Low concentrations of negatively charged sub-micron particles alter the microstructure of DPPC at the air–water interface

Impact of Polymer Nanoparticles on DPPC Monolayer Properties

The application of surface rheology and Brewster angle microscopy on mixed monolayers of DPPC and polymeric nanoparticles (cationic and anionic) showed that the sign of the particle charge affects the dynamic properties of the monolayers less than the nanoparticles’ ability to aggregate. Under almost physiological conditions, the effect of nanoparticles on the elasticity of DPPC monolayer is insignificant. However, the particles prevent the surface tension from decreasing to extremely low values. This effect could affect the functionality of pulmonary surfactants.

Fluid Films as Models for Understanding the Impact of Inhaled Particles in Lung Surfactant Layers

Pollution is currently a public health problem associated with different cardiovascular and respiratory diseases. These are commonly originated as a result of the pollutant transport to the alveolar cavity after their inhalation. Once pollutants enter the alveolar cavity, they are deposited on the lung surfactant (LS) film, altering their mechanical performance which increases the respiratory work and can induce a premature alveolar collapse. Furthermore, the interactions of pollutants with LS can induce the formation of an LS corona decorating the pollutant surface, favoring their penetration into the bloodstream and distribution along different organs. Therefore, it is necessary to understand the most fundamental aspects of the interaction of particulate pollutants with LS to mitigate their effects, and design therapeutic strategies. However, the use of animal models is often invasive, and requires a careful examination of different bioethics aspects. This makes it necessary to design in vitro models mimicking some physico-chemical aspects with relevance for LS performance, which can be done by exploiting the tools provided by the science and technology of interfaces to shed light on the most fundamental physico-chemical bases governing the interaction between LS and particulate matter. This review provides an updated perspective of the use of fluid films of LS models for shedding light on the potential impact of particulate matter in the performance of LS film. It should be noted that even though the used model systems cannot account for some physiological aspects, it is expected that the information contained in this review can contribute on the understanding of the potential toxicological effects of air pollution.

Unmodified Clay Nanosheets at the Air-Water Interface

Quasi-two-dimensional (2D) nanolayers, such as graphene oxide or clay layers, adhere to gas-liquid or liquid-liquid interfaces. Particularly, clays are of wide general interest in this context because of their extensive and crucial use as Pickering emulsion stabilizers, as well as for their ability to provide colloidosome capsules. So far, clays could only be localized at oil-water or air-saline-water interfaces in aggregated states, while our results now show that clay nanosheets without any modification can be located at air-deionized-water interfaces. The clay mineral used in the present work is synthetic fluorohectorite with a very high aspect ratio and superior quality in homogeneity and charge distribution compared to other clay minerals. This clay mineral is more suitable for achieving unmodified clay anchoring to fluid interfaces compared to other clay minerals used in previous works. In this context, we studied clay nanosheet organization at the air-water interface by combining different experimental methods: Langmuir-Blodgett trough studies, scanning electron microscopy (SEM) studies of film deposits, grazing-incidence X-ray off-specular scattering (GIXOS), and Brewster angle microscopy (BAM). Clay films formed at the air-water interface could be transferred to solid substrates by the Langmuir-Schaefer method. The BAM results indicate a dynamic equilibrium between clay sheets on the interface and in the subphase. Because of this dynamic equilibrium, the Langmuir monolayer surface pressure does not change significantly when pure clay sheets are spread on the liquid surface. However, also, GIXOS results confirm that there are clay nanosheets at the air-water interface. In addition, we find that clay sheets modified by a branched polymer are much more likely to be confined to the interface.

Interaction of Particles with Langmuir Monolayers of 1,2-Dipalmitoyl-Sn-Glycero-3-Phosphocholine: A Matter of Chemistry?

