VTechWorks Repository :: Browsing by Author "De Vita, Raffaella"

Browsing by Author "De Vita, Raffaella"

Now showing 1 - 20 of 69

Advanced Echocardiographic Imaging In Dogs With Myxomatous Mitral Valve Disease
Menciotti, Giulio (Virginia Tech, 2017-05-23)
Myxomatous mitral valve degeneration (MMVD) is the most common canine cardiac disease. In the studies presented in this dissertation, we used advanced echocardiographic techniques to elucidate several aspects of MMVD in dogs. Our hypothesis was that the mitral valve (MV) morphology could have a role in the development of MMVD. First, we tested whether we could use real time three-dimensional transthoracic echocardiography (RT-3DTTE), and an offline software for MV analysis to evaluate canine MV. We described that the technique was feasible and repeatable, we evaluated the morphology of the MV in healthy dogs, and we provided reference values for MV morphologic variables in this species. Then, we used the same technique to compare healthy dogs to dogs affected by MMVD. We found that dogs affected by MMVD have more circular and flatter valve. We then analyzed the MV of healthy Cavalier King Charles Spaniels (CKCSs), given the high predisposition of this breed for MMVD. Our findings indicate that compared to healthy dogs of other breeds, the MV of healthy CKCSs is flatter and has less leaflet tenting, corroborating our hypothesis that an altered MV morphology could represent a predisposing factor for disease development. We also used RT–3DTTE to characterize the area of the regurgitant MV orifice of dogs affected with MMVD, finding that the technique requires further standardization in order to become clinically useful. The elevation of pulmonary venous pressure caused by MMVD can, in some dogs, cause pulmonary arterial hypertension (PH), which is a risk factor associated with worse outcome in dogs with MMVD. Diagnosis of PH in dogs with MMVD is usually made by estimating pulmonary pressure using Doppler echocardiography. We are currently evaluating the accuracy of this technique, compared to invasive measurement of pulmonary pressure. Only preliminary data are presented regarding this study, as the disclosure of the blinding would have infringed the power of the study. Our preliminary results demonstrate that there is only moderate agreement between the two techniques, indicating that caution should be used when deriving the non-invasive estimation of systolic pulmonary pressure in order to make clinical decisions.
Advancing Maternal Health through Projection-based and Machine Learning Strategies for Reduced Order Modeling
Snyder, William David (Virginia Tech, 2024-06-12)
High-fidelity computer simulations of childbirth are time consuming, making them impractical for guiding decision-making during obstetric emergencies. The complex geometry, micro-structure, and large finite deformations undergone by the vagina during childbirth result in material and geometric nonlinearities, complicated boundary conditions, and nonhomogeneities within finite element (FE) simulations. Such nonlinearities pose a significant challenge for numerical solvers, increasing the computational time. Simplifying assumptions can reduce the computational time significantly, but this usually comes at the expense of simulation accuracy. The work herein proposed the use of reduced order modeling (ROM) techniques to create surrogate models that capture experimentally-measured displacement fields of rat vaginal tissue during inflation testing in order to attain both the accuracy of higher-fidelity models and the speed of lower-fidelity simulations. The proper orthogonal decomposition (POD) method was used to extract the significant information from FE simulations generated by varying the luminal pressure and the parameters that introduce the anisotropy in the selected constitutive model. In our first study, a new data-driven (DD) variational multiscale (VMS) ROM framework was extended to obtain the displacement fields of rat vaginal tissue subjected to ramping luminal pressure. For comparison purposes, we also investigated the classical Galerkin ROM (G-ROM). In our numerical study, both the G-ROM and the DD-VMS-ROM decreased the FE computational cost by orders of magnitude without a significant decrease in numerical accuracy. Furthermore, the DD-VMS-ROM improved the G-ROM accuracy at a modest computational overhead. Our numerical investigation showed that ROM had the potential to provide efficient and accurate computational tools to describe vaginal deformations, with the ultimate goal of improving maternal health. Our second study compared two common computational strategies for surrogate modeling, physics-based G-ROM and data-driven machine learning (ML), for decreasing the cost of FE simulations of the ex vivo deformations of rat vaginal tissue subjected to inflation testing to study the effect of a pre-imposed tear. Since there are many methods associated with each modeling approach, to provide a fair and natural comparison, we selected a basic model from each category. From the ROM strategies, we considered a simplified G-ROM that is based on the linearization of the underlying nonlinear FE equations. From the ML strategies, we selected a feed-forward dense neural network (DNN) to create mappings from constitutive model parameters and luminal pressure values to either the FE displacement history (in which case we denote the resulting model ML) or the POD coefficients of the displacement history (in which case we denote the resulting model POD-ML). The numerical comparisons of G-ROM, ML, and POD-ML took place in the reconstructive regime. The numerical results showed that the G-ROM outperformed the ML model in terms of offline central processing unit (CPU) time for model training, online CPU time required to generate approximations, and relative error with respect to the FE models. The POD-ML model improved on the speed performance of the ML, having online CPU times comparable to those of the G-ROM given the same size of POD bases. However, the POD-ML model did not improve on the error performance of the ML. In our last study, we expanded our investigation of ML methods for surrogate modeling by comparing the performance of a DNN similar to what was used previously to that of a convolutional neural network (CNN) using 1-D convolution on the input parameters from FE simulations of active vaginal tearing. The new FE simulations utilized a custom continuum damage model that provided material damage and failure properties to an existing anisotropic hyperelastic constitutive model to replicate experimentally-observed tear propagation behaviors. We employed our DNN and CNN models to create mappings from constitutive model parameters, geometric properties of the propagating tear, and luminal pressure values to either the full FE displacement history or the POD coefficients of the displacement history. The root-mean-square error (RMSE) with respect to the FE displacement history achieved by full order output ML predictions was reproducible with POD-ML using a basis of only dimension l=10. Additionally, an order of magnitude reduction in offline time was observed using POD-ML over full-order ML with minimal difference between DNN and CNN architectures. Differences in online computational costs between ML and POD-ML were found to be negligible, but the DNNs produced predictions slightly faster than the CNNs, though both online times were on the same order of magnitude. While convolution did not significantly aid the regression task at hand, POD-ML was demonstrated to be an efficient and effective approach for surrogate modeling of the FE tear propagation model, approximating the displacement history with RMSE less than 0.1 mm and generating results 7 orders of magnitude faster than the FE model. This set of baseline numerical investigations serves as a starting point for future computer simulations that consider state-of-the-art G-ROM and ML strategies, and the in vivo geometry, boundary conditions, material properties, and tissue damage mechanics of the human vagina, as well as their changes during labor.
