected it to accelerations of the sort that could cause mild TBI in a living subject. They 3D-printed a skull in rigid plastic based on the dimensions of a real skull. They made the brain from a gelatin solution, while the eyeballs were composed of polydimethylsiloxane.
Microelectromechanical inertial measurement units (IMUs) placed on a diagonal line through the brain recorded how different regions responded to the acceleration of the phantom. Additional IMUs on the surfaces of the eyeballs (held underneath an artificial lower eyelid) measured the ocular response, while reference IMUs on the outside of the phantom measured the acceleration of the rigid skull.
The researchers placed the head phantom on a rotating vestibulo-ocular response (VOR) chair to induce rotational accelerations, and swung it on an inverted pendulum (like a falling tree) to induce linear accelerations. They found that the onset of acceleration in the brain occurred slightly later than that of the skull, and that the length of the delay depended on where the IMU was located relative to the rotation axis or direction of impact.
The phantom’s eyes experienced accelerations largely comparable to those recorded in the brain. In the rotational acceleration tests, despite the eyes being furthest from the axis of rotation, the delay before acceleration onset was in the middle of the range seen in the brain. Meng and colleagues attribute this to the effect of the small rigid eye socket.
The VOR chair test apparatus. Left: the phantom skull on the chair with all inertial measurement units connected. Right: a volunteer has one inertial measurement unit inserted in the lower eyelid of the left eye and another attached on the forehead. (Courtesy: Physiol. Meas. 10.1088/1361-6579/ab78bd)
To demonstrate the feasibility of the technique in vivo, the researchers subjected a human volunteer to (non-injurious) rotational accelerations in the VOR chair, with an IMU placed beneath their lower eyelid. Although the researchers could not compare the measured eye movement to the motion of the subject’s brain, they confirmed that the eye’s passive response to acceleration was detectable before it was arrested by the ocular muscles.
The next step for the researchers is to miniaturize the IMUs and integrate them into a smart contact lens, which Meng says has already begun.
“We intend to test the prototype on more human volunteers and develop an entire real-time, on-field concussion monitoring system,” says Meng. “A wearable device based on this concept has a promising future in health monitoring in the context of sports, crashes and even on the battlefield.”
Light-emitting nanosensors have been used to measure chemical signals that propagate through living plants in response to damage. Tedrick Lew and Michael Strano at the Massachusetts Institute of Technology and an international team created the new devices to observe variations in waves of hydrogen peroxide as they passed through the tissues of different species of plant. Their approach represents an important step forward for the integration of nanotechnology and plant science and could have numerous benefits for agriculture.
When plants are damaged, recent studies have shown that cells surrounding wound sites release hydrogen peroxide. This triggers the release of calcium ions in neighbouring cells and these ions stimulate the release of more hydrogen peroxide. The result is a chemical wave that propagates throughout the plant. Through this sophisticated communication system, plant cells are instructed to produce the molecules required to repair the damage. Until now, however, there has been no way for biologists to reliably observe how these waves propagate in real time.
Nanobionic plants
Lew, Strano and colleagues observed these intricate waveforms using a technique first developed by Strano. The team integrated plant tissues with specialized carbon nanotubes that emit near-infrared light specifically on contact with hydrogen peroxide, which can then be imaged directly with an inexpensive camera. The result is that these “nanobionic” plants allow the team to map the time-varying concentrations of hydrogen peroxide throughout entire plants.
Importantly, Lew’s team found that their nanosensors could be integrated harmlessly into a diverse range of plants. This allowed them to investigate hydrogen peroxide propagation in six different plant species including lettuce, spinach, and strawberries, without the need for genetic manipulation. They observed the waves travelling at speeds in the range 0.4-3.1 m/s, depending on the species. In addition, information about the different types of stress causing the damage, including infection, heat, and mechanical injury, were encoded into the shapes of the waves.
The technology could have numerous applications in agriculture; potentially enabling farmers to screen their crops for their ability to resist damage from factors like water shortages and extreme heat. It could also lead to studies of how certain crops respond to pathogens which are currently unleashing significant damages, including citrus greening and coffee rust. Lew’s team now has numerous plans for their future research; through further upgrades to their nanosensors, they hope to investigate how signals propagate at cellular levels, and to decipher their complex dynamics in more detail.
