## 2 Materials and Methods  
Selective plated copper coatings were meticulously deposited on two types of metallic substrates made of either medical-grade stainless steel (SS) 1.4021 or nickel alloy Ni200. The substrates were cut to dimensions of 20 mm \(\times\) 30 mm \(\times\) 3 mm from sheets using wire electro-cutting. The dimensions were provided by a surgical instruments manufacturer (Chirmed, Rudniki, Poland). Following the cut-out process, the steel substrates were submitted to heat treatment, performed to relieve stresses. Finally, the surfaces of the substrates were finished using peening with glass micro-beads of gradation 40-70 \(\upmu\)m. These well-defined and standardized procedures were implemented to ensure consistent and reliable samples for further investigation and analysis. During the selective plating process, two types of copper electrolyte solutions--acidic and alkaline--were used. Prior to initiating the copper-plating process, a series of preparatory operations were performed. These included substrate masking, specific preparatory steps depending on the substrate material, and application of a bonding layer. First, a rectangular area of 20 mm \(\times\) 25 mm (500 mm\({}^{2}\)) was carefully masked on the surface of each substrate. The preparatory steps for both stainless steel and nickel alloy substrates included electrochemical cleaning, electrochemical etching, and activating the masked surface. Following this, a bonding nickel layer of 1 \(\upmu\)m thickness was deposited via selective plating. While the presence of such a layer is in favor of the adhesive strength, it simultaneously serves as a hydrogen and oxidation barrier. Finally, a copper layer of 40 \(\upmu\)m thickness of was plated onto the surfaces. Immediately after the plating procedure, all samples were thoroughly rinsed with deionized water and carefully dried to ensure their readiness for subsequent analysis and evaluation. All utilized chemical reagents, purchased from Marvin Ltd. (Varna, Bulgaria), were of analytical grade quality, ensuring their reliability and accuracy. Two distinct plating electrolytes were used for copper deposition: an acidic solution with a pH value of 0.5 \(\pm\) 0.1, and an alkaline solution with a pH value of 8.5 \(\pm\) 0.1. It is worth noting that all experimental procedures were meticulously carried out under conditions of a controlled laboratory environment. The solutions were maintained at a constant temperature of 20 \({}^{\circ}\)C \(\pm\) 1 \({}^{\circ}\)C throughout the selective plating process. The setup configuration for selective plating is illustrated schematically in Figure 1.  
In order to thoroughly assess and analyze the properties of the coatings, a comprehensive range of characterization techniques was employed. The microstructure and thickness of the coatings were investigated using an upright metallurgical microscope equipped with a digital camera and dedicated software (LM-308, L&D, DGT-SRT200A, Dandong, China). Cross-sections of the plated samples (including substrates) were created using a low-speed cutting machine (DTQ-5, LaiZhou Weiyi Experimental Machinery Manufacture Co., Yantai, China). All samples used for optical microscopy were molded in black resin, grinded with alumina sandpaper, and polished with diamond suspension. X-ray diffraction (XRD) analysis was carried out to evaluate the phase composition of the plated layers using a diffractometer (D8 Discover, Bruker, Billerica, MA, USA) with CoK\(\alpha\) radiation (\(\lambda\) = 1.79026 A) at room temperature. The XRD patterns were collected in 2-Theta coupled mode in the range 20-138\({}^{\circ}\), identifying and matching the peaks utilizing XRD analysis software (X'pert HighScore Plus, PANalytical B.V., Almelo, The Netherlands). The  
Figure 1: Schematic illustration of the selective plating process (**left**) and a plated sample (**right**).