In scaffold-based tissue engineering, control of scaffold degradation turns out to be a critical issue for reliable clinical applications. Degradation in this thesis refers to mass loss. Most of the present control methods take the approach of scaffold material modification and/or scaffold work environment adjustment to address this issue. The latter can easily get to its limit, and the former is not promising in the in-vivo implementation. This thesis proposed a new approach to control of scaffold degradation, that is, closed-loop and real-time control. To realize this approach, this thesis has tackled three important problems, namely (1) effects on degradation, (2) modeling of degradation, and (3) real-time measurement of degradation. This thesis is grounded to a biomaterial called chitosan, as it is widely used for building scaffolds. For the first problem, a statistical experiment was designed and a factorial analysis was conducted. For the second problem, a combined empirical-based and probabilistic-based approach was taken. For the third problem, a prototype of a sensor, which is based on the concept of carbon nanotube (CNT) conductive polymer, was built and tested. This thesis concludes (1) a joint thermal and enzymatic effect is significant on chitosan degradation, (2) the model for chitosan degradation is accurate, and (3) real-time measurement of mass loss of scaffold by means of carbon nanotube film is feasible. The major contributions of this thesis are (i) the proposal of the concept of the closed-loop control of degradation, (ii) a finding that there is a significant conjoint thermal and enzymatic effect on chitosan degradation in terms of mass loss, and (iii) a prototype of the novel CNT (carbon nanotube) chitosan film sensor for real-time measurement of mass loss of the scaffold. The significance of these contributions is that they give us confidence to a full development of the closed-loop and real-time degradation control approach. This approach appears promising to bring forth a transformative impact to clinic applications of scaffold-based tissue regeneration.

Nitroxide-mediated Controlled Degradation of Polypropylene <br /> Controlled-rheology polypropylene resins (CRPP) have been produced industrially for years using reactive extrusion processes employing peroxides as free radical initiators. The molecular weight characteristics of CRPP materials can be tailor-made depending on the final application in a very efficient and economic manner. <br /><br /> A PP-based nitroxide (NOR) with the trade name Irgatec CR76 has been recently developed by CIBA Chemicals, as a source of radicals and it is currently being evaluated for the production of CRPP. NORs are well-known as powerful stabilizers to protect plastics from the negative influence of light and heat, and easy in handling during processing. Preliminary experimental results exhibit a qualitative difference between Irgatec CR76 and other commonly used peroxides. <br /><br /> The purpose of this research work is to evaluate this new material as a potential replacement of commonly used peroxides in the production of CRPP. CRPP will be produced by reactive processing using varying amounts of Irgatec CR76 and the rheological properties of the materials produced will be investigated. By comparing results with Irgatec CR76 to those from other typical initiators, the effectiveness of Irgatec CR76 as an initiator will also be assessed. <br /><br /> The results of this research will have an impact not only on the potential extension of uses of the specific PP-based NOR (Irgatec CR76), but also, more generally, on the wider application of additives during degradation of PP. Potentially new materials can be produced with enhanced heat and light protection along with the other benefits inherent to standard CRPP.

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Polymeric materials are increasingly being used in diverse, very demanding applications. Either pre- or post- application environments may require exposures to conditions hostile to the polymer's integrity. Frequently, these demanding conditions result in degradation of the polymer and subsequent decreases in desirable properties. Clearly then, a methodology to predict important properties, such as Tg, molecular weight, and tensile strength, from knowledge of the environmental history of a polymeric-based specimen is beneficial. The current study focuses on bisphenol A polycarbonate and tracks changes in the properties of this material as a function of the degree of degradation, t. For the purposes of the present research, the environmental effects have been limited to those associated with elevated temperature, although the methodology is general. This t parameter is a product of the kinetic rate constant, k, found from isothermal kinetics, and the time of degradation, t. Elucidation of t has been linked to measurement of the molecular weight distribution which in turn can be related to various properties to yield predictive relationships for these properties. Only the thermal history of the polymer and its initial properties are required for the model. This technique is not limited to a specific polymer or even to thermal degradation. As long as the kinetics of the process can be mathematically modeled, this approach should apply to a host of other situations, providing property prediction simply from knowledge of the material history. The research seeks to better understand the thermal degradation of polycarbonate. Kinetics of the process was explored, and the chemical mechanisms were examined. A key part of the project was the determination of the molecular weights and molecular weight distributions at each level of degradation. Furthermore, mechanical stress-strain properties, glass transition temperatures, and melt viscosities were also measured. This information, together with the kinetic expressions, facilitated prediction of these types of material properties for a known thermal history. / Ph. D.

The convening of the Earth Summit in Rio de Janeiro in June 1992 represented a major milestone in the global debate on environment and development. This thesis, however, is not concerned about the broad views and perspectives advanced by developed and developing countries. Rather, the focus is at a much more micro level. The thesis examines the relationship between the environment and one aspect of development, namely, housing. The discussion centres on environmental conditions in low-income communities in a developing country: St. Lucia. This is really the reality of the debate within the boundaries of Small-Island Developing States (SIOS). As Dr. Mahathir Mohamad, Prime Minister of Malaysia states " ... peasants are likely to be far less concerned about holes in the ozone layer than about holes in their rooft" (in Main, 1994:3). It is within this context that the thesis is set. Brown agenda issues are becoming increasingly more evident in the cities of the developing world. In St. Lucia this is most visible in low-income housing communities. This thesis examines the underlying reasons for the neglect of low-income community environments and assesses whether there is a direct correlation between the level of government intervention in the establishment of these communities and the state of their household and neighbourhood environments. It also provides a comprehensive understanding of the operations of the formal and informal housing sectors in respect of low-income housing. The thesis argues that the State has failed in its attempts to provide lowincome housing and that low-income households have taken the provision of their shelter needs literally into their own hands. However, construction within the informal sector has resulted in serious environmental degradation. While households themselves are making some effort to address their environmental conditions, their actions tend to be reactive and ad-hoc, with little improvements being realized. These households have however acknowledged that they are unable by themselves to ameliorate their environmental conditions to any significant extent. They emphasize that the State must playa facilitating role in the process. This thesis is therefore concerned about the ways in which these stakeholders can work together to ensure the delivery of low-income housing within an environmentally sustainable framework. The argument put forward is that this can be achieved through an aided self-help approach, which will signal a new orientation towards the provision of low-income housing in St. Lucia and implicitly, a new environmental agenda for low-income communities.

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