Aging Behavior of Water-Based YSZ Suspensions for Plasma Spraying of Thermal Barrier Coatings.

Suspension stability is a key parameter that should be considered in any coating process utilizing a suspension as the main feedstock. Application of water as the liquid phase for suspension preparation is promising due to its availability, low cost and no toxicity. In the present study, the effects of three surfactants, polyethyleneimine (PEI), 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) and α-Terpineol, on the functional stability of yttria-stabilized zirconia (YSZ) water-based suspensions were investigated. The changes in the suspensions' zeta potential, pH, viscosity and Turbiscan Stability Index (TSI) were monitored over their aging time of 1 to 7 days. The results showed that α-Terpineol is the most effective surfactant to produce functionally stable suspensions with low viscosity and surface tension values. Plasma spraying of such stable suspensions results in the formation of thermal barrier coatings (TBCs) with columnar morphology having porosity in the range of 17 to 18 vol.%.

Dimensional Changes of Yttria-stabilized Zirconia under Different Preparation Designs and Sintering Protocols.

PURPOSE: To evaluate the linear and volumetric dimensional changes that occur throughout the fabrication process of monolithic 4.5-6% yttria-stabilized zirconia copings under the influence of different preparation designs and two sintering protocols. MATERIALS AND METHODS: A titanium master die was fabricated using Atlantis core file implant-abutment. Six copings were designed virtually according to different finish line offsets and coping thicknesses, with four equidistant occlusal posts for linear measurements. Zirconia copings were milled using IPS e.max ZirCAD LT zirconia blanks. The experimental groups according to the coping designs were the following: G1: 0.5 mm finish line offset, 0.5 mm thickness; G2: 0.5 mm finish line offset, 1.0 mm thickness; G3: 0.5 mm finish line offset, 1.5 mm thickness; G4: 1.2 mm finish line offset, 0.5 mm thickness; G5: 1.2 mm finish line offset, 1.0 mm thickness; G6: 1.2 mm finish line offset, 1.5 mm thickness. Six samples per group were sintered by standard sintering (SS) and the other six by fast sintering (FS). Linear and volumetric measurements were taken at the three fabrication stages (virtual design, milling stage, and sintering) by using an intraoral scanner and imported as the .stl file to Meshmixer software for analysis. Statistical analysis was performed by a linear mixed effect model via statistical software R (R Core team, 2018). RESULTS: There was a significant interaction between the coping design group, stage of fabrication and sintering protocol on the linear (F = 4.451, p < 0.001) and volumetric (F = 2.716; p < 0.001) dimensional changes. Standard sintering G1 showed the smallest linear and dimensional changes among the groups compared to the reference design. Sintered copings had shrunk on average 1.32% within SS and 1.54% within FS linearly and 3.82% within SS and 3.90% within FS volumetrically compared to the initial design parameters. CONCLUSION: The linear and volumetric dimensional changes did not differ significantly between standard and fast sintering protocols, and the preparation designs had more influence on the dimensional changes compared to sintering protocols.

Biomimetically Mineralized Alginate Nanocomposite Fibers for Bone Tissue Engineering: Mechanical Properties and in Vitro Cellular Interactions.

We report herein the structural and mechanical properties and in vitro cellular response of hydroxyapatite (HAp)/alginate nanocomposite fibrous scaffolds mimicking the mineralized collagen fibrils of bone tissue. The biomimetically "engineered" nanocomposites, fabricated by electrospinning and in situ synthesis strategy, were compared with pure alginate nanofibers and micrometer-level HAp/alginate composite fibers. The tensile strength and elastic modulus of the nanocomposites increased by 79.3 and 158.4%, respectively, compared to those of alginate. The uniform nucleation and HAp nanocrystal growth on the alginate nanofibers resulted in such enhancement of the mechanical properties via a stress-transfer effect. Rat calvarial osteoblasts were stably attached and stretched more extensively on the nanocomposites' surface than on the pristine alginate. The controlled deposition of the HAp nanophase contributed to a much faster cell proliferation rate on the nanocomposites than on the others. The improved structural stability and osteoblast interactions suggest the fibrous nanocomposite scaffold's potential advantages for bone tissue regeneration.

