Thermo-responsive hydrogels based on highly branched poly(ethylene glycol)-poly(l-lactide) copolymers

Highly branched poly(ethylene glycol)-poly(l-lactide) (PEG-PLLA) copolymers were synthesized by a coupling reaction of eight-arm amine-functionalized PEG and macromonomers having 2 PLLA arms and a N-hydroxysuccinimide activated ester group at the branching point. A coupling efficiency of approximately 50% was achieved. Copolymer solutions in water (⩾4 wt.%) showed a phase transition from gel to sol upon increasing the temperature. The critical gelation concentration at 20 °C decreased with increasing PLLA block length. The gel-sol transition of a 12 wt.% aqueous copolymer solution increased from 4 °C for a copolymer with PLLA blocks with 3 repeating lactide units per arm to 38 °C for a copolymer with PLLA blocks with 5 repeating lactide units per arm. Furthermore, the gel-sol transition temperature increased with increasing copolymer concentration, and could be tuned closely to body temperature, which makes these hydrogels interesting as injectable systems for in situ gel formation.