NCCR Bio-Inspired Materials researchers have developed a simple method to create dazzling hues using polymers, mimicking the behaviour of cuttlefish and octopuses. Potential 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.

NCCR researchers at the University of Fribourg and colleagues have now developed ellipsoidal microparticles that work along the same principle as the 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, by applying a magnetic field, the particles rotate and align themselves with the field. 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, published in the journal Advanced Functional Materials, show that higher molecular-weight polymers are widely spaced, reflecting longer wavelengths towards the red end of the visible spectrum of light. 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 at the University of Fribourg’s Adolphe Merkle Institute.

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. Because the particles form through self-assembly in an emulsion process, the approach also points towards realistic scalability for future manufacturing.

Potential applications:

Adaptive camouflage: coatings or textiles that change appearance on command to match their surroundings by applying a magnetic field.

Smart displays: low-power color elements that switch and hold states without continuous energy input.

Sensors: optical indicators that respond to magnetic changes by altering their reflected color.

Switchable coatings: protective layers for vehicles, buildings, or devices whose appearance can be changed remotely.

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. https://doi.org/10.1002/adfm.202528686  (open access)