  The role of pulmonary surfactant is to reduce the surface tension in the
lungs and to facilitate breathing. Surfactant replacement therapy (SRT) aims at
bringing a substitute by instillation into the airways, a technique that has
proven to be efficient and lifesaving for preterm infants. Adapting this
therapy to adults requires to scale the administered dose to the patient body
weight and to in-crease the lipid concentration, whilst maintaining its surface
and flow properties similar. Here, we exploit a magnetic wire-based
microrheology technique to measure the viscosity of the exogenous pulmonary
surfactant Curosurf in various experimental conditions. The Curosurf viscosity
is found to increase exponentially with lipid concentration following the
Krieger-Dougherty law of colloids. The Krieger-Dougherty behavior also predicts
a divergence of the viscosity at the liquid-to-gel transition. For Curosurf the
transition concentration is found close to the concentration at which it is
formulated (117 g L-1 versus 80 g L-1). This outcome suggests that for SRT the
surfactant rheological properties need to be monitored and kept within a
certain range. The results found here could help in producing suspensions for
respiratory distress syndrome adapted to adults. The present work also
demonstrates the potential of the magnetic wire microrheology tech-nique as an
accurate tool to explore biological soft matter dynamics.
