Fabrication of Silk Microneedles for Controlled-Release Drug Delivery

Microneedles are emerging as a minimally invasive drug delivery alternative to hypodermic needles. Current material systems utilized in microneedles impose constraints hindering the further development of this technology. In particular, it is difficult to preserve sensitive biochemical compounds (such as pharmaceuticals) during processing in a single microneedle system and subsequently achieve their controlled release. A possible solution involves fabricating microneedles systems from the biomaterial silk fibroin. Silk fibroin combines excellent mechanical properties, biocompatibility, biodegradability, benign processing conditions, and the ability to preserve and maintain the activity of biological compounds entrained in its material matrix. The degradation rate of silk fibroin and the diffusion rate of the entrained molecules can be controlled simply by adjusting post-processing conditions. This combination of properties makes silk an ideal choice to improve on existing issues associated with other microneedle-based drug delivery system. In this study, a fabrication method to produce silk biopolymer microstructures with the high aspect ratios and mechanical properties required to manufacture microneedle systems is reported. Room temperature and aqueous-based micromolding allows for the bulk loading of these microneedles with labile drugs. The drug release rate is decreased 5.6-fold by adjusting the post-processing conditions of the microneedles, mainly by controlling the silk protein secondary structure. The release kinetics are quantified in an in vitro collagen hydrogel model, which allows tracking of the model drug. Antibiotic loaded silk microneedles are manufactured and used to demonstrate a 10-fold reduction of bacterial density after their application. The processing strategies developed in this study can be expanded to other silk-based structural formats for drug delivery and biologicals storage applications.