Bilateral corneal blindness represents a quarter of the total blind, world-wide. The artificial cornea in assorted forms, was developed to replace opaque non-functional corneas and to return sight in otherwise hopeless cases that were not amenable to corneal grafts; believed to be 2% of corneal blind. Despite technological advances in materials design and tissue engineering no artificial cornea has provided absolute, long-term success. Formidable problems exist, due to a combination of unpredictable wound healing and unmanageable pathology. To have a solid guarantee of reliable success an artificial cornea must possess three attributes: an optical window to replace the opaque cornea; a strong, long term union to surrounding ocular tissue; and the ability to induce desired host responses. A unique artificial cornea possesses all three functional attributes- the Osteo-odonto-keratoprosthesis (OOKP). The OOKP has a high success rate and can survive for up to twenty years, but it is complicated both in structure and in surgical procedure; it is expensive and not universally available. The aim of this project was to develop a synthetic substitute for the OOKP, based upon key features of the tooth and bone structure. In doing so, surgical complexity and biological complications would be reduced. Analysis of the biological effectiveness of the OOKP showed that the structure of bone was the most crucial component for implant retention. An experimental semi-rigid hydroxyapatite framework was fabricated with a complex bone-like architecture, which could be fused to the optical window. The first method for making such a framework, was pressing and sintering of hydroxyapatite powders; however, it was not possible to fabricate a void architecture with the correct sizes and uniformity of pores. Ceramers were synthesised using alternative pore forming methods, providing for improved mechanical properties and stronger attachment to the plastic optical window. Naturally occurring skeletal structures closely match the structural features of all forms of natural bone. Synthetic casts were fabricated using the replamineform process, of desirable natural artifacts, such as coral and sponges. The final method of construction by-passed ceramic fabrication in favour of pre-formed coral derivatives and focused on methods for polymer infiltration, adhesion and fabrication. Prototypes were constructed and evaluated; a fully penetrative synthetic OOKP analogue was fabricated according to the dimensions of the OOKP. Fabrication of the cornea shaped OOKP synthetic analogue was also attempted.

This work sets out to evaluate the potential benefits and pit-falls in using a priori information to help solve the Magnetoencephalographic (MEG) inverse problem. In chapter one the forward problem in MEG is introduced, together with a scheme that demonstrates how a priori information can be incorporated into the inverse problem. Chapter two contains a literature review of techniques currently used to solve the inverse problem. Emphasis is put on the kind of a priori information that is used by each of these techniques and the ease with which additional constraints can be applied. The formalism of the FOCUSS algorithm is shown to allow for the incorporation of a priori information in an insightful and straightforward manner. In chapter three it is described how anatomical constraints, in the form of a realistically shaped source space, can be extracted from a subject’s Magnetic Resonance Image (MRI). The use of such constraints relies on accurate co-registration of the MEG and MRI co-ordinate systems. Variations of the two main co-registration approaches, based on fiducial markers or on surface matching, are described and the accuracy and robustness of a surface matching algorithm is evaluated. Figures of merit introduced in chapter four are shown to given insight into the limitations of a typical measurement set-up and potential value of a priori information. It is shown in chapter five that constrained dipole fitting and FOCUSS outperform unconstrained dipole fitting when data with low SNR is used. However, the effect of errors in the constraints can reduce this advantage. Finally, it is demonstrated in chapter six that the results of different localisation techniques give corroborative evidence about the location and activation sequence of the human visual cortical areas underlying the first 125ms of the visual magnetic evoked response recorded with a whole head neuromagnetometer.

No description available.

