Durability of Ultem 9085 in Marine Environments: A Consideration in Fused Filament Fabrication of Structural Components.

The long-term durability of polymer components produced by additive manufacturing (AM) in marine conditions is poorly understood. Here, fused filament fabrication (FFF) of Ultem 9085 was conducted and accelerated aging was performed. Two printing orientations (-45/45° and 0/90°) and two sample types (ASTM D638 Type 1 and Type 4) were produced and subjected to accelerated aging in either seawater or air. Results from tensile tests showed that the elastic modulus, yield strength and ultimate tensile strength increased after seawater aging, whereas the elongation to failure decreased. Results of thermogravimetric analysis (TGA) and derivative-TGA curves indicated that hydrolysis occurred after seawater exposure to the polycarbonate (PC) component and changes in structure or hydrogen bonds formed in the polyetherimide (PEI) component. Differential scanning calorimetry showed that physical aging occurred after short exposure periods and low temperature. Longer exposures and higher temperatures resulted in increasing plasticization by water and scission of the PC molecules. Results from Raman suggest that hydrolysis of the PC occurred, with a reduction in free volume produced by physical aging or hydrogen bonding with water molecules. These results highlight that Ultem 9085 is susceptible to degradation in marine environments, and there are two primary mechanisms, including physical and chemical aging. Their specific contribution is highly sensitive to the aging temperature and require careful selection in accelerated aging evaluations.

How mangabey molar form differs under routine vs. fallback hard-object feeding regimes.

Background: Components of diet known as fallback foods are argued to be critical in shaping primate dental anatomy. Such foods of low(er) nutritional quality are often non-preferred, mechanically challenging resources that species resort to during ecological crunch periods. An oft-cited example of the importance of dietary fallbacks in shaping primate anatomy is the grey-cheeked mangabey Lophocebus albigena. This species relies upon hard seeds only when softer, preferred resources are not available, a fact which has been linked to its thick dental enamel. Another mangabey species with thick enamel, the sooty mangabey Cercocebus atys, processes a mechanically challenging food year-round. That the two mangabey species are both thickly-enameled suggests that both fallback and routine consumption of hard foods are associated with the same anatomical feature, complicating interpretations of thick enamel in the fossil record. We anticipated that aspects of enamel other than its thickness might differ between Cercocebus atys and Lophocebus albigena. We hypothesized that to function adequately under a dietary regime of routine hard-object feeding, the molars of Cercocebus atys would be more fracture and wear resistant than those of Lophocebus albigena. Methods: Here we investigated critical fracture loads, nanomechanical properties of enamel, and enamel decussation in Cercocebus atys and Lophocebus albigena. Molars of Cercopithecus, a genus not associated with hard-object feeding, were included for comparison. Critical loads were estimated using measurements from 2D µCT slices of upper and lower molars. Nanomechanical properties (by nanoindentation) and decussation of enamel prisms (by SEM-imaging) in trigon basins of one upper second molar per taxon were compared. Results: Protocone and protoconid critical fracture loads were significantly greater in Cercocebus atys than Lophocebus albigena and greater in both than in Cercopithecus. Elastic modulus, hardness, and elasticity index in most regions of the crown were greater in Cercocebus atys than in the other two taxa, with the greatest difference in the outer enamel. All taxa had decussated enamel, but that of Cercocebus atys uniquely exhibited a bundle of transversely oriented prisms cervical to the radial enamel. Quantitative comparison of in-plane and out-of-plane prism angles suggests that decussation in trigon basin enamel is more complex in Cercocebus atys than it is in either Lophocebus albigena or Cercopithecus cephus. These findings suggest that Cercocebus atys molars are more fracture and wear resistant than those of Lophocebus albigena and Cercopithecus. Recognition of these differences between Cercocebus atys and Lophocebus albigena molars sharpens our understanding of associations between hard-object feeding and dental anatomy under conditions of routine vs. fallback hard-object feeding and provides a basis for dietary inference in fossil primates, including hominins.

Biomechanical perspectives on dentine cracks and fractures: Implications in their clinical management.

