Three-dimensional printing and in vitro evaluation of poly(3-hydroxybutyrate) scaffolds functionalized with osteogenic growth peptide for tissue engineering

Bio-mechanical analysis of porous Ti-6Al-4V scaffold: a comprehensive review on unit cell structures in orthopaedic application

A scaffold is a three-dimensional porous structure that is used as a template to provide structural support for cell adhesion and the formation of new cells. Metallic cellular scaffolds are a good choice as a replacement for human bones in orthopaedic implants, which enhances the quality and longevity of human life. In contrast to conventional methods that produce irregular pore distributions, 3D printing, or additive manufacturing, is characterized by high precision and controlled manufacturing processes. AM processes can precisely control the scaffold's porosity, which makes it possible to produce patient specific implants and achieve regular pore distribution. This review paper explores the potential of Ti-6Al-4V scaffolds produced via the SLM method as a bone substitute. A state-of-the-art review on the effect of design parameters, material, and surface modification on biological and mechanical properties is presented. The desired features of the human tibia and femur bones are compared to bulk and porous Ti6Al4V scaffold. Furthermore, the properties of various porous scaffolds with varying unit cell structures and design parameters are compared to find out the designs that can mimic human bone properties. Porosity up to 65% and pore size of 600 μm was found to give optimum trade-off between mechanical and biological properties. Current manufacturing constraints, biocompatibility of Ti-6Al-4V material, influence of various factors on bio-mechanical properties, and complex interrelation between design parameters are discussed herein. Finally, the most appropriate combination of design parameters that offers a good trade-off between mechanical strength and cell ingrowth are summarized.

Production and functionalization strategies for superior polyhydroxybutyrate blend performance

This study investigates the effects of blending poly(3-hydroxybutyrate) (PHB) with microcrystalline cellulose (MCC), polylactic acid (PLA), lignin, and polyethylene glycol (PEG) on the properties of the resulting composite materials. Using a melt blending method, the composites were characterized by scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA). The results reveal that blending PHB with MCC, PLA, lignin, and PEG significantly enhances the thermal stability, mechanical strength, and biodegradability of the composites compared to pure PHB. Specifically, the tensile strength of PHB-PLA blends increased by up to 47.77 MPa, compared to 27.16 MPa for pure PHB. The blend with 50 % cellulose content showed the highest tensile strength of 54.91 MPa. TGA results show that the PHB-MCC and PHB-lignin blends exhibit improved thermal stability, with onset degradation temperatures rising to 294.8 °C, compared to 275 °C for pure PHB. Moreover, the PHB-lignin blend demonstrated a gradual weight loss starting at 200 °C and continuing until about 350 °C. SEM images of the blends indicate a uniform microstructure, contributing to the improved mechanical properties. The PHB-PEG blend demonstrated an elongation at break of 4.34 %, significantly higher than the 2.15 % for pure PHB, highlighting its suitability for applications requiring pliable materials. The biodegradability tests showed that PHB-PLA blends maintained consistent degradation rates, making them advantageous for applications needing controlled biodegradability. These findings suggest that blending PHB with MCC, PLA, lignin, and PEG can produce materials with enhanced properties suitable for applications in packaging, biomedical devices, and other areas where both performance and sustainability are essential.

3D-Printed Silk Proteins for Bone Tissue Regeneration and Associated Immunomodulation

Current bone repair methods have limitations, prompting the exploration of innovative approaches. Tissue engineering emerges as a promising solution, leveraging biomaterials to craft scaffolds replicating the natural bone environment, facilitating cell growth and differentiation. Among fabrication techniques, three-dimensional (3D) printing stands out for its ability to tailor intricate scaffolds. Silk proteins (SPs), known for their mechanical strength and biocompatibility, are an excellent choice for engineering 3D-printed bone tissue engineering (BTE) scaffolds. This article comprehensively reviews bone biology, 3D printing, and the unique attributes of SPs, specifically detailing criteria for scaffold fabrication such as composition, structure, mechanics, and cellular responses. It examines the structural, mechanical, and biological attributes of SPs, emphasizing their suitability for BTE. Recent studies on diverse 3D printing approaches using SPs-based for BTE are highlighted, alongside advancements in their 3D and four-dimensional (4D) printing and their role in osteo-immunomodulation. Future directions in the use of SPs for 3D printing in BTE are outlined.

3D Melt-Extrusion Printing of Medium Chain Length Polyhydroxyalkanoates and Their Application as Antibiotic-Free Antibacterial Scaffolds for Bone Regeneration

In this work, we investigated, for the first time, the possibility of developing scaffolds for bone tissue engineering through three-dimensional (3D) melt-extrusion printing of medium chain length polyhydroxyalkanoate (mcl-PHA) (i.e., poly(3-hydroxyoctanoate-co-hydroxydecanoate-co-hydroxydodecanoate), P(3HO-co-3HD-co-3HDD)). The process parameters were successfully optimized to produce well-defined and reproducible 3D P(3HO-co-3HD-co-3HDD) scaffolds, showing high cell viability (100%) toward both undifferentiated and differentiated MC3T3-E1 cells. To introduce antibacterial features in the developed scaffolds, two strategies were investigated. For the first strategy, P(3HO-co-3HD-co-3HDD) was combined with PHAs containing thioester groups in their side chains (i.e., PHACOS), inherently antibacterial PHAs. The 3D blend scaffolds were able to induce a 70% reduction of Staphylococcus aureus 6538P cells by direct contact testing, confirming their antibacterial properties. Additionally, the scaffolds were able to support the growth of MC3T3-E1 cells, showing the potential for bone regeneration. For the second strategy, composite materials were produced by the combination of P(3HO-co-3HD-co-HDD) with a novel antibacterial hydroxyapatite doped with selenium and strontium ions (Se-Sr-HA). The composite material with 10 wt % Se-Sr-HA as a filler showed high antibacterial activity against both Gram-positive (S. aureus 6538P) and Gram-negative bacteria (Escherichia coli 8739), through a dual mechanism: by direct contact (inducing 80% reduction of both bacterial strains) and through the release of active ions (leading to a 54% bacterial cell count reduction for S. aureus 6538P and 30% for E. coli 8739 after 24 h). Moreover, the composite scaffolds showed high viability of MC3T3-E1 cells through both indirect and direct testing, showing promising results for their application in bone tissue engineering.

