A review of bioceramic porous scaffolds for hard tissue applications: Effects of structural features

Tissue engineering has acquired remarkable attention as an alternative strategy to treat and restore bone defects during recent years. A scaffold is a fundamental component for tissue engineering, on which cells attach, proliferate and differentiate to form new desirable functional tissue. The composition, and structural features of scaffolds, including porosity and pore size, play a fundamental role in the success of tissue-engineered construct. This review summarizes the effect of porosity and pore size of bioceramic-based scaffolds on their mechanical properties and biological performances. The focus of this review is on scaffolds with porosities 40% and above. From the mechanical point of view, the degree of porosity is a more important factor than pore size and scaffolds with porosities greater than 40% were more likely to substitute trabecular bones. While for in vitro and in vivo performances, pore size appeared more influential feature and co-existence of macropores and micropores led to better bone formation.     

Additive manufacturing of SiOC scaffolds with tunable structure-performance relationship

Aiming at optimizing the performance of porous ceramics through structural optimization, this work explored the properties variation achieved by designing different patterns in SiOC log-pile structures fabricated by direct ink writing. Specifically, we investigated the effect of filament diameter, spacing between filaments and angle of deflection between adjacent layers on the compression strength and gas permeability of these structures. Results confirm that mechanical performance could be tuned by designing the structures’ architectural features, such as the spacing between filaments and the angle of deflection between layers, leading to changes in the contact area of filaments belonging to adjacent layers. Permeability decreased with varying angle of deflection from 90 ° to 15 °, due to the higher tortuosity of the flow paths. This enables to optimize the strength and permeability of the structure without reducing the porosity of the component.     

Improving mechanical properties of porous calcium phosphate scaffolds by constructing elongated gyroid structures using digital light processing

The present study reports the manufacturing of a novel type of porous calcium phosphate scaffolds with elongated gyroid structures using digital light processing (DLP), in order to offer significantly enhanced mechanical properties. In particular, solid camphor was employed as the diluent, in order to offer sufficiently low viscosity at high solid loading for conventional layer-by-layer DLP process. Four types of porous CaP scaffolds with different percent elongation (%EL = 0, 20, 40, and 60) were manufactured, and their porous structures and mechanical properties were characterized. All porous CaP scaffolds showed that CaP walls were elongated along the z-direction, while the degree of pore elongation increased with an increase in the designed %EL. Owing to the use of controlled processing parameters, such as layer thickness and exposure time for layer-by-layer photocuring process, and carefully designed debinding process, the photocured layers could be completely bonded together with high densification after sintering at 1,200 °C for 3 h. Such elongation of a gyroid structure offered significantly enhanced mechanical properties ? compressive strengths of 4.33 ± 0.26 MPa and 11.51 ± 1.75 MPa were obtained for the porous CaP scaffold with the %EL of 0 and 60, respectively.     

Additive manufacturing of tissues and organs

Additive manufacturing techniques offer the potential to fabricate organized tissue constructs to repair or replace damaged or diseased human tissues and organs. Using these techniques, spatial variations of cells along multiple axes with high geometric complexity in combination with different biomaterials can be generated. The level of control offered by these computer-controlled technologies to design and fabricate tissues will accelerate our understanding of the governing factors of tissue formation and function. Moreover, it will provide a valuable tool to study the effect of anatomy on graft performance. In this review, we discuss the rationale for engineering tissues and organs by combining computer-aided design with additive manufacturing technologies that encompass the simultaneous deposition of cells and materials. Current strategies are presented, particularly with respect to limitations due to the lack of suitable polymers, and requirements to move the current concepts to practical application.     

Additive manufacturing of lead-free KNN by binder jetting

Additive manufacturing of lead-free piezoceramics is of great interest, given the large request of application-oriented designs with optimal performances and reduced material consumption. Binder Jetting (BJ) is an additive manufacturing technique potentially suited to the production of ceramic components, however the number of feasibility studies on BJ of piezoceramics is extremely limited and totally lacking in the case of sodium-potassium niobate (KNN). In this work, as-synthesised powders are employed in the BJ 3D printing process. Microstructural properties, such as porosity, grain size distributions, and phase composition are studied by SEM, XRD and MIP (Mercury Intrusion Porosimetry) and compared to die-pressed pellets. Analyses reveal considerable residual porosity (~40%) regardless of the printing parameters, with a weak preferential orientation parallel to the printing plane. The piezoelectric characterization demonstrates an outstanding d₃₃ value of 80–90 pC N^?1. Finally, Figures of Merits for the employment as porous piezoceramics in the direct mode are presented.     

