Optimization of Indentation for the Material Characterization of Soft PVA-Cryogels

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

Nonlinear effects in finite elements analysis of colorectal surgical clamping

Minimal Invasive Surgery (MIS) is a procedure that has increased its applications in past few years in different types of surgeries. As number of application fields are increasing day by day, new issues have been arising. In particular, instruments must be inserted through a trocar to access the abdominal cavity without capability of direct manipulation of tissues, so a loss of sensitivity occurs. Generally speaking, the student of medicine or junior surgeons need a lot of practice hours before starting any surgical procedure, since they have to difficulty in acquiring specific skills (hand–eye coordination among others) for this type of surgery. Here is what the surgical simulator present a promising training method using an approach based on Finite Element Method (FEM). The use of continuum mechanics, especially Finite Element Analysis (FEA) has gained an extensive application in medical field in order to simulate soft tissues. In particular, colorectal simulations can be used to understand the interaction between colon and the surrounding tissues and also between colon and instruments. Although several works have been introduced considering small displacements, FEA applied to colorectal surgical procedures with large displacements is a topic that asks for more investigations. This work aims to investigate how FEA can describe non-linear effects induced by material properties and different approximating geometries, focusing as test-case application colorectal surgery. More in detail, it shows a comparison between simulations that are performed using both linear and hyperelastic models. These different mechanical behaviours are applied on different geometrical models (planar, cylindrical, 3D-SS and a real model from digital acquisitions 3D-S) with the aim of evaluating the effects of geometric non-linearity. Final aim of the research is to provide a preliminary contribution to the simulation of the interaction between surgical instrument and colon tissues with multi-purpose FEA in order to help the preliminary set-up of different bioengineering tasks like force-contact evaluation or approximated modelling for virtual reality (surgical simulations). In particular, the contribution of this work is focused on the sensitivity analysis of the nonlinearities by FEA in the tissue-tool interaction through an explicit FEA solver. By doing in this way, we aim to demonstrate that the set-up of FEA computational surgical tools may be simplified in order to provide assistance to non-expert FEA engineers or medicians in more precise way of using FEA tools

The development of a soft tissue mimicking hydrogel: Mechanical characterisation and 3D printing

Accurate tissue phantoms are difficult to design due to the complex hyperelastic, viscoelastic and biphasic properties of real soft tissues. The aim of this work is to demonstrate the tissue mimicking ability of a composite hydrogel (CH), constituting of poly(vinyl alcohol) (PVA) and phytagel (PHY), as a soft tissue phantom over a range mechanical properties, for a variety of biomedical and tissue engineering applications. Its compressive stress-strain behaviour, relaxation response, tensile impact stresses and surgical needle-tissue interactions were mapped and characterised with respect to its constituent hydrogel formulation. The mechanical characterisation of biological tissues was also investigated and the results were used as the ground truth for mimicking. The best mimicking hydrogel compositions were determined by combining the most relevant mechanical properties for each desired application. This thesis demonstrates the use of the tissue mimicking composite hydrogel formulations as tissue phantoms for various surgical procedures, including convection enhanced drug delivery, and traumatic brain injury studies. To expand the applications of the CH, a preliminary biological evaluation of the hydrogel was performed using human dermal fibroblasts. Cell seeded on the collagen-coated composite hydrogel showed good attachment and viability. Finally, a novel fabrication method with the aim of creating samples that replicate the anisotropic properties of biological tissues was developed. A cryogenic 3D printing method utilising the liquid to solid phase change of the composite hydrogel ink was achieved by rapidly cooling the ink solution below its freezing point. The setup was able to successfully create complex 3D brain mimicking material. The method was validated by showing that the mechanical and microstructural properties of the 3D printed material was well matched to its cast-moulded equivalent. This greatly widens the applications of the CH as a mechanically accurate tool for in-vitro testing and also demonstrates promise for future mechanobiology and tissue engineering studies.Open Acces

