Confined Intra-Arm Communication and Medical
Applications – Extended Abstract
MSR-TR-2012-74
Trang Thai
Gerald DeJean
Ran Gilad-Bachrach
Georgia Institute of Technology
85 5th St., NW
Atlanta, GA 30308 U.S.A.
001-404-944-0845
Microsoft Research
One Microsoft Way
Redmond, WA 98052 U.S.A.
001-425-722-6400
Microsoft Research
One Microsoft Way
Redmond, WA 98052 U.S.A.
001-425-706-7901
trang.thai@gatech.edu
dejean@microsoft.com
rang@microsoft.com
ABSTRACT
Personal area networks are enablers for many new medical
applications. In this work, we present an implementation of such a
network through guided wave on body communication channel.
This method allows the creation of high bandwidth
communication channels which are confined to the body and
improve on previous technologies in terms of privacy and resilient
to interference and bandwidth. The technology proposed can also
be used for other applications such as tracking infections and
especially Nosocomial infections. Nosocomial infections
(hospital-acquired infections) are believed to be linked to the
death of around 100,000 patients each year in the U.S. only.
Improving this situation requires monitoring the interactions
between patients, staff members and objects in the hospital. The
technology proposed here allows the detection of a hand shake
between people and the interaction with other objects and thus
registering them for analysis of the root cause of an infection.
Categories and Subject Descriptors
B.4.3 [Interconnections (Subsystems)]: Physical structures J.3
[Life and Medical Sciences]: health
General Terms
Measurement, Performance, Design, Experimentation, Human
Factors
and less privacy and security concerns. They also enable a set of
applications such as transactions approved by handshakes [4] and
more.
In this work, we present a novel body-wave method that improves
on previous on-body communication solutions. The main
observation is that the skin is conductive and the arm is
cylindrical. Therefore, we design a prototype arm band that
transforms the arm into a modified coaxial line. In a nut-shell, the
antenna uses the arm as the inner lid in a coaxial cable. The arm
band itself injects the signal to the arm and acts as an excitation
source and part of the coaxial channel. In the modified form, the
channel is a leaky coaxial cable on which the signal is highly
confined. Note that coaxial cables are the most shielded
transmission lines for high frequency signals. The resulting
channel works at higher frequencies then previous solutions [3, 4]
and thus, is less likely to interfere with natural body signals. It
provides an order of magnitude higher bandwidth and is much
more confined to the arm.
The proposed solution allows implementation of many
applications. For example, a temperature sensor placed on the arm
pit can communicate with a wrist watch over this channel. If two
people wearing the arm band shake hands, a communication
channel is established which allows for applications such as
money transfer, business card sharing or signing agreements [3].
Many evolving health care application use a network of sensors
and actuators spread on the subjects’ body. Personal area
networks (PANs) allow sensors and actuators to communicate
with each other and with other components such as gateways and
computing providers [1]. In this work, we discuss a special type of
network in which the communication channel is the body itself, as
opposed to the air as in Bluetooth, IEEE 802.15.4 and similar
technologies. Traditional, air-based networks, suffer from several
limitations. The gain of the network is severely affected by the
posture of the subject [2]. They are subject to interference and
require sophisticated privacy preserving protocols in the logical
layers.
One of the interesting applications of this technology in the health
domain is tracking the propagation of infections, especially in the
hospital environment. Nosocomial infections (also known as
hospital acquired infections or HAI) are a major cause for death
and other complications. For example, around 20% of patients
who go through a bone marrow transplant die due to infections
and bacteria [5]. According to the report of the International
Nosocomial Infection Control Consortium [6], around 7.6% of the
patients in intensive care units (ICUs) suffer from HAI. They also
report on a significant increase in the mortality rate in patients
with HAI. For example, the crude mortality rate among patients
without HAI is around 14:4% while the crude mortality rate for
patients with HAI ranges from 18:5% to 42:7% depending on the
type of infection [6] (Table 12 therein). Similar affects have been
measured in infants [6] (Table 13 therein). Moreover, acquiring an
infection increases the number of days a patient stays for
treatment and those increase costs.