Lipid layers are considered among the first protective barriers of the human body against pollutants, e.g., skin, lung surfactant, or tear film. This makes it necessary to explore the physico-chemical bases underlying the interaction of pollutants and lipid layers. This work evaluates using a pool of surface-sensitive techniques, the impact of carbon black and fumed silica particles on the behavior of Langmuir monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The results show that the incorporation of particles into the lipid monolayers affects the surface pressure–area isotherm of the DPPC, modifying both the phase behavior and the collapse conditions. This is explained considering that particles occupy a part of the area available for lipid organization, which affects the lateral organization of the lipid molecules, and consequently the cohesion interactions within the monolayer. Furthermore, particles incorporation worsens the mechanical performance of lipid layers, which may impact negatively in different processes presenting biological relevance. The modification induced by the particles has been found to be dependent on their specific chemical nature. This work tries to shed light on some of the most fundamental physico-chemical bases governing the interaction of pollutants with lipid layers, which plays an essential role on the design of strategies for preventing the potential health hazards associated with pollution.

Evolution and mechanics of mixed phospholipid fibrinogen monolayers

All mammals depend on lung surfactant (LS) to reduce surface tension at the alveolar interface and facilitate respiration. The inactivation of LS in acute respiratory distress syndrome (ARDS) is generally accompanied by elevated levels of fibrinogen and other blood plasma proteins in the alveolar space. Motivated by the mechanical role fibrinogen may play in LS inactivation, we measure the interfacial rheology of mixed monolayers of fibrinogen and dipalmitoylphosphatidylcholine (DPPC), the main constituent of LS, and compare these to the single species monolayers. We find DPPC to be ineffective at displacing preadsorbed fibrinogen, which gives the resulting mixed monolayer a strongly elastic shear response. By contrast, how effectively a pre-existing DPPC monolayer prevents fibrinogen adsorption depends upon its surface pressure. At low DPPC surface pressures, fibrinogen penetrates DPPC monolayers, imparting a mixed viscoelastic shear response. At higher initial DPPC surface pressures, this response becomes increasingly viscous-dominated, and the monolayer retains a more fluid, DPPC-like character. Fluorescence microscopy reveals that the mixed monolayers exhibit qualitatively different morphologies. Fibrinogen has a strong, albeit preparation-dependent, mechanical effect on phospholipid monolayers, which may contribute to LS inactivation and disorders such as ARDS.

Polyelectrolyte Coatings Can Control Charged Fluorocarbon Nanodroplet Stability and Their Interaction with Macrophage Cells

Fluorocarbon nanodroplets, ∼100 to ∼400 nm in diameter, are of immense interest in a variety of medical applications including the imaging and therapy of cancer and inflammatory diseases. However, fluorocarbon molecules are both hydrophobic and lipophobic; therefore, it is challenging to synthesize fluorocarbon nanodroplets with the optimal stability and surface properties without the use of highly specialized surfactants. Here, we hypothesize that we can decouple the control of fluorocarbon nanodroplet size and stability from its surface properties. We use a simple, two-step procedure where standard, easily available anionic fluorosurfactants are used to first stabilize the fluorocarbon nanodroplets, followed by electrostatically attaching functionalized polyelectrolytes to the nanodroplet surfaces to independently control their surface properties. Herein, we demonstrate that PEGylated polyelectrolyte coatings can effectively alter the fluorocarbon nanodroplet surface properties to reduce coalescence and its uptake into phagocytic cells in comparison with non-PEGylated polyelectrolyte coatings and uncoated nanodroplets, as measured by flow cytometry and fluorescence microscopy. In this study, perfluorooctyl bromide (PFOB) was used as a representative fluorocarbon material, and PEGylated PFOB nanodroplets with diameters between 250 and 290 nm, depending on the poly(ethylene glycol) block length, were prepared. The PEGylated PFOB nanodroplets had superior size stability in comparison with uncoated and non-PEGylated polyelectrolyte nanodroplets in saline and within macrophage cells. Of significance, non-PEGylated nanodroplets were rapidly internalized by macrophage cells, whereas PEGylated nanodroplets were predominantly colocalized on the cell membrane. This suggests that the PEGylated-polyelectrolyte coating on the charged PFOB nanodroplets may afford adjustable shielding from cells of the reticuloendothelial system. This report shows that using the same fluorosurfactant as a base layer, modularly assembled PFOB nanodroplets tailored for a variety of end applications can be created by selecting different polyelectrolyte coatings depending on their unique requirements for stability and interaction with phagocytic cells.

