Polyetheretherketone (PEEK) is a high-performance polymer that has been widely employed in orthopedic implants since the late 1990s due to its excellent mechanical strength, chemical stability, biocompatibility, and an elastic modulus comparable to that of natural bone (Zheng et al., 2022; Chen X. et al., 2022). PEEK has demonstrated promising clinical potential in various orthopedic procedures, including fracture fixation, bone defect reconstruction, spinal fusion, and joint replacement, particularly in load-bearing applications where biomechanical compatibility and long-term stability are essential. Compared to conventional metallic or ceramic implant materials, PEEK exhibits a more bone-like elastic modulus, effectively minimizing stress shielding effects and thereby reducing the risks of implant loosening and bone resorption. In addition, PEEK possesses favorable radiolucency (X-ray transparency), which facilitates postoperative follow-up and clinical imaging assessments (Kim et al., 2020; Vanaei et al., 2021). Although it has many advantages above, the widespread clinical use of PEEK is restricted by its hydrophobicity, poor osseointegration and microbial infection susceptibility which pose a significant risk of implant failure (Zheng et al., 2022; Sacks et al., 2024). Giving PEEK implants biological activity to address these drawbacks, including better cell binding, increased osteogenic potential, and even anti-infection properties to thwart the spread of harmful bacteria, have a significant clinical impact on the long-term success of implant surgery (He et al., 2023).

In-depth study has found that immune and inflammatory responses following implantation can obstruct the osteogenesis process, therefore, the concept of osteoimmunology was proposed (Yu et al., 2022; Zhang et al., 2023). When PEEK is implanted, it causes a fibrous capsule to form, triggers the body’s immune system to produce a foreign body reaction (FBR), and prevents PEEK from adhering to bone tissue, which is a multifactorial immune regulation process that involves several cells and cytokines (Zhang et al., 2023). Macrophages are a crucial element of the immune system. They become activated when they participate in inflammatory reactions in local tissue and are categorised as M1 and M2 macrophages according to their polarisation characteristic (Mantovani et al., 2004; Zhao et al., 2024). One factor thought to influence FBR is macrophage polarisation, which is the result of an interplay between M1 and M2 macrophage phenotypes (Zhang et al., 2023). M1 macrophages have a pro-inflammatory phenotype and can produce cytokines, TNF-α, and IL-6, among other harmful reactive oxygen intermediates, which enhancing inflammatory response and suppressing tissue repair (Fontana et al., 2024; Wang T. et al., 2022). On the contrary, M2 macrophages present an anti-inflammatory phenotype and producing bone morphogenetic protein (BMP), VEGF, and TGF-β to reduce inflammation reaction and promoting tissue repair (Zhang et al., 2023; Wang T. et al., 2022). Even though M1 and M2 macrophages are usually thought of as separate phenotypes, alterations brought about by implants may cause both M1 and M2 markers to express simultaneously in the local microenvironment (Zhang et al., 2023; Yang C. et al., 2024). Consequently, “smart” bone repair materials with immunoregulatory effects ought to be able to control macrophages’ transition to the M1/M2 phenotype and promote the regeneration of bone tissue.

To enhance the osseointegration performance of polyetheretherketone (PEEK), various effective strategies have been proposed, including the incorporation of metal ions, pharmaceuticals, bioactive proteins, or functional peptides. These modifications aim to improve the bioactivity of PEEK surfaces and modulate the local immune microenvironment, thereby promoting the polarization of macrophages toward the M2 phenotype, which is associated with tissue repair. Such immunoregulatory effects play a critical role in facilitating bone regeneration and improving the integration of the implant with surrounding osseous tissue (Zhang et al., 2023; Xu et al., 2019; Du et al., 2024). Compared to other bioactive metal ions in repairing bone tissue such as magnesium (Mg), calcium (Ca), silicon (Si), strontium (Sr) getting more and more attention, which may be a more suitable candidate for “immune osteogenesis” for the following reasons: (1) strontium salt (strontium) as a commercial anti-osteoporosis drug is FDA-approved clinical drug for osteoporosis therapy, which has been reported to have modulate bone metabolism effects via promoting osteoblastogenesis and inhibiting osteoclastogenesis (Jacobs et al., 2020; Sun Y. et al., 2021). (2) Sr²⁺ can control the immunological milieu in bone tissue, polarize macrophages towards M2, and upregulate the expression of molecules including vascular endothelial-derived growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2) to facilitate both bone tissue and vascular regeneration, thus forming an osteogenic microenvironment conducive to bone tissue regeneration (Wang et al., 2025; Mo et al., 2023; Wang L. et al., 2023). (3) According to pertinent studies, strontium can block the NF-κB signalling pathway, which lowers the expression of proinflammatory factors including IL-1β and TNF-α, thus reducing population of M1 macrophages and preventing the inflammatory response from progressing (Wang L. et al., 2023; Huang et al., 2020). Overall, strontium can potentially promote bone tissue repair through both ways of direct osteogenesis and immune osteogenesis. Therefore, the two effects of “direct osteogenesis” and “immune osteogenesis” that strontium ions have at the same time can form a synergistic effect on PEEK materials to achieve the best osteogenesis effect.

