1,721,168 research outputs found
Biomechanical Properties of the Ocular Globe Based on Ex Vivo Testing and Multiscale Numerical Modelling
No full textThe present study is the culmination of quantifying and qualitative experimental and
numerical research representing biomechanical behaviour of the human eye.
A new experimental technique for testing intact eye globes in a form that is representative
of in vivo conditions is developed which is suitable for determining the material
properties of the complete outer ocular tunic.
A test rig has been developed to provide closed-loop control of either applied intraocular
pressure or resulting apical displacement, measurement of displacements across
the external surface of eye globe using high-resolution digital cameras and digital image
correlation software, prevention of rigid-body motion and protection of ocular surface
from environmental drying. The method has been demonstrated on one human and
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one porcine eye globe, which were cyclically loaded. Finite element models based on
specimen specific tomography, free from rotational symmetry, were used along with
experimental pressure-displacement data in an inverse analysis process to derive the
mechanical properties of tissue in different regions of the eye’s outer tunic.
The test method enabled monitoring of mechanical response to intraocular pressure
variation across the surface of the eye globe. For the two eyes tested, the method
showed a gradual change in the sclera’s stiffness from a maximum at the limbus to
a minimum at the posterior pole, while in the cornea the stiffness was highest at the
centre and lowest in the peripheral zone. Further, for both the sclera and cornea, the
load-displacement behaviour did not vary significantly between loading cycles.
The first methodology capable of mechanically testing intact eye globes, with applied
loads and boundary conditions that closely represent in vivo conditions has been introduced.
The method enables determination of the regional variation in mechanical
behaviour across the ocular surface.
Two numerical models based in continuum mechanics theory have been developed
which represent the 3D anisotropic behaviour of the corneal stroma. Experimental
data has been gathered from a number of previous studies to provide the basis and
calibration parameters for the numerical modelling. The resulting models introduce
numerical representation of collagen fibril density and its related regional variation, interlamellar
cohesion and age-related stiffening in anisotropic and viscoelastic models of
the human cornea. Further, the models incorporate previous modelling developments
including representation of lamellae anisotropy and stiffness of the underlying matrix.
Wide angle X-ray scattering has provided measured data which quantifies relative fibril
anisotropy in the 2D domain. Accurate numerical description of material response to
deformation is essential to providing representative simulations of corneal behaviour.
Representing experimentally obtained 2D anisotropy and regional density variation in
the 3D domain is an essential component of this accuracy. The constitutive model was
incorporated into finite element analyses. Combining with inverse analysis, the model
was calibrated to an extensive experimental database of ex vivo corneal inflation tests
and ex vivo corneal shear tests. This model represented stiffness of the underlying matrix
which is 2−3 orders of magnitude lower than the mechanical response representing
the collagen fibrils in the lamellae. The presented model, along with its age dependent
material coefficients, allows finite element modelling for an individual patient with material
stiffness approximated based on their age. This has great potential to be used in
both daily clinical practice for the planning and optimisation of corrective procedures
and in pre-clinical optimisation of diagnostic procedures.
The second constitutive numerical model based on the continuum mechanics theory
was developed which extended the representation of the model above to include both
age-related viscoelastic stiffening behaviour of the human cornea. Experimental data
gathered from a number of previous studies on 48 ex vivo human cornea (inflation and
shear tests) enabled numerical model calibration. The present study suggests that stiffness
parallel to the lamellae of the cornea approximately doubles from an increase in
strain-rate of 0.5 − 5%/min. While the underlying stromal matrix provides a stiffness
2−3 orders of magnitude lower than the lamellae. The model has been simultaneously
calibrated to within 5% error across three age groups ranging from 50 − 95, multiple
strain-rates and multiple loading scenarios. Age and strain-rate dependent material
coefficients allow finite element modelling for an individual patient with material stiffness
approximated by their age under varying loading scenarios. This present study
addresses a significant gap in numerical representation of the cornea and has great
potential in both daily clinical practice particularly in highly viscoelastic dependent
simulations such as non-contact tomometry.
