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Improving the Compressive Strength of Bioceramic Robocast Scaffolds by Polymer Infiltration
Abstract
The effect of polymer infiltration on the compressive strength of β-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.
... d A fast fabrication method of high-resolution features using SLA printing [34]. e A DLP printer for the fabrication of a high-resolution 3D solid object and the composition of a photocurable resin [37] Recently, the projection type SLA printers are developed, where the pattern is formed when the UV light goes through a transparent window and solidifies the resin layerby-layer, as shown in Fig. 1.1b. In the DLP process, a digital mirror device is used to project the UV light on a whole layer to solidify the design pattern in one shot, as shown in Fig. 1.1c. ...
... One way is to print intrinsically stretchable materials such as polymers and composites containing nanomaterials. The other is to print stretchable architectures such as serpentine structure [64], wavy structure [47], web/mesh structure [65,66], origami/kirigami structure [67], scaffold/matrix structure [37,[68][69][70] and helix/spiral structure [64,71,72]. The 3D-printed materials can either be self-standing or on a stretchable substrate. ...
... Adjusting the composition of hard segments and soft segments in polymers can tailor the mechanical properties of the polymer to better fit with the human skin. For example, soft segments such as poly-(tetrahydrofuran) (PTMG), poly(propylene glycol) (PPG), and polycaprolactone diol (PCL-diol) were added to synthesize polyurethane acrylate oligomers (PUAs) for UV curable 3D patterning process to modify the tensile strength, elongation at break, and Young's modulus [37]. A homemade DLP 3D printer was used to print the modified ink layer by layer, as shown in Fig. 1.14a. ...
Four-dimensional (4D) printing, an innovative extension of three-dimensional (3D) printing, is defined as the additive manufacturing technology of intelligent components with controllable stimuli-responsive characteristics. The 4D-printed components can automatically and controllably change their shapes, properties, and/or functionalities with time in response to external stimuli, such as heat, moisture, light, PH, magnetism, and electricity. 4D printing integrates the design of structures’ intelligent behaviors into the fabrication processes, realizing integrated manufacturing of materials, structures, and functionalities. It has aroused worldwide attention in the academic and industrial communities since it was first proposed in 2013. Stimuli-response materials play a critical role in the relation of 4D printing. The stimuli-responsive characteristics of the 4D-printed components mainly depend on the properties of used stimuli-responsive materials and their combination and arrangement in 3D space. However, so far only limited stimuli-responsive materials for 4D printing have been developed, which greatly restricts to tap of the application potential of 4D printing. Thus, stimuli-responsive materials for 4D printing have become the hot and key spots of research in academia and engineering fields. In this chapter, after an introduction of the definition and prospect of 4D printing, the research advances in available stimuli-responsive materials for 4D printing were reviewed in detail in three categories: polymers and their composite materials, metals and their composite materials, and ceramics and their composite materials. This chapter may enhance our understanding of the 4D printing of stimuli-responsive materials and inspire further innovative ideas for the design of materials for 4D printing.
... Assuming that the whole system has anisotropic elastic behavior, the RVE is tested in compression experiments. The material's elastic modulus as the input parameter is shown in Table 1 [39,40]. The RVE is compressed in the thickness direction under a compression force in the normal direction of the two rigid plates. ...
... The XRD patterns of CS and PCL-coated scaffolds are indicated in Fig. 3 together with that of PCL. The diffraction peaks of the CS Table 1 Parameter settings for finite element analysis [39,40]. ...
... FEM can predict the mechanical behavior of complex structures like multilayer systems by providing the mechanical properties of the materials contained in the structure [41]. As shown in Fig. 7, under the same strain, the stress value of the CS/PCL1 is significantly higher than that of the CS, indicating that the PCL filled inside has the stress shielding effect on the CS scaffold [40]. Although CS/PCL1 showed higher stress values, the stress distribution of CS/PCL1 was more uniform than that of CS, indicating that the internally filled PCL had the effect of defect repair and reduced stress concentration [42]. ...
Calcium silicate (CS) is a suitable substrate for bone tissue engineering because it can provide bioactive
ions like Si4+ and Ca2+ to promote bone regeneration. However, the rapid degradation of CS leads to pH
problems and does not match the rate of osteogenesis. The 3D printed CS scaffolds were immersed in
Polycaprolactone (PCL) solution to obtain PCL-coated scaffolds with improved mechanical and biological
properties. Finite element method (FEM) analysis found that PCL impregnation has the effect of stress
shielding and defect healing, effectively improving the mechanical properties of CS porous scaffolds.
The degradation rate of PCL-coated scaffolds was significantly slowed down in Tris buffer. After 4 weeks
of degradation, the compressive strength of PCL-coated scaffolds remained at 23.34 MPa, maintaining
reliable mechanical properties. PCL coating significantly reduced the degradation rate of CS scaffolds,
and the pH value and Si ion concentration of Dulbecco’s modified Eagle’s medium (DMEM) soaked by
PCL-coated scaffolds were more favorable for cell survival. The results show that PCL-coated scaffolds
enhance cell proliferation and osteogenic differentiation, demonstrating the potential applicability of
bone tissue engineering applications.
... Infiltration of biodegradable polymers into the macropores of the scaffolds is one approach to the fabrication of composite scaffolds with improved mechanical performance (refer to Fig. 7D(i)). Martínez-Vázquez et al. [105] infiltrated robocast β-TCP scaffolds by immersing them into a melt of poly(e-caprolactone) (PCL) and polylactic acid (PLA) respectively. The compressive strength was increased by three (PCL) and six times (PLA), respectively, and the compressive strength of the PLAinfiltrated scaffolds was found comparable to that of cortical bone. ...
... The compressive strength was increased by three (PCL) and six times (PLA), respectively, and the compressive strength of the PLAinfiltrated scaffolds was found comparable to that of cortical bone. This is because the infiltrated polymer phase filled up the defects on strut surfaces and assisted in stress distribution upon loading, as suggested by finite element analysis [105]. Other than melt-immersion, an in-situ catalytic polymerization process has also been adopted to infiltrate β-TCP scaffolds. ...
... Full polymer infiltration into the macropores can lead to the most significant improvement in mechanical strength, as denoted by the green diamonds at low porosities. The maximum compressive strength is 105 MPa by the infiltration of PLA, accompanied by a significant reduction of porosity to 13% [105]. Polymer coating, on the other hand, preserves the porous structure and the strengthening effect is more moderate with compressive strength slightly higher than that of cancellous bone. ...
Among different treatments of critical-sized bone defects, bone tissue engineering (BTE) is a fast-developing strategy centering around the fabrication of scaffolds that can stimulate tissue regeneration and provide mechanical support at the same time. This area has seen an extensive application of bioceramics, such as calcium phosphate, for their bioactivity and resemblance to the composition of natural bones. Moreover, recent advances in additive manufacturing (AM) have unleashed enormous potential in the fabrication of BTE scaffolds with tailored porous structure as well as desired biological and mechanical properties. Robocasting is an AM technique that has been widely applied to fabricate calcium phosphate scaffolds, but most of these scaffolds do not meet the mechanical requirements for load-bearing BTE scaffolds. In light of this challenge, various approaches have been utilized to mechanically strengthen the scaffolds. In this review, the current state of knowledge and existing research on robocasting of calcium phosphate scaffolds are presented. Applying the Gibson-Ashby model, this review provides a meta-analysis from the published literature of the compressive strength of robocast calcium phosphate scaffolds. Furthermore, this review evaluates different approaches to the mechanical strengthening of robocast calcium phosphate scaffolds. The aim of this review is to provide insightful data and analysis for future research on mechanical strengthening of robocast calcium phosphate scaffolds and ultimately for their clinical applications.
... Despite the good biological properties of BAG, the major disadvantage of BAG is brittleness and insufficient elasticity, which limits its application to repair bone defects [10]. Composite fabrication is a current approach for the improvement of the mechanical properties of BAG [2,[11][12][13]. Kim et al. by fabricating PCL/bioactive glass (CaO-SiO 2 -P 2 O 5 -B 2 O 3 ) composites showed that the mechanical properties of the composites were significantly improved compared to pure bioactive glass scaffolds with the same porosity [14]. ...