This thesis describes the development of a microcomputer controlled 16 electrode Impedance Imaging System which can be used by the orthopaedic clinician, to measure resistivity changes associated with diaphyseal fractures of the human limb. The system is also designed to reconstruct two dimensional images of the approximate distribution of these changes. Electrical Impedance Tomography is a relatively new technique. It has attracted a lot of clinical interest since the technique is inexpensive, repeatable, portable and as far as is known, not harmful. Though spatial resolution is poor, temporal resolution is excellent. Research has been directed towards a number of clinical applications such as gastric emptying, dynamic cardiac imaging and monitoring hyperthermia treatment. Since bone is highly resistive, the occurrence of a diaphyseal fracture and the associated oedema should result in a decrease in resistivity in the region of the fracture. This particular application of the impedance technique was investigated here. An impedance measuring instrument was built in the first instance. It was designed to be able to inject a current of 1 milliampere at a frequency of 10 kHz by a chosen pair of electrodes into a region surrounded by 16 electrodes. The resulting voltages on the boundary were then measured. It was interfaced with a BBC Master microcomputer which was programmed to collect these boundary measurements, analyse the data and reconstruct images of the distribution of log(resistivity) or resistivity in the region. The instrument was tested for its linearity, stability and accuracy. Subsequently phantom tests were carried out to assess its performance. Phantom experiments showed that this system was capable of both measuring resistivity changes and imaging phantom objects adequately. The system could image a resistive object of ~ 15% of the diameter of the phantom. It could resolve two resistive objects spaced one phantom radius apart in the central region. The battery of tests performed on the phantom indicated that greater changes in peripheral voltage gradient measurements would occur if the background resistivity changed as compared to changes in a small region of the phantom. The experiments also underlined the fact that the shape of the region was of great importance in the boundary voltage gradient profile. The property of bilateral symmetry of human limbs was utilised to compare resistivity measurements and images of one limb with that of the other. In the normal individual they underlined the similarity of the limbs. Resistivity measurements in volunteer patients with limb fractures confirmed the results of phantom tests. Hence soon after a fracture the oedema of the surrounding tissues causes a large drop in resistivity superimposed on a smaller drop due to the fracture. Similar measurements in an un-united fracture, where there is virtually no oedema, showed that there is significantly lower resistivity in the injured limb compared to the normal limb. Measurements on a volunteer with a united fracture showed greater resistivity in the injured limb compared to the normal limb. Three volunteers with upper arm fractures at different levels showed increasing resistivities towards normal levels during the healing process. Static impedance images of the distribution of log(resistivity) were found to contain a number of artefacts because the geometry of the circular reference and the upper arm did not match. However since bilaterally symmetrical limbs were being compared these artefacts appeared to be duplicated in the two and changes associated with the healing process were still evident in the images. Differential images appeared to reduce the problem of inexact geometry resulting in fewer artefacts. Accuracy of electrode positioning was still found to be critical and the images therefore were less than ideal. Though the number of subjects was limited, the results of this study were promising. They indicated that electrical impedance measurements comparing the fractured limb with the contralateral normal limb could offer the orthopaedic clinician unique information about the changing electrical characteristics of the fracture region. Given the possibility of better electrode positioning and correction for geometric shape, the improvement in impedance images would be a useful adjunct to clinical monitoring of fracture healing.

Laser speckle is an interference effect, which can be observed either in free space or on the image plane of a diffuse object illuminated by coherent laser light. The possibility of using the spatial statistics of time-integrated laser speckle pattern for monitoring capillary blood flow in quasi real-time has been demonstrated recently. However, there are still several important unresolved issues. First, the validity of the fundamental laser speckle statistical techniques used still requires verification. Second, the existing algorithms and software need to be re-examined and improved for achieving a real-time solution and new algorithms and software need to be introduced with a higher statistical accuracy and higher spatial resolution. Finally, a comprehensive theory and model(s) are needed to guide any further development. This thesis begins by studying the laser and laser speckle concepts and principles. Then the original laser speckle contrast analysis (LASCA) model and algorithms have been evaluated and re-examined. A real-time solution has been presented by using improved computational algorithms. The study of LASCA theory starts by exploring the biomedical structure of skin and tissue optics. A more comprehensive LASCA theory has then been established, with which three new LASCA computational models have been developed, based on temporal speckle statistics, spatial speckle statistics and temporal-spatial speckle statistics, respectively. To obtain a real-time solution, a class of highly efficient algorithms has been designed and developed, based on the three new computational models. To implement these algorithms, the LASCA foundation classes library and its application software have been developed accordingly, with the characteristics of portability, reusability, and modularity. To build up an optimised LASCA system, the LASCA related experimental parameters have also been analysed and discussed in detail and solutions have been proposed. As a result, the enhanced LASCA theory, LASCA system design techniques, LASCA related algorithms and software should become valuable tools in' guiding and assisting a more extensive study on the LASCA techniques.

