But where did the color come from? It is harder to make blue pigments through biochemistry than, say, yellow or brown pigments. Blue pigments require an arrangement of high-energy electrons that is expensive for an animal or plant to produce. The excess energy of formation of a blue pigment would have to come from somewhere, which suggests that a jump to a substantially different biochemistry would be required to suddenly start making a blue pigment – an unlikely evolutionary direction.
Nature tends to reuse its abilities whenever possible, and that is the route taken by the Bastard Hogberry. On examining the microscopic structure of a typical BH fruit, the color of the fruit is found to be caused by optical interference of incoming light interacting with a highly complex nanostructure.
At the bottom left is a view about 50 microns across showing a cross-section of a single tissue cell, revealing its concentric, slightly flattened cylindrical multilayer structure. Finally, at bottom right is a 2-micron wide view showing details of the stacked layers. In this final image it becomes clear that the layers are actually bilayers – one material upon another, and that bilayer then makes up the cylindrical multilayer structure of the hard inner layer of the BH fruit.
This was the key observation. The fruit cells look like a periodic stack of bilayers from every direction. As each of the materials making up the bilayers has a different index of refraction, the multilayers form Fabry-Perot dichroic filters which reflect one characteristic color. The color of the light depends on the thickness of the bilayers and the difference of optical properties in the two materials making up the bilayers.
In the case of BH fruit, the fibers reflect blue light and transmit other colors to be absorbed in the brown underlying plant material. The color is the same from any angle around the axis of the cylinder, but varies when the viewpoint is not perpendicular to the cylinder axis. This leads to the iridescence of the blue color.
“Our new fiber is based on a structure we found in nature, and through clever engineering we’ve taken its capabilities a step further,” says lead author Mathias Kolle, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS). “The plant, of course, cannot change color. By combining its structure with an elastic material, however, we’ve created an artificial version that passes through a full rainbow of colors as it’s stretched.”
To make the artificial color-changing photonic fibers, the members of the Harvard-Exeter team mimicked the optical structure of the fruit cells using an novel roll-up mechanism perfected in the Harvard laboratories.
As one of the new fibers is stretched, the bilayers become thinner, so to keep the optical conditions that produce the fiber color the same, the reflected color will shift to having a shorter wavelength. If an unstretched fiber has a red color, stretching the fiber will change its color to orange, yellow, green, and then finally blue as seen above. In that example, the wavelength becomes about 35 percent shorter as the fiber's length is doubled by stretching.
Other optical effects occur in the new fibers, based on the same principle. For example, if you apply pressure perpendicular to the fibers to compress them, two things happen. One is that the bilayers that are being squeezed become thinner, so their color in the direction of the pressure moves toward the blue. On the sides, however, the bilayers actually become a little thicker, which moves the color perpendicular to the pressure toward the red.
So, pressure causes the fiber colors to shift with the viewing angle, an effect that could be fascinating in a "smart cloth" made of these photonic fibers. The color will also change with the temperature of the fiber, due to thermal expansion. Higher temperatures cause a shift to redder colors.
“Our fiber-rolling technique allows the use of a wide range of materials, especially elastic ones, with the color-tuning range exceeding by an order of magnitude anything that has been reported for thermally drawn fibers,” says coauthor Joanna Aizenberg, Amy Smith Berylson Professor of Materials Science at Harvard SEAS.
Athletic spandex that change color as you move? Superhero costumes whose color changes to tell you who is winning the latest battle? Adaptive camouflage that can change patterns by controlling a heating grid within the fabric? Or (more likely), packaging materials whose color tells you if the wrappings are about to burst? There are a wide range of applications for this new class of photonic fibers just awaiting further development and commercialization. I can't wait!
Source: Harvard University