Improving osteointegration and osteogenesis of three-dimensional porous Ti6Al4V scaffolds by polydopamine-assisted biomimetic hydroxyapatite coating

Simvastatin/hydrogel-loaded 3D-printed titanium alloy scaffolds suppress osteosarcoma via TF/NOX2-associated ferroptosis while repairing bone defects

Postoperative anatomical reconstruction and prevention of local recurrence after tumor resection are two vital clinical challenges in osteosarcoma treatment. A three-dimensional (3D)-printed porous Ti6Al4V scaffold (3DTi) is an ideal material for reconstructing critical bone defects with numerous advantages over traditional implants, including a lower elasticity modulus, stronger bone-implant interlock, and larger drug-loading space. Simvastatin is a multitarget drug with anti-tumor and osteogenic potential; however, its efficiency is unsatisfactory when delivered systematically. Here, simvastatin was loaded into a 3DTi using a thermosensitive poly (lactic-co-glycolic) acid (PLGA)-polyethylene glycol (PEG)-PLGA hydrogel as a carrier to exert anti-osteosarcoma and osteogenic effects. Newly constructed simvastatin/hydrogel-loaded 3DTi (Sim-3DTi) was comprehensively appraised, and its newfound anti-osteosarcoma mechanism was explained. Specifically, in a bone defect model of rabbit condyles, Sim-3DTi exhibited enhanced osteogenesis, bone in-growth, and osseointegration compared with 3DTi alone, with greater bone morphogenetic protein 2 expression. In our nude mice model, simvastatin loading reduced tumor volume by 59%-77 % without organic damage, implying good anti-osteosarcoma activity and biosafety. Furthermore, Sim-3DTi induced ferroptosis by upregulating transferrin and nicotinamide adenine dinucleotide phosphate oxidase 2 levels in osteosarcoma both in vivo and in vitro. Sim-3DTi is a promising osteogenic bone substitute for osteosarcoma-related bone defects, with a ferroptosis-mediated anti-osteosarcoma effect.

Preparation of Cu2+/TA/HAP composite coating with anti-bacterial and osteogenic potential on 3D-printed porous Ti alloy scaffolds for orthopedic applications

Because of stress shielding effects, traditional titanium (Ti) alloy scaffolds have a high elastic modulus, which might promote looseness and bone disintegration surrounding the implant, increasing the likelihood of a second surgery. In contrast, 3D-printed porous Ti alloy scaffolds can reduce the scaffold weight while enhancing biocompatibility. Further, these scaffolds' porous nature allows bone tissue ingrowth as well as strong pore connectivity, which can improve nutrient absorption. Nevertheless, bare Ti alloy implants may fail because of inadequate bone integration; hence, adding a coating on the implant surface is an effective technique for improving implant stability. In this study, a composite coating comprising hydroxyapatite (HAP), chitosan (CS), tannic acid (TA) and copper ions (Cu2+) (Cu2+/TA/HAP composite coating) was prepared on the surface of 3D printed porous Ti alloy scaffolds using electrophoretic deposition. Using the standard plate count method, Live/Dead bacteria staining assay, FITC Phalloidin and 4',6-diamidino-2-phenylindole staining assay, and live/dead staining of cells we determined that the composite coating has better antibacterial properties and cytocompatibility as well as lower cytotoxicity. The Alkaline Phosphatase assay revealed that the coating results showed good osteogenesis potential. Overall, the composite coatings produced in this investigation give new potential for the application of Ti alloys in clinics.

Incomplete insertion of pedicle screws triggers a higher biomechanical risk of screw loosening: mechanical tests and corresponding numerical simulations

Screw loosening is a widely reported issue after spinal screw fixation and triggers several complications. Biomechanical deterioration initially causes screw loosening. Studies have shown that incomplete insertion of pedicle screws increases the risk of screw breakage by deteriorating the local mechanical environment. However, whether this change has a biomechanical effect on the risk of screw loosening has not been determined. This study conducted comprehensive biomechanical research using polyurethane foam mechanical tests and corresponding numerical simulations to verify this topic. Pedicle screw-fixed polyurethane foam models with screws with four different insertion depths were constructed, and the screw anchoring ability of different models was verified by toggle tests with alternating and constant loads. Moreover, the stress distribution of screw and bone-screw interfaces in different models was computed in corresponding numerical mechanical models. Mechanical tests presented better screw anchoring ability with deeper screw insertion, but parameters presented no significant difference between groups with complete thread insertion. Correspondingly, higher stress values can be recorded in the model without complete thread insertion; the difference in stress values between models with complete thread insertion was relatively slight. Therefore, incomplete thread insertion triggers local stress concentration and the corresponding risk of screw loosening; completely inserting threads could effectively alleviate local stress concentration and result in the prevention of screw loosening.

Advanced micro-/nanotechnologies for exosome encapsulation and targeting in regenerative medicine

Exosomes, a subset of vesicles generated from cell membranes, are crucial for cellular communication. Exosomes' innate qualities have been used in recent studies to create nanocarriers for various purposes, including medication delivery and immunotherapy. As a result, a wide range of approaches has been designed to utilize their non-immunogenic nature, drug-loading capacity, or targeting ability. In this study, we aimed to review the novel methods and approaches in exosome engineering for encapsulation and targeting in regenerative medicine. We have assessed and evaluated each method's efficacy, advantages, and disadvantages and discussed the results of related studies. Even though the therapeutic role of non-allogenic exosomes has been demonstrated in several studies, their application has certain limitations as these particles are neither fully specific to target tissue nor tissue retainable. Hence, there is a strong demand for developing more efficient encapsulation methods along with more accurate and precise targeting methods, such as 3D printing and magnetic nanoparticle loading in exosomes, respectively.

A drug-loaded composite coating to improve osteogenic and antibacterial properties of Zn-1Mg porous scaffolds as biodegradable bone implants

Zinc (Zn) alloy porous scaffolds produced by additive manufacturing own customizable structures and biodegradable functions, having a great application potential for repairing bone defect. In this work, a hydroxyapatite (HA)/polydopamine (PDA) composite coating was constructed on the surface of Zn-1Mg porous scaffolds fabricated by laser powder bed fusion, and was loaded with a bioactive factor BMP2 and an antibacterial drug vancomycin. The microstructure, degradation behavior, biocompatibility, antibacterial performance and osteogenic activities were systematically investigated. Compared with as-built Zn-1Mg scaffolds, the rapid increase of Zn²⁺, which resulted to the deteriorated cell viability and osteogenic differentiation, was inhibited due to the physical barrier of the composite coating. In vitro cellular and bacterial assay indicated that the loaded BMP2 and vancomycin considerably enhanced the cytocompatibility and antibacterial performance. Significantly improved osteogenic and antibacterial functions were also observed according to in vivo implantation in the lateral femoral condyle of rats. The design, influence and mechanism of the composite coating were discussed accordingly. It was concluded that the additively manufactured Zn-1Mg porous scaffolds together with the composite coating could modulate biodegradable performance and contribute to effective promotion of bone recovery and antibacterial function.

Antimicrobial and Biodegradable 3D Printed Scaffolds for Orthopedic Infections

In bone tissue engineering, the performance of scaffolds underpins the success of the healing of bone. Microbial infection is the most challenging issue for orthopedists. The application of scaffolds for healing bone defects is prone to microbial infection. To address this challenge, scaffolds with a desirable shape and significant mechanical, physical, and biological characteristics are crucial. 3D printing of antibacterial scaffolds with suitable mechanical strength and excellent biocompatibility is an appealing strategy to surmount issues of microbial infection. The spectacular progress in developing antimicrobial scaffolds, along with beneficial mechanical and biological properties, has sparked further research for possible clinical applications. Herein, the significance of antibacterial scaffolds designed by 3D, 4D, and 5D printing technologies for bone tissue engineering is critically investigated. Materials such as antibiotics, polymers, peptides, graphene, metals/ceramics/glass, and antibacterial coatings are used to impart the antimicrobial features for the 3D scaffolds. Polymeric or metallic biodegradable and antibacterial 3D-printed scaffolds in orthopedics disclose exceptional mechanical and degradation behavior, biocompatibility, osteogenesis, and long-term antibacterial efficiency. The commercialization aspect of antibacterial 3D-printed scaffolds and technical challenges are also discussed briefly. Finally, the discussion on the unmet demands and prevailing challenges for ideal scaffold materials for fighting against bone infections is included along with a highlight of emerging strategies in this field.

