http://hdl.handle.net/1721.1/7889 Search the Channel search http://dspace.mit.edu/simple-search http://hdl.handle.net/1721.1/33843 Title: Novel polypyrrole derivatives to enhance conductive polymer-tissue interactions <br/> <br/>Authors: George, Paul M. (Paul Matthew) <br/> <br/>Abstract: Developing materials that interact effectively with surrounding tissue is a major obstacle in sensor and drug delivery research. The body's natural immune response prevents foreign objects from easily integrating with an organism. Without an intimate link between a biomedical device and the proximate environment, reliable measurements or delivery of molecules is not possible. Many of the current materials used for biomedical applications are centered on inert substances and polymers that degrade in the body but have limited functional capabilities. This thesis work addresses the need to develop materials that are capable of interacting in biological environments. Polypyrrole (PPy) is a conducting polymer that is a promising biomaterial for drug delivery and sensing applications. Because PPy is a polymer that can be made in degradable forms and because it can be stimulated electrically, it is an interactive platform for biomedical applications. By accomplishing the following research objectives, this thesis work could help develop an effective polymeric paradigm for tissue interactions: 1) Develop a new method to effectively micro-pattern electrodeposited polymers and metals for in vivo devices 2) Determine the optimal synthesis conditions of the conductive polymer, PPy, for sensor and implant applications.; (cont.) 3) Fabricate PPy tubes to be used as nerve guides to promote nerve regeneration 4) Modify PPy for neurotrophic factor drug delivery devices and antibody-based sensing applications Through the use of standard microfabrication techniques, the patterning template upon which PPy is electrodeposited can be controlled precisely. By utilizing the growth mechanism of PPy on these templates, three-dimensional polymer objects can be created. Being able to micropattern the PPy and release the polymer generates the ability to create implants and devices that are completely erodible in the body. To develop the optimum conditions for sensor and drug delivery applications, PPy implants were fabricated and implanted into rat cortical tissue. Compared to similar Teflon implants, the electrically conductive PPy had preferable characteristics for material integration in the cortex. Additionally, PPy tubes have been designed and promoted peripheral nerve growth after tissue injury. By controlling the shape and morphology of PPy, the polymer implants formed an interactive bridge with their biological environment. By incorporating bioactive molecules into the PPy matrix, materials for externally controlled drug release and sensing devices can be designed.; (cont.) Drug delivery was demonstrated through the integration of nerve growth factor (NGF), a neurotrophic factor, into the PPy followed by triggered pulsatile release. Such neurotrophic factors can be used to promote neural growth in peripheral and central nervous system injury. Because PPy is easily modifiable through the use of dopants and control of its shape, PPy provides a flexible platform for novel polymeric-tissue interactions. <br/> <br/>Description: Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2005.; Vita.; Includes bibliographical references. http://hdl.handle.net/1721.1/33842 Title: Involvement of TGF-beta in skin photoaging <br/> <br/>Authors: Choi, Won Seon, 1975- <br/> <br/>Abstract: The goal of this thesis study was to understand the role of TGF-[beta] in skin photoaging, especially in solar elastosis. Solar elastosis, the accumulation of elastotic material in the dermal extracelluar matrix, is a major hallmark of photoaging. However, the mechanisms by which UV radiation causes solar elastosis are poorly understood. TGF-[beta] is a multifunctional cytokine involved in the regulation of extracelluar matrix and is known to be up-regulated by UVR. Involvement of reactive oxygen species (ROS) in the development of solar elastosis has been demonstrated by many studies using antioxidants and anti-inflammatory agents in the mouse skin in vivo. We hypothesized that ROS produced by TGF-[beta] are key components in the tropoelastin (TE, a soluble precursor of elastin) up-regulation in dermal fibroblasts, and that TGF-[beta] is a major regulator in the photoaging processes. Using human skin fibroblasts system in vitro, we found that ROS generated from NADPH oxidase and mitochondria are involved in the TGF-[beta] induced elastin production, and intracellular sources of ROS vary with time. We showed that both Smad and non-Smad pathways, e.g. MAPK and PKC pathways, are required for TE mRNA up-regulation by TGF-[beta].; (cont.) However, ROS were not involved in some of the important steps in these pathways, such as phosphorylations of p38 or ERK or Smad2, suggesting that ROS acts downstream of these pathways. The in vivo chronic UVB irradiation study using a Skh- 1 hairless mouse model with a small molecule inhibitor for the TGF-[beta] type I receptor showed that the TGF-[beta] receptor inhibitor increased the number of mast cells, but decreased the levels of active TGF-[beta] protein, and mRNA levels for collagen III and IV, MMP-2 and 9, and TE in the chronically UVB irradiated mouse skin. However, those responses did not result in the changes in the collagen and elastin content, or the wrinkle formation. Overall, this work indicates that TGF-[beta] contributes to the solar elastosis, through the effects on the TE mRNA level in skin. Implication of this role of TGF-[beta] in the elastin fiber deposition or visible changes of photoaged skin requires further investigation. <br/> <br/>Description: Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2005.; Vita.; Includes bibliographical references. http://hdl.handle.net/1721.1/33841 Title: The role of HIF-1 alpha in the localization of embryonic stem cells with respect to hypoxia within teratomas <br/> <br/>Authors: Cochran, David M., Ph. D. Massachusetts Institute of Technology <br/> <br/>Abstract: