259 research outputs found
Picosecond laser procedures to enhance the efficacy of tissue resection
No full textThe fundamental goal of this work was to develop an improved surgical modality in
tissue, where minimising thermal damage is paramount, using an ultrashort pulse
picosecond laser. Additionally, an investigation into flexibly delivering such pulses via a
hollow core negative curvature fibre, in order to enable future minimally invasive
endoscopic procedures, was conducted.
Initially, the analysis of colon tissue resection in a porcine model based on plasma
mediated laser ablation (at 1030 nm and 515 nm) using a scanning galvanometer is
presented. A minimal thermally damaged region (<60 µm) and the ability to finely tune
the depth of ablation using different scanning strategies, pulse repetition rate, pulse
energy and laser fluences are demonstrated. These desirable surgical effects on the tissue
were confirmed using surface profilometry and histological analysis. The picosecond
laser ablation of healthy and cancerous lung tissue in an ovine model was also
investigated. It has been observed that the ablation depth of cancerous tissue is
approximately equal to half of the ablation depth of healthy tissue using the same laser
parameters. This thesis also demonstrates that secondary effects of plasma formation such
as shock wave induced mechanical damage, cavitation/gas bubble formation, can occur
dependent on the parameters used. An appropriate scanning strategy (where there is little
or no overlap between consecutive laser pulses) therefore needs to be implemented to
minimise these detrimental effects. A laser scanning methodology (0% and 20% overlap
with consecutive pulses) with enhanced reduction in thermal injury is presented using 20
kHz pulse repetition rate, 1030 nm wavelength and 13 J/cm2
laser fluence with a
maximum ablation rate of 6 (0% Overlap) and 4 (20% overlap) mm3
/minute.
The development of novel hollow core microstructured fibres has enabled the potential
for delivery of ultrashort pulse picosecond laser radiation throughout the body. Therefore,
in this thesis ultrashort laser pulses suitable for precision porcine colon resection were
flexibly delivered via a hollow core negative curvature fibre. The fibre was manipulated
via multi-axis robotic device to mimic movements expected during a practical surgical
procedure. Again, a controllable change in ablation depth and with a minimum thermally
damage region (< 85 µm) is observed. Furthermore, ablation depths are of comparable
scale to that of early stage lesions/polyps in the inner lining of the colon and hence provide
a level of control of resection suited to surgical application to thin walled structures such
as the bowel.James Watt Scholarshi
Precision laser micromachining of hollow core negative curvature fibres
No full textThe principal aim of this work was to develop a novel micromachining strategy for a new
class of hollow core silica optical fibre, the Negative Curvature Fibre (NCF). Processing
techniques were investigated to increase the physical access to the hollow core (along the
length of the fibre) in order to enhance the interaction of chemical species with the light
and hence enable practical sensing devices. Because of the unique internal structure of
these NCFs, consisting of a fine (sub-micron) silica webbing, a highly precise and
controllable machining process was required. Due to the well-known advantages of
femtosecond laser machining such as the ability for inscription in any material, small
volume removal and the non-thermal nature of the process, resulting in machined
structures with an almost negligible heat-affected zone, a new femtosecond laser
micromachining process was developed.
A methodology was successfully demonstrated which gives the capability to precisely
machine away the solid outer cladding fibre and then controllably remove the silica
webbing and expose the hollow core of the fibre. This single step process provides a
more direct way of machining a fibre (compared to previously reported hybrid techniques
such as laser machining plus chemical etching). Parameter optimisation allowed control
of the removal depth, minimisation of re-deposited material and avoidance of damage to
the remaining silica web which is very important for NCF due to its guidance mechanism.
An NCF, which was machined through to the hollow core, exhibited no significant
disruption to the guidance preserving the confinement of light to the hollow core. Hence
the laser micromachining strategy presented in this work paves the way to develop new
optical sensing devices exploiting the unique properties of these novel fibre geometries
Application of a liquid crystal spatial light modulator to laser marking
No full textLaser marking is demonstrated using a nanosecond (ns) pulse duration laser in combination with a liquid crystal spatial light modulator to generate two-dimensional patterns directly onto thin films and bulk metal surfaces. Previous demonstrations of laser marking with such devices have been limited to low average power lasers. Application in the ns regime enables more complex, larger scale marks to be generated with more widely available and industrially proven laser systems. The dynamic nature of the device is utilized to improve mark quality by reducing the impact of the inherently speckled intensity distribution across the generated image and reduce thermal effects in the marked surface. (C) 2011 Optical Society of America</p
Compensation for time fluctuations of phase modulation in a liquid-crystal-on-silicon display by process synchronization in laser materials processing
No full textWe demonstrate the adverse influence of temporal fluctuations of the phase modulation of a spatial light modulator (SLM) display device on nanosecond laser micromachining. We show that active cooling of the display reduces the amplitude of these fluctuations, and we demonstrate a process synchronization technique developed to compensate for these fluctuations when applying the SLM to laser materials processing. For alternative SLM devices developed specifically for laser wavefront control (which do not exhibit such flickering problems), we show that our process synchronization approach is also beneficial to avoid machining glitches when switching quickly between different phase profiles (and hence beam patterns). (C) 2011 Optical Society of America</p
Computationally-intensive Econometrics using a Distributed Matrix-programming Language
No full textThis paper reviews the need for powerful facilities in econometrics, focusing on concrete problems which arise in financial economics and in macroeconomics. We argue that the profession is being held back by the lack of easy to use generic software which is able to exploit the availability of cheap clusters of distributed computers. Our response is to extend, in a number of directions, the well known matrix-programming interpreted language Ox developed by the first author. We note three possible levels of extensions: (i) Ox with parallelization explicit in the Ox code; (ii) Ox with a parallelized run-time library; (iii) Ox with a parallelized interpreter. This paper studies and implements the first case, emphasizing the need for deterministic computing in science. We give examples in the context of financial economics and time-series modelling.Distributed computing; Econometrics; High-performance computing; Matrix-programming language