OBJECTIVES: The present review discussed the biomechanical properties of cracks and fractures in crown and root dentine and attempted to explain why cracked teeth and vertical root fractures are so frequent despite the existence of multiple crack toughening mechanisms in dentine. The implications of this knowledge were used to justify how these defects are managed clinically. DATA, SOURCES AND STUDY SELECTION: Literature search was conducted on PubMed, Web of Science, and Scopus for a narrative review on fracture mechanics of crown and root dentine as well as the clinical management of cracked teeth and teeth with vertical root fracture. CONCLUSIONS: Although dentine is tougher and less brittle than enamel, it's facture toughness is considerably lower than most ductile metals. Because the initiation toughness of dentine is very low, cracks initiate from incipient damage under low stress While crack toughening mechanisms exist that enable dentine to resist crack extension, these mechanisms are often inadequate for protecting dentine from crack propagation that ultimately leads to catastrophic failure. Additional factors such as ageing also reduces the resistance of dentine to crack growth. Because dentine cracks are eventually filled with bacteria biofilms upon exposure to oral fluids, they enable rapid bacteria ingress into the dental pulp via open dentinal tubules. To date, treatment options for cracked teeth are limited. While most teeth with vertical root fracture are recommended for extraction, new strategies have been reported that appeared to achieve short-term success in preserving these teeth. CLINICAL SIGNIFICANCE: Current strategies for the management for dentine cracks and fractures are limited and their long-term effectiveness remain uncertain. Understanding the characteristics, toughening mechanism and weakening factors of tooth cracks is helpful in designing better treatment.

Preparation of collagen fibrils from mineralized tissues and evaluation by atomic force microscopy.

Mineralized tissues like bone and dentin are materials that support the distribution of mechanical loads through the body of humans and other animals. While their organic content plays a critical role on the structural behavior of these materials, investigations that quantify the structural properties of collagen fibrils in mineralized tissues at the nanoscale are rather limited. We report a new experimental methodology to prepare samples of dentinal collagen fibrils for evaluation by atomic force microscopy and characterize their mechanical behavior. Specifically, a Dynamic Mechanical Analysis (DMA) of the collagen fibrils was performed to study their viscoelastic behavior. The capacity for viscous dampening in the fibrils was characterized in terms of measures of the energy dissipation, phase angle and loss modulus in both the peak and trough regions of the fibrils. According to the phase angle and the loss modulus, the peak regions of the fibrils exhibit significantly greater stiffness and capacity for dampening than the trough regions. This new approach will help in exploring the role of collagen fibrils in the mechanical behavior of dentin and other mineralized tissues as well as help to understand the potential effects from changes in fibril confirmation with tissue treatments, aging or that result from chronic disease.

Additive manufacturing of lithium disilicate glass-ceramic by vat polymerization for dental appliances.

OBJECTIVES: The objectives of this study were to evaluate the mechanical properties of lithium disilicate components produced by additive manufacturing (AM) and to assess the effect of build orientation on the resistance to fracture. METHODS: Oversized bars were printed with a glass-filled photoactive resin using a digital light processing technique. After sintering and post-processing, flexure and chevron notch fracture toughness bars were obtained in three principal orientations (0°, 45°, and 90°) with respect to the build direction. Mechanical properties were obtained according to the relevant ASTM standards. The hardness, indentation fracture resistance, and elastic modulus were measured for each orientation, and a Weibull analysis was conducted with the flexure responses. Fractography of the fracture surfaces was performed to identify the failure origins. RESULTS: The 0° orientation exhibited characteristic strength, Weibull modulus, and elastic modulus of 313 MPa, 4.42, and 168 ± 3 GPa, respectively, which are comparable to lithium disilicate materials from traditional processes. However, build orientation contributed significantly to the flexure strength, elastic modulus, and Weibull modulus; the characteristic strengths for the 45° and 90° build orientations were 86 MPa and 177 MPa, respectively. The primary contribution to the orientation dependence was the number of residual build layer-related flaws from incomplete union between printed layers. Of note, hardness and the fracture toughness were not dependent on build orientation. SIGNIFICANCE: AM of lithium disilicate materials can achieve the mechanical properties of materials produced by traditionally processing. Thus, while further process development is warranted, the outlook for dentistry is promising.

