Release and bioactivity of bone morphogenetic protein-2 are affected by scaffold binding techniques in vitro and in vivo

Leveraging the Recent Advancements in GelMA Scaffolds for Bone Tissue Engineering: An Assessment of Challenges and Opportunities

The field of bone tissue engineering has seen significant advancements in recent years. Each year, over two million bone transplants are performed globally, and conventional treatments, such as bone grafts and metallic implants, have their limitations. Tissue engineering offers a new level of treatment, allowing for the creation of living tissue within a biomaterial framework. Recent advances in biomaterials have provided innovative approaches to rebuilding bone tissue function after damage. Among them, gelatin methacryloyl (GelMA) hydrogel is emerging as a promising biomaterial for supporting cell proliferation and tissue regeneration, and GelMA has exhibited exceptional physicochemical and biological properties, making it a viable option for clinical translation. Various methods and classes of additives have been used in the application of GelMA for bone regeneration, with the incorporation of nanofillers or other polymers enhancing its resilience and functional performance. Despite promising results, the fabrication of complex structures that mimic the bone architecture and the provision of balanced physical properties for both cell and vasculature growth and proper stiffness for load bearing remain as challenges. In terms of utilizing osteogenic additives, the priority should be on versatile components that promote angiogenesis and osteogenesis while reinforcing the structure for bone tissue engineering applications. This review focuses on recent efforts and advantages of GelMA-based composite biomaterials for bone tissue engineering, covering the literature from the last five years.

Carbon Nanomaterial-Based Hydrogels as Scaffolds in Tissue Engineering: A Comprehensive Review

Carbon-based nanomaterials (CBNs) are a category of nanomaterials with various systems based on combinations of sp2 and sp3 hybridized carbon bonds, morphologies, and functional groups. CBNs can exhibit distinguished properties such as high mechanical strength, chemical stability, high electrical conductivity, and biocompatibility. These desirable physicochemical properties have triggered their uses in many fields, including biomedical applications. In this review, we specifically focus on applying CBNs as scaffolds in tissue engineering, a therapeutic approach whereby CBNs can act for the regeneration or replacement of damaged tissue. Here, an overview of the structures and properties of different CBNs will first be provided. We will then discuss state-of-the-art advancements of CBNs and hydrogels as scaffolds for regenerating various types of human tissues. Finally, a perspective of future potentials and challenges in this field will be presented. Since this is a very rapidly growing field, we expect that this review will promote interdisciplinary efforts in developing effective tissue regeneration scaffolds for clinical applications.

Enhanced Osteogenic Activity and Bone Repair Ability of PLGA/MBG Scaffolds Doped with ZIF-8 Nanoparticles Loaded with BMP-2

Background

Tissue engineering scaffolds are porous and can be loaded with growth factors to promote osteogenesis and bone repair, which can solve the problem of clinical bone defects. The direct loading of growth factors on scaffolds is hindered by the disadvantages of low loading capacities, and uncontrollable burst release. Zeolitic imidazolate framework-8 (ZIF-8) has osteoinductive activity and drug-loading potential and can be loaded with growth factors to achieve sustained release. In this study, we aimed to establish a sustained release system of composite scaffolds loaded with growth factors to achieve the goal of slow controlled release and effective bone repair.

Methods

ZIF‑8 nanoparticles loaded with bone morphogenetic protein-2 (BMP-2) were incorporated into poly-(lactide-co-glycolide)/mesoporous bioactive glass (PLGA/MBG) porous scaffolds by a 3D-printing method. The surface morphology, chemical properties and BMP-2 release of the prepared scaffold were investigated. The osteoblast adhesion, proliferation, spreading, and osteogenic differentiation in vitro and the bone repair ability in vivo of the PLGA/MBG/ZIF-8/BMP-2 (PMZB) scaffold were evaluated, and compared with those of PLGA/MBG (PM) and PLGA/MBG/ZIF-8 (PMZ) scaffolds.

Results

The results showed that the PMZB scaffold exhibited a slow and continuous BMP-2 release pattern, enhanced osteoblast adhesion, proliferation, spreading and osteogenic differentiation in vitro, and promoted new bone formation and bone repair in vivo.

Conclusion

The PLGA/MBG/ZIF-8/BMP-2 porous scaffold could continuously and slowly release BMP-2, enhance osteogenic activity, and promote new bone formation and bone repair at bone defects. The PMZB scaffold can be used as a bone graft material to repair bone defect at non-weight-bearing sites.

Optimizing Delivery of Therapeutic Growth Factors for Bone and Cartilage Regeneration

Bone- and cartilage-related diseases, such as osteoporosis and osteoarthritis, affect millions of people worldwide, impairing their quality of life and increasing mortality. Osteoporosis significantly increases the bone fracture risk of the spine, hip, and wrist. For successful fracture treatment and to facilitate proper healing in the most complicated cases, one of the most promising methods is to deliver a therapeutic protein to accelerate bone regeneration. Similarly, in the setting of osteoarthritis, where degraded cartilage does not regenerate, therapeutic proteins hold great promise to promote new cartilage formation. For both osteoporosis and osteoarthritis treatments, targeted delivery of therapeutic growth factors, with the aid of hydrogels, to bone and cartilage is a key to advance the field of regenerative medicine. In this review article, we propose five important aspects of therapeutic growth factor delivery for bone and cartilage regeneration: (1) protection of protein growth factors from physical and enzymatic degradation, (2) targeted growth factor delivery, (3) controlling GF release kinetics, (4) long-term stability of regenerated tissues, and (5) osteoimmunomodulatory effects of therapeutic growth factors and carriers/scaffolds.

High Molecular Weight Poly(glutamic acid) to Improve BMP2-Induced Osteogenic Differentiation

FDA-approved bone morphogenetic protein 2 (BMP2) has serious side effects due to the super high dose requirement. Heparin is one of the most well-studied sulfated polymers to stabilize BMP2 and improve its functionality. However, the clinical use of heparin is questionable because of its undesired anticoagulant activity. Recent studies suggest that poly(glutamic acid) (pGlu) has the potential to improve BMP2 bioactivity with less safety concerns; however, the knowledge on pGlu's contribution remains largely unknown. Therefore, we aimed to study the role of pGlu in BMP2-induced osteogenesis and its potential application in bone tissue engineering. Our data, for the first time, indicated that both low (L-pGlu) and high molecular weight pGlu (H-pGlu) were able to significantly improve the BMP2-induced early osteoblastic differentiation marker (ALP) in MC3T3-E1 preosteoblasts. Importantly, the matrix mineralization was more rapidly enhanced by H-pGlu compared to L-pGlu. Additionally, our data indicated that only α-H-pGlu could significantly improve BMP2's activity, whereas γ-H-pGlu failed to do so. Moreover, both gene expression and mineralization data demonstrated that α-H-pGlu enabled a single dose of BMP2 to induce a high level of osteoblastic differentiation without multiple doses of BMP2. To study the potential application of pGlu in tissue engineering, we incorporated the H-pGlu+BMP2 nanocomplexes into the collagen hydrogel with significantly elevated osteoblastic differentiation. Furthermore, H-pGlu-coated 3D porous gelatin and chitosan scaffolds significantly enhanced osteogenic differentiation through enabling sustained release of BMP2. Thus, our findings suggest that H-pGlu is a promising new alternative with great potential for bone tissue engineering applications.

