Madison L. Childress, Russell E. Bruhnke, Clint R. Frandsen, Mark A. Benscoter
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Advancements in implantable medical devices have created the need for more robust continuous data transmission abilities. Implantable medical device wireless data transmission within a health-care setting is beneficial, as it has the potential to increase safety, broaden independence, and increase mobility for patients. Minimizing the need for an individual to be tethered to electrical equipment restores movement, improves the quality of life for him or her, and may reduce the safety risk profile.
Here, we study data transmission within the health-care environment at 433 MHz to understand the effectiveness of this at a frequency range that prevents patient harm. A three-turn loop antenna was used as the transmit antenna, and a three-element Yagi–Uda array was employed as the receive antenna. The antennas were characterized to determine their behavior, and transmission experiments were conducted in an operating room (OR). The results of this study show that using this antenna combination results in a signal strength that enables robust data transmission within a health-care environment.
Implantable devices are a mainstay in patient care and commonly used in cardiac and neurologic applications. The use of an implantable device influences a patient’s quality of life. For example, an early design of the pacemaker had to be plugged into an ac outlet for power, severely limiting patient movement (Aquilina, 2006). The invention of the implantable pacemaker allowed patients to move around with much more freedom. Additionally, the risks associated with wired power were reduced through the use of a battery (Aquilina, 2006).
Similarly, the ability to wirelessly transmit data from an implantable device would increase a patient’s comfort by allowing him or her to be more mobile. Due to conflicting electrical signals and the presence of other electrical equipment, the ability to successfully transmit and receive data from wireless devices in the health-care environment can be challenging. This article studies the use of radio frequencies (RFs) for wireless communication in implantable devices.
The effectiveness of data transmission for an implantable wireless device is directly influenced by the antenna design. The antenna used for transmitting patient data plays a crucial role in the strength of a transmitted signal as well as the size of the device (Yazdandoost and Kohno, 2007). The receive antenna also influences the system performance; a high-gain antenna could counteract the effects of a low-gain one on the transmission side. Furthermore, the number and location of receive antennas could improve the ability to maintain a reliable signal with patient movement. Research has been done on possible antenna designs (Rosen et al., 2002; Yazdandoost and Kohno, 2007; Kiourti and Nikita, 2021; Nguyen and Jung, 2016; Huang et al., 2011; Duan et al., 2014; Yakovlev et al., 2012; Kim and Rahman-Samii, 2004), but few have been tested in an actual health-care environment.
The purpose of this study is to examine a system where the transmit antenna has been designed with the intent to minimize the transmit power to send a signal to the receiver while remaining above the noise level. Both the transmit antenna (a three-turn loop) and receive antenna (a Yagi–Uda array) were modeled and simulated in the antenna modeling software, 4nec2, before being physically created and tested. The three-turn loop antenna for the transmit one was designed to fit within a traditional 8-cm-diameter implantable case. The specific absorption rate (SAR) was also considered throughout the design process.
The amount of power the body will absorb plays an important role in antenna design. The power absorbed by the body from an electromagnetic field can be represented by the equation [Federal Communications Commission (FCC), 1997; IEEE, 2005] \[{P}_{\text{abs}} = \frac{1}{2}\int{{\sigma}\left|{{E}^{2}}\right|}{dV}{,}\] where ${\sigma}$ is the conductivity of the tissue (S/m), and E is the electric field intensity in the body (V/m).
The SAR is the amount of energy absorbed per unit mass of biologic tissue. The exposure to RF energy causes heating of the tissue in what are called thermal effects. Due to the rapid heating of tissue at high frequency levels, the FCC instilled limits on the SAR for parts of the body. In the case of implantable medical devices, IEEE Standard c95.1 (FCC; IEEE, 2005) states that the maximum level of the SAR should be treated as a partial body exposure in an uncontrolled environment (1.6 W/kg per 1 cm3 of tissue).
The SAR can be calculated by relating it to the electric field by (IEEE, 2005) \[{\text{SAR}} = \frac{{\sigma}{\left|{E}\right|}^{2}}{\rho}\] where ${\sigma}$ is the conductivity of the tissue (S/m), ${\rho}$ is the density of the tissue (kg/m3), and E is the root-mean-square electric field strength in tissue (V/m).
The antenna chosen for the transmit side can affect the amount of power absorbed in the body from the electromagnetic field as well as the overall efficiency of the system. The operating frequency, transmit power, and antenna size are parameters that need to be taken into consideration when designing the transmit antenna.
