Effect of Slurry Viscosity and Dispersant Agent on the Sintering of 3Y-TZP Ceramics Fabricated by Slip Casting.

Dental zirconia implants are typically manufactured by mechanical machining of a zirconia block into a tooth shape. Many cracks are created by this machining process on the surface of the green body, which induces deterioration of mechanical strength and reliability of the sintered zirconia implant. In this study, we fabricated a dense zirconia specimen by slip casting and sintering. The zirconia slurry for slip casting was prepared by mixing yttria-stabilized zirconia powder with an average particle size of 20 nm, distilled water, and dispersant. Slurry viscosity was controlled by varying pH, dispersant concentration, and solid content; the lower viscosity being achieved at pH 11 controlled by ammonium hydroxide. With a dispersant content of 0.4-0.8 wt%, the viscosity was reduced from 190 cP to 40 cP at pH 11. After casting and sintering at 1550 °C for 2 h, the sintered body reached a density of 5.70-6.01 g/cm³ and a grain size of 300-700 nm, depending on the slurry preparation conditions.

Effect of Solid Loading on the Sintered Properties of 3 mol% Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP) Ceramics via Slip Casting.

Dental zirconia implants fabricated by the mechanical machining and sintering of zirconia blocks have many surface cracks that lead to the deterioration of mechanical strength and the failure of the implant in the body. In this study, we attempted to manufacture an extremely dense and crack-free zirconia specimen by slip casting and pressureless sintering. After the preparation of zirconia slurry by control of its viscosity and by solid loading, highly dense zirconia specimens could be obtained by pressureless sintering at 1450 °C for 2 h. Slurry viscosity was controlled by adjusting the mixing ratio of 3Y-TZP powder, a dispersant, and a pH adjustment agent. Highly dense 3Y-TZP specimens with a relative density of 99% and small grain size of 200-400 nm could be obtained at a solid loading of 50-65 wt%. An optimally dense specimen was fabricated from zirconia slurry with 60 wt% solid loading that had the highest apparent density of 6.07 g/cm³ (99.5%).

In-Vitro Mechanical Performance Study of Biodegradable Polylactic Acid/Hydroxyapatite Nanocomposites for Fixation Medical Devices.

Osteoconductive, biocompatible, and resorbable organic/inorganic composites are most commonly used in fixation medical devices, such as suture anchors and interference screws, because of their unique physical and chemical properties. Generally, studies on biodegradable composites have focused on their mechanical properties based on the composition and the individual roles of organic and inorganic biomaterials. In this study, we prepared biodegradable organic/inorganic nanocomposite materials using the solvent mixing process and conventional molding. We used polylactic acid (PLA) as the matrix and nano-sized hydroxyapatite (nano-HAp) as the osteoconductive filler. The content of nano-HAp was varied in 0-30 wt% and its influence on the In-Vitro mechanical performance of PLA/HAp nanocomposites was evaluated. The In-Vitro mechanical properties of nanocomposites were evaluated using standardized tensile and flexural tests after different immersion times in simulated body fluid.

Polylactic Acid/Nanostructured Si-Substituted ß-Tricalcium Phosphate Composites for Biodegradable Fixation Medical Devices.

Organic/inorganic biocomposite materials for biodegradable fixation medical devices require osteoconductivity, biocompatibility, and adequate mechanical properties with biodegradation behavior. The objective of this study was to investigate the effect of Si ions substituted in ß-tricalcium phosphate (ß-TCP) on the mechanical properties of organic/inorganic biocomposites. Biodegradable composite materials were prepared with polylactic acid (PLA) as the matrix and nano Si-substituted ß-TCP as the osteoconductive filler by solvent mixing and conventional molding. The nanostructured Si-substituted ß-TCP powders were synthesized by co-precipitation, controlling the quantity of Si ions. The amount of nanostructured Si-substituted ß-TCP powders in composites was varied in the 0-40 wt% range and the material properties were compared with those of pure ß-TCP/PLA composites. The influence of Si ions on the mechanical properties of the composites was evaluated by tensile and flexural tests.

Interfacial Property of Dental Cobalt­Chromium Alloys and Their Bonding Strength with Porcelains.

