Osteoimmunomodulation by bone implant materials: harnessing physicochemical properties and chemical composition

Chronic inflammation at bone defect sites can impede regenerative processes, but local immune responses can be adjusted to promote healing. Regulating the osteoimmune microenvironment, particularly through macrophage polarization, has become a key focus in bone regeneration research. While bone implants are crucial for addressing significant bone defects, they are often recognized by the immune system as foreign, triggering inflammation that leads to bone resorption and implant issues like fibrous encapsulation and aseptic loosening. Developing osteoimmunomodulatory implants offers a promising approach to transforming destructive inflammation into healing processes, enhancing implant integration and bone regeneration. This review explores strategies based on tuning the physicochemical attributes and chemical composition of materials in engineering osteoimmunomodulatory and pro-regenerative bone implants.

A dual-functional biodegradable composite coating fabricated on sulfonated PEEK via vacuum cold spraying: immunomodulation-driven osteointegration

Polyetheretherketone (PEEK) is a promising orthopedic implant alternative to metals due to its bone-mimetic modulus and biocompatibility; yet, its bioinert nature often triggers fibrous encapsulation and impedes osteointegration. Here, a biodegradable calcium silicate/β-tricalcium phosphate (CS/TCP) composite coating was fabricated on sulfonated PEEK (SP) via vacuum cold spraying to address these limitations. The CS/TCP coating exhibited robust bonding strength, enhanced hydrophilicity, and sustained release of Ca/Si ions, fostering apatite deposition in simulated body fluid. This bioactive interface promoted an immunomodulatory microenvironment by polarizing macrophages toward the anti-inflammatory M2 phenotype. Synergistically, ionic release and cytokine secretion enhanced MC3T3-E1 cell adhesion, proliferation, and osteogenic differentiation. In vivo, CS/TCP-SP reduced fibrous tissue thickness in a rat air-pouch model and improved bone-implant integration in rabbit cranial defects. The scalable coating strategy transforms inert PEEK into a bioactive, immunoregulatory implant, demonstrating potential to mitigate aseptic loosening and revision surgeries.

Kaempferol regulating macrophage foaming and atherosclerosis through Piezo1-mediated MAPK/NF-κB and Nrf2/HO-1 signaling pathway

INTRODUCTION

Antioxidants represented by kaempferol have been shown to be effective against atherosclerosis (AS). However, the underlying mechanisms still remain unclear.

OBJECTIVES

The aim of this research was to reveal the mechanism of kaempferol regarding the treatment of AS and accumulation of foam cell.

METHODS

We explored the contribution of kaempferol to the levels of inflammatory factors, scavenger receptor CD36, mitochondrial membrane potential, ROS, MAPK/NF-κB, Nrf2/HO-1, Ca2+ and Piezo1 levels in RAW264.7 macrophages exposed to ox-LDL. In addition, to explore whether kaempferol inhibits ox-LDL-induced foamy macrophage through Piezo1, we extracted macrophages from Piezo1 macrophage-specific knockout (Piezo1ΔLysM) mice. For further validation, ApoE-/- and Piezo1 macrophage-specific knockout mice (Piezo1ΔLysM/ ApoE-/-) were generated.

RESULTS

The results showed that kaempferol notably suppressed inflammatory response, CD36 expression, mitochondrial membrane potential elevation, ROS production, MAPK/NF-κB expression, Ca2+ influx, and increased Nrf2/HO-1 levels in RAW264.7. In addition, depletion of macrophage Piezo1 also effectively reduced lipid droplet deposition, inflammatory factor expression, oxidative damage, MAPK/NF-κB, Ca2+ influx, and increased Nrf2/HO-1 expression in mouse BMDMs, and the results were still consistent after kaempferol treatment. In vivo studies have shown that kaempferol significantly reduces atherosclerotic plaque formation. However, the beneficial effect of kaempferol was attenuated in Piezo1 depletion mice.

CONCLUSIONS

These results collectively provide compelling evidence that kaempferol regulates CD36-mediated mitochondrial ROS production by inhibiting the Piezo1 channels and Ca2+ influx, and then regulates the downstream pathways of NF-κB/MAPK and HO-1/Nrf2, inhibiting to the formation of foam cells. In conclusion, this study revealed a potential mechanism by which the natural antioxidant kaempferol prevents foamy macrophage.

