A team of engineers at the University of California San Diego (UCSD) have developed a humidity-based image encoder that looks straight out of James Bond’s Q-Lab. The postage stamp-sized chip can store a hidden message that is only revealed when exterior humidity levels surpass 60 percent. The image can then be concealed again by bringing humidity back down. In practice, that means someone handed an object with the chip on it could simply breathe on it to unveil its secret message.
While it’s a potentially nifty tool for an undercover spy, the researchers say the encoder could also be used to reveal a security code on a credit card, or even serve as a visual indicator of climate changes in a particular area. In all of these cases, humidity essentially acts as a key. The findings were recently published in the journal Light: Science & Applications.
“You can imagine using this as a built-in security feature with the environment acting like a key that unlocks different pieces of information,” study co-author and UC San Diego electrical and computer engineering postdoctoral researcher Asad Nauman said in a statement.
In a video demonstration, a clear blue image of a UCSD trident logo appears and then quickly begins to fade as the area around it brightens. After only a few seconds in, the UCSD library logo emerges. The image then fades back to the man with the trident before switching back once more to the library logo.
Optical device displays different images in response to humidity
Hiding a message in plain sight
The chip consists of two separate hydrogel layers. The bottom layer, made of a phase-changing material called antimony trisulfide, essentially acts as a canvas onto which lasers can etch messages. These can be text or, as in the example above, full images. The top layer is made of a softer hydrogel material called azido-grafted carboxymethyl cellulose. This layer swells in humid conditions and shrinks in dry ones, which is why the hidden message becomes visible.
The first, low-humidity image or message is visible when humidity levels are at or below 40 percent. As humidity levels approach 60 percent, the hidden message starts taking shape. It is then fully visible at 80 percent humidity. The image reveal is also accompanied by a color shift due to small gaps between the two hydrogel layers. When the top layer swells and expands, the increased space between the layers alters the way light reflects off them, resulting in a shift from blue to red.
Of course, for any of this to work, a spy or other user would need to operate in an area with a predictable climate. Blowing on a message in a tropical environment where the air is already thick with moisture probably won’t do the trick. Still, in a pinch, it might beat having to write out long, intricate messages on finicky invisible ink.
