Stimulation of bone regeneration following the controlled release of water-insoluble oxysterol from biodegradable hydrogel

The role of micro-vibration parameters in inflammatory responses of macrophages cultured on biphasic calcium phosphate ceramics and the resultant influence on osteogenic differentiation of mesenchymal stem cells

Although in vitro studies have shown that biomaterials and mechanical stimuli can mediate inflammatory responses or regulate osteogenesis of MSCs, the underlying behaviour of the inflammatory response of macrophages on biomaterials mediated by mechanical stimuli, which regulates osteogenesis, is relatively unknown. Thus, it is imperative to explore the role of bionic mechanical stimulation in the biomaterial-mediated inflammatory response of macrophages. In this study, we used osteoinductive biphasic calcium phosphate (BCP) ceramics as the model biomaterial and chose micro-vibration stimulation (MVs) with three variable parameters (frequency, magnitude, and time). Based on orthogonal experiments, nine combinations of MVs parameters were generated, and their effects on the BCP-mediated macrophage inflammatory response were investigated. MVs significantly affected the gene expression and cytokine secretion of macrophages grown on BCP ceramics and further influenced the behaviour of bone marrow mesenchymal stem cells (BMMSCs) in a paracrine manner. Moreover, frequency seemed to be the most dominant factor (compared with magnitude and time) in regulating the inflammatory response of macrophages. The optimal combination of MVs parameters (frequency 10 Hz, magnitude 0.45 g, and time 60 min) could induce a healing-associated M2 phenotype, as evidenced by the downregulated pro-inflammatory gene (Il-1β, and Tnf-α) expression, the upregulated anti-inflammatory gene (Il10) expression, and the inhibited pro-inflammatory cytokine (Il-1β and Tnf-α) secretion of macrophages grown on BCP ceramics, and its conditioned medium (CM) could further promote osteogenic differentiation of BMMSCs. These findings provide valuable insights into the mechanical stimulus-mediated macrophage inflammatory response and osteogenesis in the presence of osteoinductive BCP ceramics and allow accurate evaluation of the biological performance of biomaterials in vitro, in order to optimize bone substitute materials to achieve the desired clinical performance.

Enhancement of in vivo supraspinatus tendon-to-bone healing with an alginate-chitin scaffold and rhBMP-2

INTRODUCTION

Rotator cuff disorders present a high retear rate despite advances in surgical treatment. Tissue engineering could therefore be interesting in order to try to enhance a more biological repair. RhBMP-2 is one of the most osteogenic growth factors and it also induces the formation of collagen type I. However, it has a short half-life and in order to get a more stable release over time it could be integrated in a more slowly degradable carrier, such as an alginate-chitin scaffold. The aim of this study was to investigate the role of the alginate-chitin scaffold alone and in combination with different concentrations of rhBMP-2 when applied on chronic rotator cuff lesions in a rat model.

MATERIALS AND METHODS

We performed an experimental study with 80 Sprague-Dawley rats, 8 months old, with a chronic rupture of the supraspinatus tendon that was repaired with a modified Mason Allen suture. A scaffold was applied over the suture and 4 groups were obtained; suture (S) only suture, double control (DC) alginate and chitin scaffold, single sample (SS) scaffold of alginate with rhBMP-2 (20 µg rhBMP-2) and chitin, double sample (DS) a scaffold containing alginate with rhBMP-2 and chitin with rhBMP-2 (40 µg rhBMP-2). Macroscopic, histological and biomechanical studies were performed at 4 months after reparation.

RESULTS

The modified Åström and Rausing's histological scale (the higher the score the worse outcome, 0 points=native tendon) was applied: S got 52 points compared to DC 30 (p = 0,034), SS 22 (p = 0,009) and DS 16 (p = 0,010). Biomechanically the maximum load was highest in DC (63,05 N), followed by DS (61,60 N), SS (52,35 N) and S (51,08), p = 0,025 DS vs S. As to the elastic constant a higher value was obtained in DC (16,65), DS (12,55) and SS (12,20) compared to S (9,33), p = 0,009 DC vs S and 0,034 DS vs S.