Anisotropy of Passive and Active Rat Vagina under Biaxial Loading
Huntington, Alyssa Joan (Virginia Tech, 2018-06-11)
Pelvic organ prolapse, the decent of the pelvic organs from their normal anatomical position, is a common condition among women that is associated with mechanical alterations of the vaginal wall. In order to characterize the complex mechanical behavior of the vagina, we performed planar biaxial tests of vaginal specimens in both the passive (relaxed) and active (contracted) states. Specimens were isolated from virgin, female Long-Evans rats (n=16) and simultaneously stretched along the longitudinal direction (LD) and circumferential direction (CD) of the vagina. Tissue contraction was induced by electric field stimulation (EFS) at incrementally increasing values of stretch and, subsequently, by KCl. On average, the vagina was stiffer in the CD than in the LD (p<0.001). The mean maximum EFS-induced active stress was significantly higher in the CD than in the LD (p<0.001). On the contrary, the mean KCl-induced active stress was lower in the CD than in the LD (p<0.01). When comparing the mean maximum EFS-induced active stress to the mean KCl-induced active stress, no differences were found in the CD (p=0.404) but, in the LD, the mean active stress was much higher in response to the KCl stimulation (p<0.001). Collectively, these results demonstrate that the anisotropic behavior of the vaginal tissue is determined not only by the collagen and smooth muscle fiber organization but also by the innervation. The findings of this study may contribute to the development of more effective treatments for pelvic organ prolapse.
Biaxial Mechanical Behavior of Swine Pelvic Floor Ligaments: Experiments and Modeling
Becker, Winston Reynolds (Virginia Tech, 2014-06-08)
Although mechanical alterations to pelvic floor ligaments, such as the cardinal and uterosacral ligaments, are one contributing factor to the development and progression of pelvic floor disorders, very little research has examined their mechanical properties. In this study, the first biaxial elastic and viscoelastic tests were performed on uterosacral and cardinal ligament complexes harvested from adult female swine. Biaxial elastic testing revealed that the ligaments undergo large strains and are anisotropic. The direction normal to the upper vagina was typically stiffer than the transverse direction. Stress relaxation tests showed that the relaxation was the same in both directions, and that more relaxation occurred when the tissue was stretched to lower initial strains. In order to describe the experimental findings, a three-dimensional constitutive model based on the Pipkin-Rogers integral series was formulated and the parameters of such model were determined by fitting the model to the experimental data. In formulating the model, it was assumed that the tissues consist of a ground substance with two embedded families of fibers oriented in two directions and that the ligaments are incompressible. The model accounts for finite strains, anisotropy, and strain-dependent stress relaxation behavior. This study provides information about the mechanical behavior of female pelvic floor ligaments, which should be considered in the development of new treatment methods for pelvic floor disorders.
Biaxial Mechanical Evaluation of Uterosacral and Cardinal Ligaments
Baah-Dwomoh, Adwoa Sarpong (Virginia Tech, 2018-03-06)
The uterosacral ligament (USL) and the cardinal ligament (CL) are two major suspensory tissues that provide structural support to the vagina/cervix/uterus complex. These ligaments have been studied mainly due for their role in the surgical repair for pelvic organ prolapse (POP). POP, which is the descent of a pelvic organ from its normal place towards the vaginal walls and into the vaginal cavity, affects an estimated 3.3 million women in the United States annually. Despite their important mechanical function, little is known about the elastic and viscoelastic properties of the USL and CL due to ethical concerns with in vivo testing of human tissues and the lack of accepted animal models. The goal of this first study is to help establish an appropriate animal model for studying the mechanics of these pelvic supportive ligaments. To achieve this, the first rigorous comparison of histological and planar equi-biaxial mechanical properties of the swine and human USLs was completed. Relative collagen, smooth muscle, and elastin contents were quantified from histological sections and the USL was found to have similar components in both species, with a comparable relative collagen content. Using the digital image correlation (DIC) method to calculate the in-plane Lagrangian strain, no differences in the peak strain during precon- ditioning/cyclic loading tests, secant modulus of the pre-creep/elastic response, and strain at the end of creep tests were detected in the USLs from the two species along both axial loading directions (the main in vivo loading direction and the direction that is perpendicular to it). Because these ligaments are subjected to repeated constant loads in vivo, the effect of re- peated biaxial loads at three different load levels (1 N, 2 N, or 3 N) on elastic and creep properties of the swine CL was investigated. The results showed that CL was elastically anisotropic, as statistical differences were found between the mean strains along the two axial loading directions for specimens at all three different load levels. The increase in strain over time by the end of the 3rd creep test was comparable along the axial loading direc- tions. The greatest mean normalized strain (or, equivalently, the largest increase in strain over time) was measured at the end of the 1st creep test, regardless of the equi-biaxial load magnitude or loading direction. Overall, these experimental findings validate the use of swine as an appropriate animal model and offer new knowledge of the mechanical properties of the USL and CL that can guide the development of better treatment methods such as surgical reconstruction for POP.
