Cryogenic 3D printing of dual-delivery scaffolds for improved bone regeneration with enhanced vascularization

Advancing bone regeneration: Unveiling the potential of 3D cell models in the evaluation of bone regenerative materials

Bone, a rigid yet regenerative tissue, has garnered extensive attention for its impressive healing abilities. Despite advancements in understanding bone repair and creating treatments for bone injuries, handling nonunions and large defects remains a major challenge in orthopedics. The rise of bone regenerative materials is transforming the approach to bone repair, offering innovative solutions for nonunions and significant defects, and thus reshaping orthopedic care. Evaluating these materials effectively is key to advancing bone tissue regeneration, especially in difficult healing scenarios, making it a critical research area. Traditional evaluation methods, including two-dimensional cell models and animal models, have limitations in predicting accurately. This has led to exploring alternative methods, like 3D cell models, which provide fresh perspectives for assessing bone materials' regenerative potential. This paper discusses various techniques for constructing 3D cell models, their pros and cons, and crucial factors to consider when using these models to evaluate bone regenerative materials. We also highlight the significance of 3D cell models in the in vitro assessments of these materials, discuss their current drawbacks and limitations, and suggest future research directions. STATEMENT OF SIGNIFICANCE: This work addresses the challenge of evaluating bone regenerative materials (BRMs) crucial for bone tissue engineering. It explores the emerging role of 3D cell models as superior alternatives to traditional methods for assessing these materials. By dissecting the construction, key factors of evaluating, advantages, limitations, and practical considerations of 3D cell models, the paper elucidates their significance in overcoming current evaluation method shortcomings. It highlights how these models offer a more physiologically relevant and ethically preferable platform for the precise assessment of BRMs. This contribution is particularly significant for "Acta Biomaterialia" readership, as it not only synthesizes current knowledge but also propels the discourse forward in the search for advanced solutions in bone tissue engineering and regeneration.

Imidazole functionalized photo-crosslinked aliphatic polycarbonate drug-eluting coatings on zinc alloys for osteogenesis, angiogenesis, and bacteriostasis in bone regeneration

Zinc (Zn) alloys have demonstrated significant potential in healing critical-sized bone defects. However, the clinical application of Zn alloys implants is still hindered by challenges including excessive release of zinc ions (Zn2+), particularly in the early stage of implantation, and absence of bio-functions related to complex bone repair processes. Herein, a biodegradable aliphatic polycarbonate drug-eluting coating was fabricated on zinc-lithium (Zn-Li) alloys to inhibit Zn2+ release and enhance the osteogenesis, angiogenesis, and bacteriostasis of Zn alloys. Specifically, the photo-curable aliphatic polycarbonates were co-assembled with simvastatin and deposited onto Zn alloys to produce a drug-loaded coating, which was crosslinked by subsequent UV light irradiation. During the 60 days long-term immersion test, the coating showed distinguished stable drug release and Zn2+ release inhibition properties. Benefiting from the regulated release of Zn2+ and simvastatin, the coating facilitated the adhesion, proliferation, and differentiation of MC3T3-E1 cells, as well as the migration and tube formation of EA.hy926 cells. Astonishingly, the coating also showed remarkable antibacterial properties against both S. aureus and E. coli. The in vivo rabbit critical-size femur bone defects model demonstrated that the drug-eluting coating could efficiently promote new bone formation and the expression of platelet endothelial cell adhesion molecule-1 (CD31) and osteocalcin (OCN). The enhancement of osteogenesis, angiogenesis, and bacteriostasis is achieved by precisely controlling of the released Zn2+ at an appropriate level, as well as the stable release profile of simvastatin. This tailored aliphatic polycarbonate drug-eluting coating provides significant potential for clinical applications of Zn alloys implants.

Conducting biointerface of spider-net-like chitosan-adorned polyurethane/SPIONs@SrO2-fMWCNTs for bone tissue engineering and antibacterial efficacy

In pursuit of enhancing bone cell proliferation, this study delves into the fabrication of porous scaffolds through the integration of nanomaterials. Specifically, we present the development of highly conductive chitosan (CS) nanonets on fibro-porous polyurethane (PU) bio-membranes. These nanofibers comprise functionalized multiwall carbon nanotubes (fMWCNTs), well-dispersed superparamagnetic iron oxide (SPIONs), and strontium oxide (SrO2) nanoparticles. The resulting porous scaffold exhibits remarkable interfacial biocompatibility, antibacterial properties, and load-bearing capability. Through meticulous in vitro investigations, the CS-PU/SPIONs/SrO2-fMWCNTs nanofibrous scaffolds have demonstrated a propensity to promote bone cell regeneration. Notably, the integration of these nanomaterials has been found to upregulate crucial bone-related markers, including ALP, ARS, COL-I, RUNX2, and SPP-I. The evaluation of these markers, conducted through quantitative real-time polymerase chain reaction (qRT-PCR) and immunocytochemistry, substantiates the improved cell survival and enhanced osteogenic differentiation facilitated by the integrated nanomaterials. This comprehensive analysis underscores the efficacy of CS-PU/SPIONs/SrO2-fMWCNTs bioscaffolds in promoting MC3T3-E1 cell regeneration within, thereby holding promise for advancements in bone tissue engineering and regenerative medicine.

Crosslinking strategies for biomimetic hydrogels in bone tissue engineering

Bone tissue engineering has become a popular area of study for making biomimetic hydrogels to treat bone diseases. In this work, we looked at biocompatible hydrogels that can be injected into bone defects that require the smallest possible surgery. Mineral ions can be attached to polymer chains to make useful hydrogels that help bones heal faster. These ions are very important for the balance of the body. In the chemically-triggered sector, advanced hydrogels cross-linked by different molecular agents have many advantages, such as being selective, able to form gels, and having mechanical properties that can be modified. In addition, different photo-initiators can be used to make photo cross linkable hydrogels react quickly and moderately under certain light bands. Enzyme-triggered hydrogels are another type of hydrogel that can be used to repair bone tissue because they are biocompatible and gel quickly. We also look at some of the important factors mentioned above that could change how well bone tissue engineering works as a therapy. Finally, this review summarizes the problems that still need to be solved to make clinically relevant hydrogels.

PP2 alleviates the progression of osteoarthritis by inhibiting Wnt/β-catenin and activating TGF-β/Smad signaling

OBJECTIVE

We aimed to explore the effect and mechanism of the Src inhibitor PP2 on osteoarthritis (OA) progression.

METHODS

The protein expressions of Src, p-Src (y418) and p-FAK in normal and OA human chondrocytes were detected by immunofluorescence (IF) analysis. Chondrocytes from the femur and tibial plateau of 3-day-old mice were extracted and inoculated into 6-well plates. The chondrocytes were co-cultured with IL-1β and different doses of PP2, and then the degeneration of extracellular matrix was analyzed. A mouse OA model was induced by destabilizing medial meniscectomy of the right knee. Two weeks after the operation, different doses of PP2 were injected intraperitoneally. The drug was given three times a week for 6 weeks, and then the mice were sacrificed. Histopathological, IF and immunoblotting analyses were used to detect key OA catabolic markers and protein expression and related signaling.