Importance of Build Design Parameters to the Fatigue Strength of Ti6Al4V in Electron Beam Melting Additive Manufacturing.

The fatigue properties of metals resulting from Powder Bed Fusion (PBF) is critically important for safety-critical applications. Here, the fatigue life of Grade 5 Ti6Al4V from Electron Beam PBF was investigated with respect to several build and component design parameters using a design of experiments (DOE). Part size (i.e., diameter), part proximity, and part location within the build envelope were considered. Overall, metal in the as-built condition (i.e., no post-process machining) exhibited a significantly lower fatigue life than the machined surface condition. In both conditions, the fatigue life decreased significantly with the decreasing part diameter and increasing radial distance; height was not a significant effect in the machined condition. Whereas the surface topography served as the origin of failure for the as-built condition, the internal lack of fusion (LOF) defects, exposed surface LOF defects, and rogue defects served as the origins for the machined condition. Porosity parameters including size, location, and morphology were determined by X-ray micro-computed tomography (XCT) and introduced within regression models for fatigue life prediction. The greatest resistance to fatigue failure is obtained when parts are placed near the center of the build plane to minimize the detrimental porosity. Machining can improve the fatigue life, but only if performed to a depth that minimizes the underlying porosity.

Microstructure, mechanical properties and elemental composition of the terrestrial isopod Armadillidium vulgare cuticle.

The exoskeletons of crustaceans are essential for providing protection from predators and other environmental threats. Understanding the structure and mechanical behavior of their natural armor could inspire the design of lightweight and high toughness synthetic materials. Most published work has focused on marine crustacea rather than their terrestrial counterparts, which are exposed to a multitude of unique threats. The interest in the terrestrial isopod Armadillidium vulgare (A. vulgare) has grown but the interrelationship between the microstructure, chemical composition, and mechanical properties has not been thoroughly investigated. Thus, this study aims to elucidate missing details concerning this biological mineralized composite. Exoskeleton specimens were fixated to preserve the intrinsic protein structure. We utilize scanning electron microscopy for microstructure analysis, Raman spectroscopy for elemental analysis, and nanoindentation property mapping to achieve mechanical characterization. The naturally fractured A. vulgare exoskeleton cross-section reveals four subregions with the repeating helicoidal 'Bouligand' arrangement most prominent in the endocuticle. The hardness and reduced modulus distributions exhibit a through-thickness exponential gradient with decreasing magnitudes from the outermost to the innermost layers of the exoskeleton. The Raman spectra show a graded spatial distribution of key constituents such as calcium carbonate across the thickness, some of which are consistent with the mechanical property gradient. Potential microstructure, elemental composition, and mechanical property relationships are discussed to explain how the hierarchical structure of this nanolaminate armor protects this species.

Contributions to enamel durability with aging: An application of data science tools.

Understanding aging of tooth tissues is the first step to developing robust treatments that support lifelong oral health. In this study selected nanomechanical, compositional and structural parameters of human enamel were characterized to assess the effects of aging on its durability in terms of the apparent fracture toughness (KApp) and brittleness (B). The interdependencies between aging and the enamel properties were assessed using a combination of traditional Pearson's correlation coefficient matrices and self-organizing maps (SOMs) via unsupervised machine learning. To consider age effects, the enamel of three age groups of donor teeth was studied, including primary (donor age ≤10), young (20 age ≤ age ≤50), and old (55 ≤ age) and differences in properties and correlations were identified. Results showed that KApp was negatively correlated to the E, H, degree of crystallinity, and fluoridation, but positively correlated with carbonate content; the opposite trends were observed in B. Interestingly, the SOMs showed that the outer enamel of the old group underwent a degradation in durability (decrease in KApp and increase in B) that was related to multiple contributions, whereas the inner enamel did not undergo this change. Application of K-means clustering on the trained SOMs offered novel insights into the contributions of enamel durability with aging, unique visualization of high-dimensional data onto 2D plots and identified new research directions that would not have otherwise been discovered. Overall, the findings demonstrate the opportunities for understanding aging of enamel using machine learning techniques to pursue age-targeted oral health care.

Odontoblast apoptosis and intratubular mineralization of sclerotic dentin with aging.

