Every year, 800,000 Americans elect to have a tiny metal-mesh tube inserted into their coronary artery to prop it open and improve blood flow to cardiac muscle tissue. It's an easy choice — the alternative entails cracking open the chest and operating on a stopped heart. The tube, or stent, is permanent, but the vessel hardens over it within months. After that, it becomes a nuisance. The metal blocks x-rays and MRI scans, and it can catch blood cells and form a dangerous clot. Now medical-equipment manufacturer Abbot Laboratories has developed a stent that opens the artery and then simply disintegrates.