Laser-Based Selective Material Processing for Next-Generation Additive Manufacturing

The connection between laser-based material processing and additive manufacturing is quite deeply rooted. In fact, the spark that started the field of additive manufacturing is the idea that two intersecting laser beams can selectively solidify a vat of resin. Ever since, laser has been accompanying the field of additive manufacturing, with its repertoire expanded from processing only photopolymer resin to virtually any material, allowing liberating customizability. As a result, additive manufacturing is expected to take an even more prominent role in the global supply chain in years to come. Herein, an overview of laser-based selective material processing is presented from various aspects: the physics of laser-material interactions, the materials currently used in additive manufacturing processes, the system configurations that enable laser-based additive manufacturing, and various functional applications of next-generation additive manufacturing. Additionally, current challenges and prospects of laser-based additive manufacturing are discussed.

Life Cycle of Functional All-Green Biocompatible Fibrous Materials Based on Biodegradable Polyhydroxybutyrate and Hemin: Synthesis, Service Life, and the End-of-Life via Biodegradation

This article addresses the entire life cycle of the all-green fibrous materials based on poly(3-hydroxybutyrate) (PHB) containing a natural biocompatible additive Hemin (Hmi): from preparation, service life, and the end of life upon in-soil biodegradation. Fibrous PHB/Hmi materials with a highly developed surface and interconnected porosity were prepared by electrospinning (ES) from Hmi-containing feed solutions. Structural organization of the PHB/Hmi materials (porosity, uniform structure, diameter of fibers, surface area, distribution of Hmi within the PHB matrix, phase composition, etc.) is shown to be governed by the ES conditions: the presence of even minor amounts of Hmi in the PHB/Hmi (below 5 wt %) serves as a powerful tool for the control over their structure, performance, and biodegradation. Service characteristics of the PHB/Hmi materials (wettability, prolonged release of Hmi, antibacterial activity, breathability, and mechanical properties) were studied by different physicochemical methods (scanning electron microscopy, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, differential scanning calorimetry, contact angle measurements, antibacterial tests, etc.). The effect of the structural organization of the PHB/Hmi materials on their in-soil biodegradation at the end of life was analyzed, and key factors providing efficient biodegradation of the PHB/Hmi materials at all stages (from adaptation to mineralization) are highlighted (high surface area and porosity, thin fibers, release of Hmi, etc.). The proposed approach allows for target-oriented preparation and structural design of the functional PHB/Hmi nonwovens when their structural supramolecular organization with a highly developed surface area controls both their service properties as efficient antibacterial materials and in-soil biodegradation upon the end of life.

Recent Biotechnological Applications of Polyhydroxyalkanoates (PHA) in the Biomedical Sector-A Review

Petroleum-derived plastics are materials of great importance for the contemporary lifestyle, and are widely used commercially because they are low cost, resistant, malleable, and weightless, in addition to their hydrophobic character. However, some factors that confer the qualities of these materials also cause problems, mainly environmental, associated with their use. The COVID-19 pandemic aggravated these impacts due to the high demand for personal protective equipment and the packaging sector. In this scenario, bioplastics are environmentally positive alternatives to these plastics due to their applicability in several areas ranging from packaging, to biomedicine, to agriculture. Polyhydroxyalkanoates (PHAs) are biodegradable biopolymers usually produced by microorganisms as an energy reserve. Their structural variability provides a wide range of applications, making them a viable option to replace polluting materials. PHAs can be applied in various biotechnology sectors, such as producing drug carriers and scaffolds for tissue engineering. This review aimed to survey works published in the last five years on the study and biotechnological application of PHAs in the biomedical sector, exploring the versatility and advantages of their use and helping to understand how to enhance their application.

Three-Dimensional Printing of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] Biodegradable Scaffolds: Properties, In Vitro and In Vivo Evaluation

The results of constructing 3D scaffolds from degradable poly(3-hydrosbutyrpate-co-3-hydroxyvalerate) using FDM technology and studying the structure, mechanical properties, biocompatibility in vitro, and osteoplastic properties in vivo are presented. In the process of obtaining granules, filaments, and scaffolds from the initial polymer material, a slight change in the crystallization and glass transition temperature and a noticeable decrease in molecular weight (by 40%) were registered. During the compression test, depending on the direction of load application (parallel or perpendicular to the layers of the scaffold), the 3D scaffolds had a Young's modulus of 207.52 ± 19.12 and 241.34 ± 7.62 MPa and compressive stress tensile strength of 19.45 ± 2.10 and 22.43 ± 1.89 MPa, respectively. SEM, fluorescent staining with DAPI, and calorimetric MTT tests showed the high biological compatibility of scaffolds and active colonization by NIH 3T3 fibroblasts, which retained their metabolic activity for a long time (up to 10 days). The osteoplastic properties of the 3D scaffolds were studied in the segmental osteotomy test on a model defect in the diaphyseal zone of the femur in domestic Landrace pigs. X-ray and histological analysis confirmed the formation of fully mature bone tissue and complete restoration of the defect in 150 days of observation. The results allow us to conclude that the constructed resorbable 3D scaffolds are promising for bone grafting.

Recent Advances in the Production of Pharmaceuticals Using Selective Laser Sintering

Selective laser sintering (SLS) is an additive manufacturing process that has shown promise in the production of medical devices, including hip cups, knee trays, dental crowns, and hearing aids. SLS-based 3D-printed dosage forms have the potential to revolutionise the production of personalised drugs. The ability to manipulate the porosity of printed materials is a particularly exciting aspect of SLS. Porous tablet formulations produced by SLS can disintegrate orally within seconds, which is challenging to achieve with traditional methods. SLS also enables the creation of amorphous solid dispersions in a single step, rather than the multi-step process required with conventional methods. This review provides an overview of 3D printing, describes the operating mechanism and necessary materials for SLS, and highlights recent advances in SLS for biomedical and pharmaceutical applications. Furthermore, an in-depth comparison and contrast of various 3D printing technologies for their effectiveness in tissue engineering applications is also presented in this review.