OBJECTIVES: The aims of the study were to evaluate the roles of odontoblast apoptosis in the progression of tubular sclerosis of teeth from donors at different ages and assess its correlation to chemical composition and mechanical properties. DESIGN: Healthy human teeth were obtained and divided into young (age ≤ 25, n = 12) and old (age ≥ 60, n = 12) groups. Odontoblasts were counted with standard hematoxylin and eosin staining. Odontoblast apoptosis within dentinal tubules was determined by cleaved caspase-3 immunostaining. Teeth in each group were evaluated by dynamic nanoindentation and energy-dispersive X-ray spectroscopy (EDS). RESULTS: The number of odontoblasts decreased significantly with age. The most prominent change occurred in the apical third of roots. Odontoblastic apoptosis was visualized within dentinal tubules. The apoptosis staining fraction was significantly higher in the outer and inner dentin of old teeth when compared with young teeth (p < 0.05). EDS showed increased calcium content in peritubular dentin but a decrease in the intertubular dentin with increasing age. Scanning based nanoindentation showed that the old intertubular dentin exhibited a significantly higher elastic modulus. CONCLUSIONS: Odontoblast apoptosis, starting at the cell extension in dentinal tubules and proceeding from outer to inner dentin, contributes to the stoichiometric Ca/P ratio in peritubular dentin, which is potentially responsible for intratubular mineralization due to an imbalance of calcium and phosphorous ions.

A machine learning approach to investigate the materials science of enamel aging.

Understanding aging of tooth tissues is critical to the development of patient-centric oral healthcare. Yet, the traditional methods for analyzing the composition-structure-property relationships of hard tissues have limitations when considering aging and other factors. OBJECTIVE: To apply unsupervised machine learning tools to pursue an understanding of relationships between the composition and mechanical behavior of aging enamel. METHODS: Molar teeth were collected from primary (age ≤ 8), young adult (24 ≤ age ≤ 46) and old adult (55 ≤ age) donors. The hardness and elastic modulus were quantified using nanoindentation as a function of distance from the Dentin Enamel Junction (DEJ) within the cervical, cuspal and inter-cuspal regions of the enamel crown. Similarly, a co-located analysis of the chemical composition and structure was performed using Raman spectroscopy. A Self-Organizing Maps (SOMs) algorithm was implemented to identify multi-dimensional composition-property relationships. RESULTS: The hardness and elastic modulus are positively correlated to crystallinity and negatively correlated with carbonate substitution. Furthermore, the effects from fluoridation on the age-dependent properties of enamel is non-linear and depends on its location. The contributions of fluoridation to the enamel properties are different in the cervical and non-cervical regions and appear to be unique within primary and senior adult teeth. SIGNIFICANCE: Based on the findings, unsupervised learning methods can reveal complicated non-linear structure-property relationships in tooth tissues and help to understand the materials science of aging and its consequences.

Effect of Airborne-Particle Abrasion Protocols and MDP-based Primer on the Bond Strength of Highly Translucent Zirconia.

PURPOSE: To evaluate the effects of airborne-particle abrasion and MDP (methacryloyloxydecyl dihydrogen phosphate)-based primer treatment on the strength of resin bonds to highly translucent zirconia. MATERIALS AND METHODS: Eight groups (n = 20 per group) of specimens were prepared with airborne-particle abrasion treatments (0.1-, 0.3-, or 0.6-MPa pressure) or not (untreated control) and MDP-based primer (treated) or not (untreated). Shear bond strength (SBS) tests were performed on the composite-to-ceramic bonded specimens either with or without thermocycling. After airborne-particle abrasion, the surface topography was evaluated by white light interferometry, and a phase analysis was conducted with x-ray diffraction (XRD). Surface roughness (Ra), surface energy (SE), and SBS measurements were statistically analyzed using either Tukey's HSD or the Kruskal-Wallis test, based on applicability. Lastly, the failure mode was observed by optical microscope and scanning electron microscope. RESULTS: Airborne-particle abrasion resulted in significantly larger Ra (p < 0.05), especially with higher treatment pressures. Treatment with MDP-based primer caused significantly higher SE and SBS than airborne-particle abrasion alone (p < 0.05), both with and without aging. CONCLUSION: MDP-based primer can enhance the bond strength and reduce hydrolytic aging of the bonded interface for highly translucent zirconia, exceeding the effects of airborne-particle abrasion. It is recommended that MDP-based primer treatment be applied with a composite cement containing adhesive phosphate monomer.

