Advanced Biomaterials and Devices in Medicine
December 2014, Volume 1, Issue 1, pp 11-17

Release kinetics from porous xerogels determined by sol-gel synthesis, porous nanostructure and immersion

H. Qu, S. Radin, P. Ducheyne*

Center for Bioactive Materials and Tissue Engineering, Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104 USA

* Corresponding author: Prof. Paul Ducheyne, e-mail: ducheyne@seas.upenn.edu

Abstract
Room temperature processed silica sol-gel materials are bioresorbable, biocompatible, controlled release materials. The benefits of porous xerogels for the controlled delivery of drugs have been well documented. However, the relationship among synthesis conditions, nanostructure, and release kinetics has not been addressed in any significant way. This relationship needs to be thoroughly analyzed in order to control release kinetics precisely.
We address this relationship among synthesis, nanostructure and drug release kinetics of xerogels. The effects of sol-gel process parameters such as solvent (for drug introduction), pH of the sol, water to alkoxide molar ratio R, and drug payload on xerogel nanostructure and release kinetics were determined.
Using Brunauer–Emmett–Teller (BET) sorption analysis, it is shown that the above mentioned processing parameters affect the nanostructure of xerogels. Varying solvent and pH of the sol produces xerogels with different specific surface areas and pore volumes. Both the specific surface area and pore volume vary linearly with water to alkoxide ratio. The Higuchi rate coefficient as a measure for release kinetics is linearly related with variations of these parameters associated with the nanostructure. It is noteworthy that the porous xerogel appeared non-porous with negligible specific surface area and pore volume when drug was introduced. The release kinetics varied linearly with drug payload. Thus, high drug payload into xerogels results in pore filling, but the pore filling does not affect release kinetics. There are also nanostructural variations, mainly silica restructuring, arising from immersion. Release kinetics can be estimated by drug payload and the nanostructural parameters which vary with sol-gel synthesis. A general equation that takes into consideration the effects of both nanostructure and drug payload on release kinetics is formulated.

Keywords: sol-gel, controlled release, nanostructure, in vitro