Effect of pores on cell adhesion to additively manufactured titanium implants: A systematic review

STATEMENT OF PROBLEM

Titanium dental implants produced by additive manufacturing have pores that, depending on their size and quantity, may improve osteogenic cell adhesion without impairing mechanical properties. A systematic review of in vitro studies on this topic is lacking.

PURPOSE

The purpose of this systematic review was to answer the question "What is the influence of pores on osteogenic cell adhesion on titanium surfaces produced by additive manufacturing?".

MATERIAL AND METHODS

The study was designed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) 2020 standards and registered in the Open Science Framework (OSF) (osf.io/baw59). A manual search of published articles without language or time restrictions was conducted in November 2022 in the electronic databases PubMed, Scopus, ScienceDirect, Embase, and in the nonpeer-reviewed literature via Google Scholar.

RESULTS

A total of 1338 initial results were found, and after removing duplicates and applying eligibility criteria, 13 articles were included in this review that, according to the Joanna Briggs Institute (JBI) tool, presented a low risk of bias. Pores with larger diameters provide greater a surface area that favors cell filopodia adhesion and has interconnection that optimizes the transport of nutrients and oxygen and bone cell activity.

CONCLUSIONS

The presence of pores on the surface of titanium produced by additive manufacturing increases the adhesion, migration, proliferation, and viability of osteogenic cells.

Improving osteoinduction and osteogenesis of Ti6Al4V alloy porous scaffold by regulating the pore structure

Titanium alloy scaffolds with a porous structure have attracted much attention in bone defect repair. However, which pore structure is more beneficial to bone defect repair is controversial. In the present research, the Ti6Al4V alloy porous scaffolds with gradient pore sizes were designed and fabricated. The microstructure characterization, tests of mechanical properties, and in vitro and in vivo experiments have been performed to systematically evaluate the effect of pore size on osteoinduction and osteogenesis. The results revealed that the contact angle with water, compressive strength, and elastic modulus of the Ti6Al4V alloy porous scaffolds decreased gradually with the increase of pore size. However, there were obvious drops when the pore size of the porous scaffold was around 600 μm. As the pore size increased, the proliferation and integrin β1 of RAW 264.7 macrophages seeded on Ti6Al4V alloy porous scaffolds increased at first, reaching a maximum value at a pore size of around 600 μm, and then decreased subsequently. The proliferation, integrin β1, and osteogenic gene-related expressions of Bone marrow mesenchymal stem cells (BMSCs) seeded on Ti6Al4V alloy porous scaffolds with different pore sizes all exhibited similar variations which rose with increased pore size firstly, obtaining the maximum value at pore size about 600 μm, and then declined. The in vivo experiments confirmed the in vitro results, and the Ti6Al4V alloy porous scaffold with a pore size of 600 μm possessed the better capability to induce new bone formation. Therefore, for the design of Ti6Al4V alloy with a regular porous scaffold, the surface morphology, porosity, strength, and elastic modulus should be considered systematically, which would determine the capability of osteoinduction and osteogenesis.

Geopolymer Materials for Bone Tissue Applications: Recent Advances and Future Perspectives

With progress in the bone tissue engineering (BTE) field, there is an important need to develop innovative biomaterials to improve the bone healing process using reproducible, affordable, and low-environmental-impact alternative synthetic strategies. This review thoroughly examines geopolymers' state-of-the-art and current applications and their future perspectives for bone tissue applications. This paper aims to analyse the potential of geopolymer materials in biomedical applications by reviewing the recent literature. Moreover, the characteristics of materials traditionally used as bioscaffolds are also compared, critically analysing the strengths and weaknesses of their use. The concerns that prevented the widespread use of alkali-activated materials as biomaterials (such as their toxicity and limited osteoconductivity) and the potentialities of geopolymers as ceramic biomaterials have also been considered. In particular, the possibility of targeting their mechanical properties and morphologies through their chemical compositions to meet specific and relevant requirements, such as biocompatibility and controlled porosity, is described. A statistical analysis of the published scientific literature is presented. Data on "geopolymers for biomedical applications" were extracted from the Scopus database. This paper focuses on possible strategies necessary to overcome the barriers that have limited their application in biomedicine. Specifically, innovative hybrid geopolymer-based formulations (alkali-activated mixtures for additive manufacturing) and their composites that optimise the porous morphology of bioscaffolds while minimising their toxicity for BTE are discussed.

Electrical stimulation of piezoelectric BaTiO3 coated Ti6Al4V scaffolds promotes anti-inflammatory polarization of macrophages and bone repair via MAPK/JNK inhibition and OXPHOS activation

Bone regeneration is a highly synchronized process that requires multiple biochemical, bioelectrical, mechanical, and other physiological cues. The restoration and delivery of electrical cues locally through piezoelectric materials have been demonstrated to facilitate osteogenesis in vitro and bone repair in vivo. However, the underlying mechanism by which piezoelectric stimulation promotes osteogenesis and bone repair remains unclear yet, limiting the design and clinical application of piezoelectric materials for bone repair. Herein, a piezoelectric BaTiO3/Ti6Al4V (BT/Ti) scaffold was prepared by hydrothermal synthesis of a uniform BaTiO3 layer on three dimensionally printed Ti6Al4V scaffold. The BT/Ti scaffolds exhibited piezoelectricity and favorable biocompatibility with RAW264.7 macrophages after polarization. In vitro results demonstrated that the piezoelectric effects of the poled BT/Ti scaffolds promoted M2 polarization of macrophages and immunoregulatory osteogenesis of MC-3T3 osteoblasts. In a subcutaneous implantation model, a higher proportion of CD68⁺ CD206⁺ M2 macrophages was observed in the tissues around the poled BT/Ti scaffolds under low intensity pulsed ultrasound (LIPUS) stimulation. Improvements in macrophage M2 polarization and bone regeneration were further identified in a sheep cervical corpectomy model. RNA sequencing and mechanistic investigation revealed that the piezoelectric BT/Ti (poled) scaffolds inhibited the inflammatory MAPK/JNK signaling cascade and activated oxidative phosphorylation (OXPHOS) and ATP synthesis in macrophages. Collectively, our study provides a promising method for regulating the immune microenvironment and enhancing bone regeneration using polarized piezoelectric BT/Ti scaffolds.

Hierarchy of hybrid materials. Part-II: The place of organics-on-inorganics in it, their composition and applications

Hybrid materials or hybrids incorporating organic and inorganic constituents are emerging as a very potent and promising class of materials due to the diverse but complementary nature of their properties. This complementarity leads to a perfect synergy of properties of the desired materials and products as well as to an extensive range of their application areas. Recently, we have overviewed and classified hybrid materials describing inorganics-in-organics in Part-I (Saveleva, et al., Front. Chem., 2019, 7, 179). Here, we extend that work in Part-II describing organics-on-inorganics, i.e., inorganic materials modified by organic moieties, their structure and functionalities. Inorganic constituents comprise of colloids/nanoparticles and flat surfaces/matrices comprise of metallic (noble metal, metal oxide, metal-organic framework, magnetic nanoparticles, alloy) and non-metallic (minerals, clays, carbons, and ceramics) materials; while organic additives can include molecules (polymers, fluorescence dyes, surfactants), biomolecules (proteins, carbohydtrates, antibodies and nucleic acids) and even higher-level organisms such as cells, bacteria, and microorganisms. Similarly to what was described in Part-I, we look at similar and dissimilar properties of organic-inorganic materials summarizing those bringing complementarity and composition. A broad range of applications of these hybrid materials is also presented whose development is spurred by engaging different scientific research communities.

Evaluation of cell adhesion and osteoconductivity in bone substitutes modified by polydopamine

Bones damaged due to disease or accidents can be repaired in different ways. Tissue engineering has helped with scaffolds made of different biomaterials and various methods. Although all kinds of biomaterials can be useful, sometimes their weakness in cellular activity or osteoconductivity prevents their optimal use in the fabrication of bone scaffolds. To solve this problem, we need additional processes, such as surface modification. One of the common methods is coating with polydopamine. Polydopamine can not only cover the weakness of the scaffolds in terms of cellular properties, but it can also create or increase osteoconductivity properties. Polydopamine creates a hydrophilic layer on the surface of scaffolds due to a large number of functional groups such as amino and hydroxyl groups. This layer allows bone cells to anchor and adheres well to the surfaces. In addition, it creates a biocompatible environment for proliferation and differentiation. Besides, the polydopamine coating makes the surfaces chemically active by catechol and amine group, and as a result of their presence, osteoconductivity increases. In this mini-review, we investigated the characteristics, structure, and properties of polydopamine as a modifier of bone substitutes. Finally, we evaluated the cell adhesion and osteoconductivity of different polydopamine-modified bone scaffolds.

