Controlled multiple growth factor delivery from bone tissue engineering scaffolds via designed affinity

Subperiosteal delivery of transforming growth factor beta 1 and human growth hormone from mineralized PCL films

The ability to locally deliver bioactive molecules to distinct regions of the skeleton may provide a novel means by which to improve fracture healing, treat neoplasms or infections, or modulate growth. In this study, we constructed single-sided mineral-coated poly-ε-caprolactone membranes capable of binding and releasing transforming growth factor beta 1 (TGF-β1) and human growth hormone (hGH). After demonstrating biological activity in vitro and characterization of their release, these thin bioabsorbable membranes were surgically implanted using an immature rabbit model. Membranes were circumferentially wrapped under the periosteum, thus placed in direct contact with the proximal metaphysis to assess its bioactivity in vivo. The direct effects on the metaphyseal bone, bone marrow, and overlying periosteum were assessed using radiography and histology. Effects of membrane placement at the tibial growth plate were assessed via physeal heights, tibial growth rates (pulsed fluorochrome labeling), and tibial lengths. Subperiosteal placement of the mineralized membranes induced greater local chondrogenesis in the plain mineral and TGF-β1 samples than the hGH. More exuberant and circumferential ossification was seen in the TGF-β1 treated tibiae. The TGF-β1 membranes also induced hypocellularity of the bone marrow with characteristics of gelatinous degeneration not seen in the other groups. While the proximal tibial growth plates were taller in the hGH treated than TGF-β1, no differences in growth rates or overall tibial lengths were found. In conclusion, these data demonstrate the feasibility of using bioabsorbable mineral coated membranes to deliver biologically active compounds subperiosteally in a sustained fashion to affect cells at the insertion site, bone marrow, and even growth plate.

Efforts to promote osteogenesis-angiogenesis coupling for bone tissue engineering

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

Enzyme-Mediated Nerve Growth Factor Release from Nanofibers Using Gelatin Microspheres

Spinal cord injury is a complex environment, with many conflicting growth factors present at different times throughout the injury timeline. Delivery of multiple growth factors has received mixed results, highlighting a need to consider the timing of delivery for possibly antagonistic growth factors. Cell-mediated degradation of delivery vehicles for delayed release of growth factors offers an attractive way to exploit the highly active immune response in the spinal cord injury environment. In this study, growth factor-loaded gelatin microspheres (GMS) combined with methacrylated hyaluronic acid (MeHA) were electrospun to create GMS fibers (GMSF) for delayed release of growth factors (GFs). GMS were successfully combined with MeHA while electrospinning, with an average fiber diameter of 365 ± 10 nm and 44% ± 8% fiber alignment. GMSF with nerve growth factor (NGF) was tested on dissociated chick dorsal root ganglia cells. We further tested the effect of M1 macrophage-conditioned media (M1CM) to simulate macrophage invasion after spinal cord injury for cell-mediated degradation. We hypothesized that neurons grown on GMSF with loaded NGF would exhibit longer neurites in M1CM, showing a release of functional NGF, as compared with controls. GMSF in M1CM was significantly different from MeHA in serum-free media (SFM) and M0-conditioned media (M0CM), as well as GMSF in M0CM (p < 0.05). Moreover, GMSF + NGF in all media conditions were significantly different from MeHA in SFM and M0CM (p < 0.05). The goal of this study was to develop a biomaterial system where drug delivery is triggered by immune response, allowing for more control and longer exposure to encapsulated drugs. The spinal cord injury microenvironment is known to have a robust immune response, making this immune-medicated drug release system particularly significant for directed repair.

Reducing retraction in engineered tissues through design of sequential growth factor treatment

Heart valve disease is associated with high morbidity and mortality worldwide, resulting in hundreds of thousands of heart valve replacements each year. Tissue engineered heart valves (TEHVs) have the potential to overcome the major limitations of traditional replacement valves; however, leaflet retraction has led to the failure of TEHVs in preclinical studies. Sequentially varying growth factors over time has been utilized to promote maturation of engineered tissues and may be effective in reducing tissue retraction, yet it is difficult to predict the effects of such treatments due to complex interactions between the cells and the extracellular matrix (ECM), biochemical environment, and mechanical stimuli. We hypothesize that sequential treatments of fibroblast growth factor 2 (FGF-2) and transforming growth factor beta 1 (TGF-β1) can be used to minimize cell-generated tissue retraction by decreasing active cell contractile forces exerted on the ECM and by inducing the cells to increase the ECM stiffness. Using a custom culturing and monitoring system for 3D tissue constructs, we designed and tested various TGF-β1 and FGF-2 based growth factor treatments, and successfully reduced tissue retraction by 85% and increased the ECM elastic modulus by 260% compared to non-growth factor treated controls, without significantly increasing the contractile force. We also developed and verified a mathematical model to predict the effects of various temporal variations in growth factor treatments and analyzed relationships between tissue properties, the contractile forces, and retraction. These findings improve our understanding of growth factor-induced cell-ECM biomechanical interactions, which can inform the design of next generation TEHVs with reduced retraction. The mathematical models could also potentially be applied toward fast screening and optimizing growth factors for use in the treatment of diseases including fibrosis.

Effectiveness of biomechanically stable pergola-like additively manufactured scaffold for extraskeletal vertical bone augmentation

Objective: Extraskeletal vertical bone augmentation in oral implant surgery requires extraosseous regeneration beyond the anatomical contour of the alveolar bone. It is necessary to find a better technical/clinical solution to solve the dilemma of vertical bone augmentation. 3D-printed scaffolds are all oriented to general bone defect repair, but special bone augmentation design still needs improvement.

Methods: This study aimed to develop a structural pergola-like scaffold to be loaded with stem cells from the apical papilla (SCAPs), bone morphogenetic protein 9 (BMP9) and vascular endothelial growth factor (VEGF) to verify its bone augmentation ability even under insufficient blood flow supply. Scaffold biomechanical and fluid flow optimization design by finite element analysis (FEA) and computational fluid dynamics (CFD) was performed on pergola-like additive-manufactured scaffolds with various porosity and pore size distributions. The scaffold geometrical configuration showing better biomechanical and fluid dynamics properties was chosen to co-culture for 2 months in subcutaneously into nude mice, with different SCAPs, BMP9, and (or) VEGF combinations. Finally, the samples were removed for Micro-CT and histological analysis.

Results: Micro-CT and histological analysis of the explanted scaffolds showed new bone formation in the “Scaffold + SCAPs + BMP9” and the “Scaffold + SCAPs + BMP9 + VEGF” groups where the VEGF addition did not significantly improve osteogenesis. No new bone formation was observed either for the “Blank Scaffold” and the “Scaffold + SCAPs + GFP” group. The results of this study indicate that BMP9 can effectively promote the osteogenic differentiation of SCAPs.

