Regulation of an osteon-like concentric microgrooved surface on osteogenesis and osteoclastogenesis

Bone microphysiological models for biomedical research

Bone related disorders are highly prevalent, and many of these pathologies still do not have curative and definitive treatment methods. This is due to a complex interplay of multiple factors, such as the crosstalk between different tissues and cellular components, all of which are affected by microenvironmental factors. Moreover, these bone pathologies are specific, and current treatment results vary from patient to patient owing to their intrinsic biological variability. Current approaches in drug development to deliver new drug candidates against common bone disorders, such as standard two-dimensional (2D) cell culture and animal-based studies, are now being replaced by more relevant diseases modelling, such as three-dimension (3D) cell culture and primary cells under human-focused microphysiological systems (MPS) that can resemble human physiology by mimicking 3D tissue organization and cell microenvironmental cues. In this review, various technological advancements for in vitro bone modeling are discussed, highlighting the progress in biomaterials used as extracellular matrices, stem cell biology, and primary cell culture techniques. With emphasis on examples of modeling healthy and disease-associated bone tissues, this tutorial review aims to survey current approaches of up-to-date bone-on-chips through MPS technology, with special emphasis on the scaffold and chip capabilities for mimicking the bone extracellular matrix as this is the key environment generated for cell crosstalk and interaction. The relevant bone models are studied with critical analysis of the methods employed, aiming to serve as a tool for designing new and translational approaches. Additionally, the features reported in these state-of-the-art studies will be useful for modeling bone pathophysiology, guiding future improvements in personalized bone models that can accelerate drug discovery and clinical translation.

Crystallographic plane-induced selective mineralization of nanohydroxyapatite on fibrous-grained titanium promotes osteointegration and biocorrosion resistance

The (002) crystallographic plane-oriented hydroxyapatite (HA) and anatase TiO2 enable favorable hydrophilicity, osteogenesis, and biocorrosion resistance. Thus, the crystallographic plane control in HA coating and crystalline phase control in TiO2 is vital to affect the surface and interface bioactivity and biocorrosion resistance of titanium (Ti) implants. However, a corresponding facile and efficient fabrication method is absent to realize the HA(002) mineralization and anatase TiO2 formation on Ti. Herein, we utilized the predominant Ti(0002) plane of the fibrous-grained titanium (FG Ti) to naturally form anatase TiO2 and further achieve a (002) basal plane oriented nanoHA (nHA) film through an in situ mild hydrothermal growth strategy. The formed FG Ti-nHA(002) remarkably improved hydrophilicity, mineralization, and biocorrosion resistance. Moreover, the nHA(002) film reserved the microgroove-like topological structure on FG Ti. It could enhance osteogenic differentiation through promoted contact guidance, showing one order of magnitude higher expression of osteogenic-related genes. On the other hand, the nHA(002) film restrained the osteoclast activity by blocking actin ring formation. Based on these capacities, FG Ti-nHA(002) improved new bone growth and binding strength in rabbit femur implantation, achieving satisfactory osseointegration within 2 weeks.

Biomaterial Cues for Regulation of Osteoclast Differentiation and Function in Bone Regeneration

Tissue regeneration involves dynamic dialogue between and among different cells and their surrounding matrices. Bone regeneration is specifically governed by reciprocity between osteoblasts and osteoclasts within the bone microenvironment. Osteoclast-directed resorption and osteoblast-directed formation of bone are essential to bone remodeling, and the crosstalk between these cells is vital to curating a sequence of events that culminate in the creation of bone tissue. Among bone biomaterial strategies, many have investigated the use of different material cues to direct the development and activity of osteoblasts. However, less attention has been given to exploring features that similarly target osteoclast formation and activity, with even fewer strategies demonstrating or integrating biomaterial-directed modulation of osteoblast-osteoclast coupling. This review aims to describe various biomaterial cues demonstrated to influence osteoclastogenesis and osteoclast function, emphasizing those that enhance a material construct's ability to achieve bone healing and regeneration. Additionally discussed are approaches that influence the communication between osteoclasts and osteoblasts, particularly in a manner that takes advantage of their coupling. Deepening our understanding of how biomaterial cues may dictate osteoclast differentiation, function, and influence on the microenvironment may enable the realization of bone-replacement interventions with enhanced integrative and regenerative capacities.

Manipulation of signaling pathways in bone tissue engineering and regenerative medicine: Current knowledge, novel strategies, and future directions

During osteogenesis, a large number of bioactive molecules, macromolecules, cells, and cellular signals are activated to induce bone growth and development. The activation of molecular pathways leads to the occurrence of cellular events, ultimately resulting in observable changes. Therefore, in the studies of bone tissue engineering and regenerative medicine, it is essential to target fundamental events to exploit the mechanisms involved in osteogenesis. In this context, signaling pathways are activated during osteogenesis and trigger the activation of numerous other processes involved in osteogenesis. Direct influence of signaling pathways should allow to manipulate the signaling pathways themselves and impact osteogenesis. A combination of sequential cascades takes place to drive the progression of osteogenesis. Also, the occurrence of these processes and, more generally, cellular and molecular processes related to osteogenesis necessitate the presence of transcription factors and their activity. The present review focuses on outlining several signaling pathways and transcription factors influencing the development of osteogenesis, and describes various methods of their manipulation to induce and enhance bone formation.

Regulation of mitochondrial network architecture and function in mesenchymal stem cells by micropatterned surfaces

Mitochondrial network architecture, which is closely related to mitochondrial function, is mechanically sensitive and regulated by multiple stimuli. However, the effects of microtopographic cues on mitochondria remain poorly defined. Herein, polycaprolactone (PCL) surfaces were used as models to investigate how micropatterns regulate mitochondrial network architecture and function in rat adipose-derived stem cells (rASCs). It was found that large pit (LP)-induced rASCs to form larger and more complex mitochondrial networks. Consistently, the expression of key genes related to mitochondrial dynamics revealed that mitochondrial fusion (MFN1 and MFN2) and midzone fission (DRP1 and MFF) were increased in rASCs on LP. In contrast, the middle pit (MP)-enhanced mitochondrial biogenesis, as evidenced by the larger mitochondrial area and higher expression of PGC-1. Both LP and MP promoted ATP production in rASCs. It is likely that LP increased ATP levels through modulating mitochondrial network architecture while MP stimulated mitochondria biogenesis to do so. Our study clarified the regulation of micropatterned surfaces on mitochondria, highlighting the potential of LP and MP as a simple platform to stimulate mitochondria and the subsequent cellular function of MSCs.