Definition, measurement, and function of pore structure dimensions of bioengineered porous bone tissue materials based on additive manufacturing: A review

Bioengineered porous bone tissue materials based on additive manufacturing technology have gradually become a research hotspot in bone tissue-related bioengineering. Research on structural design, preparation and processing processes, and performance optimization has been carried out for this material, and further industrial translation and clinical applications have been implemented. However, based on previous studies, there is controversy in the academic community about characterizing the pore structure dimensions of porous materials, with problems in the definition logic and measurement method for specific parameters. In addition, there are significant differences in the specific morphological and functional concepts for the pore structure due to differences in defining the dimensional characterization parameters of the pore structure, leading to some conflicts in perceptions and discussions among researchers. To further clarify the definitions, measurements, and dimensional parameters of porous structures in bioengineered bone materials, this literature review analyzes different dimensional characterization parameters of pore structures of porous materials to provide a theoretical basis for unified definitions and the standardized use of parameters.

Surface analysis of (Ti,Mg)N coated bone fixation devices following the rabbit femur surgery

BACKGROUND

Magnesium (Mg) enhances the bone regeneration, mineralization and attachment at the tissue/biomaterial interface.

OBJECTIVE

In this study, the effect of Mg on mineralization/osseointegration was determined using (Ti,Mg)N thin film coated Ti6Al4V based plates and screws in vivo.

METHODS

TiN and (Ti,Mg)N coated Ti6Al4V plates and screws were prepared using arc-PVD technique and used to fix rabbit femur fractures for 6 weeks. Then, mineralization/osseointegration was assessed by surface analysis including cell attachment, mineralization, and hydroxyapatite deposition on concave and convex sides of the plates along with the attachment between the screw and the bone.

RESULTS

According to Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analyses; cell attachment and mineralization were higher on the concave sides of the plates from both groups in comparison to the convex sides. However, mineralization was significantly higher on Mg-containing ones. The mean gray value indicating mineralized area after von Kossa staining was found as 0.48 ± 0.01 and 0.41 ± 0.04 on Mg containing and free ones respectively. Similarly, Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD) analyses showed that hydroxyapatite growth was abundant on the Mg-containing and concave sides of the plates. Enhanced mineralization and strong attachment to bone were also detected in EDS and SEM analyses of Mg-containing screws.

CONCLUSION

These findings indicated that (Ti,Mg)N coatings can be used to increase attachment at the implant tissue interface due to accelerated mineralization, cell attachment, and hydroxyapatite growth.

A review on in vitro/in vivo response of additively manufactured Ti-6Al-4V alloy

Bone replacement using porous and solid metallic implants, such as Ti-alloy implants, is regarded as one of the most practical therapeutic approaches in biomedical engineering. The bone is a complex tissue with various mechanical properties based on the site of action. Patient-specific Ti-6Al-4V constructs may address the key needs in bone treatment for having customized implants that mimic the complex structure of the natural tissue and diminish the risk of implant failure. This review focuses on the most promising methods of fabricating such patient-specific Ti-6Al-4V implants using additive manufacturing (AM) with a specific emphasis on the popular subcategory, which is powder bed fusion (PBF). Characteristics of the ideal implant to promote optimized tissue-implant interactions, as well as physical, mechanical/chemical treatments and modifications will be discussed. Accordingly, such investigations will be classified into 3B-based approaches (Biofunctionality, Bioactivity, and Biostability), which mainly govern native body response and ultimately the success in implantation.

Functional engineering strategies of 3D printed implants for hard tissue replacement

Three-dimensional printing technology with the rapid development of printing materials are widely recognized as a promising way to fabricate bioartificial bone tissues. In consideration of the disadvantages of bone substitutes, including poor mechanical properties, lack of vascularization and insufficient osteointegration, functional modification strategies can provide multiple functions and desired characteristics of printing materials, enhance their physicochemical and biological properties in bone tissue engineering. Thus, this review focuses on the advances of functional engineering strategies for 3D printed biomaterials in hard tissue replacement. It is structured as introducing 3D printing technologies, properties of printing materials (metals, ceramics and polymers) and typical functional engineering strategies utilized in the application of bone, cartilage and joint regeneration.

MOF-derived CuO@ZnO modified titanium implant for synergistic antibacterial ability, osteogenesis and angiogenesis

Surface modification of titanium implants with antibacterial, osteogenic and even angiogenic capabilities are essential to enhance their clinical applicability. Herein, metal-organic framework (MOF) derived CuO@ZnO composite was grafted onto the polydopamine (PDA) modified titanium alloy to achieve vascularized bone regeneration. The CuO@ZnO-coated titanium effectively inhibits the formation of bacterial biofilms and the sterilization rate of Staphylococcus aureus (S. aureus) reaches 99%. Benefitting from the intrinsic porous architecture of MOFs, the Zn2+ and Cu2+ could be controllably released to facilitate the production of excess intracellular reactive oxygen species (ROS) inside the bacteria, which ensures the excellent antibacterial performance of the composite coating. The CuO@ZnO-coated titanium also exhibits good cytocompatibility, effectively promotes the adhesion and proliferation of the human bone marrow mesenchymal stem cells (hBMSCs) and reduces the level of the cell apoptosis. The up-regulated expression of the osteogenesis-related genes and the superior extracellular matrix mineralization reveals that the CuO@ZnO coating possesses fantastic osteoinductive properties. In addition, the transwell and tube formation assays of the human umbilical vein endothelial cells (HUVECs) suggest the superior angiogenesis ability of the CuO@ZnO-coated titanium. The released Cu2+ stimulated the angiogenesis of the HUVECs in vitro by up-regulating the expression of the vascular endothelial growth factor (VEGF). These findings will provide new insight into the development of multifunctional titanium implants for clinical applications.

Smart biomaterials and their potential applications in tissue engineering

Smart biomaterials have been rapidly advancing ever since the concept of tissue engineering was proposed. Interacting with human cells, smart biomaterials can play a key role in novel tissue morphogenesis. Various aspects of biomaterials utilized in or being sought for the goal of encouraging bone regeneration, skin graft engineering, and nerve conduits are discussed in this review. Beginning with bone, this study summarizes all the available bioceramics and materials along with their properties used singly or in conjunction with each other to create scaffolds for bone tissue engineering. A quick overview of the skin-based nanocomposite biomaterials possessing antibacterial properties for wound healing is outlined along with skin regeneration therapies using infrared radiation, electrospinning, and piezoelectricity, which aid in wound healing. Furthermore, a brief overview of bioengineered artificial skin grafts made of various natural and synthetic polymers has been presented. Finally, by examining the interactions between natural and synthetic-based biomaterials and the biological environment, their strengths and drawbacks for constructing peripheral nerve conduits are highlighted. The description of the preclinical outcome of nerve regeneration in injury healed with various natural-based conduits receives special attention. The organic and synthetic worlds collide at the interface of nanomaterials and biological systems, producing a new scientific field including nanomaterial design for tissue engineering.

Additively manufactured metallic biomaterials

Metal additive manufacturing (AM) has led to an evolution in the design and fabrication of hard tissue substitutes, enabling personalized implants to address each patient's specific needs. In addition, internal pore architectures integrated within additively manufactured scaffolds, have provided an opportunity to further develop and engineer functional implants for better tissue integration, and long-term durability. In this review, the latest advances in different aspects of the design and manufacturing of additively manufactured metallic biomaterials are highlighted. After introducing metal AM processes, biocompatible metals adapted for integration with AM machines are presented. Then, we elaborate on the tools and approaches undertaken for the design of porous scaffold with engineered internal architecture including, topology optimization techniques, as well as unit cell patterns based on lattice networks, and triply periodic minimal surface. Here, the new possibilities brought by the functionally gradient porous structures to meet the conflicting scaffold design requirements are thoroughly discussed. Subsequently, the design constraints and physical characteristics of the additively manufactured constructs are reviewed in terms of input parameters such as design features and AM processing parameters. We assess the proposed applications of additively manufactured implants for regeneration of different tissue types and the efforts made towards their clinical translation. Finally, we conclude the review with the emerging directions and perspectives for further development of AM in the medical industry.

