Showing papers in "Journal of The Mechanical Behavior of Biomedical Materials in 2020"

Functionally graded and multi-morphology sheet TPMS lattices: Design, manufacturing, and mechanical properties.

Abstract: Functionally graded and multi-morphology lattices are gaining increased attention recently in the tissue engineering research community because of the ability to control their physical, mechanical and geometrical properties spatially. In this work, relative density grading, cell size grading, and multi-morphology (lattice type grading) are mechanically investigated for sheet-based lattices with topologies based on triply periodic minimal surfaces (TPMS), namely; the Schoen Gyroid, and Schwarz Diamond minimal surfaces. To investigate the role of loading direction on the exhibited deformation mechanism, tests were performed parallel and perpendicular to the grading direction. For relative density grading, testing parallel to grading direction exhibited a layer-by-layer deformation mechanism with a lower Young's Modulus as compared to samples tested perpendicular to grading direction which exhibited a shear band deformation. Moreover, multi-morphology lattices exhibited a shift in deformation mechanism from layer-by-layer to the formation of a shear band at the interface between the different TPMS morphologies when tested parallel and perpendicular to hybridization direction, respectively. FE analysis revealed that sheet-networks multi-morphology lattices exhibit higher elastic properties as compared to solid-networks multi-morphology lattices.

Comprehensive review on nanocellulose: Recent developments, challenges and future prospects

Abstract: Cellulose constitutes most of a plant's cell wall, and it is the most abundant renewable polymer source on our planet. Given the hierarchical structure of cellulose, nanocellulose has gained considerable attention as a nano-reinforcement for polymer matrices in various industries (medical and healthcare, oil and gas, packaging, paper and board, composites, printed and flexible electronics, textiles, filtration, rheology modifiers, 3D printing, aerogels and coating films). Herein, nanocellulose is considered as a sustainable nanomaterial due to its substantial strength, low density, excellent mechanical performance and biocompatibility. Indeed, nanocellulose exists in several forms, including bacterial cellulose, nanocrystalline cellulose and nanofibrillated cellulose, which results in biodegradable and environmentally friendly bionanocomposites with remarkably improved material properties. This paper reviews the recent advances in production, physicochemical properties, and structural characterization of nanocelluloses. It also summarises recent developments in several multifunctional applications of nanocellulose with an emphasis on bionanocomposite properties. Besides, various challenges associated with commercialisation and economic aspects of nanocellulose for current and future markets are also discussed inclusively.

Mechanical performance of highly permeable laser melted Ti6Al4V bone scaffolds.

Abstract: Critically engineered stiffness and strength of a scaffold are crucial for managing maladapted stress concentration and reducing stress shielding. At the same time, suitable porosity and permeability are key to facilitate biological activities associated with bone growth and nutrient delivery. A systematic balance of all these parameters are required for the development of an effective bone scaffold. Traditionally, the approach has been to study each of these parameters in isolation without considering their interdependence to achieve specific properties at a certain porosity. The purpose of this study is to undertake a holistic investigation considering the stiffness, strength, permeability, and stress concentration of six scaffold architectures featuring a 68.46-90.98% porosity. With an initial target of a tibial host segment, the permeability was characterised using Computational Fluid Dynamics (CFD) in conjunction with Darcy's law. Following this, Ashby's criterion, experimental tests, and Finite Element Method (FEM) were employed to study the mechanical behaviour and their interdependencies under uniaxial compression. The FE model was validated and further extended to study the influence of stress concentration on both the stiffness and strength of the scaffolds. The results showed that the pore shape can influence permeability, stiffness, strength, and the stress concentration factor of Ti6Al4V bone scaffolds. Furthermore, the numerical results demonstrate the effect to which structural performance of highly porous scaffolds deviate, as a result of the Selective Laser Melting (SLM) process. In addition, the study demonstrates that stiffness and strength of bone scaffold at a targeted porosity is linked to the pore shape and the associated stress concentration allowing to exploit the design freedom associated with SLM.

Microarchitected 3D printed polylactic acid (PLA) nanocomposite scaffolds for biomedical applications.

Abstract: Anti-bacterial scaffolds made of copper, bronze and silver particles filled PLA nanocomposites were realized via fused filament fabrication (FFF), additive manufacturing. The thermal, mechanical and biological characteristics including bioactivity and bactericidal properties of the scaffolds were evaluated. The incorporation of bronze particles into the neat PLA increases the elastic modulus up to 10% and 27% for samples printed in 0° and 90° configurations respectively. The stiffness increases, up to 103% for silver filled PLA nanocomposite scaffolds. The surface of scaffolds was treated with acetic acid to create a thin porous network. Significant increase (~20-25%) in the anti-bacterial properties and bioactivity (~18-100%) is attributed to the synergetic effect of reinforcement of metallic/metallic alloy particles and acid treatment. The results indicate that PLA nanocomposites could be a potential candidate for bone scaffold applications.

