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Passive optical concentrator could boost solar-cell efficiency

A new optical lens harvests and concentrates scattered light from multiple directions without any moving components, raising hopes that it could help make future solar cells more efficient. Designed by Nina Vaidya and Olav Solgaard at Stanford University, US, the lens relies solely on the increasing refractive indices of successive glass layers to redirect light. The success of a prototype suggests that it could be used as a tile-able surface on solar panels.

To improve the efficiency of solar cells, many researchers are working on techniques to concentrate incoming sunlight onto smaller areas. This can be done using a wide range of advanced optical setups – but for optimum efficiency, these devices must move to face the Sun at all times, which requires costly and complex tracking systems.

As an alternative, Vaidya and Solgaard designed a lens that collects scattered sunlight passively, over a broad range of incident angles, and concentrates it onto a single spot. Dubbed the Axially Graded Index Lens (AGILE) by its designers, the device is shaped like an inverted square pyramid with the apex cut off. It is composed of eight glass layers, with refractive indices that increase progressively towards the bottom.

Thanks to this arrangement, when a beam of light enters the larger square at the top of AGILE, its path curves downwards as it progresses through the pyramid. Regardless of the beam’s angle of incidence at the top, it will thus be almost vertical once it reaches the smaller square at the bottom. Vaidya and Solgaard also coated the sloping sides of their pyramid with a mirror, so light that might otherwise escape from the lens gets reflected inside.

Search for the right materials

To build a prototype version of AGILE, Vaidya and Solgaard carried out an extensive search of possible glass materials. These glasses would need to satisfy a stringent set of requirements, including the ability to transmit a broad range of wavelengths from the solar spectrum, which spans roughly 300 to 1200 nm. The materials would also need to display similar rates of thermal expansion, while still encompassing a broad range of refractive indices.

Once the duo identified a set of optical glasses that fulfilled these conditions, they fabricated a prototype by bonding the layers together into a vertical stack, before carving out the lens’ pyramid shape and coating it with reflective aluminium.

Kirigami solar cells follow the Sun

In their initial experiments, which they describe in Microsystems and Nanoengineering, the researchers showed that AGILE transmitted over 90% of incoming scattered light, concentrated on a spot a third of the size of the upper square surface. Based on this result, they suggest that solar panels could be coated with arrays of AGILE tiles, which would not only allow the panels to capture the Sun’s light passively throughout the day, but also permit them to harvest the diffuse light scattered by Earth’s atmosphere.

The duo report that the next step will be to show how AGILE could be manufactured on large scales, through techniques including spray coating, moulding and 3D printing.

Celebrating the life of the pioneering nuclear physicist Gertrude Goldhaber

In this episode of the Physics World Weekly podcast, we explore the life and scientific legacy of Gertrude Goldhaber, who overcame great adversity to become a pioneering nuclear physicist and advocate for women in science.

Born in 1911 into a Jewish family, Goldhaber fled Nazi Germany in 1935, eventually settling in the US where she became the first female physicist at Brookhaven National Laboratory. Goldhaber died in 1998 and now archivists at the Center for Jewish History in New York City have finished processing her papers – with the aim of making them available online.

In this podcast you will hear the recollections of Goldhaber’s sons Michael and Fred along with Fred’s wife Suzan Goldhaber. Renate Evers, The Bruno and Suzanne Scheidt Director of Collections at the Leo Baeck Institute New York, also joins the conversation.

A description of the Gertrude Goldhaber archive is available online. The first part of the collection has already been digitized and is accessible online. Additional articles, photos, and sources related to Gertrude Goldhaber can be found here. You can also find out more about her work in this Physics World feature by Sidney Perkowitz.

FLASH proton therapy: uncovering the optimal delivery technique

Proton FLASH delivery modes — Distinct dose-rate structures: The researchers simulated four different modes of proton FLASH delivery. (Courtesy: Eric Diffenderfer)

FLASH radiotherapy – the delivery of therapeutic radiation at ultrahigh dose rates – offers the potential to vastly reduce normal tissue toxicity while maintaining anti-tumour activity. While almost all studies to date have been pre-clinical, the first patient treatment with FLASH was performed at Lausanne University Hospital in 2019, and the first clinical trial in humans completed accrual last year.