On-body channels have several advantages [3]. They are confined,
by design, to the body and hence are subject to less interference
To restrict the risk of antibiotic resistance, it is extremely
important to restrict the use of anti-infectives [6]. Thus, it has
Keywords
Personal area network, antenna design, infection tracking, sensor
network
1. INTRODUCTION
been found that targeted programs for improving hygiene are very
efficient in reducing HAI rates without increasing the risks of
antibiotic resistance [7, 8]. These studies have shown that the
problem of sustaining high degree hygiene levels is not just a
matter of education. It turns out that among the hospital staff, the
worst hygiene levels are maintained by physician. This is despite
the fact that they are well educated and know about the
significance of it, as found by Ignaz Phillip Semmelweis in the
19th century [9]. Therefore, continuous targeted performance
feedback should be used to complement the educational tools [6].
The communication channel described in this work can be used
for tracking the in hospital interactions such that once an infection
is discovered it will be possible to trace back its root cause,
whether it is a human or an object that was not treated properly.
The focus of this paper is the introduction of our technology,
especially the design of the arm band and the physics behind its
capability. As discussed above, the motivation for the
development of this technology stems from its medical
applications. Unfortunately, at this stage we have not had the
opportunity to conduct clinical trials.
2. RELATED WORK
One of the biggest challenges of using the human arm as a
medium for signal transmission is properly injecting a strong
signal to the arm for propagation. Past research has extensively
studied some techniques that can be used to excite signals into the
human arm. Most of these methods focus on placing transmit and
receive electrodes on the biological entity for transmitting and
receiving the signal electrostatically [1, 4, 10]. In these cases, the
communication is facilitated through electric field between two
grounded electrodes (Fig. 1). Most of these studies have been
performed at frequency ranges around 10 MHz and have
transmission gains in the neighborhood of -30 to -25 dB. These
are not guided waves but rather radiated waves or leaky waves,
which are not to be confused with leaky channels.
In addition, there was a brief explanation of a method that treats
the human body as a waveguide (Fig. 2) with high frequency
electromagnetic waves generated at a terminal propagating
through the body that is captured by another terminal at a finite
distance away [3]. However, the concept of utilizing the arm into
part of the coaxial transmission line topology introduces a new
mechanism of transmission as well as a channel platform with
significantly altered signal distribution and signal confinement. To
the authors’ knowledge, this concept has never been observed,
and at the same time there have not been much results published
based on body channel either. The objective of this method is that
using the human arm as the inner conductor of the coaxial
waveguide will allow the electromagnetic energy to be confined
Figure 2. Communication through wave propagation.
mostly on the skin [11]. The conductivity of the skin medium is
much higher than the fat layer residing underneath it.
In this paper, we present a method of coupling an electromagnetic
signal to the human arm through the use of a coaxial transmission
line where the human arm acts as an inner conducting medium
allowing an injected signal to propagate tangential to the outer
circumference of the arm. The human arm is loaded with an openended coaxial transmission line arm band that is worn on the arm.
A voltage signal is excited between the outer and inner conductors
of an accompanying arm band. The inner conductor of this arm
band is in direct contact with the arm, and the signal from the
inner conductor is coupled to the arm for transmission. As will be
seen later, a direct transmission line mode will be present on the
arm in the 400-500 MHz range allowing for a deliberate means of
wireless on skin communications. This presents a great
opportunity for a myriad of applications such as health monitoring
and tracking of infections in a hospital environment.
3. ARM BAND DESIGN
The arm band is designed as an open-ended circular coaxial cuff
that is placed around a human arm. The basic components of a
standard coaxial cable are an outer metallic shield, a center core
metallic inner conductor, and a dielectric insulator between the
outer shield and inner conductor (Fig. 3). Electric field lines
propagate in the dielectric medium (including air) between the
conductors. Fig. 4 shows an illustration of this modified arm band
used for the human arm. The inner conductor is in direct contact
with the arm. Its diameter is 5 cm, while that of the outer
conductor is 6 cm. Therefore, when this band is worn around the
arm, a coaxial mode is initially excited between the inner and
outer conductors.
An illustration of the arm band around a cylinder modeled as a
human arm is displayed in Fig. 5. The length of the inner
conductor is designed to be shorter than that of the outer
conductor. In turn, the conductive properties of the human arm
replace the metallic inner conductor and become the new inner
conductor for the coaxial transmission as the electric fields
propagate down the length of the arm.
Figure 1. Static electric field communication through
electrodes.
Figure 3. Communication through wave propagation.
Figure 4. Model of arm band.
