A vaso-occlusive device includes an inner coil made from inner coil wire having a first diameter and formed from a first material, and an outer coil disposed at least partially around the inner coil and made from outer coil wire having a second diameter and formed from a second material. The inner coil has a first pitch and the outer coil has a second pitch. The first diameter, material, and pitch are different from the second diameter, material, and pitch, respectively. In some embodiments, the second diameter is less than the first diameter, the second material is softer than the first material, and the second pitch is more open than the first pitch. In some embodiments, the inner coil extends proximally beyond the outer coil. In one embodiment, a vaso-occlusive device includes an outer coil defining a lumen and an inner coil disposed at least partially in the lumen, wherein the vaso-occlusive device also includes an intermediate layer disposed between the inner coil and the outer coil, wherein the intermediate layer includes a biocompatible metal or a biocompatible, swellable polymer. The vaso-occlusive device may also include a biodegradable coating disposed on the outer surface of the inner coil. In one embodiment, a vaso-occlusive device includes an outer coil and a plurality of inner coils of increasing size each disposed immediately inside of the outer coil or one of the inner coils.
