A novel two-level microstructured poly(N-isopropylacrylamide) hydrogel for controlled release.

The aim of this work was to demonstrate that conventional poly(N-isopropylacrylamide) (PNIPAAm) hydrogels can improve their shrinkage and release properties solely due to the introduction of a heterogeneous density fluctuation-based microstructure. To this end, a novel structurally engineered PNIPAAm hydrogel was designed and compared with a chemically similar, but homogeneous, PNIPAAm hydrogel reference. For the two-step preparation PNIPAAm microgels were firstly synthesized with surface amine groups and further functionalized with polymerizable acrylate groups. In the second step the microgels, themselves acting as crosslinkers, were crosslinked to form a bulk network by inter-connecting the microgels with linear PNIPAAm chains. Although the chemical composition of the newly prepared hydrogel was generally the same as conventional PNIPAAm hydrogels (a relative control), significantly improved shrinkage properties and a more efficient "on-off" switching induced by temperature modulations were observed for the novel gel as compared with the homogeneous reference. These improved shrinkage properties were ascribed to the novel structure, which is believed to enable rapid shrinking of the small microgel crosslinkers and, thereupon, the generation of a sufficient number of diffusion channels for quick water release. Rhodamine B and ibuprofen (IBU) as model compounds were completely released from this novel gel at 20 degrees C, whereas at temperatures above the lower critical solution temperature release stopped after initial 40% and 70% "bursts" for rhodamine B and IBU, respectively, due to shrinkage of the gel network. This approach may provide an avenue to design temperature-sensitive drug delivery systems with state of the art switching properties and fast release kinetics by combining the here presented innovative strategy with complementary enhancements, such as the introduction of porosity. 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.