| Based on thermosensitivity, biodegradability, hydrogel properties and thermoplasticity, new injectable implant systems were developed. The aqueous solutions of the poly(ethylene glycol-b-(DL-lactic acid-co-glycolic acid)-b-ethylene glycol) (PEG-PLGA-PEG) triblock copolymers are free-flowing sols at room temperature; at body temperature a gel is formed.; The triblock copolymers form micelles in water. In the low concentration range, micellar expansion and an increase in polymer-polymer attraction were observed with increasing temperature. Core-shell structure is preserved in the sol to gel transition as well as through the gel phase. Small endothermic processes are involved in the sol to gel transition. Further increases in temperature enhance the PLGA mobility and dehydrate the PEG block followed by phase mixing between PEG and PLGA. Therefore, micellar structure seems to be broken and the system flows at the gel to sol transition temperature.; The sol-gel transition temperature and critical gel concentration (CGC) could be controlled by changing molecular parameters of the PEG-PLGA-PEG triblock copolymers such as PLGA length, PEG length, and composition of DLLA and GA. By increasing the hydrophobicity of the polymers, both sol-gel transition temperature and the CGC decreased. Water structure making salt decreases the sol-to-gel transition temperature while water structure breaking salt increases the transition temperature.; The in-vivo gel formation was confirmed by subcutaneous injection of aqueous solutions of PEG-PLGA-PEG triblock copolymers into rats. The transparent gels initially formed were degraded to opaque gels at a later stage. PEG-rich blocks were preferentially diffused out of the gel during degradation.; Two model drugs that differ in hydrophobicity, ketoprofen and spironolactone, were released from the PEG-PLGA-PEG hydrogel system. The release profiles of the hydrophilic drug showed a one-stage diffusion profile over two weeks while those of the hydrophobic drug were released over two months and the release rate could be controlled by initial polymer concentration and polymer structure.; To conclude, this system not only has many a possible applications in drug delivery but also expands the understanding of thermoreversible gelation. |