Most pre-clinical FLASH studies, as well as the patient treatment, used electrons. But proton therapy systems can also deliver FLASH dose rates, and could prove particularly promising for clinical use, offering more conformal dose distribution than electrons and the ability to treat deeper tumours. Proton beams can be delivered using various techniques that create distinct spatial-temporal dose-rate structures. So which is the most optimal modality for delivering FLASH proton beams?

Project leader: Eric Diffenderfer from the University of Pennsylvania. (Courtesy: Eric Diffenderfer)

A team led by Eric Diffenderfer from the University of Pennsylvania is using computational modelling to find out. Diffenderfer (presenting on behalf of first author Ray Yang from BC Cancer) described the group’s work to quantitatively determine which aspects of the proton dose-rate structure maximize the FLASH effect.

The researchers simulated four modes of proton FLASH delivery: pencil-beam scanning (PBS), which provides the highest instantaneous focal dose rate; double-scattering using a ridge filter; range-modulated double-scattering using a rotating modulator wheel; and a hybrid PBS-RF approach in which the pencil beam is delivered through a ridge filter to irradiate all depths simultaneously.

They then compared the impact of these different FLASH delivery modes on normal tissue sparing. In particular, they examined three surrogate metrics of tissue sparing: the oxygen depletion effect; kinetics of organic radical species formation; and survival of circulating immune cells.

To model these metrics, each technique was used to deliver a spatially equivalent spread-out Bragg peak plan with 11 energy layers to a 5x5x5 cm target. The cyclotron output for FLASH was defined as a beam current of 500 nA, which gives a dose rate of approximately 2 Gy/ms at the Bragg peak.

The model calculates spatial dose distributions using machine data from the IBA proton therapy system at Penn. The team then used the model outputs to quantify the abovementioned radiophysical, radiochemical and radiobiological parameters, on a voxel-by-voxel basis. Diffenderfer noted that the model’s flexibility enables parameters to be refined for comparison with new experimental evidence.

The researchers first examined radiosensitivity modulation via the oxygen effect: the hypothesis that oxygen depletion at ultrahigh dose rates mimics hypoxia in normal tissues, making them more radioresistant. Diffenderfer showed how at ultrahigh dose rates, transient oxygen depletion occurs differentially over space and time and reduces the effective dose deposition.

The team calculated the dose rate-dependent oxygen depletion and recovery, and determined energy deposition versus oxygen concentration for all four delivery modes. The hybrid PBS-RF technique exhibited the most significant downward shift in oxygen concentration.

Oxygen is just one of several dose rate-dependent species that facilitate the formation of organic radicals, a known precursor to DNA damage. So next, the researchers used radiochemical rate equations to determine the concentration of organic radicals over time, with the cumulative area under the curve a surrogate metric for DNA damage. For all four delivery methods, FLASH reduced the level of damage compared with the corresponding conventional irradiation.

Another potential mechanism proposed to explain FLASH’s tissue-sparing effect is the reduction in radiation-induced death of circulating immune cells at ultrahigh dose rates. To investigate this, the team implemented a radiobiological model that considers how radiation intersects with the circulating blood pool to quantify the survival of immune cells.

Plotting the proportion of immune cells killed as function of dose rate for the four techniques revealed that PBS causes the greatest cell death, likely because it allows the most time for different parts of the blood pool to be exposed to radiation.

Overall, all three mechanistic models agreed on their rankings, with the most tissue sparing seen for the PBS-RF model. The least effective delivery techniques was PBS, likely due to its inherent long slew times (particularly for energy-layer switching) allowing significant oxygen replenishment, increased retention of radicals and reduced immune cell survival.