Realistic tool-tissue interaction models for surgical simulation and planning

Surgical simulators present a safe and potentially effective method for surgical training, and can also be used in pre- and intra-operative surgical planning. Realistic modeling of medical interventions involving tool-tissue interactions has been considered to be a key requirement in the development of high-fidelity simulators and planners. The soft-tissue constitutive laws, organ geometry and boundary conditions imposed by the connective tissues surrounding the organ, and the shape of the surgical tool interacting with the organ are some of the factors that govern the accuracy of medical intervention planning.\ud \ud This thesis is divided into three parts. First, we compare the accuracy of linear and nonlinear constitutive laws for tissue. An important consequence of nonlinear models is the Poynting effect, in which shearing of tissue results in normal force; this effect is not seen in a linear elastic model. The magnitude of the normal force for myocardial tissue is shown to be larger than the human contact force discrimination threshold. Further, in order to investigate and quantify the role of the Poynting effect on material discrimination, we perform a multidimensional scaling study. Second, we consider the effects of organ geometry and boundary constraints in needle path planning. Using medical images and tissue mechanical properties, we develop a model of the prostate and surrounding organs. We show that, for needle procedures such as biopsy or brachytherapy, organ geometry and boundary constraints have more impact on target motion than tissue material parameters. Finally, we investigate the effects surgical tool shape on the accuracy of medical intervention planning. We consider the specific case of robotic needle steering, in which asymmetry of a bevel-tip needle results in the needle naturally bending when it is inserted into soft tissue. We present an analytical and finite element (FE) model for the loads developed at the bevel tip during needle-tissue interaction. The analytical model explains trends observed in the experiments. We incorporated physical parameters (rupture toughness and nonlinear material elasticity) into the FE model that included both contact and cohesive zone models to simulate tissue cleavage. The model shows that the tip forces are sensitive to the rupture toughness. In order to model the mechanics of deflection of the needle, we use an energy-based formulation that incorporates tissue-specific parameters such as rupture toughness, nonlinear material elasticity, and interaction stiffness, and needle geometric and material properties. Simulation results follow similar trends (deflection and radius of curvature) to those observed in macroscopic experimental studies of a robot-driven needle interacting with gels

Cartilage contact pressure in the knee during walking in healthy and degenerated conditions: a subject-specific Finite Element modeling analysis

L’osteoartrosi inficia gravemente la biomeccanica di ginocchio. L’individuazione precoce è cruciale per limitare danni alla cartilagine e il suo processo degenerativo. Nella ricerca sull’osteoartrosi, i modelli computazionali hanno un ruolo importante nell’analisi quantitativa della distribuzione in-vivo dei carichi nei tessuti. Lo scopo di questo studio è analizzare le pressioni di contatto sulla cartilagine di ginocchio nel cammino in condizioni sane e patologiche, utilizzando un modello agli elementi finiti personalizzato da MRI. Il modello con cartilagine sana include: articolazione tibiofemorale con relative ossa, cartilagini, legamenti e muscoli, e articolazione d’anca. La cartilagine è modellata come iperelastica, e il contatto tra cartilagini senza attrito. Sono stati sviluppati due modelli con cartilagine degenerata: con difetto cartilagineo, e con materiale più cedevole intorno al difetto. L’analisi è stata condotta per la fase di appoggio del ciclo del passo. Sono state confrontate le pressioni di contatto del modello sano con quelle ottenute da un modello multi-corpo precedentemente sviluppato, e sono state analizzate le differenze tra modelli con cartilagine sana e degenerata. Abbiamo ottenuto pressioni di contatto simili tra il nostro modello sano e quello multicorpo (R^2=0.94), e ciò valida indirettamente quest’ultimo. Abbiamo riscontrato che un difetto cartilagineo induce un significativo aumento di pressione fino al 75% in confronto a condizioni sane, in particolare intorno al difetto. L’indebolimento nelle proprietà materiali induce poi una diversa distribuzione di pressione a seguito di una maggiore area di contatto. Nonostante i limiti, questo studio risulta rilevante nella comprensione dei meccanismi di degenerazione cartilaginea. La forza del modello risiede nell’approccio MRI-based ed open-source e nella parametrizzazione del modello per studiare molteplici attività motorie ed interazioni tra i tessuti