CONCLUSIONS

The alginate-chitin scaffold seems to promote a more biological response after the reparation of a chronic rotator cuff lesion. Its effect is further enhanced by the addition of rhBMP-2 since the osteotendinous junction is more native-like and has better biomechanical properties.

Bottom-Up Self-assembled Hydrogel-Mineral Composites Regenerate Rabbit Ulna Defect without Added Growth Factors

Hydrogel-based biomaterials have advanced bone tissue engineering approaches in the last decade, through their ability to serve as a carrier for potent growth factor, bone morphogenic protein-2 (BMP-2). However, biophysical properties of hydrogels such as multiscale structural hierarchy and bone extracellular matrix (ECM)-mimetic microarchitecture are underutilized while designing current bone grafts. Incorporation of these properties offers great potential to create a favorable biomimetic microenvironment to harness their regenerative potential. Here, we present our approach to fabricate collagen-inspired bioactive hydrogel scaffolds (referred to as "RegenMatrix") to guide and enhance bone regeneration in a rabbit ulna defect model through the mimicry of multiscale architecture of bone ECM, i.e., native collagen. Specifically, we employed polyelectrolyte complexation to promote bottom-up self-assembly of oppositely charged polysaccharides (chitosan and kappa-carrageenan) at multiple length scales forming fibrils, which further assemble into fibers. The self-assembly and bioinspired scaffold fabrication method resulted in robust cylindrical RegenMatrix with excellent retention of the multiscale architecture and uniform mineral deposition throughout the scaffolds. RegenMatrix, in both nonmineralized and mineralized forms, enhanced bone regeneration in the semiload-bearing ulna defect when compared to the empty defect. RegenMatrix also showed greater histocompatibility without any fibrous tissue formation. Collectively, the RegenMatrix developed in this study has a great potential as a bioactive bone graft without any added growth factors.

Oxysterols as promising small molecules for bone tissue engineering: Systematic review

BACKGROUND

Bone tissue engineering is an area of continued interest within orthopaedic surgery, as it promises to create implantable bone substitute materials that obviate the need for autologous bone graft. Recently, oxysterols – oxygenated derivatives of cholesterol – have been proposed as a novel class of osteoinductive small molecules for bone tissue engineering. Here, we present the first systematic review of the in vivo evidence describing the potential therapeutic utility of oxysterols for bone tissue engineering.

AIM

To systematically review the available literature examining the effect of oxysterols on in vivo bone formation.

METHODS

We conducted a systematic review of the literature following PRISMA guidelines. Using the PubMed/MEDLINE, Embase, and Web of Science databases, we queried all publications in the English-language literature investigating the effect of oxysterols on in vivo bone formation. Articles were screened for eligibility using PICOS criteria and assessed for potential bias using an expanded version of the SYRCLE Risk of Bias assessment tool. All full-text articles examining the effect of oxysterols on in vivo bone formation were included. Extracted data included: Animal species, surgical/defect model, description of therapeutic and control treatments, and method for assessing bone growth. Primary outcome was fusion rate for spinal fusion models and percent bone regeneration for critical-sized defect models. Data were tabulated and described by both surgical/defect model and oxysterol employed. Additionally, data from all included studies were aggregated to posit the mechanism by which oxysterols may mediate in vivo bone formation.

RESULTS

Our search identified 267 unique articles, of which 27 underwent full-text review. Thirteen studies (all preclinical) met our inclusion/exclusion criteria. Of the 13 included studies, 5 employed spinal fusion models, 2 employed critical-sized alveolar defect models, and 6 employed critical-sized calvarial defect models. Based upon SYRCLE criteria, the included studies were found to possess an overall “unclear risk of bias”; 54% of studies reported treatment randomization and 38% reported blinding at any level. Overall, seven unique oxysterols were evaluated: 20(S)-hydroxycholesterol, 22(R)-hydroxycholesterol, 22(S)-hydroxycholesterol, Oxy4/Oxy34, Oxy18, Oxy21/Oxy133, and Oxy49. All had statistically significant in vivo osteoinductive properties, with Oxy4/Oxy34, Oxy21/Oxy133, and Oxy49 showing a dose-dependent effect in some cases. In the eight studies that directly compared oxysterols to rhBMP-2-treated animals, similar rates of bone growth occurred in the two groups. Biochemical investigation of these effects suggests that they may be primarily mediated by direct activation of Smoothened in the Hedgehog signaling pathway.

