published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

published_or_final_version / Dentistry / Master / Master of Dental Surgery

Dental restorative cements are placed in a harsh oral environment where they are subjected to thermal shock, chemical degradation, and repeating masticatory force. The ideal restorative dental cements should have superior mechanical properties, chemical stability, aesthetic, good handling properties, biocompatibility, antibacterial properties, and preferably bioactivity. This thesis presents research on dental restorative cements with enhanced properties. The overall aim was to increase the bioactivity and antibacterial properties of dental restorative cements without affecting their other properties. The effect from adding calcium silicate to glass ionomer cement (GIC) was investigated. The results showed that calcium silicate could increase the bioactivity and reduce the cytotoxicity of conventional glass ionomer cement without compromising its setting and mechanical properties. Hydroxyapatite (HA) with a high aspect ratio and thin nacreous-layered monetite sheets were also synthesized. Nano HA particles with an aspect ratio of 50 can be synthesized by both precipitation and hydrothermal methods. The aspect ratio was controlled via the pH of reaction medium. Thin nacreous-layered monetite sheets were synthesized through a self-assembly process in the presence of an amine based cationic quaternary surfactant. Temperature, pH, and presence of surfactant played essential roles in forming the nacreous-layered monetite sheets. Then the effect from adding silver doped HA and monetite particles was investigated. The results showed that the antibacterial properties of GIC could be increased by incorporating silver doped HA and monetite particles. Further examination showed that the pH change, F- ion release, and concentration of released Ag+ ions were not responsible for the improved antibacterial properties. The quasi-static strengths and compressive fatigue limits of four types of the most commonly used dental restorations were evaluated. In our study, resin modified GIC and resin-based composite showed superior static compressive strength and fatigue limits compared to conventional GIC. The static compressive strength of dental cements increased with the aging time. However, aging had no effect on the compressive fatigue limit of resin modified GIC and resin-based composite. The compressive fatigue limit of conventional GIC even showed a drastic decrease after aging.

