Granular honeycomb scaffolds composed of carbonate apatite for simultaneous intra- and inter-granular osteogenesis and angiogenesis

Fabrication and in vitro cytocompatibility evaluation of porous bone scaffold based on cuttlefish bone-derived nano-carbonated hydroxyapatite reinforced with polyethylene oxide/chitosan fibrous structure

A novel porous bone scaffold based on nano-carbonated hydroxyapatite reinforced with fibrous-like structured polyethylene oxide/chitosan network (nCHA/PEO/CS) was introduced and fabricated via freeze-drying. Prior to this, the nCHA was synthesized through a hydrothermal reaction based on cuttlefish bone (CFB, Sepia officinalis). The raw cuttlefish bone (raw-CFB) was first decomposed to obtain cuttlefish bone-derived calcium oxide (CaO-CFB) by calcination at 1000 °C, which was used for synthesizing nCHA. The chemical composition analysis showed that the nCHA formed AB-type CHA with a high carbonate content of 7.38 wt%, which is in the range of carbonate content in native bone (2-9 wt%). The Ca/P molar ratio of nCHA was 1.712, very close to the Ca/P of biological apatite of 1.71. Morphological analysis revealed that nCHA consists of nanosized particles, potentially offering a large surface area to volume to promote ion exchange and cell interaction. The excellent physicochemical and morphological properties of nCHA proposed suitability as a bone scaffold precursor combined with PEO and CS. The nCHA/PEO/CS scaffolds were freeze-dried with varying PEO/CS concentrations. Physicochemical analysis indicated that increasing the PEO/CS concentration decreased the crystallinity of the scaffold, causing it to be lower than the nCHA crystallinity, which may be beneficial for cell growth. Morphological analysis revealed that the scaffold structure comprised nCHA cross-linked within a fibrous-like structured PEO/CS network, which appropriately mimics the fibrous structure of extracellular matrix (ECM) in natural bone. However, the nCHA/PEO/CS-11 scaffold formed more appropriate pores with suitable porosity for cell development, blood vessel formation, and nutrient perfusion. The nCHA/PEO/CS-11 scaffold also demonstrated sufficient compressive strength and good swelling behavior, which may favor bone regeneration. The nCHA/PEO/CS-11 scaffold demonstrated high cytocompatibility and facilitated the adherence of MC3T3E1 cells on the scaffold surface. The nCHA/PEO/CS-11 scaffold also promoted cell osteogenic differentiation. Owing to its desirable and suitable characteristics, the nCHA/PEO/CS-11 scaffold is promising in bone tissue engineering.

Controlling the pore size of carbonate apatite honeycomb scaffolds enhances orientation and strength of regenerated bone

To restore functions of long bones and avoid reconstruction failure, segmental defects should be quickly repaired using abundant amounts of regenerated bone with high mechanical strength and orientation along the bone axis. Although both bone volume and bone matrix orientation are important for faster restoration of long bones with segmental defects, researchers have primarily focused on the former. Artificial bone scaffolds with uniaxial channels, (e.g., honeycomb (HC) scaffolds), are considered adequate for regenerating bone oriented along the bone axis. The channel size may affect the orientation, amount, and strength of the regenerated bone. In this study, we investigated the effects of channel size in carbonate apatite HC scaffolds on the orientation of bones regenerated in segmental bone defects and determined the adequate channel size. Carbonate apatite HC scaffolds, with different channel sizes (350, 550, 730, and 890 μm in length on the side of the square aperture), were fabricated by extrusion molding of a mixture of calcium carbonate and organic binder, debinding, and subsequent phosphatization to convert the composition from calcium carbonate to carbonate apatite. No significant difference in the amounts of regenerated bones was observed for different channel sizes. However, bone along the bone axis was formed in the channels ≤550 μm in size but not in channels ≥730 μm. The HC scaffolds with a channel size of 350 μm regenerated bone with higher bending strength than those with a channel size of 890 μm. However, bone regenerated with the HC scaffolds having channel sizes of 350, 550, and 730 μm showed equal bending strength. Thus, the adequate channel size for fast regeneration of high-strength bone, oriented to the bone axis, is ≤730 μm. To the best of our knowledge, this is the first study to report the effect of channel size on bone orientation and strength. The findings of this study are relevant to the fast repair of segmental bone defects.

