The insight on the interfacial and bulk rheology of lung surfactants is crucial in designing a surfactant replacement therapy that is effective in overcoming respiratory distress syndromes. In this work, a torsional rheometer with a cone-and-plate fixture is used to measure the bulk viscosity of three clinical lung surfactants: Survanta, Curosurf, and Infasurf. It is shown that all three of them exhibit shear thinning behaviors, with Survanta demonstrating the strongest shear thinning trend and the highest viscosity, with orders of magnitude differences from the other two. Recognizing the plausibility of an air-water interfacial film formation with a cone-and-plate fixture, it is natural to consider that a coupling between the bulk and the interface takes place for the surface-active lung surfactants. 1
In particular, Survanta also demonstrates significantly strong interfacial viscosity and elasticity as opposed to Curosurf and Infasurf, which show very weak viscoelasticity behaviors. This suggests a likelihood that the measured viscosity from the cone-and-plate may not be the real bulk viscosity, but rather the combination of the bulk and interfacial viscosity, with the interfacial viscosity giving the non-Newtonian apparent measured bulk viscosity.
Furthermore, a nonadsorbing hydrophilic polymer, polyethylene glycol (PEG), reverses lung surfactant inactivation by enhancing its adsorption to the interface. Cone-and-plate measurements of PEG-mixed lung surfactants also show decreased viscosity. Both these phenomena can be explained by depletion attraction that happens between lung surfactant colloids at the interface. This consistency thus gives stronger evidence of a coupling between the bulk and the interface of clinical lung surfactant systems.
(1) Sharma, V.; Jaishankar, A.; Wang, Y.-C.; Mckinley, G. H. Rheology of Globular Proteins: Apparent Yield Stress, High Shear Rate Viscosity and Interfacial Viscoelasticity of Bovine Serum Albumin Solutions.