Stereotactic radiosurgery provides precision irradiation of intracranial and skull-base tumours. (Courtesy: the Radiosurgery Society)
“The thing that becomes overwhelming is that we are used to considering cancer care as one of the highest priorities of the healthcare system. To be in a position where we have to fight for every single aspect of cancer care is a very new position for all of us. It’s like all of a sudden, the diagnosis and treatment of cancer doesn’t matter anymore.”
Shalom Kalnicki, chairman of the radiation oncology department at Montefiore Medical Center in New York, highlighted the challenges faced by radiation oncologists, physicians and medical physicists looking to treat cancer patients in the time of COVID-19.
Kalnicki was one of a panel of experts speaking in a webinar hosted by the Radiosurgery Society last week. The webinar focused on how radiation treatments have had to adapt to the current pandemic. In particular, the participants considered stereotactic radiosurgery (SRS), which delivers precise radiation doses to brain tumours in up to five fractions, and stereotactic body radiotherapy (SBRT), a similar approach used for extracranial targets.
Hopefully, this will encourage additional studies and investigations into SBRT management of cancer patients
John Kresl, webinar co-chair
One specific concern of any radiation treatment is that it is delivered in a series of fractions over several days, requiring patients to attend repeat hospitals visits and increasing the associated risks. Kalnicki emphasized that each visit must be treated as the first, with patients screened for potential symptoms of COVID-19. He noted that screening patients and staff can add up to 50% extra time to each visit. “Everything takes much longer, nothing is simple,” he said.
So how does this new scenario impact patient scheduling? Helen Shih from Massachusetts General Hospital explained that it’s important to assess which patients really need to be seen in person for evaluation, adding that 90–95% can be assessed via telephone or video consultations. It’s also possible to delay in-person visits for patients with benign diagnoses or non-urgent surveillance cases.
“Cancer is a high priority disease, and anyone who is having active treatment, post-treatment symptoms or concerns for residual disease are still prioritized and seen, remotely or in person as needed,” Shih added.
If patients must attend the hospital, steps are taken for their protection. Kalnicki said that all patients are given surgical masks and gloves when they arrive and are screened for symptoms. At MGH, meanwhile, Shih noted that patients are phoned and asked about any symptoms before each visit.
In some cases, however, rules may be too strict. Michael Schulder, director of the Brain Tumor Center at Northwell Health, NY, described how one fairly disabled patient whom he treated was not allowed to be accompanied by her husband. “I think this is taking restriction to an extreme, common sense has to apply as well,” he said.
It’s equally essential to protect the healthcare workers. Iris Gibbs, at Stanford University Medical Center, CA, explained that the centre has separate entrances for staff and patients, with screening in place at both, as well as a screening app for workers to declare their health each day. But the most important factor in protecting healthcare workers and patients, she emphasized, is testing.
Gibbs explained that Stanford Medicine developed its own test for COVID-19, which was FDA-approved by the end of February. “On March 3, we launched widespread testing within our healthcare community and we’ve been collecting data since,” she said. “When quick 15-minute tests became available, our radiation oncology department had access to that; now all our patients are tested at the time of simulation.”
The situation in New York is less favourable. “As opposed to Stanford, who were fortunate to have their own kits, for now, we are dependent on kits available from industry,” said Schulder. “We only test patients who have a particular set of symptoms, have been exposed to someone known to be infected or are having trans-nasal or sinus surgery. We’re certainly not testing everybody, so that adds a measure of risk.”
Gibbs noted that all treatment planning and dosimetry is performed remotely at Stanford, and for treatment delivery, there’s an emphasis on physical distancing. For SRS, for example, the CyberKnife system has a mode in which the physician can view the treatment console from a different room. For SBRT cases delivered on a linac, however, the physician still needs to be close to the machine.
Site specific changes
The panel also considered how radiation treatments of specific disease sites are being adapted, starting with tumours of the central nervous system (CNS) and brain. Simon Lo from the University of Washington explained that for high-grade gliomas with poor prognosis and elderly patients, they are using more hypofractionation – in which larger dose-per-fraction is delivered over fewer treatments. At the other end of the scale, patients with low-grade gliomas will have their treatment delayed where possible.
