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Can state-of-the-art linacs boost the uptake of 4D-CBCT in radiotherapy?

**Simulated scans** Reconstructed CBCT images with variable and static angular gaps, using various reconstruction algorithms. (Courtesy: CC BY 4.0/*Med. Phys.*10.1002/mp.16585)

Four-dimensional cone-beam computed tomography (4D-CBCT) is an imaging technique employed during radiotherapy to help ensure the safe and accurate delivery of therapeutic radiation. Using the on-board kilovoltage imager found on most linear accelerator (linac) gantries, 4D-CBCT acquires X-ray projections and then groups the images into respiratory phase bins (usually 10 phases) to create a respiratory-correlated CBCT image. In this way, 4D-CBCT can capture breathing patterns and track the movement of the targeted tumour and nearby organs.

The use of 4D-CBCT for image guidance of lung cancer radiotherapy is increasing. However, clinical adoption is limited by inconsistent image quality, streaking artefacts and high imaging dose, as well as long scan times. The introduction of new linacs that can acquire 4D-CBCT scans in as little as 9.2 s, (compared with around 240 s for earlier models) could help. But it’s important to assess whether such fast gantry rotations can achieve suitable projection spacing across all of the required respiratory phases.

With this aim, researchers at the University of Sydney’s Image X Institute have analysed the impact of gantry velocity and angular separation between X-ray projections on 4D-CBCT image quality. They investigated whether constant and adaptive gantry velocity acquisitions are viable with advanced linacs and state-of-the-art reconstruction methods. Their findings, reported in Medical Physics, provide a guide for the future development of 4D-CBCT acquisition protocols on the newest generation of linacs.

Simulation studies

PhD student Benjamin Lau and his supervisor team used a 4D extended cardiac-torso (XCAT) digital phantom to simulate fast 4D-CBCT acquisitions (200 projections in 60–80 s). The XCAT contains a spherical tumour in the lung and was programmed with a sinusoidal breathing trace. The breathing trace was separated into 10 respiratory phases, and an XCAT volume generated for each.

The researchers first simulated patient scans on a constant velocity gantry system (the current clinical standard) using respiratory phase data from 30 patients participating in the ADAPT (adaptive CT acquisition for personalized thoracic imaging) clinical trial. To study the effects of rotation speed on image quality, they simulated four acquisition times (9.2, 60, 120 and 240 s), encompassing both the fastest possible gantry rotation speed (200° rotation in 9.2 s on the Varian Halcyon) and the current standard (200° in 240 s).

After simulating the projections, the team performed image reconstruction using three algorithms: Feldkamp–Davis-Kress (FDK), McKinnon-Bates (MKB), and motion compensated McKinnon Bates (MCMKB).

Simulations showed that reconstructions from constant gantry velocity acquisitions were streakier and contained more motion blur than ideal reconstructions with even angular spacing. The fastest acquisitions (9.2 and 60 s) with FDK or MKB reconstruction suffered the greatest reduction in image quality. MCMKB reconstruction reduced some streaks, noise and blur. Faster MCMKB acquisitions, however, still exhibited motion blur around the diaphragm and tumour, while slower acquisitions reduced blur but increased streak artefacts.

The researchers attribute these effects to inconsistent sampling of projections across each individual respiratory phase. In general, all constant gantry velocity reconstructions produced lower image quality than seen with ideal angular separation, for all acquisition times and reconstruction techniques.

To understand the impact of angular separation on image quality, the researchers simulated a series of acquisitions using variable angular gaps (in which fast or rapidly changing irregular breathing causes the gantry to miss a respiratory bin) and static angular gaps (where patient breath-holds result in missed data acquisition over a particular angular range) of 20°, 30° and 40°.

Static angular gaps had the greatest impact on image quality, with streak and noise artefacts increasing with greater gap size. Variable angular gaps, on the other hand, resulted in visually similar images to the ideal reconstructions, with MCMKB reconstructions having the least streaks and noise.

Adaptive acquisitions

The team also analysed the impact of adaptive gantry rotations – in which the gantry rotation speed is adapted to the patient’s breathing – using the angular position of X-ray projections from breathing traces of patients in the ADAPT trial. In general, projection angles from ADAPT patients produced image quality metrics similar to those from the ideal angular separation for all reconstructions.

Lau and colleagues note that this study is the first to investigate the impact of angular separation between projections on adaptive acquisition. “It is challenging to predict whether adaptive acquisition in clinical practice can achieve precise gantry control and its impact on image quality,” they write. Their analysis showed that image quality starts to degrade with an angular gap of 20° or larger, but that motion compensated reconstruction using the MCMKB algorithm could compensate for small to moderate deviations in angular separation.

On-board cone-beam CT visualizes delivered dose

Based on their findings, the researchers advise that acquiring fast low-dose 4D-CBCT acquisitions using a constant velocity gantry is not a feasible way to produce reconstructions that are similar to the ideal. However, they point out that precise angular spacing during fast low-dose adaptive 4D-CBCT may not be necessary to achieve high image quality. “The results demonstrate adaptive velocity gantry systems in conjunction with motion-compensated reconstruction may be a good option for fast low-dose 4D-CBCT to make full use of the emerging generation of faster rotating linear accelerators,” they write.

