Mechanical Properties of Robust Ultrathin Silk Fibroin Films *

Biocompatible materials have received increased interest due to their combination of unique physical, chemical, and biological properties and their potential in the fields of drug delivery, medical treatments, and other biological applications. Among the natural polymers, silkworm silk fibroin has been of interest for its use in textiles but also in consumer products such as cosmetic creams, lotions, makeup, and pharmaceuticals. Silkworm silk fibroin has been used commercially as biomedical sutures for decades. The incorporation of silk into medical textiles such as artificial tendons, blood vessels, and skin grafts is attractive because the protein exhibits excellent biocompatibility in vivo. Promising results regarding this feature have been demonstrated both in vitro and in vivo. Silk fibroin from the silkworm B. mori has been dissolved and then reformulated into new materials, with the ability to control the crystalline state (beta-sheet content) and morphology, in order to modulate the mechanical properties and the rate of extent of degradation. The outstanding mechanical properties of silk fibers are characterized by high strength (ca. 3 GPa) combined with high extensibility (ca. 30 %), and good compressibility. Moreover, they are mechanically stable up to 200 °C under dynamic mechanical evaluations. With these excellent mechanical properties, silk proteins have become candidates as strong biomaterials that can be applied in various fields of controlled release and scaffolds for tissue engineering, where a combination of high strength and elasticity are often required. To fabricate uniform thin and ultrathin polymeric films spin casting and layer-by-layer (LbL) assembly are widely utilized. Spin casting represents the easier fabrication method (100– 1000 nm thick films) for ultrathin silk films while LbL assembly allows for the fabrication of ultrathin (1–100 nm) multilayered films in a step-wise manner similarly to that developed for electrostatically driven LbL. Depending on the nature of components and fabrication conditions, inter-layer interactions may be electrostatic, hydrogen bonding, van der Waals interactions, and short-range hydrophobic interactions. Stepwise deposition, an expansion of the classical LbL assembly, was demonstrated for the fabrication of silk fibroin multilayer films because of their short-range hydrophobic interactions. Although LbL assembly has become a versatile approach widely applied to the fabrication of biosensors, controlled drug delivery, superhydrophobic surfaces, fuel-cell membranes, and elec-