Hybrid scaffolds based on PLGA and silk for bone tissue engineering

Advanced Applications of Silk-Based Hydrogels for Tissue Engineering: A Short Review

Silk has been consistently popular throughout human history due to its enigmatic properties. Today, it continues to be widely utilized as a polymer, having first been introduced to the textile industry. Furthermore, the health sector has also integrated silk. The Bombyx mori silk fibroin (SF) holds the record for being the most sustainable, functional, biocompatible, and easily produced type among all available SF sources. SF is a biopolymer approved by the FDA due to its high biocompatibility. It is versatile and can be used in various fields, as it is non-toxic and has no allergenic effects. Additionally, it enhances cell adhesion, adaptation, and proliferation. The use of SF has increased due to the rapid advancement in tissue engineering. This review comprises an introduction to SF and an assessment of the relevant literature using various methods and techniques to enhance the tissue engineering of SF-based hydrogels. Consequently, the function of SF in skin tissue engineering, wound repair, bone tissue engineering, cartilage tissue engineering, and drug delivery systems is therefore analysed. The potential future applications of this functional biopolymer for biomedical engineering are also explored.

In Vitro and Anti-Inflammatory Activity Evaluation Nanofibers from a Breath Mask and Filter Based on Polyurethane and Polyvinylidene Fluoride

Nanofiber (NF) products exhibit outstanding performances in materials science, textiles, and medicine that cannot be realized using conventional technologies. However, the safety of such products is debated because of the potential health risks that nanomaterials pose and the lack of standardized guidelines for the safety evaluation of NF products. The global safety evaluations of nanomaterials have focused on evaluating the cytotoxicity of low-dimensional materials, including nanoparticles and nanotubes, based on OECD (Organization for Economic Co-operation and Development) criteria. NFs are one-dimensional materials with nanometer diameters and considerable lengths. Many fibers are applied in a densely woven web-like form, so assessing cellular penetration and fiber toxicity using the same methods is inappropriate. This study verifies the safety of the polyurethane (PU) and polyvinylidene fluoride (PVDF) polymers currently applied in filters and masks. To this end, polymer NFs were collected from each product, and the NFs were compared with reference samples using FT-IR and Raman spectroscopy. For the safety evaluation, DMSO stocks of varying concentrations of PVDF and PU NFs (at 0.5, 1, 5, and 10 μg/mL) were prepared. The cytotoxicity and inhibitory effects on nitric oxide production and protein expression obtained via Western blot were identified.

Plasma-Treated Electrospun PLGA Nanofiber Scaffold Supports Limbal Stem Cells

The corneal epithelial layer is continuously replaced by limbal stem cells. Reconstructing this layer in vitro using synthetic scaffolds is highly needed. Poly-lactic-co-glycolic acid (PLGA) is approved for human use due to its biocompatibility and biodegradability. However, PLGA is hydrophobic, preventing cell adherence to PLGA membranes. PLGA scaffolds were prepared by electrospinning on a custom-made target drum spinning at a rate of 1000 rpm with a flow rate of 0.5 mL/h and voltage at 20 kV, then treated with oxygen plasma at 30 mA using a vacuum coater. Scaffolds were characterized by SEM, mechanically by tensile testing, and thermally by DSC and TGA. In vitro degradation was measured by weight loss and pH drop. Wettability was assessed through water uptake and contact angles measurements. Human limbal stem cells (hLSCs) were isolated and seeded on the scaffolds. Cell attachment and cytotoxicity assay were evaluated on day 1 and 5 after cell seeding. SEM showed regular fiber morphology with diameters ranging between 150 nm and 950 nm. Tensile strength demonstrated similar average stress values for both plasma- and non-plasma-treated samples. Scaffolds also showed gradual degradability over a period of 7-8 weeks. Water contact angle and water absorption were significantly enhanced for plasma-treated scaffolds, indicating a favorable increase in their hydrophilicity. Scaffolds have also supported hLSCs growth and attachment with no signs of cytotoxicity. We have characterized a nanofiber electrospun plasma-treated PLGA scaffold to investigate the mechanical and biological properties and the ability to support the attachment and maintenance of hLSCs.

3D and 4D printing hydroxyapatite-based scaffolds for bone tissue engineering and regeneration

The osseous tissue can be classified as a nanocomposite that encompasses a complex interweaving of organic and inorganic matrices. This intricate amalgamation consists of a collagen component and a mineral phase that are intricately arranged to form elaborate and perforated configurations. Hydroxyapatite, whether synthesized artificially or obtained from natural sources, has garnered considerable attention as a composite material in the field of bone tissue engineering due to its striking resemblance to bone in terms of structure and characteristics. Hydroxyapatite (HA) constitutes the predominant ceramic biomaterial for biomedical applications due to its ability to replicate the mineral composition of vertebrate bone. Nonetheless, it is noteworthy that the present biomimetic substance exhibits unfavorable mechanical characteristics, characterized by insufficient tensile and compressive strength, thus rendering it unsuitable for effective employment in the field of bone tissue engineering. Due to its beneficial attributes, hydroxyapatite (HA) is frequently employed in conjunction with various polymers and crosslinkers as composites to enhance mechanical properties and overall efficacy of implantable biomaterials engineered. The restoration of skeletal defects through the use of customized replacements is an effective way to replace damaged or lost bone structures. This method not only restores the bones' original functions but also reinstates their initial aesthetic appearance. The utilization of hydroxyapatite-polymer composites within 3D-printed grafts necessitates meticulous optimization of both mechanical and biological properties, in order to ensure their suitability for employment in medical devices. The utilization of 3D-printing technology represents an innovative approach in the manufacturing of HA-based scaffolds, which offers advantageous prospects for personalized bone regeneration. The expeditious prototyping method, with emphasis on the application of 3D printing, presents a viable approach in the development of bespoke prosthetic implants, grounded on healthcare data sets. 4D printing approach is an evolved form of 3D printing that utilizes programmable materials capable of altering the intended shape of printed structures, contingent upon single or dual stimulating factors. These factors include aspects such as pH level, temperature, humidity, crosslinking degree, and leaching factors.