“With the emergence of fast gantry rotation systems, very fast 4D-CBCT scans can be acquired provided that the entire scan range is adaptively sampled, and motion-compensated reconstruction is performed in the context of patient positioning in radiotherapy,” the researchers conclude.

Partially submerged objects experience more drag than expected

Researchers in the US have made several unexpected discoveries when observing how water flows around partially submerged spheres. Their simple experiments could lead to a better understanding of fluid flow in biological systems – and result in better designs of ships and aircraft.

Interactions between flowing fluids and solid objects play crucial roles in a wide range of phenomena from biological functions to the movement of goods and people around the world. These interactions are often studied by placing an object in an air or water flow and measuring the forces on it.

When an object is partially submerged in flowing water, it can experience greater drag than if that object is fully submerged. One reason for this is that at modest flow speeds greater than about 23 cm/s, a semi submerged object will create a wake of waves that dissipate energy – therefore increasing drag. As the flow speed increases, this results in the familiar wakes created by ships, which increase drag.

In a new study of how water flows past spheres, Robert Hunt and Daniel Harris at Brown University in the US and colleagues have discovered that semi-submerged objects are subject to greater drag even at flow speeds of less than 23 cm/s. They also made the counterintuitive discovery that semi-submerged spheres with a water-repellent (hydrophobic) coating experience greater drag than spheres with water-attracting (hydrophilic) coatings. This came as a surprise because hydrophobic coatings are often used to decrease drag.

Deep dipping

The researchers did their experiments by dipping small spheres (8–13 mm diameter) into flowing water and measuring the drag. They found as the spheres dropped deeper into the water, the drag increased and did not start to fall until the spheres were fully submerged.

This increase is not unexpected because as a sphere goes deeper in the water it presents a greater surface area to the flow – and therefore the drag should increase. What surprised the team was that the drag on partially submerged objects was three or four times greater than that measured on a fully submerged object.

“You might naively approximate the drag by saying that if the sphere is almost 100% in the water, the drag is going to be almost the same as if it was fully immersed beneath the surface,” says Hunt. “What we found is the drag can actually can be much larger than that — and not like 50% but more like 300% or 400%.”

Low-speed drag

One surprising observation made by the team is that significant drag enhancement was observed for flow speeds below the minimum capillary-gravity wave speed of 23 cm/s. Below this speed, waves are not expected to be created around the object – and these waves tend to increase the drag. “This points to the possibility of other drag mechanisms that should be considered when operating at a free surface,” Harris told Physics World.

The researchers also looked how coating the surfaces of the spheres affected the drag. To do this they used spheres coated with three different materials. One coating was the superhydrophobic material NeverWet, which is very good at repelling water. Another coating was agar, which is a superhydrophilic material that attracts water. The third sphere was coated in an acrylic that has a middling affinity for water.

The team found that the superhydrophobic sphere experienced about 50% more drag just as the spheres were being fully submerged. Harris explains this using simple physics. “Essentially, the water that piles up in front of the object exerts an extra pressure on the front of the sphere and leads to an extra drag force,” explains Harris. This can be seen in the above figure, in which the flow is going left to right.

He adds, “A hydrophilic sphere more readily allows the water to flow over the top, minimizing the pile-up effect and letting the pressures in the front and the back become more equalized. A hydrophobic spheres more strongly resists the fluid flowing over the top, and thus maintains a larger pile-up for longer.” Harris says that this finding runs counter to some current thought on how to reduce drag. “Superhydrophobic coatings are often developed and proposed for drag reduction, and thus this finding was initially counter-intuitive,” observes Harris. “In trying to decrease the drag, you might actually increase it substantially.”

The researchers says that they are surprised that these simple measurements had not already been done before. Now, they hope to expand their research to study shapes that are relevant to biology and technology.

“Starting from the simplest point, we look at what are the physics here and then as a next step we begin to apply our knowledge to more realistic structures, whether it’s emulating a biological structure or looking at artificial propulsive structures,” Harris says. “We are interested in the design of small-scale autonomous robots that operate and propel at or near an air-water interface. Many of the existing designs of such devices are bioinspired, borrowing geometric or kinematic designs from nature, such as from the water strider or whirligig beetle.”

The research is described Physical Review Fluids.

So you think hard disks are boring? It’s time to get real

In 2022 I stood for election to the magnetism group of the Institute of Physics (IOP) as I always think the best way to be involved in anything is simply to get stuck in. I’d only joined the group the year before after starting to work at a company specializing in permanent magnetic materials. Despite not knowing much about the group, I was – to my surprise – elected.

The magnetism group is one of the IOP’s larger special-interest groups and, among other things, runs a successful and widely attended annual conference, which this year was held in Manchester in April. The meeting covered topics stretching from spintronics, 2D physics, domain walls and dynamics to vortices, skyrmions, topological physics and intelligent computing. It also spanned a wide range of materials from biological and organic systems to spin ices, superconductors and magnetocalorics.

But the IOP’s magnetism group isn’t the only UK outfit involved in this field: there’s also the UK Magnetics Society (UKMagSoc), of which the company I work for is a corporate member. UKMagSoc’s activities overlap to some extent with those of the IOP, but the society is more focused on the application and commercialization of magnets, such as electrical machines, instruments and hard disks. The society does, though, have individual members as well – in fact, in June I attended a joint IOP/MagSoc event at the IOP’s headquarters in London, where speakers spelled out the challenges over the next five or 10 years in a range of magnetic materials and devices.

