Reinforcing 13-93 bioglass scaffolds fabricated by robocasting and pressureless spark plasma sintering with graphene oxide.

13-93 bioglass (BG) scaffolds reinforced with graphene oxide (GO) were fabricated by robocasting (direct-ink-writing) technique. Composite scaffolds with 0-4 vol% content of GO platelets were printed, and then consolidated by pressureless spark plasma sintering at 650 °C. It was found that, despite hampering densification of the bioglass, the addition of GO platelets up to a certain content enhanced the mechanical performance of the 13-93 bioglass scaffolds in terms of strength and, especially, toughness. Best performance was obtained for 2 vol.% GO, which increased strain energy density (toughness) of the scaffolds by ∼894%, and their compressive strength by ∼26%. At higher contents, agglomeration of the nanoplatelets and increased porosity significantly reduced the mechanical enhancement obtained. Implications of the results on the fabrication of novel bioglass scaffolds that may find use in load-bearing bone tissue engineering applications are discussed.

Enhancing the mechanical and in vitro performance of robocast bioglass scaffolds by polymeric coatings: Effect of polymer composition.

The effect of different polymeric coatings, including natural and synthetic compositions, on the mechanical performance of 45S5 bioglass robocast scaffolds is systematically analyzed in this work. Fully amorphous 45S5 bioglass robocast scaffolds sintered at 550 °C were impregnated with natural (gelatin, alginate, and chitosan) and synthetic (polycaprolactone, PCL and poly-lactic acid, PLA) polymers through a dip-coating process. Mechanical enhancement provided by these coatings in terms of both compressive strength and strain energy density was evaluated. Natural polymers, in general, and chitosan, in particular, were found to produce the greater reinforcement. The effect of these coatings on the in vitro bioactivity and degradation behavior of 45S5 bioglass robocast scaffolds was also investigated through immersion tests in simulated body fluid (SBF). Coatings from natural polymers, especially chitosan, are shown to have a positive effect on the bioactivity of 45S5 bioglass, accelerating the formation of an apatite-like layer. Besides, most coating compositions reduced the degradation (weight loss) rate of the scaffold, which has a positive impact on the evolution of their mechanical properties.

Case study: Reinforcement of 45S5 bioglass robocast scaffolds by HA/PCL nanocomposite coatings.

The purpose of this study is to analyze the mechanical enhancement provided by nanocomposite coatings deposited on robocast 45S5 bioglass (BG) scaffolds for bone tissue regeneration. In particular, a nanocomposite layer consisting of hydroxyapatite (HA) nanoparticles, as reinforcing phase, in a polycaprolactone (PCL) matrix was deposited onto the surface of the BG struts conforming the scaffold. Three different HA nanopowders were used in this study. The effect of particle size and morphology of these HA nanopowders on the mechanical performance of 45S5 BG scaffolds is evaluated.

Understanding the role of dip-coating process parameters in the mechanical performance of polymer-coated bioglass robocast scaffolds.

The effect of different dip-coating variables-solvent, deposition temperature and polymer concentration-on the mechanical performance of polycaprolactone-coated 45S5 bioglass robocast scaffolds is systematically analyzed in this work. The reproducible geometry of the scaffolds produced by this additive manufacturing technique makes them an optimal model system and facilitates the analysis. The results suggest that the mechanical performance of the hybrid scaffolds is improved monotonically with polymer concentration, but this concentration cannot be increased indefinitely if the macroporosity interconnectivity, and thus the scaffold׳s capacity to promote tissue ingrowth, are to be preserved. An optimal concentration, and therefore viscosity (~1-4Pas in the present case), exists for any given set of process variables (scaffold geometry and material, polymer, solvent and process temperature) that yields coatings with optimal reinforcement and minimal reduction of scaffold functionality. Solvent and process temperature do not directly affect the strengthening provided by the polymeric coating. However they can determine the maximum concentration at the critical viscosity, and thereby the maximum achievable mechanical performance of the resulting hybrid scaffold.

Simulation of enamel wear for reconstruction of diet and feeding behavior in fossil animals: A micromechanics approach.

