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

Current Perspectives of Protein in Bone Tissue Engineering: Bone Structure, Ideal Scaffolds, Fabrication Techniques, Applications, Scopes, and Future Advances

In view of their exceptional approach, excellent inherent biocompatibility and biodegradability properties, and interaction with the local extracellular matrix, protein-based polymers have received attention in bone tissue engineering, which is a multidisciplinary field that repairs and regenerates fractured bones. Bone is a multihierarchical complex structure, and it performs several essential biofunctions, including maintaining mineral balance and structural support and protecting soft organs. Protein-based polymers have gained interest in developing ideal scaffolds as emerging biomaterials for bone fractured healing and regeneration, and it is challenging to design ideal bone substitutes as perfect biomaterials. Several protein-based polymers, including collagen, keratin, gelatin, serum albumin, etc., are potential materials due to their inherent cytocompatibility, controlled biodegradability, high biofunctionalization, and tunable mechanical characteristics. While numerous studies have indicated the encouraging possibilities of proteins in BTE, there are still major challenges concerning their biodegradability, stability in physiological conditions, and continuous release of growth factors and bioactive molecules. Robust scaffolds derived from proteins can be used to replace broken or diseased bone with a biocompatible substitute; proteins, being biopolymers, provide excellent scaffolds for bone tissue engineering. Herein, recent developments in protein polymers for cutting-edge bone tissue engineering are addressed in this review within 3-5 years, with a focus on the significant challenges and future perspectives. The first section discusses the structural fundamentals of bone anatomy and ideal scaffolds, and the second section describes the fabrication techniques of scaffolds. The third section highlights the importance of proteins and their applications in BTE. Hence, the recent development of protein polymers for state-of-the-art bone tissue engineering has been discussed, highlighting the significant challenges and future perspectives.

Development of Biocompatible 3D-Printed Artificial Blood Vessels through Multidimensional Approaches

Within the human body, the intricate network of blood vessels plays a pivotal role in transporting nutrients and oxygen and maintaining homeostasis. Bioprinting is an innovative technology with the potential to revolutionize this field by constructing complex multicellular structures. This technique offers the advantage of depositing individual cells, growth factors, and biochemical signals, thereby facilitating the growth of functional blood vessels. Despite the challenges in fabricating vascularized constructs, bioprinting has emerged as an advance in organ engineering. The continuous evolution of bioprinting technology and biomaterial knowledge provides an avenue to overcome the hurdles associated with vascularized tissue fabrication. This article provides an overview of the biofabrication process used to create vascular and vascularized constructs. It delves into the various techniques used in vascular engineering, including extrusion-, droplet-, and laser-based bioprinting methods. Integrating these techniques offers the prospect of crafting artificial blood vessels with remarkable precision and functionality. Therefore, the potential impact of bioprinting in vascular engineering is significant. With technological advances, it holds promise in revolutionizing organ transplantation, tissue engineering, and regenerative medicine. By mimicking the natural complexity of blood vessels, bioprinting brings us one step closer to engineering organs with functional vasculature, ushering in a new era of medical advancement.

Progress in Surface Modification of Titanium Implants by Hydrogel Coatings

Although titanium and titanium alloys have become the preferred materials for various medical implants, surface modification technology still needs to be strengthened in order to adapt to the complex physiological environment of the human body. Compared with physical or chemical modification methods, biochemical modification, such as the introduction of functional hydrogel coating on implants, can fix biomolecules such as proteins, peptides, growth factors, polysaccharides, or nucleotides on the surface of the implants, so that they can directly participate in biological processes; regulate cell adhesion, proliferation, migration, and differentiation; and improve the biological activity on the surface of the implants. This review begins with a look at common substrate materials for hydrogel coatings on implant surfaces, including natural polymers such as collagen, gelatin, chitosan, and alginate, and synthetic materials such as polyvinyl alcohol, polyacrylamide, polyethylene glycol, and polyacrylic acid. Then, the common construction methods of hydrogel coating (electrochemical method, sol-gel method and layer-by-layer self-assembly method) are introduced. Finally, five aspects of the enhancement effect of hydrogel coating on the surface bioactivity of titanium and titanium alloy implants are described: osseointegration, angiogenesis, macrophage polarization, antibacterial effects, and drug delivery. In this paper, we also summarize the latest research progress and point out the future research direction. After searching, no previous relevant literature reporting this information was found.