Conclusion: The pergola-like scaffold can be used as an effective carrier and support device for new bone regeneration and mineralization in bone tissue engineering, and can play a crucial role in obtaining considerable vertical bone augmentation even under poor blood supply.

Recent progress in the manipulation of biochemical and biophysical cues for engineering functional tissues

Tissue engineering (TE) is currently considered a cutting-edge discipline that offers the potential for developing treatments for health conditions that negatively affect the quality of life. This interdisciplinary field typically involves the combination of cells, scaffolds, and appropriate induction factors for the regeneration and repair of damaged tissue. Cell fate decisions, such as survival, proliferation, or differentiation, critically depend on various biochemical and biophysical factors provided by the extracellular environment during developmental, physiological, and pathological processes. Therefore, understanding the mechanisms of action of these factors is critical to accurately mimic the complex architecture of the extracellular environment of living tissues and improve the efficiency of TE approaches. In this review, we recapitulate the effects that biochemical and biophysical induction factors have on various aspects of cell fate. While the role of biochemical factors, such as growth factors, small molecules, extracellular matrix (ECM) components, and cytokines, has been extensively studied in the context of TE applications, it is only recently that we have begun to understand the effects of biophysical signals such as surface topography, mechanical, and electrical signals. These biophysical cues could provide a more robust set of stimuli to manipulate cell signaling pathways during the formation of the engineered tissue. Furthermore, the simultaneous application of different types of signals appears to elicit synergistic responses that are likely to improve functional outcomes, which could help translate results into successful clinical therapies in the future.

Dual drug delivery platforms for bone tissue engineering

The dual delivery platforms used in bone tissue engineering provide supplementary bioactive compounds that include distinct medicines and growth factors thereby aiding enhanced bone regeneration. The delivery of these compounds can be adjusted for a short or prolonged time based on the requirement by altering various parameters of the carrier platform. The platforms thus used are fabricated to mimic the niche of the bone microenvironment, either in the form of porous 3D structures, microspheres, or films. Thus, this review article focuses on the concept of dual drug delivery platform and its importance, classification of various platforms for dual drug delivery specific to bone tissue engineering, and finally highlights the foresight into the future direction of these techniques for better clinical applications.

Surface engineering of 3D-printed scaffolds with minerals and a pro-angiogenic factor for vascularized bone regeneration

Scaffolds functionalized with biomolecules have been developed for bone regeneration but inducing the regeneration of complex structured bone with neovessels remains a challenge. For this study, we developed three-dimensional printed scaffolds with bioactive surfaces coated with minerals and platelet-derived growth factor. The minerals were homogeneously deposited on the surface of the scaffold using 0.01 M NaHCO₃ with epigallocatechin gallate in simulated body fluid solution (M2). The M2 scaffold demonstrated enhanced mineral coating amount per scaffold with a greater compressive modulus than the others which used different concentration of NaHCO₃. Then, we immobilized PDGF on the mineralized scaffold (M2/P), which enhanced the osteogenic differentiation of human adipose derived stem cells in vitro and promoted the secretion of pro-angiogenic factors. Cells cultured in M2/P showed remarkable ratio of osteocalcin- and osteopontin-positive nuclei, and M2/P-derived medium induced endothelial cells to form tubule structures. Finally, the implanted M2/P scaffolds onto mouse calvarial defects had regenerated bone in 80.8 ± 9.8% of the defect area with the arterioles were formed, after 8 weeks. In summary, our scaffold, which composed of minerals and pro-angiogenic growth factor, could be used therapeutically to improve the regeneration of bone with a highly vascularized structure. STATEMENT OF SIGNIFICANCE: Surface engineered scaffolds have been developed for bone regeneration but inducing the volumetric regeneration of bone with neovessels remains a challenge. In here, we developed 3D printed scaffolds with bioactive surfaces coated with bio-minerals and platelet-derived growth factors. We proved that the 0.01 M NaHCO₃ with polyphenol in simulated body fluid solution enhanced the deposition of bio-minerals and even distribution on the surface of scaffold. The in vitro studies demonstrated that the attached cells on the bioactive surface showed the enhanced osteogenic differentiation and secretion of pro-angiogenic factors. Finally, the scaffold with bioactive surface not only improved the regenerated volume of bone tissues but also increased neovessel formation after in vivo implantation onto mouse calvarial defect.

Vascularization Strategies in Bone Tissue Engineering

Bone is a highly vascularized tissue, and its development, maturation, remodeling, and regeneration are dependent on a tight regulation of blood vessel supply. This condition also has to be taken into consideration in the context of the development of artificial tissue substitutes. In classic tissue engineering, bone-forming cells such as primary osteoblasts or mesenchymal stem cells are introduced into suitable scaffolds and implanted in order to treat critical-size bone defects. However, such tissue substitutes are initially avascular. Because of the occurrence of hypoxic conditions, especially in larger tissue substitutes, this leads to the death of the implanted cells. Therefore, it is necessary to devise vascularization strategies aiming at fast and efficient vascularization of implanted artificial tissues. In this review article, we present and discuss the current vascularization strategies in bone tissue engineering. These are based on the use of angiogenic growth factors, the co-implantation of blood vessel forming cells, the ex vivo microfabrication of blood vessels by means of bioprinting, and surgical methods for creating surgically transferable composite tissues.

Multi-Dimensional Printing for Bone Tissue Engineering

The development of 3D printing has significantly advanced the field of bone tissue engineering by enabling the fabrication of scaffolds that faithfully recapitulate desired mechanical properties and architectures. In addition, computer-based manufacturing relying on patient-derived medical images permits the fabrication of customized modules in a patient-specific manner. In addition to conventional 3D fabrication, progress in materials engineering has led to the development of 4D printing, allowing time-sensitive interventions such as programed therapeutics delivery and modulable mechanical features. Therapeutic interventions established via multi-dimensional engineering are expected to enhance the development of personalized treatment in various fields, including bone tissue regeneration. Here, recent studies utilizing 3D printed systems for bone tissue regeneration are summarized and advances in 4D printed systems are highlighted. Challenges and perspectives for the future development of multi-dimensional printed systems toward personalized bone regeneration are also discussed.

