Wearable Sensor Detects Toxic Gases, With a Hologram

Billy Hurley, Digital Editorial Manager

Researchers from the Pohang University of Science & Technology (POSTECH)  have created a tiny gas sensor that you can place on your glasses or gloves like a sticker. With a hologram display, the sensor immediately notifies the user of volatile gas detection.

The wearable technology could someday help to prevent gas accidents related to toxic gas leakage in factories, carbon monoxide leakage of boilers, or toxic gas suffocation during manhole cleaning.

How the Wearable Gas Sensor Works

The 300 x 300 µm sensor is made up of a metasurface and liquid crystal (LC) cell, which is sensitive to the toxic gas. When gas is detected, the orientation of the cells change to form a customized warning.

The holograms are produced by the metasurface, which makes visible objects disappear by controlling the refractive index of light.

The change in orientation of the liquid crystal in the cell reverses the circular polarization of the incoming light from right to left.

This circularly polarized light then interacts with the metasurface, which is designed to produce two distinct holograms — one for each light polarization.

In the "safe state" the LCs are configured to produce the polarization of light that generates the "safe" hologram. The exposure to a toxic gas influences the LCs to orientate in a way to produce the opposite circular polarization, generating the "alarm" hologram signal.

"We obtain a phase distribution that produces the desired image and arrange the nanostructures on the metasurface accordingly," said POSTECH researcher Trevon Badloe. "Then, when the metasurface is illuminated with circularly polarized light, the holographic images are displayed."

Badloe and his team attached the metasurface to the curved surface of a pair of safety glasses.

With the metasurface, the gas sensor can float a holographic image alarm in space in just a few seconds by controlling the polarization of transmitted light. The holograms can be made to appear above a sample, or wherever the wearer decides to put it, says Badloe.

"We used laser illumination to produce the holograms, however we plan to integrate the device with a small light source such as OLEDs or MicroLEDs to create a complete on-chip device in the future," said the researcher.

Unlike other conventional commercial gas sensors, the POSTECH device requires no support from external mechanical or electronic devices.

In tests, the researchers used isopropyl alcohol, a toxic substance that causes stomach pain, headache, and dizziness, as the target hazardous gas. According to the team's study, the sensor detected even the minute amount of gas of about 200ppm.

The flexible and wearable gas sensor is made quickly, through a one-step nanocomposite printing method. The metasurface structure, previously processed on a hard substrate, is nanocasted on a curved or flexible substrate in one step.

What's Next?

Next, the researchers will develop a high-performance environmental sensor that displays the type and concentration level of gases or biochemicals in the surroundings with a holographic alarm. The engineers are also studying optical design techniques that can encode various holographic images. The studies, if successful, can be used to reduce accidents caused by biochemical or gas leaks.

The sensor makers, according to Badloe and his team, led by POSTECH professor Professor Junsuk Rho, hopes to integrate the technology with glass-type AR display systems under development at Apple, Samsung, Google, and Facebook.

Badloe, a member of Professor Rho’s group and a co-author of the researchers' recent paper in Science Advances , spoke with Tech Briefs via email about how he envisions a future of holographic images. Badloe communicated on behalf of his team and in coordination with Dr. Inki Kim, lead author of the paper.

Tech Briefs: What does an "immediate visual holographic alarm” look like?

Trevon Badloe: The visual holographic alarm can display any chosen images for the safe and warning states; this could include text, images, or symbols. The method of designing and encoding the images into the holographic metasurface is well-known, so they are chosen before the fabrication process.

Tech Briefs: What gases can be detected with this sensor?

Trevon Badloe: Since the functionality of the gas sensing is only reliant on the liquid crystal cell, the selectivity of the response to certain toxic gases could also be fairly easily integrated into the device. We tested and verified the response times when sensing chloroform, acetone, toluene, IPA, p-xylene, methanol, and DMF. The response time of the "safe" to "alarm" signal depends on the dosage and type of gas.

Tech Briefs: How do you design a distinct hologram in the metasurface?

Trevon Badloe: The process is fairly involved, but well-understood.

First, we need to digitally generate a phase map that produces the required interference pattern (i.e., the image that we want to display) at the desired location. The design of a hologram is done computationally, known as computer generated holography (CGH). Here we used "phase only" CGH. The generation of the phase map for the required image is fairly straightforward and well-understood, most commonly using the Gerchberg-Saxton algorithm . Then, this phase map is physically encoded onto a metasurface using nanostructures that are designed to exhibit the required phase characteristics.

Tech Briefs: Is it a challenging process to create two separate holograms?

Trevon Badloe: We take advantage of the fact that we can induce equal and opposite polarizations of light (left and right circularly polarized). Then, we can combine the two distinct phase maps in a way that allows us to encode the nanostructures using their inherent phase characteristics, plus an extra compensation that is only related to the rotation of the nanostructures (known as the propagation and geometric phase, respectively).

Tech Briefs: Can you take me through an example application of this sensor?

Trevon Badloe: Using a flexible liquid crystal cell and metasurface can allow us to apply the gas sensing visual alarms to any surface, regardless of it being smooth and flat. As an example, we attached the metasurface to the curved surface of a pair of safety glasses. They could also be attached somewhere such as the gloves of a user, or anywhere that they can see easily to provide a visual alarm.

Tech Briefs: What’s next with your research?

Trevon Badloe: As follow-up research we are going to produce a device that not only provides safe and alarm signals, but can also give the user information on the level of toxic gas that is present. For example, it could be useful to know the existence of toxic gas before it gets to dangerous levels. Then, the correct actions can be undertaken to stop the flow of the unwanted gas.

Tech Briefs: What is most exciting to you about this technology and its possibilities?

Trevon Badloe: This technology proves the impact that metasurfaces could have in real-life applications. With the simple integration of metasurfaces with liquid crystals numerous functionalities can be realized. Holographic visual alarms could prove extremely useful in various fields where instantaneous alarms are required for the safety of the users, such as in labs or even military environments for the rapid detection of biohazardous gases.

What do you think? Do you see other uses for holographic warnings? Share your questions and comments below.

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