It has significant clinical implications to preparing bone implants with antimicrobial properties to prevent and treat implant-associated infections (IAI) (Fu et al., 2021). At present, administering high concentrations of antibiotics locally over an extended period during IAI treatment results in both bacterial resistance to antibiotics and cytotoxic damage to bone tissue, which ultimately hinders the healing process of the damaged bone tissue (Shan et al., 2023; Gao and Ma, 2022; Wu et al., 2023). To treat IAI more effectively, it is critical to develop antimicrobial substances with good biocompatibility in replace of antibiotics. AMPs, or antimicrobial peptides, are an element of the biological innate immune system, which shows strong antibacterial activity against a variety of harmful bacteria, biofilms, and prevents bacteria from developing drug resistance (Wu et al., 2023; Lan et al., 2023). The antimicrobial peptide known as porcine cathelicidin PMAP-36, which was initially identified in 1994 and has a proline-induced hinge region and 36 amino acids, is strongly cationic (13⁺), α-helical, and amphipathic (Storici et al., 1994). Based on porcine antimicrobial peptide PMAP-36, monomeric and dimeric forms of PMAP-36 were chemically synthesized. Relative to dimeric forms of PMAP-36, monomeric forms of antimicrobial peptide PMAP-36, which displayed good antibacterial activity and less cytotoxicity (Scocchi et al., 2005). Because of their poor absorption and distribution, which results in limited bioavailability, AMPs are typically unstable in the gastrointestinal tract and other bodily fluids when administered systemically (Costa et al., 2019). Therefore, AMPs were immobilized on the surface of the biomaterials for prevent and treat IAI is a feasible and effective application strategy (Wu et al., 2023).

In recent years, mussel adhesion proteins have drawn a lot of interest because of their capacity to rapidly solidify in humid environments and form strong adhesion interfaces on the surfaces of various substrate materials (Lee et al., 2024; Jia et al., 2019). Further study revealed that mussel foot gland cells could secrete a super powerful slime, which mainly consisted of Mytilus edulis foot proteins (Mefps), which were rich in levodopa (3, 4-dihydroxy-L-phenylalanine, L-DOPA). Catechol in the structure of L-DOPA can be oxidized to form quinone, which in turn causes its own cross-linking, and these components can explain the observed adhesion properties (Lin et al., 2007). Dopamine has become a new coating material because of its molecular structure like 3, 4-dihydroxy-phenylalanine. Dopamine forms a polydopamine (PDA) coating through oxidative self-polymerization under weakly alkaline conditions. Currently, the PDA biomimetic coating has been successfully used to modify the surface of biological materials (Jia et al., 2019). Almost any kind of inorganic or organic material can have the PDA coating applied to its surface because of its long-lasting stability and controlled coating thickness (Lee et al., 2024). At the same time, The PDA coating can be used to chelating metal ions and covalently bind biomolecular without the need for extra catalysts or hazardous reagents because of its unique chemical structure and exceptional biocompatibility. It contains numerous functional groups, including as catechols, amines, and imines (Wang L. et al., 2023; Jia et al., 2019; Qin et al., 2021). In addition, the evidence supporting PDA coating’s ability to enhance cell adhesion, distribution, proliferation, and differentiation is mounting (Wang L. et al., 2023; Wang et al., 2019). Due to these outstanding properties such as simplicity, mildness and environmental friendliness, PDA biomimetic chemical modification technology has been widely used to regulate the reaction of cells and tissues to biomaterials and has shown good application prospects in the fields of surface coating and molecular imprinting (Jia et al., 2019; Qin et al., 2021; Wang et al., 2019).

