## 6 PLGA Nanoparticles Functionalization
### Pretranscytosis Circulation
#### 6.1.2 Hydrophilic Surface  
Due to their hydrophobic nature, PLGA NPs are recognized as foreign substances by the cells of the mononuclear phagocyte system, which are responsible for the clearance of particles from the blood stream [5]. As a result, PLGA NPs experience high rates of opsonization and thus their accumulation in target cells and tissues is reduced. Therefore, in order to reduce PLGA NPs immunogenicity, several strategies to create a hydrophilic surroundings of these NPs have been developed [97]. Examples of PLGA NPs functionalization to provide such hydrophilic coatings include:  
* Polyethylene glycol (PEG): in 2007, [PERSON] et al. [119] demonstrated that PEG-based NPs present a tightly packed composite for a longer circulation time in plasma. * Human serum albumin (HSA): which is the most abundant native protein in the human body and shows several advantages including high availability, biodegradability, and low toxicity and immunogenicity [120]. In 2015, [PERSON] et al. [121] decorated PLGA NPs with HSA, which resulted in a longer blood circulation of NPs. * Polyethylene oxide (PEO): in contact with an aqueous environment, highly hydrated and flexible PEO chains form dense \"conformational clouds\" over the particle surface, impairing interactions with approaching opsonins as well as phagocytic cells [122]. In 1999, [PERSON] et al. [123] demonstrated a clear relationship between PEO content and the decrease of uptake by the MPS cells. * Poloxamers and poloxamines, also known as Pluronic(r) and Tetronic(r) macromolecules, respectively [124]: they strongly adsorb onto the surface of hydrophobic nanospheres, protecting them from quick engulfing by macrophages [125]. * Polysorbate 80, d-\(\alpha\)-tocopherol PEG 1000 succinate (TPGS) and polysaccharides like dextran [74; 126]. PEG is the most commonly used polymer used to create a hydrophilic shell surrounding NPs by grafting the PEG chains onto the surface of NPs and thus hindering biomacromolecules from penetrating into the polymer layer by stearic stabilization, and by binding to the underlying core via hydrophobic or electrostatic interactions [127]. As a result, PEG coatings on NPs increase the hydrophilicity and solubility of the formulation, shielding the surface from aggregation, opsonization and phagocytosis by the reticuloendothelial system, thereby prolonging blood circulation time [128]. Indeed, [PERSON] et al. [110] developed mementine-PEG-PLGA NPs formulation for the treatment of AD. The resulting NPs had a mean particle size below 200 nm and were able to overcome the BBB both in vitro and in vivo, reaching the hippocampus and increasing drug concentration at the target site, and thereby enhancing mementine's effects against AD. This is consistent with what other authors observed when PEGylating polymeric NPs. For instance, [PERSON] et al. [129] also developed PEGylated NPs with an average size smaller than 200 nm and demonstrated that they were able to effectively overcome the BBB. Moreover, [PERSON] et al. [130] prepared PEGylated NPs with a mean size of about 160 nm and demonstrated that PEG entrapment provided additional protection against enzyme-induced aggregation and degradation in simulated gastrointestinal fluids in vitro. In addition, mice treated with these NPs revealed a decrease in memory impairment when compared to mice treated with the free drug solution. Moreover, histological studies confirmed that mementine-PEG-PLGA NPs reduced \(\beta\)-amyloid plaques and the associated inflammation characteristic of AD. Therefore, the loading of mementine into these particles could be a promising alternative to improve the treatment of AD patients [110]. To escape from interaction with serum proteins and with mononuclear phagocyte system cells, the PEG chains surrounding nanoparticles should provide a sufficiently thick layer for steric hindrance. As the molecular weight of grafted PEG chains is directly proportional to the length of the polymeric chain, that parameter is instrumental for effective surface shielding [128].