Composite PLGA–Nanobioceramic Coating on Moxifloxacin-Loaded Akermanite 3D Porous Scaffolds for Bone Tissue Regeneration

Silica-based ceramics doped with calcium and magnesium have been proposed as suitable materials for scaffold fabrication. Akermanite (Ca2MgSi2O7) has attracted interest for bone regeneration due to its controllable biodegradation rate, improved mechanical properties, and high apatite-forming ability. Despite the profound advantages, ceramic scaffolds provide weak fracture resistance. The use of synthetic biopolymers such as poly(lactic-co-glycolic acid) (PLGA) as coating materials improves the mechanical performance of ceramic scaffolds and tailors their degradation rate. Moxifloxacin (MOX) is an antibiotic with antimicrobial activity against numerous aerobic and anaerobic bacteria. In this study, silica-based nanoparticles (NPs) enriched with calcium and magnesium, as well as copper and strontium ions that induce angiogenesis and osteogenesis, respectively, were incorporated into the PLGA coating. The aim was to produce composite akermanite/PLGA/NPs/MOX-loaded scaffolds through the foam replica technique combined with the sol–gel method to improve the overall effectiveness towards bone regeneration. The structural and physicochemical characterizations were evaluated. Their mechanical properties, apatite forming ability, degradation, pharmacokinetics, and hemocompatibility were also investigated. The addition of NPs improved the compressive strength, hemocompatibility, and in vitro degradation of the composite scaffolds, resulting in them keeping a 3D porous structure and a more prolonged release profile of MOX that makes them promising for bone regeneration applications.

Characteristic Evaluation of Recombinant MiSp/Poly(lactic-co-glycolic) Acid (PLGA) Nanofiber Scaffolds as Potential Scaffolds for Bone Tissue Engineering

Biomaterial-based nanofibrous scaffolds are the most effective alternative to bone transplantation therapy. Here, two recombinant minor ampullate spidroins (spider silk proteins), R1SR2 and NR1SR2C, were blended with Poly(lactic-co-glycolic) Acid (PLGA), respectively, to generate nanofiber scaffolds by electrospinning. The N-terminal (N), C-terminal (C), repeating (R1 and R2) and spacer (S) modules were all derived from the minor ampullate spidroins (MiSp). The physical properties and structures of the blended scaffolds were measured by scanning electron microscopy (SEM), water contact angle measurement, Fourier transform infrared spectroscopy (FTIR), Differential scanning calorimetry (DSC), and Tensile mechanical testing. The results showed that blending of MiSp (R1SR2 and NR1SR2C) reduced the diameter of nanofibers, increased the porosity and glass transition temperatures of nanofibrous scaffolds, and effectively improved the hydrophilicity and ultimate strain of scaffolds. It is worth noting that the above changes were more significant in the presence of the N- and C-termini of MiSp. In cell culture assays, human bone mesenchymal stem cells (HBMSCs) grown on NR1SR2C/PLGA (20/80) scaffolds displayed markedly enhanced proliferative and adhesive abilities compared with counterparts grown on pure PLGA scaffolds. Jointly, these findings indicated recombinant MiSp/PLGA, particularly NR1SR2C/PLGA (20/80) blend nanofibrous scaffolds, is promising for bone tissue engineering.

Bioessential Inorganic Molecular Wire-Reinforced 3D-Printed Hydrogel Scaffold for Enhanced Bone Regeneration

Materials with physicochemical properties and biological activities similar to those of the natural extracellular matrix are in high demand in tissue engineering. In particular, Mo₃ Se₃ ^- inorganic molecular wire (IMW) is a promising material composed of bioessential minerals and possess nanometer-scale diameters, negatively charged surfaces, physical flexibility, and nanotopography characteristics, which are essential for interactions with cell membrane proteins. Here, an implantable 3D Mo₃ Se₃ ^- IMW enhanced gelatin-GMA/silk-GMA hydrogel (IMW-GS hydrogel) is developed for osteogenesis and bone formation, followed by biological evaluations. The mechanical properties of the 3D printed IMW-GS hydrogel are improved by noncovalent interactions between the Mo₃ Se₃ ^- IMWs and the positively charged residues of the gelatin molecules. Long-term biocompatibility with primary human osteoblast cells (HOBs) is confirmed using the IMW-GS hydrogel. The proliferation, osteogenic gene expression, collagen accumulation, and mineralization of HOBs improve remarkably with the IMW-GS hydrogel. In in vivo evaluations, the IMW-GS hydrogel implantation exhibits a significantly improved new bone regeneration of 87.8 ± 5.9% (p < 0.05) for 8 weeks, which is higher than that from the gelatin-GMA/silk-GMA hydrogel without Mo₃ Se₃ ^- IMW. These results support a new improved strategy with in vitro and in vivo performance of 3D IMW enhanced scaffolds in tissue engineering.

Functionalized multidimensional biomaterials for bone microenvironment engineering applications: Focus on osteoimmunomodulation

As bone biology develops, it is gradually recognized that bone regeneration is a pathophysiological process that requires the simultaneous participation of multiple systems. With the introduction of osteoimmunology, the interplay between the immune system and the musculoskeletal diseases has been the conceptual framework for a thorough understanding of both systems and the advancement of osteoimmunomodulaty biomaterials. Various therapeutic strategies which include intervention of the surface characteristics or the local delivery systems with the incorporation of bioactive molecules have been applied to create an ideal bone microenvironment for bone tissue regeneration. Our review systematically summarized the current research that is being undertaken in the field of osteoimmunomodulaty bone biomaterials on a case-by-case basis, aiming to inspire more extensive research and promote clinical conversion.

Polylactide Perspectives in Biomedicine: From Novel Synthesis to the Application Performance

The incessant developments in the pharmaceutical and biomedical fields, particularly, customised solutions for specific diseases with targeted therapeutic treatments, require the design of multicomponent materials with multifunctional capabilities. Biodegradable polymers offer a variety of tailored physicochemical properties minimising health adverse side effects at a low price and weight, which are ideal to design matrices for hybrid materials. PLAs emerge as an ideal candidate to develop novel materials as are endowed withcombined ambivalent performance parameters. The state-of-the-art of use of PLA-based materials aimed at pharmaceutical and biomedical applications is reviewed, with an emphasis on the correlation between the synthesis and the processing conditions that define the nanostructure generated, with the final performance studies typically conducted with either therapeutic agents by in vitro and/or in vivo experiments or biomedical devices.