Porcelain-fused-to-metal crown is one of the widely-used prostheses among the dental porcelain restorations. Nonprecious metals like Ni­Cr and Co­Cr have extensively been used for metal-ceramic restorations due to advantages such as inexpensive price, hardness, durability, resistance to deformation, thin thickness of metal of porcelain area, and other mechanical and physical properties. However, the immediate advantage of the Co­Cr alloy is comparable performance to other base metal alloys, but without an allergenic nickel component. To achieve clinical longevity of porcelain-fused-to-metal (PFM) crowns, it is essential to have adequate bond strength between the metal substrate and porcelain. Any type of metal-ceramic fracture failure can become a costly and timeconsuming problem, both in the clinic and laboratory. Therefore, the suitability of the Co­Cr alloy for dental applications is critically associated with its ceramic bonding capacity. In this study, Co­Cr metal alloys modified by acid-etching and sandblasting, oxide layer was formed for subsequent bonding to porcelain ceramics. By both acid-etching and sandblasting oxide layer was formed and showed higher bonding strength at a proper condition, but debonding was occurred at porcelain layer so that they showed highest bonding strength by combined these two kind of surface treatment. Because the oxide film was formed more densely in a vacuum at the portions where more sophisticated concavo-convex were formed on the surface of a metal.

Surface Corrosion of Nanoscaled Hydroxyapatite During an In Vivo Experiment.

Hydroxyapatite (HA) is widely used as a bioactive ceramics as it forms a chemical bond with bone. However, the drawback to using this material is its inferior mechanical properties. In this research, surface corrosion and disintegration of nanoscaled HA in a dog were studied, and the mechanism by which phase-pure HA dissolved in vivo was investigated. Biological properties of HA in vivo are affected by the grain-boundary dissolution followed by a surface corrosion and microstructural disintegration. This kind of dissolution process, apparently evidenced at the grain boundary, causes particle generation, which indicates that both long-term bone in-growth and mechanical properties can dramatically deteriorate. Implant dissolution by osteoclasts in vivo is also observed on the surface of hydroxyapatite. Implant surface showed an aggressive corrosion by an osteoclast resorption. Severe and deeper dissolution underwent close to osteoclast resulting in formation of smaller and more round particle shape.

Fabrication and Microstructure of Hydroxyapatite Coatings on Zirconia by Room Temperature Spray Process.

Hydroxyapatite coatings were fabricated on zirconia substrates by a room temperature spray process and were investigated with regards to their microstructure, composition and dissolution in water. An initial hydroxyapatite powder was prepared by heat treatment of bovine-bone derived powder at 1100 °C for 2 h, while dense zirconia substrates were fabricated by pressing 3Y-TZP powder and sintering it at 1350 °C for 2 h. Room temperature spray coating was performed using a slit nozzle in a low pressure-chamber with a controlled coating time. The phase composition of the resultant hydroxyapatite coatings was similar to that of the starting powder, however, the grain size of the hydroxyapatite particles was reduced to about 100 nm due to their formation by particle impaction and fracture. All areas of the coating had a similar morphology, consisting of reticulated structure with a high surface roughness. The hydroxyapatite coating layer exhibited biostability in a stimulated body fluid, with no severe dissolution being observed during in vitro experimentation.

Influence of Starting Powders on Hydroxyapatite Coatings Fabricated by Room Temperature Spraying Method.

Three types of raw materials were used for the fabrication of hydroxyapatite coatings by using the room temperature spraying method and their influence on the microstructure and in vitro characteristics were investigated. Starting hydroxyapatite powders for coatings on titanium substrate were prepared by a heat treatment at 1100 °C for 2 h of bovine bone, bone ash, and commercial hydroxyapatite powders. The phase compositions and Ca/P ratios of the three hydroxyapatite coatings were similar to those of the raw materials without decomposition or formation of a new phase. All hydroxyapatite coatings showed a honeycomb structure, but their surface microstructures revealed different features in regards to surface morphology and roughness, based on the staring materials. All coatings consisted of nano-sized grains and had dense microstructure. Inferred from in vitro experiments in pure water, all coatings have a good dissolution-resistance and biostability in water.

The Role of Magnesium Ion Substituted Biphasic Calcium Phosphate Spherical Micro-Scaffolds in Osteogenic Differentiation of Human Adipose Tissue-Derived Mesenchymal Stem Cells.

This study was investigated the role of magnesium (Mg2+) ion substituted biphasic calcium phosphate (Mg-BCP) spherical micro-scaffolds in osteogenic differentiation of human adipose tissue-derived mesenchymal stem cells (hAT-MSCs). Mg-BCP micro-scaffolds with spherical morphology were successfully prepared using in situ co-precipitation and spray drying atomization process. The in vitro cell proliferation and differentiation of hAT-MSCs were determined up to day 14. After in vitro biological tests, Mg-BCP micro-scaffolds with hAT-MSCs showed more enhanced osteogenicity than pure hAT-MSCs as control group by unique biodegradation of TCP phase and influence of substituted Mg2+ ion in biphasic nanostructure. Therefore, these results suggest that Mg-BCP micro-scaffolds promote osteogenic differentiation of hAT-MSCs.

