Enhancing Cell Adhesion on Polyetheretherketone (PEEK) through Gold Coating and Argon Plasma Treatment

Background

Age‑related joint degeneration leads to fractures, arthritis, and bone tumors, driving the demand for reliable orthopedic implants. Traditional metal alloys such as titanium and cobalt‑chrome are favored for their strength and corrosion resistance, yet their high modulus often results in stress shielding and subsequent bone resorption [1,2,3]. Polymer alternatives—UHMWPE, PTFE, PMMA, PLA, PGA, PHB—are widely used but generally lack the mechanical robustness required for load‑bearing applications [4,5]. Polyetheretherketone (PEEK), a semi‑crystalline aromatic thermoplastic, bridges this gap by offering a modulus close to cortical bone, excellent chemical and thermal stability, and proven biocompatibility [6–13]. However, its inherent hydrophobicity and bioinertness limit protein adsorption and cell adhesion, necessitating surface modification strategies.

Surface tailoring can be achieved through plasma activation or metal sputtering. Plasma treatment introduces reactive species that alter surface chemistry without compromising bulk properties [16–17], while gold sputtering provides a biologically active layer that enhances cell attachment and serves as a platform for further functionalization [18–20]. In particular, gold nanoparticles below 100 nm are known to boost cell adhesion and proliferation [20]. The present work investigates the combination of argon plasma activation and gold sputter‑coating to fabricate a nanostructured PEEK surface optimized for fibroblast growth, with potential implications for spinal and orthopedic implants.

Methods

Materials and Surface Modifications

PEEK foils (50 µm thick, density 1.26 g cm⁻³) were sourced from Goodfellow Ltd. Each 2 cm diameter sample was first exposed to argon plasma (Balzers SCD 050) for 60 s or 240 s at 8.3 W. Subsequently, half of each sample was coated with gold using DC argon plasma sputtering (current 40 mA, 15 W) for 30 s, 150 s, or 300 s, achieving gold layers of 30–300 s deposition time. Samples were stored at 24 °C, 40–60 % humidity until testing.

Surface Characterization

Gravimetry

Gold thickness was quantified by mass gain measured with a Mettler Toledo UMX2 microbalance, using gold’s bulk density. Ten replicates per condition yielded <15 % measurement error.

Water Contact Angle

Wettability was measured using a Drop Shape Analysis System DSA 100. Two‑µL droplets were placed on aged samples (14 days) and contact angles recorded after a 2‑s delay. Each sample was measured at least seven times on three replicates, and the mean ± SD reported.

X‑ray Photoelectron Spectroscopy

XPS spectra were obtained with an Omicron ESCAProbeP spectrometer. A 2 × 3 mm² area was analyzed under ultra‑light vacuum, focusing on C 1s, O 1s, and Au 4f peaks. Three independent measurements per sample provided <10 % error.

Zeta Potential

Electrokinetic potential was measured in 1 mM KCl and phosphate‑buffered saline (PBS, pH 7.4) using an Anton Paar SurPASS. Two‑film pairs (20 × 10 mm²) were placed in a 100 µm gap cell. Streaming current was recorded three times per sample, and the Helmholtz–Smoluchowski equation applied. All measurements were performed on 14‑day aged samples.

Atomic Force Microscopy

AFM imaging (Bruker VEECO CP II) in tapping mode with RTESPA‑CP probes revealed surface morphology over 1 × 1 µm² regions. Multiple scans confirmed reproducibility. Samples were 14‑days aged.

Inductively Coupled Plasma Mass Spectrometry

Gold leaching into PBS (pH 7.4) was quantified by ICP‑MS (Agilent 8800). Samples were incubated at 37 °C for 6, 24, and 72 h in humidified 5 % CO₂. Leachates were diluted 1:8 and analyzed with <3 % uncertainty.

Cell Culture and Cytocompatibility Testing

Mouse embryonic fibroblasts (L929) were cultured in DMEM with 10 % FBS and 2 mM L‑glutamine at 37 °C, 5 % CO₂. Sterilized PEEK discs were mounted in 12‑well plates; 30 000 cells per well were seeded. Cell adhesion was assessed at 6 h, while proliferation was monitored at 24 h and 72 h. Experiments were performed in triplicate.

Fluorescence Microscopy

After incubation, cells were fixed with 4 % formaldehyde, stained for F‑actin (phalloidin‑Atto 565) and nuclei (DAPI), and imaged. Images were captured on 14‑day aged samples.