In current study, we design multi-functional PEEK orthopedic implants with immunomodulatory, osteogenicity as well as antibacterial properties. The hydrophilic feature of the PEEK surface is achieved through PDA chemical treatment, which is further chelating Sr ion and covalently immobilize monomeric forms of antimicrobial peptide PMAP-36 to fabricate the multifunctional PEEK implants (PEEK-PDA-Sr/AMP). To assess PEEK-PDA-Sr/AMP’s possible immunomodulatory capacity as well as macrophages’ function in osteogenesis, mouse bone marrow macrophages (BMMs) was chosen as immune cells for in vitro investigation (Chen Q. et al., 2022). Then, investigations were conducted on the cooperative impact of immunoactive Sr²⁺ on macrophage polarisation and the in vitro osteogenesis effect. Next, the antibacterial qualities of the PEEK-PDA-Sr/AMP in vitro were further systematically studied. Finally, using a rat femoral implant-related osteomyelitis model, the multifunctionality of the PEEK-PDA-Sr/AMP implant was examined using macrophage polarisation, antibacterial characteristics, and interfacial osteointegration (Figure 1). To the best of our knowledge, this is the first investigation on PEEK implants that have been co-modified with strontium and AMP with multifunction. In addition, this work may offer a potential strategy for surface bioengineering of other inert biomaterials, particularly the need to create multi-functional surfaces for diverse clinical requirements.

In orthopedic surgery, biomaterials are frequently utilized as bone implant materials to treat patients with injuries, tumors, infections, osteonecrosis, and other pathological conditions. Good osseous integration, or the capacity of implant materials to adhere to the surrounding new bone tissue without producing fibrous tissue, is the most important component for the long-term durability of orthopedic implants in vivo (Torstrick et al., 2018; Chen et al., 2023). Due to PEEK is a bioinert material, fiber cysts formed upon implantation prevent the implant from making direct touch with the surrounding bone tissue, which eventually causes the implant to loosen or perhaps collapse (Chen et al., 2023; Yuan et al., 2018). Postoperative implant-associated infections (IAI) are another serious complication which make bone integration process more complex, treatment IAI often includes antibiotic system administration, debridement surgery, and implant removal, which has negative effects on the operative effect and patient’s quality of life (Heras et al., 2020). Based on modelling data, it is predicted that by 2030, the prevalence of prosthetic infections resulting from hip and knee replacements will surpass 6%. This growth can be attributed to various variables, including an ageing population, growing surgical demand, and the obesity pandemic (Heras et al., 2020; Peel, 2019). Prior research mostly concentrated on enhancing PEEK’s osteogenic qualities and antibacterial potential using a variety of modification strategies. However, the field of osteoimmunology has garnered a lot of interest because of the inconsistent results of direct osteogenic biomaterials in vitro and in vivo (Liu et al., 2018). Further research suggests that the immune response triggered by biomaterials is the cause of this phenomenon, which has been shown that the bone immune cells can affect bone remodeling and absorption since immune cells in bone and the skeletal system share lot of receptors, cytokines, and signaling molecules (Yu et al., 2022; Zhang et al., 2023; Liu et al., 2018). Research has confirmed that excessive immune and inflammatory responses after implantation surgery can result in the growth of fibrous tissue and the creation of fibrous capsules, which in turn cause the PEEK and bone tissue to become loosely connected and cause implantation failure (Zhang et al., 2023). These results imply that to create a favorable immune microenvironment and achieve satisfactory osseointegration results, cutting-edge methods for bone regeneration materials should concentrate on the development of immunomodulatory materials in addition to osteogenesis and antimicrobial resistance.

