Influence of Surface Roughness on Mechanical Properties of Two Computer-aided Design/Computer-aided Manufacturing (CAD/CAM) Ceramic Materials

SUMMARY The aim of this study was to evaluate the influence of surface roughness on surface hardness (Vickers; VHN), elastic modulus (EM), and flexural strength (FLS) of two computer-aided design/computer-aided manufacturing (CAD/CAM) ceramic materials. One hundred sixty-two samples of VITABLOCS Mark II (VMII) and 162 samples of IPS Empress CAD (IPS) were ground according to six standardized protocols producing decreasing surface roughnesses (n=27/group): grinding with 1) silicon carbide (SiC) paper #80, 2) SiC paper #120, 3) SiC paper #220, 4) SiC paper #320, 5) SiC paper #500, and 6) SiC paper #1000. Surface roughness (Ra/Rz) was measured with a surface roughness meter, VHN and EM with a hardness indentation device, and FLS with a three-point bending test. To test for a correlation between surface roughness (Ra/Rz) and VHN, EM, or FLS, Spearman rank correlation coefficients were calculated. The decrease in surface roughness led to an increase in VHN from (VMII/IPS; medians) 263.7/256.5 VHN to 646.8/601.5 VHN, an increase in EM from 45.4/41.0 GPa to 66.8/58.4 GPa, and an increase in FLS from 49.5/44.3 MPa to 73.0/97.2 MPa. For both ceramic materials, Spearman rank correlation coefficients showed a strong negative correlation between surface roughness (Ra/Rz) and VHN or EM and a moderate negative correlation between Ra/Rz and FLS. In conclusion, a decrease in surface roughness generally improved the mechanical properties of the CAD/CAM ceramic materials tested. However, FLS was less influenced by surface roughness than expected.

The long-term mechanical integrity of non-reinforced PEEK-OPTIMA polymer for demanding spinal applications: experimental and finite-element analysis

Polyetheretherketone (PEEK) is a novel polymer with potential advantages for its use in demanding orthopaedic applications (e.g. intervertebral cages). However, the influence of a physiological environment on the mechanical stability of PEEK has not been reported. Furthermore, the suitability of the polymer for use in highly stressed spinal implants such as intervertebral cages has not been investigated. Therefore, a combined experimental and analytical study was performed to address these open questions. A quasi-static mechanical compression test was performed to compare the initial mechanical properties of PEEK-OPTIMA polymer in a dry, room-temperature and in an aqueous, 37 degrees C environment (n=10 per group). The creep behaviour of cylindrical PEEK polymer specimens (n=6) was measured in a simulated physiological environment at an applied stress level of 10 MPa for a loading duration of 2000 hours (12 weeks). To compare the biomechanical performance of different intervertebral cage types made from PEEK and titanium under complex loading conditions, a three-dimensional finite element model of a functional spinal unit was created. The elastic modulus of PEEK polymer specimens in a physiological environment was 1.8% lower than that of specimens tested at dry, room temperature conditions (P<0.001). The results from the creep test showed an average creep strain of less than 0.1% after 2000 hours of loading. The finite element analysis demonstrated high strain and stress concentrations at the bone/implant interface, emphasizing the importance of cage geometry for load distribution. The stress and strain maxima in the implants were well below the material strength limits of PEEK. In summary, the experimental results verified the mechanical stability of the PEEK-OPTIMA polymer in a simulated physiological environment, and over extended loading periods. Finite element analysis supported the use of PEEK-OPTIMA for load-bearing intervertebral implants.

Bovine primary chondrocyte culture in synthetic matrix metalloproteinase-sensitive poly(ethylene glycol)-based hydrogels as a scaffold for cartilage repair

A poly(ethylene glycol) (PEG)-based hydrogel was used as a scaffold for chondrocyte culture. Branched PEG-vinylsulfone macromers were end-linked with thiol-bearing matrix metalloproteinase (MMP)-sensitive peptides (GCRDGPQGIWGQDRCG) to form a three-dimensional network in situ under physiologic conditions. Both four- and eight-armed PEG macromer building blocks were examined. Increasing the number of PEG arms increased the elastic modulus of the hydrogels from 4.5 to 13.5 kPa. PEG-dithiol was used to prepare hydrogels that were not sensitive to degradation by cell-derived MMPs. Primary bovine calf chondrocytes were cultured in both MMP-sensitive and MMP-insensitive hydrogels, formed from either four- or eight-armed PEG. Most (>90%) of the cells inside the gels were viable after 1 month of culture and formed cell clusters. Gel matrices with lower elastic modulus and sensitivity to MMP-based matrix remodeling demonstrated larger clusters and more diffuse, less cell surface-constrained cell-derived matrix in the chondron, as determined by light and electron microscopy. Gene expression experiments by real-time RT-PCR showed that the expression of type II collagen and aggrecan was increased in the MMP-sensitive hydrogels, whereas the expression level of MMP-13 was increased in the MMP-insensitive hydrogels. These results indicate that cellular activity can be modulated by the composition of the hydrogel. This study represents one of the first examples of chondrocyte culture in a bioactive synthetic material that can be remodeled by cellular protease activity.