Regulation of Macrophage Polarization Through Periodic Photo-Thermal Treatment to Facilitate Osteogenesis

The richened reactive oxygen species (ROS) and their derived excessive inflammation at bone injured sites hinder osteogenesis of endosseous Ti-based implants. Herein, anti-oxidized polydopamine (PDA) is deposited on hydrothermal growth formed hydroxyapatite (HA) nanorods on Ti to form a core-shell structural nanorod-like array with HA as a core and PDA as an amorphous shell (PDA@HA), showing not only ROS scavenging ability but also near-infrared (NIR) light derived photo-thermal effects. PDA@HA suppresses inflammation based on its ROS scavenging ability to a certain extent, while periodic photo-thermal treatment (PTT) at a mild temperature (41 ± 1 °C) further accelerates the transition of the macrophages (MΦs) adhered to PDA@HA from the pro-inflammatory (M1) phenotype to the anti-inflammatory (M2) phenotype in vitro and in vivo. Transcriptomic analysis reveals that the activation of the PI3K-Akt1 signaling pathway is responsible for the periodic PTT induced acceleration of the M1-to-M2 transition of MΦs. Acting on mesenchymal stem cells (MSCs) with paracrine cytokines of M2 macrophages, PDA@HA with mild PTT greatly promote the osteogenetic functions of MSCs and thus osteogenesis. This work paves a way of employing mildly periodic PTT to induce a favorable immunomodulatory microenvironment for osteogenesis and provides insights into its underlying immunomodulation mechanism.

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.

Metallic Scaffold with Micron-Scale Geometrical Cues Promotes Osteogenesis and Angiogenesis via the ROCK/Myosin/YAP Pathway

The advent of precision manufacturing has enabled the creation of pores in metallic scaffolds with feature size in the range of single microns. In orthopedic implants, pore geometries at the micron scale could regulate bone formation by stimulating osteogenic differentiation and the coupling of osteogenesis and angiogenesis. However, the biological response to pore geometry at the cellular level is not clear. As cells are sensitive to curvature of the pore boundary, this study aimed to investigate osteogenesis in high- vs low-curvature environments by utilizing computer numerical control laser cutting to generate triangular and circular precision manufactured micropores (PMpores). We fabricated PMpores on 100 μm-thick stainless-steel discs. Triangular PMpores had a 30° vertex angle and a 300 μm base, and circular PMpores had a 300 μm diameter. We found triangular PMpores significantly enhanced the elastic modulus, proliferation, migration, and osteogenic differentiation of MC3T3-E1 preosteoblasts through Yes-associated protein (YAP) nuclear translocation. Inhibition of Rho-associated kinase (ROCK) and Myosin II abolished YAP translocation in all pore types and controls. Inhibition of YAP transcriptional activity reduced the proliferation, pore closure, collagen secretion, alkaline phosphatase (ALP), and Alizarin Red staining in MC3T3-E1 cultures. In C166 vascular endothelial cells, PMpores increased the VEGFA mRNA expression even without an angiogenic differentiation medium and induced tubule formation and maintenance. In terms of osteogenesis-angiogenesis coupling, a conditioned medium from MC3T3-E1 cells in PMpores promoted the expression of angiogenic genes in C166 cells. A coculture with MC3T3-E1 induced tubule formation and maintenance in C166 cells and tubule alignment along the edges of pores. Together, curvature cues in micropores are important stimuli to regulate osteogenic differentiation and osteogenesis-angiogenesis coupling. This study uncovered key mechanotransduction signaling components activated by curvature differences in a metallic scaffold and contributed to the understanding of the interaction between orthopedic implants and cells.

Silk Fibroin/Hydroxyapatite Coating Improved Osseointegration of Porous Titanium Implants under Diabetic Conditions via Activation of the PI3K/Akt Signaling Pathway

The application of three-dimensional printed porous titanium implants (TIs) is compromised in patients suffering from diabetes mellitus (DM), which disturbs the normal process of implant osseointegration, resulting in fixation failure. It was possibly because of reactive oxygen species (ROS) overproduction at the bone-implant interface. A silk fibroin-based hydroxyapatite (SF/HA) hybrid material emerged as a novel biological material for accelerating new bone formation. We proposed that the SF/HA hybrid coated titanium implant (SHT) could mitigate DM-mediated impaired osseointegration, which had never been reported previously. To test this assumption and further elucidate the mechanisms, primary rabbit osteoblasts were seeded on TIs or SHTs and cultured with normal serum, diabetic serum (DS), DS + N-acetyl-L-cysteine (NAC) (a potent ROS inhibitor), and DS + LY294002 (a specific PI3K/Akt inhibitor) for osteoblast behavior examinations. An animal study was performed on diabetic rabbits implanted with the two kinds of implants for osseointegration tests. DM-mediated ROS overproduction caused osteoblastic biological dysfunctions and apoptotic injury, associated with suppression of PI3K/Akt signaling in osteoblasts cultured on a TI substrate. Of note, the SHT substrate significantly suppressed ROS overproduction under diabetic conditions, improved osteoblast functional recovery including ameliorative osteoblast adhesion and morphology, improved cellular proliferation and differentiation, and abrogated apoptosis, which exhibited the same effect as NAC administration on the TI. The in vitro results were further corroborated in vivo by enhanced osteogenesis and osseointegration of SHTs in diabetic rabbits. Moreover, the aforesaid promotive effects afforded by the SF/HA coating were totally abolished with administration of LY294002 for blocking PI3K/Akt signaling. The above results collectively demonstrated that the SF/HA hybrid coating significantly ameliorated DM-mediated impaired osseointegration of the TI via reactivation of the ROS-mediated PI3K/Akt signaling pathway. The hybrid coating elicited a novel surface biofunctionalization strategy to attain favorable clinical performance of TI in diabetics.

Surface coating of orthopedic implant to enhance the osseointegration and reduction of bacterial colonization: a review

The use of orthopedic implants in surgical technology has fostered restoration of physiological functions. Along with successful treatment, orthopedic implants suffer from various complications and fail to offer functions correspondent to native physiology. The major problems include aseptic and septic loosening due to bone nonunion and implant site infection due to bacterial colonization. Crucial advances in material selection in the design and development of coating matrixes an opportunity for the prevention of implant failure. However, many coating materials are limited in in-vitro testing and few of them thrive in clinical tests. The rate of implant failure has surged with the increasing rates of revision surgery creating physical and sensitive discomfort as well as economic burdens. To overcome critical pathogenic activities several systematic coating techniques have been developed offering excellent results that combat infection and enhance bone integration. This review article includes some more common implant coating matrixes with excellent in vitro and in vivo results focusing on infection rates, causes, complications, coating materials, host immune responses and significant research gaps. This study provides a comprehensive overview of potential coating technology, with functional combination coatings which are focused on ultimate clinical practice with substantial improvement on in-vivo tests. This includes the development of rapidly growing hydrogel coating techniques with the potential to generate several accurate and precise coating procedures.

A two-phase and long-lasting multi-antibacterial coating enables titanium biomaterials to prevent implants-related infections

In clinical work, the main challenges for titanium (Ti) implantation are bacterial infection and aseptic loosening, which severely affect the survival rate of implants. The first 4 weeks post-operation is the infection peak phase of implants. Inhibiting implant infection caused by bacteria adhesion and proliferation during the early phase as well as promoting subsequent osteointegration is essential for implant success. Herein, we constructed a quaternary ammonium carboxymethyl chitosan (QCMC), collagen (COL Ⅰ) and hydroxyapatite (HAP) multilayers coating on Ti substrates via a modified layer-by-layer (LBL) technique and polymerization of dopamine. The QCMC/COL/HAP coating exhibited a multi-antibacterial property with a two-phase function: (1) At the first 4 weeks post-operation, the covalently bonded QCMC could be slowly degraded and demonstrated both contact-killing and release-killing properties during the infection peak phase; (2) At the second phase, osteogenesis and osseointegration-promotion capabilities were enhanced by HAP under the effective control of infection. The multifilm coating was degraded for more than 45 days under the action of collagenase Ⅰ, and displayed good biocompatibility in vivo and in vitro. Most importantly, the coating exhibited a long-lasting antibacterial activity for more than 3 months, against the main pathogenic bacteria of peri-implant infections. Both in vitro studies and in vivo animal models revealed a desirable osteogenic differentiation capacity of Ti-CCH. Therefore, our study reports a two-phase, long-lasting multi-antibacterial coating on Ti-CCH and indicates potential applications of the modified LBL strategy in orthopaedic fields, which is enlightening for developing practical implant and scaffold materials.