RESULTS

The levels of Src, p-Src (y418) and p-FAK in the knee cartilage tissue of patients with OA were abnormally increased. After chondrocytes were co-treated with IL-1β and different doses of PP2, the results showed that PP2 reduced the abnormal increase of β-catenin, p-β-catenin and other proteins induced by IL-1β, and reversed the decrease of p-Smad3, aggrecan and collagen Ⅱ protein levels. Meanwhile, intraperitoneal injection of PP2 in vivo significantly reduced the degeneration of articular cartilage in the OA mouse model.

CONCLUSION

Our data indicate that targeting Src with PP2 protected against cartilage destruction in an OA mouse model by inhibiting Wnt/β-catenin and activating TGF-β/Smad signaling, suggesting that Src may be a potential therapeutic target for OA treatment.

Intelligent Vascularized 3D/4D/5D/6D-Printed Tissue Scaffolds

Blood vessels are essential for nutrient and oxygen delivery and waste removal. Scaffold-repairing materials with functional vascular networks are widely used in bone tissue engineering. Additive manufacturing is a manufacturing technology that creates three-dimensional solids by stacking substances layer by layer, mainly including but not limited to 3D printing, but also 4D printing, 5D printing and 6D printing. It can be effectively combined with vascularization to meet the needs of vascularized tissue scaffolds by precisely tuning the mechanical structure and biological properties of smart vascular scaffolds. Herein, the development of neovascularization to vascularization to bone tissue engineering is systematically discussed in terms of the importance of vascularization to the tissue. Additionally, the research progress and future prospects of vascularized 3D printed scaffold materials are highlighted and presented in four categories: functional vascularized 3D printed scaffolds, cell-based vascularized 3D printed scaffolds, vascularized 3D printed scaffolds loaded with specific carriers and bionic vascularized 3D printed scaffolds. Finally, a brief review of vascularized additive manufacturing-tissue scaffolds in related tissues such as the vascular tissue engineering, cardiovascular system, skeletal muscle, soft tissue and a discussion of the challenges and development efforts leading to significant advances in intelligent vascularized tissue regeneration is presented.

Mechanism and application of 3D-printed degradable bioceramic scaffolds for bone repair

Bioceramics have attracted considerable attention in the field of bone repair because of their excellent osteogenic properties, degradability, and biocompatibility. To resolve issues regarding limited formability, recent studies have introduced 3D printing technology for the fabrication of bioceramic bone repair scaffolds. Nevertheless, the mechanisms by which bioceramics promote bone repair and clinical applications of 3D-printed bioceramic scaffolds remain elusive. This review provides an account of the fabrication methods of 3D-printed degradable bioceramic scaffolds. In addition, the types and characteristics of degradable bioceramics used in clinical and preclinical applications are summarized. We have also highlighted the osteogenic molecular mechanisms in biomaterials with the aim of providing a basis and support for future research on the clinical applications of degradable bioceramic scaffolds. Finally, new developments and potential applications of 3D-printed degradable bioceramic scaffolds are discussed with reference to experimental and theoretical studies.

Novel scaffold platforms for simultaneous induction osteogenesis and angiogenesis in bone tissue engineering: a cutting-edge approach

Despite the recent advances in the development of bone graft substitutes, treatment of critical size bone defects continues to be a significant challenge, especially in the elderly population. A current approach to overcome this challenge involves the creation of bone-mimicking scaffolds that can simultaneously promote osteogenesis and angiogenesis. In this context, incorporating multiple bioactive agents like growth factors, genes, and small molecules into these scaffolds has emerged as a promising strategy. To incorporate such agents, researchers have developed scaffolds incorporating nanoparticles, including nanoparticulate carriers, inorganic nanoparticles, and exosomes. Current paper provides a summary of the latest advancements in using various bioactive agents, drugs, and cells to synergistically promote osteogenesis and angiogenesis in bone-mimetic scaffolds. It also discusses scaffold design properties aimed at maximizing the synergistic effects of osteogenesis and angiogenesis, various innovative fabrication strategies, and ongoing clinical studies.

Recent Advances in Formulations for Long-Acting Delivery of Therapeutic Peptides

Recent preclinical and clinical studies have focused on the active area of therapeutic peptides due to their high potency, selectivity, and specificity in treating a broad range of diseases. However, therapeutic peptides suffer from multiple disadvantages, such as limited oral bioavailability, short half-life, rapid clearance from the body, and susceptibility to physiological conditions (e.g., acidic pH and enzymolysis). Therefore, high peptide dosages and dose frequencies are required for effective patient treatment. Recent innovations in pharmaceutical formulations have substantially improved therapeutic peptide administration by providing the following advantages: long-acting delivery, precise dose administration, retention of biological activity, and improvement of patient compliance. This review discusses therapeutic peptides and challenges in their delivery and explores recent peptide delivery formulations, including micro/nanoparticles (based on lipids, polymers, porous silicon, silica, and stimuli-responsive materials), (stimuli-responsive) hydrogels, particle/hydrogel composites, and (natural or synthetic) scaffolds. This review further covers the applications of these formulations for prolonged delivery and sustained release of therapeutic peptides and their impact on peptide bioactivity, loading efficiency, and (in vitro/in vivo) release parameters.

Functionalized 3D-Printed PLA Biomimetic Scaffold for Repairing Critical-Size Bone Defects

The treatment of critical-size bone defects remains a complicated clinical challenge. Recently, bone tissue engineering has emerged as a potential therapeutic approach for defect repair. This study examined the biocompatibility and repair efficacy of hydroxyapatite-mineralized bionic polylactic acid (PLA) scaffolds, which were prepared through a combination of 3D printing technology, plasma modification, collagen coating, and hydroxyapatite mineralization coating techniques. Physicochemical analysis, mechanical testing, and in vitro and animal experiments were conducted to elucidate the impact of structural design and microenvironment on osteogenesis. Results indicated that the PLA scaffold exhibited a porosity of 84.1% and a pore size of 350 μm, and its macrostructure was maintained following functionalization modification. The functionalized scaffold demonstrated favorable hydrophilicity and biocompatibility and promoted cell adhesion, proliferation, and the expression of osteogenic genes such as ALP, OPN, Col-1, OCN, and RUNX2. Moreover, the scaffold was able to effectively repair critical-size bone defects in the rabbit radius, suggesting a novel strategy for the treatment of critical-size bone defects.

3D-printed porous functional composite scaffolds with polydopamine decoration for bone regeneration

Large size bone defects affect human health and remain a worldwide health problem that needs to be solved immediately. 3D printing technology has attracted substantial attention for preparing penetrable multifunctional scaffolds to promote bone reconditioning and regeneration. Inspired by the spongy structure of natural bone, novel porous degradable scaffolds have been printed using polymerization of lactide and caprolactone (PLCL) and bioactive glass 45S5 (BG), and polydopamine (PDA) was used to decorate the PLCL/BG scaffolds. The physicochemical properties of the PLCL/BG and PLCL/BG/PDA scaffolds were measured, and their osteogenic and angiogenic effects were characterized through a series of experiments both in vitro and in vivo. The results show that the PLCL/BG2/PDA scaffold possessed a good compression modulus and brilliant hydrophilicity. The proliferation, adhesion and osteogenesis of hBMSCs were improved in the PDA coating groups, which exhibited the best performance. The results of the SD rat cranium defect model indicate that PLCL/BG2/PDA obviously promoted osteointegration, which was further confirmed through immunohistochemical staining. Therefore, PDA decoration and the sustained release of bioactive ions (Ca, Si, P) from BG in the 3D-printed PLCL/BG2/PDA scaffold could improve surface bioactivity and promote better osteogenesis and angiogenesis, which may provide a valuable basis for customized implants in extensive bone defect repair applications.

