Researchers at Stanford University have developed a new flexible material, described as an artificial "photonic skin," capable of rapidly altering both its color and surface texture independently. This advancement replicates the sophisticated camouflage mechanisms observed in cephalopods like octopuses and cuttlefish, which dynamically change their appearance by manipulating pigment cells for color and muscle-controlled papillae for texture. The findings were published in the journal Nature on January 7.
Material Capabilities and Design
The developed soft polymer film can swell and modify its texture and color within seconds, creating patterns with resolutions finer than a human hair. This capability allows for the independent control of both color and surface topography, addressing a long-standing challenge in synthetic material development.
The material's foundation is the polymer PEDOT:PSS, which swells upon contact with water. Electron-beam lithography, a technique commonly used in semiconductor manufacturing, is employed to modify specific areas of the polymer film. This modification controls the film's swelling capacity, allowing for the creation of detailed patterns when the material is wet. The initial observation that an electron beam could alter the polymer's absorbency and create patterns was noted as a serendipitous discovery during earlier experiments by Siddharth Doshi, a doctoral student in materials science and engineering and the paper's first author.
Texture and Color Control Mechanisms
For texture control, electron-beam patterning allows for precision, enabling the creation of nanoscale replicas, such as a three-dimensional representation of Yosemite National Park's El Capitan formation when wet. These textures can revert to a flat state upon the addition of an alcohol-based solvent. The technique also permits the production of fine-scale textures that adjust light scattering, allowing the surface finish to range from glossy to matte.
Color control is achieved by integrating thin metallic layers on each side of the patterned polymer film, forming Fabry-Pérot resonators. As the polymer films swell to varying widths, they display a range of colors. This method enables a single-colored sheet to exhibit diverse colorful patterns when exposed to an appropriate mix of water and solvent.
Professor Mark Brongersma, a senior author of the paper, emphasized that dynamically controlling the thickness and topography of a polymer film facilitates the production of various colors and textures. Benjamin Renz and Na Liu from the University of Stuttgart, in an accompanying article, highlighted the researchers' achievement of a photonic skin with separately controlled color and texture, mirroring natural octopus regulation mechanisms.
Independent Control and Potential Applications
By combining different films into a multilayer device, the research team achieved the ability to manipulate both color and texture simultaneously and independently. Nicholas Melosh, a professor of materials science and engineering and a senior author, noted the material's properties, including its softness, swellability, and nanoscale patterning capabilities.
Potential applications identified for this technology include:
- Enhanced dynamic camouflage for both human and robotic systems.
- Development of flexible, color-changing displays for wearable technologies and soft machines.
- Advancements in nanophotonics for electronics, encryption, and biology.
- Adaptive architectural facades that can adjust reflectivity based on environmental stimuli.
Future Directions
Future research aims to integrate a computer vision system, potentially an AI-based system, to automatically adjust the material's swelling for real-time background matching without human intervention. Beyond visual camouflage, the team is exploring additional applications such as altering friction for small robots and bioengineering uses, considering that nanoscale structures can influence cell responses. Artistic applications are also under investigation.