Additive manufacturing of zirconia ceramics by material jetting

The possibility of additive manufacturing of ceramics has been reported widely in scientific literature. This study investigates the potential of direct inkjet printing or material jetting of 3Y-TZP ceramics by assessing the microstructure and mechanical properties of the sintered printed parts. The technique allows to print in layers of 10.5 μm, with an as-printed green density of 58 % and nearly fully sintered density of 6.03 ± 0.1 g/cm³ (99.7 % TD). The dimensions of the green and sintered parts were highly accurate but showed an anisotropic roughness in function of the building direction, mainly due to the support structures. The biaxial bending and 4-point bending strength of the sintered material was found to be substantially higher in the XY direction than in the building (Z) direction. SEM and X-Ray computed tomography revealed the presence of delamination cracks, agglomerates and spherical pores, which were identified as fracture origins on fractured surfaces.     

Fabrication of bioceramic scaffolds with pre-designed internal architecture by gel casting and indirect stereolithography techniques

A new technique of combining the gel casting and indirect rapid prototyping methods was utilized to fabricate macroporous β-tricalcium phosphate (β-TCP) scaffolds, which provided an excellent control over the internal architecture of scaffolds and enhanced their mechanical properties. A stereolithography apparatus was used to produce resin molds for ceramic gel casting. These molds were filled with a water based thermosetting ceramic slurry which solidifies inside the mold. After burning the resin mold and sintering, the β-TCP scaffolds with designed pore architecture were obtained. The pore morphology, size, and distribution of the resulting scaffolds were characterized using a scanning electron microscope. X-ray diffraction was used to determine the crystal structure and chemical composition of scaffolds. The mechanical measurements showed that the average compressive strength was 16.1 ± 0.8 MPa.     

Dual-scale porous biphasic calcium phosphate gyroid scaffolds using ceramic suspensions containing polymer microsphere porogen for digital light processing

This study demonstrates a novel type of biphasic calcium phosphate (BCP) gyroid scaffolds featuring of gyroid macroporous structure and micropous BCP walls using poly(methyl methacrylate) (PMMA) microspheres as the porogen for ceramic digital light processing (DLP) technique. To tailor the microporosity of the BCP walls and the overall porosity of the dual-scale porous BCP scaffolds, the PMMA content with regard to the BCP powder was controlled in the range of 40 vol% to 70 vol%. After debinding at 600 °C and sintering at 1200 °C for 3 h, micropores were uniformly created throughout each BCP framework, while preserving 3?dimensional gyroid macroporous structures. As the PMMA content increased from 40 vol% to 70 vol%, the microporosity remarkably increased from 31.9 (±2.5) vol% to 55.2 (±1.4) vol%. This approach allowed the achievement of very high overall porosities (82.2–89.7 vol%) for the dual-scale porous scaffolds. However, all the scaffolds showed reasonable compressive strengths (0.8 MPa ?2.1 MPa), which are comparable to those of cancellous bones.     

Additive manufacturing of green ceramic by selective laser gasifying of frozen slurry

The slurry-based additive manufacturing (AM) of ceramics involves a drying process to form solid support; however, the drying process is time-consuming, and the support is not easily removed. We propose a new AM process for green ceramic that includes freezing a layer of aqueous ceramic slurry, laser gasifying of the frozen-layer ice to process 2D green ware, and removing the support in water to release the 3D ceramic part. With a suitable laser power and scanning speed, this approach can yield a layer that has a thickness of 90 μm, a cantilever structure with a wall thickness of 115 μm and a span of 30 mm without deflection. The casting layer cannot be damaged by using a cryopanel to rapidly freeze the slurry, and redundant frozen materials can be melted in water without swelling. Therefore, this new process can rapidly form a solid support and has a high removal efficiency.     

Additive manufacturing of zirconia parts by indirect selective laser sintering

Thermally induced phase separation (TIPS) was used to produce spherical polypropylene–zirconia composite powder for selective laser sintering (SLS). The influence of the composition of the composite starting powder and the SLS parameters on the density and strength of the composite SLS parts was investigated, allowing realizing SLS parts with a relative density of 36%. Pressure infiltration (PI) and warm isostatic pressing (WIPing) were applied to increase the green density of the ZrO₂–PP SLSed parts. Infiltrating the SLS parts with an aqueous 30 vol.% ZrO₂ suspension allowed to increase the sintered density from 32 to 54%. WIPing (135 °C and 64 MPa) of the SLS and SLS/infiltrated complex shape green polymer–ceramic composite parts prior to debinding and sintering allowed raising the sintered density of the 3 mol Y₂O₃ stabilized ZrO₂ parts to 92 and 85%, respectively.     