CONCLUSION

Present preclinical evidence suggests oxysterols significantly augment in vivo bone formation. However, clinical trials are necessary to determine which have the greatest therapeutic potential for orthopaedic surgery patients.

Cell sheets of co-cultured BMP-2-modified bone marrow stromal cells and endothelial progenitor cells accelerate bone regeneration in vitro

Bone tissue engineering provides a substitute for bone transplantation to address various bone defects. However, bone regeneration involves a large number of cellular events. In addition, obtaining sufficient source material for autogenous bone or alloplastic bone substitutes remains an unsolved issue. In previous studies, it was confirmed that bone marrow stromal cells (BMSCs) and endothelial progenitor cells (EPCs) had the capacity to promote bone regeneration. Additionally, bone morphogenetic protein-2 (BMP-2) has been demonstrated to be an active inducer of osteoblast differentiation. Therefore, the aim of the present study was to produce an effective integration system, including a scaffold, reparative cells and growth factors, that may enhance bone regeneration. Firstly, bone marrow-derived BMSCs and EPCs were isolated and identified by flow cytometry. Cell proliferation ability, secreted BMP-2 levels and alkaline phosphatase (ALP) activity were highest in the cell sheets containing BMP-2-modified BMSCs and EPCs. In addition, the expression levels of osteogenesis-associated genes, including runt related transcription factor 2 (Runx2), distal-less homeobox 5 (Dlx5), ALP and integrin-binding sialoprotein (Ibsp), and osteogenesis-associated proteins, including Runx2, Dlx, ALP, Ibsp, vascular endothelial growth factor, osteonectin, osteopontin and type I collagen, gradually increased during the co-culture of ad-BMP-2-BMSCs/EPCs. The levels of these genes and proteins were increased compared with those observed in the BMSC, EPC and BMP-2-modified BMSC groups. Finally, scanning electron microscopy observation also demonstrated that the BMP2-modified BMSCs were able to combine well with EPCs to construct a cell sheet for bone formation. Collectively, these results describe an adenovirus (ad)-BMP2-BMSCs/EPCs co-culture system that may significantly accelerate bone regeneration compared with a BMSCs/EPCs co-culture system or ad-BMP2-BMSCs alone.

Nanoparticle-modified chitosan-agarose-gelatin scaffold for sustained release of SDF-1 and BMP-2

Background

Stromal cell-derived factor 1 (SDF-1) is an important chemokine for stem cell mobilization, and plays a critical role in mobilization of mesenchymal stem cells (MSCs). Bone morphogenetic protein 2 (BMP-2) plays a critical role in osteogenesis of MSCs. However, the use of SDF-1 and BMP-2 in bone tissue engineering is limited by their short half-lives and rapid degradation in vitro and in vivo.

Methods

The chitosan oligosaccharide/heparin nanoparticles (CSO/H NPs) were first prepared via self-assembly. Chitosan-agarose-gelatin (CAG) Scaffolds were then synthesized via gelation technology using cross-linked chitosan, agarose, and gelatin, and were modified by CSO/H NPs. The encapsulation efficiency and release kinetics of SDF-1 and BMP-2 were quantified using an enzyme-linked immunosorbent assay. A CCK-8 assays were used to evaluate biocompatibility of NP-modified scaffolds. The biological activity of the loaded SDF-1 and BMP-2 was evaluated using the transwell migration assay and osteogenic induction assay. An animal MSC recruitment model was used to study the ability of SDF-1 released from NP-modified scaffolds to induce migration of MSCs.

