1. Introduction
In recent years, the emergence of electric vehicles (EVs) has eased the pressure of environmental pollution caused by the exhaust of conventional fuel vehicles and dependence on fossil fuels. To improve the safety, reliability, and convenience of EV charging [
1,
2], various research teams have been working on wireless power transfer (WPT) technologies [
3,
4]. High-performance WPT devices are required to meet the needs of consumers and commercial users in the EV market [
5,
6]. The EV-WPT systems with a transmission power of 30 kW at a transmission frequency of 20 kHz and a transmission power of 52 kW at a transmission frequency of 40 kHz were established [
7,
8]; however, leaked electromagnetic fields (EMFs) of uneven strength are generated around high-power EV-WPT devices, and human electromagnetic safety when exposed to leaked EMFs from these high-power devices needs to be evaluated [
9,
10]. For leaked EMFs at different frequencies, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) is concerned with different indicators when the frequency is less than 100 kHz to prevent the human nerve stimulation effects considering induced electric field intensity (induced
E-field) [
11]; when the frequency is higher than 100 kHz, the specific absorption rate (SAR) is considered to prevent the local temperature increase in human tissues [
12]. The high-power leakage of electromagnetic radiation (EMR) generated by the EV-WPT device impacts molecular ions and electrons, as well as reactive oxygen species, protein, and DNA/RNA levels. Furthermore, the EMR has cytotoxic effects on cells by causing degeneration, apoptosis, and necrosis. EMR has a substantial impact on the central nervous system, reproductive system, cardiovascular system, and hematological system. Biological systems that are continuously exposed to electromagnetic radiation over a long period of time may experience an increase in tissue temperature [
13]. Therefore, this paper performs a quantitative assessment of the induced
E-field of the human body exposed to a high-power, 85 kHz transmission frequency EV-WPT device to determine human electromagnetic safety.
The electromagnetic safety of the surrounding human body during the operation of EV-WPT devices has been extensively studied. For example, [
14] quantified the induced
E-field value of the human body for an EV-WPT device with a transmission power of 7 kW and a transmission frequency of 85 kHz in an electromagnetic exposure environment, considering the WPT device misalignments generated and adult human models in different postures. The author of [
15] quantified the induced
E-field value of the human body and the safety distance under EV-WPT devices with a transmission frequency of 85 kHz and different transmission powers. The heart is one of the most essential organs in the human body, and its main role is to drive blood flow, deliver oxygen and nutrients to all organs and tissues of the body, and carry away metabolites. However, cardiovascular diseases cause one-third of all deaths annually and are expected to account for 24 million cases by 2030 [
16]. Coronary artery stenosis often complicates coronary artery disease and leads to myocardial infarction. Coronary stents are metal mesh tubular structures that are permanently implanted into narrowed arteries to improve the arteries and restore blood supply to the heart [
17]. Existing studies have shown that when a human body with a metallic medical implant is placed under an EMF, the implant has the potential to alter the EMF distribution and absorb electromagnetic energy that is absorbed by local tissues and organs of the body, leading to a significant increase in electromagnetic exposure indicators in the vicinity of the implant and the risk of damage to human tissues [
18].
There are also a small number of studies related to the quantification of electromagnetic exposure of a human body containing implants under the leaked EMFs of WPT devices. In [
19], a quantitative assessment of the SAR of a human body under the leaked EMF of a small radiofrequency WPT antenna implant device at the transmission frequency of 1470 MHz was performed. In [
20], the induced
E-field value of the human body was studied considering the transmission frequency of 85 kHz and the leaked EMF of EV-WPT devices with different transmission powers on human bodies containing implants such as tibial intramedullary nails and hip joints. In [
21], the SAR values of the surrounding human tissues were quantitatively evaluated using an EV-WPT system with a transmission frequency of 13.56 MHz and a transmission power of 5.3 kW, considering implants such as nerve stimulators, hip joints, etc. However, there are fewer studies on the human electromagnetic safety of precision implants, such as those containing coronary stents. In view of the fact that coronary stent implants often have tips and sharp edges, human tissues located in the vicinity of tips and sharp edges of implants under EMFs have higher values of electromagnetic exposure indicators, which may affect the normal physiological function of the heart. At the same time, due to the Society of Automotive Engineers (SAE) standard, there will be a coil offset in the actual operation of the WPT device [
22]. When the coil is misaligned, it will generate unevenly distributed leaked EMFs in space, affecting the internal EMF distribution of the human body containing the implant. Therefore, it is important to consider the leaked EMFs of EV-WPT devices for different operating conditions to quantify the electromagnetic exposure of human bodies containing coronary stent implants.
