Bio-inspired dental multilayers: effects of layer architecture on the contact-induced deformation

Methodological approaches in graded dental ceramics

BACKGROUND

Functionally graded materials (FGM) with indistinct boundaries potentially eliminate the damaging stresses occurring at the interfaces. FGM applications in dental ceramics have enhanced their fatigue resistance and interfacial toughness.

OBJECTIVES

This scoping review aims to map graded designs in dental ceramics, distinguish their methodological approaches with their material characteristics and properties, and understand the factors affecting the outcomes of each of the graded approaches.

METHODS

A systematic electronic search was performed with the databases MEDLINE (PubMed), Scopus, Cochrane Library, EBSCO, and Google Scholar along with a manual search.

RESULTS

About 2675 articles were initially found from all the searches with no date restriction till July 2023. After rejecting duplicates and based on exclusion criteria, about 52 articles were included.

SIGNIFICANCE

Methodological approaches in grading such as glass-infiltration and silica-infiltration have been investigated on pre-sintered zirconia. The type of infiltration and the method of infiltrate application significantly influenced the phase transformation of zirconia, its microstructure, surface hardness, fracture toughness, flexural strength, wear, and fatigue strength of graded dental zirconia. Interlayers were accommodated between metal-ceramic and veneer-core all-ceramic layers. Fractions of zirconia-porcelain and alumina-porcelain showed high bending strength and better stress distribution. The results of finite element analysis studies predicted that using 10-layered graded layers reduced the stresses at the crown-cement-dentin interface.

A parametrical finite element analysis for functionally graded material overlay restoration

The main cause of failure in bonded ceramic restorations is material fracture due to excessive stress concentration at the base of the prosthesis. The design of restorative functionally graded materials (FGM) could represent a major advance in dissipating mechanical stresses during occlusal contacts. The aim of this paper is to carry out a complete factorial design of finite element analyses to optimize a multilayer FGM introduced at the bottom of an overlay prosthesis. The number and thickness of layers vary within a spectrum compatible with ceramic shaping processes whereas Young's moduli variations are set in the range of dental tissues. For a 1.5-mm thick prosthesis, the optimal FGM configuration appears to be a 5 layers of 0.2 mm thickness with a linear distribution of Young's modulus from 30 to 70 GPa. This configuration was implemented in a 3D model of a restored tooth with realistic geometry to validate the proof-of-concept.

Bioinspired Polymers: Transformative Applications in Biomedicine and Regenerative Medicine

Bioinspired polymers have emerged as a promising field in biomaterials research, offering innovative solutions for various applications in biomedical engineering. This manuscript provides an overview of the advancements and potential of bioinspired polymers in tissue engineering, regenerative medicine, and biomedicine. The manuscript discusses their role in enhancing mechanical properties, mimicking the extracellular matrix, incorporating hydrophobic particles for self-healing abilities, and improving stability. Additionally, it explores their applications in antibacterial properties, optical and sensing applications, cancer therapy, and wound healing. The manuscript emphasizes the significance of bioinspired polymers in expanding biomedical applications, addressing healthcare challenges, and improving outcomes. By highlighting these achievements, this manuscript highlights the transformative impact of bioinspired polymers in biomedical engineering and sets the stage for further research and development in the field.

Principles of elastic bridging in biological materials

Load-bearing biological materials employ specialized elastic bridging regions to connect material parts with substantially different properties. While such bridging regions emerge in diverse systems of biological systems, their functional-mechanical origins are yet disclosed. Here, we hypothesize that these elastic bridging regions evolved primarily to minimize the near-interface stress effects in the biological material and, supported by experiments and simulations, we develop a simple theoretical model for such stress-minimizing bridging modulus. Our theoretical model describes well extensive experimental data of diverse biomechanical systems, suggesting that despite their compositionally distinct bridging regions, they share a similar mechanical adaptation strategy for stress minimization. The theoretical model developed in this study may directly serve as a design guideline for bio-inspired materials, biomedical applications, and advanced interfacial architectures with high resilience to mechanical failure. STATEMENT OF SIGNIFICANCE: Biological materials exhibit unconventional structural-mechanical strategies allowing them to attain extreme load-bearing capabilities. Here, we identify the strategy of biological materials to connect parts of distinct elastic properties in an optimal manner of stress minimization. Our findings are compatible with broad types of biological materials, including biopolymers, biominerals, and their bio-composite combinations, and may promote novel engineering designs of advanced biomedical and synthetic materials.

