  Inhaled nanoparticles (< 100 nm) reaching the deep lung region first interact
with the pulmonary surfactant, a thin lipid film lining the alveolar
epithelium. To date, most biophysical studies have focused on particle induced
modifications of the film interfacial properties. In comparison, there is less
work on the surfactant bulk properties, and on their changes upon particle
exposure. Here we study the viscoelastic properties of a biomimetic pulmonary
surfactant in the presence of various engineered nanoparticles. The
microrheology technique used is based on the remote actuation of micron-sized
wires via the application of a rotating magnetic field and on time-lapse
optical micros-copy. It is found that particles strongly interacting with lipid
vesicles, such as cationic silica (SiO2, 42 nm) and alumina (Al2O3, 40 nm)
induce profound modifications of the surfactant flow proper-ties, even at low
concentrations. In particular, we find that silica causes fluidification, while
alumi-na induces a liquid-to-soft solid transition. Both phenomena are
described quantitatively and ac-counted for in the context of colloidal physics
models. It is finally suggested that the structure and viscosity changes could
impair the fluid reorganization and recirculation occurring during breath-ing.
