Hollow hydroxyapatite microspheres: a novel bioactive and osteoconductive carrier for controlled release of bone morphogenetic protein-2 in bone regeneration

Combination of a Synthetic Bioceramic Associated with a Polydioxanone-Based Membrane as an Alternative to Autogenous Bone Grafting

The purpose of this study was to evaluate the repair process in rat calvaria filled with synthetic biphasic bioceramics (Plenum® Osshp-70:30, HA:βTCP) or autogenous bone, covered with a polydioxanone membrane (PDO). A total of 48 rats were divided into two groups (n = 24): particulate autogenous bone + Plenum® Guide (AUTOPT+PG) or Plenum® Osshp + Plenum® Guide (PO+PG). A defect was created in the calvaria, filled with the grafts, and covered with a PDO membrane, and euthanasia took place at 7, 30, and 60 days. Micro-CT showed no statistical difference between the groups, but there was an increase in bone volume (56.26%), the number of trabeculae (2.76 mm), and intersection surface (26.76 mm2) and a decrease in total porosity (43.79%) in the PO+PG group, as well as higher values for the daily mineral apposition rate (7.16 µm/day). Histometric analysis presented material replacement and increased bone formation at 30 days compared to 7 days in both groups. Immunostaining showed a similar pattern between the groups, with an increase in proteins related to bone remodeling and formation. In conclusion, Plenum® Osshp + Plenum® Guide showed similar and sometimes superior results when compared to autogenous bone, making it a competent option as a bone substitute.

Chitin microspheres: From fabrication to applications

Chitin microspheres (CMs) have attracted increasing attention due to their biocompatibility, uniform size and shape, large surface area, and porous structure. Considerable research efforts have been focused on developing CMs and promoting their applications in various areas. In this context, this review aims to describe the most recent progress in the fabrication and application of CMs. Different routes that can be used to prepare CMs, such as the drip method and the emulsion method, are emphatically introduced. Moreover, the applications of CMs as drug delivery systems, wound dressings, three-dimensional (3D) scaffolds, water purification, and functional supporting materials in the fields of biomedicine, tissue engineering, environmental protection, and energy storage are also highlighted. We hope this review can provide a comprehensive and useful database for further innovation of CMs.

Glycosaminoglycans' Ability to Promote Wound Healing: From Native Living Macromolecules to Artificial Biomaterials

Glycosaminoglycans (GAGs) are important for the occurrence of signaling molecules and maintenance of microenvironment within the extracellular matrix (ECM) in living tissues. GAGs and GAG-based biomaterial approaches have been widely explored to promote in situ tissue regeneration and repair by regulating the wound microenvironment, accelerating re-epithelialization, and controlling ECM remodeling. However, most approaches remain unacceptable for clinical applications. To improve insights into material design and clinical translational applications, this review highlights the innate roles and bioactive mechanisms of native GAGs during in situ wound healing and presents common GAG-based biomaterials and the adaptability of application scenarios in facilitating wound healing. Furthermore, challenges before the widespread commercialization of GAG-based biomaterials are shared, to ensure that future designed and constructed GAG-based artificial biomaterials are more likely to recapitulate the unique and tissue-specific profile of native GAG expression in human tissues. This review provides a more explicit and clear selection guide for researchers designing biomimetic materials, which will resemble or exceed their natural counterparts in certain functions, thereby suiting for specific environments or therapeutic goals.

Hollow Hydroxyapatite Microspheres Loaded with rhCXCL13 to Recruit BMSC for Osteogenesis and Synergetic Angiogenesis to Promote Bone Regeneration in Bone Defects

Introduction

Bone tissue engineering is a promising method to treat bone defects. However, the current methods of preparing composite materials that mimic the complex structure and biological activity of natural bone are challenging for recruitment of bone marrow mesenchymal stem cells (BMSCs), which affects the application of these materials in situ bone regeneration. Hollow hydroxyapatite microspheres (HHMs) possess a natural porous bone structure, good adsorption, and slow release of chemokines, but have low ability to recruit BMSCs and induce osteogenesis. In this study, The HHM/chitosan (CS) and recombinant human C-X-C motif chemokine ligand 13 (rhCXCL13)-HHM/CS biomimetic scaffolds that optimize bone regeneration and investigated their mechanism of BMSC recruitment and osteogenesis through cell and animal experiments and transcriptomic sequencing.

Methods

Evaluate the physical characteristics of the HHM/CS and rhCXCL13-HHM/CS biomimetic scaffolds through Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and the cumulative release curve of rhCXCL13. Transwell migration experiments and co-culture with BMSCs were conducted to study the recruitment ability and osteogenic differentiation of the scaffolds. Transcriptomic sequencing was performed to analyze the osteogenic differentiation mechanism. The osteogenesis and bone healing performance were evaluated using a rabbit radial defect model.

Results

SEM demonstrated that the rhCXCL13-HHM/CS scaffold comprised hydroxyapatite microspheres in a porous three-dimensional network. The rhCXCL13 showed excellent sustained release capability. The rhCXCL13-HHM/CS scaffold could recruit BMSCs and induce bone regeneration. Transcriptome sequencing and experimental results showed that the osteogenesis mechanism of rhCXCL13-HHM/CS was through the PI3K-AKT pathway. In vivo, the rhCXCL13-HHM/CS scaffold significantly promoted osteogenesis and angiogenesis at 12 weeks after surgery.

Conclusion

The rhCXCL13-HHM/CS scaffold demonstrates excellent potential for BMSC recruitment, osteogenesis, vascularized tissue-engineered bone reconstruction, and drug delivery, providing a theoretical basis for material osteogenesis mechanism study and promising clinical applications for treating large bone defects.

Hydroxyapatite-based carriers for tumor targeting therapy

At present, targeted drug delivery is regarded as the most effective means of tumor treatment, overcoming the lack of conventional chemotherapeutics that are difficult to reach or enter into cancer cells. Hydroxyapatite (HAP) is the main component of biological hard tissue, which can be regarded as a suitable drug carrier due to its biocompatibility, nontoxicity, biodegradation, and absorbability. This review focuses on the cutting edge of HAP as a drug carrier in targeted drug delivery systems. HAP-based carriers can be obtained by doping, modification, and combination, which benefit to improve the loading efficiency of drugs and the response sensitivity of the microenvironment in the synthesis process. The drug adsorbed or in situ loaded on HAP-based carriers can achieve targeted drug delivery and precise treatment through the guidance of the in vivo microenvironment and the stimulation of the in vitro response. In addition, HAP-based drug carriers can improve the cellular uptake rate of drugs to achieve a higher treatment effect. These advantages revealed the promising potential of HAP-based carriers from the perspective of targeted drug delivery for tumor treatment.

