Described herein are apparatus, compositions, systems and associated methods to occlude structures and malformations of the vasculature with radiopaque hydrogel filaments with delayed controlled rates of expansion. Further described is a device for implantation in an animal comprising a difunctional, low molecular weight ethylenically unsaturated shapeable macromer; an ethylenically unsaturated monomer; and a radiopaque element, wherein said device contains no support members. Methods of forming such devices are also disclosed.