Additive manufacturing of electrofused mullite slurry by digital light processing

Mullite, one of the main refractory materials, has several applications that may demand tiny structures with complex geometries, and digital light processing (DLP) can produce such parts with outstanding dimensional precision and surface quality. In this work, electrofused mullite powder was used as a raw material for additive manufacturing by DLP. Photosensitive mullite suspensions were developed and their rheological behavior, stability, and thermal decomposition were investigated. Mullite parts were printed from suspensions with different ceramic loadings, debound, and sintered at different temperatures (from 1500 to 1650 °C). Density and strength increased with an increase in both solid loading and sintering temperature. Printed parts from slurry with 50 vol% of solid loading sintered at 1650 °C reached a relative density of 97.7 ± 0.3 % and flexural strength of 95.2 ± 5.0 MPa.     

Additive manufacturing of fiber reinforced ceramic matrix composites: Advances,challenges, and prospects

Fiber reinforced ceramic matrix composites (FRCMCs) have been used in various engineering fields. Additive manufacturing (AM) technologies provide new methods for fabricating FRCMCs and their structures. This review systematically reviews the additive manufacturing technologies of FRCMCs. In this review, the progress for additive manufacturing of FRCMCs were summarized firstly. The key scientific and technological challenges, and prospects were also discussed. This review aims to motivate the future research of the additive manufacturing of FRCMCs.     

Additive manufacturing of ceramics: Advances,challenges, and outlook

Additive manufacturing (AM) of ceramics is relatively more challenging with respect to polymers and metals, owing to their high melting temperatures and inherent brittleness. Thus, this review aims to provide a comprehensive survey of recent AM technologies successfully employed to produce net shape ceramic components. In recent years, several techniques have been developed and the latest progress in this field are highlighted, as well as the current challenges in the complex shaped ceramic parts production via AM technologies. The state of the art concerning the various 3D printing processes applied to the fabrication of ceramic components is discussed with, for each method, the presentation of its advantages, disadvantages, and possible applications. The potential of AM for producing complex shape ceramic components and the challenges to overcome are discussed as well.     

Sintering parameters study of a biphasic calcium phosphate bioceramic synthesized by alcoholic sol-gel technique

In the sintering of biphasic calcium phosphate bioceramics (BCP) can occur phases transformations accompanied by a sudden thermal expansion due to different coefficients of thermal expansion of each phase, which generates internal stress concentrations inducing undesired cracks within the sample. Therefore, this work aimed to study the sintering parameters of a BCP, composed of hydroxyapatite (HAp) and β-tricalcium phosphate (β-TCP), synthesized by the alcoholic sol-gel technique, in order to evaluate the better conditions for avoiding defect generation. BCP powders were uniaxially cold-pressed at 300 MPa and air-sintered at 1070 °C/2 h (BCP1070 sample) and 1130 °C/2 h (BCP1130 sample), with heating rates of 10 °C/min and 5 °C/min, respectively. Samples were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, relative density determination, Vickers hardness test, and nanohardness tests. The results displayed that the assessed sintering parameters were suitable for promoting a satisfactory particle consolidation. The transformation from HAp to β-TCP occurred simultaneously with grain growth and material densification under all conditions. The mechanical tests revealed that BCP1070 and BCP1130 samples have different behaviour when analyzed micro or nanostructurally. But, the heating rate (10 °C/min) combined with the sintering temperature (1070 °C) anabled to obtain a suitable sinterability with samples presenting smaller grain size and without defects. Further, it kept the microporosity which is an essential property for application in bioceramic materials.     

3D-printed barium strontium titanate-based piezoelectric scaffolds for bone tissue engineering

In order to promote bone healing, new generations of biomaterials are under development. These biomaterials should demonstrate proper biological and mechanical properties preferably similar to the natural bone tissue. In this research, 3D-printed barium strontium titanate (BST)/β-tricalcium phosphate (β-TCP) composite scaffolds have been synthesized as an alternative strategy for bone regeneration to not only induce appropriate bioactive characteristics but also piezoelectric behavior. The physical, chemical and biological performance of the scaffolds have been examined in terms of mechanical, dielectric properties, apatite-forming ability, Alizarin Red Staining (ARS), Alkaline Phosphatase activity (ALP), and cytotoxicity. The samples composed of 60% BST and 40% β-TCP showed the highest compressive strength, bending module, elastic modulus and the Young's modulus. The dielectric constant increased with further addition of the BST phase in the constructs. Scanning Electron Microscope (SEM) and energy dispersive X-ray (EDX) analyses showed that 60% BST/40% β-TCP sample had the highest amount of bone-like apatite formation after 28 days in simulated body fluid (SBF). Moreover, the results of ARS proved that 60% BST/40% β-TCP composite could present higher quantities of mineral deposition. The ALP activity of osteosarcoma cells on 60% BST/40% β-TCP sample showed higher activities compare with the other composites. None of the samples demonstrated any sign of toxicity using MTT test. It can be suggested that BST/β-TCP composite scaffolds can be potentially used as the next generation of bone tissue engineering scaffold materials.     