Mechanical aspects of dental implants and osseointegration: A narrative review

Abstract: With the need of rapid healing and long-term stability of dental implants, the existing Ti-based implant materials do not meet completely the current expectation of patients. Low elastic modulus Ti-alloys have shown superior biocompatibility and can achieve comparable or even faster bone formation in vivo at the interface of bone and the implant. Porous structured Ti alloys have shown to allow rapid bone ingrowth through their open structure and to achieve anchorage with bone tissue by increasing the bone-implant interface area. In addition to the mechanical properties of implant materials, the design of the implant body can be used to optimize load transfer and affect the ultimate results of osseointegration. The aim of this narrative review is to define the mechanical properties of dental implants, summarize the relationship between implant stability and osseointegration, discuss the effect of metallic implant mechanical properties (e.g. stiffness and porosity) on the bone response based on existing in vitro and in vivo information, and analyze load transfer through mechanical properties of the implant body. This narrative review concluded that although several studies have presented the advantages of low elastic modulus or high porosity alloys and their effect on osseointegration, further in vivo studies, especially long-term observational studies are needed to justify these novel materials as a replacement for current Ti-based implant materials.

Laser powder bed fusion of titanium-tantalum alloys: Compositions and designs for biomedical applications.

Abstract: In this study, laser powder bed fusion (L-PBF), also known as selective laser melting (SLM), was used to fabricate samples of titanium-tantalum (TiTa) alloys with 0, 10, 30 and 50 wt% of tantalum using in-situ alloying. As-fabricated samples comprised of randomly-dispersed pure tantalum particles in a titanium-tantalum matrix. Porosity and unmelted tantalum particles of the samples were revealed using an optical microscope (OM). The microstructure of the alloys were determined by combination of field emission scanning electron microscopy (FE-SEM), electron back scatter diffraction (EBSD) and X-ray diffraction (XRD). The mechanical properties of the alloys were investigated with tensile and Vickers hardness tests. To ascertain the suitability of these alloys as biomaterials, Ti50Ta scaffolds with 60% porosity were characterized biologically. This study further shows that porous TiTa scaffolds fabricated using L-PBF are biocompatible with comparable biological results and manufacturability as Ti6Al4V and commercially pure titanium, based on the results obtained from cell culture with human osteosarcoma cell line SAOS-2.

Rational design, bio-functionalization and biological performance of hybrid additive manufactured titanium implants for orthopaedic applications: A review

Abstract: Evolution of metallurgy and biomaterials has progressively shifted the focus of metallic bone-interfacing implant design from adequate mechanical strength and biocompatibility to rapid osseointegration and infection inhibition. The now relatively well-established technology - powder bed additive manufacturing (AM), offers the ability to fabricate porous implants with precise mechanical properties, topological pore architectures and patient-specific design functions, has revolutionized the production of customized multifunctional metallic implants for the individual patient with anatomic-specific requirement. Even though AM titanium and its alloy Ti-6Al-4V have been investigated and adopted for clinical application for decades, the development of porous AM titanium implants is far from complete and further research is required to achieve excellent long-term clinical performance. In this review, we summarize the current status of AM in bone-interfacing implant fabrication, with particular focus on the experimental outcomes of various factors that influence osseointegration, bone and vascular ingrowth as well as hybrid strategies to combat infection, including: pore size, porosity, pore structure, surface modification techniques and incorporation of biological factors. In addition, we also discuss the osteogenic capacity of constructs fabricated through different manufacturing methods and titanium alloys. To this end, we highlight the exciting prospect of AM for bone-interfacing implant manufacture through optimization via material development, implant design, bio-functionalization to clinical evaluation to provide enhanced patient specificity and long-term function.

3D-printed PLA/HA composite structures as synthetic trabecular bone: A feasibility study using fused deposition modeling.

Abstract: 3D-printed PLA/HA composite structures as synthetic trabecular bone: a feasibility study using Fused Deposition Modelling

3D printed biodegradable functional temperature-stimuli shape memory polymer for customized scaffoldings.

Abstract: Shape memory polymers (SMPs) and their composites have become the prominent choice of the various industries owing to the unique inherent characteristics which can be stimulated through the exposure of external stimuli. The use of SMPs in the three-dimensional (3D) technologies has produced enormous advantages. However, the potential of SMPs in 3D printing has limitedly explored. In the present study, an investigation was performed to study the shape memory effect (SME) of the fused filament fabricated (FFF) chitosan (CS) reinforced poly-lactic-acid (PLA) based porous scaffolds. Firstly, the composite filaments, with 1, 1.5, and 2% wt. of CS, were fabricated by using the twin-screw extrusion process, which was later used to print the test specimens at different infill density. The printed samples were selectively pre-elongated to 2.5 mm and then processed through direct heating, at 60-70 °C, for enabling the SME. It has been observed that the CS particles acted as rigid phases and interrupted the re-ordering of PLA chain. However, the scaffoldings showed 18.8% shape recovery at optimized process parametric settings. In addition, wettability and biocompatibility analyses of developed scaffoldings have also been performed to investigate the biological aspects of the developed scaffoldings. The stimulated samples found to be possessed with good wettability and cell proliferation. Overall, the 3D printed PLA/CS porous scaffoldings have shown significant shape recovery characteristics and are biologically active to be used as self-healing implants for acute bone deficiencies.