Related to this thesis, the author has either primarily or secondarily authored the
following related journal articles which are included in this thesis in modified forms:
Whitford C. & Elsheikh A., Corneal Biomechanics Testing Methods, May 2014,
Chinese Journal of Optometry and Ophthalmology Visual Science;
Whitford C., Joda A., Jones S., Bao F., Rama P. & Elsheikh A., Ex-vivo Test-
ing of Intact Eye Globes Under Inflation Conditions to Determine Regional
Variation of Mechanical Stiffness, July 2016, Eye and Vision.
Elsheikh, A., Whitford, C., Hamarashid, R., Kassem, W., Joda, A., B¨uchler, P.,
Stress free configuration of the human eye. Febuary 2013, Medical Engineering
& Physics.
Yu J., Bao F., Feng Y., Whitford C., Ye T., Huang Y., Wang Q., Elsheikh A.,
Assessment of Corneal Biomechanical Behavior Under Posterior and Ante-
rior Pressure. January 2013, Journal of Refractive Surgery.
Whitford C., Studer H., Boote K., Meek K.M. & Elsheikh A., Biomechanical
Model of the Human Cornea: Considering Shear Stiffness and Regional
Variation of Collagen Anisotropy and Density, Feb 2015, Journal of the Mechanical
Behavior of Biomedical Materials.
Elsheikh A., McMonnies C.W., Whitford C. & Boneham G.C., In-vivo study of
Corneal Responses to Increased Intraocular Pressure, 2015, Eye and Vision.
An additional journal publication has been prepared from the content in this present
study:
Whitford C., Movchan N. & Elsheikh A., A Viscoelastic Hyperelastic Anisotropic
Model of the Human Cornea.
Further, two book chapters have been published which related to this thesis:
Whitford C., Studer H., Boote C., Meek K. & Elsheikh A., Modelo Biomecnico de
la Crnea Humana Considerando la Variacin Regional de la Anisotropa, la
Densidad y la Cohesin Interlaminar de las Fibrillas de Colgeno, in Biomec-
nica y Arquitectura Corneal, May 2014.
Geraghty B., Whitford C., Boote C., Akhtar R,. & Elsheikh A., Age-Related
Variation in the Biomechanical and Structural Properties of the Corneo-
Scleral Tunic, in Mechanical Properties of Ageing Soft Tissues, January 2015.
In addition, a number of conference proceedings have been published
Projection-based Embedded Discrete Fracture Model (pEDFM) on Corner-point Grid Geometry for Subsurface Flow and Geothermal Modeling
No full textWe develop projection-based embedded discrete fracture model (pEDFM) on corner-point grids (CPG) for fluid flow and heat transfer in subsurface geological formations. The coupling between the flow and heat transfer is fully-implicit, to allow for stable simulations, specially in presence of highly contrasting fractures. We define independent CPG-based mesh for matrix rock and all 3D fractures, which allows for capturing geologically complex geometries. The connectivities between the non-neighbouring cells are described such that a consistent discrete representation of the embedded fractures are developed within the CPG geometry. Numerical rests are developed first to verify the CPG grid implementation compared with the Cartesian structured ones, and then to illustrate the applicability of the pEDFM for field-scale geologically complex reservoirs.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Numerical AnalysisReservoir Engineerin
Age-related variations in the biomechanical properties of human sclera
No full texthttp://hdl.handle.net/20.500.11768/9689
Characterization of age- related variation in corneal biomechanical properties
No full texthttp://hdl.handle.net/20.500.11768/96859An experimental study has been conducted to determine the stress-strain behaviour of human corneal tissue and how the behaviour varies with age. Fifty-seven well-preserved ex vivo donor corneas aged between 30 and 99 years were subjected to cycles of posterior pressure up to 60 mmHg while monitoring their behaviour. The corneas were mechanically clamped along their ring of scleral tissue and kept in physiological conditions of temperature and hydration. The tissue demonstrated hyper-elastic pressure-deformation and stress-strain behaviour that closely matched an exponential trend. Clear stiffening (increased resistance to deformation) with age was observed in all loading cycles, and the rate of stiffness growth was nonlinear with bias towards older specimens. With a strong statistical association between stiffness and age (p < 0.05), it was possible to develop generic stress-strain equations that were suitable for all ages between 30 and 99 years. These equations, which closely matched the experimental results, depicted corneal stiffening with age in a form suitable for implementation in numerical simulations of ocular biomechanical behaviour
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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