Results

In this study, we developed a novel nanoparticle-modified CAG scaffold for the delivery of SDF-1 and BMP-2. CCK-8 assays demonstrated excellent biocompatibility of NP-modified scaffolds. In addition, we investigated the release of SDF-1 and BMP-2 from NP-modified scaffolds, and evaluated the effect of released SDF-1 on MSC migration. The effect of released BMP-2 on MSC osteogenesis was also examined. In vitro cell migration assays showed that SDF-1 released from NP-modified scaffolds retained its migration activity; osteogenesis studies demonstrated that released BMP-2 exhibited a strong ability to induce differentiation towards osteoblasts. Our in vivo recruitment assays showed continuous chemotactic response of MSCs to SDF-1 released from the NP-modified scaffold.

Conclusion

The simplicity of synthesizing CSO/H NP-modified CAG scaffolds, combined with its high cytokine loading capacity and sustained release effect, renders NP-modified CAG scaffold an attractive candidate for sustained release of SDF-1 and BMP-2 to promote bone repair and regeneration.

Bone marrow concentrate promotes bone regeneration with a suboptimal-dose of rhBMP-2

Bone marrow concentrate (BMC), which is enriched in mononuclear cells (MNCs) and platelets, has recently attracted the attention of clinicians as a new optional means for bone engineering. We previously reported that the osteoinductive effect of bone morphogenetic protein-2 (BMP-2) could be enhanced synergistically by co-transplantation of peripheral blood (PB)-derived platelet-rich plasma (PRP). This study aims to investigate whether BMC can effectively promote bone formation induced by low-dose BMP-2, thereby reducing the undesirable side-effects of BMP-2, compared to PRP. Human BMC was obtained from bone marrow aspirates using an automated blood separator. The BMC was then seeded onto β-TCP granules pre-adsorbed with a suboptimal-dose (minimum concentration to induce bone formation at 2 weeks in mice) of recombinant human (rh) BMP-2. These specimens were transplanted subcutaneously to the dorsal skin of immunodeficient-mice and the induction of ectopic bone formation was assessed 2 and 4 weeks post-transplantation. Transplantations of five other groups [PB, PRP, platelet-poor plasma (PPP), bone marrow aspirate (BM), and BM-PPP] were employed as experimental controls. Then, to clarify the effects on vertical bone augmentation, specimens from the six groups were transplanted for on-lay placement on the craniums of mice. The results indicated that BMC, which contained an approximately 2.5-fold increase in the number of MNCs compared to PRP, could accelerate ectopic bone formation until 2 weeks post-transplantation. On the cranium, the BMC group promoted bone augmentation with a suboptimal-dose of rhBMP-2 compared to other groups. Particularly in the BMC specimens harvested at 4 weeks, we observed newly formed bone surrounding the TCP granules at sites far from the calvarial bone. In conclusion, the addition of BMC could reduce the amount of rhBMP-2 by one-half via its synergistic effect on early-phase osteoinduction. We propose here that BMC transplantation facilitates the clinical use of rhBMP-2 as an alternative strategy for bone engineering.

Osteoinductivity of nanostructured hydroxyapatite-functionalized gelatin modulated by human and endogenous mesenchymal stromal cells

The demand of new strategies for the induction of bone regeneration is continuously increasing. Biomimetic porous gelatin-nanocrystalline hydroxyapatite scaffolds with tailored properties were previously developed, showing a positive response in terms of cell adhesion, proliferation, and differentiation. In the present paper, we focused on their osteoinductive properties. The effect of scaffolds on osteogenic differentiation of human mesenchymal stromal cells (hMSCs) was investigated in vitro. hMSCs were seeded on GEL (type A gelatin) and GEL containing 10 wt% hydroxyapatite (GEL-HA) and cultured in osteogenic medium. Results showed that GEL and GEL-HA10 sustained hMSC differentiation, with an increased ALP activity and a higher expression of bone specific genes. The osteoinductive ability of these scaffolds was then studied in vivo in a heterotopic bone formation model in nude mice. The influence of hMSCs within the implants was examined as well. Both GEL and GEL-HA10 scaffolds mineralized when implanted without hMSCs. On the contrary, the presence of hMSC abolished or reduced mineralization of GEL and GEL-HA10 scaffolds. However, we could observe a species-specific response to the presence of HA, which stimulated osteogenic differentiation of human cells only. In conclusion, the scaffolds showed promising osteoinductive properties and may be suitable for use in confined critical defects. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 914-923, 2018.