“We identified differences in spatial-temporal dose-rate structure for different delivery techniques and how that influences tissue sparing at ultrahigh dose rates, in a more subtle way than just looking at the field-averaged dose rate,” Diffenderfer concluded. The team’s findings could pave the way to better understanding and adapting the spatial-temporal structure of proton treatment plans to maximize the FLASH effect.

How to become a better PhD supervisor

As an early career nanoscientist, I have so far worked in research groups in four different countries. The culture and supervision style in each group were never quite alike, but what they had in common was that PhD students often faced the same mental and emotional challenges. Sadly, research is rarely the hardest part about doing a PhD – rather, it is managing the dynamics between a supervisor and their students and wider research group. Difficult situations often – but not always – arise due to dysfunctional leadership styles and toxic work environments.

There is a scary and common misconception in physics that students work better if their supervisors and other senior group members pressure them with negative, condescending language. The students might be undermined and made to feel inferior to their peers, but – so the thinking goes – it’ll spur them on. However, this leadership style drains students and makes them less engaged, worsening their performance. In contrast, PhD students who feel respected and encouraged have a higher work output, share their work more openly and progress faster.

Supervisors must take responsibility for their role as leaders and assess whether they are helping or hindering their students. Improving academia’s work culture is crucial for the future of physics – and supervisors play a vital role in fostering a welcoming environment for all. Any supervisor who thinks they don’t influence their research group’s culture is underestimating their role as the captain of the ship.

To keep PhD students motivated and hardworking, supervisors and other group members need to encourage, guide and support one another. So, what can supervisors do – and more importantly not do – to improve the wellbeing of their students so that they can raise their own work standards and that of the research group?

Setting the standard

PhD students will be happy and productive if they are made to feel understood and respected. Supervisors need to remember that students are highly motivated researchers and – driven by that motivation – they are working as hard as they can. Any comments that suggest otherwise can be manipulative, signal a lack of trust, and can cause the student to feel undervalued, misunderstood and demotivated.

If there is a concern that a student is not working hard enough, their supervisor should schedule a meeting to discuss the situation in an honest and respectful way. Negative comments and jokes about a particular student not working hard enough usually do more harm than good. Positively influencing their motivation, for example by encouraging students to overcome their research challenges, is the best way forward. After all, a student’s output is heavily guided by their supervisor.

In my experience, the best research groups are those where people help each other to progress, stand up for each other and have a healthy dynamic. Supervisors can create this kind of environment by leading by example: being supportive and encouraging in meetings, and suggesting open collaboration. This kind of culture will not be fostered by comparing students’ progress against each another, or putting down group members to others behind their backs, or rewarding those who succeeded but left a mess behind for others to clean up. The language a supervisor uses when talking to students will rub off on them so that when they talk to each other it will be done in an equally respectful (or disrespectful) way.

I also encourage supervisors to find out what their PhD students need, rather than what they think they need. The quickest way to optimize supervision style is to ask for feedback – and to listen to it. Sure, there are sometimes good arguments for running things the way they always have been done, but it might create a huge positive change if supervision style is adapted even slightly to such feedback.

People are often more motivated by clear, well-defined tasks rather than a lack of direction. Although research is by its very nature undefined, which makes it easy to get bogged down in a particular problem, students still need input from their supervisors on the “bigger picture” progress of their project as well as reassurance about the goal that everyone is working towards.

For those who could have a future in academia, give them a fair and realistic picture of what it is like to work there. For those who hope to forge a career path in industry, teaching, finance, engineering, IT or the many other areas that physicists go into, students still need to be respected and encouraged so that they part ways on good terms. How previous group members have been treated will heavily influence the dynamics of the group.

Many physicists go into the subject because they love science and not necessarily because they wanted to be leaders of people. But the fact remains that supervisors are crucial to nurturing the next generation of scientists – and that power and responsibility must be used for good.