Anti-infective composite cryogel scaffold treats osteomyelitis and augments bone healing in rat femoral condyle

Bone and joint infections pose a serious challenge in the orthopedic medical condition which presents a major health care problem and economic burden to the patients. The current treatment strategies adopted have a very limited successful outcome in majority of the cases and need serious reconsiderations in terms of management, diagnosis and effective treatment approach. Herein, we have developed a composite cryogel scaffold from nanohydroxyapatite and collagen mimicking natural bone composition for the local delivery of antibiotic to treat osteomyelitis. The biomimetic and biodegradable antibiotic-loaded composite scaffold was found to be biocompatible with potent osteogenic capacity and anti-infective characteristics under in vitro conditions. Moreover, the anti-infective potency of the antibiotic-loaded composite cryogel was also evaluated in rat osteomyelitis model to cure the infection and promote bone healing. It was observed that anti-infective collagen-nanohydroxyapatite composite cryogel when loaded with bone morphogenetic protein-2 (BMP-2) and zoledronic acid (ZA) could completely eradicate the infection in rat femoral condyle and simultaneously, accelerate bone healing at the dead space created during surgical procedures. The approach developed in this study is the development of biomimetic and bioactive composite carrier of antibiotics for the treatment of bone infection. The findings of this study insinuate that this antibiotic-loaded composite cryogel scaffold could potentially be used as an anti-infective biomaterial for the treatment of bone infections which will simultaneosuly promote bone healing at the dead space created during surgical procedures.

Effects of Substitution Ratios of Zinc-Substituted Hydroxyapatite on Adsorption and Desorption Behaviors of Bone Morphogenetic Protein-2

Understanding interactions between bone morphogenetic proteins (BMPs) and biomaterials is of great significance in preserving the structure and bioactivity of BMPs when utilized in clinical applications. Currently, bone morphogenetic protein-2 (BMP-2) is one of the most important growth factors in bone tissue engineering; however, atomistic interactions between BMP-2 and zinc-substituted hydroxyapatite (Zn-HAP, commonly used in artificial bone implants) have not been well clarified until now. Thus, in this work, the interaction energies, binding/debinding states, and molecular structures of BMP-2 upon a series of Zn-HAP surfaces (Zn-HAPs, 1 at%, 2.5 at%, 5 at%, and 10 at% substitution) were investigated by hybrid molecular dynamics (MD) and steered molecular dynamics (SMD) simulations. Meanwhile, cellular studies including alkaline phosphatase (ALP) activity and reverse transcription-polymerase chain reaction (RT-PCR) assay were performed to verify the theoretical modeling findings. It was found that, compared to pure HAP, Zn-HAPs exhibited a higher binding affinity of BMP-2 at the adsorption process; meanwhile, the detachment of BMP-2 upon Zn-HAPs was more difficult at the desorption process. In addition, molecular structures of BMP-2 could be well stabilized upon Zn-HAPs, especially for Zn10-HAP (with a 10 at% substitution), which showed both the higher stability of cystine-knots and less change in the secondary structures of BMP-2 than those upon HAP. Cellular studies confirmed that higher ALP activity and osteogenic marker gene expression were achieved upon BMP-2/Zn-HAPs than those upon BMP-2/HAP. These findings verified that Zn-HAPs favor the adsorption of BMP-2 and leverage the bioactivity of BMP-2. Together, this work clarified the interaction mechanisms between BMP-2 and Zn-HAPs at the atom level, which could provide new molecular-level insights into the design of BMP-2-loaded biomaterials for bone tissue engineering.

Immune-instructive copolymer scaffolds using plant-derived nanoparticles to promote bone regeneration

Background

Age-driven immune signals cause a state of chronic low-grade inflammation and in consequence affect bone healing and cause challenges for clinicians when repairing critical-sized bone defects in elderly patients.

Methods

Poly(l-lactide-co-ɛ-caprolactone) (PLCA) scaffolds are functionalized with plant-derived nanoparticles from potato, rhamnogalacturonan-I (RG-I), to investigate their ability to modulate inflammation in vitro in neutrophils and macrophages at gene and protein levels. The scaffolds’ early and late host response at gene, protein and histological levels is tested in vivo in a subcutaneous rat model and their potential to promote bone regeneration in an aged rodent was tested in a critical-sized calvaria bone defect. Significant differences were tested using one-way ANOVA, followed by a multiple-comparison Tukey’s test with a p value ≤ 0.05 considered significant.

Results

Gene expressions revealed PLCA scaffold functionalized with plant-derived RG-I with a relatively higher amount of galactose than arabinose (potato dearabinated (PA)) to reduce the inflammatory state stimulated by bacterial LPS in neutrophils and macrophages in vitro. LPS-stimulated neutrophils show a significantly decreased intracellular accumulation of galectin-3 in the presence of PA functionalization compared to Control (unmodified PLCA scaffolds). The in vivo gene and protein expressions revealed comparable results to in vitro. The host response is modulated towards anti-inflammatory/ healing at early and late time points at gene and protein levels. A reduced foreign body reaction and fibrous capsule formation is observed when PLCA scaffolds functionalized with PA were implanted in vivo subcutaneously. PLCA scaffolds functionalized with PA modulated the cytokine and chemokine expressions in vivo during early and late inflammatory phases. PLCA scaffolds functionalized with PA implanted in calvaria defects of aged rats downregulating pro-inflammatory gene markers while promoting osteogenic markers after 2 weeks in vivo.

Conclusion

We have shown that PLCA scaffolds functionalized with plant-derived RG-I with a relatively higher amount of galactose play a role in the modulation of inflammatory responses both in vitro and in vivo subcutaneously and promote the initiation of bone formation in a critical-sized bone defect of an aged rodent. Our study addresses the increasing demand in bone tissue engineering for immunomodulatory 3D scaffolds that promote osteogenesis and modulate immune responses.

Bone Healing Materials in the Treatment of Recalcitrant Nonunions and Bone Defects

The usual treatment for bone defects and recalcitrant nonunions is an autogenous bone graft. However, due to the limitations in obtaining autogenous bone grafts and the morbidity associated with their procurement, various bone healing materials have been developed in recent years. The three main treatment strategies for bone defects and recalcitrant nonunions are synthetic bone graft substitutes (BGS), BGS combined with bioactive molecules, and BGS and stem cells (cell-based constructs). Regarding BGS, numerous biomaterials have been developed to prepare bone tissue engineering scaffolds, including biometals (titanium, iron, magnesium, zinc), bioceramics (hydroxyapatite (HA)), tricalcium phosphate (TCP), biopolymers (collagen, polylactic acid (PLA), polycaprolactone (PCL)), and biocomposites (HA/MONs@miR-34a composite coating, Bioglass (BG)-based ABVF-BG (antibiotic-releasing bone void filling) putty). Bone tissue engineering scaffolds are temporary implants that promote tissue ingrowth and new bone regeneration. They have been developed to improve bone healing through appropriate designs in terms of geometric, mechanical, and biological performance. Concerning BGS combined with bioactive molecules, one of the most potent osteoinductive growth factors is bone morphogenetic proteins (BMPs). In recent years, several natural (collagen, fibrin, chitosan, hyaluronic acid, gelatin, and alginate) and synthetic polymers (polylactic acid, polyglycolic acid, polylactic-coglycolide, poly(e-caprolactone) (PCL), poly-p-dioxanone, and copolymers consisting of glycolide/trimethylene carbonate) have been investigated as potential support materials for bone tissue engineering. Regarding BGS and stem cells (cell-based constructs), the main strategies are bone marrow stromal cells, adipose-derived mesenchymal cells, periosteum-derived stem cells, and 3D bioprinting of hydrogels and cells or bioactive molecules. Currently, significant research is being performed on the biological treatment of recalcitrant nonunions and bone defects, although its use is still far from being generalized. Further research is needed to investigate the efficacy of biological treatments to solve recalcitrant nonunions and bone defects.