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Developing a QCMC/COL/HAP multifilm coating via modified layer-by-layer technique and self-polymerization of dopamine.

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The QCMC/COL/HAP coating exhibited desirable mechanical properties and excellent biocompatibility.

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The release kinetics endowed the QCMC/COL/HAP coating with multi-antibacterial activity at the first phase after operation.

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The QCMC/COL/HAP coating could improve osseointegration at the second phase of post-operation.

Advanced Surface Modification for 3D-Printed Titanium Alloy Implant Interface Functionalization

With the development of three-dimensional (3D) printed technology, 3D printed alloy implants, especially titanium alloy, play a critical role in biomedical fields such as orthopedics and dentistry. However, untreated titanium alloy implants always possess a bioinert surface that prevents the interface osseointegration, which is necessary to perform surface modification to enhance its biological functions. In this article, we discuss the principles and processes of chemical, physical, and biological surface modification technologies on 3D printed titanium alloy implants in detail. Furthermore, the challenges on antibacterial, osteogenesis, and mechanical properties of 3D-printed titanium alloy implants by surface modification are summarized. Future research studies, including the combination of multiple modification technologies or the coordination of the structure and composition of the composite coating are also present. This review provides leading-edge functionalization strategies of the 3D printed titanium alloy implants.

3D-printed porous Ti6Al4V scaffolds for long bone repair in animal models: a systematic review

BACKGROUND

Titanium and its alloys have been widely employed for bone tissue repair and implant manufacturing. The rapid development of three-dimensional (3D) printing technology has allowed fabrication of porous titanium scaffolds with controllable microstructures, which is considered to be an effective method for promoting rapid bone formation and decreasing bone absorption. The purpose of this systematic review was to evaluate the osteogenic potential of 3D-printed porous Ti6Al4V (Ti64) scaffold for repairing long bone defects in animal models and to investigate the influential factors that might affect its osteogenic capacity.

METHODS

Electronic literature search was conducted in the following databases: PubMed, Web of Science, and Embase up to September 2021. The SYRCLE's tool and the modified CAMARADES list were used to assess the risk of bias and methodological quality, respectively. Due to heterogeneity of the selected studies in relation to protocol and outcomes evaluated, a meta-analysis could not be performed.

RESULTS

The initial search revealed 5858 studies. Only 46 animal studies were found to be eligible based on the inclusion criteria. Rabbit was the most commonly utilized animal model. A pore size of around 500-600 µm and porosity of 60-70% were found to be the most ideal parameters for designing the Ti64 scaffold, where both dodecahedron and diamond pores optimally promoted osteogenesis. Histological analysis of the scaffold in a rabbit model revealed that the maximum bone area fraction reached 59.3 ± 8.1% at weeks 8-10. Based on micro-CT assessment, the maximum bone volume fraction was found to be 34.0 ± 6.0% at weeks 12.

CONCLUSIONS

Ti64 scaffold might act as a promising medium for providing sufficient mechanical support and a stable environment for new bone formation in long bone defects. Trail registration The study protocol was registered in the PROSPERO database under the number CRD42020194100.

Conductive Scaffolds for Bone Tissue Engineering: Current State and Future Outlook

Bone tissue engineering strategies attempt to regenerate bone tissue lost due to injury or disease. Three-dimensional (3D) scaffolds maintain structural integrity and provide support, while improving tissue regeneration through amplified cellular responses between implanted materials and native tissues. Through this, scaffolds that show great osteoinductive abilities as well as desirable mechanical properties have been studied. Recently, scaffolding for engineered bone-like tissues have evolved with the use of conductive materials for increased scaffold bioactivity. These materials make use of several characteristics that have been shown to be useful in tissue engineering applications and combine them in the hope of improved cellular responses through stimulation (i.e., mechanical or electrical). With the addition of conductive materials, these bioactive synthetic bone substitutes could result in improved regeneration outcomes by reducing current factors limiting the effectiveness of existing scaffolding materials. This review seeks to overview the challenges associated with the current state of bone tissue engineering, the need to produce new grafting substitutes, and the promising future that conductive materials present towards alleviating the issues associated with bone repair and regeneration.

Biomimetic cuttlebone polyvinyl alcohol/carbon nanotubes/hydroxyapatite aerogel scaffolds enhanced bone regeneration

Inspired by the ordered porous nanostructure of bone, biomimetic functionalization porous biomaterial could be considered as promising substitutes for bone regeneration. To realize the relevant biomimetic porous structure, polyvinyl alcohol (PVA)-based biomimetic cuttlebone aerogel scaffold which simultaneously contained modified carbon nanotubes (MCNTs) and hydroxyapatite (HAP) was first prepared using a one-step rapid freeze-drying method. By adjusting the MCNTs contents, both the surface hydrophilicity and the mechanical properties of the scaffold could be improved concurrently. Besides, the PVA/MCNTs/HAP enhanced the adhesion, differentiation and gene expression of osteogenic markers performances of MC3T3-E1 cells. Furthermore, the aerogel scaffolds were implanted into the calvarial defect model of SD IGS Rat to evaluate osteogenic performance in vivo. The Micro-CT characterization and bone content theoretical analysis after 8 weeks together indicated that the PVA/MCNTs/HAP aerogel scaffolds could accelerate bone regeneration without the contribution of endogenous cytokines. The unique biomimetic porous structure, superior mechanical properties and excellent bone regeneration capacity of PVA/MCNTs/HAP aerogel scaffolds made them potential materials for bone regeneration.

Properties improvement of titanium alloys scaffolds in bone tissue engineering: a literature review

Owing to their excellent biocompatibility and corrosion-resistant properties, titanium (Ti) (and its alloy) are essential artificial substitute biomaterials for orthopedics. However, flaws, such as weak osteogenic induction ability and higher Young's modulus, have been observed during clinical application. As a result, short- and long-term postoperative follow-up has found that several complications have occurred. For decades, scientists have exerted efforts to compensate for these deficiencies. Different modification methods have been investigated, including changing alloy contents, surface structure transformation, three-dimensional (3D) structure transformation, coating, and surface functionalization technologies. The cell-surface interaction effect and imitation of the natural 3D bone structure are the two main mechanisms of these improved methods. In recent years, significant progress has been made in materials science research methods, including thorough research of titanium alloys of different compositions, precise surface pattern control technology, controllable 3D structure construction technology, improvement of coating technologies, and novel concepts of surface functionalization. These improvements facilitate the possibility for further research in the field of bone tissue engineering. Although the underlying mechanism is still not fully understood, these studies still have some implications for clinical practice. Therefore, for the direction of further research, it is beneficial to summarize these studies according to the basal method used. This literature review aimed to classify these technologies, thereby providing beginners with a preliminary understanding of the field.

Mussel patterned with 4D biodegrading elastomer durably recruits regenerative macrophages to promote regeneration of craniofacial bone

Crosstalk between bone marrow mesenchymal stem cells (BMSCs) and macrophages plays vital role in bone healing. By investigating the mechanism of collagen membrane-guided bone regeneration, we found compact structure and rapid membrane degradation compromised the duration of M2 macrophages influx, which restricts the recruitment of BMSCs that is essential for bone healing. To tackle this issue, a biodegrading elastomeric compound consisting of poly(glycerol sebacate) (PGS) and polycaprolactone (PCL) was fabricated into hierarchically porous membrane. The rational design of 3D microstructure enabled sufficient polydopamine (PDA) coating. Without any addition of growth factors, the 3D-patterned PDA membrane enables early and durable influx of M2 macrophages, which in turn promotes BMSCs recruitment and osteogenic differentiation. Furthermore, 4D-morphing of the membrane fully regenerates the dome shaped calvarial bone as well as arc-shape bone in peri-implant alveolar defect without filling xenogenous substitute. This study revealed the superiority of 3D printed microstructures in immunomodulatory materials. The availability of 4D-morphing for PGS/PCL construct expanded their advantages in reconstructing craniofacial bone.