Mechanical performance of auxetic meta-biomaterials.

Abstract: The innovative design of orthopedic implants could play an important role in the development of life-lasting implants, by improving both primary and secondary implant fixations. The concept of meta-biomaterials aims to achieve a unique combination of mechanical, mass transport, and biological properties through optimized topological design of additively manufactured (AM) porous biomaterials. In this study, we primarily focused on a specific class of meta-biomaterials, namely auxetic meta-biomaterials. Their extraordinary behavior of lateral expansion in response to axial tension could potentially improve implant-bone contact in certain orthopedic applications. In this work, a multitude of auxetic meta-biomaterials were rationally designed and printed from Ti-6Al-4V using a commercially available laser powder bed fusion process called selective laser melting. The re-entrant hexagonal honeycomb unit cell was used as a starting point, which was then parametrically tuned to obtain a variety of mechanical and morphological properties. In this two-step study, the morphology and quasi-static properties of the developed meta-biomaterials were assessed using mechanical experiments accompanied with full-field strain measurements using digital image correlation. In addition, all our designs were computationally modelled using the finite element method. Our results showed the limits of the AM processes for the production of auxetic meta-biomaterials in terms of which values of the design parameters (e.g., re-entrant angle, relative density, and aspect ratio) could be successfully manufactured. We also found that the AM process itself imparts significant influence on the morphological and mechanical properties of the resulting auxetic meta-biomaterials. This further highlights the importance of experimental studies to determine the actual mechanical properties of such metamaterials. The elastic modulus and strength of many of our designs fell within the range of those reported for both trabecular and cortical bone. Unprecedented properties like these could be used to simultaneously address the different challenges faced in the mechanical design of orthopedic implants.

Mechanical properties of polymeric implant materials produced by extrusion-based additive manufacturing.

Abstract: The application of material extrusion-based additive manufacturing methods has recently become increasingly popular in the medical sector. Thereby, thermoplastic materials are likely to be used. However, thermoplastics are highly dependent on the temperature and loading rate in comparison to other material classes. Therefore, it is crucial to characterise these influences on the mechanical properties. On this account, dynamic mechanical analyses to investigate the application temperature range, and tensile tests at different crosshead speeds (103, 101, 10-1 and 10-3 mms-1) were performed on various 3D-printable polymers, namely polyetheretherketone (PEEK), polylactide (PLA), poly(methyl methacrylate) (PMMA), glycol-modified poly(ethylene terephthalate) (PETG), poly(vinylidene fluoride) (PVDF) and polypropylene (PP). It was found that the mechanical properties of PEEK, PLA, PMMA and PETG hardly depend on temperature changes inside the human body. PVDF and PP show a significant decrease in stiffness with increasing body temperatures. Additionally, the dependency of the stiffness on the strain-rate is increasing between PLA, PP, PEEK, PETG, PMMA and PVDF. Besides the mechanical integrity of these materials (strength, stiffness and its strain-rate and temperature dependency inside the body), the materials were further ranked considering their filling density as a measure of their processability. Hence, useful information for the selection of possible medical applications for each material and the design process of 3D-printed implants are provided.

Rationally designed functionally graded porous Ti6Al4V scaffolds with high strength and toughness built via selective laser melting for load-bearing orthopedic applications.

Abstract: Functionally graded materials (FGMs) with porosity variation strategy mimicking natural bone are potential high-performance biomaterials for orthopedic implants. The architecture of FGM scaffold is critical to gain the favorable combination of mechanical and biological properties for osseointegration. In this study, four types of FGM scaffolds with different structures were prepared by selective laser melting (SLM) with Ti6Al4V as building material. All the scaffolds were hollow cylinders with different three-dimensional architectures and had gradient porosity resembling the graded-porous structure of human bone. Two unit cells (diamond and honeycomb-like unit cells) were used to construct the cellular structures. Solid support structures were embedded into the cellular structures to improve their mechanical performances. The physical characteristics, mechanical properties, and deformation behaviors of the scaffolds were compared systematically. All the as-built samples with porosities of ~52–67% exhibited a radial decreasing porosity from the inner layer to the outer layer, and their pore sizes ranged from ~420 to ~630 μm. The compression tests showed the Young's moduli of all the as-fabricated samples (~3.79–~10.99 GPa) were similar to that of cortical bone. The FGM structures built by honeycomb-like unit cells with supporting structure in outer layer exhibited highest yield strength, toughness and stable mechanical properties which is more appropriate to build orthopedic scaffolds for load-bearing application.