Calcium Phosphate Delivery Systems for Regeneration and Biomineralization of Mineralized Tissues of the Craniofacial Complex

Calcium phosphate (CaP)-based materials have been extensively used for mineralized tissues in the craniofacial complex. Owing to their excellent biocompatibility, biodegradability, and inherent osteoconductive nature, their use as delivery systems for drugs and bioactive factors has several advantages. Of the three mineralized tissues in the craniofacial complex (bone, dentin, and enamel), only bone and dentin have some regenerative properties that can diminish due to disease and severe injuries. Therefore, targeting these regenerative tissues with CaP delivery systems carrying relevant drugs, morphogenic factors, and ions is imperative to improve tissue health in the mineralized tissue engineering field. In this review, the use of CaP-based microparticles, nanoparticles, and polymer-induced liquid precursor (PILPs) amorphous CaP nanodroplets for delivery to craniofacial bone and dentin are discussed. The use of these various form factors to obtain either a high local concentration of cargo at the macroscale and/or to deliver cargos precisely to nanoscale structures is also described. Finally, perspectives on the field using these CaP materials and next steps for the future delivery to the craniofacial complex are presented.

Safety and Efficacy of Recombinant Human Bone Morphogenetic Protein-2 in Multilevel Posterolateral Lumbar Fusion in a Prospective, Randomized, Controlled Trial

OBJECTIVE

This study is an investigator-initiated, prospective, randomized, controlled study to evaluate the efficacy and safety of the combined use of recombinant human BMP-2 (rhBMP-2) and a hydroxyapatite (HA) carrier in multilevel fusion in patients with adult spinal deformity (ASD).

METHODS

Thirty patients underwent posterolateral fusion for lumbar spinal deformities at 3 to 5 segments between L1 and S1. The patients received rhBMP-2+HA or HA on the left or right side of the transverse processes. They were followed up regularly at 1, 3, 6, and 12 months postoperatively. Fusion was defined according to the bone bridging on computed tomography scans. The fusion rate per segment was subanalyzed. Function and quality of life as well as pain in the lower back and lower extremities were evaluated.

RESULTS

The union rate for the rhBMP-2+HA group was 100% at 6 and 12 months. The union rate for the HA group was 77.8% (21 of 27) at 6 months and 88.0% (22 of 25) at 12 months (p = 0.014 at 6 months; not significant at 12 months). All segments were fused at 6 and 12 months in the rhBMP-2+HA group (p < 0.001). In the HA group, 108 of 115 segments (93.5%) were fused at 6 months and 105 of 109 segments (96.3%) at 12 months. Other clinical parameters (visual analogue scale, 36-item Short Form Health Survey, and Scoliosis Research Society-22 scores) improved compared to baseline.

CONCLUSION

Combining rhBMP-2 and an HA carrier is a safe and effective method to achieve multilevel fusion in patients with ASD.

Functional microspheres for tissue regeneration

As a new type of injectable biomaterials, functional microspheres have attracted increasing attention in tissue regeneration because they possess some advantageous properties compared to other biomaterials, including hydrogels. A variety of bio-inspired microspheres with unique structures and properties have been developed as cellular carriers and drug delivery vehicles in recent years. In this review, we provide a comprehensive summary of the progress of functional and biodegradable microspheres that have been used for tissue regeneration over the last two decades. First, we briefly introduce the biomaterials and general methods for microsphere fabrication. Next, we focus on the newly developed technologies for preparing functional microspheres, including macroporous microspheres, nanofibrous microspheres, hollow microspheres, core-shell structured microspheres, and surface-modified functional microspheres. After that, we discuss the application of functional microspheres for tissue regeneration, specifically for bone, cartilage, dental, neural, cardiac, and skin tissue regeneration. Last, we present our perspectives and future directions of functional microspheres as injectable carriers for the future advancement of tissue regeneration.

Highly bioactive bone cement microspheres based on α-tricalcium phosphate microparticles/mesoporous bioactive glass nanoparticles: Formulation, physico-chemical characterization and in vivo bone regeneration

Calcium phosphate cement (CPC) is a self-setting, biocompatible and osteoconductive bone cement, however its use as a bone substitute is still limited owing to its low bioactivity (i.e. its slow in vivo resorption and slow new bone formation rate) which is a challenging issue to be addressed. Herein, we report for the first time highly bioactive bone cement microspheres formulated from a cement paste containing α-tricalcium phosphate microparticles (α-TCP) and mesoporous calcium silicate bioactive glass nanoparticles (mesoporous BGn) using a water-in-oil emulsion method. Indeed, bioactive microspheres possess high potential as bone defect fillers for bone regeneration. The α-TCP microparticles were prepared by a solid state synthesis at 1400 ºC while mesoporous BGn were synthesized by template-assissted ultrasound-mediated sol-gel method. The particle size distribution of as-prepared cement microspheres was in the range of 200 - 450 µm with a sphericity index in the range of 0.92 - 0.94. The surface morphology of α-TCP microspheres revealed α-TCP micoparticles with smooth surfaces whereas α-TCP/BGn microspheres unveiled nano-roughened α-TCP microparticles. The as-prepared α-TCP/BGn cement microspheres exhibited larger specific surface area ca 18.6 m²/g, sustained release of soluble silicate (SiO₄^4-) ions (118 ppm within a week) and high protein adsorption capacity (252 mg/g). Notably, the α-TCP/BGn cement microspheres showed excellent in vitro surface bioactivity via formation of massive amounts of bone-like hydroxyapatite spherules and aggregates on their surfaces after soaking in simulated body fluid. Importantly, the in vivo implantation of as-prepared α-TCP/BGn cement microspheres in rat calvarial critical size bone defects for 6 weeks unveiled high in vivo bioactivity in terms of substantial new bone ingrowth and significant new bone formation within the bone defect as evidenced by histological analyses, X-ray radiography and micro-computed tomography evaluations.

