The Production of Porous Hydroxyapatite Scaffolds with Graded Porosity by Sequential Freeze-Casting

A Gellan Gum, Polyethylene Glycol, Hydroxyapatite Composite Scaffold with the Addition of Ginseng Derived Compound K with Possible Applications in Bone Regeneration

Engineered bone scaffolds should mimic the natural material to promote cell adhesion and regeneration. For this reason, natural biopolymers are becoming a gold standard in scaffold production. In this study, we proposed a hybrid scaffold produced using gellan gum, hydroxyapatite, and Poly (ethylene glycol) within the addition of the ginseng compound K (CK) as a candidate for bone regeneration. The fabricated scaffold was physiochemically characterized. The morphology studied by scanning electron microscopy (SEM) and image analysis revealed a pore distribution suitable for cells growth. The addition of CK further improved the biological activity of the hybrid scaffold as demonstrated by the MTT assay. The addition of CK influenced the scaffold morphology, decreasing the mean pore diameter. These findings can potentially help the development of a new generation of hybrid scaffolds to best mimic the natural tissue.

Biocompatibility and antibacterial activity of MgO/Ca3(PO4)2 composite ceramic scaffold based on vat photopolymerization technology

Recent advancements in medical technology and increased interdisciplinary research have facilitated the development of the field of medical engineering. Specifically, in bone repair, researchers and potential users have placed greater demands on orthopedic implants regarding their biocompatibility, degradation rates, antibacterial properties, and other aspects. In response, our team developed composite ceramic samples using degradable materials calcium phosphate and magnesium oxide through the vat photopolymerization (VP) technique. The calcium phosphate content in each sample was, respectively, 80 %, 60 %, 40 %, and 20 %. To explore the relationship between the biocompatibility, antibacterial activity, and MgO content of the samples, we cultured them with osteoblasts (MC3T3-E1), Escherichia coli (a gram-negative bacterium), and Staphylococcus aureus (a gram-positive bacterium). Our results demonstrate that as the MgO content of the sample increases, its biocompatibility improves but its antibacterial activity decreases. Regarding the composite material samples, the 20 % calcium phosphate content group exhibited the best biocompatibility. However, after 0.5 h of co-cultivation, the antibacterial rates of all groups except the 20 % calcium phosphate content group co-cultured with S. aureus exceed 80 %. Furthermore, after 3 h, the antibacterial rates against E. coli exceed 95 % in all groups. This is because higher levels of MgO correspond to lower pH values and Mg2+ concentrations in the cell and bacterial culture solutions, which ultimately promote cell and bacterial proliferation. This elevates the biocompatibility of the samples, albeit at the expense of their antimicrobial efficacy. Thus, modulating the MgO content in the composite ceramic samples provides a strategy to develop gradient composite scaffolds for better control of their biocompatibility and antibacterial performance during different stages of bone regeneration.

Sintering densification mechanism and mechanical properties of the 3D-printed high-melting-point-difference magnesium oxide/calcium phosphate composite bio-ceramic scaffold

Over the past few years, biodegradable ceramic scaffolds have gained significant attention in the field of bone repair. Calcium phosphate (Ca3(PO4)2)- and magnesium oxide (MgO)-based ceramics are biocompatible, osteogenic, and biodegradable, making them attractive for potential applications. However, the mechanical properties of Ca3(PO4)2 are limited. We developed a magnesium oxide/calcium phosphate composite bio-ceramic scaffold characterized by a high melting point difference, using vat photopolymerization (VP) technology to address this issue. The primary goal was to fabricate high-strength ceramic scaffolds using biodegradable materials. In this study, we investigated ceramic scaffolds with varying MgO contents and sintering temperatures. We also discussed the co-sintering densification mechanism of high and low melting-point materials associated with composite ceramic scaffolds. During sintering, a liquid phase was generated, which filled up the pores generated during the vaporization of additives (such as resin) under the influence of capillary force. This led to an increase in the extent of ceramic densification realized. Moreover, we found ceramic scaffolds with 80 wt% MgO exhibited the best mechanical performance. This kind of composite scaffold performed better than pure MgO scaffold. The results reported herein highlight that high-density composite ceramic scaffolds can be potentially used in the field of bone repair.

