## 2 Materials and Methods  
The UHMWPE samples used in this study had a molecular weight range of 5.2 \(\times\) 10\({}^{6}\) g/mol to 6.2 \(\times\) 10\({}^{6}\) g/mol as obtained from Mitsubishi Chemical Advanced Materials Inc. The UHMWPE resin used to make the purchased polymer sheet was manufactured in accordance with ASTM D4020. Other material properties of this polymer can be found in the data sheet in reference [15]. Thirteen-mm diameter circular samples with a 3-mm thickness were cut from the polymer sheet. The samples were then attached with epoxy to metal disks, which were needed later for compatibility with the atomic force microscope (AFM) machinery. These metal disks were then fixed to the top of 32-mm diameter epoxy pucks sized to fit into the polishing machine so that the samples could all be polished simultaneously with an automatic polisher. This setup is clarified in Figure 3.  
A Buehler AutoMet Variable Speed Grinder Polisher was used with grit paper and diamond suspensions to polish the polymer samples down to a 0.05 \(\upmu\)m diamond suspension. After each of the samples was sufficiently polished, each was separated from the epoxy pucks and the bottom of the metal disk was sanded to remove excess epoxy and ensure they could be properly inserted into the AFM. The samples then underwent an additional cleaning treatment in a CD-4800 Ultrasonic Cleanser. This allowed the samples to be pristine enough to be analyzed by the AFM without risk of causing damage to the machine. The CAP plasma source that was used for the various plasma treatments is shown in Figure 1. The source consists of a 4 mm inner diameter glass tube with a thickness of 1 mm and an AC voltage provided through a copper electrode rounded externally on the tube to excite the helium atoms and generate the plasma inside the tube. The generated plasma filled the entirety of the glass tube with its plume extending outside the tubing up to a few centimeters depending on the gas flow rate. The plasma was created with an AC voltage of about 15 kV peak to peak, an RMS current of about 0.5 A, and a frequency of 20 kHz supplied to the electrode, which was placed 11.4 cm from the opening of the glass tube. The samples were treated with different regions of the plasma plume, at different gas flow rates, and for different treatment times. Figure 4 shows a schematic of the two treatment locations of the UHMWPE samples used in this study, one at the tip of the plume and the other at the base of the plume near the opening of the tube. It is important to reference again that spectra were taken of the plasma plume with the UHMWPE sample present at these locations in the previous study by the authors and are presented in Figure 2.  
Figure 3: UHMWPE sample setup used for surface polishing and cleaning preparation. The various source parameters used for treatment are summarized in Table 1. The table also shows the measurements performed for each sample after each treatment. The contact angle on each sample was measured after each plasma treatment to help characterize the wettability and adhesion properties of the polymer surface. Higher wettability and hydrophilic properties are related to better adhesion to the UHMWPE surface and a decrease in the wear on the polymer because of the increased lubrication, which stems from the improved interaction with the synovial fluid in the joint [10]. A decrease in the contact angle is an indication of an increase in the hydrophilicity of the surface [6,16]. To perform the contact angle measurements, a 0.5 \(\upmu\)L drop of deionized water was gently placed onto the UHMWPE samples using a Fisherbrand Finnpipette II micropipette, rated between 0.5 and 10 \(\upmu\)L and imaged using a digital camera. The image was then analyzed with ImageJ ([PERSON]. _Image J._ version 1.52a; National Institutes of Health: USA, 2018. ), which is a common software used for contact angle measurements [17]. The contact angle measurements were performed on the pristine samples as well as after each subsequent plasma treatment.