Galactic remnant of the universe’s ‘dark ages’ is rotating, say astronomers

One of the most distant galaxies ever observed is very likely to be rotating, say astronomers. An international team led by Tsuyoshi Tokuoka of Waseda University, Japan, discovered the motion using observations from the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile. The result offers crucial new insights into the evolution of newly formed galaxies and could provide useful guidance for upcoming observations with the James Webb Space Telescope (JWST).

When galaxies first began to form, the universe was in its “dark ages” – a period when virtually all matter was cool and transparent. As matter collapsed under gravity, galaxies formed, kicking off star formation in nascent galactic centres and triggering the so-called “epoch of reionization” that ended the dark ages. From there, star formation spread out into rotating galactic discs, where newer stars now reside.

Astronomers still have much to learn about the physics that governed these ancient galaxies. To shed new light on these questions, including the origins of the galactic rotation, Tokuoka and colleagues turned to observations from ALMA. This instrument has revolutionized the observation of distant, highly redshifted galaxies, owing to its impressive spatial and frequency resolutions.

In the latest study, the researchers used ALMA to study MACS1149-JD1: a gravitationally lensed galaxy that lies over 10 billion light-years away, making it one of the most distant objects ever confirmed. Through spectroscopy, astronomers have discovered that JD1 contains a population of stars roughly 300 million years old, placing its origins well inside the universe’s dark ages – just 270 million years after the Big Bang.

Different redshifts

The team examined the characteristic wavelengths emitted by doubly ionized oxygen (O III) in JD1. This gas is widely found in supernova remnants, making it a key component of material in the interstellar medium. Thanks to ALMA’s resolution, the team was able to identify variations in the redshift of O III emissions in different parts of the galaxy. This revealed a gradient in the velocity of material in JD1’s interstellar medium – with one side of the galaxy displaying a distinctly different redshift.

This observation satisfied nearly all criteria that must be met to confirm that a galaxy is rotating, making it the earliest example of a rotating disc ever discovered. Its rotational speed was also far slower than is found in other galaxies, including our own – suggesting that JD1’s rotational motion is still in its early stages.

The result, which is described in The Astrophysical Journal Letters, means that astronomers have a record of galactic rotation speeds spanning over 95% of the universe’s total history, which members of the team say is an important step in understanding of how the physical characteristics of galaxies evolve. Tokuoka and colleagues now hope that many remaining questions will soon be answered with the help of the JWST, which should enable them to identify the ages of specific stellar populations inside the galaxy.

Could this revolutionary plane turn air travel green?

I recently booked my first flight since the COVID-19 pandemic devastated the aviation industry. Who can forget the airports full of grounded planes, with staff and pilots laid off? According to the International Civil Aviation Organization (ICAO), the number of air passengers worldwide fell by 60% between 2019 and 2020. And although numbers climbed back to 2.3bn in 2021, they were still 49% below pre-pandemic levels.

But despite the problems, sustainable air travel has made big progress over the last two years. Many airlines and carriers have exploited the opportunity afforded by the drop in passenger numbers to scrap older, less economic and less efficient planes. Planes spew out carbon dioxide (CO₂) and nitrous oxides (NOx), which also helps form ozone in the upper troposphere. They also emit particulates and leave water-vapour trails (contrails), both of which trap heat.

Sustainable air travel has made big progress over the last two years.

With airlines and plane manufacturers keen to improve their environmental credentials, one simple solution is to power aircraft with bio-fuel, known as sustainable aviation fuel (SAF) in the trade. Existing aircraft can use jet fuel mixed with 50% SAF without needing to be modified in any way. Doing so can slash emissions by up to 80% compared to ordinary jet fuel, with Rolls-Royce and Boeing having already carried out test flights on 747s fitted with Trent engines using 100% SAF.

Alternative fuels

Unfortunately, these greener fuels are up to five times more expensive than jet fuels so they won’t succeed without tax incentives or investment by the fuel industry to make them cheaper. The Finnish firm Neste, for example, is using old cooking oils as a feed stock for its SAF, claiming that more than 370,000 commercial flights have used SAF since 2016. Neste can currently make about 150 m litres of the fuel a year, but that’s still a tiny fraction of what’s needed – and there’s only so much used cooking fat available before SAFs compete with global food supplies.