Adjustment of Micro- and Macroporosity of ß-TCP Scaffolds Using Solid-Stabilized Foams as Bone Replacement

To enable rapid osteointegration in bioceramic implants and to give them osteoinductive properties, scaffolds with defined micro- and macroporosity are required. Pores or pore networks promote the integration of cells into the implant, facilitating the supply of nutrients and the removal of metabolic products. In this paper, scaffolds are created from ß-tricalciumphosphate (ß-TCP) and in a novel way, where both the micro- and macroporosity are adjusted simultaneously by the addition of pore-forming polymer particles. The particles used are 10-40 wt%, spherical polymer particles of polymethylmethacrylate (PMMA) (Ø = 5 µm) and alternatively polymethylsilsesquioxane (PMSQ) (Ø = 2 µm), added in the course of ß-TCP slurry preparation. The arrangement of hydrophobic polymer particles at the interface of air bubbles was incorporated during slurry preparation and foaming of the slurry. The foam structures remain after sintering and lead to the formation of macro-porosity in the scaffolds. Furthermore, decomposition of the polymer particles during thermal debindering results in the formation of an additional network of interconnecting micropores in the stabilizing structures. It is possible to adjust the porosity easily and quickly in a range of 1.2-140 μm with a relatively low organic fraction. The structures thus prepared showed no cytotoxicity nor negative effects on the biocompatibility.

Bone Tissue Engineering Scaffolds: Function of Multi-Material Hierarchically Structured Scaffolds

Bone tissue engineering (BTE) is a topic of interest for the last decade, and advances in materials, processing techniques, and the understanding of bone healing pathways have opened new avenues of research. The dual responsibility of BTE scaffolds in providing load-bearing capability and interaction with the local extracellular matrix to promote bone healing is a challenge in synthetic scaffolds. This article describes the usage and processing of multi-materials and hierarchical structures to mimic the structure of natural bone tissues to function as bioactive and load-bearing synthetic scaffolds. The first part of this literature review describes the physiology of bone healing responses and the interactions at different stages of bone repair. The following section reviews the available literature on biomaterials used for BTE scaffolds followed by some multi-material approaches. The next section discusses the impact of the scaffold's structural features on bone healing and the necessity of a hierarchical distribution in the scaffold structure. Finally, the last section of this review highlights the emerging trends in BTE scaffold developments that can inspire new tissue engineering strategies and truly develop the next generation of synthetic scaffolds.

Improved Mechanical Amplification of Monolithic PZT and PZT Composite via Optimized Honeycomb Macrostructures

Honeycomb-based, modular composites with a relative density of 0.3948 and a slenderness ratio L_ges/t of 6.48 were fabricated on PZT building blocks connected with a PZT-filled phenyl silicone resin. The macro- and micro-structure, phase composition, and the interface between the two materials were analyzed by SEM and image analysis techniques. The mechanical in-plane strain response was determined with uniaxial compression tests and the transversal piezoelectric strain response was determined by applying an electric field. These deformations were analyzed by a 2D digital image correlation analysis to calculate the mechanical strain amplification of monolithic and composite PZT lattice structures. Compared to bulk PZT, the piezoelectric strain amplification in the Y-direction |a_y^piezo| was higher by a factor of 69 for the composite and by a factor of 12 for the monolithic cellular PZT lattice, when it was assumed that the ratio of the deformation of the bulk material to bulk material was 1. The mechanical amplification of the composite lattices increased up to 73 and that of the cellular PZT lattices decreased to 12. Special focus was given to the fracture behavior and the interface of the PZT/PZT-filled phenyl silicone resin interface.

Porous biomaterials for tissue engineering: a review

The field of biomaterials has grown rapidly over the past decades. Within this field, porous biomaterials have played a remarkable role in: (i) enabling the manufacture of complex three-dimensional structures; (ii) recreating mechanical properties close to those of the host tissues; (iii) facilitating interconnected structures for the transport of macromolecules and cells; and (iv) behaving as biocompatible inserts, tailored to either interact or not with the host body. This review outlines a brief history of the development of biomaterials, before discussing current materials proposed for use as porous biomaterials and exploring the state-of-the-art in their manufacture. The wide clinical applications of these materials are extensively discussed, drawing on specific examples of how the porous features of such biomaterials impact their behaviours, as well as the advantages and challenges faced, for each class of the materials.