A Surface-Mediated Biomimetic Porous Polyether-Ether-Ketone Scaffold for Regulating Immunity and Promoting Osteogenesis

The repair of critical-sized bone defects remains a major challenge for clinical orthopedic surgery. Here, we develop a surface biofunctionalized three-dimensional (3D) porous polyether-ether-ketone (PEEK) scaffold that can simultaneously promote osteogenesis and regulate macrophage polarization. The scaffold is created using polydopamine (PDA)-assisted immobilization of silk fibroin (SF) and the electrostatic self-assembly of nanocrystalline hydroxyapatite (nano-HA) on a 3D-printed porous PEEK scaffold. The SF/nano-HA functionalized surface provides a bone-like microenvironment for osteoblastic cells' adhesion, proliferation, mineralization and osteogenic differentiation. Moreover, the biofunctionalized surface can effectively drive macrophages polarization from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype. Integrin β1-specific cell-matrix binding and the activation of Ca2+ receptor-mediated signaling pathway play critical roles in the regulation of macrophage polarization. Compared with the as-printed scaffold, the SF/nano-HA functionalized porous PEEK scaffold induces minimal inflammatory response, enhanced angiogenesis, and substantial new bone formation, resulting in improved osseointegration in vivo. This study not only develops a promising candidate for bone repair but also demonstrates a facile surface biofunctionalization strategy for orthopedic implants to improve osseointegration by stimulating osteogenesis and regulating immunity.

Advances in Mussel Adhesion Proteins and Mussel-Inspired Material Electrospun Nanofibers for Their Application in Wound Repair

Mussel refers to a marine organism with strong adhesive properties, and it secretes mussel adhesion protein (MAP). The most vital feature of MAP is the abundance of the 3,4-dihydroxyphenylalanine (DOPA) group and lysine, which have antimicrobial, anti-inflammatory, antioxidant, and cell adhesion-promoting properties and can accelerate wound healing. Polydopamine (PDA) is currently the most widely used mussel-inspired material characterized by good adhesion, biocompatibility, and biodegradability. It can mediate various interactions to form functional coatings on cell-material surfaces. Nanofibers based on MAP and mussel-inspired materials have been exerting a vital role in wound repair, while there is no comprehensive review presenting them. This Review introduces the structure of MAPs and their adhesion mechanisms and mussel-inspired materials. Second, it introduces the functionalized modification of MAPs and their inspired materials in electrospun nanofibers and application in wound repair. Finally, the future development direction and coping strategies of MAP and mussel-inspired materials are discussed. Moreover, this Review can offer novel strategies for the application of nanofibers in wound repair and bring about new breakthroughs and innovations in tissue engineering and regenerative medicine.

Collagen patches releasing phosphatidylserine liposomes guide M1-to-M2 macrophage polarization and accelerate simultaneous bone and muscle healing

Bilateral communication between bones and muscles is essential for healing composite bone-muscle injuries from orthopedic surgeries and trauma. However, these injuries are often characterized by exaggerated inflammation, which can disrupt bone-muscle crosstalk, thereby seriously delaying the healing of either tissue. Existing approaches are largely effective at healing single tissues. However, simultaneous healing of multiple tissues remains challenging, with little research conducted to date. Here we introduce collagen patches that overcome this overlooked issue by harnessing the plasticity of macrophage phenotypes. Phosphatidylserine liposomes (PSLs) capable of shifting the macrophage phenotype from inflammatory M1 into anti-inflammatory/prohealing M2 were coated on collagen patches via a layer-by-layer method. Original collagen patches failed to improve tissue healing under inflammatory conditions coordinated by M1 macrophages. In contrast, PSL-coated collagen patches succeeded in accelerating bone and muscle healing by inducing a microenvironment dominated by M2 macrophages. In cell experiments, differentiation of preosteoblasts and myoblasts was completely inhibited by secretions of M1 macrophages but unaffected by those of M2 macrophages. RNA-seq analysis revealed that type I interferon and interleukin-6 signaling pathways were commonly upregulated in preosteoblasts and myoblasts upon stimulation with M1 macrophage secretions, thereby compromising their differentiation. This study demonstrates the benefit of PSL-mediated M1-to-M2 macrophage polarization for simultaneous bone and muscle healing, offering a potential strategy toward simultaneous regeneration of multiple tissues. STATEMENT OF SIGNIFICANCE: Existing approaches for tissue regeneration, which primarily utilize growth factors, have been largely effective at healing single tissues. However, simultaneous healing of multiple tissues remains challenging and has been little studied. Here we demonstrate that collagen patches releasing phosphatidylserine liposomes (PSLs) promote M1-to-M2 macrophage polarization and are effective for simultaneous healing of bone and muscle. Transcriptome analysis using next-generation sequencing reveals that differentiation of preosteoblasts and myoblasts is inhibited by the secretions of M1 macrophages but promoted by those of M2 macrophages, highlighting the importance of timely regulation of M1-to-M2 polarization in tissue regeneration. These findings provide new insight to tissue healing of multiple tissues.

