Thesis (S.M.)--Harvard-MIT Division of Health Sciences and Technology, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 35-38). / Background: The optimal long-term antiplatelet therapy (APT) that balances the benefit of preventing myocardial infarction (MI) with the risk of severe bleeding is unknown in patients greater than one year after drug-eluting stent (DES) placement. Methods: We modeled life expectancy (LE) using published data by building a Markov model to compare several APT strategies composed of aspirin and clopidogrel, both as monotherapy and in various clinically plausible combinations. The base case examined a 65-year old person treated with a DES then continuous aspirin plus clopidogrel (Dual-Rx) for one year without complications. We considered risk of mortality from myocardial infarction and severe bleeding. We used a lifetime horizon and projected LE without quality-adjustment. Results: In the base-case analysis, APT yielding greatest LE was a toss-up between Dual-Rx indefinitely (LE of 13.48 years), clopidogrel indefinitely (LE of 13.45 years), and aspirin indefinitely (LE of 13.42 years); of the strategies considered, no APT was least preferred (LE of 13.36 years). All parameters were varied over plausible ranges in sensitivity analyses, including the duration of future treatment with clopidogrel (base-case, life long). The choice of APT remained a toss-up unless: the annual probability of MI fell below 0.0087 (base-case, 0.013) or the relative risk of systemic bleeding exceeded 1.52 (base case, 1.00), in which case clopidogrel indefinitely was preferred; or the efficacy of clopidogrel to prevent MI fell below 0.09 (base case, 0.20) or the relative risk of clopidogrel for severe gastrointestinal hemorrhage exceeded 3.33 (base case, 2.01), in which case aspirin indefinitely was preferred. Conclusions: For patients with a drug-eluting stent placed greater than one year ago, the antiplatelet therapy which yields the greatest life expectancy is a toss-up between dual antiplatelet therapy (clopidogrel plus aspirin indefinitely), clopidogrel indefinitely, and aspirin indefinitely. However, additional research (including a clinical trial, subgroup analysis, and modeling) is needed. / by J. Stewart Evans. / S.M.

Thesis (Sc. D.)--Harvard-MIT Division of Health Sciences and Technology, 2005. / Includes bibliographical references (p. 185-195). / Space-suited activity is critical for human spaceflight, and is synonymous with human planetary exploration. Space suits impose kinematic and kinetic boundary conditions that affect movement and locomotion, and in doing so modify the metabolic cost of physical activity. Metabolic requirements, found to be significantly elevated in space-suited activity, are a major driver of the allowable duration and intensity of extravehicular activity. To investigate how space suited locomotion impacts the energetics of walking and running, I developed a framework for analyzing energetics data, derived from basic thermodynamics, that clearly differentiates between muscle efficiency and energy recovery. The framework, when applied to unsuited locomotion, revealed that the human run-walk transition in Earth gravity occurs when energy recovery for walking and running are approximately equal. The dependence of muscle efficiency on gravity -during locomotion and under a particular set of assumptions- was derived as part of the framework. Next, I collected and transformed data from prior studies of suited and unsuited locomotion into a common format, and performed regression analysis. This analysis revealed that in reduced gravity environments, running in space suits is likely to be more efficient, per unit mass and per unit distance, than walking in space suits. Second, the results suggested that space suits may behave like springs during running. To investigate the spring-like nature of space suit legs, I built a lower-body exoskeleton to simulate aspects of the current NASA spacesuit, the Extravehicular Mobility Unit (EMU). / (cont.) Evaluation of the exoskeleton legs revealed that they produce knee torques similar to the EMU in both form and magnitude. Therefore, space suit joints such as the EMU knee joint behave like non-linear springs, with the effect of these springs most pronounced when locomotion requires large changes in knee flexion such as during running. To characterize the impact of space suit legs on the energetics of walking and running, I measured the energetic cost of locomotion with and without the lower-body exoskeleton in a variety of simulated gravitational environments at specific and self- selected Froude numbers, non-dimensional parameters used to characterize the run-walk transition. Exoskeleton locomotion