Development, Characterization and In Vitro Biological Properties of Scaffolds Fabricated From Calcium Phosphate Nanoparticles

Efforts to promote osteogenesis-angiogenesis coupling for bone tissue engineering

Repair of bone defects exceeding a critical size has been always a big challenge in clinical practice. Tissue engineering has exhibited great potential to effectively repair the defects with less adverse effect than traditional bone grafts, during which how to induce vascularized bone formation has been recognized as a critical issue. Therefore, recently many studies have been launched to attempt to promote osteogenesis-angiogenesis coupling. This review summarized comprehensively and explored in depth current efforts to ameliorate the coupling of osteogenesis and angiogenesis from four aspects, namely the optimization of scaffold components, modification of scaffold structures, loading strategies for bioactive substances, and employment tricks for appropriate cells. Especially, the advantages and the possible reasons for every strategy, as well as the challenges, were elaborated. Furthermore, some promising research directions were proposed based on an in-depth analysis of the current research. This paper will hopefully spark new ideas and approaches for more efficiently boosting new vascularized bone formations.

Processing and characterization of egg shell derived nano-hydroxyapatite synthetic bone for Orthopaedic and Arthroscopy implants and substitutes in dentistry

The present work is focused on the nano-Hydroxyapatite (nHAp) synthesis with two different Indian breed Aseel and Kadaknath eggshells. The alloplast implants were developed through the foam replica method with polyurethane 45-PPI as a porous template. The synthesized nHAp was characterized by Field Emission Scanning Electron Microscopy (FE-SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The FE-SEM images of the nHAp showed the one dimensional clustered nanoparticles and the X-ray diffraction spectrum confirms that the major phase was hydroxyapatite with a small trace of β-tricalcium phosphate. The maximum compression strength of the sample was 5.49 ± 0.12 MPa which is in the range of the compression strength of human trabecular bone. The thermal and degradability studies results confirmed that these are highly stable and provides necessary a resorption needed for new bone tissue formation. Besides, the antimicrobial activity against tested human microbiome are satisfactory and the cell viability towards MG 63 human osteoblast-like cells provides a potential pathway for developing the nHAp implants for bone tissue engineering.

Stem cell-derived exosomes in bone healing: focusing on their role in angiogenesis

Fractures in bone, a tissue critical in protecting other organs, affect patients' quality of life and have a heavy economic burden on societies. Based on regenerative medicine and bone tissue engineering approaches, stem cells have become a promising and attractive strategy for repairing bone fractures via differentiation into bone-forming cells and production of favorable mediators. Recent evidence suggests that stem cell-derived exosomes could mediate the therapeutic effects of their counterpart cells and provide a cell-free therapeutic strategy in bone repair. Since bone is a highly vascularized tissue, coupling angiogenesis and osteogenesis is critical in bone fracture healing; thus, developing therapeutic strategies to promote angiogenesis will facilitate bone regeneration and healing. To this end, stem cell-derived exosomes with angiogenic potency have been developed to improve fracture healing. This review summarizes the effects of stem cell-derived exosomes on the repair of bone tissue, focusing on the angiogenesis process.

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.

Calcium Orthophosphate (CaPO4)-Based Bioceramics: Preparation, Properties, and Applications

Various types of materials have been traditionally used to restore damaged bones. In the late 1960s, a strong interest was raised in studying ceramics as potential bone grafts due to their biomechanical properties. A short time later, such synthetic biomaterials were called bioceramics. Bioceramics can be prepared from diverse inorganic substances, but this review is limited to calcium orthophosphate (CaPO4)-based formulations only, due to its chemical similarity to mammalian bones and teeth. During the past 50 years, there have been a number of important achievements in this field. Namely, after the initial development of bioceramics that was just tolerated in the physiological environment, an emphasis was shifted towards the formulations able to form direct chemical bonds with the adjacent bones. Afterwards, by the structural and compositional controls, it became possible to choose whether the CaPO4-based implants would remain biologically stable once incorporated into the skeletal structure or whether they would be resorbed over time. At the turn of the millennium, a new concept of regenerative bioceramics was developed, and such formulations became an integrated part of the tissue engineering approach. Now, CaPO4-based scaffolds are designed to induce bone formation and vascularization. These scaffolds are usually porous and harbor various biomolecules and/or cells. Therefore, current biomedical applications of CaPO4-based bioceramics include artificial bone grafts, bone augmentations, maxillofacial reconstruction, spinal fusion, and periodontal disease repairs, as well as bone fillers after tumor surgery. Prospective future applications comprise drug delivery and tissue engineering purposes because CaPO4 appear to be promising carriers of growth factors, bioactive peptides, and various types of cells.