Electronic cigarette vapor alters the lateral structure but not tensiometric properties of calf lung surfactant

Background

Despite their growing popularity, the potential respiratory toxicity of electronic cigarettes (e-cigarettes) remains largely unknown. One potential aspect of e-cigarette toxicity is the effect of e-cigarette vapor on lung surfactant function. Lung surfactant is a mixture of lipids and proteins that lines the alveolar region. The surfactant layer reduces the surface tension of the alveolar fluid, thereby playing a crucial role in lung stability. Due to their small size, particulates in e-cigarette vapor can penetrate the deep lungs and come into contact with the lung surfactant. The current study sought to examine the potential adverse effects of e-cigarette vapor and conventional cigarette smoke on lung surfactant interfacial properties.

Methods

Infasurf^®, a clinically used and commercially available calf lung surfactant extract, was used as lung surfactant model. Infasurf^® films were spread on top of an aqueous subphase in a Langmuir trough with smoke particulates from conventional cigarettes or vapor from different flavors of e-cigarettes dispersed in the subphase. Surfactant interfacial properties were measured in real-time upon surface compression while surfactant lateral structure after exposure to smoke or vapor was examined using atomic force microscopy (AFM).

Results

E-cigarette vapor regardless of the dose and flavoring of the e-liquid did not affect surfactant interfacial properties. In contrast, smoke from conventional cigarettes had a drastic, dose-dependent effect on Infasurf^® interfacial properties reducing the maximum surface pressure from 65.1 ± 0.2 mN/m to 46.1 ± 1.3 mN/m at the highest dose. Cigarette smoke and e-cigarette vapor both altered surfactant microstructure resulting in an increase in the area of lipid multilayers. Studies with individual smoke components revealed that tar was the smoke component most disruptive to surfactant function.

Conclusions

While both e-cigarette vapor and conventional cigarette smoke affect surfactant lateral structure, only cigarette smoke disrupts surfactant interfacial properties. The surfactant inhibitory compound in conventional cigarettes is tar, which is a product of burning and is thus absent in e-cigarette vapor.

Effect of the Incorporation of Nanosized Titanium Dioxide on the Interfacial Properties of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine Langmuir Monolayers

The effect of the incorporation of hydrophilic titanium dioxide (TiO₂) nanoparticles on the interfacial properties of Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) has been evaluated combining interfacial thermodynamic studies, dilatational rheology, and Brewster angle microscopy (BAM). The results show that the TiO₂ nanoparticles are able to penetrate DPPC layers, modifying the organization of the molecules and, consequently, the phase behavior and viscoelastic properties of the systems. Measurements of dilational viscoelasticity against the frequency have been performed, using the oscillatory barrier method, at different values of the surface pressure corresponding to different degrees of compression of the monolayer. The presence of TiO₂ nanoparticles also affects the dynamic response of the monolayer modifying both the quasi-equilibrium dilatational elasticity and the high frequency limit of the viscoelastic modulus. The principal aim of this work is to understand the fundamental physicochemical bases related to the incorporation of specific nanoparticles of technological interest into the interfacial layer with biological relevance such as phospholipid layers. This can provide information on potential adverse effects of nanoparticles for health and the environment.

Multi-scale mechanical characterization of prostate cancer cell lines: Relevant biological markers to evaluate the cell metastatic potential

BACKGROUND

Considering the importance of cellular mechanics in the birth and evolution of cancer towards increasingly aggressive stages, we compared nano-mechanical properties of non-tumoral (WPMY-1) and highly aggressive metastatic (PC-3) prostate cell lines both on cell aggregates, single cells, and membrane lipids.

METHODS

Cell aggregate rheological properties were analyzed during dynamic compression stress performed on a homemade rheometer. Single cell visco-elasticity measurements were performed by Atomic Force Microscopy using a cantilever with round tip on surface-attached cells. At a molecular level, the lateral diffusion coefficient of total extracted lipids deposited as a Langmuir monolayer on an air-water interface was measured by the FRAP technique.

RESULTS

At cellular pellet scale, and at single cell scale, PC-3 cells were less stiff, less viscous, and thus more prone to deformation than the WPMY-1 control. Interestingly, stress-relaxation curves indicated a two-step response, which we attributed to a differential response coming from two cell elements, successively stressed. Both responses are faster for PC-3 cells. At a molecular scale, the dynamics of the PC-3 lipid extracts are also faster than that of WPMY-1 lipid extracts.

