Comparison of the knitted silk vascular grafts coated with fibroin sponges prepared using glycerin, poly(ethylene glycol diglycidyl ether) and poly(ethylene glycol) as porogens

Development of a small-diameter artificial vascular graft is urgent because existing materials often occlude within a short time. We have shown that small-diameter vascular graft using Bombyx mori silk fibroin is a potential candidate. Silk fibroin grafts are fabricated by coating silk fibroin on the knit tube prepared from silk fibroin fibers. However, there is a serious problem that the coated silk fibroin portion hardens when alcohol is used for insolubilization of the coated silk fibroin. This hardening prevents the desired biodegradation of the coated silk fibroin. In this study, we improved the silk fibroin coating method of the knit silk fibroin tube. Namely, the silk fibroin sponge coating was performed using glycerin, poly(ethylene glycol diglycidyl ether) or poly(ethylene glycol). In addition, silk fibroin grafts were prepared avoiding dryness during the coating process and were kept in the hydrated state until implantation into the abdominal aorta was complete. After implantation of the hydrated silk fibroin grafts, grafts were taken out at two weeks or three months, and histopathological examination was performed. The grafts coated with three types of silk fibroin sponges had a higher tissue infiltration rate than alcohol-treated grafts and were superior in the formation of smooth muscle cell and vascular endothelial cell remodeling. Biodegradations of the silk fibroin grafts prepared using the three types of silk fibroin sponge coatings and alcohol-treated silk fibroin grafts were also examined with protease XIV in vitro, and the grafts were observed by scanning electron microscopy before and 24 h after biodegradation. Faster biodegradations were observed for grafts coated with the three types of silk fibroin sponges. C-13 solid-state nuclear magnetic resonance studies showed that the conformation of the silk fibroin sponge prepared using porogen was a random coil with high mobility in the hydrated state. We believe that small-diameter silk fibroin vascular grafts coated with quick biodegradable silk fibroin sponges can be developed based on these findings.