Rapid Nanoimprinting of Silk Fibroin Films for Biophotonic Applications

With soft microand nanopatterned materials becoming increasingly useful for various optical, mechanical, electronic, microfluidic, and optofluidic devices, the extension of this paradigm to a pure protein-based material substrate would provide entirely new options for such devices. Silk fibroin is an appealing biopolymer for forming such devices because of its optical properties, mechanical properties, all aqueous processing, relatively easy chemical and biological functionalization, and biocompatibility. Biologically functionalized silk fibroin films can be patterned on the micro and nanoscale using a soft lithography casting technique while maintaining the biological activity of the embedded proteins. The combination of these properties could enable a new class of active optofluidic devices that merge high-quality photonic structures whose very material constituent responds, through the embedded proteins, to analytes infused through integrated microfluidics. However, the silk fibroin casting process takes 12–36 h, hindering the ability to rapidly produce multiple devices and the resulting silk structures contain artifacts due to drying and liftoff. In this communication, we will show that silk has the properties of an ideal nanoimprint resist enabling rapid device fabrication, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines nanophotonics, biopolymeric and biocompatible materials. Optofluidics, though a relatively new field, is already undergoing evolution, finding applications to an ever-increasing range of problems, including varieties of biological sensing and detection. Initially optofluidics was developed as a fusion of microfluidics and photonics to enable compact, novel optical modulation technologies. The union of optical and fluidic confining structures, however, led optofluidic devices to be applied to sensing problems especially looking toward highly parallel, sensitive and low analyte volume applications. A further development of the optofluidic paradigm, introduced here through the use of silk, is to ‘‘activate’’ the constituent material of the device to make it chemically sensitive to species flowed past it. Typically, optofluidic devices are fabricated from materials usually found in photonics or microfluidics such as silica, silicon, polydimethylsiloxane or polymethacrylmethacrylate and other polymers. These materials, while possessing suitable and well-characterized optical and material properties are not inherently chemically sensitive or specific. It is possible to functionalize the surfaces of these materials with chemical reagents, however, a much broader range of sensitivities and specificities can be achieved if proteins or enzymes are used as the sensitizing agents. The use of proteins presents an issue in itself. Binding proteins (or chemicals receptive to them) to inorganic or synthetic polymer surfaces is complex. Ideally, a material such as silk fibroin that posseses excellent optical and mechanical qualities can be formed into a variety of optofluidic geometries and maintains the activity of embedded proteins is needed for realizing active optofluidic devices. A proof of concept presented here is to build a self-sensing nanoscale imprinted optofluidic device based on imprinted silk doped with lysed red blood cells. The device can be thought of as ‘‘self-analyzing’’ in that the single optofluidic component provides both chemical and spectral analysis due to the activation of the constituent imprinted silk. Nanoimprinting is a high-throughput lithography technique in which a mold is pressed onto a thermoplastic material heated above its glass-transition temperature. The softened material conforms to the mold due to applied pressure. Sub-100 nm structures by nanoimprint lithography were first demonstrated in polymethylmethacrylate (PMMA) and now structures as small as 10 nm are routinely achieved in PMMA. An ideal nanoimprint resist combines rapid imprinting times with low temperature and pressure as well as low surface energy to aid in mold removal. As such, the mold is often coated with a low surface energy surfactant. Nanoimprinting of biopolymers presents additional challenges because of a restricted parameter space that limits the ranges of temperature and pressures usable. However, in this communication, we demonstrate that silk fibroin films exhibit many characteristics of an ideal nanoimprint resist, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines