Design and synthesis of proteoglycan analogues for tissue repair and regeneration

This thesis is concerned with the design and synthesis of a novel, injectable proteoglycan analogue for tissue repair. This is of particular relevance to the restoration of disc height to a degraded nucleus pulposus of the intervertebral disc. The focus is on the use of sulfonate monomers as proteoglycan analogues, in particular sodium 2-acrylamido-2-methylpropane sulfonic acid and the potassium salt of 3-sulfopropyl acrylate. For most biomedical applications, synthetic hydrogels need to show dimensional stability to changes in pH, osmolarity, and temperature. This is readily achieved by neutral structures however ionic sulfonate containing hydrogels are responsive to environmental change which renders them difficult to manage in most tissue replacement applications. In this case osmotic responsiveness rather than stability is desirable. Therefore sulfonate based materials possess advantageous properties. This is a result of the sulfonate becoming an ideal surrogate for the sulfate group present within the structure of natural proteoglycans. This thesis reports polymerisation studies based on the production of a redox initiated copolymer system capable of polymerising in situ within a timescale of circa. 5-7 minutes. The rheological properties, osmotic drive, and residual monomer content of successful compositions is analysed. Properties are adapted to mimic those of the target natural tissue. The adaptation of the material for use as an injectable intra-ocular lens, with hyaluronic acid as an interpenetrate is reported. The synthesis of a radiopaque macromer to allow visibility of the repair system once in situ is investigated and discussed. The results presented in this thesis describe a suitable proteoglycan tissue analogue which is injectable, biomimetic, osmotically responsive and mechanically stable in its desired application.

The oxidation behaviour of sintered iron

The oxidation behaviour of porous, sintered iron was studied by thermo-gravimetric analysis (TGA), at temperatures between 300oC and 700oC, in a flowing atmosphere of 20% O2/80% N2. Samples for TGA tests were compacted from pure iron powder, at 150MPa to 550MPa, and vacuum sintered at 1120oC. The mass gain of samples during oxidation was recorded continuously for a period of 24 hours. It was found that the oxidation mass gain of PM samples depended on the permeability of the pore structure and the temperature. At low temperatures, the oxidising gas was able to permeate through the pore structure, causing the oxidation of a large active surface area. At high temperatures the active surface area was smaller, because oxygen diffusing into the pore structure, from the external atmosphere, was adsorbed by pore surfaces close to the external surface of the compact. Although the weight of the external oxide scale on compacts increased with increasing oxidation temperature, the absence of oxide in the core porosity in compacts oxidised at higher temperatures resulted in smaller mass gains than were observed for compacts oxidised at lower temperatures. The heat generated by the oxidation of the large active surface areas of porous samples was studied by thermo-calorimetric analysis (TCA). It was determined that this phenomenon could raise the core temperature of samples significantly above the ambient furnace temperature, and affecting the morphology of the oxide scale formed. The effects (on oxidation behaviour at 500oC) of small, elemental alloy additions of Al, Cu, P and Si to pure iron powder were studied. It was found that elements that promote pore rounding during sintering caused a significant reduction in the mass gain rate of the PM alloys, compared to the PM pure iron. The oxidation resistance due to these elements prevented pore closure by oxide growth, so that the active surface area of these PM alloys remained high. The PM alloys were also studied by thermo-mechanical analysis (TMA, dilatometry), to determine their dimensional stability during sintering and subsequent elevated temperature service. The oxidation experiment was augmented with optical and electron microscopy, and X-ray analysis of alloy and scale compositions.

