*2.1. MSC-Sca*ff*old Pre-Prototypes*

Surface modification of the MSC-Scaffold prototype for non-cemented resurfacing joint endoprostheses was carried out on the MSC-Scaffold pre-prototypes designed as fragments of the central part of the femoral component of the TRHA endoprosthesis. The multilateral spikes of the MSC-Scaffold pre-prototypes were arranged in concentric parallel rings around the central spike with axes parallel to each other, whereas the central spike was coincident with the femoral head axis of symmetry. The length of the square side in the spike pyramid's base was 0.5 mm in the MSC-Scaffold CAD model. The prototype THRA endoprosthesis with the MSC-Scaffold manufactured using selective laser melting (SLM) of Ti4Al6V powder is presented in Figure 1A. In Figure 1B, CAD models of the MSC-Scaffold pre-prototypes designed for this research are presented. The two design variants vary by the distance between spike bases, 200 μm (PSc200) and 350 μm (PSc350), both circumferentially and radially, which corresponds to the thickness of bone trabeculae of cancellous bone. In Figure 1C, the MSC-Scaffold pre-prototypes manufactured using SLM are shown. The manufacturing was subcontracted to the Centre of New Materials and Technologies at the West Pomeranian University of Technology in Szczecin, Poland. The process parameters applied during the SLM manufacturing were: laser power 100 W, layer thickness 30 μm, laser spot size 0.2 mm, scan speed 0.4 m/s and laser energy density 70 J/mm3.

**Figure 1.** (**A**) Prototype of the entirely cementless total resurfacing hip arthroplasty (TRHA) endoprosthesis with the multi-spiked connecting scaffold (MSC-Scaffold) manufactured using selective laser melting (SLM) of Ti4Al6V powder; (**B**) CAD models of the MSC-Sca ffold pre-prototypes for RHA endoprostheses designed in two geometrical configuration variants, which vary by the distance between the spike bases, 200 μm (PSc200) and 350 μm (PSc350), both circumferentially and radially, and (**C**) the MSC-Sca ffold pre-prototypes manufactured on the basis of these CAD models using SLM.

### *2.2. Preparation of the MSC-Sca*ff*old Pre-Prototypes' Surfaces*

After SLM manufacturing, to remove the adhered powder aggregates from the spike surfaces, a manual blasting treatment was carried out using an experimentally customized abrasive mixture composed of equal proportions of white aloxiteF220 (~53–75 μm), white aloxite F320 (~29.2 μm ± 1.5%), and blasting micro glass beads (~30 μm ± 10%) [57]. Cleaning in an ultrasonic bath (Sonic 3, Polsonic, Poland) was applied using the following agents distilled water, ethanol, acetone and, again, distilled water three more times; each stage of cleaning was carried out for 15 min. After that, the MSC-Sca ffold pre-prototypes were dried at room temperature and the initial weight was measured using a precise analytical balance (AS 110/X, Radwag, Poland).

### *2.3. Determination of the Most Suitable Range of Conditions for the ECDV*=*constProcess*

To determine the most suitable range of potential (VECD) for the ECDV=const process, a total of 56 MSC-Sca ffold pre-prototypes (28 of each variant) were subjected to surface modification. To search for the appropriate conditions of the ECDV=const process, VECD values from −9 to −3V were investigated using a potentiostat-galvanostat apparatus (PGSTAT 302N, Metrohm Autolab, Ultrecht, The Netherlands). The CaP ions were deposited from a solution composed of 0.042 M calcium nitrate, Ca(NO3)2, and 0.025 M ammonium dihydrogen phosphate, NH4H2PO4, with pH=6. The ECD process was performed in a two-electrode system. The process was carried out for one hour at room temperature. A gold plate anode was used as the counter electrode. After the ECDV=const process, the MSC-Sca ffold pre-prototypes, playing the role of working electrode, were rinsed with distilled water and, to convert the deposited amorphous CaP coating into the bone-like biomineral coating, they were immersed for 48 h in an SBF solution composed of 6.8 g/<sup>L</sup> NaCl, 0.4 g/<sup>L</sup> KCl, 0.2 g/<sup>L</sup> CaCl2, 0.2048 g/<sup>L</sup> MgSO4·7H2O, 0.1438 g/<sup>L</sup> NaH2PO4·H2O and 1.0 g/<sup>L</sup> NaHCO3 at 37 ◦C. After the incubation in SBF, the MSC-Sca ffold pre-prototypes were dried at room temperature and their final weight was measured. The weight increase due to surface coatings deposited on the spikes of the MSC-Sca ffold pre-prototypes was calculated as the di fference between the initial and final weights of the modified MSC-Sca ffold pre-prototype. An analysis of chemical composition of the coating deposited on the lateral spike surfaces of the MSC-Sca ffold pre-prototypes was performed using a scanning electron microscope (Hitachi TM-3030, Hitachi High-Tech Technologies Europe GmbH, Krefeld, Germany) equipped with the energy dispersive X-Ray (EDS) system (Oxford Instruments, Abingdon, UK). The results can be used for calculating the Ca/P ratios.

### *2.4. The Influence of the AAT Pretreatment*

The same ECDV=const surface modification process was performed to examine the influence of the AAT pretreatment on the final surface modification. In this step, the VECD values that provided the highest weight increase simultaneous with Ca and P contents with Ca/P ratios corresponding to the Ca/P ratios of native osseous CaP there were applied to new pre-prototypes. A total number of 36 MSC-Sca ffold pre-prototypes were modified, 12 for each VECD value identified as favorable; half of the pre-prototypes underwent AAT pretreatment. The AAT process was conducted in 40% H2SO4 for 40 min at 60 ◦C and subsequently in 1 mol/L NaOH for 40 min at 80 ◦C.

EDS surface mapping of three randomly selected subareas of the lateral spike surfaces of each MSC-Sca ffold pre-prototype was performed using a specialized software analyser in the EDS system used. Based on mapping analysis, the regions with CaP deposited on the spikes' lateral surface were indicated and the coverage degree of the lateral spike surfaces was determined. In each of the analysed subareas, 10 pointwise measurements of the chemical composition were made, and the Ca/P ratios were calculated. The analyses of the coverage degree of the lateral surface of spikes and the deposited coating uniformity were made using the professional software tool ImageJ (National Institutes of Health, Bethesda, Maryland, USA). Structure and phase composition of the deposited coating was identified by XRD on a PANalytical EMPYREAN X-ray di ffractometer (Malvern, UK) at a scanning

speed of 0.02◦/s with Cu Kα radiation (λ = 0.15405 nm, 40 mA, 40 kV) at a 2θ range of 30–70◦. Since, there was no technical possibility to analyze the surface of MSC-Scaffold pre-prototypes directly, so to obtain the XRD roentgenograms we had to use the deposits detached mechanically from the spikes as a powder sample.
