# On the rheology of pulmonary surfactant: effects of concentration and consequences for the surfactant replacement therapy

## TL;DR

This study investigates how pulmonary surfactant viscosity varies with concentration using magnetic microrheology, revealing a critical transition point that informs better dosing strategies for adult surfactant replacement therapy.

## Contribution

It introduces a magnetic wire microrheology method to measure surfactant viscosity and identifies a concentration-dependent rheological transition relevant for clinical applications.

## Key findings

- Viscosity increases exponentially with concentration following Krieger-Dougherty law.
- A divergence in viscosity occurs near the gel transition point.
- Results can guide formulation adjustments for adult surfactant therapy.

## Abstract

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.

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Source: https://tomesphere.com/paper/1903.03654