**1. Introduction**

Polyetheretherketone (PEEK) is a high-performance thermoplastic polymer that has found increasing application in orthopaedic implant devices (such as spinal fusion cages), whereby its properties have been shown to outperform those of traditional metallic biomaterials, namely titanium alloys and stainless steel. PEEK is biocompatible, has enhanced resistance to in vivo degradation, favourable mechanical properties (such as sti ffness, which is close to human bone, thereby reducing stress shielding), and it can be imaged without introducing artefacts (such as in X-Rays, Computed Tomography (CT) scanning and Magnetic Resonance Imaging (MRI)) [1]. Furthermore, PEEK has been shown to be a much easier material to work with than metals/metal alloys in terms of manufacturing, processability, cost and its ability to be easily 3D printed [2]. As such, it shows a lot of promise for orthopaedics (namely spinal fusion cages), and other 'made-to-measure' implants produced through additive manufacturing approaches [3]. A range of other approaches have also been considered for spinal repair, including studies by Gloria et al, whereby a polyetherimide (PEI)-based fusion cage reinforced with carbon fibres through filament winding and compression moulding provided structures with appropriate mechanical properties that would avoid stress shielding problems and other issues about metal ion release [4] In another study, poly(ε–caprolactone) intervertebral discs produced via additive manufacturing were shown to have favourable mechanical and in vitro properties [5]. Finally, work by Duarte et al developed 3D foams of polycaprolactone doped with polydopamine and polymethacrylic acid (PCL pDA pMAA) with appropriate mechanical properties which could be deployed without the use of instrumentation. [6] However, a key limitation of PEEK is that it is bioinert [7]. There is, therefore, a need to provide a mechanism to functionalise its surface, especially with respect to bone, to make the material at least osteoconductive to ensure a more rapid, improved, and stable fixation that will last longer in vivo. It has been considered that one way in which this can be achieved is through the modification of the PEEK implant with bioactive calcium phosphate (CaP) materials, such as hydroxyapatite (HA–Ca10(PO4)6(OH)2). HA is a highly valuable bone repair and regeneration material because of its similarity to the inorganic phase of human bone. It is bioactive, osteoconductive, and can form a direct chemical bond with human bone [8].

Several technologies have been utilised for the deposition of HA onto metals, including plasma spraying, electrophoretic deposition, pulsed laser deposition, sol-gel, biomimetic, and radio frequency (RF) magnetron sputtering [9]. Several methods have also been investigated as a means to deposit a bioactive HA coating onto PEEK, namely plasma spraying, [10] Ion Bean Assisted Deposition (IBAD) [11], aerosol deposition [12], spin coating [13], and RF magnetron sputtering [14]. Of these techniques, RF magnetron sputtering has shown significant promise for the deposition of CaP coatings due to the ability of the technique to provide greater control of the coating's properties and improved biological performance [15]. However, the previous studies, whereby HA was sputtered onto PEEK [16] (or IBAD onto PEEK) [17], required the use of an intermediate layer of yttria-stabilised zirconia (YSZ), thereby introducing additional processing steps, which adds complexity and enhanced cost to the method. Titanium dioxide and magnesium has also been sputter-deposited onto PEEK to alter the surface chemistry and morphology and influence their osteoblast cell behaviour and corrosion resistance, respectively [18]. There have also been studies whereby CaP materials have been sputtered onto polytetrafluorethane, polystyrene, polyethylene, polydimethylsiloxane, polylactic acid, and a copolymer of vinilidene fluoride and tetrafluoroethylene [19]. However, there have been no reports in the literature detailing the direct deposition of CaP materials onto PEEK using RF magnetron sputtering, to the knowledge of the authors.

This work was undertaken to study the RF magnetron sputter deposition of CaP materials onto PEEK, via a single step (direct) process, with the primary objective of creating a surface with specific chemistry and morphology commensurate with making the PEEK osteoconductive. Ideally, here the aim would be to deposit a HA coating, with properties commensurate with the requirements for HA coatings as laid out in the ISO (International Organisation for Standards) 13779-2 (2018) and ASTM (American Society for Testing and Materials (ASTM) F1609 standards. The work was completed using a custom designed RF magnetron sputtering facility utilising two sputtering targets (referred to as sources), operating at a low discharge power level (150 W). A low discharge power level was chosen for this study to prevent damage to the underlying polymer substrates and to ensure that the quality and consistency of the targets used could be guaranteed throughout the sputter deposition process. The e ffect of the deposition time on the surface morphology and chemistry were investigated here. All the surfaces produced were characterised using X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), optical profilometry, and Time of Flight Secondary Ion Mass Spectrometry (ToFSIMS). Therefore, this study represents the first attempt to deposit CaP materials directly onto PEEK using RF magnetron sputtering, and their subsequent surface characterisation.

### **2. Materials and Methods**
