Structure and Properties of Urea-Crosslinked Star Poly[(ethylene oxide)-ran-(propylene oxide)] Hydrogels Paul D. Dalton, Carolin Hostert, Krystyna Albrecht, Martin Moeller, Juergen Groll * Introduction Poly(ethylene oxide) (PEO, also called poly(ethylene glycol) or PEG for molecular weights lower than 20 kDa) is a unique and versatile synthetic polymer which has found uses in various fields and applications. [1–3] The backbone of PEO strongly hydrogen bonds with water, rendering this molecule considerably inert to biological systems. [4] Indeed, modification with PEO increases the circulation time of drugs and shields them from proteolytic enzymes, while maintaining their pharmacochemical nature. [5,6] In addition, when appropriately bound to surfaces, PEO has high protein resistance and is useful in bioengineering to reduce non-specific adsorption of proteins on surfaces. [7,8] The chemical and physical properties of PEO can be transformed through the functional group at the end of the polymer chain, providing a multitude of variations and specific chemistries such as alcohols, amines, carboxylic acids, thiols and others. Another possibility to alter the properties of PEO is control over the molecular architecture. Branched struc- tures with multiple end groups allow gelation of PEO and can utilize specific conjugation chemistry to encapsulate and gel around cells as a matrix for tissue engineering. [9–13] The defined molecular architecture of star shaped mole- cules as precursors offers the possibility to control the structure of the resulting three dimensional gels on a molecular level. The first attempts to obtain hydrogels from star shaped PEO molecules by unspecific radiation Full Paper P. D. Dalton School of Biological Sciences, University of Southampton, Bassett Cr East, Southampton SO16 7PX, UK P. D. Dalton, C. Hostert, K. Albrecht, M. Moeller, J. Groll DWI e.V. and Institute of Technical and Macromolecular Chem- istry, RWTH Aachen University, Pauwelsstraße 8, 52074 Aachen, Germany Fax: þ49 241 802 3301; E-mail: groll@dwi.rwth-aachen.de Isocyanate-terminated six armed star shaped macromers with a statistical copolymer back- bone consisting of 80% EO and 20% PO have previously demonstrated excellent protein and cell repellence as nano-layered surfaces. In this study, various macromers are mixed with water and provide a spectrum of materials that range from particles to uniform hydrogels. Due to hydrophobic end groups, 3kDa molecular weight macromers result in micro and nano- particles, while 18 kDa macromers completely dissolve and consequently uniform, transparent, high water content hydrogels are formed. Oriented channels may be induced into these hydrogels through the controlled freezing of water in the preformed hydrogel. Macromol. Biosci. 2008, 8, 923–931 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mabi.200800080 923