Future challenges

For a newcomer like myself, the meeting was a really helpful overview of magnetics and I finally found out from a talk by Thorsten Hesjedal from the University of Oxford what a skyrmion really is. It’s a whirling, nanometre-sized topological defect in a thin magnetic film that could provide a new form of computer memory. Skyrmions could, I learned, be particularly useful out in space where conventional computer memory can easily get corrupted by cosmic radiation.

There were also some excellent talks of direct interest to my day job on various aspects of motor design and electrical machines by John Reeve from drive manufacturer FluxSys Ltd, by Dean Evans of motor and generator firm NEMA, and by Juliette Soulard of the Warwick Manufacturing Group. Particularly interesting to me was a lecture by Robert Hicken, a condensed-matter physicist from the University of Exeter. He examined the challenges of keeping up with the ever-increasing demand for data storage, a topic I reported on in December 2020.

Now you’ve certainly heard of Moore’s law, which – as I mentioned in another recent article – states that the number of transistors on a microchip doubles about every two years. It is named in honour of the late Gordon Moore, the co-founder of Intel who came up with the concept in 1965. However, Hicken introduced a related concept that was new to me, which is that the density of data storage on magnetic media (such as hard drives) should increase at a rate of about 40% per year.

The demand for data storage will certainly rise in the long term and companies like Seagate, Toshiba and Western Digital will be doing all they can meet the huge demand

Known as “Kryder’s law”, it’s named after Mark Kryder — a former chief technology officer at disk-drive manufacturer Seagate. Kryder, who has a PhD in physics, first mentioned the notion in an article in Scientific American in 2005. Sadly, Kryder’s law has been rather less successful than Moore’s law because the growth in storage density on magnetic materials rose by only 15% in the five years ending in 2014.

Even if Kryder’s bold predicition didn’t hold true, the demand for data storage will certainly rise in the long term and companies like Seagate, Toshiba and Western Digital – who together make 80% of all hard-disk drives (HDDs) in the world – will be doing all they can meet the huge demand. Worth $35bn per year in 2021 according to a report from Future Market Insights, the data-storage market is set to grow to $80bn by 2029.

A staggering 259 million HDDs were shipped in 2021, with most going into data centres. With 44% of the market, Seagate is currently the largest manufacturer of HDDs — in fact, its technology facility in Northern Ireland alone makes almost 30% of the global supply of read/write heads. The company also seems to be leading the storage-density race thanks to its work on “heat-assisted magnetic recording” (HAMR). It’s allowed Seagate to boost the storage density of its hard disks to more than 2 x 10¹²bytes (2 TB) per square inch, with the bits written via a laser and a plasmonic near-field transducer integrated into the read/write head.

Some 21 years after the idea was conceived, the first HAMR HDDs have recently started shipping with a massive 32 TB capacity (10 platters of 3.2 TB each). Seagate’s road-map suggests we will end up with 120 TB drives with this HAMR technology by the end of the decade, with more than 10 TB data storage per disk. This will mostly be down to major improvements in magnetic media and growth technology plus associated alignment challenges.

Smaller, brighter, better

Today each hard-disk track – of which there are about a million per inch – contains bits of information recorded into tiny areas just 42 nm wide (roughly six to eight magnetic grains) and 10 nm long (barely two to three grains). But if we are to meet growth targets, these tracks will have to get much, much smaller. Single bit-patterned magnetic media will be required to achieve the end goal of more than 8 TB of data per square inch using a technology called heated dot magnetic recording (HDMR).

As Hicken went on to explain, boosting storage densities is not as simple as just making bits as small as possible: you also have to ensure that data can be written on to the drives quickly enough and that they are thermally stable too. Companies, in other words, face a trilemma of conflicting requirements that are a challenge to solve. Thankfully, they have other solutions up their sleeve. Optical techniques in particular could push HDDs beyond current limits with “all-optical” disk writing and switching.

So if you thought hard disks were boring objects locked away in shed-like data centres, it’s time to think again. They hide a huge amount of cutting-edge physics and engineering and we can expect many more exciting developments if we are to reach the 120 TB per HDD and keep up with the demand for “cloud” storage. If there’s another joint IOP/UKMagSoc event next year, I strongly urge you attend.

Recreating the visual effects in Oppenheimer, hearing-impaired music lovers prefer different mixes

If you have seen the film Oppenheimer, you may have enjoyed the visual effects that were used in the film to illustrate esoteric physical concepts – I certainly did. There has been a bit of a debate in the movie press about whether computer-generated imagery (CGI) was used to create the effects. As far as I can tell, visual effects such as the conflagration used to illustrate the detonation of the first atomic bomb where real images of burning fuel. But, digital techniques were used to put the images together to create the final effect. So the images were not computer generated.

Now, the independent filmmaker William H Baker and colleagues have had a go at recreating some of the visual effects in Oppenheimer without using CGI. Check out the above video to see how they have done.

Different perceptions

Musicians and producers put a great deal of effort into getting the right mix of instruments and vocals in their music. But it is often the case that these mixes are fine-tuned for listeners with “normal” hearing. However, many people around the world suffer from hearing loss related to age, disease and exposure to loud noises. This loss can be more pronounced at some frequencies than at others, meaning that different people will perceive the same music in different ways.