Practical strategy to construct anti-osteosarcoma bone substitutes by loading cisplatin into 3D-printed titanium alloy implants using a thermosensitive hydrogel

Surgical resection and perioperative adjuvant chemotherapy-based therapies have improved the prognosis of patients with osteosarcoma; however, intraoperative bone defects, local tumour recurrence, and chemotherapy-induced adverse effects still affect the quality of life of patients. Emerging 3D-printed titanium alloy (Ti₆Al₄V) implants have advantages over traditional implants in bone repair, including lower elastic modulus, lower stiffness, better bone conduction, more bone in-growth, stronger mechanical interlocking, and lager drug-loading capacity by their inherent porous structure. Here, cisplatin, a clinical first-line anti-osteosarcoma drug, was loaded into Ti₆Al₄V implants, within a PLGA-PEG-PLGA thermo-sensitive hydrogel, to construct bone substitutes with both anti-osteosarcoma and bone-repair functions. The optimal concentrations of cisplatin (0.8 and 1.6 mg/mL) were first determined in vitro. Thereafter, the anti-tumour effect and biosafety of the cisplatin/hydrogel-loaded implants, as well as their bone-repair potential were evaluated in vivo in tumour-bearing mouse, and bone defect rabbit models, respectively. The loading of cisplatin reduced tumour volume by more than two-thirds (from 641.1 to 201.4 mm³) with negligible organ damage, achieving better anti-tumour effects while avoiding the adverse effects of systemic cisplatin delivery. Although bone repair was hindered by cisplatin loading at 4 weeks, no difference was observed at 8 weeks in the context of implants with versus without cisplatin, indicating acceptable long-term stability of all implants (with 8.48%–10.04% bone in-growth and 16.94%–20.53% osseointegration). Overall, cisplatin/hydrogel-loaded 3D-printed Ti₆Al₄V implants are safe and effective for treating osteosarcoma-caused bone defects, and should be considered for clinical use.

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Vehiculated within PLGA-PEG-PLGA hydrogel, cisplatin can be conveniently loaded into 3D-printed Ti₆Al₄V implants.

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The cisplatin/hydrogel-loaded implants are safe and show a good anti-tumour potential both in vitro and in vivo.

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This strategy has better anti-osteosarcoma effects and fewer side effects than the conventional cisplatin delivery method.

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Cisplatin loading does not decrease the bone repair effect of 3D-printed Ti₆Al₄V implants 8 weeks after surgery.

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As the components of the implants are non-toxic, this strategy has great potential for clinical translation.

Hydroxyapatite-modified micro/nanostructured titania surfaces with different crystalline phases for osteoblast regulation

Surface structures and physicochemical properties critically influence osseointegration of titanium (Ti) implants. Previous studies have shown that the surface with both micro- and nanoscale roughness may provide multiple features comparable to cell dimensions and thus efficiently regulate cell-material interaction. However, less attention has been made to further optimize the physicochemical properties (e.g., crystalline phase) and to further improve the bioactivity of micro/nanostructured surfaces. Herein, micro/nanostructured titania surfaces with different crystalline phases (amorphous, anatase and anatase/rutile) were prepared and hydroxyapatite (HA) nanorods were deposited onto the as-prepared surfaces by a spin-assisted layer-by-layer assembly method without greatly altering the initial multi-scale morphology and wettability. The effects of crystalline phase, chemical composition and wettability on osteoblast response were investigated. It is noted that all the micro/nanostructured surfaces with/without HA modification presented superamphiphilic. The activities of MC3T3-E1 cells suggested that the proliferation trend on the micro/nanostructured surfaces was greatly influenced by different crystalline phases, and the highest proliferation rate was obtained on the anatase/rutile surface, followed by the anatase; but the cell differentiation and extracellular matrix mineralization were almost the same among them. After ultrathin HA modification on the micro/nanostructured surfaces with different crystalline phases, it exhibited similar proliferation trend as the original surfaces; however, the cell differentiation and extracellular matrix mineralization were significantly improved. The results indicate that the introduction of ultrathin HA to the micro/nanostructured surfaces with optimized crystalline phase benefits cell proliferation, differentiation and maturation, which suggests a favorable biomimetic microenvironment and provides the potential for enhanced implant osseointegration in vivo.

Strategies for Bone Regeneration: From Graft to Tissue Engineering

Bone is a regenerative organ characterized by self-renewal ability. Indeed, it is a very dynamic tissue subjected to continuous remodeling in order to preserve its structure and function. However, in clinical practice, impaired bone healing can be observed in patients and medical intervention is needed to regenerate the tissue via the use of natural bone grafts or synthetic bone grafts. The main elements required for tissue engineering include cells, growth factors and a scaffold material to support them. Three different materials (metals, ceramics, and polymers) can be used to create a scaffold suitable for bone regeneration. Several cell types have been investigated in combination with biomaterials. In this review, we describe the options available for bone regeneration, focusing on tissue engineering strategies based on the use of different biomaterials combined with cells and growth factors.

A systematic review of preclinical in vivo testing of 3D printed porous Ti6Al4V for orthopedic applications, part I: Animal models and bone ingrowth outcome measures

For Ti6Al4V orthopedic and spinal implants, osseointegration is often achieved using complex porous geometries created via additive manufacturing (AM). While AM porous titanium (pTi) has shown clinical success, concerns regarding metallic implants have spurred interest in alternative AM biomaterials for osseointegration. Insights regarding the evaluation of these new materials may be supported by better understanding the role of preclinical testing for AM pTi. We therefore asked: (a) What animal models have been most commonly used to evaluate AM porous Ti6Al4V for orthopedic bone ingrowth; (b) What were the primary reported quantitative outcome measures for these models; and (c) What were the bone ingrowth outcomes associated with the most frequently used models? We performed a systematic literature search and identified 58 articles meeting our inclusion criteria. We found that AM pTi was evaluated most often using rabbit and sheep femoral condyle defect (FCD) models. Additional ingrowth models including transcortical and segmental defects, spinal fusions, and calvarial defects were also used with various animals based on the study goals. Quantitative outcome measures determined via histomorphometry including ''bone ingrowth'' (range: 3.92-53.4% for rabbit/sheep FCD) and bone-implant contact (range: 9.9-59.7% for rabbit/sheep FCD) were the most common. Studies also used 3D imaging to report outcomes such as bone volume fraction (BV/TV, range: 4.4-61.1% for rabbit/sheep FCD), and push-out testing for outcomes such as maximum removal force (range: 46.6-3092 N for rabbit/sheep FCD). Though there were many commonalities among the study methods, we also found significant heterogeneity in the outcome terms and definitions. The considerable diversity in testing and reporting may no longer be necessary considering the reported success of AM pTi across all model types and the ample literature supporting the rabbit and sheep as suitable small and large animal models, respectively. Ultimately, more standardized animal models and reporting of bone ingrowth for porous AM materials will be useful for future studies.

Comparative Study on 3D Printed Ti6Al4V Scaffolds with Surface Modifications Using Hydrothermal Treatment and Microarc Oxidation to Enhance Osteogenic Activity

Titanium (Ti) and its alloys have been widely used in clinics as preferred materials for bone tissue repair and replacement. However, the lack of biological activity of Ti limits its clinical applications. Surface modification of Ti with bioactive elements has always been a research hotspot. In this study, to promote the osseointegration of Ti6Al4V (Ti64) implants, calcium (Ca), oxygen (O), and phosphorus (P) codoped multifunctional micro–nanohybrid coatings were prepared on a three-dimensional (3D) printed porous Ti64 surface by microarc oxidation (MAO) and a hydrothermal method (HT). The surface morphologies, chemical compositions, and surface/cell interactions of the obtained coatings were studied. In vitro experiments indicated that all hybrid coating-modified Ti64 implants could enhance protein adsorption and MC3T3 osteoblasts’ activity, adhesion, and differentiation ability. In vivo experiments showed that the hybrid coating promoted early osseointegration. By comparison, microarc oxidation-treated Ti64 (M-Ti) has the best biological activity and the strongest ability of osseointegration. It provides important theoretical significance and potential application prospects for improving the biological activity of Ti implants.