The frequency used will need to be low enough to pass through the human body, as biologic tissue is a poor transmission medium due to losses caused by absorption or changes to the radiation pattern (Duan et al., 2014). If the frequency is too high, the body will cause the transmitted signal to scatter, resulting in a reduction in the amount that would leave the body (Skrivervik et al., 2001). A lower transmit power will also result in the SAR being less problematic (Yazdandoost, 2007; IEEE, 2005).
The antenna size needs to be miniaturized to accommodate a small, hermetically sealed case. The overall efficiency of the antenna will decrease due to the small transmit antenna. To counteract the lack of transmit efficiency, an efficient receive antenna will be used.
The Friis equation is used to calculate the theoretical values of what the power received should be \[{P}_{\text{r}} = \frac{{P}_{\text{t}}{G}_{\text{t}}{G}_{\text{r}}{c}^{2}}{{(}{4}{\pi}{Rf}{)}^{2}}\] where ${P}_{\text{t}}$ is the power transmitted, ${G}_{\text{t}}$ is the gain of the transmit antenna, ${G}_{\text{r}}$ is the gain of the receive antenna, R is the distance between the antennas, c is the speed of light, and f is the frequency of operation.
The wireless transmission system is composed of one transmit station and one receive station, an Agilent Technologies E4438C signal generator for the transmit station, and an Agilent Technologies N9010A spectrum analyzer (Santa Clara, California) for the receive station, as seen in Fig. 1.
Fig 1 The test setup for the antenna characterization. Rx: receiver; Tx: transmitter.
The system was used to determine the minimal transmit power required to stay above the noise level and maintain a reliable signal. The amplitude on the signal generator was set to 0 dBm.
The transmit antenna was a three-turn loop with a diameter of approximately 3.5 cm. This antenna was created with 16-gauge electrical wire and soldered to a SubMiniature version A connector for use with a Tektronix TTR506A vector network analyzer (VNA) (Beaverton, Oregon) and the signal generator.
The receive antenna was a Yagi–Uda antenna. It was created as a linear array using 14-gauge wire with a horizontal half-wave dipole antenna as the driven element. An antenna stand made of acrylic was used to hold the elements in place and was constructed so the director and reflector could be moved closer or farther away from the driver.
The VNA was used to determine the impedance of each of the antennas and if a matching network was needed. Inductors and capacitors were added to form the matching network for the three-turn loop antenna to resonate at 433 MHz.
The spacing between the elements of the Yagi were adjusted until the reactance was at a minimum. The antennas were also created and simulated in 4nec2 to compare the results.
The transmit antenna was characterized inside a 10-m-long lecture hall consisting of tables, chairs, a whiteboard, a computer, a projector, and a projector screen. A metal box with dimensions of approximately 30 × 30 cm was present in the middle of the floor, and the noise level of the room was –80 dBm. A horizontal half-wave dipole was also used as a receiver for the initial characterization testing.
Another three-turn loop antenna was created to resonate in water and saline at 433 MHz. This antenna was then tested while submerged in tap water and then saline to determine how the use of a human phantom would affect the function of antenna transmission.
The transmit antenna, the three-turn loop, was connected to a signal generator to emit a signal at 433 MHz, and the receive antenna was connected to the spectrum analyzer to determine the strength of the signal received. The space within two wavelengths was cleared of all objects to observe normal antenna behavior.
Both antennas were attached to stands (see Fig. 1) so that they were upright and at least two wavelengths off the ground. The transmit antenna remained in place for the duration of the experiment, and the receive antenna started 1 m away from the transmit antenna. Both of them directly faced each other. The receive antenna was then moved away from the transmit antenna at increments of 10 cm until a total distance of 4.4 m between the antennas was reached. A total of 45 data points were collected for each trial, and two trials were conducted.
The received signal strength from the spectrum analyzer was recorded at each increment, and the averages at each interval were calculated to factor out any differences in reflections present in the room. The receive antenna was also characterized using the same process—with the three-turn loop as the transmit antenna. The Friis equation for transmission loss was used to calculate all of the theoretical values for the power received. These theoretical values assume that the experiment was run in free space with no cable loss.
The same basic process was carried out inside an OR. The OR consisted of equipment used during a standard operation, most of which was located in the upper half of the room. In the lower left corner, there were several metal tables throughout the room as well as a steel door in the lower right corner (C4 in Fig. 2). The receive antenna was placed on a nonconductive tripod, while the transmit antenna was put on an acrylic stand in the middle of the operating table, parallel to the surface of the bed.
Fig 2 The OR test setup.
