Bone Regeneration in Critical-Sized Bone Defects Treated with Additively Manufactured Porous Metallic Biomaterials: The Effects of Inelastic Mechanical Properties

A chitosan-coated PCL/nano-hydroxyapatite aerogel integrated with a nanofiber membrane for providing antibacterial activity and guiding bone regeneration

A guided bone regeneration (GBR) membrane can act as a barrier to prevent the invasion and interference from foreign soft tissues, promoting infiltration and proliferation of osteoblasts in the bone defect area. Herein, a composite scaffold with dual functions of osteogenesis and antibacterial effects was prepared for GBR. A polycaprolactone (PCL)/nano-hydroxyapatite (n-HA) aerogel produced by electrospinning and freeze-drying techniques was fabricated as the loose layer of the scaffold, while a PCL nanofiber membrane was used as the dense layer. Chitosan (CS) solution served as a middle layer to provide mechanical support and antibacterial effects between the two layers. Morphological results showed that the loose layer had a porous structure with n-HA successfully dispersed in the aerogels, while the dense layer possessed a sufficiently dense structure. In vitro antibacterial experiments illustrated that the CS solution in the middle layer stabilized the scaffold structure and endowed the scaffold with good antibacterial properties. The cytocompatibility results indicated that both fibroblasts and osteoblasts exhibited superior cell activity on the dense and loose layers, respectively. In particular, the dense layer made of nanofibers could work as a barrier layer to inhibit the infiltration of fibroblasts into the loose layer. In vitro osteogenesis analysis suggested that the PCL/n-HA aerogel could enhance the bone induction ability of bone mesenchymal stem cells, which was confirmed by the increased expression of the alkaline phosphatase activity. The loose structure facilitated the infiltration and migration of bone mesenchymal stem cells for better osteogenesis. In summary, such a composite scaffold exhibited excellent osteogenic and antibacterial properties as well as the barrier effect, thus holding promising potential for use as GBR materials.

Flavonoid-Loaded Biomaterials in Bone Defect Repair

Skeletons play an important role in the human body, and can form gaps of varying sizes once damaged. Bone defect healing involves a series of complex physiological processes and requires ideal bone defect implants to accelerate bone defect healing. Traditional grafts are often accompanied by issues such as insufficient donors and disease transmission, while some bone defect implants are made of natural and synthetic polymers, which have characteristics such as good porosity, mechanical properties, high drug loading efficiency, biocompatibility and biodegradability. However, their antibacterial, antioxidant, anti-inflammatory and bone repair promoting abilities are limited. Flavonoids are natural compounds with various biological activities, such as antitumor, anti-inflammatory and analgesic. Their good anti-inflammatory, antibacterial and antioxidant activities make them beneficial for the treatment of bone defects. Several researchers have designed different types of flavonoid-loaded polymer implants for bone defects. These implants have good biocompatibility, and they can effectively promote the expression of angiogenesis factors such as VEGF and CD31, promote angiogenesis, regulate signaling pathways such as Wnt, p38, AKT, Erk and increase the levels of osteogenesis-related factors such as Runx-2, OCN, OPN significantly to accelerate the process of bone defect healing. This article reviews the effectiveness and mechanism of biomaterials loaded with flavonoids in the treatment of bone defects. Flavonoid-loaded biomaterials can effectively promote bone defect repair, but we still need to improve the overall performance of flavonoid-loaded bone repair biomaterials to improve the bioavailability of flavonoids and provide more possibilities for bone defect repair.

Open-porous magnesium-based scaffolds withstand in vitro corrosion under cyclic loading: A mechanistic study

The successful application of magnesium (Mg) alloys as biodegradable bone substitutes for critical-sized defects may be comprised by their high degradation rate resulting in a loss of mechanical integrity. This study investigates the degradation pattern of an open-porous fluoride-coated Mg-based scaffold immersed in circulating Hanks' Balanced Salt Solution (HBSS) with and without in situ cyclic compression (30 N/1 Hz). The changes in morphological and mechanical properties have been studied by combining in situ high-resolution X-ray computed tomography mechanics and digital volume correlation. Although in situ cyclic compression induced acceleration of the corrosion rate, probably due to local disruption of the coating layer where fatigue microcracks were formed, no critical failures in the overall scaffold were observed, indicating that the mechanical integrity of the Mg scaffolds was preserved. Structural changes, due to the accumulation of corrosion debris between the scaffold fibres, resulted in a significant increase (p < 0.05) in the material volume fraction from 0.52 ± 0.07 to 0.47 ± 0.03 after 14 days of corrosion. However, despite an increase in fibre material loss, the accumulated corrosion products appear to have led to an increase in Young's modulus after 14 days as well as lower third principal strain (εp3) accumulation (−91000 ± 6361 με and −60093 ± 2414 με after 2 and 14 days, respectively). Therefore, this innovative Mg scaffold design and composition provide a bone replacement, capable of sustaining mechanical loads in situ during the postoperative phase allowing new bone formation to be initially supported as the scaffold resorbs.

