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Hydroxyapatite (HAp), a type of calcium phosphate found in bones and teeth, exhibits proton conductivity. In this study, its conductivity is investigated by measuring the complex permittivity of polycrystalline hydroxyapatite samples obtained via a sintering process. The dielectric dispersions measured from samples with different grain sizes are successfully explained using the Maxwell-Wagner interfacial polarization model. The results show that polycrystalline hydroxyapatite consists of both a high impedance grain boundary and a proton-conductive grain that has a conductivity of about 1.0 × 10− 7 S cm− 1 at 573 K. The interfacial polarization is confirmed using thermally stimulated depolarization current measurements. Additionally, electrets are prepared with the polycrystalline HAp via a poling process, and it is demonstrated that the electret formation is caused by the interfacial polarization.

The objective of this study was to assess the influence of alumina sandblasting on the flexural strength of the latest generation of highly translucent yttria partially stabilized dental zirconia (Y-PSZ). Fully-sintered zirconia disk-shaped specimens (14.5-mm diameter; 1.2-mm thickness) of four Y-PSZ zirconia grades (KATANA HT, KATANA STML, KATANA UTML, all Kuraray Noritake; and Zpex Smile, Tosoh) were sandblasted at 0.2 MPa with 50-μm alumina (Al2O3) sand (Kulzer) or left as-sintered (control). For each zirconia grade, the yttria (Y2O3) content was determined using X-ray fluorescence (XRF). Surface roughness was assessed using 3D confocal laser microscopy. Micro-Raman spectroscopy (μ-Raman) and X-ray diffraction (XRD) were used to assess potentially induced residual stresses. Biaxial flexural strength (n = 20) was statistically compared by Weibull analysis. Focused ion beam - scanning electron microscopy (FIB/SEM) was used to observe the subsurface microstructure. Fracture surfaces after biaxial flexural strength testing were observed by SEM. KATANA UTML had the highest Y2O3 content (6 mol%), followed by KATANA STML and Zpex Smile (5 mol%), and KATANA HT (4 mol%). Al2O3-sandblasting significantly increased surface roughness of KATANA UTML and Zpex Smile. μRaman and XRD revealed the presence of residual compressive stress on all Al2O3-sandblasted surfaces. FIB/SEM revealed several sub-surface microcracks in the sandblasted specimens. Weibull analysis revealed that Al2O3-sandblasting increased the characteristic strength of KATANA HT, KATANA STML, whereas it decreased the strength of KATANA UTML. The strength enhancement after Al2O3-sandblasting of KATANA HT was the highest, followed by KATANA STML. For Zpex Smile, the influence was statistically insignificant. The impact of Al2O3-sandblasting on the Weibull modulus was controversial. The strength of zirconia after Al2O3-sandblasting is determined by the balance between microcrack formation (decreased strength) and surface compressive stress build-up (increased strength).

In this paper, we examine the power generation performance of a rotational electret power generator theoretically and experimentally using electrostatic induction based on the properties of hydroxyapatite (HAp) ceramic electrets; the two surfaces of the electrets have opposing polarities. The HAp electrets have long-term stability in comparison with polymer electrets, which are more commonly used. The theoretical power output is proportional to the square of the total surface area of electrets per block of the stator, frequency of the rotor, and surface electric potential of the electret. Moreover, the power output is proportional to the square of the inverse of the air gap between the rotor and electrets. The behavior of power outputs obtained under different experimental conditions correlated with our theoretical prediction. The maximum output power generated by our prototype electret generator was 3.6 μW with the frequency of the rotor being 24 Hz.

We studied theoretically and experimentally the effects of the surface electric field generated by polarization and polyacrylic acid (PAA) additives on the heterogeneous nucleation of calcite on the calcite single crystal substrates with (10.4), (10.0) and (00.1) orientations. A set of “in-situ” experiments with optical microscopy was performed to determine the waiting time of CaCO3 nucleation, defined as the time interval between the onset of the diffusion of CO2 and the appearance of the first visible precipitation. Calcite was nucleated on the oriented calcite substrates through diffusion of NH3 and CO2 gas from a solid ammonium carbonate into calcium chloride solutions. A theoretical analysis showed that the surface electric field of the polarized calcite substrate decrease the activation energy for nucleation and consequently promotes nucleation. Experimentally, the surface electric field and PAA addition were found to decrease both contact angles and waiting times, and as a result, promote the heterogeneous nucleation. Combined effect of PAA and surface electric field further reduced contact angles and waiting times regardless of orientation differences of the calcite substrates. The cooperation acts remarkably on N-surface of the respective calcite substrates. The results were explained by the Cassie׳s equation, a classical heterogeneous nucleation theory under a surface electric field, and matching of the charged sites on the PAA chain with the ion arrangement on the calcite substrate.

