When again placed in thrombin-containing serum after 4 h, the hydrogels continued to release t-PA at the same rates as in the first two hours. These data suggest that t-PA release may be‘‘turned on’’ (triggered) when thrombosis occurs and ‘‘turned off’’ when thrombosis stops, and that by controlling the propertiesofthegelandthet-PAloading,thefibrinolyticresponsecan be regulated with some degree of precision