Novel fundamental building blocks and site dependent isomorphism in the first actinide borophosphates.

Three novel uranyl borophosphates, Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]H2O (AgNBPU-1), Ag(2-x)(NH4)3[(UO2)2{B2P5O(20-x)(OH)x}] (x = 1.26) (AgNBPU-2), and Ag(2-x)(NH4)3[(UO2)2{B2P(5-y)AsyO(20-x)(OH)x}] (x = 1.43, y = 2.24) (AgNBPU-3), have been prepared by the H3BO3-NH4H2PO4/NH4H2AsO4 flux method. The structure of AgNBPU-1 has an unprecedented fundamental building block (FBB), composed of three BO4 and six PO4 tetrahedra which can be written as 9□:[Φ] □<3□>□|□<3□>□|□<3□>□|. Two Ag atoms are linearly coordinated; the coordination of a third one is T-shaped. AgNBPU-2 and AgNBPU-3 are isostructural and possess a FBB of two BO4 and five TO4 (T = P, As) tetrahedra (7□:□<4□>□|□). AgNBPU-3 is a solid solution with some PO4 tetrahedra of the AgNBPU-2 end-member being substituted by AsO4. Only two out of the three independent P positions are partially occupied by As, resulting in site dependent isomorphism. The three compounds represent the first actinide borophosphates.

Octahedral tilting in Pb-based relaxor ferroelectrics at high pressure.

We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc(0.5)Ta(0.5)O(3) (PST) and Pb(0.78)Ba(0.22)Sc(0.5)Ta(0.5)O(3) (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is Fm3m as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h = 2n + 1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the 111 cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either R3c or R3 symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a(-)a(-)a(-) that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3 GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35 GPa.

New insights into structural alteration of enamel apatite induced by citric acid and sodium fluoride solutions.

Attenuated total reflectance infrared spectroscopy and complementary scanning electron microscopy were applied to analyze the surface structure of enamel apatite exposed to citric acid and to investigate the protective potential of fluorine-containing reagents against citric acid-induced erosion. Enamel and, for comparison, geological hydroxylapatite samples were treated with aqueous solutions of citric acid and sodium fluoride of different concentrations, ranging from 0.01 to 0.5 mol/L for citric acid solutions and from 0.5 to 2.0% for fluoride solutions. The two solutions were applied either simultaneously or consecutively. The citric acid-induced structural modification of apatite increases with the increase in the citric acid concentration and the number of treatments. The application of sodium fluoride alone does not suppress the atomic level changes in apatite exposed to acidic agents. The addition of sodium fluoride to citric acid solutions leads to formation of surface CaF2 and considerably reduces the changes in the apatite P-O-Ca framework. However, the CaF2 globules deposited on the enamel surface seem to be insufficient to prevent the alteration of the apatite structure upon further exposure to acidic agents. No evidence for fluorine-induced recovery of the apatite structure was found.

Locally preserved α → ß phase transition in natural radiation-damaged titanite (CaTiSiO5): evidence from laser-induced photoluminescence and dielectric measurements.

In situ temperature-dependent laser-induced photoluminescence and dielectric measurements provide new evidence for the local occurrence of the α → ß phase transition near 500 K in the preserved crystalline parts of natural radiation-damaged titanite (sample E2335 with ~24% amorphous fraction, containing Fe and Al impurities). Photoluminescence spectroscopic measurements show an anomaly in the vicinity of 500 K. The temperature-dependent evolution of the real part of the electrical conductivity (σ) and the real (ε') and the imaginary (ε″) part of the complex dielectric permittivity (ε *) of titanite have been measured at various AC frequencies (~1.2-96.8 kHz). Despite the masking and smearing effect of impurities and defects, the temperature-dependent behaviour of ε' and ε″ around the transition temperature of the investigated natural titanite E2335 shows a remarkable similarity to that of the synthetic end-member material (see Zhang et al (1995 Phys. Chem. Miner. 22 41-9)). This study indicates the suitability of photoluminescence and impedance spectroscopy for the detection of phase transitions, even in heavily disordered systems.

CO2 laser-induced zonation in dental enamel: a Raman and IR microspectroscopic study.