Analytical Techniques for the Characterization of Bioactive Coatings for Orthopaedic Implants

The development of bioactive coatings for orthopedic implants has been of great interest in recent years in order to achieve both early- and long-term osseointegration. Numerous bioactive materials have been investigated for this purpose, along with loading coatings with therapeutic agents (active compounds) that are released into the surrounding media in a controlled manner after surgery. This review initially focuses on the importance and usefulness of characterization techniques for bioactive coatings, allowing the detailed evaluation of coating properties and further improvements. Various advanced analytical techniques that have been used to characterize the structure, interactions, and morphology of the designed bioactive coatings are comprehensively described by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), 3D tomography, quartz crystal microbalance (QCM), coating adhesion, and contact angle (CA) measurements. Secondly, the design of controlled-release systems, the determination of drug release kinetics, and recent advances in drug release from bioactive coatings are addressed as the evaluation thereof is crucial for improving the synthesis parameters in designing optimal bioactive coatings.

Injectable and self-healing nanocomposite hydrogel loading needle-like nano-hydroxyapatite and graphene oxide for synergistic tumour proliferation inhibition and photothermal therapy

Non-chemotherapeutic tumour treatment has received extensive attention due to its having fewer side effects as compared to chemotherapy. However, nanomaterials-based non-chemotherapy still faces limitations such as poor targeting and low retention. Therefore, a Schiff base cross-linked hydrogel was designed and prepared using aldehyde-modified polyethylene glycol (PEG) and carboxymethyl chitosan (CMC). This hydrogel has good injectable and self-healing properties and can carry graphene oxide (GO) as a photothermal agent and needle-like nano-hydroxyapatite (HAP) as a tumour inhibitor. Combined with tumour proliferation inhibition therapy and photothermal therapy, the nanocomposite hydrogel system can avoid the side effects of chemotherapy and improve the accuracy of tumour treatment. The PEG-CMC/HAP/GO nanocomposite hydrogel system has a porous structure, good injectability and self-healing properties to meet the mechanical requirements. In vitro cell characterization showed that GO is phototoxic to tumour cells, HAP can inhibit the proliferation of tumour cells, the nanocomposite hydrogel remained in the tumour site, and the encapsulated GO and HAP did not transfer to the normal site and cause cell damage. In the in vivo investigation, the breast cancer tumour-bearing mice, the model animals for tumour treatment, were treated with an intratumoral injection of the PEG-CMC/HAP/GO nanocomposite hydrogel. This functional self-healing hydrogel loaded with GO and HAP effectively inhibited tumour cell proliferation and realized the synergistic effect of photothermal therapy, which is expected to become a new effective treatment approach for tumours.

The Role of Growth Factors in Bioactive Coatings

With increasing obesity and an ageing population, health complications are also on the rise, such as the need to replace a joint with an artificial one. In both humans and animals, the integration of the implant is crucial, and bioactive coatings play an important role in bone tissue engineering. Since bone tissue engineering is about designing an implant that maximally mimics natural bone and is accepted by the tissue, the search for optimal materials and therapeutic agents and their concentrations is increasing. The incorporation of growth factors (GFs) in a bioactive coating represents a novel approach in bone tissue engineering, in which osteoinduction is enhanced in order to create the optimal conditions for the bone healing process, which crucially affects implant fixation. For the application of GFs in coatings and their implementation in clinical practice, factors such as the choice of one or more GFs, their concentration, the coating material, the method of incorporation, and the implant material must be considered to achieve the desired controlled release. Therefore, the avoidance of revision surgery also depends on the success of the design of the most appropriate bioactive coating. This overview considers the integration of the most common GFs that have been investigated in in vitro and in vivo studies, as well as in human clinical trials, with the aim of applying them in bioactive coatings. An overview of the main therapeutic agents that can stimulate cells to express the GFs necessary for bone tissue development is also provided. The main objective is to present the advantages and disadvantages of the GFs that have shown promise for inclusion in bioactive coatings according to the results of numerous studies.

Biomimetic Mineralization on 3D Printed PLA Scaffolds: On the Response of Human Primary Osteoblasts Spheroids and In Vivo Implantation

This study aimed to assess the response of 3D printed polylactic acid (PLA) scaffolds biomimetically coated with apatite on human primary osteoblast (HOb) spheroids and evaluate the biological response to its association with Bone Morphogenetic Protein 2 (rhBMP-2) in rat calvaria. PLA scaffolds were produced via 3D printing, soaked in simulated body fluid (SBF) solution to promote apatite deposition, and characterized by physical-chemical, morphological, and mechanical properties. PLA-CaP scaffolds with interconnected porous and mechanical properties suitable for bone repairing were produced with reproducibility. The in vitro biological response was assessed with human primary osteoblast spheroids. Increased cell adhesion and the rise of in vitro release of growth factors (Platelet-Derived Growth Factor (PDGF), Basic Fibroblast Growth Factor (bFGF), Vascular Endothelial Growth Factor (VEGF) was observed for PLA-CaP scaffolds, when pre-treated with fetal bovine serum (FBS). This pre-treatment with FBS was done in a way to enhance the adsorption of serum proteins, increasing the number of bioactive sites on the surface of scaffolds, and to partially mimic in vivo interactions. The in vivo analysis was conducted through the implantation of 3D printed PLA scaffolds either alone, coated with apatite (PLA-CaP) or PLA-CaP loaded with rhBMP-2 on critical-sized defects (8 mm) of rat calvaria. PLA-CaP+rhBMP2 presented higher values of newly formed bone (NFB) than other groups at all in vivo experimental periods (p < 0.05), attaining 44.85% of NFB after six months. These findings indicated two new potential candidates as alternatives to autogenous bone grafts for long-term treatment: (i) 3D-printed PLA-CaP scaffold associated with spheroids, since it can reduce the time of repair in situ by expression of biomolecules and growth factors; and (ii) 3D-printed PLA-CaP functionalized rhBMP2 scaffold, a biocompatible, bioactive biomaterial, with osteoconductivity and osteoinductivity.

