Synthesis of Poly(ethylene glycol)-based Hydrogels via Amine-Michael Type Addition with Tunable Stiffness and Postgelation Chemical Functionality

The synthesis of a novel PEG-based hydrogel formed by amine-Michael type addition is reported. Star-shaped PEG molecules (having 8 arms with acrylate end groups; 8PEG) are utilized as macromonomer, and NH3 in ammonia solution (30%) is used as cross-linker, a small and volatile molecule, which unreacted remains can be easily removed from the gel matrix. A distinct relationship between hydrogel structure and properties has been obtained, that is, higher amounts of NH3 in the reaction lead to higher cross-linking density, higher bulk, and surface elasticity and smoother surface morphologies. Moreover, it is demonstrated that the incomplete amine-Michael type addition chemistry leads to gel formation with plenty of residual acrylate groups that are verified by LC-MS analysis and Raman spectra. Those residual acrylate groups enable us on the one hand to (bio)functionalize the gels, for example, via a second Michael-type addition reaction, employing thiol-reactive (bio)functional molecules and/or to perform additional UV-curing (photoinitiated cross-linking) to further stabilize the preformed gels with higher mechanical integrity. The proof of principle of the postgelation reactivity is demonstrated by fabricating nanoscale precise 3D patterns and by using a thiolated fluorescent dye, which reacts at the surface of the cross-linked gel and shows a small but clear penetration profile of around SO pm from the surface into the bulk of the reactive gel.