Establishing physiologically relevant conditions for measuring the interfacial rheology of lung surfactants

Lung surfactants are essential in the human respiratory system due to their role in lowering surface tension and preventing alveoli from collapsing. The main component that serves this purpose is dipalmitoylphosphatidylcholine (DPPC), a type of phospholipid that is insoluble in water. In this work, we modified an interfacial dilational rheometer (IDR) to measure the dilational modulus of DPPC at physiologically relevant temperatures and employed it at frequencies close to those relevant to breathing. Our analysis reveals that in the liquid-expanded phase, the dilational modulus is governed solely by surface pressure and is independent of temperature. This key insight is supported by the close agreement between our interfacial dilational rheometer measurements and data from other techniques. We demonstrate that the method of interface formation, compression versus direct deposition, significantly impacts the measured interfacial rheology of DPPC, underscoring the importance of deformation history in replicating physiologically relevant lung conditions. Additionally, we demonstrate the need for further work to develop improved equations of state describing DPPC behavior in the liquid-expanded phase.