# Wear Effect on the Contact between a Metallic Pin and a Rotating Polymeric Specim
## 2 Materials and Methods
### Experimental Methods  
After thermoforming, UHWMPE was characterized by thermal and mechanical tests; a differential scanning calorimetric (Q-100 DSC analyzer, supplied by TA Instruments, New Castle, DE, USA) and a Shore-D microhardness testing (PCE-DDD 10 analyzer, supplied by PCE Instruments, Meschede, Germany).  
For DSC analysis, 6-8 mg specimens were prepared and heated from 25 \({}^{\circ}\)C to 200 \({}^{\circ}\)C at 10 \({}^{\circ}\)C min\({}^{-1}\) under an inert atmosphere, according to ASTM DF2625 standard. DSC analysis provided the melting temperature (\(T_{m}\)) and enthalpy \(\Delta H_{m}\) of the samples. The crystallinity degree \(X_{c}\) was calculated by the following equation:  
\[X_{c}(\%)=\left(\frac{\Delta H_{m}}{\Delta H_{m}^{0}}\right)\cdot\!100 \tag{1}\]  
where \(\Delta H_{m}\) [J/g] is the melting enthalpy, \(\Delta H^{0}{}_{m}\) is equal to 291 J/g and represents the hypothetical enthalpy of fusion of UHMWPE crystal of infinite extension, according to the literature ([PERSON] et al., 2018).  
The surface hardness was measured according to the standard ASTM-2240 with shore-D scale and a 5 kg load. Six samples of UHMWPE were analyzed on their surface, and the final value was the average of ten measurements for each sample.  
Wear tests were carried out at room temperature through a pin-on-disc device, both in dry and under lubrication with simulated synovial fluid (SSF) containing 0.3 wt % of Hyaluronic Acid (HA) in phosphate-buffered saline solution (pH 7.4). The inorganic electrolyte concentration in the SSF was: 153.1 mM of Na\({}^{+}\), 4.2 mM of K\({}^{+}\), 139.6 mM of Cl\({}^{+}\), and 9.6 mM of phosphate buffer.  
As shown in Figure 2, the pin was mounted with an offset r with respect to the rotation axis and loaded with 30 N. The rotating speed (n = 60 rpm) was constant in all the tests. A single specimen for each test was placed in a containment box fixed on the rotating disc and lubricated drop by drop. The working parameters were chosen in accordance with those commonly adopted by other authors ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018). In particular, following a method developed in ([PERSON] et al., 2018) to simulate the real conditions in the hip joint, the load applied in the pin-on-disc test can be evaluated in the range of 20-30 N.  
The tribometric trials were performed on different specimens at various times up to 240 min. Two different pins were utilized for the two series of tests (under lubrication with SSF and in dry conditions) for a total duration of each series equal to 525 min.  
The metallic tip and the polymeric surface were observed using a digital microscope (Hirox KH-8700, Tokyo, Japan). Each polymeric specimen was examined using a confocal microscope (Leica DCM 3D, Leica Microsystems, Wetzlar, Germany). 3D images of grooves and plastic deformation accompanying the formation of wear tracks were obtained by an EPI 20X-L objective in LeicaScan DCM 3D software. To capture all points on the surface specimen, the z-scan covered a height of 224 um, with z representing the orthogonal direction to the specimen surface. Post-processing of the images was performed using LeicaMap 6.2 software.  
The scans were performed in slit confocal contrasting mode by illuminating the specimens with a high-power white LED (\(\lambda\) = 530 nm). The aperture of the objective was 0.5 in confocal mode, and the volume of the acquired image had the following sizes: 636.61 \(\times\) 477.25 \(\upmu\)m\({}^{2}\) in the xy plane and 224 \(\upmu\)m along the \(z\)-axis. The axial optical resolution (\(x\), \(y\) directions) is r\({}_{\rm x}\) = r\({}_{\rm y}\) = 0.28 \(\upmu\)m, and the vertical resolution (\(z\)-axis) is r\({}_{\rm z}\) < 15 nm. Finally, the relative error referred to the acquisition volume is given by the sum of the relative errors [33], resulting \(\varepsilon_{\rm r}\) = 0.28/636.61 + 0.28/477.25 + 0.015/224 = 1.1 \(\times\) 10\({}^{-3}\).