An expandable stent having a covering which exhibits the characteristic of being normally dissolvable in blood, but upon being activated, becomes inert to blood. The stent may be placed across the neck of an aneurysm to seal the aneurysm and thereafter the portion of the covering of the stent across the neck of the aneurysm is activated to become inert to blood and the balance of the covering dissolves to permit blood to flow to adjacent vessels.