A vaso-occlusive device includes a first coil made from a first substantially non-expansile material, and a second coil at least partially co-wound with the first coil and made from a second expansile material. The second coil has a contracted state and an expanded state, in which the second coil expands radially and/or longitudinally. The vaso-occlusive device has increased stiffness when the second coil is in its expanded state.