Then, at the termination of the outer conductor, the coaxial line is
open and two types of electromagnetic phenomena occur at
different frequencies. Within the frequency range of
approximately 400-500 MHz, the electric fields at the open ended
termination of the outer conductor stay closely confined to the
conductive arm as shown in Fig. 6. This facilitates non-TEM
(transverse electromagnetic) transmission line propagation of
fields that begin to gradually lose intensity with the increasing
length of the human arm. On the other hand, at the higher
frequency of 1.5 GHz, the channel appears like a dielectric
waveguide and the electric fields are distributed mostly over the
inner region of the arm (the fat layer) rather than concentrated at
the skin as in the case of 400 MHz signals as shown in Fig. 7 [12].
Notice that the field at 1.5 GHz is present at the center of the
arm’s cross section, while the field at 400 MHz is scarcely present
inside the arm [Fig. 6]. The electric fields terminated at the open
ended termination are radiated into the air that generates an
“antenna-like” mode. This mode offers an opportunity to
communicate to base stations and other mobile devices and opens
the door to several applications. Using the human arm as an inner
conductive medium of a coaxial transmission allows for multiple
modes of communication.
Figure 5. Model of human arm loaded with arm bands.
Figure 6. Electric fields at 400 MHz as probe plane travels
from the source starting at ‘a’ and moving to ‘c’.
Figure 7. Electric fields at 1.5 GHz as probe plane travels
from the source starting at ‘a’ and moving to ‘c’.
4. HUMAN ARM SIMULATION SETUP
To get an idea of the relative received power of one arm band in
receive mode when another arm band transmits a signal, it is
necessary to examine the path loss between the two devices. The
path loss is a measurement of the amount of power that is loss
when one device transmits a signal to a second receiving device.
The path loss typically does not take into account any power
losses due to imperfect conductors or lossy dielectric mediums.
To examine the path loss, the authors performed simulations using
two electromagnetic software packages: Microstripes and
Microwave Studio (both developed by CST). The setup for this
simulation consists of a cylinder modeled as a human arm that has
a diameter of 5 cm. The biological materials used in the
simulation are dry skin, fat, and muscle. The most inner material
of the arm model is muscle followed by fat, and finally skin as the
outermost layer. Table 1 illustrates the thicknesses and the
electrical parameters of each of these biological materials. In
simulation, these materials were treated as 2 nd order dispersive
Debye materials. A dispersive material is one in which the
electrical properties change with frequency. The two arm bands
are in direct contact with the arm and physically separated by a
finite distance denoted L. In addition, the arm bands were excited
Material
Muscle
Inner/Outer
Diameter
NA/19mm
Dielectric Constant
Fat
19mm/24mm
Dry Skin
24mm/25mm
472 @ 2.3 MHz,
1.37 @ 1.15 GHz
9282 @ 1 MHz,
8.75 @ 712 MHz
24.71 @ 356 MHz,
1.6 @ 4.4 GHz
in simulation by a linear 20 ohm reference discrete port between
the inner and outer conductors. The figure of merit in the
simulation in the S21 scattering parameter which measures the
voltage ratio of signal received from one arm band in receive
mode (called port 2) when the other arm band is in transmit mode
(port 1):
𝑆21 =
𝑉2 −
𝑉1 +
,
where V is the voltage (proportional to the square root of power),
numeral 1 is one arm band, numeral 2 is another arm band, ‘+’
means transmit mode, and ‘-’ means receive mode. This
parameter is used to represent the path loss between the two
devices. Three simulations were performed: 1.) a S21 measurement
between the two arm bands when L=40 cm; 2.) a S 21 measurement
between the two arm bands when L=30 cm; and 3.) a S21
measurement between the two arm bands when L=40 cm, but the
arm-modeled cylinder has been split with a 5 cm gap. This
signifies a scenario of two separate arms with one arm band being
worn on each arm. Fig. 8 displays a plot of this S21 measurement
versus frequency. In this plot, it can be seen that the S 21
experiences maximum voltage ratios between 375 – 526 MHz for
the three simulations. This frequency range corresponds to the
range of transmission line signal propagation. As expected, the
closer the arm bands are to each other without discontinuity, the
higher the S21 parameter (meaning the lower the path loss). A low
path loss symbolizes that a large amount of transmitted signal
from one device has been received by a second device assuming
only two devices are present. The presence of the gap in scenario
#3 results in the lowest S21 (or the highest path loss). This is
understandable since the current through the arm is discontinued
at the gap. Since the gap is not very large, the electromagnetic
fields at the gap are responsible for the magnitude of signal in this
scenario.
