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    How Cocoons Are Turned Into Optical Devices

    A simple process turns cocoons into optical devices with biological applications.
    December 22, 2008
    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.

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