The dimensions of the room were measured. The locations to collect data for this particular room consisted of two circles—one at a distance of 1.52 m and another at 3.04 m from the transmit antenna. The four corners of the room were also chosen to collect data points (see Fig. 2).
Each spot was measured and marked with tape to identify its exact location. Eight points were chosen for each circle—four of them were exactly in line with the transmit antenna, and another four were at a 45° angle from the transmit. One spot in the room was chosen as the starting point, denoted S1 in Fig. 2. The receive antenna was moved to each point to collect data, while the transmit antenna remained fixed.
The three-turn loop antenna was tested on the VNA to determine the impedance of the antenna, which was 27 – j245 X before adding the matching network. Based on the 4nec2 simulation, the impedance of the antenna was 13.2-j1355 X. This difference can be explained by the fact that the antenna from the simulation was not constructed using perfect circles. Additionally, the spacing of the wire in between each turn in the simulation had to be larger, as 4nec2 assumes that wires are overlaid if they are too close to each other (Fig. 3).
Fig 3 The transmitter antenna: (a) a 433-MHz three-turn loop antenna and (b) the 4nec2 gain simulation.
The physical antenna was also tested in an environment that consisted of objects such as metal cabinets and lab equipment. After adding the matching network, the impedance was 47 + j2. The impedance was also checked while attached to the stands that were used during testing, and it did not change.
The Yagi antenna was found to have minimal reactance when the elements were spaced approximately 4 cm apart. The impedance at this distance was 52 –j4 X on the VNA and 45.2 –j5 X in the 4nec2 simulation. To attach the stand for the Yagi to the nonconductive tripod, a plastic holder was 3D printed with a threaded hole at the bottom to connect to the tripod. Two acrylic screws placed between the elements were used to connect the tripod holder to the Yagi stand. The impedance was measured with the screws in place and antenna on the tripod. The impedance stayed within a range of ±5 X resistance and ±10 X reactance (Fig. 4).
Fig 4 The receiver antenna: (a) a 433-MHz Yagi and (b) the 4nec2 gain simulation results.
After the antennas were closely matched to 50 X, they were characterized in the lecture hall. The data presented in Fig. 5 were collected from the three-turn transmit loop antenna and the receive horizontal half-wave dipole in air, water, and saline. While the water and saline did have an effect on the received power, the signal-to-noise ratio was acceptable.
Fig 5 A comparison of the power received at 433 MHz for dry (blue) versus tap water (red) versus saline (yellow).
The data in Fig. 6 show the data three-turn loop and Yagi antenna combination, and the results were compared against the theoretical values from the Friis equation. The actual values do not account for power loss due to the cable.
Fig 6 A characterization of the actual values collected (blue) versus theoretical ones calculated (red) in a 433-MHz loop.
Overall, the results resembled the theoretical values, with the received power decreasing as the distance increased. This experiment was not completed in an open area, which resulted in reflections within the room causing interference. Although using a wide, open space would be best to truly see the behavior of these antennas, the realistic application for this experiment would be in a room that has reflections and points of interference.
After characterizing the antennas, an experiment was conducted in an OR to study the environmental effects on the system. Tables 1 and 2 show the results of this experiment. The noise level of the OR was –88 dBm, and the data from each of the locations were well above the noise level. The worst value was at –61 dBm, which is more than adequate for the purpose of this project. The Friis equation for transmission loss was once again used to calculate all of the theoretical values for the power received. As shown in both Tables 1 and 2, the results from the 3.04-m spots were much closer to the theoretical values than from those at 1.52 m. The results from the 3.04-m spots were also much more consistent than those at the 1.52-m distance.
Table 1. The 433-MHz circle results—Theoretical versus actual.
Table 2. The 433-MHz corner results—Actual versus theoretical.
A model of an antenna used on a commercially available implantable device was created and tested. This is a type of folded monopole (Fig. 7). The performance of this antenna was measured in the lecture hall using the same setup as previously described. The three-turn loop was also measured, so a direct comparison could be made. The model antenna was formed and situated like it would be when implanted in a patient, and the loop was placed in the horizontal plane. The receive antenna was a commercially available monopole.
Fig 7 The (a) edge view of the antenna on a commercially available device and (b) test model.
The transmit and receive antennas were placed 3.5 m apart. The signal generator was set to 433 MHz at a 0-dBm output. Table 3 shows the results; as can be seen, the loop significantly outperforms the folded monopole.
Table 3. A comparison of the 433-MHz antenna results.