Now, Aravindan Benjamin and Kai Siedenburg at Germany’s University of Oldenburg have investigated how the mix of a musical track affects how it is perceived by people with hearing loss. The duo played music with different mixes to subjects with normal and impaired hearing.

They found that hearing-impaired listeners preferred louder vocals, higher frequencies and sparser mixes. “Generally, hard-of-hearing listeners have reduced frequency selectivity and impaired level perception,” explains Benjamin. “They tend to prefer louder levels of lead vocals compared to normal listeners.”

Headphones and equalization

While the listening experience can be enhanced by using hearing aids, the team point out that these devices also have their downside for music listeners. Instead Siedenburg recommends another solution. “Getting good headphones, for example, and then playing around with the equalization might be a better approach than trying to squeeze everything through the hardware of the hearing aid.”

He also has advice for artists, “One approach could be to offer a couple of different mixes, one for the general public and one for people who are moderately hard of hearing,” says Siedenburg. “Certain adjustments to the mix might help to cater to the needs of this group of people in a better way.”

The research is described in an open access paper in The Journal of the Acoustical Society of America.

The physics of hand clapping: here’s how to do it best

It might be hard to imagine yourself in ancient Rome, but if you were dropped into the audience of a play 2000 years ago, you’d probably know what to do when it finished – start clapping. Making a sound by putting your hands together is in fact such a long-standing practice that no-one knows quite when or how it started. Clapping certainly seems to have been well established when the Romans were around.

Some people have even made a profession of it, notably the “claqueurs” of 19th-century France, who received money and free tickets in return for particularly zealous applause. But I wonder if any of these entrepreneurs ever considered the physics of their trade. Imagine if they tested various clapping techniques to find out which would please their client most. After all, there’s more than one way to crash two hands into each other, so which is best?

The loudest configuration is one in which the hands are held at about 45 degrees to one another and the palms partially overlap

It’s a question that inspired Nikolaos Papadakis and Georgios Stavroulakis – two engineers at the Technical University of Crete – to investigate. While teaching acoustics, Papadakis found that his students often wanted to know how they could measure sound without using any expensive equipment. For acoustic measurements like these, you generally need a short but loud sound source – and there’s nothing cheaper than a handclap.

11 photos of hands clapping — **Hear, hear** The loudest claps occur when you hold your hands at about 45 degrees to one another with palms partially overlapped (A2), while the lowest tones occur with the hands at 45 degrees but with palms fully overlapped (A1+). (CC BY Nikolaos Papadakis and Georgios Stavroulakis, from *Acoustics* 2 224)

To see how well handclap measurements fared against those made with expensive acoustic kit, the researchers got a group of 24 students to perform single handclaps at various venues in no fewer than 11 different hand configurations. Each of these was defined by a unique combination of the angle at which the hands are held to one another and how much the fingers of one hand overlap with the fingers or palm of the other.

Although it might be a bit late to help the claqueurs, the results are in (Acoustics 2 224). The loudest configuration, generating an average sound pressure of 85.2 dB, is one in which the hands are held at about 45 degrees to one another and the palms partially overlap (A2 in the figure “Hear, hear”).

But decibels aren’t everything when it comes to sound: the frequency distribution is vital too. So what works best there? Turns out there is one mode of clapping that produces particularly low tones. This involves keeping the hands at 45 degrees, but with the palms fully overlapped and slightly domed to enclose a pocket of air (A1+ in the figure).

While both flat and domed handclaps disturb the air and create pressure waves that our ears detect as sounds, they do so in slightly different ways. As two flat hands collide, the air between them is forced out increasingly quickly, ultimately exceeding the speed of sound. This creates an abrupt pressure change, resulting in shock waves that make up a large part of the noise we hear.

With cupped palms, meanwhile, there is usually a gap left around the thumbs, so not all the air is expelled. This makes for slightly gentler pressure changes that do not create much of a shock wave, but which produce what’s known as a Helmholtz resonance.

“In general, a Helmholtz resonator is a container of gas with an open hole,” says Papadakis. “At the Helmholtz resonance, a volume of air in and near the open hole vibrates because of the ‘springiness’ of the air inside. This vibration creates sound at sufficiently low frequencies that other handclap configurations cannot produce at such volume.”

This is the same phenomenon underlying the hum you get from blowing across a bottletop, as well as that uncanny “sound of the sea” in a seashell. In the latter case, environmental fluctuations in sound pressure enter the shell and get reflected off its hard inner surfaces, with resonant frequencies getting amplified in the enclosed pocket of air and simulating the whooshing of ocean waves.

As poetic as it might be to use a seashell, you can actually recreate this effect using just your hands. If you put your domed palms together, leaving a gap where your thumbs overlap, and hold this gap to your ear, you may well hear that familiar hiss. You can even play around with varying how domed your hands are and hear a noticeable change in frequency as you do. When you clap your hands together into this shape, you generate a brief, loud pulse at these resonances.

So, equipped with these insights, has Papadakis changed the way he claps? “Surprisingly, yes!” he says. “Especially at concerts that I have really enjoyed and when I want to express my enthusiasm and appreciation to the artist, I prefer to do the domed handclap with the Helmholtz resonance. This is probably because I can more easily distinguish the sound of my own handclap among the overall sound of clapping, and because the richer frequency content with more volume in the low-frequency range expresses my enthusiasm better.”