Mechanical properties and failure mechanisms of polyamide 12 gradient scaffolds developed with selective laser sintering

Developing a functional gradient scaffold compatible with the fantastic biological and mechanical properties of natural bone tissue is imperative in bone tissue engineering. In this work, the stretch-dominated (cubical and circular) and bending-dominant (diamond and gyroid) pore styles were employed to design custom-graded scaffolds based on the curve interference method and then were fabricated by selective laser sintering (SLS) using polyamide 12 (PA12) powder. Subsequently, the mechanical behavior, failure mechanism, and energy absorption performance of porous structures were investigated via compression experiments and finite element (FE) simulation. The results indicated that the stretch-dominated radial gradient structures entire exhibited transverse shear failure and the bending-dominant radial gradient structures whole exhibited progressive destruction, while all of the axial gradient scaffolds suffered a predictable layer-by-layer fracture. Among them, the bending-dominated radial gradient structure of gyroid had been proven to sustain stronger deformability and energy absorption capacity. Meanwhile, the FE method powerfully predicted the mechanical behavior of the scaffold, and this research thereby possessed significant implications for the development of bone tissue engineering.

Poly(3-hydroxybutyrate) 3D-Scaffold-Conduit for Guided Tissue Sprouting

Scaffold biocompatibility remains an urgent problem in tissue engineering. An especially interesting problem is guided cell intergrowth and tissue sprouting using a porous scaffold with a special design. Two types of structures were obtained from poly(3-hydroxybutyrate) (PHB) using a salt leaching technique. In flat scaffolds (scaffold-1), one side was more porous (pore size 100-300 μm), while the other side was smoother (pore size 10-50 μm). Such scaffolds are suitable for the in vitro cultivation of rat mesenchymal stem cells and 3T3 fibroblasts, and, upon subcutaneous implantation to older rats, they cause moderate inflammation and the formation of a fibrous capsule. Scaffold-2s are homogeneous volumetric hard sponges (pore size 30-300 μm) with more structured pores. They were suitable for the in vitro culturing of 3T3 fibroblasts. Scaffold-2s were used to manufacture a conduit from the PHB/PHBV tube with scaffold-2 as a filler. The subcutaneous implantation of such conduits to older rats resulted in gradual soft connective tissue sprouting through the filler material of the scaffold-2 without any visible inflammatory processes. Thus, scaffold-2 can be used as a guide for connective tissue sprouting. The obtained data are advanced studies for reconstructive surgery and tissue engineering application for the elderly patients.

Bioactive peptides for boosting stem cell culture platform: Methods and applications

Peptides, short protein fragments, can emulate the functions of their full-length native counterparts. Peptides are considered potent recombinant protein alternatives due to their specificity, high stability, low production cost, and ability to be easily tailored and immobilized. Stem cell proliferation and differentiation processes are orchestrated by an intricate interaction between numerous growth factors and proteins and their target receptors and ligands. Various growth factors, functional proteins, and cellular matrix-derived peptides efficiently enhance stem cell adhesion, proliferation, and directed differentiation. For that, peptides can be immobilized on a culture plate or conjugated to scaffolds, such as hydrogels or synthetic matrices. In this review, we assess the applications of a variety of peptides in stem cell adhesion, culture, organoid assembly, proliferation, and differentiation, describing the shortcomings of recombinant proteins and their full-length counterparts. Furthermore, we discuss the challenges of peptide applications in stem cell culture and materials design, as well as provide a brief outlook on future directions to advance peptide applications in boosting stem cell quality and scalability for clinical applications in tissue regeneration.

Fine Tuning of the Mechanical Properties of Bio-Based PHB/Nanofibrillated Cellulose Biocomposites to Prevent Implant Failure Due to the Bone/Implant Stress Shielding Effect

A significant mechanical properties mismatch between natural bone and the material forming the orthopedic implant device can lead to its failure due to the inhomogeneous loads distribution, resulting in less dense and more fragile bone tissue (known as the stress shielding effect). The addition of nanofibrillated cellulose (NFC) to biocompatible and bioresorbable poly(3-hydroxybutyrate) (PHB) is proposed in order to tailor the PHB mechanical properties to different bone types. Specifically, the proposed approach offers an effective strategy to develop a supporting material, suitable for bone tissue regeneration, where stiffness, mechanical strength, hardness, and impact resistance can be tuned. The desired homogeneous blend formation and fine-tuning of PHB mechanical properties have been achieved thanks to the specific design and synthesis of a PHB/PEG diblock copolymer that is able to compatibilize the two compounds. Moreover, the typical high hydrophobicity of PHB is significantly reduced when NFC is added in presence of the developed diblock copolymer, thus creating a potential cue for supporting bone tissue growth. Hence, the presented outcomes contribute to the medical community development by translating the research results into clinical practice for designing bio-based materials for prosthetic devices.

Review on Bioinspired Design of ECM-Mimicking Scaffolds by Computer-Aided Assembly of Cell-Free and Cell Laden Micro-Modules

Tissue engineering needs bioactive drug delivery scaffolds capable of guiding cell biosynthesis and tissue morphogenesis in three dimensions. Several strategies have been developed to design and fabricate ECM-mimicking scaffolds suitable for directing in vitro cell/scaffold interaction, and controlling tissue morphogenesis in vivo. Among these strategies, emerging computer aided design and manufacturing processes, such as modular tissue unit patterning, promise to provide unprecedented control over the generation of biologically and biomechanically competent tissue analogues. This review discusses recent studies and highlights the role of scaffold microstructural properties and their drug release capability in cell fate control and tissue morphogenesis. Furthermore, the work highlights recent advances in the bottom-up fabrication of porous scaffolds and hybrid constructs through the computer-aided assembly of cell-free and/or cell-laden micro-modules. The advantages, current limitations, and future challenges of these strategies are described and discussed.

Personalized 3D printed bone scaffolds: A review

3D printed bone scaffolds have the potential to replace autografts and allografts because of advantages such as unlimited supply and the ability to tailor the scaffolds' biochemical, biological and biophysical properties. Significant progress has been made over the past decade in additive manufacturing techniques to 3D print bone grafts, but challenges remain in the lack of manufacturing techniques that can recapitulate both mechanical and biological functions of native bones. The purpose of this review is to outline the recent progress and challenges of engineering an ideal synthetic bone scaffold and to provide suggestions for overcoming these challenges through bioinspiration, high-resolution 3D printing, and advanced modeling techniques. The article provides a short overview of the progress in developing the 3D printed scaffolds for the repair and regeneration of critical size bone defects. STATEMENT OF SIGNIFICANCE: Treatment of critical size bone defects is still a tremendous clinical challenge. To address this challenge, diverse sets of advanced manufacturing approaches and materials have been developed for bone tissue scaffolds. 3D printing has sparked much interest because it provides a close control over the scaffold's internal architecture and in turn its mechanical and biological properties. This article provides a critical overview of the relationships between material compositions, printing techniques, and properties of the scaffolds and discusses the current technical challenges facing their successful translation to the clinic. Bioinspiration, high-resolution printing, and advanced modeling techniques are discussed as future directions to address the current challenges.