Effect of polydimethylsiloxane surface morphology on osteogenic differentiation of mesenchymal stem cells through SIRT1 signalling pathway

Constructing surface topography with a certain roughness is a widely used, non-toxic, cost-effective and effective method for improving the microenvironment of cells, promoting the proliferation and osteogenic differentiation of mesenchymal stem cells (MSCs), and promoting the osseointegration of grafts and further improving their biocompatibility under clinical environmental conditions. SIRT1 plays an important regulatory role in the osteogenic differentiation of bone marrow-derived MSCs (BM-MSCs). However, it remains unknown whether SIRT1 plays an important regulatory role in the osteogenic differentiation of BM-MSCs with regard to surface morphology. Polydimethylsiloxane (PDMS) with different surface morphologies were prepared using different grits of sandpaper. The value for BMSCs added on different surfaces was detected by cell proliferation assays. RT-qPCR and Western blotting were performed to detect SIRT1 activation and osteogenic differentiation of MSCs. Osteogenesis of MSCs was detected by alkaline phosphatase (ALP) and alizarin red S staining. SIRT1 inhibition experiments were performed to investigate the role of SIRT1 in the osteogenic differentiation of MSCs induced by surface morphology. We found that BM-MSCs have better value and osteogenic differentiation ability on a surface with roughness of PDMS-1000M. SIRT1 showed higher gene and protein expression on a PDMS-1000M surface with a roughness of 13.741 ± 1.388 µm. The promotion of the osteogenic differentiation of MSCs on the PDMS-1000M surface was significantly decreased after inhibiting SIRT1 expression. Our study demonstrated that a surface morphology with certain roughness can activate the SIRT1 pathway of MSCs and promote the osteogenic differentiation of BMSCs via the SIRT1 pathway.

Apoptotic extracellular vesicles restore homeostasis of the articular microenvironment for the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) is a severe autoimmune disease with symptoms including synovial inflammation, cartilage erosion, and bone loss in RA lesions, which eventually lead to joint deformity and function loss. Most current treatments fail to achieve satisfying therapeutic outcomes with some adverse effects. Extracellular vesicles derived from apoptotic cells (apoEVs) have emerged as important mediators in intercellular communication regulating diverse physiological and pathological processes. In this study, we investigated the therapeutic efficacy of macrophage-derived and osteoclast-derived apoEVs (Mφ-apoEVs and OC-apoEVs) on RA. The in vitro results showed that both Mφ-apoEVs and OC-apoEVs induced macrophage repolarization toward the anti-inflammatory M2 phenotype, promoted chondrocyte functions and chondrogenesis, and inhibited osteoclast formation and maturation. In addition, OC-apoEVs promoted osteogenic differentiation. The in vivo study on the CIA mouse model further demonstrated that apoEVs could couple various functions and exert synergistic effects on the joint with RA, as evidenced by the regression of synovial inflammation, the reversal of cartilage damage and bone erosion, and the preservation of joint structure. These findings demonstrated that Mφ-apoEVs and OC-apoEVs contributed to restoring the homeostasis of the overall microenvironment in the RA joint and highlighted their potential application as a promising alternative to treat RA.

Micro/nano topological modification of TiO2 nanotubes activates Thy-1 signaling to control osteogenic differentiation of stem cells

Micro/nano topological modification is critical for improving the in vivo behaviors of bone implants, regulating multiple cellular functions. Titania (TiO2) nanotubes show the capacity of promoting osteoblast-related cell differentiation and induce effective osseointegration, serving as a model material for studying the effects of micro/nano-topological modifications on cells. However, the intracellular signaling pathways by which TiO2 nanotubes regulate the osteogenic differentiation of stem cells are not fully defined. Thy-1 (CD90), a cell surface glycoprotein anchored by glycosylphosphatidylinositol, has been considered a key molecule in osteoblast differentiation in recent years. Nevertheless, whether the micro/nano topology of the implant surface leads to changes in Thy-1 is unknown, as well as whether these changes promote osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). Here, TiO2 nanotubes of various diameters were prepared by adjusting the anodizing voltage. qPCR and immunoblot were carried out to assess the mechanism by which TiO2 nanotubes regulate Thy-1. The results revealed Ti plates harboring TiO2 nanotubes ∼100-nm diameter (TNT-100) markedly upregulated Thy-1. Subsequently, upregulated Thy-1 promoted the activation of Fyn/RhoA/MLC Ⅱ/F-actin axis, which enhanced the nuclear translocation of YAP. After Thy-1 knockdown by siRNA, the Fyn/RhoA/MLC Ⅱ/F-actin axis was significantly inhibited and TiO2 nanotubes showed decreased effects on osteogenic differentiation. Therefore, Thy-1 upregulation might be a major mechanism by which micro/nano-topological modification of TiO2 nanotubes promotes osteogenic differentiation in BMSCs. This study provides novel insights into the molecular mechanism of TiO2 nanotubes, which may help design improved bone implants for clinical application.

Suppression of LPS-activated inflammatory responses and chromosomal histone modifications in macrophages by micropattern-induced nuclear deformation

Macrophages are important immune effector cells which participate various physiological and pathological conditions. Numerous studies have demonstrated the regulation of macrophage phenotype by micropatterns. It is well accepted that micropatterns affect cellular behaviors through changing cell shape and modulating the associated mechanical sensors on the plasma membrane and cytoskeleton. However, the role of nucleus, which serves as a critical physical sensing device, is often ignored. Herein, we found the nuclear deformation and the subsequently increased chromosomal histone methylation (H3K36me2) may contribute to the micropattern-induced suppression of macrophage inflammatory responses. Specifically, macrophages on micropatterned surfaces expressed lower levels of key inflammatory genes, compared with those on flat surfaces. Further investigation on macrophage nuclei showed that micropatterned surfaces cause shrinkage of nucleus volume and compaction of chromatin. Moreover, micropatterned surfaces elevated the methylation level of H3K36me2 in macrophages, while decreased the methylation level of H3K4me3. Our study provides new mechanistic insight into how micropatterns affect macrophage phenotype and highlights the importance of nuclear shape and chromatin histone modification in mediating micropattern-induced change in cell behaviors.

Effect of Anisotropic Structural Depth on Orientation and Differentiation Behavior of Skeletal Muscle Cells

Extensive research has been conducted to examine how substrate topological factors are involved in modulating the cell behavior. Among numerous topological factors, the vital influence of the touchable depth of substrates on cell behaviors has already been extensively characterized, but the response of cells to the topological structure at untouchable depth is still elusive. Herein, the influences of substrate depth on myoblast behaviors are systematically investigated using substrates with depths ranging from touchable depth (microgrooved) to untouchable depth (microbridges). The results show that an increase in microgroove depth is accompanied by an inhibited cell spreading, an enhanced elongation, and a more obvious orientation along microgrooves. Interestingly, myoblasts located on microbridges show a more pronounced elongation with increasing culture time but a position-dependent orientation. Myoblasts on the center and parallel boundary of microbridges orient along the bridges, while myoblasts on the vertical boundary align perpendicular to the microbridges. Moreover, the differentiation results of the myoblasts indicate that the differentiated myotubes can maintain this position-dependent orientation. The simulation of the stress field in cell monolayers suggests that the position-dependent orientation is caused by the comprehensive response of myoblasts to the substrate discontinuity and substrate depth. These findings provide valuable insights into the mechanism of cell depth sensing and could inform the design of tissue engineering scaffolds for skeletal muscle and biohybrid actuation.

