In order to develop an injectable material for drug delivery that has both formulation advantages of a sol-to-gel transition system and minimal burst release of a drug, a soft thermogel of poly(ethylene glycol)-sebacic acid polyester was synthesized. The polymer aqueous solution (25 wt%) undergoes 'clear sol-to-gel' transition as the temperature increases from 5 to 65 degrees C. The drug can be mixed in a low viscous sol state at low temperature (< 15 degrees C). In particular, the thermogel is soft enough to be injected through a 21-gauge syringe needle even as a gel state. The model hydrophilic drug, FITC-dextran (molecular weight: 40,000 Da), was released from the gel over 24 h. The biodegradable poly(ethylene glycol)sebacic acid polyester soft thermogel is believed to be promising for the hydrophilic drug delivery where an initial burst of a drug might be a concern. (c) 2006 Elsevier Ltd. All rights reserved.