Statistical mechanics of columnar DNA assemblies

Many physical systems can be mapped onto solved or "solvable" models of magnetism. In this work, we have mapped the statistical mechanics of columnar phases of ideally helical rigid DNA -- subject to the earlier found unusual, frustrated pair potential [A. A. Kornyshev and S. Leikin, J. Chem. Phys. 107, 3656 (1997)] -- onto an exotic, unknown variant of the XY model on a fixed or restructurable lattice. Here the role of the 'spin' is played by the azimuthal orientation of the molecules. We have solved this model using a Hartree-Fock approximation, ground state calculations, and finite temperature Monte Carlo simulations. We have found peculiar spin order transitions, which may also be accompanied by positional restructuring, from hexagonal to rhombohedric lattices. Some of these have been experimentally observed in dense columnar aggregates. Note that DNA columnar phases are of great interest in biophysical research, not only because they are a useful in vitro tool for the study of DNA condensation, but also since these structures have been detected in living matter. Within the approximations made, our study provides insight into the statistical mechanics of these systems.Comment: 19 pages, 18 figure

Cobalt-Chromium alloys in fixed prosthodontics

Gold and its alloys have been the dominant framework materials in metal-ceramic constructions when substantial loss of tooth substance or missing teeth needs to be replaced. However, base metal alloys, such as cobalt-chromium (Co-Cr), have been introduced due to increased gold price. Co-Cr alloys possess beneficial mechanical properties because the alloys can, even with small dimensions, resist high chewing forces and exhibit an acceptable bond to the surface porcelain layer. Yet, the biocompatibility of Co-Cr alloys has been questioned. The overall aim of the present thesis is to increase our knowledge about Co-Cr alloys that are used in fixed prosthodontics. Results from Study I, which was a survey directed to all dental laboratories in Sweden, demonstrated that more than thirty different Co-Cr alloys were reported to be used in fixed prosthodontics. Besides the various chemical composition among the reported alloys, they were also manufactured by four different techniques; cast, milled, laser melted and pre-sintered milled. Based on the results from Study I, five Co-Cr alloys manufactured by four techniques, together with commercially pure titanium (c.p. Ti) grade 4 and Ti6Al4V ELI, were further included in Studies II-IV. In vitro studies were conducted in order to evaluate ion release, cell viability and inflammatory response. The effect of material combinations was also investigated. The mechanical properties and material structure with regards to yield strength, elongation after fracture, hardness, elastic modulus, surface roughness and microstructure were evaluated. The effect of heat treatment was also investigated on the above-mentioned parameters. The total ion release from all materials was extremely low, yet highest for the cast Co-Cr alloys in acidic conditions. The combination of Co-Cr, Ti6Al4V ELI and c.p. Ti showed lower total ion release compared with the non-presence of c.p. Ti. All tested materials demonstrated non-cytotoxic effects, although the highest inflammatory response from cells exposed to the materials was observed for the cast and pre-sintered milled Co-Cr materials. Overall, the laser melted Co-Cr demonstrated the highest values in mechanical properties. Conclusion: In vitro biological aspects and mechanical properties are influenced by the choice of manufacturing technique, heat treatment and microstructure of the materials. In order to evaluate these findings, more clinical studies are needed