Evaluation of synergistic osteogenesis between icariin and BMP2 through a micro/meso hierarchical porous delivery system

BMP2 is well known as an outstanding growth factor for inducing new bone formation. However, improvements are still required to use BMP2 effectively and expand its clinical application due to the potential side effects at high doses. In this study, icariin (IC), a type of traditional Chinese medicine, was originally proposed to be a cooperative factor for BMP2. An alkaline phosphatase (ALP) activity assay showed that IC promoted BMP2 osteogenesis in a concentration-dependent manner with significant enhancement at 38.4 µM versus that for BMP2 at 0.8 µg/mL. Furthermore, we developed a composite hierarchical porous scaffold (SF/SBA15; composed of micropores of silk fibroin [SF] scaffold and mesopores of SBA15) for the controlled delivery of BMP2 and IC. This composite scaffold was investigated by a series of physical characterizations and displayed good in vitro cell biocompatibility. In addition, the composite scaffold also showed the degradation rate of 12% dry weight loss and a slight change in the microstructures within 10 days. Moreover, BMP2 and IC were loaded into the SF and SBA15 structures, respectively, of the SF/SBA15 scaffold. This protein/drug loading system (SFBMP2/SBA15IC) provided delivery of BMP2 with an initial burst release of 60.9%±0.9% in the first 24 hours and a gradual release over the subsequent 6 days to 97.9%±0.8%, whereas IC exhibited a burst release of 64.2%±0.7% in the first 24 hours, followed by a sustained release to 92.4%±0.8% over 10 days. With the prolonged local retention and interaction duration of BMP2 and IC, the SFBMP2/SBA15IC scaffold provided better osteogenic differentiation than other groups with different loading modes of BMP2 or IC, as determined by ALP staining and quantitation and Alizarin red staining. Finally, the results of quantitative real-time polymerase chain reaction analysis indicated that the SFBMP2/SBA15IC scaffold induced a significantly higher increase in the RUNX2, ALP, COL I, and OCN expression levels of cocultured bone marrow mesenchymal stem cells than other payload composite scaffolds. This study suggests that a micro/meso hierarchical porous delivery system of BMP2 and IC ensures osteogenic synergy and demonstrates promise for bone tissue engineering.

Design and Characterization of a Therapeutic Non-phospholipid Liposomal Nanocarrier with Osteoinductive Characteristics To Promote Bone Formation

Sterosomes are recently developed types of non-phospholipid liposomes formed from single-chain amphiphiles and high content of sterols. Although sterosomes presented significantly increased stability compared to conventional phospholipid liposomes, current sterosome biomaterials are not truly bioactive and have no intrinsic therapeutic effects. The purpose of this study was to develop a sterosome formulation with osteoinductive properties by an effective selection of sterol, one of the sterosome components. Oxysterols are oxidized derivatives of cholesterol and are known to stimulate osteogenesis and bone formation. Thus, 20S-hydroxycholesterol (Oxy), one of the most potent oxysterols for bone regeneration, was examined as a promising candidate molecule to form fluid lamellar phases with a single-chain amphiphile, namely, stearylamine (SA). First, the optimal composition was identified by investigating the phase behavior of SA/Oxy mixtures. Next, the capacity of the optimized SA/Oxy sterosomes to promote osteogenic differentiation of bone marrow stromal cells was assessed in vitro in a hydrogel environment. Furthermore, we explored the effects of osteogenic oxysterol sterosomes in vivo with the mouse critical-sized calvarial defect model. Our results showed that SA/Oxy sterosomes induced osteogenic differentiation in vitro and enhanced calvarial healing without delivery of additional therapeutic agents, indicating their intrinsic bone-forming potential. This study suggests a promising non-phospholipid liposomal platform with osteoinductive properties for delivery of small molecular drugs and/or other therapeutic genes for enhanced bone formation.