Antimicrobial HPA3NT3 peptide analogs: placement of aromatic rings and positive charges are key determinants for cell selectivity and mechanism of action.

In an earlier study, we determined that HP(2-20) (residues 2-20 of parental HP derived from the N-terminus of the Helicobacter pylori ribosomal protein L1) and its analog, HPA3NT3, had potent antimicrobial effects. However, HPA3NT3 also showed undesirable cytotoxicity against HaCaT cells. In the present study, we designed peptide analogs including HPA3NT3-F1A (-F1A), HPA3NT3-F8A (-F8A), HPA3NT3-F1AF8A (-F1AF8A), HPA3NT3-A1 (-A1) and HPA3NT3-A2 (-A2) in an effort to investigate the effects of amino acid substitutions in reducing their hydrophobicity or increasing their cationicity, and any resulting effects on their selectivity in their interactions with human cells and pathogens, as well as their mechanism of antimicrobial action. With the exception of HPA3NT3-A1, all of these peptides showed potent antimicrobial activity. Moreover, substitution of Ala for Phe at positions 1 and/or 8 of the HPA3NT3 peptides (-F1A, -F8A and -F1AF8A) dramatically reduced their cytotoxicity. Thus the cytotoxicity of HPA3NT3 appears to be related to its Phe residues (positions 1 and 8), which strongly interact with sphingomyelin in the mammalian cell membrane. HPA3NT3 exerted its bactericidal effects through membrane permeabilization mediated by pore formation. In contrast, fluorescent dye leakage and nucleic acid gel retardation assays showed that -A2 acted by penetrating into the cytoplasm, where it bound to nucleic acids and inhibited protein synthesis. Notably, Staphylococcus aureus did not develop resistance to -A2 as it did with rifampin. These results suggest that the -A2 peptide could potentially serve as an effective antibiotic agent against multidrug-resistant bacterial strains.

Comparison of microstructures and phase compositions of artificial and bone-derived hydroxyapatites by transmission electron microscopy and energy dispersive electron spectroscopy.

Biological problems associated with sintered implants fabricated from artificially synthesized hydroxyapatite include selective dissolution at grain boundaries, microstructural disintegration in the body due to particle loosening, and slow crack growth at the implant surface. In addition, mechanical degradation has been shown to be significant, thus limiting their application as load-bearing medical implants. In contrast, bone-derived hydroxyapatite bioceramics have highly dissolution-resistant properties and excellent biocompatibility in the body. They are also easy to synthesize by thermal decomposition of animal bone. In this study, microstructural observations and crystal phase analysis of bone-derived hydroxyapatite were investigated by TEM and EDS using sintered hydroxyapatite samples. In addition, a comparative investigation into elemental distributions and the microstructures of artificial hydroxyapatite, bovine, and tuna bone-derived hydroxyapatites was performed. Bone-derived HA consists mainly of HA and a small amount of MgO. Hot-pressed HA compacts showed homogeneous microstructures and densities of 95-97%, however, grain sizes and microstructures varied with the starting powders.

Effect of repetitive lysine-tryptophan motifs on the eukaryotic membrane.

In a previous study, we synthesized a series of peptides containing simple sequence repeats, (KW)(n)-NH(2) (n = 2,3,4 and 5) and determined their antimicrobial and hemolytic activities, as well as their mechanism of antimicrobial action. However, (KW)(5) showed undesirable cytotoxicity against RBC cells. In order to identify the mechanisms behind the hemolytic and cytotoxic activities of (KW)(5), we measured the ability of these peptides to induce aggregation of liposomes. In addition, their binding and permeation activities were assessed by Trp fluorescence, calcein leakage and circular dichrorism using artificial phospholipids that mimic eukaryotic liposomes, including phosphatidylcholine (PC), PC/sphingomyelin (SM) (2:1, w/w) and PC/cholesterol (CH) (2:1, w/w). Experiments confirmed that only (KW)(5) induced aggregation of all liposomes; it formed much larger aggregates with PC:CH (2:1, w/w) than with PC or PC:SM (2:1, w/w). Longer peptide (KW)(5), but not (KW)(3) or (KW)(4), strongly bound and partially inserted into PC:CH compared to PC or PC:SM (2:1, w/w). Calcein release experiments showed that (KW)(5) induced calcein leakage from the eukaryotic membrane. Greater calcein leakage was induced by (KW)(5) from PC:CH than from PC:SM (2:1, w/w) or PC, whereas (KW)(4) did not induce calcein leakage from any of the liposomes. Circular dichroism measurements indicated that (KW)(5) showed higher conformational transition compared to (KW)(4) due to peptide-liposome interactions. Taken together, our results suggest that (KW)(5) reasonably mediates the aggregation and permeabilization of eukaryotic membranes, which could in turn explain why (KW)(5) displays efficient hemolytic activity.