Thermally stimulated depolarization current (TSDC) measurements were performed on 3 mol% and 8 mol% yttria stabilized zirconia ceramics (3YSZ and 8YSZ), in order to observe defect associations. Several TSDC peaks were observed, and the characteristics suggested that the peaks were attributed to space charge polarization due to long range migration of oxygen vacancies and electric dipole polarization due to trimeric defect association of Y at Zr site (Y Zr ') and oxygen vacancy (Vo). The activation energies for the dipole polarizations were different between 3YSZ and 8YSZ. The difference was caused by the difference of the structure of defect association. In addition, in 3YSZ, peaks with high activation energy of 1.24 eV were observed. The high activation energy suggests the presence of oxygen migration across Y–Y edges, in other word, indicating that Y–Y pairs exist in 3YSZ.

We demonstrated the crystallization of calcium carbonate (CaCO3) on polarized calcite single crystal substrates with three different orientations characterized by the (10.0), (00.1), and (10.4) planes in the presence of polyaspartic acid (PAsp) as an organic additive. The precipitates had a multiple phase structure with thin-film-like and hemispherical forms and were a mixture of calcite, aragonite, and vaterite. The main component of the thin-film-like layers was calcite elongated along the c-axis, regardless of the differently oriented calcite substrates, while the main component of the hemispherical layers was aragonite. The individual crystals of each CaCO3 polymorph shaped the morphology of the mesocrystals. Calcite-aragonite complex films with two distinct structures are similar to those found in pearls and bivalve shells. The combined effect of a surface electric field by virtue of the polarized calcite substrates and PAsp promoted the formation of the thin-film-like layers and moreover, acted remarkably on the negatively charged surface of each oriented calcite substrate. The matching between the β-sheets of PAsp and the calcium-ion arrangement on top of the oriented calcite substrates explained the preferential calcite crystallization in the form of the thin-film-like layers and why calcite precipitates are in the following sequence: (00.1)>(10.4)>(10.0).

New tissue engineering techniques based on embryonic stem (ES) cells and artificial scaffolds are required for regenerative medicine. Because artificial scaffolds can regulate the differentiation states of ES cells, compatibility between the cells and artificial scaffolds is important. To our knowledge, this study is the first report showing that mouse ES (mES) cells can be maintained in undifferentiated state on hydroxyapatite coated with gelatin. In contrast to previous studies, our culture medium was serum-free and included a GSK-3 inhibitor. Under these conditions, mES colony morphology was similar to that of an undifferentiated state; mES cells expressed the pluripotent-specific factors Oct-3/4 and Nanog, and they maintained the ability to differentiate into the three germ layers. Moreover, a GSK-3 inhibitor blocked the expression of integrin subunits that bind to laminin which are known to induce the differentiation of mES cells. These findings indicate that mES cells can be cultured under serum- and feeder-free conditions and maintained in an undifferentiated state on a composite with hydroxyapatite and that this composite can be used to control the differentiation of stem cells.

Micro-arc oxidation (MAO) is one of the useful surface modifications of titanium implants to improve bioactivity. Also, electric polarization treatment enhances bioactivity of calcium phosphate. The aim of this study was to evaluate the effect of the combination of two surface modifications, micro-arc oxidation (MAO) with electric polarization, on the behavior of osteoblast-like osteosarcoma MG63 cells. MAO-treated materials had a surface geometry that was favored by MG63 cells as determined using scanning electron microscopy and X-ray diffraction; additionally, electric polarization induced surface electric fields, which were measured using thermally stimulated depolarization currents. The results of assays to study cell–material interactions suggest that these two approaches could regulate cell attachment, spreading, proliferation, and differentiation without the addition of other reagents. This new surface modification processes produce materials with a good surface geometry, generate surface electric fields and enhance the osteopromotive ability of osteoblasts.