Nanostructured Carbonated Hydroxyapatite Associated to rhBMP-2 Improves Bone Repair in Rat Calvaria

Many biomaterials are used for Bone Morphogenetic Proteins (BMPs) delivery in bone tissue engineering. The BMP carrier system's primary function is to hold these growth factors at the wound's site for a prolonged time and provide initial support for cells to attach and elaborate the extracellular matrix for bone regeneration. This study aimed to evaluate the nanostructured carbonated hydroxyapatite microspheres (nCHA) as an rhBMP-2 carrier on rats calvaria. A total of fifteen male Wistar rats were randomly divided into three groups (n = 5): clot (control group), rhBMP-2 associated with collagen membrane (COL/rhBMP-2) or associated with the microspheres (nCHA/rhBMP-2). After 45 days, the calvaria defect samples were evaluated through histological, histomorphometric, and SR-µCT analyses to investigate new-formed bone and connective tissue volume densities. The descriptive histological analysis showed that nCHA/rhBMP-2 improved bone formation compared to other groups. These results were confirmed by histomorphometric and SR-µCT analysis that showed substantially defect area filling with a higher percentage of newly formed (36.24 ± 6.68) bone than those with the COL/rhBMP-2 (0.42 ± 0.40) and Clot (3.84 ± 4.57) (p < 0.05). The results showed that nCHA is an effective carrier for rhBMP-2 encouraging bone healing and an efficient alternative to collagen membrane for rhBMP-2 delivery.

Chitosan-coated hydroxyapatite and drug-loaded polytrimethylene carbonate/polylactic acid scaffold for enhancing bone regeneration

Biocompatible polymers and drug-delivery scaffolds have driven development in bone regeneration. In this study, we fabricated a chitosan (CS)-coated polytrimethylene carbonate (PTMC)/polylactic acid (PLLA)/oleic acid-modified hydroxyapatite (OA-HA)/vancomycin hydrochloride (VH) microsphere scaffold for drug release with excellent biocompatibility. The incorporation of PLLA, OA-HA, and VH into PTMC microspheres not only slowed the biodegradability of the scaffold but also enhanced its mechanical properties and surface properties. Moreover, the CS coating stimulated extensive adhesion of osteoblasts before OA-HA incorporation, which facilitated the controlled release of OA-HA. The scaffolds were characterized via scanning electron microscopy, in vitro comprehensive performance testing, cell culturing, and microcomputer tomography scanning. The results indicated that the surface of the composite microsphere scaffold was suitable for osteoblast adhesion. Additionally, the release of OA-HA stimulated osteogenic proliferation. Our findings suggest that the CS-PTMC/PLLA/OA-HA/VH microsphere scaffold is promising for bone tissue engineering applications.

Effect of bone morphogenetic protein-2/hydroxyapatite on ankle fusion with bone defect in a rabbit model: a pilot study

Background

Revision ankle-fusion surgery after a failure of total ankle arthroplasty has a problem with bone-defect management by implant removal. For the reconstruction of bone defects, autogenous bone often causes minor and major complications. Recombinant human-bone morphogenetic protein-2 (rhBMP-2) plays essential roles in bone regeneration strategies, and hydroxyapatite (HA) is beneficial as the rhBMP-2 carrier. In this study, we evaluate whether rhBMP-2/HA can replace autogenous bone in a rabbit ankle-fusion model with distal tibia bone defect.

Methods

The bone defect was created in the distal tibia. The ankle fusion was performed by a cannulated screw from lateral malleolus and various treatments on bone defect. Thirty male white New Zealand rabbits were divided into three groups of 10 animals on each group dependent on treatment methods as control group (no treatment into defect), auto-bone group (autogenous bone treatment), and rhBMP-2/HA group (40 μL of 1 μg/mL rhBMP-2/100 μL HA). Bone formation on defect and the union of the ankle joint were evaluated by X-ray, micro-CT, and histological analysis at 8 weeks and 12 weeks, postoperatively.

Results

Radiographic assessment found the control and auto-bone groups still had the bone defect present, but rhBMP-2/HA group showed complete replacement of the defect with newly formed bone at 12 weeks. Micro-CT showed significantly higher new bone formation within the defect in the rhBMP-2/HA group than in the auto-bone and control groups at 8 weeks (p > 0.05 and p < 0.01, respectively) and 12 weeks (p < 0.05, p < 0.001, respectively). Fusion rate (%) analysis of micro-CT showed a higher percentage of union in the rhBMP-2/HA group than in the auto bone and control groups at 8 weeks (p > 0.05, p < 0.001, respectively) and 12 weeks (p < 0.001 and p < 0.001, respectively). The histological showed the highest osteointegration between distal tibia and talus in the rhBMP-2/HA group at 12 weeks.

Conclusions

This study indicated that rhBMP-2/HA showed much better bone fusion than did the autogenous bone graft and was effective in promoting fusion rate and improving the quality of the ankle joint fusion.

Investigation of nature of starting materials on the construction of hydroxyapatite 1D/3D morphologies

With the increasing requirement of bone repair materials, hydroxyapatite (HA) has been paid widely attention to investigation because of its good bioactivity and osteoconductivity. The structure of HA is a vital factor to expand its application in the field of hard tissue therapy. Thus, many strategies have been utilized in fabricating one-dimensional (1D) and three-dimensional (3D) nanostructured HA. In this paper, we successful synthesize HA with 1D nanofibers and 3D nanostructured microspheres using stearic acid as a template and different phosphates as phosphorus sources under the same synthetic system. The morphology of HA changes from nanofibers with high flexibility to nanostructured microspheres with good sphericity under the synergistic effect of stearic acid and various phosphates. The HA nanofibers and microspheres are promising for applications in biomedical fields. Base on characterization results, the formation mechanisms of HA nanofibers and HA microspheres self-assembled by nanorods are proposed. Furthermore, the HA morphology transition from nanofibers to nanostructured microspheres may be attributed to the formation of polyphosphate-induced water-in-oil microemulsion system in the synthesis process. The finding may provide a new direction to control HA morphology from 1D nanofibers to 3D microspheres based on previous strategies.

3D Printing of Hot Dog-Like Biomaterials with Hierarchical Architecture and Distinct Bioactivity

Hierarchical structure has exhibited an important influence in the fields of supercapacitors, catalytic applications, and tissue engineering. The hot dog, a popular food, is composed of bread and sausage with special structures. In this study, inspired by the structure of a hot dog, the strategy of combining direct ink writing 3D printing with bidirectional freezing is devised to prepare hot dog-like scaffolds with hierarchical structure. The scaffolds are composed of hollow bioceramic tubes (mimicking the "bread" in hot dogs, pore size: ≈1 mm) embedded by bioceramic rods (mimicking the "sausage" in hot dogs, diameter: ≈500 µm) and the sausage-like bioceramic rods possess uniformly aligned lamellar micropores (lamellar pore size: ≈30 µm). By mimicking the functions of hierarchical structure of bone tissues for transporting and storing nutrients, the prepared hot dog-like scaffolds show excellent properties for loading and releasing drugs and proteins as well as for improving the delivery and differentiation of tissue cells. The in vivo study further demonstrates that both the hierarchical structure itself and the controlled drug delivery in hot dog-like scaffolds significantly contribute to the improved bone-forming bioactivity. This study suggests that the prepared hot dog-like scaffolds are a promising biomaterial for drug delivery, tissue engineering, and regenerative medicine.

