Pal Jen Wei

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.

Adhesion Forces and Contact Angles of Water Strider Legs

This study investigated the adhesion (pull-off) force and contact angles of a water striders leg. During hydrostatic experiments, the adhesion force was found to be 2 dyn. The image of a cross section of a live leg contacted with a deformed water surface provided the contact angle of 168.8 degrees . A numerical scheme was proposed to determine the water surface on a groove wall of a seta. The results showed that the asperities of a seta are almost wetted, and the fraction of the wetted projection area was 0.69. Thus, the contact angle of a seta was 124.8 degrees .

A new method for determining the strain energy release rate of an interface via force-depth data of nanoindentation tests

Indentation forces, including constant rate and oscillating mode, were applied to SiO(2)/Si and diamond-like carbon (DLC)/Si specimens. A two-stage behavior was exhibited in the force-depth results after delamination occurred. When the depth was smaller than the threshold value, a linear load-depth relationship was exhibited because the debonded film was suspended over the substrate. Membrane theory was applied to analyze the deflection of the suspended film, and thus the in-plane stress exhibited in the debonded film was evaluated. Through the proposed method, the strain energy release rate of the interface can be directly evaluated by analyzing the force-depth data of the indentation tests.

Characteristics of water strider legs in hydrodynamic situations.

This study investigated the forces and the cross-section images of a water striders leg through experimental observations. In the vertical direction, the spring coefficients were found to be 0.6 N/m for the leg and 0.3 N/m for the water, which provide a water-treading stiffness of 0.2 N/m. In the horizontal directions, besides the alignment of the microsetae, the large cuticle forces were also related to the resistant side in a Wenzel state.

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.

Electric contact resistance for monitoring nanoindentation-induced delamination

This study applied an in situ electric contact resistance technique to monitor delamination induced by indentation loads. A suddenly increasing indentation depth, together with a simultaneous drop in monitoring contact current, suggests that delamination occurred. During unloading processes, the rapid decrease in both contact depth and current imply that the delaminated film was suspended as long as the indentation load became sufficiently small. When delamination occurred during oscillating processes, the contact current was found to drop from an initial value to a steady value, which is related to a loss of interfacial contact.

Initial and adhesive contact between a diamond indenter and polydimethylsiloxane.

Detailed observations for the initial and adhesive contact of polydimethylsiloxane indentations with sharp indenters are proposed and discussed in this study. In dry experiments, the load-depth results revealed an almost reversible feature, which indicated elastic deformation. Significant initial penetration depths, created during the finding surface process, were found. A power-law relationship was used to illustrate the initial portion of the loading curve and to evaluate the initial penetration depth for correcting depth measurements. An axisymmetrical indenter with a sticky boundary condition was applied to illustrate the results of dry experiments. When the load exceeded a specific value, both loading and unloading results showed an invariant slope. The analysis of the sticky indenter provided a reasonable explanation for the linear load-depth results. By correcting the initial penetration depth, the evaluated Youngs modulus values, obtained from the indenters with different geometries and under different environments, were shown to be unique and accurate.

Analyses of the work required to delaminate a coating film from a substrate under oscillating load conditions and the film-substrate contact behavior after delamination

Nanoindentation tests in which oscillating loads were applied to specimens were carried out to study the work (Wf) required for a coating film to delaminate from its substrate. A sharp increase in the indentation depth occurred during coating film delamination. A theoretical model was developed for the present study to evaluate the difference between the work in the load-depth profiles obtained with and without delamination. Arranging that the maximum load was reached at the end of the loading process allowed us to obtain an approximately constant value for the indentation depth propagation rate during in one cycle. This allowed a determination of the number of oscillating cycles and, thus, the work required by assuming that there was no delamination during this delamination period. The depth propagation rates and the work required for coating film delamination were investigated by varying Pmax, Pmean, P0, and frequency (f). The depth propagation rates were slightly increased by increasing the Pmax value ...

A Model Developed for the Adhesion Forces Formed between an Atomic Force Microscopy Tip and a Rough Surface under Different Humidity Levels

This study proposed a sophisticated numerical model based on geometry constraints and force equilibrium on the water/air interface in order to compute the meniscus profile and the adhesion forces between a semispherical probe tip and a rough surface under humid conditions. Through the proposed method, the unusual concave curve for the adhesion force results can be understood in detail. The effects of the contact angles of the probe tip and the asperity on the adhesion force were also discussed.

The reflectivity of an etched silicon surface with pyramids: I. Theoretical model and its predictions

In the present paper, a 3D model is developed to evaluate the reflectivity of an etched surface with pyramids. This model determines primary and secondary reflections by tracing parallel light beams from the location of each surface element. The model is then applied to predict total reflectivity and area fraction of illumination as a function of the angle of incidence and mean pyramid height. The reduction in the reflectivity of a surface is obtained by increasing the mean pyramid height, or by increasing the angle of incidence, irrespective of the mean pyramid height. However, the reflectivity is asymptotic to a constant value as the angle of incidence is lowered to a sufficiently small value.