## 2 Materials and Methods
### Laser-Structuring  
Ultrashort pulsed fs lasers enable the creation of extremely fine surface structures, as surface thermal melting typically does not occur during laser processing. In this study, laser processing was carried out using an Ytterbium-based amplified Spirit 1040-16 HE femtosecond laser system (Spectra Physics, Rankweil, Austria). This laser had a wavelength, \(\lambda\), of 1040 nm; a pulse length, \(\tau\), of 350 fs; a typical laser power, \(p\), between 5 and 50 mW; and a repetition frequency, \(f\), of 500 Hz to 5 kHz. A lens with a focal length of 100 mm (LB1676-B-ML, Thorlabs, Bergkirchen, Germany) was used to focus the linearly polarized laser beam onto the sample. The resulting beam profile was Gaussian-shaped with a beam waist, \(2w_{0}\), of 75 \(\upmu\)m. Before the lens, we measured the power, \(p\), using a power-meter TPM-A.1 (Gentec, Olching, Germany). The calculation of the corresponding pulse energy, \(E\), and peak fluence, \(F_{0}\), took into account a loss of approximately 6.7% due to the passage through the lens. To obtain the lateral movement of the samples (fixed with double-sided tape) into the x- and y-directions during the irradiation, we used two linear mechanical positioning stages. This setup could be used either in a scanning mode or in a burst pulse mode. The active mode was chosen by a switch box. For line scanning, stage velocities \(v_{scan}=1000\) to 2000 \(\upmu\)m/s were used. Each line could be scanned once or several times in a sequence (in the so called \"overscan\" mode). The line distance \(\Delta S\) between neighboring lines was typically 20 to 70 \(\upmu\)m. On each sample, several regions with laser-induced structures were generated. The samples were cleaned before the laser treatment with ethanol and dried by nitrogen gas. After the laser treatment, this procedure was repeated. For CAB, this cleaning step was performed in an ultrasonic bath (for a duration of 600 s).  
The laser beam waist, \(2w_{0}\), at the focus was determined using measurements obtained with the 14-bit digital camera system WinCamD (DataRay, Boulder Creek, CA, USA) at various positions relative to the focus, and its value was extrapolated from measurements taken some distance away from the focal point. We assumed a Gaussian beam profile, which was fulfilled according to our beam profile measurements. The formulas describing Gaussian beams are well known in the literature [25; 26]. The radial fluence distribution is given by  
\[F(r)\ =\ F_{0}\ \mathrm{exp}\Big{(}{-2\Big{(}{r^{2}}/{w_{0}}^{2}\Big{)}}\Big{)}. \tag{1}\]  
Given a lens transmission \(T_{lens}=92.3\%\), the following relationship holds for the dependance of the laser pulse energy, \(E\), on the laser power, \(p\), measured before the lens and the frequency,\(f\)  
\[E\ =\ (p/f)\cdot T_{lens}. \tag{2}\]  
Figure 1: Chemical structure formula of (**a**) polycarbonate (PC) and (**b**) cellulose acetate butyrate (CAB). Images taken from [23; 24], respectively. Images show simple structural formulas ineligible for copyright and therefore in public domain.  
According to this definition, the peak fluence, \(F_{0}\), and the laser pulse energy, \(E\), are linearly related through the following equation  
\[F_{0}\ =\ E/\Big{(}0.5\pi{w_{0}}^{2}\Big{)}. \tag{3}\]  
For the pulse burst mode, the number of pulses, \(N\), per area is defined by the irradiation time and the pulse repetition frequency, \(f\), while for line scanning at a constant velocity, \(v_{scan}\), an effective irradiation time is given be the ratio of the beam waist, \(2w_{0}\), and \(v_{scan}\). This results in an effective pulse number, \(N_{line}\), expressed as  
\[N_{line}\ =\ 2w_{0}f/v_{scan}. \tag{4}\]  
In the case of multiple overscans, \(N_{line}\) has to be multiplied by the number of overscans #os. Please also keep in mind that non-uniformities in pulse distribution arise due to the use of laser spots with Gaussian beam profiles. However, Equation (4) assumes uniform pulse overlap in two dimensions.