Protein-protein nanoimprinting of silk fibroin films.

In this context, the glass transition temperature (Tg) of the protein is a parameter of particular relevance. In a silk film dried under ambient conditions, the water retained by the film acts as a plasticizer, significantly lowering the glass transition from 178°C to ~78°C.[26] The actual Tg depends inversely on the water content and can be modeled as a function of the fractions of silk and water in the dried construct.[27] In imprinting, the application of heat and pressure to a silk fibroin film layered on a hard mask rapidly pushes the film above Tg, causing it to transition from a glassy state to a liquid-like rubber, allowing reflow of polymer on the nanoscale.[25] In this work, we expand upon this technique by exchanging the conventionally fabricated hard silicon or glass masters with a second silk fibroin film, in a process we term protein-protein imprinting (or PiP for short). This is accomplished by control of the water content and beta-sheet crystallinity of each of the two films. Though the heat and pressure application are similar to the previous technique, using a dry, crystallized fibroin master and a wet, uncrystallized “blank” film allows for reflow of the template film only and limits the adhesion between the two layers for easy release. As an improvement to the current technique, this method significantly increases throughput, reduces dependence on the hard mask, and, additionally, allows for direct conformal imprinting on curved surfaces due to the flexibility of the nanoimprinted silk master. Nanoimprint lithography on nonplanar surfaces has been investigated recently for applications in spectroscopy,[28] superhydrophobic structures,[29] and microlens arrays,[30][31] and has shown resolution on the order of ~100 nm. Here, a wide variety of approaches have been attempted, ranging from nanoimprint lithography with flexible materials to chemical vapor deposition assisted methods. Most of these currently available techniques suffer from complex procedures or destruction of the imprint master.[32]-[34] The required photoresists and imprinted materials are often not biocompatible, limiting their use in biomedical applications.[34] Furthermore, most of those processes are relatively time consuming, with a required imprinting duration ranging from a few minutes to several hours. For example, a common conformal printing technique using thin poly(dimethylsiloxane) (PDMS) membranes usually requires ~5 min imprint times for a resolution of ~500 nm, and lacks long term repeatability due to master deformation after repeated heating for the long imprinting time.[32] Further, none of these prior approaches provides facile modes to incorporate bioactive components with retention of function, a feature that permits broader utility in medical devices and other domains. For the application of PiP to nonplanar surfaces we demonstrate resolution on the order of currently available nonplanar imprint technologies, with higher throughput and fewer concerns about master durability. In addition, due to the biocompatibility of silk, nonplanar PiP can be used to apply nanophotonic structures to curved biological surfaces.[1][3] Together, this approach provides a versatile and simple fabrication method for biointegrated silk devices, further expanding the utility of silk fibroin as a bridge between high technology and biomedical applications.