Decreased bacteria activity on Si₃N₄ surfaces compared with PEEK or titanium

Abstract

A significant need exists for orthopedic implants that can intrinsically resist bacterial colonization. In this study, three biomaterials that are used in spinal implants--titanium (Ti), polyether-ether-ketone (PEEK), and silicon nitride (Si₃N₄)--were tested to understand their respective susceptibility to bacterial infection with Staphylococcus epidermidis, Staphlococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Enterococcus. Specifically, the surface chemistry, wettability, and nanostructured topography of respective biomaterials, and the effects on bacterial biofilm formation, colonization, and growth were investigated. Ti and PEEK were received with as-machined surfaces; both materials are hydrophobic, with net negative surface charges. Two surface finishes of Si₃N₄ were examined: as-fired and polished. In contrast to Ti and PEEK, the surface of Si₃N₄ is hydrophilic, with a net positive charge. A decreased biofilm formation was found, as well as fewer live bacteria on both the as-fired and polished Si₃N₄. These differences may reflect differential surface chemistry and surface nanostructure properties between the biomaterials tested. Because protein adsorption on material surfaces affects bacterial adhesion, the adsorption of fibronectin, vitronectin, and laminin on Ti, PEEK, and Si₃N₄ were also examined. Significantly greater amounts of these proteins adhered to Si₃N₄ than to Ti or PEEK. The findings of this study suggest that surface properties of biomaterials lead to differential adsorption of physiologic proteins, and that this phenomenon could explain the observed in-vitro differences in bacterial affinity for the respective biomaterials. Intrinsic biomaterial properties as they relate to resistance to bacterial colonization may reflect a novel strategy toward designing future orthopedic implants.

Keywords: anti-infective; biofilm; nanostructure; protein adsorption; silicon nitride.

Figure 1

Scanning electron microscopy surface microstructures of PEEK Optima^®, Ti, and Si₃N₄: (A1) PEEK 1000×; (A2) PEEK 5000×; (B1) Ti 1000×; (B2) Ti 5000×; (C1) Si₃N₄ 1000×; (C2) Si₃N₄ 5000×. Abbreviation: PEEK, poly-ether-ether-ketone.

Figure 2

Scanning electron microscopy surface microstructure of polished and as-fired Si₃N₄: (A) polished surface 20,000×; (B) as-fired surface 20,000×.

Figure 3

Biofilm formation: (A) Staphylococcus epidermidis, (B) S. aureus, (C) Pseudomonas aeruginosa, (D) Escherichia coli, (E) Enterococcus. Note: Dashed lines represent confidence intervals at P < 0.05. Abbreviations: SN, Si₃N₄; PEEK, poly-ether-ether-ketone.

Figure 4

Count of live bacteria: (A) Staphylococcus epidermidis, (B) S. aureus, (C) Pseudomonas aeruginosa, (D) Escherichia coli, (E) Enterococcus. Note: Dashed lines represent confidence intervals at P < 0.05. Abbreviations: SN, Si₃N₄; PEEK, poly-ether-ether-ketone.

Figure 5

Protein adsorption: (A) 20 minutes, (B) 60 minutes, (C) 240 minutes. Notes: *P < 0.01 compared with all others; **P < 0.05 compared with PEEK. Abbreviations: Arb, arbitrary; SN, Si₃N₄; PEEK, poly-ether-ether-ketone.