The major protein in silkworm silk is being used by Peter Domachuk and his colleagues at the University of Sydney as a platform for sophisticated new health tests.
The protein, fibroin, is extremely strong and so bio-friendly that it allows long-term studies of the interactions of molecules which, until now, have been too sensitive to handle in the laboratory outside of cells. Fibroin is also transparent, and can be spun into structures that manipulate light.
Peter already has sensors for oxygen, and is planning a system that allows dozens of simultaneous blood tests.
Biopolymer Photonics: Unnatural Uses for Natural Materials
School of Physics, University of Sydney, NSW 2006, Australia
The use of silk fibroin as a biophotonic material will be presented.
Silk’s properties will be discussed in the context as a photonic material, as well as demonstrations of silk-based photonics components.
Silk fibroin is a naturally occurring protein polymer (or biopolymer) , composed of s ome 5000 peptides, found in conjunction with thebinding protein sericin in structural fibres spun by spiders and silk-moth pupae . Recent advances have allowed silk fibroin protein to be efficiently and reliably extracted from the cocoon of the Bombyx mori silk worm . This extraction is straightforward: silk-worm cocoons are boiled in water and various salt solutions then dialysed. The extraction process is entirely aqueous and, compared to other optical materials, almost entirely natural, non toxic and environmentally friendly. Once the fibroin protein is in aqueous solution it can be spin coated and air-dried into a thin film  with thicknesses ranging from tens of nm to microns . Once the films are dried they can be fortified by dehydration using organic solvents (ethanol, methanol or acetone). This process, as well as removing water, induces a change in the secondary structure of the fibroin, from an alphahelix to beta-sheets . This change of conformation renders the silk film stiffer and significantly less soluble in water.
Silk fibroin films have material and optical properties that, in total combination, are unique to the materi al. Mechanically, fibroin films have very high tensile strength, over 100 MPa, and can be made very fl at, with RMS surface roughness o f under 0.5 nm . Silk films may be cast, or imprinted , to replicate structures with a resolution of 20 nm, such as holographic diffraction gratings . The imprinting process is of particular interest in the fabrication of photonics. Provided a high quality mould is available, a dry silk film will adopt its features with high fidelity using only pressure generated directly by human hands at room temperature and pressure . It is possible to imagine this method allowing the massproduction of high-quality optical structures given a mould of sufficient area.
The silk fibroin films are transparent and clear, with optical transmission of over 95% in the visible and over 80% for most of the near infrared , aside from regions of water absorption. Silk fibroin’s refractive index is 1.54, close to that of silica glass, siloxane polymers and other typical photonic materials . Silk fibroin also possesses the unique ability to preserve the activity of bio-molecules, such as proteins  and complex reagents , far beyond their typical life in aqueous solution in lab conditions. Tests are currently being performed on glucose oxidase, whose activity in-vitro is difficult to maintain, whereby the silk fibroin matrix preserves the activity of the enzyme for six months thus far. These bio-molecules require no additional chemistry for preservation, other than to be mixed into the silk solution prior to processing. Silk fibroin enables active, bio-molecule doped, high transparency optical structures can be made based around silk fibroin. This unique combination of properties in one material makes silk fibroin protein exemplary for realising highly integrated and multifunctional optofluidic chips.
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Peter Domachuk, firstname.lastname@example.org