The effect of preparation taper on the resistance to fracture of monolithic zirconia crowns.

OBJECTIVE: Monolithic zirconia crowns have become a viable alternative to conventional layered restorations. The aim of this study was to evaluate whether the taper, and thus wall thickness, of the abutment or pre-defined cement space affect the fracture resistance or fracture mode of monolithic zirconia crowns. METHODS: A model tooth was prepared with a taper of 15° and a shallow circumferential chamfer preparation (0.5 mm). Two additional models were made based on the master model with a taper of 10° and 30° using computer-aided design software. Twenty monolithic 3rd generation translucent zirconia crowns were produced for each model with pre-defined cement space set to either 30 µm or 60 µm (n = 60). The estimated cement thickness was assessed by the replica method. The cemented crowns were loaded centrally in the occlusal fossa at 0.5 mm/min until fracture. Fractographic analyses were performed on all fractured crowns. RESULTS: The load at fracture was statistically significant different between the groups (p < 0.05). The crowns with 30° taper fractured at lower loads than those with 10° and 15° taper, regardless of the cement space (p < 0.05). The fracture origin for 47/60 crowns (78%) was in the cervical area, close to the top of the curvature in the mesial or distal crown margin. The remaining fractures started at the internal surface of the occlusal area and propagated cervically. SIGNIFICANCE: The fracture resistance of the monolithic zirconia crowns was lower for crowns with very large taper compared to 10 and 15° taper even though the crown walls were thicker.

Shrinkage Strains in the Dentin of Endodontically Treated Teeth with Water Loss.

INTRODUCTION: Dehydration has been considered as a potential contributor to vertical root fractures (VRFs) after root canal treatment (RCT). A loss of water could cause embrittlement of dentin and detrimental shrinkage strains. Senior patients have the highest risk of VRF. In this study, we characterized the spatial distribution in shrinkage of tooth roots with respect to donor age and prior RCT. METHODS: Single-rooted human teeth with and without prior RCT were collected from young (age <25 years) and old (age >60 years) adults. Transverse slices were sectioned from the apical, middle, and coronal thirds of the roots, and digital image correlation was used to evaluate shrinkage during free convection. Crack initiation and growth analysis was performed via optical microscopy, and bound water in dentin was characterized by Raman spectroscopy. RESULTS: The rate of shrinkage was significantly higher (p ≤ .05) in the apical third than in the middle and coronal thirds of all teeth regardless of donor age. The highest shrinkage strain occurred in the apical third of old donor teeth with prior RCT. In addition, the RCT-treated old teeth suffered the highest percentage of water loss with dehydration. Cracks initiated from the root surface and extended toward the canal with loss of water and shrinkage. CONCLUSIONS: The apical third undergoes significantly larger shrinkage strains with dehydration than the remainder of the root. Prior RCT exacerbates the extent of shrinkage, particularly in the teeth of seniors and after clinical function, which could increase the propensity for VRF.

Long-term antibacterial activity and cytocompatibility of novel low-shrinkage-stress, remineralizing composites.

A low-shrinkage-stress (LSS), antibacterial and remineralizing nanocomposite was recently developed; however, validation of its long-term antibacterial potency in modulating human salivary-derived biofilm is an unmet need. This study aimed to evaluate the antibacterial effect of the bioactive LSS composite before and after aging in acidic solution for 90 days using a multi-species biofilm model, and to evaluate its cytotoxicity. The LSS composite consisted of urethane dimethacrylate (UDMA) and triethylene glycol divinylbenzyl ether (TEG-DVBE), 3% dimethylaminohexadecyl methacrylate (DMAHDM) and 20% nanoparticles of amorphous calcium phosphate (NACP). Biofilm colony-forming units (CFU), lactic acid production, and confocal laser scanning microscopy (3D biofilm) were evaluated before and after three months of aging. Cytotoxicity was assessed against human gingival fibroblasts (HGF). The new LSS composite presented the lowest biofilm CFU, lactic acid and biofilm biomass, compared to controls (n = 6, p < 0.05). Importantly, the new composite exhibited no significant difference in antibacterial performance before and after 90-day-aging, demonstrating long-term antibacterial activity (p > 0.1). The LSS antibacterial and remineralizing composite presented a low cell viability at original extract that has increased with further dilutions. In conclusion, this study spotlighted that the new bioactive composite not only had a low shrinkage stress, but also down-regulated the growth of oral biofilms, reduced acid production, maintained antibacterial activity after the 90-day-aging, and did not compromise the cytocompatibility.