Bone repair scaffold coated with bone morphogenetic protein-2 for bone regeneration in murine calvarial defect model: Systematic review and quality evaluation

To systematically assess the effects of hydroxyapatite bone repair scaffold coated with bone morphogenetic protein-2 on murine calvarial defect models and to determine the quality of studies according to the Animal Research Reporting in In Vivo Experiments guidelines. Internet search was performed in duplicate using PubMed, MEDLINE, Ovid and Embase databases (without restrictions on publication date). The Animal Research Reporting in In Vivo Experiments guidelines were used to evaluate the quality of selected studies. Following screening, 12 studies were eligible for the review. Studies with average quality coefficients predominated (66.67%), followed by poor (25%) and excellent (8.33%) quality coefficients. Minimum quality scores were assigned to the Animal Research Reporting in In Vivo Experiments guideline items: housing and husbandry (9), allocation (11), outcomes (12), interpretation (18) and generalizability (19). Sprague–Dawley rats were the most frequently used (50%) species, and most studies had a sample size of more than 30 (58.33%). A defect dimension of 5 mm was the most common (33.33%). The biological hydroxyapatite composite scaffold was common (50%), and the bioactive factors were bone morphogenetic protein-2 (50%) and recombinant human bone morphogenetic protein-2 (50%). Histomorphometric results showed that bone morphogenetic protein-2 enhanced the capacity to regenerate bone considerably. In addition, scaffolds with bone morphogenetic protein-2 resulted in a significant increase in the blood vessel in the new bone. The findings suggested that data on animal experiments of hydroxyapatite scaffold coated with bone morphogenetic protein-2 in murine calvarial defect models lack homogeneity. Animal experiment should follow the Animal Research Reporting in In Vivo Experiments guidelines to promote the high quality, integrity and reproducibility. This systematic review suggested that bone morphogenetic protein-2 enhanced the capacity to regenerate bone and the angiogenesis in the new bone.

Antitumor Effect by Hydroxyapatite Nanospheres: Activation of Mitochondria-Dependent Apoptosis and Negative Regulation of Phosphatidylinositol-3-Kinase/Protein Kinase B Pathway

Hydroxyapatite nanoparticles (HA NPs) have been acknowledged for their benign biocompatibility and proliferation inhibition effect on tumor cells, attracting considerable attention for tumor therapeutics without late effects. However, unnoticeable tumor cytotoxicity of HA NPs limited the final clinical therapeutic efficacy. Herein, a two-phase synthetic approach was developed to synthesize sphere-like HA NPs by varying the conventional growth habit of HA precipitate. We present our in vitro and in vivo experimental evidence that spherical HA NPs have surprisingly high inhibitory activities against tumor cells. We demonstrate further, based on our experimental data, that the underlying cause for the death of the tumor cells is related to two concurrent pathways, the mitochondria-dependent apoptosis pathway and negative regulation of the phosphatidylinositol-3-kinase/protein kinase B (PIK3/AKT) pathway. The present study indicated that HA nanospheres can be engineered as nontoxic specific inhibitors for efficient tumor therapeutics with nanobiomaterials.

Preparation and characterization of mesoporous hydroxyapatite with non-cytotoxicity and heavy metal adsorption capacity

Mesoporous hydroxyapatite (MPHA) particles have recently gained a great deal of interest in a broad range of fields including biomedical fields, wastewater treatment and catalysis.

Histological Evaluation of Bone Repair with Hydroxyapatite: A Systematic Review

The aim of this study was to evaluate the morphological bone response in animal experiments by applying hydroxyapatite grafts in critical and non-critical size bone defects. Current report followed the guidelines established by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. Animal experiments were selected by assessing repair of bone defects with hydroxyapatite as bone graft and with blood clot only as control. Eight articles were identified in specialized literature and included in the meta-analysis. Statistical analysis was carried out with a random-effect model (p = 0.05). Subgroup analyses were further performed to investigate bone repair in critical and non-critical bone defects. Comprehensive analysis of bone repair outcome showed a statistically significant difference between hydroxyapatite and blood clot control (p < 0.05). Subgroup analyses showed statistically significant difference for critical bone defects (p < 0.05). No statistically significant difference was reported in non-critical bone defects (p > 0.05). Although animal studies revealed a high risk of bias and results should be interpreted with caution, the literature suggests that non-critical bone defects may heal spontaneously and without the need of a bone graft. Conversely, when critical-size defects are present, the use of hydroxyapatite bone graft improves the bone repair process.

Biomimetically grown apatite spheres from aggregated bioglass nanoparticles with ultrahigh porosity and surface area imply potential drug delivery and cell engineering applications

Here we communicate the generation of biomimetically grown apatite spheres from aggregated bioglass nanoparticles and the potential properties applicable for drug delivery and cell/tissue engineering. Ion releasing nanoparticulates of bioglass (85%SiO₂-15%CaO) in a mineralizing medium show an intriguing dynamic phenomenon - aggregation, mineralization to apatite, integration and growth into micron-sized (1.5-3μm) spheres. During the progressive ionic dissolution/precipitation reactions, nano-to-micro-morphology, glass-to-crystal composition, and the physico-chemical properties (porosity, surface area, and charge) change dynamically. With increasing reaction period, the apatite becomes more crystallized with increased crystallinity and crystal size, and gets a composition closer to the stoichiometry. The developed microspheres exhibit hierarchical surface nanostructure, negative charge (ς-potential of -20mV), and ultrahigh mesoporosity (mesopore size of 6.1nm, and the resultant surface area of 63.7m²/g and pore volume of 0.153cm³/g) at 14days of mineralization, which are even higher than those of its precursor bioglass nanoparticles. Thanks to these properties, the biomimetic mineral microspheres take up biological molecules effectively, i.e., loading capacity of positive-charged protein is over 10%. Of note, the release is highly sustainable at a constant rate, i.e., profiling almost 'zero-order' kinetics for 4weeks, suggesting the potential usefulness as protein delivery systems. The biomimetic mineral microspheres hold some remnant Si in the core region, and release calcium, phosphate, and silicate ions over the test period, implying the long-term ionic-related therapeutic functions. The mesenchymal stem cells favour the biomimetic spheres with an excellent viability. Due to the merit of sizes (a few micrometers), the spheres can be intercalated into cells, mediating cellular interactions in 3D cell-spheroid engineering, and also can stimulate osteogenic differentiation of cells when incorporated into cell-laden gels. The intriguing properties observed in this study, including biomimetic composition, high mesoporosity, release of therapeutic ions, effective loading and long-term release of proteins, and diverse yet favorable 3D cellular interactions, suggest great potential of the newly developed biomimetic microspheres in biomedical applications, such as drug delivery and cell/tissue engineering.