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First report on in vitro cyclic loading of MgF₂ coated open-porous Mg scaffolds in HBSS simulating 2–3 months in humans.

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Fluoride-coating slows down corrosion under cyclic loading in vitro.

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Entangled scaffold structure accumulates local corrosion debris which keeps the mechanical integrity over 14 days in vitro.

Metals Biotribology and Oral Microbiota Biocorrosion Mechanisms

During the last decades, metal-based biomaterials have been extensively explored to be used as biocompatible metals for biomedical applications, owing to their superior mechanical properties and corrosion resistance. Consequently, for long-term implanted medical devices, to assure the biomaterials' reliability, functionality, and biocompatibility, studying the various bio-tribological damage mechanisms to obtain the optimum properties is one of the most important goals. In this review, we consider the most important metal-based biomaterials such as stainless steel, alloys of titanium (Ti), cobalt-chromium (Co-Cr), and Nichel-Titatium (Ni-Ti), as well Magnesium (Mg) alloys and with Tantalum (Ta), emphasizing their characteristics, clinical applications, and deterioration over time. The influence of metal elements on biological safety, including significant effects of metal-based biomaterials in dentistry were discussed, considering the perspectives of surface, mechanical properties, corrosion behaviors, including interactions, bio-mechanisms with tissues, and oral environments. In addition, the role of the oral microbiota was explored due to its role in this erosion condition, in order to further understand the mechanism of metal-based biomaterials implanted on the microflora balance of aerobic and anaerobic bacteria in an oral environment.

The Promotion of Mechanical Properties by Bone Ingrowth in Additive-Manufactured Titanium Scaffolds

Although the initial mechanical properties of additive-manufactured (AM) metal scaffolds have been thoroughly studied and have become a cornerstone in the design of porous orthopaedic implants, the potential promotion of the mechanical properties of the scaffolds by bone ingrowth has barely been studied. In this study, the promotion of bone ingrowth on the mechanical properties of AM titanium alloy scaffolds was investigated through in vivo experiments and numerical simulation. On one hand, the osseointegration characteristics of scaffolds with architectures of body-centred cubic (BCC) and diamond were compared through animal experiments in which the mechanical properties of both scaffolds were not enhanced by the four-week implantation. On the other hand, the influences of the type and morphology of bone tissue in the BCC scaffolds on its mechanical properties were investigated by the finite element model of osseointegrated scaffolds, which was calibrated by the results of biomechanical testing. Significant promotion of the mechanical properties of AM metal scaffolds was only found when cortical bone filled the pores in the scaffolds. This paper provides a numerical prediction method to investigate the effect of bone ingrowth on the mechanical properties of AM porous implants, which might be valuable for the design of porous implants.

Porous construction and surface modification of titanium-based materials for osteogenesis: A review

Titanium and titanium alloy implants are essential for bone tissue regeneration engineering. The current trend is toward the manufacture of implants from materials that mimic the structure, composition and elasticity of bones. Titanium and titanium alloy implants, the most common materials for implants, can be used as a bone conduction material but cannot promote osteogenesis. In clinical practice, there is a high demand for implant surfaces that stimulate bone formation and accelerate bone binding, thus shortening the implantation-to-loading time and enhancing implantation success. To avoid stress shielding, the elastic modulus of porous titanium and titanium alloy implants must match that of bone. Micro-arc oxidation technology has been utilized to increase the surface activity and build a somewhat hard coating on porous titanium and titanium alloy implants. More recently, a growing number of researchers have combined micro-arc oxidation with hydrothermal, ultrasonic, and laser treatments, coatings that inhibit bacterial growth, and acid etching with sand blasting methods to improve bonding to bone. This paper summarizes the reaction at the interface between bone and implant material, the porous design principle of scaffold material, MAO technology and the combination of MAO with other technologies in the field of porous titanium and titanium alloys to encourage their application in the development of medical implants.