increased energy recovery and significantly improved the efficiency of locomotion, per unit mass and per unit distance, in reduced gravity but not in Earth gravity. The framework was used to predict, based on Earth gravity data, the metabolic cost of unsuited locomotion in reduced gravity; there were no statistical differences between the predictions and the observed values. The results suggest that the optimal space-suit knee-joint torque may be non-zero: it may be possible to build a 'tuned space suit' that minimizes the energy cost of locomotion. Furthermore, the observed lowering of the self-selected run-walk transition Froude number during exoskeleton locomotion is consistent with the hypothesis that the run-walk transition is mediated by energy recovery. The major contributions of the dissertation include: 1. A model that predicts metabolic cost in non-dimensional form for unsuited locomotion across running and walking and across gravity levels, 2. / (cont.) An assessment of historical data that reveals the effect of pressure suits on work output and the metabolic cost of locomotion, 3. A method of simulating a space suit using a lower-body exoskeleton, and methods for designing and characterizing the exoskeleton, 4. An explanation for the differences in the energetic costs of walking and running in space suits, 5. Evidence that there is an optimal space suit leg stiffness, perhaps an optimal space suit leg stiffness for a given gravity environment, 6. Evidence, mostly indirect, that energy recovery plays a role in gait switching. / by Christopher Edward Carr. / Sc.D.

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2003. / Page 286 blank. / Includes bibliographical references. / Polyethylene wear is a significant clinical problem limiting the long-term survival of joint replacement prostheses, particularly in total hip arthroplasty (THA) and total knee arthroplasty (TKA). Although the tribology of joint replacement has consequently become an area of significant research, the effect of joint fluid on lubrication in the replaced joint has been largely overlooked. Several factors that affect the tribology of metal on polyethylene articulation in joint prostheses stem from the fluid lubricating the joint. In particular, the properties and composition of joint fluid likely contribute to fluid film lubrication and boundary lubrication in joint replacements, as they do in natural joints. The primary objective of this thesis is to examine the effect of natural variation in joint fluid composition and properties on friction, lubrication, and wear in joint arthroplasty. To achieve this goal, several parameters relating to the composition and mechanical properties of joint fluid are determined. Steady shear viscosity and linear viscoelastic properties of joint fluid are evaluated as indicators of its mechanical properties. Furthermore, concentrations of the hyaluronic acid, protein, and phospholipid in joint fluid are measured using standard biochemical techniques. / (cont.) The molecular weight of hyaluronic acid is also determined using size exclusion chromatography. These properties and components are evaluated in joint fluid from patients undergoing TKA and from patients undergoing surgical revision of an existing TKA (as well as from other patient groups). Results are considered in the context of previous studies of healthy and diseased synovial fluid. Correlations between and among components and flow properties are determined. Friction tests are performed on articulations between ultra-high molecular weight polyethylene (PE) and cobalt-chromium-molybdenum alloy (Co-Cr), materials commonly used in total joint replacement prostheses. These tests evaluate joint fluid samples as well as synthetic joint lubricants that are composed based on the range of compositions and properties determined. Certain components are found to increase friction in this articulation relative to water lubrication, but some joint fluid samples performed as well as bovine serum. / (cont.) Significant differences in tribology demonstrated by these experiments indicate that the composition of joint fluid affects the tribology of Co- Cr on PE joint prostheses, though the variability in friction could not be explained by physiological variation in the components examined. In related work, the relative importance of contact area and normal load is evaluated in the wear of a Co-Cr on PE articulation. Within a relevant range of contact stress, volumetric wear rate increased with increasing contact area, and was independent of normal load. The results of these tribological investigations are brought together in a conceptual framework under which to consider the wear of PE in TJA. / by Dan Mazzucco. / Ph.D.