The purpose of this paper is to analyze the electromagnetic safety of humans with coronary stent implants in the vicinity of EV-WPT devices. The electromagnetic exposure of an EV-WPT device with a transmission frequency of 85 kHz and transmission powers of 11 kW [
5] and 22 kW [
23] and a human body with a coronary stent implantation are modeled. Under the leakage EMFs of the EV-WPT device, the induced
E-field of the human body implanted with a coronary stent is quantified using the magnetic quasi-static (MQS) approach, taking into account the different locations of the human body in the inside and at the rear of the vehicle and the misalignment generated by the WPT device. The rest of this paper is as follows:
Section 2 establishes the numerical simulation model of a human containing a coronary stent implant under the electromagnetic exposure of a leaked EMF of an EV-WPT device and introduces the numerical simulation method of human electromagnetic exposure adopted in this paper.
Section 3 discusses the different electromagnetic exposure scenarios for the EV-WPT device considered in this paper and quantifies the induced
E-field in different postures of the human body containing a coronary stent implant under electromagnetic exposure scenarios. Finally,
Section 4 concludes the paper.
3. Numerical Simulation Calculation and Discussion
The above established numerical simulation model of human electromagnetism includes a coronary stent implant in the electromagnetic exposure scenario of an EV-WPT device, at a transmission frequency of 85 kHz and transmission powers of 11 kW and 22 kW. When the WPT device coils
x0 and
z0 have no misalignment and misalignments of 75 mm, the obtained 11 kW and 22 kW EV-WPT devices in the plane of the main visual cross-section
E-field and magnetic flux density distributions are as shown in
Figure 5 and
Figure 6. The figures show that the EV-WPT device generates a very uneven
E-field and magnetic flux density between TC and RC and also in the surrounding space, and due to the shielding effect of the vehicle body, the
E-field intensity inside the vehicle is much smaller than that outside the vehicle. But numerically, the maximum values of the
E-field in the 11 kW and 22 kW spaces are 332.1 V/m and 234.5 V/m, respectively, which are much higher than the 83 V/m of the ICNIRP reference level. By comparison, it is found that when the WPT device is misaligned, the leakage
E-field and magnetic flux density are generated with an uneven intensity distribution in the room, while the exposed
E-field generated by the EV-WPT device is significantly enhanced at higher power levels, so it is necessary to quantitatively evaluate the induced
E-field of the human body containing coronary stents exposed to this condition.
According to ICNIRP guidelines, the safety limit for the induced E-field value of the human body at a transmission frequency of 85 kHz is 11.475 V/m. It is not appropriate to use the 99th percentile as a result of this numerical calculation of the induced E-field value of the human body with medical implants because the 99th percentile value is not appropriate for quantifying the induced E-field value of the human body with implants. When quantifying the induced E-field value of the human body with an implant, the values are more widely distributed in the vicinity of human tissues around the implant, and taking the 99th percentile value and ignoring the top 1% may underestimate the true exposure; so, in situ values are used to quantify the calculated results to reflect the true exposure of human tissues near the sharp edges of the implant.
The standing human body is located at the rear of the vehicle, and the sitting human body is located in the driver’s position. There is no bit error in the
x0 and
z0 of the coil of the WPT device, the induced
E-field distributions of the standing human body under the WPT device with transmission powers of 11 kW and 22 kW are shown in
Figure 7 and
Figure 8, and the induced
E-field distributions of the sitting human body under the WPT device with transmission powers of 11 kW and 22 kW are shown in
Figure 9 and
Figure 10. As shown in
Figure 9 and
Figure 10, comparative numerical simulations are performed using the same grids for different electromagnetic exposure scenarios: a human body without implants and a human body with implants. According to the numerical simulation, the coronary stent implant has a significant effect on the distribution of the
E-field in human tissues in different postures. The induced
E-field of human tissues near the sharp edges of the implants is significantly larger than that of the body without implants. The larger values of the induced
E-field are distributed at both ends of the coronary stents. Wave diffraction may be the mechanism that causes a significant enhancement of the induced
E-field around the sharp edges and tips of implants. The presence of metallic implants in vivo causes scattering of incident electromagnetic waves, and sharp edges have the potential for high electromagnetic energy dissipation, which alters the spatial distribution of the electric field. Thus, the coronary stent implant significantly affects the induced
E-field in the human heart, especially around the sharp edges of the coronary stent implant. According to
Figure 7,
Figure 8,
Figure 9 and
Figure 10, the induced
E-field distribution near the apical edge of the implant is significantly enhanced regardless of the posture of the human body compared to the induced
E-field distribution of the coronary stent implant in other regions of the human body, and the maximum values of the cardiac induced
E-field values are distributed at the sharp ends of the coronary stent implant. As shown in
Table 2, the cardiac induced
E-field
max values are also greatly enhanced when a coronary stent is implanted in the body. Comparison of the induced
E-field
max value of the human body in
Table 2 shows that the higher the power of the nonimplanted WPT device, the higher the induced
E-field
max value of the human body, and the induced
E-field
max value of the human body with an implanted coronary stent is significantly enhanced compared to that of the nonimplanted one. The induced
E-field
max value of the human body with an implant for a WPT device with a transmission power of 22 kW is 1.758 V/m, which is well below the safe limit. Still, when the WPT device is misaligned, it generates a relatively large leakage of EMFs in space, and the value may exceed the limit, which causes the induced
E-field value of the human heart tissue with a coronary stent implant to exceed the safety limit, and there may be a potential safety hazard of EMR to the human heart organ. So, it is necessary to quantify the misalignment situation.