The influence of differently functionalized nanodiamonds on proliferation, apoptosis and EMT/MET phenomena in 2D and 3D tumor cell cultures

Nanodiamonds (ND) have been suggested to have several potential uses in biomedicine, since they are seemingly biocompatible. However, data about the biological effects of ND in physiological conditions are scarce. In this study, we observed that prostate cancer cells (LNCaP) and breast cancer cells (MDA-MB-231 and MCF-7) cultured with ND show morphological changes and altered gene and protein expression. In 2D we could detect only slight effects of ND on cell growth and apoptosis induction. Therefore, we applied different functionalized ND in a novel 3D cell culture model that reflects better tissue conditions compared to conventional 2D cell cultures. In 3D proliferation was reduced by all nanoparticles and benzoquinone functionalized ND induced cell death. As the used decellularized scaffold maintains the tissue architecture, we could also functionally investigate if nanoparticles induce cell migration into deeper layers and if they display markers of Mesenchymal Epithelial Transition (MET). We detected in more mesenchymal and invasive growing MDA-MB-231 cells less vimentin and increased levels of pan-cytokeratin expression after ND treatment, which indicates a MET induction. Our observations suggest that the presence of ND stimulates MET, with varying degrees of transition. The observation that ND do not support the opposite, EMT, is beneficial, since EMT is known to play a major role in tumor metastasis. However, a special focus should be placed on the characterization of biological effects to be able to guarantee the safety of ND in clinical use.

Osteoinductive Moldable and Curable Bone Substitutes Based on Collagen, BMP-2 and Highly Porous Polylactide Granules, or a Mix of HAP/β-TCP

In dentistry, maxillofacial surgery, traumatology, and orthopedics, there is a need to use osteoplastic materials that have not only osteoinductive and osteoconductive properties but are also convenient for use. In the study, compositions based on collagen hydrogel were developed. Polylactide granules (PLA) or a traditional bone graft, a mixture of hydroxyapatite and β-tricalcium phosphate (HAP/β-TCP), were used for gel filling to improve mechanical osteoconductive properties of compositions. The mechanical tests showed that collagen hydrogels filled with 12 wt% highly porous PLA granules (elastic modulus 373 ± 55 kPa) or 35 wt% HAP/β-TCP granules (elastic modulus 451 ± 32 kPa) had optimal manipulative properties. All composite components were cytocompatible. The cell’s viability was above 90%, and the components’ structure facilitated the cell’s surface adhesion. The bone morphogenetic protein-2 (BMP-2) provided osteoinductive composition properties. It was impregnated directly into the collagen hydrogel with the addition of fibronectin or inside porous PLA granules. The implantation of a collagen hydrogel with BMP-2 and PLA granules into a critical-size calvarial defect in rats led to the formation of the most significant volume of bone tissue: 61 ± 15%. It was almost 2.5 times more than in the groups where a collagen-fibronectin hydrogel with a mixture of HAP/β-TCP (25 ± 7%) or a fibronectin-free composition with porous PLA granules impregnated with BMP-2 (23 ± 8%) were used. Subcutaneous implantation of the compositions also showed their high biocompatibility and osteogenic potential in the absence of a bone environment. Thus, the collagen-fibronectin hydrogel with BMP-2 and PLA granules has optimal biocompatibility, osteogenic, and manipulative properties.

Strategies for inclusion of growth factors into 3D printed bone grafts

There remains a critical need to develop new technologies and materials that can meet the demands of treating large bone defects. The advancement of 3-dimensional (3D) printing technologies has allowed the creation of personalized and customized bone grafts, with specific control in both macro- and micro-architecture, and desired mechanical properties. Nevertheless, the biomaterials used for the production of these bone grafts often possess poor biological properties. The incorporation of growth factors (GFs), which are the natural orchestrators of the physiological healing process, into 3D printed bone grafts, represents a promising strategy to achieve the bioactivity required to enhance bone regeneration. In this review, the possible strategies used to incorporate GFs to 3D printed constructs are presented with a specific focus on bone regeneration. In particular, the strengths and limitations of different methods, such as physical and chemical cross-linking, which are currently used to incorporate GFs to the engineered constructs are critically reviewed. Different strategies used to present one or more GFs to achieve simultaneous angiogenesis and vasculogenesis for enhanced bone regeneration are also covered in this review. In addition, the possibility of combining several manufacturing approaches to fabricate hybrid constructs, which better mimic the complexity of biological niches, is presented. Finally, the clinical relevance of these approaches and the future steps that should be taken are discussed.

Functionalization of Synthetic Bone Substitutes

Bone substitutes have been applied to treat osseous defects for a long time. To prevent implant related infection (IRI) and enhance bone healing functionalized biomaterials, antibiotics and osteoinductive substances have been introduced. This study gives an overview of the current available surface-coated bone substitutes and provides an outlook for future perspectives.

Advances in Growth Factor Delivery for Bone Tissue Engineering

Shortcomings related to the treatment of bone diseases and consequent tissue regeneration such as transplants have been addressed to some extent by tissue engineering and regenerative medicine. Tissue engineering has promoted structures that can simulate the extracellular matrix and are capable of guiding natural bone repair using signaling molecules to promote osteoinduction and angiogenesis essential in the formation of new bone tissues. Although recent studies on developing novel growth factor delivery systems for bone repair have attracted great attention, taking into account the complexity of the extracellular matrix, scaffolding and growth factors should not be explored independently. Consequently, systems that combine both concepts have great potential to promote the effectiveness of bone regeneration methods. In this review, recent developments in bone regeneration that simultaneously consider scaffolding and growth factors are covered in detail. The main emphasis in this overview is on delivery strategies that employ polymer-based scaffolds for spatiotemporal-controlled delivery of both single and multiple growth factors in bone-regeneration approaches. From clinical applications to creating alternative structural materials, bone tissue engineering has been advancing constantly, and it is relevant to regularly update related topics.

Sequential dual-drug delivery of BMP-2 and alendronate from hydroxyapatite-collagen scaffolds for enhanced bone regeneration

The clinical use of bioactive molecules in bone regeneration has been known to have side effects, which result from uncontrolled and supraphysiological doses. In this study, we demonstrated the synergistic effect of two bioactive molecules, bone morphogenic protein-2 (BMP-2) and alendronate (ALN), by releasing them in a sequential manner. Collagen-hydroxyapatite composite scaffolds functionalized using BMP-2 are loaded with biodegradable microspheres where ALN is encapsulated. The results indicate an initial release of BMP-2 for a few days, followed by the sequential release of ALN after two weeks. The composite scaffolds significantly increase osteogenic activity owing to the synergistic effect of BMP-2 and ALN. Enhanced bone regeneration was identified at eight weeks post-implantation in the rat 8-mm critical-sized defect. Our findings suggest that the sequential delivery of BMP-2 and ALN from the scaffolds results in a synergistic effect on bone regeneration, which is unprecedented. Therefore, such a system exhibits potential for the application of cell-free tissue engineering.

The Body's Cellular and Molecular Response to Protein-Coated Medical Device Implants: A Review Focused on Fibronectin and BMP Proteins

Recent years have seen a marked rise in implantation into the body of a great variety of devices: hip, knee, and shoulder replacements, pacemakers, meshes, glucose sensors, and many others. Cochlear and retinal implants are being developed to restore hearing and sight. After surgery to implant a device, adjacent cells interact with the implant and release molecular signals that result in attraction, infiltration of the tissue, and attachment to the implant of various cell types including monocytes, macrophages, and platelets. These cells release additional signaling molecules (chemokines and cytokines) that recruit tissue repair cells to the device site. Some implants fail and require additional revision surgery that is traumatic for the patient and expensive for the payer. This review examines the literature for evidence to support the possibility that fibronectins and BMPs could be coated on the implants as part of the manufacturing process so that the proteins could be released into the tissue surrounding the implant and improve the rate of successful implantation.