Reinforced thermoplastics for engineering applications

The effect of mechano-chemically bound polypropylene modifiers on the mechanical performance and thermal-oxidative stability of polypropylene composites has been studied. The mechanical performance of unmodified polypropylene containing silane coupled glass and Rockwool (mineral) fibre was poor by comparison with a similar commercially produced glass reinforced composite; this was attributed to poor fibre-matrix adhesion. Mechano-chemical binding with unsaturated additives was obtained in the presence of a free radical initiator (di-cumyl peroxide). This process was inhibited by stabilisers present in commercial grades of polypropylene composites by chemical bond formation between the chemically bound modifier and the silane coupling agent on the fibre surface, resulting in a dramatic improvement in the mechanical properties, dimensional stability and retention of mechanical performance after immersion in fluids typically found in under-bonnet environments.A feature unique to some of these modifiers was their ability not only to enhance the mechanical properties of polypropylene composites to levels substantially in excess of currently available commercial materials, but their ability to act as effective thermal-oxidative polypropylene stabilisers. The mode of action was shown to be a chain-breaking mechanism and as a result of the high binding levels achieved during melt processing, these modifiers were able to efficiently stabilise polypropylene in the most severe volatilising and solvent-extracting environments, thus giving much better protection to the polymer than currently available commercially stabilised grades of polypropylene.

Uncertainty evaluation of an axi-symmetric measurement machine

This paper describes a method of uncertainty evaluation for axi-symmetric measurement machines which is compliant with GUM and PUMA methodologies. Specialized measuring machines for the inspection of axisymmetric components enable the measurement of properties such as roundness (radial runout), axial runout and coning. These machines typically consist of a rotary table and a number of contact measurement probes located on slideways. Sources of uncertainty include the probe calibration process, probe repeatability, probe alignment, geometric errors in the rotary table, the dimensional stability of the structure holding the probes and form errors in the reference hemisphere which is used to calibrate the system. The generic method is described and an evaluation of an industrial machine is described as a worked example. Type A uncertainties were obtained from a repeatability study of the probe calibration process, a repeatability study of the actual measurement process, a system stability test and an elastic deformation test. Type B uncertainties were obtained from calibration certificates and estimates. Expanded uncertainties, at 95% confidence, were then calculated for the measurement of; radial runout (1.2 µm with a plunger probe or 1.7 µm with a lever probe); axial runout (1.2 µm with a plunger probe or 1.5 µm with a lever probe); and coning/swash (0.44 arc seconds with a plunger probe or 0.60 arc seconds with a lever probe).

Spatiotemporal optical bullets in two-dimensional fiber arrays and their stability

Long-lived light bullets fully localized in both space and time can be generated in novel photonic media such as multicore optical fiber or waveguide arrays. In this paper we present detailed theoretical analysis on the existence and stability of the discrete-continuous light bullets using a very generic model that occurs in a number of applications.

Bifurcation and stability in a low-dimensional model for multiple-frequency mode-locked lasers

Recent theoretical investigations have demonstrated that the stability of mode-locked solutions of multiple frequency channels depends on the degree of inhomogeneity in gain saturation. In this article, these results are generalized to determine conditions on each of the system parameters necessary for both the stability and the existence of mode-locked pulse solutions for an arbitrary number of frequency channels. In particular, we find that the parameters governing saturable intensity discrimination and gain inhomogeneity in the laser cavity also determine the position of bifurcations of solution types. These bifurcations are completely characterized in terms of these parameters. In addition to influencing the stability of mode-locked solutions, we determine a balance between cubic gain and quintic loss, which is necessary for the existence of solutions as well. Furthermore, we determine the critical degree of inhomogeneous gain broadening required to support pulses in multiple-frequency channels. © 2010 The American Physical Society.

Three-dimensional fluid pressure mapping in porous media using magnetic resonance imaging with gas-filled liposomes

This paper presents and demonstrates a method for using magnetic resonance imaging to measure local pressure of a fluid saturating a porous medium. The method is tested both in a static system of packed silica gel and in saturated sintered glass cylinders experiencing fluid flow. The fluid used contains 3% gas in the form of 3-μm average diameter gas filled 1,2-distearoyl-sn-glycero-3-phosphocholine (C18:0, MW: 790.16) liposomes suspended in 5% glycerol and 0.5% Methyl cellulose with water. Preliminary studies at 2.35 T demonstrate relative magnetic resonance signal changes of 20% per bar in bulk fluid for an echo time TE=40 ms, and 6-10% in consolidated porous media for TE=10 ms, over the range 0.8-1.8 bar for a spatial resolution of 0.1 mm3 and a temporal resolution of 30 s. The stability of this solution with relation to applied pressure and methods for improving sensitivity are discussed. © 2007 Elsevier Inc. All rights reserved.