A proposta deste trabalho experimental in vitro foi avaliar se a incorporação de 1% - em peso - do alcalóide teobromina (Sigma Aldrich, Darmstadt, Alemanha) ao cimento de ionômero de vidro (CIV) convencional (GC Gold Fuji 9, GC Corp, Japão) tem a capacidade de alterar as propriedades físico-químicas desse material. Para tanto, dois grupos experimentais foram propostos: G1 - CIV convencional e G2 - CIV com adição de teobromina. Foram confeccionados 160 discos de CIV de acordo com as instruções do fabricante, utilizando matrizes circulares. Para analisar as mencionadas propriedades, os discos foram submetidos a testes específicos, de acordo com as normas da International Standard Organization (ISO) para cada uma das propriedades. Discos de 15mmx1mm de diâmetro foram utilizados para as provas de sorção (n=5) e de solubilidade (n=5), com o auxílio de balança analítica, dissecadores e estufa a 23 e 37ºC por várias semanas. O ensaio de microdureza foi realizado em amostras (n=20) de 15mm×1mm submetidas a cinco edentações, com carga de 25 gramas e 30 segundos, à temperatura ambiente. Para a avaliação da cor, discos (n=20) de 15mm×1mm foram submetidos ao espectrofotômetro, adotando-se a guia colorimétrica da Comissão internacional de Iluminação (CIE). No ensaio de resistência flexural, os espécimes (n=60) de 12mm×1mm foram armazenados em estufa a 37ºC durante 24 horas para posterior analise na máquina de ensaios universal. Para avaliar a influência da teobromina adicionada ao CIV na formação de biofilme por Streptococcus mutans,, sobre os corpos da prova (n=40) de 12mm×1mm biofilmes foram desenvolvidos. As cepas de S.mutans foram cultivadas em Tryptic Soy Agar (TSA, Difco) a 37°C. Também a dosagem de flúor foi avaliada, em discos de prova (n=10) armazenados em saliva artificial e submersos em solução TISAB para posterior análise com eletrodo de flúor e obtenção da curva da liberação dessa substância. Os dados obtidos nos testes de sorção e solubilidade, microdureza, cor e resistência flexural foram submetidos à análise de variância ANOVA um fator e ao teste de Tukey para comparação entre os grupos, adotando-se 5% de nível de significância (p<0,05). O ensaio da influência da teobromina adicionada ao CIV na quantidade de biofilme de Streptococcus mutans formado e o teste de dosagem de flúor foram submetidos à análise de variância two-way ANOVA e ao teste de Tukey, com nível de significância de 5% para comparação entre os grupos experimentais. O segundo fator avaliado nestes dois ensaios foi o tempo. Os resultados não revelaram alteração da sorção e da solubilidade no CIV que recebeu teobromina (p>0,05). A microdureza aumentou com a adição de teobromina ao CIV (p<0,05). Não houve alteração de cor do CIV que recebeu teobromina (p>0,05). A resistência na flexão biaxial diminuiu quando da adição de teobromina ao CIV (p<0,05). Já a quantidade de biofilme formado foi menor em G2 (p<0,05). Em relação à liberação de flúor, observou-se que a adição de teobromina não altera essa propriedade do CIV (p>0,05). Com base em tais achados, conclui-se que a adição de teobromina a 1% ao cimento do ionômero de vidro convencional não produz alterações significativas nas propriedades desse material, podendo até mesmo otimizar algumas dessas propriedades. Ainda assim, estudos adicionais sobre o assunto devem ser realizados. / The purpose of this experimental in virtro work, was to evaluate if the incorporation of the 1% in weight of an alkaloid: theobromine (Sigma Aldrich Darmstadt, Germany) to the conventional glass ionomer cement (GIC) (GC Gold Fuji 9; GCC Japan Corp) has the capacity to change the properties physicochemical of this material. Whereby, it was proposed two experimental groups: G1 - Conventional GIC and G2 GIC incorporated with theobromine. According to the instructions of the creator, there were created 160 specimens, for which it was used circular matrices. To analyze the mentioned properties, the specimens were analyzed under the International Standard Organization (ISO) for each property. It was used matrices of 15mmx1mm diameter for the sorption (n=5) solubility (n=5) tests. With the help of an analytical balance, desiccators and a stove at 23º and 37° for various weeks; it was made essays of micro firmness using 20 matrices of 15mmx1mm, were submitted to five indentations with a charge of 26 grams and 30 seconds in environmental temperature. For the color evaluation it was used 20 matrices of 15mmx1mm, that were submitted to a spectrophotometer following the colorimetric guide of the International Commission of Illumination (CIE). An essay of biaxial flexural strength was made in 60 matrices of 12mmx1mm that were stored in a stove at 37º for 24 hours, for a later analysis in a universal testing machine. To evaluate the influence of the theobromine incorporated to the glass ionomer cement (GIC) in the creation of biofilm Streptococcus mutans, there were developed 40 matrices of 12mmx1mm of biofilm. The strains of the Streptococcus mutans were cultivated in Tryptic Soy-Agar at 37°. It was also evaluated the fluorine release capacity using 10 test matrices stored in artificial saliva and submerged in TISAB substance for a later analysis with an fluorine electrode, to obtain the release curve of that substance. The obtained information from the test of sorption, solubility, micro strength, color and biaxial flexural strength, were submitted to an analysis of variance ANOVA one factor and Tukey\'s test for a comparison between groups assuming the 5% level of significance (p <0,05). The essay of the influence of the theobromine added to the GIC in the amount of biofilm Streptococcus mutans formed and the test of fluorine release were submitted to the Bidirectional Variance Analysis ANOVA and Tukey\'s test, with a significant level of 5% to the comparison between experimental groups; the second factor evaluated in this two essays was time. The results didn\'t show an alteration in the sorption and solubility in the GIC that received theobromine (p >0,05). The micro strength increased with the addition of the theobromine to the GIC (p< 0,05). There were not any alterations in the color of the GIC that received theobromine (p >0.05). The resistance to the biaxial flexural strength decreased when the theobromine was added to the GIC (p <0.05). The amount of formed biofilm was less in the G2. In relation to the fluorine release it was observed that the addition of theobromine does not change the properties of GIC (p >0.05). Based on these findings, it is concluded that the addition of theobromine in 1% to the conventional glass ionomer cement, does not produce significant changes over this material properties and also, it can optimize some of these properties. However, more studies should be done about this topic.