... Consequently, preparing a composite of BAG with a biodegradable polymer can enable the scaffolds to possess pH resistance, adjusting the degradation of the polymer [15]. Another limitation of BAG is the fabrication of scaffolds with controlled porous structures by conventional techniques (solvent casting, fiber meshing, gas foaming, etc.) [13]. An appropriate three-dimensional (3D) scaffold with a well-designed porous structure, which promotes good interactions between bone tissue and biomaterials, can be produced by a new method such as 3D printing (fused deposition modeling and robocasting based on a computer-aided design (CAD) model) [1,16]. ...
In this study, three-dimensional polycaprolactone (PCL)-based scaffolds with controlled pore architecture were fabricated from sol-gel-derived bioactive glass containing 2% mol Ag (BAG) via robocasting technique. This method was implemented due to its advantageous features, including its high reproducibility, versatility in shapes and sizes, and customizability. The Taguchi method was employed to determine the experimental parameters for preparing optimized printable BAG /PCL nanocomposite inks, with five groups of printable inks. The printed scaffolds were characterized by scanning electron microscopy, simultaneous thermal analysis, Fourier transforms infrared spectroscopy and X-ray diffraction. The heat-treated BAG nanopowder at 550 °C exhibited an average particle size diameter of less than 15 nm with a homogenous silver distribution without any additional phase. Based on SEM images of BAG /PCL nanocomposite scaffolds, the regularity of printed structure depends on the weight% of powder and PCL. The BAG75P30 and BAG65P50 with 65 and 75%wt of BAG powder possessed the best regular structures (microscopic rods and also the well-designed macropores, lumen about 500 μm) with higher porosity (61–64%). All the fabricated scaffolds provided acceptable cell viability according to the MTT assay. The cells cultured on BAG75P30, BAG65P40, and BAG65P50 showed the highest ALP activity compared to other groups. Also, these three groups represented significant antibacterial properties among the groups. The 3D-printed BAG /PCL nanocomposite scaffolds with macro and micropores in the structure can be a promising candidate for bone tissue engineering to promote tissue restoration due to their structure and also antibacterial properties resulting from silver in the composition.
... However, the polymer layer may reduce the biological performance of the ceramic scaffolds and even change their absorbability because the polymer layer prevents calcium, phosphate and other appropriate ions from coming into direct contact with cells and body fluids [14,18,19]. To overcome this problem, full polymer infiltration, a process in which a polymer is introduced into a porous ceramic structure to form a co-continuous phase composite instead of a polymer coating, is receiving increasing attention [10,[20][21][22][23]. Indeed, co-continuous phase ceramic/polymer composites show not only a remarkable increase in strength, but also a toughness orders of magnitude higher than that of bare ceramic scaffolds due to the interconnectivity of the 3D polymeric phase interpenetrating structure. ...
... The compressive strength (σ c ) and flexural strength (σ f ) were evaluated by the maximum stress values obtained from the nominal stress-strain curves. The toughness of the compressed samples was estimated as the strain energy density for three values: the strain at the maximum compressive strength (G max ), at 10% strain (G 0.1 ) and at 20% strain (G 0.2 ) [20], obtained from the corresponding integrals of the nominal stress-strain curves. Similarly, the toughness of the three-point bending samples was estimated as the fracture energy density for two values: the strain at the maximum compressive strength (G max ) and at 5% strain (G 0.05 ) [9]. ...
Strong and tough β-TCP/PCL composite scaffolds with interconnected porosity were developed by combining digital light processing and vacuum infiltration. The composite scaffolds were comprised of pure β-TCP, β-TCP matrix composite and PCL matrix composite. The porous β-TCP/PCL composite scaffolds showed remarkable mechanical advantages compared with ceramic scaffolds with the same macroscopic pore structure (dense scaffolds). The composite scaffolds exhibited a significant increase in strain energy density and fracture energy density, though with similar compressive and flexural strengths. Moreover, the composite scaffolds had a much higher Weibull modulus and longer fatigue life than the dense scaffolds. It was revealed that the composite scaffolds with interconnected porosity possess comprehensive mechanical properties (high strength, excellent toughness, significant reliability and fatigue resistance), which suggests that they could replace the pure ceramic scaffolds for degradable bone substitutes, especially in complex stress environments.
... Robocasting is currently considered one of the most effective and powerful SFF methods for the processing of ceramics [5][6][7][8], glasses [9][10][11][12] and relevant composites [13,14]. This technique is based on the continuous extrusion of a filament through a nozzle onto a building platform on which the 3D structure of the product is layer-wise built [15]. ...
... In this regard, a great achievement related to the application of robocasting to BG-based materials was reported in 2014 by Eqtesadi et al. [18] who successfully produced vitreous grid-like 45S5 Bioglass® scaffolds with compressive strength (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13) and total porosity (60-80 vol%) suitable for BTE applications. ...
Robocasting is universally recognized as an affordable and reproducible manufacturing strategy to process glass and glass-ceramic materials in the form of highly ordered porous scaffolds for bone tissue engineering (BTE) applications. Nevertheless, while being widely applied to melt-derived bioactive glasses, this technique was seldom implemented with sol-gel materials due to the intrinsic difficulties in producing suitable inks for extrusion and thus, good printing outcomes. The present experimental work describes a new and relatively easy method to manufacture multicomponent sol-gel bioactive silicate scaffolds (oxide system: 47.5SiO2–20CaO–10MgO–10Na2O-10 K2O-2.5P2O5, mol.%) using dried gels as basic material within the ink composition, which allows by-passing intermediate heat treatments that are usually detrimental to the bioactive potential of the material in physiological environment. The scaffolds, exhibiting a total porosity of 81 vol.%, were characterized in terms of morphological-compositional features and bioactivity in simulated body fluid (SBF), paying special attention to ion release and surface modifications occurring upon soaking (apatite-forming ability). The compressive strength of the scaffolds (around 5 MPa) was comparable to that of human cancellous bone. Collectively, the results supported the possibility of using robocast sol-gel–derived scaffolds in BTE approaches. Furthermore, a comparison with robocast scaffolds based on a melt-derived glass with the same composition was reported in order to investigate the effect of the synthesis route on the dissolution behavior and morphological features of the final biomaterials.
... Essential in the construction of structural elements made of polymers[78].2.Compressive Stress Strength at Break: determines the maximum pressure a polymer can withstand before breaking. Important for assessing the resilience of polymer structures to mechanical forces[79].3.Compressive Stress Strength at Yield: measures the strength of a polymer under pressure before plastic deformation begins. Important for the preliminary evaluation of the material's structural reliability[80].4.Dynamic Compressive Properties Storage Modulus: characterizes the material's ability to store energy under dynamic loading. ...
This article investigates the utility of machine learning (ML) methods for predicting and analyzing the diverse physical characteristics of polymers. Leveraging a rich dataset of polymers’ characteristics, the study encompasses an extensive range of polymer properties, spanning compressive and tensile strength to thermal and electrical behaviors. Using various regression methods like Ensemble, Tree-based, Regularization, and Distance-based, the research undergoes thorough evaluation using the most common quality metrics. As a result of a series of experimental studies on the selection of effective model parameters, those that provide a high-quality solution to the stated problem were found. The best results were achieved by Random Forest with the highest R2 scores of 0.71, 0.73, and 0.88 for glass transition, thermal decomposition, and melting temperatures, respectively. The outcomes are intricately compared, providing valuable insights into the efficiency of distinct ML approaches in predicting polymer properties. Unknown values for each characteristic were predicted, and a method validation was performed by training on the predicted values, comparing the results with the specified variance values of each characteristic. The research not only advances our comprehension of polymer physics but also contributes to informed model selection and optimization for materials science applications.
... The interpenetrating polymer phase prevents the growth or propagation of cracks and keeps the broken ceramics together, even after large strains. This observation indicates the importance of polymer phases in biomimetic composite materials [57,58]. ...
Ceramic materials are highly regarded for their exceptional chemical and mechanical stability; however, their inherent brittleness limits their resistance to impact fractures. Researchers have investigated the design of biomimetic ceramic structures to overcome this problem, demonstrating the ability to toughen ceramics and expand their applications. However, the complexity of these structures makes the implementation of traditional processing methods challenging. To address this problem, a new preparation method for high-strength and high-toughness biomimetic ceramic structures is proposed. First, the primitive structure with the best mechanical performance was selected as the experimental object. Second, the vat photopolymerization printing process was optimized; further, primitive structures were designed with different wall thicknesses to investigate the effects of graded and variable-graded structures on the performance of triply periodic minimal surface structures. Finally, a polymer was used to impregnate the ceramic parts to prepare biomimetic structures. In the experiments, the biomimetic structures outperformed their pure ceramic counterparts in terms of toughness and avoided catastrophic failure. In particular, when the ceramic had graded structures, the energy absorption capability of the part increased by 202%. Finite-element modeling was used to analyze the stress concentration and distribution. Peridynamic simulations provided insights into the strengthening and toughening mechanisms at the micro level and guidance for developing damage-resistant, high-energy-absorbing materials.