The deformation and wear events that underlie microwear and macrowear signals commonly used for dietary reconstruction in fossil animals can be replicated and quantified by controlled laboratory tests on extracted tooth specimens in conjunction with fundamental micromechanics analysis. Key variables governing wear relations include angularity, stiffness (modulus), and size of the contacting particle, along with material properties of enamel. Both axial and sliding contacts can result in the removal of tooth enamel. The degree of removal, characterized by a "wear coefficient," varies strongly with particle content at the occlusal interface. Conditions leading to a transition from mild to severe wear are discussed. Measurements of wear traces can provide information about contact force and particle shape. The potential utility of the micromechanics methodology as an adjunct for investigating tooth durability and reconstructing diet is explored.

Mechanics of microwear traces in tooth enamel.

It is hypothesized that microwear traces in natural tooth enamel can be simulated and quantified using microindentation mechanics. Microcontacts associated with particulates in the oral wear medium are modeled as sharp indenters with fixed semi-apical angle. Distinction is made between markings from static contacts (pits) and translational contacts (scratches). Relations for the forces required to produce contacts of given dimensions are derived, with particle angularity and compliance specifically taken into account so as to distinguish between different abrasives in food sources. Images of patterns made on human enamel with sharp indenters in axial and sliding loading are correlated with theoretical predictions. Special attention is given to threshold conditions for transition from a microplasticity to a microcracking mode, corresponding to mild and severe wear domains. It is demonstrated that the typical microwear trace is generated at loads on the order of 1N - i.e. much less than the forces exerted in normal biting - attesting to the susceptibility of teeth to wear in everyday mastication, especially in diets with sharp, hard and large inclusive intrinsic or extraneous particulates.

A model for predicting wear rates in tooth enamel.

It is hypothesized that wear of enamel is sensitive to the presence of sharp particulates in oral fluids and masticated foods. To this end, a generic model for predicting wear rates in brittle materials is developed, with specific application to tooth enamel. Wear is assumed to result from an accumulation of elastic-plastic micro-asperity events. Integration over all such events leads to a wear rate relation analogous to Archard׳s law, but with allowance for variation in asperity angle and compliance. The coefficient K in this relation quantifies the wear severity, with an arbitrary distinction between 'mild' wear (low K) and 'severe' wear (high K). Data from the literature and in-house wear-test experiments on enamel specimens in lubricant media (water, oil) with and without sharp third-body particulates (silica, diamond) are used to validate the model. Measured wear rates can vary over several orders of magnitude, depending on contact asperity conditions, accounting for the occurrence of severe enamel removal in some human patients (bruxing). Expressions for the depth removal rate and number of cycles to wear down occlusal enamel in the low-crowned tooth forms of some mammals are derived, with tooth size and enamel thickness as key variables. The role of 'hard' versus 'soft' food diets in determining evolutionary paths in different hominin species is briefly considered. A feature of the model is that it does not require recourse to specific material removal mechanisms, although processes involving microplastic extrusion and microcrack coalescence are indicated.

Effect of Polymer Infiltration on the Flexural Behavior of ß-Tricalcium Phosphate Robocast Scaffolds.

The influence of polymer infiltration on the flexural strength and toughness of ß-tricalcium phosphate (ß-TCP) scaffolds fabricated by robocasting (direct-write assembly) is analyzed. Porous structures consisting of a tetragonal three-dimensional lattice of interpenetrating rods were impregnated with biodegradable polymers (poly(lactic acid) (PLA) and poly(ε-caprolactone) (PCL)) by immersion of the structure in a polymer melt. Infiltration increased the flexural strength of these model scaffolds by a factor of 5 (PCL) or 22 (PLA), an enhancement considerably greater than that reported for compression strength of analogue materials. The greater strength improvement in bending was attributed to a more effective transfer of stress to the polymer under this solicitation since the degree of strengthening associated to the sealing of precursor flaws in the ceramic rod surfaces should remain unaltered. Impregnation with either polymer also improved further than in compression the fracture energy of the scaffolds (by more than two orders of magnitude). This increase is associated to the extraordinary strengthening provided by impregnation and to a crack bridging toughening mechanism produced by polymer fibrils.

Reinforcing bioceramic scaffolds with in situ synthesized ε-polycaprolactone coatings.

