**4. Discussion**

In this study, the experimental CaP modification of the bone-contacting surfaces of MSC-Scaffold pre-prototypes was undertaken in search of the suitable range of conditions for CaP deposition in the ECDV=constprocess. Since the MSC-Scaffold prototype, as the original concept of a multi-spiked (needle-palisade) fixation of RA endoprostheses components, was developed within the frames of two of our research projects, its most suitable geometrical properties evolved based on the findings of bioengineering research in relation to that primarily suggested in the patented version [3–5]. Additionally, the conditions ECD modifications have changed accordingly. Attempts to modify the bone-contacting surfaces of MSC-Scaffold prototypes initially undertaken at constant current densities were satisfactory [52], i.e., it is possible to control the deposition of CaPs on the bone-contacting surfaces of MSC-Scaffold pre-prototypes by adjusting the current density [52].Significant enhancement of the osteoinduction/osseointegration potential of the MSC-Scaffold prototype was confirmed in pilot experimental studies in animal models and in osteoblast cultures [53]. After the structural-geometric functionalization of the additively manufactured prototype MSC-Scaffold, we observed that the results of CaP modifications of the bone-contacting surfaces of MSC-Scaffold prototypes carried out during the potentiostatic process (ECDV=const) showed much higher repeatability compared to those of the galvanostatic process (ECDj=const).

Experimental CaP modification of the MSC-Scaffold pre-prototypes was carried out in two steps. In the first step, the purpose was to determine the most suitable range of conditions for the ECDV=const process. VECD values from the −9 to −3V range were applied. This stage of investigations showed that the suitable conditions for the ECDV=const process of CaP modification of complex-shaped bone-contacting surfaces of the MSC-Sca ffold prototype are strongly influenced by the geometrical features of the sca ffold prototype, i.e., by the distance between the spikes. In the case of insu fficient room between the MSC-Sca ffold's spikes, the CaP deposits are found between the spikes instead of on their lateral surface. Hence, the PSc200variant of the MSC-Sca ffold pre-prototype was excluded from further research. Based on the characterization of the coating's physiochemical properties—in terms of structural (EDS) and morphological (SEM) properties, and the weight increase of the deposits—the VECD range from −5.25 to −4.75V was determined to provide the expected CaP modification of the bone-contacting surfaces of the PSc350 variant of the MSC-Sca ffold pre-prototype.

In the second step, the influence of AAT pretreatment was examined by applying the previously determined range of VECD values that achieve the native biomineral Ca/P ratio in coatings on the lateral surface of the MSC-Sca ffold pre-prototypes. The investigation procedure applied in the first step was extended to include EDS surface mapping and quantitative analysis of crystalline phases (XRD). The enhancement of the coverage degree of the lateral spike surfaces and the coverage uniformity were ascertained. AAT pretreatment prevents micro-crack formation on the bone-contacting surfaces of the MSC-Sca ffold and also a ffects the increase of the spikes' lateral surface coverage. The Ca/P ratios of deposits on the lateral spike surfaces in all modified MSC-Sca ffold pre-prototypes are consistent with the Ca/P ratios of native osseous CaPs, and plate-like and needle-like CaP crystals appeared on the bone-contacting surface of the MSC-Sca ffold pre-prototypes undergoing the AAT pretreatment.

The best overall results for CaP modification of the bone-contacting surfaces of the MSC-Sca ffold pre-prototypes were obtained for the VECD = −5.00 V—the native biomineral Ca/P ratio of deposits (i.e., the closest values to the Ca/P ratio native osseous CaP) was achieved, as well as the highest average mass growth of the coating and the highest coverage degree of spikes' lateral surface (even in the case of the MSC-Sca ffold prototypes without AAT pretreatment). The numerous micro-cracks observed on the MSC-Sca ffold pre-prototypes CaP modified at VECD= −5.00V were prevented by applying the AAT pretreatment, finally providing the highest uniformity in comparison to the other VECD values.
