A vaso-occlusive device includes a first wire having a first cross-sectional geometry and a second wire having a second cross-sectional geometry, where the first cross-sectional geometry is different from the second cross-sectional geometry. The first wire may be wound to form a first coil, where the second wire is wound to form a second coil defining a lumen therein, and where the first coil is disposed at least partially in the lumen. The first wire and the second wire may be co-wound to form a single coil.
