In vitro assessment of the biopersistence of vitreous fibers: state of the art from the physical-chemical point of view.

Biopersistence is a function of different parameters: low solubility of the vitreous phase in physiological media, good mechanical properties of altered fibers, limited ability of phagocytosis to digest residual fragments. This article emphasizes solubility problems. From studies related to nuclear waste storage and other industrial problems, the mechanisms (formation of a leached layer of variable thickness and structure) and the kinetic laws describing the dissolution of vitreous fibers are now fairly well known. Appropriate methods depend only on the composition of the vitreous fibers that have to be chosen to determine intrinsic dissolution rates. All other parameters influencing the dissolution rate have to be fixed radius of the fibers, composition of the saline solution) or within a convenient range (flow-rate, s/v ratio). Alternatively, physicochemical parameters may be derived from a known relation (Arrhenius plot for T, kinetic equation for pH, geometrical equation for S). In spite of their widespread use, flow-through systems, in our opinion, give less precise kinetic results than large-volume closed systems with small s/v ratios (less than 0.5 cm-1). In closed and open systems, we suggest the use of two parameters for describing the dissolution rates at 37 degrees C: the initial rate constant, vo, and the time constant, k, for a rate decreasing at variable S.