Strength-fracture toughness synergy strategy in ostrich tibia's compact bone: Hierarchical and gradient

Ostriches are the fastest bipeds in the world, but their tibias are very thin. How the thin tibia can withstand the huge momentum impacts of the heavy body during running? The present work revealed that the combination of hierarchical and gradient design strategies was the main reason for their high strength and fracture toughness. The microstructure of ostrich's tibias compact bone was self-assembled into the 6-level hierarchical structure from the hydroxyapatite (HAP) crystals, collagen fiber (sub-nano), mineralized collagen fiber (nano-), mineralized collagen fiber bundle (sub-micro), lamellae (micro-) and osteon (macro-scales). The most distinctive design in the ostrich compact bone was that the HAP crystals were embedded in collagen fibers as well as wrapped in the outer layer of mineral collagen fibers (MCFs) in the form of HAP nanocrystals, thus achieving a high degree of soft and hard combination from the nanoscale. The bending strength was gradient-structure dependent and up to 787.2 ± 40.5 MPa, 4 times that of a human's compact bone. The fracture toughness (K_Jc) is 5.8 ± 0.1 MPa m^1/2. Several toughening mechanisms, such as crack deflection/twist, bridging, HAP fibers pulling-out, and fracture of the MCF bundles were found in the compact bone.

Two-layer additively manufactured crown: Proof of concept

OBJECTIVES

To assess the feasibility of additively manufacturing a crown with a 2-layer design.

METHODS

A mandibular first molar tooth preparation titanium die for a full coverage restoration was obtained. The die was used to design a monolayer (ML group) and 2-layer (2L group) anatomically contoured crown. In the ML group, the specimen was manufactured with a hard polymer (Rigur RGD450; Stratasys). In the 2L group, the crown was splinted into 2 parts: the intaglio that represented 25% of the total crown volume that was manufactured with a resilient polymer (Vero; Stratasys) and the exterior that represented the remaining crown volume that was manufactured with a hard polymer (Rigur RGD450; Stratasys). Specimens were manufactured using a material jetting printer (Connex3 Object260; Stratasys). The marginal and internal discrepancies of ML and 2L specimens were visually assessed.

RESULTS

The ML and 2L specimens were manufactured using a material jetting printer that obtained a visually acceptable marginal and internal discrepancy.

CONCLUSIONS

The 2-layer dental crown can be manufactured using a material jetting printer.

CLINICAL SIGNIFICANCE

Material jetting technology has the capability to fabricate a 2-layer dental crown design which can be fabricated using materials with different properties.

Failure probability and stress distribution of milled porcelain-zirconia crowns with bioinspired/traditional design and graded interface

OBJECTIVE

To evaluate the failure probability and stress distribution of traditional and bioinspired porcelain-zirconia milled crowns, with and without silica infiltration (graded zirconia).

METHODS

Traditional crown design had a zirconia infrastructure veneered with porcelain; Bioinspired, had a porcelain infrastructure with translucent-zirconia veneer; Graded and Graded Bioinspired crowns had their zirconia layer infiltrated by silica (n = 25). The cameo surface of each crown (porcelain or zirconia) was glazed. The restoration layers were fused by a vitreous connector and the crowns were adhesively cemented to dies. The specimens were then mechanically cycled in a sliding machine using 100 N load at 4 Hz. The specimens were tested until 2 × 10⁶ cycles, and every 0.5 × 10⁶ cycles the crowns were evaluated under stereomicroscopy for the presence of failures. The stress distribution was inspected with Finite Element Analyses.

RESULTS

The predominant failure modes for the Traditional and Graded crowns were delamination and cracking, respectively. The Weibull parameters beta and eta were, respectively: Traditional 1.30 and 2.3 × 10⁶ cycles, and Graded 1.95 and 2.3 × 10⁶ cycles. Thus, the Traditional and Graded crowns presented greater susceptibility to failure due to fatigue, while the Bioinspired and Graded Bioinspired crowns showed no fatigue effect using 100N load, showing beta = 1 and eta of approximately 17 × 10⁶ cycles. Also, through finite element analyses, it was verified that the Bioinspired and Graded Bioinspired crowns presented the best stress distribution on both crowns and dental structures.

SIGNIFICANCE

Bioinspired and Graded Bioinspired crowns had the lowest failure probability and better stress distribution and may be considered robust long lasting restorations.