Ultraviolet Light-Based Micropattern Printing on Titanium Surfaces to Promote Early Osseointegration

Patterned interfaces are widely used for surface modification of biomaterials because of a morphological unit similar to that of native tissue. However, engineering fast and cost-effective high-resolution micropatterns directly onto titanium surfaces remains a grand challenge. Herein, a simply designed ultraviolet (UV) light-based micropattern printing to obtain geometrical patterns on implant interfaces is fabricated by utilizing customized photomasks and titanium dioxide (TiO2 ) nanorods as a photo-responsive platform. The technique manipulates the cytoskeleton of micropatterning cells on the surface of TiO2 nanorods. The linear pattern surface shows the elongated morphology and parallel linear arrangements of human mesenchymal stem cells (hMSCs), significantly enhancing their osteogenic differentiation. In addition to the upregulated expression of key osteo-specific function genes in vitro, the accelerated osseointegration between the implant and the host bone is obtained in vivo. Further investigation indicates that the developed linear pattern surface has an outstanding effect on the cytoskeletal system, and finally activates Yes-Associated Protein (YAP)-mediated mechanotransduction pathways, initiating hMSCs osteogenic differentiation. This study not only offers a microfabrication method that can be extended to fabricate various shape- and size-controlled micropatterns on titanium surfaces, but also provides insight into the surface structure design for enhanced bone regeneration.

3D-Printing of succulent plant-like scaffolds with beneficial cell microenvironments for bone regeneration

Biomimetic materials with complicated structures inspired by natural plants play a critical role in tissue engineering. The succulent plants, with complicated morphologies, show tenacious vitality in extreme conditions due to the physiological functions endowed by their unique anatomical structures. Herein, inspired by the macroscopic structure of succulent plants, succulent plant-like bioceramic scaffolds were fabricated via digital laser processing 3D printing of MgSiO₃. Compared with conventional scaffolds with interlaced columns, the structures could prevent cells from leaking from the scaffolds and enhance cell adhesion. The scaffold morphology could be well regulated by changing leaf sizes, shapes, and interlacing methods. The succulent plant-like scaffolds show excellent properties for cell loading as well as cell distribution, promoting cellular interplay, and further enhancing the osteogenic differentiation of bone marrow stem cells. The in vivo study further illustrated that the succulent plant-like scaffolds could accelerate bone regeneration by inducing the formation of new bone tissues. The study suggests that the obtained succulent plant-like scaffold featuring the plant macroscopic structure is a promising biomaterial for regulating cell distribution, enhancing cellular interactions, and further improving bone regeneration.

Mechanical and morphological responses of osteoblast-like cells to two-photon polymerized microgrooved surfaces

Microgrooved surfaces are recognized as an important strategy of tissue engineering to promote the alignment of bone cells. In this work, we have investigated the mechanical and morphological aspects of osteoblasts cells after interaction with different micro-structured polymeric surfaces. Femtosecond laser writing technique was used for the construction of circular and parallel microgrooved patterns in biocompatible polymeric surfaces based on pentaerythritol triacrylate. Additionally, we have studied the influence of the biocompatible TiO₂ nanocrystals (NCs) related to the cell behavior, when incorporated to the photoresin. The atomic force microscopy technique was used to investigate the biomechanical reaction of the human osteoblast-like MG-63 cells for the different microgroove. It was demonstrated that osteoblasts grown on circular microgrooved surfaces exhibited significantly larger Young's modulus compared to cells sown on flat films. Furthermore, we could observe that TiO₂ NCs improved the circular microgrooves effects, resulting in more populated sites, 34% more elongated cells, and increasing the cell stiffness by almost 160%. These results can guide the design and construction of effective scaffold surfaces with circular microgrooves for tissue engineering and bone regeneration.

Electric Spark Deposition of Antibacterial Silver Coating on Microstructured Titanium Surfaces with a Novel Flexible Brush Electrode

Infection caused by orthopedic titanium implants, which results in tissue damage, is a key factor in endosseous implant failure. Given the seriousness of implant infections and the limitations of antibiotic therapy, surface microstructures and antimicrobial silver coatings have emerged as prominent research areas and have displayed certain antimicrobial effects. Researchers are now working to combine the two to produce more effective antimicrobial surfaces. However, building robust and homogeneous coatings on complex microstructured surfaces is a tough task due to the limits of surface modification techniques. In this study, a novel flexible electrode brush (silver brush) instead of a traditional hard electrode was designed with electrical discharge machining, which has the ability to adapt to complex groove interiors. The results showed that the use of flexible electrode brush allowed silver to be deposited uniformly in titanium alloy microgrooves. On the surface of Ag-TC4, a uniformly covered deposit was visible, and it slowly released silver ions into a liquid environment. In vitro bacterial assays showed that a Ag-TC4 microstructured surface reduced bacterial adhesion and bacterial biofilm formation, and the antibacterial activity of Ag-TC4 against Staphylococcus aureus and Escherichia coli was 99.68% ± 0.002 and 99.50% ± 0.007, respectively. This research could lay the groundwork for the study of antimicrobial metal bound to microstructured surfaces and pave the way for future implant surface design.

Poly (Butylene Succinate)/Silicon Nitride Nanocomposite with Optimized Physicochemical Properties, Biocompatibility, Degradability, and Osteogenesis for Cranial Bone Repair

Congenital disease, tumors, infections, and trauma are the main reasons for cranial bone defects. Herein, poly (butylene succinate) (PB)/silicon nitride (Si₃N₄) nanocomposites (PSC) with Si3N4 content of 15 w% (PSC15) and 30 w% (PSC30) were fabricated for cranial bone repair. Compared with PB, the compressive strength, hydrophilicity, surface roughness, and protein absorption of nanocomposites were increased with the increase in Si₃N₄ content (from 15 w% to 30 w%). Furthermore, the cell adhesion, multiplication, and osteoblastic differentiation on PSC were significantly enhanced with the Si₃N₄ content increasing in vitro. PSC30 exhibited optimized physicochemical properties (compressive strength, surface roughness, hydrophilicity, and protein adsorption) and cytocompatibility. The m-CT and histological results displayed that the new bone formation for SPC30 obviously increased compared with PB, and PSC30 displayed proper degradability (75.3 w% at 12 weeks) and was gradually replaced by new bone tissue in vivo. The addition of Si₃N₄ into PB not only optimized the surface performances of PSC but also improved the degradability of PSC, which led to the release of Si ions and a weak alkaline environment that significantly promoted cell response and tissue regeneration. In short, the enhancements of cellular responses and bone regeneration of PSC30 were attributed to the synergism of the optimized surface performances and slow release of Si ion, and PSC30 were better than PB. Accordingly, PSC30, with good biocompatibility and degradability, displayed a promising and huge potential for cranial bone construction.

A bioactive poly(ether-ether-ketone) nanocomposite scaffold regulates osteoblast/osteoclast activity for the regeneration of osteoporotic bone

Due to the lower regeneration capacity of the osteoporotic bone, the treatment of osteoporotic defects is extremely challenging in clinics. In this study, strontium-doped bioactive glass nanoparticles loaded with sodium alendronate (ALN), namely A-SrBG, were incorporated into the poly(ether-ether-ketone) matrix to fabricate a bioactive composite scaffold (ASP), which was expected to both inhibit bone resorption and promote bone regeneration. The results showed that such a composite scaffold with interconnected macropores (200-400 μm) could release Ca²⁺, Sr²⁺, and ALN in vitro. The proliferation, alkaline phosphatase (ALP) activity, expression of osteogenesis-related genes, and formation of calcified nodules of rat bone marrow stromal cells (rBMSCs) were clearly evidenced, and the reduction in the proliferation, tartrate-resistant acid phosphatase (TRAP) activity, cell fusion, and expression of osteoclastogenesis-related genes of osteoclasts was observed as well. In the presence of the ASP scaffold, enhanced osteogenesis along with inhibiting osteoclastogenesis was observed by modulating the osteoprotegerin (OPG)/receptor activator for nuclear factor κB ligand (RANKL) ratio. The efficacy of the composite scaffold in the regeneration of osteoporotic critical-sized cranial defect in a rat model was evaluated. Therefore, the bioactive composite scaffold with excellent biocompatibility and osteogenic potential could be a promising material for the repair of osteoporotic bone defects.

