Preston Murray and Dr. Scott Thomson, Department of Mechanical Engineering
Synopsis
The purpose of this research project was to identify a fluid that has similar properties as the airway surface liquid (ASL) that covers vocal folds, and then apply it to a vibrating synthetic vocal fold model to measure its effects.
Background
The ASL protects the vocal folds as they vibrate and rapidly collide (on the order of a hundred or more times per second). Because of these functions the ASL must perform, it has unique characteristics. The ASL is comprised of two distinct layers (Button and Boucher, 2008). Also, as frequency increases the ASL viscosity decreases (King and Macklem, 1977). In other words, the ASL is a multilayered fluid, with one layer that is shear thinning (decreasing viscosity with increasing shear stress).
Methods
Six candidate shear-thinning fluids were studied in order to find the best approximation of the ASL: xanthan gum, tragacanth gum, guar gum, carob bean, tapioca starch, and sodium alginate. In each case, the additive was a powder which, when mixed with water, produced a fluid with shear thinning properties. The best way to compare these fluids was through rheological measurements (rheology is the study of the flow of matter). King and Macklem (1977) were able to obtain a small amount of ASL and measured its rheological properties. This work was used as the foundation for this research of identifying which fluid best approximates the ASL. Each fluid was mixed with varying weight percents of powder and its rheological properties were measured. After testing and comparison, only xanthan gum was found to be a suitable candidate based on comparison of its trends with those reported by King and Macklem (1977) (see Figure 1).
The experimental design to discover the effects of applying xanthan gum to a vibrating vocal fold model was patterned after a previous study (Nakagawa et. al, 1998). In the experiment completed at BYU, the same metrics of onset pressure (pressure at which vibration begins), frequency, and radiated sound were obtained. Additionally, the experiment performed at BYU obtained the open quotient (ratio of the time the vocal folds are open to the time of one cycle) and force of the impact from the vocal fold hitting an acrylic plate. Newtonian fluids (fluids that have the same viscosity with varying applied shear stress) were included in the experiment to see if there was a difference between the effects of Newtonian versus shear thinning fluids.
In the experiment, a vibrating vocal fold model was coated with olive oil, glycerin, water, or one of three different viscosities of xanthan gum. A run with no fluid was completed between each fluid run to determine if there was evidence of residual fluid on the vocal fold model. The fluid runs were randomly assigned and the experiment was replicated 5 times so that variation could be quantified. To ensure the acoustic measurement devices were not affected by external sources (e.g. compressors operating in the adjacent lab), calibration runs were completed every six runs. In total, 74 runs were tested and the above mentioned metrics were obtained.
Results
The data from the experiment were sorted and the effects of each fluid were compared. If only the mean values are considered, then there the effects of the different fluids can be seen (see Figure 2). Unfortunately, the variation in the runs of the individual fluids is so great that the effect of the fluids on the vibratory response of the vocal folds is not statistically significant (P < 0.05). This lack of significance prompted a closer investigation of the high speed images and it was discovered that the fluid did not remain of the vocal fold during vibration. Because of this, nothing useful can be determined from this experiment.
In order to obtain definitive conclusions, the fluid must be maintained between the vocal fold model and the acrylic plate in the experiment. Further studies will be done as part of my master’s work and will incorporate methods to ensure the fluid remains on the vocal fold and that the variation in the process is minimized.
References
- Button, B., Boucher, R. C., (2008). “Role of mechanical stress in regulating airway surface hydration and mucus clearance rates,” Resp. Physiol. & Neurobio. 163 (2008):189–201.
- Malcom, K., Macklem, P., (1977). “Rheological properties of microliter quantities of normal mucus,” J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 42(6):797-802.
- Nakagawa, H, Fukuda, H., Kawaida, M., Akihiro, S., Kanzaki, J., “Lubrication Mechanism of the Larynx during Phonation: An experiment in Excised Canine Larynges,” Folia Phoniatr Logop. 50:183-194.