The effects of high fat diet, bone healing, and BMP-2 treatment on endothelial cell growth and function

Angiogenesis is a vital process during the regeneration of bone tissue. The aim of this study was to investigate angiogenesis at the fracture site as well as at distal locations from obesity-induced type 2 diabetic mice that were treated with bone morphogenetic protein-2 (BMP-2, local administration at the time of surgery) to heal a femoral critical sized defect (CSD) or saline as a control. Mice were fed a high fat diet (HFD) to induce a type 2 diabetic-like phenotype while low fat diet (LFD) animals served as controls. Endothelial cells (ECs) were isolated from the lungs (LECs) and bone marrow (BMECs) 3 weeks post-surgery, and the fractured femurs were also examined. Our studies demonstrate that local administration of BMP-2 at the fracture site in a CSD model results in complete bone healing within 3 weeks for all HFD mice and 66.7% of LFD mice, whereas those treated with saline remain unhealed. At the fracture site, vessel parameters and adipocyte numbers were significantly increased in BMP-2 treated femurs, irrespective of diet. At distal sites, LEC and BMEC proliferation was not altered by diet or BMP-2 treatment. HFD increased the tube formation ability of both LECs and BMECs. Interestingly, BMP-2 treatment at the time of surgery reduced tube formation in LECs and humeri BMECs. However, migration of BMECs from HFD mice treated with BMP-2 was increased compared to BMECs from HFD mice treated with saline. BMP-2 treatment significantly increased the expression of CD31, FLT-1, and ANGPT2 in LECs and BMECs in LFD mice, but reduced the expression of these same genes in HFD mice. To date, this is the first study that depicts the systemic influence of fracture surgery and local BMP-2 treatment on the proliferation and angiogenic potential of ECs derived from the bone marrow and lungs.

3D printed micro-chambers carrying stem cell spheroids and pro-proliferative growth factors for bone tissue regeneration

Three-dimensional (3D)-printed scaffolds have proved to be effective tools for delivering growth factors and cells in bone-tissue engineering. However, delivering spheroids that enhance cellular function remains challenging because the spheroids tend to suffer from low viability, which limits bone regenerationin vivo. Here, we describe a 3D-printed polycaprolactone micro-chamber that can deliver human adipose-derived stem cell spheroids. Anin vitroculture of cells from spheroids in the micro-chamber exhibited greater viability and proliferation compared with cells cultured without the chamber. We coated the surface of the chamber with 500 ng of platelet-derived growth factors (PDGF), and immobilized 50 ng of bone morphogenetic protein 2 (BMP-2) on fragmented fibers, which were incorporated within the spheroids as a new platform for a dual-growth-factor delivery system. The PDGF detached from the chamber within 8 h and the remains were retained on the surface of chamber while the BMP-2 was entrapped by the spheroid. In vitro osteogenic differentiation of the cells from the spheroids in the micro-chamber with dual growth factors enhanced alkaline phosphatase and collagen type 1A expression by factors of 126.7 ± 19.6 and 89.7 ± 0.3, respectively, compared with expression in a micro-chamber with no growth factors. In vivo transplantation of the chambers with dual growth factors into mouse calvarial defects resulted in a 77.0 ± 15.9% of regenerated bone area, while the chamber without growth factors and a defect-only group achieved 7.6 ± 3.9% and 5.0 ± 1.9% of regenerated bone areas, respectively. These findings indicate that a spheroid-loaded micro-chamber supplied with dual growth factors can serve as an effective protein-delivery platform that increases stem-cell functioning and bone regeneration.

An "all-in-one" scaffold targeting macrophages to direct endogenous bone repair in situ

Scaffolds for tissue repair are designed in an increasingly complicated manner to meet multi-facet biological needs during the healing process. However, overly sophisticated design, especially the use of multiple components and delivery of exogenous cells, hampers the bench-to-bedside translation. Here, a multi-functional - yet mono-compositional - bioactive scaffold is devised to mediate the full-range, endogenous bone repair. Based on immunoactivity screening, a chemically-modified glucomannan polysaccharide is selected and processed into an anisotropic porous scaffold, which accurately stimulates macrophages to produce pro-regenerative cytokines. These cytokines effectively enhance the recruitment ("R") and induced osteogenesis ("IO") of the bone progenitor cells in situ. Meanwhile, the anisotropic porosity and carbohydrate signal of the scaffold facilitate differential adhesion ("A") and distribution ("D") of the macrophages and bone progenitor cells - enabling the former's accumulation at the surface while encouraging the latter's infiltration into the scaffold. Implanted in a rat calvarial defect model, this "RADIO" system effectively promotes healing over 12 weeks, with the obvious formation of hard callus through the scaffold. In summary, RADIO integrates multiple functions into one single scalable system ("all-in-one") to govern the dynamic bone-repair process, by harnessing the power of host macrophages. RADIO represents an open platform to solving the long-lasting complexity-versus-simplicity dilemma in biomaterials design. STATEMENT OF SIGNIFICANCE: Biomaterials as versatile tools for tissue repair are becoming increasingly complicated, yet overly sophisticated design - especially the use of multiple components, exogenous cells, and overdosed growth factors - hampers their clinical application. The pre-requisite for designing a successful integrative scaffold is to identify an inherent biological target responding to biomaterial signals, thereby efficiently and safely promoting tissue repair via the endogenous healing capability instead of extra multifarious biochemical components. For bone regeneration, the pivotal regulator is macrophages. Through activating host macrophages, our single-component scaffold system coordinates the entire bone regenerative cascade in situ and induces successful bone regeneration in a calvarial defect model. This scaffold represents a scalable and multi-functional approach to effectively simplify the sophisticated design in regenerative medicine.

Single-dose mRNA therapy via biomaterial-mediated sequestration of overexpressed proteins

Nonviral mRNA delivery is an attractive therapeutic gene delivery strategy, as it achieves efficient protein overexpression in vivo and has a desirable safety profile. However, mRNA's short cytoplasmic half-life limits its utility to therapeutic applications amenable to repeated dosing or short-term overexpression. Here, we describe a biomaterial that enables a durable in vivo response to a single mRNA dose via an "overexpress and sequester" mechanism, whereby mRNA-transfected cells locally overexpress a growth factor that is then sequestered within the biomaterial to sustain the biologic response over time. In a murine diabetic wound model, this strategy demonstrated improved wound healing compared to delivery of a single mRNA dose alone or recombinant protein. In addition, codelivery of anti-inflammatory proteins using this biomaterial eliminated the need for mRNA chemical modification for in vivo therapeutic efficacy. The results support an approach that may be broadly applicable for single-dose delivery of mRNA without chemical modification.