Evaluation of the Properties of PHB Composite Filled with Kaolin Particles for 3D Printing Applications Using the Design of Experiment

In the presented work, poly(3-hydroxybutyrate)-PHB-based composites for 3D printing as bio-sourced and biodegradable alternatives to synthetic plastics are characterized. The PHB matrix was modified by polylactide (PLA) and plasticized by tributyl citrate. Kaolin particles were used as a filler. The mathematical method "Design of Experiment" (DoE) was used to create a matrix of samples for further evaluation. Firstly, the optimal printing temperature of the first and upper layers was determined. Secondly, the 3D printed samples were tested with regards to the warping during the 3D printing. Testing specimens were prepared using the determined optimal printing conditions to measure the tensile properties, impact strength, and heat deflection temperature (HDT) of the samples. The results describe the effect of adding individual components (PHB, PLA, plasticizer, and filler) in the prepared composite sample on the resulting material properties. Two composite samples were prepared based on the theoretical results of DoE (one with the maximum printability and one with the maximum HDT) to compare them with the real data measured. The tests of these two composite samples showed 25% lower warping and 8.9% higher HDT than was expected by the theory.

Bioinspired Protein/Peptide Loaded 3D Printed PLGA Scaffold Promotes Bone Regeneration

Background: This study was aimed to investigate the effect of three dimensional (3D)printed poly lactide-co-glycolide (PLGA) scaffolds combined with Gly-Phe-Hyp-Gly-Arg (GFOGER) and bone morphogenetic protein 9 (BMP-9) on the repair of large bone defects.

Methods: 3D printing method was used to produce PLGA scaffolds, and the sample was viewed by both optical microscopy and SEM, XRD analysis, water absorption and compressive strength analysis, etc. The rabbits were divided into six groups randomly and bone defect models were constructed (6 mm in diameter and 9 mm in depth): control group (n = 2), sham group (n = 4), model group (n = 4) and model + scaffold group (n = 4 rabbits for each group, 0%,2% and 4%). The rabbits were sacrificed at the 4th and 12th weeks after surgery, and the samples were collected for quantitative analysis of new bone mineral density by micro-CT, histopathological observation, immunohistochemistry and Western blot to detect the protein expression of osteoblast-related genes.

Results: This scaffold presented acceptable mechanical properties and slower degradation rates. After surface modification with GFOGER peptide and BMP-9, the scaffold demonstrated enhanced new bone mineral deposition and density over the course of a 12 week in vivo study. Histological analysis and WB confirmed that this scaffold up-regulated the expression of Runx7, OCN, COL-1 and SP7, contributing to the noted uniform trabeculae formation and new bone regeneration.

Conclusions: The application of this strategy in the manufacture of composite scaffolds provided extensive guidance for the application of bone tissue engineering.

The fabrication of multifunctional sodium alginate scaffold incorporating ibuprofen-loaded modified PLLA microspheres based on cryogenic 3D printing

A strategy to develop a multifunctional sodium alginate personalized scaffold with enhanced mechanical stability, osteogenesis activity and excellent anti-inflammatory activity by cryogenic 3 D printing combined with subsequent crosslinking with Sr²⁺ is proposed in this study. The ink for 3 D printing was prepared by dispersing modified PLLA droplets containing ibuprofen into sodium alginate aqueous solution using lecithin as stabilizer. The results showed that the drug-loaded microspheres formed from the low-temperature solidifying of the modified PLLA droplets were homogeneously dispersed in sodium alginate substrate, and the scaffold displayed a sustained drug release performance toward ibuprofen which endowed the scaffold with persistent anti-inflammatory effects. In vitro cell culture indicated that the lecithin not only acted as the stabilizer, but also stimulated the proliferation and mineralization of osteoblastic cells on the scaffold. Sr²⁺-crosslinking improved the mechanical properties and osteogenic activity of the scaffold.

Additive Manufacturing of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/Poly(D,L-lactide-co-glycolide) Biphasic Scaffolds for Bone Tissue Regeneration

Polyhydroxyalkanoates are biopolyesters whose biocompatibility, biodegradability, environmental sustainability, processing versatility, and mechanical properties make them unique scaffolding polymer candidates for tissue engineering. The development of innovative biomaterials suitable for advanced Additive Manufacturing (AM) offers new opportunities for the fabrication of customizable tissue engineering scaffolds. In particular, the blending of polymers represents a useful strategy to develop AM scaffolding materials tailored to bone tissue engineering. In this study, scaffolds from polymeric blends consisting of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(D,L-lactide-co-glycolide) (PLGA) were fabricated employing a solution-extrusion AM technique, referred to as Computer-Aided Wet-Spinning (CAWS). The scaffold fibers were constituted by a biphasic system composed of a continuous PHBV matrix and a dispersed PLGA phase which established a microfibrillar morphology. The influence of the blend composition on the scaffold morphological, physicochemical, and biological properties was demonstrated by means of different characterization techniques. In particular, increasing the content of PLGA in the starting solution resulted in an increase in the pore size, the wettability, and the thermal stability of the scaffolds. Overall, in vitro biological experiments indicated the suitability of the scaffolds to support murine preosteoblast cell colonization and differentiation towards an osteoblastic phenotype, highlighting higher proliferation for scaffolds richer in PLGA.

Additive Manufacturing Technologies for Fabrication of Biomaterials for Surgical Procedures in Dentistry: A Narrative Review

PURPOSE

To screen and critically appraise available literature regarding additive manufacturing technologies for bone graft material fabrication in dentistry.

MATERIAL AND METHODS

PubMed and Scopus were searched up to May 2021. Studies reporting the additive manufacturing techniques to manufacture scaffolds for intraoral bone defect reconstruction were considered eligible. A narrative review was synthesized to discuss the techniques for bone graft material fabrication in dentistry and the biomaterials used.