Releasable, Immune-Instructive, Bioinspired Multilayer Coating Resists Implant-Induced Fibrosis while Accelerating Tissue Repair

Implantable biomaterials trigger foreign body reactions (FBRs), which reduces the functional life of medical devices and prevents effective tissue regeneration. Although existing therapeutic approaches can circumvent collagen-rich fibrotic encapsulation secondary to FBRs, they disrupt native tissue repair. Herein, a new surface engineering strategy in which an apoptotic-mimetic, immunomodulatory, phosphatidylserine liposome (PSL) is released from an implant coating to induce the formation of a macrophage phenotype that mitigates FBRs and improves tissue healing is described. PSL-multilayers constructed on implant surfaces via the layer-by-layer method release PSLs over a 1-month period. In rat muscles, poly(etheretherketone) (PEEK), a nondegradable polymer implant model, induces FBRs with dense fibrotic scarring under an aberrant cellular profile that recruits high levels of inflammatory infiltrates, foreign body giant cells (FBGCs), scar-forming myofibroblasts, and inflammatory M1-like macrophages but negligible amounts of anti-inflammatory M2-like phenotypes. However, the PSL-multilayer coating markedly diminishes these detrimental signatures by shifting the macrophage phenotype. Unlike other therapeutics, PSL-multilayered coatings also stimulate muscle regeneration. This study demonstrates that PSL-multilayered coatings are effective in eliminating FBRs and promoting regeneration, hence offering potent and broad applications for implantable biomaterials.

A novel method to fabricate monetite granules for bone graft applications

Monetite granules were reported to be able to balance osteoclastic resorption and new bone formation. However, to date, the dehydration of preset brushite has been the well-known method for preparing monetite granules. In the present study, for the first time, monetite granules could be prepared from the phase transformation of calcium sulfate dihydrate (CSD) granules through immersion in NaH2PO4 solution under hydrothermal conditions. CSD granules could be fully transformed into monetite granules at a reaction temperature of 125°C for 24 h. The obtained monetite granules were eight times more soluble in acetate buffer than in Tris-HCl buffer. Furthermore, monetite granules were two times more soluble in acetate buffer but comparable in Tris-HCl buffer compared to xenograft HA. The initial cytotoxicity test indicated that the novel monetite granules were nontoxic. In short, novel monetite granules were successfully prepared, exhibited better solubility in osteoclastic simulation than xenograft HA and were nontoxic.

Latest advances: Improving the anti-inflammatory and immunomodulatory properties of PEEK materials

Excellent biocompatibility, mechanical properties, chemical stability, and elastic modulus close to bone tissue make polyetheretherketone (PEEK) a promising orthopedic implant material. However, biological inertness has hindered the clinical applications of PEEK. The immune responses and inflammatory reactions after implantation would interfere with the osteogenic process. Eventually, the proliferation of fibrous tissue and the formation of fibrous capsules would result in a loose connection between PEEK and bone, leading to implantation failure. Previous studies focused on improving the osteogenic properties and antibacterial ability of PEEK with various modification techniques. However, few studies have been conducted on the immunomodulatory capacity of PEEK. New clinical applications and advances in processing technology, research, and reports on the immunomodulatory capacity of PEEK have received increasing attention in recent years. Researchers have designed numerous modification techniques, including drug delivery systems, surface chemical modifications, and surface porous treatments, to modulate the post-implantation immune response to address the regulatory factors of the mechanism. These studies provide essential ideas and technical preconditions for the development and research of the next generation of PEEK biological implant materials. This paper summarizes the mechanism by which the immune response after PEEK implantation leads to fibrous capsule formation; it also focuses on modification techniques to improve the anti-inflammatory and immunomodulatory abilities of PEEK. We also discuss the limitations of the existing modification techniques and present the corresponding future perspectives.