Mesoporous Hydroxyapatite Nanoparticles Mediate the Release and Bioactivity of BMP-2 for Enhanced Bone Regeneration

Efficient delivery of bone morphogenetic protein-2 (BMP-2) with desirable bioactivity is still a great challenge in the field of bone regeneration. In this study, a silk fibroin/chitosan scaffold incorporated with BMP-2-loaded mesoporous hydroxyapatite nanoparticles (mHANPs) was prepared (SCH-L). BMP-2 was preloaded onto mHANPs with a high surface area before mixing with a silk fibroin/chitosan composite. Bare (without BMP-2) silk fibroin/chitosan/mHANP (SCH) scaffolds and SCH scaffolds with directly absorbed BMP-2 (SCH-D) were investigated in parallel for comparison. In vitro release kinetics indicated that BMP-2 released from the SCH-L scaffold showed a significantly lower initial burst release, followed by a more sustained release over time than the SCH-D scaffold. In vitro cell viability, osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), and the in vivo osteogenic effect of scaffolds in a rat calvarial defect were evaluated. The results showed that compared with bare SCH and SCH-D scaffolds, the SCH-L scaffold significantly promoted the osteogenic differentiation of BMSCs in vitro and induced more pronounced bone formation in vivo. Further studies demonstrated that the mHANP-mediated satisfactory conformational change and sustained release benefited the protection of the released BMP-2 bioactivity, as confirmed by alkaline phosphatase (ALP) activity and a mineralization deposition assay. More importantly, the interaction of BMP-2/mHANPs enhanced the binding ability of BMP-2 to cellular receptors, thereby maintaining its biological activity in osteogenic differentiation and osteoinductivity well, which contributed to the markedly promoted in vitro and in vivo osteogenic efficacy of the SCH-L scaffold. Taken together, these results provide strong evidence that mHANPs represent an attractive carrier for binding BMP-2 to scaffolds. The SCH-L scaffold shows promising potential for bone tissue regeneration applications.

Mineral-Coated Microparticles Enhance mRNA-Based Transfection of Human Bone Marrow Cells

The regenerative potential of bone marrow cells could be harnessed for tissue engineering applications. Bone marrow can be easily collected from patients, providing a valuable autologous source of therapeutic cells. However, years of delivery of bone marrow cells have highlighted the need for their genetic manipulation to overcome heterogeneity and to confer specificity to the regenerative process. In this study, we optimized the use of condensed mRNA as a non-viral alternative. As a proof of concept, we used mRNA encoding for reporter proteins such as EGFP or Firefly luciferase, which was condensed by complexing agents and delivered to human bone marrow cells using mineral-coated microparticles. We demonstrated that human bone marrow cells could be transfected with complexed mRNA, and that this approach was more efficient than the delivery of complexed plasmid DNA. In addition, human bone marrow cells were vulnerable to the toxicity of mRNA complexing agents, but these deleterious effects were mitigated by using mineral-coated microparticles as a carrier of complexed mRNA. Microparticle-mediated delivery of complexed mRNA also enabled higher cell metabolic activity and higher transfection in multiple in vitro culture conditions, including suspension culture and three-dimensional culture.

Tissue engineered bone mimetics to study bone disorders ex vivo: Role of bioinspired materials

Recent advances in materials development and tissue engineering has resulted in a substantial number of bioinspired materials that recapitulate cardinal features of bone extracellular matrix (ECM) such as dynamic inorganic and organic environment(s), hierarchical organization, and topographical features. Bone mimicking materials, as defined by its self-explanatory term, are developed based on the current understandings of the natural bone ECM during development, remodeling, and fracture repair. Compared to conventional plastic cultures, biomaterials that resemble some aspects of the native environment could elicit a more natural molecular and cellular response relevant to the bone tissue. Although current bioinspired materials are mainly developed to assist tissue repair or engineer bone tissues, such materials could nevertheless be applied to model various skeletal diseases in vitro. This review summarizes the use of bioinspired materials for bone tissue engineering, and their potential to model diseases of bone development and remodeling ex vivo. We largely focus on biomaterials, designed to re-create different aspects of the chemical and physical cues of native bone ECM. Employing these bone-inspired materials and tissue engineered bone surrogates to study bone diseases has tremendous potential and will provide a closer portrayal of disease progression and maintenance, both at the cellular and tissue level. We also briefly touch upon the application of patient-derived stem cells and introduce emerging technologies such as organ-on-chip in disease modeling. Faithful recapitulation of disease pathologies will not only offer novel insights into diseases, but also lead to enabling technologies for drug discovery and new approaches for cell-based therapies.

Recent Progress in the Construction of Functional Artificial Bone by Cytokine-Controlled Strategies

OBJECTIVE

Combining artificial scaffolds with stimulatory factors to reconstruct lost bone tissues is one of the hottest research directions. The purpose of this review was to conduct a retrospective survey on the latest reports on artificial bone fabrication with functional cytokines.

DATA SOURCES

The status of related scientific research from the year 2005 to 2018 was analyzed through the mode of literature retrieval in PubMed and VIP Database. The retrieval words are as follows: "bone tissue engineering," "angiogenesis," "cytokines," "osteogenesis," "biomimetic bone marrow," "sol-gel," "delivery system," and the corresponding Chinese words.

STUDY SELECTION

After reading through the title and abstract for early screening, the full text of relevant studies was evaluated and those not related with this review had been ruled out.

RESULTS

According to the literature retrospective survey, there were three key points for the successful construction of functional artificial bones: (1) the continuous supply of relatively low concentration of cytokines during the required period; (2) the delivery of two or more cytokines essential to the process and ensure the relatively spatial independence to reduce the unnecessary interference; and (3) supporting the early-stage angiogenesis and late-stage osteogenesis, respectively, regulating and balancing the crosslinking of both to avoid the surface ossification that would probably block the osteogenesis inside.

CONCLUSIONS

The synergistic effect of both angiogenic factors and osteogenic factors applied in bone regeneration is a key point in the combined functional artificial bone. Through analysis, comparison, and summary of the current strategies, we proposed that the most promising one is to mimic the natural bone marrow function to facilitate the regeneration process and ensure the efficient repair of large weight-bearing bone defect.

Bone Morphogenetic Protein-Based Therapeutic Approaches

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor (TGF)-β family of ligands and exert most of their effects through the canonical effectors Smad1, 5, and 8. Appropriate regulation of BMP signaling is critical for the development and homeostasis of numerous human organ systems. Aberrations in BMP pathways or their regulation are increasingly associated with diverse human pathologies, and there is an urgent and growing need to develop effective approaches to modulate BMP signaling in the clinic. In this review, we provide a wide perspective on diseases and/or conditions associated with dysregulated BMP signal transduction, outline the current strategies available to modulate BMP pathways, highlight emerging second-generation technologies, and postulate prospective avenues for future investigation.