Recent Advances in a Polydopamine-Mediated Antimicrobial Adhesion System

The drug resistance developed by bacteria during antibiotic treatment has been a call to action for researchers and scientists across the globe, as bacteria and fungi develop ever increasing resistance to current drugs. Innovative antimicrobial/antibacterial materials and coatings to combat such infections have become a priority, as many infections are caused by indwelling implants (e.g., catheters) as well as improving postsurgical function and outcomes. Pathogenic microorganisms that can exist either in planktonic form or as biofilms in water-carrying pipelines are one of the sources responsible for causing water-borne infections. To combat this, researchers have developed nanotextured surfaces with bactericidal properties mirroring the topographical features of some natural antibacterial materials. Protein-based adhesives, secreted by marine mussels, contain a catecholic amino acid, 3,4-dihydroxyphenylalanine (DOPA), which, in the presence of lysine amino acid, empowers with the ability to anchor them to various surfaces in both wet and saline habitats. Inspired by these features, a novel coating material derived from a catechol derivative, dopamine, known as polydopamine (PDA), has been designed and developed with the ability to adhere to almost all kinds of substrates. Looking at the immense potential of PDA, this review article offers an overview of the recent growth in the field of PDA and its derivatives, especially focusing the promising applications as antibacterial nanocoatings and discussing various antimicrobial mechanisms including reactive oxygen species-mediated antimicrobial properties.

Electroactive barium titanate coated titanium scaffold improves osteogenesis and osseointegration with low-intensity pulsed ultrasound for large segmental bone defects

For large segmental bone defects, porous titanium scaffolds have some advantages, however, they lack electrical activity which hinders their further use. In this study, a barium titanate (BaTiO3) piezoelectric ceramic was used to modify the surface of a porous Ti6Al4V scaffold (pTi), which was characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and roughness and water contact angle analyses. Low intensity pulsed ultrasound (LIPUS) was applied in vitro and in vivo study. The activity of bone marrow mesenchymal stem cells, including adhesion, proliferation, and gene expression, was significantly superior in the BaTiO3/pTi, pTi + LIPUS, and BaTiO3/pTi + LIPUS groups than in the pTi group. The activity was also higher in the BaTiO3/pTi + LIPUS group than in the BaTiO3/pTi and pTi + LIPUS groups. Additionally, micro-computed tomography, the mineral apposition rate, histomorphology, and the peak pull-out load showed that these scaffold conditions significantly enhanced osteogenesis and osseointegration 6 and 12 weeks after implantation in large segmental bone defects in the radius of rabbits compared with those resulting from the pTi condition. Consequently, the improved osteogenesis and osseointegration make the BaTiO3/pTi + LIPUS a promising method to promote bone regeneration in large segmental bone defects for clinical application.

Biological Effects of a Three-Dimensionally Printed Ti6Al4V Scaffold Coated with Piezoelectric BaTiO3 Nanoparticles on Bone Formation

Bone defect repair at load-bearing sites is a challenging clinical problem for orthopedists. Defect reconstruction with implants is the most common treatment; however, it requires the implant to have good mechanical properties and the capacity to promote bone formation. In recent years, the piezoelectric effect, in which electrical activity can be generated due to mechanical deformation, of native bone, which promotes bone formation, has been increasingly valued. Therefore, implants with piezoelectric effects have also attracted great attention from orthopedists. In this study, we developed a bioactive composite scaffold consisting of BaTiO₃, a piezoelectric ceramic material, coated on porous Ti6Al4V. This composite scaffold showed not only appropriate mechanical properties, sufficient bone and blood vessel ingrowth space, and a suitable material surface topography but also a reconstructed electromagnetic microenvironment. The osteoconductive and osteoinductive properties of the scaffold were reflected by the proliferation, migration, and osteogenic differentiation of mesenchymal stem cells. The ability of the scaffold to support vascularization was reflected by the proliferation and migration of human umbilical vein endothelial cells and their secretion of VEGF and PDGF-BB. A well-established sheep spinal fusion model was used to evaluate bony fusion in vivo. Sheep underwent implantation with different scaffolds, and X-ray, micro-computed tomography, van Gieson staining, and elemental energy-dispersive spectroscopy were used to analyze bone formation. Isolated cervical angiography and visualization analysis were used to assess angiogenesis at 4 and 8 months after transplantation. The results of cellular and animal studies showed that the piezoelectric effect could significantly reinforce osteogenesis and angiogenesis. Furthermore, we also discuss the molecular mechanism by which the piezoelectric effect promotes osteogenic differentiation and vascularization. In summary, Ti6Al4V scaffold coated with BaTiO₃ is a promising composite biomaterial for repairing bone defects, especially at load-bearing sites, that may have great clinical translation potential.

Schwann Cell-derived exosomes promote bone regeneration and repair by enhancing the biological activity of porous Ti6Al4V scaffolds

Implants made of porous titanium alloy and fabricated by 3D printing are increasingly used in clinical research. However, porous titanium alloys do not integrate very well with surrounding bone tissue, and bone ingrowth into the implants is not substantial. Schwann cells (SCs) and SC-derived exosomes can effectively promote nerve regeneration, but their role in bone tissue regeneration and repair has not been studied. Therefore, we added SC-derived exosomes to bone marrow stromal cell (BMSC) cultures and observed their effect on BMSCs in vitro; then, we combined exosomes with porous Ti6Al4V scaffolds and observed their effects on bone regeneration and repair in vivo. We found that SC-derived exosomes could promote the migration, proliferation and differentiation of BMSCs and that combining exosomes with porous titanium alloy can effectively improve the efficacy of titanium alloy scaffolds in bone repair. The combination of exosomes and porous Ti6Al4V implants may constitute a new therapeutic strategy for treating bone defects.

In Situ Generation of Hydroxyapatite on Biopolymer Particles for Fabrication of Bone Scaffolds Owning Bioactivity

Hydroxyapatite (HAP) can endow a biopolymer scaffold with good bioactivity and osteoconductive ability, while the interfacial bonding is fairly weak between HAP and biopolymers. In this study, HAP was in situ generated on poly(l-lactic acid) (PLLA) particles, and then they were used to fabricate a scaffold by selective laser sintering. Detailedly, PLLA particles were first functionalized by dopamine oxide polymerization, which introduced abundance active catechol groups on the particle surface, and subsequently, the catechol groups concentrated Ca²⁺ ions by chelation in a simulated body fluid solution, and then, Ca²⁺ ions absorbed PO₄^3- ions through electrostatic interactions for in situ nucleation of HAP. The results indicated that HAP was homogeneously generated on the PLLA particle surface, and HAP and PLLA exhibited good interfacial bonding in the HAP/PLLA scaffolds. Meanwhile, the scaffolds displayed excellent bioactivity by inducing apatite precipitation and provided a good environment for human bone mesenchymal stem cell attachment, proliferation, and osteogenic differentiation. More importantly, the ingrowth of blood vessel and the formation of new bone could be stimulated by the scaffolds in vivo, and the bone volume fraction and bone mineral density increased by 44.44 and 41.73% compared with the pure PLLA scaffolds, respectively. Serum biochemical indexes fell within the normal range, which indicated that there was no harmful effect on the normal functioning of the body after implanting the scaffold.

Novel design of additive manufactured hollow porous implants

OBJECTIVE

Our aim is to examine the mechanical properties of two types of additive manufactured hollow porous dental implants and 6 and 12-week bone ingrowth after insertion in animals. A 3D numerical model is also developed to show detailed tissue differentiation and to provide design guidelines for implants.

METHODS

The two porous and a commercial dental implant were studied by series of in vitro mechanical tests (three-point bending, torsional, screwing torque, and sawbone pull-out tests). They also evaluated by in vivo animal tests (micro-CT analysis) and ex vivo pull-out tests. Moreover, the mechano-regulation algorithm was implemented by the 3D finite element model to predict the history of tissue differentiation around the implants.

RESULTS

The results showed that the two porous implants can significantly improve osseointegration after 12-week bone healing. This resulted in good fixation and stability of implants, giving very high maximum pull-out strength 413.1 N and 493.2 N, compared to 245.7 N for the commercial implant. Also, several features were accurately predicted by the mechano-regulation model, such as transversely connected bone formation, and bone resorption occurred in the middle of implants.