Pellet-Based Fused Filament Fabrication (FFF)-Derived Process for the Development of Polylactic Acid/Hydroxyapatite Scaffolds Dedicated to Bone Regeneration

Scaffolds can be defined as 3D architectures with specific features (surface properties, porosity, rigidity, biodegradability, etc.) that help cells to attach, proliferate, and to differentiate into specific lineage. For bone regeneration, rather high mechanical properties are required. That is why polylactic acid (PLA) and PLA/hydroxyapatite (HA) scaffolds (10 wt.%) were produced by a peculiar fused filament fabrication (FFF)-derived process. The effect of the addition of HA particles in the scaffolds was investigated in terms of morphology, biological properties, and biodegradation behavior. It was found that the scaffolds were biocompatible and that cells managed to attach and proliferate. Biodegradability was assessed over a 5-month period (according to the ISO 13781-Biodegradability norm) through gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and compression tests. The results revealed that the presence of HA in the scaffolds induced a faster and more complete polymer biodegradation, with a gradual decrease in the molar mass (Mn) and compressive mechanical properties over time. In contrast, the Mn of PLA only decreased during the processing steps to obtain scaffolds (extrusion + 3D-printing) but PLA scaffolds did not degrade during conditioning, which was highlighted by a high retention of the mechanical properties of the scaffolds after conditioning.

Polylactic acid scaffold with directional porous structure for large-segment bone repair

Biodegradable porous scaffolds with different structure, porosity, and strength play a critical role in the repair and regeneration of defects in bone tissue engineering by changing the proliferation condition for cell. In this study, polylactic acid (PLA) scaffold with directional porous structure is designed and fabricated using the method of ice template and phase inversion for speeding up bone repair by promoting the growth and proliferation of bone cells. The morphology, mechanical properties, hydrophilicity, and wicking properties of PLA scaffolds were characterized by scanning electron microscope, universal testing machine, contact angle tester and wicking rate test, respectively. In vitro biocompatibility has been investigated through measuring cell adhesion, proliferation, and viability on PLA scaffold with directional porous structure. Prepared PLA scaffold was implanted into animals to observe the repair mechanism of large-sized bone defects. This study proposes a novel bioporous scaffold design to induce osteocyte growth at the rat calvaria with a directional pore structure, and the scaffold edges were integrated with the calvaria at week 12, effectively promoting the repair and regeneration of defective bone tissue.

Organic acid cross-linked 3D printed cellulose nanocomposite bioscaffolds with controlled porosity, mechanical strength, and biocompatibility

Herein, we fabricated chemically cross-linked polysaccharide-based three-dimensional (3D) porous scaffolds using an ink composed of nanofibrillated cellulose, carboxymethyl cellulose, and citric acid (CA), featuring strong shear thinning behavior and adequate printability. Scaffolds were produced by combining direct-ink-writing 3D printing, freeze-drying, and dehydrothermal heat-assisted cross-linking techniques. The last step induces a reaction of CA. Degree of cross-linking was controlled by varying the CA concentration (2.5–10.0 wt.%) to tune the structure, swelling, degradation, and surface properties (pores: 100-450 μm, porosity: 86%) of the scaffolds in the dry and hydrated states. Compressive strength, elastic modulus, and shape recovery of the cross-linked scaffolds increased significantly with increasing cross-linker concentration. Cross-linked scaffolds promoted clustered cell adhesion and showed no cytotoxic effects as determined by the viability assay and live/dead staining with human osteoblast cells. The proposed method can be extended to all polysaccharide-based materials to develop cell-friendly scaffolds suitable for tissue engineering applications.