Green synthesis of calcium silicate bioceramic powders

Calcium silicates have proven to be potential candidates for biomedical applications because of their osteogenic properties. Sol–gel methods are typically used for the preparation of calcium silicate powders. However, in the sol–gel route, an acid or base and ethanol are used to catalyze the precursors. From the perspective of green chemistry, it is better to avoid the use of organic solvents. The objective of this study was to prepare calcium silicate powders using a green synthesis route (hydrothermal method) without organic solvents. The powders were also prepared via the sol–gel process using tetraethoxysilane (TEOS) and calcium nitrate as the raw materials for the purpose of comparison. The powders were sintered at temperatures ranging from 600 to 1000 °C after the application of both methods. To understand the feasibility of using the resulting materials in medical applications for bone repair, the powders were mixed with water to form cements. The results indicated that the powder composition was not significantly affected by the different techniques but was dependent on the Ca:Si ratio of the precursors and on the sintering temperature. The different techniques produced no differences in powder morphology. In addition, the setting times of the powder-derived cements were found to be independent of the sintering temperature and synthesis technique, but it was affected by the Ca:Si ratio of the precursors. The mechanical strength of the cements was similar. These encouraging results suggest that the hydrothermal method is a potentially beneficial alternative to the sol–gel route for the production of calcium silicate powders.     

Additive manufacturing,quasi-static and dynamic compressive behaviours of ceramic lattice structures

Ceramic lattice structures (CLSs) are used for construction in common and extreme environments because of the extraordinary properties of ceramics. In this study, we designed and additively manufactured CLSs with distinct structural parameters to explore their quasi-static and dynamic compressive behaviours in detail. It was demonstrated that both the relative density (?ρ) and inclination angle (ω) had a significant impact on the quasi-static and dynamic mechanical properties of the CLSs. Furthermore, the mathematical relationships between the quasi-static compressive properties, including quasi-static compressive strength (QS), quasi-static Young’s modulus (QY), and quasi-static energy absorption (QE), versus ?ρ and ω obeyed the Gibson–Ashby and Deshpande and Fleck models, respectively. It was revealed by experiment and simulation that as the stiffness increased, the quasi-static failure mode of the CLSs changed from a parallel-vertical-inclined mixed mode to a parallel-vertical mode. In addition, the relationship between the dynamic mechanical properties of the CLSs versus ?ρ and ω also followed the Gibson–Ashby and Deshpande and Fleck models. The exceptional dynamic increase factor indicated that CLSs are highly suitable for extreme environments. These findings will aid in the research and development of customised additively manufactured CLSs.     

Additive manufacturing of ceramic materials for energy applications: Road map and opportunities

Among engineering materials, ceramics are indispensable in energy applications such as batteries, capacitors, solar cells, smart glass, fuel cells and electrolyzers, nuclear power plants, thermoelectrics, thermoionics, carbon capture and storage, control of harmful emission from combustion engines, piezoelectrics, turbines and heat exchangers, among others. Advances in additive manufacturing (AM) offer new opportunities to fabricate these devices in geometries unachievable previously and may provide higher efficiencies and performance, all at lower costs. This article reviews the state of the art in ceramic materials for various energy applications. The focus of the review is on material selections, processing, and opportunities for AM technologies in energy related ceramic materials manufacturing. The aim of the article is to provide a roadmap for stakeholders such as industry, academia and funding agencies on research and development in additive manufacturing of ceramic materials toward more efficient, cost-effective, and reliable energy systems.     

Additive manufacturing of silicon carbide by selective laser sintering of PA12 powders and polymer infiltration and pyrolysis

In this work, we propose a novel hybrid additive manufacturing technique, which combines selective laser sintering (SLS) of polyamide powders and subsequent preceramic polymer infiltration and pyrolysis to manufacture Silicon Carbide components for complex architectures. By controlling the porosity of the sintered polymeric preform we are able to control the shrinkage upon the first infiltration and pyrolysis. This enabled the manufacturing of smaller features than those achievable with other manufacturing techniques. The mechanical strength of the resulting ceramic increased with the number of reinfiltration cycles up to 24 MPa, inversely the residual porosity decreased to 10 vol%. The microstructure showed two distinct phases of SiOC and SiC. The first was attributed to the interaction between the porous polyamide and the ceramic precursor during the first infiltration. SiC derived from the pyrolysis of the preceramic precursor alone.