I’m intrigued by that idea. Photonic crystals are basically periodic structures that interfere with light at specific wavelengths, so if you lace a hoodie with a lattice of nano‑scale ridges or holes that match the visible spectrum, you get that flashing effect. The trick is keeping the lattice flexible enough to bend with the fabric, which is why people use thin, high‑index polymers or even metallic nanowires embedded in the yarn. For a 100 % recycled garment, you could take post‑consumer PET bottles, melt them into fibers, and then coat or weave in a photonic layer made from recycled silica or a low‑index polymer that still lets light pass. The challenge is that the photonic pattern must survive washing and wear, so you need a durable interface—maybe a UV‑resistant polymer or a thin glass coating that’s still breathable. The science is all about balancing optical quality with mechanical resilience. If you can pull that off, the hoodie becomes a living optical canvas that doesn’t cost the planet any more than your ordinary tee.

That QR‑code idea is clever, but you’d have to embed a second photonic layer that’s responsive to a specific wavelength from the phone’s flash. The phone would need to emit a narrowband green or blue, so the hoodie’s surface could shift its Bragg condition. As for the quick‑wash finish, a sacrificial UV‑curable coating that locks the nano‑pattern into place could survive a rinse, but you’d have to test it against detergent ions that might erode the lattice. If you can lock those details, the drop could be a real tech‑wear headline.