The elaboration of nanoparticles aqueous suspensions aimed at the drug delivery and related process development appear as difficult tasks, due to specificities related to the nanometric size. Such small sizes are required for specific applications in pharmacy. The switch of micrometric to nanometric field represents an actual challenge and cannot be considered as a simple scaling-down of chemical engineers. Ideas and concepts developed first in nanosciences have been adapted to the pharmaceutical application in drug delivery. In spite of drastic constraints due to pharmaceutical application, some parameters allow the control. A brief and not exhaustive review of the state-of-the-art on polymer particles used in the drug delivery field is presented. Attention was more particularly paid on preparation processes and their constraints by describing advantages and drawbacks of each process. The adaptation to the pharmaceutical field, the difficulties and pitfalls which are shared, with most research works in nanoscience, are illustrated thanks to our own results on nanocapsules obtained by "emulsion-diffusion" process presented as a case-study. Thanks to these results, we illustrate the peculiar features and difficulties encountered regarding nanocapsules preparation and characterization. Indeed, such a process allows to prepare nanocapsules of few hundreds nanometers diameter having an oil core surrounded by a polymeric membrane. The characterization of such soft particles colloidal suspensions is often difficult and involves heavy investigation techniques in order to highlight physical mechanisms leading to the nanocapsule properties. This is a key step regarding the final properties as a drug delivery system.

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