Publication Date

2017-04-07

Availability

Embargoed

Embargo Period

2019-04-07

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Mechanical Engineering (Engineering)

Date of Defense

2017-03-10

First Committee Member

Landon R. Grace

Second Committee Member

Michael R. Swain

Third Committee Member

Emrah Celik

Fourth Committee Member

Leonard Pinchuk

Abstract

Poly(Styrene-block-Isobutylene-block-Styrene)(SIBS) is a biocompatible thermoplastic elastomer used extensively in the biomedical field. The outstanding biocompatibility and tailorability of this material make it an ideal candidate for a wide range applications. From coating applications in coronary stents to ophthalmic devices for glaucoma treatment, SIBS has shown great potential in many devices. In this work, the use of SIBS in prosthetic applications is investigated, along with the requisite analysis of material relaxation to better understand long-term performance. An efficient, low-cost, automated, novel method to fabricate orbital prosthesis based on non-contact facial topography mapping and 3-D printing is proposed and validated. The virtual anatomy map is reconstructed and manipulated in order to create a perfectly symmetric, custom fitting virtual prosthesis and its subsequent negative mold for injection molding. This method is the first of its kind to allow the use of thermoplastic elastomers for the fabrication of orbital prosthesis, allowing the prosthesis to last longer and be repaired instead of having to be discarded and refabricated. A SIBS thermoplastic elastomer is selected for the application due to its exceptional biocompatibility. The patient commented on the more comfortable fit and the closer matching symmetry of the 3D printed prosthesis compared to the conventional silicone casted prosthesis. A fluid-immersed, electromagnetically driven, dynamic ocular piece is also proposed for the orbital prosthesis. An infrared camera that captures the contralateral pupil is used to detect the position of the healthy eye and maps the dynamic ocular piece for real-time synchronized movement of the prosthesis. The device showed potential for usage in ocular prosthesis, surpassing human saccade speeds and achieving the necessary rotations to simulate the human eye. Fabrication and material plasticization were also analyzed in order to maximize material performance for prosthetic applications. A Taguchi method coupled with a Response Surface Methodology and Artificial Neural Network are used to determine the optimal injection molding parameters of SIBS for maximum tensile strength. Optimized validation samples tensile strengths are predicted with errors of less than 3% and 2.55% respectively. Lipid uptake is analyzed via gravimetric measurements in SIBS by simulated immersion in triglycerides. The effect of molecular weight and relative styrene content are used to predict diffusion behavior of SIBS based on a first order polynomial model. Saturation lipid content was inversely related to polystyrene content and molecular weight. Diffusion showed slight non-Fickian behavior due to relaxation and plasticization effects of the polymer network. Swelling and degradation due to polymer relaxation and plasticization are analyzed in order to predict performance in long-term in vivo applications. SIBS is contaminated up to various lipid saturation contents in order to characterize mechanical and viscoelastic behavior with lipid exposure. Plasticization is fully analyzed via uniaxial tension, stress relaxation, dynamic creep, microscopy and spectroscopy. New degradation mechanisms are observed and described that explain the mechanics of lipid-induced plasticization in SIBS. Higher molecular weight showed slower lipid-induced plasticization, potentially allowing an ultra-high molecular weight SIBS be used for load bearing applications without sacrificing biocompatibility with particle reinforcements. A hyperelastic material model is used to model the mechanical behavior of contaminated SIBS and used to predict long-term performance in lipid-rich environments using a coupled diffusion-swelling finite element method approach. This novel method to calculate swelling coefficient can be readily expanded to different material systems and allows more accurate evaluation of lipid-induced relaxation.

Keywords

Orbital Prosthesis; SIBS; Plasticization; Lipids; Difussion; Swelling

Available for download on Sunday, April 07, 2019

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