The objective of this work is the proposal of a fabrication method capable of yielding an antenna directly integrated in the metal substrate of an implant, during its fabrication, without significant modifications to its design and structure. For this, it is desired the design of a small antenna with little impact on the structure of the implant. Therefore, a wire antenna inside the substrate presents itself as a viable option that meets the aforementioned requirements. The design of such an antenna is presented in the next section of this work.
2.1 Proposed antenna
Bearing in mind that the preservation of the structural integrity of the substrate is of paramount importance, an antenna with a low impact on the substrate is proposed. Such antenna can be, for example, a loop antenna, and this antenna architecture will be considered in this work. To build such an antenna, it is necessary to deposit a conductive metal wire in the metal substrate and guarantee its electrical insulation from the substrate. This can be achieved by creating a small cavity in the substrate that is filled with an electrical insulator, i.e. a dielectric material, followed by the creation of a conducting wire in the insulator.
The antenna proposed in this work is presented in Figure 2. The dielectric material is zirconia, while the conducting metal of the antenna is silver.
2.2 Simulation performance of the proposed antenna
The model of the proposed loop antenna was constructed using Ansys HFSS, and the antenna was simulated to determine its potential performance. The antenna model was also inserted inside a box with the same dielectric properties of muscle tissue, in order to mimic the implantation environment of the device, in accordance to Figure 3.
The antenna's return loss and impedance are presented in Figure 4, while its radiation pattern at 2.6 GHz and 8.4 GHz are presented in Figure 5.
Firstly, the return loss also presents an unusual wavy pattern, which is also observed in its impedance, leading to the conclusion that such an effect occurs due to the characteristics of the structure being simulated, and not a simulation error such as a poorly placed radiation absorption boundary.
Analysing the return loss, one could infer that the antenna operates best at lower frequencies, as the return loss is lowest in this region, as opposite to higher frequencies, where a significant part of the signal that is injected into the antenna is immediately reflected to the source. Nevertheless, when comparing the radiation patterns at 2.6 GHz and 8.4 GHz, it is possible to see that the antenna presents the highest gain at the higher frequency (-44 dB versus -26 dB). These contradictory results lead to the conclusion that, despite the fact that the antenna accepts the most power at the lower frequencies, this power is not radiated, as it is lost to the substrate through the dielectric insulator. On the other hand, at higher frequencies the dielectric material becomes thicker, electrically speaking, and is capable of better insulating the antenna from the metal substrate. Consequently, even though it doesn't accept as much power at high frequencies, the power it accepts is radiated rather than lost to the substrate. This claim is supported by the analysis of the efficiency of the antenna at each of the studied frequencies, as it increases from -44.7 dB at 2.6 GHz to -29.2 dB at 8.4 GHz, a 15 dB increase. Additional support for this conclusion comes from the analysis of the surface currents at the dielectric material, presented in Figure 6. As it can be seen, the current that travels through the dielectric is higher at lower frequencies, thus validating the previous claim.
The next section of this paper presents a fabrication method that can be used to produce antennas such as the one designed here.
2.3 Antenna Fabrication
In this work, commercial Yttria-stabilized zirconia (3Y-TZP) powder with dispersion of 3 mol% Yttria (Tosoh Corporation, Japan), with particle size of 40 nm (in agglomerated size of 60 µm) was used as the dielectric material and silver powder (≥ 99% pure) with an average grain size of 230 nm, from Metalor Technologies-USA, was used as the conductor path of the antenna.
The proposed fabrication procedure consists of three steps, which are described below:
- Deposition and laser sintering of silver powder
- Spray deposition and laser sintering of zirconia layer;
- Micro-cavity creation in Ti6Al4V substrate by laser;
i) Micro-cavity creation
In this step, a micro-cavity is created on the surface of a circular Ti6Al4V titanium alloy target with 8 mm diameter and a thickness of 2 mm for subsequently receiving the insulating layer and then, the silver powder. For this purpose, a pulsed Nd:YAG laser (OEM Plus) with a wavelength of 1064 nm and 6 W of maximum power has been used. To create the micro-cavity, a strategy consisting of overlapping lines with different wobble amplitudes and scanning speeds was performed. Thus, a sequence of 10 wobbles with different widths and scanning speed was undertaken, aiming to create a rounded shape cavity through more even distribution of laser energy. In additional, each wobble amplitude was repeated in ascending sequence, in order to achieve greater depth, which resulted in 110 laser passages. Figure 7 shows a scheme of the laser strategy above mentioned.
ii) Zirconia layer deposition
In this step, 3Y-TZP powder was used as a dielectric material. After micro-cavity creation on titanium alloy surface, a zirconia layer was deposited on the surface by means of spray deposition process. Zirconia powder was dispersed in acetone and the suspension was sprayed using an air-brush on top of the titanium substrate. Before spraying, the solution was ultrasonically dispersed to avoid particle agglomeration. The sample was sprayed three times on the surface, in order to obtain a thick and dense layer. Then, the Ti6Al4V/ZrO2 layer was heated by using a CO2 laser (Bende CO2 laser marking) with an output power of 40 W and a spot size of 200 µm for zirconia layer consolidation. The great advantage of this process is the possibility to sinter specific zones without compromising the substrate. Thus, a power of 20 W and a scanning speed of 200 mm/s was applied on the surface, in a frequency of 20 Hz. Figure 2b schematically illustrates the sintering process of zirconia layer.
iii) Silver powder deposition and laser sintering
To print the silver-based antenna, silver powder (≥ 99% pure) with an average grain size of 230 nm, (Metalor Technologies-USA) was compacted into the cavity created in step (i) on Ti6Al4V substrate. A pressure of 8 MPa was applied to ensure the total accommodation of the powder. Then, a Nd:YAG laser (Sisma - 1064 nm of wavelength) with a spot size of 0.2 mm and 100 J of maximum energy has been used to sinter the silver powder and thus, generate the conductor path of antenna. An energy of 10 J was used in the sintering process. In this last step, a support Figure 2c illustrates this process. Each aforementioned step is schematically illustrated in Figure 8.
The cross-section of the produced antenna was analysed using scanning electron microscopy (SEM - Nova NanoSEM 200, FEI, Netherlands) equipped with an Energy Dispersive Spectrometer (EDS). EDS analysis was performed using an energy of 15 keV by means of a probe with spot size of 5.5 mm, corresponding to a current of 9.2 nA.