MMP pretreated particles were injected into healthy rat myocardium (A) and tissue was evaluated 1 minute, 1 hour, 1 day, 2 days, and 7 days post-injection for presence of the fluorescent aggregates (B). Rhodamine labeled, responsive particles (B, top row) were observed at the site of injection up to 7 days post-injection, while the nonresponsive particles (B, bottom row) were cleared after 1 hour. Scale bar: 1 mm (A); 50 µm (B).