Structural optimization of fiber-reinforced composite dental bridges

The much improved strength of dental restorative materials has brought more flexibility to the design of dental prosthesis, leading to the concept of minimally invasive dentistry. Fiber-reinforced composite (FRC) systems, for example, which provide excellent tooth-colored appearance, allow the amount of tooth removal to be minimized. Clinically, the ideal indication of FRC restoration is single tooth replacement. Depending on the position of the missing tooth, the periodontal status of the neighboring teeth, and the patient’s occlusal force or any existing parafunctional habits, the following two fixed partial denture (FPD) designs can be prescribed: a 3-unit dental bridge supported by two abutments, one at each end of the edentulous area, or a cantilevered dental bridge supported by only one abutment. However, according to a 2009 systemic review, the mean survival rate of FRC restorations was only 73.4% at 4.5 years. The two major failure modes were reported as debonding at the tooth-retainer interface and structural fracture; the latter could occur at the loading point, the pontic and the connectors linking the retainer and the pontic. Several studies have shown that the main contributing factor of clinical failure is suboptimal fiber position and orientation. In the present project, we apply a bio-inspired stress-induced material transformation (SMT) technique to the two main FRC designs, i.e. the 3-unit bridge and 2-unit cantilever. Using this technique, the mechanical property of a structure under optimization can be modified according to the local stresses in an evolutionary manner. Structural optimization is performed using ABAQUS via a user-defined material subroutine. For the 3-unit FRC bridge, regions with high stresses are iteratively reinforced with stronger fiber materials and, to reduce the risk of delamination, reinforcing fibers are closely aligned with the plane of the maximum principal stress in all locations. For the more challenging cantilevered design with only a single retainer, a two-step approach is adopted. The first step involves optimizing the shape of the cavity preparation/retainer on the abutment to lower the interfacial tensile stress at the abutment-pontic connection to reduce the risk of debonding. Then, with the optimized retainer, the user subroutine is applied to the restoration to seek an optimal fiber layout. Results from the optimization suggest that the fiber has to be placed at the top of the connector in the cantilevered design and a U-shaped fiber substructure has to be placed at the bottom of the pontic in the 3-unit bridge. Compared to the conventional designs, the peak tensile stress is reduced by ~30% and ~45% for the 3-unit and cantilevered FRC FPD, respectively. Furthermore, the accompanying cavity design for the cantilevered bridge reduces the peak tensile stress at the tooth-denture interface by ~70%. In the in vitro validation tests, both optimized designs demonstrate higher fracture resistance. For the 3-unit bridge, acoustic emission measurement shows that the optimized design has, on average, fewer micro-cracking events than the conventional design during loading (38 vs. 2969). For the 2-unit cantilevered design, the optimized design has a ~108% higher mean failure load than the conventional step-box design (203.35 ± 28.02 N vs. 97.32 ± 21.10 N)

BODY MECHANICS OF TAI CHI CHUAN

The purpose of this study was to analyse the fundamental movements of Tai Chi and to identify their correct execution, so as to increase their effectiveness, by using modern motion analysis techniques. In this preliminary study, we focused on the power generated from the forward push movement. The process, which involved the subject pushing a punch bag using stands of different lengths and widths, was videotaped. Reflective markers were placed on essential joints of the subject to allow the creation of a spatial model using motion analysis software. The results demonstrated that the length of the stand had more effect than its width on the power generated, and the standard as recommended by certain schools of Tai Chi might not be optimum

The Roles of Transport and Wave-Particle Interactions on Radiation Belt Dynamics

Particle fluxes in the radiation belts can vary dramatically during geomagnetic active periods. Transport and wave-particle interactions are believed to be the two main types of mechanisms that control the radiation belt dynamics. Major transport processes include substorm dipolarization and injection, radial diffusion, convection, adiabatic acceleration and deceleration, and magnetopause shadowing. Energetic electrons and ions are also subjected to pitch-angle and energy diffusion when interact with plasma waves in the radiation belts. Important wave modes include whistler mode chorus waves, plasmaspheric hiss, electromagnetic ion cyclotron waves, and magnetosonic waves. We investigate the relative roles of transport and wave associated processes in radiation belt variations. Energetic electron fluxes during several storms are simulated using our Radiation Belt Environment (RBE) model. The model includes important transport and wave processes such as substorm dipolarization in global MHD fields, chorus waves, and plasmaspheric hiss. We discuss the effects of these competing processes at different phases of the storms and validate the results by comparison with satellite and ground-based observations. Keywords: Radiation Belts, Space Weather, Wave-Particle Interaction, Storm and Substor

