Engineering

Publication Search Results

Now showing 1 - 10 of 13
  • (2006) Huang, Henry Yen-Chin
    Thesis
    The success of endoluminal stent-graft treatment for abdominal aortic aneurysm relies on maintenance of an effective seal when the stent expands into the healthy artery. Clinical observation of aortic neck dilation following endoluminal grafting has led to the hypothesis that excessive stent expansion forces may cause remodelling and dilation of the artery to accommodate the strong forces. This may lead to failure of the seal, hence so-called endoleak. In this research, we analysed the force field generated by aortic stent-grafts and investigated in vitro approaches for studying the effects of these forces on cells within the vascular wall. The pressure-deformation behaviour of ovine arteries was examined experimentally and was found to vary with artery type. A finite element model of abdominal aorta (AA) characterised by Mooney-Rivlin hyperelastic material properties was validated. The property inputs were derived from the polynomial form of the strain energy density function proposed by Patel and Vaishnav. Stent-artery contact simulations revealed stresses 1.2-19 times higher than within a normal vessel at 120 mmHg when contacted by a zig-zag, square cross-section stent that expanded the AA by 3-16%. Streses 1.3-23 times normal were predicted for circular cross-section stents at the same range of expansions. The stress distribution was determined to be concentrated at the contacting surface and within the inner region of the aortic wall. These results confirmed that the forces within the vessel wall are likely to place unnatural physiological demands on the cells within. We then developed an in vitro system for studying the impact of this mechanical stress on cells within a three dimensional (3D) structure. A 20 wt% poly(vinyl alcohol) (PVA) - 5 wt% collagen tubular construct was developed to support cells, and was shown to sustain physiological blood pressures. Two cell-seeding techniques were examined, direct cell encapsulation and surface cell-seeding. Both demonstrated the capability of entrapping viable cells within the construct that remained viable for up to 4 days. In conclusion, stent contact does create abnormal stress concentrations within the vessel wall with a magnitude severely higher than physiological levels. A feasible tubular construct and an in vitro system were developed, enabling further assessments on the effects of these abnormality on the cells.

  • (2006) Walter, William Lindsay
    Thesis
    Hip simulators are designed to reproduce the forces and motion patterns of normal walking. In vivo demands on total hip replacements, however, are varied and often more severe than normal walking conditions. It is these severe conditions that often lead to implant failure. This is clinically based research aimed at understanding some of the more severe conditions in hips and the effect that these have on the performance of the total hip replacement. The polyethylene liner can act as a pump in an acetabular component, forcing fluid and wear particles through the holes to the retroacetabular bone causing osteolysis. Ten patients were studied at revision surgery. Pressures were measured in retroacetabular osteolytic lesions while performing pumping manouvers with the hip. Two laboratory experiments were then designed to study pumping mechanisms in vitro. In patients with contained osteolytic lesions, fluid pressure fluctuations could be measured in the lesion in association with the pumping action. Patients with uncontained osteolytic lesions showed no such pressure fluctuations. In the laboratory we identified 3 distinct mechanisms whereby fluid can be pumped from the hip joint to the retroacetabular bone. These pumping effects could be mitigated by improved implant design. Loading of the femoral head against the edge of the acetabular component produces dramatically increased contact pressures particularly in hard-on-hard bearings. In an analysis of 16 retrieved ceramic-on-ceramic bearings we were able to characterise the mechanism of edge loading based on the pattern of edge loading wear on the bearing surface. Finally in a radiographic study of patients with squeaking ceramic-on-ceramic hips. Squeaking was found to be associated with acetabular component malposition. It seems that edge loading or impingement may be an associated factor in these cases.