The 4nec2 software was also used to model the field strength of the three-turn loop antenna in various tissues. Muscle, fat, and skin were considered, and a transmit power of 0 dBm was used. Using the SAR equation, the maximum limit for the intensity of the electric field was found at 433 MHz. Table 4 shows the electric field limits for each of the tissues.
Table 4. The electric field intensity limits.
The environment in 4nec2 was adjusted to match the conductivity and dielectric constant of the tissue. The electric field in the region surrounding the antenna was measured to be less than that of the limit needed to meet the SAR requirements for all three tissues.
Fig. 8 shows the electric field intensity for muscle. The middle of the picture denotes the center of the antenna, and the ends of the antenna are the edges of the blue/green band. While the antenna itself has a field intensity larger than the limits for the SAR at 0 dBm, the electric field in the surrounding tissue remains well below the required level. The field intensity can be further reduced by using a lower output power from the transmitter. The power absorbed in a given volume of tissue can then be found using the field intensity and conductivity of the tissue.
Fig 8 The electric field intensity in muscle at 3.051 V/m for x = –0.1, y = –0, and z = 0.87.
Another frequency range was also characterized and tested for possible use. The 610-MHz frequency band is allocated specifically for medical use. Therefore, a 610-MHz two-turn loop was constructed with a diameter of about 3.5 cm for the transmit antenna, and the receive antenna was a three-element Yagi antenna with a horizontal half-wave dipole as the driven element.
When tested in the OR, the 610-MHz antenna combination was checked in corners 2 and 3 against the 433-MHz combination. The power level reading at C2, the upper left corner in Fig. 2, of –45 dBm was slightly better at 610 MHz than the –49-dBm reading at 433 MHz, but the results were the same at C3, the lower left corner in Fig. 2, for a reading of –40 dBm. As choosing one frequency band over the other does not result in any significant difference in the results, it may be beneficial to examine the possibility of using the 610-MHz band because it is specifically dedicated for this purpose. However, the three-turn loop shows great promise, as it outperforms the folded monopole currently in use.
These experiments and simulations were carried out with a transmit power of 0 dBm. The results showed that significantly less transmit power can be used while still getting a reasonable signal-to-noise ratio at the receiver. As an example, the Silicon Labs Si446X transceivers specify a typical sensitivity of –97 dBm for a bit rate of 500 kb/s. Lowering the transmit power by 10 dB will leave a signal that is well above this limit and greater than the measured noise level in the OR.
Additional work will be carried out using commercially available transceiver modules. The production cost of this technology was not part of the scope of this effort. However, a number of currently commercialized solutions include similar abilities, and we feel they are a predictive indicator for the costs to produce this system. Additional work is needed to examine the application of this capability in other environments, such as a patient’s home and the clinical monitoring room.
This study has demonstrated the antenna configurations of a three-turn loop antenna for transmission and three-element Yagi for receiving that enable a wireless system to function properly within a health-care environment. Based on these results, it can be determined that only one receive antenna may be necessary and that a high-gain antenna for the receiver would not be needed. The SAR will also not be an issue, as 0 dBm is below the power limit needed to stay within the 1.6-W/kg limit. It also compared the three-turn loop against a folded monopole antenna that is used on a commercially available device.
Madison L. Childress (mlchildress107@gmail.com) received the B.S. degree in both computer engineering and electrical engineering from the Missouri University of Science and Technology (S&T), Rolla, MO 65401 USA. She previously worked as an undergraduate research assistant at S&T and, most recently, completed a co-op at the Mayo Clinic in Rochester, Minnesota.
Russell E. Bruhnke (bruhnke.russell@mayo.edu) is a principal engineer II in the Electronics Development Unit in the Division of Engineering, Mayo Clinic, Rochester, NY 55902 USA.
Clint R. Frandsen (clint.frandsen@gmail.com) received the B.S. degree in computer engineering from Brigham Young University (BYU), Provo, UT 684602 USA. He previously worked as an undergraduate research assistant at BYU’s magnetic resonance imaging facility and, most recently, completed a co-op at the Mayo Clinic in Rochester, Minnesota. He is a Member of IEEE.
Mark A. Benscoter (mbenscoter@gofast.am) earned his M.S. and Ph.D. degrees. He is the unit head for electronics development in the Division of Engineering, Rochester, NY 55902, USA. In addition, he serves as an assistant professor in neurology and biomedical engineering in the Mayo Clinic College of Medicine. He is a Member of IEEE.
Digital Object Identifier 10.1109/MPOT.2019.2911319