Development of Bio-Inspired Hierarchical Fibres to Tailor the Fibre/Matrix Interphase in (Bio)composites

Several naturally occurring biological systems, such as bones, nacre or wood, display hierarchical architectures with a central role of the nanostructuration that allows reaching amazing properties such as high strength and toughness. Developing such architectures in man-made materials is highly challenging, and recent research relies on this concept of hierarchical structures to design high-performance composite materials. This review deals more specifically with the development of hierarchical fibres by the deposition of nano-objects at their surface to tailor the fibre/matrix interphase in (bio)composites. Fully synthetic hierarchical fibre reinforced composites are described, and the potential of hierarchical fibres is discussed for the development of sustainable biocomposite materials with enhanced structural performance. Based on various surface, microstructural and mechanical characterizations, this review highlights that nano-objects coated on natural fibres (carbon nanotubes, ZnO nanowires, nanocelluloses) can improve the load transfer and interfacial adhesion between the matrix and the fibres, and the resulting mechanical performances of biocomposites. Indeed, the surface topography of the fibres is modified with higher roughness and specific surface area, implying increased mechanical interlocking with the matrix. As a result, the interfacial shear strength (IFSS) between fibres and polymer matrices is enhanced, and failure mechanisms can be modified with a crack propagation occurring through a zig-zag path along interphases.

PICN Nanocomposite as Dental CAD/CAM Block Comparable to Human Tooth in Terms of Hardness and Flexural Modulus

Polymer infiltrated ceramic network (PICN) composites are an increasingly popular dental restorative material that offer mechanical biocompatibility with human enamel. This study aimed to develop a novel PICN composite as a computer-aided design and computer-aided manufacturing (CAD/CAM) block for dental applications. Several PICN composites were prepared under varying conditions via the sintering of a green body prepared from a silica-containing precursor solution, followed by resin infiltration. The flexural strength of the PICN composite block (107.8–153.7 MPa) was similar to a commercial resin-based composite, while the Vickers hardness (204.8–299.2) and flexural modulus (13.0–22.2 GPa) were similar to human enamel and dentin, respectively. The shear bond strength and surface free energy of the composite were higher than those of the commercial resin composites. Scanning electron microscopy and energy dispersive X-ray spectroscopic analysis revealed that the microstructure of the composite consisted of a nanosized silica skeleton and infiltrated resin. The PICN nanocomposite block was successfully used to fabricate a dental crown and core via the CAD/CAM milling process.

Natural protein bioinspired materials for regeneration of hard tissues

This review describes the protein bioinspired materials for the repair of hard tissues such as enamel, dentin and bone.

Spatial and temporal tunability of magnetically-actuated gradient nanocomposites

Natural biological materials usually adopt functional gradient designs within interfacial regions to fulfil unusual mechanically-challenging demands. Manufacturing analogous gradients to alleviate premature failures for synthetic interfaces has remained challenging until recently, where magnetically-actuated gradient nanocomposites (MA-G-NCs) have emerged as a promising processing technique. The essence of this technique lies in controlling the spatial distribution of nanoreinforcements (usually particles) inside a polymer matrix through a magnetophoresis process. Herein, we present a theory-experiment-combined study on the evolution kinetics and equilibrium distribution of the nanoparticles during the magnetophoresis process and consequently to explore the spatial and temporal tunability of the MA-G-NCs. Using a simplified drift-diffusion theory as the guide, we determine two critical processing parameters for the MA-G-NCs: the applied magnetic field and the actuation duration. By systematically varying these two parameters independently, we experimentally demonstrate that the profile of the nanoparticle distribution inside the MA-G-NCs can be finely tuned both spatially and temporally. In order to quantify the volume fraction of the nanoparticles along the cross section of the MA-G-NCs, we propose a mechanics-based method by site-specifically measuring the local elastic modulus and converting back to the volume fractions based on an established modulus-fraction correlation. The nanoparticle concentration profiles obtained thereby are validated by morphological characterizations and also agree well with theoretical predictions based on the drift-diffusion theory. Our combined results indicate that the magnetophoresis-induced evolution of the nanoparticles follows approximately the drift-diffusion transport process and the gradient profile of the MA-G-NCs is highly controllable and programmable. The presented study not only advances the fundamental understanding of the evolution kinetics of the nanoparticles under the effect of magnetophoresis, but also establishes the critical processing-structure-property relationships for the MA-G-NCs that should guide future development of customized interfaces with desired mechanical and physical property gradients.