... Numerous studies have recently concentrated on enhancing the bioceramics scaffolds' toughness by introducing a biodegradable polymeric phase through full impregnation or coating [109,145,160,[259][260][261][262][263][264]. While these methods undoubtedly improve mechanical properties, they also have drawbacks in terms of the biological performance of the scaffold. ...
Bone healing is an impressive ability of the human body, but critical-sized bone defects require external intervention. Bioceramic scaffolds with excellent biocompatibility and bioactivity have been developed to treat non-healing bone defects because of their unique features for bone repair. Meanwhile, the mechanical properties of the material continue to be disadvantageous. This review focuses on (i) essential factors in affecting and improving bioceramic-based scaffolds' mechanical properties, including porosity, pore size, methods, and material composition, and (ii) summarizing previous studies and highlighting strategies to fabricate scaffolds with improved mechanical properties such as using nano-particles, using a combination of bioceramics and polymers, and modifying scaffold surfaces. Further research is necessary to improve bioceramic scaffolds for bone repair applications.
... There are relatively few studies in the literature reporting the Weibull modulus for scaffolds based on other bioceramics or glass compositions by additive manufacturing technologies. Martinez-Vazquez et al. [41] increased the Weibull modulus of robocast phosphate scaffolds from 3.0 to more than 7.5 by polymer infiltration. The same strategy was reported by Eqtesadi et al. [42] in the case of robocast 13-93 scaffolds, revealing an analogous percentage improvement in terms of Weibull modulus after polymer infiltration (from 8.0 to 15.0). ...
... In FDM, a filament of the biomaterial is used for extrusion through a heated nozzle . Other frequently used extrusion-based techniques for bone tissue engineering are robocasting and direct writing, which use pastes of polymers such as polycaprolactone (PCL) and polylactic acid (PLA), ceramics such as bioglass and TCP and their composites for 3D printing (Dellinger et al., 2007;Heo et al., 2009;Martínez-Vázquez et al., 2010;Fu et al., 2011;Seyednejad et al., 2012;Serra et al., 2013). ...
... In FDM, a filament of the biomaterial is used for extrusion through a heated nozzle . Other frequently used extrusion-based techniques for bone tissue engineering are robocasting and direct writing, which use pastes of polymers such as polycaprolactone (PCL) and polylactic acid (PLA), ceramics such as bioglass and TCP and their composites for 3D printing (Dellinger et al., 2007;Heo et al., 2009;Martínez-Vázquez et al., 2010;Fu et al., 2011;Seyednejad et al., 2012;Serra et al., 2013). ...
... Another method to improve the mechanical properties of CaPO4 bioceramics is to cover the items by polymeric coatings [366][367][368] or infiltrate porous structures by polymers [369][370][371]; however, this is another topic. Other approaches are also possible [154]. ...
Various types of materials have been traditionally used to restore damaged bones. In the late 1960s, a strong interest was raised in studying ceramics as potential bone grafts due to their biomechanical properties. A short time later, such synthetic biomaterials were called bioceramics. Bioceramics can be prepared from diverse inorganic substances, but this review is limited to calcium orthophosphate (CaPO4)-based formulations only, due to its chemical similarity to mammalian bones and teeth. During the past 50 years, there have been a number of important achievements in this field. Namely, after the initial development of bioceramics that was just tolerated in the physiological environment, an emphasis was shifted towards the formulations able to form direct chemical bonds with the adjacent bones. Afterwards, by the structural and compositional controls, it became possible to choose whether the CaPO4-based implants would remain biologically stable once incorporated into the skeletal structure or whether they would be resorbed over time. At the turn of the millennium, a new concept of regenerative bioceramics was developed, and such formulations became an integrated part of the tissue engineering approach. Now, CaPO4-based scaffolds are designed to induce bone formation and vascularization. These scaffolds are usually porous and harbor various biomolecules and/or cells. Therefore, current biomedical applications of CaPO4-based bioceramics include artificial bone grafts, bone augmentations, maxillofacial reconstruction, spinal fusion, and periodontal disease repairs, as well as bone fillers after tumor surgery. Prospective future applications comprise drug delivery and tissue engineering purposes because CaPO4 appear to be promising carriers of growth factors, bioactive peptides, and various types of cells.
... Overcoming this limitation is of great importance since one of the main drawbacks of bioceramic scaffolds is their low strength. In this sense, a simple way to increase the scaffolds strength, without adding additional polymeric phases [18][19][20], would be to reduce the diameter of the bioceramic struts. In this way, Weibull size effect [21], associated to a reduction in the population of large precursor defects responsible for the fracture, would produce an increase in the ceramic rod intrinsic strength and, consequently, improve the overall strength of the scaffold. ...
A novel method for obtaining ceramic (tricalcium phosphate, TCP) fibres with a small diameter (below 0.1 mm) is proposed and its potential use in the 3D printing of scaffolds for biomedical applications is explored. An ink consisting of a high solid content (40 vol%) ceramic slurry in a photocurable resin was prepared and extruded using near-field electrospinning. The influence of the electric potential, flow rate, and distance between tip and collector on the fabrication process in static mode were studied and the role played by unidirectional motion of the collector was also analyzed. A one order of magnitude reduction in the diameter of the jet to around 30 μm is demonstrated under static conditions, which increased to around 100 μm when collector was displaced. Continuous fibres were deposited but the slurry spread over the collector. The method was implemented on a DIW system, using in-flight UV light curing to prevent the spreading of the ink upon deposition. The feasibility of the strategy was demonstrated, although challenges remain for the optimization and control of the fabrication process. Nevertheless, these preliminary results suggest this could be a promising alternative to produce 3D ceramic scaffolds for biomedical applications with improved spatial resolution.
... For this reason, research is currently relevant, according to the results of which it will be possible to create composite materials for biomedical purposes, which will combine biological activity and mechanical properties comparable to bone tissue [19]. Composite materials of hydroxyapatite with synthetic biocompatible polymers can improve the mechanical and biocompatible properties of hydroxyapatite [20,21]. ...
At the moment, the field of biomedical materials science is actively developing, which aims at creating new functional materials. A developing direction in biomedical materials science is that towards the treatment of diseases associated with bone tissue disorders, using biodegradable composite materials based on polymer and calcium phosphate materials. We developed a material based on polyvinyl alcohol cryogel, mineralized with calcium phosphate. A material based on cryogel of polyvinyl alcohol mineralized with calcium phosphate was developed. The composites were obtained by the method of cyclic freezing–thawing, and the synthesis of calcium phosphates was carried out in situ with heating, stirring, and exposure to microwave radiation. The phase composition, as well as the composition of functional groups, was determined by IR spectroscopy and X-ray phase analysis. Monocytes isolated from human blood showed higher viability compared to the controls.
... Scaffold strength is also critically important and may be in the range of 14 to 59 MPa for cartilage [41], 1 to 12 MPa for cancellous bone [42], and 50 to 190 MPa for cortical bone [42]. Tissue engineering scaffold strengths may range from 1 to 9 MPa for polycaprolactone [9,26,27,45], 60 to 130 MPa for ceramic polymer composites [46], and 16 to 180 MPa for ceramic [6,10,44]. As with scaffold stiffness, choosing an appropriate material is essential to achieve an appropriate strength, but it is also critical to effectively design the scaffold geometry since this can affect strength by almost an order of magnitude [26]. ...
... In recent years, progressive has been made in LEF research and application, including slurry preparation mechanism and expansion of application fields to include biology, ceramic, food, medical and electronics, etc [98][99][100][101][102][103][104] . LEF enables a cheaper and faster manufacturing process compared with other AM methods, and shows advantages such as suitability for various materials, and reduced size shrinkage [105][106] . ...