Response Surface Methodology for Statistical Optimization of Chitosan/Alginate Nanoparticles as a Vehicle for Recombinant Human Bone Morphogenetic Protein-2 Delivery

Purpose

In this study, chitosan/alginate nanoparticles are prospected as a carrier for controlled release of recombinant human bone morphogenetic protein-2 (rhBMP-2).

Materials and Methods

The rhBMP-2-loaded chitosan/alginate nanoparticles (Cs/Alg/B NPs) were prepared using the ionic gelation (IG) method. The current research was conducted to optimize the effective factors for entrapping rhBMP-2 in Cs/Alg NPs using response surface methodology (RSM) and the Box–Behnken design (BBD). The variables were the Cs/Alg molecular weight (Mw) ratios (1–3), pH (4.8–5.5), stirring rates (900–1300 rpm) and the responses included size, ζ-potential, polydispersity index (PDI), loading efficacy (LE), cumulative release (CR), and morphological degradation time (MDE). Then, the morphological properties of optimum formulation were studied for post-characterization. In the next step, the MTT assay for the optimized run was done for 24 and 48 hours.

Results

The results revealed that the optimum conditions for the mentioned variables were stirring rate=1100 rpm, pH=5.15, and Cs/Alg Mw ratio=1.75 based on numerical optimization. It was shown that the average particle size and loading efficacy at optimum conditions were 253 nm and 67%, respectively. Other responses were as follows: CR=66%, ζ-potential=+35mV, PDI=0.5, and MDT=7 days.

Conclusion

The results have suggested that the statistical optimization of rhBMP-2 offers the possibility of preparing Cs/Alg/B NPs with a favorable size, controlled release characteristics, and high loading efficiency. It is expected that the acquired optimum conditions will be useful for efficient rhBMP-2 delivery.

Bone morphogenetic protein-2 delivery from polyelectrolyte multilayers enhances osteogenic activity on nanostructured titania

Incomplete osseointegration is primary cause of failure for orthopedic implants. New biomaterials that present stable signals promoting osteogenesis could reduce failure rates of orthopedic implants. In this study bone morphogenetic protein-2 (BMP-2) was delivered from titania nanotubes (Nt) modified with chitosan/heparin polyelectrolyte multilayers (PEMs). The surfaces were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. BMP-2 release from the surfaces was measured in vitro for up to 28 days. After an initial burst release of BMP-2 during the first 2 days, most of the BMP-2 remained on the surface. To determine the osteogenic properties of these surfaces, they were seeded with rat bone marrow cells; alkaline phosphatase (ALP) activity, total protein, calcium deposition, and osteocalcin were measured up to 4 weeks in vitro. When compared to Nt surfaces, the surfaces with BMP-2 induce greater osteocalcin and calcium deposition. PEMs provide sustained presentation of BMP-2, from a biomimetic surface. This enhances the osteogenic properties of the surface without requiring supraphysiologic growth factor dose. This growth factor delivery strategy could be used to improve bone healing outcomes and reduce complications for recipients of orthopedic implants.

Concurrent multi-lineage differentiation of mesenchymal stem cells through spatial presentation of growth factors

Severe tendon and ligament injuries are estimated to affect between 300 000 and 400 000 people annually. Surgical repairs of these injuries often have poor long-term clinical outcomes because of resection of the interfacial tissue-the enthesis-and subsequent stress concentration at the attachment site. A healthy enthesis consists of distinct regions of bone, fibrocartilage, and tendon, each with distinct cell types, extracellular matrix components, and architecture, which are important for tissue function. Tissue engineering, which has been proposed as a potential strategy for replacing this tissue, is currently limited by its inability to differentiate multiple lineages of cells from a single stem cell population within a single engineered construct. In this study, we develop a multi-phasic gelatin methacrylate hydrogel construct system for spatial presentation of proteins, which is then validated for multi-lineage differentiation towards the cell types of the bone-tendon enthesis. This study determines growth factor concentrations for differentiation of mesenchymal stem cells towards osteoblasts, chondrocytes/fibrochondrocytes, and tenocytes, which maintain similar differentiation profiles in 3D hydrogel culture as assessed by qPCR and immunofluorescence staining. Finally, it is shown that this method is able to guide heterogeneous and spatially confined changes in mesenchymal stem cell genes and protein expressions with the tendency to result in osteoblast-, fibrochondrocyte-, and tenocyte-like expression profiles. Overall, we demonstrate the utility of the culture technique for engineering other musculoskeletal tissue interfaces and provide a biochemical approach for recapitulating the bone-tendon enthesis in vitro.

Bio-inspired polymeric iron-doped hydroxyapatite microspheres as a tunable carrier of rhBMP-2

Hybrid superparamagnetic microspheres with bone-like composition, previously developed by a bio-inspired assembling/mineralization process, are evaluated for their ability to uptake and deliver recombinant human bone morphogenetic protein-2 (rhBMP-2) in therapeutically-relevant doses along with prolonged release profiles. The comparison with hybrid non-magnetic and with non-mineralized microspheres highlights the role of nanocrystalline, nanosize mineral phases when they exhibit surface charged groups enabling the chemical linking with the growth factor and thus moderating the release kinetics. All the microspheres show excellent osteogenic ability with human mesenchymal stem cells whereas the hybrid mineralized ones show a slow and sustained release of rhBMP-2 along 14 days of soaking into cell culture medium with substantially bioactive effect, as reported by assay with C2C12 BRE-Luc cell line. It is also shown that the release extent can be modulated by the application of pulsed electromagnetic field, thus showing the potential of remote controlling the bioactivity of the new micro-devices which is promising for future application of hybrid biomimetic microspheres in precisely designed and personalized therapies.

In vitro characterization of osteoblast cells on polyelectrolyte multilayers containing detonation nanodiamonds

Nanoparticle-enhanced coatings of bone implants are a promising method to facilitate sustainable wound healing, leading to an increase in patient well-being. This article describes the in vitro characterization of osteoblast cells interacting with polyelectrolyte multilayers, which contain detonation nanodiamonds (NDs), as a novel class of carbon-based coating material, which presents a unique combination of photoluminescence and drug-binding properties. The cationic polyelectrolyte, namely polydiallyldimethylammonium chloride (PDDA), has been used to immobilize NDs on silica glass. The height of ND-PDDA multilayers varies from a minimum of 10 nm for one bilayer to a maximum of 90 nm for five bilayers of NDs and PDDA. Human fetal osteoblasts (hFOBs) cultured on ND-PDDA multilayers show a large number of focal adhesions, which were studied via quantitative fluorescence imaging analysis. The influence of the surface roughness on the filopodia formation was assessed via scanning electron microscopy and atomic force microscopy. The nano-rough surface of five bilayers constrained the filopodia formation. The hFOBs grown on NDs tend to show not only a similar cell morphology compared to cells cultured on extracellular matrix protein-coated silica glass substrates, but also increased cell viability by about 40%. The high biocompatibility of the ND-PDDA multilayers, indicated via high cell proliferation and sound cell adhesion, shows their potential for biomedical applications such as drug-eluting coatings and biomaterials in general.