Promises of Functionally Graded Material in Bone Regeneration: Current Trends, Properties, and Challenges

Functionally graded materials (FGMs) are emerging materials systems, with structures and compositions gradually changing in a particular direction. Consequently, the properties of the materials gradually change in the desired direction to achieve particular nonhomogeneous service demands without abrupting the compositional and behavioral interface at the macroscale. FGMs have been found to have high potential as orthopedic implants; because the functional gradient can be adapted in such a manner that the core of FGM should be compatible with the density and strength of bone, interlayers can maintain the structural integrity and outermost layers would provide bioactivity and corrosion resistance, thus overall tailoring the stress shielding effect. This review article discusses the typical FGM systems existing in nature and the human body, focusing on bone tissue. Further, the reason behind the application of these FGMs systems in orthopedic implants is explored in detail, considering the physical and biological necessities. The substantial focus of the present critical review is devoted to two primary topics related to the usage of FGMs for orthopedic implants: (1) the synthesizing techniques currently available to produce FGMs for load-bearing orthopedic applications and (2) the properties, such as mechanical, structural, and biological behavior of the FGMs. This review article gives an insight into the potential of FGMs for orthopedic applications.

Endosseous Dental Implant Materials and Clinical Outcomes of Different Alloys: A Systematic Review

In recent years, implantology has made significant progress, as it has now become a safe and predictable practice. The development of new geometries, primary and secondary, of new surfaces and alloys, has made this possible. The purpose of this review is to analyze the different alloys present on the market, such as that in zirconia, and evaluate their clinical differences with those most commonly used, such as those in grade IV titanium. The review, conducted on major scientific databases such as Scopus, PubMed, Web of Science and MDPI yielded a startling number of 305 results. After the application of the filters and the evaluation of the results in the review, only 10 Randomized Clinical Trials (RCTs) were included. Multiple outcomes were considered, such as Marginal Bone Level (MBL), Bleeding on Probing (BoP), Survival Rate, Success Rate and parameters related to aesthetic and prosthetic factors. There are currently no statistically significant differences between the use of zirconia implants and titanium implants, neither for fixed prosthetic restorations nor for overdenture restorations. Only the cases reported complain about the rigidity and, therefore, the possibility of fracture of the zirconium. Certainly the continuous improvement in these materials will ensure that they could be used safely while maintaining their high aesthetic performance.

Characterization of porous fluorohydroxyapatite bone-scaffolds fabricated using freeze casting

With the increasing demand for orthopedic and dental reconstruction surgeries, there comes a shortage of viable bone substitutes. This study was therefore designed to assess the efficacy of porous fluorohydroxyapatite (FHA) as a potential bone substitute. For this, porous FHA scaffolds were fabricated using the freeze casting technique. They were then sintered at 1250, 1350 and, 1450 °C, and microstructural, mechanical, and in vitro properties were analyzed. The microstructure analyses revealed the porosity remained constant within the temperature range. However, the pore size decreased with increasing sintering temperature. The greatest compressive strength and elastic modulus were obtained at 1450 °C, which were 13.5 ± 4.0 MPa and 379 ± 182 MPa, respectively. These are comparable values to human trabecular bone and other porous scaffolds made using hydroxyapatite. This analysis has thus helped to attain an understanding of the mechanical and material properties of freeze-cast FHA scaffolds that have not been presented before. In vitro studies revealed an increasing rate of human osteoblast cell proliferation on freeze-cast FHA scaffolds with increasing sintering temperature, suggesting improved osteogenic properties. Additionally, osteoblasts cells were also shown to proliferate into the interior pores of all freeze-cast FHA scaffolds. These results indicate the potential of porous FHA scaffolds fabricated using the freeze-casting technique to be utilized clinically as bone substitutes.

