Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 769

small particles undergo more diffusive transport, 20 whereas large particles interact

with a porous, charged, viscoelastic network.
When a particle set has a large particle size range, the average particle transport
rate is influenced by transport rates of large particles. Assessed ensemble-average
diffusivities of 100-, 200-, and 500-nm particles in cystic fibrotic sputum range from
about 1.5

10 − 2 to 3.8 10 − 4 m 2 /s [ 1561 ]. 21 × ×

12.5.2 Mucus Contact Properties

Wettability and adhesiveness govern contact properties with the respiratory epithe-
lium. These 2 properties must be high enough in order to avoid sedimentation down
to the distal airways in lower lung lobes, and low enough for mucus mobilization by
air flow during coughing. Surface-active phospholipids, such as phosphatidylcholine
and phosphatidylglycerol, improve mucus wettability [ 1562 ].

12.5.3 Mucus Rheology

Biological materials are commonly viscoelastic. When they are subjected to a small-
amplitude, sinusoidal load, and display a linear response, they can be characterized
by 2 frequency-dependent properties: (1) elastic or storage modulus , a measure of
material stiffness or tendency for mucus non-Newtonian gel (with a viscosity that
varies non-linearly with shear rate) to recover its original configuration following
load-induced deformation, and (2) viscous or loss modulus , i.e., extent to which
the gel resists the tendency to flow that is also a measure of energy dissipation.
Additional rheological quantities can also describe biological materials. Creep
describes strain as a function of time when a constant stress is applied. It quantifies
the tendency to deform permanently. Spinnability measures the capacity of fluids to
be shaped into threads.
At the macroscopic scale that yields averaged measures of rheological features,
mucus is a viscoelastic, thixotropic, non-Newtonian gel, as its viscosity varies non-
linearly with shear rate (strong and weak resistance to deformation at low and
high shear rate, respectively). Mucus is indeed characterized by a shear thinning
behavior. Viscosity of human respiratory mucus of 12 to 15 Pa s with a relaxation

time of about 40 s and elastic modulus of 1 Pa is supposed to represent an optimal
rheological profile for mucociliary clearance [ 1557 ]. The rheological properties

·

20 Small particles (100-200 nm) can move almost linearly with time and more freely than larger
particles.
21 In water, diffusion coefficients of 100-, 200-, and 500-nm particles are equal to 4.5, 2.3, and
0.9 m 2 /s, respectively.