Instrumentation for multiaxial mechanical testing of inhomogeneous elastic membranes

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 93-100).This thesis presents the design, development, and construction of an instrument for biaxial mechanical testing of inhomogeneous elastic membranes. The instrument incorporates an arrangement of linear motion stages for applying arbitrary deformation profiles on the material under test, purpose-built two-axis force transducers for high-resolution measurement of applied loads, and a digital imaging system for full-field strain measurement. The components described herein provide the foundation for a sophisticated biaxial testing platform for determining the mechanical properties of anisotropic, inhomogeneous membrane materials.by Ariel Marc Herrmann.S.M

Doctor of Philosophy

dissertationDespite the progress that has been made since the inception of the finite element method, the field of biomechanics has generally relied on software tools that were not specifically designed to target this particular area of application. Software designed specifically for the field of computational biomechanics does not appear to exist. To overcome this limitation, FEBio was developed, an acronym for “Finite Elements for Biomechanics”, which provided an open-source framework for developing finite element software that is tailored to the specific needs of the biomechanics and biophysics communities. The proposed work added an extendible framework to FEBio that greatly facilitates the implementation of novel features and provides an ideal platform for exploring novel computational approaches. This framework supports plugins, which simplify the process of adding new features even more since plugins can be developed independently from the main source code. Using this new framework, this work extended FEBio in two important areas of interest in biomechanics. First, as tetrahedral elements continue to be the preferred modeling primitive for representing complex geometries, several tetrahedral formulations were investigated in terms of their robustness and accuracy for solving problems in computational biomechanics. The focus was on the performance of quadratic tetrahedral formulations in large deformation contact analyses, as this is an important area of application in biomechanics. Second, the application of prestrain to computational models has been recognized as an important component in simulations of biological tissues in order to accurately predict the mechanical response. As this remains challenging to do in existing software packages, a general computational framework for applying prestrain was incorporated in the FEBio software. The work demonstrated via several examples how plugins greatly simplify the development of novel features. In addition, it showed that the quadratic tetrahedral formulations studied in this work are viable alternatives for contact analyses. Finally, it demonstrated the newly developed prestrain plugin and showed how it can be used in various applications of prestrain

Biomechanical Models of Human Upper and Tracheal Airway Functionality

The respiratory tract, in other words, the airway, is the primary airflow path for several physiological activities such as coughing, breathing, and sneezing. Diseases can impact airway functionality through various means including cancer of the head and neck, Neurological disorders such as Parkinson\u27s disease, and sleep disorders and all of which are considered in this study. In this dissertation, numerical modeling techniques were used to simulate three distinct airway diseases: a weak cough leading to aspiration, upper airway patency in obstructive sleep apnea, and tongue cancer in swallow disorders. The work described in this dissertation, therefore, divided into three biomechanical models, of which fluid and particulate dynamics model of cough is the first. Cough is an airway protective mechanism, which results from a coordinated series of respiratory, laryngeal, and pharyngeal muscle activity. Patients with diminished upper airway protection often exhibit cough impairment resulting in aspiration pneumonia. Computational Fluid Dynamics (CFD) technique was used to simulate airflow and penetrant behavior in the airway geometry reconstructed from Computed Tomography (CT) images acquired from participants. The second study describes Obstructive Sleep Apnea (OSA) and the effects of dilator muscular activation on the human retro-lingual airway in OSA. Computations were performed for the inspiration stage of the breathing cycle, utilizing a fluid-structure interaction (FSI) method to couple structural deformation with airflow dynamics. The spatiotemporal deformation of the structures surrounding the airway wall was predicted and found to be in general agreement with observed changes in luminal opening and the distribution of airflow from upright to supine posture. The third study describes the effects of cancer of the tongue base on tongue motion during swallow. A three-dimensional biomechanical model was developed and used to calculate the spatiotemporal deformation of the tongue under a sequence of movements which simulate the oral stage of swallow