Chitosan/Polycyclodextrin (CHT/PCD)-Based Sponges Delivering VEGF to Enhance Angiogenesis for Bone Regeneration

Vascularization is one of the main challenges in bone tissue engineering (BTE). In this study, vascular endothelial growth factor (VEGF), known for its angiogenic effect, was delivered by our developed sponge, derived from a polyelectrolyte complexes hydrogel between chitosan (CHT) and anionic cyclodextrin polymer (PCD). This sponge, as a scaffold for growth factor delivery, was formed by freeze-drying a homogeneous CHT/PCD hydrogel, and thereafter stabilized by a thermal treatment. Microstructure, water-uptake, biodegradation, mechanical properties, and cytocompatibility of sponges were assessed. VEGF-delivery following incubation in medium was then evaluated by monitoring the VEGF-release profile and its bioactivity. CHT/PCD sponge showed a porous (open porosity of 87.5%) interconnected microstructure with pores of different sizes (an average pore size of 153 μm), a slow biodegradation (12% till 21 days), a high water-uptake capacity (~600% in 2 h), an elastic property under compression (elastic modulus of compression 256 ± 4 kPa), and a good cytocompatibility in contact with osteoblast and endothelial cells. The kinetic release of VEGF was found to exert a pro-proliferation and a pro-migration effect on endothelial cells, which are two important processes during scaffold vascularization. Hence, CHT/PCD sponges were promising vehicles for the delivery of growth factors in BTE.

Tissue engineering applications in otolaryngology-The state of translation

While tissue engineering holds significant potential to address current limitations in reconstructive surgery of the head and neck, few constructs have made their way into routine clinical use. In this review, we aim to appraise the state of head and neck tissue engineering over the past five years, with a specific focus on otologic, nasal, craniofacial bone, and laryngotracheal applications. A comprehensive scoping search of the PubMed database was performed and over 2000 article hits were returned with 290 articles included in the final review. These publications have addressed the hallmark characteristics of tissue engineering (cellular source, scaffold, and growth signaling) for head and neck anatomical sites. While there have been promising reports of effective tissue engineered interventions in small groups of human patients, the majority of research remains constrained to in vitro and in vivo studies aimed at furthering the understanding of the biological processes involved in tissue engineering. Further, differences in functional and cosmetic properties of the ear, nose, airway, and craniofacial bone affect the emphasis of investigation at each site. While otolaryngologists currently play a role in tissue engineering translational research, continued multidisciplinary efforts will likely be required to push the state of translation towards tissue-engineered constructs available for routine clinical use.

LEVEL OF EVIDENCE

NA.

Electrospun, synthetic bone void filler promotes human MSC function and BMP-2 mediated spinal fusion

INTRODUCTION

Synthetic bone grafts are often used to achieve a well-consolidated fusion mass in spinal fusion procedures. These bone grafts function as scaffolds, and ideally support cell function and facilitate protein binding.

OBJECTIVE

The aim was to characterize an electrospun, synthetic bone void filler (Reb) for its bone morphogenetic protein (BMP)-2 release properties and support of human mesenchymal stem cell (hMSC) function in vitro, and its efficacy in promoting BMP-2-/bone marrow aspirate-(BMA)-mediated posterolateral spinal fusion (PLF) in vivo.

METHODS

BMP-2 release kinetics from Reb versus standard absorbable collagen sponge (ACS) was determined. hMSC adhesion and proliferation on Reb was tested using cell counting, fluorescence microscopy and MTS. Cell osteogenic differentiation was quantified via cellular alkaline phosphatase (ALP) activity. For in vivo analysis, 18 Lewis rats were treated during PLF surgery with the following groups: (I) Reb + BMA, (II) Reb + BMA + BMP-2 and (III) BMA. A safe, minimally effective dose of BMP-2 was used. Fusion consolidation was followed for 3 months using radiography and micro-CT. After sacrifice, fusion rate and biomechanical stiffness was determined using manual palpation, biomechanical tests and histology.

RESULTS

In vitro, BMP-2 release kinetics were similar between Reb versus ACS. MSC proliferation and differentiation were increased in the presence of Reb. At 3 months post-surgery, fusion rates were 29% (group I), 100% (group II), and 0% (group III). Biomechanical stiffness was higher in group II versus I. Micro-CT showed an increased bone volume and connectivity density in group II. Trabecular thickness was increased in group I versus II. H&E staining showed newly formed bone in group II only.

CONCLUSIONS

Reb possesses a high protein binding affinity and promotes hMSC function. Combination with BMA and minimal dose BMP-2 allowed for 100% bone fusion in vivo. This data suggests that a minimally effective dose of BMP-2 can be used when combined with Reb.

A multifaceted biomimetic interface to improve the longevity of orthopedic implants

The rise of additive manufacturing has provided a paradigm shift in the fabrication of precise, patient-specific implants that replicate the physical properties of native bone. However, eliciting an optimal biological response from such materials for rapid bone integration remains a challenge. Here we propose for the first time a one-step ion-assisted plasma polymerization process to create bio-functional 3D printed titanium (Ti) implants that offer rapid bone integration. Using selective laser melting, porous Ti implants with enhanced bone-mimicking mechanical properties were fabricated. The implants were functionalized uniformly with a highly reactive, radical-rich polymeric coating generated using a unique combination of plasma polymerization and plasma immersion ion implantation. We demonstrated the performance of such activated Ti implants with a focus on the coating's homogeneity, stability, and biological functionality. It was shown that the optimized coating was highly robust and possessed superb physico-chemical stability in a corrosive physiological solution. The plasma activated coating was cytocompatible and non-immunogenic; and through its high reactivity, it allowed for easy, one-step covalent immobilization of functional biomolecules in the absence of solvents or chemicals. The activated Ti implants bio-functionalized with bone morphogenetic protein 2 (BMP-2) showed a reduced protein desorption and a more sustained osteoblast response both in vitro and in vivo compared to implants modified through conventional physisorption of BMP-2. The versatile new approach presented here will enable the development of bio-functionalized additively manufactured implants that are patient-specific and offer improved integration with host tissue. STATEMENT OF SIGNIFICANCE: Additive manufacturing has revolutionized the fabrication of patient-specific orthopedic implants. Although such 3D printed implants can show desirable mechanical and mass transport properties, they often require surface bio-functionalities to enable control over the biological response. Surface covalent immobilization of bioactive molecules is a viable approach to achieve this. Here we report the development of additively manufactured titanium implants that precisely replicate the physical properties of native bone and are bio-functionalized in a simple, reagent-free step. Our results show that covalent attachment of bone-related growth factors through ion-assisted plasma polymerized interlayers circumvents their desorption in physiological solution and significantly improves the bone induction by the implants both in vitro and in vivo.

Burst, Short, and Sustained Vitamin D3 Applications Differentially Affect Osteogenic Differentiation of Human Adipose Stem Cells

Incorporation of 1,25(OH)₂ vitamin D₃ (vitD₃) into tissue-engineered scaffolds could aid the healing of critical-sized bone defects. We hypothesize that shorter applications of vitD₃ lead to more osteogenic differentiation of mesenchymal stem cells (MSCs) than a sustained application. To test this, release from a scaffold was mimicked by exposing MSCs to exactly controlled vitD₃ regimens. Human adipose stem cells (hASCs) were seeded onto calcium phosphate particles, cultured for 20 days, and treated with 124 ng vitD₃, either provided during 30 min before seeding ([200 nM]), during the first two days ([100 nM]), or during 20 days ([10 nM]). Alternatively, hASCs were treated for two days with 6.2 ng vitD₃ ([10 nM]). hASCs attached to the calcium phosphate particles and were viable (~75%). Cell number was not affected by the various vitD₃ applications. VitD₃ (124 ng) applied over 20 days increased cellular alkaline phosphatase activity at Days 7 and 20, reduced expression of the early osteogenic marker RUNX2 at Day 20, and strongly upregulated expression of the vitD₃ inactivating enzyme CYP24. VitD₃ (124 ng) also reduced RUNX2 and increased CYP24 applied at [100 nM] for two days, but not at [200 nM] for 30 min. These results show that 20-day application of vitD₃ has more effect on hASCs than the same total amount applied in a shorter time span.

