Functionalization of microstructured open-porous bioceramic scaffolds with human fetal bone cells.

Bone substitute materials allowing trans-scaffold migration and in-scaffold survival of human bone-derived cells are mandatory for development of cell-engineered permanent implants to repair bone defects. In this study, we evaluated the influence on human bone-derived cells of the material composition and microstructure of foam scaffolds of calcium aluminate. The scaffolds were prepared using a direct foaming method allowing wide-range tailoring of the microstructure for pore size and pore openings. Human fetal osteoblasts (osteo-progenitors) attached to the scaffolds, migrated across the entire bioceramic depending on the scaffold pore size, colonized, and survived in the porous material for at least 6 weeks. The long-term biocompatibility of the scaffold material for human bone-derived cells was evidenced by in-scaffold determination of cell metabolic activity using a modified MTT assay, a repeated WST-1 assay, and scanning electron microscopy. Finally, we demonstrated that the osteo-progenitors can be covalently bound to the scaffolds using biocompatible click chemistry, thus enhancing the rapid adhesion of the cells to the scaffolds. Therefore, the different microstructures of the foams influenced the migratory potential of the cells, but not cell viability. Scaffolds allow covalent biocompatible chemical binding of the cells to the materials, either localized or widespread integration of the scaffolds for cell-engineered implants.

Dealloying of platinum-aluminum thin films: dynamics of pattern formation.

The application of focused ion beam (FIB) nanotomography and Rutherford backscattering spectroscopy (RBS) to dealloyed platinum-aluminum thin films allows for an in-depth analysis of the dominating physical mechanisms of nanoporosity formation during the dealloying process. The porosity formation due to the dissolution of the less noble aluminum in the alloy is treated as result of a reaction-diffusion system. The RBS and FIB analysis yields that the porosity evolution has to be regarded as superposition of two independent processes, a linearly propagating diffusion front with a uniform speed and a slower dissolution process in regions which have already been passed by the diffusion front. The experimentally observed front evolution is captured by the Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP). The slower dissolution is represented by a zero-order rate law which causes a gradual porosity in the thin film.

Controlling phase distributions in macroporous composite materials through particle-stabilized foams.

Aqueous foams stabilized by ceramic and thermoplastic polymeric particles provide a general method for producing novel porous materials because their extraordinary stability against disproportionation and drainage allows them to be dried and sintered into solid materials. Here, we report the different microstructures that can be obtained from liquid foams stabilized by binary mixtures of particles when the interfacial energies between the particles and the air-liquid interfaces are manipulated to promote either preferential or competitive self-assembly of the particles at the foam interface. Modification of the interfacial energies was accomplished through surface modification of the particles or by decreasing the surface tension of the aqueous phase. Materials derived from liquid foams stabilized by poly(vinylidene fluoride) (PVDF) and alumina (Al(2)O(3)) particles are investigated. However, as is shown, the method can be extended to other polymeric and ceramic particles and provides the possibility to manufacture a wide range of porous composite materials.

Unifying model for the electrokinetic and phase behavior of aqueous suspensions containing short and long amphiphiles.

Aqueous suspensions containing oppositely charged colloidal particles and amphiphilic molecules can form fluid dispersions, foams, and percolating gel networks, depending on the initial concentration of amphiphiles. While models have been proposed to explain the electrokinetic and flotation behavior of particles in the presence of long amphiphilic molecules, the effect of amphiphiles with less than six carbons in the hydrocarbon tail on the electrokinetic, rheological, and foaming behavior of aqueous suspensions remains unclear. Unlike conventional long amphiphiles (≥10 carbons), short amphiphiles do not exhibit increased adsorption on the particle surface when the number of carbons in the molecule tail is increased. On the basis of classical electrical double layer theory and the formerly proposed hemimicelle concept, we put forward a new predictive model that reconciles the adsorption and electrokinetic behavior of colloidal particles in the presence of long and short amphiphiles. By introducing in the classical Gouy-Chapman theory an energy term associated with hydrophobic interactions between the amphiphile hydrocarbon tails, we show that amphiphilic electrolytes lead to a stronger compression of the diffuse part of the electrical double layer in comparison to hydrophilic electrolytes. Scaling relationships derived from this model provide a quantitative description of the rich phase behavior of the investigated suspensions, correctly accounting for the effect of the alkyl chain length of short and long amphiphiles on the electrokinetics of such colloidal systems. The proposed model contributes to our understanding of the stabilization mechanisms of particle-stabilized foams and emulsions and might provide new insights into the physicochemical processes involved in mineral flotation.