Carbonate Apatite Honeycomb Scaffold-Based Drug Delivery System for Repairing Osteoporotic Bone Defects

Osteoporotic bone defects are difficult to repair in elderly patients. This study aimed to repair osteoporotic bone defects using a combination of bone tissue engineering (BTE) and drug delivery systems (DDS). Herein, honeycomb granules (HCGs) composed of carbonate apatite microspheres were fabricated as BTE scaffolds. Each HCG possesses hexagonal macropores and abundant interconnected micropores between the microspheres. Owing to these multiscale interconnected pores, HCGs can readily contain antibodies against sclerostin (Scl), which causes imbalances in bone homeostasis. Anti-Scl antibody-loaded HCGs (Scl-Ab-HCGs) regulate the release of Scl-Abs in response to the pH of the osteoporotic environment. In ovariectomized rabbit osteoporotic femurs, HCG monotherapy forms new bone with less osteocyte damage (fewer empty bone lacunae) and fewer osteoclasts than osteoporotic bone; however, it is insufficient to prevent receptor activator of nuclear factor-kappa B ligand (RANKL) overexpression. Consequently, HCG monotherapy restores bone quantity better than no treatment but not to normal levels. In contrast, new bone tissue formed by Scl-Ab-HCG-based DDS predominantly expresses osteocalcin rather than RANKL, similar to normal bone, and shows a similar osteocyte apoptosis level, bone quantity, and osteoclast number as normal bone. Thus, Scl-Ab-HCG-based DDS is a promising approach for osteoporotic bone defect repair.

In vivo trial of bioresorbable mesh cages contained bone graft granules in rabbit femoral bone defects

Bone graft granules implanted in bone defects come into physical contact with the host bone and form interconnected porous structure. However, there exists an accidental displacement of granules to unintended locations and leakage of granules from bone defects. Although covering the defect with a barrier membrane prevents granule emanation, this procedure is troublesome. To resolve these problems, we fabricated bioresorbable mesh cages (BRMc) in this study. Bone graft granules composed of carbonate apatite alone (Gr) and bioresorbable mesh cages (BRMc/Gr) introduced the bone graft granules and were implanted into the bone defect in the rabbit femur. Micro-computed tomography and histological analysis were conducted at 4 and 12 weeks after implantation. Osteoprogenitors in the bloodstream from the host bone passed through the pores of BRMc, penetrated the porous structure of graft granules, and might interact with individual granules. Then bone remodeling could progress actively and new bone was formed. The new bone formation was similar to the host bone at 12 weeks and there were minimal signs of local tissue inflammation. BRMc/Gr could reduce the risk of unwanted new bone formation occurring due to loss of granules from the bone defects compared with Gr because BRMc enclosed granules and prevent granules leakage from bone defects and BRMc could not induce unfavorable effects to forme new bone. Additionally, BRMc/Gr could keep granules assembled in one place, avoid displacement of granules to unintended locations, and carry easily. These results demonstrated that BRMc/Gr was effective in bone regeneration and improved clinical handling.

Transformable Carbonate Apatite Chains as a Novel Type of Bone Graft

The aging global population is generating an ever-increasing demand for bone regeneration. Various materials, including blocks, granules, and sponges, are developed for bone regeneration. However, blocks require troublesome shaping and exhibit poor bone-defect conformities; granules migrate into the surrounding tissues during and after filling of the defect, causing handling difficulties and complications; and sponges contain polymers that are subject to religious restrictions, lack osteoconductivity, and may cause inflammation and allergies. Herein, carbonate apatite chains that overcome the limitations of conventional materials are presented. Although carbonate apatite granules migrate, causing inflammation and ectopic calcification, the chains remain in the defects without causing any complications. The chains conform to the defect shape and transform into 3D porous structures, resulting in faster bone regeneration than that observed using granules. Thus, these findings indicate that even traditional calcium phosphates materials can be converted to state-of-the-art materials via shape control.

