Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Biomedical Engineering

Supervisor

Samani, Abbas

2nd Supervisor

Lacefield, James

Joint Supervisor

Abstract

Over the past few years a variety of clinical procedures aiming at tissue repair and other relevant therapies have been under active investigation [12,32]. Success of procedures aimed at soft tissue repair depend on the combined response of biochemical and biomechanical properties of the organs neighbouring the tissue [53]. Using human or animal cadaveric tissue for this purpose is very challenging due to issues pertaining to biodegradability and infection or biohazard risk factors [135,205].As such, tissue mimicking materials (e.g. Polyvinyl Alcohol Cryo-gel (PVA-C)) have been investigated to satisfy the need for the said clinical applications. Advantages of using tissue-mimicking materials include (a) biocompatibility, (b) being not biodegradable and long term shape preservation and (c) having similar biomechanical properties of human tissue [76,126]. To assess biomechanical compatibility of tissue mimicking materials, various mechanical testing techniques have been proposed. Among them, indentation testing has shown great potential for this purpose and it has been used broadly for tissue biomechanical characterization [158]. This method has become more popular because it allows for cost effective, non-destructive, quick, and quantitative assessment of soft tissue biomechanics [64,193]. Soft tissue is idealized as non-linear [46], isotropic [72] and incompressible [198] material. Given its interesting properties and biocompatibility, PVA-C has attracted a great deal of attention as a biocompatible material suitable for clinical applications such as tissue repair, tissue engineering etc. As such, many studies have been conducted to understand this material’s mechanical properties and its suitability for fabricating artificial cornea replacement [54], heart valve [90], lung [164], breast [167], kidney [169], brain [195], stomach [160], bladder [18], prostate [36] and articular cartilage [20] This stems from that this material has similar characteristics to human soft tissue [44,46,129]. Similar to biological tissues, the internal structure of PVA-C leads to nonlinear behavior [66, 80]. This nonlinearity becomes predominant while it undergoes large deformation [205]. Several analytical, semi-analytical and computational models have been proposed to understand tissue mechanical behavior, including its linear and nonlinear behavior, under indentation testing[60]. These include the methods proposed by Boussinesq [27],Sneddon[176],Hayes[77], Cao [34]. This thesis aims at gaining in-depth insight into the mechanical behavior of PVA-C under indentation testing. To this end it presents development of an inverse Finite Element (FE) techniques solved using numerical optimization to characterize the mechanical properties of PVA-C specimens. used to understand the indentation response of PVA-C at different thickness and conditions. The investigation reported in this thesis includes numerical analysis where displacement influence factor was employed in conjunction with linear elastic model of finite thickness. In the analysis, effects of Poisson’s ratio, specimen aspect ratio and relative indentation depth were investigated and a novel mathematical term was introduced to Sneddon’s equation. Results indicate that the developed models have been successful to characterize PVA-C material while they can be used effectively in characterizing the mechanical behavior of biological tissue specimens obtained from medical intervention.

Summary for Lay Audience

A variety of clinical procedures for assessing structural and functional damage of tissues and treating the effectiveness of tissue therapeutics are under active investigation. Therapeutics of soft tissues depends on the mechanical response of the organs and neighboring tissues. Indentation techniques can be used to probe the local mechanical properties of soft tissues and tissue mimics. Although there are some distinct advantages of using indentation testing, the interpretation of the force-displacement behavior of very soft materials is less straight-forwards. Non-linear, hyper-elastic models have been used previously to characterize sources of non linearities (ie. material and geometrical) in this type of problem but have presented some problems. Indentation responses from cylindrical indenters are investigated in this study using numerical methods to develop and optimize new techniques for characterizing nonlinear material properties using an Ogden and Mooney Rivlin hyper-elastic models.

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