Does annealing improve the interlayer adhesion and structural integrity of FFF 3D printed PEEK lumbar spinal cages

Abstract: Polyaryletheretherketone (PEEK) has been commonly used for interbody fusion devices because of its biocompatibility, radiolucency, durability, and strength. Although the technology of PEEK Additive Manufacturing (AM) is rapidly developing, post-processing techniques of 3D printed PEEK remain poorly understood. AM of PEEK has been challenging because of its high melt temperature (over 340 °C) and requires specialized equipment which was not commercially available until recently. A lumbar fusion cage design, used in ASTM interlaboratory studies, was 3D printed with a medical grade PEEK filament via Fused Filament Fabrication (FFF) under two different print speeds. PEEK cages were then annealed above the PEEK's glass transition temperature, at 200 °C or 300 °C. AM cages were CT scanned to determine the porosity before and after annealing. Mechanical tests were conducted on cages according to ASTM F2077 (ASTM F2077, 2014). SEM images helped to evaluate the cages' surface morphology before and after heat treatment. It was observed that annealing did not produce markedly better mechanical properties at either temperature, however, it had an effect on the cages’ mechanical properties at lower printing speed under all loading conditions. Although the structure of the pores changed after annealing, annealing conditions examined here as a post-processing method were not able to decrease the undesired porosity formed during the 3D printing process or change the failure mechanism, which is due interlayer debonding.

3D printed porous PEEK created via fused filament fabrication for osteoconductive orthopaedic surfaces.

Abstract: Due to its unique and advantageous material properties, polyetheretherketone (PEEK) is an attractive biomaterial for implantable devices. Though concerns exist regarding PEEK for orthopaedic implants due to its bioinertness, the creation of porous networks has shown promising results for interaction with surrounding tissue. In this study, we created porous PEEK via clinically-available fused filament fabrication (FFF, 3D printing) and assessed the pore structure morphology, mechanical properties, and biologic response. The designs of the porous structures were based on a simple rectilinear pattern as well as triply periodic minimal surfaces (TPMS), specifically gyroid and diamond types. The material characteristics, including porosity, yield strength, and roughness, were evaluated using μCT, static compression testing, and optical profilometry. The porous PEEK, along with 3D printed solid PEEK, was then seeded with MC3T3-E1 preosteoblast cells for evaluation of cell proliferation and alkaline phosphatase (ALP) activity. The samples were then imaged via scanning electron microscopy (SEM) to observe cell morphology. μCT imaging showed the porous networks to be open and interconnected, with porous sizes similar (p > 0.05) to the as-designed size of 600 μm. Average compressive properties ranged from 210 to 268 MPa for elastic modulus and 6.6–17.1 MPa for yield strength, with strength being greatest for TPMS constructs. SEM imaging revealed cells attaching to and bridging micro-topological features of the porous constructs, and cell activity was significantly greater for the porous PEEK compared to solid at multiple time points.

In vitro comparison of 3D printed polylactic acid/hydroxyapatite and polylactic acid/bioglass composite scaffolds: Insights into materials for bone regeneration.

Abstract: 3D printing of polylactic acid (PLA) and hydroxyapatite (HA) or bioglass (BG) bioceramics composites is the most promising technique for artificial bone construction. However, HA and BG have different chemical composition as well as different bone regeneration inducing mechanisms. Thus, it is important to compare differentiation processes induced by 3D printed PLA + HA and PLA + BG scaffolds in order to evaluate the strongest osteoconductive and osteoinductive properties possessing bioceramics. In this study, we analysed porous PLA + HA (10%) and PLA + BG (10%) composites' effect on rat's dental pulp stem cells fate in vitro. Obtained results indicated, that PLA + BG scaffolds lead to weaker cell adhesion and proliferation than PLA + HA. Nevertheless, osteoinductive and other biofriendly properties were more pronounced by PLA + BG composites. Overall, the results showed a strong advantage of bioceramic BG against HA, thus, 3D printed PLA + BG composite scaffolds could be a perspective component for patient-specific, cheaper and faster artificial bone tissue production.

Biological and mechanical property analysis for designed heterogeneous porous scaffolds based on the refined TPMS.

Abstract: For tissue engineering (TE), triply periodic minimal surfaces (TPMSs) have received widespread application, as they produce smooth surfaces and pore interconnectivity, which can satisfy the biological/mechanical requirements and efficiently construct many complex bone scaffolds. To control the microstructure of the scaffold and mimic the anisotropy of native tissue, a design approach for heterogeneous porous scaffolds is proposed in this paper. It is carried out by discretizing the original model using the conformal refinement of an all-hexahedral mesh and mapping the TPMS units to the mesh elements with the help of a shape function. Another aim of this work is to assess the impact on the biologic/mechanical properties of the model, as it is discretized with different mesh patterns. It is found that the mesh pattern has a non-obvious effect on the surface curvature distribution that is a crucial factor to osteoblast proliferation in the TPMS scaffold. Nevertheless, the comparison presents that the mechanical properties of the refined scaffolds model exhibited anisotropy and improvements in elasticity, strengths, and especially energy absorption. With the help of conformal remeshing, the local density of architecture can be conveniently controlled and the elastic modulus of scaffold can be designed to the appropriate range in a specified area in order to mimic the actual cancellous bone.

