  Inhaled airborne particles smaller than 100 nm entering the airways have been
shown to deposit in significant amount in the alveolar region of the lungs. The
interior of the alveoli is covered with a ~ 1 micron thick lining fluid, called
pulmonary surfactant. Inhaled nanoparticles are susceptible to interact with
the lung fluid and modify pulmonary functions. Here we evaluate the structural
and rheological properties of the pulmonary surfactant substitute Curosurf
which is administered to premature babies for the treatment of respiratory
distress syndrome. Curosurf is considered a reliable model of endogenous
pulmonary surfactant in terms of composition, structure and function. Using
active microrheology based on magnetically actuated wires, we find that
Curosurf dispersions exhibit a Newtonian behavior at lipid concentration from 0
to 80 g L-1, and that the viscosity follows the Krieger-Dougherty law observed
for a wide variety of colloids. Upon addition of 40 nm alumina nanoplatelets, a
significant change of the Curosurf rheology is noticed. The dispersions then
enter a soft solid phase characterized by an infinite viscosity and a non-zero
equilibrium elastic modulus. The sol-gel transition induced by the
nanoparticles is interpreted as the result of the alumina/vesicle interaction,
which are illustrated by transmission electron microscopy. It also suggests a
potential toxicity associated with the modification of the lung fluid
structural and dynamical properties.