RESULTS

The databases search resulted in 933 articles. After removing duplicate articles (128 articles), the titles and abstracts of the remaining articles (805 articles) were evaluated. A total of 89 articles were included in this review. Reading these articles, 5 categories of additive manufacturing techniques were identified: material jetting, powder bed fusion, vat photopolymerization, binder jetting, and material extrusion.

CONCLUSIONS

Additive manufacturing technologies for bone graft material fabrication in dentistry, especially 3D bioprinting approaches, have been successfully used to fabricate bone graft material with distinct compositions.

Scaffolds the backbone of tissue engineering: Advancements in use of polyhydroxyalkanoates (PHA)

Our body is built to heal from inside out naturally but wide-ranging medical conditions necessitate the need for artificial assistance, and therefore, something that can assist the body to heal wounds and damaged tissues quickly and efficiently is of utmost importance. Tissue engineering technology helps to regenerate new tissue to replace the diseased or injured one. The technology uses biodegradable porous three-dimensional scaffolds for mimicking the structure and functions of the natural extracellular matrix. The material and design of scaffolds are critical areas of biomaterial research. Biomaterial-based three-dimensional structures have been the most promising material to serve as scaffolds for seeding cells, both in vivo and in vitro. One such material is polyhydroxyalkanoates (PHAs) which are thermoplastic biopolyesters that are highly suitable for this purpose due to their enhanced biocompatibility, biodegradability, thermo-processability, diverse mechanical properties, non-toxicity and natural origin. Moreover, they have tremendous possibilities of customization through biological physical and chemical modification as well as blending with other materials. They are being used for several tissue engineering applications such as bone graft substitute, cardiovascular patches, stents, for nerve repair and in implantology as valves and sutures. The present review overviews usage of a multitude of PHA-based biomaterials for a wide range of tissue engineering applications, based on their properties suitable for the specific applications.

Advances in 3D printing of composite scaffolds for the repairment of bone tissue associated defects

The conventional methods of using autografts and allografts for repairing defects in bone, the osteochondral bone and the cartilage tissue have many disadvantages, like donor site morbidity and shortage of donors. Moreover, only 30% of the implanted grafts are shown to be successful in treating the defects. Hence, exploring alternative techniques such as tissue engineering to treat bone tissue associated defects is promising as it eliminates the above-mentioned limitations. To enhance the mechanical and biological properties of the tissue engineered product, it is essential to fabricate the scaffold used in tissue engineering by the combination of various biomaterials. Three-dimensional (3D) printing, with its ability to print composite materials and with complex geometry seems to have a huge potential in scaffold fabrication technique for engineering bone associated tissues.This review summarizes the recent applications and future perspectives of 3D printing technologies in the fabrication of composite scaffolds used in bone, osteochondral and cartilage tissue engineering. Key developments in the field of 3D printing technologies involves the incorporation of various biomaterials and cells in printing composite scaffolds mimicking physiologically relevant complex geometry & gradient porosity. Much recently, the emerging trend of printing smart scaffolds which can respond to external stimulus such as temperature, pH and magnetic field, known as 4D printing is gaining immense popularity and can be considered as the future of 3D printing applications in the field of tissue engineering. This article is protected by copyright. All rights reserved.

Design and properties of graded polyamide12/hydroxyapatite scaffolds based on primitive lattices using selective laser sintering

Scaffolds with favorable biological characteristics and controlled functional gradient architectures are preferable for the repair of damaged tissues in bone tissue engineering. In this study, the triply periodic minimal surfaces (TPMS) were introduced to design functional gradient porous scaffolds based on Primitive lattices which were then manufactured by selective laser sintering (SLS) using pure polyamide12 (PA12) material and PA12/hydroxyapatite (HA) composite material. The mechanical properties and permeability of the scaffolds were then evaluated by mechanical compression experiments and computational fluid dynamics (CFD) analysis. The radial-graded scaffold was found to have superior good mechanical properties and permeability and be favorable for the subsequent growth of bone tissue. Further, the optimal PA12/HA composition was determined by analyzing the effect of the addition of HA particles on the hydrophilicity and mechanical properties of the composite scaffold. Additionally, the cytotoxicity tests were performed to evaluate the effects of PA12/HA gradient scaffold on cell growth. The obtained results demonstrate that the radial gradient scaffold with 15% HA addition exhibits a feasible combination of comprehensive performance and biological activity, indicating a great application potential in the field of bone tissue engineering.

[Electrospun PLGA scaffold loaded with osteogenic growth peptide accelerates cranial bone repair in rats]

OBJECTIVE

To study the feasibility of electrospun poly(D, L-lactide-co-glycolide) (PLGA) scaffold loaded with osteogenic growth peptide (OGP) for bone tissue engineering.

METHODS

PLGA scaffolds were prepared by electrospinning PLGA solution without OGP(control group)or with 0.1%, 0.2%and 0.4%OGP(0.1%OGP@PLGA, 0.2%OGP@PLGA, and 0.4%OGP@PLGA scaffolds, respectively).The microstructure of the scaffolds was observed by scanning electron microscopy(SEM).The scaffolds were soaked in PBS to confirm the release pattern of OGP.The biocompatibility of the scaffolds was evaluated using CCK-8 assay and live/dead staining after a 7-day coculture with rat bone marrow-derived mesenchymal stem cells(BMSCs).ALP assay and ARS staining were used to evaluate osteoinduction capacity of the scaffolds co-cultured with rat BMSCs for 14 days.In a male SD rat model of skull defect(5 mm in diameter), bone defect repair was evaluated 8 weeks after implantation of the scaffolds using Micro-CT, HE and Masson staining.

RESULTS

The electrospun scaffolds had a fibrous structure similar to extracellular matrix(ECM)and were capable of sustained release of OGP for at least one month.Co-culture with 0.2%OGP@PLGA and 0.4%OGP@PLGA scaffolds, as compared with pure PLGA scaffold, significantly promoted the growth of rat BMSCs ((P < 0.01).The cells co-cultured with 0.4%OGP@PLGA scaffold showed the highest ALP activity and the greatest number of calcium nodules, indicating its strong osteoinduction ability (P < 0.01).Micro-CT and HE and Masson staining results showed that 0.4%OGP@PLGA scaffold had significantly better ability for promoting bone repair than the other two OGP-loaded scaffolds(P < 0.01).

CONCLUSION

The electrospun PLGA scaffold loaded with OGP effectively mimics the structure of ECM and has a good biocompatibility and osteoinduction ability, suggesting its potential as a new bone tissue engineering scaffold for bone defect repair.