Bone-inspired enhanced fracture toughness of de novo fiber reinforced composites

Amplification in toughness and balance with stiffness and strength are fundamental characteristics of biological structural composites, and a long sought-after objective for engineering design. Nature achieves these properties through a combination of multiscale key features. Yet, emulating all these features into synthetic de novo materials is rather challenging. Here, we fine-tune manual lamination, to implement a newly designed bone-inspired structure into fiber-reinforced composites. An integrated approach, combining numerical simulations, ad hoc manufacturing techniques, and testing, yields a novel composite with enhanced fracture toughness and balance with stiffness and strength, offering an optimal lightweight material solution with better performance than conventional materials such as metals and alloys. The results also show how the new design significantly boosts the fracture toughness compared to a classic laminated composite, made of the same building blocks, also offering an optimal tradeoff with stiffness and strength. The predominant mechanism, responsible for the enhancement of fracture toughness in the new material, is the continuous deviation of the crack from a straight path, promoting large energy dissipation and preventing a catastrophic failure. The new insights resulting from this study can guide the design of de novo fiber-reinforced composites toward better mechanical performance to reach the level of synergy of their natural counterparts.

Bioinspired Wear-Resistant and Ultradurable Functional Gradient Coatings

For mechanically protective coatings, the coating material usually requires sufficient stiffness and strength to resist external forces and meanwhile matched mechanical properties with the underneath substrate to maintain the structural integrity. These requirements generate a conflict that limits the coatings from achieving simultaneous surface properties (e.g., high wear-resistance) and coating/substrate interfacial durability. Herein this conflict is circumvented by developing a new manufacturing technique for functional gradient coatings (FGCs) with the material composition and mechanical properties gradually varying crossing the coating thickness. The FGC is realized by controlling the spatial distribution of magnetic-responsive nanoreinforcements inside a polymer matrix through a magnetic actuation process. By concentrating the reinforcements with hybrid sizes at the surface region and continuously diminishing toward the coating/substrate interface, the FGC is demonstrated to exhibit simultaneously high surface hardness, stiffness, and wear-resistance, as well as superb interfacial durability that outperforms the homogeneous counterparts over an order of magnitude. The concept of FGC represents a mechanically optimized strategy in achieving maximal performances with minimal use and site-specific distribution of the reinforcements, in accordance with the design principles of many load-bearing biological materials. The presented manufacturing technique for gradient nanocomposites can be extended to develop various bioinspired heterogeneous materials with desired mechanical performances.

A Functionally Graded PICN Material for Biomimetic CAD-CAM Blocks

The objective of this study was to introduce a functionally graded (FG) polymer-infiltrated ceramic network (PICN) block, characterized by a gradient of mechanical properties, as a biomimetic material for computer-aided design and manufacturing (CAD-CAM) prostheses. FG-PICN blocks were manufactured from a slurry of glass-ceramic powder, which was subsequently centrifuged and sintered. The ceramic network was infiltrated with urethane dimethacrylate and polymerized under high temperature-pressure. Blocks were sectioned into 9 layers, and each layer was subsequently cut into 3 samples. Samples were loaded into a 3-point bending device and tested for flexural strength, flexural load energy, and flexural modulus. The volume percentage of glass-ceramic, hardness, and brittleness index were also measured and scanning electron microscopy (SEM) observations were performed. Katana translucent zirconia (HT-ZIR) and e.max-CAD (EMX) were tested for comparison. Flexural strength, flexural load energy, and Weibull modulus of FG-PICN were shown to increase from the first (enamel-like zone) to the ninth layer (dentin-like zone), while, on the contrary, flexural modulus, hardness, brittleness index, and ceramic volume percentage decreased. SEM characterization highlighted a higher porosity in layer 9 than in layer 1. Flexural strength of the dentin-like zone (372.7 ± 27.8 MPa) was similar to EMX and lower than HT-ZIR. Flexural modulus was shown to vary from 41.9 ± 5.1 to 28.6 ± 2.0 GPa from surface to depth. Flexural load energy in the dentin-like zone (27.1 ± 4.9 mJ) was significantly superior to EMX and HT-ZIR. Hardness gradient was shown to be close to tooth tissues. This work introduces FG-PICN blocks, with a gradient of mechanical and optical properties through the entire thickness of the block designed to mimic dental tissues. FG-PICN demonstrated a favorable gradient of flexural strength, elastic modulus, and, most of all, flexural load energy and hardness compared to other CAD-CAM materials, which can promote the biomechanical behavior of single-unit restorations on teeth and implants.

