Mucus Rheology
frequencer v2
Mucus Rheology

A viscoelastic gel (called a Maxwellian liquid because its stress-strain curve is not linear) such as mucus initially stores energy, but with continued stress, it will begin to flow like liquid(1). When the flow stops, it returns to its original state. A well-known example of viscoelastic gel is dripless paint. When it is pulled along by a brush or roller, it spreads well. However, when the stretching stops, it “gels” in place. This behaviour, although desirable in paint, can cause problems in mucus clearing. Since viscoelastic gels move only when applied stress is greater than a threshold level, they may not reach the threshold stress value required for flow if they are very viscous, and may simply deform without flowing.

There are two layers of intrabronchial mucus: a low viscosity, high elasticity periciliary liquid (PCL), and a higher, more viscous airway surface fluid (ASF). The elasticity of mucus appears to change with the application of stress, and may be important to the rate of beating of bronchial epithelial cilia.

Both bronchiectasis and cystic fibrosis (CF) patients have excessively large amounts of sputum. Because this sputum is thick and viscous, it does not flow easily. Furthermore, in CF, the PCL liquid layer is thinner, and the thicker ASF tends to impede the movement of the cilia(2). In healthy humans, the concentration of mucins in the mucus is about 1% by weight. In CF patients, the mucin content increases to as much as 3-4%. This results is increased viscosity, reduced elasticity and increased adhesiveness, which hinders movement of the mucus(3). The absolute viscosity of normal mucus is generally 1 kg/m-sec, but can be as much as 500 kg/m-sec in CF patients(4).

The more we agitate mucus, the less tacky, sticky and stiff it is and the more like water it is. This is comparable to shaking a bottle of ketchup (another common viscoelastic fluid) to help it flow more easily.

A test showed that the application of The Frequencer™ to a simulated mucus preparation of 40 mg/ml mucin resulted in a significant acceleration of the flow of the mucin solution as measured by a capillary rheometer. The Frequencer™ treated mucin preparations reached flow rates similar to those treated with a vortex mixer (Figure 4; control 1.02 ± 0.01 mm/sec, vortex 1.22 ± 0.02 mm/sec, Frequencer™ 1.22 ± 0.02 mm/sec, P < 0.001 each vs. control; P > 0.05 vortex vs Frequencer™, n 8).

  1. Bruce K. Rubin, “An In Vitro Comparison of the Mucoactive Properties of Guaifenesin, Iodinated Glycerol, Surfactant, and Albuterol.” Chest (July 1999): 2.
  2. D.J. Lane. “The Clinical Presentation of Chest Diseases,” in the Oxford Textbook of Medicine (London: Oxford University Press: 2003), 2.
  3. Richard Boucher, C. William Davis and Garrett Matthews. Mucociliary Clearance for CysticFibrosis. (April 29, 2004).
  4. Robert A. Freitas Jr. “Navigational Bronchography,” chap. 8.2.2 in Nanomedicine, Volume 1: Basic Capabilities. Georgetown: Landes Bioscience, 1999, 1.
  5. André M. Cantin, Marc Bacon, Yves Berthiaume 2005. Mechanical airway clearance using the Frequencer™ electro-acoustical transducer in cystic fibrosis.

mucus rheology

Figure 4 - In vitro rheological properties of 40 mg/ml mucin solution without control and with 5 second treatment using either a vortex stirrer (vortex) or the Frequencer™ at 40 Hz and 50% maximal power. Results are expressed as the mean ± sem of the flow rate of 5 ul mucin solution through a glass capillary under a constant pressure of 0.5 cm H2O (n = 8, *P < 0.001 vs control). (5)

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