Dual non-viral gene delivery from microparticles within 3D high-density stem cell constructs for enhanced bone tissue engineering

High-density mesenchymal stem cell (MSC) aggregates can be guided to form bone-like tissue via endochondral ossification in vitro when culture media is supplemented with proteins, such as growth factors (GFs), to first guide the formation of a cartilage template, followed by culture with hypertrophic factors. Recent reports have recapitulated these results through the controlled spatiotemporal delivery of chondrogenic transforming growth factor-β1 (TGF-β1) and chondrogenic and osteogenic bone morphogenetic protein-2 (BMP-2) from microparticles embedded within human MSC aggregates to avoid diffusion limitations and the lengthy, costly in vitro culture necessary with repeat exogenous supplementation. However, since GFs have limited stability, localized gene delivery is a promising alternative to the use of proteins. Here, mineral-coated hydroxyapatite microparticles (MCM) capable of localized delivery of Lipofectamine-plasmid DNA (pDNA) nanocomplexes encoding for TGF-β1 (pTGF-β1) and BMP-2 (pBMP-2) were incorporated, alone or in combination, within MSC aggregates from three healthy porcine donors to induce sustained production of these transgenes. Three donor populations were investigated in this work due to the noted MSC donor-to-donor variability in differentiation capacity documented in the literature. Delivery of pBMP-2 within Donor 1 aggregates promoted chondrogenesis at week 2, followed by an enhanced osteogenic phenotype at week 4. Donor 2 and 3 aggregates did not promote robust glycosaminoglycan (GAG) production at week 2, but by week 4, Donor 2 aggregates with pTGF-β1/pBMP-2 and Donor 3 aggregates with both unloaded MCM and pBMP-2 enhanced osteogenesis compared to controls. These results demonstrate the ability to promote osteogenesis in stem cell aggregates through controlled, non-viral gene delivery within the cell masses. These findings also indicate the need to screen donor MSC regenerative potential in response to gene transfer prior to clinical application. Taken together, this work demonstrates a promising gene therapy approach to control stem cell fate in biomimetic 3D condensations for treatment of bone defects.

Injectable Shear-Thinning CaSO4/FGF-18-Incorporated Chitin-PLGA Hydrogel Enhances Bone Regeneration in Mice Cranial Bone Defect Model

For craniofacial bone regeneration, shear-thinning injectable hydrogels are favored over conventional scaffolds because of their improved defect margin adaptability, easier handling, and ability to be injected manually into deeper tissues. The most accepted method, after autografting, is the use of recombinant human bone morphogenetic protein-2 (BMP-2); however, complications such as interindividual variations, edema, and poor cost-efficiency in supraphysiological doses have been reported. The endogenous synthesis of BMP-2 is desirable, and a molecule which induces this is fibroblast growth factor-18 (FGF-18) because it can upregulate the BMP-2 expression  by supressing noggin. We developed a chitin-poly(lactide-co-glycolide) (PLGA) composite hydrogel by regeneration chemistry and then incorporated CaSO₄ and FGF-18 for this purpose. Rheologically, a 7-fold increase in the elastic modulus was observed in the CaSO₄-incorporated chitin-PLGA hydrogels as compared to the chitin-PLGA hydrogel. Shear-thinning Herschel-Bulkley fluid nature was observed for both hydrogels. Chitin-PLGA/CaSO₄ gel showed sustained release of FGF-18. In vitro osteogenic differentiation showed an enhanced alkaline phosphatase (ALP) expression in the FGF-18-containing chitin-PLGA/CaSO₄ gel when compared to cells alone. Further, it was confirmed by studying the expression of osteogenic genes [RUNX2, ALP, BMP-2, osteocalcin (OCN), and osteopontin (OPN)], immunofluorescence staining of BMP-2, OCN, and OPN, and alizarin red S staining. Incorporation of FGF-18 in the hydrogel increased the endothelial cell migration. Further, the regeneration potential of the prepared hydrogels was tested in vivo, and longitudinal live animal μ-CT was performed. FGF-18-loaded chitin-PLGA/CaSO₄ showed early and almost complete bone healing in comparison with chitin-PLGA/CaSO₄, chitin-PLGA/FGF-18, chitin-PLGA, and sham control systems, as confirmed by hematoxylin and eosin and osteoid tetrachrome stainings. This shows that the CaSO₄ and FGF-18-incorporated hydrogel is a potential candidate for craniofacial bone defect regeneration.

Biomimetic Materials and Fabrication Approaches for Bone Tissue Engineering

Various strategies have been explored to overcome critically sized bone defects via bone tissue engineering approaches that incorporate biomimetic scaffolds. Biomimetic scaffolds may provide a novel platform for phenotypically stable tissue formation and stem cell differentiation. In recent years, osteoinductive and inorganic biomimetic scaffold materials have been optimized to offer an osteo-friendly microenvironment for the osteogenic commitment of stem cells. Furthermore, scaffold structures with a microarchitecture design similar to native bone tissue are necessary for successful bone tissue regeneration. For this reason, various methods for fabricating 3D porous structures have been developed. Innovative techniques, such as 3D printing methods, are currently being utilized for optimal host stem cell infiltration, vascularization, nutrient transfer, and stem cell differentiation. In this progress report, biomimetic materials and fabrication approaches that are currently being utilized for biomimetic scaffold design are reviewed.

* Calvarial Bone Regeneration Is Enhanced by Sequential Delivery of FGF-2 and BMP-2 from Layer-by-Layer Coatings with a Biomimetic Calcium Phosphate Barrier Layer

