The amount of heparin released and immobilized on PCL mats were determined by the modified toluidine blue assay[11]. Toluidine blue(TB) solution (0.005w/v%) was prepared in a mixed aqueous solution(0.01MHCl,0.2w/v%NaCl). 250μl heparin solutions and samples with 250μl PBS were separately incubated with 250μl TB solution at room temperature for 30 min to form heparin/TB complex. The solution was then collected and vigorously mixed with 500μl n-hexane.