The impact of aerobic exercise on brain's white matter integrity in the Alzheimer's disease and the aging population

The brain is the most complex organ in the body. Currently, its complicated functionality has not been fully understood. However, in the last decades an exponential growth on research publications emerged thanks to the use of in-vivo brain imaging techniques. One of these techniques pioneered for medical use in the early 1970s was known as nuclear magnetic resonance imaging based (now called magnetic resonance imaging [MRI]). Nowadays, the advances of MRI technology not only allowed us to characterize volumetric changes in specific brain structures but now we could identify different patterns of activation (e.g. functional MRI) or changes in structural brain connectivity (e.g. diffusion MRI). One of the benefits of using these techniques is that we could investigate changes that occur in disease-specific cohorts such as in the case of Alzheimer’s disease (AD), a neurodegenerative disease that affects mainly older populations. This disease has been known for over a century and even though great advances in technology and pharmacology have occurred, currently there is no cure for the disease. Hence, in this work I decided to investigate whether aerobic exercise, an emerging alternative method to pharmacological treatments, might provide neuroprotective effects to slow down the evident brain deterioration of AD using novel in-vivo diffusion imaging techniques. Previous reports in animal and human studies have supported these exercise-related neuro-protective mechanisms. Concurrently in AD participants, increased brain volumes have been positively associated with higher cardiorespiratory fitness levels, a direct marker of sustained physical activity and increased exercise. Thus, the goal of this work is to investigate further whether exercise influences the brain using structural connectivity analyses and novel diffusion imaging techniques that go beyond volumetric characterization. The approach I chose to present this work combined two important aspects of the investigation. First, I introduced important concepts based on the neuro-scientific work in relation to Alzheimer’s diseases, in-vivo imaging, and exercise physiology (Chapter 1). Secondly, I tried to describe in simple mathematics the physics of this novel diffusion imaging technique (Chapter 2) and supported a tract-specific diffusion imaging processing methodology (Chapter 3 and 4). Consequently, the later chapters combined both aspects of this investigation in a manuscript format (Chapter 5-8). Finally, I summarized my findings, include recommendations for similar studies, described future work, and stated a final conclusion of this work (Chapter 9)

Electro-mechanical characterization of piezo-metallic cellular solids for spine implants

Abstract Many different electrical stimulation methods are currently used to enhance bone growth in spine fusion. In this study, the feasibility of a novel electrical stimulation method using piezoelectric materials embedded into metallic cellular solid structures was presented. The aim of this study was to proof the feasibility to create a new generation of electrically stimulated implants that will mimic and enhance bone osteogenesis in the implanted area while preserving the mechanical characteristics of the environment where are implanted. Cellular composites with different geometric and dimensions were handcrafted and characterized mechanically and electrically. The following study was divided in two parts and was presented in two chapters with the mechanical and electro-mechanical characterization of the structures. First, structures with no piezoelectric plates were mechanically characterized. Non-linearity at small strain, negative compressive strain ratios (CSR), stress strain curves, modulus of elasticity and their relationship with relative densities were investigated. The feasibility of tailoring the mechanical parameters of the implants to mimic the characteristics of the replaced tissue by controlling its geometry, dimension and aspect ratio was investigated. Secondly, electromechanical structures (with embedded piezoelectric ceramics) were characterized when compressed axially. Electrical signals, force and displacements were recorded. Alternated electrical signals generated by the piezoelectric ceramics were electrically rectified and then compared to previous direct electrical current stimulators that have proven to enhance bone osteogenesis [1]. The feasibility to create implants that mimic the mechanical behavior of its environment and present embedded electrical stimulation was validated in this study. Additionally, finite element analysis (FEA) was used to validate the experimental results, design of optimal structures, and understanding in the influence on manufacturing parameters. Models with the same dimensions and geometries were created in FEA and compared to physically tested structures. After the experimental methods were finalized, the feasibility of this investigation and its potential use was discussed while conclusions were brought