Some of the brightest and most beautiful colors we see in nature — the iridescence of peacock feathers, the changing colors of the chameleon, and the bright surface of the Morpho butterfly — are the result not of pigment, but of microstructure.

Through a process known as structural coloration, surface nanostructures cause interference effects in light, resulting in a variety of colors when viewed macroscopically.

Peacock feathers, for instance, feature long hollows, which absorb the light of certain wavelengths, disperse reflected light, and result in the bird's familiar blues and greens.

Now researchers have found a way to fabricate nanostructures that employ structural coloration.

Computer scientists from the Institute of Science and Technology Austria (IST Austria) and the King Abdullah University of Science and Technology (KAUST) have built a design tool that creates 3D-printed nanostructure templates which correspond to user-defined colors.

Rather than attempting to reproduce structures found in nature, however, the approach is a “freeform” one: The user specifies a desired color, and the computer then creates a nanostructure pattern that offers the appropriate shade.

The template’s nanostructures appear to be randomly composed and do not follow a particular pattern — an advantage, explains lead author and IST postdoc Thomas Auzinger.

“When looking at the template produced by the computer, I cannot tell by the structure alone if I see a pattern for blue or red or green,” said Auzinger. “But that means the computer is finding solutions that we, as humans, could not.”

The flexible approach to making printable templates opens up possibilities for additional coloring effects, said the author. The design tool, for example, could be used to print a square that appears red from one angle, and blue from another.

Auzinger spoke with Tech Briefs about how manufacturers can take advantage of such freeform structural-coloring capabilities.

Tech Briefs: For industry, why is structural coloration a valuable approach?

Thomas Auzinger: Structural coloration has several properties that make it highly useful for practical applications. First, the process does not require the many — potentially toxic — pigmentation chemicals that conventional colors employ; instead, it can be directly applied to the base material or a thin layer of a benign substance. Furthermore, structural coloration promises exceptional lightfastness because only structural damage can cause a discoloration.

Conventional pigment-based colors generally suffer from chemical bleaching. Thus, structural color can allow permanent and non-degrading colorization of manufactured objects.

Tech Briefs: What kinds of new applications can you envision with this type of structural-coloration approach?

Auzinger: Developing the ability to design and manufacture is an exciting goal for both artistic and industrial applications, and allow for new use cases for a wide variety of applications. Imagine street or hallway markings that indicate, through color, which way to go. The creation of color through structure also allows novel sensors that indicate, through color changes, if the underlying material is stretched or compressed.

Tech Briefs: How is structural coloration traditionally achieved in manufacturing?

Auzinger: Generally, advanced fabrication methods are employed to create structural coloration. Often, one uses either chemical methods to distribute special nanoscale particles in a base material, or one uses nanofabrication methods similar to computer chip manufacturing. Both approaches usually require expensive machinery, harmful chemicals, and a lot of expertise to handle correctly.

Tech Briefs: How does your approach differ?

Auzinger: In contrast, we use multiphoton stereolithography — a recent technological advancement that is comparatively easy to use. It works quite similarly to industrial 3D printers but at a sub-micrometer scale. Inside a liquid, a small laser focus exposes the specified structure and after washing away the non-solidified liquid, the structure is ready to use. This allows for quick testing and prototyping of structural coloration effects.

Light hits the 3D-printed nanostructures from below. After it is transmitted through, the viewer sees only green light—the remaining colors are redirected. (Image Credit: Thomas Auzinger)

Tech Briefs: In the press release, you said "our design tool is completely automatic…no extra effort is required on the part of the user.” How are these structures manufactured, and how is this more “automatic” than traditional manufacturing methods?

Auzinger: Since we use multiphoton stereolithography, we need to design structures that can be fabricated with this method. There are, for example, constraints on how thin and how high such a structure can be without collapsing. To account for this in a manual design process is difficult. Furthermore, many of the known recipes that create a certain color are not applicable to every fabrication method.

Thus, the user faces the challenge of designing a structure that fits our fabrication method but that also realizes the desired color effect. This task is completely performed by our computational design tool; the user just specifies the desired color, and our algorithm finds a suitable design. This is achieved through physical simulation and numeric optimization. Over many iterations, the structural color of a temporary structure is simulated using electromagnetic methods, and a modification of the structure is computed that improves the color. These modifications are applied to the structure step after step until an optimal design is achieved. Our system also ensures that the structure is always able to be fabricated with multiphoton stereolithography.

In conventional applications, such structures are often manually created and then adapted. We do this fully automatically.

Tech Briefs: What is most exciting to you about a growing role of computational tools in fabrication?

Auzinger: So far, most fabrication-oriented design relied heavily on human expertise and intuition. While this is undeniably extremely successful, the increasing complexity of technological advancements makes this task harder and harder. Computational design tools provide an alternative strategy, where only the ultimate design goals and the potential fabrication constraints are specified. The actual designs are then automatically discovered by simulating the underlying physical and chemical systems.

As in our method, this can lead to the creation of non-intuitive structures that are very hard to design by a person. I am especially excited to see how far this fusion of advanced algorithms and fabrication methods can take us — both in terms of technological advancement and industrial applications.

Auzinger presented the project at this year’s SIGGRAPH 2018 computer graphics conference in Vancouver, British Columbia.

What do you think? Will computational design tools lead to “non-intuitive” structures? Share your comments and questions below.