SIGNIFICANCE

Systematic studies on dental implants with multiple approaches, including new design, mechanical tests, animal tests, and numerical modeling, were performed. Two hollow porous implants significantly improved bone ingrowth compared with commercial implants, while maintaining mechanical strength. Also, the numerical model was verified by animal tests. It improved the efficiency of design and reduce the demand for animal sacrifice.

Strategies for Using Polydopamine to Induce Biomineralization of Hydroxyapatite on Implant Materials for Bone Tissue Engineering

In the field of tissue engineering, there are several issues to consider when designing biomaterials for implants, including cellular interaction, good biocompatibility, and biochemical activity. Biomimetic mineralization has gained considerable attention as an emerging approach for the synthesis of biocompatible materials with complex shapes, categorized organization, controlled shape, and size in aqueous environments. Understanding biomineralization strategies could enhance opportunities for novel biomimetic mineralization approaches. In this regard, mussel-inspired biomaterials have recently attracted many researchers due to appealing features, such as strong adhesive properties on moist surfaces, improved cell adhesion, and immobilization of bioactive molecules via catechol chemistry. This molecular designed approach has been a key point in combining new functionalities into accessible biomaterials for biomedical applications. Polydopamine (PDA) has emerged as a promising material for biomaterial functionalization, considering its simple molecular structure, independence of target materials, cell interactions for adhesion, and robust reactivity for resulting functionalization. In this review, we highlight the strategies for using PDA to induce the biomineralization of hydroxyapatite (HA) on the surface of various implant materials with good mechanical strength and corrosion resistance. We also discuss the interactions between the PDA-HA coating, and several cell types that are intricate in many biomedical applications, involving bone defect repair, bone regeneration, cell attachment, and antibacterial activity.

Functionalization of 3D-printed titanium alloy orthopedic implants: a literature review

Titanium alloy orthopedic implants produced by 3D printing combine the dual advantages of having a complex structure that cannot be manufactured by traditional techniques and the excellent physical and chemical properties of titanium and its alloys; they have been widely used in the field of orthopedics in recent years. The inherent porous structure of 3D-printed implants and the original modification processes for titanium alloys provide conditions for the functionalization of implants. To meet the needs of orthopedic surgeons and patients, functionalized implants with long-term stability and anti-infection or anti-tumor properties have been developed. The various methods of functionalization deserve to be summarized, compared and analyzed. Therefore, in this review, we will collect and discuss existing knowledge on the functionalization of 3D-printed titanium alloy orthopedic implants.

Promotion of osteointegration by silk fibroin coating under diabetic conditions on 3D printed porous titanium implants via ROS-mediated NF-κB pathway

The clinical evidence indicates the compromised application of titanium implants (TI) in diabetics, associated with reactive oxygen species (ROS) overproduction at the bone-implant interface. Silk fibroin has exerted impressive biocompatibility in application of biomedical material and optimal anti-diabetic effect as oriental medicine. We proposed that SF coated titanium implant (STI) could alleviate diabetes-induced compromised osteointegration, which had been rarely reported before. To confirm the hypothesis and explore the underlying mechanisms, rat osteoblasts cultured on 3-dimensional (3D) printed TI and STI were subjected to normal serum (NS), diabetic serum (DS), DS with NAC (a ROS inhibitor) or SN50 (a NF-κB inhibitor). In vivo study was performed on diabetic sheep implanted with TI or STI into the bone defect on crista iliaca. Results demonstrated that ROS overproduction induced by diabetes lead to osteoblast dysfunctions and cellular apoptosis on TI substrate, associated with activation of NF-κB signaling pathway in osteoblasts. Importantly, STI substrate significantly attenuated ROS production and NF-κBp65 phosphorylation, through which the osteoblast biological dysfunctions were ameliorated. These results were further confirmed in vivo by the improved osteointegration of STI evidenced by Micro-CT and histological examinations compared with TI. These results demonstrated that ROS-mediated NF-κB signaling pathway played a crucial role in diabetes-induced implant destabilization. Importantly, SF coating as a promising material for biomaterial-engineering markedly improved clinical treatment effect of TI under diabetic conditions, possibly associated with the suppression of NF-κB pathway.

Fabrication of piezoelectric porous BaTiO3 scaffold to repair large segmental bone defect in sheep

Porous titanium scaffolds can provide sufficient mechanical support and bone growth space for large segmental bone defect repair. However, they fail to restore the physiological environment of bone tissue. Barium titanate (BaTiO3) is considered a smart material that can produce an electric field in response to dynamic force. Low-intensity pulsed ultrasound stimulation (LIPUS), as a kind of micromechanical wave, can not only promote bone repair but also induce BaTiO3 to generate an electric field. In our studies, BaTiO3 was coated on porous Ti6Al4V and LIPUS was externally applied to observe the influence of the piezoelectric effect on the repair of large bone defects in vitro and in vivo. The results show that the piezoelectric effect can effectively promote the osteogenic differentiation of bone marrow stromal cells (BMSCs) in vitro as well as bone formation and growth into implants in vivo. This study provides an optional alternative to the conventional porous Ti6Al4V scaffold with enhanced osteogenesis and osseointegration for the repair of large bone defects.

Multi-layered polydopamine coatings for the immobilization of growth factors onto highly-interconnected and bimodal PCL/HA-based scaffolds

For bone tissue engineering applications, scaffolds that mimic the porous structure of the extracellular matrix are highly desirable. Herein, we employ a PCL/HA-based scaffold with a double-scaled architecture of small pores coupled to larger ones. To improve the osteoinductivity of the scaffold, we incorporate two different growth factors via polydopamine (PDA) coating. As a first step, we identify the maximum amount of PDA that can be deposited onto the scaffold. Next, to allow for the deposition of a second PDA layer which, in turn, will allow to increase the loading of growth factors, we incorporate a dithiol connecting layer. The thiol groups covalently react with the first PDA coating through Michael addition while also allowing for the incorporation of a second PDA layer. We load the first and second PDA layers with bone morphogenic protein-2 (BMP2) and vascular endothelial growth factor (VEGF), respectively, and evaluate the osteogenic potential of the functionalised scaffold by cell viability, alkaline phosphatase activity and the expression of three different osteogenesis-related genes of pre-seeded human mesenchymal stem cells. Through these studies, we demonstrate that the osteogenic activity of the scaffolds loaded with both BMP2 and VEGF is greater than scaffolds loaded only with BMP2. Importantly, the osteoinductivity is higher when the scaffolds are loaded with BMP2 and VEGF in two different PDA layers. Taken together, these results indicate that the as-prepared scaffolds could be a useful construct for bone-tissue applications.

Duration of electrochemical deposition affects the morphology of hydroxyapatite coatings on 3D-printed titanium scaffold as well as the functions of adhered MC3T3-E1 cells

BACKGROUND

The use of 3D-printed scaffolds in repairing bone defects remains unexplored. We aimed to determine whether the duration of electrochemical deposition (ECD) affects the properties of hydroxyapatite (HA) coatings on 3D-printed titanium (TI) scaffolds as well as the corresponding phenotype of MC3T3-E1 cells seeded on these surfaces.

METHODS

Five groups of HA-coated TI scaffolds were produced using different durations of ECD (0, 5, 10, 20, and 30 min) and examined under scanning electron microscopy (SEM). MC3T3-E1 cell adhesion to the HA-coated scaffolds and subsequent proliferation and viability were assessed using SEM, DAPI staining, EdU staining, and Alamar Blue assay, respectively. MC3T3-E1 cell expression of osteogenic genes was analyzed by fluorescence RT-PCR.

RESULTS

On SEM, longer ECD durations resulted in more compact HA crystals of differing morphology coated onto the TI scaffolds. MC3T3-E1 cell adhesion differed among the five groups (p < 0.05), with the largest number of cells adhered to the scaffolds prepared with 30 min of ECD, followed by the group prepared with 20 min of ECD. However, the ECD duration of 20 min was associated with the highest cell viability and proliferation rate (both p < 0.05) as well as the highest mRNA expression levels of alkaline phosphatase, collagen I, osteocalcin and runt-related transcription factor 2 among the five groups (p < 0.05).

CONCLUSIONS

In the fabrication of HA-coated 3D printed TI scaffolds, an ECD duration of 20 min resulted in scaffolds that best promoted MC3T3-E1 cell viability, proliferation and osteogenic gene expression.