Engineers at the University of California San Diego have developed an optical device that reveals hidden images and changes colors in response to different levels of humidity. The technology, published in Light: Science & Applications , could lead to the development of new anti-counterfeiting labels, secure data storage, interactive displays and environmental sensors.
The device works by displaying different images depending on moisture levels in the air. Under normal conditions or low humidity levels, one image (UC San Diego Triton logo) is visible. When humidity increases, a second image (UC San Diego library logo) emerges and conceals the first. This transition can be triggered even when a person breathes on the device. It happens in a fraction of a second and can be repeated many times.
"You can imagine using this as a built-in security feature with the environment acting like a key that unlocks different pieces of information," said study first author Asad Nauman, an electrical and computer engineering postdoctoral researcher at UC San Diego. "One example would be something like a credit card security tag, where you can blow on it and reveal a hidden code. Another application would be an environmental sensor that changes color as the humidity changes."
The new optical device is a postage stamp-sized chip that consists of two layers. The bottom layer is what is known as a phase-change material called antimony trisulfide, which functions as a reusable storage medium. Images can be written, erased and rewritten on this layer using a laser. The top layer is a hydrogel made of azido-grafted carboxymethyl cellulose, a soft material that swells in humid conditions and shrinks in drier ones. An image can be permanently patterned into this layer using UV light.
This work emerged from an interdisciplinary effort within the Nano Devices and Applied Optics (NDAO) lab , led by Abdoulaye Ndao, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering and senior author of the study. Members of the NDAO lab combined their expertise in phase-change photonic materials and responsive hydrogels to synergistically integrate these fundamentally different material frameworks to create the bilayer device.
This layered approach could enable new forms of data storage for computing applications, Ndao noted. "You could encode different pieces of information in different layers stacked on a single space and choose which one to access based on the humidity."
Different humidity levels also induce color changes in the device. This is due to a tiny gap between the two layers. Though the spacing between them is small, it plays a big role. As the hydrogel layer swells and shrinks, it changes the spacing between the layers. This alters how light reflects off the material, which in turn changes the color that appears.
Both the image and color changes happen instantly, within about 300 milliseconds of the humidity changing. Plus, the layers can be produced at low-cost and over large areas, which makes them easy to scale.
Researchers are planning to explore new versions of the device that could be controlled electrically.
Full study: "Reversible Optical Data Storage and Encryption Enabled by Phase-Change and Hydrogel Integration"
This work is supported by an Alfred P. Sloan Research Fellowship, a 2023 Beckman Young Investigator Award from the Arnold and Mabel Beckman Foundation, the Chan Zuckerberg Initiative, the Moore Foundation to the PAIR-UP Imaging Science Program, Air Force Office of Scientific Research MURI (award no. FA9550-22-1-0312), UC San Diego Materials Research Science and Engineering Center (MRSEC), and Silicon Valley Community Foundation (grant DAF2023-331948).