“For post-operative SRS of surgical cavities, I usually favour treatments of 3–5 fractions,” said Lo. “But a lot of our patients come from surrounding states, so to minimize visits, I would do a single fraction treatment instead, knowing that the BED [biologically effective dose] may be lower. But we need to strike a balance between virus exposure versus BED delivered to the cavity.”
At Northwell Health, said Schulder, all benign SRS has been deferred. Patients with malignant tumours are still being treated, but with 3-week rather than 6-week fractionated radiation. He also noted that all non-COVID-19 clinical trials have been shut down.
Some centres are now treating lung cancers with a single fraction. (Courtesy: the Radiosurgery Society)
Shankar Siva, who leads the SABR programme at the Peter MacCallum Cancer Centre in Australia, explained that lung cancer treatments have also changed dramatically. The centre has switched all patients with peripheral early-stage primary lung cancer to receive a single fraction of 30 Gy. “There are three randomized phase 2 studies in lung that give us some confidence that moving to single fraction schedules is the way to go,” he said, adding that fractionated SBRT is still used for centrally located lung tumours.
Jun Yang, chief medical physicist at Philadelphia CyberKnife, noted that his practice is hesitant to move to single-fraction treatments. “Even in this critical time, we pretty much still stick to 3–5 fractions,” he said. “For a lung cancer patient, we actually would stick to the SBRT plan as much as we can to speed up the treatment and lower the patient’s chance of getting the virus.”
Patients with prostate cancer face another hurdle. Radiotherapy of such tumours usually requires implantation of fiducial markers to ensure accurate targeting. But in New York, said Kalnicki, state law precludes the placement of fiducial markers as it is considered an elective procedure, which is not allowed.
Najeeb Mohideen from Northwest Community Hospital, IL, is still performing fiducial placements for prostate cancer radiotherapy, but only under local anaesthetic, which does not require anaesthesiology services. Indrin Chetty, a medical physicist at Henry Ford Health System, MI, pointed out that cone-beam CT could provide an alternative, even in the SBRT setting, if slightly larger margins and lower fractionations are used.
What does the future hold?
Finally, the panel considered whether the approaches currently in place will impact radiation treatments in the future. Gibbs suggested that we are “forever changed”, and that risk mitigation and protection measures will continue. Hypofractionated treatment could also remain. Some centres have introduced hypofractionation for the first time, and if similar clinical outcomes are seen, may adopt this in the long term, she explained.
Siva agreed that some habits are likely to stick, such as electronic rather than paper consenting. He thinks that the Peter Maccallum Cancer Centre may continue to use a single radiation fraction for treating primary lung cancer, but that this decision will be centre-dependent.
John Kresl, from the Phoenix CyberKnife and Banner Good Samaritan Medical Center, AZ, pointed out that several surgeons have contacted him to ask about non-surgical options for their patients at this time. “I’ve had referring physicians call that maybe weren’t aware of, or were resistant to, SBRT and were asking questions about having patients treated. Hopefully, this will also encourage additional studies and investigations into SBRT management of cancer patients,” said Kresl, who co-chaired the webinar with Ben Slotman from Amsterdam UMC in the Netherlands.
“Amidst all the destruction the virus will leave in its wake, inevitably there will be opportunities to do things better,” concluded Schulder. “There are things that we’ve already adjusted to and should keep going – related to patient selection, treatment planning and delivery. I’m sure we’ll be able to make some good out of all this.”
I effectively have multiple jobs – including teaching, writing, administrative work and some research – so I tap into a range of skills gained from different parts of my education. Probably the most important skill across the board, though, is being somewhat obsessive about checking and rechecking things. The worst nightmare for a research scientist is to discover that your “big result” is actually just a statistical fluke, so you need as much confirming evidence as possible, in the form of different experiments or calculations that all arrive at the same conclusion.
That general approach turns out to be useful across all sorts of fields. When teaching a class, it’s good to have multiple ways of explaining the same thing, in case some students don’t follow the details of the first one you try. When writing it’s important to make sure that all your stories fit together into a coherent whole, and also that the colourful anecdote that perfectly illustrates some point is actually true. And when doing administrative work to support academic programmes and institutions, it’s important to cross-check all the numbers that justify a particular decision. So, with all my tasks, I spend a lot of time asking, “Is there another path to this same conclusion?” as a way of making sure that it’s the right thing to do.