Engineering disorder in precipitation-based nano-scaled metal oxide thin films.

Distinctive microstructure engineering of amorphous to nanocrystalline functional metal oxide thin films for MEMS devices is of high relevance to allow for new applications, quicker response times, and higher efficiencies. Precipitation-based thin-film techniques are first choice. However, these often involve organic solvents in preparation. Their relevance on the disorder states of amorphous to fully crystalline metal oxides is unclear, especially during crystallization. In this study the impact of organic solvents on the as-deposited amorphous state and crystallization of the metal oxide, CeO(2), is reported for thin-film preparation via the precipitation-based method spray pyrolysis. The choice of organic solvent for film preparation, i.e. glycols of different chain lengths, clearly affects the structural packing and Raman bond length of amorphous states. Organic residues act as space fillers between the metal oxide molecules in amorphous films and affect strongly the thermal evolvement of microstructure, i.e. microstrain, crystallization enthalpy, crystallographic density, grain size during crystallization and grain growth. Once the material is fully crystalline, equal near- and long-range order characteristics result independent of organic solvent choice. However, the fully crystalline films still show decreased crystallographic densities, presence of microstrain, and lower Raman shifts compared to microcrystalline bulk material. The defect state of amorphous and fully crystalline thin-film microstructures can actively be modified via explicit use of organic glycols with different chain lengths for metal oxide films in MEMS.

Electrical conductivity and crystallization of amorphous bismuth ruthenate thin films deposited by spray pyrolysis.

Amorphous oxide thin films with tailored functionality will be crucial for the next generation of micro-electro-mechanical-systems (MEMS). Due to potentially favorable electronic and catalytic properties, amorphous bismuth ruthenate thin films might be applied in this regard. We report on the deposition of amorphous bismuth ruthenate thin films by spray pyrolysis, their crystallization behavior and electrical conductivity. At room temperature the 200 nm thin amorphous films exhibit a high electrical conductivity of 7.7 × 10(4) S m(-1), which was found to be slightly thermally activated (E(a) = 4.1 × 10(-3) eV). It follows that a long-range order of the RuO(6) octahedra is no precondition for the electrical conductivity of Bi(3)Ru(3)O(11). Upon heating to the temperature range between 490 °C and 580 °C the initially amorphous films crystallize rapidly. Simultaneously, a transition from a dense and continuous film to isolated Bi(3)Ru(3)O(11) particles on the substrate takes place. Solid-state agglomeration is proposed as the mechanism responsible for disintegration. The area specific resistance of Bi(3)Ru(3)O(11) particles contacted by Pt paste on gadolinia doped ceria electrolyte pellets was found to be 7 Ω cm(2) at 607 °C in air. Amorphous bismuth ruthenate thin films are proposed for application in electrochemical devices operating at low temperatures, where a high electrical conductivity is required.

General route for the assembly of functional inorganic capsules.

Semipermeable, hollow capsules are attractive materials for the encapsulation and delivery of active agents in food processing, pharmaceutical and agricultural industries, and biomedicine. These capsules can be produced by forming a solid shell of close packed colloidal particles, typically polymeric particles, at the surface of emulsion droplets. However, current methods to prepare such capsules may involve multistep chemical procedures to tailor the surface chemistry of particles or are limited to particles that exhibit inherently the right hydrophobic-hydrophilic balance to adsorb around emulsion droplets. In this work, we describe a general and simple method to fabricate semipermeable, inorganic capsules from emulsion droplets stabilized by a wide variety of colloidal metal oxide particles. The assembly of particles at the oil-water interface is induced by the in situ hydrophobization of the particle surface through the adsorption of short amphiphilic molecules. The adsorption of particles at the interface leads to stable capsules comprising a single layer of particles in the outer shell. Such capsules can be used in the wet state or can be further processed into dry capsules. The permeability of the capsules can be modified by filling the interstices between the shell particles with polymeric or inorganic species. Functional capsules with biocompatible, bioresorbable, heat-resistant, chemical-resistant, and magnetic properties were prepared using alumina, silica, iron oxide, or tricalcium phosphate as particles in the shell.

Quantification of the heterogeneity of particle packings.