The application of additive manufacturing technology is one of the main approaches to achieving the rapid casting. Additive manufacturing technology can directly prepare casting molds (cores) with no need of patterns, and quickly cast complex castings. The combination of additive manufacturing and traditional casting technology can break the constraint of traditional casting technology, improve casting flexibility, and ameliorate the working environment. Besides, additive manufacturing promotes the realization of “free casting”, greatly simplifying the processing procedures and shortening the manufacturing cycle. This paper summarizes the basic principle of additive manufacturing technology and its development situation domestically and overseas, mainly focusing on the development status of several main additive manufacturing technologies applicable to the foundry field, including three-dimensional printing, selective laser sintering, stereolithography, layered extrusion forming, etc. Finally, the future development trend of additive manufacturing technology in the foundry field is prospected.
... DIW is also pushing the boundaries when it comes to the fabrication of bioceramic implants since the porosity helps growing body tissues for the implant [48,110]. Fabrication of artificial bone scaffolds using hydroxyapatite and calcium phosphate materials via DIW method has Fig. 17. ...
Along with vast research on the additive manufacturing (AM) of polymeric and metallic materials, three-dimensional (3D) manufacturing of ceramic materials is now the modern trend. Among all the additive manufacturing techniques, Direct Ink Writing (DIW) permits the ease of design and rapid manufacturing of ceramic-based materials in complicated geometries. This paper presents an outline of the contributions and tasks in fabrication 3D ceramic parts by the DIW technique. The current state-of-the-art manufacturing of various ceramics such as alumina, zirconia, and their composites through Direct Ink Writing (DIW) is described in detail. Moreover, this review paper aims at the innovations in the DIW approach of ceramic materials and introduces the progression of the DIW for the manufacturing of ceramics. Most importantly, the DIW technique has been explained in detail with illustrations. The prospects and challenges related to the DIW technique are also underscored.
... Multiple techniques reported in the literature have attempted to enhance the mechanical properties as well as the bioactivity of β-TCP-based materials, including doping with other ceramics (such as magnesia, zinc oxide, and silica) [37] and infiltration with various polymer solutions [58,59]. The approach of 3D printing by direct-ink writing and then sintering the resultant scaffolds designed, developed, and tested in the present study lends itself well to introducing dopants either during the preparation of the bioink or during the printing process. ...
Beta-tricalcium phosphate (β-TCP)-based bioinks were developed to support direct-ink 3D printing-based manufacturing of macroporous scaffolds. Binding of the gelatin:β-TCP ink compositions was optimized by adding carboxymethylcellulose (CMC) to maximize the β-TCP content while maintaining printability. Post-sintering, the gelatin:β-TCP:CMC inks resulted in uniform grain size, uniform shrinkage of the printed structure, and included microporosity within the ceramic. The mechanical properties of the inks improved with increasing β-TCP content. The gelatin:β-TCP:CMC ink (25:75 gelatin:β-TCP and 3% CMC) optimized for mechanical strength was used to 3D print several architectures of macroporous scaffolds by varying the print nozzle tip diameter and pore spacing during the 3D printing process (compressive strength of 13.1 ± 2.51 MPa and elastic modulus of 696 ± 108 MPa was achieved). The sintered, macroporous β-TCP scaffolds demonstrated both high porosity and pore size but retained mechanical strength and stiffness compared to macroporous, calcium phosphate ceramic scaffolds manufactured using alternative methods. The high interconnected porosity (45–60%) and fluid conductance (between 1.04 ×10 ⁻⁹ and 2.27 × 10 ⁻⁹ m ⁴ s/kg) of the β-TCP scaffolds tested, and the ability to finely tune the architecture using 3D printing, resulted in the development of novel bioink formulations and made available a versatile manufacturing process with broad applicability in producing substrates suitable for biomedical applications.
... This technique has been mostly used to fabricate a ceramic-based scaffold for orthopaedic applications (Miranda et al., 2006;Miranda et al., 2008;Miranda et al., 2007). The strength of the ceramic scaffold has also been increased using polymer infiltration (Martínez-V azquez et al., 2010). Recently, Shao et al. (2017) developed a magnesium-doped Wollastonite/TCP biphasic scaffold to increase the rate of new bone growth and regeneration capacity as compared to pure ceramic scaffolds. ...
Purpose
Additive manufacturing (AM) or solid freeform fabrication (SFF) technique is extensively used to produce intrinsic 3D structures with high accuracy. Its significant contributions in the field of tissue engineering (TE) have significantly increased in the recent years. TE is used to regenerate or repair impaired tissues which are caused by trauma, disease and injury in human body. There are a number of novel materials such as polymers, ceramics and composites, which possess immense potential for production of scaffolds. However, the major challenge is in developing those bioactive and patient-specific scaffolds, which have a required controlled design like pore architecture with good interconnectivity, optimized porosity and microstructure. Such design not only supports cell proliferation but also promotes good adhesion and differentiation. However, the traditional techniques fail to fulfill all the required specific properties in tissue scaffold. The purpose of this study is to report the review on AM techniques for the fabrication of TE scaffolds.
Design/methodology/approach
The present review paper provides a detailed analysis of the widely used AM techniques to construct tissue scaffolds using stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), binder jetting (BJ) and advanced or hybrid additive manufacturing methods.
Findings
Subsequently, this study also focuses on understanding the concepts of TE scaffolds and their characteristics, working principle of scaffolds fabrication process. Besides this, mechanical properties, characteristics of microstructure, in vitro and in vivo analysis of the fabricated scaffolds have also been discussed in detail.
Originality/value
The review paper highlights the way forward in the area of additive manufacturing applications in TE field by following a systematic review methodology.
In addition to extensive research on polymer and metal three-dimensional (3D) printing, ceramic 3D printing has recently been highlighted in various fields. The biggest advantage of 3D printing has the ability to easily create any complex shape. This review introduces the 3D printing technology of ceramics according to the type of material and deals with the latest related research in the industrial field including the biomedical engineering field. Finally, the future of ceramic 3D printing technology available in dentistry will be discussed.
3D‐printed bioceramic scaffolds offer great potential for bone tissue engineering (BTE) but their inherent brittleness and reduced mechanical properties at high porosities can easily result in catastrophic fractures. Herein, this study presents a hierarchical hydrogel impregnation strategy, incorporating poly(vinyl alcohol) (PVA) hydrogel into the macro‐ and micropores of bioceramic scaffolds and synergistically reinforcing it via freeze‐casting assisted solution substitution (FASS) in a tannic acid (TA)–glycerol solution. By effectively mitigating catastrophic brittle failures, the hydrogel‐impregnated scaffolds showcase three‐ and 100‐fold enhancement in mechanical energy absorption under compression (5.05 MJ m⁻³) and three‐point bending (3.82 MJ m⁻³), respectively. The reinforcement mechanisms are further investigated by experimental and simulation analyses, revealing a multi‐scale synergy of fracture and fragmentation resistance through macro and micro‐scale fiber bridging, and nano and molecular‐scale hydrogel reinforcement. Also, the scaffolds acquire additional antibacterial and drug‐loading capabilities from the hydrogel phase while maintaining favorable cell biocompatibility. Therefore, this study demonstrates a facile yet effective approach for preparing brittle‐failure‐free bioceramic scaffolds with enhanced biological functionalities, showcasing immense potential for BTE applications.
Due to their unique and desirable characteristics, ceramic composites have gained significant recognition as a versatile and promising class of materials. These materials offer lightweight design, high strength, high-temperature resistance, and notable corrosion and wear resistance, making them highly suitable for various applications. Additionally, their bioadaptability has opened up exciting possibilities in the biomedical field. This review study examines the recent additive manufacturing techniques employed for producing ceramic composite parts. The discussion encompasses identifying major areas where these materials can be customized to withstand high-velocity impacts, serve as protective coatings for industrial gas turbine applications, facilitate the production of aircraft brakes and engine components, and find application in the biomedical field. This review also explores the latest advancements in these research areas, highlighting the advantages of using ceramic composites while addressing potential drawbacks to identify the areas to be improved in the next generation of ceramic materials. In summary, this comprehensive review contributes to the progress and innovation of ceramic composite technology based on recent advancements found in the literature.