Lamellar Septa-like Structured Carbonate Apatite Scaffolds with Layer-by-Layer Fracture Behavior for Bone Regeneration

Generally, ceramics are brittle, and porosity is inversely correlated with strength, which is one of the challenges of ceramic scaffolds. Here, we demonstrate that lamellar septum-like carbonate apatite scaffolds have the potential to overcome these challenges. They were fabricated by exploiting the cellular structure of the cuttlebone, removing the organic components from the cuttlebone, and performing hydrothermal treatment. Scanning electron microscopy revealed that the scaffolds had a cellular structure with walls between lamellar septa. The interwall and interseptal sizes were 80-180 and 300-500 μm, respectively. The size of the region enclosed by the walls and septa coincided with the macropore size detected by mercury intrusion porosimetry. Although the scaffold porosity was extremely high (93.2%), the scaffold could be handled without disintegration. The compressive stress-strain curve demonstrated that the scaffolds showed layer-by-layer fracture behavior, which seemed beneficial for avoiding catastrophic failure under impact. When the scaffolds were implanted into rabbit femurs, new bone and blood vessels formed within the scaffold cells at 4 weeks. At 12 weeks, the scaffolds were almost entirely replaced with new bone. Thus, the lamellar septum-like cellular-structured carbonate apatite is a promising scaffold for achieving early bone regeneration and compression resistance.

Preferentially Biodegradable Gypsum Fibers Endowing Invisible Microporous Structures and Enhancing Osteogenic Capability of Calcium Phosphate Cements

It is known that hydroxyapatite-type calcium phosphate cement (CPC) shows appreciable self-curing properties, but the phase transformation products often lead to slow biodegradation and disappointing osteogenic responses. Herein, we developed an innovative strategy to endow invisible micropore networks, which could tune the microstructures and biodegradation of α-tricalcium phosphate (α-TCP)-based CPC by gypsum fibers, and the osteogenic capability of the composite cements could be enhanced in vivo. The gypsum fibers were prepared via extruding the gypsum powder/carboxylated chitosan (CC) slurry through a 22G nozzle (410 μm in diameter) and collecting with a calcium salt solution. Then, the CPCs were prepared by mixing the α-TCP powder with gypsum fibers (0-24 wt %) and an aqueous solution to form self-curing cements. The physicochemical characterizations showed that injectability was decreased with an increase in the fiber contents. The μCT reconstruction demonstrated that the gypsum fiber could be distributed in the CPC substrate and produce long-range micropore architectures. In particular, incorporation of gypsum fibers would tune the ion release, produce tunnel-like pore networks in vitro, and promote new bone tissue regeneration in rabbit femoral bone defects in vivo. Appropriate gypsum fibers (16 and 24 wt %) could enhance bone defect repair and cement biodegradation. These results demonstrate that the highly biodegradable cement fibers could mediate the microstructures of conventional CPC biomaterials, and such a bicomponent composite strategy may be beneficial for expanding clinical CPC-based applications.

Crystalline Micro-Sized Carbonated Apatites: Chemical Anisotropy of the Crystallite Surfaces, Biocompatibility, Osteoconductivity, and Osteoinductive Effect Enhanced by Poly(ethylene phosphoric acid)

