Nano‐ and Micropatterning of Optically Transparent, Mechanically Robust, Biocompatible Silk Fibroin Films

Abstract

Biopolymers, such as silk fibroin, collagen, and chitosan, are promising candidates for a variety of applications that merge the fields of biomedical optics and biomaterials. Biocompatible silk fibroin, in particular, shows promise as a biomaterial, based on a number of attributes.[1] Silk fibroin is the strongest and toughest natural fiber known and is easily formed into robust films of thermodynamically stable beta-sheets of a controllable range of thicknesses (between tens of nanometers and hundreds of micrometers). [1][2][3] These films have excellent (ca. 95%) optical transparency across the visible range and can be easily characterized and biochemically functionalized because of the all-aqueous processing, broadening their overall value. It is possible to form such silk fibroin films, having intricate 2D or 3D nano-or micropatterns, through a softlithography-based simple casting technique. This technique enables the fabrication of (at least) sub-30 nm transverse features in silk fibroin films, when cast at ambient conditions from an aqueous silk solution. The elegance of this method is in its simplicity; the fabrication of such features is completed in the absence of additional harsh chemicals, salts, or high pressures that traditionally accompany most micro-and nanofabrication techniques. By employing this simple casting technique, high-quality films that contain a wide spectrum of nano-and micropatterns can be fabricated. These films are of great consequence for use in a variety of studies based in biomedical optics.In this Communication we report on a process developed for the construction of silk fibroin-based nano-and micropatterned films. This process includes the methods for producing ultrapure silk fibroin solution, the aqueous casting process for patterning silk fibroin films, and the characterization of the smallest transverse nanopatterns realized in silk fibroin films to date.Production of the silk fibroin solution begins with the purification of harvested Bombyx mori cocoons. Sericin, a water-soluble glycoprotein that binds fibroin filaments, is removed from the fibroin strands by boiling the cocoons in a 0.02 M aqueous solution of sodium carbonate for 45 min. [4] Upon completion of this step, the remaining fibroin bundle is rinsed thoroughly in Milli-Q water and allowed to dry overnight. The dry fibroin bundle is then dissolved in a 9.3 M aqueous solution of lithium bromide at 60 8C for 4 h. The lithium bromide salt is then extracted from the solution over the course of three days, through a water-based dialysis process. The resulting solution is extracted from the dialysis cassette (Slide-a-Lyzer, Pierce, molecular weight cut-off (MWCO) 3500) and remaining particulates are removed through centrifugation and syringe-based microfiltration (5 mm pore size, Millipore Inc, Bedford, MA). This process enables the production of 8-10% w/v silk fibroin solution of excellent quality and stability. The purification step is particularly important for the generation of high-quality optical films with maxim...

Implantable, multifunctional, bioresorbable optics

“…Further, silk has been shown to be suitable for use as a material platform for sophisticated optical and opto-electronic components with features on the micro-and nanoscale (7)(8)(9). The resulting free-standing devices formed from silk are refractive or diffractive and comprise elements ranging from microlens arrays and white-light holograms to diffraction gratings and planar photonic crystals with minimum feature sizes of less than 20 nm (7,9,10). These components provide mechanically stable, high-quality optical elements that are fully degradable, biocompatible, and implantable (11).…”

Toward biomaterial-based implantable photonic devices

“…These gratings enabled sensing of proteins with sensitivity down to 5 nM by their intercalation into lipid multilayers, which lowered diffraction efficiency of the gratings. Silk has been a popular material for fabrication of optical diffractive elements [38,81,82]. Gratings are produced by pouring silk fibroin solution to a master mold, allowing it to dry, and detaching the silk layer to form a freestanding grating.…”

Rapid Nanoimprinting of Doped Silk Films for Enhanced Fluorescent Emission

“…[ 1 , 2 ] The water-based silk solution can be used as a precursor, much like polydimethylsiloxane (PDMS), in soft lithography approaches [ 3 ] and cast on any number of micro-and nano-structured molds, resulting in free-standing silk fi lms that replicate surface topographies and incorporate holograms, diffraction patterns or microlens arrays. [ 4 ] By controlling their degree of crystallinity, the fi lms can be made stable in water and used in diverse biological applications such as implantable devices or cell scaffolds. [ 5 ] In the visible wavelength range, silk fi lms have very good optical transparency [ 6 ] and have been used to make silk waveguides.…”