CONCLUSIONS

As the evolution of cancer towards increasingly aggressive stages is accompanied by alterations both in membrane composition and in cytoskeleton dynamical properties, we attribute differences in viscoelasticity between PC-3 and WPMY-1 cells to modifications of both elements.

GENERAL SIGNIFICANCE

A decrease in stiffness and a less viscous behavior may be one of the diverse mechanisms that cancer cells adopt to cope with the various physiological conditions that they encounter.

Effect of clay nanoparticles on model lung surfactant: a potential marker of hazard from nanoaerosol inhalation

This work investigates influence of different aluminosillicate nanoparticles (NPs) which are found in air in selected workplaces on the properties of the phospholipid (DPPC) monolayer at air-saline interface considered as ex vivo model of the lung surfactant (LS). The measurements were done under physiological-like conditions (deformable liquid interface at 37 °C) for NP concentrations matching the calculated lung doses after exposure in the working environment. Measured surface pressure-area (π-A) isotherms and compressibility curves demonstrated NP-induced changes in the structure and mechanical properties of the lipid monolayer. It was shown that hydrophilic nanomaterials (halloysite and bentonite) induced concentration-dependent impairment of DPPC's ability of attaining high surface pressures on interfacial compression, suggesting a possibility of reduction of physiological function of natural LS. Hydrophobic montmorillonites affected DPPC monolayer in the opposite way; however, they significantly changed the mechanical properties of the air-liquid interface during compression. The results support the hypothesis of possible reduction or even degradation of the natural function of the lung surfactant induced by particle-phospholipid interactions after inhalation of nanoclays. Presented data do not only supplement the earlier results obtained with another LS model (animal-derived surfactant in oscillating bubble experiments) but also offer an explanation of physicochemical mechanisms responsible for detrimental effects which arise after deposition of inhaled nanomaterials on the surface of the respiratory system.

Effects of nanoparticles on the mechanical functioning of the lung

Nanotechnology is a rapidly expanding field that has very promising applications that will improve industry, medicine, and consumer products. However, despite the growing widespread use of engineered nanoparticles in these areas, very little has been done to assess the potential health risks they may pose to high-risk areas of the body, particularly the lungs. In this review we first briefly discuss the structure of the lungs and establish that the pulmonary surfactant (PS), given its vulnerability and huge contribution to healthy lung function, is a mechanism of great concern when evaluating potential nanoparticle interactions within the lung. To warrant that these interactions can occur, studies on the transport of nanoaerols are reviewed to highlight that a plethora of factors contribute to a nanoparticle's ability to travel to the deep regions of the lung where PS resides. The focus of this review is to determine the extent that physicochemical characteristics of nanoparticles such as size, hydrophobicity, and surface charge effect PS function. Numerous nanoparticle types are taken into consideration in order to effectively evaluate observed consistencies across numerous nanoparticle types and develop general trends that exist among the physicochemical characteristics of interest. Biological responses from other mechanisms/components of the lung are briefly discussed to provide further insights on how the toxicology of different nanoparticles is determined. We conclude by discussing general trends that summarize consistencies observed among the studies in regard to physicochemical properties and their effects on monolayer function, addressing current gaps in our understanding, and discussing the future outlook of this field of research.

Investigating the effect of particle size on pulmonary surfactant phase behavior

We study the impact of the addition of particles of a range of sizes on the phase transition behavior of lung surfactant under compression. Charged particles ranging from micro- to nanoscale are deposited on lung surfactant films in a Langmuir trough. Surface area versus surface pressure isotherms and fluorescent microscope observations are utilized to determine changes in the phase transition behavior. We find that the deposition of particles close to 20 nm in diameter significantly impacts the coexistence of the liquid-condensed phase and liquid-expanded phase. This includes morphological changes of the liquid-condensed domains and the elimination of the squeeze-out phase in isotherms. Finally, a drastic increase of the domain fraction of the liquid-condensed phase can be observed for the deposition of 20-nm particles. As the particle size is increased, we observe a return to normal phase behavior. The net result is the observation of a critical particle size that may impact the functionality of the lung surfactant during respiration.

