Temperature-responsive biodegradable injectable polymer systems with conveniently controllable properties

Biodegradable injectable polymers (IPs) that exhibit temperature-responsive sol-gel transitions have recently drawn much attention as promising biomedical materials, such as in drug delivery systems, cell implantation, and tissue engineering. Typical examples of temperature-responsive IPs are block copolymers of poly(ethylene glycol) (PEG) and aliphatic polyesters. The use of biodegradable IP systems in clinical applications has faced several issues. In this review, after a brief introduction of IP systems, our recent progress on controllable IP systems using simple mixing strategies is introduced. Control of the gelation pH region of a dual-stimuli (both temperature and pH)-responsive IP system was achieved by changing the mixing ratio of cationic and anionic polymers. Temperature-triggered covalent (irreversible) gelation systems were developed by mixing IPs with reactive termini and appropriate cross-linker molecules. The duration of the gel state (decomposition period) of IP hydrogels was easily controlled by changing the mixing ratio of the components. The developed biodegradable IP systems with controllable properties are promising for future applications in clinical stages.