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Chemically cross-linked polysaccharide-based 3D porous scaffolds were fabricated

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Physicochemical and mechanical properties increased with cross-linker concentration

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Lower cross-linker concentration led to higher porosity and interconnected pores

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Scaffolds promoted clustered cell adhesion and showed no cytotoxic effects

The Effect of Whitlockite as an Osteoconductive Synthetic Bone Substitute Material in Animal Bony Defect Model

This study aimed to evaluate the biomechanical properties in vitro and the bone regeneration of whitlockite (WH) compared with hydroxyapatite (HA) or β-tricalcium phosphate (β-TCP)-based material. We investigated the morphology and phase composition of the bone grafts using a scanning electron microscope and X-ray diffractometer patterns and tested the compressive strength. Four circular defects of 8 mm in diameter were created on the calvaria of twelve rabbits. One defect was left empty, and each of the other defects was filled with WH, HA, and β-TCP. At 4 and 8 weeks, the specimens were harvested to evaluate for the new bone formation and the remaining bone grafts. Regarding the biomechanical properties, the three grafts had a similar micropore size, and WH showed nanopores. The compressive strength of WH was higher than HA and β-TCP without statistical significance. The radiological and histomorphometric analyses demonstrated that the new bone formation was similar among the groups. The remaining bone graft of the WH group was greater than that of the HA and β-TCP groups at 4 weeks (p < 0.05), and the total bone area of the WH, HA, and β-TCP groups was greater than that of the other (p < 0.01). WH has excellent volumetric stability and osteoconductivity compared with HA and β-TCP.

Evaluation of Osteogenic Differentiation of Bone Marrow-Derived Mesenchymal Stem Cell on Highly Porous Polycaprolactone Scaffold Reinforced With Layered Double Hydroxides Nanoclay

In recent decades, bone tissue engineering has had an effective role in introducing orthopedic implants. In this regard, polymeric scaffolds reinforced with bioactive nanomaterials can offer great potential in tissue engineering implants for replacing bone loss in patients. In this study, the thermally induced phase separation method was used to fabricate three-dimensional highly porous scaffolds made of layered double hydroxide (LDH)/polycaprolactone (PCL) nanocomposites with varied LDH contents ranging from 0.1 wt.% to 10 wt.%. The Phase identification, morphology, and elemental composition were studied using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, respectively. Interconnected pores ranging from 5 to 150 µm were detected in all samples. The results revealed that the inclusion of LDH to PCL scaffold reinforced mechanical strength and compressive modulus increased from 0.6418 to 1.3251 for the pure PCL and PCL + LDH (1 Wt.%) scaffolds, respectively. Also, thermal stability, degradation rate, and biomineralization especially in comparison with the pure PCL were enhanced. Adhesion, viability, and proliferation of human bone marrow-derived mesenchymal stem cells (hBMSCs) seeded on PCL + LDH scaffolds were improved as compared to the pure PCL. Furthermore, the addition of LDH resulted in the increased mineral deposition as well as expression of ALP and RUNX2 osteogenic genes in terms of differentiation. All in all, our findings revealed that PCL + LDH (1 Wt.%) scaffold might be an ideal choice for 3D scaffold design in bone tissue engineering approaches.

Hydroxyapatite coating on an aluminum/bioplastic scaffold for bone tissue engineering

Three-dimensional printing can produce scaffolds with shapes and dimensions tailored for practical clinical applications. Enhanced osteoconductivity of such scaffolds is generally desired. Hydroxyapatite (HA) is an inorganic ceramic that can be used to coat such scaffolds and to accelerate healing during the bone restoration process. In this study, HA-coated aluminum/bioplastic scaffolds were fabricated, and their structural characteristics and osteoconductivity were evaluated. Aluminum/bioplastic scaffolds were fabricated by three-dimensional printing, and HA slurries with solids loadings of 10–20 vol% were used for coating. As solids loadings increased, the thickness of the coating layers slightly increased, whereas pore sizes decreased. The average compressive strength was comparable to that of cancellous bone. Potential osteoconductivity was tested by simulated body fluid immersion for 28 days, and the formation of the HA phase on the surface along with a weight increase indicates the potential bioactivity of the samples.