Review on Additives in Hydrogels for 3D Bioprinting of Regenerative Medicine: From Mechanism to Methodology

The regeneration of biological tissues in medicine is challenging, and 3D bioprinting offers an innovative way to create functional multicellular tissues. One common way in bioprinting is bioink, which is one type of the cell-loaded hydrogel. For clinical application, however, the bioprinting still suffers from satisfactory performance, e.g., in vascularization, effective antibacterial, immunomodulation, and regulation of collagen deposition. Many studies incorporated different bioactive materials into the 3D-printed scaffolds to optimize the bioprinting. Here, we reviewed a variety of additives added to the 3D bioprinting hydrogel. The underlying mechanisms and methodology for biological regeneration are important and will provide a useful basis for future research.

Strategies for in situ tissue engineering of vascularized bone regeneration (Review)

Numerous physiological processes occur following bone fracture, including inflammatory cell recruitment, vascularization, and callus formation and remodeling. In particular circumstances, such as critical bone defects or osteonecrosis, the regenerative microenvironment is compromised, rendering endogenous stem/progenitor cells incapable of fully manifesting their reparative potential. Consequently, external interventions, such as grafting or augmentation, are frequently necessary. In situ bone tissue engineering (iBTE) employs cell-free scaffolds that possess microenvironmental cues, which, upon implantation, redirect the behavior of endogenous stem/progenitor cells towards a pro-regenerative inflammatory response and reestablish angiogenesis-osteogenesis coupling. This process ultimately results in vascularized bone regeneration (VBR). In this context, a comprehensive review of the current techniques and modalities in VBR-targeted iBTE technology is provided.

Revascularization and angiogenesis for bone bioengineering in the craniofacial region: a review

The revascularization of grafted tissues is a complicated and non-straightforward process, which makes it challenging to perform reconstructive surgery for critical-sized bone defects. This challenge is combined with the low vascularity that results from radiotherapy. This low vascularity could result from ischemia-reperfusion injuries, also known as ischemia which may happen upon grafting. Ischemia may affect the hard tissue during reconstruction, and this can often cause resorption, infections, disfigurement, and malunion. This paper therefore reviews the clinical and experimental application of procedures that were employed to improve the reconstructive surgery process, which would ensure that the vascularity of the tissue is maintained or enhanced. It also presents the key strategies that are implemented to perform tissue engineering within the grafted sites aiming to optimize the microenvironment and to enhance the overall process of neovascularization and angiogenesis. This review reveals that the current strategies, according to the literature, are the seeding of the mature and progenitor cells, use of extracellular matrix (ECM), co-culturing of osteoblasts with the ECM, growth factors and the use of microcapillaries incorporated into the scaffold design. However, due to the unstable and regression-prone capillary structures in bone constructs, further research focusing on creating long-lasting and stable blood vessels is required.

Strategies of immobilizing BMP-2 with 3D-printed scaffolds to improve osteogenesis

The management and definitive treatment of critical-size bone defects in severe trauma, tumor resection and congenital malformation are troublesome for orthopedic surgeons and patients worldwide without recognized good treatment strategies. Researchers and clinicians are working to develop new strategies to treat these problems. This review aims to summarize the techniques used by additive manufacturing scaffolds loaded with BMP-2 to promote osteogenesis and to analyze the current status and trends in relevant clinical translation. Optimize composite scaffold design to enhance bone regeneration through printing technology, material selection, structure design and loading methods of BMP-2 to advance the clinical therapeutic bone repair field.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Evaluation of Natural Protein-based Nanofiber Composite Photocrosslinking Hydrogel for skin wound regeneration

Protein based photocrosslinking hydrogels with nanofiber dispersions were reported to be an effective wound dressing. In this study, two kinds of protein (gelatin and decellularized dermal matrix) were modified to obtain GelMA and ddECMMA, respectively. Poly(ε-caprolactone) nanofiber dispersions (PCLPBA) and thioglycolic acid-modified chitosan (TCS) were added into GelMA solution and ddECMMA solution, respectively. After photocrosslinking, four kinds of hydrogel (GelMA, GTP4, DP and DTP4) were fabricated. The hydrogels showed excellent physico-chemical property, biocompatibility and negligible cytotoxicity. When applied on the full-thickness cutaneous deficiency of SD rats, hydrogel treated groups exhibited an enhanced wound healing effect than Blank group. Besides, the histological staining of H&E and Masson's showed that hydrogels groups with PCLPBA and TCS (GTP4 and DTP4) improved wound healing. Furthermore, GTP4 group performed better healing effect than other groups, which had great potential in skin wound regeneration.

Optimization of stress distribution of bone-implant interface (BII)

Many studies have found that the threshold of occlusal force tolerated by titanium-based implants is significantly lower than that of natural teeth due to differences in biomechanical mechanisms. Therefore, implants are considered to be susceptible to occlusal trauma. In clinical practice, many implants have shown satisfactory biocompatibility, but the balance between biomechanics and biofunction remains a huge clinical challenge. This paper comprehensively analyzes and summarizes various stress distribution optimization methods to explore strategies for improving the resistance of the implants to adverse stress. Improving stress resistance reduces occlusal trauma and shortens the gap between implants and natural teeth in occlusal function. The study found that: 1) specific implant-abutment connection design can change the force transfer efficiency and force conduction direction of the load at the BII; 2) reasonable implant surface structure and morphological character design can promote osseointegration, maintain alveolar bone height, and reduce the maximum effective stress at the BII; and 3) the elastic modulus of implants matched to surrounding bone tissue can reduce the stress shielding, resulting in a more uniform stress distribution at the BII. This study concluded that the core BII stress distribution optimization lies in increasing the stress distribution area and reducing the local stress peak value at the BII. This improves the biomechanical adaptability of the implants, increasing their long-term survival rate.

Hydrogels-A Promising Materials for 3D Printing Technology

Hydrogels are a promising material for a variety of applications after appropriate functional and structural design, which alters the physicochemical properties and cell signaling pathways of the hydrogels. Over the past few decades, considerable scientific research has made breakthroughs in a variety of applications such as pharmaceuticals, biotechnology, agriculture, biosensors, bioseparation, defense, and cosmetics. In the present review, different classifications of hydrogels and their limitations have been discussed. In addition, techniques involved in improving the physical, mechanical, and biological properties of hydrogels by admixing various organic and inorganic materials are explored. Future 3D printing technology will substantially advance the ability to pattern molecules, cells, and organs. With significant potential for producing living tissue structures or organs, hydrogels can successfully print mammalian cells and retain their functionalities. Furthermore, recent advances in functional hydrogels such as photo- and pH-responsive hydrogels and drug-delivery hydrogels are discussed in detail for biomedical applications.

