## 2 Materials and Methods  
Sample preparation required the following reagents: zirconium oxide (Aldrich, 99%, St. Louis, MI, USA), magnesium oxide (Aldrich, 99.9% USA), calcium silicate (Aldrich, 99% USA), tricalcium phosphate (Carlo Erba Reagenti, Cornaredo, Italy, Analytical Grade), ethanol (PanReac, 99.8%, Barcelona, Spain), polyvinyl butyral (PVB, Sigma-Aldrich USA), and Beycostat C213 (CECA, Chevagnes, France). A powder mixture was prepared according to the reported eutectic composition (53 MgO; 47 ZrO\({}_{2}\) in mol%) by using commercial powders of ZrO\({}_{2}\) and MgO [10]. These ceramic powders were used to fabricate cylindrical precursors by isostatic pressing for 3 min at 200 MPa. The rods of about 3 mm in diameter and 7 cm long were subsequently sintered at 1500 \({}^{\circ}\)C during 12 h.  
The eutectic composite of wollastonite (W) and tricalcium phosphate (TCP) was prepared by isostatic pressing of the powder mixture followed by a sintering process at 1200 \({}^{\circ}\)C for 12 h, with resulting precursor rods of diameter and length ranging 3 mm and 50-100 mm, respectively. Eutectic composition, 80 CaSiO\({}_{3}\); 20 Ca\({}_{3}\)(PO\({}_{4}\))\({}_{2}\) in mol%, and melting point were obtained from the phase diagram of calcium silicate (CS)-Tricalcium phosphate (TCP) reported by [PERSON] et al. [11]. Directionally solidified eutectic samples of MgO-MgSZ (magnesium stabilized zirconia) and W-TCP were grown with the LFZ technique as described elsewhere [12]. As the heating source a continuous wave CO\({}_{2}\) laser was utilized, and the growth process was carried out in air. Densification stages were applied to eliminate the porosity of the precursor rod using pulling rates of 200 mm/h and a counter rotation in the liquid zone of 50 rpm. To avoid bubbles in the eutectic samples, the growing process was performed downwards. Specifically, for the MgO-MgSZ samples, rates of 50, 100, 300, and 750 mm/h were used without rotation to avoid phase segregation, and 20, 100, and 150 mm/h with 50 rpm counter rotation for W-TCP. The final diameter for the whole samples ranged from 1-2 mm. The W-TCP samples were annealed for 2 h at 600 \({}^{\circ}\)C to relieve internal residual stresses. The surface remelting process of the eutectic rods was performed by pulling the rod downwards with rotation to homogenize the heating generated by the laser beam focused on the sample surface. The W-TCP biocearamic coatings on MgO-MgSZ eutectic rods were prepared by dip coating technique. The substrate rods were dipped and withdrawn several times from the ceramic solution at a fixed speed. The composition of the solution was in wt%: 42.37 W-TCP; 44.49 ethanol; 0.42 Beycostat C213 (as the dispersant agent); 12.72 PVB (as the binder). The thickness layer was controlled by adjusting the concentration of the solution and number of dips. Ten dips were used at a speed of 3 mm/s. No crack formation or spallation were observed on the coating surface after the sintering treatment at 1200 \({}^{\circ}\)C during 12 h, below the melting point of 1402 \({}^{\circ}\)C. This treatment was necessary to improve the densification and adherence of the coating to the substrate. The coated cylinders were superficially melted using the LFZ technique. The power was adjusted to melt the coating with minimum dilution with the substrate. A laser power of 20 W was enough to melt a W-TCP layer of about 200 \(\upmu\)m.  
Characterization of the microstructure and analyses of the composition were performed by using Field Emission Scanning Electron Microscopy, FESEM (FESEM, model Carl Zeiss MERLIN), and the Energy Dispersive X-ray Spectroscopy (EDS) detector coupled to the microscope. Analyses were carried out in polished samples in both transversal and longitudinal configurations. In the case of crystalline W-TCP eutectics, etching was used to reveal the structure. Due the poor contrast between both phases, the samples were etched with dilute acetic acid to remove the TCP phase.