On the permeability of TPMS scaffolds.

Abstract: This study presents an experimental evaluation of permeability of triply periodic minimal surfaces (TPMS). Permeability is widely used to characterize scaffolds for Tissue Engineering (TE) applications as it gives information about the structure porosity, pore size, tortuosity and pore interconnectivity which have an important impact in cell seeding and proliferation. Three different TPMS structures were used: Schwartz Diamond (SD), Gyroid (SG) and Schwartz Primitive (SP), in four different porosity levels (50, 60, 70 and 80%). Overall, the SG scaffold type was determined to be the most permeable one while the SD was the least permeable. Furthermore, the presence of microscopic inertial pressure losses was verified and the Forchheimer's law proved to be a good mathematical tool as a Darcy's law expansion for the calculation of the structure's permeability while the weak-inertia regime was hard to detect or quantify.

Effect of heat treatment on the anisotropic microstructural and mechanical properties of Co–Cr–Mo alloys produced by selective laser melting

Abstract: The purpose of this study was to evaluate the effect of heat treatment on the anisotropy of the microstructure and mechanical properties of cobalt–chromium–molybdenum (Co–Cr–Mo) alloys fabricated by selective laser melting (SLM). Dumbbell samples were fabricated with the axes deviating from the build direction by 0° (0°-sample), 45° (45°-sample), or 90° (90°-sample) and were subjected to heat treatment at various temperatures (750, 900, 1050, or 1150 °C) for 6 h. In samples heat-treated at 750, 900, and 1050 °C, the microstructures exhibited columnar grains with a fiber texture along the build direction, the same as in the as-built state. The mechanical properties showed anisotropy; the 0.2% offset yield strengths (YS) of the 0°-samples were lower than those of the 90°-samples, and the elongations of the 0°-samples were significantly higher than those of the 45°- and 90°-samples. By contrast, in samples heated to 1150 °C for 6 h, the anisotropic columnar grains completely disappeared, and equiaxed grains with random orientations were found in all samples, indicating that recrystallization had occurred. Moreover, the specific microstructures and texture generated during SLM disappeared. Regarding tensile properties, the initially strong anisotropy exhibited by the as-SLM samples was significantly reduced. Thus, heat treatment at the recrystallization temperature produced uniform equiaxed grains with random texture, which contributed to reducing the mechanical anisotropy of the SLMed Co–Cr–Mo alloys.

3D printing and characterization of a soft and biostable elastomer with high flexibility and strength for biomedical applications

Abstract: Recent advancements in 3D printing have revolutionized biomedical engineering by enabling the manufacture of complex and functional devices in a low-cost, customizable, and small-batch fabrication manner. Soft elastomers are particularly important for biomedical applications because they can provide similar mechanical properties as tissues with improved biocompatibility. However, there are very few biocompatible elastomers with 3D printability, and little is known about the material properties of biocompatible 3D printable elastomers. Here, we report a new framework to 3D print a soft, biocompatible, and biostable polycarbonate-based urethane silicone (PCU-Sil) with minimal defects. We systematically characterize the rheological and thermal properties of the material to guide the 3D printing process and have determined a range of processing conditions. Optimal printing parameters such as printing speed, temperature, and layer height are determined via parametric studies aimed at minimizing porosity while maximizing the geometric accuracy of the 3D-printed samples as evaluated via micro-CT. We also characterize the mechanical properties of the 3D-printed structures under quasistatic and cyclic loading, degradation behavior and biocompatibility. The 3D-printed materials show a Young's modulus of 6.9 ± 0.85 MPa and a failure strain of 457 ± 37.7% while exhibiting good cell viability. Finally, compliant and free-standing structures including a patient-specific heart model and a bifurcating arterial structure are printed to demonstrate the versatility of the 3D-printed material. We anticipate that the 3D printing framework presented in this work will open up new possibilities not only for PCU-Sil, but also for other soft, biocompatible and thermoplastic polymers in various biomedical applications requiring high flexibility and strength combined with biocompatibility, such as vascular implants, heart valves, and catheters.