Modeling of the Convection and Interaction of Ring Current, Plasmaspheric and Plasma Sheet Plasmas in the Inner Magnetosphere

Distinctive sources of ions reside in the plasmasphere, plasmasheet, and ring current regions at discrete energies constitute the major plasma populations in the inner/middle magnetosphere. They contribute to the electrodynamics of the ionosphere-magnetosphere system as important carriers of the global current system, in triggering; geomagnetic storm and substorms, as well as critical components of plasma instabilities such as reconnection and Kelvin-Helmholtz instability at the magnetospheric boundaries. Our preliminary analysis of in-situ measurements shoves the complexity of the plasmas pitch angle distributions at particularly the cold and warm plasmas, vary dramatically at different local times and radial distances from the Earth in response to changes in solar wind condition and Dst index. Using an MHD-ring current coupled code, we model the convection and interaction of cold, warm and energetic ions of plasmaspheric, plasmasheet, and ring current origins in the inner magnetosphere. We compare our simulation results with in-situ and remotely sensed measurements from recent instrumentation on Geotail, Cluster, THEMIS, and TWINS spacecraft

Biomimetic Mineralization of Recombinamer-Based Hydrogels toward Controlled Morphologies and High Mineral Density

Producción CientíficaThe use of insoluble organic matrices as a structural template for the bottom-up fabrication of organic−inorganic nanocomposites is a powerful way to build a variety of advanced materials with defined and controlled morphologies and superior mechanical properties. Calcium phosphate mineralization in polymeric hydrogels is receiving significant attention in terms of obtaining biomimetic hierarchical structures with unique mechanical properties and understanding the mechanisms of the biomineralization process. However, integration of organic matrices with hydroxyapatite nanocrystals, different in morphology and composition, has not been well-achieved yet at nanoscale. In this study, we synthesized thermoresponsive hydrogels, composed of elastin-like recombinamers (ELRs), to template mineralization of hydroxyapatite nanocrystals using a biomimetic polymer-induced liquid-precursor (PILP) mineralization process. Different from conventional mineralization where minerals were deposited on the surface of organic matrices, they were infiltrated into the frameworks of ELR matrices, preserving their microporous structure. After 14 days of mineralization, an average of 78 μm mineralization depth was achieved. Mineral density up to 1.9 g/cm3 was found after 28 days of mineralization, which is comparable to natural bone and dentin. In the dry state, the elastic modulus and hardness of the mineralized hydrogels were 20.3 ± 1.7 and 0.93 ± 0.07 GPa, respectively. After hydration, they were reduced to 4.50 ± 0.55 and 0.10 ± 0.03 GPa, respectively. These values were lower but still on the same order of magnitude as those of natural hard tissues. The results indicated that inorganic−organic hybrid biomaterials with controlled morphologies can be achieved using organic templates of ELRs. Notably, the chemical and physical properties of ELRs can be tuned, which might help elucidate the mechanisms by which living organisms regulate the mineralization process.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA244U13

Simulating the Outer Radiation Belt During the Rising Phase of Solar Cycle 24

After prolonged period of solar minimum, there has been an increase in solar activity and its terrestrial consequences. We are in the midst of the rising phase of solar cycle 24, which began in January 2008. During the initial portion of the cycle, moderate geomagnetic storms occurred follow the 27 day solar rotation. Most of the storms were accompanied by increases in electron fluxes in the outer radiation belt. These enhancements were often preceded with rapid dropout at high L shells. We seek to understand the similarities and differences in radiation belt behavior during the active times observed during the of this solar cycle. This study includes extensive data and simulations our Radiation Belt Environment Model. We identify the processes, transport and wave-particle interactions, that are responsible for the flux dropout and the enhancement and recovery