Vat Polymerization-Printed Partially Stabilized Zirconia: Mechanical Properties, Reliability and Structural defects.

Additive manufacturing (AM) of ceramics, particularly of zirconia, is becoming of increasing interest due to the substantial freedom available in the design and fabrication process. However, due to the novelty of the field and the challenges associated with printing dense bulk ceramics suitable for structural applications, thorough investigations that explore the effects of printing on the mechanical performance are limited. Previous work has identified anisotropy in the mechanical properties and attributed it to the layer-by-layer deposition. However, substantiated fractographic evidence detailing the origins and effects of layer lines on the probability of failure are limited. This study investigates the mechanical properties of a dense (>99 %TD), partially stabilized zirconia fabricated by a digital light projection printing method following ASTM standards. Hardness and strength evaluations were conducted, followed by a Weibull analysis and fractography. The investigation entailed five unique build directions and a conventionally manufactured reference material that was used as a control. Although the strengths were comparable to the reference material for some orientations, fracture frequently initiated at layer lines and related defects in all orientations. The findings indicate that if the layer lines can be prevented or engineered, the strength of vat printed ceramics can be improved substantially.

Bioactive low-shrinkage-stress nanocomposite suppresses S. mutans biofilm and preserves tooth dentin hardness.

Recurrent dental caries is one of the main reasons for resin composite restoration failures. This study aimed to: (1) develop a bioactive, low-shrinkage-stress, antibacterial and remineralizing composite and evaluate the sustainability of its antibacterial effect against Streptococcus mutans (S. mutans) biofilms; and (2) evaluate the remineralization and cariostatic potential of the composite containing nanoparticles of amorphous calcium phosphate (NACP) and dimethylaminohexadecyl methacrylate (DMAHDM), using dentin hardness measurement and a biofilm-induced recurrent caries model. The antibacterial and remineralizing low-shrinkage-stress composite consisted of urethane dimethacrylate (UDMA) and triethylene glycol divinylbenzyl ether (TEG-DVBE), 3% DMAHDM and 20% NACP. S. mutans biofilm was used to evaluate antibiofilm activity, before and after 3 months of composite aging in acidic solution. Human dentin was used to develop a recurrent caries biofilm-model. Adding DMAHDM and NACP into low shrinkage-stress composite did not compromise the flexural strength. The low-shrinkage-stress composite with DMAHDM achieved substantial reductions in biofilm colony-forming units (CFU), lactic acid production, and biofilm biomass (p < 0.05). The low-shrinkage-stress DMAHDM+NACP composite exhibited no significant difference in antibacterial performance before and after 3 months of aging, demonstrating long-term antibacterial activity. Under S. mutans biofilm acidic attack, dentin hardness (GPa) was 0.24 ± 0.04 for commercial control, and 0.23 ± 0.03 for experimental control, but significantly higher at 0.34 ± 0.03 for DMAHDM+NACP group (p < 0.05). At an instrumental compliance of 0.33 µm/N, the polymerization shrinkage stress of the new composite was 36% lower than that of a traditional composite (p < 0.05). The triple strategy of antibacterial, remineralization and lower shrinkage-stress has great potential to inhibit recurrent caries and increase restoration longevity. Statement of Significance Polymerization shrinkage stress, masticatory load over time as well as biochemical degradation can lead to marginal failure and secondary caries. The present study developed a new low-shrinkage-stress, antibacterial and remineralizing dental nanocomposite. Polymerization shrinkage stress was greatly reduced, biofilm acid production was inhibited, and tooth dentin mineral and hardness were preserved. The antibacterial composite possessed a long-lasting antibiofilm effect against cariogenic bacteria S. mutans. The new bioactive nanocomposite has the potential to suppress recurrent caries at the restoration margins, protects tooth structures, and increases restoration longevity.