When choosing surface modification techniques for PEEK implants, simple yet efficient surface modification techniques documented in the literature are favored to minimize complexity and guarantee repeatability, dependability, and safety, making them more promising for clinical translation (Zheng et al., 2022; Zhang et al., 2023). PDA coatings have been employed extensively recently for implant material surface biofunctionalization because of their high adherence, easy fabrication, and excellent biocompatibility (Jia et al., 2019; Cheng et al., 2019). For most of the bone implants, PDA often acts as a link and reaction platform, which can chelate metal ions or graft biomolecular via Schiff base reaction and Michael addition for biomedical applications (Wang T. et al., 2022; Wang L. et al., 2023; Zhang X. et al., 2022; Wang et al., 2024). Here, we create co-modified bone PEEK implants with immunomodulatory active ions (Sr²⁺) and antibacterial active molecules (AMP) via mussel-inspired PDA surface modification. PDA-coated PEEK implants have a hydrophilic surface in contrast to bare PEEK implants, although their surface shape does not alter much. Furthermore, in comparison to implants without surface treatment, the PDA, Sr, and AMP deposits result in a rougher surface on the PEEK implants. Other studies corroborated this as well that PDA coating, active metal ions, and/or peptide immobilization could promote the surface hydrophilicity and toughness of bone implants, which induce cell adhesion and growth (Wang T. et al., 2022; Jia et al., 2019; Yang X. et al., 2024). The EDS and XPS results demonstrated that Sr ions and AMP were successfully deposited on the PEEK-PDA-Sr/AMP implants. Furthermore, the release processes of Sr²⁺ and AMP in PEEK-PDA-Sr/AMP materials were evaluated by ICP-AES. In PEEK-PDA-Sr/AMP implants, Sr²⁺ and AMP were released for 7 days, and then consistently for up to 21 days. This is because the oxygen atom in the phenolic hydroxyl group of PDA typically coordinates with Sr²⁺, forming metal-oxygen bonds. The amino group (-NH2) can also form coordination bonds with Sr²⁺ through its lone pair of electrons. Additionally, PDA interacts with AMP through hydrogen bonding and electrostatic interactions. As a result, the loading and sustained release of Sr and AMP are achieved. To put it succinctly, all these findings suggested that the PEEK-based surfaces could achieve bioactivity for a fair amount of time and were effectively co-modified with Sr/AMP.

The biocompatibility of the implant is significantly impacted by the way cells interact with the surface of the biomaterial, and optimal host-implant response depends upon the cell’s ability to adhere to the implant surface (Wei et al., 2023). Based on SEM images, BMSCs and BMMs showed superior morphology and dispersion on the PEEK materials with a PDA coating than bare PEEK implants following a 24-h co-culturing period. Further studies with cytoskeleton staining revealed that BMSCs were a shape of spindle on the bare PEEK surface with a limited expression of F-actin after 24 h. In contrast, on the surface of PDA-treated PEEK implants, BMSCs were polygonal and filamentous, widely distributed, and expressed high levels of F-actin, suggesting improved cell adherence on these samples. Meanwhile, there were no discernible variations in the conditions of cell adhesion and distribution on PDA-modified PEEK implants. The above findings matched previous research, which demonstrating that PDA coatings can increase the hydrophilicity and surface roughness of implants, promote cell adherence and distribution (Wang L. et al., 2023; Jia et al., 2019; Yang X. et al., 2024). Hee K et al. reported that biosafety of PDA surface modification, which does not hinder the proliferation rate or cell activity types of kinds of mammalian cells, including osteoblasts, fibroblasts, endothelial cells, and neurons (Wei et al., 2023). Using CCK-8, the cytotoxicity of several PEEK implant groups was examined, and RGR values were determined. Following 1, 4, and 7 days of co-culture, there was no discernible variation in the OD value across all PEEK implants. Furthermore, all groups’ RGRs were greater than 75%, indicating that all PEEK implants are biocompatible. According to previous research, 0.32–78.34 μg/mL of Sr ions are safe range for BMSCs and BMMs following 7 days of incubation (Wang L. et al., 2023; Xing et al., 2018). Taken together, the findings suggested that the Sr²⁺ and AMP co-modified PEEK surfaces via PDA surface modification do not hinder the cell proliferation of both macrophages and BMSCs. A good milieu for bone tissue regeneration would be built on the immune cells’ and BMSCs’ enhanced adhesion, spreading, and decreased cytotoxicity (Wang T. et al., 2022).