Titanium Lattice Structures Produced via Additive Manufacturing for a Bone Scaffold: A Review

The progress in additive manufacturing has remarkably increased the application of lattice materials in the biomedical field for the fabrication of scaffolds used as bone substitutes. Ti6Al4V alloy is widely adopted for bone implant application as it combines both biological and mechanical properties. Recent breakthroughs in biomaterials and tissue engineering have allowed the regeneration of massive bone defects, which require external intervention to be bridged. However, the repair of such critical bone defects remains a challenge. The present review collected the most significant findings in the literature of the last ten years on Ti6Al4V porous scaffolds to provide a comprehensive summary of the mechanical and morphological requirements for the osteointegration process. Particular attention was given on the effects of pore size, surface roughness and the elastic modulus on bone scaffold performances. The application of the Gibson–Ashby model allowed for a comparison of the mechanical performance of the lattice materials with that of human bone. This allows for an evaluation of the suitability of different lattice materials for biomedical applications.

Reconstruction of massive bone defects after femoral tumor resection using two new-designed 3D-printed intercalary prostheses: a clinical analytic study with the cooperative utilization of multiple technologies

BACKGROUND

To reconstruct massive bone defects of the femoral diaphysis and proximal end with limited bilateral cortical bone after joint-preserving musculoskeletal tumor resections, two novel 3D-printed customized intercalary femoral prostheses were applied.

METHODS

A series of nine patients with malignancies who received these novel 3D-printed prostheses were retrospectively studied between July 2018 and November 2021. The proximal and diaphyseal femur was divided into three regions of interest (ROIs) according to anatomic landmarks, and anatomic measurements were conducted on 50 computed tomography images showing normal femurs. Based on the individual implant-involved ROIs, osteotomy level, and anatomical and biomechanical features, two alternative 3D-printed prostheses were designed. In each patient, Hounsfield Unit (HU) value thresholding and finite element analysis were conducted to identify the bone trabecula and calcar femorale and to determine the stress distribution, respectively. We described the characteristics of each prosthesis and surgical procedure and recorded the intraoperative data. All patients underwent regular postoperative follow-up, in which the clinical, functional and radiographical outcomes were evaluated.

RESULTS

With the ROI division and radiographic measurements, insufficient bilateral cortical bones for anchoring the traditional stem were verified in the normal proximal femur. Therefore, two 3D-printed intercalary endoprostheses, a Type A prosthesis with a proximal curved stem and a Type B prosthesis with a proximal anchorage-slot and corresponding locking screws, were designed. Based on HU value thresholding and finite element analysis, the 3D-printed proximal stems in all prostheses maximally preserved the trabecular bone and calcar femorale and optimized the biomechanical distribution, as did the proximal screws. With the 3D-printed osteotomy guide plates and reaming guide plates, all patients underwent the operation uneventfully with a satisfactory duration (325.00 ± 62.60 min) and bleeding volume (922.22 ± 222.36 ml). In the follow-up, Harris Hip and Musculoskeletal Tumor Society scores were ameliorated after surgery (P < 0.001 and P < 0.001, respectively), reliable bone ingrowth was observed, and no major complications occurred.

CONCLUSIONS

Two novel 3D-printed femoral intercalary prostheses, which achieved acceptable overall postoperative outcomes, were used as appropriate alternatives for oncologic patients with massive bone defects and limited residual bone and increased the opportunities for joint-preserving tumor resection. Several scientific methodologies utilized in this study may promote the clinical design proposals of 3D-printed implants.