Experimental composites of polyacrilonitrile-electrospun nanofibers containing nanocrystal cellulose.

OBJECTIVE: To test the effects of addition of polyacrilonitrile (PAN) nanofibers and nanocrystal cellulose (NCC)-containing PAN nanofibers on flexural properties of experimental dental composites. METHODS: 11wt% PAN in dimethylformamide (DMF) solution was electrospun at 17.2kVA and 20cm from the collector drum. NCC was added to the solution at 3wt%. Fiber mats were produced in triplicates and tested as-spun. Strips (5cm×0.5cm) were cut from the mat in an orientation parallel and perpendicular to the rotational direction of the collector drum. Tensile tests were performed and ultimate tensile strength (UTS), elastic modulus (E) and elongation at maximum stress (%) were calculated from stress/strain plots. Fiber mats were then infiltrated by resin monomers (50/50 BisGMA/TEGDMA wt%), stacked in a mold (2×15×25) and light-cured. Beams (2×2×25mm) were cut from the slabs and tested in a universal testing machine. Data were analyzed by multiple t-test and one-way ANOVA (α=0.05). RESULTS: Addition of 3% NCC resulted in higher tensile properties of the fibers. Fibers presented anisotropic behavior with higher UTS and E when tested in perpendicular orientation. The incorporation of 3% NCC-PAN nanofibers resulted in significant increase in work of fracture and flexural strength of experimental dental composite beams. SIGNIFICANCE: NCC was found to be a suitable nanoparticle to reinforce experimental dental composites by incorporation via nanofiber. This fundamental study warrants future investigation in the use of electrospun nanofibres as a way to reinforce dental composites.

The influence of altering sintering protocols on the optical and mechanical properties of zirconia: A review.

OBJECTIVE: As a result of advancements in chairside technology and speed sintering techniques and increased esthetic demands of patients, efforts have been made to produce monolithic zirconia restorations that are highly translucent, strong, and dense. While methods for processing zirconia are well known, there is a tendency to modify the process parameters with the aim of decreasing the overall processing time and, in particular, the sintering time. This review provides clinicians with scientific evidence of the effects of altering sintering parameters used for dental zirconia on its microstructure, phase transformation, and mechanical and optical properties. MATERIALS AND METHODS: A systematic search of Embase and Medline using Boolean operators was performed to locate relevant articles. RESULTS: Eleven articles were selected for this review. The following characteristics of monolithic zirconia have been confirmed to be affected by alterations in sintering: the microstructure, mechanical properties, optical properties, wear behavior, and low thermal degradation. CONCLUSIONS: The alteration of sintering parameters has been found to alter the grain size, wear behavior, and translucency of zirconia. There is a lack of clinical studies that investigate the influence of altering sintering parameters or methods on the clinical performance of monolithic zirconia restorations. CLINICAL SIGNIFICANCE: Alteration of sintering parameters alters the microstructural, mechanical, and optical properties of zirconia. This will consequently impact the clinical performance of zirconia prostheses. Future clinical investigations are encouraged to support these in vitro findings.

Marginal Discrepancies of Monolithic Zirconia Crowns: The Influence of Preparation Designs and Sintering Techniques.

