Implantable medical devices, such as neurostimulators, need to be wirelessly controlled from outside the body. Many of these devices use high-frequency signals in the 400 MHz, 900 MHz, and 2.45 GHz bands to communicate with external devices. However, high-frequency bands can suffer from signal attenuation in biological tissues and from electromagnetic interference with surrounding devices. In contrast, human body communication (HBC) uses relatively low frequencies in the 3–30 MHz band, which can alleviate signal attenuation and prevent emissions outside the body during communication. In this study, we investigated the use of HBC for implantable medical devices in the abdomen using electromagnetic field simulations. The results showed that the transmission between the transmitter inside the body and the receiver outside the body was adequate for stable communication. However, when the receiver was detached from the skin surface, the transmission characteristics rapidly decreased. Moreover, when the receiver and skin were separated by 1 mm, the transmission degraded by 34 dB, indicating that electromagnetic interference between the implanted transmitter and surrounding devices (including other medical devices) is rare. Finally, we evaluated the proposed system from the perspective of medical electromagnetic compatibility and human safety. The simulation results demonstrated that the radiated emissions and human exposure of the HBC system meet international standards.

Chronic pain affects 1.5 billion people worldwide.1,2 Spinal cord stimulation (SCS) is a treatment that alleviates pain by applying electrical stimulation to the spinal cord.3,4 To perform SCS, a stimulator is implanted in the lower abdomen, and it must be wirelessly controlled from outside the body, as shown in Fig. 1(a). Many implantable medical devices use high-frequency signals in the 400 MHz, 900 MHz, and 2.45 GHz bands to communicate with external devices.5–8 However, these high-frequency bands suffer from signal attenuation in biological tissues9–14 and from electromagnetic interference with other electrical devices.15–20, Table I lists the wireless medical devices and home appliances that can potentially interfere with implantable medical devices. Clearly, unconventional wireless communication technologies that do not use airborne electromagnetic waves are needed. Human body communication (HBC) is a wireless communication technology that uses the human body as a transmission path for electrical signals.21,22 In HBC, electrical signals are transmitted through electrodes that are placed on the human body. HBC enables low-power, low-noise, and secure communications because the electric field is distributed only in the vicinity of the human body. Unlike conventional far-field antenna systems, HBC generally uses a quasistatic near-field23 excited at relatively low frequencies in the 3–30 MHz band, considerably lower than the resonant frequency of HBC electrodes. Accordingly, HBC can alleviate signal attenuation and prevent emissions outside the body during communication.24 Although studies have been conducted on HBC with mobile and wearable devices, such as smartphones and smartwatches,25–28 HBC for implantable medical devices has not been sufficiently studied.29,30

FIG. 1.

(a) Conventional wireless communication system for an implantable stimulator. (b) Proposed human body communication system for an implantable stimulator.

FIG. 1.

(a) Conventional wireless communication system for an implantable stimulator. (b) Proposed human body communication system for an implantable stimulator.

Close modal
TABLE I.

Wireless medical devices and home appliances that can interfere with implantable medical devices.

StandardFrequencyDevice example
⋯ 20–100 kHz Induction cooker 
Industrial, scientific, and medical (ISM) 13.56 MHz band 13.56 MHz RFID and wireless charging devices 
Medical Device Radiocommunications Service (MedRadio) 401–457 MHz Medical telemetry systems, pacemakers, capsule endoscopy systems, and defibrillators 
ISM 2.45 GHz band 2.4–2.5 GHz Microwave ovens, Bluetooth, insulin pumps, and neurostimulators 
IEEE 802.11n 2.4–2.5 GHz WiFi devices 
 5.15–5.35 GHz  
 5.47–5.725 GHz  
StandardFrequencyDevice example
⋯ 20–100 kHz Induction cooker 
Industrial, scientific, and medical (ISM) 13.56 MHz band 13.56 MHz RFID and wireless charging devices 
Medical Device Radiocommunications Service (MedRadio) 401–457 MHz Medical telemetry systems, pacemakers, capsule endoscopy systems, and defibrillators 
ISM 2.45 GHz band 2.4–2.5 GHz Microwave ovens, Bluetooth, insulin pumps, and neurostimulators 
IEEE 802.11n 2.4–2.5 GHz WiFi devices 
 5.15–5.35 GHz  
 5.47–5.725 GHz  

