Abstract:?
The relationship between the attenuation of communication signals and different coupling methods in implantable human communication is studied.
The relationship between the attenuation of communication signals and different coupling methods in implantable human communication is studied. The coupling methods involved include capacitive coupling, current coupling, forward capacitive coupling and reverse capacitive coupling. The communication attenuation of different coupling methods at 20 MHz communication frequency is compared by building an equivalent circuit model for simulation and experimental measurement in a water model simulating the in vivo environment. The results of both simulation and experiment show that the forward capacitive coupling method has the smallest attenuation, which is 26 dB (calculated value) and 28 dB (measured value), respectively, while the attenuation of capacitive coupling, current coupling, and inverse capacitive coupling methods increases in turn. This result reflects the mechanism difference between the different coupling methods and implies that the forward capacitive coupling method would be the best choice if the human body communication method is used in implantable medical devices.
Introduction
Human body communication is a novel non-radio-frequency (RF) wireless communication technology that utilizes the human body as a signaling pathway, and is designed to communicate between networks of wearable and implantable devices and to collect physiological parameters distributed in the human body to aid in immediate medical diagnosis. This approach was initially proposed by Zimmerman of the Massachusetts Institute of Technology (MIT) in the U.S. The principle is to utilize a pair of electrodes on or near the surface of the human body at the transmitter end to input communication signals to the human body, and the signals are transmitted through the human body and then received by a pair of electrodes at the receiver end to complete the communication process. After years of research and development, the wireless body area network standard IEEE802. 15. 6 established in 2012, the physical layer of human body communication and media access control layer have formulated detailed requirements, so that the further application of this technology has a clear basis? . At present, there are studies on human body communication technology all over the world, and it is already possible to accomplish high-speed communication with a communication rate of 160 Mb/s and low-power communication with a power consumption of 37.5 μW by using human body communication. Due to the small size and low power consumption of the equipment required for human communication, it will not be affected by the shadow effect, and the communication rate is guaranteed to a certain extent, so it is very suitable for application in implantable devices that are used under harsh conditions, and it can fulfill the function of transmitting signals from the body to the outside of the body. It has been proved that it is feasible to use human body communication to accomplish ex vivo communication over short distances, but there is no clear conclusion as to which coupling method of human body communication is more suitable for implantable device communication. Therefore, in this study, the effect of different coupling methods on signal attenuation is tested in a water model simulating the human body environment through equivalent circuit model simulation, and it is concluded that forward capacitive coupling is the most suitable coupling method for implantable device human body communication, which provides a theoretical basis for the further application of human body communication methods in implantable devices.
1. Methods
Due to the special characteristics of implantable medical devices, it is not convenient to study the coupling method of implantable human body communication technology directly on the human body or living organisms, therefore, the main method of the experiment is to test the magnitude of the communication attenuation of different coupling methods in the water model that simulates the in vivo environment. Before that, the experimental results are analyzed and predicted with the established equivalent circuit model of human communication to make it more convincing. Based on the requirements and regulations of IEEE802. 15. 6 for human communication, the typical human communication frequency of 20 MHz, which has been proved to be more suitable for human communication by many researches, is selected for the experiment.
1.
1.1 Equivalent Circuit Model
The purpose of establishing an equivalent circuit model is to predict the experimental results, verify its feasibility and provide a theoretical basis, and for this reason, an equivalent circuit model of the impedance network is chosen to simulate the communication process. It has been shown that the model can accurately simulate the size of the channel attenuation when the human body communicates, with less complexity and easy for subsequent analysis, as shown in Fig. 1.
In Fig. 1, the four positions on the human body in contact with the electrodes are regarded as four circuit nodes (A, B, C, and D), and the impedance parameters Z I, Z T, Z B, and Z O are set by the resistors and capacitors connected in parallel, while the contact impedance Z C exists between the human body and the electrodes (E, F, G, and H). The impedance parameters in the model are calculated by the following formula:
Z =l/(j2πfεA + σA?) (1)
where: l represents the distance between the two nodes of the calculated impedance; A represents the area of the electrodes; σ and ε are the electrical conductivity and permittivity of the human body tissues, respectively, and the relevant parameters of different types of tissues at different frequencies can be found in the literature; f is the communication frequency, which is 20% in this experiment.
