Self-oscillating synthetic vocal fold (VF) models are often used to study human voice production. In this thesis, a method for fabricating multi-layer self-oscillating synthetic VF models using silicone 3D printing is presented. Multi-material 3D printing enables faster fabrication times with more complex geometries than traditional casting methods and builds a foundation for producing VF models with potentially more life-like geometries, materials, and vibratory characteristics. The printing method in this study used a custom dual extruder and slicing software to print UV-curable liquid silicone into a gel-like support matrix. The extruder was fabricated using high-torque stepper motors with high resolution leadscrews for precise extrusion and retraction. The custom slicing software accounted for challenges with printing a low-viscosity uncured silicone and was capable of allowing the user to visually observe the effects of print settings on print paths before finalizing the g-code. Three validation tests were conducted to demonstrate the 3D printer’s ability to print ultra-soft silicone with the desired range of stiffness, change between materials quickly, and print a material stiffness gradient. Two types of VF models were printed in this study, a previously-designed model with multiple distinct layers (“EPI” model), and the same model but with a vertical stiffness gradient (VSG) in the superficial lamina propria layer. The EPI model was chosen to demonstrate the ability to 3D print a multi-layer model, and the VSG model was chosen to demonstrate the ability to print multi-material VFs with geometric and material properties that are difficult to fabricate using traditional casting methods. Sixteen VFs (i.e., eight pairs) of each model type were printed, and their vibratory responses were recorded, including onset pressure, frequency, and glottal width. A micro-CT scanner was used to evaluate the external geometric accuracy of the models. One-centimeter cubes were 3D printed and tensile tested to characterize the material properties of each set of VF models. The material and phonatory properties of both the EPI and VSG VF models were found to be comparable to human data and to previous data acquired using synthetic VF models fabricated via other methods. In this thesis, the 3D printing methodology is summarized, the setup and results of the validation and VF model tests are reported and discussed, and recommendations for future work are provided.

One of the primary objectives of voice research is to better understand the biomechanics of voice production and how changes in properties of the vocal folds (VFs) affect voice ability and quality. Synthetic VF models provide a way to observe how changes in geometry and material property affect voice biomechanics. This thesis seeks to evaluate an approach of using a genetic algorithm to design synthetic VF models in three ways: first, through the development of a computationally cost-effective 3D vocal fold model; second, by creating and optimizing a variation of this model; and third, by validating the approach. To reduce computation times, a user-defined function (UDF) was implemented in low-fidelity 2D and 3D computational VF models. The UDF replaced the conventional meshed fluid domain with the mechanical energy equation. The UDF was implemented in the commercial finite element code ADINA and verified to produce results that were similar to those of 2D and 3D VF models with meshed fluid domains. Computation times were reduced by 86% for 2D VF models and 74% for 3D VF models while core vibratory characteristic changes were less than 5%. The results from using the UDF demonstrate that computation times could be reduced while still producing acceptable results. A genetic algorithm optimizer was developed to study the effects of altering geometry and material elasticity on frequency, closed quotient (CQ), and maximum flow declination rate (MFDR). The objective was to achieve frequency and CQ values within the normal human physiological range while maximizing MFDR. The resulting models enabled an exploration of trends between objective and design variables. Significant trends and aspects of model variability are discussed. The results demonstrate the benefit of using a structured model exploration method to create models with desirable characteristics. Two synthetic VF models were fabricated to validate predictions made by models produced by the genetic algorithm. Fabricated models were subjected to tests where frequency, CQ, and sound pressure level were measured. Trends between computational and synthetic VF model responses are discussed. The results show that predicted frequency trends between computational and synthetic models were similar, trends for closed quotient were inconclusive, and relationships between MFDR and sound pressure level remained consistent. Overall, while discrepancies between computational and synthetic VF model results were observed and areas in need of further study are noted, the study results provide evidence of potential for using the present optimization method to design synthetic VF models.

Synthetic, self-oscillating vocal fold (VF) models are physical models whose life-like vibration is induced and perpetuated by fluid flow. Self-oscillating VF models, which are often fabricated life-size from soft silicone elastomers, are used to study various aspects of voice biomechanics. Despite their many advantages, the development and use of self-oscillating VF models is limited by the casting process used to fabricate the models. Consequently, this thesis focuses on the development of 3D printing processes for fabricating silicone VF models. A literature review is first presented which describes three types of material extrusion 3D printing processes for silicone elastomers, namely direct ink writing (DIW), embedded 3D printing, and removable-embedded 3D printing. The review describes each process and provides recent examples from literature that show how each has been implemented to create silicone prints. An embedded 3D printing process is presented wherein a set of multi-layer VF models are fabricated by extruding silicone ink within a VF-shaped reservoir filled with a curable silicone support matrix. The printed models successfully vibrated during testing, but lacked several desirable characteristics which were present in equivalent cast models. The advantages and disadvantages of using this fabrication process are explored. A removable-embedded 3D printing process is presented wherein shapes were fabricated by extruding silicone ink within a locally-curable support matrix then curing the silicone ink and proximate matrix. The printing process was used to fabricate several geometries from a variety of silicone inks. Tensile test results show that printed models exhibit relatively high failure strains and a nearly isotropic elastic modulus in directions perpendicular and parallel to the printed layers. A set of single-material VF models were printed and subjected to vibration testing. The printed models exhibited favorable vibration characteristics, suggesting the continued use of this printing process for VF model fabrication. A micro-slicing process is presented which is capable of creating gcode for 3D printing multiple materials in discrete and mixed ratios by utilizing a previously-sliced single-material shape and a material definition. An important advantage of micro-slicing is its ability to create gcode with a mixed-material gradient. Initial test results and observations are included. This micro-slicing process could be used in material extrusion 3D printing