Schematic representation of hydroxyapatite synthesis, 3D printing of Al/PLA scaffolds, and hydrothermal coating of the scaffolds. The best uniformity of coating and the greatest compressive strength were observed in samples coated with 10 vol% slurry.

ADSC-Based Cell Therapies for Musculoskeletal Disorders: A Review of Recent Clinical Trials

Recently published clinical trials involving the use of adipose-derived stem cells (ADSCs) indicated that approximately one-third of the studies were conducted on musculoskeletal disorders (MSD). MSD refers to a wide range of degenerative conditions of joints, bones, and muscles, and these conditions are the most common causes of chronic disability worldwide, being a major burden to the society. Conventional treatment modalities for MSD are not sufficient to correct the underlying structural abnormalities. Hence, ADSC-based cell therapies are being tested as a form of alternative, yet more effective, therapies in the management of MSDs. Therefore, in this review, MSDs subjected to the ADSC-based therapy were further categorized as arthritis, craniomaxillofacial defects, tendon/ligament related disorders, and spine disorders, and their brief characterization as well as the corresponding conventional therapeutic approaches with possible mechanisms with which ADSCs produce regenerative effects in disease-specific microenvironments were discussed to provide an overview of under which circumstances and on what bases the ADSC-based cell therapy was implemented. Providing an overview of the current status of ADSC-based cell therapy on MSDs can help to develop better and optimized strategies of ADSC-based therapeutics for MSDs as well as help to find novel clinical applications of ADSCs in the near future.

The future of basic science in orthopaedics and traumatology: Cassandra or Prometheus?

Orthopaedic and trauma research is a gateway to better health and mobility, reflecting the ever-increasing and complex burden of musculoskeletal diseases and injuries in Germany, Europe and worldwide. Basic science in orthopaedics and traumatology addresses the complete organism down to the molecule among an entire life of musculoskeletal mobility. Reflecting the complex and intertwined underlying mechanisms, cooperative research in this field has discovered important mechanisms on the molecular, cellular and organ levels, which subsequently led to innovative diagnostic and therapeutic strategies that reduced individual suffering as well as the burden on the society. However, research efforts are considerably threatened by economical pressures on clinicians and scientists, growing obstacles for urgently needed translational animal research, and insufficient funding. Although sophisticated science is feasible and realized in ever more individual research groups, a main goal of the multidisciplinary members of the Basic Science Section of the German Society for Orthopaedics and Trauma Surgery is to generate overarching structures and networks to answer to the growing clinical needs. The future of basic science in orthopaedics and traumatology can only be managed by an even more intensified exchange between basic scientists and clinicians while fuelling enthusiasm of talented junior scientists and clinicians. Prioritized future projects will master a broad range of opportunities from artificial intelligence, gene- and nano-technologies to large-scale, multi-centre clinical studies. Like Prometheus in the ancient Greek myth, transferring the elucidating knowledge from basic science to the real (clinical) world will reduce the individual suffering from orthopaedic diseases and trauma as well as their socio-economic impact.

Biological and Medical Applications of Calcium Phosphate Nanoparticles

Calcium phosphate nanoparticles have a high biocompatibility and biodegradability due to their chemical similarity to human hard tissue, for example, bone and teeth. They can be used as efficient carriers for different kinds of biomolecules such as nucleic acids, proteins, peptides, antibodies, or drugs, which alone are not able to enter cells where their biological effect is required. They can be loaded with cargo molecules by incorporating them, unlike solid nanoparticles, and also by surface functionalization. This offers protection, for example, against nucleases, and the possibility for cell targeting. If such nanoparticles are functionalized with fluorescing dyes, they can be applied for imaging in vitro and in vivo. Synthesis, functionalization and cell uptake mechanisms of calcium phosphate nanoparticles are discussed together with applications in transfection, gene silencing, imaging, immunization, and bone substitution. Biodistribution data of calcium phosphate nanoparticles in vivo are reviewed.