Review on Computer-Aided Design and Manufacturing of Drug Delivery Scaffolds for Cell Guidance and Tissue Regeneration

In the last decade, additive manufacturing (AM) processes have updated the fields of biomaterials science and drug delivery as they promise to realize bioengineered multifunctional devices and implantable tissue engineering (TE) scaffolds virtually designed by using computer-aided design (CAD) models. However, the current technological gap between virtual scaffold design and practical AM processes makes it still challenging to realize scaffolds capable of encoding all structural and cell regulatory functions of the native extracellular matrix (ECM) of health and diseased tissues. Indeed, engineering porous scaffolds capable of sequestering and presenting even a complex array of biochemical and biophysical signals in a time- and space-regulated manner, require advanced automated platforms suitable of processing simultaneously biomaterials, cells, and biomolecules at nanometric-size scale. The aim of this work was to review the recent scientific literature about AM fabrication of drug delivery scaffolds for TE. This review focused on bioactive molecule loading into three-dimensional (3D) porous scaffolds, and their release effects on cell fate and tissue growth. We reviewed CAD-based strategies, such as bioprinting, to achieve passive and stimuli-responsive drug delivery scaffolds for TE and cancer precision medicine. Finally, we describe the authors' perspective regarding the next generation of CAD techniques and the advantages of AM, microfluidic, and soft lithography integration for enhancing 3D porous scaffold bioactivation toward functional bioengineered tissues and organs.

Recent approaches for enhanced production of microbial polyhydroxybutyrate: Preparation of biocomposites and applications

In modern decades, an increase in environmental awareness has attracted the keen interest of researchers to investigate eco-sustainable, recyclable materials to minimize reliance on petroleum-based polymeric compounds. Poly (3-hydroxybutyrate) is amorphous, linear, and biodegradable bacterial polyesters that belong to the polyhydroxyalkanoates family with enormous applications in many fields. The present review provides comprehensive information on polyhydroxybutyrate production from different biomass feedstock. Various studies on PHB production by genetically engineered bacterial cells and optimization of parameters have been discussed. Recent technological innovation in processing polyhydroxybutyrate-based biocomposite through the different process has also been examined. Besides this, the potential applications of the derived competent biocomposites in the other fields have been depicted.

Three-Dimensional Printing of Click Functionalized, Peptide Patterned Scaffolds for Osteochondral Tissue Engineering

Osteochondral repair remains a significant clinical challenge due to the multiple tissue phenotypes and complex biochemical milieu in the osteochondral unit. To repair osteochondral defects, it is necessary to mimic the gradation between bone and cartilage, which requires spatial patterning of multiple tissue-specific cues. To address this need, we have developed a facile system for the conjugation and patterning of tissue-specific peptides by melt extrusion of peptide-functionalized poly(ε-caprolactone) (PCL). In this study, alkyne-terminated PCL was conjugated to tissue-specific peptides via a mild, aqueous, and Ru(II)-catalyzed click reaction. The PCL-peptide composites were then 3D printed by multimaterial segmented printing to generate user-defined patterning of tissue-specific peptides. To confirm the bioactivity of 3D printed PCL-peptide composites, bone- and cartilage-specific scaffolds were seeded with mesenchymal stem cells and assessed for deposition of tissue-specific extracellular matrix in vitro. PCL-peptide scaffolds successfully promoted osteogenic and chondrogenic matrix deposition, with effects dependent on the identity of conjugated peptide.

Microbial Polyhydroxyalkanoates Granules: An Approach Targeting Biopolymer for Medical Applications and Developing Bone Scaffolds

Microbial polyhydroxyalkanoates (PHA) are proteinaceous storage granules ranging from 100 nm to 500 nm. Bacillus sp. serve as unique bioplastic sources of short-chain length and medium-chain length PHA showcasing properties such as biodegradability, thermostability, and appreciable mechanical strength. The PHA can be enhanced by adding functional groups to make it a more industrially useful biomaterial. PHA blends with hydroxyapatite to form nanocomposites with desirable features of compressibility. The reinforced matrices result in nanocomposites that possess significantly improved mechanical and thermal properties both in solid and melt states along with enhanced gas barrier properties compared to conventional filler composites. These superior qualities extend the polymeric composites' applications to aggressive environments where the neat polymers are likely to fail. This nanocomposite can be used in different industries as nanofillers, drug carriers for packaging essential hormones and microcapsules, etc. For fabricating a bone scaffold, electrospun nanofibrils made from biocomposite of hydroxyapatite and polyhydroxy butyrate, a form of PHA, can be incorporated with the targeted tissue. The other methods for making a polymer scaffold, includes gas foaming, lyophilization, sol-gel, and solvent casting method. In this review, PHA as a sustainable eco-friendly NextGen biomaterial from bacterial sources especially Bacillus cereus, and its application for fabricating bone scaffold using different strategies for bone regeneration have been discussed.

Recent Advances in 3D Printing of Polyhydroxyalkanoates: A Review

In the 21st century, additive manufacturing technologies have gained in popularity mainly due to benefits such as rapid prototyping, faster small production runs, flexibility and space for innovations, non-complexity of the process and broad affordability. In order to meet diverse requirements that 3D models have to meet, it is necessary to develop new 3D printing technologies as well as processed materials. This review is focused on 3D printing technologies applicable for polyhydroxyalkanoates (PHAs). PHAs are thermoplastics regarded as a green alternative to petrochemical polymers. The 3D printing technologies presented as available for PHAs are selective laser sintering and fused deposition modeling. Stereolithography can also be applied provided that the molecular weight and functional end groups of the PHA are adjusted for photopolymerization. The chemical and physical properties primarily influence the processing of PHAs by 3D printing technologies. The intensive research for the fabrication of 3D objects based on PHA has been applied to fulfil criteria of rapid and customized prototyping mainly in the medical area.

