Chang Fu Han

Riboflavin-ultraviolet-A-induced collagen cross-linking treatments in improving dentin bonding.

OBJECTIVES To evaluate the collagen cross-linkers, riboflavin-ultraviolet-A (RF/UVA) and glutaraldehyde, with regard to their efficacy in cross-linking the dentinal collagen and improving dentin bonding. METHODS Glutaraldehyde and different RF/UVA protocols (0.1%RF/1-minUV, 0.1%RF/2-minUV, and 1%RF/1-minUV) were first evaluated by gel electrophoresis to determine their abilities of collagen cross-linking. The mechanical properties of acid-etched dentin receiving these cross-linking treatments were examined in either dry or wet condition by a nanoindentation test. Fifteen teeth with exposed occlusal dentin received the microtensile bond strength (μTBS) test. The teeth were primed either with RF/UVA or glutaraldehyde, followed by adhesive treatment and composite restorations, and then cut into resin-dentin microbeams. Half of the microbeams received the μTBS test after 24h, and the other half received test after 5000 thermocycles. Nanoleakage at the bond interface was examined under TEM. The alignments of collagen fibrils in the hybrid layers were also defined by an image analysis. RESULTS Gel electrophoresis showed that glutaraldehyde induced strong collagen gelation, while RF/UVA generated milder collagen cross-linking. Glutaraldehyde, 0.1%RF/2-min-UVA, and 1%RF/1-minUV showed higher stiffness compared to untreated and 0.1%RF/1-minUV in wet condition. All the crosslinking treatments improved early μTBS, but 0.1%RF/2-minUVA treatment maintained high μTBS after theromocycles. Under TEM, glutaraldehyde-treated dentin showed dense and enclosed collagen network on the adhesive interface. 0.1%RF/2-minUVA showed the least nanoleakage, and this could be associated with the suspended collagen fibrils in the hybrid layer. SIGNIFICANCE 0.1%RF/2-minUVA treatment enhanced resin-dentin bond possibly through enhancing the stiffness and maintaining the expanding collagen matrix in the hybrid layer.

Stress-strain analysis for evaluating the effect of the orientation of dentin tubules on their mechanical properties and deformation behavior.

A model whose porosity does not vary with compression depth is developed for evaluating the mechanical properties of dentin tubules with various orientation angles from micro-pillar nanocompression tests. Experimental results for a range of loading rates indicate that the yielding parameters and the elastic modulus are little affected by the creep behavior. For a given compression depth, the hardness, elastic modulus, and yielding strength decrease with increasing orientation angle of dentin. The mechanical properties obtained using the proposed model are consistent with the reported data, and are actually more precise since they consider the orientation angle. The proposed testing method can be applied to materials that yield a negative value of the elastic modulus due to creep behavior.

Fatigue Life Study of ITO/PET Specimens in Terms of Electrical Resistance and Stress/Strain Via Cyclic Bending Tests

Bending tester in reciprocating motion is adopted to study the fatigue resistance and fatigue life of indium tin oxide (ITO) films deposited on poly(ethylene terephthalate) (PET) substrate with various prestrain levels. A commercial micro-compression tester and ANSYS-Workbench software are used to determine the stress versus strain curves, and thus the strain energy of a fixed volume in the specimen. A bending tester without a cycle limitation is designed to carry out ITO-film fatigue life tests. Two points symmetric with respect to the specimens central line are set to measure the films electrical resistance variation with the number of cycles in reciprocating motion. The number of cycles in the bending tests corresponding to the fatigue life of ITO film is determined on the bases of both the strain at which 63% of the material fails and the electrical resistance which has 10% rise w.r.t. the initial one before bending test. The mean electrical resistance increases with increasing number of cycles and prestrain applied to the PET substrate. Of the four substrate prestrains tested (0%, 2%, 4%, and 6%), the change in electrical resistance between the beginning and end of the test was highest for the PET-6%/ITO specimen regardless of the maximum acceleration of the tester. With the same substrates prestrain and at the same maximum acceleration, the number of cycles for fatigue life predicted by these two models are fairly close. Increasing either the substrate prestrain or the jigs acceleration decreases the alternating stress and increases the strain during reciprocating motion, lowering the fatigue life of the film. The strain energy defined for a fixed volume in the specimen is found always asymptotic to a constant value. The number of cycles corresponding to the beginning of this constant strain energy is found close to those of fatigue cycles predicted by the models mentioned above. The strain energy method is thus provided as an effective way to predict the fatigue life of the specimens without fatigue limit in the curve.

Characterization of the elastic and viscoelastic properties of dentin by a nanoindentation creep test

Dentin is the main supporting structure of teeth, but its mechanical properties may be adversely affected by pathological demineralization. The purposes of this study were to develop a quantitative approach to characterize the viscoelastic properties of dentin after de- and re-mineralization, and to examine the elastic properties using a nanoindentation creep test. Dentin specimens were prepared to receive both micro- and nano-indentation tests at wet and dry states. These tests were repeatedly performed after demineralization (1% citric acid for 3 days) and remineralization (artificial saliva immersion for 28 days). The nanoindentation test was executed in a creep mode, and the resulting displacement-time responses were disintegrated into primary (transient) and secondary (viscous) creep. The structural changes and mineral densities of dentin were also examined under SEM and microCT, respectively. The results showed that demineralization removed superficial minerals of dentin to the depth of 400 μm, and affected its micro- and nano-hardness, especially in the hydrate state. Remineralization only repaired the minerals at the surface layer, and partially recovered the nanohardness. Both the primary the secondary creep increased in the demineralized dentin, while the hydration further enhanced creep deformation of untreated and remineralized dentin. Remineralization reduced the primary creep of dentin, but did not effectively increase the viscosity. In conclusion, water plasticization increases the transient and viscous creep strains of demineralized dentin and reduces load sustainability. The nanoindentation creep test is capable of analyzing the elastic and viscoelastic properties of dentin, and reveals crucial information about creep responses.

