PEG-based amphiphilic copolymers such as PLGA-PEG-PLGA (PLGA: poly(D,L
-lactide-co-glycolide) and PEG-PPO-PEG (PPO: poly(propylene oxide)) are promising candidates for biomedical applications due to the thermoresponsive sol-gel transition properties and the biocompatibility originating from the PEG block in the PEG-based amphiphilic copolymers. For the maximum utilization of the highly biocompatible PEG block, the hydrophilic PEG block should be relatively longer than the hydrophobic block. In other words, the ratio of PEG to the hydrophobic block (the PEG/hydrophobic-block ratio) should be higher. However, it is known that PEG-based copolymers with a high PEG/hydrophobic-block ratio lacked the thermoresponsive sol-gel transition properties. In fact, it was found that the aqueous solution of PLGA-PEG-PLGA with a high PEG/PLGA ratio of above 0.5 would not exhibit thermoresponsive sol-gel transitions. In this work, PLGA-PEG-PLGA with higher PEG/PLGA ratios were synthesized and laponite clay nanoparticles were added to the PLGA-PEG-PLGA solutions to realize the thermoresponsive sol-gel transition in the temperature range between 25 and 37°C . We comprehensively studied the sol-gel transition behavior by varying the molecular weights of PLGA-PEG-PLGA, the PEG/PLGA block ratios of PLGA-PEG-PLGA, and the solute concentrations . The sol-gel transition behavior was studied by the test-tube inversion method and the dynamic mechanical analysis (DMA). It was found that PLGA-PEG-PLGA with a high PEG/PLGA ratio of 3.0 could exhibit the sol-gel transition by adding 1.0 wt% of laponite. It was also found that the sol-gel transition temperature and the rheological behavior were effectively regulated by varying the molecular weight of PLGA-PEG-PLGA from 2.0 k to 6.0 k, and the laponite concentration from 0.75 wt% to 1.5 wt%. Moreover, in order to discuss the thermoresponsive sol-gel transition, PEG-PLGA diblock copolymers were also synthesized and the sol-gel transition behavior was studied.
 Naho Oyama, Hiromasa Minami, Daichi Kawano, Makoto Miyazaki, Tomoki Maeda, Kazunori Toma, Atsushi Hotta, Koji Nagahama, A nanocomposite approach to develop biodegradable thermogels exhibiting excellent cell-compatibility for injectable cell delivery, Biomaterials Science, Vol.2 (8), 1057-1062 (2014).
 Makoto Miyazaki, Tomoki Maeda, Kenji Hirashima, Naruki Kurokawa, Koji Nagahama, Atsushi Hotta, PEG-based nanocomposite hydrogel: Thermoresponsive sol-gel transition controlled by PLGA-PEG-PLGA molecular weight and solute concentration, Polymer, Vol.115, 246-254 (2017).