Recently, inspired by bird feathers, American scientists have developed an “ultrablack” fabric that can absorb nearly 99.9% of light while also being washable and flexible, unlike the toxic and fragile carbon nanotube-based blackest pigment with a light absorption rate of 99.995%. This breakthrough opens up possibilities for applications in clothing, cameras, solar panels, and telescopes.
“Ultrablack” is defined as a material with a light reflectance rate lower than 0.5%. Although in 2019, MIT created the ultrablack material “VACNT” with a light absorption rate of 99.995%.
However, these colors are often difficult to produce and apply in textiles due to their toxicity, high manufacturing costs, and lack of the necessary flexibility and breathability required for textiles. Moreover, other ultrablack materials may not appear black enough at specific angles, making them unsuitable for use in textiles, cameras, solar panels, and telescopes.
Researchers from Cornell University’s College of Human Ecology, responding to the limitations, developed an industrially viable process and successfully applied the newly developed ultrablack material to clothing. The study was published in the journal “Nature” at the end of November.
Inspired by the plumage of the “Riflebird” found in the rainforests of New Guinea and northeastern Australia, the researchers studied how these birds achieve “ultrablack.” They found that the feathers of the bird have a hierarchical arrangement of small branches that curve upwards at about 30°, forming a dense tilted array. The feathers contain nanostructures with a large amount of black melanin in grooves and edges, allowing them to absorb most of the light and appear extremely black.
Using synthetic melanin called polydopamine as a dye, the researchers dyed white merino wool to create polydopamine black wool. They then subjected the fabric to plasma etching at 40 watts for 30 to 110 minutes to reveal the protein structure inside the wool, creating special spiky nanoscale growths. This structure, similar to the interior structure of Riflebird feathers, can capture a large amount of light.
The research team further mentioned that simply coating the wool with a layer of polydopamine on the surface is not dark enough. It is essential to allow the polydopamine to penetrate the fibers of the fabric, dyeing each fiber black, while plasma etching removes surface substances from the outer layer of fibers, leaving behind sharp nanofibers.
Subsequently, the research team analyzed the wool treated with ultrablack through microscopy, spectrophotometric analysis, surface characterization, and optical simulations.
After testing, it was found that wool treated for 80 minutes with plasma etching produced the best results, resulting in “ultrablack wool” with an average total visible light reflectance of only 0.13%, making it the blackest fabric to date, surpassing even Riflebird feathers in darkness. Additionally, it can maintain its “ultrablack” status within a range of 120 degrees of viewing angles.
The researchers also conducted tests on the ultrablack wool for wash fastness, light fastness, and mechanical durability. The results showed a slight decrease in light absorption rate but no significant decline in wash fastness or mechanical durability. This demonstrates that the “ultrablack wool” exhibits excellent wash fastness and mechanical durability, proving its flexibility, breathability, superiority to similar textiles on the market, and potential as wearable optical materials.
The researchers have applied for patent protection through the Cornell Technology Licensing Center (CTL) and established a company to utilize this technology on natural materials such as wool, silk, and cotton.
Last fall, Zoe Alvarez, a fashion design management student, designed a black strapless dress with rainbow blue accents near the neckline inspired by Riflebirds. The dress was embellished with ultrablack wool around the edges.
Contrast, hue, saturation, and brightness tests were conducted on photos of the dress, revealing that while other colors changed, the color of the ultrablack fabric remained deep black, resembling a “black hole”.
Assistant Professor Larissa Shepherd from Cornell University’s CHE department expressed excitement about the results from a design perspective, highlighting the durability and visual effects of the material, which maintains its ultrablack effect even at larger angles.
Shepherd further explained, “Riflebird feathers have an interesting hierarchical structure with numerous small barbs inside and black melanin, which we have applied to textiles.” Researcher Hansadi Jayamaha added, “The primary cause of the ultrablack effect is the back-and-forth reflection of light between fibers, rather than reflecting light outward.”
Dr. Kyuin Park, another researcher, pointed out the enormous potential of the newly developed technology in the field of solar heat regulation, converting absorbed light energy into heat. “In the future, this ultrablack fabric can be used to create camouflage clothing with temperature-regulating functions.”