PURPOSE: The marginal fit is an essential component for the clinical success of prosthodontic restorations. The aim of this study was to investigate the influence of different abutment finish line widths and crown thicknesses on the marginal fit of zirconia crowns fabricated using either standard or fast sintering protocols. MATERIALS AND METHODS: Six titanium abutments were fabricated for receiving zirconia molar crowns. Crowns were designed virtually and milled from partially sintered zirconia blanks and divided into 12 groups (n = 10/group). Crowns in groups 1 to 6 were sintered by standard sintering, while those in groups 7 to 12 were sintered by fast sintering. Groups were further categorized according to abutment finish line and crown thickness: G1/G7 (0.5 mm chamfer, 0.8 mm thick); G2/G8 (0.5 mm chamfer, 1.5 mm thick); G3/G9 (1.0 mm chamfer, 0.8 mm thick); G4/10 (1.0 mm chamfer, 1.5 mm thick); G5/G11 (1.2 mm chamfer, 0.8 mm thick); G6/G12 (1.2 mm chamfer, 1.5 mm thick). The marginal gaps were assessed at 8 locations using digital microscopy. The linear mixed effect model analysis was performed at a significance level of 0.05. RESULTS: All vertical marginal gaps were within the clinically acceptable range (∼11-52 µm). G8 (FS, 0.5 mm chamfer, 1.5 mm thick) demonstrated the largest gaps (47.95 µm, 95% CI: 44.57-51.23), whereas G3 (SS, 1.0 mm chamfer, 0.8 thick) had the smallest marginal gap (14.43 µm, 95% CI: 11.15-17.71). A linear mixed effect models showed significant differences for the interaction between finish line × crown thickness × sintering (F = 18.96, p < 0.001). The lingual surfaces showed the largest gaps in both sintering protocols, while the mesial and mesiobuccal surfaces demonstrated the smallest gaps. CONCLUSIONS: There was a significant interaction between finish line widths, crown thickness, and sintering protocol on the marginal gaps in both sintering protocols; 1.0 mm finish line preparations with either 0.8 mm or 1.5 mm occlusal reduction had better marginal fit in both sintering protocols compared to 0.5 mm or 1.2 mm finish lines. Smaller marginal discrepancies were observed for standard sintering crowns with a 0.5 mm finish line and 1.5 mm occlusal reduction. Conservative occlusal reduction should be accompanied with a 1.2 mm finish line to obtain better marginal fit for full-contoured zirconia crowns.

Osteogenic and anti-osteoporotic effects of risedronate-added calcium phosphate silicate cement.

Osteoporosis greatly impairs bone fracture restoration with bone cement because the accelerated resorption decreases the osseointegration between bone and implants. In this study, we designed a new drug delivery system based on the third generation bisphosphonate risedronate (RA) and the osteogenic calcium phosphate silicate cement (CPSC). The impact of RA on CPSC's material properties and microstructure was evaluated by different characterization methods (µCT, XRD, FTIR, SEM and gas sorption). In addition, in vitro biocompatibility of RA-added CPSC was evaluated (MTT assay, flow cytometry, real-time PCR). In an in vivo study of osteoporotic rabbits, osteoporosis- and bone resorption-related biomarkers were measured over time (ELISA) and local osteogenic and anti-osteoporotic effects investigated (x-ray, CT, histology, PCR arrays). RA decreased the setting rate and compressive strength of CPSC by impeding the hydration of calcium silicate. The overall porosity of CPSC was also decreased with RA. The RA-added CPSC was biocompatible and improved osteoblast proliferation and differentiation. The slow release of RA from CPSC reduced the prevalence of osteoporosis in rabbits and improved peri-implant bone formation and osseointegration. In conclusion, RA-containing CPSC demonstrates its potentials to improve fractural restoration under osteoporotic conditions and should be further engineered to increase its effectiveness in fractural restoration.

A Comprehensive Study of Osteogenic Calcium Phosphate Silicate Cement: Material Characterization and In Vitro/In Vivo Testing.

Vertebral compression fractures can be successfully restored by injectable bone cements. Here the as-yet unexplored in vitro cytotoxicity, in vivo biodegradation, and osteoconductivity of a new calcium phosphate silicate cements (CPSC) are studied, where monocalcium phosphate (MCP; 5, 10, and 15 wt%) is added to calcium silicate cement (CSC). Setting rate and compressive strength of CPSC decrease with the addition of MCP. The crystallinity, microstructure, and porosity of hardened CPSC are evaluated by X-ray diffractometer, Fourier transform infrared spectroscopy, and microcomputed tomography (CT). It is found that MCP reacts with calcium hydroxide, one of CSC hydration products, to precipitate apatite. While the reaction accelerates the hydration of CSC, the formation of calcium silicate hydrate gel is disturbed and highly porous microstructures form, resulting in weaker compressive strength. In vitro studies demonstrate that CPSC is noncytotoxic to osteoblast cells and promotes their proliferation. In the rabbit tibia implantation model, clinical X-ray and CT scans demonstrate that CPSC biodegrades slower and osseointegrates better than clinically used calcium phosphate cement (CPC). Histological studies demonstrate that CPSC is osteoconductive and induces higher bone formation than CPC, a finding that might warrant future clinical studies.