In embryonic stem (ES) cell tumors, the hypoxia-inducible transcription factor, HIF- 1[alpha], has been shown to be a tumor suppressor, and HIF-1[alpha]-expressing cells have been shown to localize preferentially in vivo to regions near tumor vasculature. These differences were proposed to be due to increased hypoxia-induced apoptosis and growth arrest of HIF-1[alpha]-expressing ES cells. This thesis presents a careful investigation into the localization of ES cells in vitro and in vivo with respect to hypoxia. A sandwich culture system was utilized in which controlled gradients of oxygen and nutrients are developed in the vicinity of the tumor cells. A diffusion-consumption model was utilized to predict the oxygen and glucose concentration profiles within the system. Oxygen and glucose consumption rates were measured and used as inputs into the model, and the concentration profiles were found to depend on a single experimental parameter, the cell density within the system. The optimum cell density was found in which stable, measurable oxygen gradients develop over 2-3 mm. The model demonstrated excellent agreement between the predicted oxygen concentration profiles and experimentally determined oxygen gradients. In vitro, there was no difference in localization with respect to hypoxia between tumor cells expressing or lacking HIF-1[alpha].; (cont.) In addition, there was no difference in apoptosis, proliferation, or migration of the tumor cells in vitro based on HIF-1[alpha] expression. Likewise, a quantitative study on localization of tumor cells within tumors in vivo demonstrated no difference between localization of HIF-1[alpha]-expressing vs. HIF-1[alpha]-lacking ES cells within tumors with respect to blood vessels or hypoxia. These results differ from previous studies, perhaps due to clonal variation of the cell phenotype or the interplay of other complex environmental factors that were not considered in this study. Interestingly, the HIF-1[alpha]-lacking cells were found to exhibit increased tumor growth relative to the HIF-1[alpha]-expressing cells, perhaps due to a normalization of the blood vessels within the HIF-1[alpha]-lacking tumors. These studies reveal the complex role of HIF-1[alpha] in tumor growth and tumor cell localization, as well as develop a useful quantitative experimental model for studying the role of the microenvironment in tumors or in embryonic stem cell biology. <br/> <br/>Description: Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2005.; Includes bibliographical references (leaves 172-183). http://hdl.handle.net/1721.1/33840 Title: Using stimulus frequency otoacoustic emissions to study basic properties of the human medial olivocochlear reflex <br/> <br/>Authors: Backus, Bradford Clark <br/> <br/>Abstract: The medial olivocochlear reflex (MOCR) is a brainstem-based neural feedback circuit by which mammals adaptively adjust the gain of their ears in response to changing environmental conditions. Activating the reflex with sound reduces cochlear gain, but the mechanisms by which the reflex produces its cochlear effects, the role(s) the reflex plays in hearing and many basic reflex properties are not well-understood. This thesis quantifies four basic properties of the reflex in humans using stimulus-frequency-otoacoustic-emissions (SFOAEs) that address the following issues: (1) The relative strengths of ipsilateral and contralateral reflex pathways (2) The reflex time-course (3) The response of the reflex to amplitude modulated (AM) noise (4;) The distribution of reflex strengths across a normal-hearing population Activating the reflex with ipsilateral or contralateral noise produced, on average, the same effect in cochlea at the 1 kHz place, contrary to expectations based upon animal studies. Simultaneous bilateral activation produced an effect that was equivalent to the sum of ipsilateral and contralateral activations, on average. Thus, no prevailing binaural interaction took place for our stimulus. Activating the reflex caused detectable changes in the cochlea within 25 ms; the changes continued to develop for 100's of milliseconds.; (cont.) The decay rate upon reflex deactivation was generally faster than the onset rate ([tau]decay= 159 ± 54 ms, [tau]onset =277 ± 62 ms). In addition, our characterization of onset and decay time-courses suggested that a single second order cellular process (probably in outer hair cells) may govern the bulk of both time-courses. The reflex is not fast enough to protect the ear against loud impulse sounds such as gunshots. Amplitude modulating a wideband noise used to activate the reflex did not, in general, produce larger effects as had been previously reported. The question of whether AM can enhance MOCR responses under some circumstances for some subjects remains unanswered. AM rates important for information in speech (2 - 11 Hz) produced a DC MOCR response. It is possible that conversational speech primes the MOCR to a level conducive to detecting speech in noise. Inter-subject differences were found in the cochlear effects at the 1 kHz place when the MOCR was activated. One difference was a subject-specific rapid frequency variation. This finding called into question basic assumptions of how MOCR activation changes SFOAEs. Averaging across frequencies revealed a second subject-specific difference that was attributed to differences in the regional strength of the reflex (near 1 kHz) between subjects.; (cont.) Regional strength varied by a factor of 7 across 24 subjects. Since a strong MOCR has been shown to protect the ear against acoustic trauma in animals, otoacoustic emission-based tests of reflex strength may help predict susceptibility to acoustic trauma in humans; this study demonstrates that such tests are feasible. The basic properties of the MOCR quantified by this thesis contribute to our understanding of the cellular mechanism that generate the reflex's effects, provides insight into the role(s) the reflex plays in hearing, and may eventually lead to clinically useful tests. <br/> <br/>Description: Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2005.; Vita.; Includes bibliographical references.