It is also important to examine the S21 response at the frequency
range around 1.5 GHz. Here, the S21 magnitude is the same for all
three scenarios. This is because the mechanism of signal
propagation at this frequency is dominantly due to radiation.
Therefore, the presence of a gap does not affect the transmission.
Also, a plot of S11 is shown in Fig. 8 which signifies the ratio of
signal received from a device to signal transmitted from the same
device. This result is exactly the same for all three scenarios.
Typically in antenna design, a S11 minimum below -6 dB for a
single device signifies a radiation mode. Since there are two
devices, one needs to analyze the S11 and the S21 response to
determine if a radiation mode exists. Although the minimum dip
in the 1.5 GHz range does not meet typical standards, combining
this response with an S21 response lower than -40 dB allows one
to logically assume that a significant amount of energy is radiated
into the atmosphere.
5. UNDERSTANDING THROUGH THE
EXAMINING ELECTRIC FIELDS
To supplement the understanding of the transmission line mode
(at around 400 MHz) and the radiation mode (at around 1.5 GHz)
for this setup, the electric fields are displayed in Figs. 6,7 and 9.
Fig. 6 shows the electric fields at 400 MHz as a probe plane
travels down the length of the arm model starting at the source of
the arm band where the field strength is the strongest. As this
probe plane travels down the arm away from the arm band, a
small quantity of electric fields continue to propagate tightly to
the arm model. The intensity of electric fields is very small
radially away from the arm model throughout the propagation.
This behavior suggests that transmission line properties are indeed
exhibited around 400 MHz. Note that in the simulation depicted in
Fig. 7a, in the cross section plane located along the arm band, the
field drops from -30 dB at the outer surface of the arm band to 50dB at 8mm away from this surface (in air). Compared to the
field distribution generated by method reported in [4], the field
drops about 10 dB between from the electrode surface and
approximately 2 cm away into the air. Therefore, our on-skin
channel platform presented here shows to produce highly confined
signals allowing extra security and privacy at the physical layer of
a communication network. Fig. 7 depicts a different type of wave
propagation at 1.5 GHz. In this figure, a strong intensity of
electric fields is present radially away from the arm model. This
intensity remains strong radially away from the model as the
probe plane travels down the arm. It is concluded that the signal
propagation at this frequency is due to radiation. Finally, Fig. 9
shows the electric fields when a 5 cm gap is inserted into the
model. At 400 MHz, the signal is tightly confined to the model
without radial propagation, but at 1.5 GHz, the signal is radiated
away from the arm regardless of the existence of a gap. Therefore,
the transmission line properties at 400 MHz and the radiation
properties at 1.5 GHz are maintained regardless of any
discontinuities in the model.
6. MEASUREMENT SETUP
Figure 8. S21 measurement of path loss simulation for
modeling a human arm loaded with arm bands.
To validate the simulations of the transmission line on-body
propagation of signals, we designed prototype arm bands to be
worn by actual human arms. Prototypes of the arm band are
illustrated in Fig. 10. The arm band consists of circular copper
strips – an inner strip connected to the arm and an outer strip to
serve as the ground – separated by styrofoam cut into strips to
allow for bending of the arm band. The copper strips have
adhesive on one side to adhere to the styrofoam. Since most
measurement equipment that is used to transmit signals are 50
ohm systems, a quarter-wave transformer were included to the
VNA
Spectrum
Analyzer
Figure 9. Electric fields at 400 MHz and 1.5 GHz as probe
plane travels from the source starting at ‘a’ and moving to ‘c’
for human arm model with a loaded gap.
physical design to transform the 20 ohm used in simulation to 50
ohms. The quarter-wave transformer is a microstrip line that has
an electrical length of λ/4 where λ is the wavelength = the speed
of light in free space divided by the frequency. The microstrip line
was soldered to the inner conductor of the arm band while the
backside ground of the transformer was soldered to the outer
conductor of the arm band.
An HP 8510 vector network analyzer was utilized to send signals
to the arm bands. Coaxial connectors were used as an interface
between the network analyzer and the arm band. Since these are
the beginning stages of this research, we are only interested at this
time in setting up an experiment to justify that a signal can be
obtained from a human arm when the arm bands are worn. To
complete this experiment, two of the authors, Trang and Gerald,
were used, and each of us wore an arm band that was excited with
a network analyzer. With both arm bands excited, a coaxial probe
is used to capture the change in power along the length of the arm.
