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When many people think of silk, they think of a material that can be used to make clothing garments (or other fabric-based products). However, silk can be functionalized to make a wide range of optical and waveguide devices. In this article we look at a few different areas of optics and waveguide technologies where silk-based materials have been used.
Silk has become an active research area in the medical field because of it biocompatibility. It has also emerged as a useful material for optical and biophotonic applications because of its optical transparency (visible and near-infrared), low optical loss, biocompatibility (and biodegradability) and an easy fabrication process.
Silk can also be easily functionalized to create highly ordered and tailored structures that are crucial for many waveguide applications. In addition to their structural properties, silk structures are mechanically and chemically stable and can be fabricated into nanoscale sub-micron structures.
Optical Waveguides
With respect to optics and photonics applications, optical waveguides are one of the most widely researched areas for silk-based materials; with planar waveguides being the most common waveguide to be fabricated from silk. The ability to fabricate a material into small and defined structures and to control the morphology are important factors for optical waveguides as their internal make-up is often complex.
The ease of functionalization and tailoring of silk enables it to be formed into diffractive optics such as diffraction gratings, pattern generators and lenses; all of which are used in optical waveguides. The biocompatibility of silk structures also enables biological molecules to be incorporated into the gratings, where upon the biological molecules retain their activity and can be used to enhance the optical properties of the waveguide.
While many optical waveguides produced today require etching or lithography methods (with harsh chemicals), silk waveguide structures can be formed using greener methods, such as direct ink writing and post-crystallization in methanol reservoirs, making the process more feasible for waveguides to include biological components, as the conditions are not harsh enough to break biomolecules down.
Complex waveguide structures produced from silk are known to have a similar optical loss to other fabricated silk components.
To date, silk-based optical waveguides have been used in a wide range of applications, including in biocompatible sensors, imaging, therapeutics and bio-microelectromechanical systems (MEMS) devices.
Optical Fibers
While most silk-based optical devices use silk fibroin from silkworms, Bombyx mori¸ optical fibers can be produced using spider silk from Nephila clavipes. Silk is not used as a stand-alone material for optical fibers, rather it is part of a polymer composite. These are usually photoresist polymers such as SU-8.
The incorporation of silk into polymer composites brings a lot of benefits. Alongside making the fiber more biocompatible, they also make the fibers more bioresorbable, biodegradable, flexible and provide the fiber with a greater tensile resistance.
The combination of polymers and silk makes them an ideal candidate for transmitting light when integrated into photonic chips, in biological media, in air and when the fiber is in a flexible conformation.
Optofluidic Devices
Another application of functionalized silk is in it use within optofluidic devices. For this use, the silk is purified and formed into composite structures with polymers and are known to be very stable. The matrix of silk-polymer optofluidic devices can accept other molecules into the matrix and retain them. Because of this, these devices have found great use as sensors, particularly as pH sensors.
The silk molecules can be modified with organic groups to increase the spectral-color-responsive pH sensitivity of the fibers and are often combined with polymers to form a single microfluidic channel.
These silk devices could be of great use in analytical laboratories as they enable spatial and temporal control when the liquid is delivered into the channel, and the device provides an optical response to categorize the pH. Because these silk optofluidic devices are chemically stable, they can be used over a wide pH range and for both acidic and alkaline solutions.
Sources:
- “Bioactive Silk Protein Biomaterial Systems for Optical Devices”- Lawrence B. D. et al, Biomacromolecules, 2008, DOI: 10.1021/bm701235f
- “The Investigation of the Waveguiding Properties of Silk Fibroin from the Visible to Near-Infrared Spectrum”- Prajzler V et al, Materials, 2018, DOI: 10.3390/ma11010112
- “Biocompatible Silk Printed Optical Waveguides”- Parker S. T. et al, Advanced Materials, 2009, DOI: 10.1002/adma.200801580
- “Native spider silk as a biological optical fiber.”- Huby N. et al, Applied Physics Letters, 2013, DOI: 10.1063/1.4798552
- “Functionalized-Silk-Based Active Optofluidic Devices”- Tsioris K. et al, Advanced Functional Materials, 2010, DOI: 10.1002/adfm.200902050
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