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Water-Activated Holographic Images Can Be Permanently Imprinted

Technology changes in brightness and color depending on degree of humidity and can be used in holographic images to protect against counterfeit currency, goods.

Elizabeth Montalbano

February 7, 2023

3 Min Read
holographic images GettyImages
Kwangmoozaa/iStock / Getty Images Plus via Getty Images

A novel humidity-responsive display projects permanent holographic images that change in brightness and color depending on their contact with water. The technology potentially can be used in applications such as currency and commercial goods to prevent against the counterfeiting or trafficking fake products.

Researchers at Pohang University of Science and Technology (POSTECH), led by Professor Junsuk Rho in the institution's Department of Mechanical Engineering and Department of Chemical Engineering, developed the imaging technology, which uses polyvinyl alcohol (PVA) to deliver images with tunable brightness, researchers said.

The flexible material, which is typically used for liquid glue or slime, features as a key property the tendency to swell as humidity increases, researchers said. In their application, this means that a holographic image that's clear at a low degree of humidity gradually becomes unclear as humidity increases, they said.

The holographic system also includes a display on which structural colors can be tuned by humidity. For example, a blue image at low humidity turns red as humidity increases, researchers said. With further finetuning based on humidity, the images can express all RGB colors in addition to red and blue, they said.

Expanding Material Limits

The technology demonstrates the application of hydrogels in nanophotonics, which previously has been limited due to the low fabrication feasibility and refractive index of this type of materials, researchers wrote in a paper on their report in the journal Nature Communications. What they found in their latest work is that the swelling characteristics of hydrogels in response to the relative humidity are well suited for use in tunable nanophotonics, the basis for the holographic images, they said.

Researchers printed the images using a single-step nanoimprinting technique, they said. The method uses highly secure multiplexed optical encryption metasurfaces to display, hide, or destroy encrypted information based on the relative humidity—both irreversibly and reversibly, they said.

"The resolution of the PVA nanoimprinting reaches sub-100 nm (nanometers), with aspect ratios approaching 10," they wrote in the paper. "In response to changes in the relative humidity, the PVA nanostructures swell by up to ~35.5%, providing precise wavefront manipulation of visible light."

Commercial Application of Holographic Images

The images could have a "a significant impact" on optically variable devices and secure optical-information sharing applications for goods such as whiskey; official documents such as passports; and paper bills used in currency, providing protection against counterfeiting, researchers said. This is because of how technology works, providing an instant alert to whoever implemented the hologram that it may have been tampered with, they said.

"Since revealing the password encrypted in the nanoprint of the humidity-sensitive metasurface destroys the encoded holographic information, the lack of a reconstruction of the holographic image instantly notifies the user that their sensitive information may have been compromised, while the information in the color print can only be observed a few times and destroyed at will," researchers wrote in the paper.

The POSTECH team already has been working with Korea Minting and Security Printing Corporation (KOMSCO) to apply the technology to commercial products, researchers said. Moreover, the holographic images have application in developing a hydrogel macromolecule-based display that responds to external stimuli such as heat, acidity/pH, and fine-dust pollution they said.

About the Author(s)

Elizabeth Montalbano

Elizabeth Montalbano has been a professional journalist covering the telecommunications, technology and business sectors since 1998. Prior to her work at Design News, she has previously written news, features and opinion articles for Phone+, CRN (now ChannelWeb), the IDG News Service, Informationweek and CNNMoney, among other publications. Born and raised in Philadelphia, she also has lived and worked in Phoenix, Arizona; San Francisco and New York City. She currently resides in Lagos, Portugal. Montalbano has a bachelor's degree in English/Communications from De Sales University and a master's degree from Arizona State University in creative writing.

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