Fiber-Templated 3D Calcium-Phosphate Scaffolds for Biomedical Applications: The Role of the Thermal Treatment Ambient on Physico-Chemical Properties

A successful bone-graft-controlled healing entails the development of novel products with tunable compositional and architectural features and mechanical performances and is, thereby, able to accommodate fast bone in-growth and remodeling. To this effect, graphene nanoplatelets and Luffa-fibers were chosen as mechanical reinforcement phase and sacrificial template, respectively, and incorporated into a hydroxyapatite and brushite matrix derived by marble conversion with the help of a reproducible technology. The bio-products, framed by a one-stage-addition polymer-free fabrication route, were thoroughly physico-chemically investigated (by XRD, FTIR spectroscopy, SEM, and nano-computed tomography analysis, as well as surface energy measurements and mechanical performance assessments) after sintering in air or nitrogen ambient. The experiments exposed that the coupling of a nitrogen ambient with the graphene admixing triggers, in both compact and porous samples, important structural (i.e., decomposition of β-Ca3(PO4)2 into α-Ca3(PO4)2 and α-Ca2P2O7) and morphological modifications. Certain restrictions and benefits were outlined with respect to the spatial porosity and global mechanical features of the derived bone scaffolds. Specifically, in nitrogen ambient, the graphene amount should be set to a maximum 0.25 wt.% in the case of compact products, while for the porous ones, significantly augmented compressive strengths were revealed at all graphene amounts. The sintering ambient or the graphene addition did not interfere with the Luffa ability to generate 3D-channels-arrays at high temperatures. It can be concluded that both Luffa and graphene agents act as adjuvants under nitrogen ambient, and that their incorporation-ratio can be modulated to favorably fit certain foreseeable biomedical applications.

Preliminary Studies on Graphene-Reinforced 3D Products Obtained by the One-Stage Sacrificial Template Method for Bone Reconstruction Applications

The bone remodeling field has shifted focus towards the delineation of products with two main critical attributes: internal architectures capable to promote fast cell colonization and good mechanical performance. In this paper, Luffa-fibers and graphene nanoplatelets were proposed as porogen template and mechanical reinforcing agent, respectively, in view of framing 3D products by a one-stage polymer-free process. The ceramic matrix was prepared through a reproducible technology, developed for the conversion of marble resources into calcium phosphates (CaP) powders. After the graphene incorporation (by mechanical and ultrasonication mixing) into the CaP matrix, and Luffa-fibers addition, the samples were evaluated in both as-admixed and thermally-treated form (compact/porous products) by complementary structural, morphological, and compositional techniques. The results confirmed the benefits of the two agents' addition upon the compact products' micro-porosity and the global mechanical features, inferred by compressive strength and elastic modulus determinations. For the porous products, overall optimal results were obtained at a graphene amount of <1 wt.%. Further, no influence of graphene on fibers' ability to generate at high temperatures internal interconnected-channels-arrays was depicted. Moreover, its incorporation led to a general preservation of structural composition and stability for both the as-admixed and thermally-treated products. The developed CaP-reinforced structures sustain the premises for prospective non- and load-bearing biomedical applications.

Bioinspired Manufacturing of Aerogels with Precisely Manipulated Surface Microstructure through Controlled Local Temperature Gradients

The freeze casting process has been widely used for fabricating aerogels due to its versatile and environmentally friendly nature. This process offers a variety of tools to tailor the entire micropore morphology of the final product in a monolithic fashion through manipulation of the freezing kinetics and precursor suspension chemistry. However, aerogels with nonmonolithic micropore morphologies, having pores of various sizes located in certain regions of the aerogels, are highly desired by certain applications such as controlled drug-delivery, bone tissue engineering, extracellular simulation, selective liquid sorption, immobilized catalysts, and separators. Furthermore, aerogels composed of micropores with predesigned size, shape, and location can open up a new paradigm in aerogel design and lead to new applications. In this study, a general manufacturing approach is developed to control the size, shape, and location of the pores on the aerogel surface by applying a precise control on the local thermal conductivity of the substrate used in a unidirectional freeze casting process. With our method, we created patterned low and high thermal conductivity regions on the substrate by depositing patterned photoresist polymer features. The photoresist polymer has a much lower thermal conductivity, which resulted in lower cooling/freezing rates compared to the silicon substrate. Patterned thermal conductivity created a designed temperature profile yielding to local regions with faster and slower freezing rates. Essentially, we fabricated aerogels whose micropore morphology on their surface was a replica of the patterned substrates in terms of size and location of the micropores. Using the same substrates, we further showed the possibility of 3D printed aerogels with precisely controlled, surface micropore morphologies. To the best of our knowledge, this is the first study that reports aerogels having micropore morphologies (e.g., size, shape, and location) that are precisely controlled through locally controlled thermal conductivity of the substrates.