Scanning Electron Microscopy

SEM (Tescan LYRA3 GMU) in secondary electron mode visualized cell morphology on pristine PEEK, plasma‑treated/gold‑coated PEEK, and glass coverslips (control). Cells were fixed with Karnovsky solution, dehydrated through graded ethanol, coated with 10 nm gold, and imaged at 10 µm scale bars.

Results and Discussion

All analyses were conducted on 14‑day aged samples to capture the stabilized surface state post‑plasma and gold deposition. Plasma treatment induced a moderate increase in surface roughness and generated polar functional groups, as evidenced by AFM and XPS data. Notably, contact angles rose slightly after plasma exposure (from 79.5° to ~95°) but decreased substantially after gold coating, indicating enhanced hydrophilicity.

Gravimetric measurements revealed a 2‑fold increase in PEEK ablation when plasma exposure was extended from 60 s to 240 s, reflecting the higher energy input. Conversely, gold sputtering at 150 s and 300 s resulted in continuous layers that improved adhesion of gold to the polymer.

XPS analysis showed a clear trend: plasma treatment reduced surface carbon content while increasing oxygen, confirming the introduction of polar groups. Subsequent gold deposition decreased oxygen signals and elevated gold signals, with the strongest gold signatures observed for the 300‑s sputtered samples.

AFM images demonstrated that longer plasma exposure produced a rougher surface, which in turn facilitated the formation of larger, more uniform gold clusters during sputtering. The 30‑s gold layers formed discontinuous islands, whereas 150‑s and 300‑s coatings yielded continuous films with increased roughness.

Zeta potential measurements revealed that plasma activation raised the surface charge (more negative) in both KCl and PBS, while gold coating shifted the potential toward less negative or slightly positive values. The effect was more pronounced in PBS due to the higher ionic strength, indicating that the engineered surface chemistry is responsive to physiological conditions.

ICP‑MS data indicated that thin gold layers (30 s) released higher gold concentrations into PBS, likely due to their discontinuous nature. In contrast, continuous 300‑s coatings released negligible gold, reducing potential cytotoxicity.

Cellular assays confirmed the superior performance of plasma‑treated and gold‑coated PEEK. At 6 h, cell adhesion on pristine PEEK was comparable to tissue culture plastic, but adhesion on plasma‑treated surfaces increased. After 72 h, the best results were observed for 60‑s plasma + 300‑s gold and 240‑s plasma + 300‑s gold, where continuous gold films provided a conducive substrate for fibroblast proliferation. Thin gold layers (30 s) yielded the lowest cell counts, correlating with higher gold leaching.

SEM images corroborated these findings, showing well‑spread, elongated fibroblasts on the optimized surfaces, whereas cells on thin‑layered gold exhibited rounded morphologies indicative of poor adhesion.

Conclusions

Argon plasma activation followed by gold sputter‑coating effectively tailors PEEK surfaces for biomedical use. Plasma exposure for 240 s doubles surface ablation and introduces polar groups, while gold layers of 150–300 s deposit continuous nanostructured films that significantly improve wettability, surface charge, and fibroblast adhesion. Thin gold layers are unsuitable due to higher metal release and reduced cell proliferation. Overall, the combined plasma‑gold approach yields a PEEK material with enhanced cytocompatibility, suitable for orthopedic and spinal implant applications.

Abbreviations

AFM:

Atomic force microscopy

CO₂:

Carbon dioxide

DAPI:

4′,6‑Diaminido‑2‑phenylindole dihydrochloride

DMEM:

Dulbecco’s modified Eagle’s medium

FBS:

Fetal bovine serum

ICP‑MS:

Inductively coupled plasma mass spectrometry

KCl:

Potassium chloride

L929:

Mouse embryonic fibroblasts

PBS:

Phosphate‑buffered saline

PEEK:

Polyetheretherketone

PGA:

Polyglycolide

PHB:

Polyhydroxybutyrate

PLA:

Poly(l‑lactide)

PMMA:

Polymethylmethacrylate

PTFE:

Polytetrafluorethylene

SEM:

Scanning electron microscopy

TCPS:

Tissue culture polystyrene

UHMWPE:

Ultra‑high‑molecular‑weight polyethylene

WCA:

Water contact angle

XPS:

X‑ray photoelectron spectroscopy