Sintered Characteristics of 3 Mole% Yttria-Stabilized Zirconia Polycrystals (3Y-TZP) Implants Manufactured by Slip-Casting and Computer Aided Design/Computer Aided Manufacturing (CAD/CAM).

Two types of dense and toughened 3Y-TZP zirconia implants were fabricated by the sintering of green compacts. Then, the sintered properties of the two implants were compared. Slip-cast and post-sintered all-ceramic zirconia implants were fabricated by the heat treatment at 1450 °C for 2 h using optimal slurry conditions (60 wt% solid content, 1 wt% dispersant, and pH 12). Computer-aided design and manufacturing (CAD/CAM)-machined and post-sintered zirconia crowns, supplied by a dental hospital, were obtained by sintering at 1650 °C for 5 h. The X-ray diffraction results indicated that the phase composition of the slip-casted specimen was completely tetragonal, but the CAD/CAM machined sample was composed of mixed phases of main tetragonal and minor monoclinic crystals. The sintered density and Vickers hardness of the slip-casted specimen were 6.07 g/cm³ and 1367 Hv, respectively, and these were higher than those of the CAD/CAM machined specimen. From the comparative results of the surface microstructure, hardness, and roughness between the two sintered specimens, the slip-casted specimen was found to have higher surface roughness and mechanical hardness, smaller grain size, and less surface micro-cracks than the CAD/CAM machined specimen.

Bioactivity Improvement of Zirconia Substrate by Hydroxyapatite Coating Using Room Temperature Spray Processing.

Zirconia ceramics has a bioinert property with low bioactivity. So, it is necessary to improve its low bioactivity by the surface modification using effective coating methods. In this study, we fabricated the hydroxyapatite-coated zirconia substrate by room temperature spray processing to improve the bioactivity of the zirconia implant and investigated its coating effect on the biological performance of zirconia substrate via an in vitro test in simulated body fluid (SBF) solution. Before the room temperature spray coating was completed, size-controlled hydroxyapatite powder that had an average size of 4.5 µm, was obtained by the calcination and milling of a commercial powder. By controlling the processing parameters, such as spraying distance, and deposition cycles, we fabricated homogeneous and dense hydroxyapatite coatings on zirconia substrate. Surface morphology, coating thickness, and microstructure were dependent on deposition cycles, and were related to surface roughness and bioactivity. Zirconia substrates with wave-patterned and roughened hydroxyapatite coatings demonstrated high bioactivity in their in vitro tests. Via the immersion test in an SBF solution, surface dissolution and new precipitates of hydroxyapatite were observed on coated zirconia substrate, indicating the degree of bioactivity.

Slurry Coating of Hydroxyapatite on Zirconia Substrate.

Zirconia dental implants require excellent biocompatibility and high bonding strength. In this study, we attempted to fabricate biocompatible zirconia ceramics through surface modification by hydroxyapatite (HA) slurry coating. A hydroxyapatite slurry for spin coating was prepared using two sizes of hydroxyapatite particles. The hydroxyapatite slurry was obtained by adjusting the solid loading, pH range, and dispersant content. The surface roughness of the HA-coated layers on the zirconia substrate depended on the change in microstructural evolution and coating thickness. With repeated coating, the coating thickness gradually increased for both small and large particles. The specimen with two coatings had the maximum surface roughness but displayed different values depending on the size of the HA particles. High surface roughness (Ra; 0.49 µm) could be obtained from the slurry of small particles compared with that of the large particles (Ra; 0.35 µm). During a 14 days in vitro experiment in SBF solution at pH 7.4, no changes were observed in the surface microstructure of the HA coating layer on the zirconia substrate.

Sol-Gel Coating of Hydroxyapatite on Zirconia Substrate.

The zirconia used in dental implants requires excellent mechanical and chemical properties such as high strength, high biological performance, corrosion resistance, and phase stability. In this study, after we prepared a highly fluidized solution of calcium phosphate, we fabricated a hydroxyapatite (HA) coating layer on a zirconia substrate using the sol-gel method to enhance its biocompatibility and bone-bonding ability. We dipped the zirconia substrate into the calcium phosphate sol to obtain the HA-coated film, which was dried at room temperature. The phase change and microstructural evolution were examined while the coating dried and during heat treatment. The biological activity of the coated and as-received substrates was evaluated using an in vitro experiment and the results were compared. The HA-coated film showed a highly dense and uniform layer structure, while its physical and biological properties depended on the starting substrate, coating times, and processing conditions.