Effects of Prestrain Applied to a Polyethylene Terephthalate Substrate Before the Coating of Al-Doped ZnO Film on Film Quality, Electrical Properties, and Pop-In Behavior During Nanoindentation

Four kinds of polyethylene-terephthalate (PET)/Al-doped zinc oxide (AZO) specimen are prepared to examine the effects of prestrain applied to the PET substrate before the coating of the AZO film on the mechanical, material, and electrical properties, and the start of pop-in in the loading phase of nanoindentation. The electrical contact resistance function is used to measure the variations of electrical current during the nanoindentation process. With the aid of the stress-strain profile, the inflection point of the load-depth profile in the loading phase is identified as the start of pop-in, at or nearby which the electrical current sharply increases due to the significant increase in the indenter-film contact area. The pop-in depth decreases with increasing prestrain. The behavior demonstrated in the pop-in depth due to the change in the prestrain is exactly opposite to those of the quantity and the mean size of submicrometer voids/cracks. Increase in the quantity/size of film voids/cracks generally reduces the specimens carrier mobility. The carrier mobility is presented to be inversely proportional to the sheet resistance. The electrical current created at the end of the loading process has its value inversely proportional to the number/size of film voids/cracks.

Effects of sputtering-deposition inclination angle on the IGZO film microstructures, optical properties and photoluminescence

Abstract: IGZO/PET specimens were respectively prepared with 0°, 15°, 30°, 45°, and 60° as the inclination angle of the thin film deposition. The n-type conductivity was identified in these specimens. The following parameters, including the film thickness and morphologies of the top and lateral surfaces, the mechanical properties, chemical compounds and their primary lattices in X-ray diffraction (XRD) patterns, and the X-ray photoelectron spectrometries (XPS) for Ga2p, Ga3d, In3d, Zn2p, and O1s are presented. IR is defined as the intensity ratio of the InGaO3(ZnO)3 peak value to the sum of the peak values of InGaO3(ZnO)3 and InGaZnO4. Decompositions of the O1s and Ga3d spectra provide nearly Gaussian profiles of Ga-Ga, In4d, Ga-O, O1, O2, and O3. The O2 intensity ratio IRO2, which is defined as the ratio of O2 peak intensity to the peak intensity sum of O1 and O2, was evaluated for these five specimens. Depth profiles of the distributions of O, Zn, Ga, and In ions were obtained by secondary ion mass spectrometry (SIMS), and the slope (Go) of the O-ion profile in the decaying region was obtained as a function of inclination angle. Increases in the inclination angle can effectively reduce surface roughness. The peak intensities of Ga2p, Ga3d, In3d, Zn2p, and O1s formed in the specimen preparations with a nonzero inclination angle were always lower than those of the IGZO_0° specimen. Although an increase in inclination angle can raise the IRO2 value, a nonzero inclination angle yielded an IR value lower than that of the IGZO_0° specimen. Moreover, the IRO2 value increased with (negative) Go decrease as the response. The combined result of increasing IRO2 and the In-O and Ga-O bonds, and decreasing the IGZO film thickness can increase light transmission. Specimens with a larger surface roughness result in a higher reflection. The IR parameter for the specimens with a nonzero inclination angle has a value smaller than that of the IGZO_0° specimen; as such, an increase in IR is advantageous for increasing the n-type conductivity. The decreasing rate of extinction coefficient k with respect to wavelength in the visible light region increases significantly with increasing IRO2. IR and IRO2 are therefore the governing factors of the peak intensities for the three decomposed profiles of micro-photoluminescence (PL). Increasing IR and IRO2 or decreasing film thickness is advantageous for increasing the PL peak intensities. Increases in the product values of IR and IRO2 are favorable for increasing the peak intensities of violet and ultraviolet.

Nano-structure and nano-mechanical properties of human teeth

In this study, we investigate the nanostructure and nano-mechanical properties of human teeth. A nano-indenter is adopted to acquire the nano-mechanical properties of teeth, while a focused ion beam system (FIB) is used to prepare high quality transmission electron microscopy specimen. For nanomechanical properties, the tooth specimens are cut into horizontal and vertical sections separately within 1mm thick. All the sections contain dentin, dentinenamel junction (DEJ) and enamel. The hardness and Youngs modulus of each compartment are systematically measured, both showing an increasing trend from the dentin to the enamel. As to the preparation for transmission electronic microscope (TEM) specimen, the teeth are cut into 50 nm thickness. High resolution images and diffraction patterns reveal that dentin is poly-crystallized and present anisotropic properties.

Use of Nanoindentation for Investigating the Nanostructure of Dentin Tissue

Human dentin contains millions of tubules, and the distribution and size of tubule is none-homogeneous, which resulting in various mechanical properties. In this study, we utilize a nanoindentor on the human dentin, and a focused ion-beam (FIB) is adopted to prepare the dentin specimen for transmission electron microscopy (TEM) to evaluate the nanostructure of human dentin after the residual impression of the nanoindents. The nanoindentation results show that the loading–unloading curve is continuous and smooth in both the loading and the unloading steps, which suggests that no cracking occurs. As to the TEM specimen, 50 nm thick specimens are prepared by FIB cross-sectioning. The nano-structural observations reveal that nanoindentation induces an atomic re-orientation, and the indent results in the formation of highstress deformation regions beneath the indenter. High resolution images via TEM and diffraction patterns indicate the dentin is poly-crystallized and present anisotropic properties.