  • (2006) Alirezaye-Davatgar, Mohammad Taghi
    Thesis
    Background: Extensive studies have been conducted to simulate blood flow in the human vasculature using nonlinear equations of pulsatile flow in collapsible tube plus a network of vessels to represent the whole vasculature and the cerebral circulation. For non-linear models numerical solutions are obtained for the fluid flow equations. Methods: Equations of fluid motion in collapsible tubes were developed in the presence of gravitational force (Gforce). The Lax-Wendroff and MacCormack methods were used to solve the governing equations and compared both in terms of accuracy, convergence, and computer processing (CPU) time. A modified vasculature of the whole body and the cerebral circulation was developed to obtain a realistic simulation of blood flow under different conditions. The whole body vasculature was used to validate the simulation in terms of input impedance and wave transmission. The cerebral vasculature was used to simulate conditions such as presence of G-force, blockage of Internal Carotid Artery (ICA), and the effects on cerebral blood flow of changes in mean and pulse pressure. Results: The simulation results for zero G-force were in very good agreement with published experimental data as was the simulation of cerebral blood flow. Both numerical methods for solutions of governing equations gave similar results for blood flow simulations but differed in calculation performance and stability depending on levels of G-force. Simulation results for uniform and sinusoidal G-force are also in good agreement with published experimental results. Blood flow was simulated in the presence of a single (left) carotid artery obstruction with varying morphological structures of the Circle of Willis (CoW). This simulation showed significant differences in contralateral blood flow in the presence or absence of communicating arteries in the CoW. It also was able to simulate the decreases in blood flow in the cerebral circulation compartment corresponding to the visual cortex in the presence of G-force. This is consistent with the known loss of vision under increased acceleration. Conclusions: This study has shown that under conditions of gravitational forces physiological changes in blood flow in the systemic and cerebral vasculature can be simulated realistically by solving the one-dimentional fluid flow equations and non-linear vascular properties numerically. The simulation was able to predict changes in blood flow with different configurations and properties of the vascular network.

  • (2006) Martin, Christopher
    Thesis
    This research investigated if exogenous growth factors (GFs), in particular platelet derived growth factor (PDGF), has an in vivo effect on the healing response of normal healthy bone. The research was orientated to study whether a clinical beneficial effect could be demonstrated. To achieve this two animal models were utilised, namely, a rabbit tibial osteotomy model and an ovine tibial defect and porous implant ingrowth model. The rabbit model comprised of a unilateral V-shaped tibial osteotomy, stabilised with an absorbable intramedullary pin and figure-of-eight tension band suture, with a 3 week survival period. The GFs tested in this model were 3 concentrations of PDGF, a single dose of insulin-like growth factor binding protein (IGF-BP) and a combination of the two. Each osteotomy was injected with a single bolus of collagen (control) or collagen containing GF (treatment) during surgery. After sacrifice tibiae were CT-scanned in situ, harvested and subject to 4-point bend testing. The callus, underlying bone and contralateral bone's greyscales and mechanical testing results were used for comparative analysis. The ovine model consisted of implanting 6 small rectangular shaped titanium alloy porous implants and one empty defect bilaterally in sheep's tibiae, for 4 and 6 weeks. The sheep were injected with tetracycline bone marker at 2 week intervals. The model's characteristics and any positional effects were initially investigated. Followed by an investigation into the influence of various exogenous GFs on the healing response and ingrowth characteristics of bone into the porous implants. The GFs investigated were PDGF, IGF-BP and fibroblast growth factor impregnated into the porous implants in a collagen carrier. Comparative analysis was done on results from 3-point bend testing of the bone/implant interface, image analysis to quantify percentage of bone, from scanning electron microscopy images of implant sections and confocal microscopy images of tibial defect sections. Analyses indicate that the GFs investigated have a direct and quantifiable positive in vivo effect. The more significant finding is that the growth factors have a potent systemic effect. These results were confirmed by both the sheep porous bone plug model and the rabbit tibial osteotomy model used within this research.