As for me, I have found myself trying out the different handclaps while I’ve been writing this article (apologies to my co-workers) and I’ve already noticed myself applauding more consciously at concerts. Who knows? Maybe if I cultivate a suitably conspicuous clap I’ll convince someone to give me free tickets. Taylor Swift, can you hear me?

Cement-based supercapacitor makes a novel energy storage system

A new cost-effective and efficient supercapacitor made from carbon black and cement could store a day’s worth of energy in the concrete foundation of a building or provide contactless recharging for electric cars as they travel across it. The device could also facilitate the use of renewable energy sources such as solar, wind and tidal power, according to the researchers at the Massachusetts Institute of Technology (MIT) and the Wyss Institute, both in the US, who developed it.

Supercapacitors are technically known as electric double-layer or electrochemical capacitors, and their capabilities fall somewhere between those of batteries and conventional (dielectric) capacitors. Though less good at storing charge than batteries, supercapacitors are better than conventional capacitors in this respect thanks to their porous electrodes, which have surface areas as large as several square kilometres. The double layer that forms at the electrolyte-electrode interface of such devices when a voltage is applied further increases the amount of charge they can store.

Supercapacitors also have some advantages over batteries. Whereas batteries can take hours to charge and discharge, supercapacitors do it in minutes. They also have a much longer lifespan, lasting for millions of cycles rather than thousands. And unlike batteries, which work through chemical reactions, supercapacitors store energy in the form of electrically charged ions that assemble on the surfaces of their electrodes.

Extremely high internal surface area

The new device, developed by a team led by Franz-Josef Ulm, Admir Masic and Yang-Shao Horn, contains a cement-based material that boasts an extremely high internal surface area. The researched achieved this by starting with a dry cement mix containing carbon black, which resembles very fine charcoal. To this mix, they added water and superplasticizers – a standard water-reducing admixture in concrete production. As the water reacts with the cement, it naturally forms a branching network of pores within the structure, and the carbon migrates into these pores to form wiry filaments with a fractal-like structure. It is this dense, interconnected, network structure that provides the material with its extremely large surface area.

“We fill the fresh material into plastic tubes and let them harden for at least 28 days,” explains Ulm. “We then cut the samples into electrode-sized chunks, soak these electrodes in a standard electrolyte solution (potassium chloride) and build a supercapacitor out of two electrodes separated by an insulating membrane.”

The researchers then polarize the electrodes by connecting one electrode to a positive charge and the other to a negative charge. During charging, positively charged ions from the electrolyte accumulate on the negatively-charged volumetric carbon wire, while negatively-charged ions accumulate on the positively-charged carbon wire.

A day’s worth of energy

With the membrane in the way, the charged ions cannot move between electrodes. This imbalance produces the electric field that charges the superconductor. “The fact that the volumetric wire fills the space available to it – something we confirmed with EDS-Raman spectroscopy – allows us to store a lot of energy on the extremely large surface of the carbon black,” says Ulm. “When we then disconnect the energy source from the supercapacitor, the stored energy is released, and can thus provide the power for a variety of applications.”

According to their calculations, which they detail in PNAS, a block of the material measuring 45 m³ (equivalent to a 3.55 m cube), would be able to store about 10 kWh of energy. This is about the same as the average daily electricity consumption of a typical household. A house built with foundations that contain this carbon-concrete composite could therefore store a day’s worth of energy – produced by solar panels, for example – and release it when needed. The material might also be incorporated into intermittent electricity generators such as wind turbines, which could then store energy in their bases and release it during down periods.

Another potential application for the supercapacitor – albeit a high-end one – would be to add it to concrete roadways. These super-roads could then store energy (perhaps produced by solar panels located alongside them) and deliver it to passing electric vehicles via electromagnetic induction. This technology is fundamentally the same as that used to wirelessly recharge mobile phones, and the researchers say it could also be used to recharge electric vehicles when they are not moving — in a car park, for instance.

More near-term uses, they add, might be in buildings far from the electricity grid, which could be powered using solar panels attached to the supercapacitors.

Very scalable system

The system is very scalable, says Ulm, since the energy-storage capacity increases in proportion with the volume of the electrodes. “You can go from 1-millimetre-thick electrodes to 1-metre-thick electrodes, and by doing so basically you can scale the energy storage capacity from lighting an LED for a few seconds, to powering a whole house,” he explains. Depending on the properties required for a given application, the system could be tuned by adjusting the mixture, he adds. For a vehicle-charging road, very fast charging and discharging rates would be needed, while for powering a home “you have the whole day to charge it up,” so slower-charging material could be used.

“The fact that the constituent materials are so readily available opens up a new way to rethink energy storage solutions,” Ulm tells Physics World. “Concrete is, after water, the most consumed material on Earth, but it comes at a non-negligible environmental cost, since roughly 8% of worldwide CO₂ emissions result from the 4 gigatons of the annual global worldwide production. Our overall focus was therefore to make concrete a multifunctional material that could provide an additional useful societal function.”

Energy storage is of critical importance today if we are to curb the impact of climate change, he notes, and previous studies have shown that a cement-carbon mixture can be used to make an electron-conducting cement. Electrical conductivity is not enough, however, to store energy. “We hypothesized that hydrating the hydrophilic cement in the presence of the hydrophobic carbon black should naturally provide the two other criteria that are needed: storage- and transport-porosity,” Ulm says.