Relaxation procedures for an iterative MFS algorithm for the stable reconstruction of elastic fields from Cauchy data in two-dimensional isotropic linear elasticity

We investigate two numerical procedures for the Cauchy problem in linear elasticity, involving the relaxation of either the given boundary displacements (Dirichlet data) or the prescribed boundary tractions (Neumann data) on the over-specified boundary, in the alternating iterative algorithm of Kozlov et al. (1991). The two mixed direct (well-posed) problems associated with each iteration are solved using the method of fundamental solutions (MFS), in conjunction with the Tikhonov regularization method, while the optimal value of the regularization parameter is chosen via the generalized cross-validation (GCV) criterion. An efficient regularizing stopping criterion which ceases the iterative procedure at the point where the accumulation of noise becomes dominant and the errors in predicting the exact solutions increase, is also presented. The MFS-based iterative algorithms with relaxation are tested for Cauchy problems for isotropic linear elastic materials in various geometries to confirm the numerical convergence, stability, accuracy and computational efficiency of the proposed method.

Magnetohydrodynamic stability of stochastically driven accretion flows

We investigate the evolution of magnetohydrodynamic (or hydromagnetic as coined by Chandrasekhar) perturbations in the presence of stochastic noise in rotating shear flows. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, however, are Rayleigh stable but must be turbulent in order to explain astrophysical observed data and, hence, reveal a mismatch between the linear theory and observations and experiments. The mismatch seems to have been resolved, at least in certain regimes, in the presence of a weak magnetic field, revealing magnetorotational instability. The present work explores the effects of stochastic noise on such magnetohydrodynamic flows, in order to resolve the above mismatch generically for the hot flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect, mimicking a small section of an astrophysical accretion disk around a compact object. It is found that such stochastically driven flows exhibit large temporal and spatial autocorrelations and cross-correlations of perturbation and, hence, large energy dissipations of perturbation, which generate instability. Interestingly, autocorrelations and cross-correlations appear independent of background angular velocity profiles, which are Rayleigh stable, indicating their universality. This work initiates our attempt to understand the evolution of three-dimensional hydromagnetic perturbations in rotating shear flows in the presence of stochastic noise. © 2013 American Physical Society.

Two-dimensional viscoelastic discrete triangular system with negative-stiffness components

Potential applications of high-damping and high-stiffness composites have motivated extensive research on the effects of negative-stiffness inclusions on the overall properties of composites. Recent theoretical advances have been based on the Hashin-Shtrikman composite models, one-dimensional discrete viscoelastic systems and a two-dimensional nested triangular viscoelastic network. In this paper, we further analyze the two-dimensional triangular structure containing pre-selected negative-stiffness components to study its underlying deformation mechanisms and stability. Major new findings are structure-deformation evolution with respect to the magnitude of negative stiffness under shear loading and the phenomena related to dissipation-induced destabilization and inertia-induced stabilization, according to Lyapunov stability analysis. The evolution shows strong correlations between stiffness anomalies and deformation modes. Our stability results reveal that stable damping peaks, i.e. stably extreme effective damping properties, are achievable under hydrostatic loading when the inertia is greater than a critical value. Moreover, destabilization induced by elemental damping is observed with the critical inertia. Regardless of elemental damping, when the inertia is less than the critical value, a weaker system instability is identified.

Linear stability of flow past two circular cylinders in a side-by-side arrangement

The linear stability of flow past two circular cylinders in a side-by-side arrangement is investigated theoretically, numerically and experimentally under the assumption of a two-dimensional flow field, in order to explore the origin of in-phase and antiphase oscillatory flows. Steady symmetric flow is realized at a small Reynolds number, but becomes unstable above a critical Reynolds number though the solution corresponding to the flow still satisfies the basic equations irrespective of the magnitude of the Reynolds number. We obtained the solution numerically and investigated its linear stability. We found that there are two kinds of unstable modes, i.e., antisymmetric and symmetric modes, which lead to in-phase and antiphase oscillatory flows, respectively. We determined the critical Reynolds numbers for the two modes and evaluated the critical distance at which the most unstable disturbance changes from the antisymmetric to the symmetric mode, or vice versa. ©2005 The Physical Society of Japan.