Graduate Thesis Or Dissertation
 

Mechanical Behavior of Elastomer Polymer under Shear and Compression: Experimental Investigation Using Digital Image Correlation

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v405sg84q

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  • Due to their viscoelastic properties, polymers and hydrogels are widely used in many applications. In this study, the viscoelastic properties of elastomer polymers were investigated for a series of hydrogel, hydrogel composites and silicon materials. A primary application for hydrogels and hydrogel composites is as a replacement for the degenerated nucleus pulposus (NP). A common source of orthopedic pathology is degeneration and herniation of the gel-like NP component of intervertebral discs in the spine. Hydrogels are a logical choice for replacement or augmentation of the NP as a therapeutic intervention, but alone lack strength for long-term utility. In the first study of this research program, we investigated a novel class of hydrogel/foam composites to identify materials with similar viscoelastic characteristics to the native tissue, but greater strength. Low acyl gellan gum and agarose in varying weight percentages were infused into a cellulose sponge material to produce three candidate composites. Dynamic oscillatory shear tests and dynamic oscillatory axial compression tests were conducted on the materials at frequencies between (f = 0.1- 10 Hz) to measure elastic (storage) and viscous (loss) moduli. The results show that infusing hydrogels into cellulose foam significantly affects viscoelastic properties compared with the hydrogel alone, in particular increasing the loss modulus by a factor of 20. One gel foam composite, 2 wt. % low acyl gellan gum with 1 wt. % agarose, was closely matched with the viscoelastic properties of native NP tissue. In the second study, we developed a disc emulator device that replicates the biomechanically complex environment within lumbar intervertebral discs, motivated by the need to study nucleus pulposus replacement materials under realistic loading conditions. The device is composed of: 1) an anatomically shaped artificial annulus fibrosus made of silicone, 2) a nucleus pulposus molded from hydrogel or hydrogel/foam composites, and 3) vertebral body analogues 3D printed from combinations of hard (acrylonitrile butadiene styrene, ABS) and soft (thermoplastic polyurethane, TPU) polymers. The capability to alter the mechanical properties of each component of the disc emulator to match the desired physiological response of the spinal motion segment, and the ability to alter these designs quickly and inexpensively using 3D printing, make the disc emulator unique. Mechanical compression experiments show good agreement with available human lumbar spine motion segment mechanical test data, for both intact and de-nucleated segments. The disc emulator provides a simple and accessible means of testing nucleus pulposus replacement materials, examining the effects of age related degeneration on annulus fibrosus properties, and validating finite element models of motion segment behavior. In a third study, we expanded the mechanical evaluation of the disc emulator by using digital image correlation (DIC) to quantify disc bulging and a transducer to measure pressure when a nucleus pulposus material is included in the disc construct. The disc emulator was used to evaluate a range of annulus fibrosus stiffness representations, both with and without a nucleus pulposus present. Disc bulging patterns documented with the emulator match patterns shown in studies of lumbar cadaver samples, and nucleus pressure are similar to documented clinical values. The results obtained from this research have contributed to our understanding of treating disc degeneration with composite gel/foam materials, and validated a novel device for studying this complex biomechanical system.
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  • 2018-08-30 to 2019-03-28

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