*2.1. Substrate Preparation*

Circular Coupons of PEEK-OPTIMA ™ LT1 (13 mm diameter and 2 mm thick) supplied by Invibio Ltd. (Thornton Cleveleys, UK) were abraded using 1200 P grade Silicon Carbide paper. Abrasion was carried out at approximately 250 revolutions per minute (RPM) for 3 min, removing all inhomogeneity from the substrate surface. The substrates were then twice sonicated in acetone (Sigma-Aldrich 99.5%, St. Louis, MI, USA) for 8 min and once in de-ionised water (DI) for 8 min and dried thoroughly in a convection oven at 70 ◦C for 12 h.

### *2.2. RF Magnetron Sputter Deposition*

Sputtering targets were manufactured by dry pressing the hydroxyapatite (HA-(Plasma Biotal Captal-R), Tideswell, Buxton, UK) powder into low oxygen copper troughs (76 mm diameter and 5 mm thick) at a load of 40 kN for 10 min. RF Magnetron sputtering was undertaken using a designed Kurt J. Lesker Ltd., system (Hastings, UK), which was custom designed, operating with two Torus 3M sputtering sources operating at 13.56 MHz. The break-in prior to deposition from the HA target was conducted at a ramp rate of 5 watts (W) per minute up to the operating power of 150 W, whereby the source shutters were kept closed. The base pressure was below 5 × 10−<sup>6</sup> Pa, with an argon gas flow rate (BOC, 99.995%) of between 15 and 20 Sccm, and a throw distance of 100 mm. During sputter deposition, the chamber pressure was maintained at 2 Pa. Table 1 outlines the sample nomenclature and key deposition parameters during the sputtering runs, with the deposition time being the main operating parameter varied during the experiments. The power density for these HA targets was approximately 3.3 W·cm<sup>−</sup><sup>2</sup> during each deposition.


**Table 1.** Sputter deposition operational parameters and sample nomenclature.

### *2.3. X-ray Photoelectron Spectroscopy*

X-ray Photoelectron Spectroscopy (XPS) of the samples was undertaken out using a Kratos Axis Ultra DLD spectrometer (Manchester, UK). Spectra were recorded by employing monochromated Al K α X-rays (hν = 1486.6 electron volts (eV)) operating at 15 kV and 10 mA (150 W). Wide energy survey scans (WESS) were obtained at a pass energy of 160 eV. High resolution spectra were recorded for O 1*s*, Ca 2*p*,P2*p*, and C 1*s* at a pass energy of 20 eV. The Kratos charge neutraliser system was used on all samples with a filament current of 2.05 A and a charge balance of 3.8 V. Sample charging e ffects on the measured binding energy (BE) positions were corrected by setting the lowest BE component of the C 1*s* spectral envelope to 285.0 eV, [20–22]. Photoelectron spectra were further processed by subtracting a linear background and using the peak area for the most intense spectral line of each of the detected elemental species to determine the % atomic concentration. In total, 3 areas were analysed from each sample. Peak fitting was carried out using a mixed Gaussian–Lorentzian (GL (30)) synthetic peak function using the Kratos Vision software (version 2.3.0).

### *2.4. Time of Flight Secondary Ion Mass Spectrometry (ToFSIMS)*

ToFSIMS data was obtained using a ToFSIMS IV instrument(ION-TOF GmbH, Münster, Germany) equipped with a 25 keV Bismuth (Bi) liquid metal ion gun (primary ion source) with a pulsed target current of 0.3 pico Amps (pA) and a post accelerator voltage of 10 kV, both with an incident angle of 45◦ to the sample surface normal. A base pressure of 6.66 × 10−<sup>6</sup> Pa was maintained in the UHV analyser chamber during the analyses by the ToF method. The negative and positive secondary ion spectra were recorded with a Primary Ion Dose Density of 1 × 10<sup>13</sup> ions/cm<sup>−</sup>2. Charge compensation on the polymer surface was achieved using a low energy (21 eV) electron flood gun source, with the data acquired over a *m*/*z* range 0–200 for both positive and negative secondary ions. Data was presented by plotting *m*/*z* against intensity (counts/second). Ion images containing 256 × 256 pixels with 15 shots/pixel were acquired randomly, using Bi3<sup>+</sup> primary ions in the high current bunched mode (HC-BU) over a 500 μm diameter area on the sample surface. Data acquisition and data processing and analysis were performed using SurfaceLab 6 (ION-TOF). The ToFSIMS IV instrument (ION-TOF GmbH, Münster, Germany) was also used to acquire depth profiles from the CaP modified PEEK samples, the instrument was equipped with a 20 keV Argon (Ar1900+) gas cluster ion gun which was rastered over the sample, with a crater size of 400 μm diameter, for 1000 secs. Analysis was completed using a Bi3<sup>+</sup> liquid metal ion gun (primary ion beam) with an energy of 25 keV in a field of view (FOV) of 150 μm<sup>2</sup> and a raster size of 128 × 128 pixels (random mode). Due to the insulating nature of the samples, a low energy (21 eV) electron flood gun source was applied for the purposes of charge compensation with a filament current of 2.5 A. In total, 3 areas were analysed from each sample.
