## 1 Introduction
For example, advances in this material have been investigated by modifying the surface of the polymer with electron beam, CAP plasma in the ambient air, and with carbon ion immersion [7]. Additionally, experiments have been performed to improve both the surface as well as the bulk properties of UHMWPE by reinforcing the polymer matrix with various composite materials [7]. All of these advances can be referenced in more detail in [PERSON] et al. [7]. Along with increased research in improving the polymer properties, there have also been studies concerning characterizing the mechanical failure of UHMWPE in applications such as the laser welding of lap joints investigated in [8; 9]. Yet, it has been shown that there are shortcomings associated with implementing UHMWPE as well, as it is still in many cases a major factor contributing to the failure of joint replacements [2]. The first downside of UHMWPE is that it does not have very good wear characteristics [4; 10]. When a joint is in use over time, small particles of the polymer wear off into the surrounding tissue, which can eventually cause failure of joint replacement because the wear debris can trigger a response by the immune system, causing the implant to loosen [2; 5; 10; 11]. When it loosens too much, it can cause pain and may create a necessity for the joint to be replaced again. Another related issue is the adhesion of UHMWPE to the bone and lubrication by the synovial fluid in the joint [4; 10; 12]. These issues result because of the polymer's inertness and nonpolar properties [12]. Furthermore, the low compatibility of the UHMWPE surface to the synovial fluid in the joint causes an inability of the fluid to cover the polymer [10]. The negative effects of the low wettability of the surface outweigh the positive effects of the low friction coefficient of UHMWPE and can cause more wear [10]. Improvement of the wettability by making the surface more hydrophilic can increase the lubrication in the joint and therefore decrease the wear of the polymer [10]. The increased wettability as well as increased roughness can also positively affect the adhesion of the polymer surface to the adjacent bone, further improving the performance of the joint replacement [6; 12]. Due to the suboptimal wear, adhesion, and lubrication properties of UHMWPE, the goal of this study was to investigate the effects of a helium tubular CAP plasma treatment for the improvement of the hardness of the surface of the polymer to characterize its mechanical wear, and the surface roughness and contact angle to characterize the adhesion and lubrication properties. Many other studies have used plasma to modify the surface of UHMWPE or other polymers for this desired purpose. It is widely accepted that plasma treatment decreases the contact angle on the surface of a polymer, thereby increasing its hydrophilicity [4; 12; 13]. For roughness on the other hand, there seems to be some discrepancy on the ability of different plasma sources to have a sizable effect as [PERSON] et al. [12] reported no significant change in the root mean square roughness of the surface with its argon CAP source, while references [13; 14] showed significant increase in the R\({}_{\rm{q}}\) value of the surface after plasma treatment with their respective oxygen and nitrogen sources. Furthermore, in the aforementioned previous study, [PERSON] et al. found that both the location in the plasma plume used for treatment and the helium source flow rate had different magnitudes of effects on the roughness of UHMWPE samples that were preliminarily treated. This discovery added to the motivation for this study to correlate the type and intensity of plasma species to the chemical and physical changes of the treated surface. The effects of the length of time of the plasma treatment, the part of the plasma plume used for treatment, and the flow rate of the helium gas on the surface properties of UHMWPE were studied.