An experimental fibre-reinforced dental resin composite

PhD ThesisFibre-reinforced dental resin composites (FRCs) have shown increased fracture resistance and tensile strength compared with particulate filled composites (PFC). However, clinically successful restorative materials require adequate bond strength and wear resistance along with high strength. An experimental FRC (ST) was developed and tested as a dentine replacement. It has randomly distributed E-glass fibres above their critical length of 0.5-1.6 mm. This work aimed to evaluate the possibility of using ST as a single restorative material by assessing its three-body wear resistance and surface contact fatigue. The polymerisation shrinkage, water sorption, and bond strength of ST were also assessed. Two commercially available materials; an FRC (Build It FR) and PFC (Z250) were used as comparators. ST showed significantly lower wear resistance and higher contact fatigue. No significant difference was found regarding polymerisation shrinkage but ST had significantly higher water sorption, lower shear bond strength (SBS) to human dentine. SBS of the interfacial layers within and between the dental resin composites was evaluated after 24 hours and 1 year of water storage in the absence of an oxygen inhibition layer. Build It/Z250 showed a significantly higher SBS at both time intervals. The presence of an oxygen inhibited layer increased the interfacial strength in all groups except ST/Z250. ST formulations were varied in resin/diluent (Bis-GMA/TEGDMA) ratios, filler loading and fibre lengths for development. Wear testing found changing the Bis-GMA/TEGDMA ratio from 60/40 to 70/30 decreased the wear resistance regardless of filler loading and fibre length. In summary, wear resistance of ST and its variants was insufficient to recommend its use as a single restorative material without a surface veneer of PFC. As a dentine replacement, ST was only comparable with Z250 and Build It in polymerisation shrinkage and SBS between composites in the absence of an oxygen inhibition layer

Reinforcement Learning based Gateway Selection in VANETs

In vehicular ad hoc networks (VANETs), providing the Internet has become an urgent necessity, where mobile gateways are used to ensure network connection to all customer vehicles in the network. However, the highly dynamic topology and bandwidth limitations of the network represent a significant issue in the gateway selection process. Two objectives are defined to overcome these challenges. The first objective aims to maximize the number of vehicles connected to the Internet by finding a suitable gateway for them depending on the connection lifetime. The second objective seeks to minimize the number of connected vehicles to the same gateway to overcome the limitation of gateways\u27 bandwidth and distribute the load in the network. For this purpose, A gateway discovery system assisted by the vehicular cloud is implemented to find a fair trade-off between the two conflicting objectives. Proximal Policy Optimization, a well-known reinforcement learning strategy, is used to define and train the agent. The trained agent was evaluated and compared with other multi-objective optimization methods under different conditions. The obtained results show that the proposed algorithm has better performance in terms of the number of connected vehicles, load distribution over the mobile gateways, link connectivity duration, and execution time

IGF axis expression and activity in differentiating dental pulp cells

The insulin-like growth factor (IGF) axis comprises two growth factors (IGF-I and IGF-II), two cell surface receptors (IGF1R and IGF2R), six high affinity soluble IGF binding proteins (IGFBP1-6) and various families of IGFBP proteases. The IGF axis acts co-ordinately to control several cellular processes including mitogenesis, apoptosis, cell migration and differentiation. In this latter respect the IGFs are the most abundant growth factors present in bone matrix and the IGF axis is believed to play an important role in the differentiation of osteoblast precursors and also to control bone accretion and resorption which occurs throughout adult life. There is much interest in the use of mesenchymal stem cells (MSCs) as a resource for tissue engineering approaches in the restoration of bone and other hard tissue lesions. As such a thorough knowledge of the effects of the IGF axis on osteogenic differentiation is essential to the success of such tissue engineering approaches. In our laboratories we use dental pulp/stromal cells (DPCs) as a source of precursor cells which can be differentiated to an osteogenic phenotype by culture under appropriate conditions. In order to address some of the issues raised above we have examined the expression and activity of the IGF axis in DPCs which have been induced to differentiate to an osteogenic/matrix mineralising phenotype in vitro. We found that DPCs express all components of the IGF axis (except IGF-I and IGFBP-1) under both basal and osteogenic conditions. With respect to IGFBPs we report that IGFBP protein concentrations in DPC conditioned medium closely follow mRNA levels prepared from DPCs. IGFBP-4 is the most abundant IGFBP in both basal and osteogenic DPCs and levels are not altered between basal and osteogenic cultures. IGFBP-4 and -5 inhibit IG-I and IGF-II stimulated osteogenic differentiation in DPCs. Of some interest although IGFBP-4 expression is not altered by differentiation of DPCs IGFBP-4 proteolysis is enhanced under osteogenic conditions. Subsequently we found that pregnancy associated plasma protein-A (PAPP-A) is the sole IGFBP-4 protease in DPC conditioned medium and its concentration is increased under osteogenic conditions. Further IGF-II (an activator of PAPP-A) and stanniocalcin-2 (STC2- an inhibitor of PAPP-A) concentrations are respectively increased and decreased during osteogenic differentiation of DPCs. We suggest that our data may represent a series of coordinated changes in IGF axis expression and activity which represent a novel osteogenic “signature” of differentiating DPCs

Electrical transport and optical studies of ferromagnetic Cobalt doped ZnO nanoparticles exhibiting a metal-insulator transition

The observed correlation of oxygen vacancies and room temperature ferromagnetic ordering in Co doped ZnO1-o nanoparticles reported earlier (Naeem et al Nanotechnology 17, 2675-2680) has been further explored by transport and optical measurements. In these particles room temperature ferromagnetic ordering had been observed to occur only after annealing in forming gas. In the current work the optical properties have been studied by diffuse reflection spectroscopy in the UV-Vis region and the band gap of the Co doped compositions has been found to decrease with Co addition. Reflections minima are observed at the energies characteristic of Co+2 d-d (tethrahedral symmetry) crystal field transitions, further establishing the presence of Co in substitutional sites. Electrical transport measurements on palletized samples of the nanoparticles show that the effect of a forming gas is to strongly decrease the resistivity with increasing Co concentration. For the air annealed and non-ferromagnetic samples the variation in the resistivity as a function of Co content are opposite to those observed in the particles prepared in forming gas. The ferromagnetic samples exhibit an apparent change from insulator to metal with increasing temperatures for T>380K and this change becomes more pronounced with increasing Co content. The magnetic and resistive behaviors are correlated by considering the model by Calderon et al [M. J. Calderon and S. D. Sarma, Annals of Physics 2007 (Accepted doi: 10.1016/j.aop.2007.01.010] where the ferromagnetism changes from being mediated by polarons in the low temperature insulating region to being mediated by the carriers released from the weakly bound states in the higher temperature metallic region.Comment: 7 pages, 6 figure