FDM 3D Printed Composites for Bone Tissue Engineering Based on Plasticized Poly(3-hydroxybutyrate)/poly(d,l-lactide) Blends

Tissue engineering is a current trend in the regenerative medicine putting pressure on scientists to develop highly functional materials and methods for scaffolds' preparation. In this paper, the calibrated filaments for Fused Deposition Modeling (FDM) based on plasticized poly(3-hydroxybutyrate)/poly(d,l-lactide) 70/30 blend modified with tricalcium phosphate bioceramics were prepared. Two different plasticizers, Citroflex (n-Butyryl tri-n-hexyl citrate) and Syncroflex (oligomeric adipate ester), both used in the amount of 12 wt%, were compared. The printing parameters for these materials were optimized and the printability was evaluated by recently published warping test. The samples were studied with respect to their thermal and mechanical properties, followed by biological in vitro tests including proliferation, viability, and osteogenic differentiation of human mesenchymal stem cells. According to the results from differential scanning calorimetry and tensile measurements, the Citroflex-based plasticizer showed very good softening effect at the expense of worse printability and unsatisfactory performance during biological testing. On the other hand, the samples with Syncroflex demonstrated lower warping tendency compared to commercial polylactide filament with the warping coefficient one third lower. Moreover, the Syncroflex-based samples exhibited the non-cytotoxicity and promising biocompatibility.

Drug Delivery Applications of Three-Dimensional Printed (3DP) Mesoporous Scaffolds

Mesoporous materials are structures characterized by a well-ordered large pore system with uniform porous dimensions ranging between 2 and 50 nm. Typical samples are zeolite, carbon molecular sieves, porous metal oxides, organic and inorganic porous hybrid and pillared materials, silica clathrate and clathrate hydrates compounds. Improvement in biochemistry and materials science led to the design and implementation of different types of porous materials ranging from rigid to soft two-dimensional (2D) and three-dimensional (3D) skeletons. The present review focuses on the use of three-dimensional printed (3DP) mesoporous scaffolds suitable for a wide range of drug delivery applications, due to their intrinsic high surface area and high pore volume. In the first part, the importance of the porosity of materials employed for drug delivery application was discussed focusing on mesoporous materials. At the end of the introduction, hard and soft templating synthesis for the realization of ordered 2D/3D mesostructured porous materials were described. In the second part, 3DP fabrication techniques, including fused deposition modelling, material jetting as inkjet printing, electron beam melting, selective laser sintering, stereolithography and digital light processing, electrospinning, and two-photon polymerization were described. In the last section, through recent bibliographic research, a wide number of 3D printed mesoporous materials, for in vitro and in vivo drug delivery applications, most of which relate to bone cells and tissues, were presented and summarized in a table in which all the technical and bibliographical details were reported. This review highlights, to a very cross-sectional audience, how the interdisciplinarity of certain branches of knowledge, as those of materials science and nano-microfabrication are, represent a growing valuable aid in the advanced forum for the science and technology of pharmaceutics and biopharmaceutics.

Biodegradable Polymers as the Pivotal Player in the Design of Tissue Engineering Scaffolds

Biodegradable polymers play a pivotal role in in situ tissue engineering. Utilizing various technologies, researchers have been able to fabricate 3D tissue engineering scaffolds using biodegradable polymers. They serve as temporary templates, providing physical and biochemical signals to the cells and determining the successful outcome of tissue remodeling. Furthermore, a biodegradable scaffold also presents the fourth dimension for tissue engineering, namely time. The properties of the biodegradable polymer change over time, presenting continuously changing features during the degradation process. These changes become more complicated when different materials are combined together to fabricate a composite or heterogeneous scaffold. This review undertakes a systematic analysis of the basic characteristics of biodegradable polymers and describe recent advances in making composite biodegradable scaffolds for in situ tissue engineering and regenerative medicine. The interaction between implanted biodegradable biomaterials and the in vivo environment are also discussed, including the properties and functional changes of the degradable scaffold, the local effect of degradation on the contiguous tissue and their evaluation using both in vitro and in vivo models.

Properties of scaffolds prepared by fused deposition modeling of poly(hydroxyalkanoates)

Poly(hydroxyalkanoates) are biodegradable and biocompatible polymers suitable for tissue engineering. Fused deposition modeling (FDM) belongs to modern rapid prototyping techniques for the fabrication of scaffolds. In this work, poly(3-hydroxybutyrate (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) were tested for FDM. Thermal and rheological properties of industrial PHAs were compared with poly(lactic acid) (PLA), which is a biodegradable polymer commonly used for FDM. The massive decrease in viscosity and loss of molecular weight of PHB and PHBV precluded their use for FDM. On the other hand, the thermal stability of PHBH was comparable to that of PLA. PHBH scaffolds prepared by FDM exhibited excellent mechanical properties, no cytotoxicity and large proliferation of mouse embryonic fibroblast cells within 96 h. The hydrolytic degradation of PHBH and PLA scaffolds tested in synthetic gastric juice for 52 days confirmed a faster degradation of PHBH than PLA. The decrease in molecular weight confirmed the first-order kinetics with a slightly higher (0.0169 day^-1) degradation rate constant for PHBH as compared to the value (0.0107 day^-1) obtained for PLA. These results indicate that PHBH could be used to produce scaffolds by FDM with application in tissue engineering.

Two-Step Solvent-Free Synthesis of Poly(hydroxybutyrate)-Based Photocurable Resin with Potential Application in Stereolithography

A bio-based polymeric ink for stereolithography developed through a two-step solvent-free process is herein proposed. Specifically, low-molecular-weight poly(hydroxybutyrate) (PHB)-diol oligomers are prepared via molten transesterification of bacterial PHB with 1,4-butanediol. Transesterification conditions such as diol concentration, catalyst amount, and reaction time are studied for optimizing the final oligomers' molecular weight and structural features. In the second step, the oligomeric hydroxyl terminals are converted into methacrylate moieties through a solvent-free end-capping reaction and diluted in propylene carbonate in order to obtain a photo-polymerizable ink with suitable viscosity. The ink is UV-cured, and the obtained material properties are investigated by FT-IR and differential scanning calorimetry measurements. The proposed method provides a valuable and environmentally friendly alternative to currently available synthetic routes, overcoming their typical disadvantages related to the used solvents and harsh conditions. Moreover, it opens up a sustainable route for converting polyesters into functionalized oligomeric derivatives, which can potentially find application in 3D printing of customized biomedical devices.

