In Vitro Behavior of Silica‐Based Xerogels Intended as Controlled Release Carriers

Abstract

We report on the in vitro behavior of room‐temperature‐prepared xerogels upon immersion in typical physiologic solutions. The effect of various parameters was studied: xerogel composition and particle size; solution composition; and immersion protocol. Three xerogel compositions were tested: a silica xerogel (S100); a Vancomycin–silica xerogel composite (S100V); and a calcium‐ and phosphorus‐containing silica xerogel (S70). The immersion was per‐formed integrally (i.e., without solution exchange) or dif‐ferentially (i.e., with solution exchange) to model the continuous flow of body fluids past implant surfaces. Four different solutions were used: tris(hydroxymethyl)amino‐methane hydrochloric acid buffer solution plus electrolytes (TE); TE supplemented with 3% H2O2; TE supplemented with 10% serum; and serum. Carbonated hydroxyapatite formed on all xerogels. The reaction was faster on S70 than on S100. The silica network fully dissolved for all xerogels in the differential experiment. S70 particles transformed to calcium‐ and phosphorus‐containing “shells” upon differ‐ential immersion. The presence of H2 O2 and serum proteins in the immersion solution slowed down the silicon dissolu‐tion of both xerogels. Xerogel dissolution was associated with decreased BET surface area and increased mean pore size. The data showed a linear relationship between the dissolution rate and the time‐dependent surface area. Re‐gardless of this relationship, our data also suggested that the rate‐limiting step of the dissolution during integral im‐mersion was the diffusion of the dissolved species to the bulk solution.