Bioglass/collagen scaffolds combined with bone marrow stromal cells on bone healing in an experimental model in cranial defects in rats

This study aimed to develop bone regenerative therapeutic strategies, based on the addition of bone marrow stromal cells (BMSC) on bioglass/collagen (BG/COL) scaffolds. For this purpose, an in vivo study was conducted using tissue response of the BG/COL scaffolds combined with BMSC in a critical-size defects. Wistar rats were submitted to the surgical procedure to perform the cranial critical size bone defects and distributed in four groups (20 animals per group): Control Group (CG) (rats submitted to the cranial bone defect surgery without treatment), Bioglass Group (BG) (rats treated with BG), BG/COL Group (rats treated with BG/COL) and Bioglass/Collagen and BMSC Group (BG/COL/BMSC) (rats treated with BG/COL scaffolds enriched with BMSCs). Animals were euthanized 15 and 30 days after surgery. Scanning electron microscopy, histopathological and immunohistochemistry analysis were used. SEM analysis demonstrated that porous scaffolds were obtained, and Col fibers were successfully impregnated to BG matrices. The implantation of the BMSC on BG/COL based scaffolds was effective in stimulating newly bone formation and produced an increased immunoexpression of markers related to the bone repair. These results highlight the potential of BG/COL scaffolds and BMSCs to be used as a therapeutic approach for bone regeneration.

Osteoregenerative Potential of 3D-Printed Poly ε-Caprolactone Tissue Scaffolds In Vitro Using Minimally Manipulative Expansion of Primary Human Bone Marrow Stem Cells

The repair of orthopedic and maxillofacial defects in modern medicine currently relies heavily on the use of autograft, allograft, void fillers, or other structural material composites. This study examines the in vitro osteo regenerative potential of polycaprolactone (PCL) tissue scaffolding, fabricated via a three-dimensional (3D) additive manufacturing technology, i.e., a pneumatic micro extrusion (PME) process. The objectives of this study were: (i) To examine the innate osteoinductive and osteoconductive potential of 3D-printed PCL tissue scaffolding and (ii) To perform a direct in vitro comparison of 3D-printed PCL scaffolding with allograft Allowash® cancellous bone cubes with regards to cell-scaffold interactions and biocompatibility with three primary human bone marrow (hBM) stem cell lines. This study specifically examined cell survival, cell integration, intra-scaffold cell proliferation, and differentiation of progenitor cells to investigate the potential of 3D-printed PCL scaffolds as an alternative to allograft bone material for the repair of orthopedic injuries. We found that mechanically robust PCL bone scaffolds can be fabricated via the PME process and the resulting material did not elicit detectable cytotoxicity. When the widely used osteogenic model SAOS-2 was cultured in PCL extract medium, no detectable effect was observed on cell viability or proliferation with multiple test groups showing viability ranges of 92.2% to 100% relative to a control group with a standard deviation of ±10%. In addition, we found that the honeycomb infill pattern of the 3D-printed PCL scaffold allowed for superior mesenchymal stem-cell integration, proliferation, and biomass increase. When healthy and active primary hBM cell lines, having documented in vitro growth rates with doubling times of 23.9, 24.67, and 30.94 h, were cultured directly into 3D-printed PCL scaffolds, impressive biomass increase values were observed. It was found that the PCL scaffolding material allowed for biomass increase values of 17.17%, 17.14%, and 18.18%, compared to values of 4.29% for allograph material cultured under identical parameters. It was also found that the honeycomb scaffold infill pattern was superior to the cubic and rectangular matrix structures, and provided a superior microenvironment for osteogenic and hematopoietic progenitor cell activity and auto-differentiation of primary hBM stem cells. Histological and immunohistochemical studies performed in this work confirmed the regenerative potential of PCL matrices in the orthopedic setting by displaying the integration, self-organization, and auto-differentiation of hBM progenitor cells within the matrix. Differentiation products including mineralization, self-organizing "proto-osteon" structures, and in vitro erythropoiesis were observed in conjunction with the documented expression of expected bone marrow differentiative markers including CD-99 (>70%), CD-71 (>60%), and CD-61 (>5%). All of the studies were conducted without the addition of any exogenous chemical or hormonal stimulation and exclusively utilized the abiotic and inert material polycaprolactone; setting this work apart from the vast majority of contemporary investigations into synthetic bone scaffold fabrication In summary, this study demonstrates the unique clinical potential of 3D-printed PCL scaffolds for stem cell expansion and incorporation into advanced microstructures created via PME manufacturing to generate a physiologically inert temporary bony defect graft with significant autograft features for enhanced end-stage healing.

Strontium and simvastatin dual loaded hydroxyapatite microsphere reinforced poly(ε-caprolactone) scaffolds promote vascularized bone regeneration

The promotion of vascular network formation in the early stages of implantation is considered a prerequisite for successful functional bone regeneration. In this study, we successfully constructed 3D printed scaffolds with strong mechanical strength and a controllable pore structure that can sustainably release strontium (Sr) ions and simvastatin (SIM) for up to 28 days by incorporation of Sr²⁺ and SIM-loaded hydroxyapatite microspheres (MHA) into a poly(ε-caprolactone) (PCL) matrix. In vitro cell experiments showed that Sr-doped scaffolds were beneficial to the proliferation and osteogenic differentiation of bone mesenchymal stem cells (BMSCs), an appropriate dose of SIM was beneficial to cell proliferation and angiogenesis, and a high dose of SIM was cytotoxic. The Sr- and SIM-dual-loaded scaffolds with an appropriate dose significantly induced osteogenic differentiation of BMSCs and tube formation of human umbilical vein endothelial cells (HUVECs) in vitro and promoted vascular network and functional bone formation in vivo. Ribose nucleic acid (RNA) sequencing analysis suggested that the mechanism of promotion of vascularized bone regeneration by fabricated scaffolds is that dual-loaded Sr²⁺ and SIM can upregulate osteogenic and vasculogenic-related genes and downregulate osteoclast-related genes, which is beneficial for vascular and new bone regeneration. The 3D printed composite scaffolds loaded with high-stability and low-cost inorganic Sr²⁺ ions and SIM small-molecule drugs hold great promise in the field of promoting vascularized bone regeneration.

Nanoparticles based composite coatings with tunable vascular endothelial growth factor and bone morphogenetic protein-2 release for bone regeneration

Bone healing is a complex cascade involving precisely coordinated spatiotemporal presentation of multiple growth factors (GFs), including osteogenic and angiogenic GFs, and each stage of bone healing requires varying types and content of GFs. In this study, we fabricated a composite nanocoating with tunable vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2) that was coated on the surface of a polydopamine (PDA)-decorated tertiary calcium phosphate (TCP) scaffold using VEGF-loaded chitosan/bovine serum albumin nanoparticles (CS/BSA-NPs) and BMP-2-loaded poly-L-lysine/oxidized alginate nanoparticles (PLL/OALG-NPs). It was found that VEGF could be efficiently released to promote vascularization in early bone repair stages due to the rapid biodegradation of CS/BSA-NPs, while bone formation can be promoted by a sustained release of BMP-2 from the slowly degrading PLL/OALG-NPs. The composite coating and TCP scaffold can be conjugated due to the excellent adhesive property of PDA. The composite coating can achieve the rapid release of VEGF and sustained release of BMP-2, which can activate GFs for accelerating bone healing.