Additive Manufacturing of Biomaterials-Design Principles and Their Implementation

Additive manufacturing (AM, also known as 3D printing) is an advanced manufacturing technique that has enabled progress in the design and fabrication of customised or patient-specific (meta-)biomaterials and biomedical devices (e.g., implants, prosthetics, and orthotics) with complex internal microstructures and tuneable properties. In the past few decades, several design guidelines have been proposed for creating porous lattice structures, particularly for biomedical applications. Meanwhile, the capabilities of AM to fabricate a wide range of biomaterials, including metals and their alloys, polymers, and ceramics, have been exploited, offering unprecedented benefits to medical professionals and patients alike. In this review article, we provide an overview of the design principles that have been developed and used for the AM of biomaterials as well as those dealing with three major categories of biomaterials, i.e., metals (and their alloys), polymers, and ceramics. The design strategies can be categorised as: library-based design, topology optimisation, bio-inspired design, and meta-biomaterials. Recent developments related to the biomedical applications and fabrication methods of AM aimed at enhancing the quality of final 3D-printed biomaterials and improving their physical, mechanical, and biological characteristics are also highlighted. Finally, examples of 3D-printed biomaterials with tuned properties and functionalities are presented.

A simple, safe and easily accessible polyvinyl alcohol hydrogel for wound cleaning

Acute wounds are often contaminated by some kind of filth, and fluids are usually used to wash away the dirt, but the force of the fluid may cause secondary injury at the wound site or even increase the risk of infection. Hydrogels have several advantages over liquid scouring since they are less intense, more portable, and easier to control. In this study, poly(vinyl alcohol) was used to prepared a series of hydrogels, which were tested in terms of their properties and abilities to clean simulated dirty wounds. Simulated dirt and bacterial (Serratia marcescens) adhesion experiments demonstrated that they could effectively adhere to a certain amount of dirt or bacteria to achieve the purpose of wound cleaning. In addition to the bacterial adhesion, the antibacterial experiments also proved that the hydrogels have a certain inhibitory effect on the proliferation of E.coli and S.aureus. The hydrogels could change shape freely and exhibited excellent biocompatibility, ductility, and self-healing capabilities, which increase their service life and make them more suitable for treating wounds or acting as protection buffers. Based on all these characteristics, the developed hydrogel may be a potentially valuable material for wound cleaning.

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.

Three-Dimensional Porous Scaffolds Derived from Bovine Cancellous Bone Matrix Promote Osteoinduction, Osteoconduction, and Osteogenesis

The use of three-dimensional porous scaffolds derived from decellularized extracellular matrix (ECM) is increasing for functional repair and regeneration of injured bone tissue. Because these scaffolds retain their native structures and bioactive molecules, in addition to showing low immunogenicity and good biodegradability, they can promote tissue repair and regeneration. Nonetheless, imitating these features in synthetic materials represents a challenging task. Furthermore, due to the complexity of bone tissue, different processes are necessary to maintain these characteristics. We present a novel approach using decellularized ECM material derived from bovine cancellous bone by demineralization, decellularization, and hydrolysis of collagen to obtain a three-dimensional porous scaffold. This study demonstrates that the three-dimensional porous scaffold obtained from bovine bone retained its osteoconductive and osteoinductive properties and presented osteogenic potential when seeded with human Wharton's jelly mesenchymal stromal cells (hWJ-MSCs). Based on its characteristics, the scaffold described in this work potentially represents a therapeutic strategy for bone repair.

Recent Advances in the Treatment of Bone Metastases and Primary Bone Tumors: An Up-to-Date Review

In the last decades, the treatment of primary and secondary bone tumors has faced a slow-down in its development, being mainly based on chemotherapy, radiotherapy, and surgical interventions. However, these conventional therapeutic strategies present a series of disadvantages (e.g., multidrug resistance, tumor recurrence, severe side effects, formation of large bone defects), which limit their application and efficacy. In recent years, these procedures were combined with several adjuvant therapies, with different degrees of success. To overcome the drawbacks of current therapies and improve treatment outcomes, other strategies started being investigated, like carrier-mediated drug delivery, bone substitutes for repairing bone defects, and multifunctional scaffolds with bone tissue regeneration and antitumor properties. Thus, this paper aims to present the types of bone tumors and their current treatment approaches, further focusing on the recent advances in new therapeutic alternatives.