In order to obtain the effect of generated EV-WPT device misalignments on the electromagnetic exposure of a human body containing medical implants, considering the EV-WPT device in operation, we must consider that the driver’s parking operation allows for the WPT device coil to be out of alignment, so there is a certain amount of misalignments generated; and the reference standard sets the range of variation in misalignments
x0 and
z0 of the WPT device coil in 2.1 to be [−75 mm, 75 mm] [
22], with a sweep parameter step size of 15 mm, which are calculated for comparison of the induced
E-field
max value of a human body without implants and with a coronary stent, respectively. The results of the induced
E-field
max value of the human body without implants and with a coronary stent in different postures under the leaked EMF of the EV-WPT device with different powers are shown in
Figure 11,
Figure 12,
Figure 13 and
Figure 14. According to the Figures, when the sitting human body without an implant is in the driving position and the standing human body is at the rear of the vehicle, when the WPT device generates misalignments, the maximum values of the induced
E-field
max value of the human body are 0.039 V/m and 0.091 V/m, respectively, at a transmission power of 11 kW. The maximum values of the induced
E-field
max value of the human body are 0.050 V/m and 0.124 V/m, respectively, at a transmission power of 22 kW. The numerical value does not exceed the ICNIRP safety limit. Still, the obtained results of the induced
E-field
max are not symmetrically distributed with the misalignments generated because the human body in different postures is not directly above the WPT device, and the degree of influence of the misalignments in the
x- and
z-directions is inconsistent when the misalignments are generated. For a human body containing a coronary stent implant and a WPT device generating misalignments, the induced
E-field
max values for a sitting human body exposed to a WPT device with transmission powers of 11 kW and 22 kW are 3.087 V/m and 5.790 V/m, respectively, neither of which exceeds the ICNIRP safety limit, partly because of the weak EMFs within the shielding effect of the EV body and partly because the human body is further away from the WPT device and receives weaker electromagnetic energy. However, the induced
E-field
max of the standing human body under the WPT device with the transmission power of 11 kW will exceed the ICNIRP safety limit in 8.3% of the misalignment range in the case of misalignments, with the maximum value of 11.839 V/m, which is 103.2% of the ICNIRP safety limit. The induced
E-field
max of the standing human body under the WPT device with the transmission power of 22 kW will also exceed the ICNIRP safety limit when the misalignments are small; 46.4% of the range of misalignments will exceed the ICNIRP safety limit value; the maximum value is 17.242 V/m, 150.3% of the ICNIRP safety limit value; and generally, the calculation results of the human body in the standing position are more significant than those in the sitting position. This is because the WPT device is located at the rear of the vehicle, and the closer the relative distance between the human body and the WPT device, the larger the leaked EMF is. The reason is that the WPT device is located at the rear of the vehicle, and the closer the human body is to the WPT device, the greater the leakage of EMFs and the greater the induced
E-field
max of the human body. When the human body is implanted with a coronary implant, the WPT misalignments generated have a more significant effect on the human body’s induced
E-field
max value. The human body in the standing position has a different degree of exceeding the ICNIRP safety limit; the greater the power, the greater the misalignment distance, and the greater the degree of exceeding the limit. In addition, the coronary stent implant has a significant influence on the distribution of the induced
E-field in the human body. From the perspective of the geometry of the implant, the coronary stent has sharper edges, which have a higher electromagnetic energy dissipation capacity, and the surrounding human body tissues have a higher energy absorption capacity.
By analyzing and discussing the results of the numerical simulation calculations, it can be concluded that the main factors affecting the value of the induced E-fieldmax in the human body are the degree of the WPT device misalignments generated, the structural shape of the implant, the relative position of the human body containing the implant and the WPT device, and the power of the WPT device. Therefore, in order to ensure the electromagnetic safety of the human body containing medical implants under the high-power EV-WPT device, attention should be paid to the misalignments of the WPT device to try to ensure that it operates in a non-misalignment situation and the relative position of the human body to the WPT device should be ensured to be in a safe range when charging the EV-WPT device, as well as strengthening related research on the optimal design of the leaked EMF shielding of the WPT device.