Carbonated hydroxyapatites (CAp) are very close to natural bone apatite in chemical composition and are regarded as a prospective bone mineral substitute for bone surgery and orthopedics. However, until now, the studies and applications of CAp were limited because of the amorphous nature of the synthetic CAp. In the present work, microsized highly crystalline carbonated apatites with uniform hexagonal (hCAp) or platelike (pCAp) morphology have been studied for the first time in vitro and in vivo, comparing against commercial hydroxyapatite (HAp) and β-tricalcuim phosphate (βTCP). In vitro experiments on dissolution of those calcium phosphate ceramics (CPCs) in acetate (pH 5.5) and Tris (pH 7.3) buffer solutions showed the following rank order of the dissolution rates: βTCP > hCAp > pCAp > HAp. The higher dissolution rate of hCAp in comparison with pCAp is explained by chemical anisotropy of the crystallite surfaces, which was proven by SEM studies of the changes in the morphology of hCAp and pCAp crystallites during hydrolysis. A 5-week experiment on subcutaneous implantation of CPC species showed the following rank order of bioresorption rates: βTCP > pCAp > hCAp > HAp. pCAp matrixes exhibited the highest biocompatibility, confirmed by histomorphological analysis. Three-month bone regeneration experiments involving a rat tibial defect model were conducted with 250-500 μm granules of pCAp and pCAp-PEPA [pCAp, pretreated with 2 wt % poly(ethylene phosphoric acid)]. Notably, pCAp-PEPA implants were resorbed at higher rates and induced the formation of more mature osseous tissue, a compact bone with Haversian systems.

Superiority of Triply Periodic Minimal Surface Gyroid Structure to Strut-Based Grid Structure in Both Strength and Bone Regeneration

The aging population has rapidly driven the demand for bone regeneration. The pore structure of a scaffold is a critical factor that affects its mechanical strength and bone regeneration. Triply periodic minimal surface gyroid structures similar to the trabecular bone structure are considered superior to strut-based lattice structures (e.g., grids) in terms of bone regeneration. However, at this stage, this is only a hypothesis and is not supported by evidence. In this study, we experimentally validated this hypothesis by comparing gyroid and grid scaffolds composed of carbonate apatite. The gyroid scaffolds possessed compressive strength approximately 1.6-fold higher than that of the grid scaffolds because the gyroid structure prevented stress concentration, whereas the grid structure could not. The porosity of gyroid scaffolds was higher than that of the grid scaffolds; however, porosity and compressive strength generally have a trade-off relationship. Moreover, the gyroid scaffolds formed more than twice the amount of bone as grid scaffolds in a critical-sized bone defect in rabbit femur condyles. This favorable bone regeneration using gyroid scaffolds was attributed to the high permeability (i.e., larger volume of macropores or porosity) and curvature profile of the gyroid structure. Thus, this study validated the conventional hypothesis using in vivo experiments and revealed factors that led to this hypothetical outcome. The findings of this study are expected to contribute to the development of scaffolds that can achieve early bone regeneration without sacrificing the mechanical strength.

Gear-shaped carbonate apatite granules with a hexagonal macropore for rapid bone regeneration

Synthetic bone grafts are in high demand owing to increased age-related bone disorders in the global aging population. Here, we report fabrication of gear-shaped granules (G-GRNs) for rapid bone healing. G-GRNs possessed six protrusions and a hexagonal macropore in the granular center. These were composed of carbonate apatite, i.e., bone mineral, microspheres with ∼1-μm micropores in the spaces between the microspheres. G-GRNs formed new bone and blood vessels (both on the granular surface and within the macropores) 4 weeks after implantation in the rabbit femur defects. The formed bone structure was similar to that of cancellous bone. The bone percentage in the defect recovered to that in a normal rabbit femur at week-4 post-implantation, and the bone percentage remained constant for the following 8 weeks. Throughout the entire period, the bone percentage in the G-GRN-implanted group was ∼10% higher than that of the group implanted with conventional carbonate apatite granules. Furthermore, a portion of the G-GRNs resorbed at week-4, and resorption continued for the following 8 weeks. Thus, G-GRNs are involved in bone remodeling and are gradually replaced with new bone while maintaining a suitable bone level. These findings provide a basis for the design and fabrication of synthetic bone grafts for achieving rapid bone regeneration.