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Growth and division are essential biological processes of cellular life. A crucial question concerning the origin of cellular life is how primitive cells (protocells) lacking complex biological machinery could grow and divide. To address this question, we first developed an effective method for preparing large monodisperse (uniform-sized) vesicles through a combination of extrusion and large-pore dialysis. The development of this preparation method has led us to the discovery of a simple but efficient pathway for the growth and division of the membrane envelope of a model protocell: growth of a large multilamellar fatty acid vesicle after being fed with fatty acid micelles leads to a series of remarkable shape transformations, from an initially spherical state to a long thread-like vesicle; under modest shear forces, the thread-like vesicle divides into multiple daughter vesicles. We have also discovered a different pathway that allows the long thread-like vesicles to divide without relying on external forces. Furthermore, in the course of studying fatty acid vesicles, we have discovered a striking phenomenon: intense illumination causes dye-packed vesicles of a few microns in diameter to explode, rapidly and locally releasing the encapsulated contents. The photoactivated release of substances from exploding vesicles in a highly spatio-temporally controlled manner suggests potential applications of this phenomenon in many areas across disciplines. / Ph.D.

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2004. / Includes bibliographical references. / Non-muscle cell shape change and motility depend primarily on the dynamics and distributions of cytoplasmic actin. In cells, actin cycles between monomeric and polymeric phases tightly regulated by actin binding proteins that control cellular architecture and movement. Here, we characterize actin remodeling in shear stress stimulated endothelial cells and in actin networks reconstituted with purified proteins. Fluid shear stress stimulation induces endothelial cells to elongate and align in the direction of applied flow. Alignment requires 24 h of exposure to flow, but the cells respond within minutes to flow by diminishing their movements by 50%. Although movement slows, actin filament turnover times and the amount of polymerized actin in cells decreases, increasing actin filament remodeling in individual cells composing a confluent endothelial monolayer to levels used by disperse, non-confluent cells for rapid movement. Hours later, motility returns to pre-shear stress levels, but actin remodeling remains highly dynamic in many cells. We conclude that shear stress initiates a cytoplasmic actin remodeling response that is used to modify endothelial cell shape instead of bulk cell translocation. We determine the steady state dynamics of purified actin filament networks in the entangled state and after orthogonal cross-linking with filamins using a novel, non-perturbing fluorescence system. Human filamin A or Dictyosteliun discoidium filamin slow actin filament turnover by [approximately] 50% and recruit much of a significant population of actin oligomers that we measure are present in polymerized purified actin solutions into the immobile filament fraction. Surprisingly, these observations occur at very low stoichiometry to actin, approximately requiring only one / (cont.) filamin molecule bound per actin filament, similar to the amount required for actin filament gelation in vitro. Networks formed with filamin truncates localize this activity to the actin binding domain and reveal that dimerization and orthogonal cross-linking are not required for dynamic stabilization. Re-expression of filamin A with or without the actin binding domain in human melanoma cells that naturally lack this protein support the findings in purified actin networks. These results indicate that filamin cross-linking stabilizes filament dynamics by, slowing filament subunit cycling rates and by either decreasing spontaneous filament fragmentation or promoting filament annealing. / by Eric A. Osborn. / Ph.D.

