A foam model highlights the differences of the macro- and microrheology of respiratory horse mucus.

This research looked at the unique nature of horse mucus, comparing its properties on a small and large scale to a hydroxyethylcellulose (HEC) gel model. The findings indicate that the mucus behaves much differently than the HEC, suggesting a foam-like microstructure that could drastically affect how particles interact with it.

Analysis of Horse Mucus and HEC Gel

• The study started with an examination of horse mucus at both a macroscopic (large) and microscopic (small) scale. This was then compared with hydroxyethylcellulose (HEC) gel, a material often used as a model for studying similar substances.
• At the microscale, the mucus and HEC demonstrated similar viscoelastic properties, which refers to their ability to exhibit both elastic (spring-like) and viscous (resistive) characteristics when under stress.
• However, on a larger scale, the viscoelastic properties of the mucus vastly surpassed those of the HEC model, indicating significant differences in how the substances behave when forces are applied over a greater area.

Differences in Response to Deformation

• You can find another key difference in the reactions of the mucus and HEC to large amplitude oscillatory shear experiments. These experiments involve applying a varying stress to a material to observe its response. The mucus showed a nonlinear response at much smaller deformations than the HEC, pointing to a unique property of the biological substance.
• This behavior led researchers to believe that, rather than the mesh-like microstructure observed in HEC, horse mucus has a foam-like structure. This foam-like structure was later supported by exploring cryogenic-scanning-electron-microscopy (CSEM) images.

Foam-like Microstructure of Mucus

• CSEM gave researchers a closer look at the mucus, which confirmed their foam-like microstructure theory. These images also provided insight into the relative amount of space within the mucus occupied by pores and scaffold structures.
• With these details, researchers were then able to estimate the elastic modulus—the measure of a substance’s ability to resist deformation under an applied force—of the mucus scaffold. This information could provide important perspective on the mechanical characteristics of the mucus.

Implications for Particle Transport

• The discovery of this unusual foam-like microstructure could hold important implications for the mucus’s function with respect to particle penetration. The specific structure of the mucus might allow it to more effectively transport or trap particulate matter, an essential element of mucous function in respiratory systems.