  • (2006) Tan, Ju Chiat
    Thesis
    Heart failure has a significant impact on mortality and morbidity. Dilated cardiomyopathy (DCM) is the third most common cause of heart failure and the most common reason for heart transplantation. Familial DCM is known to be caused by mutations in the LMNA gene encoding lamins A and C. New methods to enhance cardiac contractility would be beneficial in the treatment or prevention of heart failure. The focus of this thesis was to evaluate the mechanisms of altered contractility in two mouse models: the LMNA knockout model (homozygous, Lmna-/-; heterozygous, Lmna+/-) generated by targeted deletion of the lmna gene, and the model of enhanced contractility due to cardiac alpha1A-adrenergic receptor (1A-AR) overexpression (A1A1). Previous studies have found altered nuclear-desmin connections in lamin A/C deficient mice. It was proposed that these alterations result in defective force transmission , which leads to DCM. Studies in this thesis have supported this hypothesis. Studies of isolated single cardiomyocytes from mice aged 4-6 weeks demonstrated abnormal cell morphology and contractile dysfunction in Lmna-/- cardiomyocytes, while Lmna+/- cells showed no overt phenotype. Excitation-contraction coupling experiments and forcecalcium studies in skinned fibers excluded altered calcium kinetics as a primary cause of DCM in this model, but there was evidence of reduced sarcomere numbers and reduced sarcomere lengths as a contributor to reduce force generation in Lmna-/- and Lmna+/- mice. Previous in vivo studies showed that A1A1 mice had enhanced contractility with the absence of hypertrophy. Studies on isolated single cardiomyocytes from A1A1 mice aged 8-12 weeks showed reduced contractility in the absence of 1A-AR stimulation, but an exaggerated response to 1A-AR stimulation. In contrast isolated isovolumic Langendorff perfused A1A1 hearts without 1A-AR stimulation replicated the enhanced contractility observed in vivo. These studies are consistent with down-regulation of contractility due to the hyperactivity of the overexpressed 1A-AR in vivo, which only becomes evident in isolated cells without 1A-AR stimulation due to the loss of functional receptor numbers during isolation. Sufficient spontaneously active 1A-ARs are preserved in the isolated Langendorff heart preparation to ensure maximum contractility driven by increase calcium release.

  • (2006) Camacho, Fernando
    Thesis
    With the rise in prevalence of cardiovascular (CV) disease, risk stratification is becoming increasingly important. Accurate characterization of the CV system is required, for which central aortic blood pressure (BP) parameters form an integral part. However, invasive measurement of central aortic BP parameters (aP) is difficult. Therefore, non-invasive methods to estimate aP from the radial pressure pulse (rPulse) have been proposed. To analyze accuracy of estimated aP (aPhat) and applicability in risk stratification and diagnosis, this study presents: (1) a novel representation of the rPulse with minimal loss of information, (2) a framework for strict definition and statistical analysis of aPhat, and (3) a dynamic analysis of effects of mean BP (MP) and heart rate (HR) in the rPulse shape. Methods: (1) 2671 rPulse s measured by applanation tonometry were represented using the first eight principal components (PC) scores after standard PC transformation. rPulse shapes were compared in three subpopulations. (2) The concept of "estimation option" (EO) for aP estimation was presented. A framework for strict definition of aPhat and the comparison of EOs was proposed, and 7 different EOs compared. (3) A sequence of rPulse s was analyzed during soft exhalation maneuver (SEM) %, a mild Valsalva type maneuver, in eight healthy subjects. Radial BP and respiration pressure were continuously measured. The effects of MP and HR in the rPulse parameters were analyzed by standard linear regression for each subject. Results: (1) PC representation of the rPulse improves accuracy of the estimation of aPhat compared with the simple use of rPulse parameters. Subpopulations have distinctive rPulse shapes. (2) No single EO was better for the estimation of all aPhat. Inclusion of MP improves estimation accuracy. Despite further improvement when rPulse is included, the general transfer function EO is a biased estimator. (3) The dynamic analysis of the rPulse provides information of the effects of MP and HR in the rPulse not available in static analysis. The effects were specific for each individual and different from the results obtained from a general population. Conclusions: For accurate CV risk stratification, future studies should include a dynamic measurement of calibrated radial pressure pulse during SEM maneuver. Risk analysis and diagnosis should be based on representations of the rPulse with minimum loss of information. aPhat should be used for better understanding of the underlying physiological principles.

