Colloid Polym Sci (2006) 284: 1121–1129 DOI 10.1007/s00396-006-1489-4 ORIGINAL CONTRIBUTION Nurettin Sahiner W. T. Godbey Gary L. McPherson Vijay T. John Received: 24 January 2006 Accepted: 21 March 2006 Published online: 9 May 2006 # Springer-Verlag 2006 Microgel, nanogel and hydrogel–hydrogel semi-IPN composites for biomedical applications: synthesis and characterization Abstract Quaternary ammonium salt hydrogels from a cationic monomer, (3-acrylamidopropyl)-trimethylam- monium chloride (APTMACl), in a variety sizes such as bulk, micro- and nano- has been prepared. The synthe- sis of micro- and nanogels were carried out in the microenvironment of water-in-oil microemulsions using two types of surfactants, namely, L-α- phosphatidylcholine (lecithin) and di- octyl sulfosuccinate sodium salt (AOT). Additionally, hydrogel–hy- drogel composite semi-interpenetrat- ing polymer networks (semi-IPN) was synthesized by dispersing previously prepared micro/nanogel into neutral monomers such as acrylamide (AAm) or 2-hydroxylethyl methcarylate (HEMA) before network formation. Hydrogel swelling and pH response behaviors have been investigated for bulk gels. Morphology, structure, and size of nano-, micro- and bulk mate- rials were explored utilizing trans- mission electron microcopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was confirmed with gel electropho- resis that completely charged nanogel form a strong complex with DNA. Keywords Nanogel . Nanocomposite . Hydrogels . Polymeric biomaterials . pH-sensitive nanohydrogel Introduction Hydrogels are an extremely important class of materials with tremendous application potential in biology and pharmaceutical sciences [1–6]. Synthetic hydrogels are polymeric networks of hydrophilic groups containing chains that are swollen in water. Polyelectrolyte hydrogels are especially useful as they either carry, or are able to develop charges on the chain. Hydrogels can be designed to exhibit significant volume changes in response to small changes in their environment such as pH, ionic strength, temperature [4, 7, 8] electric field [5], solvent [9], or magnetic field [10]. Many biological applications of polyelectrolytes are due to their ability to bind oppositely charged species to form complexes. Recently, numerous studies have utilized cationic systems for gene and antisense therapies and bile acid sequestrants [11] and have devoted to the development of viral and nonviral vectors for DNA and oligonucleotide delivery [12, 13]. Among the synthetic carriers, major attention is paid to cationic polymers which are able to do both condense large structure into smaller ones and mask the negative DNA charges, which is necessary for transfecting most types of cells [14]. A good delivery system should provide resistance to premature enzymatic degradation and aggregation, be able to target specific N. Sahiner . W. T. Godbey . V. T. John Department of Chemical & Biomolecular Engineering, Tulane University, New Orleans, LA 70118, USA G. L. McPherson Department of Chemistry, Tulane University, New Orleans, LA 70118, USA N. Sahiner (*) Materials Science and Engineering, University of Delaware, 201 DuPont Hall, Newark, DE 19716, USA e-mail: sahiner@udel.edu Tel.: +302-831-6194 Fax: +302-831-4545