Finite-Size Effects and Optimal System Sizes in Simulations of Surfactant Micelle Self-Assembly.

The spontaneous formation of micelles in aqueous solutions is governed by the amphipathic nature of surfactants and is practically interesting due to the regular use of micelles as membrane mimics, for the characterization of protein structure, and for drug design and delivery. We performed a systematic characterization of the finite-size effect observed in single-component dodecylphosphocholine (DPC) micelles with the coarse-grained MARTINI model. Of multiple coarse-grained solvent models investigated using large system sizes, the nonpolarizable solvent model was found to most accurately reproduce SANS spectra of 100 mM DPC in aqueous solution. We systematically investigated the finite-size effect at constant 100 mM concentration in 23 systems of sizes 40-150 DPC, confirming the finite-size effect to manifest as an oscillation in the mean micelle aggregation number about the thermodynamic aggregation number as the system size increases. The oscillations in aggregation number mostly diminish once the system supports the formation of three micelles. Similar oscillations were observed in the estimated critical micelle concentration with a mean value of 1.10 mM, which is in agreement with experiment to 0.1 mM. The accuracy of using a multiscale simulation approach to avoid finite-size effects in the micelle size distribution and SANS spectra using MARTINI and CHARMM36 was explored using multiple long time scale 500 DPC coarse-grained simulations, which were back-mapped to CHARMM36 all-atom systems. It was found that the MARTINI model generally occupies more volume than the all-atom model, leading to the formation of micelles that are of a reasonable radius of gyration but are smaller in aggregation number. The systematic characterization of the finite-size effect and exploration of multiscale modeling presented in this work provide guidance for the accurate modeling of micelles in simulations.

Energetics of cyclohexene adsorption and reaction on Pt(111) by low-temperature microcalorimetry.

The heat of adsorption and sticking probability of cyclohexene on Pt(111) were measured as a function of coverage using single-crystal adsorption calorimetry in the temperature range from 100 to 300 K. At 100 K, cyclohexene adsorbs as intact di-sigma bonded cyclohexene on Pt(111), and the heat of adsorption is well described by a second-order polynomial (130 - 47 theta - 1250 theta(2)) kJ/mol, yielding a standard enthalpy of formation of di-sigma bonded cyclohexene on Pt(111) at low coverages of -135 kJ/mol and a C-Pt sigma bond strength of 205 kJ/mol. At 281 K, cyclohexene dehydrogenates upon adsorption, forming adsorbed 2-cyclohexenyl (c-C6H(9,a)) and adsorbed hydrogen, and the heat of adsorption is well described by another second-order polynomial (174 - 700 theta + 761 theta(2)) kJ/mol. This yields a standard enthalpy of formation of adsorbed 2-cyclohexenyl on Pt(111) at a low coverage of -143 kJ/mol. At coverages below 0.10 ML, the sticking probability of cyclohexene on Pt(111) is close to unity (>0.95), independent of temperature.

An initial evaluation of gellan gum as a material for tissue engineering applications.

Alpha-modified minimum essential medium (alphaMEM) has been found to cross-link a 1% gellan gum solution, resulting in the formation of a self-supporting hydrogel in 1:1 and 5:1 ratios of polysaccharide: alphaMEM. Rheological data from temperature sweeps confirm that in addition to orders of magnitude differences in G' between 1% gellan and 1% gellan with alphaMEM, there is also a 20 degrees C increase in the temperature at which the onset of gelation takes place when alphaMEM is present. Frequency sweeps confirm the formation of a true gel; mechanical spectra for mixtures of gellan and alphaMEM clearly demonstrate G' to be independent of frequency. It is possible to immobilize cells within a three-dimensional (3D) gellan matrix that remain viable for up to 21 days in culture by adding a suspension of rat bone marrow cells (rBMC) in alphaMEM to 1% gellan solution. This extremely simple approach to cell immobilization within 3D constructs, made possible by the fact that gellan solutions cross-link in the presence of millimolar concentrations of cations, poses a very low risk to a cell population immobilized within a gellan matrix and thus indicates the potential of gellan for use as a tissue engineering scaffold.