Surface polydopamine modification of bone defect repair materials: Characteristics and applications

Bone defects are a common challenge for clinical orthopedic surgeons. The existing bone defect repair materials are difficult to achieve satisfactory osseointegration between the material and the bone. Therefore, it is increasingly important to find effective methods to improve the integration of the materials with the bone and thus facilitate bone defect repair. Researchers have found that polydopamine (PDA) has a structure and properties similar to the adhesive proteins secreted by mussels in nature, with good biocompatibility, bioactivity, hydrophilicity, bio-adhesion and thermal stability. PDA is therefore expected to be used as a surface modification material for bone repair materials to improve the bonding of bone repair materials to the bone surface. This paper reviews research related to PDA-modified bone repair materials and looks at their future applications.

Application of 3D-Printed, PLGA-Based Scaffolds in Bone Tissue Engineering

Polylactic acid–glycolic acid (PLGA) has been widely used in bone tissue engineering due to its favorable biocompatibility and adjustable biodegradation. 3D printing technology can prepare scaffolds with rich structure and function, and is one of the best methods to obtain scaffolds for bone tissue repair. This review systematically summarizes the research progress of 3D-printed, PLGA-based scaffolds. The properties of the modified components of scaffolds are introduced in detail. The influence of structure and printing method change in printing process is analyzed. The advantages and disadvantages of their applications are illustrated by several examples. Finally, we briefly discuss the limitations and future development direction of current 3D-printed, PLGA-based materials for bone tissue repair.

Optimization of a Tricalcium Phosphate-Based Bone Model Using Cell-Sheet Technology to Simulate Bone Disorders

Bone diseases such as osteoporosis, delayed or impaired bone healing, and osteoarthritis still represent a social, financial, and personal burden for affected patients and society. Fully humanized in vitro 3D models of cancellous bone tissue are needed to develop new treatment strategies and meet patient-specific needs. Here, we demonstrate a successful cell-sheet-based process for optimized mesenchymal stromal cell (MSC) seeding on a β-tricalcium phosphate (TCP) scaffold to generate 3D models of cancellous bone tissue. Therefore, we seeded MSCs onto the β-TCP scaffold, induced osteogenic differentiation, and wrapped a single osteogenically induced MSC sheet around the pre-seeded scaffold. Comparing the wrapped with an unwrapped scaffold, we did not detect any differences in cell viability and structural integrity but a higher cell seeding rate with osteoid-like granular structures, an indicator of enhanced calcification. Finally, gene expression analysis showed a reduction in chondrogenic and adipogenic markers, but an increase in osteogenic markers in MSCs seeded on wrapped scaffolds. We conclude from these data that additional wrapping of pre-seeded scaffolds will provide a local niche that enhances osteogenic differentiation while repressing chondrogenic and adipogenic differentiation. This approach will eventually lead to optimized preclinical in vitro 3D models of cancellous bone tissue to develop new treatment strategies.

PLGA-Based Composites for Various Biomedical Applications

Polymeric materials have been extensively explored in the field of nanomedicine; within them, poly lactic-co-glycolic acid (PLGA) holds a prominent position in micro- and nanotechnology due to its biocompatibility and controllable biodegradability. In this review we focus on the combination of PLGA with different inorganic nanomaterials in the form of nanocomposites to overcome the polymer’s limitations and extend its field of applications. We discuss their physicochemical properties and a variety of well-established synthesis methods for the preparation of different PLGA-based materials. Recent progress in the design and biomedical applications of PLGA-based materials are thoroughly discussed to provide a framework for future research.

In vitro drug release and antibacterial activity evaluation of silk fibroin coated vancomycin hydrochloride loaded poly (lactic-co-glycolic acid) (PLGA) sustained release microspheres

Currently, the treatment of osteomyelitis poses a great challenge to clinical orthopedics. The use of biodegradable materials combined with antibiotics provides a completely new option for the treatment of osteomyelitis. In this study, vancomycin hydrochloride (VANCO) loaded poly (lactic-co-glycolic acid) (PLGA) microspheres were prepared by a double emulsion solvent evaporation method, and the in vitro drug release behaviors of the drug loaded microspheres were explored after coating with different concentrations of silk fibroin (SF). Drug loading, encapsulation efficiency, Scanning electron microscopy, particle size analysis, Fourier transform infrared spectroscopy, hydrophilicity, in vitro drug release, and in vitro antibacterial activity were evaluated. The results showed that the drug loading of vancomycin loaded PLGA microspheres was (24.11 ±1.72)%, and the encapsulation efficiency was (48.21 ±3.44)%. The in vitro drug release indicated that the drug loaded microspheres showed an obvious initial burst release, and the drug loaded microspheres coated with SF could alleviate the initial burst release in varying degrees. It also can reduce the amount of cumulative drug release, and the effect of microspheres coated with 0.1% concentration of SF is the best. The time of in vitro drug release in different groups of drug loaded microspheres can be up to 28 days. The microspheres coated with (0.1%SF) or without (0%SF) SF showed a cumulative release of (82.50±3.51)% and (67.70±3.81)%,respectively. Therefore, the surface coating with SF of vancomycin loaded microspheres can alleviate the initial burst release, reduce the cumulative drug release, potentially prolong the drug action time, and improve the anti-infection effect.

In situ mineralized PLGA/zwitterionic hydrogel composite scaffold enables high-efficiency rhBMP-2 release for critical-sized bone healing

Osteoconductive and osteoinductive scaffolds are highly desirable for functional restoration of large bone defects. Here, we report an in situ mineralized poly(lactic-co-glycolic acid)/poly[2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide hydrogel (PLGA/PSBMA) scaffold as a novel high-efficiency carrier for recombinant human bone morphogenetic protein-2 (rhBMP-2) for bone tissue regeneration. By virtue of the oppositely charged structure, the zwitterionic PSBMA component is able to template well-integrated dense mineralization of calcium phosphate throughout the PLGA/PSBMA scaffold. The high affinity between rhBMP-2 and the mineralized matrix, combined with the capability of the zwitterionic hydrogel to sequester and to enable sustained release of ionic proteins, endows the mineralized PLGA/PSBMA scaffolds with high-efficiency sustained release of rhBMP-2 (only 1.7% release within 35 days), thus enabling robust healing of critical-sized (5 mm) nonunion calvarial defects in rats at an ultralow dosage of rhBMP-2 (150 ng per scaffold), at which level successful healing of critical-sized bone defects has never been reported. These findings show that the mineralized PLGA/PSBMA scaffold is promising for bone defect repair.

