BRIEF COMMUNICATION

Interface of data transmission for a transcutaneouscommunication system using the human body as transmissionmedium

Eiji Okamoto • Yoshikuni Kato • Kazuyuki Seino •

Yoshinori Mitamura

Received: 14 January 2011 / Accepted: 2 August 2011 / Published online: 21 August 2011

� The Japanese Society for Artificial Organs 2011

Abstract We have been developing a new transcutaneous

communication system (TCS) that uses the human body as

an electrical conductive medium. We studied an interface

circuit of the TCS in order to optimize the leading data

current into the human body effectively. Two types of LC

circuits were examined for the interface circuit, one was an

LC series-parallel circuit, and the other was a parallel-

connected LC circuit. The LC series-parallel circuit con-

nected to the body could be tuned to a resonant frequency,

and the frequency was determined by the values of an

external inductor and an external capacitor. Permittivity of

the body did not influence the electrical resonance. Con-

nection of the LC series-parallel circuit to the body

degraded the quality factor Q because of the conductivity

of the body. However, the LC parallel-connected circuit

when connected to the body did not indicate electrical

resonance. The LC series-parallel circuit restricts a direct

current and a low-frequency current to flow into the body;

thus, it can prevent a patient from getting a shock.

According to the above results, an LC series-parallel circuit

is an optimum interface circuit between the TCS and the

body for leading data current into the body effectively and

safely.

Keywords Artificial heart � Intra-body communication �Transcutaneous data communication � Monitoring

Introduction

A transcutaneous communication system (TCS) is one of

the key technologies for monitoring and controlling artifi-

cial hearts and other artificial organs that are implanted

inside the body. The TCS needs to have stable bi-direc-

tional communication through the skin without interference

from other patient devices.

Intra-body communication using a human body as a

transmission conductor has been investigated in order to

achieve personal networks for communication between

wearable information devices and sensors [1–3].

We have been developing a new TCS that uses the

intra-body communication technology [4]. The new TCS

was able to transmit data concurrently and bi-direc-

tionally on the surface of a human body under full

duplex communication at a transmission rate of

115 kbps. The performance of the TCS is superior not

only to that of the traditional transcutaneous information

systems using electromagnetic or optical data transmis-

sion, but also to the reported intra-body communication

systems [5–7].

The electrical properties of the body are represented by

an equivalent circuit with resistors and capacitors. In a

previous study, we could obtain an electrical resonance

while leading ASK-modulated current into a human body

through an inductor. However, the mechanism of an L–C

resonance incorporating the human body has not been

studied. Hwang et al. already reported the design of a

data-receiving circuit of intra-body communication [8],

but an optimal interface for data transmission has not been

found.

In this study, we studied an interface of the TCS with a

human body in order to understand the electrical properties

of the human body when leading a data current and in order

E. Okamoto (&) � Y. Kato � K. Seino � Y. Mitamura

Department of Human Science and Informatics,

School of Biological Science and Engineering,

Tokai University, 5-1-1-1 Minami-sawa,

Minami-ku, Sapporo, Japan

e-mail: okamoto@tspirit.tokai-u.jp

123

J Artif Organs (2012) 15:99–103

DOI 10.1007/s10047-011-0600-x

to optimize the interface circuit for data output into the

human body.

The aim of this article was to study an interface circuit

of the TCS in order to achieve a suitable interface circuit

with the human body for stable and efficient data

transmission.

Method

A TCS has an external and an internal unit, and each unit

consists mainly of a data transmitter and a data receiver

[4]. The data transmitter has an oscillator (carrier fre-

quency: 4 and 10 MHz), an amplitude shift keying (ASK)

modulator, and Ag–AgCl electrodes. The ASK-modulated

data current is led into a human body through a trans-

former, and it flows back to the ground terminal of an

energy source through the body, space around the patient

and a data receiver. The data receiver consists of an Ag–

AgCl electrode, an LC parallel-tuned circuit, and an ASK

demodulator.

In this study, two types of LC circuits were examined

as data transmission interface: one was an LC series-par-

allel circuit in which an inductor was connected in parallel

with series-connected capacitors, and the other was a

parallel-connected LC circuit. A junction node between

the capacitors in the LC series-parallel circuit was con-

nected to the left forearm of a subject through an ECG

Ag–AgCl electrode [Life Med Co., Soft-E S30/A (L),

Tokyo, Japan] removing conductive paste spread on the

contact interface, and alternate current was led into the

human body. Voltage on the transmission electrode was

measured by an oscilloscope (TTF Co., Tektronix MSO

4032, Tokyo, Japan) using a high-input impedance FET

probe (TTF Co., Tektronix p6243, Tokyo, Japan) to

evaluate frequency characteristics under changing capaci-

tance of the capacitors.

The parallel-connected LC circuit was also connected to

a human body in parallel through an Ag–AgCl electrode,

and frequency characteristics were also measured.

All procedures in this study were approved by the Tokai

University institutional review board for human research.

Results

Figure 1a shows tuning characteristics of the LC series-

parallel circuit at a carrier frequency of 4 MHz. Two

capacitors having the same capacitance (C1 = C2) were

connected in series, and three kinds of voltage were

measured under changing capacitance of both capacitors.

The terminal voltage of an inductor under connecting

capacitors to the body Vo2 showed characteristics of a

parallel resonance circuit like the voltage Vo1, and the

voltage on an electrode Vo3, which was the node voltage

of the series-connected capacitors, also showed a resonant

characteristic.