During the fabrication of scaffolds for bone regeneration, it is difficult to concurrently achieve bioactivity, mechanical performance, and ease of fabrication. Additionally, implant surface functionalization is a topic of intense research to improve bone-to-implant interaction and augment bone repair. Accordingly, this study reports additively manufactured (AM) 45S5 Bioglass scaffolds reinforced with functionalized multi-walled carbon nanotubes (CNTs) that were dip-coated with cellulose nanowhiskers (CNWs). Carboxymethyl cellulose (CMC) was used as an ink carrier that showed suitable shear thinning behavior. The fabricated scaffolds were characterized using x-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), and energy dispersive x-ray spectroscopy (EDS). 45S5 Bioglass reinforcement with CNTs and coating with CNWs led to an increase in the compressive strength from 20.5 to 27 MPa (∼32% enhancement), while the toughness increased from 2.08 to 3.92 MJ/m³ (∼88% enhancement). Additionally, structural analysis based on microcomputed tomography images showed that the AM-fabricated scaffolds exhibited suitable porosity, pore size, pore throat size, and interconnectivity. Moreover, the coating of the scaffold with CNWs increased the surface roughness, which may aid in bone cell attachment on the scaffold surface. Finally, these scaffolds were found to be bioactive, as revealed by in vitro studies in simulated body fluid (SBF). These results show the potential for efficient fabrication of hybrid scaffolds with controlled structure, bioactivity, and required toughness as well as strength for bone tissue engineering.
Additive manufacturing (AM) has attracted increasing attention from the scientific community due to its huge potential to fabricate ceramic components without the use of expensive tooling. In addition, it has a layer-by-layer building principle, making it a suitable candidate for the creation of geometrically complex, functionalized, customized architectures that could not be realized at all, or could only be produced at great cost, using conventional technology. However, there has been a slower industrial adoption of additively manufactured ceramic components compared to polymers and metals. Several limitations must be addressed to extend the widespread adoption of AM by industrial end-users. This study provides a review covering the current state of the AM of advanced ceramics, culminating in a comprehensive evaluation of both the advantages and the limitations of each AM process and emphasising the achievements of advanced ceramics and ceramic matrix composites made using AM in terms of characterization and applications. The outcome is the provision of guidelines for ceramicists for optimal AM process selection considering a given material, and most suitable ceramic materials for AM specialists considering a specific application. Furthermore, this chapter also presents case studies illustrating the advantages of ceramic AM.
Bioceramics have been widely used in the substitution of diseased bone and the repair of bone defects. Bioinert ceramics, such as alumina and zirconia, are traditionally applied in hip and knee replacement benefiting from their superb wear resistance, corrosion resistance, and crack propagation resistance. Bioactive ceramics and biodegradable ceramics, such as hydroxyapatite, bioactive glass, tricalcium phosphate, and silicates, are developed as bone fillers, tissue engineering scaffolds, coatings, and bone cements because of their outstanding bioactivity, osteoconductivity, and bioresorbability. Herein, various bioceramics and their mechanical properties and biological properties in the field of bone regeneration are presented. The biological properties of bioceramics such as protein adsorption, immunomodulation, vascularization, and osteogenesis will be highlighted.
The scaffold is essential to tissue engineering. In particular, the mechanical property of scaffolds has a significant impact on the success rate of regeneration. While numerous techniques exist for measuring mechanical properties, Compression test, three-point bending test, and nano-indentation test are the most common. Nevertheless, the mechanical property of porous structures cannot be accurately measured by previous testing methods. Combining superposition principles with the Flamant solution, this study developed semi-analytical solutions. Through compression testing and FEM simulation, the semi-analytical solution was fully validated. The solution can calculate not only the maximum stress of layer-by-layer construction of complex 3D scaffolds, but also the maximum load-bearing capacity if the mechanical property of the material is known.
Damage-tolerant ceramics with great toughness are highly required for a variety of practical applications owing to their outstanding chemical and mechanical stability, but current processing strategies are impossible to create parts with complex or customized geometries due to restrictions on the shape of mold. This work reports a promising method to fabricate geometrically complex ceramic composite components with exceptional damage tolerance by exploiting additive manufacturing (AM) and novel biomimetic toughening design. As-fabricated ceramic composites avoid catastrophic failure and exhibit remarkable improvements in toughness (≈116 times) comparable to those of pure ceramics, and possess customized geometries that cannot be created by conventional method. Such bioinspired ceramic composites processed by AM create exciting opportunities for the customization applications, such as dental restorations, which are demonstrated in this work.
Additive manufacturing (AM) of ceramic matrix composites (CMCs) has enabled the production of highly customized, geometrically complex and functionalized parts with significantly improved properties and functionality, compared to single-phase ceramic components. It also opens up a new way to shape damage-tolerant ceramic composites with co-continuous phase reinforcement inspired by natural materials. Nowadays, a large variety of AM techniques has been successfully applied to fabricate CMCs, but variable properties have been obtained so far. This article provides a comprehensive review on the AM of ceramic matrix composites through a systematic evaluation of the capabilities and limitations of each AM technique, with an emphasis on reported results regarding the properties and potentials of AM manufactured ceramic matrix composites.
The need for implants to repair and regenerate damaged hard tissues, particularly bone, is driving the development of bioactive glass and composites with the requisite mechanical strength and toughness. The use of design principles and novel fabrication technologies are paving the way to create synthetic biomaterials with promising potential for reconstituting bone in load‐bearing sites. This chapter focuses on recent advances in the development and use of bioactive glass scaffolds for bone tissue engineering applications. The state‐of‐the‐art in the design and fabrication of bioactive glass and composite scaffolds that have improved mechanical properties for structural bone repair is reviewed. Potential toughening mechanisms to enable tough glass and composites are also discussed here.
Innovations in Graphene-Based Polymer Composites reviews recent developments in this important field of research. The book's chapters focus on processing methods, functionalization, mechanical, electrical and thermal properties, applications and life cycle assessment. Leading researchers from industry, academia and government research institutions from across the globe have contributed to the book, making it a valuable reference resource for materials scientists, academic researchers and industrial engineers working on recent developments in the area of graphene-based materials, graphene-based polymer blends and composites. Readers will gain insights into what has been explored to-date, along with associated benefits and challenges for the future.
Calcium silicate (CS) is a suitable substrate for bone tissue engineering because it can provide bioactive ions like Si4+ and Ca2+ to promote bone regeneration. However, the rapid degradation of CS leads to pH problems and does not match the rate of osteogenesis. The 3D printed CS scaffolds were immersed in Polycaprolactone (PCL) solution to obtain PCL-coated scaffolds with improved mechanical and biological properties. Finite element method (FEM) analysis found that PCL impregnation has the effect of stress shielding and defect healing, effectively improving the mechanical properties of CS porous scaffolds. The degradation rate of PCL-coated scaffolds was significantly slowed down in Tris buffer. After 4 weeks of degradation, the compressive strength of PCL-coated scaffolds remained at 23.34 MPa, maintaining reliable mechanical properties. PCL coating significantly reduced the degradation rate of CS scaffolds, and the pH value and Si ion concentration of Dulbecco's modified Eagle's medium (DMEM) soaked by PCL-coated scaffolds were more favorable for cell survival. The results show that PCL-coated scaffolds enhance cell proliferation and osteogenic differentiation, demonstrating the potential applicability of bone tissue engineering applications.
A frequent complication with intersomatic implants (e.g. cages) is still subsidence. The knowledge of the mechanism of these subsidences is not yet well established. The preparation of the endplate is still controversially discussed. The strength of the natural endplate is provided by the combination of a compact cover layer as a shell over underlying spongious bone as an elastic foundation. In the sense of a trade-off, cuts into the endplate are considered necessary to assure biological bridging but shall have minimal extensions to prevent loss of strength. A personal conceptual model of the head and its cervical vertebral column reveals static loads on an exemplary C5 vertebra of 131 N normal and 32 N shear load. Increasing these loads by a load factor of 1.4 to reflect activities of daily living, lifts these loads to 183 N and 45 N respectively. The order of magnitude of these loads is considered rather small and cannot confirm lack of strength as the only (or main) reason for subsidence. Other factors must also be looked at. In the case of a cage, it must be positioned in such way that the shear loads will still be transferred by the facet joints, since shear may be detrimental for an implant-/bone-interface in the early post-operative phase, especially when shear leads to micromotion. The implant must be prevented from tilting with resulting local stress concentration. Due to the new load distribution and the surgery, bone will be subject to remodeling and should be protected in this critical transition phase. Finally also the consequences of the disturbance of the once healthy biological balance between spongious bone, endplates, nucleus pulposus, endplates, spongious bone must be kept in mind.KeywordsSubsidenceShellElastic foundationBone remodelingShear loads
There are more than 2 million bone grafting procedures performed annually in the US alone. Despite significant efforts, the repair of large segmental bone defects is a substantial clinical challenge which requires bone substitute materials or a bone graft. The available biomaterials lack the adequate mechanical strength to withstand the static and dynamic loads while maintaining sufficient porosity to facilitate cell in-growth and vascularization during bone tissue regeneration. A wide range of advanced biomaterials are being currently designed to mimic the physical as well as the chemical composition of a bone by forming polymer blends, polymer-ceramic and polymer-degradable metal composites. Transforming these novel biomaterials into porous and load-bearing structures via three-dimensional printing (3DP) has emerged as a popular manufacturing technique to develop engineered bone grafts. 3DP has been adopted as a versatile tool to design and develop bone grafts that satisfy porosity and mechanical requirements while having the ability to form grafts of varied shapes and sizes to meet the physiological requirements. In addition to providing surfaces for cell attachment and eventual bone formation, these bone grafts also have to provide physical support during the repair process. Hence, the mechanical competence of the 3D-printed scaffold plays a key role in the success of the implant. In this review, we present various recent strategies that have been utilized to design and develop robust biomaterials that can be deployed for 3D-printing bone substitutes. The article also reviews some of the practical, theoretical and biological considerations adopted in the 3D-structure design and development for bone tissue engineering.