Biaxial Response of Individual Bonds in Thermomechanically Bonded Nonwoven Fabrics
Wijeratne, Roshelle Sumudu (Virginia Tech, 2017-06-29)
Thermomechanically bonded spunbond nonwoven fabrics contain discrete bonds that are formed by melted and fused fibers. Through equi-biaxial tensile testing and simultaneous image capture, the mechanical response of individual bonds was studied through loading in the preferential fiber direction, the machine direction, and in the direction that is perpendicular, the cross direction, of the fabric web. Independent biaxial force and displacement data were collected and analyzed, and the maximum force and stiffness of the bonds in the machine and cross directions were found to be statistically different. After scaling the maximum force and stiffness by a relative basis weight parameter, a fiber orientation parameter, and the width of the bond itself, the peak force and stiffness in the machine and cross directions were found to no longer be statistically different. This indicates that basis weight, fiber orientation, and bond size dictate the biaxial mechanical behavior of the bonds. Furthermore, significant fiber debonding was observed in all the bonds tested, effectively suggesting bond disintegration into the individual component fibers during testing. Digital image correlation, using the captured images, was utilized to calculate local and average Eulerian strains of the bond during the initial stages of the test. The strain experienced by the bonds in the machine direction was always positive and increasing as the biaxial load increased. The strain in the cross direction, however, experienced increasing and decreasing strain. Local strain maps revealed the highly inhomogeneous strain response of the bonds under biaxial loading.
Bifurcations, Multi-stability, and Localization in Thin Structures
Yu, Tian (Virginia Tech, 2020-01-22)
Thin structures exist as one dimensional slender objects (hairs, tendrils, telephone cords, etc.) and two dimensional thin sheets (tree leaves, Mobius bands, eggshells, etc.). Geometric and material nonlinearities can conspire together to create complex phenomena in thin structures. This dissertation studies snap-through, multi-stability, and localization in thin rods and sheets through a combination of experiments and numerics. The first work experimentally explores the multi-stability and bifurcations of buckled elastic strips subject to clamping and lateral end translations, and compares these results with numerical continuation of a perfectly anisotropic Kirchhoff rod model. It is shown that this naive Kirchhoff rod model works surprisingly well as an organizing framework for thin bands with various widths. Thin sheets prefer to bend rather than to stretch because of the high cost of stretching energy. Knowing the bending response of thin sheets can aid in simulating deformations such as creasing. The second work introduces an exact pure bending linkage mechanism for potential use in a bend tester that measures the moment-curvature relationship of soft sheets and filaments. Mechanical rotary pleating is a bending-deformation-dominant process that deforms nonwoven materials into zigzag filter structures. The third work studies what combinations of processing and material parameters lead to successful rotary pleating. The rotary pleating process is formulated as a multi-point variable-arc-length boundary value problem for an inextensible rod, with a moment-curvature constitutive law, such as might be measured by a bend tester, as input. Through parametric studies, this work generates pleatability surfaces that may help avoid pleating failure in the real pleating process. Creased thin sheets are generally bistable. The final work of this dissertation studies bistability of creased thin disks under the removal of singularities. A hole is cut in the disk and, through numerical continuation of an inextensible strip model, this work studies how the crease stiffness, crease angle, and hole geometry affect the bistability.
Bilayer Network Modeling
Creasy, Miles Austin (Virginia Tech, 2011-08-08)
This dissertation presents the development of a modeling scheme that is developed to model the membrane potentials and ion currents through a bilayer network system. The modeling platform builds off of work performed by Hodgkin and Huxley in modeling cell membrane potentials and ion currents with electrical circuits. This modeling platform is built specifically for cell mimics where individual aqueous volumes are separated by single bilayers like the droplet-interface-bilayer. Applied potentials in one of the aqueous volumes will propagate through the system creating membrane potentials across the bilayers of the system and ion currents through the membranes when proteins are incorporated to form pores or channels within the bilayers. The model design allows the system to be divided into individual nodes of single bilayers. The conductance properties of the proteins embedded within these bilayers are modeled and a finite element analysis scheme is used to form the system equations for all of the nodes. The system equation can be solved for the membrane potentials through the network and then solve for the ion currents through individual membranes in the system. A major part of this work is modeling the conductance of the proteins embedded within the bilayers. Some proteins embedded in bilayers open pores and channels through the bilayer in response to specific stimuli and allow ion currents to flow from one aqueous volume to an adjacent volume. Modeling examples of the conductance behavior of specific proteins are presented. The examples demonstrate aggregate conductance behavior of multiple embedded proteins in a single bilayer, and at examples where few proteins are embedded in the bilayer and the conductance comes from a single-channel or pore. The effect of ion gradients on the single channel conductance example is explored and those effects are included in the single-channel conductance model. Ultimately these conductance models are used with the system model to predict ion currents through a bilayer or through part of a bilayer network system. These modeling efforts provide a modeling tool that will assist engineers in designing bilayer network systems.
