Aging resistance of the titanium coating

After a successful application of a resistant adhesion promoter to the PEEK surface, the aging resistance of the titanium coating was investigated for an aqueous solution and under human body conditions. For this, the simulated body fluid solution was used according to Kokubo et al. [210]. The solution contained similar inorganic ion concentrations as the extracellular body fluid and simulated human body conditions. The compositions in comparison to the blood plasma as well as the used amounts are listed in Tab. 20. The samples were outsourced in SBF solution for six months in a closed beaker filled in a warmed oven at 310 K.

Chapter 4: Biocompatibility

Tab. 20 Comparison of the composition of simulated body fluid and human blood plasma with the as used chemicals

Ion Concentration /mmol·dm^-3 Ammount /g·L^-1

Human blood plasma SBF Chemical

Na⁺ 142 142 NaCl 7.669

K⁺ 5 5 KCl 0.224

Mg²⁺ 1.5 1.5 MgCl·6H2O 0.305

Ca²⁺ 2.5 2.5 CaCl2 0.278

Cl^- 147.8 103 NaCl, KCl, CaCl2 -

HCO3- 4.2 27 NaHCO3 0.350

HPO42- 1 1 K2HPO4·3H2O 0.174

SO42- 0.5 0.5 Na2SO4 0.071

(CH2OH)3CNH2 Tris(hydroxymethyl)aminomethan: Buffer for pH 6.057

The four pretreated PEEK samples after six months are shown in Fig. 44. After six months, all the samples with a previous pretreatment of the PEEK substrates showed no visible spalling of the titanium coating. Without any pretreatment, the titanium coating clearly pulls off from the surface of the PEEK substrate. This indicates the necessity of pretreatment before an application of titanium. A pretreatment promotes a successful implantation due to a failure of wear to appear after implantation. PEEK includes ether- and carbonyl groups, which can improve the bonding of titanium ions. Titanium has a high affinity to oxygen molecules, which can catalyze the bonding of titanium to the functional groups of PEEK and the added SiO2 layer.

Fig. 44 Different pretreated PEEK substrates outsourced in simulated body fluid (SBF) for six months at 37 °C in an

Chapter 4: Biocompatibility

Plasma electrolytic oxidation is a high-energy performing surface treatment due to the resulting micro-discharges on the surface. These energy-rich discharges have a strong influence on the surface composition, which can also have an influence on the polymer substrate underneath the titanium layer. Therefore, the PEO-treated sputtered PEEK substrate was measured with XPS measurement with regards to the molecules on the surface. The XPS measurement can help to exclude a transport of PEEK molecules into the titanium oxide layer.

Three samples were measured: A pure PEEK substrate, a titanium-sputtered PEEK substrate, and a PEO-treated PEEK substrate (electrolyte E2, 300 V). The XPS spectra can be seen in the following figures (Fig. 45 - Fig. 47), and the atomic compositions at the top of the surfaces are summarized in the following tables (Tab. 21 - Tab. 23). The important atoms are marked in the tables.

Fig. 45 XPS spectra of C1s (left) and O1s (right) mode of a pure PEEK-substrate and the molecular structure of polyetheretherketone (bottom)

Tab. 21 Atomic percent of the included atoms on the measured pure PEEK substrate: Marked columns show the important atoms for the sought substance

(at %)

C O N Si Ti Ca P Cl S Na K

Position 1 82.4 15.1 0.9 0.4 - 0.7 - 0.1 0.3 0.2 - Position 2 82.5 14.2 1.1 0.5 - 1.0 - 0.2 0.2 0.2 -

Chapter 4: Biocompatibility

Fig. 46 XPS spectra of C1s (left), O1s (right), and Ti2p (bottom) modes of a PEEK substrate with an applied titanium layer

Tab. 22 Atomic percent of the included atoms on the measured PEEK substrate with the applied titanium layer: Marked columns show the important atoms for the sought substance

(at %)

C O N Si Ti Na K

Position 1 24.7 50.5 1.2 0.2 22.7 0.4 0.3 Position 2 24.6 50.1 1.3 0.5 22.8 0.4 0.3

Chapter 4: Biocompatibility

Fig. 47 XPS spectra of and O1s (left, bottom), C1s (right) modes of the PEEK-substrate with a PEO oxide layer and a reference spectra, according to Barker et al. [164] (bottom)

Fig. 48 Crystal structure of hydroxyapatite with the natural and synthesized ratio of calcium and phosphorus contained in the oxide layer

Tab. 23 Atomic percent of the included atoms on the measured PEEK substrate with the treated PEO-coating: Marked columns show the important atoms for s the sought substance

(at %)

C O N Si Ti Ca P

Position 1 23.3 52.5 0.2 0.2 6.7 8.9 7.6 Position 2 23.4 51.5 0.6 0.3 10.7 6.3 6.

5

Chapter 4: Biocompatibility

The XPS spectra for the pure PEEK substrate show all structure related organic bonds. The characteristic carbonyl group at 289 eV and the ether bonds at 286 eV in the C1s spectra can be observed. The O1s spectra of the PEEK substrate show the related C-O bonding at 532 eV and the carbonyl group band at 533 eV. These peaks show the corresponding bonds in a PEEK molecule and should not be identified in the XPS spectra of the PEO-treated samples.

However, an absence of other atoms, except oxygen and carbon, leads to a pure PEEK substrate. In the next step, a PEEK substrate with a sputtered titanium coating was measured and the resulting XPS spectra of the C1s, O1s, and Ti2p modes and the atomic-% of the contained atoms on the surface can be seen in Fig. 46 and Tab. 22. The spectra of the C1s and O1s modes show no organic compounds on the surface of the sample. The visible and measured carbon atomic percent represents an acceptable amount of impurities over the entire treatment time. The oxygen atomic percent in Tab. 22 show a titanium to oxygen ratio of 1:2 and the Ti2p spectra show Ti⁴⁺ bonds at 458 eV and 464 eV at top of the surface. A naturally formed titanium dioxide bonding can still be confirmed with the O^2--peak in the O1s spectra at 530 eV. The titanium spectra also show a metallic titanium peak at 453 eV, which relates to the sputtered coating. In the measurement of the PEO-treated PEEK substrates, there a C1s spectra similar to those of the pure PEEK substrate can be seen. On the other hand, the O1s spectra contain a shifted double peak with a shoulder at the left side of the peak. The shoulder at 533 eV normally relates to the organic C-O double bond. However, the peak at 530 eV relates to an O^2--species that is the part of the titanium dioxide layer. In comparison with some XPS measurements by Baker et al. [164], the shoulder at 533 eV relates to some H2O on the surface, which can be explained by the occurrence of some stored water molecules inside of the PEO-pores due to the high hydrophilicity of titanium dioxide. The O1s peak at 531 eV shows the OH^- and PO43- species at the surface. Therefore, the whole O1s spectrum isn’t similar to the organic compounds of the PEEK substrate. The O1s spectra show the presence of hydroxyapatite at the PEO-surfaces, which can be also seen in Tab. 23.

10 + 6 + 2 → ( ) ( ) (1)

The surface contains calcium and phosphate atoms, which confirm the assumption of the presence of hydroxyapatite. the formation of hydroxyapatite HA is shown in equation (1) and illustrated in Fig. 48. The measured ratio of calcium to phosphate atoms is nearly 1.0 whereby natural stoichiometric HA has a ratio of Ca/P of 1.67 [211]. At the very least, the PEO-process produces HA in the titanium dioxide layer which can improve its biocompatibility.

Chapter 4: Biocompatibility