MIT boffins create device that 'paints' iridescent structural color in real time

From adaptive wearables to light-based signaling ideas, researchers are exploring what comes next

The feathers of a hummingbird, the wings of a butterfly, and the sparkle of an opal are all examples of nature's ability to produce structural, iridescent colors that typically require lab-grade materials and techniques to replicate. An MIT team says it has found a way to make that process far more accessible.

Naming their tech MorphoChrome, the team from MIT's Computer Science and Artificial Intelligence Laboratory (CSAIL) has created a handheld system that uses laser light to paint iridescent programmable structural colors onto commercially available holographic photopolymer film that can then be integrated into both flexible and rigid objects. 

If you're not familiar with structural color and the iridescence it creates, you might not understand why that's such a big deal, so here's a quick explanation. 

Iridescent, shifting colors found in nature and in man-made objects are the result not of pigments, but physical nanostructures that allow them to reflect certain wavelengths of light depending on the angle from which they're viewed, among other factors. Structural colors tend not to fade over time because there's no pigment involved, giving them a variety of useful applications beyond simply looking cool. 

As the CSAIL team noted, human methods of producing iridescent structural color have long been restricted to the domain of the laboratory. 

"Scientists have created structurally colored pigments and metamaterials, but these methods require advanced chemical and material complexity, limiting access to select individuals and laboratories," the team wrote in their paper. "Creative fabrication with structural color has historically been limited to using existing natural materials, such as select shells, feathers, or gemstones."

Looking back to MorphoChrome, the CSAIL team appears to have circumvented that limitation, with the added benefit of the process being real-time and achievable with equipment limited to the aforementioned handheld device and a paired laptop. 

MorphoChrome uses a 3D-printed device equipped with red, green, and blue laser diodes that are mixed in an optical prism based on the color selection a user makes in an accompanying Python app on a computer attached to the device via USB-C. The commercially available photopolymer film - the same type used to create holographic effects on passports, debit cards, and other objects - can then be "painted" using the MorphoChrome device. 

You may be thinking that structural iridescence already exists on the aforementioned objects, and you'd be correct, but those effects are typically produced as fixed designs using specialized manufacturing methods, not something users can alter or "paint" in real time.

"By adjusting color intensities in real time, users can create and modulate structurally colored brushstrokes directly by hand," the team said. "Each laser's intensity is digitally controlled through pulse-width modulation, allowing fine-grained adjustment of power and color balance." 

The MorphoChrome handheld device and the demo butterfly pendant - Click to enlarge

As shown in the example image from the paper in this story, the team experimented with a butterfly-shaped pendant that had a piece of the photopolymer attached to it with resin. 

MIT News explained that they also used the technology to modify a pair of golf gloves that would only turn the correct color - green - when a user was holding their club in the correct position, suggesting there are other practical uses for the tech beyond just making art pieces and jewelry. 

"We are interested in visually adaptive on-body wearables (i.e. passive color sensors), as our handheld system allows for customized, on-body designs," lead author Paris Myers, an MIT PhD student in the Electrical Engineering and Computer Science department and CSAIL researcher, told The Register in an email. Such passive color sensors could be used, for example, to tell if a wound dressing was swelling (coauthor Ben Miller has worked on such a project) or if there were changes in environmental conditions. 

"Additionally, inspired by how cephalopods use polarized light reflections to communicate, we are interested in expanding this technology to integrate programmable, discrete communication and sensing via optical techniques like phase encoding," Myers added. 

With such applications possible, Myers told us the next thing her team is working on is developing a reprogrammable variant of MorphoChrome, as the current version of the photopolymer film can't be reexposed to change colors. A paper on that is forthcoming, we're told. 

As for commercialization of such a technology, that's not on the table right now - but it could be. 

"We do not have current commercialization plans, but would love to collaborate with industry and government in the meantime," Myers explained. ®