COHESIVE FINITE ELEMENT SIMULATIONS OF CONTRACTION AND SHAPE EFFECTS ON CELL DE-ADHESION

Cohesive-interface-based finite element simulations were conducted to investigate the critical shear stress required for cell de-adhesion from extracellular substrates. The interface ligand–receptor bonds are modeled by a cohesive interface model with initial stiffness, interface strength, and fracture energy as the governing parameters. The ratio of the cell modulus to the interface stiffness defines a length scale. If this length is much less than the contact size, the de-adhesion process can be modeled by the linear elastic fracture mechanics, while the opposite limit leads to the concurrent sliding of the cell or, equivalently, debonding of all the interface ligand–receptor pairs. Since it generates additional shear-stress concentration along the interface, cell contraction generally reduces the critical de-adhesion stress. Cell de-adhesion is more prone to take place for three-dimensional irregular cell shapes because of the much easier failure in the anti-plane Mode III shear, as well as the additional stress concentration in these geometric irregularities.

Effects of two grading techniques of zirconia material on the fatigue limit of full-contour 3-unit fixed dental prostheses

OBJECTIVE

This study evaluated the effects of two grading techniques of zirconia material on the fatigue limit of full-contour 3-unit fixed dental prostheses (FDPs).

METHODS

Presintered blocks of 3Y-TZP were milled to obtain sixty-nine 3-unit FDPs, which were divided into three groups (n=23). The control group (CTL) was sintered and glazed following manufacturer's instructions. In the two experimental groups presintered FDPs received a surface silica/glass infiltration treatment before the sintering process. Silica sol-gel group (SSG) was graded by the sol-gel processing route, while the glass-zirconia-glass group (GZG) was graded by an enameling technique. Graded groups did not receive a glaze layer after sintering. All FDPs were then luted with a dual-curing resin cement on composite abutments, embedded in polyurethane and stored in water for five days. The initial load of the fatigue test was calculated based on the results of the monotonic testing applied on three specimens of each group. To determine the fatigue limit, 20 samples of each group were subjected to staircase testing (100,000 cycles/5Hz).

RESULTS

The fatigue limits (in Newtons) were CTL=1607.27, SSG=1824.31, and GZG=2006.57, and the Dixon and Mood test indicated statistically significant differences among groups (95% confidence interval) (GZG > SSG > CTL).

SIGNIFICANCE

The infiltration of silica and glass on bulk zirconia, by two different grading methods, increased the fatigue limits of monolithic zirconia FDPs.

Can hyperelastic material parameters be uniquely determined from indentation experiments?

Uniqueness of hyperelastic parameters depends on a simple criterion: whether dimensionless material parameters are coupled with indentation displacement.

Nanoforest of hierarchical core/shell CuO@NiCo2O4 nanowire heterostructure arrays on nickel foam for high-performance supercapacitors

Nickel foam-supported hierarchical core/shell CuO@NiCo₂O₄ nanowire heterostructure arrays with excellent supercapacitive performance are reported.

Fracture mode control: a bio-inspired strategy to combat catastrophic damage

The excellent mechanical properties of natural biomaterials have attracted intense attention from researchers with focus on the strengthening and toughening mechanisms. Nevertheless, no material is unconquerable under sufficiently high load. If fracture is unavoidable, constraining the damage scope turns to be a practical way to preserve the integrity of the whole structure. Recent studies on biomaterials have revealed that many structural biomaterials tend to be fractured, under sufficiently high indentation load, through ring cracking which is more localized and hence less destructive compared to the radial one. Inspired by this observation, here we explore the factors affecting the fracture mode of structural biomaterials idealized as laminated materials. Our results suggest that fracture mode of laminated materials depends on the coating/substrate modulus mismatch and the indenter size. A map of fracture mode is developed, showing a critical modulus mismatch (CMM), below which ring cracking dominates irrespective of the indenter size. Many structural biomaterials in nature are found to have modulus mismatch close to the CMM. Our results not only shed light on the mechanics of inclination to ring cracking exhibited by structural biomaterials but are of great value to the design of laminated structures with better persistence of structural integrity.