Topographic Orientation of Scaffolds for Tissue Regeneration: Recent Advances in Biomaterial Design and Applications

Tissue engineering to develop alternatives for the maintenance, restoration, or enhancement of injured tissues and organs is gaining more and more attention. In tissue engineering, the scaffold used is one of the most critical elements. Its characteristics are expected to mimic the native extracellular matrix and its unique topographical structures. Recently, the topographies of scaffolds have received increasing attention, not least because different topographies, such as aligned and random, have different repair effects on various tissues. In this review, we have focused on various technologies (electrospinning, directional freeze-drying, magnetic freeze-casting, etching, and 3-D printing) to fabricate scaffolds with different topographic orientations, as well as discussed the physicochemical (mechanical properties, porosity, hydrophilicity, and degradation) and biological properties (morphology, distribution, adhesion, proliferation, and migration) of different topographies. Subsequently, we have compiled the effect of scaffold orientation on the regeneration of vessels, skin, neural tissue, bone, articular cartilage, ligaments, tendons, cardiac tissue, corneas, skeletal muscle, and smooth muscle. The compiled information in this review will facilitate the future development of optimal topographical scaffolds for the regeneration of certain tissues. In the majority of tissues, aligned scaffolds are more suitable than random scaffolds for tissue repair and regeneration. The underlying mechanism explaining the various effects of aligned and random orientation might be the differences in “contact guidance”, which stimulate certain biological responses in cells.

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

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

Osteoclast formation from mouse bone marrow cells on micro/nano-scale patterned surfaces

OBJECTIVES

Osteoclasts can sense the surface topography of materials. However, it is difficult to identify the structural factors that affect osteoclast formation and its function. Furthermore, we hypothesized that the type of osteoclast precursor cells also affects osteoclastogenesis in the materials. In this study, we investigated the effects of defined micro/nanoscale patterns on osteoclastogenesis from bone marrow cells (BMCs).

METHODS

Various cyclo-olefin polymer (COP) patterns were prepared using nanoimprinting. The effects of shape, size, and height of the patterns, and the wettability of the patterned surfaces on osteoclastogenesis from BMCs were evaluated in vitro.

RESULTS

Osteoclast formation was promoted on pillars (diameter, 1 μm or 500 nm; height, 500 nm). Notably, osteoclastogenesis from BMCs was better promoted on hydrophobic pillars than on hydrophilic pillars. In contrast, decreased osteoclast formation was observed on the nanopillars (diameter, 100 nm; height, 200 nm).

CONCLUSIONS

We demonstrated the promotion of osteoclast formation from BMCs on hydrophobic pillars with diameters of 1 μm and 500 nm. Some cellular behaviors in the patterns were dependent on the type of osteoclast precursor cells. The designed patterns are useful for designing the surface of dental implants or bone replacement materials with a controllable balance between osteoblast and osteoclast activities.

The Bumpy Road to Stem Cell Therapies: Rational Design of Surface Topographies to Dictate Stem Cell Mechanotransduction and Fate

Cells sense and respond to a variety of physical cues from their surrounding microenvironment, and these are interpreted through mechanotransductive processes to inform their behavior. These mechanisms have particular relevance to stem cells, where control of stem cell proliferation, potency, and differentiation is key to their successful application in regenerative medicine. It is increasingly recognized that surface micro- and nanotopographies influence stem cell behavior and may represent a powerful tool with which to direct the morphology and fate of stem cells. Current progress toward this goal has been driven by combined advances in fabrication technologies and cell biology. Here, the capacity to generate precisely defined micro- and nanoscale topographies has facilitated the studies that provide knowledge of the mechanotransducive processes that govern the cellular response as well as knowledge of the specific features that can drive cells toward a defined differentiation outcome. However, the path forward is not fully defined, and the "bumpy road" that lays ahead must be crossed before the full potential of these approaches can be fully exploited. This review focuses on the challenges and opportunities in applying micro- and nanotopographies to dictate stem cell fate for regenerative medicine. Here, key techniques used to produce topographic features are reviewed, such as photolithography, block copolymer lithography, electron beam lithography, nanoimprint lithography, soft lithography, scanning probe lithography, colloidal lithography, electrospinning, and surface roughening, alongside their advantages and disadvantages. The biological impacts of surface topographies are then discussed, including the current understanding of the mechanotransductive mechanisms by which these cues are interpreted by the cells, as well as the specific effects of surface topographies on cell differentiation and fate. Finally, considerations in translating these technologies and their future prospects are evaluated.

Osteon-mimetic 3D nanofibrous scaffold enhances stem cell proliferation and osteogenic differentiation for bone regeneration

The scaffold microstructure is important for bone tissue engineering. Failure to synergistically imitate the hierarchical microstructure of the components of bone, such as an osteon with concentric multilayers assembled by nanofibers, hinders the performance for guiding bone regeneration. Here, a 2D bilayer nanofibrous membrane (BLM) containing poly(lactide-co-glycolide) (PLGA)/polycaprolactone (PCL) composite membranes in similar compositions (PCL15 and PCL20), but possessing different degrees of shrinkage, was fabricated via sequential electrospinning. Upon incubation in phosphate buffered saline (PBS) (37 °C), the 2D BLM spontaneously deformed into a 3D shape induced by PCL crystallization within the PLGA matrix, and the PCL15 and PCL20 layer formed a concave and convex surface, respectively. The 3D structure contained curved multilayers with an average diameter of 776 ± 169 μm, and on the concave and convex surface the nanofiber diameters were 792 ± 225 and 881 ± 259 nm, respectively. The initial 2D structure facilitated the even distribution of seeded cells. Adipose-derived stem cells from rats (rADSCs) proliferated faster on a concave surface than on a convex surface. For the 3D BLM, the osteogenic differentiation of rADSCs was significantly higher than that on 2D surfaces, even without osteogenic supplements, which resulted from the stretched cell morphology on the curved sublayer leading to increased expression of lamin-A. After being implanted into cranial defects in Sprague Dawley (SD) rats, 3D BLM significantly accelerated bone formation. In summary, 3D BLM with an osteon-like structure provides a potential strategy to repair bone defects.