The adaptive foam reticulation technique combines the foam reticulation and freeze casting methodologies of fabricating bone reparative scaffolds to offer a potential alternative to autografts. For the first time this paper studies the effect of processing on the mechanical properties and in vitro cell growth of controllably generating a hierarchical structure of macro- (94 ± 6 to 514 ± 36 μm) and microporosity (2–30 μm) by the inclusion of camphene as a porogen during processing. Scaffolds were produced with porogen additions of 0–25 wt%. Porosity values of the structures of 85–96% were determined using the Archimedes technique and verified using X-ray Computed Tomography. The strength of the hydroxyapatite scaffolds, 5.70 ± 1.0 to 159 ± 61 kPa, correlated to theoretically determined values, 3.71 ± 0.8 to 134 ± 12 kPa, calculated by the novel incorporation of a shape factor into a standard equation. Fibroblast (3T3) and pre-osteoblast (MC3T3) cell growth was found to be significantly (P < 0.005) improved using 25 wt% porogen. This was supported by increased levels of alkaline phosphatase and was thought to result from greater dissolution as quantified by increased calcium levels in incubating media. The combination of these properties renders adaptive foam reticulation-fabricated scaffolds suitable for non-structural bone regenerative applications in non-load bearing bone defects.
This chapter provides a review of the main types of bioactive ceramics and glasses used or under development for tissue engineering applications. Bioactive ceramics and glasses have been researched and developed mainly for the repair and regeneration of bone. However, increasing studies have shown the capacity of bioactive glasses to heal soft tissue wounds. The most widely used bioactive ceramics are the calcium phosphate bioceramics such as hydroxyapatite, beta-tricalcium phosphate, and biphasic calcium phosphate. Increasing research interest has been observed regarding the incorporation of therapeutic ions within the composition of calcium phosphate ceramics to enhance their functionality. While silicate-based bioactive glasses, such as the compositions designated 45S5 and 13–93, have received considerable attention, other compositions, such as borate and phosphate bioactive glasses, are also being studied and applied. In addition to providing a three-dimensional scaffold to guide tissue regeneration, bioactive ceramics and glasses are also being developed as delivery systems for cells, growth factors, synthetic drugs, natural herbal compounds and ions to enhance tissue regeneration. Conventional bioactive composites, composed of a discrete bioactive ceramic or glass phase dispersed in a biodegradable matrix, and inorganic–organic hybrids, composed of an inorganic bioactive component and a biodegradable organic component that interact at the nanoscale, will also be described. Properties of these bioactive ceramics and glasses, as well as their performance in vitro and in vivo are reviewed.
Endometrial injury and intrauterine adhesions are increasingly reported in recent years; however, treatment options remain limited. Intravenous injection of mesenchymal stem cells (MSCs) for endometrium regeneration has limited effectiveness as the retention rate of transplanted cells is low. Hydrogel‐based tissue‐engineered solutions, such as MSC‐seeded bioscaffolds, are reported to increase retention rates; however, a less invasive alternative is still desirable. 560‐µm homogeneous Matrigel microspheres are fabricated, loading them with about 1500 MSCs and injecting them into the injured endometria of rats’ uteri. This minimally invasive procedure is proved to significantly increase endometrium thickness by over onefold after 21 d (p < 0.0001) and fertility rates from 25% to 75% in impaired and repaired uteri (p < 0.001), respectively. This study provides a minimally invasive alternative to endometrium repair with the promise to establish a broad‐spectrum technique for MSC transplantation.
Recent experimental and simulation studies have shown that polymer-nanoparticle (NP) composites (PNCs) with ultra-high NP loading (>50%) exhibit remarkable mechanical properties and dramatic increases in polymer glass-transition temperature, viscosity, and thermal stability compared to the bulk polymer. These deviations in macroscopic properties suggest a slowdown in both segmental and chain-scale polymer dynamics due to confinement. In this work, we examine the polymer conformations and dynamics in these PNCs using molecular dynamics simulations of both unentangled and entangled coarse-grained polymers in random-close-packed NP packings with varying polymer fill fractions. We find that the changes in the polymer dynamics depend on the number of NPs in contact with a polymer segment. Using the number of polymer-NP contacts and different polymer chain conformations as criteria for categorization, we further examine the polymer dynamics at multiple length scales to show the high level of dynamic heterogeneity in PNCs with ultra-high NP loading.
The inherent brittleness of bioceramics restricts their applications in load-bearing implant, although they possess good biocompatibility and bioactivity. ZnO, MgO and 58S bioglass (BG) were incorporated as additives to further improve the mechanical properties and biocompatibility of β-TCP and ZnO/MgO/BG-β-TCP composite scaffolds were manufactured via digital light processing (DLP). The composite with the best comprehensive performance was selected for degradation behavior and biocompatibility evaluation. The effects of different proportions of ZnO/MgO/BG on mechanical strength were analyzed and ZnO0.5/MgO1/BG2-β-TCP (ZMBT) samples exhibited superior mechanical strength. The improvement by 272% and 99% respectively was achieved in fracture toughness and compressive strength with the optimal recipe. The enhancement effect is realized through phase transition, alterative sliding actions and transgranular fracture to effectively prevent the load transfer combining the functions of bioglass and metal oxide. ZMBT scaffolds exhibited a more desirable pH environment and an enhanced ability of apatite-mineralization formation, meanwhile Si⁴⁺, Mg²⁺ and Zn²⁺ were gradually released from scaffolds. Furthermore, in vitro evaluation indicated that ZMBT scaffolds presented not only excellent cell attachment, proliferation, alkaline phosphatase (ALP) activity, but they up-regulated osteogenic gene (ALP, OCN, Runx2). These results suggest that the addition of ZnO/MgO/BG to DLP-printed β-TCP scaffolds offer a smart strategy to fabricate porous scaffolds with conspicuously better biological and physicochemical properties including compressive strength, bioactivity, osteogenesis and osteogenesis-related gene expression. Metal-oxide and BG synergistically enhanced the mechanical and biological properties which make the ZMBT scaffolds a strong candidate for bone repair applications.
Tissue engineering is a new and exciting technique which has the potential to create tissues and organs de novo. It involves the in vitro seeding and attachment of human cells onto a scaffold. These cells then proliferate, migrate and differentiate into the specific tissue while secreting the extracellular matrix components required to create the tissue. It is evident, therefore, that the choice of scaffold is crucial to enable the cells to behave in the required manner to produce tissues and organs of the desired shape and size. Current scaffolds, made by conventional scaffold fabrication techniques, are generally foams of synthetic polymers. The cells do not necessarily recognise such surfaces, and most importantly cells cannot migrate more than 500 microm from the surface. The lack of oxygen and nutrient supply governs this depth. Solid freeform fabrication (SFF) uses layer-manufacturing strategies to create physical objects directly from computer-generated models. It can improve current scaffold design by controlling scaffold parameters such as pore size, porosity and pore distribution, as well as incorporating an artificial vascular system, thereby increasing the mass transport of oxygen and nutrients into the interior of the scaffold and supporting cellular growth in that region. Several SFF systems have produced tissue engineering scaffolds with this concept in mind which will be the main focus of this review. We are developing scaffolds from collagen and with an internal vascular architecture using SFF. Collagen has major advantages as it provides a favourable surface for cellular attachment. The vascular system allows for the supply of nutrients and oxygen throughout the scaffold. The future of tissue engineering scaffolds is intertwined with SFF technologies.