Manufacturing techniques, biological and mechanical aspects

Gold and its alloys have been the dominant framework materials in metal-ceramic constructions when substantial loss of tooth substance or missing teeth needs to be replaced. However, base metal alloys, such as cobalt-chromium (Co-Cr), have been introduced due to increased gold price. Co-Cr alloys possess beneficial mechanical properties because the alloys can, even with small dimensions, resist high chewing forces and exhibit an acceptable bond to the surface porcelain layer. Yet, the biocompatibility of Co-Cr alloys has been questioned. The overall aim of the present thesis is to increase our knowledge about Co-Cr alloys that are used in fixed prosthodontics. Results from Study I, which was a survey directed to all dental laboratories in Sweden, demonstrated that more than thirty different Co-Cr alloys were reported to be used in fixed prosthodontics. Besides the various chemical composition among the reported alloys, they were also manufactured by four different techniques; cast, milled, laser melted and pre-sintered milled. Based on the results from Study I, five Co-Cr alloys manufactured by four techniques, together with commercially pure titanium (c.p. Ti) grade 4 and Ti6Al4V ELI, were further included in Studies II-IV. In vitro studies were conducted in order to evaluate ion release, cell viability and inflammatory response. The effect of material combinations was also investigated. The mechanical properties and material structure with regards to yield strength, elongation after fracture, hardness, elastic modulus, surface roughness and microstructure were evaluated. The effect of heat treatment was also investigated on the above-mentioned parameters. The total ion release from all materials was extremely low, yet highest for the cast Co-Cr alloys in acidic conditions. The combination of Co-Cr, Ti6Al4V ELI and c.p. Ti showed lower total ion release compared with the non-presence of c.p. Ti. All tested materials demonstrated non-cytotoxic effects, although the highest inflammatory response from cells exposed to the materials was observed for the cast and pre-sintered milled Co-Cr materials. Overall, the laser melted Co-Cr demonstrated the highest values in mechanical properties. Conclusion: In vitro biological aspects and mechanical properties are influenced by the choice of manufacturing technique, heat treatment and microstructure of the materials. In order to evaluate these findings, more clinical studies are needed

Melting of columnar hexagonal DNA liquid crystals

The persistence length DNA hexagonal-cholesteric phase transition upon dilution and/or increase in solvent ionic strength is investigated with polarized light microscopy. The ionic strength dependence of the transition follows Lindemann criterion $C_L = 0.098 \pm 0.003$, i.e., the hexagonal lattice melts when the root-mean-square fluctuations in transverse order exceed 10% of the interaxial spacing. The spacings are derived from density and the fluctuations are estimated with a theory of undulation enhanced electrostatic interactions. Additional support for this theory is given by the DNA equation of state and anisotropic neutron radiation scattering from magnetically aligned cholesteric samples just below the phase transition

Liquid crystal phase transitions in suspensions of polydisperse plate-like particles

Colloidal suspensions that form periodic self-assembling structures on sub-micrometre scales are of potential technological interest; for example, three-dimensional arrangements of spheres in colloidal crystals might serve as photonic materials, intended to manipulate light. Colloidal particles with non-spherical shapes (such as rods and plates) are of particular interest because of their ability to form liquid crystals. Nematic liquid crystals possess orientational order; smectic and columnar liquid crystals additionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering may be inhibited in polydisperse colloidal suspensions. Here we describe a suspension of plate-like colloids that shows isotropic, nematic and columnar phases on increasing the particle concentration.We find that the columnar two-dimensional crystal persists for a polydispersity of up to 25%, with a cross-over to smectic-like ordering at very high particle concentrations. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought

Cobalt-chromium alloys in fixed prosthodontics in Sweden

Aim: The aim of this study was to compile the usage of Co-Cr alloys in fixed prosthodontics (FP) among dental laboratories in Sweden. Methods: From March to October 2015, questionnaires were sent to 542 registered dental laboratories in Sweden. The questionnaires were divided in two parts, one for fixed dental-supported prosthodontics (FDP) and one for fixed implant-supported prosthodontics (FIP). Reminders were sent three times. Results: In total of 542 dental laboratories, 55% answered the questionnaires. Most dental laboratories use Co-Cr in FP, 134 (74%) in FDP and 89(66%) in FIP. The laboratories used Co-Cr alloys of various compositions in the prostheses, 35 for FDP and 30 for FIP. The most commonly used Co-Cr alloys for tooth-supported FDPs were (a) Wirobond® 280, (b) Cara SLM and (c) Wirobond® C. For implant-supported frameworks the frequently used alloys were: (a) Cara SLM, (b) Cara Milled and (c) Wirobond® 280. Except for the difference in composition of these alloys, they were also manufactured with various techniques. In tooth-supported prostheses the dominating technique was the cast technique while newer techniques as laser-sintering and milling were more commonly reported for implant-supported constructions. A fourth technique; the ‘pre-state’ milling was reported in FDP. Conclusion: More than 30 different Co-Cr alloys were reported as being used in FP. Thus, there is a need for studies exploring the mechanical and physical behavior and the biological response to the most commonly used Co-Cr alloys