Implantable medical devices having double walled microspheres

A method of promoting nerve generation comprising implanting an implantable nerve guide comprising a biodegradable polymer tube having a lumen comprising:

• (a) an inner layer comprising a biodegradable polymer, into which are embedded double-walled microspheres containing glial cell-line derived neurotrophic factor, wherein a poly(lactic-co-glycolic acid) layer forms a core and a poly(lactide) layer forms a shell of the double-walled microsphere and the double-walled microspheres provide sustained release of the glial cell-line derived neurotrophic factor over at least seven days in an amount effective in promoting nerve regeneration;
• and
• (b) an outer layer comprising a biodegradable polymer without double-walled microspheres, wherein said outer layer encapsulates the inner layer.