Composite microgranular scaffolds with surface modifications for improved initial osteoblastic cell proliferation

Polyester-based granular scaffolds are a potent material for tissue engineering due to their porosity, controllable pore size, and potential to be molded into various shapes. Additionally, they can be produced as composite materials, e.g., mixed with osteoconductive β-tricalcium phosphate or hydroxyapatite. Such polymer-based composite materials often happen to be hydrophobic, which disrupts cell attachment and decreases cell growth on the scaffold, undermining its primary function. In this work, we propose the experimental comparison of three modification techniques for granular scaffolds to increase their hydrophilicity and cell attachment. Those techniques include atmospheric plasma treatment, polydopamine coating, and polynorepinephrine coating. Composite polymer/β-tricalcium phosphate granules have been produced in a solution-induced phase separation (SIPS) process using commercially available biomedical polymers: poly(lactic acid), poly(lactic-co-glycolic acid), and polycaprolactone. We used thermal assembly to prepare cylindrical scaffolds from composite microgranules. Atmospheric plasma treatment, polydopamine coating, and polynorepinephrine coating showed similar effects on polymer composites' hydrophilic and bioactive properties. All modifications significantly increased human osteosarcoma MG-63 cell adhesion and proliferation in vitro compared to cells cultured on unmodified materials. In the case of polycaprolactone/β-tricalcium phosphate scaffolds, modifications were the most necessary, as unmodified polycaprolactone-based material disrupted the cell attachment. Modified polylactide/β-tricalcium phosphate scaffold supported excellent cell growth and showed ultimate compressive strength exceeding this of human trabecular bone. This suggests that all investigated modification techniques can be used interchangeably for increasing wettability and cell attachment properties of various scaffolds for medical applications, especially those with high surface and volumetric porosity, like granular scaffolds.

Biomimetic Strategy to Enhance Epithelial Cell Viability and Spreading on PEEK Implants

Polyetheretherketone (PEEK) is a biocompatible material widely used in spinal and craniofacial implants, with potential use in percutaneous implants. However, its inertness prevents it from forming a tight seal with the surrounding soft tissue, which can lead to infections and implant failure. Conversely, the surface chemistry of percutaneous organs (i.e., teeth) helps establish a strong interaction with the epithelial cells of the contacting soft tissues, and hence a tight seal, preventing infection. The seal is created by adsorption of basement membrane (BM) proteins, secreted by epithelial cells, onto the percutaneous organ surfaces. Here, we aim to create a tight seal between PEEK and epithelial tissues by mimicking the surface chemistry of teeth. Our hypothesis is that collagen I, the most abundant tooth protein, enables integration between the epithelial tissue and teeth by promoting adsorption of BM proteins. To test this, we immobilized collagen I via EDC/NHS coupling on a carboxylated PEEK surface modified using diazonium chemistry. We used titanium alloy (Ti-6Al-4V) for comparison, as titanium is the most widely used percutaneous biomaterial. Both collagen-modified PEEK and titanium showed a larger adsorption of key BM proteins (laminin, nidogen, and fibronectin) compared to controls. Keratinocyte epithelial cell viability on collagen-modified PEEK was twice that of control PEEK and ∼1.5 times that of control titanium after 3 days of cell seeding. Both keratinocytes and fibroblasts spread more on collagen-modified PEEK and titanium compared to controls. This work introduces a versatile and biomimetic surface modification technique that may enhance PEEK-epithelial tissue sealing with the potential of extending PEEK applications to percutaneous implants, making it competitive with titanium.

Phosphatidylserine liposome multilayers mediate the M1-to-M2 macrophage polarization to enhance bone tissue regeneration