A paradigm shift is taking place in medicine from using synthetic implants and tissue grafts to a tissue engineering approach that uses degradable porous material scaffolds integrated with biological cells or molecules to regenerate tissues. This new paradigm requires scaffolds that balance temporary mechanical function with mass transport to aid biological delivery and tissue regeneration. Little is known quantitatively about this balance as early scaffolds were not fabricated with precise porous architecture. Recent advances in both computational topology design (CTD) and solid free-form fabrication (SFF) have made it possible to create scaffolds with controlled architecture. This paper reviews the integration of CTD with SFF to build designer tissue-engineering scaffolds. It also details the mechanical properties and tissue regeneration achieved using designer scaffolds. Finally, future directions are suggested for using designer scaffolds with in vivo experimentation to optimize tissue-engineering treatments, and coupling designer scaffolds with cell printing to create designer material/biofactor hybrids.
In a rapid prototyping system, a part is formed by depositing a bead of slurry that has a sufficient high concentration of particles to be pseudoplastic and almost no organic binders. After deposition the bead is heated to drive off sufficient liquid to cause the bead to become dilatant.
Ceramics used for the repair and re- construction of diseased or damaged parts of the musculo-skeletal sys- tem, termed bioceramics, may be bio- inert (alumina, zirconia), resorbable (tricalcium phosphate), bioactive (hy- droxyapatite, bioactive glasses, and glass-ceramics), or porous for tissue ingrowth (hydroxyapatite-coated met- als, alumina). Applications include re- placements for hips, knees, teeth, tendons, and ligaments and repair for periodontal disease, maxillofacial re- construction, augmentation and stabi- lization of the jaw bone, spinal fusion, and bone fillers after tumor surgery. Carbon coatings are thromboresistant and are used for prosthetic heart valves. The mechanisms of tissue bonding to bioactive ceramics are be- ginning to be understood, which can result in the molecular design of bio- ceramics for interfacial bonding with hard and soft tissues. Composites are being developed with high toughness and elastic modulus match with bone. Therapeutic treatment of cancer has been achieved by localized delivery of radioactive isotopes via glass beads. Development of standard test methods for prediction of long-term (20-year)
In a rapid prototyping system, a part is formed by depositing a bead of slurry that has a sufficient high concentration of particles to be pseudoplastic and almost no organic binders. After deposition the bead is heated to drive off sufficient liquid to cause the bead to become dilatant.
The modulus−volume fraction relationship for a poly(ε-caprolactone)−montmorillonite nanocomposite follows composite materials theory provided the clay volume fraction is correctly calculated. Thus, for interpretation of mechanical properties, nanocomposites do not have to be treated as a separate class of material. The tensile modulus of biodegradable poly(ε-caprolactone) was increased by 50% at 8 wt % clay addition (as corrected for surfactant), but the more dramatic improvement was in tensile elongation at break which increased from 165% to 550% for additions of up to 10 wt % clay. Poly(ε-caprolactone) nanocomposites with various clay volume fractions were produced with two organo-modified montmorillonites. Untreated montmorillonite was used as an experimental control to compare the properties with a conventional composite over the same clay volume fraction range, The composites were confirmed and characterized by XRD, DSC, NMR, and TEM.
This paper discusses the applicability of statistics to a wide field of problems. Examples of simple and complex distributions are given.
A study is made of the role of flaw state on the strength properties of brittle ceramic coating layers bonded to soft polycarbonate substrates. We introduce Vickers radial cracks at prescribed loads into the coating undersurfaces prior to bonding to control the sizes and locations of the starting flaws. A spherical indenter is then loaded on the top bilayer surfaces, directly above the Vickers indentation sites, subjecting the radial cracks to flexural tensile stress. Radial crack responses are monitored in situ, using a camera located below the transparent substrate. Critical loads to cause radial crack instability, and ensuing growth of the arrested cracks, are recorded. Conventional biaxial flexure tests on corresponding monolith coating materials provide a baseline for data comparison. Relative to the monolith flexure specimens, the bilayers show higher strengths, the more so the larger the flaw, indicating enhanced flaw tolerance. A simple fracture mechanics analysis of the radial crack evolution in the concentrated-load field, with due account for distribution of flexural tensile stresses at the coating undersurface, is unable to account completely for the enhanced bilayer strengths for the larger Vickers flaws. It is hypothesized that the epoxy used to bond the bilayer components enters the cracks, causing crack-wall adherence and providing an increased resistance to radial crack instability. The fracture mechanics are nevertheless able to account for the arrest and subsequent stable extension of the radial cracks beyond the critical loads once this extraneous adherence has been overcome.
Mesoscale periodic structures have been fabricated via directed assembly of colloidal inks. Concentrated colloidal gels with tailored viscoelastic properties were designed to form self-supporting features. The inks were deposited in a layer-by-layer sequence to directly write the desired 3-D pattern. Periodic structures with spanning features that vary between ∼100 µm and 1 mm were assembled. Shear rate profiles were calculated on the basis of the measured rheological properties of the inks under slip and no-slip boundary conditions during flow through a cylindrical deposition nozzle. Deflection measurements of spanning elements were used to probe the relationship between gel strength, deposition speed, and shear rate profiles in the nozzle. These observations revealed that the ink adopted a rigid (gel) core-fluid shell architecture during assembly, which simultaneously facilitated bonding and shape retention of the deposited elements.
The pressure dependence of the elastic constants of various halides, hard tissues and synthetic apatites have been examined in the 0- to 50-kilobar range using a solid media, pressure apparatus coupled with an ultrasonic interferometer. The samples includ: hydroxy-, fluor-, and chlor-apatite, NaCl, CaF2, mollusc shell (aragonite), ivory, dentin and enamel, and fish, bovine, and human bones. High pressures were used in order to assess the effects of porosity in aggregate samples and to make measurements on specimens of ideal density. Computer analysis of the measured longitudinal and shear ultrasonic velocities yields the pressure dependence of the bulk, shear, and Young's moduli and Poisson's ratio. Atmospheric pressure values at ideal density are obtained by back extrapolation from the high pressure measurements.
A study was performed on the strength enhancement of porous hydroxyapatite (HA) ceramics by polymer impregnation. The effect of several parameters such as concentration of solution, porosity, level of vacuum and time, on tensile strength of the composites was estimated. The tensile strength of porous HA ceramics was measured using a diametral compression method.
High-tech ceramics have always been associated to medical devices: they are used today as femoral heads and acetabular cups for total hip replacement, dental implants and restorations, bone fillers and scaffolds for tissue engineering. Here, we describe their current clinical use and propose a picture of their evolutions for the next 20 years. The need for tough, strong and stable bioinert ceramics should be met by either nano-structured, alumina and zirconia based ceramics and composites or by non-oxide ceramics. Nano-structured calcium phosphate ceramics and porous bioactive glasses, possibly combined with an organic phase should present the desired properties for bone substitution and tissue engineering. The position of ceramics in a gradual medical approach, from tissue regeneration to conventional implants, is discussed.
The purpose of this study was to fabricate composites consisting of three interpenetrating networks: tricalcium phosphate (TCP), hydroxyapatite (HA), and poly(dl-lactide-co-glycolide) (PLGA). The porous TCP network was first produced by coating a polyurethane (PU) foam with hydrolysable alpha-TCP slurry. The HA network was derived from a calcium phosphate cement (CPC) filled in the porous TCP network. The remaining open pore network in the HA/TCP composite was further infiltrated with a PLGA network. The three sets of spatially continuous networks would have different biodegradation rates and thus bone tissue would grow towards the fastest biodegrading network while the remaining networks still maintaining their geometrical shape and carrying the physiological load for the tissue ingrowth.
In this work, the sintering and grain growth of hydroxyapatite green bodies are analyzed in order to identify the optimum heat treatments for the preparation of porous hydroxyapatite scaffolds. Sintering in air at temperatures ranging between 1100 and 1200 °C yields dense materials with narrow grain-size distributions. The scaffolds are formed by the infiltration of polymer foams with hydroxyapatite slurries or by robocasting, a novel rapid-prototyping technique. Examples of the microstructures achieved with each approach are presented. It is observed that both techniques can be used to fabricate scaffolds with adequate pore size to promote bone ingrowth.
