Occlusion devices and delivery systems are described in which a polymerizing oil is used to protect a hydrogel from premature swelling. In an embodiment, a delivery system includes an occlusion device releasably coupled within a catheter sheath. The occlusion device includes an expandable implant and a hydrogel. An at least partially oxidized polymerizing oil is inside a lumen of the catheter sheath between the distal end of the catheter sheath and the hydrogel.
