**1. Introduction**

There is no uniform definition of drug delivery systems (DDSs). Generally, a drug delivery system consists of one or more drug compounds, the technology which carries out the drug(s) inside of the body (medical device or dosage form) and the drug-release mechanism [1]. According to the European Pharmacopoeia (Ph. Eur. 8.0; pg.777), conventional-release (or immediaterelease) dosage forms are preparations showing a release of the active substance(s) which

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is not deliberately modified by a special formulation design and/or manufacturing method. These forms often suffer from some drawbacks in terms of higher dose of the active pharmaceutical ingredients (APIs), lower effectiveness, toxicity and adverse side effects. Modifiedrelease drug delivery systems (MRDDS) have been developed to overcome the disadvantages of the conventional-release dosage forms. They could provide increased efficacy of the API and decreased toxicity/side effects, controlled and/or site-specific delivery, enhanced convenience, lower healthcare cost and better patient compliance.

Roughly, the drug substances suffer from two major problems: solubility and permeability. These two characteristics are responsible for the drug bioavailability upon oral administration and are the basis used to classify the APIs into four fundamental classes; a methodology known as the Biopharmaceutical Classification System (BCS), launched by Amidon and co-workers [2]. One of the most promising strategies to improve their bioavailability is based on the association of API with colloidal carriers, for example, polymeric nanoparticles (NPs), which are stable in biological environment, protective for encapsulated substances and able to modulate physicochemical characteristics, drug release and biological behaviour. Particular attention has to be paid to NPs made by synthetic polymers. These polymers possess unique properties due to their chemical structure, the type of the functional groups in the molecule, the degree of polymerization, the method of synthesis etc. [3–9]. For example, acrylates are pharmacologically inactive and due to their film-forming characteristics, these possess a good compatibility with mucosal membranes. Some of them are insoluble at physiological pH values and capable of swelling as opposed to the others, pH-responsive polymers, which are soluble only at pH 6–7. Those of them which are polycationic polymers are characterized by better mucoadhesive properties [10]. Furthermore, these are suitable vehicles of nucleic acids, oligonucleotides, DNA, peptides and proteins [11]. Aliphatic polyesters, such as poly(*ε*-caprolactone) (PCL), poly(lactic acid) (PLA) and their co-polymers, are the most exploited polymers because of their biodegradability, biocompatibility and versatility [12–14].

This chapter aims to look at the 'hot spots' in the design of synthetic polymer NPs as an intelligent drug delivery system in terms of biopharmaceutical challenges and in relation to the route of their administration: the non-invasive—oral, transdermal, transmucosal (nasal, buccal/sublingual, vaginal, rectal and ocular) and inhalation routes—and the invasive parenteral route.