Since the main difference between different coupling modes is whether the electrode is in contact with the human body or not, different coupling modes can be distinguished by adjusting the values of contact impedance Z C1 and Z C2 at the signal transmitting end: the electrode is not in contact with the human body under the capacitive coupling mode, and therefore Z C1 and Z C2 are mainly shown as capacitive; while the electrode is directly in contact with the human body under the current coupling mode, and Z C1 and Z C2 are mainly shown as capacitive; the electrode is in direct contact with the human body under the current coupling mode, and the electrode is in direct contact with the human body under the current coupling mode. In the capacitive coupling method, the electrodes are not in contact with the human body, so Z C1 and Z C2 are mainly capacitive; while in the current coupling method, the electrodes are in direct contact with the human body, so Z C1 and Z C2 are resistive. In addition, the different coupling modes of the two electrodes at the signal transmitting end are also analyzed, including the forward capacitive-resistive coupling mode when Z C1 is resistive and Z C2 is capacitive, and the reverse capacitive-resistive coupling mode when Z C1 is capacitive and Z C2 is resistive, as shown in Figure 2.
1. 2 Water Modeling Approach
Since the study is an implantable human communication coupling approach, it is inconvenient because it requires the transmitter to be placed within the human body or organism. In the research, it is common to simulate the human body environment by building a water model for experiments, and the author has also adopted this approach. The conduction pathways of an organism for electric current are shown in Fig. 3, including the extracellular pathway via the extracellular fluid resistance R ext and the intracellular pathway via the cell membrane capacitance C m and the intracellular fluid resistance R int . In this experiment, since the communication frequency used was 20 MHz and the impedance of the cell membrane capacitance C m is negligible at this frequency, the signal can be considered to propagate in the electrolyte solution. Based on this, the electrolyte environment in the human body was simulated using physiological salt solution, and a Plexiglas container with a thickness of 2 mm was used to simulate the high impedance played by the skin in the communication, and the transmitting end and the receiving end of the communication end were placed inside and outside of the container, respectively, as shown in Fig. 4.
In the human body communication experiment, if the sending end and receiving end of the communication use grounding equipment at the same time, it will introduce the interference of the ground loop, resulting in inaccurate measurement results? . Therefore, experiments were conducted using a separate signal transmitting end powered by a battery, which enables a sinusoidal signal output with a frequency of 20 MHz and a peak-to-peak value of 1 V. The signal output was generated using a battery-powered signal transmitting end. The circuit and the battery were waterproofed by placing them together in a small space cup, which could be submerged in physiological salt solution during the formal experiments, as shown in Fig. 5. Meanwhile, corresponding to the four different coupling methods, namely, capacitive coupling, current coupling, forward resistive coupling, and reverse resistive coupling, three different electrodes were fabricated to be connected with the transmitter circuit: a capacitive coupling electrode with both electrodes unexposed, a current coupling electrode with both electrodes exposed, and a capacitive resistive coupling electrode with only one electrode exposed. The electrodes are 2 cm × 2 cm copper foils, and the substrate material is epoxy resin, as shown in Fig. 6.
In the experiment, the space cup with the communication transmitter is fixed at the bottom of the container, and the electrodes are spaced 30 cm apart from the communication receiver, which is located outside of the container. the electrodes of the receiver are attached to the outer surface of the Plexiglas container, and the received signals are filtered and recorded by an oscilloscope, and the attenuation value is calculated by the formula, which has
? G = - 20 lgV rec/V tra ? (2)
Where:G is the attenuation value, dB; V rec and V tra represent the amplitude of the received and transmitted voltages respectively.
2 Results
2. 1 Equivalent Circuit Simulation
By changing the impedance characteristics of the input electrode, Multisim software is used to establish the corresponding equivalent circuit model, and simulate and test the attenuation of four different coupling modes under 20 MHz communication frequency, and the computer simulation results are shown in Table 1. The simulation results are shown in Table 1. The results show that the attenuation value of communication is the smallest in the forward capacitive coupling mode, followed by the capacitive coupling mode, current coupling mode and reverse capacitive coupling mode.