The Stability Maintenance of Protein Drugs in Organic Coatings Based on Nanogels

Protein drugs are often loaded on scaffolds with organic coatings to realize a spatiotemporal controlled release. The stability or activity of protein drugs, however, is largely affected by the organic coating, particularly with organic solvents, which can dramatically reduce their delivery efficiency and limit their application scope. In spite of this, little attention has been paid to maintaining the stability of protein drugs in organic coatings, to date. Here, we used catalase as a model protein drug to exploit a kind of chemically cross-linked nanogel that can efficiently encapsulate protein drugs. The polymeric shells of nanogels can maintain the surface hydration shell to endow them with a protein protection ability against organic solvents. Furthermore, the protection efficiency of nanogels is higher when the polymeric shell is more hydrophilic. In addition, nanogels can be dispersed in polylactic acid (PLA) solution and subsequently coated on scaffolds to load catalase with high activity. To the best of our knowledge, this is the first use of hydrophilic nanogels as a protection niche to load protein drugs on scaffolds through an organic coating, potentially inspiring researchers to exploit new methods for protein drug loading.

Human stem cell response to layered zirconium phosphate

This study aims to evaluate the in vitro cytocompatibility of layered zirconium phosphate (ZP) and its derivative material that was organically modified using glycerophosphate (ZGP). The ZP and ZGP particles were prepared via a reflux method in an aqueous solution containing phosphoric acid. The field emission scanning electron microscopy showed the prepared samples were fine particles with 70–100 nm diameter. X-ray diffraction and Raman spectrometry indicated the presence of a layered crystal structure. The interlayer distance of ZP was estimated to be 0.76 nm from the 002 diffraction. Modification of ZP with β-glycerophosphate, lead to expansion of the interlayer distance of 0.85 nm. Grazing incidence X-ray diffraction and Raman spectrometry showed that the crystal structures of ZP and ZGP were maintained even after the samples were coated onto polyethylene (PE) substrates via hot pressing. The water droplet contact angles on the PE substrates coated with the ZP and ZGP particles (ZP/PE and ZGP/PE) were 2 ∼ 6° lesser than that on the uncoated PE substrate. After human adipose-derived stem cells (hASCs) were cultured on the substrates, 2.5–3.5 times higher numbers of adhered cells were observed on the substrates coated with ZP and ZGP than on the uncoated PE substrates and 1.1–1.6 times higher than on the substrate coated with hydroxyapatite particles (HAp/PE). Increasing cell numbers were observed after culturing for 24 h, indicating that the ZP/PE and ZGP/PE showed low cytotoxicity to the hASCs. Furthermore, the ZP/PE showed the highest area of hASC adhesion among all the samples. These results highlight the possibility that layered zirconium phosphate and its organically modified substances can be applied to biomaterials for tissue repair.

This study aims to evaluate the in vitro cytocompatibility of layered zirconium phosphate (ZP) and its derivative material that was organically modified using glycerophosphate (ZGP).

A Flexible Method for Covalent Double Functionalization of Graphene Oxide

A method for the double functionalization of graphene oxide (GO) under mild alkaline conditions has been developed. Two functional groups were covalently linked to GO in two steps: the first group was attached by an epoxide ring-opening reaction and the second, bearing an amine function, was covalently conjugated to benzoquinone attached to the GO. The doubly functionalized GO was characterized by several techniques, confirming the sequential covalent modification of the GO surface with two different functional groups. This method is straightforward and the reaction conditions are mild, allowing preservation of the structure and properties of GO. This strategy could be exploited to prepare multifunctional GO conjugates with potential applications in many fields ranging from materials science to biomedicine.

Osterix-mCherry Expression Allows for Early Bone Detection in a Calvarial Defect Model

The process of new bone formation following trauma requires the temporal recruitment of cells to the site, including mesenchymal stem cells, preosteoblasts, and osteoblasts, the latter of which deposit minerals. Hence, bone repair, a process that is assessed by the extent of mineralization within the defect, can take months before it is possible to determine if a treatment is successful. Here, a fluorescently tagged Osterix, an early key gene in the bone formation cascade, is used as a predictive measure of bone formation. Using a calvarial defect model in mice, the ability to noninvasively track the Osterix transcription factor in an Osterix-mCherry mouse model is evaluated as a measure for bone formation following treatment with recombinant human Bone-Morphogenetic-Protein 2 (rhBMP-2). Two distinct delivery materials are utilized, an injectable nanocomposite hydrogel and a collagen sponge, that afford distinct release kinetics and it is found that cherry-fluorescent protein can be detected as early as 2 weeks following treatment. Osterix intensity correlates with subsequent bone formation and hence can serve as a rapid screening tool for osteogenic drugs or for the evaluation and optimization of delivery platforms.

BMP-2-Loaded HAp:Ln3+ (Ln = Yb, Er, Gd) Nanorods with Dual-Mode Imaging for Efficient MC3t3-E1 Cell Differentiation Regulation

Effective bone tissue reconstitution improves the treatment success rate of dental implantation and preserves natural teeth during periodontal tissue repair. Hydroxyapatite (HAp) has received much attention in bone remodeling field because its mineralized structure is similar to that of the natural bone tissue. For this reason, it has been used as a carrier for growth factors. Although HAp possesses outstanding biomedical properties, its capacity of loading and releasing bone growth factors and promoting osteogenesis is not well understood. In this study, Ln³⁺ (Ln = Yb³⁺, Er³⁺, Gd³⁺)-doped HAp (HAp:Ln³⁺) nanorods were synthesized by one-step hydrothermal method. To improve its biocompatibility and surface properties, bone morphogenetic protein-2 (BMP-2) was loaded onto the surface of HAp:Ln³⁺ nanorods. The results showed that BMP-2 incorporation promoted bone formation and enhanced the expression of early bone-related gene and protein (RunX2, SP7, OPN). In addition, Yb³⁺- and Er³⁺-doped HAp nanorods were examined by upconversion luminescence with 980 nm near-infrared laser irradiation to monitor the delivery position of BMP-2 protein. Furthermore, due to the positive magnetism correlated with the concentration of Gd³⁺, HAp:Ln³⁺ with enhanced contrast brightening can be deemed as T₁ MIR contrast agents. These findings indicate that HAp doped with rare-earth ions and loaded with BMP-2 has the potential to promote bone tissue repair and execute dual-mode imaging.

Novel non-resorbable polymeric-nanostructured scaffolds for guided bone regeneration

OBJECTIVE

The aim of this study was to evaluate the bone-regeneration efficiency of novel polymeric nanostructured membranes and the effect of zinc, calcium, titanium, and bone morpho-protein loading on membranes, through an in vivo rabbit model.

MATERIAL AND METHODS

Nanostructured membranes of methylmethacrylate were loaded with zinc, calcium, TiO₂ nanoparticles, and bone-morphogenetic protein (BMP). These membranes covered the bone defects prepared on the skulls of six rabbits. Animals were sacrificed 6 weeks after surgery. Micro computed tomography was used to evaluate bone architecture through BoneJ pluging and ImageJ script. Three histological processing of samples, including von Kossa silver nitrate, toluidine blue, and fluorescence by the deposition of calcein were utilized.

RESULTS

Zn-membranes (Zn-Ms) promoted the highest amount of new bone and higher bone perimeter than both unloaded and Ti-membranes (Ti-Ms). Ca-membranes (Ca-Ms) attained higher osteoid perimeter and bone perimeter than Zn-Ms. The skeleton analysis showed that Zn-Ms produced more branches and junctions at the trabecular bone than BMP-loaded membranes (BMP-Ms). Samples treated with Ti-Ms showed less bone formation and bony bridging processes. Both Zn-Ms and Ca-Ms achieved higher number of osteoblasts than the control group. BMP-Ms and Ca-Ms originated higher number of blood vessels than Ti-Ms and control group.