Extracellular oxygen concentrations affect cellular metabolism and influence tissue function. Detection methods for these extracellular oxygen concentrations currently have poor spatial resolution and are frequently invasive. Fluorescence Lifetime Imaging Microscopy (FLIM) offers a non-invasive method for quantifying local oxygen concentrations. However, existing FLIM methods also show limited spatial resolution >1 μm and low time-resolved accuracy and precision, due to widefield time-gate. This study describes a new optimised approach using FLIM to quantity extracellular oxygen concentration with high accuracy (±7 μmol/kg) and spatial resolution ( ≅ 0.3 μm). An oxygen sensitive fluorescent dye, tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate [Ru(bipy)3]+2, was excited with a multi-photon laser and fluorescence lifetime was measured using time-correlated single photon counting (TCSPC). The system was fully calibrated with optimised techniques developed for avoiding artefacts associated with photon pile-up and phototoxicity, whilst maximising spatial and temporal resolution. An extended imaging protocol (1800 sec) showed no phototoxic effects on cells at dye concentrations of <0.4 mM. Extracellular spatial oxygen gradients were identified around isolated chondrocytes, seeded in three-dimensional agarose gel. The technique was validated by regulating oxygen cellular consumption and thus confirming that the oxygen gradient was governed by cellular consumption. The technique identified a subpopulation of cells exhibiting statistically significant spatial oxygen gradients at the cell perihery. The subpopulation was shown to be significantly larger in cell diameter correlating with what that expected from chondrocytes in the deep zone. This technique provides an exciting opportunity to non-invasively quantify pericellular spatial oxygen gradients from within three-dimensional cellular constructs without prior manipulation of the cells. Thus by examining cellular metabolisms it will advance our understanding of the optimal cellular environment for tissue engineering and regenerative medicine.

In order to develop a successful soft lining material various factors have to be considered; physical strength, adhesion to the denture base ( or prosthetic) and the durability of the material's properties when in the mouth. It has been recognised that in order to fulfil these criteria the material must be stable and have a low water uptake from the aqueous environments of the mouth. In the dental field comparatively little work has focused on how soft lining materials behave in water where as water in polymers has received a considerable amount of interest, with many different types of behaviour being observed and explained. It has been realised by previous authors that the water uptake of elastomers is primarily driven by soluble impurities, these form solution droplets within the material. The nature of the growth is somewhat more debatable, with both Fickian and dual sorption kinetics being reported. Two basic types of materials were used in the study; silicone polymers and elastomer / methacrylate materials. Silicone polymers are characterised by a low water uptake and form the basis of perhaps the most successful soft lining material ('Molloplast B'). The elastomer / methacrylate materials were based on those developed by Parker (1982), Parker and Braden (1990) which showed considerable promise but suffered from an extensive protracted uptake. Water uptake at 370C in conjunction with the tensile strength were used to evaluate the materials produced as these simple tests enabled the behaviour of the material in service to estimated. Three different types of silicone polymers were used during the study classified by the curing mechanism (condensation, peroxide and hydrosilanised), various fillers and additives (such as calcium stearate) were incorporated into the materials and different uptakes observed. The condensation silicones demonstrated large weight losses (up to 20 wt%) in water which is attributed to hydrolytic instability of the siloxane bridge in the presence of an organo tin compound leading to a leaching of siloxane. The pure peroxide and hydrosilanised materials both demonstrated a low water uptake but when doped they form solution droplets in a similar way to that described in the literature. Other additives showed different behaviour with the formation of cracks within the silicone due to failure of the material around the droplets, the action of hydrophilic but insoluble fillers also promotes the uptake. The hydrosilanised silicone polymers showed considerable promise as soft lining materials with low water uptake and good tensile strength. 11 The elastomer / methacrylate materials were based initially on butadiene styrene copolymer and a higher methacrylate monomer which formed a gel this was then free radically cured. The water uptake of these materials was attributed to soluble separating agent added to the butadiene styrene (to prevent particle agglomeration) during the production of the powdered elastomer. The extent of the uptake could be controlled by improving the strength of the material but the overall uptake remained too high. When the material was placed in an osmotic solution (Na CI or glucose) the water uptake was significantly reduced and the behaviour could be described by a modified (for small strains) version of the Thomas and Muniandy (1987) theory for the growth of water droplets in a elastomer. In order to reduce the water uptake of the elastomer / methacrylate materials butadiene styrene copolymers without separating agent was used. The emulsion polymerised material contained soluble impurities from the polymerisation (i.e. soap)which acted to drive the water uptake. Solution polymerised butadiene styrene also demonstrated a high uptake but this is attributed to a clustering behaviour of carboxylic and hydroxyl groups which formed post production. Similar behaviour is also seen for a solution polymerised isoprene styrene elastomer. The role of crosslinking the material in restraining the growth of the droplets is also investigated with dramatic reductions in the uptake being observed as the crosslink density increased. The employment of a reinforcing silica filler proved more effective than simply using a dimethacrylate. Oxidation is another problem (characterised by an upturn in the absorption), although not observed in every case it was a problem for all of the unsaturated elastomers and was found to be promoted by ions present within saliva. Saturated butyl based (including chloro and bromo butyl) elastomers were used instead and did not show any tendency for oxidation but they still showed an uptake of approximately 3 to 4 wt%. Their stability however and reasonable strength makes them suitable for further development as soft lining materials. Theoretical considerations were investigated by HI NMR imaging with the formation of droplets being observed, the profiles seen indicating the absorption to be two stage rather than Fickian. The role of creep or stress relaxation is also identified as a mechanism for extending the uptake by reducing the restraining force. Further reasoning on all the data presented here concluded the role of chemical potential change associated with the water into the matrix or the droplets will determine the nature of the uptake observed.

