CONICET | Buscador de Institutos y Recursos Humanos

he stability of silica hydrogels and the possibility of tuning the synthesis within the range of biocompatibility provide optimal conditions for designing Biologically Active Materials (BAMs). Mechanical stability is an important requirement especially when BAMs are conceived for long term operation, as in the case of encapsulated vegetal cells[1]. Transport of nutrients and metabolites are of vital importance for survival of encapsulated biological entities, as well as for extraction of valuable products from metabolic processes. For measurement of transport properties on hydrogels it is necessary to know, at least, the texture and adsorption of diffusing species at the pores walls. These parameters are not easily measured, and experimental methods give empirical diffusion parameters expressed in terms of tortuosity. In this communication we present values for effective diffusion coefficient (Deff) of small anionic or cationic dyes in hydrogels prepared from silicate and preformed SiO2 nanoparticles (LUDOX® HS40) at different pH. Measurements were performed by simple experimental methods especially designed for this purpose. The diffusion profile of dyes was determined from processed images taken with a scanner. In all cases the isoelectric point of silica is below the working pH, thus for cationic dyes it was necessary to consider adsorption. The corresponding isotherms were obtained by absorptivity of the solution in equilibrium with the gel. Our results show that for pH =4.5, Deff of dyes is lower than at pH > 7, as expected for the smaller average pore diameter of more polymeric gels resulting when the synthesis is made at lower pHs.