Emerging zero-dimensional to four-dimensional biomaterials for bone regeneration

Bone is one of the most sophisticated and dynamic tissues in the human body, and is characterized by its remarkable potential for regeneration. In most cases, bone has the capacity to be restored to its original form with homeostatic functionality after injury without any remaining scarring. Throughout the fascinating processes of bone regeneration, a plethora of cell lineages and signaling molecules, together with the extracellular matrix, are precisely regulated at multiple length and time scales. However, conditions, such as delayed unions (or nonunion) and critical-sized bone defects, represent thorny challenges for orthopedic surgeons. During recent decades, a variety of novel biomaterials have been designed to mimic the organic and inorganic structure of the bone microenvironment, which have tremendously promoted and accelerated bone healing throughout different stages of bone regeneration. Advances in tissue engineering endowed bone scaffolds with phenomenal osteoconductivity, osteoinductivity, vascularization and neurotization effects as well as alluring properties, such as antibacterial effects. According to the dimensional structure and functional mechanism, these biomaterials are categorized as zero-dimensional, one-dimensional, two-dimensional, three-dimensional, and four-dimensional biomaterials. In this review, we comprehensively summarized the astounding advances in emerging biomaterials for bone regeneration by categorizing them as zero-dimensional to four-dimensional biomaterials, which were further elucidated by typical examples. Hopefully, this review will provide some inspiration for the future design of biomaterials for bone tissue engineering.

Increased Osteogenic Activity of Dynamic Cultured Composite Bone Scaffolds: Characterization and In Vitro Study

PURPOSE

The purpose of this study was to develop and characterize beta-tricalcium phosphate (β-TCP)/polycaprolactone (PCL) scaffolds, with 2 different ratios (50/50% and 65/35%), using 3-dimensionally (3D) printed dissolvable molds, and to evaluate cellular growth and osteogenic differentiation of both groups seeded with porcine bone marrow stem cells (pBMSCs) under dynamic culture in vitro.

MATERIALS AND METHODS

Two different groups of scaffolds were produced: group 1 (n = 40) with a ratio (wt%) of 50/50% and group 2 (n = 40) with 65/35% of β-TCP/PCL. Physicochemical, morphological, and mechanical characterization of the scaffolds were performed. Scaffolds were seeded with pBMSCs and differentiated osteogenically in dynamic culture. Cell density, distribution, and viability were assessed. Osteogenic differentiation was examined through alkaline phosphatase (ALP) staining, immunofluorescence, and photospectrometry.

RESULTS

Osteogenic differentiated constructs showed homogenous and viable cell distribution. Cell density was significantly higher (P < .05) for 65/35% scaffolds at 10 days postseeding, whereas at 6 weeks, cell number equalized for both groups. ALP activity increased over time and was significantly higher (P < .05) for 65/35% scaffolds at 14 days postseeding.

CONCLUSIONS

The mechanical properties of the developed 65/35% scaffolds were within the range of natural trabecular bone. Moreover, the 65/35% scaffolds showed biological advantages, such as higher cell growth and higher ALP activity.

Polymer Scaffolds-Enhanced Bone Regeneration in Osteonecrosis Therapy

Osteonecrosis without effective early treatment eventually leads to the collapse of the articular surface and causes arthritis. For the early stages of osteonecrosis, core decompression combined with bone grafting, is a procedure worthy of attention and clinical trial. And the study of bone graft substitutes has become a hot topic in the area of osteonecrosis research. In recent years, polymers have received more attention than other materials due to their excellent performance. However, because of the harsh microenvironment in osteonecrosis, pure polymers may not meet the stringent requirements of osteonecrosis research. The combined application of polymers and various other substances makes up for the shortcomings of polymers, and to meet a broad range of requirements for application in osteonecrosis therapy. This review focuses on various applying polymers in osteonecrosis therapy, then discusses the development of biofunctionalized composite polymers based on the polymers combined with different bioactive substances. At the end, we discuss their prospects for translation to clinical practice.

Fracture Healing Research-Shift towards In Vitro Modeling?

Fractures are one of the most frequently occurring traumatic events worldwide. Approximately 10% of fractures lead to bone healing disorders, resulting in strain for affected patients and enormous costs for society. In order to shed light into underlying mechanisms of bone regeneration (habitual or disturbed), and to develop new therapeutic strategies, various in vivo, ex vivo and in vitro models can be applied. Undeniably, in vivo models include the systemic and biological situation. However, transferability towards the human patient along with ethical concerns regarding in vivo models have to be considered. Fostered by enormous technical improvements, such as bioreactors, on-a-chip-technologies and bone tissue engineering, sophisticated in vitro models are of rising interest. These models offer the possibility to use human cells from individual donors, complex cell systems and 3D models, therefore bridging the transferability gap, providing a platform for the introduction of personalized precision medicine and finally sparing animals. Facing diverse processes during fracture healing and thus various scientific opportunities, the reliability of results oftentimes depends on the choice of an appropriate model. Hence, we here focus on categorizing available models with respect to the requirements of the scientific approach.

Injectable Mesenchymal Stem Cell-Laden Matrigel Microspheres for Endometrium Repair and Regeneration

Endometrial injury and intrauterine adhesions are increasingly reported in recent years; however, treatment options remain limited. Intravenous injection of mesenchymal stem cells (MSCs) for endometrium regeneration has limited effectiveness as the retention rate of transplanted cells is low. Hydrogel-based tissue-engineered solutions, such as MSC-seeded bioscaffolds, are reported to increase retention rates; however, a less invasive alternative is still desirable. 560-µm homogeneous Matrigel microspheres are fabricated, loading them with about 1500 MSCs and injecting them into the injured endometria of rats' uteri. This minimally invasive procedure is proved to significantly increase endometrium thickness by over onefold after 21 d (p < 0.0001) and fertility rates from 25% to 75% in impaired and repaired uteri (p < 0.001), respectively. This study provides a minimally invasive alternative to endometrium repair with the promise to establish a broad-spectrum technique for MSC transplantation.

