## 2 Experimental Details
### Methods  
(a)  Zirconia substrates preparation  
In this study, 3 mol% yttria-stabilized zirconia powder (from Tosoh Corporation, Tokyo, Japan) with an average size of 60 \(\upmu\)m was used to produce zirconia substrates. Zirconia disks were produced by cold press and sinter technique by using a cylindrical stainless steel mold of 18 mm in diameter, where zirconia powder was compacted by a pressure of 200 MPa for 30 s. Hereinafter, zirconia disks were sintered using a high-temperature furnace (Zirkonofen 700, Zirkonzahn, Gais, Italy) with a sintering temperature of 1500 \({}^{\circ}\)C, a heating and cooling rate of 8 \({}^{\circ}\)C/min and 2 h of holding time. Zirconia samples 14.4 mm in diameter and 2 mm thick were obtained after sintering. (b)  Antenna production  
The antenna printing was performed in three steps:  
1. Micro-cavity creation through laser machining;
2. Silver powder deposition into the cavity;
3. Laser sintering of the silver powder. The schematic representation of laser machining is illustrated in Figure 3.  
Figure 2: SEM micrographs of silver powder: (**a**) 25,000\(\times\) and (**b**) 100,000\(\times\) of amplification. 1. Micro-cavity creation through laser machining  
In this step, a pulsed Nd:YAG laser (OEM Plus, Italy) with 1064 nm of wavelength, a spot size of 3 \(\upmu\)m, 6 W of power and a focal length of 160 mm with a gaussian beam mode was applied to create the micro-cavity on zirconia surface (as a subtractive process). In order to create the micro-cavity, a sequence of lines was performed with a distance of 10 \(\upmu\)m between them, forming a cavity with a width of approximately 350 \(\upmu\)m, as illustrated in Figure 3a. The cavity was created using a laser fluence of 0.72 J/mm\({}^{2}\) and a scanning speed of 50 mm/s, and the laser machining parameters were chosen based on the optimal conditions studied in previously published works by the authors [32,35]. 2. Silver powder deposition into the cavity  
After the micro-cavity creation on the zirconia surface, the silver powder was placed and compacted into the cavity with the help of a spatula, and then a pressure of 8 MPa was applied in a hydraulic press to ensure the total accommodation of the powder, as shown in Figure 3b. During the preparation, the excess silver powder was removed from the surface by using a P800 grit SiC paper. After these processes, the micro-cavities were completely filled with silver powder. 3. Laser sintering of the silver powder  
For the silver powder consolidation, an Nd:YAG laser (Sisma, Piovene Rocchette, Italy, 1064 nm of wavelength) with 0.3 mm of spot size and 40 W of power with a gaussian beam mode was used as an additive process for sintering the silver path. Laser energy of 4 Joules was used for sintering silver powder, and this condition was chosen based on previous works [32,35], considering the best condition to melt the powder with fewer defects for the ceramic substrate. In the sintering process, a dual-axis automatic worktable was used to adjust the sample position. 4. Antenna test configuration  
Characterization and testing of the fabricated antenna began by interfacing it with standard measurement equipment. For that, a U.FL cable was cut in half, and its ground and signal lines were separated, as shown in Figure 4a. Each of these terminations was then connected to the antenna resorting to silver conductive paint, resulting in the device of Figure 4b. Figure 3: Schematic illustration of the antenna design and manufacturing (**a–c**), highlighting the laser strategy used to create the micro-cavity. The antenna was connected to a Vector Network Analyzer (model Keysight E5071C ENA) to measure its properties and determine its suitability for use inside the human body as a communication antenna. This is an important step to test the antenna performance when it is involved in a biological environment since these surroundings have a profound effect on the radiation characteristics of an antenna.