A biomimetic piezoelectric scaffold with sustained Mg2+ release promotes neurogenic and angiogenic differentiation for enhanced bone regeneration

Natural bone is a composite tissue made of organic and inorganic components, showing piezoelectricity. Whitlockite (WH), which is a natural magnesium-containing calcium phosphate, has attracted great attention in bone formation recently due to its unique piezoelectric property after sintering treatment and sustained release of magnesium ion (Mg²⁺). Herein, a composite scaffold (denoted as PWH scaffold) composed of piezoelectric WH (PWH) and poly(ε-caprolactone) (PCL) was 3D printed to meet the physiological demands for the regeneration of neuro-vascularized bone tissue, namely, providing endogenous electric field at the defect site. The sustained release of Mg²⁺ from the PWH scaffold, displaying multiple biological activities, and thus exhibits a strong synergistic effect with the piezoelectricity on inhibiting osteoclast activation, promoting the neurogenic, angiogenic, and osteogenic differentiation of bone marrow mesenchymal stromal cells (BMSCs) in vitro. In a rat calvarial defect model, this PWH scaffold is remarkably conducive to efficient neo-bone formation with rich neurogenic and angiogenic expressions. Overall, this study presents the first example of biomimetic piezoelectric scaffold with sustained Mg²⁺ release for promoting the regeneration of neuro-vascularized bone tissue in vivo, which offers new insights for regenerative medicine.

BMSC exosome-enriched acellular fish scale scaffolds promote bone regeneration

Tissue engineering scaffolds are essential for repairing bone defects. The use of biomimetic scaffolds for bone tissue engineering has been investigated for decades. To date, the trend in this area has been moved toward the construction of biomimetic acellular scaffolds with effective modification to enhance the osteogenic differentiation efficiency of bone marrow mesenchymal stem cells (BMSCs). The exosomes derived from BMSCs have been shown as a potential therapeutic tool for repairing bone defects. In this study, we demonstrated the pro-osteogenic effects of exosomes derived form osteogenic differentiated BMSCs (OBMSC) and presented a novel exosmes-functionalized decellularized fish scale (DE-FS) scaffold for promoting bone regeneration in vivo. The DE-FS scaffolds were obtained through decellularization and decalcification processes, which exhibited high biocompatibility and low immunological rejection. The intrinsic anisotropic structures of DE-FS could enhance the adhesion and proliferation ability of BMSCs in vitro. In addition, we demonstrated that the porous structure of DE-FS endowed them with the capacity to load and release exosomes to BMSCs, resulting in the enhanced osteogenic differentiation of BMSCs. Concerning these pro-osteogenic effects, it was further proved that OBMSC exosome-modified DE-FS scaffolds could effectively promote bone regeneration in the mouse calvarial defect models. In conclusion, our work provided a new insight to design exosome-riched biomimetic scaffolds for bone tissue engineering and clinical applications.

Bottom-up design of hydrogels for programmable drug release

Hydrogels are a promising drug delivery system for biomedical applications due to their biocompatibility and similarity to native tissue. Programming the release rate from hydrogels is critical to ensure release of desired dosage over specified durations, particularly with the advent of more complicated medical regimens such as combinatorial drug therapy. While it is known how hydrogel structure affects release, the parameters that can be explicitly controlled to modulate release ab initio could be useful for hydrogel design. In this review, we first survey common physical models of hydrogel release. We then extensively go through the various input parameters that we can exercise direct control over, at the levels of synthesis, formulation, fabrication and environment. We also illustrate some examples where hydrogels can be programmed with the input parameters for temporally and spatially defined release. Finally, we discuss the exciting potential and challenges for programming release, and potential implications with the advent of machine learning.

Therapeutic application of hydrogels for bone-related diseases

Bone-related diseases caused by trauma, infection, and aging affect people’s health and quality of life. The prevalence of bone-related diseases has been increasing yearly in recent years. Mild bone diseases can still be treated with conservative drugs and can be cured confidently. However, serious bone injuries caused by large-scale trauma, fractures, bone tumors, and other diseases are challenging to heal on their own. Open surgery must be used for intervention. The treatment method also faces the problems of a long cycle, high cost, and serious side effects. Studies have found that hydrogels have attracted much attention due to their good biocompatibility and biodegradability and show great potential in treating bone-related diseases. This paper mainly introduces the properties and preparation methods of hydrogels, reviews the application of hydrogels in bone-related diseases (including bone defects, bone fracture, cartilage injuries, and osteosarcoma) in recent years. We also put forward suggestions according to the current development status, pointing out a new direction for developing high-performance hydrogels more suitable for bone-related diseases.

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.

Remotely Temporal Scheduled Macrophage Phenotypic Transition Enables Optimized Immunomodulatory Bone Regeneration

Precise timing of macrophage polarization plays a pivotal role in immunomodulation of tissue regeneration, yet most studies mainly focus on M2 macrophages for their anti-inflammatory and regenerative effects while the essential proinflammatory role of the M1 phenotype on the early inflammation stage is largely underestimated. Herein, a superparamagnetic hydrogel capable of timely controlling macrophage polarization is constructed by grafting superparamagnetic nanoparticles on collagen nanofibers. The magnetic responsive hydrogel network enables efficient polarization of encapsulated macrophage to the M2 phenotype through the podosome/Rho/ROCK mechanical pathway in response to static magnetic field (MF) as needed. Taking advantage of remote accessibility of magnetic field together with the superparamagnetic hydrogels, a temporal engineered M1 to M2 transition course preserving the essential role of M1 at the early stage of tissue healing, as well as enhancing the prohealing effect of M2 at the middle/late stages is established via delayed MF switch. Such precise timing of macrophage polarization matching the regenerative process of injured tissue eventually leads to optimized immunomodulatory bone healing in vivo. Overall, this study offers a remotely time-scheduled approach for macrophage polarization, which enables precise manipulation of inflammation progression during tissue healing.

Advances in 3D bioprinting of tissues/organs for regenerative medicine and in-vitro models

Tissue/organ shortage is a major medical challenge due to donor scarcity and patient immune rejections. Furthermore, it is difficult to predict or mimic the human disease condition in animal models during preclinical studies because disease phenotype differs between humans and animals. Three-dimensional bioprinting (3DBP) is evolving into an unparalleled multidisciplinary technology for engineering three-dimensional (3D) biological tissue with complex architecture and composition. The technology has emerged as a key driver by precise deposition and assembly of biomaterials with patient's/donor cells. This advancement has aided in the successful fabrication of in vitro models, preclinical implants, and tissue/organs-like structures. Here, we critically reviewed the current state of 3D-bioprinting strategies for regenerative therapy in eight organ systems, including nervous, cardiovascular, skeletal, integumentary, endocrine and exocrine, gastrointestinal, respiratory, and urinary systems. We also focus on the application of 3D bioprinting to fabricated in vitro models to study cancer, infection, drug testing, and safety assessment. The concept of in situ 3D bioprinting is discussed, which is the direct printing of tissues at the injury or defect site for reparative and regenerative therapy. Finally, issues such as scalability, immune response, and regulatory approval are discussed, as well as recently developed tools and technologies such as four-dimensional and convergence bioprinting. In addition, information about clinical trials using 3D printing has been included.