Gellan gum based physical hydrogels incorporating highly valuable endogen molecules and associating BMP-2 as bone formation platforms

Physical hydrogels have been designed for a double purpose: as growth factor delivery systems and as scaffolds to support cell colonization and formation of new bone. Specifically, the polysaccharide gellan gum and the ubiquitous endogenous molecules chondroitin, albumin and spermidine have been used as exclusive components of these hydrogels. The mild ionotropic gelation technique was used to preserve the bioactivity of the selected growth factor, rhBMP-2. In vitro tests demonstrated the effective delivery of rhBMP-2 in its bioactive form. In vivo experiments performed in the muscle tissue of Wistar rats provided a proof of concept of the ability of the developed platforms to elicit new bone formation. Furthermore, this biological effect was better than that of a commercial formulation currently used for regenerative purposes, confirming the potential of these hydrogels as new and innovative growth factor delivery platforms and scaffolds for regenerative medicine applications.

Porous 3D implants of degradable poly-3-hydroxybutyrate used to enhance regeneration of rat cranial defect

The study describes preparation and testing of porous 3D implants of natural degradable polymer of 3-hydroxybutyric acid P(3HB) for regeneration of bone tissue defects. The ability of the P(3HB) implants to favor attachment and facilitate proliferation and directed differentiation of mesenchymal stem cells (MSCs) was studied in the culture of MSCs isolated from bone marrow and adipose tissue. Tissue-engineered hybrid systems (grafts) constructed using P(3HB) and P(3HB) in combination with osteoblasts were used in experiments on laboratory animals (n = 48) with bone defect model. The defect model (5 mm in diameter) was created in the rat parietal bone, and filling of the defect by the new bone tissue was monitored in the groups of animals with P(3HB) implants, with commercial material, and without implants (negative control). Computed tomography (CT) and histologic examination showed that after 120 days, in the group with the osteoblast-seeded P(3HB) implants, the defect was completely closed; in the group with the cell-free P(3HB) implants, the remaining defect was no more than 10% of the initial one (0.5 mm); in both the negative and positive controls, the size of the defect was about 1.0-1.2 mm. These results suggest that P(3HB) has good potential as osteoplastic material for reconstructive osteogenesis. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 566-577, 2017.

Low-magnitude, high-frequency vibration promotes the adhesion and the osteogenic differentiation of bone marrow-derived mesenchymal stem cells cultured on a hydroxyapatite-coated surface: The direct role of Wnt/β-catenin signaling pathway activation

The positive effect of low-magnitude, high‑frequency (LMHF) vibration on implant osseointegration has been demonstrated; however, the underlying cellular and molecular mechanisms remain unknown. The aim of this study was to explore the effect of LMHF vibration on the adhesion and the osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) cultured on hydroxyapatite (HA)-coated surfaces in an in vitro model as well as to elucidate the molecular mechanism responsible for the effects of LMHF vibration on osteogenesis. LMHF vibration resulted in the increased expression of fibronectin, which was measured by immunostaining and RT-qPCR. Stimulation of BMSCs by LMHF vibration resulted in the rearrangement of the actin cytoskeleton with more prominent F-actin. Moreover, the expression of β1 integrin, vinculin and paxillin was notably increased following LMHF stimulation. Scanning electron microscope observations revealed that there were higher cell numbers and more extracellular matrix attached to the HA-coated surface in the LMHF group. Alkaline phosphatase activity as well as the expression of osteogenic-specific genes, namely Runx2, osterix, collagen I and osteocalcin, were significantly elevated in the LMHF group. In addition, the protein expression of Wnt10B, β-catenin, Runx2 and osterix was increased following exposure to LMHF vibration. Taken together, the findings of this study indicate that LMHF vibration promotes the adhesion and the osteogenic differentiation of BMSCs on HA-coated surfaces in vitro, and LMHF vibration may directly induce osteogenesis by activating the Wnt/β‑catenin signaling pathway. These data suggest that LMHF vibration enhances the osseointegration of bone to a HA-coated implant, and provide a scientific foundation for improving bone-implant osseointegration through the application of LMHF vibration.