Biomechanical analysis of a novel suture pattern for repair of equine tendon lacerations
Everett, Eric K. (Virginia Tech, 2011-04-14)
Flexor tendon lacerations in horses are traumatic injuries that can be career ending and life threatening. In the horse, a tendon repair must withstand the strains placed on the tenorrhaphy by immediate weight bearing and locomotion post-operatively. Despite the use of external coaptation, such strains can lead to significant gap formation, construct failure, longer healing time and poor quality of the healed tendon. Similar to equine surgery, gap formation and construct failure are common concerns in human medicine, with early return to post-operative physiotherapy challenging the primary repair. Early return to exercise and decreased gap formation has been shown to reduce adhesion formation. Based on these concerns, the ideal tenorrhaphy suture pattern for equines would provide: 1) high ultimate failure load, 2) resistance to gap formation, 3) minimal alteration in blood supply, and 4) minimal adhesion formation. Historically, various suture patterns and materials have been evaluated for human and equine flexor tendon repair. Results of equine studies suggest the three-loop pulley pattern (3LP) compares favorably to other patterns and is recommended for primary tenorrhaphy. However, this pattern still experiences significant gap formation and can result in failure. As a result, a technique which decreases the problems inherent in the 3LP is warranted for tenorrhaphy of equine flexor tendons. A review of the human literature highlights certain characteristics of the tenorrhaphy that may improve results including core purchase length and suture loop characteristics. Optimization of these tenorrhaphy characteristics can increase tenorrhaphy performance and patient outcome. The six-strand Savage technique (SSS) is a pattern routinely used in human hand surgery for tendon repair, and possesses high ultimate failure load and resistance to gap formation that may be beneficial for application in equine tendon repair. This study compared a novel tenorrhaphy pattern for horses, the SSS, with the currently recommended pattern, the 3LP, in an in vitro model. We hypothesize the SSS will fail at a higher ultimate load, resist pull through, and resist gap formation better than the 3LP. All testing used cadaveric equine superficial digital flexor tendons from horses euthanized for reasons other than musculoskeletal injury. All testing was approved by the IACUC. The two techniques were applied to cadaveric equine superficial digital flexor tendons. The same investigator performed all repairs (EE). Biomechanical properties were determined in a blinded, randomized pair design. Ultimate failure load, mode of failure and load required to form a 3mm gap were recorded on an Instron Electropuls materials testing system. Gap formation was determined using synchronized high-speed video analysis. Results are reported as mean + standard deviation. Statistical comparisons were made using Student's T test, with significance set at p<0.05. The tenorrhaphies were tested for their ultimate failure load and failure mode. The mean failure load for the SSS construct (421.1 ± 47.6) was significantly greater than that for the 3LP repaired tendons (193.7 ± 43.0). Failure mode was suture breakage for the SSS constructs (13/13) and suture pull through for the 3LP constructs (13/13). The maximum load to create a 3mm gap in the SSS repair (102.0N ± 22.4) was not significantly different from the 3LP repair (109.9N ± 16.0). The results of the current study demonstrate that the SSS tenorrhaphy has a higher ultimate failure load and resistance to pull through than the 3LP. The biomechanical properties of the SSS technique show promise as a more desirable repair for equine flexor tendons. However, in vivo testing of the effects of the pattern on live tissue and in a cyclic loading environment is necessary before clinical application of the pattern is recommended.
The Biomechanics of Tracheal Compression in the Darkling Beetle, Zophobas morio
Adjerid, Khaled (Virginia Tech, 2019-11-05)
In this dissertation, we examine mechanics of rhythmic tracheal compression (RTC) in the darkling beetle, Zophobas morio. In Chapter 2, we studied the relationship between hemolymph pressure and tracheal collapse to test the hypothesis that pressure is a driving mechanism for RTC. We found that tracheae collapse as pressure increases, but other physiological factors in the body may be affecting tracheal compression in live beetles. Additionally, as the tracheae compress, they do so in varying spatial patterns across the insect body. In chapter 3, we examined spatial variations in the taenidial spacing, stiffness, and tracheal thickness along the length of the tracheae. We related variations in Young's modulus and taenidial spacing with measurements of collapse dimples and found that spatial patterns of Young's modulus correlate with dimensions of collapse dimples. This correlation suggests an intuitive link between tracheal stiffness variations and the unique patterns observed in compressing tracheae. Lastly, in chapter 4, we studied the non-uniform collapse patterns in 3-D. By manually pressurizing the hemocoel and imaging using synchrotron microcomputed tomography (SR-µCT), we reconstructed the tracheal system in its compressed state. While previous studies used 2-D x-ray images to examine collapse morphology, ours is the first to quantify collapse patterns in 3-D and compare with previous 2-D quantification methods. Our method is also the first to make a direct measure of tracheal volume as the tracheal system compresses, similar to the phenomenon that occurs during rhythmic tracheal compression.
