INVESTIGATION OF THE BIODURABILITY OF WOLLASTONITE AND XONOTLITE

The in vivo durability of wollastonite materials, coated and uncoated, and of xonotlite was tested. Wollastonite is an anhydrous natural sil icate and xonotlite is a hydrated synthetic calcium silicate. UICC cro cidolite was used as a positive control with high durability. Using a dry-sizing technique, fractions from the stock materials were prepared according to the definition of ''thoracic particulate mass'' and ''re spirable particulate mass'' of the American Conference of Governmental Industrial Hygienists. Fibers were instilled intratracheally into fem ale Wistar rats, and the evenness of their distribution in the lung wa s checked by scanning electron microscopy (SEM). After serial sacrific es at 2 and 14 days, 1, 3, and 6 months, and low temperature ashing of the lung, the fibers were analyzed by SEM. The number and size distri bution of fibers were investigated. The total number of crocidolite fi bers decreased with a half-time of 240 days, but the number of fibers >5 mu m in length was unchanged after 6 months. The elimination kineti cs of wollastonite fibers from the lung were relatively fast, with hal f-limes of 15 to 21 days. The coating of wollastonite in Wollastocoat had no effect on this elimination process. For the thoracic fraction o f wollastonite, the elimination from the lung was as fast as for the r espirable particulate fraction. The elimination kinetics of xonotlite from the lung was Very fast. This material consisted of single crystal s of acicular morphology with a median length of 1.3 mu m and of agglo merates of these crystals. More than 99% of single crystals and about 85 to 89% of the agglomerates were already eliminated 2 days after ins tillation.