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2008. / Includes bibliographical references. / At the present time, there are many hurdles to overcome in order to create a long-lasting and engineered tissue for tissue transplant in patients. The challenges include the isolation and expansion of appropriate cells, the arrangement of assorted cells into correct spatial organization, and the development of proper growth conditions. Furthermore, the creation of a three dimensional engineered tissue is limited by the fact that tissue assemblies greater than 100-200 micrometers, the limit of oxygen diffusion, require a perfused vascular bed to supply nutrients and to remove waste products and metabolic intermediates. To overcome this limitation, this thesis aims to pre-seed a tissue engineered construct with vascular cells (both endothelial and perivascular cells), so the vascular cells could readily form functional vessels in situ. Previous work in the laboratory had successfully demonstrated the formation of functional microvascular network by co-implantation of human umbilical cord vein endothelial cells (HUVECs) and 10 T 1/2 cells, a line of mouse embryonic fibroblasts. To translate this concept to the clinic, we need to utilize cells that can be secured and used in clinic. To this end, we systematically replace each individual vascular cell type with a readily available source of cells. First, we investigated human embryonic stem cells (hESCs) derived endothelial cells. We demonstrated that when hESCs derived endothelial cells were implanted into SCID mice, they formed blood vessels that integrated into the host circulatory system and served as blood conduits. Second, we compared the formation and function of engineered blood vessels generated from circulating endothelial progenitor cells (EPCs) derived from either adult peripheral blood or umbilical cord blood. / (cont.) We found that adult peripheral blood EPCs formed blood vessels that were unstable and regressed within three weeks. In contrast, umbilical cord blood EPCs formed normal-functioning blood vessels that lasted for more than four months. These vessels exhibited normal blood flow, perm-selectivity to macromolecules and induction of leukocyte-endothelial interactions in response to cytokine activation similar to normal vessels. Third, we evaluated human bone marrow-derived mesenchymal stem cells (hMSCs) as a source of vascular progenitor cells. hMSCs expressed a panel of smooth muscle markers in vitro and cell-cell contact between endothelial cells and hMSCs up-regulated the transcription of smooth muscle markers. hMSCs efficiently stabilized nascent blood vessels in vivo by functioning as perivascular precursor cells. The engineered blood vessels derived from HUVECs and hMSCs remained stable and functional for more than 130 days in vivo. On the other hand, we could not detect differentiation of hMSCs to endothelial cell in vitro and hMSCs by themselves could not form conduit for blood flow in vivo. Similar to normal perivascular cells, hMSCs-derived perivascular cells contracted in response to endothelin-1 in vivo. Thus, our work demonstrates the potential to generate a patent and functional microvascular network by pre-seeding vascular cells in a tissue-engineered construct. It serves as a platform for the addition of parenchymal cells to create a functional and vascularized engineered tissue. / by Pakwai (Patrick) Au. / Ph.D.

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2009. / Includes bibliographical references (p. 141-152). / Disrupted blood-brain barrier after an ischemic attack can cause vasogenic edema and increase the risk of hemorrhagic transformation. Therefore, early detection and monitoring of BBB damage is important in the pathological understanding and therapeutic treatment of stroke. Currently, MR contrast agents have been widely used in clinics for disease diagnosis and treatment evaluation, and in basic research to achieve better anatomical structure visualization and to understand pathological mechanisms of various human diseases in animal models. Thus, the central theme of this thesis to exploit the use of MR contrast agents in the study of ischemic stroke using both in vivo and ex vivo MR techniques. Specifically, the overall goals of this thesis are twofold: (1) to exploit the multiple relaxation mechanisms and varying tissue-dependent affinities of different MR contrast agents for better structure delineation, tissue differentiation, and image contrast manipulation in magnetic resonance microscopy (MRM) staining, and (2) to develop an MRI technique that employs intrinsic water as a biomarker for qualitative and quantitative monitoring of blood-brain barrier (BBB) integrity alteration in a mouse model of stroke using an intravascular long-circulating MRI contrast agent. Despite the great success of MRM in anatomical studies, MRM images based on intrinsic tissue contrast lack the flexibility and target-specificity offered by conventional histological staining. Therefore, the first focus of this thesis was on the development of MRM staining method by utilizing the different tissue relaxation