Insights into unit cell size effect on mechanical responses and energy absorption capability of titanium graded porous structures manufactured by laser powder bed fusion

Abstract: Schwartz diamond graded porous structures (SDGPSs), constructed by a triply-periodic-minimal-surface diamond unit cell topology, were developed with various unit cell sizes and printed by laser powder bed fusion (LPBF) from a commercially pure titanium powder for bone implant applications. The effect of unit cell size on the printability, strut dimensions, stress and strain distributions, mechanical properties and energy absorption capability of SDGPSs was investigated. The results indicate the good printability of SDGPSs via LPBF with multiple unit cell sizes from 3.5 mm to 5.5 mm through the three-dimensional reconstruction from micro-computed tomography. The unit cell size plays a critical role in both strut diameters and specific surface areas of SDGPSs. An increase in the unit cell size leads to a reduction in the experimental Young's modulus from 673.08 MPa to 518.71 MPa and compressive yield strength from 11.43 MPa to 7.73 MPa. The mechanical properties of LPBF-printed SDGPSs are higher than those predicted by the finite element method, which is attributed to the higher volume fractions of the printed SDGPSs than the designed values. Furthermore, a rise in unit cell size leads to the decrease of energy absorption capability from 6.06 MJ/mm3 to 4.32 MJ/mm3 and exhibits little influence on the absorption efficiency. These findings provide a good understanding and guidance to the optimization on the unit cell size of functionally graded porous structures for desirable properties.

Chemistry of novel and contemporary resin-based dental adhesives.

Abstract: The chemistry of resin-based dental adhesives is critical for its interaction with dental tissues and long-term bonding stability. Changes in dental adhesives composition influences the materials' key physical-chemical properties, such as rate and degree of conversion, water sorption, solubility, flexural strength and modulus, and cohesive strength and improves the biocompatibility to dental tissues. Maintaining a suitable reactivity between photoinitiators and monomers is important for optimal properties of adhesive systems, in order to enable adequate polymerisation and improved chemical, physical and biological properties. The aim of this article is to review the current state-of-the-art of dental adhesives, and their chemical composition and characteristics that influences the polymerisation reaction and subsequent materials properties and performance.

Strength retention, corrosion control and biocompatibility of Mg-Zn-Si/HA nanocomposites.

Abstract: Owing to the poor load-bearing ability and apparent cytotoxicity of polymeric and ceramic materials, magnesium-based materials can be an ideal substitute for bone repair applications. Magnesium is bioresorbable, unlike other metallic materials like titanium and stainless steel, has excellent biocompatibility, compressive strengths and elastic modulus similar to the natural bone, which circumvents the need for secondary surgery post-implantation in vivo. Against this background, in this study, magnesium-based nanocomposites were developed by using hydroxyapatite bioceramic as a nano reinforcement. Magnesium-based alloys were synthesized using selective alloying elements and hydroxyapatite incorporated nanocomposites were processed using the disintegrated melt deposition technique. The microstructure characterization revealed that the addition of hydroxyapatite resulted in superior grain refinement of the magnesium alloy matrix. The addition of hydroxyapatite improved the yield strength of the alloy matrix and displayed superior strength and ductility retention post corrosion for 21 days, under compression loading. The presence of hydroxyapatite improved the hydrophilicity of the alloy matrix thereby aiding the biocompatibility properties with improved corrosion resistance, level 0 cytotoxicity, and high cell attachment. Hence, the present study strongly suggests that magnesium alloy-based hydroxyapatite nanocomposites can be a suitable candidate for bone repair applications.

In vitro characterization of a novel magnetic fibrin-agarose hydrogel for cartilage tissue engineering

Abstract: The encapsulation of cells into biopolymer matrices enables the preparation of engineered substitute tissues. Here we report the generation of novel 3D magnetic biomaterials by encapsulation of magnetic nanoparticles and human hyaline chondrocytes within fibrin-agarose hydrogels, with potential use as articular hyaline cartilage-like tissues. By rheological measurements we observed that, (i) the incorporation of magnetic nanoparticles resulted in increased values of the storage and loss moduli for the different times of cell culture; and (ii) the incorporation of human hyaline chondrocytes into nonmagnetic and magnetic fibrin-agarose biomaterials produced a control of their swelling capacity in comparison with acellular nonmagnetic and magnetic fibrin-agarose biomaterials. Interestingly, the in vitro viability and proliferation results showed that the inclusion of magnetic nanoparticles did not affect the cytocompatibility of the biomaterials. What is more, immunohistochemistry showed that the inclusion of magnetic nanoparticles did not negatively affect the expression of type II collagen of the human hyaline chondrocytes. Summarizing, our results suggest that the generation of engineered hyaline cartilage-like tissues by using magnetic fibrin-agarose hydrogels is feasible. The resulting artificial tissues combine a stronger and stable mechanical response, with promising in vitro cytocompatibility. Further research would be required to elucidate if for longer culture times additional features typical of the extracellular matrix of cartilage could be expressed by human hyaline chondrocytes within magnetic fibrin-agarose hydrogels.

Effect of hydroxyapatite concentration and size on morpho-mechanical properties of PLA-based randomly oriented and aligned electrospun nanofibrous mats.