CONCLUSIONS

Zn incorporation in novel nanostructured membranes provided the highest regenerative efficiency for bone healing at the rabbit calvarial defects.

CLINICAL RELEVANCE

Zn-Ms promoted osteogenesis and enhanced biological activity, as mineralized and osteoid new bone with multiple interconnected ossified trabeculae appeared in close contact with the membrane.

The regulatory role of sulfated polysaccharides in facilitating rhBMP-2-induced osteogenesis

Sulfated polysaccharides have received much attention in recent years due to their special biological activities, especially the regulation of the biological activity of growth factors such as the representative inductive growth factor recombinant human bone morphogenetic protein-2 (rhBMP-2). However, the regulatory mechanisms from the aspect of the molecular chain structure have rarely been reported. In this article, we selected three kinds of sulfonates containing different backbone structures and functional groups, 2-N,6-O-sulfated chitosan (26 SCS), sulfated dextran (DSS) and poly(sodium-p-styrenesulfonate) (PSS), to explore the interaction between them and rhBMP-2. From in vivo and in vitro osteogenesis-related experiments, 26 SCS showed the best promoting effect on rhBMP-2 induced osteogenic differentiation and the sulfated amino group in 26 SCS could specifically bind to rhBMP-2. These findings indicated that the polysaccharide chain structure was a prerequisite for the synergy effect between 26 SCS and rhBMP-2; the effective combination of -SO₃^- and rhBMP-2 was an important factor in protecting the bioactivity of rhBMP-2. In addition, the presence of the sulfated amino group was the key factor in the specific binding between 26 SCS and rhBMP-2 and provided the possibility of capturing factors in vivo.

Efficacy of copolymer scaffolds delivering human demineralised dentine matrix for bone regeneration

Poly(L-lactide-co-ε-caprolactone) scaffolds were functionalised by 10 or 20 µg/mL of human demineralised dentine matrix. Release kinetics up to 21 days and their osteogenic potential on human bone marrow stromal cells after 7 and 21 days were studied. A total of 390 proteins were identified by mass spectrometry. Bone regeneration proteins showed initial burst of release. Human bone marrow stromal cells were cultured on scaffolds physisorbed with 20 µg/mL and cultured in basal medium (DDM group) or physisorbed and cultured in osteogenic medium or cultured on non-functionalised scaffolds in osteogenic medium. The human bone marrow stromal cells proliferated less in demineralised dentine matrix group and activated ERK/1/2 after both time points. Cells on DDM group showed highest expression of IL-6 and IL-8 at 7 days and expressed higher collagen type 1 alpha 2, SPP1 and bone morphogenetic protein-2 until 21 days. Extracellular protein revealed higher collagen type 1 and bone morphogenetic protein-2 at 21 days in demineralised dentine matrix group. Cells on DDM group showed signs of mineralisation. The functionalised scaffolds were able to stimulate osteogenic differentiation of human bone marrow stromal cells.

Applications of Nanodiamonds in the Detection and Therapy of Infectious Diseases

We are constantly exposed to infectious diseases, and they cause millions of deaths per year. The World Health Organization (WHO) estimates that antibiotic resistance could cause 10 million deaths per year by 2050. Multidrug-resistant bacteria are the cause of infection in at least one in three people suffering from septicemia. While antibiotics are powerful agents against infectious diseases, the alarming increase in antibiotic resistance is of great concern. Alternatives are desperately needed, and nanotechnology provides a great opportunity to develop novel approaches for the treatment of infectious diseases. One of the most important factors in the prognosis of an infection caused by an antibiotic resistant bacteria is an early and rigorous diagnosis, jointly with the use of novel therapeutic systems that can specifically target the pathogen and limit the selection of resistant strains. Nanodiamonds can be used as antimicrobial agents due to some of their properties including size, shape, and biocompatibility, which make them highly suitable for the development of efficient and tailored nanotherapies, including vaccines or drug delivery systems. In this review, we discuss the beneficial findings made in the nanodiamonds field, focusing on diagnosis and treatment of infectious diseases. We also highlight the innovative platform that nanodiamonds confer for vaccine improvement, drug delivery, and shuttle systems, as well as their role in the generation of faster and more sensitive clinical diagnosis.

Bone Morphogenetic Proteins (BMPs) and Bone Regeneration

Many research methods exist to elucidate the role of BMP-2 during bone regeneration. This chapter briefly reviews important animal models used in these studies and provides details on the rat femur defect model. This animal model is frequently utilized to measure the efficacy of osteogenic factors like BMP-2. Detailed information about delivery methods, dose range, and dose duration used in BMP-2-related studies are provided.

Incorporation of Stromal Cell-Derived Factor-1α in Three-Dimensional Hydroxyapatite/Polyacrylonitrile Composite Scaffolds for Bone Regeneration

Bone regeneration strategies rely on biomaterial constructs with stem cells or growth factors. By comparison, cell homing strategies employ chemokines to recruit the host endogenous stem or progenitor cells to the defect site to support endogenous healing. In the present study, we used a novel fluffy hydroxyapatite/polyacrylonitrile (HA/PAN) composite scaffold to provide a better three-dimensional cell culture microenvironment. These HA/PAN composite scaffolds loaded with stromal cell-derived factor-1α (SDF-1α) provided a diffusion-controlled SDF-1α release profile and endowed the scaffolds with cell homing capabilities. Furthermore, the scaffolds significantly stimulated bone marrow stromal cell (BMSC) recruitment, facilitated BMSC osteogenic differentiation, and promoted ectopic bone formation. Our results suggest that a HA/PAN composite scaffold loaded with SDF-1α offers a clinically beneficial bone repair strategy.

Porous nanofibrous scaffold incorporated with S1P loaded mesoporous silica nanoparticles and BMP-2 encapsulated PLGA microspheres for enhancing angiogenesis and osteogenesis

Repair of bone defects remains a major clinical challenge due to inadequate or abnormal vascularization in bone substitutes, which commonly leads to inferior bone formation or bone nonunion. Therefore, healing of bone defects requires the coordinated processes of angiogenesis and osteogenesis. In this study, sphingosine-1-phosphate (S1P) was initially loaded into mesoporous silica nanoparticles (MSNs) to form angiogenic microcarriers, which were subsequently embedded into porous nanofibrous poly-l-lactide (PLLA) scaffolds during a thermally induced phase separation (TIPS) process, while bone morphogenetic protein-2 (BMP-2) was encapsulated into poly(lactic-co-glycolic acid) (PLGA) microspheres to obtain osteogenic microcarriers, which were then integrated onto a MSNs/PLLA nanofibrous scaffold by a post seeding method. The osteogenic and angiogenic activities of the resulting dual-bioactive factor containing scaffolds were evaluated both in vitro and in vivo. The simulated drug release studies indicated that both bioactive factors will be released simultaneously and continuously from the fabricated composite scaffold. Moreover, the ectopic bone formation results showed that the sustained release of S1P and BMP-2 from the composite scaffold resulted in a synergistic effect on blood vessel formation and bone regeneration. Taken together, the results showed the promising application of the dual-bioactive factor loaded nanofibrous scaffold for enhanced bone regeneration.