In this work, polycaprolactone-coated alumina scaffolds were produced and characterized to validate the concept of polymer–ceramic composites with increased fracture resistance. Alumina scaffolds were sintered using a foam replication technique. An open-porous structure was achieved with ∼70% porosity and 150 μm mean pore size. The polymer coating was obtained by infiltrating the scaffold with either a polycaprolactone solution or a polycaprolactone nanodispersion. The latter was obtained by an emulsion–diffusion technique. Dynamical Young modulus measurements and four-point bending tests were conducted to evaluate the mechanical properties of the composites. It was found that their elastic behaviour is controlled on the first order by the ceramic scaffold, while the fracture energy mainly depends on the polymer phase. A 10–20 vol.% addition of polycaprolactone to alumina scaffolds led to a 7- to 13-fold increase of the apparent fracture energy. SEM observations showed that toughening is due to crack bridging by polymer fibrils.
The effect of the sintering conditions (temperature and time) on the microstructure (density and grain size) and mechanical properties (hardness, elastic modulus, and strength) of β-tricalcium phosphate (β-TCP) bioceramics fabricated from Ca-deficient commercial powders is analyzed. Contrary to current general opinion, it is demonstrated that the optimal sintering temperature to maximize the mechanical performance of this β-TCP material is not necessarily below the β ↔ α transformation temperature (1125 °C). In particular, optimal performance was achieved in samples sintered at 1200 °C for 3 h, since it was not until higher temperatures or longer sintering times that microcracking develops and mechanical properties are degraded. It is argued that the residual stresses developed during this reversible transformation do not lead to microcrack propagation until sufficiently large starting flaws develop in the microstructure as a consequence of grain growth. Implications of these findings for the processing routes to improve sintering of this important bioceramic are discussed.
The principal objectives of this study were to determine the mathematical dependency of the compressive mechanical properties of human bone on several commonly used measures of bone composition, and to assess variations in this dependency based upon the composition range spanned by the data. Destructive mechanical tests were conducted along the superior-inferior axis of 496 cubic specimens of human trabecular and cortical bone from five male donors (ages 46-84 yr), including specimens from lumbar vertebrae and femoral metaphyses and diaphyses. There was over a 3000-fold variation in strength (S, ultimate stress) and over a 20,000-fold variation in stiffness (E, elastic modulus) over the range of apparent dry density (rho a = 0.05-1.89 g cm-3), apparent ash density (rho alpha = 0.03-1.22 g cm-3) and mineral content (alpha = 17.4-66.2%) examined. Both linear and power models produced very high correlations (R2 > 0.81) between mechanical properties and bone composition, but the linear models resulted in a much greater percent deviation (PD) of the predicted dependent variable with respect to the measured value, in comparison to power models. The best correlations were obtained using rho alpha as the only independent variable: S (MPa) = 117 rho alpha 1.93 +/- 0.04 (R2 = 0.969, PD = 29.9, E (GPa) = 10.5 rho alpha 2.57 +/-0.04 (R2 = 0.965, PD = 46.7). Power models of bone stiffness and strength, incorporating only low density data (rho alpha < 0.2 g cm-3, rho a < 0.3), were characterized by approximately squared exponents and these models underestimated the stiffness (five-fold) and overestimated the strength (two-fold) for higher density data, which were characterized by exponents greater than two. Using a subset of the data based upon an apparent dry density range of 0.22 < rho a < 1.89 g cm-3, it was possible to obtain a mathematical relationship in which bone stiffness and strength were precisely proportional to the cube and square, respectively, of the apparent dry density. These results indicate that the mathematical dependency of bone compressive mechanical properties on composition is closely dependent upon the density and mineral content range examined and, in terms of a single compositional measure, is best predicted by apparent ash density expressed as a power function.
To evaluate the effects of two bioceramics on bone regeneration during repair of segmental bone defects, Biocoral and tricalcium phosphate cylinders were implanted in osteotomized sheep tibial defects 16 mm in length and followed up for 16 weeks. In comparison with the TCP-implanted defect, a significant increment in area and density of external callus was quantified radiomorphometrically at 3 weeks, and a marked increase in maximal torque capacity, maximal angle of deformation and absorption of energy was demonstrated mechanically in the Biocoral-implanted tibia at 16 weeks after implantation. Better bone integration with the substratum was microscopically observed in Biocoral cylinders. With good osteointegration and biomechanical-performance, Biocoral seems to be superior to TCP in repair of segmental defects in weight-bearing limbs.
The main problem for macroporous structures used as bone substitutes is their lower resistances when compared to that of cancellous bone. The present investigation aimed to improve the strength of ceramics with 65% porosities based on beta-TCP. The initial mixtures were rendered plastic by addition of non-ionic carbohydrate binders. Macropores were created using substances which were eliminated by heat. Mechanical tests indicated that the resistance of the ceramics depended more on the quantity than the nature of the binders. Porosity measurements were done with a mercury porosimeter, and cellular biocompatibility was evaluated by performing cellular attachment tests and observing the proliferation of differentiated cells.
A novel biodegradable nanocomposite porous scaffold comprising a beta-tricalcium phosphate (beta-TCP) matrix and hydroxyl apatite (HA) nanofibers was developed and studied for load-bearing bone tissue engineering. HA nanofibers were prepared with a biomimetic precipitation method. The composite scaffolds were fabricated by a method combining the gel casting and polymer sponge techniques. The role of HA nanofibers in enhancing the mechanical properties of the scaffold was investigated. Compression tests were performed to measure the compressive strength, modulus and toughness of the porous scaffolds. The identification and morphology of HA nanofibers were determined by X-ray diffraction and transmission electron microscopy, respectively. Scanning electron microscopy was used to examine the morphology of porous scaffolds and fracture surfaces to reveal the dominant toughening mechanisms. The results showed that the mechanical property of the scaffold was significantly enhanced by the inclusion of HA nanofibers. The porous composite scaffold attained a compressive strength of 9.8 +/- 0.3 MPa, comparable to the high-end value (2-10 MPa) of cancellous bone. The toughness of the scaffold increased from 1.00+/-0.04 to 1.72+/-0.02 kN/m, as the concentration of HA nanofibers increased from 0 to 5 wt %.
Hydroxyapatite (HA) scaffolds with a 3-D periodic architecture and multiscale porosity have been fabricated by direct-write assembly. Concentrated HA inks with tailored viscoelastic properties were developed to enable the construction of complex 3-D architectures comprised of self-supporting cylindrical rods in a layer-by-layer patterning sequence. By controlling their lattice constant and sintering conditions, 3-D periodic HA scaffolds were produced with a bimodal pore size distribution. Mercury intrusion porosimetry (MIP) was used to determine the characteristic pore size and volume associated with the interconnected pore channels between HA rods and the finer pores within the partially sintered HA rods.
beta-Tricalcium phosphate (beta-TCP) scaffolds with designed, three-dimensional (3-D) geometry and mesoscale porosity have been fabricated by direct-write assembly (robocasting) techniques. Concentrated beta-TCP inks with suitable viscoelastic properties were developed to enable the fabrication of the complex 3-D structures. A comprehensive study of the sintering behavior of TCP as a function of the calcium content in the starting powder was also carried out, and the optimal heat treatment for fabricating scaffolds with dense beta-TCP rods has been determined. Such analysis provides clues to controlling the microstructure of the fabricated structures and, therefore, enabling the fabrication by robocasting of TCP scaffolds with tailored performance for bone tissue engineering applications.
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.
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.
Many studies are currently underway on the quest to make synthetic bone-like materials with composites of polymeric materials and hydroxyapatite (HA). In the present work, we use wetting experiments and surface tension measurements to determine the work of adhesion between biodegradable polymers and HA, with specific reference to the role of humid environments. All the polymers are found to exhibit low contact angles (<or=60 degrees) on the ceramic with work of adhesion values ranging between 48Jm(-2) for poly(epsilon-caprolactone) and 63Jm(-2) for polylactide; these values are associated with physical bonding across the organic/inorganic interface. The corresponding mechanical fracture strengths, measured using four-point bending tests of HA-polymer-HA bonds, scale directly with the results from the wetting experiments. Short-time aging (up to 30h) in a humid environment, however, has a dramatic influence on such HA/polymer interfacial strengths; specifically, water diffusion through the organic/inorganic interface and degradation of the polymer results in a marked decrease, by some 80-90%, in the bond strengths. These results cast doubt on the use of biodegradable polymers/ceramic composites for load-bearing synthetic bone-like materials, as desired optimal mechanical properties are unlikely to be met in realistic physiological environments.
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.
Porous scaffold design for tissue Engineering
- S J Hollister
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- L L Hench
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