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...

Mucus is a viscoelastic gel secreted by the pulmonary epithelium in the tracheobronchial region of the lungs. The coordinated beating of cilia in contact with the gel layer 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 a magnetic wire-based microrheology technique to study the viscoelastic properties of human mucus collected from human bronchus tubes. The rotation versus actuation response of magnetic micron-size wires in frequency range 0.001-10 rad s-1 reveals mucus relaxation times, ranging from one to several hundred seconds. Mucus is identified as either a viscoelastic liquid with an elastic modulus of 2.5 +/-0.5 Pa and a static viscosity of 100 +/- 40 Pa s, or a soft solid with a similar elastic modulus. Our work shows that beyond the established spatial rheological property variations due to microcavities, mucus has secondary inhomogeneities which could be important in its flow properties.