The gradient of structural alteration and molecular exchange across CO(2) laser-irradiated areas in dental enamel was analyzed by Raman and attenuated total reflectance infrared microspectroscopy. The type and the degree of structural changes in morphologically distinguishable zones within the laser spot vary depending on the laser-irradiation parameters--power (1 and 3 W), treatment time (5 and 10 s), and operational mode (super pulse and continuous wave). Using higher power, irrespective of the operation mode, the enamel tissue ablates and a crater is formed. The prevalent phase at the bottom of the crater is dehydrated O(2) (2-)-bearing apatite, that is, the fundamental framework topology is preserved. Additional nonapatite calcium phosphate phases are located mainly at the slope of the laser crater. No structural transformation of mineral component was detected aside the crater rim, only a CO(3)-CO(2) exchange, which decays with the radial distance. A lower-power laser irradiation slightly roughens the enamel surface and the structural modification of enamel apatite is considerably weaker for continuous wave than for super pulse mode. Prolonged low-power laser treatment results in recrystallization, and thus structural recovering of apatite might be of clinical relevance for enamel surface treatments.

Magnesium degradation influenced by buffering salts in concentrations typical of in vitro and in vivo models.

Magnesium and its alloys have considerable potential for orthopedic applications. During the degradation process the interface between material and tissue is continuously changing. Moreover, too fast or uncontrolled degradation is detrimental for the outcome in vivo. Therefore in vitro setups utilizing physiological conditions are promising for the material/degradation analysis prior to animal experiments. The aim of this study is to elucidate the influence of inorganic salts contributing to the blood buffering capacity on degradation. Extruded pure magnesium samples were immersed under cell culture conditions for 3 and 10 days. Hank's balanced salt solution without calcium and magnesium (HBSS) plus 10% of fetal bovine serum (FBS) was used as the basic immersion medium. Additionally, different inorganic salts were added with respect to concentration in Dulbecco's modified Eagle's medium (DMEM, in vitro model) and human plasma (in vivo model) to form 12 different immersion media. Influences on the surrounding environment were observed by measuring pH and osmolality. The degradation interface was analyzed by electron-induced X-ray emission (EIXE) spectroscopy, including chemical-element mappings and electron microprobe analysis, as well as Fourier transform infrared reflection micro-spectroscopy (FTIR).

Blood compatibility of magnesium and its alloys.

RATIONALE: Blood compatibility analysis in the field of biomaterials is a highly controversial topic. Especially for degradable materials like magnesium and its alloys no established test methods are available. OBJECTIVE: The purpose of this study was to apply advanced test methodology for the analysis of degrading materials to get a mechanistic insight into the corrosion process in contact with human blood and plasma. METHODS AND RESULTS: Pure magnesium and two magnesium alloys were analysed in a modified Chandler-Loop setup. Standard clinical parameters were determined, and a thorough analysis of the resulting implant surface chemistry was performed. The contact of the materials to blood evoked an accelerated inflammatory and cell-induced osteoconductive reaction. Corrosion products formed indicate a more realistic, in vivo like situation. CONCLUSIONS: The active regulation of corrosion mechanisms of magnesium alloys by different cell types should be more in the focus of research to bridge the gap between in vitro and in vivo observations and to understand the mechanism of action. This in turn could lead to a better acceptance of these materials for implant applications. STATEMENT OF SIGNIFICANCE: The presented study deals with the first mechanistic insights during whole human blood contact and its influence on a degrading magnesium-based biomaterial. The combination of clinical parameters and corrosion layer analysis has been performed for the first time. It could be of interest due to the intended use of magnesium-based stents and for orthopaedic applications for clinical applications. An interest for the readers of Acta Biomaterialia may be given, as one of the first clinically approved magnesium-based devices is a wound-closure device, which is in direct contact with blood. Moreover, for orthopaedic applications also blood contact is of high interest. Although this is not the focus of the manuscript, it could help to rise awareness for potential future applications.

Effect of time on bond strength in indirect bonding.

The purpose of this in vitro investigation was to determine the influence of a reduced time interval before debonding on shear bond strength of stainless steel brackets bonded with a custom base indirect technique. A total of 135 bovine permanent mandibular incisors was randomly divided into nine groups of 15 specimens each. Three base composite-sealant combinations were investigated: (1) Phase II base composite, Custom I.Q. sealant, (2) Phase II base composite, Maximum Cure sealant, and (3) Transbond XT base composite, Sondhi Rapid Set sealant. Shear bond strength was measured for three different debonding time intervals: (1) time of transfer tray removal as recommended by the manufacturer, (2) 30 minutes after bonding of the sealant, and (3) 24 hours after bonding of the sealant. For groups bonded with Maximum Cure or Sondhi Rapid Set sealants, no influence of debonding time on shear bond strength was found. The Custom I.Q. sealant groups showed significantly lower bond strength measurements when debonded at the recommended tray removal time, and the Weibull analysis indicated a higher risk of bond failure at clinically relevant levels of stress. All base composite-sealant combinations showed acceptable bond strength at 30 minutes and 24 hours after bonding of the sealant.

