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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

Implantable devices have important applications in biomedical sensor networks used for biomedical monitoring, diagnosis and treatment,

Intra-body communication (IBC) is a technology using the human body as transmission medium for electrical signals [

Like on-body IBC, the implant IBC provides benefits to many applications, such as biomedical monitoring systems [

However, the previous works in this field have some limitations, which can be summarized as follows: (1) Comparatively higher signal attenuation. Previous investigations on implant IBC mainly concentrated on the implant IBC based on galvanic coupling [

On the other hand, it has been proved that on-body IBC based on capacitive coupling has comparatively lower signal attenuation. In the on-body IBC based on capacitive coupling, only the signal electrodes of the transmitter and receiver are attached to the body skin directly, while both the transmitting ground electrode and the receiving ground electrode remain floating [

The rest of the paper is organized as follows: In Section 2, a circuit model of the implant IBC based on capacitive coupling was developed, then the corresponding transfer function was derived. Some important parameters of the transfer function were discussed and modeled in detail in Section 3. In Section 4, measurement experiments were carried out for verifying the reliability of the proposed transfer function, while some important characteristics of the proposed method were studied. Finally, Section 5 concludes this paper.

Firstly, the difference between the on-body IBC based on capacitive coupling and the implant IBC based on capacitive coupling is analyzed. In the on-body IBC based on capacitive coupling, as shown in _{O}_{O2}_{O1}_{O1}_{O2}_{O}

In the implant IBC based on capacitive coupling, both the transmitter and receiver are implanted into human body, as shown in

In the implant IBC based on capacitive coupling shown in _{I}_{O}_{I}_{I}_{I}_{I}_{I}_{I}_{I}

According to

In the circuit model of the transmitter, as shown in _{0}_{a}_{1} is the impedance between the ground electrodes and the signal electrodes, and _{k}_{1} represents the impedance of insulating shell between the ground electrode and the human tissue. On the other hand, _{b}_{11} and _{b}_{12} represent the transverse impedance between the two signal electrodes, and _{b}_{21} and _{b2}_{2} represent the impedance between the two ground electrodes. Meanwhile, the coupling capacitances between the human body and the external ground are represented as _{g}_{1} and _{g}_{2}, which affect the coupling paths between the signal electrodes (_{b}_{11} and _{b}_{12}) and that between the ground electrodes (_{b}_{21} and _{b2}_{2}), respectively. Additionally, in the circuit model of the receiver, _{in}

The equivalent circuit of the circuit model in _{in}_{out}

The transfer function of the implant IBC based on capacitive coupling can be derived by Kirchhoff voltage law (KVL) mesh equations [_{n}

It is assumed that mesh impedance matrix

On the other hand, the column matrix of the voltage sources

As a result,

Furthermore, the mesh admittance matrix is determined by inverting the mesh impedance matrix as:

Therefore, the current _{4} can be obtained by calculating the mesh current matrix

Then the output voltage of the implant IBC can be expressed as:

Finally, based on

The following is the discussion with respect to the parameters of the deduced transfer function.

Due to the fact that the human body generally consists of five layers (skin, fat, muscle, cortical bone, and bone marrow), _{b}_{s}_{n}_{nf}_{nf}_{b}_{11}, _{b}_{12}) and that between the two insulating shells (_{b}_{21}, _{b}_{22}) can be obtained.

_{k}_{1} and _{k}_{2}, which are the impedances of insulating shell between the ground electrode and the human tissue, can be obtained by _{k}_{1}, _{k}_{2} and _{k}_{1}, _{k}_{2}. The capacitances of _{k}_{1} and _{k}_{2} represent the capacitances between two coaxial cylinders, which are expressed by _{0}_{r}_{A}_{B}_{k}_{1} and _{k}_{2} can be calculated by the equation _{a}_{1} and _{a}_{2} can also be obtained by the above method.

It is assumed that if a person stands in an open space, and the human body is approximated as a conductive cylinder or sphere [_{g}_{∞}_{p}_{∞}_{e}

As a result, the capacitance between the object and the ground of infinity _{∞}_{e}

In our investigation, the arm attached with the electrodes is abstracted as a cylinder, of which the diameter is _{∞}

On the other hand, the capacitance of _{P}_{P_arm}

In order to verify the validity of the proposed models and parameters, the measurements of implant IBC and the mathematical simulations based on the proposed transfer function were carried out. Moreover, the characteristics of the implant IBC based on capacitive coupling were also analyzed.

In our investigation, the experiment setup of the implant IBC based on capacitive coupling was composed of a handheld signal generator, a ScopeMeter, a pair of implantable capacitive coupling electrodes and a rectangle tank, as shown in _{0} = 50 Ω) was used to provide the output signal at the transmitter terminal, and the ScopeMeter (Fluke 196C, _{in}_{in}

A rectangle tank with the size of 45 × 35 × 20 cm was used for simulating the human body, as shown in _{r}_{r}ε_{0}A_{∞}_{P}

In our experiment setup, the ground electrode of the capacitive coupling electrodes is cylindrical casing and packed with insulating shell (^{−14}, _{r}_{A}_{B}

Moreover, in order to verify the advantages of implant IBC based on capacitive coupling compared with the implant IBC based on galvanic coupling, the electrodes of the implant IBC based on galvanic coupling were also developed, which had two cylindrical copper endings (length 1 cm and diameter 4 mm) and the distance between them was 5 cm [

In this experiment, the separation distance between the transmitter electrode and the receiver electrode was set as 30 cm. Meanwhile, the sine wave signals with the amplitude of 4 V (peak-to-peak value) were applied on the transmitter electrodes. On the other hand, the signal frequency range of 100 kHz–40 MHz was chosen in our measurement, due to the fact that the power spectrum of the electrical signals produced by the biological processes mainly covers the low frequency range (less than 100 kHz) [

In order to verify the validity and the accuracy of the transfer function, both the measurements and the corresponding simulations with respect to the frequency-dependent characteristics of the proposed method were carried out under the conditions of the different signal transmission distances and heights.