Those concerns are one reason why the Biden administration has launched the Sustainable Aviation Fuel Grand Challenge, which aims to produce 3 billion gallons of SAF a year by 2030. But even if that ambitious goal is met, SAFs will only cut the direct CO₂ emissions from planes; they do nothing for NOx, water vapour or contrails. The obvious solution is hydrogen, which emits almost no CO₂, very little NOx and just a bit of water vapour. Its energy density (140 MJ/kg) is triple that of kerosene (43 MJ/kg) and far higher than lithium-ion batteries (0.95 MJ/kg).

On the downside, hydrogen is a gas at room temperature, which means it has to be liquefied or compressed so it can be stored in the fuselage. That’s why I think electric batteries could be an answer, for smaller planes at least. Last year Rolls-Royce broke two world records for the fastest all-electric plane, hitting speeds of 555 km/h over a distance of 3 km. The plane used a 400 kW axial flux electric motor from the Oxford-based automotive powertrain supplier YASA.

Unfortunately, today’s batteries are so heavy and bulky that battery-electric planes will probably only be useful for short flights. But according to a recent report from the UK’s Aerospace Technology Institute (ATI), the aviation sector can become carbon-neutral by 2050 using a combination of SAFs and ultimately “green” hydrogen (i.e. hydrogen not derived from fossil fuels) using fuel cells, gas turbines and hybrid systems. The ATI believes that a mid-sized hydrogen-powered plane could be flying by 2035 and a narrow-body aircraft by 2037. The former could fly 280 passengers from London to San Francisco directly.

If half of the world’s commercial planes were hydrogen-powered by 2050, the ATI reckons that the aviation sector’s carbon emissions would fall by 4 × 10⁹ tonnes (4 Gt). That would be equivalent to four years’ worth of emissions from all existing conventional planes, with potentially 14 Gt being saved by 2060. It’s not a pipe dream: many companies contributed to the ATI’s report, including Airbus, easyJet, Eaton, GE Aviation, GKN Aerospace, Reaction Engines and Rolls-Royce.

Elsewhere, Reaction Engines, IP Group and the UK’s Science and Technology Facilities Council launched an intriguing new joint venture at the COP26 conference in Glasgow. They want to see if the exhaust heat from a plane can be used to make hydrogen from ammonia fuel, creating a blend that mimics jet fuel and can be used in existing aircraft engines. Meanwhile, Aviation H2 in Australia is using liquid ammonia combustion in modified jet engines and aims to have a converted Dassault Falcon 50 plane in the skies by mid-2023.

Retrofitting existing aircraft with hydrogen-electric powertrains makes good economic and environmental sense.

Beyond this kind of “green ammonia”, perhaps the cleanest option of all is being pioneered by UK/US start-up ZeroAvia, which has re-fitted an existing turboprop Dornier 228 commuter plane with hydrogen fuel cells and electric motors. Compared to jet engines, its approach could cut operating costs by 60% and maintenance costs by 75% – and, of course, with zero emissions. ZeroAvia has already carried out the first zero-emission six-seater plane flight, signed a partnerships with British Airways and United Airlines, and raised over $130m in funding.

Retrofitting existing aircraft with hydrogen-electric powertrains makes good economic and environmental sense given that there were 23,000 commercial aircraft in service around the world in 2017. But I am also intrigued by a potentially revolutionary new plane design being developed by US start-up Otto Aviation. Its ultra-aerodynamic laminar flow Otto Celera 500L craft can – the company claims – take six passengers more than 8300 km at speeds of 740 km/h using a single propeller, flying 8–9 times further per litre of fuel than a similar jet can.

If developments like these succeed, perhaps “green flying” will one day be possible.