Fabrication and characterization of a novel carbon fiber-reinforced calcium phosphate silicate bone cement with potential osteo-inductivity.

The repair of bone defects is still a pressing challenge in clinics. Injectable bone cement is regarded as a promising material to solve this problem because of its special self-setting property. Unfortunately, its poor mechanical conformability, unfavorable osteo-genesis ability and insufficient osteo-inductivity seriously limit its clinical application. In this study, novel experimental calcium phosphate silicate bone cement reinforced by carbon fibers (CCPSC) was fabricated and characterized. First, a compressive strength test and cell culture study were carried out. Then, the material was implanted into the femoral epiphysis of beagle dogs to further assess its osteo-conductivity using a micro-computed tomography scan and histological analysis. In addition, we implanted CCPSC into the beagles' intramuscular pouches to perform an elementary investigation of its osteo-inductivity. The results showed that incorporation of carbon fibers significantly improved its mechanical properties. Meanwhile, CCPSC had better biocompatibility to activate cell adhesion as well as proliferation than poly-methyl methacrylate bone cement based on the cell culture study. Moreover, pronounced biodegradability and improved osteo-conductivity of CCPSC could be observed through the in vivo animal study. Finally, a small amount of osteoid was found at the heterotopic site one month after implantation which indicated potential osteo-inductivity of CCPSC. In conclusion, the novel CCPSC shows promise as a bioactive bone substitute in certain load-bearing circumstances.

Preparation, characterization, release kinetics, and in vitro cytotoxicity of calcium silicate cement as a risedronate delivery system.

Injectable bone cements have been well characterized and studied in non-load bearing bone fixation and bone screw augmentation applications. Current calcium phosphate cement or poly(methyl methacrylate) cement have drawbacks like low mechanical strength and in situ exothermic properties. This leads especially in patients with osteoporosis to worsening contact between implant and bone and can finally lead to implant failure. To improve these properties, a calcium silicate cement (CSC) was prepared, which additionally contained the bisphosphonate risedronate (RA) to promote osteoblast function. Cement setting rate and compressive strength were measured and found to be reduced by RA above 0.5 wt%. X-ray diffraction, Rietveld refinement analysis, scanning electron microscopy, and porosity measurements by gas sorption revealed that RA reduces calcium silicate hydrate gel formation and changes the cement's microstructure. Cumulative release profiles of RA from CSC up to 6 months into phosphate buffer solution were analyzed by high-performance liquid chromatography, and the results were compared with theoretical release curves obtained from the Higuchi equation. Fourier transform infrared spectra measurements and drug release studies indicate that calcium-RA formed within the cement, from which the drug can be slowly released over time. An investigation of the cytotoxicity of the RA-CSC systems upon osteoblast-like cells showed no toxic effects of concentrations up to 2%. The delivery of RA from within a CSC might thus be a valuable and biocompatible new approach to locally deliver RA and to reconstruct and/or repair osteoporosis-related bone fractures.

Bio-inspired dicalcium phosphate anhydrate/poly(lactic acid) nanocomposite fibrous scaffolds for hard tissue regeneration: in situ synthesis and electrospinning.

The fundamental building blocks of hierarchically structured bone tissue are mineralized collagen fibrils with calcium phosphate nanocrystals that are biologically "engineered" through biomineralization. In this study, we demonstrate an original invention of dicalcium phosphate anhydrate (DCPA)/poly(lactic acid) (PLA) composite nanofibers, which mimics the mineralized collagen fibrils via biomimetic in situ synthesis and electrospinning for hard tissue regenerative medicines. The interaction of the Ca(2+) ions and the carbonyl groups in the PLA provides nucleation sites for DCPA during the in situ synthesis process. This resulted in the improved dispersion of DCPA nanocrystallites in the intrananoporous PLA nanofibers through electrospinning, compared to the severely agglomerated clusters of DCPA nanoparticles fabricated by conventional mechanical blending/electrospinning methods. The addition of poly(ethylene glycol), as a copolymer source, generated more stable and efficient electrospun jets and aided in the electrospinability of the PLA nanofibers incorporating the nanocrystallites. It is expected that the uniformly distributed DCPA nanocrystallites and its unique nanocomposite fibrous topography will enhance the biological performance and the structural stability of the scaffolds used for hard tissue reconstruction and regeneration.

