|Ph.D Student||Dganit David|
|Subject||Biodegradable Porous Poly(urethane-urea)s through|
Step-Growth Polymerization within High Internal
|Department||Department of Materials Science and Engineering||Supervisor||Full Professor Silverstein Michael|
|Full Thesis text|
PolyHIPEs are porous polymers that are typically synthesized within the external phases of water-in-oil (w/o) high internal phase emulsions (HIPEs), emulsions containing over 74% internal phase. PolyHIPEs have unusual highly interconnected open?pore structures, low bulk densities, and high permeabilities. The polymerization mechanism used for polyHIPE synthesis is almost always conventional free radical polymerization. Porous, biodegradable poly(urethane-urea) (PUU) polyHIPEs were successfully synthesized through the step-growth polymerization of various biodegradable polyols (polycaprolactone (PCL) and polylactides (PLA)) with various diisocyanates. In addition, polyurea (PUA) polyHIPEs were synthesized through the step-growth polymerization of a lysine-based diisocyanate. Two different synthesis routes were evaluated, one-stage reactions, where all the components were added to the HIPE, and two-stage reactions, where the PCL and PLA polyols were end-capped with isocyanates before HIPE formation. The molecular structures, the porous structures, the degradation mechanisms, and the physical, thermal, and mechanical properties of these PUU and PUA polyHIPEs were investigated . In addition, their suitability for tissue engineering applications was evaluated through the growth of mouse skeletal muscle cells.
For the one-stage PUUs, the urea content increased as the polyol/diisocyanate ratio decreased. Large "craters", on the order of millimeters, were formed through the generation of CO2, a by-product of the urea-formation reaction. The walls between the craters contained emulsion-templated voids on the order of tens of micrometers. These polyHIPEs, which underwent complete degradation within 8 days in 3 M NaOH, but only limited degradation in a phosphate-buffered saline solution, did not exhibit good cell adhesion and proliferation. The two-stage PUU polyHIPEs exhibited smaller voids than those in the one-stage polyHIPEs. The cells adhered to the polyHIPEs, underwent spontaneous proliferation and myotube formation, spread on the surface, and grew into the scaffold, filling the available space. The one-stage lysine-based polyHIPEs exhibited typical polyHIPE porous structures and very good cell attachment and proliferation.
This research pioneered the development of urethane/urea reaction protocols which can be applied to a wide variety of polyols and isocyanates. Entirely new families of porous polymers can now be generated through emulsion-templating and biodegradable polymers with tunable macromolecular structures, porous structures, and properties can be produced for tissue engineering applications. In addition, this research demonstrated that the dominating factor in the determination of the goodness of cell growth in these emulsion-templated PUUs and PUAs seems to be the nature of the porous structure.