Spider Silk as a Flexible Light Waveguide for Temperature Sensing

We propose an approach to utilize the natural spider dragline silk (SDS) as a flexible light waveguide integrated into an optic device. SDS is utilized as an air-clad biocompatible light waveguide in the fabricated multimode interferometer. Based on the evanescent field coupling theory, the light propagating in the microfiber can be coupled into SDS, and then coupled to another microfiber connected with a spectrometer. The difference in the effective refractive index of the modes propagating in the silk fiber changes in different external environments, thus the output multimode interference spectrum shifts with the change of environment. Accordingly, the multimode interferometer is encapsulated in polydimethylsiloxane (PDMS), a kind of flexible and temperature-sensitive material, which makes the interferometer have competitive temperature sensitivity. With the increase in temperature, the refractive index of PDMS decreases, and thus the output multimode interference spectrum blue shifts. The interferometer shows repeatability, reversibility and stability in temperature sensing, with a sensitivity of −1.15 nm/°C. The presented interferometer proves that the SDS is a natural independent light guide with good mechanical properties to be applied in the fabrication of integrated optic devices.