# A Magnetic-Balanced Inductive Link for the Simultaneous Uplink Data and Power Telemetry
## 7 Experimental Results
### Tissue and Safety  
In order to assess the impacts of tissue on the inductive link, the implant coil is wrapped in 10 mm-thick beef tissue as seen in Figure 10. According to the work in [27], the permittivity and the conductivity of the beef muscle tissue are close to those of the human muscle tissue when the frequency is in the level of MHz. However, the tissues in the eyeball need to be further studied, because our implant coil is designed to be implanted around the eyeball. Thus, we further place the inductive link in the specific true-CAD human head model and simulate them in the ANSYS HFSS electromagnetic simulation suite as seen in Figure 16. There are tissues of skin, fat, muscle, eyeball and skull in the human head model. The true-CAD human head model is obtained from NEVA Electromagnetics [28]. The permittivity and the conductivity of the tissues are obtained from [29]. In the simulation, the implant coil is wrapped in silica gel and surrounds the periphery of the cornea hidden behind the eyelid.  
Figure 12 shows the simulated and measured coupling coefficients (\(k_{12}\) and \(k_{23}\)) over different coil separations (\(d\)). The simulated coupling coefficients are in good agreement with the measured results. Figure 12 also shows that the measured coupling coefficients when the implant coil is wrapped in the beef tissue and the simulated coupling coefficients when the implant coil is placed in the human body model. Both of the measured results and the simulated results present that the coupling coefficients when the inductive link is in the tissue are almost the same as those when the inductive link is in air. The tissues scarcely affect the coupling coefficients of this link, because the absorption of the tissues towards the low frequency magnetic field is negligible. In addition, the tissue could increase the parasitic capacitance of the implant coil [30]. This may lower the PTE due to the detuning. Benefiting from the selection of the low frequency (2 MHz) power carrier, the detuning caused by the tissue is minor and negligible in our inductive link.  
In order to assess the radio frequency (RF) exposure safety on the human body, we used the ANSYS HFSS electromagnetic simulation suite to calculate the specific absorption rate (SAR).  
Figure 15: The power transfer efficiency versus coil separation \(d\) with or without connecting the uplink circuits, which includes the isolation resistors and the uplink transmitter. PTE, power transfer efficiency.  
The simulation results show that the maximal local SAR in tissues is about three orders lower than the basic restriction of 2 W/kg [31].  
In addition, there could be temperature increase in the human body due to the power consumed by the implant coil and implant circuits. According to the comprehensive thermal analysis on an intraocular implant [32], our device with 5-mW power consumption may induce the maximum temperature increase of approximately 0.4 \({}^{\circ}\)C on the surface of the device. The temperature increase is below the guideline 1 \({}^{\circ}\)C in [31].