Fabrication of bioinspired grid-crimp micropatterns by melt electrospinning writing for bone-ligament interface study

Many strategies have been adopted to engineer bone-ligament interface, which is of great value to both the tissue regeneration and the mechanism understanding underlying interface regeneration. However, how to recapitulate the complexity and heterogeneity of the native bone-ligament interface including the structural, cellular and mechanical gradients is still challenging. In this work, a bioinspired grid-crimp micropattern fabricated by melt electrospinning writing (MEW) was proposed to mimic the native structure of bone-ligament interface. The printing strategy of crimped fiber micropattern was developed and the processing parameters were optimized, which were used to mimic the crimp structure of the collagen fibrils in ligament. The guidance effect of the crimp angle and fiber spacing on the orientation of fibroblasts was studied, and both of them showed different levels of cell alignment effect.. MEW grid micropatterns with different fiber spacings were fabricated as bone region. Both the alkaling phosphatase activity and calcium mineralization results demonstrated the higher osteoinductive ability of the MEW grid structures, especially for that with smaller fiber spacing. The combined grid-crimp micropatterns were applied for the co-culture of fibroblasts and osteoblasts. The results showed that more cells were observed to migrate into the in-between interface region for the pattern with smaller fiber spacing, suggested the faster migration speed of cells. Finally, a cylindrical triphasic scaffold was successfully generated by rolling the grid-crimp micropatterns up, showing both structural and mechanical similarity to the native bone-ligament interface. In summary, the proposed strategy is reliable to fabricate grid-crimp triphasic micropatterns with controllable structural parameters to mimic the native bone-to-ligament structure, and the generated 3D scaffold shows great potential for the further bone-ligament interface tissue engineering.

Self-Organized Spatiotemporal Mineralization of Hydrogel: A Simulant of Osteon

Nature creates fascinating self-organized spatiotemporal patterns through the delicate control of reaction-diffusion dynamics. As the primary unit of cortical bone, osteon has concentric lamellar architecture, which plays a crucial role in the mechanical and physiological functions of bone. However, it remains a great challenge to fabricate the osteon-like structure in a natural self-organization way. Taking advantage of the nonequilibrium reaction in hydrogels, a simple mineralization strategy to closely mimic the formation of osteon in a mild physiological condition is developed. By constructing two reverse concentration gradients of ions from periphery to interior of cylindrical hydrogel, spatiotemporal self-organization of calcium phosphate in concentric rings is generated. It is noteworthy that minerals in different layers possess diverse contents and crystalline phases, which further guide the adhesion and spread of osteoblasts on these patterns, resembling the architecture and cytological behavior of osteon. Besides, theoretical data indicates the predominate role of ion concentrations and pH values of solution, in good accordance with experimental results. Independent of precise instruments, this lifelike method is easily obtained, cost-efficient, and effectively imitates the mineral deposition in osteon from a physiochemical view. The strategy may be expanded to develop other functional material patterns via spatiotemporal self-organization.

Hierarchical microgroove/nanopore topography regulated cell adhesion to enhance osseointegration around intraosseous implants in vivo

Implant surface topography plays a crucial role in achieving successful implantation. Simple and controllable surface topographical modifications are considered a promising method to accelerate bone osseointegration for biomedical applications. Moreover, comprehension of the mechanism between surface topography and cell osteogenic differentiation is vital for the manipulation of these processes to promote bone tissue regeneration. In this study, we investigated the effects of implant surfaces with various sized hierarchical microgroove/nanopore topographies on cell adhesion, osteogenesis, and their underlying mechanism both in vitro and in vivo. Our findings reveal that a titanium surface with an appropriately sized microgroove/nanopore topography (SLM-1MAH) exhibits the more satisfactory adhesive and osteogenic efficiency than the clinically used sand-blasted, large-grit, and acid-etched (SLA) surface. The underlying molecular mechanism lies in the activation of the integrin α2-PI3K-Akt signaling pathway, where the SLM-1MAH surface increased the protein expressions of integrin α2 (Itga2), phosphatidylinositol 3-kinase (PI3K), and phosphorylated serine/threonine kinase Akt (p-Akt) to enhance osteogenesis and osseointegration. Furthermore, the SLM-1MAH surface also displays better osseointegration efficiency with stronger bonding strength than that on the SLA surface. This work provides a novel strategy for implant surface topography design to improve bone-implant osseointegration.

Characterization and in vitro assessment of three-dimensional extrusion Mg-Sr codoped SiO2-complexed porous microhydroxyapatite whisker scaffolds for biomedical engineering

BACKGROUND

Large bone defects have always been a great challenge for orthopedic surgeons. The use of a good bone substitute obtained by bone tissue engineering (BTE) may be an effective treatment method. Artificial hydroxyapatite, a commonly used bone defect filler, is the main inorganic component of bones. Because of its high brittleness, fragility, and lack of osteogenic active elements, its application is limited. Therefore, its fragility should be reduced, its osteogenic activity should be improved, and a more suitable scaffold should be constructed.

METHODS

In this study, a microhydroxyapatite whisker (mHAw) was developed, which was doped with the essential trace active elements Mg2+ and Sr2+ through a low-temperature sintering technique. After being formulated into a slurry, a bionic porous scaffold was manufactured by extrusion molding and freeze drying, and then SiO2 was used to improve the mechanical properties of the scaffold. The hydrophilicity, pore size, surface morphology, surface roughness, mechanical properties, and release rate of the osteogenic elements of the prepared scaffold were detected and analyzed. In in vitro experiments, Sprague-Dawley (SD) rat bone marrow mesenchymal stem cells (rBMSCs) were cultured on the scaffold to evaluate cytotoxicity, cell proliferation, spreading, and osteogenic differentiation.

RESULTS

Four types of scaffolds were obtained: mHAw-SiO2 (SHA), Mg-doped mHAw-SiO2 (SMHA), Sr-doped mHAw-SiO2 (SSHA), and Mg-Sr codoped mHAw-SiO2 (SMSHA). SHA was the most hydrophilic (WCA 5°), while SMHA was the least (WCA 8°); SMHA had the smallest pore size (247.40 ± 23.66 μm), while SSHA had the largest (286.20 ± 19.04 μm); SHA had the smallest Young's modulus (122.43 ± 28.79 MPa), while SSHA had the largest (188.44 ± 47.89 MPa); and SHA had the smallest compressive strength (1.72 ± 0.29 MPa), while SMHA had the largest (2.47 ± 0.25 MPa). The osteogenic active elements Si, Mg, and Sr were evenly distributed and could be sustainably released from the scaffolds. None of the scaffolds had cytotoxicity. SMSHA had the highest supporting cell proliferation and spreading rate, and its ability to promote osteogenic differentiation of rBMSCs was also the strongest.

CONCLUSIONS

These composite porous scaffolds not only have acceptable physical and chemical properties suitable for BTE but also have higher osteogenic bioactivity and can possibly serve as potential bone repair materials.

Heterogenous hydrogel mimicking the osteochondral ECM applied to tissue regeneration

Inspired by the intricate extracellular matrix (ECM) of natural cartilage and subchondral bone, a heterogenous bilayer hydrogel scaffold is fabricated. Gelatin methacrylate (GelMA) and acryloyl glucosamine (AGA) serve as the main components in the upper layer, mimicking the chondral ECM. Meanwhile, vinylphosphonic acid (VPA) as a non-collagen protein analogue is incorporated into the bottom layer to induce the in situ biomineralization of calcium phosphate. The two heterogenous layers are effectively sutured together by the inter-diffusion between the upper and bottom layer hydrogels, together with chelation between the calcium ions and alginate added to separate layers. The interfacial bonding between the two different layers was thoroughly investigated via rheological measurements. The incorporation of AGA promotes chondrocytes to produce collagen type II and glycosaminoglycans and upregulates the expression of chondrogenesis-related genes. In addition, the minerals induced by VPA facilitate the osteogenesis of bone marrow mesenchymal stem cells (BMSCs). In vivo evaluation confirms the biocompatibility of the scaffold with minor inflammation and confirms the best repair ability of the bilayer hydrogel. This cell-free, cost-effective and efficient hydrogel shows great potential for osteochondral repair and inspires the design of other tissue-engineering scaffolds.

