To advance current understanding of how crosslinking in solution affects physical properties of polymer gels, we tudied well-defined hydrogels prepared by linking end-functionalized linear poly(ethylene glycol)(PEG) to amine-terminated poly(amidoamine) (PAMAM) dendrimers in water, a good solvent. We characterized how initial preparation conditions and molecular architecture affect gelation, equilibrium swelling, and mechanical properties. Under appropriate preparation conditions, many of our gels exhibit superabsorbent behavior, an unusual observation for a gel prepared from neutral linear precursors. These highly swelling, non-ionic gels may have interesting applications as highly permeable scaffolds for cell culture.

Current work in our group involves studying the PAMAM-PEG gels as supports for microbial bioreactors. We are studying nitrogen-fixing soil bacteria (Rhizobia), aerobic microorganisms that form symbiotic relationships with legume plants. Bacteria are immobilized by encapsulation in highly defective (porous) hydrogel that must mimic their natural environment ("nodules" in legume roots). The hydrogels must allow nutrients, dissolved nitrogen, and a limited amount of dissolved oxygen to reach the bacteria, and permit continuous removal of fixed nitrogen as ammonia. This poster will summarize current efforts to modify the highly swelling PAMAM-PEG gels for use in our bioreactor system.