3D culture of bone-derived cells immobilised in alginate following light-triggered gelation.

Photoreactive liposomes have been exploited as a means of developing 3D tissue constructs. Liposomes formulated using the photosensitive lipid 1,2-bis(4-(n-butyl)phenylazo-4'-phenylbutyroyl)phosphatidylcholine (Bis Azo PC), which undergoes conformational change on stimulation with long wavelength ultraviolet light, were prepared with entrapped CaCl(2) before being incorporated into a 4% alginate solution. It was shown that stimulation of the photosensitive lipid using a light emitting diode (LED) (peak emission at 385 nm, dose equivalent to 9 mJ/cm(2)) caused the release of liposome-entrapped CaCl(2), resulting in cross-linking of the alginate solution and immobilisation of bone-derived cells over a range of seeding densities, approximately 97% of which remained viable for periods of up to 14 days in culture. Entrapment volumes of a variety of liposome types were evaluated and interdigitating fusion vesicles were identified as having the highest payload (24%), however the inclusion of cholesterol as a means of shifting Bis Azo PC sensitivity into the visible light wavelengths resulted in an approximately 10-fold reduction in calcium entrapment. This application of light-sensitised liposomes offers the potential to create complex tissue engineering substrates containing cells immobilised in precise locations, in contrast with substrates onto which cells are seeded post-production.

The in vitro failure of all-ceramic crowns and the connector area of fixed partial dentures using bilayered ceramic specimens: the influence of core to dentin thickness ratio.

AIMS: To assess the effect of core to dentine thickness ratio on the bi-axial flexure strength and fracture mode and failure origin using bilayered ceramic specimens as an in vitro assessment for all-ceramic crowns and the connector area of fixed partial dentures (FPDs). METHODS: Sets of 30 bilayered composite discs, with a core layer thickness of 1mm and with core to dentine thickness ratios of 2:1, 1:1 and 1:2, were tested in bi-axial flexure with both the reinforcing core and veneering dentine loaded in tension. Mean flexure strengths, standard deviations and associated Weibull moduli (m) were determined. Optical microscopy was employed for identification of the fracture mode and failure origin for the failure all-ceramic crowns and the connector area of FPDs. RESULTS: For a core thickness of 1mm the core to dentine thickness ratio failed to influence the bi-axial flexure strength data when both the reinforcing core and veneering dentine porcelain were tested in tension. The number of fracture fragments, frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc-shaped specimens were dependent on the core to dentine thickness ratio and the surface loaded in tension. SIGNIFICANCE: The fracture mode and failure origin in bilayered ceramics tested to represent the failure mode of all-ceramic crowns and FPDs was dependent upon the core to dentine thickness ratio employed. However, the conventional wisdom regarding bilayered ceramic specimens with core thicknesses greater than 1mm are not followed when the core thickness was reduced to 1mm since the fracture resistance was not dependent on the core to dentine thickness ratio.

An in-vitro investigation of the accuracy of fit of Procera and Empress crowns.

The current study aimed to investigate the accuracy of fit and the reproducibility of inner crown profile for two types of high strength ceramics, IPS Empress and Procera. Procera and Empress crowns with four different morphologies were cemented to dies using zinc phosphate dental cement. Vertical and horizontal sections were made through each of the crown/die preparations and images of the vertical sections were compared for curvature reproduction by alignment using image processing. Measurements were made on horizontal sections to determine cement layer thickness. Alignment of the crowns using image analysis identified quantifiable variations in the inner surface profile compared with the outer surface of the die. The largest differences occurred from the cusp tips to the occlusal adaptation area and differences in surface profile were less pronounced for Procera than Empress crowns. Marginal gap varied independently of ceramic or internal crown shape from 7-529 microm for Procera and 26-548 microm for Empress. IPS Empress has a superior ability to reproduce the inner surface profile of the crown morphologies investigated compared with Procera. The reduced reproduction of surface profile was associated with an increased cement thickness at the occlusal contact area that may inadvertently lead to failure of the crowns functional characteristics.

