In-situ formation of biodegradable hydrogels by stereocomplexation of PEG-(PLLA)(8) and PEG-(PDLA)(8) star block copolymers
Eight-arm poly( ethylene glycol)-poly(L-lactide), PEG-(PLLA)(8), and poly(ethylene glycol)-poly(D-lactide), PEG(PDLA)(8), star block copolymers were synthesized by ring-opening polymerization of either L-lactide or D-lactide at room temperature in the presence of a single-site ethylzinc complex and 8-arm PEG (M-n = 21.8 x 10(3) or 43.5 x 10(3)) as a catalyst and initiator, respectively. High lactide conversions (> 95%) and well-defined copolymers with PLLA or PDLA blocks of the desired molecular weights were obtained. Star block copolymers were water-soluble when the number of lactyl units per poly(lactide) (PLA) block did not exceed 14 and 17 for PEG21800-(PLA)(8) and PEG43500-(PLA)(8), respectively. PEG-(PLA)(8) stereocomplexed hydrogels were prepared by mixing aqueous solutions with equimolar amounts of PEG-(PLLA)(8) and PEG-(PDLA)(8) in a polymer concentration range of 5-25 w/v% for PEG21800-(PLA)(8) star block copolymers and of 6-8 w/v% for PEG43500-(PLA)(8) star block copolymers. The gelation is driven by stereocomplexation of the PLLA and PDLA blocks, as confirmed by wide-angle X-ray scattering experiments. The stereocomplexed hydrogels were stable in a range from 10 to 70 degrees C, depending on their aqueous concentration and the PLA block length. Stereocomplexed hydrogels at 10 w/v% polymer concentration showed larger hydrophilic and hydrophobic domains as compared to 10 w/v% single enantiomer solutions, as determined by cryo-TEM. Correspondingly, dynamic light scattering showed that 1 w/v% solutions containing both PEG-(PLLA)(8) and PEG-(PDLA)(8) have larger "micelles" as compared to 1 w/v% single enantiomer solutions. With increasing polymer concentration and PLLA and PDLA block length, the storage modulus of the stereocomplexed hydrogels increases and the gelation time decreases. Stereocomplexed hydrogels with high storage moduli (up to 14 kPa) could be obtained at 37 degrees C in PBS. These stereocomplexed hydrogels are promising for use in biomedical applications, including drug delivery and tissue engineering, because they are biodegradable and the in-situ formation allows for easy immobilization of drugs and cells.