STATEMENT OF SIGNIFICANCE

This work reports the generation of apatite spheres with a few micrometers in size biomimetically grown from bioactive glass nanoparticles, through a series of intriguing yet unprecedented phenomenon involving aggregation of nanoparticles, mineralization and sphere growth. The mineral microspheres possess some unique physico-chemical properties including mesoporosity, ultrahigh surface area, and therapeutic ionic release. Furthermore, the spheres show excellent loading and delivery capacity of protein molecules, and mediate favorable cellular interactions in 2D and 3D culture conditions, demonstrating a future multifunctional microcarrier platform for the therapeutics delivery and cell/tissue engineering.

Controlled dual delivery of low doses of BMP-2 and VEGF in a silk fibroin-nanohydroxyapatite scaffold for vascularized bone regeneration

The controlled co-release of osteoinductive and angiogenic factors is an efficient approach to promote vascularized bone regeneration, and a suitable controlled release system can largely reduce the usage of these factors to avoid cost and safety problems. In this study, a cell-free vascularized bone tissue engineering system based on a silk fibroin (SF)/nanohydroxyapatite (nHAp) scaffold was developed, in which very low doses of osteoinductive and angiogenic factors, bone morphogenetic protein-2 (BMP-2) and vascular endothelial growth factor (VEGF), were embedded and released in a controlled manner to facilitate bone formation and vascularization, respectively. BMP-2 and VEGF were adsorbed onto SF microspheres (diameter of 1.5 ± 0.3 μm) that were prepared using a co-flow capillary device, and these microspheres were subsequently incorporated within the SF/nHAp scaffolds to provide controlled release. BMP-2 and VEGF were incorporated into SF microspheres via chemical covalent bonding and physical adsorption, respectively, leading to their controlled and sustained release from the SF/nHAp scaffolds. The rapid initial release of VEGF mimicked its expression at the early bone healing stage and promoted angiogenesis, and the relatively slow and sustained release of BMP-2 facilitated osteogenic differentiation both in vitro and in vivo, and the bone completely bridged the rat calvarial defects after 12 weeks of implantation. Overall, our findings suggest that the controlled dual release of very low doses of BMP-2 (300 ng per scaffold) and VEGF (20 ng per scaffold) from SF/nHAp scaffolds results in a synergistic effect on vascularized bone regeneration; this controlled release system can largely reduce the usage of BMP-2 as compared to other systems.

Near-infrared persistent luminescence hollow mesoporous nanospheres for drug delivery and in vivo renewable imaging

The design and fabrication of trackable drug carriers is of great importance for tracking the route of drug delivery in vivo. Here, we report novel stick-shell structure, near-infrared (NIR) persistent luminescence hollow mesoporous (ZnGa₂O₄:Cr³⁺@HMS) nanospheres. Due to the hollow mesoporous SiO₂ skeleton structure, the prepared nanospheres possess a high Brunauer-Emmett-Teller (BET) surface area of 514 m² g^-1 and a pore volume of 1.16 cm³ g^-1, which make these nanospheres ideal drug carriers for drug loading and sustained release. In addition, these nanospheres exhibit strong NIR emission at 696 nm, and their NIR persistent luminescence can persist for more than three hours after excitation. Thus, we can realize autofluorescence-free and long-term tracking of the transport routes of drug carriers by using the exceptional properties of NIR persistent luminescence of nanospheres. More importantly, we found that the NIR persistent luminescence of ZnGa₂O₄:Cr³⁺@HMS nanospheres can be repeatedly excited by a 655 nm LED lamp. Thus, the drug transport route can be tracked for an extended time with high sensitivity in deep tissue via the method of in situ repeatable excitation. Our results indicate that the ZnGa₂O₄:Cr³⁺@HMS nanospheres combine the advantages of high drug storage capacity and excellent NIR persistent luminescence, and they show potential for application as trackable drug carriers.

Strategies for delivering bone morphogenetic protein for bone healing

Bone morphogenetic proteins (BMPs) are the most significant growth factors that belong to the Transforming Growth Factor Beta (TGF-β) super-family. Though more than twenty members of this family have been identified so far in humans, Food and Drug Administration (FDA) approved two growth factors: BMP-2 and BMP-7 for treatments of spinal fusion and long-bone fractures with collagen carriers. Currently BMPs are clinically used in spinal fusion, oral and maxillofacial surgery and also in the repair of long bone defects. The efficiency of BMPs depends a lot on the selection of suitable carriers. At present, different types of carrier materials are used: natural and synthetic polymers, calcium phosphate and ceramic-polymer composite materials. Number of research articles has been published on the minute intricacies of the loading process and release kinetics of BMPs. Despite the significant evidence of its potential for bone healing demonstrated in animal models, future clinical investigations are needed to define dose, scaffold and route of administration. The efficacy and application of BMPs in various levels with a proper carrier and dose is yet to be established. The present article collates various aspects of success and limitation and identifies the prospects and challenges associated with the use of BMPs in orthopaedic surgery.

Improvement in degradability of 58s glass scaffolds by ZnO and β-TCP modification

58s bioactive glass shows great potential for bone defects repair. However, at early repairing stage, the degradation rate of 58s glass is too fast due to the fast ion-exchange. At later repairing stage, the degradation rate of 58s glass is too slow due to the high dense mineral layer. In this work, Zinc oxide (ZnO) and β-tricalcium phosphate (β-TCP) were introduced into 58s glass bone scaffolds to improve the degradability. The results showed that ZnO could decrease the degradation rate and promote the stability of 58s glass at early repairing stage. Moreover, the presence of β-TCP appeared to increase the degradation rate at a later stage of repairing. Furthermore, in vitro biocompatibility study, carried out using human osteoblast-like cells (MG63), demonstrated that ZnO and β-TCP enhanced cell attachment and proliferation. The study provided a reference for further research in bone tissue engineering.