Novel biomimetic hydroxyapatite/alginate nanocomposite fibrous scaffolds for bone tissue regeneration.

Hydroxyapatite/alginate nanocomposite fibrous scaffolds were fabricated via electrospinning and a novel in situ synthesis of hydroxyapatite (HAp) that mimics mineralized collagen fibrils in bone tissue. Poorly crystalline HAp nanocrystals, as confirmed by X-ray diffractometer peak approximately at 2θ = 32° and Fourier transform infrared spectroscopy spectrum with double split bands of PO4(v 4) at 564 and 602 cm(-1), were induced to nucleate and grow at the [-COO(-)]-Ca(2+)-[-COO(-)] linkage sites on electrospun alginate nanofibers impregnated with PO4 (3-) ions. This novel process resulted in a uniform deposition of HAp nanocrystals on the nanofibers, overcoming the severe agglomeration of HAp nanoparticles processed by the conventional mechanical blending/electrospinning method. Preliminary in vitro cell study showed that rat calvarial osteoblasts attached more stably on the surface of the HAp/alginate scaffolds than on the pure alginate scaffold. In general, the osteoblasts were stretched and elongated into a spindle-shape on the HAp/alginate scaffolds, whereas the cells had a round-shaped morphology on the alginate scaffold. The unique nanofibrous topography combined with the hybridization of HAp and alginate can be advantageous in bone tissue regenerative medicine applications.

In vitro studies of calcium phosphate silicate bone cements.

A novel calcium phosphate silicate bone cement (CPSC) was synthesized in a process, in which nanocomposite forms in situ between calcium silicate hydrate (C-S-H) gel and hydroxyapatite (HAP). The cement powder consists of tricalcium silicate (C(3)S) and calcium phosphate monobasic (CPM). During cement setting, C(3)S hydrates to produce C-S-H and calcium hydroxide (CH); CPM reacts with the CH to precipitate HAP in situ within C-S-H. This process, largely removing CH from the set cement, enhances its biocompatibility and bioactivity. The testing results of cell culture confirmed that the biocompatibility of CPSC was improved as compared to pure C(3)S. The results of XRD and SEM characterizations showed that CPSC paste induced formation of HAP layer after immersion in simulated body fluid for 7 days, suggesting that CPSC was bioactive in vitro. CPSC cement, which has good biocompatibility and low/no cytotoxicity, could be a promising candidate as biomedical cement.

Oxidation of gas phase trichloroethylene and toluene using composite sol-gel TiO2 photocatalytic coatings.

The previously developed composite sol-gel (CSG) process is proposed for the deposition of thick (10-50 microm) porous films of photocatalytic TiO2. The CSG titania was developed by binding pre-calcined TiO2 particles with TiO2 sol. It had relatively high surface area (15-35 m2/g) and good resistance against mechanical stress and abrasion. Photocatalytic activity tests were carried out on trichloroethylene (TCE) and toluene, and compared with those of standard Degussa P-25 titania. The CSG photocatalyst provided good photo-efficiency in removing both pollutants from contaminated air streams. When compared with P-25 titania, the CSG photocatalyst showed a similar photo-efficiency with first-order kinetic rate constants not significantly different from that of P-25. For both photocatalysts the rate of photocatalytic oxidation of TCE was significantly greater than that obtained for toluene. Overall, the combination of better mechanical integrity, resistance against abrasion, and comparable photocatalytic efficiency of the CSG titania versus that of P-25 titania, make the composite sol-gel (CSG) photocatalyst a viable alternative for industrial applications where long term stability, superior mechanical properties, and good photo-efficiency are of critical value.

In-situ preparation of poly(propylene fumarate)--hydroxyapatite composite.