The microstructure of coagulated colloidal particles, for which the interparticle potential is described by the Derjaguin-Landau-Verweg-Overbeek theory, is strongly influenced by the particles' surface potential. Depending on its value, the resulting microstructures are either more "homogeneous" or more "heterogeneous," at equal volume fractions. An adequate quantification of a structure's degree of heterogeneity (DOH), however, does not yet exist. In this work, methods to quantify and thus classify the DOH of microstructures are investigated and compared. Three methods are evaluated using particle packings generated by Brownian dynamics simulations: (1) the pore size distribution, (2) the density-fluctuation method, and (3) the Voronoi volume distribution. Each method provides a scalar measure, either via a parameter in a fit function or an integral, which correlates with the heterogeneity of the microstructure and which thus allows to quantitatively capture the DOH of a granular material. An analysis of the differences in the density fluctuations between two structures additionally allows for a detailed determination of the length scale on which differences in heterogeneity are most pronounced.

Fatigue of zirconia under cyclic loading in water and its implications for the design of dental bridges.

OBJECTIVES: The aim of this study was to evaluate the cyclic fatigue behavior of zirconia (3Y-TZP) in water and derive guidelines for the design of zirconia-based dental bridges with extended lifetime. METHODS: The subcritical crack growth parameters under aqueous and cyclic loading conditions were determined from Weibull distributions of the initial mechanical strength and of the lifetime of TZP specimens. The strength and lifetime data were obtained using a specially designed bending machine under simple and oscillatory loading conditions, respectively. RESULTS: The TZP components submitted to cyclic loading in water exhibited subcritical crack propagation at stress levels significantly ( approximately 50%) lower than the critical stress intensity factor (K(IC)=5.6 MPam(1/2)). In spite of this susceptibility to subcritical crack growth, calculations based on the fatigue parameters and on the stress applied on the prosthesis indicate that posterior bridges with zirconia frameworks can exhibit lifetimes longer than 20 years if the diameter of the bridge connector is properly designed. SIGNIFICANCE: This in vitro study indicates that partially stabilized zirconia (3Y-TZP) can withstand the severe cyclic loading and wet conditions typically applied in the molar region of the mouth and is therefore an appropriate material for the fabrication of all-ceramic multi-unit posterior bridges.

Mechanical and fracture behavior of veneer-framework composites for all-ceramic dental bridges.

OBJECTIVES: High-strength ceramics are required in dental posterior restorations in order to withstand the excessive tensile stresses that occur during mastication. The aim of this study was to investigate the fracture behavior and the fast-fracture mechanical strength of three veneer-framework composites (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) for all-ceramic dental bridges. METHODS: The load bearing capacity of the veneer-framework composites were evaluated using a bending mechanical apparatus. The stress distribution through the rectangular-shaped layered samples was assessed using simple beam calculations and used to estimate the fracture strength of the veneer layer. Optical microscopy of fractured specimens was employed to determine the origin of cracks and the fracture mode. RESULTS: Under fast fracture conditions, cracks were observed to initiate on, or close to, the veneer outer surface and propagate towards the inner framework material. Crack deflection occurred at the veneer-framework interface of composites containing a tough framework material (TZP/Cercon S and Inceram-Zirconia/Vita VM7), as opposed to the straight propagation observed in the case of weaker frameworks (Empress 2/IPS Eris). SIGNIFICANCE: The mechanical strength of dental composites containing a weak framework (K(IC)<3 MPam(1/2)) is ultimately determined by the low fracture strength of the veneer layer, since no crack arresting occurs at the veneer-framework interface. Therefore, high-toughness ceramics (K(IC)>5 MPam(1/2)) should be used as framework materials of posterior all-ceramic bridges, so that cracks propagating from the veneer layer do not lead to a premature failure of the prosthesis.

Cyclic fatigue in water of veneer-framework composites for all-ceramic dental bridges.

OBJECTIVES: Ceramic materials applied in dentistry may exhibit significant subcritical crack growth due to the severe cyclic loading in the aqueous environment encountered in the mouth during mastication. The authors report on the subcritical crack growth behavior of three dental restoration systems (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) under cyclic loading in water, in order to establish guidelines for the use and design of long-lifetime all-ceramic posterior bridges. METHODS: Inert strength and lifetime tests under cyclic loading in an aqueous environment were performed in a mechanical bending apparatus and evaluated with Weibull statistics. RESULTS: Subcritical crack growth occurred predominantly in the outer veneer layer of the veneer-framework composites. The apatite-based veneer (IPS Eris) was more susceptible to subcritical crack propagation compared to the feldspathic glass veneers (Cercon S and Vita VM7). SIGNIFICANCE: Dental restoration systems containing apatite-based veneers and weak frameworks (Empress 2/IPS Eris) are not recommended for the fabrication of all-ceramic bridges in the molar region. Conversely, veneer-framework systems consisting of feldspathic glass veneers and tough zirconia-based frameworks (TZP/Cercon S and Inceram-Zirconia/Vita VM7) may exhibit lifetimes longer than 20 years if the bridge connector is properly designed.