A drug delivery coating for synthetic bone grafts has been developed to provide sequential delivery of multiple osteoinductive factors to better mimic aspects of the natural regenerative process. The coating is composed of a biomimetic calcium phosphate (bCaP) layer that is applied to a synthetic bone graft and then covered with a poly-l-Lysine/poly-l-Glutamic acid polyelectrolyte multilayer (PEM) film. Bone morphogenetic protein-2 (BMP-2) was applied before the coating process directly on the synthetic bone graft and then, bCaP-PEM was deposited followed by adsorption of fibroblast growth factor-2 (FGF-2) into the PEM layer. Cells access the FGF-2 immediately, while the bCaP-PEM temporally delays the cell access to BMP-2. In vitro studies with cells derived from mouse calvarial bones demonstrated that Sca-1 and CD-166 positive osteoblast progenitor cells proliferated in response to media dosing with FGF-2. Coated scaffolds with BMP-2 and FGF-2 were implanted in mouse calvarial bone defects and harvested at 1 and 3 weeks. After 1 week in vivo, proliferation of cells, including Sca-1+ progenitors, was observed with low dose FGF-2 and BMP-2 compared to BMP-2 alone, indicating that in vivo delivery of FGF-2 activated a similar population of cells as shown by in vitro testing. At 3 weeks, FGF-2 and BMP-2 delivery increased bone formation more than BMP-2 alone, particularly in the center of the defect, confirming that the proliferation of the Sca-1 positive osteoprogenitors by FGF-2 was associated with increased bone healing. Areas of bone mineralization were positive for double fluorochrome labeling of calcium and alkaline phosphatase staining of osteoblasts, along with increased TRAP+ osteoclasts, demonstrating active bone formation distinct from the bone-like collagen/hydroxyapatite scaffold. In conclusion, the addition of a bCaP layer to PEM delayed access to BMP-2 and allowed the FGF-2 stimulated progenitors to populate the scaffold before differentiating in response to BMP-2, leading to improved bone defect healing.

Functionalization of microparticles with mineral coatings enhances non-viral transfection of primary human cells

Gene delivery to primary human cells is a technology of critical interest to both life science research and therapeutic applications. However, poor efficiencies in gene transfer and undesirable safety profiles remain key limitations in advancing this technology. Here, we describe a materials-based approach whereby application of a bioresorbable mineral coating improves microparticle-based transfection of plasmid DNA lipoplexes in several primary human cell types. In the presence of these mineral-coated microparticles (MCMs), we observed up to 4-fold increases in transfection efficiency with simultaneous reductions in cytotoxicity. We identified mechanisms by which MCMs improve transfection, as well as coating compositions that improve transfection in three-dimensional cell constructs. The approach afforded efficient transfection in primary human fibroblasts as well as mesenchymal and embryonic stem cells for both two- and three-dimensional transfection strategies. This MCM-based transfection is an advancement in gene delivery technology, as it represents a non-viral approach that enables highly efficient, localized transfection and allows for transfection of three-dimensional cell constructs.

Yolk shell nanocomposite particles as bioactive bone fillers and growth factor carriers

The efficient delivery of bioactive molecules via rationally designed nanoparticles is an important focus in regenerative medicine. The yolk shell nanocomposite particles described herein are composed of silk fibroin movable cores formed within voided calcium carbonate shells to load and control the release of labile cytokines. These particles are excellent carrier vehicles of potent molecules as they sustained the release of bioactive Bone Morphogenetic Protein 2 (BMP-2) for more than 28 days in vitro. Implantation into bone defects in rabbits corroborates the in vitro results and also reveals that upon contact with phosphate containing body fluids, implanted yolk shell particles agglomerate and transform into a filler that adapts to defect contour to further act as an absorbable hemostatic agent. Taken together, the fabrication of these yolk shell particle-based "bone fillers" could expand the horizon for the development of newer generations of advanced bioactive materials in tissue regeneration applications.

Covalent Binding of Bone Morphogenetic Protein-2 and Transforming Growth Factor-β3 to 3D Plotted Scaffolds for Osteochondral Tissue Regeneration

Engineering the osteochondral tissue presents some challenges mainly relying in its function of transition from the subchondral bone to articular cartilage and the gradual variation in several biological, mechanical, and structural features. A possible solution for osteochondral regeneration might be the design and fabrication of scaffolds presenting a gradient able to mimic this transition. Covalent binding of biological factors proved to enhance cell adhesion and differentiation in two-dimensional culture substrates. Here, we used polymer brushes as selective linkers of bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β3 (TGF-β3) on the surface of 3D scaffolds fabricated via additive manufacturing (AM) and subsequent controlled radical polymerization. These growth factors (GFs) are known to stimulate the differentiation of human mesenchymal stromal cells (hMSCs) toward the osteogenic and chondrogenic lineages, respectively. BMP-2 and TGF-β3 were covalently bound both homogeneously within a poly(ethylene glycol) (PEG)-based brush-functionalized scaffolds, and following a gradient composition by varying their concentration along the axial section of the 3D constructs. Following an approach previously developed by our group and proved to be successful to generate fibronectin gradients, opposite brush-supported gradients of BMP-2 and TGF-β3 were finally generated and subsequently tested to differentiate cells in a gradient fashion. The brush-supported GFs significantly influenced hMSCs osteochondral differentiation when the scaffolds were homogenously modified, yet no effect was observed in the gradient scaffolds. Therefore, this technique seems promising to maintain the biological activity of growth factors covalently linked to 3D scaffolds, but needs to be further optimized in case biological gradients are desired.

* Injectable Graft Substitute Active on Bone Tissue Regeneration

With the aim to obtain an injectable bioactive scaffold that can accelerate bone formation in sinus lift augmentation, in bony void and fracture repair, we have developed a three-dimensional (3D) jelly collagen containing lysophosphatidic acid (LPA) and 1α,25-dihydroxyvitamin D3 (1,25D3). Using an in vitro 3D culture model of bone fracture, we show that the contraction of the collagen gel is mediated by Rho-kinase activation in osteoblasts. The gel contraction showed dependence on cell concentration and was increased by LPA, which favored apposition and fastening of bone fragments approach. LPA was shown to act through actin cytoskeleton reorganization and myosin light chain phosphorylation of human primary osteoblasts (hOB). Moreover, LPA conferred osteoconductive properties as evidenced by the induction of proliferation, differentiation, and migration of hOB. The addition of 1,25D3 did not enhance cell-mediated gel contraction, but stimulated the maturation of hOB in vitro through the production of extracellular matrix of higher quality. On the basis of these observations, the collagen gel enriched with LPA and 1,25D3 described herein can be considered an injectable natural scaffold that allows the migration of cells from the side of bone defect and a promising candidate to accelerate bone growth and fracture healing.

Europium-Doped Gd2O3 Nanotubes Increase Bone Mineral Density in Vivo and Promote Mineralization in Vitro

Europium-doped Gd₂O₃ nanotubes (Gd₂O₃:Eu³⁺ NTs) have been extensively applied in the field of bioscience for their photostability and magnetic properties. Nevertheless, the distribution and interaction between Gd₂O₃:Eu³⁺ NTs and metabolism of bone are not yet sufficiently understood. In this study, a systematic study of the toxicity and distribution of Gd₂O₃:Eu³⁺ NTs in mice after oral administration was carried out. The results showed that a small number of the Gd₂O₃:Eu³⁺ NTs could pass through biological barriers into the lung, liver, and spleen, but a high concentration was observed in bone. Furthermore, the effects of Gd₂O₃:Eu³⁺ NTs on bone metabolism were systematically studied in vitro and in vivo when accumulating in bone. After being administered to mice, the Gd₂O₃:Eu³⁺ NTs extremely enhanced the bone mineral density and bone biomechanics. In vitro the Gd₂O₃:Eu³⁺ NTs increased the alkaline phosphatase (ALP) activity and mineralization and promoted the expression of osteogenesis genes in preosteoblasts MC3T3-E1 through activation of the BMP signaling pathway. This study will be significant for appropriate application of Gd₂O₃:Eu³⁺ NTs in the biomedical field and expounding the molecular mechanism of bone metabolism.

