Natural microshells of alginateechitosan: Unexpected stability and permeability Xia Tao a, * , Xue-Jun Sun c , Jingmei Su a , Jian-Feng Chen b, * , Wilson Roa c a Key Lab for Nanomaterials, Ministry of Education, Bei San Huan East Road 15, BUCT, Beijing 100029, PR China b Research Center of the Ministry of Education for High Gravity Engineering & Technology, BUCT, Beijing 100029, PR China c Cross Cancer Institute, University of Alberta, Canada T6G 1Z2 Received 19 April 2006; received in revised form 6 June 2006; accepted 11 June 2006 Available online 7 July 2006 Abstract We utilized alginate sodium (ALG) and chitosan (CHI) as wall components to construct a natural and biodegradable polyelectrolyte micro- shell by the electrostatic self-assembly technique. The resultant ALGeCHI shells were found to maintain intact spherical shape in different poly(styrenesulfonate sodium) (PSS) bulk concentrations (maximal detection concentration 20.0 wt%). Compared with the shells composed of PSS and poly(allylamine hydrochloride) (PAH) that collapse or deform at a 4 wt% or more than 4 wt% PSS concentration, the ALGe CHI shells display higher stability. Meanwhile, the permeability of the ALGeCHI shells prior to and after incubation in PSS solutions was directly traced by the fluorescence recovery after photobleaching (FRAP). The composition of the ALGeCHI shells themselves is thought to be responsible for the differences in the stability. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Alginate sodium; Chitosan; Permeability 1. Introduction Layer-by-layer (LBL) self-assembly [1] of oppositely charged polyelectrolytes onto dissolvable colloidal particles has been utilized to create ultrathin microshells (micro- capsules) with particular advantages, e.g., the tailored wall thickness on a nanometer-scale range, the ordered wall composition, as well as the controlled size and shape [2]. Both the preparation and behavior of such shells primarily composed of synthetic polyelectrolytes such as poly(styrene- sulfonate sodium) (PSS) and poly(allylamine hydrochloride) (PAH) have been reported in much detail and reviewed [3]. For practical and biomedical applications, the use of polysac- charides instead of synthetic polyelectrolytes seems an attrac- tive alternative. Natural polysaccharides such as chitosan (CHI) and algi- nate sodium (ALG) have been widely investigated for applica- tions in coating membranes, controlled-release drug delivery and biomaterials [4e6]. CHI is a natural cationic polymer de- rived from chitin N-deacetylation. ALG is an anionic polymer composed of a naturally occurring block copolymer of two monosaccharide units obtained from marine brown algae. The ALGeCHI polyelectrolyte complex (PEC) systems are commonly developed as a complex planar membrane and an ALG gel bead is coated with CHI [7e9], which places some limitations in controlling the membrane thickness on a nano- meter-scale and characterizing the encapsulating PEC mem- brane precisely. Recently, we have constructed a natural and biocompatible polyelectrolyte microshell by using ALG and CHI as building blocks with the LBL self-assembly technique and investigated the photostability of the shells during the oxidative reaction, together with the accumulation ability of organic substances in aqueous solutions [10]. In the present study, the properties of the ALGeCHI shells were further studied and compared with the typical * Corresponding authors. Tel.: þ86 10 64448472; fax: þ86 10 64434784. E-mail addresses: taoxia@yahoo.com (X. Tao), chenjf@mail.buct.edu.cn (J.-F. Chen). 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.06.038 Polymer 47 (2006) 6167e6171 www.elsevier.com/locate/polymer