Spider silk thread as a fiber optic chemical sensor

Several research teams have recently highlighted how silk can be used as an optical material. Through a regeneration process, it is possible to obtain silk films made from fibers of silk cocoons, by casting a silk fibron solution on an appropriate surface. These transparent films have been used tomake optical devices such as diffraction gratings, phase masks, and photonic crystal lattices.1 Our research actually involves amore natural approach that uses dragline silk in its pristine condition, extracted directly from the glands of a spider, to serve as a new generation of mechanically strong, biocompatible, and biodegradable optical fibers for new applications in biophotonics.2 Silica is commonly used for sensing physical parameters such as temperature, strain, and vibrations, but it is unresponsive to chemical and biological compounds. By contrast, dragline silk fiber is a protein-based biopolymer with functional side groups. Therefore, it is intrinsically sensitive to particularmolecules such as proteins and nucleic acids. Moreover, spider silk can operate at the nanoscale level with minimal energy, while occupying a very small footprint for high sensor density, accuracy, and robustness. More specifically, it contains millions of repetitive protein sequences and domains that can interact with a multitude of chemicals via its unique molecular structure, which significantly multiplies the responsivity compared with conventional chemical sensing methods. All these qualities make spider silk an excellent candidate as a photon conveyor and sensing element in a fiber optic chemical sensor system. To experiment with silk in this way, we used dragline silk from a femaleNephila edulis spider, collected by the Oxford Silk Group at the University of Oxford (see Figure 1). By reeling under controlled conditions (reeling speed, humidity, and temperature), we can spin a uniform silk thread with a smooth surface, circular cross section, and homogeneous material properties.3 Figure 1. A dragline silk thread being collected from the major ampullate gland of a Golden Orb Weaver spider (Nephila edulis). (Photograph courtesy of Oxford Silk Group.)