On the rheology of pulmonary surfactant: Effects of concentration and consequences for the surfactant replacement therapy.

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 increase 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 technique as an accurate tool to explore biological soft matter dynamics.