Molecular dynamics simulations of adsorption and desorption of bone morphogenetic protein-2 on textured hydroxyapatite surfaces

Interactions between bone morphogenetic protein-2 (BMP-2) and biomaterial surfaces are of great significance in the fields of regenerative medicine and bone tissue engineering. In this work, the adsorption and desorption behaviors of BMP-2 on a series of nano-textured hydroxyapatite (HAP) surfaces were systematically investigated by combined molecular dynamic (MD) simulations and steered molecular dynamic (SMD) simulations. The textured HAP surfaces exhibited nanostructured topographies and played a critical role in the mediation of dynamic behaviors of BMP-2. Compared to the HAP-flat model, the HAP-1:1 group (means ridge vs groove = 1:1) showed the excellent ability to capture BMP-2, less conformation change of BMP-2 molecule, and high cysteine-knot stability during the adsorption and desorption processes. These findings suggest that nano-textured HAP surfaces are more capable of loading BMP-2 molecules, and most importantly, they can help maintain a higher biological activity of BMP-2 cargos. In the present study, for the first time, we have deeply clarified the adsorption and desorption dynamics of BMP-2 on various nano-textured HAP surfaces at the atomic level, which can provide significant guidelines for the future design of BMP-2-based tissue engineering implants/scaffolds. STATEMENT OF SIGNIFICANCE: By using combined molecular dynamic (MD) simulations and steered molecular dynamic (SMD) simulations, the adsorption and desorption dynamics of bone morphogenetic protein-2 (BMP-2) dimer on a series of nano-textured hydroxyapatite (HAP) surfaces at the atomic level were presented in details for the first time. We have proved that the HAP-1:1 model (means ridge vs groove = 1:1) possessed excellent ability to capture BMP-2, less conformation change, and high cysteine-knot stability. As a result, the nano-textured topography of HAP-1:1 could maintain a relatively high biological activity of BMP-2 cargos. This work could provide theoretical guidelines for the design of BMP-2-based implants/scaffolds for bone tissue engineering.

Pervasion of beta-tricalcium phosphate with nanodiamond particles yields efficient and safe bone replacement material amenable for biofunctionalization and application in large-size osseous defect healing

Biofunctionalization of scaffold materials can enable the healing of large bone defects. In case of minimally invasive guided-bone regeneration (GBR), limitations are however hard-to-control side effects related to the potential release of biofactors into the systemic environment. Biofactors can be stably bound to nanodiamond particles (ND) through physisorption. We therefore tested the biological and clinical effects of refining beta-tricalcium phosphate (βTCP) with ND in vitro and in vivo. In vitro, βTCP carrying 4% ND resulted in enhanced attachment of mesenchymal stem cells. When assessing GBR after lateral augmentation of the mandible in sheep showed that ND in βTCP resulted in a consistently steady bone formation when compared to pure βTCP, demonstrating the biological inert behavior and the potential clinical safety of ND.

Bone tissue engineering via growth factor delivery: from scaffolds to complex matrices

In recent years, bone tissue engineering has emerged as a promising solution to the limitations of current gold standard treatment options for bone related-disorders such as bone grafts. Bone tissue engineering provides a scaffold design that mimics the extracellular matrix, providing an architecture that guides the natural bone regeneration process. During this period, a new generation of bone tissue engineering scaffolds has been designed and characterized that explores the incorporation of signaling molecules in order to enhance cell recruitment and ingress into the scaffold, as well as osteogenic differentiation and angiogenesis, each of which is crucial to successful bone regeneration. Here, we outline and critically analyze key characteristics of successful bone tissue engineering scaffolds. We also explore candidate materials used to fabricate these scaffolds. Different growth factors involved in the highly coordinated process of bone repair are discussed, and the key requirements of a growth factor delivery system are described. Finally, we concentrate on an analysis of scaffold-based growth factor delivery strategies found in the recent literature. In particular, the incorporation of two-phase systems consisting of growth factor-loaded nanoparticles embedded into scaffolds shows great promise, both by providing sustained release over a therapeutically relevant timeframe and the potential to sequentially deliver multiple growth factors.

Biomaterials for the Delivery of Growth Factors and Other Therapeutic Agents in Tissue Engineering Approaches to Bone Regeneration

Bone fracture followed by delayed or non-union typically requires bone graft intervention. Autologous bone grafts remain the clinical "gold standard". Recently, synthetic bone grafts such as Medtronic's Infuse Bone Graft have opened the possibility to pharmacological and tissue engineering strategies to bone repair following fracture. This clinically-available strategy uses an absorbable collagen sponge as a carrier material for recombinant human bone morphogenetic protein 2 (rhBMP-2) and a similar strategy has been employed by Stryker with BMP-7, also known as osteogenic protein-1 (OP-1). A key advantage to this approach is its "off-the-shelf" nature, but there are clear drawbacks to these products such as edema, inflammation, and ectopic bone growth. While there are clinical challenges associated with a lack of controlled release of rhBMP-2 and OP-1, these are among the first clinical examples to wed understanding of biological principles with biochemical production of proteins and pharmacological principles to promote tissue regeneration (known as regenerative pharmacology). After considering the clinical challenges with such synthetic bone grafts, this review considers the various biomaterial carriers under investigation to promote bone regeneration. This is followed by a survey of the literature where various pharmacological approaches and molecular targets are considered as future strategies to promote more rapid and mature bone regeneration. From the review, it should be clear that pharmacological understanding is a key aspect to developing these strategies.

Journey into Bone Models: A Review

Bone is a complex tissue with a variety of functions, such as providing mechanical stability for locomotion, protection of the inner organs, mineral homeostasis and haematopoiesis. To fulfil these diverse roles in the human body, bone consists of a multitude of different cells and an extracellular matrix that is mechanically stable, yet flexible at the same time. Unlike most tissues, bone is under constant renewal facilitated by a coordinated interaction of bone-forming and bone-resorbing cells. It is thus challenging to recreate bone in its complexity in vitro and most current models rather focus on certain aspects of bone biology that are of relevance for the research question addressed. In addition, animal models are still regarded as the gold-standard in the context of bone biology and pathology, especially for the development of novel treatment strategies. However, species-specific differences impede the translation of findings from animal models to humans. The current review summarizes and discusses the latest developments in bone tissue engineering and organoid culture including suitable cell sources, extracellular matrices and microfluidic bioreactor systems. With available technology in mind, a best possible bone model will be hypothesized. Furthermore, the future need and application of such a complex model will be discussed.

Novel 3D Hybrid Nanofiber Aerogels Coupled with BMP-2 Peptides for Cranial Bone Regeneration

An ideal synthetic bone graft is a combination of the porous and nanofibrous structure presented by natural bone tissue as well as osteoinductive biochemical factors such as bone morphogenetic protein 2 (BMP-2). In this work, ultralight 3D hybrid nanofiber aerogels composed of electrospun PLGA-collagen-gelatin and Sr-Cu codoped bioactive glass fibers with incorporation of heptaglutamate E7 domain specific BMP-2 peptides have been developed and evaluated for their potential in cranial bone defect healing. The nanofiber aerogels are surgically implanted into 8 mm × 1 mm (diameter × thickness) critical-sized defects created in rat calvariae. A sustained release of E7-BMP-2 peptide from the degradable hybrid aerogels significantly enhances bone healing and defect closure over 8 weeks in comparison to unfilled defects. Histomorphometry and X-ray microcomputed tomography (µ-CT) analysis reveal greater bone volume and bone formation area in case of the E7-BMP-2 peptide loaded hybrid nanofiber aerogels. Further, histopathology data divulged a near complete nanofiber aerogel degradation along with enhanced vascularization of the regenerated tissue. Together, this study for the first time demonstrates the fabrication of 3D hybrid nanofiber aerogels from 2D electrospun fibers and their loading with therapeutic osteoinductive BMP-2 mimicking peptide for cranial bone tissue regeneration.