In situ ring-opening polymerization of ε-caprolactone (ε-CL) was performed to coat ß-tricalcium phosphate (ß-TCP) scaffolds fabricated by robocasting in order to enhance their mechanical performance while preserving the predesigned macropore architecture. Concentrated colloidal inks prepared from ß-TCP commercial powders were used to fabricate porous structures consisting of a three-dimensional mesh of interpenetrating rods. Then, ε-CL was in situ polymerized within the ceramic structure using a lipase as catalyst and toluene as solvent, to obtain a highly homogeneous coating and full impregnation of in-rod microporosity. The strength and toughness of scaffolds coated by ε-polycaprolactone (ε-PCL) were significantly increased (twofold and fivefold increase, respectively) over those of the bare structures. Enhancement of both properties is associated to the healing of preexisting microdefects in the bioceramic rods. These enhancements are compared to results from previous work on fully impregnated structures. The implications of the results for the optimization of the mechanical and biological performance of scaffolds for bone tissue engineering applications are discussed.

Impregnation of ß-tricalcium phosphate robocast scaffolds by in situ polymerization.

Ring-opening polymerization of ε-caprolactone (ε-CL) and L-lactide (LLA) was performed to impregnate ß-tricalcium phosphate (ß-TCP) scaffolds fabricated by robocasting. Concentrated colloidal inks prepared from ß-TCP commercial powders were used to fabricate porous structures consisting of a 3D mesh of interpenetrating rods. ε-CL and LLA were in situ polymerized within the ceramic structure by using a lipase and stannous octanoate, respectively, as catalysts. The results show that both the macropores inside the ceramic mesh and the micropores within the ceramic rods are full of polymer in either case. The mechanical properties of scaffolds impregnated by in situ polymerization (ISP) are significantly increased over those of the bare structures, exhibiting similar values than those obtained by other, more aggressive, impregnation methods such as melt-immersion (MI). ISP using enzymatic catalysts requires a reduced processing temperature which could facilitate the incorporation of growth factors and other drugs into the polymer composition, thus enhancing the bioactivity of the composite scaffold. The implications of these results for the optimization of the mechanical and biological performance of scaffolds for bone tissue engineering applications are discussed.

Improving the compressive strength of bioceramic robocast scaffolds by polymer infiltration.

The effect of polymer infiltration on the compressive strength of ß-tricalcium phosphate (TCP) scaffolds fabricated by robocasting (direct write assembly) is analyzed in this work. Porous structures consisting of a tetragonal three-dimensional mesh of interpenetrating rods were fabricated from concentrated TCP inks with suitable viscoelastic properties. Biodegradable polymers (polylactic acid (PLA) and poly(ε-caprolactone) (PCL)) were infiltrated into selected scaffolds by immersion of the structure in a polymer melt. Infiltration increased the uniaxial compressive strength of these model scaffolds by a factor of three (PCL) or six (PLA). It also considerably improved the mechanical integrity of the structures after initial cracking, with the infiltrated structure retaining a significant load-bearing capacity after fracture of the ceramic rods. The strength improvement in the infiltrated scaffolds was attributed to two different contributions: the sealing of precursor flaws in the ceramic rod surfaces and the partial transfer of stress to the polymer, as confirmed by finite element analysis. The implications of these results for the mechanical optimization of scaffolds for bone tissue engineering applications are discussed.

Finite element modeling as a tool for predicting the fracture behavior of robocast scaffolds.

The use of finite element modeling to calculate the stress fields in complex scaffold structures and thus predict their mechanical behavior during service (e.g., as load-bearing bone implants) is evaluated. The method is applied to identifying the fracture modes and estimating the strength of robocast hydroxyapatite and beta-tricalcium phosphate scaffolds, consisting of a three-dimensional lattice of interpenetrating rods. The calculations are performed for three testing configurations: compression, tension and shear. Different testing orientations relative to the calcium phosphate rods are considered for each configuration. The predictions for the compressive configurations are compared to experimental data from uniaxial compression tests.

Mechanical properties of calcium phosphate scaffolds fabricated by robocasting.

The mechanical behavior under compressive stresses of beta-tricalcium phosphate (beta-TCP) and hydroxyapatite (HA) scaffolds fabricated by direct-write assembly (robocasting) technique is analyzed. Concentrated colloidal inks prepared from beta-TCP and HA commercial powders were used to fabricate porous structures consisting of a 3-D tetragonal mesh of interpenetrating ceramic rods. The compressive strength and elastic modulus of these model scaffolds were determined by uniaxial testing to compare the relative performance of the selected materials. The effect of a 3-week immersion in simulated body fluid (SBF) on the strength of the scaffolds was also analyzed. The results are compared with those reported in the literature for calcium phosphate scaffolds and human bone. The robocast calcium phosphate scaffolds were found to exhibit excellent mechanical performances in terms of strength, especially the HA structures after SBF immersion, indicating a great potential of this type of scaffolds for use in load-bearing bone tissue engineering applications.

