Respiratory tract mucus is a viscoelastic gel, the rheological properties of which are determined mainly by its content of mucous glycoproteins and water. The rheology and quantity of mucus, in concert with ciliary factors, are the major determinants of mucociliary clearance.A wide range of animal models for studying the secretion and clearance of mucus are available. Ex vivo models, such as the frog palate or excised bovine trachea, provide direct, meaningful data regarding the clearability of mucus. Rodent models of chronic bronchitis, based on irritant gas or cigarette smoke exposure, show important features of the human condition in a relatively short time. The rheological characterization of mucus is made difficult by the small quantities obtainable, particularly from normal animals.Large animal models, such as the dog or sheep, although more expensive, offer many advantages, such as the ability to carry out long-term serial measurements, and to make integrated measurements of the clearance of mucus, ciliary function, epithelial ion transport, and the rheology of mucus in the same preparation. Eur Respir J 1998; 11: 222-228
The viscoelastic properties of mucusDue to the cross-linking of glycoproteins, the rheological behaviour of mucus is described as viscoelastic, having characteristics of both a liquid and a solid [1,2]. Viscosity is the resistance to flow, and represents the capacity of a material to absorb energy as it moves. Elasticity is the capacity of a material to store the energy used to move or deform it. With ideal fluids, viscosity is independent of the stress applied. With viscoelastic liquids, such as mucus, viscosity decreases with increasing stress or rate of strain (shear rate). Mucus responds to stress with an initial solid-like deformation, followed by a viscoelastic deformation and finally by a period of steady flow, in which the rate of deformation is constant. Only partial recovery of the strain follows removal of the stress, indicating a permanent deformation of its gel structure. Changes in the viscosity and elasticity of mucus are generally interrelated.Mucus exhibits shear-thinning following exposure to high shear forces, exhibiting a decreased viscosity at low shear rates. Some shear-thinning may be permanent, indicating altered molecular structure, whilst some shearthinning, termed "thixotropy", may be reversible.The viscoelasticity of mucus can be effectively described by two relatively independent quantities, G* and tangent (tan) δ, which vary with measurement frequency. The mechanical impedance (G*), is the vector sum of viscosity and elasticity; it can be termed the "rigidity factor". Tan δ is the ratio of viscosity to elasticity; it is also known as the loss tangent and can be considered as a "recoil factor". The relative proportions of elasticity and viscosity are important in describing how a material, such as mucus, behaves when it is subjected to external forces.
Rheological assessment of mucus
Collection of mucusTracheal mucus can be collected by a modificat...