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Abstract
In this study, the formulation and process parameters that determine successful production and long-term stability of freeze-dried poly(lactic acid) (PLA) nanoparticles with “hairy-like” poly(ethylene oxide) (PEO) surfaces were investigated. Nanoparticles with grafted (covalently bound) PEO coatings were produced by the salting-out method from blends of PLA and PLA–PEO diblock or triblock copolymers. PLA nanoparticles with physically adsorbed PEO were also produced. The redispersibility of the nanoparticles after freeze-drying under various conditions was assessed. The surface of the nanoparticles was characterized and classified in terms of “brush” and “loop” conformations. Upon freeze-drying, it appeared that the presence of PEO at the nanoparticle surface could severely impair the redispersibility of the particles, especially in the PEO-grafted systems. This effect was shown to be related to the amount and molecular weight of PEO in the various formulations. In most cases, particle aggregation was prevented by use of trehalose as lyoprotective agent. Increasing the concentration of particles in the suspension to be freeze-dried was shown to induce much less damage to the nanoparticles, and freezing the suspension at a very low temperature (−196°C) was found to further improve the lyoprotective effect. Most of the lyoprotected nanoparticles remained stable for at least 12 weeks at 4 and −25°C. The production and preservation of freeze-dried PLA–PEO diblock and triblock copolymer nanoparticles is feasible under optimized lyoprotective conditions.