An appropriate immune microenvironment, governed by macrophages, is essential for rapid tissue regeneration after biomaterial implantation. The macrophage phenotypes, M1 (inflammatory) and M2 (anti-inflammatory/healing), exert opposing effects on the repair of various tissues. In this study, a new strategy to promote tissue repair and tissue-to-biomaterial integration by M1-to-M2 macrophage transition using artificial apoptotic cell mimetics (phosphatidylserine liposomes; PSLs) was developed using bone as a model tissue. Titanium was also selected as a model substrate material because it is widely used for dental and orthopedic implants. Titanium implants were functionalized with multilayers via layer-by-layer assembly of cationic protamine and negatively charged PSLs that were chemically stabilized to prevent disruption of lipid bilayers. Samples carrying PSL multilayers could drive M1-type macrophages into M2-biased phenotypes, resulting in a dramatic change in macrophage secretion for tissue regeneration. In a rat femur implantation model, the PSL-multilayer-coated implant displayed augmented de novo bone formation and bone-to-implant integration, associated with an increased M1-to-M2-like phenotypic transition. This triggered the proper generation and activation of bone-forming osteoblasts and bone-resorbing osteoclasts relative to their uncoated counterparts. This study demonstrates the benefit of local M1-to-M2 macrophage polarization induced by PSL-multilayers constructed on implants for potent bone regeneration and bone-to-implant integration. The results of this study may help in the design of new immunomodulatory biomaterials. STATEMENT OF SIGNIFICANCE: Effective strategies for tissue regeneration are essential in the clinical practice. The macrophage phenotypes, M1 (inflammatory) and M2 (anti-inflammatory/healing), exert opposing effects on the repair of various tissues. Artificially produced phosphatidylserine-containing liposomes (PSLs) can induce M2 macrophage polarization by mimicking the inverted plasma membranes of apoptotic cells. This study demonstrates the advantages of local M1-to-M2 macrophage polarization induced by PSL-multilayers constructed on implants for effective bone regeneration and osseointegration (bone-to-implant integration). Mechanistically, M2 macrophages promote osteogenesis but inhibit osteoclastogenesis, and M1 macrophages vice versa. We believe that our study makes a significant contribution to the design of new immunomodulatory biomaterials for regenerative medicine because it is the first to validate the benefit of PSLs for tissue regeneration.

Recent developments in polynorepinephrine: an innovative material for bioinspired coatings and colloids

While applications of polydopamine (PDA) are exponentially growing, research concerning the closely related neurotransmitter derivative polynorepinephrine (PNE) is in paucity, even though norepinephrine shares dopamine's ability to self-polymerize and form a coating film that is nearly substrate-agnostic. In this review, we demonstrate that PNE can be used as an alternative to PDA with equal or ever superior performance. PNE offers a thinner and smoother coating surface and thus is capable of more effectively resisting fouling by biofoulants, enhancing cell adhesion capability, surface hydrophilicity and biomolecule immobilisation. With the abundance of catechol, amino and hydroxyl groups in PNE's structure, PNE can perform as an electron donor and receiver at the same time and initiate ring opening and redox reactions. It has also been shown that PNE has the potential to be used as a biosensor due to its bioconjugation and molecular recognition ability. Here, we summarise the applications of PNE to date and discuss its potential research directions in the near future.

Treatment methods toward improving the anti-infection ability of poly(etheretherketone) implants for medical applications

Due to its favorable chemical stability, biocompatibility, and mechanical properties, Poly(etheretherketone) (PEEK) is a promising material for repairing bone and dental hard tissue defects. However, there are critical disadvantages: PEEK is biologically and chemically inert, which influences osseointegration of implants and bonding strength of prostheses, and its mechanical properties still cannot meet the requirements for some medical applications. Furthermore, bacterial infections and inflammatory reactions often accompany bone defects caused by trauma or inflammation or teeth loss caused by periodontitis. Previous studies mainly focused on enhancing PEEK's bioactivity and mechanical performance, but PEEK also lacks effective anti-infection ability. Thus, it is necessary to improve its anti-infection ability, and this is considered in this paper from two aspects. The first is to inhibit the attachment and growth of bacteria on the material, and the second is to endow the material with immunoregulatory ability, which means mobilizing the host immune system to protect tissue from inflammation. In this review, we analyze and discuss the existing treatment methods to improve the antibacterial and immunomodulatory abilities of PEEK addressing their limitations, relevant future challenges, and required research efforts.

CaP-based anti-inflammatory HIF-1α siRNA-encapsulating nanoparticle for rheumatoid arthritis therapy

Rheumatoid arthritis (RA) is a common inflammatory disease and its treatment is largely limited by drug ineffectiveness or severe side effects. In RA progression, multiple signalling pathways, such as hypoxia-inducible factor (HIF)-1α, nuclear factor kappa B (NF-κB), and mitogen-activated protein kinase (MAPK) pathways, act synergistically to maintain the inflammatory response. To downregulate HIF-1α, NF-κB, and MAPK expression, we proposed HIF-1α siRNA-loaded calcium phosphate nanoparticles encapsulated in apolipoprotein E3-reconstituted high-density lipoprotein (HIF-CaP-rHDL) for RA therapy. Here, we evaluated the potential of CaP-rHDL nanoparticles in RA therapy using a murine macrophage line (RAW 264.7) and a collagen-induced arthritis (CIA) mouse model. The CaP-rHDL nanoparticles showed significant anti-inflammatory effects along with HIF-1α knockdown and NF-κB and MAPK signalling pathway inhibition in lipopolysaccharide-activated macrophages. Moreover, they inhibited receptor activator of NF-κB ligand (RANKL)-induced osteoclast formation. In CIA mice, their intravenous administration resulted in high accumulation at the arthritic joint sites, and HIF-CaP-rHDL effectively suppressed inflammatory cytokine secretion and relieved bone erosion, cartilage damage, and osteoclastogenesis. Thus, HIF-CaP-rHDL demonstrated great potential in RA precision therapy by inhibiting multiple inflammatory signalling pathways.