Layer-by-layer assembly of procyanidin and collagen promotes mesenchymal stem cell proliferation and osteogenic differentiation in vitro and in vivo

Collagen, commonly used in tissue engineering, is widespread in various tissues. During bone tissue regeneration, collagen can stimulate the cellular response and determine the fate of cells. In this work, we integrated collagen type II with procyanidin (PC) onto an implant coating by applying a layer-by-layer technique to demonstrate that collagen and PC can participate in the construction of new biomaterials and serve as multifunctional components. The effects of PC/collagen multilayers on the viability of cocultured bone marrow mesenchymal stem cells (BMSCs) were analyzed by cell counting kit-8 analysis and phalloidin staining. The reactive oxygen species level of BMSCs was revealed through immunofluorescent staining and flow cytometry. Osteogenesis-related genes were detected, and in vivo experiment was performed to reveal the effect of newly designed material on the osteogenic differentiation of BMSCs. Our data demonstrated that in BMSCs PC/collagen multilayers accelerated the proliferation and osteogenic differentiation through Wnt/β-catenin signaling pathway and enhanced bone generation around the implant in the bone defect model of rabbit femurs. In summary, combination of collagen and PC provided a new sight for the research and development of implant materials or coatings in the future.

Nervous System-Driven Osseointegration

Implants are essential therapeutic tools for treating bone fractures and joint replacements. Despite the in-depth study of osseointegration for more than fifty years, poor osseointegration caused by aseptic loosening remains one of the leading causes of late implant failures. Osseointegration is a highly sophisticated and spatiotemporal process in vivo involving the immune response, angiogenesis, and osteogenesis. It has been unraveled that the nervous system plays a pivotal role in skeletal health via manipulating neurotrophins, neuropeptides, and nerve cells. Herein, the research related to nervous system-driven osseointegration was systematically analyzed and reviewed, aiming to demonstrate the prominent role of neuromodulation in osseointegration. Additionally, it is indicated that the implant design considering the role of neuromodulation might be a promising way to prevent aseptic loosening.

Effect of Angiogenesis in Bone Tissue Engineering

The reconstruction of large skeletal defects is still a tricky challenge in orthopedics. The newly formed bone tissue migrates sluggishly from the periphery to the center of the scaffold due to the restrictions of exchange of oxygen and nutrition impotent cells osteogenic differentiation. Angiogenesis plays an important role in bone reconstruction and more and more studies on angiogenesis in bone tissue engineering had been published. Promising advances of angiogenesis in bone tissue engineering by scaffold designs, angiogenic factor delivery, in vivo prevascularization and in vitro prevascularization are discussed in detail. Among all the angiogenesis mode, angiogenic factor delivery is the common methods of angiogenesis in bone tissue engineering and possible research directions in the future.

Effect of Pore Size of Porous-Structured Titanium Implants on Tendon Ingrowth

Purpose

The reconstruction of a tendon insertion on metal prostheses is a challenge in orthopedics. Of the available metal prostheses, porous metal prostheses have been shown to have better biocompatibility for tissue integration. Therefore, this study is aimed at identifying an appropriate porous structure for the reconstruction of a tendon insertion on metal prostheses.

Methods

Ti6Al4V specimens with a diamond-like porous structure with triply periodic minimal surface pore sizes of 300, 500, and 700 μm and a porosity of 58% (designated Ti300, Ti500, and Ti700, respectively) were manufactured by selective laser melting and were characterized with micro-CT and scanning electron microscopy for their porosity, pore size, and surface topography. The porous specimens were implanted into the patellar tendon of rabbits. Tendon integration was evaluated after implantation into the tendon at 4, 8, and 12 weeks by histology, and the fixation strength was evaluated with a pull-out test at week 12.

Results

The average pore sizes of the Ti300, Ti500, and Ti700 implants were 261, 480, and 668 μm, respectively. The Ti500 and Ti700 implants demonstrated better tissue growth than the Ti300 implant at weeks 4, 8, and 12. At week 12, the histological score of the Ti500 implant was 13.67 ± 0.58, and it had an area percentage of type I collagen of 63.90% ± 3.41%; both of these results were significantly higher than those for the Ti300 and Ti700 implants. The pull-out load at week 12 was also the highest in the Ti500 group.

Conclusion

Ti6Al4V implants with a diamond-like porous structure with triply periodic minimal surface pore size of 500 μm are suitable for tendon integration.