In-situ precipitation of hydroxyapatite (HAp) in the presence of poly(propylene fumarate) (PPF) is investigated. Amorphous calcium phosphate (ACP) precipitates in the presence of the polymer and remains in the amorphous form for a relatively long time, e.g. even after 24 h of coexistence with the mother solution. Our observations suggest that PPF interacts with the surface of the ACP particles and prevents them from transformation to crystalline hydroxyapatite. The PPF polymer seems to be more efficient in hindering the ACP to HAp transformation at higher pH conditions. From spectroscopic observations we hypothesize that the C=O bond of the PPF molecules interact with the calcium ion of the ACP particles. In case of low molecular weight PPF this interaction may lead to the incorporation of the polymer within the growing ACP particles.

Influence of apatite seeds on the synthesis of calcium phosphate cement.

This preliminary study explores the seeding effect (using crystalline hydroxyapatite particles) on the setting time, compressive strength, phase evolution, and microstructure of calcium phosphate cements (CPC) based on monocalcium phosphate monohydrate and calcium hydroxide. Experimental results showed that the setting time varies from 5 to about 30 min, as the seed concentration increased from 0 to 20 wt%. The compressive strength of CPC increased from 4 to 17 MPa, followed by decrease to 12 MPa, for the same range of seeds content. The CPC transformed to predominantly apatitic structure within 24 h for all the samples, with or without the seeds. However, increase of the seed concentration improved the final crystallinity of the apatite phase, suggesting nucleation and growth effects during precipitation of CPC from the precursor solution. The microstructure of the resulting apatitic cement showed a change from essentially featureless (or glass-like) to thin, elongated plate-like morphology, as seeds concentration increased. Correlation between microstructural evolution and corresponding compressive strength of seeded CPC is investigated.

Structural evolution of sol-gel-derived hydroxyapatite.

Structural evolution upon transformation of sol to gel, and gel to final ceramic during the synthesis of hydroxyapatite is investigated using Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), thermal behavior (DTA and TGA), and electron microscopy examination (SEM/TEM). The sol was first thermally aged at 45 C for various time periods up to 120 min. The colloidal sol, which may have an oligomeric structure, was relatively stable against coagulation. Upon drying, the sol particles consolidated into dry gel through van der Waals attraction, and showed X-ray amorphous phosphate structure. The solid gels showed a particulate microstructure, composed of primary particles of about 8-10 nm in diameter. The amorphous gel transformed into crystalline apatite at temperatures > 300 C. The calcined gels showed a nano-scale microstructure, with grains of 20-50 nm in diameter. Through an appropriate heat treatment between 300 and 400d degrees C. the apatite prepared using current process exhibits a nano-scale, low-crystallinity, carbonated apatitic structure, which closely resembles that of human bone apatite.

Sol-gel hydroxyapatite coatings on stainless steel substrates.

Thin film hydroxyapatite deposits onto sandblasted 316L stainless steel substrates were prepared using water-based sol-gel technique recently developed in our lab. The coatings were annealed in air at 375 degrees C, 400 degrees C, and 500 degrees C. Phase formation, surface morphology, interfacial microstructure, and interfacial bonding strength of the coatings were investigated. Apatitic structure developed within the coatings while annealing at temperatures > or = 400 degrees C, while those heat-treated at 375 degrees C showed poor crystallinity. The coatings were dense and firmly attached to the underlying substrates, reaching an average bonding strength (as determined through the pull-out test) of 44 MPa. Nano-porous structure was found for the coatings annealed at 500 degrees C, believed to result from grain growth, and causing a slight decrease in the bonding strength. Surface microcracking, although not extensive, occurred after annealing at temperatures > or = 400 degrees C, and was linked to non-uniform thickness of the coating due to roughness of the substrate. A contraction of the coatings as a result of sintering, and phase transition from amorphous (or poor crystalline) to reasonably good crystalline apatite, may be responsible for the loss of structural integrity of the thicker sections of the coatings. It seems quite promising that a dense and adhesive apatite coating can be achieved through water-based sol gel technology after short-term annealing at around 400 degrees C in air.