  • (2006) Cheng, Shao Koon
    Thesis
    The intracranial system consists of three main basic components - the brain, the blood and the cerebrospinal fluid. The physiological processes of each of these individual components are complex and they are closely related to each other. Understanding them is important to explain the mechanisms behind neurostructural disorders such as hydrocephalus. This research project consists of three interrelated studies, which examine the mechanical properties of the brain at the macroscopic level, the mechanics of the brain during hydrocephalus and the study of fluid hydrodynamics in both the normal and hydrocephalic ventricles. The first of these characterizes the porous properties of the brain tissues. Results from this study show that the elastic modulus of the white matter is approximately 350Pa. The permeability of the tissue is similar to what has been previously reported in the literature and is of the order of 10-12m4/Ns. Information presented here is useful for the computational modeling of hydrocephalus using finite element analysis. The second study consists of a three dimensional finite element brain model. The mechanical properties of the brain found from the previous studies were used in the construction of this model. Results from this study have implications for mechanics behind the neurological dysfunction as observed in the hydrocephalic patient. Stress fields in the tissues predicted by the model presented in this study closely match the distribution of histological damage, focused in the white matter. The last study models the cerebrospinal fluid hydrodynamics in both the normal and abnormal ventricular system. The models created in this study were used to understand the pressure in the ventricular compartments. In this study, the hydrodynamic changes that occur in the cerebral ventricular system due to restrictions of the fluid flow at different locations of the cerebral aqueduct were determined. Information presented in this study may be important in the design of more effective shunts. The pressure that is associated with the fluid flow in the ventricles is only of the order of a few Pascals. This suggests that large transmantle pressure gradient may not be present in hydrocephalus.

  • (2006) Styan, Katie
    Thesis
    Polymer organosilicate nanocomposites have attracted significant attention over the last decade due to improved mechanical, thermal, and barrier properties. Several nanocomposite researchers have recognised potential for biomedical applications, however none have conducted biological investigations. In this project, the predicted ability of the organosilicate to enhance biostability, modulate the release of included drugs, and confer biofunctionality and control over the host response, were assessed as the three primary hypotheses. The studies were conducted with the objective being employment as urinary device biomaterials. Of prime importance was that no detrimental change in cytocompatibility was resultant. Biomedical thermoplastic elastomeric polyurethane organosilicate nanocomposites were prepared from poly(ether)urethane of 1000g/mol poly(tetramethylene oxide) polyol, 4,4 diphenylmethane diisocyanate, and 1,4 butanediol chain extender chemistry, and various organosilicates with loadings from 1w% to 15w%, using a solution casting technique. Initially, partially exfoliated nanocomposites were produced using a commercially available organosilicate, Cloisite® 30B. These nanocomposites displayed several advantageous properties, namely i) significant anti-bacterial activity, reducing S. epidermidis adherence after 24h to ~20% for a 15w% organosilicate loading, ii) enhanced biostability, with a 15w% organosilicate loading displaying only slight degradation after a 6 week subcutaneous in vivo ovine implantation, and iii) static modulation of model drug release as a factor of drug properties and organosilicate loading. The former was attributed to the Cloisite® 30B quaternary ammonium compound, while the latter two were likely primarily barrier effect related and due to changes to poly(ether)urethane permeability. Electrostatic and chemical interactivity between drug and organosilicate was also implicated in the observed drug release modulation. Unfortunately, a lack of in vitro cytocompatibility and poor in vivo inflammatory response will limit in vivo use. Utilising bioinert 1-aminoundecanoic acid as an alternative organic modification, cytocompatible intercalated nanocomposites were produced thus likely allowing in vivo nanocomposite use and exploitation of the barrier effect related properties. However, these nanocomposites were not antibacterial. Variation of the organic modification and/or use of co-modification were viable means of modulating host response and biofunctionality, however nanoscale dispersion of co-modified silicate was poor. Use of nanocomposite technology was concluded beneficial to existing biomaterials, and specifically to biomaterial application as urinary catheters / stents.

