## 3 Results and Discussion  
### Contact Angle  
The contact angle measurements of the treated samples are shown in Figure 6.  
Figure 6 shows that the contact angle significantly decreased with plasma treatment time seeming to follow a somewhat exponential decay. This was a favorable result, as it is of benefit for the wettability of biopolymers to be hydrophilic due to the positive  
Figure 6: Contact angle measurements analyzed by ImageJ ([PERSON]. _Image J_. version 1.52a; National Institutes of Health: USA, 2018.). effects relating to protein adsorption, the adhesion of cells in the body, and the increase of lubrication within joints [6; 10]. The initial contact angle measurements had a mean value of 79\({}^{\circ}\) with a standard deviation of only 5\({}^{\circ}\) supporting the repeatability of the measurements and the polishing process as the pristine polymer contact angles should have all been the same. With sequential treatments the contact angle decreased, the most dramatic changes generally occurring in the first minute of treatment. The change in the contact angle can be attributed to a two-phase process, first starting with the creation of free radicals on the polymer surface, then finishing with the interactions of certain species with the radicals producing changes to the properties of the surface [6]. The presence of oxygen and OH species for these interactions have shown to create a more hydrophilic behavior on the polymer in other studies [6; 21]. Furthermore, the OH species were shown to be most abundant at the base position of the plasma plume as the helium flow was increased, as shown in Figure 2, which displays spectra taken of the plasma plume with the UHMWPE sample present at its respective treatment locations and for the two different flow rates. It can be clearly seen that the higher flow rate with the sample at the base of the plasma plume produced the highest intensity OH emission at 308 nm, thereby supporting why such a dramatic decrease in the contact angle occurred under these treatment conditions. When interacting with the free radicals on the polymer surface, the oxygen species form polar groups such as hydroxyl and carbonyl functional groups [6; 12]. The surface then exhibits a more hydrophilic behavior as the newly integrated polar species in the polymer surface interact with the polar water droplet. The contact angle of the sample treated by the base part of the plasma plume with the higher 2.5 L/min flow rate after the 20-min treatment was measured at 27\({}^{\circ}\), which is 54\({}^{\circ}\) lower than the 81\({}^{\circ}\) angle obtained without the plasma treatment. There seems to be some debate on the influence of nitrogen species, which was found to be more dominant in the middle and lower parts of the plasma plume, on the polymer surface wettability. [PERSON] et al. [22] stated that N\({}_{2}\) and other nitrogen species can cause peptide coupling which could lead to a decrease in wettability. However, most studies liken the effect of nitrogen species with that of oxygen because they too provide polar groups on the polymer surface, increasing the hydrophilic behavior [4; 23]. This latter argument supports what was found in this study; when the sample was treated by the nitrogen dominated tip of the plasma plume, with a flow rate of 2.5 L/min, the sample showed a decrease in the contact angle from 83\({}^{\circ}\) on the untreated sample to 29\({}^{\circ}\) on the treated sample after the 20-min plasma treatment. This is a decrease of 53\({}^{\circ}\), which is only slightly less than the 54\({}^{\circ}\) found for the contact angle at the base position and can perhaps be attributed to a very slight effect of the OH and oxygen species on the contact angle of a polymer surface when compared to that of just the nitrogen species. At the lower flow rate of 1 L/min at the base position, the change in contact angle was only 32\({}^{\circ}\) after the 20-min treatment.