Gas accidents, such as toxic gas leaks in factories, carbon monoxide leaks from boilers, or toxic gas asphyxiation during manhole cleaning, continue to claim lives and cause injuries.

In the fields of public health, environmental monitoring, and military applications, developing sensors capable of rapidly detecting toxic gases or biochemical substances remains a critical challenge. Recently, a research team at POSTECH developed an inexpensive, ultra-compact wearable holographic sensor that immediately notifies users of volatile gas detection.

A joint research group was formed by Junsuk Rho, Professor in the Department of Mechanical and Chemical Engineering; Inki Kim, Ph.D., in the Department of Mechanical Engineering; and Young-Ki Kim, Professor with a Ph.D. Won-Sik Kim, a candidate in the Department of Chemical Engineering at POSTECH, integrated metasurfaces with gas-reactive liquid crystal optical modulators to develop a sensor that issues an immediate visual holographic alert upon detecting hazardous gases. The results of this study were published in Science Advances on 2021-4-7.

For those working in hazardous environments such as petrochemical plants, gas sensors are vital for safety. However, conventional gas sensing equipment is not widely used due to the high costs associated with complex machinery and electronics. Furthermore, commercial gas sensors are limited by difficulties in use, poor portability, and slow response speeds.

To address these issues, the research team utilized metasurfaces, known as future optical devices that achieve invisibility cloaking effects by controlling the refractive index of light to make visible objects disappear. Metasurfaces are specifically used to transmit bidirectional holograms or 3D video images by freely controlling light.

Using metasurfaces, the research team developed a gas sensor that projects a holographic image alert into space within seconds by controlling the polarization of transmitted light, which shifts due to changes in the orientation of liquid crystal molecules within the liquid crystal layer when exposed to gases inside the sensor device. Moreover, unlike other traditional commercial gas sensors, this gas sensor developed by the research team does not require support from external mechanical or electronic equipment. The researchers used isopropyl alcohol as the target hazardous gas, a toxic substance known to cause stomach pain, headaches, dizziness, and even leukemia.

It was confirmed that the newly developed sensor can detect trace amounts of gas as low as approximately 200ppm. In practical experiments using board-shaped markers (a source of volatile gases in daily life), a visual holographic alert popped up immediately when the marker was brought near the sensor.

Furthermore, the research team developed a single-step nanocomposite printing method to produce this flexible and wearable gas sensor. The metasurface structures, initially processed on rigid substrates, were designed for rapid production on curved or flexible substrates via a single-step nano-casting process.

When flexible sensors manufactured using this method are attached like stickers to safety glasses, they can detect gases and display holographic alerts. It is expected that they will be integrated with the glass-type AR display systems currently being developed by Apple, Samsung, Google, and Facebook.

Going further, the research team is developing a high-performance environmental sensor that can display the types and concentration levels of surrounding gases or biochemical substances via holographic alerts, and is investigating optical design techniques capable of encoding various holographic images. If these studies are successful, they could help reduce accidents caused by biochemical or gas leaks.

Professor Junsuk Rho stated, "This newly developed ultra-compact wearable gas sensor provides a more intuitive holographic visual alarm than traditional auditory or simple light alarms. This method is expected to be particularly effective in more extreme work environments with intense sound and visual noise."

This research was conducted with support from the Mid-career Researcher Program, Global Frontier, Regional Leading Research Center, Future Materials Discovery, and the Sejong Science Fellowship of the National Research Foundation of Korea under the Ministry of Science and ICT.