On the other hand, when the signal transmission distance increases from 20 cm to 40 cm, both the signal attenuations of the two results have little variation. For instance, an increase of 10 cm of the signal transmission distance only leads to an extra attenuation of 0.25 dB on average according to the measurement results. Similarly, the extra attenuation of the corresponding simulation is 0.14 dB on average, which indicates that both of them basically are not sensitive to the signal transmission distances.

Moreover, under the condition that the height between the tank and the ground was set as 1 cm, 50 cm and 80 cm, respectively, while the signal transmission distance was set as 20 cm, the implant IBC experiments as well as the corresponding simulations based on the transfer function were carried out.

It can be observed from

In order to determine the characteristics of the proposed method, the corresponding simulations of the proposed method were carried out under the conditions of the different frequencies, signal transmission distances and heights based on the transfer function which has been verified. In our simulation, it is assumed that the human body is in static state at room temperature (298.15 K), which means that the capacitance to the ground keeps unchanged, and the influence of temperature variation is ignored [

In this simulation, we assumed that the implantable capacitive coupling electrodes were embedded in the arm, of which the diameter was 10 cm. The attenuation curves corresponding to the different distances (20, 30 and 40 cm) are shown in _{g}_{1} between the arm and the ground is equal to 7.99 pF.

It can be seen from _{b}_{g}_{1} and _{k}_{1}). As a result, the signal attenuation changes little with the increase of the transmission distance, which caused the increase of _{b}

In our simulations under the conditions of different heights to the ground, the distance of transmission was set as 20 cm, while the simulation frequencies were set as 3 MHz and 10MHz, which were corresponding to the cases that _{b}_{11} = 60.9 Ω, _{b}_{11} = 116 pF as well as _{b}_{11} = 54 Ω, _{b}_{11} = 33 pF, respectively. The corresponding simulation results are shown in

This phenomenon can be interpreted as that the capacitive coupling between the arm and the external ground becomes smaller as the height of the arm increases. For example, _{g}_{1} is equal to 22.83 pF when the height is 1 cm, while it is equal to 9.07 pF when the height is increased to 30 cm. Therefore, when the height decreases to some extent (such as less than 10 cm), the comparatively higher signal power is lost to the external ground through the capacitance betwe nen the arm and the ground, which leads to the comparatively bigger increase of signal attenuation. On the other hand, the attenuation becomes basically stable when the height is higher than 30 cm. For instance, the difference between the signal attenuation corresponding to 30 cm and the signal attenuation corresponding to 100 cm is only 0.12 dB when the frequency is 10 MHz. A similar phenomenon can be found in the results corresponding to 3 MHz. The main reason for this phenomenon can be interpreted as that the additional capacitance to the ground (_{P_arm}

In this paper, we propose an implant intra-body communication (IBC) method based on capacitive coupling, and investigate its transfer function and characteristics. Firstly, we derived the transfer function of the implant IBC based on capacitive coupling. Secondly, the corresponding parameters used in the transfer function were discussed. Finally, both the measurements of the proposed method and the corresponding simulations based on the transfer function were carried out under different conditions.

From the measurement and simulation results, we find that: (1) The simulation results based on the developed transfer function basically coincide with the measurements; (2) Compared with the implant IBC based on galvanic coupling, the proposed method has comparatively lower signal attenuation and basically stable frequency response within the frequency range of 2 MHz–40 MHz; (3) In the proposed method, the signal transmission distance almost has no influence on the signal attenuation; (4) The signal attenuation of the proposed method decreases with the increase of the height between body and the ground, and it becomes basically stable when the height is higher than a certain value, such as 30 cm. The above conclusions indicate that the proposed method of the implant IBC based on capacitive coupling has the advantages of low signal attenuation, insensitivity to signal transmission distance and so on. It will help to achieve an implant communication method for e-healthcare or u-healthcare with the characteristics of low power consumption and high transmission quality,

The work was supported by the National Natural Science Foundation of China (60801050), the Excellent Talent Fund of Beijing, China (2011), Excellent Young Scholars Research Fund of Beijing Institute of Technology, China (2012).

The authors declare no conflict of interest.

Implant communications based on IBC technology.

Schematic diagram of (

Circuit model of implant IBC based on capacitive coupling.

Equivalent circuit of implant IBC based on capacitive coupling.

The parameters of

Measurement setup of implant IBC based on capacitive coupling.

Measurement setup of implant IBC based on galvanic coupling.

Measurement results of the proposed method and the implant IBC based on galvanic coupling.

Comparison between measurements and simulation results corresponding to the different signal transmission distances.

Comparison between the measurement results and simulation results corresponding to the different heights.

Simulation results corresponding to the different signal transmission distances and frequencies.

Simulation results corresponding to the different heights between the arm and the external ground.

The conductivity and relative permittivity of the materials.

Parameters | _{r} |
_{r} | ||

Values | 1.75 | 80.4 | 1 × 10^{−14} |
3 |