The researchers’ immediate focus is to make a supercapacitor that can store the same amount of charge as a 12V battery. “We consider this device as the elementary brick towards more advanced devices,” Ulm says.

Miniaturized ultrasound scanner could help detect breast cancer earlier

A wearable ultrasound scanner that could help detect breast cancer earlier – and thus improve survival rates – has been developed by a research team headed up at Massachusetts Institute of Technology (MIT).

Breast cancer is the most common cancer to affect women, with around 2.3 million new cases diagnosed worldwide in 2020. When caught in its early stages, the survival rate is nearly 100%, but this drops to 25% among patients whose tumours are detected at a late stage. Regular self-checks and mammograms can help secure positive outcomes. However, in the UK, an estimated 39% of women do not regularly check their breasts for the signs of cancer – which can include shape changes, lumps or swelling.

On the screening front, notes surgical oncologist Tolga Ozmen of the Massachusetts General Hospital, “One of the main obstacles in imaging and early detection is the commute that people have to make to an imaging centre.” Yet even when people do undergo regular mammograms, breast tumours can still sneak up between screenings. These “interval cancers” make up 20–30% of all breast cancer cases, and their tumours tend to be more aggressive than those found during routine scans.

Galvanized by the death of her aunt – who was diagnosed with late-stage breast cancer at 49, despite regular check-ups – MIT engineer Canan Dagdeviren set about designing an easy-to-use device that could enable more frequent and even at-home screening.

“My goal is to target the people who are most likely to develop interval cancer,” explains Dagdeviren. “With more frequent screening, our goal is to increase the survival rate to up to 98%.”

To realize this vision, Dagdeviren, Ozmen and colleagues developed a miniaturized ultrasound scanner using a novel piezoelectric material fashioned into a phased array. The scanner fits inside a small tracker that’s inserted into a flexible, 3D-printed patch with a honeycomb-like pattern. The tracker can be moved along a path in the patch to provide imaging from six different positions, together affording coverage of the entire breast.

The whole set-up is worn by attaching the patch, via magnets, to a special bra containing openings where the scanner can contact the skin. The researchers note that the device, which they describe in Science Advances, does not require any special expertise to operate.

Conformable ultrasound breast patch — **Travelling tracker** A small ultrasound tracker, attached via cable to a circuit board, can be inserted into the honeycomb patch and moved around a hexagonal grid of holes. (Courtesy: Canan Dagdeviren)

“We changed the form factor of the ultrasound technology so that it can be used in your home. It’s portable and easy to use, and provides real-time, user-friendly monitoring of breast tissue,” Dagdeviren explains.

To demonstrate the scanner’s potential, the team tested it on a 71-year-old patient with a known history of breast cysts. The wearable enabled the researchers to detect the cysts, which were as small as 3 mm in diameter, the same scale as early-stage tumours. Furthermore, the researchers report, the device achieved a resolution comparable to that of traditional ultrasound systems used in medical imaging centres and could image tissue to a depth of 8 cm.

At present, the scanner has to be connected to a traditional imaging system for its data to be visualized. However, the researchers are developing a miniaturized version – about the size of a smartphone – to add to their scanner. Other developments being planned include the integration of artificial intelligence-powered diagnostics and adaptation of the ultrasound technology for use on other parts of the body.

Sheng Xu – a materials scientist from the University of California San Diego who was not involved in the present study, but is also working on conformable ultrasound patch designs – called the device “impressive”. He adds: “The tracker in the bra could help standardize the ultrasound imaging procedure and minimize operator dependency, which plagues conventional ultrasound technology.”

“This would be of great benefit to individuals and health services alike,” agrees biophysicist Jeff Bamber of the Institute of Cancer Research, who says that in the UK, pressure on imaging services in hospitals has helped generate waiting lists that are millions of patients long. Bamber notes that the low profile needed for wearables makes it difficult to build ultrasound transducer arrays without compromising on image quality.

He adds: “This group has incorporated single crystal piezoelectric array technology with advanced doping of the crystal, as used in modern conventional ultrasound probes. The result is good electromechanical coupling, good sensitivity and wide bandwidth, enabling them to achieve relatively good image quality to practically useful imaging depths.”

“With further development to produce images of diagnostic quality, the potential for use in home monitoring would be considerable,” Bamber tells Physics World.

Metasurfaces simplify optical sensing systems

Conventional optical systems such as those found in cameras and microscopes use curved lenses to bend and focus light. As a result, these systems tend to be bulky and difficult to miniaturize for use in systems where space is at a premium – such as smartphones.

Flat, thin optical components based on metasurfaces offer a solution to this miniaturization problem by replacing multiple conventional lenses with a single metalens. In this episode of the Physics World Weekly podcast our guest is the co-founder and CEO of Metalenz, a US-based company that has commercialized optical metasurface technology.

Rob Devlin explains how the company’s optical components bend light; how they are made using standard semiconductor processing techniques; and how they are being used in a range of sensing applications.

Supersonic cracks break classical speed limit

Tensile cracks in brittle elastic materials can spread faster than the speed of sound – and faster than the laws of classical fracture mechanics say is possible. The new fracture mode was discovered by a team at the Racah Institute of Physics at the Hebrew University of Jerusalem, Israel, and could overturn traditional pictures of what happens when things break.