Low temperature hybrid 3D printing of hierarchically porous bone tissue engineering scaffolds with in situ delivery of osteogenic peptide and mesenchymal stem cells

Comparing to other conventional fabrication techniques, 3D printing is advantageous in producing bone tissue engineering scaffolds with customized shape, tailored pore size/porosity, required mechanical properties and even desirable biomolecule delivery capability. However, towards scaffolds with a large volume, it is highly difficult to enable seed cells to migrate to the central region of the scaffolds, resulting in an inhomogeneous cell distribution, and hence lowering the bone forming ability. To address this problem, in this investigation, cell-laden bone tissue engineering scaffolds consisting of osteogenic peptide loaded β-tricalcium phosphate/poly(lactic-co-glycolic acid) (OP/TCP/PLGA) nanocomposite scaffolds and rat bone marrow derived mesenchymal stem cells (rBMSCs)-laden Gelatin/GelMA hydrogel fillers were produced through a "dual-nozzle" cryogenic hybrid 3D printing. The cell-laden scaffolds exhibited a bi-phasic structure and were mechanically similar to human cancellous bone. OP can be released from the hybrid scaffolds in a sustained manner. rBMSCs laden in the hydrogel patterns exhibited a high viability during and after cryogenic hybrid 3D printing process and can be further released from the hydrogel struts and achieve cell anchorage on the surface of adjacent OTP struts. The OP released from OTP struts enhanced rBMSCs proliferation. Comparing to rBMSC-laden hybrid scaffolds without OP incorporation, the rBMSC-laden hybrid scaffolds incorporated with OP significantly up-regulated the osteogenic differentiation of rBMSCs, by showing a higher level of alkaline phosphatase (ALP) expression and calcium deposition. This "proof-ofconcept" study provided a facile method to form cell-laden bone tissue engineering scaffolds with not only required mechanical strength, biomimetic structure and prolonged biomolecule release but also excellent cell delivery capability with uniform cell distribution.

Strategies to Promote Vascularization in 3D Printed Tissue Scaffolds: Trends and Challenges

Three-dimensional (3D) printing techniques for scaffold fabrication have shown promising advancements in recent years owing to the ability of the latest high-performance printers to mimic the native tissue down to submicron scales. Nevertheless, host integration and performance of scaffolds in vivo have been severely limited owing to the lack of robust strategies to promote vascularization in 3D printed scaffolds. As a result, researchers over the past decade have been exploring strategies that can promote vascularization in 3D printed scaffolds toward enhancing scaffold functionality and ensuring host integration. Various emerging strategies to enhance vascularization in 3D printed scaffolds are discussed. These approaches include simple strategies such as the enhancement of vascular in-growth from the host upon implantation by scaffold modifications to complex approaches wherein scaffolds are fabricated with their own vasculature that can be directly anastomosed or microsurgically connected to the host vasculature, thereby ensuring optimal integration. The key differences among the techniques, their pros and cons, and the future opportunities for utilizing each technique are highlighted here. The Review concludes with the current limitations and future directions that can help 3D printing emerge as an effective biofabrication technique to realize tissues with physiologically relevant vasculatures to ultimately accelerate clinical translation.

Fabrication of physical and chemical crosslinked hydrogels for bone tissue engineering

Bone tissue engineering has emerged as a significant research area that provides promising novel tools for the preparation of biomimetic hydrogels applied in bone-related diseases (e.g., bone defects, cartilage damage, osteoarthritis, etc.). Herein, thermal sensitive polymers (e.g., PNIPAAm, Soluplus, etc.) were introduced into main chains to fabricate biomimetic hydrogels with injectability and compatibility for those bone defect need minimally invasive surgery. Mineral ions (e.g., calcium, copper, zinc, and magnesium), as an indispensable role in maintaining the balance of the organism, were linked with polymer chains to form functional hydrogels for accelerating bone regeneration. In the chemically triggered hydrogel section, advanced hydrogels crosslinked by different molecular agents (e.g., genipin, dopamine, caffeic acid, and tannic acid) possess many advantages, including extensive selectivity, rapid gel-forming capacity and tunable mechanical property. Additionally, photo crosslinking hydrogel with rapid response and mild condition can be triggered by different photoinitiators (e.g., I2959, LAP, eosin Y, riboflavin, etc.) under specific wavelength of light. Moreover, enzyme triggered hydrogels were also utilized in the tissue regeneration due to its rapid gel-forming capacity and excellent biocompatibility. Particularly, some key factors that can determine the therapy effect for bone tissue engineering were also mentioned. Finally, brief summaries and remaining issues on how to properly design clinical-oriented hydrogels were provided in this review.

Engineering biomaterials to 3D-print scaffolds for bone regeneration: practical and theoretical consideration

There are more than 2 million bone grafting procedures performed annually in the US alone. Despite significant efforts, the repair of large segmental bone defects is a substantial clinical challenge which requires bone substitute materials or a bone graft. The available biomaterials lack the adequate mechanical strength to withstand the static and dynamic loads while maintaining sufficient porosity to facilitate cell in-growth and vascularization during bone tissue regeneration. A wide range of advanced biomaterials are being currently designed to mimic the physical as well as the chemical composition of a bone by forming polymer blends, polymer-ceramic and polymer-degradable metal composites. Transforming these novel biomaterials into porous and load-bearing structures via three-dimensional printing (3DP) has emerged as a popular manufacturing technique to develop engineered bone grafts. 3DP has been adopted as a versatile tool to design and develop bone grafts that satisfy porosity and mechanical requirements while having the ability to form grafts of varied shapes and sizes to meet the physiological requirements. In addition to providing surfaces for cell attachment and eventual bone formation, these bone grafts also have to provide physical support during the repair process. Hence, the mechanical competence of the 3D-printed scaffold plays a key role in the success of the implant. In this review, we present various recent strategies that have been utilized to design and develop robust biomaterials that can be deployed for 3D-printing bone substitutes. The article also reviews some of the practical, theoretical and biological considerations adopted in the 3D-structure design and development for bone tissue engineering.

Poly(DL-lactic acid) scaffolds as a bone targeting platform for the co-delivery of antimicrobial agents against S. aureus-C. albicans mixed biofilms

New strategies for the treatment of polymicrobial bone infections are required. In this study, the co-delivery of two antimicrobials by poly(D,L-lactic acid) (PDLLA) scaffolds was investigated in a polymicrobial biofilm model. PDLLA scaffolds were prepared by solvent casting/particulate leaching methodology, incorporating minocycline and voriconazole as clinically relevant antimicrobial agents. The scaffolds presented a sponge-like appearance, suitable to support cell proliferation and drug release. Single- and dual-species biofilm models of Staphylococcus aureus and Candida albicans were developed and characterized. S. aureus presented a higher ability to form single-species biofilms, compared to C. albicans. Minocycline and voriconazole-loaded PDLLA scaffolds showed activity against S. aureus and C. albicans single- and dual-biofilms. Ultimately, the cytocompatibility/functional activity of PDLLA scaffolds observed in human MG-63 osteosarcoma cells unveil their potential as a next-generation co-delivery system for antimicrobial therapy in bone infections.