The moment biomaterials are inserted into the body, the host’s local immunological response is triggered. This includes the production of blood clots, the absorption of proteins that create temporary matrices, and an acute inflammatory response. Activated cytokines cause monocytes to congregate around implants during the acute inflammatory phase and undergo differentiation into macrophages, which are crucial for tissue regeneration and the creation of new bone (Zhang et al., 2023; Peng et al., 2022). When it comes to bone implants, excessive pro-inflammatory M1 subtypes result in fibrous capsule development and bone resorption, which cause the implant and bone to become loosely connected, eventually causing implantation failure (Wang L. et al., 2023). Related studies have shown that PEEK can be surface modification via techniques such as surface chemical modification, drug delivery systems, and surface porous treatment to endow PEEK implants with anti-inflammatory and immunomodulatory functions to differentiate pro-inflammatory M1 subtype into anti-inflammatory M2 subtype and promote tissue repair is an effective strategy (Zhang et al., 2023; Yang X. et al., 2024). The immunofluorescence results of this investigation demonstrated that the Sr-modified PEEK groups promoted the expression of CD206 (a marker of the M2 phenotype) than the non-Sr-modified PEEK groups. According to these findings, exogenous materials such as PEEK and PEEK-PDA groups would cause macrophages to change from M0 to M1, inducing several harmful host immunological reactions. On the other hand, the addition of Sr by PDA was a successful surface modification method that changed the intracellular milieu by triggering a signaling cascade and modulating the surface roughness of PEEK, giving PEEK osteoimmunomodulatory characteristics. Furthermore, semi-quantitative results displayed that Sr ions-modified PEEK material can differentiate BMMs into M2 subtypes with fewer M1 macrophages. Recent studies have demonstrated that the NF-kB signaling pathway is a considerable regulator of the immune response, and the receptor activator of NF-kB ligands (RANKL) is thought to be a crucial component that links the skeletal system and the immune system (Peng et al., 2022; Yang S.-Y. et al., 2024). Sr²⁺ can suppress the NF-κB signaling pathway and repolarize M1 macrophages into M2 macrophages to reduce the inflammatory response which helps with tissue repair (Yang X. et al., 2024; Zeng et al., 2020).

To investigate the immunomodulatory mechanism of PEEK implants treated with Sr ions, RT-PCR was further utilized to analyze the BMMs seed on various PEEK implants. PEEK implants with Sr ion can suppress the gene expression of pro-inflammatory factors including IL-1β and TNF-α, and promote the gene expression of anti-inflammatory factors including IL-10 and TGF-β. This is in line with earlier studies because, when macrophages polarize to the M1 subtype, NF-κB is phosphorylated and transported into nucleus, forming pro-inflammatory cytokines like IL-6, TNF-α, and IL-1β that are detrimental to tissue healing (Yang X. et al., 2024; Chen et al., 2020). Based on the above results, we can conclude that Sr ions-modified PEEK implants can create an advantageous immune microenvironment by modulating the polarization of macrophages towards M2 subtype, which benefits for subsequent bone repair.

The concept behind “osteoimmunomodulation” is to control macrophages to create an environment that is favourable for osteogenesis, thereby improving bone integration of implantable biomaterials (Zhang et al., 2023; Wang T. et al., 2022). Generally speaking, acute and uncontrollable inflammation reaction can damage osteogenic differentiation and bone regeneration, while a mild immune response to the biomaterial was favorable for promoting bone integration of the implant (Wang F. et al., 2023). Thus, we cultivated BMSCs in macrophage-conditioned media to assess the osteogenic differentiation capacity of the environment generated by Sr-modified PEEK. In current study, a Sr-modified PEEK implants was developed, which can promote the gene expression of ALP, Runx2, COL-I, OCN, and OPN in BMSCs. Runx2 is a particular transcription factor that regulates the expression of matrix proteins in osteoblasts, which is essential for osteogenic differentiation and bone production (Wei et al., 2023). Osteoblasts produce COL-I, which is a crucial component of bone matrix and is involved in adhesion, differentiation, and bone matrix production (Bou Assaf et al., 2019). The upregulation of Runx2 and COL-I gene expression indicates that Sr-modified PEEK implants can simultaneously promote early and late osteogenesis. In addition, the mineralization ability of BMSCs on various PEEK implants were determined by ALP and ARS. BMSCs co-cultured with PEEK implants modified with Sr²⁺ exhibited more extracellular matrix mineralization than PEEK implants without Sr modification, which were confirmed by ALP, Alizarin red staining and quantitative analysis. Other research has yielded comparable findings, indicating that strontium functionalized biomaterials have good osteogenic effects (Wang et al., 2025; Mo et al., 2023; Wang L. et al., 2023). The osteogenic effect of Sr ion is concentration-dependent, and the ideal concentration is between 1 and 500 mM to promote osteogenesis in vitro; if the Sr concentration exceeds this range, it would hinder its osteogenic potential and cause cytotoxicity (Wang L. et al., 2023; Naruphontjirakul et al., 2022). Sr also can inhibit osteoclast precursor differentiation by downregulating the expression of osteoclast marker genes and decreasing the osteoclast specific protein activity, thus activating the ERK signaling pathway and inhibiting the signaling pathways of p38, JNK, and AKT (Chen F. et al., 2022). In addition, by downregulating the expression of RANK, cth-k, MMP-9, and c-fos, Sr may prevent macrophages from differentiating into osteoclasts, which will decrease the quantity of osteoclasts and bone resorption (Zhu et al., 2016). According to the above findings, it can be indicated that Sr-modified PEEK implants would offer an ideal osteoimmunomodulatory microenvironment and promote immuno-enhanced osteogenesis.