In situ micropillar compression reveals superior strength and ductility but an absence of damage in lamellar bone

Ageing societies suffer from an increasing incidence of bone fractures. Bone strength depends on the amount of mineral measured by clinical densitometry, but also on the micromechanical properties of the hierarchical organization of bone. Here, we investigate the mechanical response under monotonic and cyclic compression of both single osteonal lamellae and macroscopic samples containing numerous osteons. Micropillar compression tests in a scanning electron microscope, microindentation and macroscopic compression tests were performed on dry ovine bone to identify the elastic modulus, yield stress, plastic deformation, damage accumulation and failure mechanisms. We found that isolated lamellae exhibit a plastic behaviour, with higher yield stress and ductility but no damage. In agreement with a proposed rheological model, these experiments illustrate a transition from a ductile mechanical behaviour of bone at the microscale to a quasi-brittle response driven by the growth of cracks along interfaces or in the vicinity of pores at the macroscale.

Enamel Surface Changes After Exposure to Bleaching Gels Containing Carbamide Peroxide or Hydrogen Peroxide.

OBJECTIVE This study evaluated the differences in enamel color change, surface hardness, elastic modulus, and surface roughness between treatments with four bleaching gels containing carbamide peroxide (two at 10% and one each at 35%, and 45%) and two bleaching gels containing hydrogen peroxide (two at 40%). METHODS Enamel specimens were bleached and color changes were measured. Color change was calculated using either ΔE or the Bleaching Index (BI). Then, surface hardness, elastic modulus, and surface roughness of the enamel specimens were evaluated. All measurements were performed at baseline and directly after the first bleaching treatment for all carbamide peroxide- and hydrogen peroxide-containing bleaching gels. In addition, final measurements were made 24 hours after each of a total of 10 bleaching treatments for carbamide peroxide bleaching gels, and 1 week after each of a total of three bleaching treatments for hydrogen peroxide bleaching gels. RESULTS After the last bleaching treatment, respective ΔE scores were 17.6 and 8.2 for the two 10% carbamide peroxide gels, 12.9 and 5.6 for the 45% and 35% carbamide peroxide gels, and 9.6 and 13.9 for the two 40% hydrogen peroxide gels. The respective BI scores were -2.0 and -2.0 for the two 10% carbamide peroxide gels, -3.5 and -1.5 for the 45% and 35% carbamide peroxide gels, and -2.0 and -3.0 for the two 40% hydrogen peroxide gels. Each bleaching gel treatment resulted in significant whitening; however, no significant difference was found among the gels after the last bleaching. Whitening occurred within the first bleaching treatments and did not increase significantly during the remaining treatments. Surface hardness significantly decreased after the last bleaching treatment, when 10% carbamide peroxide was used. Furthermore, significant changes in the elastic modulus or surface roughness occurred only after treatment with 10% carbamide peroxide. CONCLUSION All six bleaching gels effectively bleached the enamel specimens independent of their concentration of peroxide. Gels with low peroxide concentration and longer contact time negatively affected the enamel surface.

The Young's modulus of fetal preterm and term amniotic membranes

OBJECTIVE: To examine the Young's modulus of the human amniotic membranes, as well as its relationship to gestational age. To determine whether cellular and material-related parameters affect this modulus. STUDY DESIGN: In a prospective study at the Obstetric outpatient clinic of the University Hospital Zurich Young's modulus, thickness and mesenchymal:epithelial cell ratio of amniotic membranes of preterm (N=23) and term (N=40) placentae were examined. Significance (P<0.05) was calculated with the Mann-Whitney two-sample rank sum test and Wilcoxon signed rank test, while correlations were made using the Spearman's correlation. RESULTS: The Young's modulus of preterm amniotic membranes was significantly higher than that of term membranes. It varied within the same amniotic membrane. The thickness of the amnion in both preterm and term membranes did not differ significantly. The thinner the preterm and term amniotic membranes, the higher the Young's modulus was. There was no relation to the mesenchymal:epithelial cell ratio in the amnion. CONCLUSIONS: Preterm amniotic membranes are stiffer than term amniotic membranes. Tentatively, we hypothesise that there may be a correlation between the extracellular matrix components and the elastic properties of the membrane.

How fluids bend: the elastic expansion for higher-dimensional black holes

Hydrodynamics can be consistently formulated on surfaces of arbitrary co-dimension in a background space-time, providing the effective theory describing long-wavelength perturbations of black branes. When the co-dimension is non-zero, the system acquires fluid-elastic properties and constitutes what is called a fluid brane. Applying an effective action approach, the most general form of the free energy quadratic in the extrinsic curvature and extrinsic twist potential of stationary fluid brane configurations is constructed to second order in a derivative expansion. This construction generalizes the Helfrich-Canham bending energy for fluid membranes studied in theoretical biology to the case in which the fluid is rotating. It is found that stationary fluid brane configurations are characterized by a set of 3 elastic response coefficients, 3 hydrodynamic response coefficients and 1 spin response coefficient for co-dimension greater than one. Moreover, the elastic degrees of freedom present in the system are coupled to the hydrodynamic degrees of freedom. For co-dimension-1 surfaces we find a 8 independent parameter family of stationary fluid branes. It is further shown that elastic and spin corrections to (non)-extremal brane effective actions can be accounted for by a multipole expansion of the stress-energy tensor, therefore establishing a relation between the different formalisms of Carter, Capovilla-Guven and Vasilic-Vojinovic and between gravity and the effective description of stationary fluid branes. Finally, it is shown that the Young modulus found in the literature for black branes falls into the class predicted by this approach - a relation which is then used to make a proposal for the second order effective action of stationary blackfolds and to find the corrected horizon angular velocity of thin black rings.