Engineering Extracellular Microenvironment for Tissue Regeneration

The extracellular microenvironment is a highly dynamic network of biophysical and biochemical elements, which surrounds cells and transmits molecular signals. Extracellular microenvironment controls are of crucial importance for the ability to direct cell behavior and tissue regeneration. In this review, we focus on the different components of the extracellular microenvironment, such as extracellular matrix (ECM), extracellular vesicles (EVs) and growth factors (GFs), and introduce engineering approaches for these components, which can be used to achieve a higher degree of control over cellular activities and behaviors for tissue regeneration. Furthermore, we review the technologies established to engineer native-mimicking artificial components of the extracellular microenvironment for improved regenerative applications. This review presents a thorough analysis of the current research in extracellular microenvironment engineering and monitoring, which will facilitate the development of innovative tissue engineering strategies by utilizing different components of the extracellular microenvironment for regenerative medicine in the future.

A facile way to construct Sr-doped apatite coating on the surface of 3D printed scaffolds to improve osteogenic effect

Bone-like apatite coating fabricated by biomineralization process is a facile way for surface modification of porous scaffolds to improve interfacial behaviors and thus facilitate cell attachment, proliferation, and differentiation for bone tissue engineering. In this study, a Sr-containing calcium phosphate solution was made and used to construct a thick layer of Sr-doped bone-like apatite on the surface of 3D printed scaffolds via biomineralization process. Importantly, Sr-doped bone-like apatite could form and fully cover the 3D printed scaffolds surface in hours. The characterization results indicated that Sr2+ ions successfully replaced Ca2+ ions in bone-like apatite and the molar ratio of Sr/(Ca+Sr) was up to 8.2%. Furthermore, the Sr-doped apatite coating increased the compressive strength and Young’s modulus of composite scaffolds. The compatibility and bioactivity of mineralized scaffolds were evaluated by cell attachment, proliferation, and differentiation of MC3T3-E1 cells. It was found that Sr-doped apatite coating could gradually release Sr2+ ions and further promote cell attachment, proliferation rate, and the expression of alkaline phosphatase activity and osteogenic related genes, such as collagen type I (Col I), Runt-related transcription factor 2 (Runx-2), osteopontin, and osterix. Therefore, the Sr-doped apatite coating fabricated by this facile and rapid biomineralization process offers a new strategy to modify 3D printed porous scaffolds with significantly improved mechanical and biological properties for bone tissue engineering applications.

3D Printing for Bone-Cartilage Interface Regeneration

Due to the vasculature defects and/or the avascular nature of cartilage, as well as the complex gradients for bone-cartilage interface regeneration and the layered zonal architecture, self-repair of cartilage and subchondral bone is challenging. Currently, the primary osteochondral defect treatment strategies, including artificial joint replacement and autologous and allogeneic bone graft, are limited by their ability to simply repair, rather than induce regeneration of tissues. Meanwhile, over the past two decades, three-dimension (3D) printing technology has achieved admirable advancements in bone and cartilage reconstruction, providing a new strategy for restoring joint function. The advantages of 3D printing hybrid materials include rapid and accurate molding, as well as personalized therapy. However, certain challenges also exist. For instance, 3D printing technology for osteochondral reconstruction must simulate the histological structure of cartilage and subchondral bone, thus, it is necessary to determine the optimal bioink concentrations to maintain mechanical strength and cell viability, while also identifying biomaterials with dual bioactivities capable of simultaneously regenerating cartilage. The study showed that the regeneration of bone-cartilage interface is crucial for the repair of osteochondral defect. In this review, we focus on the significant progress and application of 3D printing technology for bone-cartilage interface regeneration, while also expounding the potential prospects for 3D printing technology and highlighting some of the most significant challenges currently facing this field.

Titanium alloy composited with dual-cytokine releasing polysaccharide hydrogel to enhance osseointegration via osteogenic and macrophage polarization signaling pathways

Titanium alloy has been widely used in orthopedic surgeries as bone defect filling. However, the regeneration of high-quality new bones is limited due to the pro-inflammatory microenvironment around implants, resulting in a high occurrence rate of implant loosening or failure in osteological therapy. In this study, extracellular matrix (ECM)-mimetic polysaccharide hydrogel co-delivering BMP-2 and IL-4 was composited with 3D printed titanium alloy to promote the osseointegration and regulate macrophage response to create a pro-healing microenvironment in bone defect. Notably, it is discovered from the bioinformatics data that IL-4 and BMP-2 could affect each other through multiple signal pathways to achieve a synergistic effect towards osteogenesis. The composite scaffold significantly promoted the osteoblast differentiation and proliferation of human bone marrow mesenchyme stem cells (hBMSCs). The repair of large-scale femur defect in rat indicated that the dual-cytokine-delivered composite scaffold could manipulate a lower inflammatory level in situ by polarizing macrophages to M2 phenotype, resulting in superior efficacy of mature new bone regeneration over the treatment of native titanium alloy or that with an individual cytokine. Collectively, this work highlights the importance of M2-type macrophages-enriched immune-environment in bone healing. The biomimetic hydrogel-metal implant composite is a versatile and advanced scaffold for accelerating in vivo bone regeneration, holding great promise in treating orthopedic diseases.

3D Printing Scaffold Vaccine for Antitumor Immunity

Cancer vaccine platform has attracted great interest in the field of cancer immunotherapy. Here, 3D printed scaffolds loaded with immunoregulators are developed for enhanced cancer immunotherapy. The rapid manufacturing and precise molding based on 3D printing can realize the mass manufacturing of cancer vaccines and personalized design. Meanwhile, compared to the traditional hydrogel, the 3D-scaffold with porous structure endows its similar functions compared with real lymphoid organs by recruitment of a great number of immune cells, leading to the formation of "artificial tertiary lymphoid structures," where there is a promising site to enhance both humoral and cellular immune responses. Efficient anticancer immunity is induced when combined with immune checkpoint blockade to inhibit the tumor growth. Personalized antitumor scaffold vaccines are further demonstrated for filling of tumor site after surgery to prevent cancer metastasis. Taken together, these results promise the 3D printing scaffold vaccine as the potential strategy for cancer vaccine therapy in the future.

Advances in Filament Structure of 3D Bioprinted Biodegradable Bone Repair Scaffolds

Conventional bone repair scaffolds can no longer meet the high standards and requirements of clinical applications in terms of preparation process and service performance. Studies have shown that the diversity of filament structures of implantable scaffolds is closely related to their overall properties (mechanical properties, degradation properties, and biological properties). To better elucidate the characteristics and advantages of different filament structures, this paper retrieves and summarizes the state of the art in the filament structure of the three-dimensional (3D) bioprinted biodegradable bone repair scaffolds, mainly including single-layer structure, double-layer structure, hollow structure, core-shell structure and bionic structures. The eximious performance of the novel scaffolds was discussed from different aspects (material composition, ink configuration, printing parameters, etc.). Besides, the additional functions of the current bone repair scaffold, such as chondrogenesis, angiogenesis, anti-bacteria, and anti-tumor, were also concluded. Finally, the paper prospects the future material selection, structural design, functional development, and performance optimization of bone repair scaffolds.