3D bioprinting of in situ vascularized tissue engineered bone for repairing large segmental bone defects

Large bone defects remain an unsolved clinical challenge because of the lack of effective vascularization in newly formed bone tissue. 3D bioprinting is a fabrication technology with the potential to create vascularized bone grafts with biological activity for repairing bone defects. In this study, vascular endothelial cells laden with thermosensitive bio-ink were bioprinted in situ on the inner surfaces of interconnected tubular channels of bone mesenchymal stem cell-laden 3D-bioprinted scaffolds. Endothelial cells exhibited a more uniform distribution and greater seeding efficiency throughout the channels. In vitro, the in situ bioprinted endothelial cells can form a vascular network through proliferation and migration. The in situ vascularized tissue-engineered bone also resulted in a coupling effect between angiogenesis and osteogenesis. Moreover, RNA sequencing analysis revealed that the expression of genes related to osteogenesis and angiogenesis is upregulated in biological processes. The in vivo 3D-bioprinted in situ vascularized scaffolds exhibited excellent performance in promoting new bone formation in rat calvarial critical-sized defect models. Consequently, in situ vascularized tissue-engineered bones constructed using 3D bioprinting technology have a potential of being used as bone grafts for repairing large bone defects, with a possible clinical application in the future.

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3D bioprinting was used to fabricate in situ vascularized tissue engineered bone.

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In situ bioprinted endothelial cells exhibited uniform distribution and greater seeding efficiency.

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3D-bioprinted scaffold produced coupling between angiogenesis and osteogenesis.

Channel Aperture Characteristics of Carbonate Apatite Honeycomb Scaffolds Affect Ingrowths of Bone and Fibrous Tissues in Vertical Bone Augmentation

Synthetic scaffolds with the ability to prevent fibrous tissue penetration and promote bone augmentation may realize guided bone regeneration without the use of a barrier membrane for dental implantation. Here, we fabricated two types of honeycomb scaffolds of carbonate apatite, a bone mineral analog, whose channel apertures were square (HC-S) and rectangular (HC-R). The side lengths of the HC-Ss and HC-Rs were 265.8 ± 8.9; 817.7 ± 2.4 and 267.1 ± 5.2 μm, respectively. We placed cylindrical HC-Ss and HC-Rs on the rabbit calvaria. At 4 weeks post-implantation, the HC-Ss prevented fibrous tissue penetration from the top face via the channels, which allowed the new bone to reach the top of the scaffold from the bottom face or the calvarium. In contrast, in the HC-Rs, fibrous tissues filled the channels in the top region. At 12 weeks post-implantation, the HC-Ss were partially replaced with new bone. In the top region of the HC-Rs, although new bone had formed, fibrous tissue remained. According to the findings here and in our previous study, the longer side length rather than the shorter side length of a rectangular scaffold channel aperture is the dominant factor that affects fibrous tissue penetration and new bone augmentation. Furthermore, even though channel aperture areas are similar, bone and fibrous tissue ingrowths are different when the aperture shapes are different.

Effects of Scaffold Shape on Bone Regeneration: Tiny Shape Differences Affect the Entire System

Although studies on scaffolds for tissue generation have mainly focused on the chemical composition and pore structure, the effects of scaffold shape have been overlooked. Scaffold shape determines the scaffold surface area (SA) at the single-scaffold level (i.e., microscopic effects), although it also affects the amount of interscaffold space in the tissue defect at the whole-system level (i.e., macroscopic effects). To clarify these microscopic and macroscopic effects, this study reports the osteogenesis abilities of three types of carbonate apatite granular scaffolds with different shapes, namely, irregularly shaped dense granules (DGs) and two types of honeycomb granules (HCGs) with seven hexagonal channels (∼255 μm in length between opposite sides). The HCGs possessed either 12 protuberances (∼75 μm in length) or no protuberances. Protuberances increased the SA of each granule by 3.24 mm2 while also widening interscaffold spaces and increasing the space percentage in the defect by ∼7.6%. Interscaffold spaces were lower in DGs than HCGs. On DGs, new bone formed only on the surface, whereas on HCGs, bone simultaneously formed on the surface and in intrascaffold channels. Interestingly, HCGs without protuberances formed approximately 30% more new bone than those with protuberances. Thus, even tiny protuberances on the scaffold surface can affect the percentage of interscaffold space, thereby exerting dominant effects on osteogenesis. Our findings demonstrate that bone regeneration can be improved by considering macroscopic shape effects beyond the microscopic effects of the scaffold.