which detected the conversion of 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide(MTT, Sigma) to formazan. Briefly, 1 cm×1 cm test samples made from the membranes were sterilized by ethylene oxide vapor and immersed in RPMI 1640 medium with 10% FBS for 24 h (n = 3).