Bioprinted nanocomposite hydrogels: A proposed approach to functional restoration of skeletal muscle and vascular tissue following volumetric muscle loss

Musculoskeletal conditions are the highest contributor to global disability, accounting for 16% of all ages lived with disability. Volumetric muscle loss (VML) is classified as significant damage to skeletal muscle compartments and motor units, leading to significant tissue loss, functional deficits, and long-term disability. In this review, the current tissue engineering approaches in terms of fabrication techniques, materials, cell sources, and growth factors for enhanced angiogenesis and neuromuscular junction (NMJ) in VML repair, are discussed. Review of results recently published in the literature suggested that bioprinted nanocomposite hydrogels (NC gels) seeded with adult muscle progenitor cells that promote secretion of endogenous vascular growth factors have potential applications in promoting skeletal muscle regeneration, revascularization, and NMJ repair (Figure 1). Despite recent advancements, future research is needed on NC gels and the complex processes underlying vascular infiltration and NMJ repair in VML injuries.

Electrospun artificial periosteum loaded with DFO contributes to osteogenesis via the TGF-β1/Smad2 pathway

Deferoxamine (DFO), an iron chelator regarded as a hypoxic analogue, has been reported to be involved in angiogenesis and osteogenic differentiation. In this study, DFO was loaded into nanospheres, Then, DFO-loaded NPs and free DFO were co-encapsulated in nanofibers through coaxial electrospinning and its effects on cell viability, migration, and osteogenic differentiation, and the potential mechanisms were investigated. The results suggested that DFO maintained cell viability and promoted the migration of human mesenchymal stem cells (hMSCs) and MC3T3-E1 cells. ALP activity, calcium deposition, and expression of osteogenesis-related markers, including collagen, osteocalcin, and osteopontin, were all increased with DFO. Moreover, hypoxia-inducible factor-1α, transforming growth factor-β, and Smad2 were upregulated with DFO, which indicated activation of the TGF-β1/Smad2 signalling pathway. This may contribute to osteogenic differentiation of cells.

Covas J. Assessment of the Mechanical and Thermal Properties of Injection-Molded Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/Hydroxyapatite Nanoparticles Parts for Use in Bone Tissue Engineering

In the present study, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] was reinforced with hydroxyapatite nanoparticles (nHA) to produce novel nanocomposites for potential uses in bone reconstruction. Contents of nHA in the 2.5-20 wt % range were incorporated into P(3HB-co-3HHx) by melt compounding and the resulting pellets were shaped into parts by injection molding. The addition of nHA improved the mechanical strength and the thermomechanical resistance of the microbial copolyester parts. In particular, the addition of 20 wt % of nHA increased the tensile (Et) and flexural (Ef) moduli by approximately 64% and 61%, respectively. At the highest contents, however, the nanoparticles tended to agglomerate, and the ductility, toughness, and thermal stability of the parts also declined. The P(3HB-co-3HHx) parts filled with nHA contents of up to 10 wt % matched more closely the mechanical properties of the native bone in terms of strength and ductility when compared with metal alloys and other biopolymers used in bone tissue engineering. This fact, in combination with their biocompatibility, enables the development of nanocomposite parts to be applied as low-stress implantable devices that can promote bone reconstruction and be reabsorbed into the human body.

Injectable and Crosslinkable PLGA-Based Microribbons as 3D Macroporous Stem Cell Niche

Poly(lactide-co-glycolide) (PLGA) has been widely used as a tissue engineering scaffold. However, conventional PLGA scaffolds are not injectable, and do not support direct cell encapsulation, leading to poor cell distribution in 3D. Here, a method for fabricating injectable and intercrosslinkable PLGA microribbon-based macroporous scaffolds as 3D stem cell niche is reported. PLGA is first fabricated into microribbon-shape building blocks with tunable width using microcontact printing, then coated with fibrinogen to enhance solubility and injectability using aqueous solution. Upon mixing with thrombin, firbornogen-coated PLGA microribbons can intercrosslink into 3D scaffolds. When subject to cyclic compression, PLGA microribbon scaffolds exhibit great shock-absorbing capacity and return to their original shape, while conventional PLGA scaffolds exhibit permanent deformation after one cycle. Using human mesenchymal stem cells (hMSCs) as a model cell type, it is demonstrated that PLGA μRB scaffolds support homogeneous cell encapsulation, and robust cell spreading and proliferation in 3D. After 28 days of culture in osteogenic medium, hMSC-seeded PLGA μRB scaffolds exhibit an increase in compressive modulus and robust bone formation as shown by staining of alkaline phosphatase, mineralization, and collagen. Together, the results validate PLGA μRBs as a promising injectable, macroporous, non-hydrogel-based scaffold for cell delivery and tissue regeneration applications.

Bone forming ability of recombinant human collagen peptide granules applied with β-tricalcium phosphate fine particles

Recombinant human collagen peptide, developed based on human collagen type I, contains an arginyl-glycyl-aspartic acid (RGD)-rich motif to enhance cell behavior and is anticipated as a xeno-free polymer material for use in tissue engineering. We fabricated granules containing recombinant human collagen peptide (RCP) applied with beta-tricalcium phosphate fine particles (RCP/β-TCP) as bone filling scaffold material and assessed the bone forming ability of RCP/β-TCP. Recombinant peptide was thermal crosslinked and freeze-dried to prepare RCP. An aqueous dispersion of β-TCP fine particles was added to RCP to obtain RCP/β-TCP. Subsequently, RCP/β-TCP were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), and cell culture assessments. Furthermore, RCP/β-TCP were implanted into rat cranial bone defects for radiographic and histological evaluations. In SEM and EDX analyses of RCP/β-TCP, β-TCP particles dose-dependently covered the surface of RCP. Cell culture tests showed that RCP/β-TCP remarkably promoted proliferation and mRNA expression of various genes, such as integrin β1 and osteogenic markers, of osteoblastic MC3T3-E1 cells. Histomorphometric assessment at 4 weeks showed that RCP/β-TCP significantly promoted new skull bone formation compared to RCP (p < 0.05) and control (no application) (p < 0.01). Accordingly, these findings suggest RCP/β-TCP possess bone forming capability and would be beneficial for bone tissue engineering therapy.