In this study, we propose and investigate the use of HBC for medical devices that are implanted in the abdomen, as shown in Fig. 1(b). We aim to clarify the mechanism for signal propagation in and around the body and analyze the basic communication characteristics, such as transmission characteristics and electric field distributions.

An electromagnetic field simulation model, shown in Fig. 2, was used to study the communication between an implantable abdominal medical device and a controller on the skin surface. The dimensions of the abdominal model were determined based on the average shape of a Japanese adult male.31 The model was composed of skin, fat, and muscle layers, and we determined the electrical properties of each tissue layer referencing our previously developed electromagnetic phantoms, which can be used at 10–30 MHz.32, Table II lists the relative permittivity and conductivity of each layer. Using the electrical properties of phantoms will facilitate a comparison between future simulations and experimental results. The overall dimensions of the abdominal model were 260 × 210 × 190 mm3, and the thicknesses of the skin, fat, and muscle layers were 2, 6, and 182 mm, respectively. To determine the thickness of each tissue layer, we considered the thickness range of each abdominal tissue.33 The thickness ranges reasonably compared with those in conventional studies of multi-layer abdominal tissue phantoms.34,35 The implantable device (sized 48 × 48 × 10 mm3) was fixed to the fascia within 2 cm of the skin surface, reproducing the situation of actual implantable stimulators.36 A free space was created in the muscle layer to accommodate the device housing.36 In this study, the implantable device was the transmitter (TX) and the controller outside the body was the receiver (RX). An 8 × 24 mm2 two-electrode structure, commonly used in wearable HBC systems,27,37 was used as the communication electrode, which was placed in the center of the model. The TX electrode was placed in contact with the lower part of the fat layer, while the RX electrode was placed on the surface of the skin layer. From a human safety perspective, implantable electrodes should be formed from biocompatible materials, such as titanium.36 We confirmed that the electrical conductivity of titanium (0.6–2.0 × 106 S/m) electrodes does not affect signal transmission in terms of an electrode material. Therefore, all electrodes were constructed from perfect electrical conductors. The impedance of the TX electrode at the feeding point and the receiving load of the RX were both 50 Ω. When selecting the signal frequency (10 MHz), we consulted previous studies38,39 and considered the 13.56-MHz industry–science–medical band.40 The finite-difference time-domain method-based electromagnetic field simulator XFdtd (Remcom Inc., PA, USA) was used to calculate the transmission characteristics, electric field distributions, and input impedance of the electrodes. The computing space of the simulation was represented by non-uniform grids, with grid sizes of 1 mm around the electrodes and 2 mm at the edge of the computing space. The distance between the model edge and the absorbing boundary was 20 cells thick. The absorbing boundary was a perfectly matched layer composed of seven sublayers. The time step of the calculation was 1.926 ps.

FIG. 2.

(a) Overview of the simulation model. (b) Side view of the simulation model in cross section including the feed point. (c) Transmitter (TX) and receiver (RX) models.

FIG. 2.

(a) Overview of the simulation model. (b) Side view of the simulation model in cross section including the feed point. (c) Transmitter (TX) and receiver (RX) models.

Close modal
TABLE II.