Fracture modes under uniaxial compression in hydroxyapatite scaffolds fabricated by robocasting.

The fracture modes of hydroxyapatite (HA) scaffolds fabricated by direct-write assembly (robocasting) are analyzed in this work. Concentrated HA inks with suitable viscoelastic properties were developed to enable the fabrication of prototype structures consisting of a 3-D square mesh of interpenetrating rods. The fracture behavior of these model scaffolds under compressive stresses is determined from in situ uniaxial tests performed in two different directions: perpendicular to the rods and along one of the rod directions. The results are analyzed in terms of the stress field calculated by finite element modeling (FEM). This analysis provides valuable insight into the mechanical behavior of scaffolds for bone tissue engineering applications fabricated by robocasting.

Fatigue and damage tolerance of Y-TZP ceramics in layered biomechanical systems.

The fatigue properties of fine-grain Y-TZP in cyclic flexural testing are studied. Comparative tests on a coarser-grain alumina provide a baseline control. A bilayer configuration with ceramic plates bonded to a compliant polymeric substrate and loaded with concentrated forces at the top surfaces, simulating basic layer structures in dental crowns and hip replacement prostheses, is used as a basic test specimen. Critical times to initiate radial crack failure at the ceramic undersurfaces at prescribed maximum surface loads are measured for Y-TZP with as-polished surfaces, mechanically predamaged undersurfaces, and after a thermal aging treatment. No differences in critical failure conditions are observed between monotonic and cyclic loading on as-polished surfaces, or between as-polished and mechanically damaged surfaces in monotonic loading, consistent with fatigue controlled by slow crack growth. However, the data for mechanically damaged and aged specimens show substantial declines in sustainable stresses and times to failure in cyclic loading, indicating an augmenting role of mechanical and thermal processes in certain instances. In all cases, however, the sustainable stresses in the Y-TZP remain higher than that of the alumina, suggesting that with proper measures to avoid inherent structural instabilities, Y-TZP could provide superior performance in biomechanical applications.

Materials design in the performance of all-ceramic crowns.

Results from a systematic study of damage in material structures representing the basic elements of dental crowns are reported. Tests are made on model flat-layer specimens fabricated from various dental ceramic combinations bonded to dentin-like polymer substrates, in bilayer (ceramic/polymer) and trilayer (ceramic/ceramic/polymer) configurations. The specimens are loaded at their top surfaces with spherical indenters, in simulation of occlusal function. The onset of fracture is observed in situ using a video camera system mounted beneath the transparent polymer substrate. Critical loads to induce fracture and deformation at the ceramic top and bottom surfaces are measured as functions of layer thickness and contact duration. Radial cracking at the ceramic undersurface occurs at relatively low loads, especially in thinner layers. Fracture mechanics relations are used to confirm the experimental data trends, and to provide explicit dependencies of critical loads in terms of key variables: material-elastic modulus, hardness, strength and toughness; geometric-layer thicknesses and contact radius. Tougher, harder and (especially) stronger materials show superior damage resistance. Critical loads depend strongly (quadratically) on crown net thickness. The analytic relations provide a sound basis for the materials design of next-generation dental crowns.

Fracture of ceramic/ceramic/polymer trilayers for biomechanical applications.

Fracture damage in trilayers consisting of outer and inner brittle layers bonded to a compliant (polycarbonate) substrate and subjected to concentrated surface loading is analyzed. The principal mode of fracture is radial cracking at the undersurface of the inner (core) layer, even in the strongest of core ceramics--other damage modes, including radial cracking in the outer (veneer) layer, are less invasive in these all-brittle coating systems. Tests on simple trilayer structures fabricated from glasses, sapphire, and dental ceramics are used to examine the dependence of the critical load for radial fracture in terms of relative outer/inner layer thickness and modulus, and inner layer strength. An explicit relation for the critical load, based on a flexing plate model in which the outer/inner bilayer is reduced to an "equivalent" monolithic coating with "effective" composite modulus, is used to examine these dependencies. The theoretical relation describes all the major trends in the critical load data over a broad range of variables, thus providing a sound basis for trilayer design. Relevance of the analysis to dental crowns and other biomechanical applications is a central theme of the study.