4D-printed material responds to environmental stimuli

Researchers in the Netherlands have produced models of a beetle that changes colour and a scallop shell that opens and closes in response to changing humidity in the surrounding air. Inspired by iridescent structures in nature, Jeroen Sol and colleagues at Eindhoven University of Technology showed that they could integrate a specialized liquid crystal into standard 3D-printing techniques, creating “4D printed” devices that react to their changing environments.

Over millions of years, many organisms have evolved micro-scale structures in their anatomies that allow them to change their vibrant iridescent colours in response to stimuli. Recently, researchers have developed inks that change colour in the same way and have begun to experiment with incorporating them into 3D-printed structures.

This technology has been dubbed 4D printing, where the fourth dimension represents reversible, time-varying changes to the structures after printing. One widely used technique in 4D printing is to deposit ink directly onto 3D printed structures. This approach can accommodate many types of material, as well as a versatile range of printing temperatures, speeds and path designs.

Liquid crystals respond to environmental changes

A particularly promising class of inks for 4D printing are cholesteric liquid crystals (ChLCs). In conventional liquid crystals, molecules flow like a liquid while still orienting themselves like a solid crystal. In ChLCs, molecules arranged across multiple vertical layers can adopt spiral structures in their orientations. Crucially, these structures can be easily and reversibly varied in response to the presence of water, certain chemical compounds and mechanical forces – all of which alter their optical characteristics.

In their study, which they describe in Advanced Functional Materials, Sol and colleagues took inspiration from a species of longhorn beetle (Tmesisternus isabellae) that changes its iridescent colour in response to humidity. To reproduce this effect, the researchers incorporated ChLC ink onto the back of a 3D-printed beetle, then treated this layer with acid in such a way that its crystal structure would respond to moisture.

In high-humidity conditions, the ink swelled. This altered its spiralling molecular structure, causing the beetle’s vibrant iridescent colour to transform from green to red. Once the humidity was removed, the ink reverted back to its original structure, and the beetle turned green again.

An open and shut case

In a parallel experiment, Sol and colleagues printed an open scallop shell from a ChLC elastomer material prone to swelling in high humidity. The team then treated one side of the shell with light to remove this humidity response, while treating the other side with acid as they had with the beetle. This meant that when exposed to dry air, the acid-treated side shrank, causing the shell to clamp shut, only to open again once humid air was restored.

Sol’s team says that these reversible, stimuli-responsive behaviours could inspire applications in robotics and sensing technologies. They could be particularly useful in healthcare, where affordable, wearable 3D printed devices would allow patients to monitor their symptoms simply by tracking the devices’ variable iridescent colours.

EEG detects hidden consciousness in brain-injured patients

Researchers in the US have shown how brainwaves detected in unresponsive patients can help predict if and when they will make a full recovery from traumatic brain injury. By analysing the electrical signals using machine learning, a team led by Jan Claassen at Columbia University Irving Medical Centre found that patients who recover faster tended to generate brainwave activity in response to verbal commands, even when their bodies couldn’t respond physically.

For clinically unresponsive patients who have suffered from traumatic brain injury, it can be incredibly difficult for doctors to predict how long it will take for them to fully recover. As patients start to show signs of recovery, rehabilitation is crucial to ensuring that their brains regain their usual function.

To maximize their chances of success, these rehabilitation programmes must be tailored to the unique rate of progress of each patient. Yet for reasons that neurologists don’t yet understand, the timescale of recovery can vary drastically between patients: ranging from just a few months to potentially several years. Ultimately, this makes it far harder for doctors to decide on how rehabilitation should proceed.

Currently, the extent of a patient’s recovery is often assessed by asking them to respond to simple verbal commands to move a certain part of their body. Those who do not respond to these commands are considered unconscious.

Recently, however, more advanced techniques have emerged, based on electroencephalography (EEG). Here, electrodes placed on a patient’s scalp pick up their brainwaves: oscillations in electrical current generated by large clusters of synchronized neurons in their brains. Studies have shown that even if patients with traumatic brain injury can’t respond to verbal commands directly, their brainwaves indicate that they are aware of them to at least some extent. In this case, patients are said to be in a state of “covert consciousness”.