Modulating immune microenvironment during bone repair using biomaterials: Focusing on the role of macrophages

Bone is a self-regenerative tissue that can repair small defects and fractures. In large defects, bone tissue is unable to provide nutrients and oxygen for repair, and autologous grafting is used as the gold standard. As an alternative method, the bone tissue regeneration approach uses osteoconductive biomaterials to overcome bone graft disadvantages. However, biomaterials are considered as foreign components that can stimulate host immune responses. Although traditional principles have been aimed to minimize immune reactions, the design of biomaterials has steadily shifted toward creating an immunomodulatory microenvironment to harness immune cells and responses to repair damaged tissue. Among immune cells, macrophages secrete various immunomodulatory mediators and crosstalk with bone-forming cells and play key roles in bone tissue engineering. Macrophage polarization toward M1 and M2 subtypes mediate pro-inflammatory and anti-inflammatory responses, respectively, which are crucial for bone repairing at different stages. This review provides an overview of the crosstalk between various immune cells and biomaterials, macrophage polarization, and the effect of physicochemical properties of biomaterials on the immune responses, especially macrophages, in bone tissue engineering.

Modification of polyetheretherketone implants: From enhancing bone integration to enabling multi-modal therapeutics

Polyetheretherketone (PEEK) is a popular thermoplastic material widely used in engineering applications due to its favorable mechanical properties and stability at high temperatures. With the first implantable grade PEEK being commercialized in 1990s, the use of PEEK has since grown exponentially in the biomedical field and has rapidly transformed a large section of the medical devices landscape. Nowadays, PEEK is a standard biomaterial used across a wide range of implant applications, however, its bioinertness remains a limitation for bone repair applications. The increasing demand for enhanced treatment efficacy/improved patient quality of life, calls for next-generation implants that can offer fast bone integration as well as other desirable therapeutic functions. As such, modification of PEEK implants has progressively shifted from offering desirable mechanical properties, enhancing bioactivity/fast osteointegration, to more recently, tackling post-surgery bacterial infection/biofilm formation, modulation of inflammation and management of bone cancers. Such progress is also accompanied by the evolution of the PEEK manufacturing technologies, to meet the ever increasing demand for more patient specific devices. However, no review has comprehensively covered the recently engaged application areas to date. This paper provides an up-to-date review on the development of PEEK-based biomedical devices in the past 10 years, with particularly focus on modifying PEEK for multi-modal therapeutics. The aim is to provide the peers with a timely update, which may guide and inspire the research and development of next generation PEEK-based healthcare products. STATEMENT OF SIGNIFICANCE: Significant progress has been made in PEEK processing and modification techniques in the past decades, which greatly contributed to its wide applications in the biomedical field. Despite the high volume of published literature on PEEK implant related research, there is a lack of review on its emerging applications in multi-modal therapeutics, which involve bone regeneration, anti-bacteria/anti-inflammation, and cancer inhibition, etc. This timely review covers the state-of-the-art in these exciting areas and provides the important guidance for next generation PEEK based biomedical device research and development.

