Transparent healthcare Illustration of the fully transparent sensor that reacts to sunlight and allows real-time monitoring of UVA exposure on the skin. The device could be integrated into wearable items, such as glasses or patches. (Courtesy: Jnnovation Studio)
A flexible and wearable sensor that allows the user to monitor their exposure to ultraviolet (UV) radiation has been unveiled by researchers in South Korea. Based on a heterostructure of four different oxide semiconductors, the sensor’s flexible, transparent design could vastly improve the real-time monitoring of skin health.
UV light in the A band has wavelengths of 315–400 nm and comprises about 95% of UV radiation that reaches the surface of the earth. Because of its relatively long wavelength, UVA can penetrate deep into the skin. There it can alter biological molecules, damaging tissue and even causing cancer.
While covering up with clothing and using sunscreen are effective at reducing UVA exposure, researchers are keen on developing wearable sensors that can monitor UVA levels in real time. These can alert users when their UVA exposure reaches a certain level. So far, the most promising advances towards these designs have come from oxide semiconductors.
Many challenges
“For the past two decades, these materials have been widely explored for displays and thin-film transistors because of their high mobility and optical transparency,” explains Seong Jun Kang at Soongsil University, who led the research. “However, their application to transparent ultraviolet photodetectors has been limited by high persistent photocurrent, poor UV–visible discrimination, and instability under sunlight.”
While these problems can be avoided in more traditional UV sensors, such as gallium nitride and zinc oxide, these materials are opaque and rigid – making them completely unsuitable for use in wearable sensors.
In their study, Kang’s team addressed these challenges by introducing a multi-junction heterostructure, made by stacking multiple ultrathin layers of different oxide semiconductors. The four semiconductors they selected each had wide bandgaps, which made them more transparent in the visible spectrum but responsive to UV light.
The structure included zinc and tin oxide layers as n-type semiconductors (doped with electron-donating atoms) and cobalt and hafnium oxide layers as p-type semiconductors (doped with electron-accepting atoms) – creating positively charged holes. Within the heterostructure, this selection created three types of interface: p–n junctions between hafnium and tin oxide; n–n junctions between tin and zinc oxide; and p–p junctions between cobalt and hafnium oxide.
Efficient transport
When the team illuminated their heterostructure with UVA photons, the electron–hole charge separation was enhanced by the p–n junction, while the n–n and p–p junctions allowed for more efficient transport of electrons and holes respectively, improving the design’s response speed. When the illumination was removed, the electron–hole pairs could quickly decay, avoiding any false detections.
To test their design’s performance, the researchers integrated their heterostructure into a wearable detector. “In collaboration with UVision Lab, we developed an integrated Bluetooth circuit and smartphone application, enabling real-time display of UVA intensity and warning alerts when an individual’s exposure reaches the skin-type-specific minimal erythema dose (MED),” Kang describes. “When connected to the Bluetooth circuit and smartphone application, it successfully tracked real-time UVA variations and issued alerts corresponding to MED limits for various skin types.”
As well as maintaining over 80% transparency, the sensor proved highly stable and responsive, even in direct outdoor sunlight and across repeated exposure cycles. Based on this performance, the team is now confident that their design could push the capabilities of oxide semiconductors beyond their typical use in displays and into the fast-growing field of smart personal health monitoring.
“The proposed architecture establishes a design principle for high-performance transparent optoelectronics, and the integrated UVA-alert system paves the way for next-generation wearable and Internet-of-things-based environmental sensors,” Kang predicts.
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Spending time in space has a big impact on the human body and can cause a range of health issues. Many astronauts develop vision problems because microgravity causes body fluids to redistribute towards the head. This can lead to swelling in the eye and compression of the optic nerve.
While eye conditions can generally be treated with medication, delivering drugs in space is not a straightforward task. Eye drops simply don’t work without gravity, for example. To address this problem, researchers in Hungary are developing a tiny dissolvable eye insert that could deliver medication directly to the eye. The size of a grain of rice, the insert has now been tested by an astronaut on the International Space Station.
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Researchers in Japan and Taiwan have captured three-dimensional images of an entire geothermal system deep in the Earth’s crust for the first time. By mapping the underground distribution of phenomena such as fracture zones and phase transitions associated with seismic activity, they say their work could lead to improvements in earthquake early warning models. It could also help researchers develop next-generation versions of geothermal power – a technology that study leader Takeshi Tsuji of the University of Tokyo says has enormous potential for clean, large-scale energy production.
“With a clear three-dimensional image of where supercritical fluids are located and how they move, we can identify promising drilling targets and design safer and more efficient development plans,” Tsuji says. “This could have direct implications for expanding geothermal power generation, reducing dependence on fossil fuels, and contributing to carbon neutrality and energy security in Japan and globally.”
In their study, Tsuji and colleagues focused on a region known as the brittle-ductile transition zone, which is where rocks go from being seismically active to mostly inactive. This zone is important for understanding volcanic activity and geothermal processes because it lies near an impermeable sealing band that allows fluids such as water to accumulate in a high-pressure, supercritical state. When these fluids undergo phase transitions, earthquakes may follow. However, such fluids could also produce more geothermal energy than conventional systems. Identifying their location is therefore important for this reason, too.
A high-resolution “digital map”
Many previous electromagnetic and magnetotelluric surveys suffered from low spatial resolution and were limited to regions relatively close to the Earth’s surface. In contrast, the techniques used in the latest study enabled Tsuji and colleagues to create a clear high-resolution “digital map” of deep geothermal reservoirs – something that has never been achieved before.
To make their map, the researchers used three-dimensional multichannel seismic surveys to image geothermal structures in the Kuju volcanic group, which is located on the Japanese island of Kyushu. They then analysed these images using a method they developed known as extended Common Reflection Surface (CRS) stacking. This allowed them to visualize deeper underground features such as magma-related structures, fracture-controlled fluid pathways and rock layers that “seal in” supercritical fluids.
“In addition to this, we applied advanced seismic tomography and machine-learning based analyses to determine the seismic velocity of specific structures and earthquake mechanisms with high accuracy,” explains Tsuji. “It was this integrated approach that allowed us to image a deep geothermal system in unprecedented detail.” He adds that the new technique is also better suited to mountainous geothermal regions where limited road access makes it hard to deploy the seismic sources and receivers used in conventional surveys.
A promising site for future supercritical geothermal energy production
Tsuji and colleagues chose to study the Kuju area because it is home to several volcanoes that were active roughly 1600 years ago and have erupted intermittently in recent years. The region also hosts two major geothermal power plants, Hatchobaru and Otake. The former has a capacity of 110 MW and is the largest geothermal facility in Japan.
The heat source for both plants is thought to be located beneath Mt Kuroiwa and Mt Sensui, and the region is considered a promising site for supercritical geothermal energy production. Its geothermal reservoir appears to consist of water that initially fell as precipitation (so-called meteoric water) and was heated underground before migrating westward through the fault system. Until now, though, no detailed images of the magmatic structures and fluid pathways had been obtained.
Tsuji says he has long wondered why geothermal power is not more widely used in Japan, despite the country’s abundant volcanic and thermal resources. “Our results now provide the scientific and technical foundation for next-generation supercritical geothermal power,” he tells Physics World.
The researchers now plan to try out their technique using portable seismic sources and sensors deployed in mountainous areas (not just along roads) to image the shallower parts of geothermal systems in greater detail as well. “We also plan to extend our surveys to other geothermal fields to test the general applicability of our method,” Tsuji says. “Ultimately, our goal is to provide a reliable scientific basis for the large-scale deployment of supercritical geothermal power as a sustainable energy source.”
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