Silk fibroin encapsulated powder reservoirs for sustained release of adenosine

Due to its unique properties, silk fibroin was studied as a biodegradable polymer vehicle for sustained, local delivery of the anticonvulsant adenosine from encapsulated reservoirs. Silk is a biologically derived protein polymer that is biocompatible, mechanically strong and degrades to non-toxic products in vivo. To achieve local, sustained, controlled adenosine release from fully degradable implants, solid adenosine powder reservoirs were coated with silk fibroin. Material properties of the silk coating including thickness, crystallinity and morphology were investigated to assess the relationships between silk coating biomaterial features and adenosine release from silk encapsulated reservoirs. Reservoir coating thickness was varied through manipulation of the silk coating solution concentration and number of coatings applied. Release studies were also performed in proteinase type XIV to model the effects of degradation. Increasing the harder to diffusion, either by increasing coating thickness or crystallinity was found to delay adenosine burst, decrease average daily release rate, and increase duration of release. In the case of encapsulated reservoirs coated with eight layers of 8% (w/v) silk, a linear release profile was observed and adenosine release was sustained for 14 days. The ability to achieve nearly constant release for 2 weeks for adenosine via control of the silk coating suggests these encapsulated reservoirs represent a novel system for delivering adenosine. We anticipate that this approach could also be extended to other implant needs and small-molecule drugs to treat a range of clinical needs. (C) 2010 Elsevier B.V. All rights reserved.