Ultrablack materials, defined as reflecting less than 0.5% of incoming light, are used in devices such as cameras, solar panels and telescopes but can appear lighter when viewed off-axis. The Cornell team targeted this limitation with a process that can be applied to wearable fabrics.
The researchers took inspiration from the magnificent riflebird, whose feathers combine melanin pigment with densely packed barbules that trap and absorb most incident light, producing an ultrablack appearance when seen head-on. At oblique angles, the bird's plumage becomes more reflective, behavior the team set out to improve upon in a textile.
Working in the Responsive Apparel Design Lab, the group treated a white merino wool knit with a polydopamine dye and then exposed the fabric to plasma etching to grow nanoscale fibrils on the fiber surfaces. These nanofibrils mimic the riflebird's hierarchical feather structures and enhance light trapping by scattering and directing light into the textile instead of allowing it to reflect away.
"From a design perspective, I think it's exciting because a lot of the ultrablack that exists isn't really as wearable as ours. And it stays ultrablack even from wider angles," said Larissa Shepherd, assistant professor in the Department of Human Centered Design and director of the Responsive Apparel Design Lab.
Shepherd, senior author of the Nature Communications paper "Ultrablack Wool Textiles Inspired by Hierarchical Avian Structure," noted that the choice of polydopamine reflects its role as a synthetic analog of melanin, the pigment associated with ultrablack coloration in birds, fish and butterflies. "Polydopamine is a synthetic melanin, and melanin is what these creatures have," she said. "And the riflebird has these really interesting hierarchical structures, the barbules, along with the melanin. So, we wanted to combine those aspects in a textile."
To ensure consistent performance, the team made sure the polydopamine dye penetrated into the wool fibers rather than remaining only on the surface, so the full cross-section of each fiber turned black. During plasma etching, some outer material was removed, leaving spiky nanofibrils that play a central role in trapping light inside the textile.
Optical measurements showed the treated wool has an average total reflectance of 0.13%, placing it firmly in the ultrablack regime and darker than previously reported fabrics. The textile maintained this ultrablack reflectance across a 120-degree angular range, appearing consistently dark from up to 60 degrees either side of a straight-on view and outperforming current commercial ultrablack products.
The researchers report that the process can be applied to other natural fibers, including silk and cotton, using methods they describe as suitable for scaling. The team has filed for patent protection on the method and is exploring commercialization through a new company focused on ultrablack textiles.
Research Report:Ultrablack Wool Textiles Inspired by Hierarchical Avian Structure
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