Biomedicine How Cocoons Are Turned Into Optical Devices A simple process turns cocoons into optical devices with biological applications. by Katherine Bourzac December 22, 2008 Sponsored by Preda stirs cut-up cocoons in a salt solution. The cocoons are boiled in a beaker over a hot plate to dissolve the protein that holds them together, sericin. The silk fibers, now pure fibroin protein, will be dissolved in another salt solution. Using a syringe, Preda loads the syrupy silk solution into a dialysis cartridge. The cartridge will be placed in a beaker of water, which will draw the salt out through the cartridge’s clear window. Finally, Preda will use a syringe to suck out the pure water-fibroin solution left behind, which she’ll store in the fridge. To make a hologram, a researcher deposits the water-based solution of pure silk fibroin onto a mold with a pipette. Fibroin makes a good optical material because it’s translucent when it dries, and it conforms well to both the nanoscale and macroscale details of molds like this one. After drying for several hours, silk optical devices like the hemoglobin-containing card in the researcher’s left hand can be peeled off their molds. Each iridescent square has been molded into a different device. One is a diffraction grating that can act as an oxygen sensor.