Glass‐ionomer cements (GICs) are acid base reaction cements used in dentistry for restorative applications. In conventional GICs aluminosilicate glass is used to react with a polyalkenoic acid such as poly acrylic acid. Glass–ionomer cements have the potential to be used for bone cement applications, however there is a concern over the neurotoxicity of aluminium in the glass component of these cements. In this work zinc containing phosphate glass based glass–ionomer cement has been developed for the bone cement application. In this relation structure of zinc containing phosphate glasses in the system Na2O:CaO:ZnO:P2O5 ranging from metaphosphate compositions to invert phosphate compositions has been examined. Various advanced techniques in solid state nuclear magnetic resonance (NMR) such as dipolar recoupling and solid state NMR of low  quadrupolar nuclei (67Zn and 43Ca) and neutron diffraction have been employed. Special emphasis has been focused on structural role of zinc in glasses of different composition and the effect of different cations on coordination of zinc along with the effect on phosphate tetrahedra. 67Zn and 43Ca solid state NMR spectra of selected glass samples have been successfully acquired at ultrahigh field 900 MHz using Solid Echo pulse sequence. In addition solid state NMR spectra of two polymorphs of crystalline Zn(PO3)2 have also been acquired at 850 MHz using DFS– WURST–QCPMG pulse sequence in order to distinguish between the magnitude and orientation of electric field gradient (EFG) of zinc in octahedral and tetrahedral coordination. Structure determined through various techniques has been related to the dissolution behaviour of glass compositions from metaphosphate to invert phosphate. A decrease in pH of surrounding medium has been observed due to the presence of zinc. Insertion of ZnO4 tetrahedra between PO4 tetrahedra has been observed in invert phosphate glasses with high zinc oxide content which renders them suitable for glass– ionomer cements. Glass–ionomer cement from glass composition 10Na2O:50ZnO:10CaO:30P2O5 and hydroxyapatite and, acrylic acid and vinyl phosphonic acid co‐polymer has been produced successfully.

Magister Scientiae Dentium - MSc(Dent) (Paediatric Dentistry) / The use of glass ionomer cement (GIC) restorative materials assists in the prevention of dental caries due to its long-term fluoride release. However, poor physical strength is one of the main drawbacks of GIC. A surface coating is recommended to improve the physical strength and is considered necessary during the overlapping stages of setting reactions of GIC restorative materials. The development of resin based coatings has improved the properties of the material but the effect on fluoride release needs investigation.