Multi-scale mechanical and morphological characterisation of sintered porous magnesium-based scaffolds for bone regeneration in critical-sized defects

Magnesium (Mg) and its alloys are very promising degradable, osteoconductive and osteopromotive materials to be used as regenerative treatment for critical-sized bone defects. Under load-bearing conditions, Mg alloys must display sufficient morphological and mechanical resemblance to the native bone they are meant to replace to provide adequate support and enable initial bone bridging. In this study, unique highly open-porous Mg-based scaffolds were mechanically and morphologically characterised at different scales. In situ X-ray computed tomography (XCT) mechanics, digital volume correlation (DVC), electron microscopy and nanoindentation were combined to assess the influence of material properties on the apparent (macro) mechanics of the scaffold. The results showed that Mg exhibited a higher connected structure (38.4mm^-3 and 6.2mm^-3 for Mg and trabecular bone (Tb), respectively) and smaller spacing (245µm and 629µm for Mg and Tb, respectively) while keeping an overall appropriate porosity of 55% in the range of trabecular bone (30-80%). This fully connected and highly porous structure promoted lower local strain compared to the trabecular bone structure at material level (i.e. -22067 ± 8409µε and -40120 ± 18364µε at 6% compression for Mg and trabecular bone, respectively) and highly ductile mechanical behaviour at apparent level preventing premature scaffold failure. Furthermore, the Mg scaffolds exceeded the physiological strain of bone tissue generated in daily activities such as walking or running (500-2000µε) by one order of magnitude. The yield stress was also found to be close to trabecular bone (2.06MPa and 6.67MPa for Mg and Tb, respectively). Based on this evidence, the study highlights the overall biomechanical suitability of an innovative Mg-based scaffold design to be used as a treatment for bone critical-sized defects. STATEMENT OF SIGNIFICANCE: Bone regeneration remains a challenging field of research where different materials and solutions are investigated. Among the variety of treatments, biodegradable magnesium-based implants represent a very promising possibility. The novelty of this study is based on the characterisation of innovative magnesium-based implants whose structure and manufacturing have been optimised to enable the preservation of mechanical integrity and resemble bone microarchitecture. It is also based on a multi-scale approach by coupling high-resolution X-ray computed tomography (XCT), with in situ mechanics, digital volume correlation (DVC) as well as nano-indentation and electron-based microscopy imaging to define how degradable porous Mg-based implants fulfil morphological and mechanical requirements to be used as critical bone defects regeneration treatment.

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.

A multifunctional silk coating on additively manufactured porous titanium to prevent implant-associated infection and stimulate bone regeneration

Despite tremendous progress in the design and manufacturing of metallic implants, they do not outlive the patient. To illustrate, more than half of hip replacements will fail, mainly due to implant infection and loosening. Surface engineering approaches and, in particular, coatings can facilitate implant bio-functionality via the recruitment of more host cells for new bone formation and inhibition of bacterial colonization. Here, we used electrophoretic deposition to apply a silk fibroin solution consisting of tricalcium phosphate (TCP) and vancomycin as a coating on the surface of additively-manufactured porous titanium. Furthermore, the surface properties of the coatings developed and the release kinetics of the vancomycin were studied to evaluate the applied coating. The in vitro antibacterial behavior of the multifunctional coating, as well as the cell viability and osteogenic differentiation of the MC3T3-E1 cell line were extensively studied. The biomaterials developed exhibited an antibacterial behavior with a reduction of up to four orders of magnitude in both planktonic and adherent bacteria for 6 h and 1 d. A live-dead assay, the Alamar Blue activity, the DNA content, and cytoskeleton staining demonstrated a significant increase in the cell density of the coated groups versus the as-manufactured ones. The significantly enhanced calcium deposition and the increase in mineralization for the groups with TCP after 21 and 28 d, respectively, demonstrate upregulation of the MC3T3 cells' osteogenic differentiation. Our results collectively show that the multifunctional coating studied here can be potentially used to develop a new generation of orthopedic implants.