## 6 PLGA Nanoparticles Functionalization
### BBB Transcytosis
#### 6.2.4 Adsorption-Mediated Transcytosis  
In addition to particle size, surface charge (i.e., zeta potential) is also expected to affect NP cellular uptake and cytotoxicity. Therefore, nanoparticles with a positive surface charge are more suitable for cellular uptake. The surface of CNS endothelial cells shows a negative charge and thus, attracts positively charged nanoparticles to interact with the BBB through adsorption-mediated endocytosis [181]. In fact, while receptor-mediated transcytosis requires the initial binding of a ligand to the membrane of the BBB endothelial cells, absorptive-mediated transcytosis relies on nonspecific charge-based interactions [100]. Consequently, poor selectivity of absorptive-mediated transcytosis is predominant, since it can be easily initiated by polycationic compounds binding the negative charges onto the membrane. Cationic surfactant coating is believed to render a positive charge to NPs, thus improving their interaction with cells and tissues [97]. For instance, dicoctadecyldimethylammonium bromide (DODAB), a quaternary ammonium surfactant, can drastically alter the negative charge of PLGA NPs by preferential adsorption [182]. In the same way, didode-cyldimethylammonium bromide (DMAB), another quaternary ammonium compound, is also used for nanoparticle stabilization. [PERSON] et al. [183] reported that DMAB enabled the interaction of NPs with a cell membrane model in a proportional fashion to their cellular uptake in vitro. Therefore, [PERSON] et al. [103] prepared DMAB-PLGA NPs loaded with coumarin-6 and observed that surface modification with DMAB notably improved cellular uptake in vitro, which was size-dependent with an optimal particle size of 100 nm. Indeed, DMAB-modified NPs showed smaller sizes and higher zeta potentials than the PVA-coated NPs, meaning that the DMAB-coated nanoparticles could be entrapped by the cells more easily, increasing cellular uptake. In addition to cationic surfactants, cationic polymers such as chitosan (CS) can be employed. As previously mentioned, CS is a natural cationic polymer which has been demonstrated to promote AMT, thereby enhancing NP cellular uptake. Indeed, [PERSON] et al. [102] investigated CS as a surface-modifying agent to improve PLGA NPs brain delivery. NP surface modification with CS increased NP concentrations in the brain compared to un modified NPs. Moreover, CS-PLGA NPs were absorbed onto the cerebral blood vessel by adhesion to endothelial cells by means of electrostatic interaction with cell membranes, thereby enhancing cellular uptake of CS-PLGA NPs, whereas unabsorbed particles were eliminated rapidly from blood circulation due to uptake by the RES. Therefore, PLGA NPs surface-modified with CS may play an interesting role for CNS drug delivery. Trimethylated chitosan (TMC) is a quaternized CS derivative that is positively charged under physiological conditions [184]. As a cationic ligand, TMC facilitates NP active transport via absorptive-mediated transcytosis. Thus, TMC-modified NPs can be exploited for delivery to the brain [185; 186]. [PERSON] et al. [100] prepared PLGA-NPs and then employed covalent binding to attach TMC to the surface of PLGA-NPs and form TMC/PLGA-NPs. The obtained particles showed a mean diameter of 150 nm and were distributed in the periventricular region of the cortex and the third ventricle extensively, while no brain uptake of unmodified PLGA-NPs was seen, showing that positively charged TMC contributed to the electrostatic interaction with the anionic binding sites of the bran capillaries. This triggered the absorptive-mediated transcytosis pathway, followed by the uptake of NPs through the BBB, reaching the brain parenchyma. The increased cellular uptake and transport into the brain after surface modification with TMC was further demonstrated by [PERSON] et al. [108]. Moreover, TMC formed a hydrophilic surrounding, which also contributed to this enhancement and avoided uptake by the mononuclear phagocytic system.