Given the poor durability of bioprosthetic heart valves and thrombogenicity of mechanical valves, recent attention has been directed towards synthetic leaflet valves. The work of this thesis forms part of a project to develop a tri-leaflet polyurethane heart valve prosthesis. Two aspects have been addressed: in vitro valve function and durability test methods, and fabrication techniques for polyurethane valves. Existing in vitro valve testing facilities, comprising a hydrodynamic function tester (pulse duplicator) and accelerated fatigue testers, have been upgraded. An improved data acquisition system combined with a computerised control system has been developed for the pulse duplicator. The new system allows valve function to be more efficiently and reliably assessed, and also provides a means for characterising the pulse duplicator and its transducers. Accelerated fatigue testing facilities have been similarly enhanced by the introduction of a computerised data acquisition system. In order to exploit the design potential offered by the use of a synthetic material, an integrated CAD/CAM system has been developed for producing sculptured valve formers. Such formers have been incorporated into injection moulding tools for tri-leaflet polyurethane valves. Polyurethane valves have been moulded, but to date not with sufficiently thin leaflets: the required leaflet thickness (< 150 gm) results in a mould cavity which presents an enormous resistance to flow in the injection moulding process. However, a finite element-based mathematical model has been used to simulate the flow of molten polyurethane into the mould cavity and initial results suggest that it should indeed be possible to injection mould a polyurethane valve and a practicable means of achieving this has been identified. The sculptured formers incorporated into the injection moulding tools have also been used to create dip moulded tri-leaflet polyurethane valves. These dip moulded valves, though difficult to produce consistently, function reasonably well in the pulse duplicator and accelerated fatigue tests are in progress.

This thesis treats biomedical ultrasonics, cavitation and sonoporation. Focussed ultrasound surgery can heat tissue to a temperature that causes protein denaturation and coagulative necrosis. For high-resolution focused ultrasound microsurgery, high working frequencies are necessary. We manufactured a highfrequency, high-intensity focussed ultrasound transducer, using lithium niobate as the active element. The transducer was capable of creating 2.5×3.4 (mm)2 lesions without affecting surrounding tissue. Such disruptive effects of ultrasound also have applications outside medicine. Since cyanobacteria contain gas vesicles, we hypothesised that these can be disrupted with the aid of ultrasound. During 1-hour sonication in the clinical diagnostic range, we forced blue-green algae to sink, thus promoting natural decay. In medical diagnostics, ultrasound contrast agents are added to the blood stream to differentiate between blood and other tissue types. We injected such lipid-shelled microbubbles into a synthetic capillary and sonicated using continuous ultrasound. The microbubbles formed clusters at a quarter wavelength apart owing to radiation forces. We observed cluster coalescence and translation towards the capillary wall. To study acoustic cavitation, we designed and built a scientific instrument combining a pulsed laser and a high-intensity focussed ultrasound transducer, capable of nucleating at precise locations. The cavitation dynamics were recorded using highspeed cameras. At high acoustic intensities, interacting cavitation clouds were formed. Microbubbles under sonication have been observed to create transient pores in adjacent cell membranes. This so called sonoporation has been associated with highly non-linear bubble phenomena. We observed lipid-shelled microbubbles near cancer cells under quasi-continuous low-amplitude sonication. Typically within a second of sonication, microbubbles were seen to enter the cells and dissolve. This new explanation of sonoporation was verified using high-speed photography and confocal fluorescence microscopy. If drug and genes can be successfully coupled to acoustically active vehicles, sonoporation might revolutionise non-invasive therapy as we know it.

No description available.