Electrospun nanofibers for the delivery of active drugs through nasal, oral and vaginal mucosa: Current status and future perspectives

Transmucosal surfaces bypass many limitations associated with conventional drug delivery (oral and parenteral routes), such as poor absorption rate, enzymatic activity, acidic environment and first-pass metabolism occurring inside the liver. However, these surfaces have several disadvantages such as poor retention time, narrow absorption window and continuous washout of the drug by the surrounding fluids. Electrospun nanofibers with their unique surface properties and encapsulation efficiency may act as novel drug carriers to overcome the challenges associated with conventional drug delivery routes, so as to achieve desired therapeutic responses. This review article provides detailed information regarding the challenges faced in the mucosal delivery of drugs, and the use of nanofiber systems as an alternative to deliver drugs to the systemic circulation, as well as local drug administration. The physiological and anatomical features of different types of mucosal surfaces and current challenges are systematically discussed. We also address future considerations in the area of transmucosal delivery of some important drugs.

Natural polypeptides-based electrically conductive biomaterials for tissue engineering

Fabrication of an appropriate scaffold is the key fundamental step required for a successful tissue engineering (TE). The artificial scaffold as extracellular matrix in TE has noticeable role in the fate of cells in terms of their attachment, proliferation, differentiation, orientation and movement. In addition, chemical and electrical stimulations affect various behaviors of cells such as polarity and functionality. Therefore, the fabrication approach and materials used for the preparation of scaffold should be more considered. Various synthetic and natural polymers have been used extensively for the preparation of scaffolds. The electrically conductive polymers (ECPs), moreover, have been used in combination with other polymers to apply electric fields (EF) during TE. In this context, composites of natural polypeptides and ECPs can be taken into account as context for the preparation of suitable scaffolds with superior biological and physicochemical features. In this review, we overviewed the simultaneous usage of natural polypeptides and ECPs for the fabrication of scaffolds in TE.

Injectable Nanocomposite Hydrogels and Electrosprayed Nano(Micro)Particles for Biomedical Applications

Polymeric scaffolds have played important roles in biomedical applications due to their potentially practical performance such as delivery of bioactive components and/or regenerative cells. These materials were well-designed to encapsulate bioactive molecules or/and nanoparticles for enhancing their performance in tissue regeneration and drug delivery systems. In the study, several multifunctional nanocomposite hydrogel and polymeric nano(micro)particles-electrosprayed platforms were described from their fabrication methods and structural characterizations to potential applications in the mentioned fields. Regarding to their described performance, these multifunctional nanocomposite biomaterials could pay many ways for further studies that enables them apply in clinical applications.

Multiscale Porosity in Compressible Cryogenically 3D Printed Gels for Bone Tissue Engineering

Three-dimensional (3D) printing technology has seen several refinements when introduced in the field of medical devices and regenerative medicines. However, it is still a challenge to 3D print gels for building complex constructs as per the desired shape and size. Here, we present a novel method to 3D print gelatin/carboxymethylchitin/hydroxyapatite composite gel constructs of a complex shape. The objective of this study is to fabricate a bioactive gel scaffold with a controlled hierarchical structure. The hierarchy ranges from 3D outer shape to macroporosity to microporosity and rough surface. The fabrication process developed here uses 3D printing in a local cryogenic atmosphere, followed by lyophilization and cross-linking. The gel instantly freezes after extrusion on the cold plate. The cooling action is not limited to the build plate, but the entire gel scaffold is cooled during the 3D printing process. This enables the construction of a stable self-sustaining large-sized 3D complex geometry. Further, lyophilization introduces bulk microporosity into the scaffolds. The outer shape and macroporosity were controlled with the 3D printer, whereas the microporous structure and desirable rough surface morphology were obtained through lyophilization. With cryogenic 3D printing, up to 90% microporosity could be incorporated into the scaffolds. The microporosity and pore size distribution were controlled by changing the cross-linker and total polymer concentration, which resulted in six times increase in surface open pores of size <20 μm on increasing the cross-linker concentration from 25 to 100 mg/mL. The introduction of bulk microporosity was shown to increase swelling by 1.8 times along with a significant increase in human umbilical cord mesenchymal stem cells and Saos-2 cell attachment (2×), proliferation (2.4×), Saos-2 cell alkaline phosphatase level (2×), and mineralization (3×). The scaffolds are spongy in nature in a wet state, thus making them potential implants for bone cavities with a small opening. The application of these cryogenically 3D printed compressible gel scaffolds with multiscale porosity extends to a small- as well as a large-sized open/partially open patient-specific bone defect.

Three-dimensional (3D) printed scaffold and material selection for bone repair

Critical-sized bone defect repair remains a substantial challenge in clinical settings and requires bone grafts or bone substitute materials. However, existing biomaterials often do not meet the clinical requirements of structural support, osteoinductive property, and controllable biodegradability. To treat large-scale bone defects, the development of three-dimensional (3D) porous scaffolds has received considerable focus within bone engineering. A variety of biomaterials and manufacturing methods, including 3D printing, have emerged to fabricate patient-specific bioactive scaffolds that possess controlled micro-architectures for bridging bone defects in complex configurations. During the last decade, with the development of the 3D printing industry, a large number of tissue-engineered scaffolds have been created for preclinical and clinical applications using novel materials and innovative technologies. Thus, this review provides a brief overview of current progress in existing biomaterials and tissue engineering scaffolds prepared by 3D printing technologies, with an emphasis on the material selection, scaffold design optimization, and their preclinical and clinical applications in the repair of critical-sized bone defects. Furthermore, it will elaborate on the current limitations and potential future prospects of 3D printing technology. STATEMENT OF SIGNIFICANCE: 3D printing has emerged as a critical fabrication process for bone engineering due to its ability to control bulk geometry and internal structure of tissue scaffolds. The advancement of bioprinting methods and compatible ink materials for bone engineering have been a major focus to develop optimal 3D scaffolds for bone defect repair. Achieving a successful balance of cellular function, cellular viability, and mechanical integrity under load-bearing conditions is critical. Hybridization of natural and synthetic polymer-based materials is a promising approach to create novel tissue engineered scaffolds that combines the advantages of both materials and meets various requirements, including biological activity, mechanical strength, easy fabrication and controllable degradation. 3D printing is linked to the future of bone grafts to create on-demand patient-specific scaffolds.