Bioactive Natural Protein-Hydroxyapatite Nanocarriers for Optimizing Osteogenic Differentiation of Mesenchymal Stem Cells

Improving the controlled release of bioactive growth factors to regulate cell behavior and tissue regeneration remains a need in tissue engineering and regenerative medicine. Inorganic and polymeric nanoparticles have been extensively fabricated as bioactive biomaterials with enhanced biocompatibility and effective carriers of therapeutic agents, however, challenges remain such as the achievement of high loading capacity and sustained release, and the bioactivity preservation of growth factors. Here, a multilayered, silk coated hydroxyapatite (HA) nanocarrier with drug loading-release capacity superior to pure silk or HA nanoparticles was developed. Bone morphogenetic protein-2 (BMP-2) was bound to the silk coatings with a high binding efficiency of 99.6%, significantly higher than that in silk or the HA nanoparticles alone. The release of BMP-2 was sustained in vitro over a period of 21 days without burst release. Compared with BMP-2 loaded silk or HA particles, bone mesenchymal stem cells (BMSCs) showed improved proliferation and osteogenesis when cultured with the BMP-2 loaded composite nanocarriers. Therefore, these silk-HA composite nanoparticles present a useful approach to designing bioactive nanocarrier systems with enhanced functions for bone tissue regeneration needs.

Preparation of resorbable carbonate-substituted hollow hydroxyapatite microspheres and their evaluation in osseous defects in vivo

Hollow hydroxyapatite (HA) microspheres, with a high-surface-area mesoporous shell, can provide a unique bioactive and osteoconductive carrier for proteins to stimulate bone regeneration. However, synthetic HA has a slow resorption rate and a limited ability to remodel into bone. In the present study, hollow HA microspheres with controllable amounts of carbonate substitution (0-12 wt.%) were created using a novel glass conversion route and evaluated in vitro and in vivo. Hollow HA microspheres with ~12 wt.% of carbonate (designated CHA12) showed a higher surface area (236 m(2) g(-1)) than conventional hollow HA microspheres (179 m(2)g(-1)) and a faster degradation rate in a potassium acetate buffer solution. When implanted for 12 weeks in rat calvarial defects, the CHA12 and HA microspheres showed a limited capacity to regenerate bone but the CHA12 microspheres resorbed faster than the HA microspheres. Loading the microspheres with bone morphogenetic protein-2 (BMP2) (1 μg per defect) stimulated bone regeneration and accelerated resorption of the CHA12 microspheres. At 12 weeks, the amount of new bone in the defects implanted with the CHA12 microspheres (73±8%) was significantly higher than the HA microspheres (59±2%) while the amount of residual CHA12 microspheres (7±2% of the total defect area) was significantly lower than the HA microspheres (21±3%). The combination of these carbonate-substituted HA microspheres with clinically safe doses of BMP2 could provide promising implants for healing non-loaded bone defects.

Biomimetic synthesis of hybrid hydroxyapatite nanoparticles using nanogel template for controlled release of bovine serum albumin

A biomimetic method was used to prepare hybrid hydroxyapatite (HAP) nanoparticles with chitosan/polyacrylic acid (CS-PAA) nanogel. The morphology, structure, crystallinity, thermal properties and biocompatibility of the obtained hybrid nanogel-HAP nanoparticles have been characterized. In addition, bovine serum albumin (BSA) was used as a model protein to study the loading and release behaviors of the hybrid nanogel-HAP nanoparticles. The results indicated that the obtained HAP nanoparticles were agglomerated and the nanogel could regulate the formation of HAP. When the nanogel concentration decreased, different HAP crystal shapes and agglomerate structures were obtained. The loading amount of BSA reached 67.6 mg/g for the hybrid nanoparticles when the mineral content was 90.4%, which decreased when the nanogel concentration increased. The release profile of BSA was sustained in neutral buffer. Meanwhile, an initial burst release was found at pH 4.5 due to the desorption of BSA from the surface, followed by a slow release. The hemolysis percentage of the hybrid nanoparticles was close to the negative control, and these particles were non-toxic to bone marrow stromal stem cells. The results suggest that these hybrid nanogel-HAP nanoparticles are promising candidate materials for biocompatible drug delivery systems.

Preparation and characterization of gelatin-hydroxyapatite composite microspheres for hard tissue repair

Gelatin-hydroxyapatite composite microspheres composed of 21% gelatin (G) and 79% hydroxyapatite (HA) with uniform morphology and controllable size were synthesized from a mixed solution of Ca(NO3)2, NH4H2PO4 and gelatin by a wet-chemical method. Material analyses such as X-ray diffraction (XRD), scanning/transmission electron microscopy examination (SEM/TEM) and inductively coupled plasma-mass spectroscopy (ICP-MS) were used to characterize G-HA microspheres by analyzing their crystalline phase, microstructure, morphology and composition. HA crystals precipitate along G fibers to form nano-rods with diameters of 6-10nm and tangle into porous microspheres after blending. The cell culture indicates that G-HA composite microspheres without any toxicity could enhance the proliferation and differentiation of osteoblast-like cells. In a rat calvarial defect model, G-HA bioactive scaffolds were compared with fibrin glue (F) and Osteoset® Bone Graft Substitute (OS) for their capacity of regenerating bone. Four weeks post-implantation, new bone, mineralization, and expanded blood vessel area were found in G-HA scaffolds, indicating greater osteoconductivity and bioactivity than F and OS.

MiRNA inhibition in tissue engineering and regenerative medicine

MicroRNAs (miRNAs) are noncoding RNAs that provide an endogenous negative feedback mechanism for translation of messenger RNA (mRNA) into protein. Single miRNAs can regulate hundreds of mRNAs, enabling miRNAs to orchestrate robust biological responses by simultaneously impacting multiple gene networks. MiRNAs can act as master regulators of normal and pathological tissue development, homeostasis, and repair, which has motivated expanding efforts toward the development of technologies for therapeutically modulating miRNA activity for regenerative medicine and tissue engineering applications. This review highlights the tools currently available for miRNA inhibition and their recent therapeutic applications for improving tissue repair.