Bioinspired hierarchical impact tolerant materials.

The quest for new light-weight materials with superior mechanical properties is a goal of materials scientists and engineers worldwide. A promising route in this pursuit is drawing inspiration from nature to design and develop materials with enhanced properties. By emulating the graded mineral content and hierarchical structure of fish scales of the Arapaima gigas from the nano to macro scales, we were able to develop bioinspired laminated composites with improved impact resistance. Activated by the addition of nano-particles of Al2O3 and nano-layers of TiN to a thermoplastic fiber substrate, new energy dissipation mechanisms operating at the nanoscale enhanced the energy absorption and stiffness of the bioinspired material. Remarkably, the newly developed materials are easily transferred to the industry with minimum associated manufacturing costs.

Does the bond strength of highly translucent zirconia show a different dependence on the airborne-particle abrasion parameters in comparison to conventional zirconia?

PURPOSE: To compare the effects of airborne-particle abrasion protocols on the surface morphology, the phase transformation and the resin bond strength of highly translucent zirconia (M) and conventional zirconia (Z). METHODS: Thirteen groups (N = 12) of Z and M specimens were prepared. Except for the control group, the specimens were sandblasted with conditions involving different grit sizes (50 µm or 110 µm), treatment times (10 s or 20 s) and pressures (0.1 MPa, 0.3 MPa or 0.6 MPa). The surface morphology was analyzed using scanning electron microscope (SEM) and the phase analysis was conducted with X-ray diffraction (XRD). The Ra and the shear bond strength (SBS) were measured and statistically analyzed, and the failure mode was determined by optical microscope. RESULTS: The surface morphologies were strongly dependent on treatment conditions. Larger particle size and higher pressure resulted in higher Ra for both materials. Longer blasting time resulted in higher Ra for Z but not M. Overall, the SBS increased with increasing Ra; the highest average SBS was achieved by M and exceeded 18 MPa. The monoclinic transformation was not found in any treatment for M, but was found in Z. CONCLUSIONS: Z and M showed different dependence on the airborne-particle abrasion parameters in terms of Ra, SBS and phase transformation. The conditions for maximizing SBS included a 110 µm particle size and 20 s treatment for both, with pressures of 0.3 MPa and 0.6 MPa for the M and Z, respectively.

On the wear behavior and damage mechanism of bonded interface: Ceramic vs resin composite inlays.

Advances in adhesive technologies have increased indications for the use of inlays. Decrease in the bonded interface integrity due to wear has been cited as the main cause of its failure. However, this process of interface degradation and the influence of inlay material on damage mechanism appear to be poorly understood. Thus, we aimed to compare the wear behavior and interface damage between ceramic and resin composite inlays bonded to enamel under sliding contact and use the experimental findings to support recommendation of the appropriate inlay material. Bonded interface specimens involving tooth enamel and either ceramic or resin composite inlays were prepared and subjected to reciprocating wear tests up to 5×104 cycles. The wear track profiles and morphologies were characterized after increments of cyclic sliding contact using white light interferometry and scanning electron microscopy, respectively. Optical microscopy was used to evaluate sub-surface cracks and their propagation within the samples. A finite element analysis was used to analyze the stress distributions of the bonded interfaces. Composite inlays showed higher wear depth than the ceramic in the early stage (N ≤ 5×102 cycles), while no significant difference was found at the later stage. For ceramic inlay a greater portion of the contact load was concentrated in the ceramic structure, which facilitated cracks and chipping of the ceramic inlay, with rather minimal damage in the adjacent interface and enamel. In contrast, for the resin composite inlay there was larger stress concentrated in the adjacent enamel, which caused the development of cracks and their propagation to the inner enamel. The restoration material could contribute to the stress distribution and extent of damage within enamel-inlay bonded interfaces. A tough ceramic appears to be more effective at protecting the residual dental tissue.