Ballistic Penetration of Highly Charged Nanoaerosol Particles through a Lipid Monolayer

To be used as a drug, inhaled nanoaerosol particles (NAPs) must first penetrate the lipid layer on top of the lung fluid before they will be able to reach the lung epithelium. We investigated how the penetration of NAPs through a model lipid monolayer (LM) depends upon their charging level and size. It was shown that deposition of NAPs 20-200 nm in diameter and charged to the Rayleigh limit gradually increased the surface tension of a dipalmitoylphosphatidylcholine monolayer (DPPC), indicating a loss of lipid molecules from the monolayer. This phenomenon was reproduced with a variety of NAPs produced from glucose, proteins, and polymers. Transfer of the lipid material into the subphase was documented by direct visualization of lipid nanoparticles in the subphase with atomic force microscopy after deposition of glucose NAPs on a DPPC monolayer, followed by collection of the lipid nanoparticles on a mica surface. Partial restoration of tension upon storage indicates that some of the lipid may return to the monolayer. Experiments with the deposition of highly charged calibrated polystyrene nanoparticles showed that the amount of lipid removed from the surface was roughly proportional to the overall surface area of the deposited NAPs. When the number of charges on the NAPs was reduced from their Rayleigh level of 10³-10⁴ units to 1-10 units, no notable changes in monolayer surface tension were observed even with prolonged deposition of such NAPs. It was therefore concluded that only highly charged NAPs of a certain size acquire sufficient speed from their attraction by mirror charges to enable ballistic penetration through a lipid monolayer.