Materials fail when cracks form and propagate within them. Classical fracture mechanics says that these tensile cracks should move in a way that dissipates the elastic energy that builds up within a point-like zone at their tips. One consequence of this is that a classical tensile crack cannot travel faster than the Rayleigh wave speed, c_R, which is related to the material’s shear-wave velocity and how much it deforms under a load (its Poisson ratio). However, Meng Wang, Songlin Shi and Jay Fineberg have now found that some cracks do not obey this rule. Instead, they smoothly accelerate to near-supersonic speeds.

“We are rather excited by this discovery,” Fineberg tells Physics World. “The very existence of these ‘supersonic failure modes’ calls into question the fundamental physical assumptions that underpin our current understanding of the fracture process. It is not that the established framework is wrong, but it implies that there is not a unique ‘set of rules’ that guide fracture.”

Cracks move unexpectedly quickly

In the new work, which is detailed in Science, Fineberg and his colleagues studied brittle gels that are “neo-Hookean”, meaning that they have a nonlinear relationship between applied stress and strain. Using soft materials like these slows the speed of crack propagation by about three orders of magnitude, allowing the team to observe crack dynamics with fast, high-resolution cameras while making precise, real-time measurements on the strain fields surrounding the crack tips. Such measurements would have been impossible in a material such as glass, Fineberg stresses.

The team’s previous work had indicated that cracks in these brittle gels do not behave any differently from cracks in standard brittle materials. This time, however, when they uniformly stretched sheets of the material and introduced a small cut to create an initial crack, the velocities of the cracks hit speeds never previously documented, with the fastest moving about 30% faster than the speed of sound.

These observations contradict earlier studies, both theoretical and experimental, showing that cracks cannot propagate faster than sound. This is because sound speed reflects how quickly mechanical energy can move from one part of a material to another – something that must occur for it to crack.

The researchers say their observations must therefore indicate the presence of “supershear” dynamics that are governed by different principles than those guiding classical cracks. Notably, the new mode of tensile fracture does not occur randomly. Instead, it is triggered at critical strain levels that depend on the material. Such effects had been suggested theoretically nearly two decades ago by Michael Marder at the University of Texas at Austin in the US, but “since they were so different to the accepted description of fracture, they were not taken too seriously by many in the field,” Fineberg explains. “The new experiments irrefutably demonstrate that such modes of fracture both can and do exist – and under what conditions.”

A new mode of fracture

Fineberg adds that he and his colleagues came upon their results by accident while trying to study an entirely different phenomenon. “The challenge was, after we had convinced ourselves that these initially unexpected effects were real, to try and obtain a physical picture of what determines the transition to this new type of fracture,” he says. “This involves relating what happens in the singular region near the crack tip to its macroscopic (large-scale) behaviour.”

The team is now working to characterize the new mode of facture it unearthed. “In parallel we will compare our observations to both Michael Marder’s original theoretical description and to new theory and calculations,” Fineberg says. “We are only just at the beginning of understanding the effects we have observed.”

How Space Pride is campaigning for change in the space sector

Milan is a city synonymous with fashion, where designers and haute couture descend every September for its annual fashion week. Shortly after next year’s event, however, Milan will host another cultural milestone – the first Space Pride Fashion Gala. Running for one day between 14 and 18 October 2024, the event promises to be “an out-of-this-world Pride parade” that builds on the latest advances in “technofabrics” – material that incorporate technology or functionality into a traditional textile.

“It’s a fashion show with the theme of Pride parade meets space,” explains Protea Vale, who founded Space Pride – a non-profit dedicated to bringing together the LGBTQ+ community within the global space sector – in October 2022 (see box below).

By staging the Space Pride Fashion Gala during the 75th annual congress of the International Astronautical Federation (IAF), which is also being held next October in Milan, the charity aims to pose “a joyful protest” on behalf of the LGBTQIA+ community working in the space industry, whose needs – or even existence – are not always well recognized. Partners for the occasion include Pride in STEM, KOSMICA, InnovaSpace, International Space University, Cumulus Association and the European Space Foundation, along with an official endorsement from the IAF Committee for the Cultural Utilisation of Space (ITACCUS).

Space Pride and the gala are motivated by a desire to shift attitudes in the space industry. Despite deploying the latest, cutting-edge technology, Vale says the space sector is “a bit of a dinosaur” that is decades behind the rest of the world on diversity issues, and in particular the rights of the LGBTQIA+ community. Tackling the space industry’s backwardness is not, however, easy given that the sector is a worldwide endeavour and operates in many countries, including those where homophobic attitudes are still enshrined in law.

Individuals from the LGBTQIA+ community who work for space companies in those nations can therefore end up suppressing a part of their identity to avoid discrimination. However, Vale believes the problems cannot simply be attributed to disparities in the accepted norms of nations on LGBTQIA+ issues. “Compared to other sectors, the space sector has only just started talking about women,” says Vale. “They’ve barely spoken about racism. And they haven’t even begun to talk about disability and neurodiversity in terms of the widespread space community.”

Vale, who personally identifies with the LGBTQIA+ community, says the archaic attitudes are not helped by the IAF conference being routinely held in countries with homophobic laws, including those where gay marriage is not permitted. “Every time such an influential conference, which has 5000+ attendees, is held in one of those countries, the queer community lags behind,” says Vale. “That’s one problem – it’s not one with an easy solution.”

