Laser Scanning Microscopy

Tribological tests were performed using a reciprocating friction and wear testing machine SRV IV (Co. Optimol Instruments Prüftechnik GmbH). A cylinder on plate geometry was used, as well as a sphere on plate geometry (Fig. S1a and b, ESI†). The average roughness R_a of the cylinder was 0.26 μm with 15 mm in diameter and 22 mm in length providing a contact length of 15 mm (standard test specimen from Optimol Instruments Prüftechnik GmbH). The balls had an average roughness of R_a = 0.26 μm and 12.7 mm diameter. Before the test, the polymeric specimen was attached on a 100Cr6 steel plate with 24 mm diameter. The tests were performed at 24 ± 1 °C, relative humidity of 30 ± 5%, reciprocating frequency of 50 Hz or 20 Hz, an initial contact pressure of 7.2 N mm⁻² for the cylinder on plate geometry and 50.5 N mm⁻² for the sphere on plate geometry with an applied load of 10 N. The length of stroke from each reciprocating cycle was 1 mm. Each test was carried out for 30 min. For more information on the chosen parameters, see Table S1 (ESI†). COF was evaluated by averaging the COF received from the software on the last 50 cycles of each measurement. For every specimen three measurements were carried out. Wear was evaluated for the specimens using a ball on plate geometry and was measured with a VK-9710K Color 3D-Laser scanning microscope (Co. Keyence Corporation).
For friction and wear tests with applied electrical field, the parameters were adjusted to a reciprocating frequency of 1 Hz and an applied load of 10 N. A ball on plate geometry was chosen as depicted in Fig. S1c (ESI†). A rectangular current collector loop in a distance of 1.0 ± 0.1 cm was melted on the surface of the specimen by heating. A VersaSTAT 3F Potentiostat Galvanostat (Co. Ametek Scientific Instruments) was used for applying electrical fields.

All surface resistance measurements were performed with a 2400 Sourcemeter (Co. Keithley Instruments) with a 4-wire sense mode configuration in combination with a custom made 4-point measuring probe. The cylindrical probe tips were arranged in line with a diameter of 0.8 mm and a distance between the probes of 2.1 mm. A constant current was applied through the outer probe tips to the sample. The voltage drop from the outer to the inner probe tips was used for resistance calculation based on a method described by Schroder et al.⁵⁶ Each measurement was performed with a constant current in the range of 1 μA to 1 mA at 2.1 V for 200 s of measurement time. The average value of 6 measurements at different locations on the sample surface is reported for each of the samples.
Attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIR) was recorded with a 1.9 cm⁻¹ spectral resolution on a 670 FT-IR spectrometer (Co. Varian Inc. (now: Agilent Technologies)). The assignment of measured vibrations were supported by DFT-based calculations on a D3(BJ)-BP86-def2-SVP level of theory.^57–61 Deviations to the measured spectra were described by Benavides-Garcia and Monroe.⁶² All calculations were carried out using the ORCA computational chemistry program.^63,64 Raman spectra were recorded on an inVia confocal (Co. Renishaw) with an excitation wavelength of 532 nm, 3 times for each specimen with 20 s exposure time. Hardness testing was performed using a Fischerscope H100C XYp Nanoindenter (Co. Helmut Fischer GmbH) using a Vickers diamond indenter. After contact with the surface, the indenter was approached into specimens at a constant rate of 300.00 mN/60 s until 150 mN of force was reached and withdrawn from the surface at the same rate as loading. At least 12 indentations were performed for each specimen and the average value was reported. Surface roughness measurements and optical imaging were performed using a VK-9700 Color 3D-Laser scanning microscope (Co. Keyence Corporation). For each sample, at least five randomly selected areas of the surface were measured and the surface roughness R_a and surface depth R_z were determined. High-resolution images of the composite material were taken using a scanning electron microscope (SEM, S-3400N, Co. Hitatchi Science Systems, Ltd) and spectral maps for sulfur and phosphorus were prepared using energy dispersive X-ray spectroscopy (EDX). The samples were fractured after storage in liquid nitrogen for at least 3 h and the exposed surface was coated with a thin platinum layer using a high vacuum platinum sputter at low voltage (brittle fractures). High-resolution transmission electron microscopy (FEI, Talos 120C, Co. Thermo Fisher Scientifics) images were taken of selected polymer compounds. Therefore, very thin lamellae were sectioned with a diatome diamond knife (Cryo-Mikrotomy, Co. Reichert-Jung Ultracut E and RMC CR-X Cryoattachment) at a temperature of −120 °C. The freshly microtomed sample surfaces were subsequently measured by AFM (MultiMode 8, Co. Brucker). The ultrathin sections (about 60 nm) were collected and used for TEM measurements. By evaluating the distribution of the added liquid and solid lubricants in the bulk material, the tribological mechanisms leading to self-lubrication will be analyzed.

The mitophagy detection kit was used to detect mitophagy in NPCs. The NPCs were seeded in the logarithmic growth period on a 6-well plate covered with cell slides at 2 × 10⁵ per well. The NPCs were cultured at 37 °C overnight under 5% CO₂ saturated humidity and treated as depicted above. Next, the cells were incubated with 100 nM Mtphagy Dye working solution for 30 min at 37 °C for mitochondrial probe staining. After that, the cells were washed twice with PBS and subjected to IL-1β for 24 h. After culturing for 24 h, 1 μmol/L LYSO DYE working solution was added to the cells and then incubated for 30 min at 37 °C. Finally, the NPCs were observed under a Laser Scanning Microscope (LSM) (ZEISSLSM780, Germany).