Regulation of an Antimicrobial Peptide GL13K-Modified Titanium Surface on Osteogenesis, Osteoclastogenesis, and Angiogenesis Base on Osteoimmunology

Creating a pro-regenerative immune microenvironment around implant biomaterial surfaces is significant to osseointegration. Immune cells, especially macrophages that participate in the osseointegration, including osteogenesis, osteoclastogenesis, and angiogenesis, should be considered when testing biomaterials. In this study, we immobilized an antimicrobial peptide GL13K with immunomodulatory properties onto a titanium surface via silanization. The modified surfaces show good biocompatibility with bone mesenchymal stromal cells (BMSCs), human umbilical vein endothelial cells (HUVECs), and RAW264.7. By co-culturing BMSCs with RAW264.7, we found that the GL13K-coated titanium surfaces could promote late-stage osteogenesis as demonstrated by the upregulated expression of recombinant collagen type I alpha 1 (COL-1α1) and more extracellular matrix mineralization, while the early phase remained unchanged. The surfaces inhibited the osteoclastogenic differentiation of RAW264.7 cells by restraining nuclear factor-activated T cells, cytoplasmic 1 (NFATc1), the main factor of the receptor activator of nuclear factor-κ B, and the receptor activator of the nuclear factor-κ B ligand signaling pathway, from entering the nucleus and further reduced the expression of the activating osteoclastogenic tartrate-resistant acid phosphatase gene. Moreover, the GL13K-coated titanium surface demonstrated significant promotion of angiogenesis differentiation of HUVECs as indicated by the upregulated expression of essential angiogenesis function genes, including hypoxia-inducible factor-1α, endothelial nitric oxide synthase, kinase insert domain receptor, and vascular endothelial growth factor A (HIF-1α, eNOS, KDR, and VEGF-A). Taken together, these results demonstrated that the GL13K coating had properties of osteogenesis, angiogenesis, and anti-osteoclastogenesis via its immunomodulatory potential.

Single-cell RNA-seq reveals functionally distinct biomaterial degradation-related macrophage populations

Macrophages play crucial roles in host tissue reaction to biomaterials upon implantation in vivo. However, the complexity of biomaterial degradation-related macrophage subpopulations that accumulate around the implanted biomaterials in situ is not fully understood. Here, using single cell RNA-seq, we analyze the transcriptome profiles of the various cell types around the scaffold to map the scaffold-induced reaction, in an unbiased approach. This enables mapping of all biomaterial degradation-associated cells at high resolution, revealing distinct subpopulations of tissue-resident macrophages as the major cellular sources of biomaterial degradation in situ. We also find that scaffold architecture can affect the mechanotransduction and catabolic activity of specific material degradation-related macrophage subpopulations in an Itgav-Mapk1-Stat3 dependent manner, eventually leading to differences in scaffold degradation rate in vivo. Our work dissects unanticipated aspects of the cellular and molecular basis of biomaterial degradation at the single-cell level, and provides a conceptual framework for developing functional tissue engineering scaffolds in future.

3D printed submicron patterns orchestrate the response of macrophages

The surface topography of engineered extracellular matrices is one of the most important physical cues regulating the phenotypic polarization of macrophages. However, not much is known about the ways through which submicron (i.e., 100-1000 nm) topographies modulate the polarization of macrophages. In the context of bone tissue regeneration, it is well established that this range of topographies stimulates the osteogenic differentiation of stem cells. Since the immune response affects the bone tissue regeneration process, the immunomodulatory consequences of submicron patterns should be studied prior to their clinical application. Here, we 3D printed submicron pillars (using two-photon polymerization technique) with different heights and interspacings to perform the first ever systematic study of such effects. Among the studied patterns, the highest degree of elongation was observed for the cells cultured on those with the tallest and densest pillars. After 3 days of culture with inflammatory stimuli (LPS/IFN-γ), sparsely decorated surfaces inhibited the expression of the pro-inflammatory cellular marker CCR7 as compared to day 1 and to the other patterns. Furthermore, sufficiently tall pillars polarized the M1 macrophages towards a pro-healing (M2) phenotype, as suggested by the expression of CD206 within the first 3 days. As some of the studied patterns are known to be osteogenic, the osteoimmunomodulatory capacity of the patterns should be further studied to optimize their bone tissue regeneration performance.

Regulation of Myogenic Differentiation by Topologically Microgrooved Surfaces for Skeletal Muscle Tissue Engineering

Inspired by the natural topological structure of skeletal muscle tissue, the topological surface construction of bionic scaffolds for skeletal muscle repair has attracted great interest. Many previous studies have focused on the effects of the topological structure on myoblasts. However, these studies used only specific repeating sizes and shapes to achieve the myoblast alignment and myotube formation; moreover, the regulatory effects of the size of a topological structure on myogenic differentiation are often neglected, leading to a lack of guidance for the design of scaffolds for skeletal muscle tissue engineering. In this study, we fabricated a series of microgroove topographies with various widths and depths via a combination of soft lithography and melt-casting and studied their effects on the behaviors of skeletal muscle cells, especially myogenic differentiation, in detail. Microgrooved poly(lactic-co-glycolic acid) substrates were found to effectively regulate the proliferation, myogenic differentiation, and myotube formation of C2C12 cells, and the degree of myogenic differentiation was significantly dependent on signals in response to the size of the microgroove structure. Compared with their depth, the width of the microgroove structures can more strongly affect the myogenic differentiation of C2C12 cells, and the degree of myoblast differentiation was enhanced with increasing groove width. Microgroove structures with relatively large groove widths and small groove depths promoted the myogenic differentiation of C2C12 cells. In addition, the integrin-mediated focal adhesion kinase signaling pathway and MAPK signaling pathway were activated in cells in response to the external topological structure, and the size of the topological structure of the material surface effectively regulated the degree of the cellular response to the external topological structure. These results can guide the design of scaffolds for skeletal muscle tissue engineering and the construction of effective bionic scaffold surfaces for skeletal muscle regeneration.