In orthopedic surgery, implants are highly susceptible to infection in the body, which is another major reason for implant failure (Zheng et al., 2022; He et al., 2023). Surgical site infections are becoming more commonplace and is estimated at 2.1%–8.5% in implant surgery (Zheng et al., 2022). Research has shown that bacteria are able to adhere to the implant’s surface, proliferate, and form biofilms, which endows the bacteria with antibacterial properties within the body (Geng et al., 2024; Yang et al., 2023). According to the study, the first 6 h following implant surgery are crucial since this is when bacteria tend to stick to the implant’s surface and multiply (Fu et al., 2021; Yang et al., 2023). Therefore, to increase the long-term success of implantation surgery, early bacterial adherence on the surface of PEEK implants must be prevented. At present, the methods to improve the antimicrobial properties of PEEK mainly include sulfonation treatment, antibiotics/metal nanoparticles/antibacterial coating, antimicrobial peptides coating and so on (Zheng et al., 2022). Antimicrobial peptides with broad spectrum and strong antibacterial activity can play an antibacterial role against a variety of microorganisms, including drug-resistant pathogenic bacteria (Wu et al., 2023; Mookherjee et al., 2020). In contrast to physical integration, covalent immobilization is an effective chemical integration method which ensures that antimicrobial peptides stay stable on the surface of the implant material and are not impacted by the surrounding environmental (Wu et al., 2023). Several studies have demonstrated that the abundant functional groups found in PDA coatings, such as the amino and phenolic hydroxyl groups, can act as links to facilitate secondary reactions with other compounds. For example, Michael addition reactions or Schiff base reactions with protein molecules that contain -NH₂ and -SH groups can be used to adsorb functional protein or peptide molecules onto the surface of biomaterials (Wu et al., 2023; Zhang Y. et al., 2022; Wang B. et al., 2023). Previous publications have found that S. aureus and E. coli are the main pathogenic bacteria causing implant infections, we used these two types of pathogens to investigate the antibacterial performance of various PEEK implants (Zheng et al., 2022; Yang et al., 2023). The results of bacterial morphology, live and death staining and spreading plate method all indicate that the Sr/AMP co-modified PEEK implants have remarkable antibacterial properties against S. aureus and E. coli. Prior studies have indicated that monomeric forms of the antimicrobial peptide PMAP-36 adopt an amphipathic α-helical conformation and kill bacteria via quickly penetrate both the outer and inner membranes of bacteria by creating toroidal pores or transient channels, which in turn cause bacterial death. At low concentrations, bacteria can be killed, and the minimum inhibitory concentration (MIC) is 1–2 µM for S. aureus and 0.5–1 µM for E. coli (Scocchi et al., 2005; Hancock and Chapple, 1999), which is consistent with our experimental results. Above results in vitro all indicate that PEEK-PDA-Sr/AMP implants can successfully combine immune regulation, osteogenesis, and antimicrobial properties together with good biocompatibility.

