In Stanford's laboratories, a team of engineers and physicists has succeeded in bringing to life materials inspired by a master of natural camouflage: the cephalopod. Their creation reinvents how a surface can interact with light and touch.
This advancement relies on combining a flexible polymer with an extremely precise etching tool, allowing the simultaneous control of the relief and color of a synthetic film. These artificial "skins," capable of rapid and reversible metamorphosis, open up prospects in various fields such as robotics, tactile interfaces, or even bio-engineering.
The principle of a controlled metamorphosis
The key to this innovation lies in a polymer film with astonishing properties: it swells in a controlled manner by absorbing water. Using an electron beam, researchers can locally modify the molecular structure of this polymer. These "treated" areas then absorb less liquid and therefore swell less. This difference in swelling between areas, imperceptible when dry, results in the appearance of microscopically fine relief patterns as soon as the material is moistened.
The polymer acts like a reactive canvas, where the artist uses not a brush, but a particle beam to etch in negative the patterns that will only reveal themselves upon contact with water.
By creating reliefs on the micrometer scale, scientists can vary how light is scattered. A surface can thus reflect light directionally or have a matte appearance by dispersing the light. This capability is important for realistic camouflage, as shine is often a factor in visual betrayal in a natural environment.
Evolution of color patterns in a sample of soft photonic skin.
Siddharth Doshi Colors without pigment and potential applications
To generate colors, the researchers added an optical dimension to the system. They deposited thin metallic layers on either side of the polymer film, creating a resonant cavity. The thickness of this cavity, which varies with the swelling of the polymer, determines the wavelength of light that is reflected. By precisely controlling the swelling according to predetermined patterns, a single, uniform surface can thus display a palette of colored spots, without any dye.
The reversibility of the process is ensured by using a solvent, such as isopropyl alcohol, which dehydrates the polymer and returns it to its flat, initial state in a few seconds. This reversibility has been tested over hundreds of cycles without notable degradation in performance, an encouraging robustness for practical applications.
The envisioned applications are numerous. In the field of robotics, such materials could allow soft robots to blend into their environment or modify their grip on a surface. In bio-engineering, the ability to dynamically modify texture on the nanoscale offers avenues for guiding cell adhesion and growth. Finally, this work paves the way for a new kind of touch screen, capable of generating raised buttons or braille characters on demand.
Article author: Cédric DEPOND