Abstract: The growing demand for nanofibrous biocomposites able to provide peculiar properties requires systematic investigations of processing-structure-property relationships. Understanding the morpho-mechanical properties of an electrospun scaffold as a function of the filler features and mat microstructure can aid in designing these systems. In this work, the reinforcing effect of micrometric and nanometric hydroxyapatite particles in polylactic acid-based electrospun scaffold presenting random and aligned fibers orientation, was evaluated. The particles incorporation was investigated trough Fourier transform infrared spectroscopy in attenuated total reflectance. The morphology of the nanofibers was analyzed through scanning electron microscopy and it was correlated with the viscosity of polymeric solutions studied by rheological measurements. Scaffolds were mechanical characterized with tensile tests in order to find a correlation between the preparation method and the strength of the mats. The influence of hydroxyapatite particles on the crystallinity of the composites was investigated by differential scanning calorimetry. Finally, cell culture assays with pre-osteoblatic cells were conducted on a selected composite scaffold in order to compare the cell proliferation and morphology with that of polylactic acid scaffolds. Based on the results, we can prove that polylactic acid/hydroxyapatite composites can be one of the biomaterials with the greatest potential for bone tissue regeneration.

Mechanical properties and energy absorption characteristics of tropical fruit durian (Durio zibethinus).

Abstract: The paper presents for the first time the material properties and energy absorption capacity of durian shells with an attempt to use as an alternative sustainable material and mimic their structural characteristics to design a bio-inspired structure for protective packaging applications. A series of quasi-static compression tests were carried out to determine Young's modulus and bioyield stress of the durian shells as well as their energy absorption capacity. The mesocarp layers and thorns are interesting parts for investigating their energy absorption characteristics because they play an important role in protecting the flesh of durians during their drop impact onto the ground. The mesocarp layers of the shell were subjected to axial and lateral compression while the thorn specimens were compressed under axial loading with an increasing number of thorns. The results showed that the densification strain, plateau stress and specific energy absorption of the mesocarp layer under lateral loading is higher than that under axial loading. Furthermore, the compression tests on the thorns demonstrated that an increase in the number of thorns helped to absorb more energy and the specific energy absorption of the thorns was nearly two times higher than that of the mesocarp layer under the axial loading. In addition, the cyclic loading of the thorns showed that the extent of reversibility of deformation in the thorns decreases from 32% at the first cycle to around 10% at the 9th-cycle. Finally, the microstructure of the thorn and mesocarp layer was investigated to explain the experimental observation. The results indicated that the spherical shape associated with the thorns and mesocarp materials displayed an excellent energy absorption efficiency that can be mimicked to design an effective bio-inspired absorber for packing applications.

Machinability: Zirconia-reinforced lithium silicate glass ceramic versus lithium disilicate glass ceramic.

Abstract: Diamond grinding used in dental adjustment of high-strength zirconia-reinforced lithium silicate glass ceramic (ZLS) and lithium disilicate glass ceramic (LDGC) is challenging in restorative dentistry. This study aimed to compare the machinability of ZLS and LDGC in diamond grinding in terms of machining forces and energy, debris, surface and edge chipping damage. Grinding experiments in simulation of dental adjustment were conducted using a computer-assisted high-speed dental handpiece and coarse diamond burs. A piezoelectric force dynamometer and a high-speed data acquisition system were used for on-processing monitoring for assessment of grinding forces and energy. Grinding debris and grinding-induced surface and edge chipping damage were examined using scanning electron microscopy. The results show that grinding of ZLS required higher tangential and normal forces and energy than LDGC (p 0.05), continued efforts are required to explore new reinforcement technologies for optimized LDGC.

Mechanical performance of additively manufactured pure silver antibacterial bone scaffolds.

Abstract: Implant infection is a serious complication resulting in pain, mortality, prolonged recovery, and antimicrobial resistance (AMR). Reducing the risk-of-infection associated with tissue implants require imminent attention, where pure silver (Ag) offers enormous potential. However, the printability, mechanical performance nor microbial resistance of additively manufactured (AM) pure Ag is unavailable in literature. This is critical as Ag is thought to play a vital role in the development of AM patient-specific infection resistant implants in the decade to come. The study therefore additively manufactured 99.9% pure-Ag through selective laser melting (SLM) and systematically investigates its mechanical performance. The validated SLM process parameters were then used to conceive two fully porous bone scaffold each at approximately 68 and 90% (wt.) porosity. While the study brings to attention the potential defects in SLM pure-Ag through X-ray nanotomography (X-ray nCT), the mechanical properties of porous Ag scaffolds were found to be similar to cancellous bone. The study achieved the highest SLM pure-Ag density of 97% with Young’s modulus (E), elastic limit ( σ e ), yield strength ( σ y ), ultimate strength ( σ u l t ) and ultimate strain ( e u l t ) in the range of 15.5–17.8 GPa, 50.7–57.7 MPa, 57.6–67.2 MPa, 82.4–95.9 MPa and 0.07–0.10 respectively. The antimicrobial efficacy of printed silver was tested against the common implant infection-causing Staphylococcus aureus and led to 90% and 99.9% kill in 4 and 14 h respectively. The study, therefore, is a first step towards achieving a new generation Ag-based AM infection resistant porous implants.