To simulate clinically relevant implant-related infection, we utilized an SD rat femur osteomyelitis model system to further validate its biological function of PEEK-PDA-Sr/AMP implants in vivo. It is well known that if bacterial infection and osteomyelitis occur, osseous integration of the implant will be inhibited (Gri and stina, 1987). Micro-CT analysis allows quantitative analysis of bone growth or in-depth observations of infected areas, allowing researchers to accurately compare the development of infection and the process of osseointegration (Wang L. et al., 2023; Lin et al., 2017). The bone tissue surrounding the implant was discontinuous four- and 8-weeks following surgery for the PEEK, PEEK-PDA, and PEEK-PDA-Sr groups, as demonstrated by 2D and 3D reconstruction pictures of the CT scan. This observation implied poor bone integration caused by bacterial infection in these three groups (Gao et al., 2021). On the contrary, PEEK-PDA-Sr/AMP group induced the highest new bone formation around the implant, indicating that Sr-AMP co-modified surface can successfully prevent bone infection and encourage bone integration (Wang et al., 2022b). The quantitative analysis of osteogenic parameters of the PEEK implants including BV/TV and Tb. Sp in different groups also showed the same trend. Extensive literature has confirmed that the increase of M2/M1 macrophages is conducive to promoting the osseous integration of implants (Wang T. et al., 2022; Li et al., 2024). In our case, the expression of CD206 in the PEEK-PDA-Sr/AMP group was highest accompanied by a decrease in CD86 expression at 4 weeks postoperatively, suggesting that the potential of Sr/AMP co-modified PEEK can inhibit inflammatory infection response and enhance interfacial osseointegration. Pathological histological analysis can directly reveal the inflammation, osteogenesis, and bacterial infections in vivo (Wang L. et al., 2023; Lin et al., 2017). In current study, H&E, Masson, and Giemsa staining were utilized to evaluate the inflammatory responses, new bone formation and bacterial infections around the implant at different time points, respectively. Similar to the results of in vitro studies, in vivo models of rat femur implant-associated infections, the PEEK-PDA-Sr/AMP group demonstrated excellent performance in eliminating bacterial infection and promoting bone integration. Previous publications have revealed that PEEK implants with immunomodulatory and antibacterial functions can establish an anti-inflammatory microenvironment, resist pathogen invasion, and increase bone integration under infection conditions (Xu et al., 2019; Su et al., 2023), this is consistent with our findings. As a bone-inducing protein, bone morphogenetic protein 2 (BMP-2) is a potent inducer of bone production and is crucial for the repair of bone tissue (Smith et al., 2023). Sun, et al. reported that successful preparation of 3D bio-printed scaffolds loaded with macrophages and BMSCs, the inflammatory response was inhibited and the secretion of BMP-2 by macrophages was facilitated by inducing the polarization of macrophages to M2 macrophages, thus further accelerating the bone repair of diabetic bone defects (Sun X. et al., 2021), which in line with our results of immunohistochemical staining. Moreover, PDA application in vivo is regarded as safe; intravenous infusion of PDA nanoparticles has an LD50 of 400.22–585.19 mg/kg, showing low toxicity (Wang et al., 2022c). In short, the in vitro and in vivo results confirm that the combination of Sr and AMP can confer multiple biological activities of immunomodulatory, osteogenic, and antibacterial activities on PEEK bone implants, which effectively promote bone integration in the presence of bacteria and may offer a promising approach to the surface bioengineering of inert medical implants, particularly by enabling the rational integration of multiple biofunctions to meet clinical requirements.

Despite the significant progress achieved in this study, several areas require further investigation. For instance, although Sr²⁺ and AMP exhibited sustained release profiles and short-term bioactivity in vitro, the long-term structural integrity and functional stability of the coating under physiological conditions—such as mechanical loading, biodegradation, and fluid shear stress—remain to be validated through extended in vivo studies and accelerated aging models. Moreover, while the SD rat model is widely accepted for preliminary evaluation of osseointegration and antibacterial performance, it differs substantially from human physiology in terms of bone metabolism, immune complexity, and infection progression. Therefore, we acknowledge that the current findings need to be further verified in large animal models (e.g., rabbits, dogs, or sheep) and in long-term implantation studies to fully assess the feasibility and safety of the proposed strategy.

To better fulfil the characteristics and requirements of PEEK materials for orthopedic applications, multifunctional PEEK implants with Sr/AMP co-modified were fabricated using mussel adhesion-mediated assembling strategy. PEEK-PDA-Sr/AMP group exhibited excellent biological functions in vitro, including promoting cell adhesion and spreading, immune regulation, osteogenic differentiation, and antibacterial effect. In vivo, the PEEK-PDA-Sr/AMP implant markedly enhance the ability of interfacial osseointegration of the implant in the presence of bone infection. This surface modification strategy has the advantages of simplicity, high efficiency, and universal application, which offer a surface modification technique with a strong potential for practical conversion to increase the clinical use of PEEK bone implant materials.