Methodological improvements in mechanical measurement of vocal fold-related tissues

Viscoelastic properties of vocal fold tissues determine phonation threshold pressure, fundamental frequency, and vibration modes. Human, animal, and biomaterial data have been reported but order of magnitude discrepancies exist. It was hypothesized that differences were due to methodology. The purpose of the present study was to investigate error sources of a stress controlled rheometer for data collection up to 150 Hz. Aim 1 was to empirically determine a motor inertia value of a stress controlled rheometer. Newtonian and non-Newtonian standards were shown to have measurement sensitivity to a 10% motor inertia error, where measurement error was as high as 1000% at 12 Hz for the Newtonian standard and 20% at 130 Hz for the non-Newtonian standard. An accurate motor inertia value was determined, where the use of two different methods and two different materials produced a value, with 3% uncertainty. A new 2-plate method was found to be superior to the traditional phase method, because it was derived explicitly from physics principles, error contributions were known, and the method required less time and fewer measurements to implement. Aim 2 was to investigate sample preshearing and its effect on viscoelastic measurements of two commonly used vocal fold injectables. Samples were extruded onto the rheometer base using three different sized hypodermic needles, and their rheologic properties were measured across frequency. Viscoelastic properties of each material were affected by preshearing, either a 10-fold increase with decreasing needle size, or a 0-50% increase with increasing needle size. The structure of the material determined how the material was affected. Aim 3 was to improve sample to hard surface adhesion, and thus avoid slippage in frequency sweeps up to 100 Hz using a stress controlled rheometer. Hard surfaces covered with 220grit sandpaper worked the best at reducing variability for viscoelastic measurement of two vocal fold injectables, but measurement accuracy requires further study. Surface analysis suggested that height information and distance between irregularities work together to grip a sample during measurement. In general, slip remains a potentially large source of rheologic error. Detecting slip and improving adhesion remains a trial and error process.