Microporosities in 3D-Printed Tricalcium-Phosphate-Based Bone Substitutes Enhance Osteoconduction and Affect Osteoclastic Resorption

Additive manufacturing is a key technology required to realize the production of a personalized bone substitute that exactly meets a patient’s need and fills a patient-specific bone defect. Additive manufacturing can optimize the inner architecture of the scaffold for osteoconduction, allowing fast and reliable defect bridging by promoting rapid growth of new bone tissue into the scaffold. The role of scaffold microporosity/nanoarchitecture in osteoconduction remains elusive. To elucidate this relationship, we produced lithography-based osteoconductive scaffolds from tricalcium phosphate (TCP) with identical macro- and microarchitecture, but varied their nanoarchitecture/microporosity by ranging maximum sintering temperatures from 1000 °C to 1200 °C. After characterization of the different scaffolds’ microporosity, compression strength, and nanoarchitecture, we performed in vivo studies that showed that ingrowth of bone as an indicator of osteoconduction significantly decreased with decreasing microporosity. Moreover, at the 1200 °C peak sinter temperature and lowest microporosity, osteoclastic degradation of the material was inhibited. Thus, even for wide-open porous TCP-based scaffolds, a high degree of microporosity appears to be essential for optimal osteoconduction and creeping substitution, which can prevent non-unions, the major complication during bone regeneration procedures.

Biomimetic Design for a Dual Concentric Porous Titanium Scaffold with Appropriate Compressive Strength and Cells Affinity

In repairing or replacing damaged bones, a dual concentric porous titanium scaffold (P-Tix-y) has emerged as a promising bio-mimic design. Herein, various P-Tix-y were made and sintered with relatively dense (x = 10, 20, or 30% porosity) and loose (y = 45, 55, or 65 porosity) structures. Firstly, NaCl was used as the pore-forming additive and followed by a hydrothermal removal method. The compressive strength of the as-formed P-Tix_y and surface morphology, nanomechanical property, and cells' affinity on the cross-sectioned surface of P-Tix_y (CP-Tix_y) were then characterized. The results demonstrate that the compressive strength of P-Ti10_45, P-Ti20_45, or P-Ti20_55 exhibits a relatively mild decline (e.g., in the range of 181 and 97 MPa, higher than the required value of 70 MPa) and suitable porosities for the intended structure. Nano-hardness on the solid surface of CP-Tix_y shows roughly consistent with that of CP-Ti (i.e., ~8.78 GPa), thus, the porous structure of CP-Tix_y remains mostly unaffected by the addition of NaCl and subsequent sintering process. Most of the surfaces of CP-Tix_y exhibit high fibroblast (L929) cell affinity with low cell mortality. Notably, in the hFOB 1.19 cell adhesion and proliferation test, CP-Ti20_55 and CP-Ti20_65 reveal high cell viability, most probably relating with the assembly of dual porosities with interconnected pores. Overall, the sample P-Ti20_55 provides a relatively load-bearable design with high cell affinity and is thus promising as a three-dimensional bio-scaffold.