Electrical properties of the abdominal model.32 

LayerRelative permittivity εrConductivity σ (S/m)
Skin 421 0.17 
Fat 71 0.028 
Muscle 184 0.67 
LayerRelative permittivity εrConductivity σ (S/m)
Skin 421 0.17 
Fat 71 0.028 
Muscle 184 0.67 

In this study, we used the transmission characteristic |S21| to express the ratio between the received power at the RX and the available power at the TX. The simulation results revealed that |S21| was −22 dB, which is sufficient for stable communication. This is because the TX and RX electrodes are positioned in close proximity to each other through the skin and fat layers. To investigate the signal propagation mechanism in greater detail, we calculated the electric field distribution around and inside the model, as shown in Fig. 3. The observational plane includes the feeding point of the TX and the receiving load of the RX. The electric field distribution demonstrated that the TX and RX electrodes are strongly coupled across the skin and fat layers. The signal from the TX propagates strongly toward the skin layer where the RX is located, whereas only a weak signal propagates toward the deeper muscle layer. The propagation strengths differ because the free space inside the implantable device insulates the TX electrodes and the muscle layer. Signal transmission in HBC is contributed by conductive currents through biological tissues. In the direction of the muscle layer, signal propagation is heavily attenuated by the insulation of free space, preventing unwanted signal propagation.

FIG. 3.

Electric field distribution in cross section, including the feed point.

FIG. 3.

Electric field distribution in cross section, including the feed point.

Close modal

The ultimate sensitivity of the commonly used Bluetooth module is approximately −70 to −80 dBm. For example, the SH-M08 Bluetooth module used in the wireless neurostimulator system has an ultimate sensitivity of −84 dBm.41 When |S21| is −22 dB in the configuration of this study, the feeding point requires 20 nW (−48 dBm) input power to satisfy the ultimate sensitivity of −70 dBm. This implies that implantable HBC can reduce power consumption compared to conventional wireless communication methods.

Moreover, we calculated the input impedance of the TX electrode to be Zin = 299 − j412 Ω, which deviates considerably from the 50 Ω output impedance at the feeding point. This is because the electrode structure was intended to be worn on the wrist. To resolve this impedance mismatch at the feeding point and to effectively input more of the signal into the body, the electrode structure must be optimized for implantable usage.

To investigate the impact of the displacement between devices inside and outside the body, we calculated |S21| by shifting the RX position horizontally and vertically. Figure 4 displays the RX’s horizontal (xy) and vertical (zx) displacements. In the horizontal plane, we displaced the RX by 24 and 48 mm in the x and y directions from the center of the abdomen model, respectively. In the vertical plane, we displaced the RX up to 10 mm in 1 mm increments while maintaining the center in the horizontal plane.

FIG. 4.

(a) Displacement of the receiver in the horizontal plane (xy). (b) Displacement of the receiver in the vertical plane (z–x).

FIG. 4.

(a) Displacement of the receiver in the horizontal plane (xy). (b) Displacement of the receiver in the vertical plane (z–x).

Close modal

As shown in Fig. 5, |S21| decreased as we displaced the RX from the center of the abdominal model, directly above the TX. This is because the electric field generated by the TX became weak as the RX moved further from the center of the TX position, as shown in Fig. 3. To prevent unintended signal interruption due to device displacement, it is desirable to fix both devices with magnets via the body during communication, as in transcutaneous optical couplers.42 To clarify the maximum allowable displacement, we calculated the displacement ranges that fulfill the ultimate sensitivity (−70 dBm) of the RX when the TX outputs 0.01 mW (−20 dBm), the power output of Bluetooth Low Energy Power Class 3 at an approximate communication distance of 1 m.43 A sufficient power in both the x and y directions was received at horizontal displacements within 24 mm. Therefore, the maximum achievable distance in both x and y directions was estimated as 24 mm.

FIG. 5.

Transmission characteristics as a function of the horizontal position of the receiver.

FIG. 5.

Transmission characteristics as a function of the horizontal position of the receiver.