The in-vitro clinical failure of all-ceramic crowns and the connector area of fixed partial dentures: the influence of interfacial surface roughness.

OBJECTIVES: To assess the effect of interfacial surface roughness on the flexure strength and fracture mode and origin utilizing an in-vitro assessment of the clinical failure conditions expected for all-ceramic crowns and the connector area of fixed partial dentures (FPDs) using bilayered ceramic specimens tested in bi-axial flexure. METHODS: Sets of 20 bilayered composite discs, with core:dentine thickness ratio of 2:1 and interfacial surface roughnesses determined by alumina abrasion with different alumina particle sizes, were tested in bi-axial flexure with both the reinforcing core and veneering dentine loaded in tension. Mean flexure strengths, standard deviations and associated Weibull Moduli (m) were determined. Optical microscopy was employed for identification of the fracture mode and origin for the failure all-ceramic crowns and the connector area of FPDs. RESULTS: The interfacial surface roughness influenced the bi-axial flexure strength and reliability of the flexure strength data when both the reinforcing core and veneering dentine porcelain were tested in tension. The number of fracture fragments, frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc-shaped specimens was also dependent on the interfacial surface roughness and the surface loaded in tension. CONCLUSIONS: The fracture resistance, failure mode and failure origin in bilayered ceramics tested to represent the clinical failure mode of all-ceramic crowns and FPDs are dependent upon the interfacial surface roughness and the modulus of the material in tension.

The effect of core:dentin thickness ratio on the bi-axial flexure strength and fracture mode and origin of bilayered dental ceramic composites.

OBJECTIVES: The aim of the current study was to assess the effect of core:dentin thickness ratio on the flexure strength, fracture mode and origin of bilayered dental ceramic composite disc specimens. METHODS: Sets of 30 bilayered composite discs with core:dentin thickness ratio of 2:1, 1:1 and 1:2 were tested in bi-axial flexure with both the reinforcing core and veneering dentin loaded in tension. Mean flexure strengths, standard deviations and associated Weibull Moduli (m) were determined. A combination of optical and scanning electron microscopy was employed for identification of the fracture mode and origin. RESULTS: The core:dentin ratio influenced the bi-axial flexure strength and reliability of the flexure strength data when both the reinforcing core and veneering dentin porcelain were tested in tension. The strength and reliability was increased for a core:dentin thickness of 2:1. The number of fracture fragments, the frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc shaped specimens was also dependent on the core/dentin ratio and the surface loaded in tension. CONCLUSIONS: Core:dentin thickness ratio influences the bi-axial flexure strength and fracture mode and origin in bilayered dental ceramic composite specimens.

The influence of interfacial surface roughness on bilayered ceramic specimen performance.

OBJECTIVES: The aim of the current study was to assess the influence of interfacial surface roughness on the performance of bilayered ceramic composite disc-shaped specimens. METHODS: Specific surface flaw distributions were introduced to a series of standard aluminous core porcelain disc specimens (12 mm diameter, 1.7 mm thickness) prior to bilayering with dentin porcelain. Mean flexure strengths, standard deviations and associated Weibull moduli (m) were determined using bi-axial flexure (ball-on-ring) for each series of 30 bilayered specimens. RESULTS: The mean bi-axial fracture strengths of bilayered specimens resulted in significant differences between specimen groups with the smoothest interfacial roughness recording the highest strength and fracture strength reliability. Specimen delamination was identified in five of the bilayered group with the smoothest interfacial roughness compared with no specimen delaminations in the rougher interface groups. However, fracture occurred in the weaker dentin layer rather than along the interface between the dentin and core porcelain for specimens in these rougher interface groups. CONCLUSIONS: A smoother interfacial surface roughness resulted in increased performance, namely increased strength and reliability amongst the bilayered dental ceramic composite specimens examined in the current investigation. The incidence of complete or partial delamination of the reinforcing core and aesthetic dentin was increased for these specimens since the reduced tortuosity of the interface between porcelain layers fails to restrict propagation of a crack front along the interface.