A Nanofiber Mat With Dual Bioactive Components and a Biomimetic Matrix Structure for Improving Osteogenesis Effect

The challenge of effectively regenerating bone tissue through tissue engineering technology is that most tissue engineering scaffolds cannot imitate the three-dimensional structure and function of the natural extracellular matrix. Herein, we have prepared the poly(L-lactic acid)–based dual bioactive component reinforced nanofiber mats which were named as poly(L-lactic acid)/bovine serum albumin/nanohydroxyapatite (PLLA/BSA/nHAp) with dual bioactive components by combining homogeneous blending and electrospinning technology. The results showed that these nanofiber mats had sufficient mechanical properties and a porous structure suitable for cell growth and migration. Furthermore, the results of cell experiments in vitro showed that PLLA/BSA/nHAp composite nanofiber mat could preferably stimulate the proliferation of mouse osteoblastic cells (MC3T3 cells) compared with pure PLLA nanofiber mats. Based on these results, the scaffolds developed in this study are considered to have a great potential to be adhibited as bone repair materials.

Local dual delivery therapeutic strategies: Using biomaterials for advanced bone tissue regeneration

Bone development is a complex process involving a vast number of growth factors and chemical substances. These factors include transforming growth factor-beta, platelet-derived growth factor, insulin-like growth factor, and most importantly, the bone morphogenetic protein, which exhibits excellent therapeutic value in bone repair. However, the spatial-temporal relationship in the expression of these factors during bone formation makes the bone repair a more complicated process to address. Thus, using a single therapeutic agent to address bone formation does not seem to provide a clinically effective option. Conversely, a dual delivery approach facilitating the co-delivery of agents has proved to be a dynamic alternative since such a strategy can provide more efficient spatial-temporal action. Such delivery systems can smartly target more than one pathway or differentiation lineage and thus offer more efficient bone regeneration. This review discusses various dual delivery strategies reported in the literature employed to achieve improved bone regeneration. These include concurrent use of different therapeutic agents (including growth factors and drugs), enhancing bone formation and cell recruitment, and improving the efficiency of bone healing.

Additive Manufacturing of Caffeic Acid-Inspired Mineral Trioxide Aggregate/Poly-ε-Caprolactone Scaffold for Regulating Vascular Induction and Osteogenic Regeneration of Dental Pulp Stem Cells

Mineral trioxide aggregate (MTA) is a common biomaterial used in endodontics regeneration due to its antibacterial properties, good biocompatibility and high bioactivity. Surface modification technology allows us to endow biomaterials with the necessary biological targets for activation of specific downstream functions such as promoting angiogenesis and osteogenesis. In this study, we used caffeic acid (CA)-coated MTA/polycaprolactone (PCL) composites and fabricated 3D scaffolds to evaluate the influence on the physicochemical and biological aspects of CA-coated MTA scaffolds. As seen from the results, modification of CA does not change the original structural characteristics of MTA, thus allowing us to retain the properties of MTA. CA-coated MTA scaffolds were shown to have 25% to 55% higher results than bare scaffold. In addition, CA-coated MTA scaffolds were able to significantly adsorb more vascular endothelial growth factors (p < 0.05) secreted from human dental pulp stem cells (hDPSCs). More importantly, CA-coated MTA scaffolds not only promoted the adhesion and proliferation behaviors of hDPSCs, but also enhanced angiogenesis and osteogenesis. Finally, CA-coated MTA scaffolds led to enhanced subsequent in vivo bone regeneration of the femur of rabbits, which was confirmed using micro-computed tomography and histological staining. Taken together, CA can be used as a potently functional bioactive coating for various scaffolds in bone tissue engineering and other biomedical applications in the future.

Co-Axial Gyro-Spinning of PCL/PVA/HA Core-Sheath Fibrous Scaffolds for Bone Tissue Engineering

The present study aspires towards fabricating core-sheath fibrous scaffolds by state-of-the-art pressurized gyration for bone tissue engineering applications. The core-sheath fibers comprising dual-phase poly-ε-caprolactone (PCL) core and polyvinyl alcohol (PVA) sheath are fabricated using a novel "co-axial" pressurized gyration method. Hydroxyapatite (HA) nanocrystals are embedded in the sheath of the fabricated scaffolds to improve the performance for application as a bone tissue regeneration material. The diameter of the fabricated fiber is 3.97 ± 1.31 µm for PCL-PVA/3%HA while pure PCL-PVA with no HA loading gives 3.03 ± 0.45 µm. Bead-free fiber morphology is ascertained for all sample groups. The chemistry, water contact angle and swelling behavior measurements of the fabricated core-sheath fibrous scaffolds indicate the suitability of the structures in cellular activities. Saos-2 bone osteosarcoma cells are employed to determine the biocompatibility of the scaffolds, wherein none of the scaffolds possess any cytotoxicity effect, while cell proliferation of 94% is obtained for PCL-PVA/5%HA fibers. The alkaline phosphatase activity results suggest the osteogenic activities on the scaffolds begin earlier than day 7. Overall, adaptations of co-axial pressurized gyration provides the flexibility to embed or encapsulate bioactive substances in core-sheath fiber assemblies and is a promising strategy for bone healing.

3D printing of dual-cell delivery titanium alloy scaffolds for improving osseointegration through enhancing angiogenesis and osteogenesis

BACKGROUND

The repair of large bone defects is a great challenge for orthopedics. Although the development of three-dimensional (3D) printed titanium alloy (Ti6Al4V) implants with optimized the pore structure have effectively promoted the osseointegration. However, due to the biological inertia of Ti6Al4Vsurface and the neglect of angiogenesis, some patients still suffer from postoperative complications such as dislocation or loosening of the prosthesis.

METHODS

The purpose of this study was to construct 3D printed porous Ti6Al4V scaffolds filled with bone marrow mesenchymal stem cells (BMSC) and endothelial progenitor cells (EPC) loaded hydrogel and evaluate the efficacy of this composite implants on osteogenesis and angiogenesis, thus promoting osseointegration.

RESULTS

The porosity and pore size of prepared 3D printed porous Ti6Al4V scaffolds were 69.2 ± 0.9 % and 593.4 ± 16.9 μm, respectively, which parameters were beneficial to bone ingrowth and blood vessel formation. The BMSC and EPC filled into the pores of the scaffolds after being encapsulated by hydrogels can maintain high viability. As a cell containing composite implant, BMSC and EPC loaded hydrogel incorporated into 3D printed porous Ti6Al4V scaffolds enhancing osteogenesis and angiogenesis to repair bone defects efficiently. At the transcriptional level, the composite implant up-regulated the expression levels of the osteogenesis-related genes alkaline phosphatase (ALP) and osteocalcin (OCN), and angiogenesis-related genes hypoxia-inducible factor 1 alpha (HIF-1α), and vascular endothelial growth factor (VEGF).

CONCLUSIONS

Overall, the strategy of loading porous Ti6Al4V scaffolds to incorporate cells is a promising treatment for improving osseointegration.