Squid-Inspired Display Shows Encrypted Images Using Only Magnets

Inspired by the color-changing skin of squid and other cephalopods, researchers have developed a flexible display that can store and display encrypted images without using electronics – just tiny magnetic particles.

For some time now, scientists have been interested in metamaterials—materials designed to have properties not typically found in nature—to create objects as disparate as invisibility cloaks or bone-penetrating ultrasound. Mechanical metamaterials, in particular, are capable of programmable behavior through the interaction between the material and the structure, enabling advanced functionalities that go beyond their mechanical properties.

Researchers are currently exploring the potential of mechanical metamaterials in information processing and computing. However, their use is limited because they rely on mechanisms that bend, curve, and deform, which are difficult to miniaturize. Now, engineers at the University of Michigan (UM) have developed a flexible display that uses magnetic fields instead of electronics to reveal images. And it’s inspired by squid skin.

“This is one of the first times that mechanical materials have used magnetic fields for encryption, information processing and system-level computation,” said Joerg Lahann, a UM professor of chemical engineering and co-corresponding author of the study. Abdon Pena-Francesch, an assistant professor of materials science and engineering and another corresponding author, explains how the researchers developed the display in the video below.

But what role do squid play? Squid and other cephalopods have chromatophores in the top layer of their skin, organs that contain pockets of pigment that expand and contract rapidly under the control of muscles. The collective actuation of the chromatophores allows the squid to adapt the color and pattern of its skin depending on its purpose: camouflage, predation, or mating. It was the contraction and expansion of the chromatophores that inspired the researchers and helped them decide on the screen resolution.

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

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

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

By holding the screen over a set of magnets of different strengths and orientations, the polarization of certain parts of the screen was selectively changed, causing some pixels to turn white and others orange under the same magnetic field. An image was thus encoded.

Because 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 certain parts of the screen flip. When the iron oxide particles are brought back to the standard magnet, they return to their original polarization and the public image is displayed.

A public image can contain multiple private images, each with a unique decryption key that can be programmed to only work with specific encryption keys, adding an extra layer of security.

“This device can be programmed to display specific information only when the right keys are pressed,” Pena-Francesch explained. “And there’s no code or electronics to hack. It could also be used to change the color of surfaces, for example on camouflaged robots.”

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

The researchers say the display is designed for use where light and electricity are impractical or undesirable, such as on clothing, stickers, ID badges, bar codes and e-book readers.

The study was published in the journal Advanced materials.

Source: Michigan Engineering