In their study, Claassen and his team worked with 193 intensive care patients with traumatic brain injury, all of whom were unresponsive to verbal commands at the start of the study. To identify covert consciousness in the patients, the researchers applied machine learning to their EEG recordings – allowing them to distinguish whether the brainwaves appearing after verbal commands to “keep moving” were different from those triggered by instructions to “stop moving”.

In total, the researchers identified brainwaves associated with covert consciousness in 27 of the patients. Out of this group, 41% had made a full recovery after just one year; while nearly all of them showed visible signs of improvement after just three months. In contrast, just 10% of patients without covert consciousness had made a full recovery over the same period.

The result is an important step forward in neurologists’ understanding of how the timescale of recovery in unresponsive patients can be predicted from their brain activity. Based on these insights, Claassen’s team hopes that doctors could develop smarter rehabilitation programmes for their patients; while also helping their families to make more informed decisions about their care.

The study is reported in The Lancet Neurology

NPL hosts international workshop on medical ultrasound tomography

Conventional ultrasound imaging – as routinely employed in hospital scans – works in reflection mode and provides qualitative images of soft tissue reflectivity, or echogenicity. In ultrasound tomography (UST), by contrast, many more measurements are taken and a computational algorithm is used to reconstruct quantitative images of acoustic properties, most commonly the sound speed.

Although UST was first proposed several decades ago, recent advances in hardware, not least in computational power, have led to a revival of interest and progress in the technique, in particular for breast imaging.

Bringing together UST research groups from around the globe, the 3^rd International Workshop on Medical Ultrasound Tomography, MUST 2022, took place last month from 27–29 June, hosted by the National Physical Laboratory (NPL) in collaboration with Imperial College London and University College London.

The talks at the workshop covered all aspects of UST, from hardware design, through image reconstruction, to application in the clinic. But there were perhaps two dominant themes: breast imaging and the use of full-waveform inversions for image reconstruction.

The workshop was opened by NPL’s Chief Scientist, JT Janssen, who described some of the illustrious history of NPL and its current roles in maintaining standards and thereby accelerating research and innovation and facilitating trade.

The first invited speaker was Jeroen Veltman, a breast radiologist from the University of Twente in the Netherlands, who gave a clear description of the clinical workflow requirements and unmet needs in breast radiology. Recently, UST scanners developed by both QT Imaging and Delphinus Medical Technologies have been approved by the US Food and Drug Administration for breast imaging, and the conference attendees heard from the chief scientists behind both these scanners, James Wiskin and Neb Duric.

The group led by Nicole Ruiter, from Karlsruhe Institute of Technology, is a long-time pioneer of fully 3D breast UST technology. Ruiter spoke about UST technical challenges and system design, including presenting details of the group’s latest breast scanner.

The topic of image reconstruction using full-waveform methods was headlined by Jeroen Tromp from Princeton University, who gave a beautifully illustrated description of the state of the field of waveform tomography in the geosciences and seismology, and noted the considerable overlaps with biomedical imaging.

Continuing this theme, there were several talks concerned with applying UST to imaging the brain through the intact skull. The skull is a major barrier to ultrasound, so this represents a considerable challenge. It will be exciting to see, at the next MUST conference, how much progress has been made.

The meeting was supported by NPL, Precision Acoustics, Blatek, the Department for Business Energy & Industrial Strategy, and the UK Acoustics Network (UKAN), who sponsored an early career researcher prize for the best poster. The winner of the UKAN prize was Martin Angerer from Karlsruhe Institute of Technology, for his poster: A new generation of transducer arrays for 3D USCT III.

Smartphone test provides cost-effective screening of middle ear function

Tympanometry is a test that measures middle ear function by examining the compliance of the eardrum to changing air pressure. The test, used to help diagnose middle ear disorders that could lead to hearing loss, is currently performed using a tympanometer, a device that costs between $2000 and $5000. A team of engineers at the University of Washington in Seattle has now designed a smartphone-based system that performs the same function using off-the-shelf components costing just $28.