Bond strength with custom base indirect bonding techniques.

Different types of adhesives for indirect bonding techniques have been introduced recently. But there is limited information regarding bond strength with these new materials. In this in vitro investigation, stainless steel brackets were bonded to 100 permanent bovine incisors using the Thomas technique, the modified Thomas technique, and light-cured direct bonding for a control group. The following five groups of 20 teeth each were formed: (1) modified Thomas technique with thermally cured base composite (Therma Cure) and chemically cured sealant (Maximum Cure), (2) Thomas technique with thermally cured base composite (Therma Cure) and chemically cured sealant (Custom I Q), (3) Thomas technique with light-cured base composite (Transbond XT) and chemically cured sealant (Sondhi Rapid Set), (4) modified Thomas technique with chemically cured base adhesive (Phase II) and chemically cured sealant (Maximum Cure), and (5) control group directly bonded with light-cured adhesive (Transbond XT). Mean bond strengths in groups 3, 4, and 5 were 14.99 +/- 2.85, 15.41 +/- 3.21, and 13.88 +/- 2.33 MPa, respectively, and these groups were not significantly different from each other. Groups 1 (mean bond strength 7.28 +/- 4.88 MPa) and 2 (mean bond strength 7.07 +/- 4.11 MPa) showed significantly lower bond strengths than groups 3, 4, and 5 and a higher probability of bond failure. Both the original (group 2) and the modified (group 1) Thomas technique were able to achieve bond strengths comparable to the light-cured direct bonded control group.

In vitro investigation of indirect bonding with a hydrophilic primer.

The aim of this in vitro investigation was to evaluate bond strength for a custom base indirect bonding technique using a hydrophilic primer on moisture-contaminated tooth surfaces. Stainless steel brackets were bonded to 100 permanent bovine incisors using a light-cured custom base composite adhesive, a chemically cured sealant, and the hydrophilic primer Transbond MIP (3M-Unitek, Monrovia, Calif). Five groups (A-E) of 20 teeth each were formed according to the time of contamination (before or after application of the primer) and the type of contaminant (distilled water or saliva): A, control group with no contamination; B, contamination with saliva before application of the primer; C, contamination with water before application of the primer; D, contamination with saliva before and after application of the primer; and E, contamination with water before and after application of the primer. Mean bond strength for the group without contamination (A) was 15.07 +/- 4.14 MPa and was not significantly different from bond strengths for groups B (14.91 +/- 3.99 MPa) and C (16.12 +/- 3.67 MPa), in which contamination occurred before application of the hydrophilic primer. Average bond strength in group D was 11.92 +/- 4.76 MPa. The lowest mean bond strength was measured for group E (9.85 +/- 3.77 MPa) and was significantly lower than for groups A, B, and C. Contamination after primer application resulted in an increased risk of bond failure at clinically relevant levels of stress.

In vitro evaluation of a moisture-active adhesive for indirect bonding.

The aim of this in vitro investigation was to evaluate bond strength for a cyanoacrylate adhesive in combination with an indirect bonding technique. Eighty bovine permanent mandibular incisors were randomly divided into four groups of 20 teeth each. The influence of two factors on shear bond strength was investigated: (1) type of adhesive (Smartbond cyanoacrylate, Sondhi Rapid Set composite sealant) and (2) time of debonding (30 minutes and 24 hours after bonding). Stainless steel mesh-based brackets were used. Although, bond strength was not significantly different for the two debonding time periods, significantly lower bond strength measurements were found for the cyanoacrylate adhesive (P < .001). The mean bond strength for the cyanoacrylate adhesive group was 5.44 +/- 1.65 MPa for debonding 30 minutes and 6.92 +/- 1.48 MPa for debonding 24 hours after the bonding procedure vs 16.16 +/- 5.25 MPa and 14.98 +/- 2.85 MPa for the composite adhesive groups debonded at 30 minutes and 24 hours, respectively. The Weibull analysis indicated that there was an increased risk of bond failure at clinically relevant levels of stress for indirect bonding with the cyanoacrylate adhesive.

Intermediate structures in radiation damaged titanite (CaTiSiO5): a Raman spectroscopic study.

