Carbohydrate Polymers 84 (2011) 1011–1018 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Preparation of biodegradable nanoporous microspherical aerogel based on alginate M. Alnaief ∗ , M.A. Alzaitoun, C.A. García-González, I. Smirnova Hamburg University of Technology, Thermal Separation Processes, Eißendorfer Straße 38, D-21073 Hamburg, Germany article info Article history: Received 4 November 2010 Received in revised form 16 December 2010 Accepted 19 December 2010 Available online 25 December 2010 Keywords: Spherical aerogel microparticles Biodegradable nanoporous materials Supercritical CO2 extraction Polysaccharides Alginate aerogels abstract Supercritical extraction of the solvent from organic gels, like polysaccharides, enables the production of highly porous biodegradable aerogel with a high surface area. The structural properties of the pro- duced aerogel depend mainly on the preparation methods and the composition of the gel phase. This work presents a new method to produce biodegradable microspherical alginate aerogels particles using an emulsion technique. Water in oil (W/O) emulsion was produced by mixing a Na–alginate solution (dispersed phase) with oil (continuous phase) followed by cross-linking the dispersed phase to form the gel particles. The gelation parameters as well as the emulsion process parameters were investigated in order to control the form and the structural properties of the produced alginate aerogel. Alginate aerogel microspherical particles with a high surface area up to 680 m 2 /g and relatively large pore volume up to 4.0 cm 3 /g and different mean particle diameters ranging from 25 m to few hundred microns were produced using the presented method. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Silica aerogels are regarded as promising drug delivery sys- tems (Patel, Purohit, & Suthar, 2009; Smirnova, Mamic, & Arlt, 2003; Smirnova, Suttiruengwong, Seiler, & Arlt, 2004; Smirnova, Turk, Wischumerski, & Wahl, 2005). Due to their large specific surface area, high drug loadings can be reached. The drug release kinetics show remarkable characteristics that can be tailored by adjusting the functional groups on the aerogel surface, based on the affinity of each specific drug (Alnaief & Smirnova, 2010; Gorle, Smirnova, & Arlt, 2009). However, for some applications in the pharmaceutical industry biodegradability is a limiting fac- tor. For those applications silica aerogels cannot be used since they are biocompatible, but not biodegradable. This demand can be met using matrices made out of organic materials or biodegradable polymers like polysaccharides (Dumitriu, 2005; Walter, 1998). Polysaccharides are one of the most abundant renewable resources on the earth. Due to their physiological com- patibility, polysaccharides are quite common additives in food and drug formulations (Qiu, 2009). The combination of the out- standing structural properties of aerogels with the biocompatibility and biodegradability of polysaccharides would result in high potential drug delivery systems (Mehling, Smirnova, Guenther, ∗ Corresponding author. Tel.: +49 40 42878 4392; fax: +49 40 42878 4072. E-mail address: mohammad.alnaief@tu-harburg.de (M. Alnaief). & Neubert, 2009; Robitzer, David, Rochas, Di Renzo, & Quignard, 2008). Alginate is a natural polysaccharide polymer, found in great abundance in brown seaweeds. Because of non-toxicity, biodegrad- ability and accessibility, alginate has been used in food industry, pharmaceutics and medicine (Domb & Kost, 1997; Dumitriu, 2005; Phillips & Williams, 2000; Walter, 1998). Alginate consists of a lin- ear copolymer composed of 1,4-linked--d-mannuronic acid (M) and -l-guluronic acid (G) residues of varying composition and sequence (Fig. 1). The presence of the carboxylate group within G blocks rings bears a global negative charge at pH 7 usually com- pensated by sodium cations. Adding divalent ions like Ca 2+ induces the cross-linking of the polymer and thus the formation of a gel (Rehm, 2009). This property was explained by the so-called “egg- box” model, Fig. 1, suggesting a possible binding site for Ca 2+ in a single alginate chain (Phillips & Williams, 2000; Rehm, 2009). Gelling of alginate depends mainly on the strength, number and length of cross-linking; hence, the composition and the sequence of G and M residues are the main properties of alginate, which influence the mechanical properties of the produced gel (Draget, Østgaard, & Smidsrød, 1989; Dumitriu, 2005; Rehm, 2009). Gen- erally, two fundamental methods are used to induce gelation of the alginate solution: (1) the diffusion method; (2) the internal setting method. In case of the diffusion method the cross-linking ion diffuses from a large reservoir into an alginate solution (Trens, Valentin, & Quignard, 2007; Valentin, Molvinger, Quignard, & Di Renzo, 2005; Valentin et al., 2006). Internal setting method dif- fers from the former one by control release of the cross-linking 0144-8617/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2010.12.060