  Mucus is a viscoelastic gel secreted by the pulmonary epithelium in the
tracheobronchial region of the lungs. The coordinated beating of cilia moves
mucus upwards towards pharynx, removing inhaled pathogens and particles from
the airways. The efficacy of this clearance mechanism depends primarily on the
rheological properties of mucus. Here we use magnetic wire based microrheology
to study the viscoelastic properties of human mucus collected from human
bronchus tubes. The response of wires between 5 and 80 microns in length to a
rotating magnetic field is monitored by optical time-lapse microscopy and
analyzed using constitutive equations of rheology, including those of Maxwell
and Kelvin-Voigt. The static shear viscosity and elastic modulus can be
inferred from low frequency (from 0.003 to 30 rad s-1) measurements, leading to
the evaluation of the mucin network relaxation time. This relaxation time is
found to be widely distributed, from one to several hundred seconds. Mucus is
identified as a viscoelastic liquid with an elastic modulus of 2.5 +/- 0.5 Pa
and a static viscosity of 100 +/- 40 Pa s. Our work shows that beyond the
established spatial variations in rheological properties due to microcavities,
mucus exhibits secondary inhomogeneities associated with the relaxation time of
the mucin network that may be important for its flow properties.