An investigation of the carbon nanotube--Lipid interface and its impact upon pulmonary surfactant lipid function

Multiwalled carbon nanotubes (MWCNTs) are now synthesized on a large scale, increasing the risk of occupational inhalation. However, little is known of the MWCNT-pulmonary surfactant (PS) interface and its effect on PS functionality. The Langmuir-Blodgett trough was used to evaluate the impact of MWCNTs on fundamental properties of PS lipids which influence PS function, i.e. compression resistance and maximum obtainable pressure. Changes were found to be MWCNT length-dependent. 'Short' MWCNTs (1.1 μm, SD = 0.61) penetrated the lipid film, reducing the maximum interfacial film pressure by 10 mN/m (14%) in dipalmitoylphosphatidylcholine (DPPC) and PS, at an interfacial MWCNT-PS lipid mass ratio range of 50:1 to 1:1. 'Long' commercial MWCNTs (2.1 μm, SD = 1.2) caused compression resistance at the same mass loadings. 'Very long' MWCNTs (35 μm, SD = 19) sequestered DPPC and were squeezed out of the DPPC film. High resolution transmission electron microscopy revealed that all MWCNT morphologies formed DPPC coronas with ordered arrangements. These results provide insight into how nanoparticle aspect ratio affects the interaction mechanisms with PS, in its near-native state at the air-water interface.

Calf Lung Surfactant Recovers Surface Functionality After Exposure to Aerosols Containing Polymeric Particles

BACKGROUND

Recent studies have shown that colloidal particles can disrupt the interfacial properties of lung surfactant and thus key functional abilities of lung surfactant. However, the mechanisms underlying the interactions between aerosols and surfactant films remain poorly understood, as our ability to expose films to particles via the aerosol route has been limited. The aim of this study was to develop a method to reproducibly apply aerosols with a quantifiable particle dose on lung surfactant films and investigate particle-induced changes to the interfacial properties of the surfactant under conditions that more closely mimic those in vivo.

METHODS

Films of DPPC and Infasurf^® were exposed to aerosols containing polystyrene particles generated using a Dry Powder Insufflator^™. The dose of particles deposited on surfactant films was determined via light absorbance. The interfacial properties of the surfactant were studied using a Langmuir-Wilhelmy balance during surfactant compression to film collapse and cycles of surface compression and expansion at a fast cycling rate within a small surface area range.

RESULTS

Exposure of surfactant films to aerosols led to reproducible dosing of particles on the films. In film collapse experiments, particle deposition led to slight changes in collapse surface pressure and surface area of both surfactants. However, longer interaction times between particles and Infasurf^® films resulted in time-dependent inhibition of surfactant function. When limited to lung relevant surface pressures, particles reduced the maximum surface pressure that could be achieved. This inhibitory effect persisted for all compression-expansion cycles in DPPC, but normal surfactant behavior was restored in Infasurf^® films after five cycles.

CONCLUSIONS

The observation that Infasurf^® was able to quickly restore its function after exposure to aerosols under conditions that better mimicked those in vivo suggests that particle-induced surfactant inhibition is unlikely to occur in vivo due to an aerosol exposure.

Two-dimensional DPPC based emulsion-like structures stabilized by silica nanoparticles

We studied the mechanical and structural properties of mixed surface layers composed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and silica nanoparticles (NPs). These layers are obtained by spreading a DPPC Langmuir monolayer on a colloidal silica dispersion. The transfer/incorporation of NPs into the DPPC monolayer, driven by electrostatic interactions, alters the molecular orientation, the mechanisms of domain formation, and consequently the phase behavior of the surface layer during compression. The investigation of these systems by means of complementary techniques (Langmuir trough, fluorescence microscopy, ellipsometry, and scanning electron microscopy (SEM)) shows that the incorporated NPs preferentially distribute along the liquid expanded phase of DPPC. The layer assumes the stable and homogeneous bidimensional structure of a two-dimensional (2D) analogue of a Pickering emulsion. In fact, the presence of particles provides a circular shape to the DPPC domains and stabilizes them against growth and coalescence during the monolayer compression.