Improved Small Extracellular Vesicle Secretion of Rat Adipose-Derived Stem Cells by Microgrooved Substrates through Upregulation of the ESCRT-III-Associated Protein Alix

Mesenchymal stem cell-derived small extracellular vesicles (MSC-sEVs) hold great potential for regenerative therapies and have received considerable research attention in recent years. However, the use of MSC-sEVs is limited by very low yield in routine culture conditions and suboptimal potency for certain diseases. Thus, strategies that enable the production of sufficient quantities of sEVs with desired therapeutic cargo in a facile and inexpensive way are in high demand. This study finds that the microgrooved substrates stimulate rat adipose-derived mesenchymal stem cells (rASCs) to produce a larger quantity of sEVs than the flat substrates. Further investigation suggests that the ESCRT-III-associated protein Alix may be involved in mediating the elevated sEV production of rASCs on the microgrooved substrates. Besides, the cargo of sEVs is altered. SEVs secreted by rASCs on the microgrooved substrates carry higher levels of proangiogenic miRNAs and growth factors than those secreted by rASCs on the flat substrates. Functional assessments demonstrate that sEVs from rASCs on microgrooved substrates enhance the angiogenic properties of Human umbilical vein endothelial cells. The findings demonstrate that substrate topography is an effective regulator of the sEVs secretion by rASCs and highlight the potential of using microgrooved substrates as a platform to produce rASC-sEVs rich in pro-angiogenic factors.

Manipulating Mesenchymal Stem Cell Differentiation on Nanopattern Constructed through Cell-Mediated Mineralization

The extracellular matrix microenvironment, including chemical constituents and topological structure, plays key role in regulating the cell behavior, such as adhesion, proliferation, differentiation, apoptosis, etc. Until now, to investigate the relationship between surface texture and cell response, various ordered patterns have been prepared on the surface of different matrixes, whereas almost all these strategies depend on advanced instruments or severe synthesis conditions. Herein, cell-mediated mineralization method has been applied to construct nanopattern on the surface of β-TCP scaffold. The formation process, morphology, and composition of the final pattern were characterized, and a possible mineralization mechanism has been proposed. Moreover, the cell behavior on the nanopattern has been investigated, and the results showed that the mouse bone marrow mesenchyme stem cells (mBMSCs) display good affinity with the nanopattern, which was manifested by the good proliferation and osteogenic differentiation status of cells. The synthetic strategy may shed light to construct advanced topological structures on other matrixes for bone repair.

Topological structure of electrospun membrane regulates immune response, angiogenesis and bone regeneration

The fate of biomaterials is orchestrated by biocompatibility and bioregulation characteristics, reported to be closely related to topographical structures. For the purpose to investigate the topography of fibrous membranes on the guided bone regeneration performance, we successfully fabricated poly (lactate-co-glycolate)/fish collagen/nano-hydroxyapatite (PFCH) fibrous membranes with random, aligned and latticed topography by electrospinning. The physical, chemical and biological properties of the three topographical PFCH membranes were systematically investigated by in vitro and in vivo experiments. The subcutaneous implantation of C57BL6 mice showed an acceptable mild foreign body reaction of all three topological membranes. Interestingly, the latticed PFCH membrane exhibited superior abilities to recruit macrophage/monocyte and induce angiogenesis. We further investigated the osteogenesis of the three topographical PFCH membranes via the critical-size calvarial bone defect model of rats and mice and the results suggested that latticed PFCH membrane manifested promising performance to promote angiogenesis through upregulation of the HIF-1α signaling pathway; thereby enhancing bone regeneration. Our research illustrated that the topological structure of fibrous membranes, as one of the characteristics of biomaterials, could regulate its biological functions, and the fibrous structure of latticed topography could serve as a favorable surface design of biomaterials for bone regeneration. STATEMENT OF SIGNIFICANCE: In material-mediated regeneration medicine, the interaction between the biomaterial and the host is key to successful tissue regeneration. The micro-and nano-structure becomes one of the most critical physical clues for designing biomaterials. In this study, we fabricated three topological electrospun membranes (Random, Aligned and Latticed) to understand how topological structural clues mediate bone tissue regeneration. Interestingly, we found that the Latticed topographical PFCH membrane promotes macrophage recruitment, angiogenesis, and osteogenesis in vivo, indicating the fibrous structure of latticed topography could serve as a favorable surface design of biomaterials for bone regeneration.

Recent advances in PLGA-based biomaterials for bone tissue regeneration

Bone regeneration is an interdisciplinary complex lesson, including but not limited to materials science, biomechanics, immunology, and biology. Having witnessed impressive progress in the past decades in the development of bone substitutes; however, it must be said that the most suitable biomaterial for bone regeneration remains an area of intense debate. Since its discovery, poly (lactic-co-glycolic acid) (PLGA) has been widely used in bone tissue engineering due to its good biocompatibility and adjustable biodegradability. This review systematically covers the past and the most recent advances in developing PLGA-based bone regeneration materials. Taking the different application forms of PLGA-based materials as the starting point, we describe each form's specific application and its corresponding advantages and disadvantages with many examples. We focus on the progress of electrospun nanofibrous scaffolds, three-dimensional (3D) printed scaffolds, microspheres/nanoparticles, hydrogels, multiphasic scaffolds, and stents prepared by other traditional and emerging methods. Finally, we briefly discuss the current limitations and future directions of PLGA-based bone repair materials. STATEMENT OF SIGNIFICANCE: As a key synthetic biopolymer in bone tissue engineering application, the progress of PLGA-based bone substitute is impressive. In this review, we summarized the past and the most recent advances in the development of PLGA-based bone regeneration materials. According to the typical application forms and corresponding crafts of PLGA-based substitutes, we described the development of electrospinning nanofibrous scaffolds, 3D printed scaffolds, microspheres/nanoparticles, hydrogels, multiphasic scaffolds and scaffolds fabricated by other manufacturing process. Finally, we briefly discussed the current limitations and proposed the newly strategy for the design and fabrication of PLGA-based bone materials or devices.

Covalent immobilization of DJK-5 peptide on porous titanium for enhanced antibacterial effects and restrained inflammatory osteoclastogenesis

Currently, implant-related bone infection characterized by aggravated infection-induced inflammatory responses and osteolysis, remains a severe challenge in orthopedic surgery, especially in patients with osteoporosis. Attempts to control such responses using biomaterials with combined immunomodulatory and anti-bacterial properties may provide novel strategies. Herein, DJK-5, a class of host defense peptides (HDPs) with established antimicrobial and immunomodulatory functions, was introduced into porous Ti alloy. Our results indicated that the DJK-5 immobilized surfaces showed intrinsically multifunctional properties, including antibacterial ability, anti-inflammation, biocompatibility and osteolysis-inhibiting properties. The results demonstrated that the antibacterial efficiency of DJK-5 functionalized surfaces was over 90 % for both Gram-positive and Gram-negative bacteria. Specifically, DJK-5 functionalized samples also possessed the excellent anti-bacterial activity against a mixture of bacterial strains, including S. aureus, S. epidermidis and P. aeruginosa, with an antibacterial rate against mixed bacteria reaching 91.36 %, as well as reduced biofilm formation. The remarkable anti-bacterial efficacy was likely based on the direct anti-bacterial effect of DJK-5, which destroyed the integrity of bacteria membranes, leading to the leakage of intracellular materials. Additionally, the immobilized DJK-5 surfaces could indirectly kill bacteria through promoted macrophage capacity to bacteria uptake. Furthermore, DJK-5 functionalized surfaces suppressed inflammatory reaction by decreasing the release of pro-inflammatory factors and increasing the secretions of anti-inflammatory factors, and thereby impeded the activation of NF-κB signal pathway, which resulted in the disruption of the actin rings and decreased Tracp5b expressions. Based on these promising findings, the multi-functional DJK-5 immobilized titanium represents an efficient alternative to realize better osseointegration in sever implant-associated bacterial infections.