Nanohydroxyapatite/cellulose nanocrystals/silk fibroin ternary scaffolds for rat calvarial defect regeneration

The purpose of this study was to design and characterise a novel biomimetic scaffold for the repair of critical size calvarial defects.

Hydrogels 2.0: improved properties with nanomaterial composites for biomedical applications

The incorporation of nanomaterials in hydrogels (hydrated networks of crosslinked polymers) has emerged as a useful method for generating biomaterials with tailored functionality. With the available engineering approaches it is becoming much easier to fabricate nanocomposite hydrogels that display improved performance across an array of electrical, mechanical, and biological properties. In this review, we discuss the fundamental aspects of these materials as well as recent developments that have enabled their application. Specifically, we highlight synthesis and fabrication, and the choice of nanomaterials for multifunctionality as ways to overcome current material property limitations. In addition, we review the use of nanocomposite hydrogels within the framework of biomedical and pharmaceutical disciplines.

Dual-controlled release system of drugs for bone regeneration

Controlled release systems have been noted to allow drugs to enhance their ability for bone regeneration. To this end, various biomaterials have been used as the release carriers of drugs, such as low-molecular-weight drugs, growth factors, and others. The drugs are released from the release carriers in a controlled fashion to maintain their actions for a long time period. Most research has been focused on the controlled release of single drugs to demonstrate the therapeutic feasibility. Controlled release of two combined drugs, so-called dual release systems, are promising and important for tissue regeneration. This is because the tissue regeneration process of bone formation is generally achieved by multiple bioactive molecules, which are produced from cells by other molecules. If two types of bioactive molecules, (i.e., drugs), are supplied in an appropriate fashion, the regeneration process of living bodies will be efficiently promoted. This review focuses on the bone regeneration induced by dual-controlled release of drugs. In this paper, various dual-controlled release systems of drugs aiming at bone regeneration are overviewed explaining the type of drugs and their release materials.

Targeted delivery as key for the success of small osteoinductive molecules

Molecules such as growth factors, peptides and small molecules can guide cellular behavior and are thus important for tissue engineering. They are rapidly emerging as promising compounds for the regeneration of tissues of the musculoskeletal system. Growth factors have disadvantages such as high cost, short half-life, supraphysiological amounts needed, etc. Therefore, small molecules may be an alternative. These molecules have been discovered using high throughput screening. Small osteoinductive molecules exhibit several advantages over growth factors owing to their small sizes, such as high stability and non-immunogenicity. These molecules may stimulate directly signaling pathways that are important for osteogenesis. However, systemic application doesn't induce osteogenesis in most cases. Therefore, local administration is needed. This may be achieved by using a bone graft material providing additional osteoconductive properties. These graft materials can also act by themselves as a delivery matrix for targeted and local delivery. Furthermore, vascularization is necessary in the process of osteogenesis. Many of the small molecules are also capable of promoting vascularization of the tissue to be regenerated. Thus, in this review, special attention is given to molecules that are capable of inducing both angiogenesis and osteogenesis simultaneously. Finally, more recent preclinical and clinical uses in bone regeneration of those molecules are described, highlighting the needs for the clinical translation of these promising compounds.

Hydrogels That Allow and Facilitate Bone Repair, Remodeling, and Regeneration

Bone defects can originate from a variety of causes, including trauma, cancer, congenital deformity, and surgical reconstruction. Success of the current "gold standard" treatment (i.e., autologous bone grafts) is greatly influenced by insufficient or inappropriate bone stock. There is thus a critical need for the development of new, engineered materials for bone repair. This review describes the use of natural and synthetic hydrogels as scaffolds for bone tissue engineering. We discuss many of the advantages that hydrogels offer as bone repair materials, including their potential for osteoconductivity, biodegradability, controlled growth factor release, and cell encapsulation. We also discuss the use of hydrogels in composite devices with metals, ceramics, or polymers. These composites are useful because of the low mechanical moduli of hydrogels. Finally, the potential for thermosetting and photo-cross-linked hydrogels as three-dimensionally (3D) printed, patient-specific devices is highlighted. Three-dimensional printing enables controlled spatial distribution of scaffold materials, cells, and growth factors. Hydrogels, especially natural hydrogels present in bone matrix, have great potential to augment existing bone tissue engineering devices for the treatment of critical size bone defects.