Influences of niobium pentoxide on roughness, hydrophilicity, surface energy and protein absorption, and cellular responses to PEEK based composites for orthopedic applications

To improve the bio-performances of polyetheretherketone (PEEK) for orthopedic applications, submicro-particles of niobium pentoxide (Nb2O5) were synthesized using a sol-gel method, and PEEK/Nb2O5 composites (PNC) with a Nb2O5 content of 25v% (PNC25) and 50v% (PNC50) were fabricated by utilizing a process of pressing-sintering. The results showed that the Nb2O5 particles were not only dispersed in the composites but also exposed on the surface of the composites, which formed submicro-structural surfaces. In addition, the hydrophilicity, surface energy, surface roughness and absorption of proteins of the composites were improved with increasing Nb2O5 content. Moreover, the release of Nb ions with the highest concentration of 5.01 × 10-6 mol L-1 from the composite into the medium displayed no adverse effects on cell proliferation and morphology, indicating no cytotoxicity. Furthermore, compared with PEEK, the composites, especially PNC50, obviously stimulated adhesion and proliferation as well as osteogenic differentiation of bone mesenchymal stem cells of rats. The results suggested that the incorporation of Nb2O5 submicro-particles into PEEK produced novel bioactive composites with improved surface properties, which played important roles in regulating cell behaviors. In conclusion, the composites, especially PNC50 with good cytocompatibility and promotion of cellular responses, exhibited great potential as implantable materials for bone repair.

A hierarchical nanostructural coating of amorphous silicon nitride on polyetheretherketone with antibacterial activity and promoting responses of rBMSCs for orthopedic applications

Silicon nitride (SN) with good osteoconductivity has been introduced as an implantable biomaterial for joint replacement and interbody fusion devices. In this study, SN was coated on a polyetheretherketone (PEEK) surface by inductively coupled plasma-enhanced chemical vapor deposition (ICPECVD). The results showed that a dense coating (thickness of about 500 nm) of amorphous SN was closely combined with a PEEK substrate (PKSN) with a binding strength of 6.88 N. In addition, the coating surface showed hierarchical nanostructures containing many spherical bulges (sizes about 150 nm), which were composed of many small humps (sizes about 10 nm). Moreover, the roughness, hydrophilicity, surface energy, surface charge and adsorption of bovine serum albumin (BSA) of PKSN were obviously higher than those of PEEK. After immersion into simulated body fluid (SBF), the Si ions were gradually released from PKSN into SBF and a weak alkaline environment was created. Antibacterial experiments showed that PKSN exhibited a greater antibacterial activity than that of PEEK. Moreover, compared with PEEK, PKSN significantly promoted adhesion, proliferation, differentiation and expression of osteogenic related genes of the rat bone marrow stromal cells (rBMSCs). In conclusion, the SN coating of PKSN with hierarchical nanostructures exhibited excellent antibacterial activity and cytocompatibility, which would make it a great candidate for orthopedic applications.

Surface Modifications of Poly(Ether Ether Ketone) via Polymerization Methods-Current Status and Future Prospects

Surface modification of poly(ether ether ketone) (PEEK) aimed at applying it as a bone implant material aroused the unflagging interest of the research community. In view of the development of implantology and the growing demand for new biomaterials, increasing biocompatibility and improving osseointegration are becoming the primary goals of PEEK surface modifications. The main aim of this review is to summarize the use of polymerization methods and various monomers applied for surface modification of PEEK to increase its bioactivity, which is a critical factor for successful applications of biomedical materials. In addition, the future directions of PEEK surface modifications are suggested, pointing to low-ppm surface-initiated atom transfer radical polymerization (SI-ATRP) as a method with unexplored capacity for flat surface modifications.

Enhanced Osseointegration Capability of Poly(ether ether ketone) via Combined Phosphate and Calcium Surface-Functionalization

Biomedical applications of poly(ether ether ketone) (PEEK) are hindered by its inherent bioinertness and lack of osseointegration capability. In the present study, to enhance osteogenic activity and, hence, the osseointegration capability of PEEK, we proposed a strategy of combined phosphate and calcium surface-functionalization, in which ozone-gas treatment and wet chemistry were used for introduction of hydroxyl groups and modification of phosphate and/or calcium, respectively. Surface functionalization significantly elevated the surface hydrophilicity without changing the surface roughness or topography. The cell study demonstrated that immobilization of phosphate or calcium increased the osteogenesis of rat mesenchymal stem cells compared with bare PEEK, including cell proliferation, alkaline phosphatase activity, and bone-like nodule formation. Interestingly, further enhancement was observed for samples co-immobilized with phosphate and calcium. Furthermore, in the animal study, phosphate and calcium co-functionalized PEEK demonstrated significantly enhanced osseointegration, as revealed by a greater direct bone-to-implant contact ratio and bond strength between the bone and implant than unfunctionalized and phosphate-functionalized PEEK, which paves the way for the orthopedic and dental application of PEEK.