Closed Loop Control of Muscle Contraction using Functional Electrical Stimulation
Jaramillo Cienfuegos, Paola (Virginia Tech, 2016-02-05)
A promising approach to treat patients with vocal fold paralysis using electrical stimulation is investigated throughout this research work. Functional Electrical Stimulation works by stimulating the atrophied muscle or group of muscles directly by current when the transmission lines between the central nervous system are disrupted. This technique helps maintain muscle mass and promote blood flow in the absence of a functioning nervous system. The goal of this work is two-fold: develop control techniques for muscle contraction to optimize muscle stimulation and develop a small-scale electromagnetic system to provide stimulation to the laryngeal muscles for patients with vocal fold paralysis. These studies; therefore, focus on assessing a linear Proportional-Integral (PI) controller and two nonlinear controllers: Model Reference Adaptive Controller (MRAC) and an Adaptive Augmented PI (ADP-PI) system to identify the most appropriate controller providing effective stimulation of the muscle. Direct stimulation is applied to mouse skeletal muscle in vitro to test the controllers along with numerical simulations for validation of these experimental tests. The experiments included muscle contractions following four distinct trajectories: a step, sine, ramp, and square wave. Overall, the closed-loop controllers followed the stimulation trajectories set for all the simulated and tested muscles. When comparing the experimental outcomes of each controller, we concluded that the ADP-PI algorithm provided the best closed-loop performance for speed of convergence and disturbance rejection. Next, the focus of the research work was on the implementation of an electromagnetic system to generate appropriate currents of stimulation using the aforementioned controllers. For this study, Nickel-Titanium shape memory alloys were used to assess activation (contraction) through a two-coil system guided by the controllers. The application of the two-coil system demonstrated the effectiveness of the approach and a main effect was observed between the PI, MRAC, and ADP-PI controllers when following the trajectories. Lastly, a small scale two-coil system is developed for animal testing in the muscle-mass-spring setup. Experiments were successful in generating the appropriate stimulation controlled by the output-based algorithms for muscle contraction. Trials conducted for this study were compared to the muscle contractions observed in the first study. The controllers were able to provide appropriate stimulation to the muscle system to follow the set trajectories: a step, ramp, and sinusoidal input. More trials are required to draw statistical conclusions about the performance of each controller. Regardless, the small-scale two-coil system along with the applied controllers can be reconfigured to be an implantable system and tested for appropriate stimulation of the laryngeal muscles.
A Comparison of Methods for Measuring Damage in Sucrose-Treated Medial Collateral Ligaments
Stewart, Victor A. (Virginia Tech, 2013-05-29)
The knee is the most complex joint in the human body. It consists of a system of muscle, bone, and ligaments that endures repetitive loading during daily and athletic activities. When this loading is excessive, damage to the knee occurs leading to a decreased quality of life.The medial collateral ligament (MCL) is one of the 4 major ligaments known to be commonly injured in the knee. The risk of injury to the knee joint increases with the elderly and individuals who experience chronic dehydration. For this reason, the focus of this study is to compare different mechanical quantities that can be used to analyze damage to the MCL. In this study, a novel mechanical testing protocol is used to progressively induce damage in dehydrated rat MCLs by performing tensile tests. This involves stretching the ligaments along their longitudinal axes to consecutive and increasing displacements starting at a 0.4 mm displacement and in increments of 0.2 mm until complete failure occurs. The load and change in length that the ligament experiences are measured at each displacement. Three different methods were evaluated to determine subfailure and damage propagation in rat MCLs: changes in tangent stiffness and chord stiffness, and changes in the load value at the 0.4 mm displacement for each load-displacement curve. The findings of this study indicate that the tangent stiffness and load at the 0.4 mm displacement provide information of the early onset of damage propagation. The decrease in chord stiffness of the ligament does not indicate damage progression in the ligament, but rather is the sign of the imminent failure of the MCL.This study provides insightful data into understanding the subfailure damage in the MCL.
Control of Nanoscale Thermal Transport for Thermoelectric Energy Conversion and Thermal Rectification
Pal, Souvik (Virginia Tech, 2013-12-18)
Materials at the nanoscale show properties uniquely different from the bulk scale which when controlled can be utilized for variety of thermal management applications. Different applications require reduction, increase or directional control of thermal conductivity. This thesis focuses on investigating thermal transport in two such application areas, viz., 1) thermoelectric energy conversion and 2) thermal rectification. Using molecular dynamics simulations, several methods for reducing of thermal conductivity in polyaniline and polyacetylene are investigated. The reduction in thermal conductivity leads to improvement in thermoelectric figure of merit. Thermal diodes allow heat transfer in one direction and prevents in the opposite direction. These materials have potential application in phononics, i.e., for performing logic calculations with phonons. Rectification obtained with existing material systems is either too small or too difficult to implement. In this thesis, a more useful scheme is presented that provides higher rectification using a single wall carbon nanotube (SWCNT) that is covalently functionalized near one end with polyacetylene (PA). Although several thermal diodes are discussed in literature, more complex phononic devices like thermal logic gates and thermal transistors have been sparingly investigated. This thesis presents a first design of a thermal AND gate using asymmetric graphene nanoribbon (GNR) and characterizes its performance.