Robocasting of Single and Multi-Functional Calcium Phosphate Scaffolds and Its Hybridization with Conventional Techniques: Design, Fabrication and Characterization

In this work, dense, porous, and, for the first time, functionally-graded bi-layer scaffolds with a cylindrical geometry were produced from a commercially available hydroxyapatite powder using the robocasting technique. The bi-layer scaffolds were made of a dense core part attached to a surrounding porous part. Subsequently, these bi-layer robocast scaffolds were joined with an outer shell of an antibacterial porous polymer layer fabricated by solvent casting/salt leaching techniques, leading to hybrid ceramic-polymer scaffolds. The antibacterial functionality was achieved through the addition of silver ions to the polymer layer. All the robocast samples, including the bi-layer ones, were first characterized through scanning electron microscopy observations, mechanical characterization in compression and preliminary bioactivity tests. Then, the hybrid bi-layer ceramic-polymer scaffolds were characterized through antimicrobial tests. After sintering at 1300 °C for 3 h, the compressive strengths of the structures were found to be equal to 29 ± 4 MPa for dense samples and 7 ± 4 MPa for lattice structures with a porosity of 34.1%. Bioactivity tests performed at 37 °C for 4 weeks showed that the precipitated layer on the robocast samples contained octacalcium phosphate. Finally, it was evidenced that the hybrid structure was effective in releasing antibacterial Ag+ ions to the surrounding medium showing its potential efficiency in limiting Staphylococcus aureus proliferation during surgery.

Recent additive manufacturing methods categorized by characteristics of ceramic slurries for producing dual-scale porous ceramics

Porous ceramics have been utilized in various fields due to their advantages derived from characteristics of ceramics and porous structure and they were produced by versatile fabricating methods. However, the adoption of differently scaled pores in the porous ceramics by conventional pore forming strategies which results in dual-scale porosity has been studied to combine the specific functional abilities of each scaled pore. Those proposed strategies were supplemented to the recent additive manufacturing methods for constructing complicated structure with precisely controlled fabricating conditions. In this review, we provide the researches creating dual-scale porous ceramics with additive manufacturing which utilized the ceramic slurries containing homogeneous solution of photocurable monomers and terpenes. Introduction of the basic way to prepare photocurable monomer and terpene incorporated ceramic slurries which are suitable for specific printing mechanism was firstly discussed. And based on the characteristics of slurries, lithography-based and extrusion-based method are discussed with the experimental results. Subsequently, the remaining challenges of the techniques are further discussed with suggesting potentially capable approaches to overcome the limitations.

Mechanical Enhancement of Cytocompatible 3D Scaffolds, Consisting of Hydroxyapatite Nanocrystals and Natural Biomolecules, Through Physical Cross-Linking

Bioinspired scaffolds mimicking natural bone-tissue properties holds great promise in tissue engineering applications towards bone regeneration. Within this work, a way to reinforce mechanical behavior of bioinspired bone scaffolds was examined by applying a physical crosslinking method. Scaffolds consisted of hydroxyapatite nanocrystals, biomimetically synthesized in the presence of collagen and l-arginine. Scaffolds were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), microcomputed tomography, and nanoindentation. Results revealed scaffolds with bone-like nanostructure and composition, thus an inherent enhanced cytocompatibility. Evaluation of porosity proved the development of interconnected porous network with bimodal pore size distribution. Mechanical reinforcement was achieved through physical crosslinking with riboflavin irradiation, and nanoindentation tests indicated that within the experimental conditions of 45% humidity and 37 °C, photo-crosslinking led to an increase in the scaffold’s mechanical properties. Elastic modulus and hardness were augmented, and specifically elastic modulus values were doubled, approaching equivalent values of trabecular bone. Cytocompatibility of the scaffolds was assessed using MG63 human osteosarcoma cells. Cell viability was evaluated by double staining and MTT assay, while attachment and morphology were investigated by SEM. The results suggested that scaffolds provided a cell friendly environment with high levels of viability, thus supporting cell attachment, spreading and proliferation.

