**1. Introduction**

Resurfacing joint endoprostheses, e.g., total resurfacing hip arthroplasty (THRA) endoprostheses, are a bone-tissue-preserving option offered for relatively young and active patients with advanced osteoarthritis (OA). In contrast to traditional long-stem total hip replacement (THR) endoprostheses requiring surgical removal of the femoral head and neck, for the current generation of THRA

endoprostheses, the femoral head is not removed, but is instead trimmed and capped with metal components fixed in subchondral bone with cement and a short stem placed in the femoral neck [1,2]. An essential innovation in the fixation technique for components of THRA endoprostheses in the periarticular trabecular bone—entirely cementless interfacing by means of the biomimetic multi-spiked connecting sca ffold (MSC-Sca ffold)—was designed, manufactured, structurally and geometrically functionalized, and tested in our previous research [3–8]. The concept of multi-spiked (needle-palisade) fixation of RA endoprostheses components in bone was invented by Rogala [9–11].

Unmodified metallic surfaces of joint endoprostheses components interacting with the bone have low osteoconductive and osseointegrative behavior; therefore, surface modifications are essential to enhance their biocompatibility and biological performance. Calcium phosphate (CaP) bioceramics are widely used in the field of bone regeneration, both in orthopaedics and in dentistry, due to their good biocompatibility, osteoconductivity, and osseointegrativity [12–17]. CaPs are of special importance since they are the most important inorganic constituents of hard tissues in vertebrates [12–14]. Coatings on orthopaedic or dental implant surface with a layer of CaP have also proven to be an effective approach in providing the base material with enhanced biocompatibility, osteoconductivity and osseointegrativity [15–17].

Synthetic CaP coatings can be prepared using a variety of processes. In general, the commonly used methods can be divided into two groups, physical deposition techniques and wet-chemical techniques [15,18]. Physical methods include plasma spraying [19], pulsed laser deposition [20], low-temperature high-speed collision [21], radio-frequency magnetron sputtering [22], gas detonation spraying [23] and ion implantation [24]. Chemical methods include chemical vapor deposition [25], biomimetic deposition [26,27], hydrothermal methods [28], sol–gel deposition [29] and electrochemical methods [27,30–38].

CaP coating deposition on flat substrates has been widely investigated, while CaP deposition on the bone-contacting surfaces of complex geometrical shapes, e.g., of porous implants or additively manufactured sca ffolds, have only been studied relatively rarely and quite recently [39–41]. Most CaP deposition methods have a line-of-sight requirement, which greatly limits choices in coating irregular shapes [36]. Only a few methods can be applied for complex-shaped or porous materials and sca ffolds. Therefore, for improving the osteoinductive and osseointegrative behavior of the bone-contacting surface of the MSC-Sca ffold, electrochemical methods are preferred due to its shape complexity. The commonly used technologies for this purpose are electrophoretic deposition (EPD) and electrochemical deposition (ECD) [30].

The ECD process can be carried out at room temperature and allows for the CaP surface modification of complex-shaped Ti-alloy implants, resulting in a non-delaminating CaP coating of ca 1 μm thickness characterized by relatively high adhesive strength in comparison with the EPD process, where the hydroxyapatite (HA) coating is obtained from a suspension containing HA particles. Without applying thermal post-processing by subsequent sintering, the EPD deposited HA coating delaminates [21,24,30,31].

In the ECD process, CaP coatings are formed from an electrolyte containing calcium nitrate, Ca(NO3)2, and ammonium dihydrogen phosphate, NH4H2PO4, wherein the weight ratio of calcium to phosphorus is ca. 1.67 and is the same as the ratio of Ca/P in the native osseous CaPs [32,42–46]. This method enables control of the properties of the deposited coatings by appropriately choosing the electric parameters of the ECD process, such as current density [47] or electric potential [29], and/or by adjusting the process time [23]. The subsequent immersion of the modified substrates in simulated body fluid (SBF) leads to the transformation of the amorphous CaP coating into a crystalline CaP coating [29,31]. Application of chemical pretreatment, like acid, alkaline or acid–alkaline treatment (AAT), may advantageously influence the outcome of the ECD process [48–53].

Attempts to modify the bone-contacting surfaces of the MSC-Sca ffold pre-prototype by ECD of CaPs has been undertaken initially at constant current densities with subsequent immersion in SBF (to transform the amorphous CaP coating into bone-like biomineral coating) [54]. It was observed that the deposition of CaPs on surfaces of the MSC-Sca ffold pre-prototypes can be controlled by adjusting the current density. These modifications were successfully performed in the galvanostatic ECDj=const process, but continued research using the potentiostatic process (ECDV=const) showed significantly higher repeatability than the galvanostatic ECD process [55]. Similarly, the poorly investigated CaP deposition on complex-shaped substrates (e.g., sca ffolds) and the unsatisfactory attempts during our preliminary research in applying ECD process parameters recommended for flat substrates to the MSC-Sca ffold pre-prototypes, strongly justify the need to take an experimental approach for finding the suitable range of conditions for the CaP ECDV=const process on the MSC-Sca ffold pre-prototypes.

The particular aim of this paper was to present the determination of the suitable range of conditions for potentiostatic electrochemical deposition of calcium phosphates (CaPs) on the MSC-Sca ffold prototypes to achieve a native biomineral Ca/P ratio in the coating, which is of grea<sup>t</sup> importance for good biocompatibility and biological performance of the implant in vivo. Good biological performance of this implant was proved in our recent investigation in swines on a partial knee arthroplasty (RKA) endoprosthesis working prototype with the CaP coated in potentiostatic ECD process MSC-Sca ffold [56].

The main aim of our work is to elaborate on the suitable MSC-Sca ffold prototypes for a new generation of entirely cementless RA endoprostheses.

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