**1. Introduction**

Polymers such as polyethylene (PE) are the most widely used materials for medical applications due to their properties (i.e., high flexibility, low density, high chemical resistance, biocompatibility) [1,2]. However, its medical application often requires a surface modification and enhancement of the surface properties (i.e., low surface hardness). Therefore, special surface treatments must be applied to improve the physicochemical properties. This can be achieved using thin layer technology, including oxygen and nitrogen plasma discharge [3], laser irradiation [4,5], deposition of anti-wear and/or functional coatings (i.e., diamond-like carbon (DLC)) [6–8] and immobilization of biopolymers (e.g., chitosan and its derivatives) [9,10]. Since plasma treatment results in the generation of high-energy species such as radicals, ions, or molecules in an excited electronic state, this enables surface reactions to take place and leads to surface activation and modification [11]. Such plasma techniques can transform PE

into a valuable material for medical applications due to the modification of the surface without any interference in the substrate interior [12].

In the case of biomedical applications, the pre-treated processes (i.e., plasma etching) have a huge impact on the coating properties, biocompatibility, and enhancement of cell attachment [13]. Plasma treatment is the most versatile surface modification technique and involves electron-induced excitations, ionization, and dissociation to facilitate the production of tailored surface properties such as wettability, roughness, and many more chemical, physical, and biological properties of the polymer surface. For enhanced adhesion, the surface free energy of the polymer should be larger than that of the substrate it will be bonded with. This is why surface modification of polyethylene is required to increase the surface free energy, which corresponds to a decrease of contact angle. In addition, it is very beneficial for many applications to obtain protective and gas barrier coatings (based on DLC structure) as well as obtaining the most biocompatible surfaces (based on DLC and chitosan structures) on the polymeric substrate [7,9,14–16]. DLC layers are characterized by high hardness (up to 30 GPa) and a high Young's modulus, but usually also high internal stresses (up to 7 GPa). These properties are related to the presence of a *sp*<sup>3</sup> C fraction in the structure [17]. However, the stresses can be reduced by incorporating other atoms into the structure (i.e., Si, O, N, F [18–20] or metals [21,22]). It is worth mentioning that the reduction in stresses is often associated with a reduction in hardness and elastic modulus of the layers [6,23].

In this paper, the RF CVD (radio frequency chemical vapor deposition) method was used to modify the physicochemical parameters of LDPE (low-density polyethylene) substrate. The experimental part consisted of different approaches to studying various multi-layer systems (i.e., DLC, N-DLC layers, or Si-DLC layers). Every time, the polymer substrate was first treated and functionalized by plasma etching using Ar+ ions. Precise characterization before and after surface modification was performed, presented, and discussed. The surface properties of the modified PE were determined by scanning electron microscopy (SEM) with chemical composition analysis (EDX). Atomic structure and topography were examined by infrared spectroscopy (Fourier transform infrared-attenuated total reflectance (FTIR-ATR)) and atomic force microscopy (AFM), respectively. The nanoindentation method was applied to assess hardness and Young's modulus profiles. Cytotoxicity in vitro against the MG-63 cell line was evaluated by Alamar Blue assay. Additionally, the influence of surface modification on wettability and surface free energy of modified polyethylene substrate was also examined.

#### **2. Materials and Methods**

#### *2.1. Sample Preparation and Surface Treatment*

The material used in this study was low-density polyethylene prepared in the form of regular samples (width/length/height—10 mm/7 mm/4 mm). The average chemical composition of this material was 99.5 at.% carbon and 0.5 at.% oxygen (according to EDS analysis, EDAX Genesis, EDAX Inc., Mahwah, NJ, USA) and provided by Sigma-Aldrich (Karlsruhe, Germany). The oxygen content determined on the LDPE surface is probably the result of the adsorption of this element on the surface in air atmosphere. Before coating deposition on PE substrates, the samples were chemically purified in isopropanol and subjected to Ar+ ion pre-treatment (etching process) in plasma conditions. The process of PE surface modification was performed in a RF CVD reactor (Elettrorava S.p.A., Turin, Italy), which generated plasma using radio-frequency discharge (13.56 MHz) under low-pressure conditions. The distance between the electrodes (cathode–anode) was 20 mm. All plasma processes (treatment and layers deposition) on the PE substrate were performed at room temperature (297 K) due to the low thermal resistance (melting point 383 K) of polyethylene. The treatment in the RF reactor of the polyethylene surface (ion etching) was conducted for all samples in order to prepare the substrates for further modification processes and surface purification from adsorbed gases (e.g., O2). This stage of processing was conducted in Ar flow conditions (75 cm3/min) under a pressure of 53 Pa and plasma density of 0.08 W/cm<sup>2</sup> for 10 min. The experiments consisted of four independent series of PE surface

modification, including the deposition of DLC, N-DLC, and Si-DLC coatings, as well as multilayer systems. These processes were carried out in accordance with the technical parameters presented in Table 1.


**Table 1.** Technical parameters applied in surface modification of LDPE (low-density polyethylene) substrates using the RF CVD (radio frequency chemical vapor deposition) method.

> Note: ρPrf.—plasma density; *p*—pressure in the chamber; *t*—process time.

In addition, the unmodified substrate (PE\_0 series) was also tested to compare the obtained test results. The obtained samples were characterized using techniques adequate for material engineering as well as the evaluation of biological activity.

#### *2.2. Surface Characterization and Mechanical Study*

The surface microstructure and chemical composition of unmodified and coated polyethylene was examined using scanning electron microscopy (NOVA NANO SEM 200, FEI, Hillsboro, OR, USA) with energy dispersive X-ray spectroscopy analysis (EDX). In the case of the detection of light elements (i.e., C, N, and O), an accelerating voltage of 5 eV was applied. In addition, the thickness of the obtained layers was examined based on the cross-section of the tested samples. Furthermore, the topography of tested samples was investigated using atomic force microscopy (AFM, Bruker, Santa Barbara, CA, USA) equipped with peak force tapping mode with a MultiMode 8 (Bruker, Santa Barbara, CA, USA) microscope with a Sb-doped silicon tip of 8 nm diameter. The chemical structure of the polymer surface was examined using FTIR-ATR (Fourier transform infrared) spectroscopy on a Bio-Rad FTS60 V device (Digilab Division, Cambridge, MA, USA). The spectra were measured within 400–4000 cm<sup>−</sup>1, 275 scans, and 4 cm<sup>−</sup><sup>1</sup> resolution.

Contact angle measurements were conducted using the sessile drop technique performed on a DSA10Mk2 (Kruss, Hamburg, Germany) analyzer. Wettability and surface energy measurements were made using ultra-high-quality water (UHQ—water produced with the use of UHQ-PS, Elga, Buckinghamshire, UK) and diiodomethane (Aldrich, Taufkichen, Germany) droplets with a volume of 0.2 μL. For each tested sample, five independent measurements were carried out on the surface.

Indentation technique with G200 Nano Indenter® (MTS, Oak Ridge, TN, USA) equipped with Berkovich-type diamond tip was used to assess the mechanical properties of the surface. Profiles of hardness and modulus of elasticity were acquired by the continuous stiffness measurement (CSM) method, where the maximum penetration depth was set to 2 μm.