Small molecules and their controlled release that induce the osteogenic/chondrogenic commitment of stem cells

Stem cell-based tissue engineering plays a significant role in skeletal system repair and regenerative therapies. However, stem cells must be differentiated into specific mature cells prior to implantation (direct implantation may lead to tumour formation). Natural or chemically synthesised small molecules provide an efficient, accurate, reversible, and cost-effective way to differentiate stem cells compared with bioactive growth factors and gene-related methods. Thus, investigating the influences of small molecules on the differentiation of stem cells is of great significance. Here, we review a series of small molecules that can induce or/and promote the osteogenic/chondrogenic commitment of stem cells. The controlled release of these small molecules from various vehicles for stem cell-based therapies and tissue engineering applications is also discussed. The extensive studies in this field represent significant contributions to stem cell-based tissue engineering research and regenerative medicine.

Biomimetic approaches in bone tissue engineering: Integrating biological and physicomechanical strategies

The development of responsive biomaterials capable of demonstrating modulated function in response to dynamic physiological and mechanical changes in vivo remains an important challenge in bone tissue engineering. To achieve long-term repair and good clinical outcomes, biologically responsive approaches that focus on repair and reconstitution of tissue structure and function through drug release, receptor recognition, environmental responsiveness and tuned biodegradability are required. Traditional orthopedic materials lack biomimicry, and mismatches in tissue morphology, or chemical and mechanical properties ultimately accelerate device failure. Multiple stimuli have been proposed as principal contributors or mediators of cell activity and bone tissue formation, including physical (substrate topography, stiffness, shear stress and electrical forces) and biochemical factors (growth factors, genes or proteins). However, optimal solutions to bone regeneration remain elusive. This review will focus on biological and physicomechanical considerations currently being explored in bone tissue engineering.

Skeletal tissue regeneration: where can hydrogels play a role?

The emerging field of tissue engineering reveals promising approaches for the repair and regeneration of skeletal tissues including the articular cartilage, bone, and the entire joint. Amongst the myriad of biomaterials available to support this strategy, hydrogels are highly tissue mimicking substitutes and thus of great potential for the regeneration of functional tissues. This review comprises an overview of the novel and most promising hydrogels for articular cartilage, osteochondral and bone defect repair. Chondro- and osteo-conductive and -instructive hydrogels are presented, highlighting successful combinations with inductive signals and cell sources. Moreover, advantages, drawbacks, and future perspectives of the role of hydrogels in skeletal regeneration are addressed, pointing out the current state of this rising approach.

Immobilizing hydroxycholesterol with apatite on titanium surfaces to induce ossification

BACKGROUND

Immobilizing bioactive molecules and osteoconductive apatite on titanium implants have investigated direct ossification. In this study, hydroxycholesterol (HC) was immobilized with apatite on titanium through simply adsorption or sandwich-like coating. Three kinds of hydroxycholesterol were chosen to induce ossification: 20α-hydroxycholesterol (20α- HC), 22(S)-hydroxycholesterol (22(S)-HC) and 25-hydroxycholesterol (25-HC).The effects of HC/apatite coating on ossification abilities were evaluated in vitro and in vivo.

RESULTS

At 6 d, adsorbed apatite/25-HC and apatite/22(S)-HC coating exhibited some cytotoxicity, while the cell viability of apatite/20α-HC coating was similar as apatite coating. Immobilizing HC with apatite significantly enhanced the ALP activities compared with apatite coating. There was no significant difference in ALP value between adsorbed apatite/HC coating and sandwich-like apatite/HC/apatite coating. When compared with apatite coating, the mineral deposition improved by adsorbed HC with apatite at higher concentration in vivo.

CONCLUSIONS

When compared with apatite coating, immobilizing HC with apatite coating induced the ossification in vitro and in vivo.