Scientists at the University of California San Diego have created a new optical device that can reveal hidden images and change colors when exposed to different levels of humidity.
The technology could one day be used in anti-counterfeiting labels, secure data storage systems, smart displays, and environmental sensors.
The small device, about the size of a postage stamp, can display different images depending on how much moisture is present in the air.
Under normal or dry conditions, one image is visible. When humidity rises, a completely different image appears and hides the first one.
Remarkably, the change can happen simply by breathing on the device.
The transformation takes place almost instantly and can be repeated many times without damaging the material.
Researchers say this technology could provide a new way to hide and reveal information using environmental conditions as a type of security key.
“You can imagine using this as a built-in security feature with the environment acting like a key that unlocks different pieces of information,” explained study first author Asad Nauman, a postdoctoral researcher in electrical and computer engineering at UC San Diego.
For example, a security label on a credit card or product package could contain a hidden code that only becomes visible when exposed to moisture from a person’s breath. The device could also be used as a sensor that changes appearance as humidity levels fluctuate.
The device is made from two different layers that work together. The bottom layer is composed of a material called antimony trisulfide, known as a phase-change material. This layer acts like a reusable storage medium because images can be written, erased, and rewritten onto it using a laser.
The top layer is a special hydrogel made from a cellulose-based material. Hydrogels are soft materials that absorb water. When humidity increases, the hydrogel swells. When the air becomes dry, it shrinks back to its original size. Researchers permanently imprint an image into this layer using ultraviolet light.
The project combined expertise from different scientific fields. Researchers brought together advanced optical materials and moisture-responsive hydrogels to create a device that can store and reveal information in new ways.
According to senior author Abdoulaye Ndao, this layered design could open the door to new forms of data storage. Different pieces of information could be stored in separate layers within the same device, with humidity determining which layer becomes visible.
The device also changes color as humidity levels rise and fall. This effect is caused by a tiny gap between the two layers. As the hydrogel expands and contracts, the gap changes slightly. These small changes alter how light reflects from the device, producing different colors.
Both the image switching and color changes occur extremely quickly—within about 300 milliseconds of a humidity change. Researchers say the materials can be manufactured at low cost and over large areas, making the technology practical for future commercial applications.
The team is now exploring ways to create versions of the device that can be controlled electrically, potentially expanding its uses even further.
The study was published in the journal Light: Science & Applications.