Micro-CT - a digital 3D microstructural voyage into scaffolds: a systematic review of the reported methods and results

BACKGROUND

Cell behavior is the key to tissue regeneration. Given the fact that most of the cells used in tissue engineering are anchorage-dependent, their behavior including adhesion, growth, migration, matrix synthesis, and differentiation is related to the design of the scaffolds. Thus, characterization of the scaffolds is highly required. Micro-computed tomography (micro-CT) provides a powerful platform to analyze, visualize, and explore any portion of interest in the scaffold in a 3D fashion without cutting or destroying it with the benefit of almost no sample preparation need.

MAIN BODY

This review highlights the relationship between the scaffold microstructure and cell behavior, and provides the basics of the micro-CT method. In this work, we also analyzed the original papers that were published in 2016 through a systematic search to address the need for specific improvements in the methods section of the papers including the amount of provided information from the obtained results.

CONCLUSION

Micro-CT offers a unique microstructural analysis of biomaterials, notwithstanding the associated challenges and limitations. Future studies that will include micro-CT characterization of scaffolds should report the important details of the method, and the derived quantitative and qualitative information can be maximized.

3D bioactive composite scaffolds for bone tissue engineering

Bone is the second most commonly transplanted tissue worldwide, with over four million operations using bone grafts or bone substitute materials annually to treat bone defects. However, significant limitations affect current treatment options and clinical demand for bone grafts continues to rise due to conditions such as trauma, cancer, infection and arthritis. Developing bioactive three-dimensional (3D) scaffolds to support bone regeneration has therefore become a key area of focus within bone tissue engineering (BTE). A variety of materials and manufacturing methods including 3D printing have been used to create novel alternatives to traditional bone grafts. However, individual groups of materials including polymers, ceramics and hydrogels have been unable to fully replicate the properties of bone when used alone. Favourable material properties can be combined and bioactivity improved when groups of materials are used together in composite 3D scaffolds. This review will therefore consider the ideal properties of bioactive composite 3D scaffolds and examine recent use of polymers, hydrogels, metals, ceramics and bio-glasses in BTE. Scaffold fabrication methodology, mechanical performance, biocompatibility, bioactivity, and potential clinical translations will be discussed.

Recent Trends in the Fabrication of Starch Nanofibers: Electrospinning and Non-electrospinning Routes and Their Applications in Biotechnology

Electrospinning a versatile and the most preferred technique for the fabrication of nanofibers has revolutionized by opening unlimited avenues in biomedical fields. Presently, the simultaneous functionalization and/or post-modification of as-spun nanofibers with biomolecules has been explored, to serve the distinct goals in the aforementioned field. Starch is one of the most abundant biopolymers on the earth. Besides, being biocompatible and biodegradable in nature, it has unprecedented properties of gelatinization and retrogradation. Therefore, starch has been used in numerous ways for wide range of applications. Keeping these properties in consideration, the present article summarizes the recent expansion in the fabrication of the pristine/modified starch-based composite scaffolds by electrospinning along with their possible applications. Apart from electrospinning technique, this review will also provide the comprehensive information on various other techniques employed in the fabrication of the starch-based nanofibers. Furthermore, we conclude with the challenges to be overcome in the fabrication of nanofibers by the electrospinning technique and future prospects of starch-based fabricated scaffolds for exploration of its applications.

Potentiation effect on accelerating diabetic wound healing using 2-N,6-O-sulfated chitosan-doped PLGA scaffold

Accelerating the wound healing of diabetes-impaired cutaneous tissue is still a challenge due to the aberrant cellular behavior, poor angiogenesis, and pathological micro-environment. Activation with growth factors and modulation of the redox micro-environment of the diabetic wound are considered to be effective strategies. Herein, we have described a highly sulfated heparin-like polysaccharide 2-N, 6-O-sulfated chitosan (26SCS)-doped poly(lactic-co-glycolic acid) scaffold (S-PLGA), which can achieve controlled and sustained release of heparin-binding epidermal growth factor (HB-EGF) owing to its affinity for heparin-binding growth factors. Interestingly, the antioxidant effect of 26SCS was confirmed and it was shown to have a strong scavenging activity towards superoxide radicals, a moderate scavenging activity towards hydroxyl radicals and a lower scavenging activity towards hydrogen peroxide. It also exhibited stronger protective effects in a human keratinocyte cell line (Ha-cat) against H₂O₂-induced oxidative damage. The Ha-cat cells cultured in the presence of the S-PLGA scaffold were significantly protected against oxidative stress during proliferation. In a full thickness excisional wound model of a diabetic rat, the wound treated with the HB-EGF-loaded S-PLGA scaffold was basically healed after 28 days. Conversely, the wounds in the other diabetic groups were not closed. The migration effect of the keratinocytes was enhanced by the 26SCS-induced sustainable release of HB-EGF and the scavenging of ROS which led to rapid re-epithelialization. Furthermore, histopathological evaluation demonstrated the positive effects on wound contraction, epithelial regeneration, and collagen deposition when treated with the HB-EGF loaded S-PLGA scaffold. These findings highlight that 26SCS may serve as a promising coagent for both controlled release of growth factors and alleviation of excessive ROS production, thus leading to increased regeneration of the diabetic wounds.

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.

A macroporous heparin-releasing silk fibroin scaffold improves islet transplantation outcome by promoting islet revascularisation and survival

Islet transplantation is considered the most promising therapeutic option with the potential to cure diabetes. However, efficacy of current clinical islet transplantation is limited by long-term graft dysfunction and attrition. We have investigated the therapeutic potential of a silk fibroin macroporous (SF) scaffold for syngeneic islet transplantation in diabetic mice. The SF scaffold was prepared via lyophilisation, which enables incorporation of active compounds including cytokines, peptide and growth factors without compromising their biological activity. For the present study, a heparin-releasing SF scaffold (H-SF) in order to evaluate the versatility of the SF scaffold for biological functionalisation. Islets were then co-transplanted with H-SF or SF scaffolds in the epididymal fat pad of diabetic mice. Mice from both H-SF and SF groups achieved 100% euglycaemia, which was maintained for 1year. More importantly, the H-SF-islets co-transplantation led to more rapid reversal of hyperglycaemia, complete normalisation of glucose responsiveness and lower long-term blood glucose levels. This superior transplantation outcome is attributable to H-SF-facilitated islet revascularisation and cell proliferation since significant increase of islet endocrine and endothelial cells proliferation was shown in grafts retrieved from H-SF-islets co-transplanted mice. Better intra-islet vascular reformation was also evident, accompanied by VEGF upregulation. In addition, when H-SF was co-transplanted with islets extracted from vegfr2-luc transgenic mice in vivo, sustained elevation of bioluminescent signal that corresponds to vegfr2 expression was collected, implicating a role of heparin-dependent activation of endogenous VEGF/VEGFR2 pathway in promoting islet revascularisation and proliferation. In summary, the SF scaffolds provide an open platform as scaffold development for islet transplantation. Furthermore, given the pro-angiogenic, pro-survival and minimal post-transplantation inflammatory reactions of H-SF, our data also support the feasibility of clinical implementation of H-SF to improve islet transplantation outcome.