Indirect selective laser sintering-printed microporous biphasic calcium phosphate scaffold promotes endogenous bone regeneration via activation of ERK1/2 signaling

The fabrication technique determines the physicochemical and biological properties of scaffolds, including the porosity, mechanical strength, osteoconductivity, and bone regenerative potential. Biphasic calcium phosphate (BCP)-based scaffolds are superior in bone tissue engineering due to their suitable physicochemical and biological properties. We developed an indirect selective laser sintering (SLS) printing strategy to fabricate 3D microporous BCP scaffolds for bone tissue engineering purposes. The green part of the BCP scaffold was fabricated by SLS at a relevant low temperature in the presence of epoxy resin (EP), and the remaining EP was decomposed and eliminated by a subsequent sintering process to obtain the microporous BCP scaffolds. Physicochemical properties, cell adhesion, biocompatibility, in vitro osteogenic potential, and rabbit critical-size cranial bone defect healing potential of the scaffolds were extensively evaluated. This indirect SLS printing eliminated the drawbacks of conventional direct SLS printing at high working temperatures, i.e. wavy deformation of the scaffold, hydroxyapatite decomposition, and conversion of β-tricalcium phosphate (TCP) to α-TCP. Among the scaffolds printed with various binder ratios (by weight) of BCP and EP, the scaffold with 50/50 binder ratio (S4) showed the highest mechanical strength and porosity with the smallest pore size. Scaffold S4 showed the highest effect on osteogenic differentiation of precursor cells in vitro, and this effect was ERK1/2 signaling-dependent. Scaffold S4 robustly promoted precursor cell homing, endogenous bone regeneration, and vascularization in rabbit critical-size cranial defects. In conclusion, BCP scaffolds fabricated by indirect SLS printing maintain the physicochemical properties of BCP and possess the capacity to recruit host precursor cells to the defect site and promote endogenous bone regeneration possibly via the activation of ERK1/2 signaling.

3D printing of bone tissue engineering scaffolds

Tissue engineering is promising in realizing successful treatments of human body tissue loss that current methods cannot treat well or achieve satisfactory clinical outcomes. In scaffold-based bone tissue engineering, a high performance scaffold underpins the success of a bone tissue engineering strategy and a major direction in the field is to produce bone tissue engineering scaffolds with desirable shape, structural, physical, chemical and biological features for enhanced biological performance and for regenerating complex bone tissues. Three-dimensional (3D) printing can produce customized scaffolds that are highly desirable for bone tissue engineering. The enormous interest in 3D printing and 3D printed objects by the science, engineering and medical communities has led to various developments of the 3D printing technology and wide investigations of 3D printed products in many industries, including biomedical engineering, over the past decade. It is now possible to create novel bone tissue engineering scaffolds with customized shape, architecture, favorable macro-micro structure, wettability, mechanical strength and cellular responses. This article provides a concise review of recent advances in the R & D of 3D printing of bone tissue engineering scaffolds. It also presents our philosophy and research in the designing and fabrication of bone tissue engineering scaffolds through 3D printing.

3D Printing for the Clinic: Examining Contemporary Polymeric Biomaterials and Their Clinical Utility

The advent of additive manufacturing offered the potential to revolutionize clinical medicine, particularly with patient-specific implants across a range of tissue types. However, to date, there are very few examples of polymers being used for additive processes in clinical settings. The state of the art with regards to 3D printable polymeric materials being exploited to produce novel clinically relevant implants is discussed here. We focus on the recent advances in the development of implantable, polymeric medical devices and tissue scaffolds without diverging extensively into bioprinting. By introducing the major 3D printing techniques along with current advancements in biomaterials, we hope to provide insight into how these fields may continue to advance while simultaneously reviewing the ongoing work in the field.

Interactions at scaffold interfaces: Effect of surface chemistry, structural attributes and bioaffinity

Effective regenerative medicine relies on understanding the interplay between biomaterial implants and the adjoining cells. Scaffolds contribute by presenting sites for cellular adhesion, growth, proliferation, migration, and differentiation which lead to regeneration of tissues over desired periods of time. The fabrication and recruitment of scaffolds often fail to consider the interactions that occur at the interfaces, thereby risking rejection. This lack of knowledge on interfacial microenvironments and related exchanges often causes reduced cellular interactions, poor cell survival and intervention failure. Successful regenerative therapy requires scaffolds with bespoke biocompatibility, optimum pore structure, and cues for cell attachments. These factors determine the development of cellular affinity in scaffolds. For biomedical applications, a detailed understanding of scaffolds and their interfaces is required for better tuning of biomaterials to suit the microenvironments. In this review, we discuss the role of biointerfaces with a focus on surface chemistry, pore structure, scaffold hydro-affinity and their biointeractions. An understanding of the effect of scaffold interfacial properties is crucial for enhancing the progress of tissue engineering towards clinical applications.

Zirconia toughened hydroxyapatite biocomposite formed by a DLP 3D printing process for potential bone tissue engineering

The construction of ceramic components with UV curing is a developing trend by an additive manufacturing (AM) technology, due to the excellent advantages of high precision selective fixation and rapid prototyping, the application of this technology to bone defect repair had become one of the hotspots of research. Hydroxyapatite (HAP) is one of the most popular calcium phosphate biomaterials, which is very close to the main ingredient of human bones. Thus, hydroxyapatite biomaterials are popular as bone graft materials. In summary, the preparation of HAP bioceramics by a 3D printing of digital light processing (DLP) is a promising work. However, the preparation of HAP hybrid suspensions with high solid loading and good fluidity that can be printed by DLP encountered some challenges. Therefore, the purpose of this work is to improve and develop a novel UV-curing suspension with a high solids loading, which the suspension with the hydrodynamic properties and stability are suitable for DLP printer, in order to compensate for the brittleness of HAP ceramics itself to a certain extent, a low amount of zirconia was added in the suspension as an additive to fabricate a zirconia toughened HAP bioceramic composite by a DLP of 3D printing. In this work, the HAP powder was pre-modified by two organic modifiers to improve the compatibility in the acrylic resin system, and the addition of the castor oil phosphate further reduced the shear stress of the suspension to ensure strong liquidity. The UV suspension with 60 wt% powder particle loading had a minimum viscosity of 7495 mPa·s at 30 rpm, which was vacuum sintered at 1100 °C, 1200 °C, and 1250 °C, respectively. The composite ceramics (with 6 wt% ZrO₂) at 1200 °C had a relative density of 90.7%, while the sintered samples at 1250 °C had stronger tensile strength and bending strength. The toughening effect of zirconia incorporation on HAP ceramics was also confirmed by the change of tensile modulus and bending modulus, whereas the corresponding mechanical properties were also significantly enhanced.