A recent study in Scientific Reports explored how adding strontium (Sr) to polyetheretherketone (PEEK) using a mussel-inspired polydopamine (PDA) coating could improve osteogenic activity. The results showed that Sr-modified PEEK released Sr ions steadily over 35 days, enhancing compatibility with MC3T3-E1 pre-osteoblasts.
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Background
PEEK is a high-performance, semicrystalline thermoplastic with excellent biocompatibility and mechanical properties comparable to human cortical bone. These characteristics make it a widely used material in orthopedic and dental implants. However, its bio-inert nature—stemming from its hydrophobic surface and chemical stability—limits direct bone integration, reducing its effectiveness in promoting osseointegration.
To enhance its osteogenic potential, researchers have explored various surface modification techniques, including phosphonation, bisphosphonation, sulfonation, and hybrid coatings. Among these, Sr incorporation is particularly promising due to its dual role in stimulating osteoblast activity and inhibiting osteoclast-mediated bone resorption. This study investigates whether Sr-coated PEEK surfaces improve bone-implant interactions by facilitating better cell adhesion, proliferation, and mineralization.
Methods
PEEK and PDA-coated PEEK samples were soaked in SrCl₂ solutions at three concentrations—5, 10, and 25 mg/mL—creating PEEK-Sr5, PEEK-Sr10, and PEEK-Sr25. A PDA-coated PEEK sample (PEEK-PDA) served as the control.
Surface changes were analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) identified the elemental composition. Researchers measured hydrophilicity by testing the contact angle of a 4 µL water droplet. Sr ion release was tracked over 35 days using inductively coupled plasma optical emission spectrometry (ICP-OES).
To assess cytocompatibility, MC3T3-E1 pre-osteoblast adhesion was observed under SEM. Researchers also tested cell viability and toxicity by incubating pre-osteoblasts on the samples for 3 and 5 days. The Cell Counting Kit-8 (CCK-8) assay measured cell growth at 1, 3, and 5 days. Additional tests looked at osteogenic gene expression, alkaline phosphatase (ALP) activity, collagen secretion, and extracellular matrix (ECM) mineralization using Alizarin Red staining.
Results and Discussion
SEM images showed that while pure PEEK had a smooth surface, PEEK-PDA and PEEK-Sr surfaces were rougher after PDA coating. AFM confirmed that PEEK-Sr10 had the most surface roughness among the modified samples.
The PDA coating significantly increased surface hydrophilicity, making it easier for biomolecules and ions to adhere. Contact angle tests showed that higher Sr concentrations reduced the angle, confirming increased hydrophilicity. Sr release followed a predictable pattern: PEEK-Sr25 had the highest initial burst, followed by a steady release phase in all samples.
Osteoblast behavior differed significantly among the samples. On pure PEEK, cells appeared thin and shrunken after four hours, highlighting its bio-inert nature. In contrast, cells on PEEK-PDA and PEEK-Sr attached better, adopting a flatter shape. After 24 hours, cells on all samples expanded, with those on PEEK-Sr10 and PEEK-Sr25 reaching near confluency.
PEEK-Sr10 stood out. It showed the highest cell proliferation, ALP activity, and collagen secretion. The findings confirmed a dose-dependent effect, but the 10 mg/mL concentration provided the best balance of performance and controlled release.
Conclusion
This study successfully developed Sr-coated PEEK surfaces using a simple, bioinspired PDA-assisted method. In vitro tests confirmed that Sr modification improved osteogenic activity, with PEEK-Sr10 delivering the best results. It outperformed the other samples in cell adhesion, proliferation, osteogenic gene expression, collagen production, ALP activity, and ECM mineralization.
Sr release from PEEK-Sr10 sustained osteoblast activity over time, making it a strong candidate for enhancing bone-implant integration. However, more in vivo research is needed to confirm long-term safety, efficacy, and potential systemic effects. Future studies should focus on translating these results into clinical applications to refine Sr-modified PEEK for real-world use.
Journal Reference
Zhang, Y., et al. (2025). Dose-dependent enhancement of in vitro osteogenic activity on strontium-decorated polyetheretherketone. Scientific Reports. DOI: 10.1038/s41598-025-86561-3, https://www.nature.com/articles/s41598-025-86561-3
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