Magnetic tunable colors inspired by nature

Adolphe Merkle Institute researchers have developed a simple method to create dazzling hues using polymers, mimicking the behaviour of cuttlefish and octopuses. Possible applications include adaptive camouflage, smart displays, and switchable coatings.

Cuttlefish and octopuses can flash vivid colors and almost instantly disappear into their surroundings thanks to special skin cells called iridophores. These specialized structures contain stacks of ultra-thin, transparent layers with alternating refractive indices. When light bounces off these nanoscale layers, they reflect only specific wavelengths, creating vivid reds, blues, and greens without the use of dyes or pigments. Cephalopods can tune their optical appearance by changing the orientation of these stacks, allowing rapid, reversible shifts in reflected color and directionality, which is central to their adaptive camouflage.

The AMI researchers and colleagues at the University of Fribourg, in Spain, and Italy have created ellipsoidal microparticles that work on the same principle as iridophores. These particles are composed of block copolymers, macromolecules formed from two or more distinct polymer chains (blocks) linked together in a linear sequence. They arrange themselves into axially stacked nanolayers during a simple “one-pot” emulsion process. The key innovation involves adding superparamagnetic nanoparticles to the polymeric matrix, which provides each particle with a built-in magnetic handle.

When a magnetic field is absent, these microparticles float in a liquid with no specific orientation. Their internal stacks still reflect light, but because every particle points in a random direction, the suspension appears whitish due to diffuse scattering. However, when a magnetic field is applied, the particles rotate and align with it. This results in the stacked layers inside each particle sharing a common orientation relative to the incoming light, and the suspension suddenly displays vivid structural colors depending on the spacing of the internal layers.

The researchers’ findings show that more widely spaced higher-molecular-weight polymers reflect longer wavelengths towards the red end of the visible light spectrum. Lower molecular-weight polymers with tighter spacing reflect shorter wavelengths on the blue side of the spectrum. By simply blending polymers of different molecular weights at specific ratios, it is possible to smoothly transition across a wide range of hues, from deep blues to vibrant reds, without the need for chemical modifications or dyes.

What makes this technology especially striking is its angle-dependent color, with the reflected wavelength shifting predictably with the orientation angle, like the iridescence seen in the wings of butterflies or hummingbirds. “Because the particles respond to the direction of the magnetic field, tilting the field provides a simple way to dynamically control the color shift in real time,” explains Dr. Andrea Dodero, a group leader in AMI’s Soft Matter Physics group.

Most previous approaches to color-changing materials have been complicated to fabricate, slow to respond, or dependent on constant power. This new strategy uses commercially available polymers, a straightforward manufacturing route, and fast, reversible switching under relatively weak magnetic fields, all the while pointing towards realistic scalability for future manufacturing.

Potential applications include smart coatings, adaptive camouflage surfaces, anti-counterfeiting features, and display-like optical elements, in which tunable appearance is a key function, according to Dodero. “These are areas where a magnetically switchable color platform may offer something conventional static pigments cannot,” he adds.

Dodero points out, though, that the work is not simply about producing colored particles, but about integrating materials design, optical behavior, and magnetic actuation into one coherent concept inspired by biology.  “The final result reflects not just multiple disciplines working side by side, but a true convergence of chemistry, physics, and bio-inspired materials design, leading to a system that is both scientifically elegant and application-oriented,” he says.

Reference:

Mazzotta, G.; Bertucci, S.; Mendoza-Carreño, J.; Mihi, A.; Lova, P.; Comoretto, D.; Steiner, U.; Lattuada, M.; Dodero, A. Bio-Inspired Magnetically Tunable Structural Colors from Elliptical Self-Assembled Block Copolymer Microparticles. Adv. Funct. Mater. 2025, e28686.