Debond Buckling of Woven E-glass/Balsa Sandwich Composites Exposed to One-sided Heating
Cholewa, Nathan (Virginia Tech, 2015-01-26)
An experimental investigation was undertaken to analyze the behavior of sandwich composite structures exposed to one-sided heating where a debond exists between the unexposed facesheet and core material. Sandwich composites of plain weave E-glass/epoxy facesheets and an end-grain balsa wood core manufactured using the Vacuum Assisted Resin Transfer Molding (VARTM) technique were the only materials analyzed. These were selected due to their current use in naval vessels and the heightened interest in the fire response properties of balsa wood and its utility as a core material. In order to better understand the interfacial behavior, Mode I Double Cantilever Beam (DCB) fracture tests were performed at ambient, 60 C, and 80 C to determine the influence of the decreased Mode I fracture toughness. While ambient testing showed that stable crack growth could be obtained, high temperature tests resulted in considerable damage occurring to the core at the crack-front preventing stable crack growth. This can be attributed to the significant decrease in the balsa core strength and material properties even for small increases in temperature. Additionally, Mode II Cracked Split Beam (CSB) tests were performed at ambient temperature to examine the sliding dominant crack-growth. Again, the occurrence of balsa core damage prevented stable crack-growth and an accurate measurement of Mode II fracture toughness was not obtained. Intermediate-scale compression testing with one-sided heating at two heat flux levels was performed with a custom designed load frame on sandwich composite columns. This enabled the influence of the debond to be measured using a 3D-Digital Image Correlation (DIC) technique spatially linked with a thermographic camera. The DIC allowed for a detailed observation of debond growth and buckling prior to global failure of the test article. A behavior similar to that observed in the Mode I DCB fracture tests occurred: as the interfacial temperature increased, the amount of crack growth decreased. This crack growth was followed by a core failure at the crack-front, triggering a global failure of the test article. This global failure for test articles containing a debond manifested itself primarily as an anti-symmetric post-buckling shape. Test articles with no debond exhibited the typical progression of the out-of-plane displacement profile for a fixed-fixed column. As the out-of-plane displacement increased, core failure ultimately occurred near the gripped region where the zero-slope condition is required, triggering global failure of the no debond test article. These tests highlight that the reduction in strength and material properties of the end-grain balsa wood core significantly outweigh the reduction in interfacial fracture toughness due to the increased temperatures.
Design and Analysis of a Collagenous Anterior Cruciate Ligament Replacement
Walters, Valerie Irene (Virginia Tech, 2011-05-02)
The anterior cruciate ligament (ACL) contributes to normal knee function, but it is commonly injured and has poor healing capabilities. Of the current treatments available for ACL reconstruction, none replicate the long-term mechanical properties of the ACL. It was hypothesized that tissue-engineered scaffolds comprised of reconstituted type I collagen fibers would have the potential to yield a more suitable treatment for ACL reconstruction. Ultra-violet (UV) radiation and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) were investigated as possible crosslinking methods for the scaffolds, and EDC crosslinking was deemed more appropriate given the gains in strength and stiffness afforded to individual collagen fibers. Scaffolds were composed of 54 collagen fibers, which were made using an extrusion process, organized in accordance with a braid-twist design; the addition of a hydrogel (gelatin) to this scaffold was also investigated. The scaffolds were tested mechanically to determine ultimate tensile strength (UTS), Young's modulus, and viscoelastic properties. Scaffolds were also evaluated for the cellular activity of primary rat lateral collateral ligament (LCL) and medial collateral ligament (MCL) fibroblast cells after 7, 14, and 21 days. The crosslinked scaffolds without gelatin exhibited mechanical and viscoelastic properties that were more similar to the human ACL. Cellular activity on the crosslinked scaffolds without gelatin was observed after 7 and 21 days, but no significant increase was observed with time. Although more studies are needed, these results indicate that a braid- twist scaffold (composed of collagen fibers) has the potential to serve as a scaffold for ACL replacement.
Developing Fast and Accurate Water Models for Atomistic Molecular Dynamics Simulations
Xiong, Yeyue (Virginia Tech, 2021-09-15)
Water models are of great importance for different fields of studies such as fluid mechanics, nano materials, and biomolecule simulations. In this dissertation, we focus on the water models applied in atomistic simulations, including those of biomolecules such as proteins and DNA. Despite water's simple structure and countless studies carried out over the decades, the best water models are still far from perfect. Water models are normally divided into two types--explicit model and implicit model. Here my research is mainly focused on explicit models. In explicit water models, fixed charge n-point models are most widely used in atomistic simulations, but have known accuracy drawbacks. Increasing the number of point charges, as well as adding electronic polarizability, are two common strategies for accuracy improvements. Both strategies come at considerable computational cost, which weighs heavily against modest possible accuracy improvements in practical simulations. With a careful comparison between the two strategies, results show that adding polarizability is a more favorable path to take. Optimal point charge approximation (OPCA) method is then applied along with a novel global optimization process, leading to a new polarizable water model OPC3-pol that can reproduce bulk liquid properties of water accurately and run at a speed comparable to 3- and 4-point non-polarizable water models. For practical use, OPC3-pol works with existing non-polarizable AMBER force fields for simulations of globular protein or DNA. In addition, for intrinsically disordered protein simulations, OPC3-pol fixes the over-compactness problem of the previous generation non-polarizable water models.