Nanocomposite scaffolds composed of Apacite (apatite-calcite) nanostructures, poly (ε-caprolactone) and poly (2-hydroxyethylmethacrylate): The effect of nanostructures on physico-mechanical properties and osteogenic differentiation of human bone marrow mesenchymal stem cells in vitro

Nanocomposite scaffolds were fabricated from poly (ε-caprolactone) (PCL), Poly (2-hydroxyethylmethacrylate) (PHEMA), and Apacite (Apatite-calcite) nanostructures (15 and 25 wt%). The nanoscale structure, physical and chemical properties, mechanical properties, hydrophilic behavior, degradability and osteogenic properties of the fabricated scaffolds were evaluated. The results showed that the mechanical strength, degradation, wetting ability, and mechanical strength of PCL-PHEMA scaffolds significantly increases upon inclusion of Apacite nanoparticles up to 25 wt%. Accordingly, the best mechanical values (E ~ 7.109 MPa and σ ~ 0.414 MPa) and highest degradability (32% within 96 h) were recorded for PCL-PHEMA scaffolds containing 25 wt% of Apacite. Furthermore, highest porosity and roughness were observed in the PCL-PHEMA/25% Apacite as a result of the Apacite nanoparticles inclusion. There was no cytotoxicity recorded for the fabricated scaffolds based on the results obtained from MTT assay and acridine orange staining. Alkaline phosphatase activity, calcium content quantification, Van Kossa staining, FESEM and real time PCR tests confirmed the biomineralization, and the differentiation potential of the nanocomposite scaffolds. Overall, the 3D structure, optimum porosity and balanced dissolution rate of PCL-PHEMA/25% Apacite providing a balanced microenvironment resulted in improved cell adhesion, cell behavior, and replication, as well as osteogenic induction of human bone-marrow-derived mesenchymal stem cells (hBM-MSCs).

Biomimetic Synthesis of Nanocrystalline Hydroxyapatite Composites: Therapeutic Potential and Effects on Bone Regeneration

The development of a novel alloplastic graft with both osteoinductive and osteoconductive properties is still necessary. In this study, we tried to synthesize a biomimetic hydroxyapatite microspheres (gelatin/nano-hydroxyapatite microsphere embedded with stromal cell-derived factor-1: GHM-S) from nanocrystalline hydroxyapatites and to investigate their therapeutic potential and effects on bone regeneration. In this study, hydroxyapatite was synthesized by co-precipitation of calcium hydroxide and orthophosphoric acid to gelatin solution. The microbial transglutaminase was used as the agent to crosslink the microspheres. The morphology, characterization, and thermal gravimetric analysis of microspheres were performed. SDF-1 release profile and in vitro biocompatibility and relative osteogenic gene expression were analyzed, followed by in vivo micro-computed tomography study and histological analysis. The synthesized hydroxyapatite was found to be similar to hydroxyapatite of natural bone tissue. The stromal cell-derived factor-1 was embedded into gelatin/hydroxyapatite microsphere to form the biomimetic hydroxyapatite microsphere. The stromal cell-derived factor-1 protein could be released in a controlled manner from the biomimetic hydroxyapatite microsphere and form a concentration gradient in the culture environment to attract the migration of stem cells. Gene expression and protein expression indicated that stem cells could differentiate or develop into pre-osteoblasts. The effect of bone formation by the biomimetic hydroxyapatite microsphere was assessed by an in vivo rats’ alveolar bone defects model and confirmed by micro-CT imaging and histological examination. Our findings demonstrated that the biomimetic hydroxyapatite microsphere can enhance the alveolar bone regeneration. This design has potential be applied to other bone defects.

Hydroxyapatite Formation on Self-Assembling Peptides with Differing Secondary Structures and Their Selective Adsorption for Proteins

Self-assembling peptides have been employed as biotemplates for biomineralization, as the morphologies and sizes of the inorganic materials can be easily controlled. We synthesized two types of highly ordered self-assembling peptides with different secondary structures and investigated the effects of secondary structures on hydroxyapatite (HAp) biomineralization of peptide templates. All as-synthesized HAp-peptides have a selective protein adsorption capacity for basic protein (e.g., cytochrome c and lysozyme). Moreover, the selectivity was improved as peptide amounts increased. In particular, peptide-HAp templated on β-sheet peptides adsorbed more cytochrome c than peptide-HAp with α-helix structures, due to the greater than 2-times carboxyl group density at their surfaces. It can be expected that self-assembled peptide-templated HAp may be used as carriers for protein immobilization in biosensing and bioseparation applications and as enzyme-stabilizing agents.