have done much work with bioresorbable hydrogel microspheres prepared from carboxymethylcellulose and chitosan. Chitosan (N-acetylglucosamine) is a linear polysaccharide derived from chitin after deacetylation.27 Introducing carboxymethyl groups into the chitosan produces chitosan derivatives that are readily soluble in a physiologic pH.28 The carboxymethyl chitosan can then be cross-linked with oxidized carboxymethylcellulose to form microspheres through an inverse emulsion method. These microspheres are degraded by lysozyme into glucosamine, which can be absorbed completely by the body. The degradation time appears to depend on the parameters of microsphere preparation, specifically on the degree of cross-linking density. These microspheres do not aggregate due to low coefficient of surface friction. In vivo evaluation comparing these microspheres with trisacryl gelatin microspheres has been performed with renal embolization in a rabbit model.29 The performance of the two agents was similar in terms of the mean size of the vessels occluded with the two microspheres. These microspheres were biocompatible and well tolerated, with a mild tissue reaction and no evidence of vessel wall disruption. The authors did demonstrate that preparing the resorbable microspheres with a higher cross-linking density makes the spheres more rigid, which subsequently causes a more proximal embolization. Of note, the carboxylic groups in the microsphere matrix and their highly porous internal structure may allow for loading and release of positively charged drugs such as doxorubicin hydrochloride.