Development of an Experimental and Computational Pipeline for Characterizing the Mechanical Properties and Micromechanical Environment within In Vitro 3D Printed Bone Tissue Engineered Scaffolds
Hunt, Elizabeth Albright (Virginia Tech, 2024-06-10)
Delayed fracture healing is the improper healing of fractures within a reasonable amount of time and is estimated to impact a sixth of all fractures that occur annually in the United States1. While blood- and imaging-based bone turnover biomarkers have been thoroughly investigated throughout the healing process of bone, there is still a lack of understanding on how well these biomarkers can predict union in individual patients. Although conventional radiography is the most common clinical practice for assessing bone healing progression, this imaging technique—as well as the other imaging methods used—fails to discern the in vivo mechanical environment of bone, and therefore the likeliness of union or nonunion. There is a need to identify mechanical biomarkers that could better differentiate between patients who undergo typical healing progression versus delayed fracture healing. In order to identify these mechanical biomarkers, a 3D in vitro cell culture platform that recapitulates the micromechanical environment must be developed and tested. Success of this in vitro platform relies on the generation of rigorous testing protocols for assessing stiffness and fluid flow within this organoid system. This study aims to develop an experimental and computational pipeline for mechanically characterizing 3D printed (3DP) scaffolds—Voronoi, IsoTruss, and Truncated Octahedron (TO) geometries—that will be the foundation for future studies to explore patient-specific mechanical biomarkers in these bone tissue engineered scaffolds A dynamic mechanical analysis (DMA) strain sweep was performed on the scaffolds (n=6 for 4- and 7-day 3T3 fibroblast seeded Voronoi and TO scaffolds, n=4 for 4- and 7-day seeded IsoTruss scaffolds, n=3 for 4- and 7-day soaked controls for each geometry) to measure storage modulus, loss modulus, and the damping coefficient. The Voronoi geometry increased significantly in storage modulus when seeded for seven days compared to four days (p=0.0293). There was also an overall significant decrease in stiffness when the scaffolds were seeded versus non-seeded (p<<0.001). Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was performed to produce fluid flow experimental validation data, and this provided insights on the micromechanical environment of the IsoTruss scaffold that were consistent with the computational fluid dynamics (CFD) simulation model. The CFD model was used to calculate wall shear stresses (WSS) for various inlet velocities (0.05, 0.10, 0.15, 0.20, and 0.25 mm/s), with 0.15 mm/s producing WSS best within the range of extracellular matrix formation. DMA, DCE-MRI, and CFD all confirmed mechanical characteristics of the IsoTruss geometry that were unique to its specific micromechanical architecture. Out of all scaffolds tested, the IsoTruss geometry achieved the maximum (3.47 MPa) and minimum (0.0631 MPa) storage modulus. The computational analysis pipeline revealed that the patterns observed in the DMA experiments could be caused by buckling due to the fourteen-strut intersections and printing infidelity issue related to the IsoTruss geometry. The protocol developed herein for the experimental and computational analyses done on the scaffolds in this thesis will be used in the future on bone organoids to study individualized fracture healing.
Dynamics of Multi-functional Acoustic Holograms in Contactless Ultrasonic Energy Transfer Systems
Bakhtiari Nejad, Marjan (Virginia Tech, 2020-08-28)
Contactless ultrasonic power transfer (UPT), using piezoelectric transducers, is based on transferring energy using acoustic waves, in which the waves are generated by an acoustic source or transmitter and then transferred through an acoustic medium such as water or human tissue to a sensor or receiver. The receiver then converts the mechanical strain induced by the incident acoustic waves to electricity and delivers to an electrical load, in which the electrical power output of the system can be determined. The execution and efficiency of this technology can be significantly enhanced through patterning, focusing, and localization of the transmitted acoustic energy in space to simultaneously power pre-determined distributed sensors or devices. A passive 3D-printed acoustic hologram plate alongside a single transducer can generate arbitrary and pre-designed ultrasound fields in a particular distance from the hologram mounted on the transmitter, i.e., a target plane. This dissertation presents the use of these simple, cost-effective, and high-fidelity acoustic holograms in UPT systems to selectively enhance and pattern the electrical power output from the receivers. Different holograms are numerically designed to create single and multi-focal pressure patterns in a target plane where an array of receivers are placed. The incident sound wave from a transmitter, after passing through the hologram, is manipulated, hence, the output field is the desired pressure field, which excites the receivers located at the pre-determined focal points more significantly. Furthermore, multi-functional holograms are designed to generate multiple images at different target planes and driving frequencies, called, respectively, multi-image-plane and multi-frequency patterning holograms. The multiple desired pressure distributions are encoded on the single hologram plate and each is reconstructed by changing the axial distance and by switching the frequency. Several proof-of-concept experiments are performed to verify the functionality of the computationally designed holograms, which are fabricated using modern 3D-printers, i.e., the desired wavefronts are encoded in the hologram plates' thickness profile, being input to the 3D-printer. The experiments include measurement of output pressure fields in water using needle hydrophones and acquisition of receivers' voltage output in UPT systems. Another technique investigated in this dissertation is the implementation of acoustic impedance matching layers deposited on the front leading surface of the transmitter and receiver transducers. Current UPT systems suffer from significant acoustic losses through the transmission line from a piezoelectric transmitter to an acoustic medium and then to a piezoelectric receiver. This is due to the unfavorable acoustic impedance mismatch between the transducers and the medium, which causes a narrow transducer bandwidth and a considerable reflection of the acoustic pressure waves at the boundary layers. Using matching layers enhance the acoustic power transmission into the medium and then reinforce the input as an excitation into the receiver. Experiments are performed to identify the input acoustic pressure from a cylindrical transmitter to a receiver disk operating in the 33-mode of piezoelectricity. Significant enhancements are obtained in terms of the receiver's electrical power output when implementing a two-layer matching structure. A design platform is also developed that can facilitate the construction of high-fidelity acoustically matched transducers, that is, the material layers' selection and determination of their thicknesses. Furthermore, this dissertation presents a numerical analysis for the dynamical motions of a high-intensity focused ultrasound (HIFU)-excited microbubble or stable acoustic cavitation, which includes the effects of acoustic nonlinearity, diffraction, and absorption of the medium, and entails the problem of several biomedical ultrasound applications. Finally, the design and use of acoustic holograms in microfluidic channels are addressed which opens the door of acoustic patterning in particle and cell sorting for medical ultrasound systems.