Hybrid fabrication of photo-clickable vascular hydrogels with additive manufactured titanium implants for enhanced osseointegration and vascularized bone formation

Bone regeneration of critical-sized bone defects, bone fractures or joint replacements remains a significant unmet clinical challenge. Although there has been rapid advancement in both the fields of bone tissue engineering and additive manufacturing (AM), functional bone implants with rapid vascularization capacity to ensure osseointegration and long-term biological fixation in large bone defects remains limited in clinics. In this study, we developed an in vitro vascularized bone implant by combining cell-laden hydrogels with direct metal printed (DMP) porous titanium alloys (Ti-6Al-4V). 5wt% allylated gelatin (GelAGE), was utilized to co-encapsulate human mesenchymal stromal cells (hMSCs) and human umbilical vein endothelial cells (HUVECs) to investigate concurrent osteogenic and vasculogenic performance. DMP macro-porous Ti-6Al-4V scaffolds were subsequently infused/enriched with cell-laden GelAGE to examine the feasibility to deliver cells and engineer vascular-like networks in the hybrid implant. Furthermore, as a proof of concept, a full-scale porous Ti-6Al-4V acetabular cup was impregnated with cell-laden hydrogel to validate the clinical potential of this strategy. The vasculogenic potential was evaluated by examining micro-capillary formation coupled with capillary network maturation and stabilization. Osteogenic differentiation was assessed via ALP activity as well as osteocalcin and osteopontin expression. Our results suggested that GelAGE supported HUVECs spreading and vascular-like network formation, along with osteogenesis of hMSCs. Titanium hybrid constructs with cell-laden hydrogel demonstrated enhanced osteogenesis with similar vasculogenic capability compared to the cell-laden hydrogel alone constructs. The full-scale implant with cell-laden hydrogel coating similarly showed cell distribution and spreading, implying the potential for further clinical application. Our study presents the feasibility of integrating bio-functional hydrogels with porous titanium implants to fabricate a vascularized hybrid construct with both mechanical support and preferable biological functionality (osteogenesis/vasculogenesis), which paves the way for improved strategies to enhance bone regeneration in complex large bone defects achieving long-term bone-implant fixation.

The Effects of Graphene on the Biocompatibility of a 3D-Printed Porous Titanium Alloy

3D-printed titanium (Ti) materials have attracted much attention in the field of bone tissue repair. However, the combination strength of traditional alloy materials with bone tissue is lower, and the elastic modulus is higher than that of natural bone tissue, which makes the titanium alloy susceptible to stress shielding phenomena after implantation. Therefore, it is urgent to find better surface modification technology. In this study, the physical and chemical properties, toxicity, and proliferation of adipose stem cells of composite graphene-coated titanium alloy (Gr–Ti) were investigated using 3D-printed titanium alloy as a material model. Physical and chemical property tests confirmed that 3D printing could produce porous titanium alloy materials; the compressive strength and elastic modulus of the titanium alloy scaffolds were 91 ± 3 MPa and 3.1 ± 0.4 GPa, matching the elastic modulus of normal bone tissue. The surface characterization shows that graphene can be coated on titanium alloy by a micro-arc oxidation process, which significantly improves the surface roughness of titanium alloy. The roughness factor (Ra) of the Ti stent was 4.95 ± 1.12 μm, while the Ra of the Gr–Ti stent was 6.37 ± 0.72 μm. After the adipose stem cells were co-cultured with the scaffold for 4 h and 24 h, it was found that the Gr–Ti scaffold could better promote the early cell adhesion. CCK-8 tests showed that the number of ADSCs on the G–Ti scaffold was significantly higher than that on the Ti scaffold (p < 0.01). The relative growth rate (RGR) of ADSCs in Gr–Ti was grade 0–1 (non-toxic). In the in vivo experiment of repairing a critical bone defect of a rabbit mandible, the bone volume fraction in the Gr–Ti group increased to 49.42 ± 3.28%, which was much higher than that in the Ti group (39.76 ± 3.62%) (p < 0.05). In conclusion, the porous graphene–titanium alloy promotes the proliferation and adhesion of adipose stem cells with multidirectional differentiation potential, which has great potential for the application of bone tissue engineering in repairing bone defects in the future.