Synergistic effect of surface phosphorylation and micro-roughness on enhanced osseointegration ability of poly(ether ether ketone) in the rabbit tibia

This study was aimed to investigate the osseointegration ability of poly(ether ether ketone) (PEEK) implants with modified surface roughness and/or surface chemistry. The roughened surface was prepared by a sandblast method, and the phosphate groups on the substrates were modified by a two-step chemical reaction. The in vitro osteogenic activity of rat mesenchymal stem cells (MSCs) on the developed substrates was assessed by measuring cell proliferation, alkaline phosphatase activity, osteocalcin expression, and bone-like nodule formation. Surface roughening alone did not improve MSC responses. However, phosphorylation of smooth substrates increased cell responses, which were further elevated in combination with surface roughening. Moreover, in a rabbit tibia implantation model, this combined surface modification significantly enhanced the bone-to-implant contact ratio and corresponding bone-to-implant bonding strength at 4 and 8 weeks post-implantation, whereas modification of surface roughness or surface chemistry alone did not. This study demonstrates that combination of surface roughness and chemical modification on PEEK significantly promotes cell responses and osseointegration ability in a synergistic manner both in vitro and in vivo. Therefore, this is a simple and promising technique for improving the poor osseointegration ability of PEEK-based orthopedic/dental implants.

Laser-assisted wet coating of calcium phosphate for surface-functionalization of PEEK

Calcium phosphate (CaP) coating is an effective method for surface-functionalization of bioinert materials and for production of osteoconductive implants. Recently, we developed a laser-assisted biomimetic process (LAB process) for facile and area-specific CaP coating. In this study, the LAB process was applied to chemically stable and mechanically durable poly(etheretherketone) (PEEK), which has become widely used as an orthopedic and dental implant material. The LAB process was carried out by irradiating pulsed Nd:YAG laser light (355 nm) onto a PEEK substrate that was immersed in supersaturated CaP solution. The CaP coating applicability depended on laser fluence, i.e., CaP successfully formed on PEEK surface after the LAB process at 2 W/cm2. Further increase in laser fluence did not result in the successful formation. At the optimal fluence of 2 W/cm2, the laser-irradiated PEEK surface was modified and heated to induce heterogeneous CaP precipitation within 10 min in CaP solution, followed by further CaP growth over the irradiation time (tested up to 30 min). The LAB process improved the cytocompatibility of PEEK surface with osteoblastic MC3T3-E1 cells. Furthermore, the LAB-processed CaP-coated PEEK substrate formed a dense hydroxyapatite layer on its surface in the simulated body fluid, suggesting the osteoconductivity of this material. The present LAB process can be a useful new tool to produce osteoconductive PEEK-based implants.

Antibacterial activity and osseointegration of silver-coated poly(ether ether ketone) prepared using the polydopamine-assisted deposition technique

PEEK-PDA-Ag substrates may be a promising orthopaedic implant material due to the outstanding biocompatibility and antibacterial properties.

A superhydrophilic titanium implant functionalized by ozone gas modulates bone marrow cell and macrophage responses

Bone-forming cells and Mϕ play key roles in bone tissue repair. In this study, we prepared a superhydrophilic titanium implant functionalized by ozone gas to modulate osteoconductivity and inhibit inflammatory response towards titanium implants. After 24 h of ozone gas treatment, the water contact angle of the titanium surface became zero. XPS analysis revealed that hydroxyl groups were greatly increased, but carbon contaminants were largely decreased 24 h after ozone gas functionalization. Also, ozone gas functionalization did not alter titanium surface topography. Superhydrophilic titanium (O3-Ti) largely increased the aspect ratio, size and perimeter of cells when compared with untreated titanium (unTi). In addition, O3-Ti facilitated rat bone marrow derived MSCs differentiation and mineralization evidenced by greater ALP activity and bone-like nodule formation. Interestingly, O3-Ti did not affect RAW264.7 Mϕ proliferation. However, naive RAW264.7 Mϕ cultured on unTi produced a two-fold larger amount of TNFα than that on O3-Ti. Furthermore, O3-Ti greatly mitigated proinflammatory cytokine production, including TNFα and IL-6 from LSP-stimulated RAW264.7 Mϕ. These results demonstrated that a superhydrophilic titanium prepared by simple ozone gas functionalization successfully increased MSCs proliferation and differentiation, and mitigated proinflammatory cytokine production from both naive and LPS-stimulated Mϕ. This superhydrophilic surface would be useful as an endosseous implantable biomaterials and as a biomaterial for implantation into other tissues.