In vitro behavior of silica-based xerogels intended as controlled release carriers

We report on the in vitro behavior of room-temperature-prepared xerogels up on immersion in typical physiologic solutions. The effect of various parame ters was studied: xerogel composition and particle size; solution compositi on; and immersion protocol. Three xerogel compositions were tested: a silic a xerogel (S100); a Vancomycin-silica xerogel composite (S100V); and a calc ium- and phosphorus-containing silica xerogel (S70), The immersion was perf ormed integrally (i.e., without solution exchange) or differentially (i.e., with solution exchange) to model the continuous flow of body fluids past i mplant surfaces. Four different solutions were used: tris(hydroxymethyl)ami nomethane hydrochloric acid buffer solution plus electrolytes (TE); TE supp lemented with 3% H2O2; TE supplemented with 10% serum; and serum. Carbonate d hydroxyapatite formed on all xerogels. The reaction was faster on S70 tha n on S100. The silica network fully dissolved for all xerogels in the diffe rential experiment. S70 particles transformed to calcium- and phosphorus-co ntaining "shells" upon differential immersion. The presence of H2O2 and ser um proteins in the immersion solution slowed down the silicon dissolution o f both xerogels. Xerogel dissolution was associated with decreased BET surf ace area and increased mean pore size. The data showed a linear relationshi p between the dissolution rate and the time-dependent surface area. Regardl ess of this relationship, our data also suggested that the rate-limiting st ep of the dissolution during integral immersion was the diffusion of the di ssolved species to the bulk solution.