2.2.4. Contact Angle Measurement

The wettability of the textures was characterized by means of a contact angle measurement device (SCA20, DataPhysics Instruments GmbH, Filderstadt, Germany). The sample surfaces were wiped with isopropanol and dried under ambient conditions prior to analysis. Next, a droplet of water of 5 μL was deposited on the surface and the angle between the surface and the droplet was measured. Three measurements were conducted for each sample.

#### 2.2.5. Biocompatability

The substrates were sterilized by immersing them in ethanol for 15 min. A simulated body fluid (SBF) solution was made according to the instructions of Kokubo et al. [22]. That is, salts were dissolved in de-ionized water such that a solution was created with ion concentrations similar to that of blood plasma. The pH value of SBF (7.40) is comparable to the pH of human blood plasma, which ranges from 7.2 to 7.4. The substrates were immersed in 40 mL of SBF solution at 37 ◦C in a shaking incubator (160 rpm). The ion release was analyzed after 1, 7, 14, 21 and 26 days respectively. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) (Optima 5300 dual view, PerkinElmer Inc., Waltham, MA, USA) was used to analyze the leaching properties.

## **3. Results and Discussion**

#### *3.1. Surface Structures Processed with Linear Polarization*

Two types of surface textures were processed using linear polarized laser irradiation, by applying increasing peak fluence levels and various number of overscans during the laser processing. These two types are low spatial frequency LIPSS (LSFL, see Figure 2) and hierarchical structures composed of micro-grooves and superimposed LSFL (see Figure 3).

**Figure 2.** Scanning electron microscope (SEM) micrographs of low spatial frequency laser-induced periodic surface structures (LSFL) processed on CoCrMo with a laser peak fluence of *F*<sup>0</sup> = 1.67 J/cm2 and number of overscans *N*OS = 1 ((**a**) 1000× magnification; and (**b**) 10,000× magnification). (**c**) 2D-fast Fourier transform (FFT) map of the SEM micrograph (**a**). The spatial periodicity of the LSFL is Λ ≈ 800 nm. The arrow in micrograph (**a**) indicates the direction of the E-field of the laser polarization.

The periodicity of the LSFL in Figure 2 was found to equal Λ ≈ 800 nm and are perpendicular to the E-field of the laser polarization direction, which is typical for LSFL on metals [23]. LSFL on the sample are spread homogeneously over the processed surface of 5 × 5 mm2.

Figure 3 shows SEM micrographs of hierarchical structures processed on CoCrMo with *N*OS = 5 and various peak fluence levels. It can be observed in this figure, that with increasing peak fluence levels, the periodicity of the micro grooves increases from ΛGrooves ≈ 3.55 μm at a peak fluence level of *<sup>F</sup>*<sup>0</sup> = 1.82 J/cm2 to <sup>Λ</sup>Grooves ≈ 7.9 <sup>μ</sup>m at *<sup>F</sup>*<sup>0</sup> = 7.07 J/cm2. The formation of micro-grooves and micro-bumps is attributed to an increased heat accumulation during processing [24–26].

The periodicity ΛLSFL ≈ 920 nm of the LSFL in Figure 3 was constant for all fluence levels within the fluence range studied. It is known that the LSFL periodicity increases with increasing fluence levels up to a certain fluence level, after which the periodicity does not vary with the fluence [27,28].

**Figure 3.** SEM micrographs of hierarchical structures processed on CoCrMo with *N*OS = 5 and various peak fluence levels at two different magnifications ((**a**–**c**) 1000×; and (**d**–**f**) 10,000×). (**g**–**i**) 2D-FFT maps of the micrographs of the processed areas (**a**–**c**). The periodicity of the micro-grooves ΛGrooves increases with increasing peak fluence levels. The periodicity of the LSFL features are constant at ΛLSFL ≈ 920 nm for all micrographs. The arrow in micrograph (**a**) indicates the direction of the E-field of the laser polarization.