The new device comprises a lightweight and portable smartphone attachment that can vary the air pressure of the ear canal and measure eardrum mobility. It automatically detects when a seal has been formed with the ear canal, safely varies air pressure, and generates a tympanogram (a plot of how the eardrum moves) on the smartphone in real time. In an initial clinical study reported in Communications Medicine, 86% of test results agreed with those produced by commercial tympanometers.

The researchers have made their hardware design and smartphone app software (designed to work with the Android operating system) free of charge and accessible to audiologists and developers for use and adaptation. They hope that the device will improve access to tympanometry, particularly in low- and middle-income countries and at geographically remote healthcare facilities.

System design

Led by principal investigator Justin Chan, a PhD candidate at the Paul G. Allen School of Computer Science and Engineering, the team created both handheld and desktop tympanometer systems.

Smartphone attachment: The circuit board contains the key acoustic and pressure sensing elements of the tympanometer, plus a microcontroller, Bluetooth antenna and micro-USB port. The syringe is used to adjust the pressure in the ear and move the eardrum. (Courtesy: Dennis Wise/University of Washington)

In the handheld version, all of the electronic components fit into a compact 3D-printed enclosure that attaches to the back of a smartphone. An ear probe incorporating pressure and acoustic sensors is connected through 1 m of lightweight air-tight silicone tubes, which provide mobility during measurements. The tip of the probe, which rests securely in a patient’s ear, is a plastic adapter that interfaces with standard tympanometer disposable rubber ear tips.

During a tympanometry test, the air pressure in the ear canal is changed to evaluate eardrum mobility. To achieve this, the system incorporates a pressure transducer made from a stepper motor that precisely moves the plunger of a 5 ml syringe. Moving the plunger by 5.3 mm changes the pressure between -400 and 200 daPa.

A fail-safe device stops the measurement in case of sensor malfunction. Also, if the probe dislodges from the ear canal during a measurement, air pressure returns to ambient pressure.

During the pressure sweep, the system sends a 226 Hz audio tone (the recommended frequency for patients over nine months of age) at 85 dB SPL (sound pressure level) and records the acoustic reflections at a microphone connected to the smartphone. After the measurement, the pressure data are sent to the smartphone using an onboard wireless Bluetooth radio. The synchronized pressure and audio data are then converted into a tympanogram.

Prior to using the system with a smartphone, a one-time sound level calibration is conducted using a sound level meter. The team notes that two individuals unfamiliar with the process were able to perform the entire calibration procedure in less than 5 min after reading instructions.

Clinical study

Tympanometry is helpful for diagnosing middle ear infections, fluid in the middle ear, a perforated tympanic membrane and issues with the Eustachian tube. Children can be especially susceptible to such middle ear problems, and for this reason the designers elected to conduct their initial clinical tests with paediatric patients at Seattle Children’s Hospital.

For the clinical study, two licensed audiologists performed tympanometry on 50 ears from a total of 28 paediatric patients ranging in age from one to 20 years, first with the smartphone device and then with one of two commercial tympanometers (the GSI TympStar Pro or GSI TympStar). Five paediatric audiologists classified the 100 randomized and anonymized tympanograms, with only patient age provided. The agreement between the two device types was on average 86±2% across all five audiologists.

“The team is currently researching the utility of the system for infants under nine months,” says senior author Shyam Gollakota. “We are testing the tool with higher frequencies that are used with newborn babies. We are also integrating it with other audiological tests, such as audiometry, to provide a smartphone-based suite for addressing all ear-related conditions.”

Studies are being planned or under way in various low-resource countries. “A new study is currently being set up in Kenya,” Gollakota tells Physics World. “We’ll be announcing details about this in the near future. We are quite excited about all of these. Given the prevalence of inexpensive budget smartphones, particularly in developing countries, our frugal system has the potential to be a screening tool for middle ear disorders in resource-constrained environments.”

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