Effects of radiation damage and thermal annealing on the crystal structure of natural titanite (CaTiSiO(5)) were studied using Raman spectroscopy. The results show that well crystallized natural titanites generally have the P2(1)/a structure at the unit cell level, in contrast to the A2/a symmetry reported previously. Radiation caused by naturally incorporated impurities (such as U and Th) leads to structural damage and amorphization in titanite, as evidenced by a significant loss of band intensity, spectral broadening and frequency shifts. Additional bands (e.g. near 574 and 650 cm(-1)) occur in weakly or partially metamict titanite due to the formation of an intermediate phase (with the A2/a symmetry). Raman spectra of titanite thermal glasses showed features different from those of metamict titanite, especially in the Ti-O and Si-O stretching regions. The effect of thermal annealing is strongly affected by the initial degrees of damage that the sample experienced. Weakly damaged titanite samples showed that annealing leads to a structural recovery, and the spectral patterns of these recovered crystals are consistent with the P2(1)/a symmetry. Highly damaged titanite starts to recrystallize into an A2/a phase near 700-800 K, and additional structural modification occurs when annealed at 1300-1400 K, which involves significant change in broad Ti-O features. However, in terms of bandwidths, the metamict samples are far from fully recovered even on being annealed at 1300-1400 K.

Chemical surface alteration of biodegradable magnesium exposed to corrosion media.

The understanding of corrosion processes of metal implants in the human body is a key problem in modern biomaterial science. Because of the complicated and adjustable in vivo environment, in vitro experiments require the analysis of various physiological corrosion media to elucidate the underlying mechanism of "biological" metal surface modification. In this paper magnesium samples were incubated under cell culture conditions (i.e. including CO(2)) in electrolyte solutions and cell growth media, with and without proteins. Chemical mapping by high-resolution electron-induced X-ray emission spectroscopy and infrared reflection microspectroscopy revealed a complex structure of the formed corrosion layer. The presence of CO(2) in concentrations close to that in blood is significant for the chemistry of the oxidised layer. The presence of proteins leads to a less dense but thicker passivation layer which is still ion and water permeable, as osmolality and weight measurements indicate.

The structural state of lead-based relaxor ferroelectrics under pressure.

The exceptional properties of lead-based perovskite-type (ABO(3)) relaxor ferroelectrics are due to their structural inhomogeneities. At ambient conditions, the average structure is pseudocubic but rich in ferroic nanoregions too small to be directly studied by conventional diffraction analysis. However, combining in situ temperature and pressure diffraction and Raman scattering allows us to resolve the structural complexity of relaxors. Because of the different length and time scales of sensitivity, diffraction probes the long-range order, i.e., the structure averaged over time and space, whereas Raman spectroscopy can detect local structural deviations from the average structure via the anomalous Raman activity of the phonon modes that, when the symmetry of the average structure is considered, should not generate Raman peaks. Hence, the combined analysis of the long-range order induced at low temperatures or high pressures and of the phonon anomalies enhanced on temperature decrease or pressure increase can reveal the energetically preferred structural nanoclusters existing at ambient conditions. In this regard, high-pressure experiments are vital for understanding the nanoscale structure of relaxors. Using X-ray diffraction, neutron diffraction, and Raman scattering on stoichiometric and doped PbSc(0.5)Ta(0.5)O(3) and PbSc(0.5)Na(0.5)O(3), we demonstrate the existence of a pressure-induced cubic-to-rhombohedral continuous phase transition. The high-pressure structure has suppressed polar shifts of B-site cations, enhanced correlation of Pb-O ferroic species, and long-range ordered antiphase BO(6) octahedral tilts. The critical pressure is preceded by an intermediate pressure at which the coupling between off-centered Pb and B-cations is suppressed and octahedral tilting detectable by neutron diffraction is developed.

Side effects of a non-peroxide-based home bleaching agent on dental enamel.

Changes in the chemistry and structure of enamel due to a non-peroxide-based home bleaching product (Rapid White) were studied in vitro using attenuated total reflectance-infrared spectroscopy, Raman spectroscopy, electron probe microanalysis, flame atomic absorption spectroscopy, and total reflection X-ray fluorescence. The results revealed that the citric-acid-containing gel-like component of the bleaching system substantially impacts on the dental hard tissue. Enamel is affected on several levels: (i) the organic component is removed from superficial and deeper enamel layers and remnants of the bleaching gel are embedded in the emptied voids; (ii) cracks and chemical inhomogeneities with respect to Ca and P occur on the surface; and (iii) within a submicron layer of enamel, the Ca-O bond strength in apatite decreases, thus enhancing calcium leakage from the bleached enamel hard tissue.