Close modal

When the distance between the RX electrodes and the skin surface z was 1 mm, |S21| was approximately −56 dB, as shown in Fig. 6. This value is 34 dB smaller than that calculated when z was 0 mm, i.e., the RX electrodes were firmly attached to the skin. As the RX moved further away from the skin surface, |S21| decreased at an approximate rate of 1.6 dB/mm. This implies that signal transmission will be lost when the RX is removed from the body. This decrease in |S21| is also illustrated by the electric field distribution shown in Fig. 3. These results indicate that the signal from the implanted TX rarely causes electromagnetic interference with surrounding devices, such as other medical devices, because the signal attenuates rapidly as the RX moves away from the body surface. In addition, based on the duality of antennas, implanted HBC devices are considered robust against electromagnetic interference from surrounding devices.

FIG. 6.

Transmission characteristics as a function of the vertical position of the receiver.

FIG. 6.

Transmission characteristics as a function of the vertical position of the receiver.

Close modal

We also evaluated the proposed system from the perspective of medical electromagnetic compatibility (EMC). The radiated emission limit for medical devices [group 1, class B] is 40 dB μV/m at a distance of 3 m from the device.44 In the proposed system, the electric field strength at 30 cm from the TX was ∼17 dB μV/m when the TX inputted 1 μW power to meet the ultimate sensitivity of the RX. This means that the radiated emission from the proposed system is clearly below the limit level. Therefore, implantable HBC can achieve superior medical EMC compared to conventional wireless communication methods.

To assess the human safety of the proposed system, we adopted the specific absorption rate (SAR), which determines human exposure to an electromagnetic field. The SAR (in W/kg) defines the power of a radio frequency electromagnetic field absorbed by 1 kg of human tissue. It is calculated as SAR = σE2/ρ, where E is the RMS electric field within the tissue, σ is the electrical conductivity of the tissue sample, and ρ is the sample density. As the averaging scheme of SAR, we selected 1 g averaging. Under the same conditions as medical EMC evaluation, the peak SAR per 1 g of abdominal tissue in direct contact with the electrodes was 1.6 × 10−4 W/kg, sufficiently smaller than the regional absorption limit (1.6 W/kg per 1 g of any tissue) in the safety guidelines.45,46 Therefore, the proposed system meets the human safety criterion within a sufficient margin.

We investigated the use of implantable HBC for medical devices implanted in the abdomen using electromagnetic field simulations. The results showed that the transmission characteristic |S21| between the transmitter (TX) and receiver (RX) was −22 dB, which is sufficient for stable communication. To understand the signal propagation mechanism of HBC, we examined the electric field distribution around and inside the abdominal model. We also investigated the influence of displacement between the devices inside and outside the body by calculating |S21| when shifting the RX position. The horizontal displacement of the RX had a considerable effect on |S21|, suggesting that the position of the devices should be fixed during communication. When the RX was moved away from the skin surface, |S21| rapidly decreased due to the signal propagation mechanism of HBC, which uses the body as a transmission path for signals. We also evaluated the proposed system from the perspective of medical electromagnetic compatibility and human safety. The results showed that the radiated emission and human exposure of the proposed HBC system was adequately small according to the limitation in international standards. These results can be used not only for the development of implantable HBC for abdominal medical devices but also for general implantable applications. HBC signals can also power implantable devices deep inside the body.47 In future work, we will investigate appropriate electrode structures and signal frequencies of implantable HBC devices through experiments on both electromagnetic phantoms and test animals.

A part of this research was funded by JSPS KAKENHI under Grant No. JP22H03996, Takahashi Industrial and Economic Research Foundation, and KDDI Foundation.

The authors have no conflicts to disclose.

Dairoku Muramatsu: Conceptualization (lead); Data curation (equal); Formal analysis (lead); Funding acquisition (lead); Investigation (lead); Methodology (equal); Project administration (lead); Resources (lead); Software (lead); Supervision (lead); Validation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead). Miyu Kodama: Conceptualization (supporting); Data curation (equal); Formal analysis (supporting); Investigation (supporting); Methodology (equal); Validation (supporting); Visualization (supporting); Writing – original draft (supporting); Writing – review & editing (supporting).

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

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