Polyetheretherketone (PEEK) is a high-temperature thermoplastic polymer belonging to the polyaryletherketone (PAEK) family. Because of its excellent mechanical properties, chemical stability and biocompatibility, PEEK has been applied from industrial to medical fields[1], [2]. Currently, the materials traditionally used in dental prostheses are mainly zirconia and metal alloys[3], [4]. Compared with zirconia and metal alloys, PEEK offers many desirable advantages. For example, low elastic modulus (3 to 4 GPa) of PEEK allows it to provide a more balanced stress distribution[5], [6], [7], [8]. It offers low thermal conductivity, thereby protecting abutment teeth from the temperature stimulus[9], [10]. It has radiopermeability, which makes it compatible with imaging techniques[1], [11]. Several potential applications in the field of dentistry have been reported in scientific literature. For instance, investigations have been conducted on the utilization of PEEK as a framework material for removable prostheses[5], [12] and fixed restorations, including crowns, fixed partial dentures, and post-and-core systems[6], [11], [12], [13]. However, the inert surface of PEEK makes the adhesion of resin-matrix composites to PEEK a challenge[12], [14], [15], which greatly limits its potential for widespread application in dentures.

To meet the clinical demand, improving the bond strength of PEEK through surface modification has become a promising research direction. According to their action mechanism, modification methods can be categorized as follows: (1) surface coating with bioactive materials[16], [17]; (2) changing the surface morphology of PEEK, such as acid etching[18], [19], sandblasting[20] and laser surface texturing[21]; and (3) grafting active functional groups onto PEEK, such as plasma[22], [23], accelerated neutral atom beam[24] and UV light[25]. Despite the progress achieved in improving the bioactivity of PEEK as an implant by surface modification[16], [26], the research for improving the adhesion of PEEK to resin composites remains insufficient.

Numerous studies have proved that the change in PEEK surface morphology affects its bonding properties[12], [19], [20]. The adhesive contactable area increases with surface roughness, and the mechanical interlocking force also increased. Consequently, sulfonation has gained increasing attention due to its reliable etching ability for PEEK[27]. In addition, plasma technology is increasingly applied in medicine and dentistry because of its efficiency, simplicity and economy[28]. Plasma can remove hydrophobic pollutants, increase the wettability and free energy of the material, and introduce active oxygen-containing groups such as hydroxyl on the surface of the material[29], [30]. In addition to promoting osteogenesis, this chemical modification has also been found to improve the adhesive properties of PEEK[31], [32]. However, the mechanical strength of PEEK has been reported to be reduced by improper sulfonation and plasma parameters[13], [33].

The silane coupling agent, a trialkoxysilane, is highly effective in bonding silica-based restorative materials such as resin composite luting cement to acid-etched porcelain surfaces, which is widely used in clinical applications[34], [35]. As mentioned above, sulfonation and plasma alter the surface structure and chemical state of PEEK, respectively [36], [37]. It has been reported that the combined use of these two treatments can endow PEEK with antimicrobial properties and osteogenic activity[38]. Regrettably, no studies have been conducted to investigate the effect of combining these two treatments on the bond strength of PEEK to date. Furthermore, the majority of existing research on the adhesive properties of PEEK is primarily limited to assessments of immediate bond strength, without considering the potential influence of humid intraoral environments. Therefore, the present study developed a novel mixed modification method involving sulfonation and plasma, and a silane coupling agent was coated on the surface of the dual-modified PEEK to test its shear bond strength with the resin material immediately and after aging (as shown in Fig. 1). The innovation of this study lies in the first combination of sulfonation, oxygen plasma and silane coupling agent to modify the surface of PEEK, thereby significantly improving the immediate and long-term bond strength between PEEK and resin. Firstly, we here hypothesized that (1) the sulfonation time and plasma parameters used in this study would not affect the mechanical strength of PEEK; (2) this mixed modification of the surface microstructure and chemical state would synergistically improve the adhesive properties of PEEK; (3) the mixed-modified PEEK would maintain desirable adhesive properties even after 10,000 thermal cycles.