Over time, veins removed from the legs (saphenous veins) and attached to the heart to replace clogged coronary arteries tend to resemble old, sluggish drain pipes. In fact, within five to 10 years of coronary artery bypass surgery, half of all vein grafts become diseased to the point of requiring re-intervention -- either repeat open-heart surgery or balloon angioplasty with the insertion of a stent. Stents can be inserted into a grafted vein or into a native coronary artery during a balloon angioplasty procedure. The balloon is used to open a plaque-clogged area, and the stent -- a tiny mesh tube -- is designed to function as a scaffold to keep the vessel open. But conventional bare-metal stents have been prone to the formation of scar tissue and subsequent re-narrowing.