Injection Molding of 3-3 Hydroxyapatite Composites

The manufacturing of ideal implants requires fabrication processes enabling an adjustment of the shape, porosity and pore sizes to the patient-specific defect. To meet these criteria novel porous hydroxyapatite (HAp) implants were manufactured by combining ceramic injection molding (CIM) with sacrificial templating. Varied amounts (Φ = 0–40 Vol%) of spherical pore formers with a size of 20 µm were added to a HAp-feedstock to generate well-defined porosities of 11.2–45.2 Vol% after thermal debinding and sintering. At pore former contents Φ ≥ 30 Vol% interconnected pore networks were formed. The investigated Young’s modulus and flexural strength decreased with increasing pore former content from 97.3 to 29.1 GPa and 69.0 to 13.0 MPa, agreeing well with a fitted power-law approach. Additionally, interpenetrating HAp/polymer composites were manufactured by infiltrating and afterwards curing of an urethane dimethacrylate-based (UDMA) monomer solution into the porous HAp ceramic preforms. The obtained stiffness (32–46 GPa) and Vickers hardness (1.2–2.1 GPa) of the HAp/UDMA composites were comparable to natural dentin, enamel and other polymer infiltrated ceramic network (PICN) materials. The combination of CIM and sacrificial templating facilitates a near-net shape manufacturing of complex shaped bone and dental implants, whose properties can be directly tailored by the amount, shape and size of the pore formers.

Comprehensive In Vitro Testing of Calcium Phosphate-Based Bioceramics with Orthopedic and Dentistry Applications

Recently, a large spectrum of biomaterials emerged, with emphasis on various pure, blended, or doped calcium phosphates (CaPs). Although basic cytocompatibility testing protocols are referred by International Organization for Standardization (ISO) 10993 (parts 1-22), rigorous in vitro testing using cutting-edge technologies should be carried out in order to fully understand the behavior of various biomaterials (whether in bulk or low-dimensional object form) and to better gauge their outcome when implanted. In this review, current molecular techniques are assessed for the in-depth characterization of angiogenic potential, osteogenic capability, and the modulation of oxidative stress and inflammation properties of CaPs and their cation- and/or anion-substituted derivatives. Using such techniques, mechanisms of action of these compounds can be deciphered, highlighting the signaling pathway activation, cross-talk, and modulation by microRNA expression, which in turn can safely pave the road toward a better filtering of the truly functional, application-ready innovative therapeutic bioceramic-based solutions.

Hydroxyapatite Microspheres as an Additive to Enhance Radiopacity, Biocompatibility, and Osteoconductivity of Poly(methyl methacrylate) Bone Cement

This study demonstrates the utility of hydroxyapatite (HA) microspheres as an additive to enhance the radiopaque properties, biocompatibility, and osteoconductivity of poly(methyl methacrylate) (PMMA)-based bone cements. HA microspheres were synthesized using spray drying. They had well-defined spherical shapes, thus allowing for the production of PMMA/HA composites with a very high HA content (20 vol % and 40 vol %). The uniform distribution of these HA microspheres in the PMMA matrix resulted in a remarkable increase in compressive modulus (p < 0.05), while preserving a reasonably high compressive strength. The PMMA/HA bone cements showed much higher radiopacity than PMMA containing BaSO₄ as the additive. This was attributed to the high HA content up to 40 vol %. In addition, the biocompatibility and osteoconductivity of PMMA/HA bone cements were significantly enhanced compared to those of PMMA bone cements containing BaSO₄, which were assessed using in vitro tests and in vivo animal experiments.

Design and Production of Continuously Gradient Macro/Microporous Calcium Phosphate (CaP) Scaffolds Using Ceramic/Camphene-Based 3D Extrusion

This study proposes a new type of calcium phosphate (CaP) scaffolds with a continuously gradient macro/microporous structure using the ceramic/camphene-based 3D extrusion process. Green filaments with a continuously gradient core/shell structure were successfully produced by extruding a bilayered feedrod comprised of a CaP/camphene mixture lower part and a pure camphene upper part. The extruded filaments were then deposited in a controlled manner to construct triangular prisms, followed by the assembly process for the production of CaP scaffolds with a gradient core/shell structure. In addition, a gradient microporous structure was created by heat-treating the green body at 43 °C to induce the overgrowth of camphene dendrites in the CaP/camphene walls. The produced CaP scaffold showed a highly macroporous structure within its inner core, where the size of macrochannels increased gradually with an increase in the distance from the outer shell, while relatively larger micropores were created in the outer shell.