Prospective clinical study of zirconia posterior fixed partial dentures: 3-year follow-up.

OBJECTIVES: The purpose of this prospective clinical cohort study was to determine the success rate of 3- to 5-unit posterior fixed partial dentures (FPDs) with zirconia frameworks after 3 years of function. METHOD AND MATERIALS: Forty-five patients in need of at least 1 FPD to replace 1 to 3 posterior teeth were included. The frameworks were produced by means of a prototype computer-assisted manufacture system. They were milled with a precisely calculated increase in size out of presintered zirconia blanks and subsequently shrunk to the required size. Fifty-seven FPDs were cemented using either Variolink or Panavia TC cement. Clinical and radiographic examinations were performed at baseline, 12, 24, and 36 months after cementation. Statistical analysis was performed by descriptive statistics and the Kaplan-Meier survival analysis. Comparisons of probing depth, Plaque Index, and bleeding on probing between test (abutment) and control (contralateral) teeth were done with the McNemar test. RESULTS: Thirty-six patients with 46 FPDs were available for examination after 36 months. No fractures occurred, rendering a 100% success rate of the zirconia frameworks. Seven FPDs had to be replaced because of biologic and technical problems. The survival rate, therefore, was 84.8%. Secondary caries was found in 10.9% of the FPDs, and chipping of the veneering ceramic was found in 13.0%. There were no significant differences regarding the probing depth in test and control teeth. CONCLUSION: Zirconia frameworks demonstrated sufficient stability for replacement of posterior teeth. However, the high rates of technical problems should be reduced by further developments of the prototype processing technology.

Lysozyme and bovine serum albumin adsorption on uncoated silica and AlOOH-coated silica particles: the influence of positively and negatively charged oxide surface coatings.

The adsorption of lysozyme and bovine serum albumin on silica and AlOOH-coated silica particles-representing negatively and positively charged oxide surfaces-was investigated. The protein-treated uncoated and completely AlOOH-coated silica particles were characterized by zeta potential analysis and UV/VIS spectroscopy. It was found that at pH 7 a protein oppositely charged to the oxide surface adsorbs in significantly higher amounts. In contrast, proteins of the same charge did not or only in very low amounts adsorbed on an oxide surface. As both oxide surfaces were measured to be very hydrophilic it can be concluded that electrostatic interactions dominate the adsorption process at the investigated experimental conditions. The pH regions where the proteins interact via attractive and repulsive coulomb interaction with the particular oxide surfaces were calculated and outlined.

Strength and reliability of four-unit all-ceramic posterior bridges.

OBJECTIVES: The purpose of this study was to determine the in vitro load bearing capacity of four-unit posterior frameworks made of glass ceramic with lithium-disilicate crystals (E2), of zirconia-reinforced glass-infiltrated alumina (ICZ) and of zirconia stabilized with 3 mol% yttria (CEZ). METHODS: All frameworks mimicked a four-unit posterior situation with 7.3 mm2 interdental cross-sections and possessed exactly the same dimensions. The load bearing capacity was measured on a special bridge test setup with 15 specimens for each of the materials. The data were analyzed with Weibull statistics giving the characteristic load bearing capacity F0 at 63% failure probability and the Weibull modulus m as indicator for the reliability and reproducibility. RESULTS: For the E2 frameworks the average load bearing capacity and the SD was 260 (+/-53) N, the characteristic load F0 282 N and the reliability m = 5.7. For the ICZ frameworks the average load bearing capacity was 470 (+/-101) N, F0 518 N and m = 4.5. CEZ frameworks revealed the highest average load bearing capacity of 706 (+/-123) N, the highest characteristic load bearing capacity F0 = 755 N and the best reliability m = 7.0. SIGNIFICANCE: CEZ frameworks showed the best mechanical properties as demonstrated by the high values of average load bearing capacity, reliability and characteristic load bearing capacity with respect to the other ceramics studied. However, for four-unit posterior CEZ frameworks the connector size of 7.3 mm2 is insufficient to withstand occlusal forces reported in the literature. Four-unit posterior frameworks require a connector size larger than 7.3 mm2.

Relation between microstructure and mechanical behavior of concentrated silica gels.

We produce concentrated (40 vol%) gels of uniformly sized silica particles by an in situ process, based on the enzyme-catalyzed hydrolysis of urea in the liquid phase of electrostatically stabilized suspensions. Two different methods are used: Either the pH of the suspensions is shifted toward the isoelectric point of the particles (delta pH method), or the ionic strength is continuously increased at constant pH (deltaI method). We compare the two kinds of gels in terms of elastic and yield behavior as well as microstructure by using rheological measurements in oscillation and high-pressure freezing in combination with cryo-SEM, respectively. Results suggest a strong increase of elastic and yield properties in concentrated particle gels with decreasing homogeneity of their microstructures.