Ovine Mesenchymal Stromal Cells: Morphologic, Phenotypic and Functional Characterization for Osteochondral Tissue Engineering

INTRODUCTION

Knowledge of ovine mesenchymal stromal cells (oMSCs) is currently expanding. Tissue engineering combining scaffolding with oMSCs provides promising therapies for the treatment of osteochondral diseases.

PURPOSE

The aim was to isolate and characterize oMSCs from bone marrow aspirates (oBMSCs) and to assess their usefulness for osteochondral repair using β-tricalcium phosphate (bTCP) and type I collagen (Col I) scaffolds.

METHODS

Cells isolated from ovine bone marrow were characterized morphologically, phenotypically, and functionally. oBMSCs were cultured with osteogenic medium on bTCP and Col I scaffolds. The resulting constructs were evaluated by histology, immunohistochemistry and electron microscopy studies. Furthermore, oBMSCs were cultured on Col I scaffolds to develop an in vitro cartilage repair model that was assessed using a modified International Cartilage Research Society (ICRS) II scale.

RESULTS

oBMSCs presented morphology, surface marker pattern and multipotent capacities similar to those of human BMSCs. oBMSCs seeded on Col I gave rise to osteogenic neotissue. Assessment by the modified ICRS II scale revealed that fibrocartilage/hyaline cartilage was obtained in the in vitro repair model.

CONCLUSIONS

The isolated ovine cells were demonstrated to be oBMSCs. oBMSCs cultured on Col I sponges successfully synthesized osteochondral tissue. The data suggest that oBMSCs have potential for use in preclinical models prior to human clinical studies.

Surface delivery of tunable doses of BMP-2 from an adaptable polymeric scaffold induces volumetric bone regeneration

The rapid and effective bone regeneration of large non-healing defects remains challenging. Bioactive proteins, such as bone morphogenetic protein (BMP)-2, are proved their osteoinductivity, but their clinical use is currently limited to collagen as biomaterial. Being able to deliver BMP-2 from any other biomaterial would broaden its clinical use. This work presents a novel means for repairing a critical size volumetric bone femoral defect in the rat by combining a osteoinductive surface coating (2D) to a polymeric scaffold (3D hollow tube) made of commercially-available PLGA. Using a polyelectrolyte film as BMP-2 carrier, we tune the amount of BMP-2 loaded in and released from the polyelectrolyte film coating over a large extent by controlling the film crosslinking level and initial concentration of BMP-2 in solution. Using microcomputed tomography and quantitative analysis of the regenerated bone growth kinetics, we show that the amount of newly formed bone and kinetics can be modulated: an effective and fast repair was obtained in 1-2 weeks in the best conditions, including complete defect bridging, formation of vascularized and mineralized bone tissue. Histological staining and high-resolution computed tomography revealed the presence of bone regeneration inside and around the tube with spatially distinct organization for trabecular-like and cortical bones. The amount of cortical bone and its thickness increased with the BMP-2 dose. In view of the recent developments in additive manufacturing techniques, this surface-coating technology may be applied in combination with various types of polymeric or metallic scaffolds to offer new perspectives of bone regeneration in personalized medicine.

A Survey of Strategies to Modulate the Bone Morphogenetic Protein Signaling Pathway: Current and Future Perspectives

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the TGF-β family of ligands and are unequivocally involved in regulating stem cell behavior. Appropriate regulation of canonical BMP signaling is critical for the development and homeostasis of numerous human organ systems, as aberrations in the BMP pathway or its regulation are increasingly associated with diverse human pathologies. In this review, we provide a wide-perspective on strategies that increase or decrease BMP signaling. We briefly outline the current FDA-approved approaches, highlight emerging next-generation technologies, and postulate prospective avenues for future investigation. We also detail how activating other pathways may indirectly modulate BMP signaling, with a particular emphasis on the relationship between the BMP and Activin/TGF-β pathways.

A controlled double-duration inducible gene expression system for cartilage tissue engineering

Cartilage engineering that combines competent seeding cells and a compatible scaffold is increasingly gaining popularity and is potentially useful for the treatment of various bone and cartilage diseases. Intensive efforts have been made by researchers to improve the viability and functionality of seeding cells of engineered constructs that are implanted into damaged cartilage. Here, we designed an integrative system combining gene engineering and the controlled-release concept to solve the problems of both seeding cell viability and functionality through precisely regulating the anti-apoptotic gene bcl-2 in the short-term and the chondrogenic master regulator Sox9 in the long-term. Both in vitro and in vivo experiments demonstrated that our system enhances the cell viability and chondrogenic effects of the engineered scaffold after introduction of the system while restricting anti-apoptotic gene expression to only the early stage, thereby preventing potential oncogenic and overdose effects. Our system was designed to be modular and can also be readily adapted to other tissue engineering applications with minor modification.

Taking cues from the extracellular matrix to design bone-mimetic regenerative scaffolds

There is an ongoing need for effective materials that can replace autologous bone grafts in the clinical treatment of bone injuries and deficiencies. In recent years, research efforts have shifted away from a focus on inert biomaterials to favor scaffolds that mimic the biochemistry and structure of the native bone extracellular matrix (ECM). The expectation is that such scaffolds will integrate with host tissue and actively promote osseous healing. To further enhance the osteoinductivity of bone graft substitutes, ECM-mimetic scaffolds are being engineered with a range of growth factors (GFs). The technologies used to generate GF-modified scaffolds are often inspired by natural processes that regulate the association between endogenous ECMs and GFs. The purpose of this review is to summarize research centered on the development of regenerative scaffolds that replicate the fundamental collagen-hydroxyapatite structure of native bone ECM, and the functionalization of these scaffolds with GFs that stimulate critical events in osteogenesis.

