Squid-skin-inspired screen displays encrypted images using only magnets

Inspired by the color-changing skin of squids and other cephalopods, researchers have developed a flexible screen capable of storing and displaying encrypted images without using electronics – just tiny magnetic particles.

For a while now, scientists have been playing around with metamaterials, materials engineered to have properties not usually found in nature, to create things as disparate as invisibility cloaks and bone-penetrating ultrasound. Mechanical metamaterials, particularly, are capable of programmable behavior through the interplay between material and structure, enabling advanced functionality that extends beyond their mechanical properties.

The potential for using mechanical metamaterials in information processing and computing is something that researchers are exploring. However, their use is constrained because they rely on mechanisms that fold, bend and buckle, which are difficult to miniaturize. Now, engineers from the University of Michigan (U-M) have developed a flexible display screen that uses magnetic fields instead of electronics to reveal images. And it was inspired by squid skin.

“It’s one of the first times where mechanical materials use magnetic fields for system-level encryption, information processing and computing,” said Joerg Lahann, professor of chemical engineering at U-M and the study’s co-corresponding author. Abdon Pena-Francesch, assistant professor of materials science and engineering and the other corresponding author, explains how the researchers developed the screen in the video below.

But how does the squid factor in? Squids and other cephalopods have chromatophores in the top skin layer, organs that contain pigment sacs that expand and contract rapidly under the control of muscles. The collective actuation of chromatophores enables the squid to adapt its skin color and pattern to suit its purpose: camouflage, predation, or mating. It was the contraction and expansion of chromatophores the researchers were inspired by, and helped them decide on the screen’s resolution.

“If you make the beads too small, the changes in color become too small to see,” said Zane Zhang, a doctoral student in materials science and engineering and lead author of the study. “The squid’s pigment sacs have optimized size and distribution to give high contrast, so we adapted our device’s pixels to match their size.”

The ‘pixels’ are, in fact, a swarm of magnetoactive Janus particles (MAJP). Janus particles are special nanoparticles whose surfaces have two or more distinct physical properties. Here, the researchers created bi-compartment MAJPs composed of neodymium (NdFeB) ferromagnetic microparticles and superparamagnetic iron oxide nanoparticles (SPIONs) in one compartment and titanium oxide (TiO₂) pigment in the other.

Leveraging the MAJP’s switching mechanisms, the researchers could use a magnetic field to program a swarm of particles into multiple states. They could, for example, define two states – ‘on’ and ‘off’ – determined by the MAJP’s orientation and color they collectively displayed: iron compartment up is orange; titanium compartment up is white. In this way, the pixel-like MAJPs flip between orange and white depending on the direction of magnetization – or polarization – of the applied magnetic field. For the MAJPs made with iron oxide magnetic particles, the polarization could be changed with relatively weak magnetic fields. However, the polarization of MAJPs that also included neodymium particles required a strong magnetic pulse.

Holding the screen over a collection of magnets of different strengths and orientations selectively changed the polarization in some parts of the screen, making some pixels flip to white and others flip to orange under the same magnetic field. An image was encoded in this way.

Because the iron oxide nanoparticles can be reprogrammed using relatively weak magnetic fields, private images can be displayed with a second magnetic grid that selectively rewrites how some parts of the screen flip. When the iron oxide particles are returned to the standard magnet, they revert to their original polarization, and the public image is displayed.

One public image can contain several private images, each with a unique decoding key that can be programmed only to work with specific encoding keys, adding an extra layer of security.

“This device can be programmed to show specific information only when the right keys are provided,” Pena-Francesch said. “And there is no code or electronics to be hacked. This could also be used for color-changing surfaces, for example, on camouflaged robots.”

Reminiscent of an Etch-A-Sketch, the red-framed mechanical drawing toy that’s been around since the late 1950s, shaking the screen erases the display. Exposing it to a magnetic field again causes the image to return.

The researchers say the screen is designed for use when light and power are impractical or undesirable, such as on clothes, stickers, ID badges, barcodes, and eBook readers.

The study was published in the journal Advanced Materials.

Source: Michigan Engineering