ability and tissue biophysical/biochemical properties of different MR contrast agents. Two common MR contrast agents, Gd-DTPA and MnCl2 were used in this thesis. The ability of MR contrast agents to increase SNR and enhance image contrast was first tested in a relatively simple in vitro glioma spheroid (diameter ' 400 um) system. / (cont.) We then fully characterized the relaxation mechanisms and tissue-dependent staining properties of these contrast agents in the brain tissue, and demonstrated that their unique relaxation and tissue properties led to differentiated MR staining in the ex vivo mouse brains, which greatly enhanced the ability of MRM to delineate tissue structures in addition to providing improved SNR. This MRM staining method was then applied to the Kif2la knockout mouse model for the anatomical phenotyping of the new born Kif2la knockout mice. The BBB damage is usually detected through the spatial leakage profiles of extrinsically administrated markers such as staining dyes, fluoresceins, radiolabeled compounds, or gadolinium based compound, which are only possible when BBB is compromised to the extent that allows extravasation of these markers. It is therefore desirable to develop a technique that allows the early detection of BBB damage. In the second part of thesis, we first presented the theoretical background of measuring trans vascular water exchange based on a two-compartment water exchange model. Parameters affecting the quantitative BBB water exchange measurement were initially characterized using computer simulations. We then performed graded hypercapnia and Mannitol-induced BBB-opening experiments to test the ability of this novel MRI technique to detect and monitor the changes of BBB integrity and cerebral blood volume (CBV). Upon the characterization of this MRI technique, we measured baseline BBB water exchange and other MRI-derived cerebrovascular parameters in the eNOS knockout mice, and showed that there is basal increase of trans vascular water exchange in addition to the morphological changes in the vasculature of eNOS knockout mice. / (cont.) After developing and characterizing these ex vivo and in vivo MR techniques, we applied the in vivo MRI BBB water exchange detection technique and the ex vivo MRM staining method to a mouse model of transient stroke. We demonstrated the importance of CBV restoration in the BBB integrity change at acute stage after reperfusion, and showed that MRM staining may have a great potential in histopathological studies of ischemic brain injury. / by Shuning Huang. / Ph.D.

Thesis (Ph. D. in Medical Engineering and Medical Physics)--Harvard-MIT Program in Health Sciences and Technology, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Spinal cord injury (SCI) is a significant health issue resulting in life-long disability and associated secondary complications, affecting approximately 300,000 individuals in the United States. Primary barriers to functional recovery after SCI include the formation of a growth inhibitory astrocyte scar at the lesion border and a lack of a supportive stroma within the defect allowing for axon regeneration. Interestingly, in animals capable of spinal cord regeneration, astrocytes create a tissue bridge across the injury site to facilitate the regeneration of axons through the defect and thus enable functional recovery. The overall goal of this thesis was to develop an injectable collagen-genipin (Col-Gen) hydrogel to facilitate the intrinsic regenerative response after SCI by promoting the population of the defect with astrocytes through a provisional scaffold pennissive of astrocyte migration. The specific aims of the thesis involved: 1) development and materials characterization of an injectable collagen hydrogel for neural tissue regeneration, capable of undergoing covalent crosslinking in vivo; 2) evaluation of the permissiveness of Col-Gen gels with and without Fibroblast growth factor-2 (FGF-2), a known astrocyte chemoattractant, incorporated within lipid microtubules (LMTs) to infiltration by primary astrocytes using an in vitro cellular outgrowth assay; 3) evaluation of select formulations of the gel, based on the in vitro findings, in a standardized hemi-resection defect in the rat spinal cord. Functional, locomotor, and histopathological outcome measures, recorded up to 4 weeks post-SCI were correlated with each other and with micro MRI studies. In vivo, the implantation of Col-Gen gels containing FGF-2 LMTs resulted in the enhancement of astrocyte, blood vessel, and laminin infiltration of the defect; increased the amount of spinal cord tissue spared from secondary degeneration; and increased functional recovery, at four weeks post injury as compared to control or Col-Gen treatment groups. Micro MRI was found to be a suitable modality to nondestructively observe the features of the injury in situ. This work commends an injectable, covalently cross-linkable formulation of collagen gel incorporating FGF-2-releasing LMTs for further investigation for the treatment of SCI. / by Daniel J. Macaya. / Ph.D.in Medical Engineering and Medical Physics

