The Design and Characterisation of Sinusoidal Toolpaths using Sub-Zero Bioprinting of Polyvinyl Alcohol

Sub-zero (°C) additive manufacturing (AM) systems present a promising solution for the fabrication of hydrogel structures with complex external geometry or a heterogeneous internal structure. Polyvinyl alcohol cryogels (PVA-C) are promising tissue-mimicking materials, with mechanical properties that can be designed to satisfy a wide variety of soft tissues. However, the design of more complex mechanical properties into AM PVA-C samples, which can be enabled using the toolpath, is a largely unstudied area. This research project will investigate the effect of toolpath variation on the elastic and viscoelastic properties of PVA-C samples fabricated using a sinusoidal toolpath. Samples were fabricated using parametric variation of a sinusoidal toolpath, whilst retaining the same overall cross-sectional area, using a sub-zero AM system. To mechanically characterise the samples, they were tested under tension in uniaxial ramp tests, and through dynamic mechanical analysis (DMA). The elastic and viscoelastic moduli of the samples are presented. No correlations between the parametric variation of the design and the Young's modulus were observed. Analysis of the data shows high intra-sample repeatability, demonstrated robust testing protocols, and variable inter-sample repeatability, indicating differences in the printability and consistency of fabrication between sample sets. DMA of the wavelength samples, show a frequency-dependent loss moduli. The storage modulus demonstrates frequency independence, and a large increase in magnitude as the sample increases to 3 wavelengths

Application of magnetic resonance elastography to atherosclerosis

Atherosclerosis is the root cause of a wide range of cardiovascular diseases. Although it is a global arterial disease, some of the most severe consequences, heart attack and stroke, are caused by ischemia due to local plaque rupture. The risk of rupture is related to the mechanical properties of the plaque. Magnetic resonance elastography (MRE) images tissue elasticity by inverting, externally excited, harmonic wave displacement into a stiffness map, known as an elastogram. The aim of this thesis is to computationally and experimentally investigate the application of MRE to image the mechanical properties of atherosclerotic plaques. The cardiac cycle, lumen boundary, size and inhomogeneous nature of atherosclerotic plaques pose additional complications compared to more well-established MRE applications. Computational modelling allowed these complications to be assessed in a controlled and simplified environment, prior to experimental studies. Computational simulation of MRE was proposed by combining steady state shear waves, yielded by finite element analysis, with the 2D Helmholtz inversion algorithm. The accuracy and robustness of this technique was ascertained through models of homogeneous tissue. A computational sensitivity study was conducted through idealised atherosclerotic plaques, incorporating the effects of disease variables and mechanical, imaging and inversion parameters on the wave images and elastograms. Subject to parameter optimisation, a change in local plaque shear modulus with composition was established. Amongst other variables, an increase of the lipid pool volume in 10mm3 increments was shown to decrease the predicted shear modulus for stenosis sizes between 50% and 80%. The limitations of the Helmholtz inversion algorithm were demonstrated. A series of arterial phantoms containing plaques of various size and stiffness were developed to test the experimental feasibility of the technique. The lumen was identifiable in the wave images and elastograms. However the experimental wave propagation, noise and resolution left the vessel wall and plaque unresolvable. A computational replica of the phantoms yielded clearer wave images and elastograms, indicating that changes to the experimental procedure could lead to more successful results. The comparison also highlighted certain areas for improvement in the computational work. Imaging protocol for in vivo MRE through the peripheral arteries of healthy volunteers and peripheral artery disease patients was developed. The presence of physiological motion and low signal to noise ratios made the vessel anatomy unidentifiable. The application of MRE to atherosclerotic plaques through simulations, arterial phantoms, healthy volunteers and patients has shown that although there is the potential to identify a change in shear modulus with composition, the addition of realistic experimental complications are severely limiting to the technique. The gradual addition of complications throughout the thesis has allowed their impact to be assessed and in turn has highlighted areas for future research

Translating the Three-Dimensional Mathematical Modelling of Plant Growth to Additive Manufacturing