Direct methanol fuel cells (DMFCs) are considered a clean source of electrical power for future energy demand, creating a potential to reduce our dependency on fossil fuels. Despite their advantages, including high energy density, efficiency and easy handling and distribution of fuel, the commercialization of DMFCs has suffered from some drawbacks, including methanol crossover and contamination of the system. Metal cation contaminants (such as Ni, Co, etc) introduced through the degradation of fuel cell components (bipolar plate and electro-catalyst layer) can significantly affect the Nafion-membrane properties and overall fuel cell performance. In the current study, a systematic approach is taken to characterize and identify the mechanism of the effect of metal solution contaminants on the activities of electro-catalysts of DMFCs. Cyclic voltammetry and rotating disk electrode (RDE) techniques were utilized in order to characterize the effect of various concentrations (i.e., 2x10-x M (x=1-7)) of six metal solution contaminants (i.e., Co, Ni and Zn with sulfate and nitrate as counter-anions) on the voltammetric properties and electro-catalytic activity of polycrystalline Pt during methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). The results showed a decrease in the MOR and ORR activities of Pt as the concentration of metal solution increased. The effect of counter-anion on the Pt activity was further investigated. The results showed that a combined effect of counter-anions and metal cations may be responsible for the decrease in the electro-catalytic activity of Pt. The effect of metal solution contaminants on the Nafion-ionomer of anode electro-catalysts was investigated using Nafion-coated Pt electrode. Voltammetric properties and MOR activities of Nafion-coated and bare Pt electrodes in the presence of Ni solution contaminants were characterized using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The overall results showed a significant negative effect of Ni solution contaminants on the electro-catalytic activity of bare Pt electrode as compared to the Nafion-coated Pt electrode. Based on the results, it appears that Nafion-ionomer film may interact with metal cations (through its sulfonate groups) and repel them away from the Pt active sites, partially inhibiting the negative effect of metal cations on the Pt activity of Nafion-coated Pt electrode. The effect of metal solution contaminants on the carbon-supported platinum nanoparticle (Pt/C) with various Nafion-ionomer distributions and contents (i.e., Nafion-incorporated Pt/C and Nafion-coated Pt/C electrodes) was further investigated. Cyclic voltammetry and EIS techniques were employed to characterize the effect of Ni solution contaminants on the voltammetric properties and MOR activities of Nafion-incorporated and Nafion-coated Pt/C electrodes. The overall results showed a stronger negative effect of Ni solution contaminants on the electro-catalytic activity of Nafion-incorporated Pt/C electrodes as compared to the Nafion-coated Pt/C electrodes. This further confirms previous observations showing the sulfonate groups of Nafion-ionomer film may attract the Ni metal cations, localize them away from the Pt active sites, and subsequently suppress the negative effect of cations on the activity of Nafion-coated Pt/C electrodes.

Thesis (M.D.S.)--University of Hong Kong, 1992. / Also available in print.

Articulated rigid-rod polymers asPBI were synthesized via polycondensation reaction. Using 2-sulfoterephthalic acid and 5-sulfoisophthalic acid in different ratios for copolycondensation reaction making the fully conjugated rigid-rod backbone became articulated. Both rigid-rod and articulated rigid-rod were further derivatized using alkane sulfonated pendants and became water-soluble rigid-rod and articulated rigid-rod polyelectrolytes. Lithium salt doped cast films of the polyelectrolytes showed a root-temperature DC conductivity parallel to film surface (

This study utilized polycondensation reaction to synthesize fully conjugated rod-like polymer dihydroxy-PBI. Chemical derivatizations were applied to attach pendants of propane sulfonic coil for dihydroxy-PBI-PS and to attach aromatic phenylene ring with Li ionic moiety for dihydroxy-PBI-AS. The attachment of pendants for dihydroxy-PBI-PS was 42.27 % and for dihydroxy-PBI-AS was only for 0.04 % causing by stereo hindrance of this molecule. These polymers seemed to have good thermal stability. Dihydroxy-PBI started to show degradation at 467.8 oC and retained 60.5 wt. % at 800 oC. Derivatized dihydroxy- PBI-PS and dihydroxy-PBI-AS lost their pendants at 295.3 oC and 314.4 oC, respectively. Dihydroxy-PBI was cast into thin film. Upon doping with lithium salt of LiClO at 2.02 wt. %, dihydroxy-PBI cast film showed the highest room-temperature dc conductivity parallel to the film (£m¡ü) of 1.71 x 10-4 S/cm and perpendicular to the film (£m¡æ) of 1.49 x 10-5 S/cm. For dihydroxy-PBI-PS cast film, the highest conductivity was at 0.49 wt. % of LiClO4 with £m¡ü of 1.05 x 10-3 S/cm and £m¡æ of 1.05 x 10-4 S/cm. For dihydroxy-PBI-AS cast film, the highest conductivity was at 2.02 wt. % of LiClO4 with £m¡ü of 1.32 x 10-3 S/cm and £m¡æ of 2.26 x 10-5 S/cm. From scanning electron microscopy and wide-angle x-ray scattering, it was learned that cast films of dihydroxy-PBI and dihydroxy-PBI-AS had anisotropic layered structure parallel to the film, and that of dihydroxy-PBI-PS showed less of this anisotropy.