Dynamics of smart materials in high intensity focused ultrasound field
Bhargava, Aarushi (Virginia Tech, 2020-05-06)
Smart materials are intelligent materials that change their structural, chemical, mechanical, or thermal properties in response to an external stimulus such as heat, light, and magnetic and electric fields. With the increase in usage of smart materials in many sensitive applications, the need for a remote, wireless, efficient, and biologically safe stimulus has become crucial. This dissertation addresses this requirement by using high intensity focused ultrasound (HIFU) as the external trigger. HIFU has a unique capability of maintaining both spatial and temporal control and propagating over long distances with reduced losses, to achieve the desired response of the smart material. Two categories of smart materials are investigated in this research; shape memory polymers (SMPs) and piezoelectric materials. SMPs have the ability to store a temporary shape and returning to their permanent or original shape when subjected to an external trigger. On the other hand, piezoelectric materials have the ability to convert mechanical energy to electrical energy and vice versa. Due to these extraordinary properties, these materials are being used in several industries including biomedical, robotic, noise-control, and aerospace. This work introduces two novel concepts: First, HIFU actuation of SMP-based drug delivery capsules as an alternative way of achieving controlled drug delivery. This concept exploits the pre-determined shape changing capabilities of SMPs under localized HIFU exposure to achieve the desired drug delivery rate. Second, solving the existing challenge of low efficiency by focusing the acoustic energy on piezoelectric receivers to transfer power wirelessly. The fundamental physics underlying these two concepts is explored by developing comprehensive mathematical models that provide an in-depth analysis of individual parameters affecting the HIFU-smart material systems, for the first time in literature. Many physical factors such as acoustic, material and dynamical nonlinearities, acoustic standing waves, and mechanical behavior of materials are explored to increase the developed models' accuracy. These mathematical frameworks are designed with the aim of serving as a basic groundwork for building more complex smart material-based systems under HIFU exposure.
Ex Vivo Deformations of the Uterosacral Ligaments
Donaldson, Kandace E. (Virginia Tech, 2023-02-24)
The uterosacral ligaments (USLs) are important anatomical structures that support the uterus and apical vagina within the pelvis. As these structures are over-stretched, become weak, and exhibit laxity, pelvic floor disorders such as pelvic organ prolapse occur. Although several surgical procedures to treat pelvic floor disorders are directed toward the USLs, there is still a lot that is unknown about their function. These surgeries often result in poor outcomes, demonstrating the need for new surgical approaches and biomaterials. The first chapter of this dissertation presents a review of the current knowledge on the mechanical properties of the USLs. The anatomy, microstructure, and clinical significance of the USLs are first reviewed. Then, the results of published experimental studies on the {emph{in vivo}} and {emph{ex vivo}}, uniaxial and biaxial tensile tests are compiled. Based on the existing findings, research gaps are identified and future research directions are discussed. The second chapter proposes the use of planar biaxial testing, digital image correlation (DIC), and optical coherence tomography (OCT) to quantify the deformations of the USLs, both in-plane and out-of-plane. Using virgin swine as an animal model, the USLs were found to deform significantly less in their main direction (MD) of {emph{in vivo}} loading than in the direction perpendicular to it (PD) at increasing equibiaxial stresses. Under constant equibiaxial loading, the USLs deformed over time equally, at comparable rates in both the MD and PD. The thickness of the USLs decreased as the equibiaxial loading increased but, under constant equibiaxial loading, the thickness increased in some specimens and decreased in others. The third chapter presents new experimental methods for testing the {emph{ex vivo}} tensile properties of the uterosacral ligaments (USLs) in rats. USL specimens were carefully dissected to preserve their anatomical attachments, and they were loaded along their main {emph{in vivo}} loading direction (MD) using a custom-built uniaxial tensile testing device. This chapter reports the first mechanical data on the rat USLs in isolation from surrounding organs. It is also the first experimental study to provide measurements of the inhomogeneous deformations of the USLs during loading along their main textit{in vivo} loading direction, revealing that the USLs may behave as auxetic structures. The fourth and final chapter presents preliminary findings on novel imaging applications to characterize the evolving structure of the USLs before, during, and after tensile pulling along the ligaments' main textit{in vivo} axis of loading. Rat USLs were excised using the proposed novel dissection method and pulled uniaxially as was performed in the previous chapter. Before and after mechanical testing, second harmonic generation (SHG) was used to image collagen and muscle within the three anatomical regions of the USLs. During mechanical testing, OCT was used to collect out-of-plane images of the cervical/intermediate regions of the USL specimens, resulting in 3D volume scans of the regions. SHG images showed the USLs to have complex microstructures with significant wavy collagen bundles interwoven with muscle bundles. Preliminary observation of the microstructure during testing revealed interwoven sections of tissue with collagenous fibers that reoriented in all directions illustrating how the USLs may expand laterally during uniaxial loading, causing the auxetic properties documented in the previous chapter. Though more quantitative work remains to be done, the findings presented in this dissertation improve our understanding of how the USLs deform with increasing load, such as what occurs during pregnancy. Together, these studies serve as a springboard for future investigations on the supportive function of the USLs in animal models by offering guidelines on testing methods that capture their complex mechanical behavior.

Browsing by Author "De Vita, Raffaella"

Results Per Page

Sort Options