2. 2 Water modeling experiments
The oscilloscope recorded the communication attenuation results under four different coupling methods, as shown in Table 2. It is easy to see from the data in the table that in the water model experiment, the coupling mode with the smallest attenuation is the forward resistive coupling mode, and the communication attenuation of the capacitive coupling mode, the current coupling mode and the reverse resistive coupling increases in turn, which is consistent with the simulation results.
2. 3 Comparison of results
The simulation results and experimental results are put together for comparison, as shown in Figure 7. It can be seen that the communication attenuation in the forward capacitive coupling mode is significantly smaller than the other three coupling modes, both in the simulated and measured values. Comparing the results of simulation and experiment, it is found that, except for the reverse capacitive-resistive coupling mode, the attenuation values of the simulation and experiment in the capacitive coupling, current coupling and forward capacitive-resistive coupling modes are basically the same, which proves that the established model is more similar to the real situation, and the experimental results are convincing to a certain extent. The difference between the two in the case of reverse capacitive coupling is mainly due to the lack of precision of the oscilloscope used in the experiment, which is unable to measure the received amplitude below 10 mV.
3 Discussion
After the simulation and experimentation of the signal attenuation of four different human communication coupling methods, it can be concluded that the forward resistive coupling method is the most suitable for the communication of implantable devices, which can provide help and support for the communication design of implantable medical devices. At present, the implantable medical devices are used in the radio frequency based communication method, the shortcomings of which lie in the short communication distance, energy consumption, which has a great impact on the service life of the implantable medical devices, and the fundamental reason for these shortcomings is precisely due to the signal attenuation of the radio frequency communication in accordance with the square of the communication distance increases. On the contrary, the human body communication, the experimental forward capacitive coupling method at a distance of 30 cm under the communication attenuation of only 28 dB, through a simple amplification filter can obtain the communication signal, indicating that this method has a very great application prospects. Fig. 8 Transmission principle of different coupling methods for human body communication. On the basis of the experimental results, the original cause of the results were also analyzed, as shown in Figure 8. In the capacitive coupling mode, the electrodes are not in direct contact with the human body, the communication signal in the form of electromagnetic waves coupled with the human body and the surrounding environment, to achieve the purpose of information transmission; current coupling mode by the electrodes directly to the human body to send signals, but due to the two electrodes are in contact with the human body, through the human body to form the current path, most of the energy directly back to the signal source, and only a small portion of the realization of the communication function, so the communication attenuation instead of higher than the capacitive coupling method. Communication attenuation is higher than the capacitive coupling method; in the forward capacitive coupling method, the sending end of the signal electrode direct contact with the human body, the reference electrode and the surrounding environment through the distribution of capacitive coupling, the human body mainly transmits communication signals, communication attenuation is minimal; and reverse capacitive coupling method is the opposite, the human body contacted the transmitting end of the reference electrode, and did not play a role in transmitting the communication signals, it is very difficult to complete communication through this way. It is difficult to complete the communication in this way. In addition, this experiment only studied the situation of sending signals from the body to the outside of the body, if you want to complete the duplex communication of implantable devices through the human body communication method, you need to carry out the experiment of sending signals from the outside of the body to the inside of the body to receive, in order to add the communication system is complete.
4 Conclusion
Human body communication is a promising non-radio frequency wireless communication method, which has potential applications in many fields. This study focuses on the application of human body communication in implantable medical devices, and explores the optimal coupling method for implantable device communication using human body communication. The communication attenuation of four different coupling methods, namely, capacitive coupling, current coupling, forward resistive coupling, and inverse resistive coupling, is examined through the simulation of the equivalent circuit model and the experimental measurements of the water model, and it is concluded that the forward resistive coupling is the most suitable coupling for ex vivo human body communication. It is concluded that the forward capacitive coupling is the most suitable coupling method for in vivo and ex vivo human body communication. In further research, we will try to design and fabricate a human communication module suitable for implantable device communication, and verify its practicability through animal experiments, so as to achieve the purpose of applying human communication to implantable devices.