Bone Regeneration Using Bone Morphogenetic Proteins and Various Biomaterial Carriers

Trauma and disease frequently result in fractures or critical sized bone defects and their management at times necessitates bone grafting. The process of bone healing or regeneration involves intricate network of molecules including bone morphogenetic proteins (BMPs). BMPs belong to a larger superfamily of proteins and are very promising and intensively studied for in the enhancement of bone healing. More than 20 types of BMPs have been identified but only a subset of BMPs can induce de novo bone formation. Many research groups have shown that BMPs can induce differentiation of mesenchymal stem cells and stem cells into osteogenic cells which are capable of producing bone. This review introduces BMPs and discusses current advances in preclinical and clinical application of utilizing various biomaterial carriers for local delivery of BMPs to enhance bone regeneration.

Hollow hydroxyapatite microspheres/chitosan composite as a sustained delivery vehicle for rhBMP-2 in the treatment of bone defects

Composite scaffold comprised of hollow hydroxyapatite (HA) and chitosan (designated hHA/CS) was prepared as a delivery vehicle for recombinating human bone morphogenetic protein-2 (rhBMP-2). The in vitro and in vivo biological activities of rhBMP2 released from the composite scaffold were then investigated. The rhBMP-2 was firstly loaded into the hollow HA microspheres, and then the rhBMP2-loaded HA microspheres were further incorporated into the chitosan matrix. The chitosan not only served to bind the HA microspheres together and kept them at the implant site, but also effectively modified the release behavior of rhBMP-2. The in vitro release and bioactivity analysis confirmed that the rhBMP2 could be loaded and released from the composite scaffolds in bioactive form. In addition, the composite scaffolds significantly reduced the initial burst release of rhBMP2, and thus providing prolonged period of time (as long as 60 days) compared with CS scaffolds. In vivo bone regenerative potential of the rhBMP2-loaded composite scaffolds was evaluated in a rabbit radius defect model. The results revealed that the rate of new bone formation in the rhBMP2-loaded hHA/CS group was higher than that in both negative control and rhBMP2-loaded CS group. These observations suggest that the hHA/CS composite scaffold would be effective and feasible as a delivery vehicle for growth factors in bone regeneration and repair.

Controlled-release of bone morphogenetic protein-2 from a microsphere coating applied to acid-etched Ti6AL4V implants increases biological bone growth in vivo

A central clinical challenge regarding the surgical treatment of bone and joint conditions is the eventual loosening of an orthopedic implant as a result of insufficient bone ingrowth at the bone-implant interface. We investigated the in vivo effectiveness of a coating containing recombinant human bone morphogenetic protein-2 (rhBMP-2)-loaded microspheres applied to acid-etched Ti6Al4V cylinders for implantation. Three groups of rabbits (24 per group) were used for implantation: (1) acid-etched Ti6Al4V implants coated with a mixture of rhBMP-2-loaded microspheres (125 ng rhBMP-2/mg microspheres) and α-butyl cyanoacrylate; (2) acid-etched, uncoated implants; and (3) bare, smooth uncoated implants. After implantation, 12 rabbits from each group were used for bone ingrowth determination at 4, 5, 6, 7, 8, and 12 weeks (2 rabbits per time point), while the remainder were used for histological analysis and push-out testing at 12 weeks. Scanning electron microscopy showed significant improvement in bone growth of the rhBMP-2 microspheres/α-butyl cyanoacrylate group compared with the other groups (p<0.01). Histological analysis and push-out testing also demonstrated enhanced bone growth of the rhBMP-2 group over that in the other two groups (p<0.01). The rhBMP-2 group showed the most significant bone growth, suggesting that coating acid-etched implants with a mixture of rhBMP-2-loaded microspheres and α-butyl cyanoacrylate may be an effective method to improve the osseointegration of orthopedic implants.

Controlled release of BMP-2 using a heparin-conjugated carrier system reduces in vivo adipose tissue formation

There is growing concern about unwanted effects associated with the clinical use of recombinant human bone morphogenetic protein-2 (rhBMP-2) at high concentrations, including cyst-like bone formation and excessive fatty marrow formation. We, therefore, evaluated the induction of mineralized/adipose tissue formation and the bone-healing pattern associated with the controlled release of E. coli-derived rhBMP-2 (ErhBMP-2) by a heparin-conjugated fibrin (HCF) system using ectopic and orthotopic in vivo models, respectively. In the ectopic transplantation model, mineralized tissue formed at the most superficial layer of the transplanted area and on the surfaces of grafted materials, and most of the interstitial space within the transplanted area was filled with excessive adipose tissue specifically at sites that received ErhBMP-2. However, sites that received ErhBMP-2 and HCF showed significantly increased mineralized tissue formation and decreased adipose tissue formation compared to the normal fibrin system with ErhBMP-2. In the orthotopic (calvarial defect) model, controlled release of ErhBMP-2 induced by HCF significantly reduced adipose tissue formation within the defect area compared to the clinically approved absorbable collagen sponge. From these results, it can be concluded that the use of a HCF system loaded with ErhBMP-2 may reduce adipose tissue formation and enhance mineralized tissue formation.

Evaluation of rhBMP-2 and bone marrow derived stromal cell mediated bone regeneration using transgenic fluorescent protein reporter mice

The aim of the study is use of transgenic fluorescent protein reporter mouse models to understand the cellular processes in recombinant human bone morphogenetic protein-2 (rhBMP-2) mediated bone formation. Bilateral parietal calvarial bone defects in Col3.6Topaz transgenic fluorescent osteoblast reporter mouse were used to understand the bone formation in the presence and absence of rhBMP2 and/or Col3.6Cyan bone marrow derived stromal cells (BMSCs), using collagen-hydroxyapatite matrix (Healos) as a biomaterial. The bone regeneration was not confined to the site of BMP-2 implantation and significant bone formation was observed in the neighboring defect site. Osteogenic cellular activity with overlying alizarin complexone staining was observed in both the defects indicating host cell induced mineralization. However, implantation of BMSCs along with rhBMP-2 demonstrated a donor cell derived bone formation. The presence of rhBMP-2 did not support host cell recruitment in the presence of donor cells. This study demonstrates the potential of multiple fluorescent reporters to understand the cellular processes involved in the bone regeneration process using biomaterials, growth factors, and/or stem cells.