Thesis: Ph. D., Harvard-MIT Program in Health Sciences and Technology, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 75-81). / Most vaccines stimulate the production of antibodies that provide a potent defense upon reinfection by the same strain of pathogen. The key process in antibody development is a stochastic process known as affinity maturation (AM) which generates strain-specific antibodies upon immunization by one antigen. A highly mutable virus like HIV evades recognition by these strain-specific antibodies via the emergence of new mutant strains within the patient. In some chronically infected patients, antibodies that can bind diverse antigens and thus protect against many HIV strains arise naturally; they are named broadly-neutralizing antibodies (bnAbs). A vaccine that elicits bnAbs could prevent HIV infections. This vaccine is expected to contain several different antigens. However, because bnAbs rarely appear in HIV patients, the complex mechanisms by which they emerge are not well understood. Theoretical models of AM could help identify promising vaccination strategies and shed light on a previously ignored problem in basic immunology; meaning how AM works with several antigens. For my thesis I investigated two pathways for breadth evolution. First, motivated by experimental findings that bnAbs have many mutations that may modify the flexibility of the binding region, I examined how flexibility influences breadth. A flexible binding region is expected to enable different conformations and therefore to allow binding to diverse antigens. Towards that goal, I developed a theoretical model of AM which, combined with Molecular Dynamics simulations, suggests that eliciting flexibility-affecting mutations is not essential for the evolution of bnAbs if proper germline B cells are first activated. This is significant as it simplifies the task of immunogen design. For my second project, I studied how separating the different antigens in time and mutational distance affects breadth of binding and antibody titers. The main observation is that introducing the antigens at different times is key to generating breadth. Furthermore, sequentially introducing one antigen per injection yields the greatest breadth and antibody titers. We also devised a prediction tool for breadth given a set of antigens and an immunization protocol. My results suggest optimal vaccination strategies which are expected to guide future in vivo investigations by our collaborators. / by Joy E. Louveau. / Ph. D.

Thesis (S.M.)--Harvard-MIT Division of Health Sciences and Technology, 2009. / Includes bibliographical references. / Clinical decision support systems (CDSS) are developed primarily from knowledge gleaned from evidence-based research, guidelines, trusted resources and domain experts. While these resources generally represent information that is research proven, time-tested and consistent with current medical knowledge, they lack some qualities that would be desirable in a CDSS. For instance, the information is presented as generalized recommendations that are not specific to particular patients and may not consider certain subpopulations. In addition, the knowledge base that produces the guidelines may be outdated and may not reflect real-world practice. Ideally, resources for decision support should be timely, patient-specific, and represent current practice. Patient-oriented clinical decision support is particularly important in the practice of pediatrics because it addresses a population in constant flux. Every age represents a different set of physiological and developmental concerns and considerations, especially in medication dosing patterns. Patient clinical data warehouses (CDW) may be able to bridge the knowledge gap. CDWs contain the collective intelligence of various contributors (i.e. clinicians, administrators, etc.) where each data entry provides information regarding medical care for a patient in the real world. CDWs have the potential to provide information as current as the latest upload, be focused to specific subpopulations and reflect current clinical practice. In this paper, I study the potential of a well-known patient clinical data warehouse to provide information regarding pediatric levothyroxine dosing as a form of clinical decision support. I study the state of the stored data, the necessary data transformations and options for representing the data to effectively summarize and communicate the findings. / (cont.) I also compare the resulting transformed data, representing actual practice within this population, against established dosing recommendations. Of the transformed records, 728 of the 854 (85.2%, [95% confidence interval 82.7:87.6]) medication records contained doses that were under the published recommended range for levothyroxine. As demonstrated by these results, real world practice can diverge from established recommendations. Delivering this information on real-world peer practice medication dosing to clinicians in real-time offers the potential to provide a valuable supplement to established dosing guidelines, enhancing the general and sometimes static dosing recommendations. / by Elisabeth Lee Scheufele. / S.M.