Small-angle static light scattering of concentrated silica suspensions during in situ destabilization.

The aggregation of concentrated aqueous silica suspensions is characterized by means of static light scattering. We use an in situ destabilization mechanism based on the enzyme-catalyzed hydrolysis of urea. This method enables us to continuously and homogeneously change the interparticle potential from repulsive to attractive without disturbing the aggregation process. Moreover, our electrostatically stabilized suspensions can be destabilized by two different methods. In the first method, the pH is shifted toward the isoelectric point of the particles ( Delta pH method), thereby leading to a decrease of their surface charge. In the second method, the ionic strength is continuously increased at constant pH ( Delta I method), leading to a compression of the electrical double layer around the charged particles. A laboratory-built flat-cell light-scattering instrument is used, which allows fast data acquisition and an adjustment of the sample cell thickness. To circumvent multiple scattering effects, we use a very small sample thickness ( approximately 13 microm). In addition, the refractive index difference between the aqueous phase and the particles is reduced by adding sucrose to the liquid phase of our suspensions. We are able to characterize the structural changes at the very early stages of the destabilization process, where no significant effects are yet detected in macroscopic rheological measurements. While during the Delta pH destabilization, the scattering curve shows significant changes only after some characteristic delay time, it changes continuously during the Delta I destabilization. The latter is attributed to the formation of a weak pre-gel structure in the suspensions, as a shallow secondary minimum appears in the interparticle potential. Data are evaluated by using a HMSA square-well structure factor model. Results are in good agreement with those predicted from DLVO theory.

Quantification of microstructures in stable and gelated suspensions from Cryo-SEM.

It is shown that the microstructures of concentrated suspensions can be analyzed in a quantitative way from cryo-SEM images of high-pressure frozen samples, both in the electrostatically stabilized and in the flocculated state. Suspensions of spherical silica particles (40 vol%) in an aqueous solution were used. The average particle radius was 525 nm with a polydispersity of below 7%. The suspensions were high-pressure frozen, which resulted in a quenching of the configuration without apparent change in volume or crack formation. After fracturing the samples at liquid nitrogen temperature, the fracture surface was etched by controlled sublimation of the frozen aqueous phase, coated with 8 nm of platinum, and examined by stereo-cryo-SEM. The 3-dimensional positions of all the visible particles were determined from the SEM images. Assuming an isotropic particle configuration in the sample before cracking, it is possible to extract the 3-dimensional pair correlation function from the particle positions on the fracture surface. A comparison to recent results from Brownian Dynamics simulations shows good agreement between our experiments and the simulations.

Engineering macroporous composite materials using competitive adsorption in particle-stabilized foams.

The high absorption energies of partially wetted particles at fluid interfaces allow the production of macroporous composite materials from particle-stabilized foams. Competition between the different particle types determines how they are distributed in the foam lamella and allow the phase distribution to be controlled; a technique that is useful in the design and engineering of porous composites. Here, we report details on the effects of preferential and competitive adsorption of poly(vinylidene fluoride) (PVDF) and alumina (Al(2)O(3)) particles at the foam interfaces on the consolidated macroporous composite materials. By varying the relative composition and surface energies of the stabilizing particles, macroporous composite materials with a broad range of phase distributions are possible.

Contact angle and adsorption behavior of carboxylic acids on α-Al2O3 surfaces.

The hydrophilic character of aluminum oxide surfaces may be altered through coating such surfaces with carboxylic acids. The initially hydrophilic nature of the solid substrate changes towards a less hydrophilic character as the bulk concentration and the chain length of the acids increases. The acids employed in this work (propionic, valeric and enanthic) show a certain affinity to the liquid-gas, solid-liquid and solid-gas interfaces, being the relative adsorption on them competitive. The adsorption behavior of these carboxylic acids is experimentally investigated combining pendant drop tensiometry, contact angle measurements on α-Al(2)O(3) polycrystalline ceramics and adsorption on particles in aqueous suspensions, as a function of the hydrocarbon chain length of the acids and their bulk concentration, at pH equal to the acids' pKa. The hydrophilic character of the coated alumina decreases with the acids concentration upon a certain concentration beyond that, it increases. The minimum of hydrophilicity is reached right before bi-layer arrangements on the adsorption pattern of the acids on the solid substrates take place.