More specifically speaking, one of the most useful skills I picked up from physics is the idea of doing quick order-of-magnitude estimates, to check whether it’s worth doing a more detailed investigation of any task. If your back-of-the-envelope numbers say you’re a factor of 100 away from having the resources you need, you shouldn’t waste any more time refining that estimate. The other absolutely essential “soft skill” is communication: having the confidence and ability to speak up in both small and large meetings, as well as clearly explaining what you’re doing (or want to do) is invaluable. No matter what line of work you end up in, you’ll always need to be able to convince other people to help you achieve your goals.
What do you like best and least about your job?
The absolute best moments come from teaching and writing – when something I said makes an idea click into place for a student or reader. It’s incredibly gratifying to see students light up when they finally get a tricky problem, or to hear from readers saying things like “I’ve heard about this topic before, but now it finally makes sense to me!” It’s an honour to be in a line of work where I get to do that on a somewhat regular basis.
I also like the variety of the job. Even when I’m teaching the same class for the fourth year in a row, there’s always some new twist, a new explanation that occurs to me on the fly, or a question that no previous student has asked. On the writing and research side, I get to pick whatever topics I want to pursue, so if I stumble across some new quirk of physics, I can go chasing down all the details of that topic in the lab or library, and usually get something out of it that I end up being able to use later.
My absolute least favourite thing in the world is grading. It’s just miserable, and I will invent elaborate ways to procrastinate on doing it, up to and including answering interview questions from physics magazines.
What do you know today that you wish you knew when you were starting out in your career?
My first few years as a faculty member, when people asked how I found the job, I would say “It’s a lot more work than it looked like from out in the classroom.” That’s still true, and something to keep in mind for anyone on the academic track: a one-hour lecture in a class takes the same amount of prep time as a one-hour seminar talk, but you’re expected to do 3–4 new class lectures per week, every week.
The other key advice I would offer is that basically every task you need to do, either for work or in life more generally, can expand to exceed the time available for doing it. It’s critically important to set boundaries, and make sure that you’re blocking out enough time to do each of the many things you’ll need to do, and not letting any one of them encroach on the time for the others. To make this a reality can involve some unexpected changes. I remain somewhat surprised that I’ve turned into a morning person. I do most of my writing between about 7 a.m. and 9 a.m., from when the kids get up to when I need to go to campus, because that’s a block of time when I know I can work uninterrupted.
I work hard to keep that block of time free and not spend it on any of my other tasks. That’s been essential to maintaining writing productivity despite having two school-age kids and expanding responsibilities at work. I don’t necessarily recommend waking before dawn as a path for everyone, but the general idea of finding a block of time for what you really need to do, and protecting that time against everything else, is critical.
Across the world, personal and professional lives have been profoundly affected during the past few months – and scientists are no exception. In this episode of the Physics World Stories podcast, we find out how physics and physicists are adapting to coronavirus-related lockdowns. Among physicists – as with many professions – there is a growing realisation that things are not about to go back to normal anytime soon.
Isolation from colleagues, facilities and important conferences bring obvious disadvantages. But as you will hear in the podcast, some physicists are also finding positive outcomes from the situation. Before the lockdown, did you ever meet a researcher who didn’t complain about being time-poor? Plenty were sick of travelling to international events because they felt they had to show their face. And you name an academic didn’t have a paper they kept meaning to write but never got around to it because of things like endless faculty meetings.
First up in the episode is the theoretical physicist and author Sabine Hossenfelder. Among other things during the pandemic, she has teamed up with climate physicist Tim Palmer to record a coronavirus-inspired reworking of the REM classic hit “It’s the End of the World as We Know It (And I Feel Fine)”. You will also hear from Bonnie Tsim and Rebecca Waters who both attended the recent Women in Graphene Career Development Day – an online event that reimagined various aspects of real-world conferences. Perhaps the success of this virtual event is a sign of what physics conferences will be like for the foreseeable future.
For more personal accounts of the impact of the COVID-19 lockdowns, take a look at the “physics in the pandemic” series on the Physics World blog.