Importance of dual delivery systems for bone tissue engineering

Bone formation is a complex process that requires concerted function of multiple growth factors. For this, it is essential to design a delivery system with the ability to load multiple growth factors in order to mimic the natural microenvironment for bone tissue formation. However, the short half-lives of growth factors, their relatively large size, slow tissue penetration, and high toxicity suggest that conventional routes of administration are unlikely to be effective. Therefore, it seems that using multiple bioactive factors in different delivery systems can develop new strategies for improving bone tissue regeneration. Combination of these factors along with biomaterials that permit tunable release profiles would help to achieve truly spatiotemporal regulation during delivery. This review summarizes the various dual-control release systems that are used for bone tissue engineering.

Biofabrication of bone tissue: approaches, challenges and translation for bone regeneration

The rising incidence of bone disorders has resulted in the need for more effective therapies to meet this demand, exacerbated by an increasing ageing population. Bone tissue engineering is seen as a means of developing alternatives to conventional bone grafts for repairing or reconstructing bone defects by combining biomaterials, cells and signalling factors. However, skeletal tissue engineering has not yet achieved full translation into clinical practice as a consequence of several challenges. The use of additive manufacturing techniques for bone biofabrication is seen as a potential solution, with its inherent capability for reproducibility, accuracy and customisation of scaffolds as well as cell and signalling factor delivery. This review highlights the current research in bone biofabrication, the necessary factors for successful bone biofabrication, in addition to the current limitations affecting biofabrication, some of which are a consequence of the limitations of the additive manufacturing technology itself.

In Vivo Assessment of Bone Regeneration in Alginate/Bone ECM Hydrogels with Incorporated Skeletal Stem Cells and Single Growth Factors

The current study has investigated the use of decellularised, demineralised bone extracellular matrix (ECM) hydrogel constructs for in vivo tissue mineralisation and bone formation. Stro-1-enriched human bone marrow stromal cells were incorporated together with select growth factors including VEGF, TGF-β3, BMP-2, PTHrP and VitD3, to augment bone formation, and mixed with alginate for structural support. Growth factors were delivered through fast (non-osteogenic factors) and slow (osteogenic factors) release PLGA microparticles. Constructs of 5 mm length were implanted in vivo for 28 days within mice. Dense tissue assessed by micro-CT correlated with histologically assessed mineralised bone formation in all constructs. Exogenous growth factor addition did not enhance bone formation further compared to alginate/bone ECM (ALG/ECM) hydrogels alone. UV irradiation reduced bone formation through degradation of intrinsic growth factors within the bone ECM component and possibly also ECM cross-linking. BMP-2 and VitD3 rescued osteogenic induction. ALG/ECM hydrogels appeared highly osteoinductive and delivery of angiogenic or chondrogenic growth factors led to altered bone formation. All constructs demonstrated extensive host tissue invasion and vascularisation aiding integration and implant longevity. The proposed hydrogel system functioned without the need for growth factor incorporation or an exogenous inducible cell source. Optimal growth factor concentrations and spatiotemporal release profiles require further assessment, as the bone ECM component may suffer batch variability between donor materials. In summary, ALG/ECM hydrogels provide a versatile biomaterial scaffold for utilisation within regenerative medicine which may be tailored, ultimately, to form the tissue of choice through incorporation of select growth factors.

3D liver models on a microplatform: well-defined culture, engineering of liver tissue and liver-on-a-chip

The liver, the largest organ in the human body, is a multi-functional organ with diverse metabolic activities that plays a critical role in maintaining the body and sustaining life. Although the liver has excellent regenerative and recuperative properties, damages caused by chronic liver diseases or viral infection may lead to permanent loss of liver functions. Studies of liver disease mechanism have focused on drug screening and liver tissue engineering techniques, including strategies based on in vitro models. However, conventional liver models are plagued by a number of limitations, which have motivated the development of 'liver-on-a-chip' and microplatform-based bioreactors that can provide well-defined microenvironments. Microtechnology is a promising tool for liver tissue engineering and liver system development, as it can mimic the complex in vivo microenvironment and microlevel ultrastructure, by using a small number of human cells under two-dimensional (2D) and three-dimensional (3D) culture conditions. These systems provided by microtechnology allow improved liver-specific functions and can be expanded to encompass diverse 3D culture methods, which are critical for the maintenance of liver functions and recapitulation of the features of the native liver. In this review, we provide an overview of microtechnologies that have been used for liver studies, describe biomimetic technologies for constructing microscale 2D and 3D liver models as well as liver-on-a-chip systems and microscale bioreactors, and introduce applications of liver microtechnology and future trends in the field.

Bone Tissue Engineering

Medical advances have led to a welcome increase in life expectancy. However, accompanying longevity introduces new challenges: increases in age-related diseases and associated reductions in quality of life. The loss of skeletal tissue that can accompany trauma, injury, disease or advancing years can result in significant morbidity and significant socio-economic cost and emphasise the need for new, more reliable skeletal regeneration strategies. To address the unmet need for bone augmentation, tissue engineering and regenerative medicine have come to the fore in recent years with new approaches for de novo skeletal tissue formation. Typically, these approaches seek to harness stem cells, innovative scaffolds and biological factors that promise enhanced and more reliable bone formation strategies to improve the quality of life for many. This review provides an overview of recent developments in bone tissue engineering focusing on skeletal stem cells, vascular development, bone formation and the translation from preclinical in vivo models to clinical delivery.

Bone morphogenetic protein-2 and vascular endothelial growth factor in bone tissue regeneration: new insight and perspectives

The study of bone tissue regeneration in orthopaedic diseases has stimulated great interest among bone tissue engineering specialists and orthopaedic surgeons. Combinations of biomaterials, growth factors and stem cells for repairing bone have been much studied and researched, yet remain a challenge for both scientists and clinicians pursuing regenerative medicine. The purpose of this review was to elucidate the role of sequential release of bone morphogenetic protein-2 and vascular endothelial growth factor in producing better outcomes in the field of bone tissue regeneration.

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.

How does the pathophysiological context influence delivery of bone growth factors?

"Orthobiologics" represents an important category of therapeutics for the regeneration of bone defects caused by injuries or diseases, and bone growth factors are a particularly rapidly growing sub-category. Clinical application of bone growth factors has accelerated in the last two decades with the introduction of BMPs into clinical bone repair. Optimal use of growth factor-mediated treatments heavily relies on controlled delivery, which can substantially influence the local growth factor dose, release kinetics, and biological activity. The characteristics of the surrounding environment, or "context", during delivery can dictate growth factor loading efficiency, release and biological activity. This review discusses the influence of the surrounding environment on therapeutic delivery of bone growth factors. We specifically focus on pathophysiological components, including soluble components and cells, and how they can actively influence the therapeutic delivery and perhaps efficacy of bone growth factors.