## 1 Introduction  
A tubular cold atmospheric pressure plasma source was investigated and characterized by the authors in a previous study [1]. The plasma produced by this source was generated by flowing helium gas through a glass tube with a copper electrode rounded externally around it to provide an electric field for gas excitation. The plasma plume extended for up to a few centimeters from the opening of the tube as shown in Figure 1.  
Optical Emission Spectroscopy (OES) was used to identify the composition of the plasma along the length of the plume. Results indicated that the type of the plasma species and the intensity of the species' emissions changed at different locations along the plume. Furthermore, spectra were taken for the plasma jet in both a free jet configuration and with an UHMWPE sample present in the plume. For the free jet configuration, one of the emissions measured was the OH line at 308 nm, present only at the base of the plasma plume near the tube opening, while the rest of the plume was dominated by nitrogen Second Positive and First Negative systems. It was also found that the flow rate of the source gas played a role in the emission type and intensity of the plasma species. Higher flow rates led to higher intensities of OH at the base of the plasma plume. Additionally, the intensity of all emissions was found to decrease moving away from base of the plume and the opening of the source tube towards the tip. The OH emission was attributed to water condensation on the inner tube surfaces, while the N\({}_{2}\) and N\({}_{2}\)\({}^{+}\) emissions came from the interaction between the plasma and the nitrogen of the ambient air. With the UHMWPE surface present in the plume, the emission spectra were similar to the free jet configuration with some increased intensity of the OH lines and a new He emission that was measured as the surface was moved closer to the base of the plasma plume. These changes were attributed to the obstruction that the UHMWPE sample provided between the plasma and the ambient air. The results obtained from the previous study can be summarized in the spectra shown in Figure 2.  
The presence of the OH and nitrogen plasma species provides a unique application of the CAP plasma source in the modification of polymeric surfaces. This modification occurs when the species of the plasma interact both mechanically and chemically to change the surface properties of the polymer, such as its morphology and the functional groups present, without affecting the bulk material. A use case where polymer surface modification can be important is for the pre-treatment of UHMWPE to be incorporated in joint implants. UHMWPE is a semi crystalline  
Figure 1: Schematic of cold atmospheric plasma (CAP) source used for the treatment of the Ultra-High-Molecular-Weight Polyethylene (UHMWPE) samples. Figure 2: Optical Emission Spectroscopy (OES) spectra of the plasma plume with the UHMWPE sample present at its treatment locations and with its treatment flow rates. polymer whose crystalline and amorphous phases both influence its mechanical properties [2; 3]. It is part of the polyethylene (PE) family of polymers and has a repeating unit of (C\({}_{2}\)H\({}_{4}\))\({}_{\rm{n}}\) in a linear configuration [2]. This polymer is often used in joint replacements in the knee and hip because of its biocompatibility, low friction coefficient, biological inertness, and inexpensive cost [2; 4; 5; 6]. With the numerous beneficial properties of UHMWPE, it makes this biopolymer a highly sought-after product. UHMWPE has been the material of choice when it comes to many biomedical applications. Not only has UHMWPE's demand in the market raised from 60.9 kilotons to 204.8 kilotons during 2015-2024, its value also stands at about $1.36 billion [7]. With this increase in demand, UHMWPE has an annual compounded growth rate of about 15% [7]. Furthermore, there have been many studies to further improve upon UHMWPE for its use in biomedical applications.