Pockets of the space sector are beginning to catch on, and for good reason since employers have a vested interest in ensuring their employees feel comfortable in the workplace however they identify. “People don’t realize how our personal lives and professional lives interact with each other,” says Dhanisha Sateesh, an aerospace engineer who works an Indian space-data company. “I believe if you are in a closeted space and are not able to express yourself freely, you will not be able to give yourself 100% all of the time.”

Ramping up efforts

This kind of “psychological safety” in the workplace is not just an abstract notion: it helps the space sector to produce more effective results. That’s one reason why Sateesh champions these issues through her work on the diversity and gender equality group at the Space Generation Advisory Council (SGAC) – a non-profit body that represents students and young professionals who are interested in working in the space industry. “It’s such a great platform because it offers access to voices across the world,” says Sateesh. “There’s no bias and you get to know about what each person is doing and contributing to the space sector.”

As a woman, Sateesh, who designs satellites, found herself something of a rarity during her studies at college in India, and welcomed the chance to share experiences with like-minded people. Yet she does not consider the lack of bias characteristic of the space industry, highlighting the “selection bias” in choosing astronauts. “There’s different levels of discrimination we all face together,” she says.

Sateesh identifies as female with an undefined sexuality, and it was her work to combat gender inequalities at SGAC that brought her into touch with Space Pride, where she is now a member. Indeed, Space Pride now has several members championing LGBTQIA+ rights within other organizations. Rynee Fandora, for example, is co-lead of the IAF’s LGBTQ+ working group. Having faced her own challenges regarding the freedom to express herself as a trans woman, she feels it is not just the LGBTQIA+ community who lose out when they are not embraced and encouraged in the space sector. “We are in a professional arena when we talk about space,” she says. “We need everyone in because their voice is important, and their ideas are important.”

Some space organizations are now trying to address the shortfall in provisions for the LGBTQIA+ community. Indeed, Neela Rajendra, who is now chief inclusion officer at NASA’s Jet Propulsion Laboratory, believes a lot of organizations have ramped up their efforts on diversity, equity and inclusivity issues in recent years. “I don’t think I am exaggerating when I say there was a world reckoning following the murder of George Floyd [in 2020],” she says. “[It led to greater] awareness and willingness and commitment to change from the perspective of diversity, equity and inclusion.”

One of the first things she did in her role was to ensure that the leaders of various employee resource groups, which represent marginalized populations in the organization, are paid for their time. As she sees it, people should not be expected to do the work for free, as is so often the case. JPL also pays for group leaders to attend diversity-related professional development conferences. She says that while the atmosphere at the organization is generally embracing of freedom of expression, many very practical, pressing and frustratingly familiar challenges remain, ranging from installing gender-neutral toilets to automating how pronouns of choice are included in JPL e-mails.

In April 2022 NASA released its diversity, equity and accessibility (DEIA) strategic plan for its workforce. Elaine Ho, the space agency’s chief diversity officer, told Physics World that this includes goals that drive equitable access to professional opportunities and support services, aiming to prevent unconscious bias, while enhancing workforce diversity, and ensuring DEIA integration into all NASA missions. Earlier this year NASA also surveyed its employees to find out what issues exist and how best to address them.

A quest for equity: Protea Vale

Protea Vale (she/they) was born in South Africa. Having led many campaigns to champion diversity and inclusion, Vale’s first forays into activism were prompted by their experiences at a summer school organized by the International Space University (ISU) in 2022. Despite many world-leading astronauts teaching at the ISU, Vale was disappointed to see a lack of African involvement. “Africa didn’t have a seat at that table,” says Vale.

In response, later that year Vale set up and ran a community-based campaign to elect the first African board of trustees representative. The campaign was met “with an outpouring of support” and an African representative was successfully elected.

Vale, who also has ADHD and a specific learning difficulty, feels that neurodiversity is disproportionately prevalent among both the Pride and space communities, but is rarely discussed – and that talking about it more could make a big difference for many people.

Vale now merges technology and creativity to tell powerful stories, describing themself as “a futurist, advocate and creative force committed to reshaping a future of hope and plurality”.

To the stars

Vale laments the lack of clear documentation of discrimination among the LGBTQIA+ community in the space sector despite the anecdotal evidence. While a key aim of Space Pride is simply getting the conversation started on LGBTQIA+ issues in the space sector through education and outreach, another is to conduct research that fills this void, providing, as they put it, “actionable data that can be used to reduce the discrimination that the queer community face”. The third is to provide a platform for the community, which is where the fashion gala comes in.

As another step towards wider acknowledgement of the LGBTQIA+ community in space, Space X’s lunar lander, which is set for launch in early 2024, will – thanks to the Interstellar Foundation – carry a digital version of Space Pride inspired artwork created by Rania Djojosugito and Khushi Shah with help from the Space Pride team (see main image). The artwork is part of a project by the foundation to create a message in a bottle as part of the cargo of an interstellar probe that presents a snapshot of Earth to any possible extraterrestrials who might find it – an upgraded version of the Golden Records borne by Voyager 1 and 2.

“’We are all made of stardust’,” says Vale, quoting the Space Pride slogan. “We are all human and we should welcome and accept everyone.”

For more information on the Space Pride Fashion Gala, e-mail spaceprideorg@gmail.com or follow Space Pride on social media

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