Improvement of the stability of hydroxyapatite through glass ceramic reinforcement.

Hydroxyapatite has achieved significant application in orthopedic and dental implants due to its excellent biocompatibility. Sintered hydroxyapatites showed significant dissolution, however, after their immersion in water or simulated body fluid (SBF). This grain boundary dissolution, even in pure hydroxyapatites, resulted in grain separation at the surfaces, and finally, in fracture. In this study, hydroxyapatite ceramics containing apatite-wollastonite (AW) or calcium silicate (SG) glass ceramics as additives were prepared to prevent the dissolution. AW and SG glass ceramics were added at 0-7 wt% and powder-compacted uniaxially followed by firing at moisture conditions. The glass phase was incorporated into the hydroxyapatite to act as a sintering aid, followed by crystallization, to improve the mechanical properties without reducing the biocompatibility. As seen in the results of the dissolution test, a significant amount of damage was reduced even after more than 14 days. TEM and SEM showed no decomposition of HA to the secondary phase, and the fracture toughness increased, becoming even higher than that of the commercial hydroxyapatite.

Effect of Powder Characteristics on Slip Casting Fabrication of Dental Zirconia Implants.

Dense zirconia compacts were fabricated by slip casting and sintering of nanoscale zirconia powders, and the effect of the powder characteristics (crystallite size, specific surface area, yttria content, and agglomeration) on the slurry and sintered properties was investigated. Three types of commercial 3 mol% yttria-stabilized tetragonal zirconia polycrystals powders were used as the starting powders after the powder characteristic analysis. A zirconia slurry for slip casting was prepared by mixing zirconia powder (solid loading of 60, 65, and 70 wt.%), distilled water, and a dispersant of Darvan C. The green compacts obtained from slip casting were cold isostatic pressed to enhance the close packing and densified by sintering at 1450 °C for 2 h. Highly dense zirconia compacts with a relative density of 99.5% and grain size of 350 nm were obtained based on the powder type and solid loading in the slurry. The microstructure and mechanical hardness of the sintered specimen after slip casting were dependent on the yttria content in the 3 mol% yttria-stabilized tetragonal zirconia polycrystal powder and the solid loading within the slurry.

Fabrication of an All-Ceramic Implant by Slip Casting of Nanoscale Zirconia Powder.

Dental implants are typically composed of 3Y-TZP (3 mol% yttria-stabilized tetragonal zirconia polycrystals). Most dental zirconia implants are currently fabricated via mechanical machining. However, during the machining of zirconia green bodies, many cracks form on the surface. To prevent surface crack formation on the implants, shape forming of the zirconia is necessary using methods such as slip casting. Herein, we fabricated green compacts using slip casting, candidate forming process to reduce surface cracking. To fabricate an optimal 3Y-TZP implant by slip casting and sintering, we prepared a suitable 3Y-TZP slurry for slip casting by adjusting the viscosity via pH, dispersant agent content, and solid loading refinement. Green compacts were prepared by the slip casting of all-ceramic zirconia implants fabricated using optimal slurry conditions, for example, 60 wt% solid content, 1 wt% dispersant, pH 12 and post-sintering at 1450 °C for 2 h. All sintered bodies contained a tetragonal phase with a high sintered density of approximately 6.07 g/cm³, good mechanical hardness of approximately 1367 Hv, grain size of 220 nm, and high surface roughness without cracks.

Influence of Processing Factors on the Surface Properties of Zirconia Coatings Fabricated by Room Temperature Spray Process.

Highly roughened surfaces on dental implants enhance the bone-bonding ability and in vivo cell adhesion on the implant surface. In this study, zirconia substrates were coated by powder coating using room temperature spray processing to improve their surface properties. Processing factors (particle size of the starting powder, number of repetitions of the deposition cycle, and spraying distance) were controlled to form a dense coating layer with high surface roughness on the zirconia substrate. Starting zirconia powders for coating were heat-treated at high temperature to control the particle size and kinetic energy. The coating layer fabricated from starting powder with a particle size of about 1.52 µm shows a homogeneous and dense microstructure, and it has a maximum surface roughness about 0.37 µm. The surface roughness of the film coatings increased with the number of times that the deposition cycle was repeated. No phase changes between the starting powder and the coating layer were observed, and all of the materials show identical tetragonal phases.