  • (2006) Wang, Wanxin
    Thesis
    The major drawback of crossflow membrane filtration is that permeate flux declines with time as a result of the increase in total membrane resistance. Pulsatile flow is well known to reduce the resistance and enhance permeate flux. This study applied pulsatile flow induced by the oscillation of a collapsible tube to microfiltration and ultrafiltration, to improve filtration performance expressed as permeate flux enhancement and backflushable resistance reduction. Three membranes (ceramic tubular microfiltration, PVDF spiral-wound microfiltration and PS hollow-fibre ultrafiltration) and two media (bentonite suspension and whey solution) were used. In bentonite pulsatile microfiltration with the tubular membrane, up to 300% of permeate flux enhancement and 90% of backflushable resistance reduction were achieved. In bentonite and whey pulsatile microfiltration with the spiral-wound membrane, moderate improvements were gained: for bentonite, the highest increase in permeate flux was 51% and decrease in backflushable resistance was 45%; for whey, the highest permeate flux enhancement and backflushable resistance reduction were 36% and 38% respectively. In ultrafiltration of both media, no significant performance improvement was found. This is thought due in the one case to the relatively minute membrane pore size, and in the other to the large irreversible resistance created by whey solution. Transmural pressure at the collapsible tube downstream end indicates the tube compression and influences the pulsation vigour. Increasing the transmural pressure was an effective way to improve filtration performance. In bentonite microfiltration with the tubular membrane, increasing crossflow velocity was also effective, but increasing transmembrane pressure was not. Analysis of pulsatility parameters showed that the pulsatile flow always resulted in enhanced wall shear, and induced pore backflush always in the tubular membrane and sometimes in the HF membrane. These mechanistic findings helped to understand the filtration performance improvements. The analysis of energy consumption in bentonite microfiltration with the tubular membrane clearly demonstrated the benefit of applying the collapsible-tube-induced pulsatile flow in energy utilisation. The system specific energy could be reduced more than 70 % relative to the equivalent steady microfiltration permeate flux. For a given specific energy, the permeate flux could be increased by a factor of nearly four.

  • (2006) Gibson, Thomas J.
    Thesis
    There is a large body of work investigating whiplash-associated injury in motor vehicles and its causation. Being unable to detect the actual injury and having to use the symptoms of the sufferer as a surrogate has made progress in understanding the injury causation slow. Still lacking are the causal relationships between the biomechanical load on the vehicle occupant in the crash, the resulting loading on the neck and the actual injuries suffered. The optimisation of the design of vehicle safety systems to minimise whiplash needs a better understanding of human tolerance to these injuries. This thesis describes the development of a mathematical multi-body C5/C6 motion segment model to investigate the causation of soft-tissue neck injury. This model was validated with available static in-vitro experimental data on excised motion-segments and then integrated into the existing, validated multi-body human head and neck model developed by van der Horst, to allow the application of realistic dynamic loads. The responses and injury sensing capability of the C5/C6 model were compared with available data for volunteers and cadavers in rear impacts. The head and neck model was applied to the investigation of a group of real rear impact crashes (n = 78) of vehicles equipped with a crash-pulse recorder and with known postcrash injury outcomes. The motion of the occupants in these crashes had previously been reconstructed with a MADYMO BioRID II dummy-in-seat model validated by sled testing. The occupant T1 accelerations from these reconstructions were used to drive the head and neck model. The soft-tissue loading at C5/C6 of the head and neck model was analysed during the early stage of the impact, prior to contact with the head restraint. The loading and the pain outcome from the vehicle occupants in the actual crash were compared statistically. For the longer-term whiplash-associated pain outcomes (of greater than 1 month duration) for these occupants, the C5/C6 model indicated good correlation with the magnitude of the shear loading on the facet capsule. In lower severity impacts, the model result supported a second hypothesis of injury to this motion segment: facet surface impingement.