STATEMENT OF SIGNIFICANCE

1) The silk fibroin scaffold presented in the present study provides an open platform for scaffold development in islet transplantation, with heparinisation as an example. 2) Both heparin and silk fibroin have been used clinically. The excellent in vivo therapeutic outcome reported here may therefore be clinically relevant and provide valuable insights for bench to bed translation. 3) Compared to conventional clinical islet transplantation, during which islets are injected via the hepatic portal vein, the physical/mechanical properties of silk fibroin scaffolds create a more accessible transplantation site (i.e., within fat pad), which significantly reduces discomfort. 4) Islet implantation into the fat pad also avoids an instant blood mediated inflammatory response, which occurs upon contact of islet with recipient's blood during intraportal injection, and prolongs survival and function of implanted islets.

Morphological Analysis of Biocompatibility of Autologous Bone Marrow Mononuclear Cells with Synthetic Polyethylene Terephthalate Scaffold

We studied the properties of a tissue-engineered trachea consisting of a polyethylene terephthalate scaffold populated with autologous bone marrow mononuclear cells. The tissue-engineered constructs were obtained before surgery, during the postoperative period, and during autopsy. Cytomorphological analysis during the postoperative period showed the presence of mesenchymal stem cells on the inner surface of the implant on day 3 after surgery and cells of the respiratory epithelium on day 10-14. In autopsy samples, single epithelial cells, endothelial cells, and basal cells were found. Biocompatibility of the tissue-engineered trachea with autologous mononuclear cells of the patient was demonstrated.

The Influence of Copolymer Composition on PLGA/nHA Scaffolds' Cytotoxicity and In Vitro Degradation

The influence of copolymer composition on Poly(Lactide-co-Glycolide)/nanohydroxyapatite (PLGA/nHA) composite scaffolds is studied in the context of bone tissue engineering and regenerative medicine. The composite scaffolds are fabricated by thermally-induced phase separation and the effect of bioactive nanoparticles on their in vitro degradation in phosphate-buffered solution at 37 °C is analyzed over eight weeks. The indirect cytotoxicity evaluation of the samples followed an adaptation of the ISO 10993-5 standard test method. Based on the measurement of their molecular weight, molar mass, pH, water absorption and dimensions, the porous scaffolds of PLGA with a lower lactide/glycolide (LA/GA) molar ratio degraded faster due to their higher hydrophilicity. All of the samples without and with HA are not cytotoxic, demonstrating their potential for tissue engineering applications.

Bioactive polymeric scaffolds for tissue engineering

A variety of engineered scaffolds have been created for tissue engineering using polymers, ceramics and their composites. Biomimicry has been adopted for majority of the three-dimensional (3D) scaffold design both in terms of physicochemical properties, as well as bioactivity for superior tissue regeneration. Scaffolds fabricated via salt leaching, particle sintering, hydrogels and lithography have been successful in promoting cell growth in vitro and tissue regeneration in vivo. Scaffold systems derived from decellularization of whole organs or tissues has been popular due to their assured biocompatibility and bioactivity. Traditional scaffold fabrication techniques often failed to create intricate structures with greater resolution, not reproducible and involved multiple steps. The 3D printing technology overcome several limitations of the traditional techniques and made it easier to adopt several thermoplastics and hydrogels to create micro-nanostructured scaffolds and devices for tissue engineering and drug delivery. This review highlights scaffold fabrication methodologies with a focus on optimizing scaffold performance through the matrix pores, bioactivity and degradation rate to enable tissue regeneration. Review highlights few examples of bioactive scaffold mediated nerve, muscle, tendon/ligament and bone regeneration. Regardless of the efforts required for optimization, a shift in 3D scaffold uses from the laboratory into everyday life is expected in the near future as some of the methods discussed in this review become more streamlined.

Approaches for building bioactive elements into synthetic scaffolds for bone tissue engineering

Bone tissue engineering (BTE) is emerging as a possible solution for regeneration of bone in a number of applications. For effective utilization, BTE scaffolds often need modifications to impart biological cues that drive diverse cellular functions such as adhesion, migration, survival, proliferation, differentiation, and biomineralization. This review provides an outline of various approaches for building bioactive elements into synthetic scaffolds for BTE and classifies them broadly under two distinct schemes; namely, the top-down approach and the bottom-up approach. Synthetic and natural routes for top-down approaches to production of bioactive constructs for BTE, such as generation of scaffold-extracellular matrix (ECM) hybrid constructs or decellularized and demineralized scaffolds, are provided. Similarly, traditional scaffold-based bottom-up approaches, including growth factor immobilization or peptide-tethered scaffolds, are provided. Finally, a brief overview of emerging bottom-up approaches for generating biologically active constructs for BTE is given. A discussion of the key areas for further investigation, challenges, and opportunities is also presented.

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

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

Biodegradable Materials for Bone Repair and Tissue Engineering Applications

This review discusses and summarizes the recent developments and advances in the use of biodegradable materials for bone repair purposes. The choice between using degradable and non-degradable devices for orthopedic and maxillofacial applications must be carefully weighed. Traditional biodegradable devices for osteosynthesis have been successful in low or mild load bearing applications. However, continuing research and recent developments in the field of material science has resulted in development of biomaterials with improved strength and mechanical properties. For this purpose, biodegradable materials, including polymers, ceramics and magnesium alloys have attracted much attention for osteologic repair and applications. The next generation of biodegradable materials would benefit from recent knowledge gained regarding cell material interactions, with better control of interfacing between the material and the surrounding bone tissue. The next generations of biodegradable materials for bone repair and regeneration applications require better control of interfacing between the material and the surrounding bone tissue. Also, the mechanical properties and degradation/resorption profiles of these materials require further improvement to broaden their use and achieve better clinical results.