Mechanical properties of diamond lattice Ti-6Al-4V structures produced by laser powder bed fusion: On the effect of the load direction.

Abstract: Laser powder bed fusion (L-PBF) techniques have been increasingly adopted for the production of highly personalized and customized lightweight structures and bio-medical implants. L-PBF can be used with a multiplicity of materials including several grades of titanium. Due to its biocompatibility, corrosion resistance and low density-to-strength ratio, Ti-6Al-4V is one of the most widely used titanium alloys to be processed via L-PBF for the production of orthopedic implants and lightweight structures. Mechanical properties of L-PBF Ti-6Al-4V lattice structures have mostly been studied in uniaxial compression and lately, also in tension. However, in real-life applications, orthopedic implants or lightweight structures in general are subjected to more complex stress conditions and the load directions can be different from the principal axes of the unit cell. In this research, the mechanical behavior of Ti-6Al-4V diamond based lattice structures produced by L-PBF is investigated exploring the energy absorption and failure modes of these metamaterials when the loading directions are different from the principal axis of the unit cell. Moreover, the impact of a heat treatment (i.e. hot isostatic pressing) on the mechanical properties of the aforementioned lattice structures has been evaluated. Results indicate that the mechanical response of the lattice structures is significantly influenced by the direction of the applied load with respect to the unit cell reference system revealing the anisotropic behavior of the diamond unit cell.

Assessment of the morphology and dimensional accuracy of 3D printed PLA and PLA/HAp scaffolds

Abstract: Introduction In complex clinical conditions when physiological bone regeneration is insufficient, there is a need to develop synthetic material-based scaffolds. The morphologic properties of porous scaffolds are of crucial importance. The dimensional accuracy of 3D printed scaffolds can be affected by a variety of factors. Materials and methods Three groups of 3D printed scaffolds were investigated: PLA1 (pure polylactic acid) printed with an FDM Ultimaker Original printer, PLA2 and composite PLA/hydroxyapatite (PLA/HAp) scaffolds printed with a Pharaoh XD 20. PLA/HAp filament was created with hot-melt extrusion (HME) equipment. The morphology of the prepared scaffolds was investigated with SEM, micro-CT and superimposition techniques, gravimetric and liquid displacement methods. Results Layer heights of PLA1 scaffolds varied the most. PLA1 scaffold volume statistically significantly differed from PLA2 (p 0.05). The total porosity of printed PLA/HAp scaffolds deviated the least from the original STL model. Conclusions This study showed that PLA/10% HAp filament fabricated with HME and printed with FFF 3D printer produced equal or even better accuracy of printed scaffolds than scaffolds printed with pure PLA filament. Further research is needed to analyze the effect of HAp on 3D scaffold morphology, accuracy, mechanical and biologic properties.

Comparison between novel strength-gradient and color-gradient multilayered zirconia using conventional and high-speed sintering

Abstract: Objectives To investigate and compare the mechanical and optical properties of novel strength- and color-gradient multilayered zirconia and the impact of conventional and high-speed sintering. Materials and methods Following zirconia materials were analyzed and compared: the high-speed sintered Katana Zirconia Block STML (4Y-TZP, KZC), the conventionally sintered Katana Zirconia Disc STML (4Y-TZP, KZL) and IPS e. max ZirCAD Prime (5Y-TZP/3Y-TZP, EZL). As control group acted the crystallized lithium disilicate ceramic IPS e. max CAD (ELC). Monolithic single molar crowns were fabricated and half of them were aged in a chewing simulator with human enamel antagonists (1.2 × 106 cycles, 50 N, lateral movement of 0.7 mm, 5/55 °C). The fracture load was tested in a universal testing machine (N = 96/n = 12). The two-body wear was determined using 3D matching of pre- and post-scans (N = 48/n = 12). Translucency (N = 36/n = 10) was evaluated with UV/Vis spectrophotometer. Data was analyzed using Kolmogorov-Smirnov test, one-way ANOVA followed by post-hoc Scheffe test, unpaired t-test and Weibull analysis (p Results Zirconia groups showed higher fracture load than ELC (p ≤ 0.001). Initially, all zirconia materials ranged in the same values (p > 0.05). After chewing simulation, EZL showed higher fracture load than KZC (p Conclusions The novel strength and color-gradient multilayered zirconia showed higher mechanical properties than lithium disilicate ceramic. The high-speed sintering of zirconia showed neither a negative impact on the fracture load nor on the two-body wear. However, the optical properties and the reliability of zirconia is lower than those of highly translucent lithium disilicate ceramic.