*6.2. Role of Particle Surface Charge and Wettability on Their Interactions with Lung Surfactant Films*

The ability of particles to interact with biological structures is strongly dependent on the particle surface charge and their hydrophilic-lipophilic balance, i.e., wettability. This is understood considering that both parameters present a very critical role in how particles are incorporated within biological interfaces and interact with biomolecules, affecting metabolic pathways of the LS and their biophysical function [65,143,221]. In particular, it has been found that charged polystyrene particles lead to a stronger inflammatory response in lung than neutral ones [222], and hence it may be expected that electrostatic interactions present a prominent role in the modification of the respiratory physiology [200,212,223]. This can be ascribed to the higher strength of electrostatic interactions in relation to other forces [224].

The incorporation of negatively charged hydrophilic silicon dioxide particles into DPPC monolayers can occur through their electrostatic interaction with the ammonium terminal group of the lipid, allowing a modification of the dipolar moment of the DPPC molecules at the interface which in turn modifies its ability for reorienting at the water/vapor interface and the molecular packing. This hinders the formation of ordered phases, and reduces the rigidity of the film. On the other side, hydrophilic silicon particles undergo an effective clearance from the interface at the highest values of the surface pressure, suggesting the formation of a lipid corona on the particle surface [136]. Similar conclusions were found by Farnoud and Fiegel [55] when analyzing the interaction of negatively charged polystyrene carboxylate with DPPC films. Furthermore, they also reported that the incorporation of charged particles into lipid monolayers affects the size of the compression-expansion hysteresis loop, which plays a critical role in the exchange of material between the interface and the adjacent subphase, affecting to the compositional remodeling of LS layers, and in turn, to its normal physiological function.

The inhibitory character of the LS function associated with its interaction with charge particles was also evidenced by molecular dynamic simulations [225]. It is worth mentioning that the specific nature of the particle charge does not present a significant effect for the interaction of monolayers formed by zwitterionic lipids such as DPPC. However, it may be expected that the effect of the cationic and anionic particles on true LS becomes more complex due to the presence of lipids and proteins with different positively and negatively charged moieties.

Negatively charged polylactide particles induce a strong inhibitory effect of the behavior of bovine LS extract (Curosurf) as a result of the association between the particles and the SP-B protein. Furthermore, the interaction of particles with SP-C protein also has a very important influence in the inhibitory effect of the negatively charged polylactide particles. However, the interaction of positively charged particles with the LS model results in a reduced inhibitory effect which confirms the importance of the charge nature on their inhibitory character [19,103,226]. This agrees with the different scenario found upon the interaction of positively and negatively charged aluminum oxide, silicon dioxide, and latex nanoparticles, with LS layers. Thus, negatively charged particles present a negligible impact on the LS function, whereas positive charged ones lead to the formation of aggregates with LS vesicles. This interaction, which occurs with an LS having an average negative charge, is enhanced as the charge density of the particles is increased [208]. The different impact of charged particles on the interfacial properties of lipid mixtures and LS formulations containing proteins was independently demonstrated by Behyan et al. [189] in their study about the interaction of positively and negatively charged silicon dioxide particles with different LS models, a DPPC/POPG mixture, and an extract from calf lung (Infasurf). They found that whereas the behavior of Infasurf was only modified by the cationic particles, that of the natural extract was influenced by both, positively and negatively charged particles.

The particle wettability also emerges as a very important parameter influencing the particle interaction with LS films [200]. Thus, the incorporation of hydrophobic particles into DPPC layers modifies lateral packing of molecules at the interface and the cohesion interactions between the lipids at the interface, which results in the inhibition of the ability of DPPC for reducing the surface tension of the interface, and the reduction of the film rigidity. Furthermore, hydrophobic particles do not undergo an effective clearance upon the compression of the interface [118,133]. On the contrary, hydrophilic particles undergo an effective clearance from the interface upon compression [200]. The above picture is in agreement with the work by Zhang et al. [214] on the interactions of hydrophobic gold particles and DPPC monolayers, in which the particle hydrophobicity was found to be critical for ensuring the retention of the particles within the alveolar lining film for long periods of time. Furthermore, the interactions of particles with the lipids result in a decrease of the layer rigidity in accordance with the finding by Guzmán et al. [118,133]. Similar results in relation to the retention of hydrophobic and hydrophilic particles were found when the interaction of particles with different hydrophilicity and natural LS models was analyzed. The enhanced retention of the hydrophobic particles is associated with the emergence of strong van der Waals interactions between particles and the hydrophobic tails of the lipid molecules [57,227], which makes the modification of the monolayer organization easy as result of the particle trapping [228].

In general, it was found that the incorporation of hydrophobic particles, e.g., hydrophobic montmorillonite, silicon dioxide, carbon black, or graphene oxide, into DPPC films shifts the surface pressure-area per molecule isotherm to more expanded states as a result of an excluded area effect induced by particles [118,129,133,229]. On the other side, hydrophilic particles, including halloysite or bentonice, induce the opposite effect, pushing DPPC monolayers towards more compressed states and reducing the average distance between lipid molecules [229]. The above situation is very different to what emerges when commercial LS formulations are used as models. In these cases, the isotherm appears shifted to more compressed states with independence of the wetting properties of the particles. This leads to a strong inhibition of the LS performance, which is characterized by a worsening of the ability of LS to reduce the surface tension, and compositional remodeling [19,227]. However, the influence of hydrophilic and hydrophobic particles follows very different pathways. Thus, hydrophilic particles undergo a direct penetration into LS layers, whereas hydrophobic particles require being wrapped by LS components to be incorporated in LS films [228,230] in agreement with the finding by Hu et al. [19]. They combined experiments and simulations, and found that hydrophilic particles undergo a fast translocation through the LS films during compression, whereas hydrophobic particles lead to the formation of structural protrusion in the LS film, and hence the inhibition induced by hydrophilic particles emerge faster than those induced for the hydrophobic one.