Much like how plants grow via the expansion and multiplication of cells, a 3D printed component is formed via the bonding of material point-by-point from the bottom-up. Exploiting this analogy, this work employs mathematical models of three-dimensional plant growth to further understand and aid implementation of additive manufacturing (AM) technologies (otherwise known as 3D printing). The resolution of these printed structures is of the upmost importance in the fabrication of tissue scaffolds or constructs that mimic the mechanical properties of tissues. As such, the overarching aim is to derive a generalised mathematical model to simulate the extrusion-based bioprinting process via manipulation of the underlying physics of the system. Such a model has the potential to theoretically identify which combinations of printing process parameters generate a successful resolution: the ‘window of printability’ of a bioink. A hydrogel typically presents a shear-thinning behaviour. In this paper we consider the simplest case: a Newtonian fluid flow far from any edge effects. An initial steady-state model for a viscous thread under extrusion using an arc-length-based coordinate system is presented. As such, this research presents a significant milestone toward representing the non-Newtonian system. This uniquely transdisciplinary methodology seeks to optimise the comparability and transferability of results across materials and laboratories and, above all, increase the efficiency of extrusion-based bioprinting and enhance design creativity by devising a user-friendly, sustainable tool for engineers to visualise AM as a process of growth

Topological analysis to enhance the understanding of transdisciplinary engineering

In engineering, the design of a product relies heavily on a design specification; a co-creation of customer and engineer which captures the requirements. Subjectivity is intrinsic to this process. Whilst engineers typically have a high appreciation of the technical aspects of design, the detailed knowledge of environmental and socioeconomic (ESE) implications are often held elsewhere. As such, efficient and effective design is critically dependent on the processes underpinning knowledge transfer. However, the information interfaces between engineering and the requirements of our swiftly changing civilisation remain indirect and suboptimal, and the unintended consequences of design choices are becoming increasingly serious.Transdisciplinary engineering bridges knowledge boundaries interfacing with engineering (e.g. social science). This paper explores whether topology (a branch of pure mathematics) presents an opportunity to analyse the complex interdependency of transdisciplinary engineering information. Topology and geometry describe the structure of objects such as connectedness or the number of holes and have recently provided a suite of powerful and robust tools for analysing high-dimensional data sets. However, the real-world implementation of the term topology is still evolving. Interviews with engineering organisations, revealed that topology is almost exclusively interpreted as ‘Topology Optimisation’ in the context of advanced design and manufacturing. To date, mathematical processes for critically and systematically examining the topology of systems have not been transferred through to the engineering industry. This paper compares how topology is interpreted by the engineering industry, compared to academic literature, and reflects on the opportunities of applying the mathematical theory of topological analysis to transdisciplinary engineering data

Topological analysis to enhance the understanding of transdisciplinary engineering

In engineering, the design of a product relies heavily on a design specification; a co-creation of customer and engineer which captures the requirements. Subjectivity is intrinsic to this process. Whilst engineers typically have a high appreciation of the technical aspects of design, the detailed knowledge of environmental and socioeconomic (ESE) implications are often held elsewhere. As such, efficient and effective design is critically dependent on the processes underpinning knowledge transfer. However, the information interfaces between engineering and the requirements of our swiftly changing civilisation remain indirect and suboptimal, and the unintended consequences of design choices are becoming increasingly serious.Transdisciplinary engineering bridges knowledge boundaries interfacing with engineering (e.g. social science). This paper explores whether topology (a branch of pure mathematics) presents an opportunity to analyse the complex interdependency of transdisciplinary engineering information. Topology and geometry describe the structure of objects such as connectedness or the number of holes and have recently provided a suite of powerful and robust tools for analysing high-dimensional data sets. However, the real-world implementation of the term topology is still evolving. Interviews with engineering organisations, revealed that topology is almost exclusively interpreted as ‘Topology Optimisation’ in the context of advanced design and manufacturing. To date, mathematical processes for critically and systematically examining the topology of systems have not been transferred through to the engineering industry. This paper compares how topology is interpreted by the engineering industry, compared to academic literature, and reflects on the opportunities of applying the mathematical theory of topological analysis to transdisciplinary engineering data