Osteon: Structure, Turnover, and Regeneration

Bone is composed of dense and solid cortical bone and honeycomb-like trabecular bone. Although cortical bone provides the majority of mechanical strength for a bone, there are few studies focusing on cortical bone repair or regeneration. Osteons (the Haversian system) form structural and functional units of cortical bone. In recent years, emerging evidences have shown that the osteon structure (including osteocytes, lamellae, lacunocanalicular network, and Haversian canals) plays critical roles in bone mechanics and turnover. Therefore, reconstruction of the osteon structure is crucial for cortical bone regeneration. This article provides a systematic summary of recent advances in osteons, including the structure, function, turnover, and regenerative strategies. First, the hierarchical structure of osteons is illustrated and the critical functions of osteons in bone dynamics are introduced. Next, the modeling and remodeling processes of osteons at a cellular level and the turnover of osteons in response to mechanical loading and aging are emphasized. Furthermore, several bioengineering approaches that were recently developed to recapitulate the osteon structure are highlighted. Impact statement This review provides a comprehensive summary of recent advances in osteons, especially the roles in bone formation, remodeling, and regeneration. Besides introducing the hierarchical structure and critical functions of osteons, we elucidate the modeling and remodeling of osteons at a cellular level. Specifically, we highlight the bioengineering approaches that were recently developed to mimic the hierarchical structure of osteons. We expect that this review will provide informative insights and attract increasing attentions in orthopedic community, shedding light on cortical bone regeneration in the future.

Tannic acid/Mg2+-based versatile coating to manipulate the osteoimmunomodulation of implants

Instead of directly stimulating osteogenesis, endowing an implant surface with a favourable osteoimmunomodulatory (OIM) function has emerged as a new effective strategy to enhance osteointegration. Though metal-phenolic coatings have demonstrated to possess an immunomodulatory function, their potential application in manipulating an osteoimmune response has not been well explored. Herein, in order to develop a simple, rapid and universal coating method to impart excellent OIM to hard tissue implants, tannic acid (TA) and Mg²⁺ were selected to form a coating on Ti plate based on metal-phenolic chemistry. Besides its virtues of simplicity, ultrafastness, low-cost, and versatility, another merit for the coating method is that it can easily combine the unique functions of metal ions and phenolic ligands. The chelated Mg²⁺ can not only activate macrophage polarization towards the anti-inflammatory phenotype but also directly stimulate the osteogenic differentiation of bone marrow-derived stem cells (BMSCs). TA motifs rendered the coating with an excellent reactive oxygen species (ROS) scavenging capacity. TA and Mg²⁺ showed synergistic effects on regulating macrophage biological behaviour, suppressing its polarization towards the M1 phenotype, and promoting its polarization towards the M2 phenotype. In vivo histological analysis also demonstrated that the TA/Mg²⁺ coating could effectively inhibit the host response. Finally, the formed osteoimmune environment obviously enhanced the osteogenic differentiation of BMSCs. The above results demonstrated that the designed TA/Mg²⁺ coating not only possessed the function of directly stimulating osteogenesis but also the function of manipulating OIM to a desired one. Hence, it has great potential to be applied on advanced hard tissue implants to enhance osteointegration.

Controlled Delivery of Growth Factor by Hierarchical Nanostructured Core-Shell Nanofibers for the Efficient Repair of Critical-Sized Rat Calvarial Defect

Electrospun nanofibers have received much attention as bone tissue-engineered scaffolds for their capacity to mimic the structure of natural extracellular matrix (ECM). Most studies have reproduced nanofibers with smooth surface for tissue engineering. This is quite different from the triple-helical nanotopography of natural collagen nanofibrils. In this study, hierarchical nanostructures were coated on the surface of drug-loaded core-shell nanofibers to mimic natural collagen nanofibrils. The nanoshish-kebab (SK) structure was decorated regularly on the surface of the nanofibers, and the inner-loaded bone morphogenetic protein 2 (BMP2) exhibited a gentle release pattern, similar to a zero-order release pattern in kinetics. The in vitro study also showed that the SK structure could accelerate cell proliferation, attachment, and osteogenic differentiation. Four groups of scaffolds were implanted in vivo to repair critical-sized rat calvarial defects: (1) PCL/PVA (control); (2) SK-PCL/PVA; (3) PCL/PVA-BMP2; and (4) SK-PCL/PVA-BMP2. Much more bone was formed in the SK-PCL/PVA group (24.57 ± 3.81%) than in the control group (1.21 ± 0.23%). The BMP2-loaded core-shell nanofibers with nanopatterned structure (SK-PCL/PVA-BMP2) displayed the best repair efficacy (76.38 ± 4.13%), followed by the PCL/PVA-BMP2 group (39.86 ± 5.74%). It was believed that the hierarchical nanostructured core-shell nanofibers could promote osteogeneration and that the SK structure showed synergistic ability with nanofiber-loaded BMP2 in vivo for bone regeneration. Thus, this BMP2-loaded core-shell nanofiber scaffold with hierarchical nanostructure holds great potential for bone tissue engineering applications.

Self-assembled structures of halloysite nanotubes: towards the development of high-performance biomedical materials

Halloysite nanotubes (HNTs), 1D natural tubular nanoparticles, exhibit a high aspect ratio, empty lumen, high adsorption ability, good biocompatibility, and high biosafety, which have attracted researchers' attention in applications of the biomedical area. HNTs can be readily dispersed in water due to their negatively charged surface and good hydrophilicity. The unique rod-like structure and surface properties give HNTs assembly ability into ordered hierarchical structures. In this review, the self-assembly approaches of HNTs including evaporation induced self-assembly by a "coffee-ring" mechanism, shear force induced self-assembly, and electric field force induced self-assembly were introduced. In addition, HNT self-assembly on polymeric substrates and biological substrates including hair, cells, and zebrafish embryos was discussed. These assembly processes are related to noncovalent interactions such as electrostatic, hydrogen bonding, and van der Waals forces or electron-transfer reactions. Moreover, the applications of self-assembled HNT patterns in biomedical areas such as capture of circulating tumor cells, guiding oriented cell growth, controlling cell germination, and delivery of drugs or nutrients were discussed and highlighted. Finally, challenges and future directions of assembly of HNTs were introduced. This review will inspire researchers in the design and fabrication of functional biodevices based on HNTs for tissue engineering, cancer diagnosis/therapy, and personal healthcare products.

Engineering topography: Effects on corneal cell behavior and integration into corneal tissue engineering

Cell-material interactions are important to tissue engineering. Inspired by the natural topographic structures on the extracellular matrix, a growing number of studies have integrated engineering topography into investigations of cell behavior on biomaterials. Engineering topography has a significant influence on cell behaviors. These cell-topography interactions play an important role in regenerative medicine and tissue engineering. Similarly, cell-topography interactions are important to corneal reconstruction and regeneration. In this review, we primarily summarized the effects of topographic cues on the behaviors of corneal cells, including cell morphology, adhesion, migration, and proliferation. Furthermore, the integration of engineering surface topography into corneal tissue engineering was also discussed.