Compressive behaviour of uniaxially aligned individual mineralised collagen fibres at the micro- and nanoscale
No full textInternational audienc
The cathedral and the bazaar of e-repository development: encouraging community engagement with moving pictures and sound
No full textThis paper offers an insight into the development, use and governance of e‐repositories for learning and teaching, illustrated by Eric Raymond's bazaar and cathedral analogies and by a comparison of collection strategies that focus on content coverage or on the needs of users. It addresses in particular the processes that encourage and achieve community engagement. This insight is illustrated by one particular e‐repository, the Education Media On‐Line (EMOL) service. This paper draws analogies between the bazaar approach for open source software development and its possibilities for developing e‐repositories for learning and teaching. It suggests in particular that the development, use and evaluation of online moving pictures and sound objects for learning and teaching can benefit greatly from the community engagement lessons provided by the development, use and evaluation of open source software. Such lessons can be underpinned by experience in the area of learning resource collections, where repositories have been classified as ‘collections‐based’ or ‘user‐based’. Lessons from the open source movement may inform the development of e‐repositories such as EMOL in the future
Two Stochastic Volatility Processes - American Option Pricing
No full textIn this paper we consider the pricing of an American call option whose underlying asset dynamics evolve under the influence of two independent stochastic volatility processes of the Heston (1993) type. We derive the associated partial differential equation (PDE) of the option price using hedging arguments and Ito's lemma. An integral expression for the general solution of the PDE is presented by using Duhamel's principle and this is expressed in terms of the joint transition density function for the driving stochastic processes. We solve the Kolmogorov PDE for the joint transition density function by first transforming it to a corresponding system of characteristic PDEs using a combination of Fourier and Laplace transforms. The characteristic PDE system is solved by using the method of characteristics. With the full price representation in place, numerical results are presented by first approximating the early exercise surface with a bivariate log linear function. We perform numerical comparisons with results generated by the method of lines algorithm and note that our approach is very competitive in terms of accuracy.American options; Fourier transform; Laplace transform; method of characteristics
The elasto-plastic nano- and microscale compressive behaviour of rehydrated mineralised collagen fibres
No full textAbstract The multiscale architectural design of bio-based nanostructured materials such as bone enables them to combine unique structure-mechanical properties that surpass classical engineering materials. In biological tissues, water as one of the main components plays an important role in the mechanical interplay, but its influence has not been quantified at the length scale of a mineralised collagen fibre. Here, we combine in situ experiments and a statistical constitutive model to identify the elasto-plastic micro- and nanomechanical fibre behaviour under rehydrated conditions. Micropillar compression and simultaneous synchrotron small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) were used to quantify the interplay between fibre, mineralised collagen fibrils and mineral nanocrystals. Rehydration led to a 65% to 75% decrease of fibre yield stress and compressive strength, and a 70% decrease of stiffness with a 3x higher effect on stress than strain values. While in good agreement with bone extracellular matrix, the decrease is 1.5-3x higher compared to micro-indentation and macro-compression. Hydration has a higher influence on mineral than fibril strain while the highest difference to the macroscale was observed comparing mineral and tissue levels. Results suggest that the effect of hydration is strongly mediated by ultrastructural interfaces while corroborating the previously reported water-mediated structuring of bone apatite providing insights towards the mechanical consequences. Results show that the missing reinforcing capacity of surrounding tissue is more pronounced in wet than dry conditions when testing an excised array of fibrils, mainly related to the swelling of fibrils in the matrix. Differences leading to higher compressive strength between mineralised tissues do not seem to depend on the rehydration state while fibril mobilisation follows a similar regime in wet and dry conditions. The lack of kink bands point towards the role of water as an elastic embedding, thus, adapting the way energy is absorbed. Statement of significance Characterising structure-property-function relationships of biomaterials helps us to elucidate the underlying mechanisms that enables the unique properties of these architectured materials. Experimental and computational methods can advance our understanding towards their complex behaviour providing invaluable insights towards bio-inspired material development. In our study, we present a novel method for biomaterials characterisation. We close a gap of knowledge at the micro- and nanometre length scale by combining synchrotron experiments and a statistical model to describe the behaviour of a rehydrated single mineralised collagen fibre. Results suggest a high influence of hydration on structural interfaces, and the role of water as an elastic embedding. Using a statistical model, we are able to deduce the differences in wet and dry elasto-plastic properties of fibrils and fibres close to their natural hydration state
How can allocative inefficiency reveal risk preference? An empirical investigation on French wheat farms
No full textWe focus on a simple framework on wheat producer behaviour in a context of price output uncertainty. More precisely, we establish a relationship between ex post output price level and allocative inefficiency that allows to characterize farmers’ risk preferences. Given this analysis, the connection between risk aversion and other socioeconomic variables (such as degree of output specialisation, total asset, debts, farmer’s age…) can furthermore empirically be explored. This relationship is empirically tested on an unbalanced panel including about 650 wheat producers located in the French Department of Meuse over 1992- 2003.Producer behaviour, allocative inefficiency, risk aversion, Crop Production/Industries, Risk and Uncertainty,
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