Figure 1b shows tuning characteristics of an LC paral-

lel-connected circuit at a carrier frequency of 4 MHz. The

terminal voltage of the LC parallel-connected circuit Vo1

showed resonant characteristics like the LC series-parallel

circuit in Fig. 1a. However, the terminal voltage of the LC

parallel-connected circuit connected to the body Vo2 did

not show resonance, and the amplitude change of the

voltage Vo2 was quite small under changing capacitance of

the tuning capacity.

From the above results, the LC series-parallel circuit is

suitable for an interface circuit of a TCS to lead commu-

nication data current effectively into the body.

Discussion

We have been developing a TCS for communication

between a host monitor outside the body and an internal

artificial heart controller inside the body using the body as

a transmission conductor. In general, the electrical char-

acteristics of tissue are determined by their conductivity

and permittivity depending on current frequency, and the

electrical properties of the body as an assembly of tissue

are represented by an equivalent circuit consisting of

resistors and capacitors.

In a previous study, we examined a series-connected LC

interface circuit connected to a human body. Development

of electrical resonance depended on the length of the signal

line, and installation of a capacitor between the signal line

and a circuit ground made a stable electrical resonance

regardless of the length of the signal line [9].

Leading current into the body under electrical reso-

nance has several advantages: effective application of

transmission current, and band limitation of the transmis-

sion current contributing to both bi-directional communi-

cation and protection against shock caused by low

frequency currents.

However, the mechanism of electrical resonance when

connecting the LC interface circuit to the human body has

not been determined. Also an optimum design of the LC

interface circuit between the TCS and the body have not

been reported. Therefore, in this article, we studied the LC

interface circuit in order to clarify the effect of the elec-

trical properties of the body on the LC electrical resonance

in order to achieve an optimum interface circuit for data

transmission.

An LC series-parallel circuit connected to a human

body showed electrical resonance (Fig. 1a). Capacitances

of the tuning capacity at resonance were the same in

100 J Artif Organs (2012) 15:99–103

123

terminal voltages Vo1, Vo2, and Vo3; thus, permittivity of

the body did not influence electrical resonance. However,

an LC parallel-connected circuit connected to the body did

not indicate electrical resonance, as shown in Fig. 1b.

The body is electrically represented by an RC parallel

circuit, as shown Fig. 2a, and the LC series-parallel circuit

connected to the body is electrically expressed as in

Fig. 2b. When the capacitance of a capacitor originated

from permittivity of the body, C3 was much smaller than

the capacitance of an external capacitor C2; an equivalent

circuit of the LC series-parallel circuit connecting to the

body is simplified in Fig. 2c. The resonant frequency fr is

determined by the inductor L and the external capacitor C1,

C2, and it is expressed as follows,

fr ¼ 1

2pffiffiffiffiffiffi

LCp ð1Þ

where

C ¼ C1 � C2

C1 þ C2

ð2Þ

Capacitor components of the body C3 are not included in

resonant frequency fr; thus, the size of the human body

does not affect resonant frequency.

The quality factor Q is a measure of the quality of a

tuned circuit, and it is determined by a ratio of resonant

frequency to the bandwidth of the tuned circuit. As shown

in Fig. 1a, the quality factor Q of the LC series-parallel

circuit decreased from Vo1 to Vo3 by connecting to the

body. The quality factor Q of the tuned circuit is also given

by the ratio of susceptance to its conductance. Therefore,

connection of the LC series-parallel circuit to the body

degrades the quality factor Q because of the increase in

conductance by adding the conductivity of the body.

As for the LC parallel-connected circuit, an increase of

conductivity by connecting to the body decreases the

quality factor Q greatly, and it causes disappearance of

0

1

2

3

4

5

6

7

8

0 50 100 150 200 250

Tuning Capacitance C (pF)

Vou

t (V

)

21

21

CC

CCC= =

+C21C C

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250

Vou

t(V

)

Tuning Capacitance C(pF)

4MHz

4MHz

(a)

(b)

Fig. 1 Frequency

characteristics of the interface

circuit for leading data current

into the body. a The LC series-

parallel circuit connected to the

body. b The LC parallel

connected circuit connected to

the body

J Artif Organs (2012) 15:99–103 101

123

electric resonance when connecting the LC parallel-con-

nected circuit to the body (Fig. 1b).

The LC series-parallel circuit applied for a data trans-

mission interface to the body has another advantage. Both a

transformer used as an inductor L and a capacitor C1,

which is connected in series between the transformer and

the body, restrict a direct current and a low-frequency

current to flow into the body, and it prevents a patient from

getting a shock.

From the above results, the series-parallel LC circuit

shown in Fig. 1a is a suitable interface circuit for leading

current into a human body effectively and safely.

Further study is required to evaluate the performance

of the TCS interface circuit under transcutaneous bi-

directional data transmission in a long-term animal

experiment.

Conclusion

We have studied an interface circuit of the TCS using a

human body as electrical conductive medium. An LC

series-parallel circuit is an optimum interface between the

electronics of the TCS and the human body for leading data

transmission current into the human body effectively and

safely, and it should promise stable communication

between an implanted artificial organ and external monitor

outside the body.

Acknowledgments This study was supported by KAKENHI

[Grant-in-Aid for Scientific Research-C (22500413), 2010] and the

program for Promotion of Fundamental Studies of the Tateishi Sci-

ence and Technology Foundation, 1091004, 2009.

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LC

1

2=

+=

21

21

CC

CCC

(a)

(b)(c)

Fig. 2 Equivalent circuit of

interface for leading data

current into the body

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