Complex polymeric micelles with a PLA core and a mixed PEG/PNIPAM shell were prepared by self-assembly of two block copolymers: poly(ethylene glycol)-b-poly(lactic acid) (PEG-b-PLA) and poly(N-isopropylacrylamide)-b-poly(lactic acid) (PNIPAM-b-PLA). Using H-1 NMR spectroscopy and dynamic light scattering, the micellization and the enzymatic degradation status were characterized. At 25 degrees C. the PNIPAM block is hydrophilic and the PLA core is prone to the enzymatic degradation, resulting in the disassembly of the micelles. While increasing the temperature to 45 degrees C, the PNIPAM collapsed onto the PLA core, protecting the PLA core from the attack by the enzyme, and the micelles exhibit a resistance to the enzymatic degradation. Furthermore, the enzymatic degradation rate of the micelles can also be tuned by changing the ratio of PEG to PNIPAM. With increasing content of PNIPAM, the conformation of the collapsed PNIPAM changes from patchy domains to a continuous and dense layer, and the enzyme accessibility to the PLA core is changed. (C) 2012 Elsevier Ltd. All rights reserved.