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

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. 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). 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 nanoor 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 microand nanofabrication techniques. By employing this simple casting technique, high-quality films that contain a wide spectrum of nanoand 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 nanoand 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