This coaxial probe is connected to the input port of a spectrum
analyzer to display the change. Since it is possible for the probe to
pick up ambient signals that are identified as noise, it is important
to remember that the relative change of the signal when the probe
touches the arm is the parameter that is being investigated. The
entire measurement setup is shown in Fig. 11.
7. MEASUREMENT RESULTS
Since the coaxial probe is merely the short signal line of a coaxial
connector, the length of the signal line is too short to capture any
Figure 10. Prototype arm band and coaxial probe.
Fig. 11. Measurement system
radiative signals. Therefore, we limit the experiments to the
transmission line characteristic of the communication channel.
The measurements that were performed include two coaxial probe
conditions: poke and non-poke. The poke condition is active when
the probe is normal to the arm and in direct contact with the arm
as the arm bands are transmitting and receiving signals. The nonpoke condition constitutes the probe being tangent to the arm at a
radial distance of approximately 5 mm away without actually
touching the arm.
Fig. 12 shows the received power from the coaxial probe for poke
and non-poke contact at two distances (15 cm and 5 mm away
along the arm axis) from each individual’s arm band when there is
a 5 cm spacing between our arms. This condition is similar to
scenario #3 used in simulations. In these plots, it is noted that the
poke condition applied to both individuals records a larger
received power than the non-poke condition on both individuals.
This can be seen across the frequency band, but the high received
power at the 1.5 GHz radiative band when the poke condition is
enforced can be attributed to transmission as well as near-field
radiation from the arm as a current-carrying source. The
difference in magnitude between Trang’s poke condition and
Gerald’s poke condition can be attributed to many factors
including differences in our body’s electrical current and ambient
sources of noise. Notice that despite the noisy signal, the
characteristics of the channel is shown clear in the case of 15 cm
away in which the difference of the power level between poke and
non-poke condition is the largest across the band.
Fig. 13 illustrates the received power from the coaxial probe for
poke and non-poke contact but for this measurement, only one
arm band was worn at a time. Additionally, Trang and Gerald
were shaking hands, thus establishing a continuous connection
and transfer of body current. This plot gives a good understanding
of the transmission line properties of the communications channel.
Again, one can see that the poke conditions result in a higher
received power that the non-poke condition. A measurement was
also recorded for a poke condition on Gerald’s arm when Trang
was wearing the arm band while shaking hands and vice versa.
Here, it is important to notice that the received power is much
larger in the 400 MHz range than in the 1.5 GHz range. Compared
the non-poke to poke on the non-equipped arm in the hand shake,
the high power received in the 400 MHz range successfully
validates the on-skin channel with its confinement characteristic.
This characteristic can be attributed to the decreasing signal
strength at the higher frequency (non-guided waves) that does not
allow for a strong radiation. Conversely, a relatively strong signal
is received from the tightly confined transmission line mode at the
lower frequency range.
Figure 13. Received power from coaxial probe when Trang
and Gerald are shaking hands with one arm band active.
Finally, Fig. 14 shows the received power from the coaxial probe
for poke and non-poke contact when Trang and Gerald are both
wearing the arm bands while shaking hands as illustrated in Fig.
15. In this measurement, the poke condition occurs at the center of
Figure 12. Received power from coaxial probe when Trang
and Gerald are not in contact with each other.
Figure 14. Received power from coaxial probe when Trang
and Gerald are shaking hands with both arm bands active.
Figure 15. Setup of hand shake condition in which both worn
arm bands are excited by a VNA while the measuring probe is
connected to a spectrum analyzer for signal level detection.
the connection of the hands. Similar to the other cases, the poke
conditions exhibit a larger received power than the non-poke
conditions, which strongly suggests that the signal at 400 MHz
range takes the guided form and well confined to the surface so
that its propagation on the surface is the strongest. The difference
in received power between poking Trang’s arm and poking
Gerald’s arm can be attributed to some unknown factors that we
did not examine in these experiments. More rigorous studies of
using the human body, particularly the arm, will be intensely
explored in future work, but one common trend that Figs. 12-14
exhibit is the increase in received power from contact with a
human when an arm band is transmitting.
8. CONCLUSIONS
In this work, we present a novel method to create on body
communication channel. Using simulations, we showed that this
channel has high capacity, compared to previous technologies and
is more confined to the body. Thus, it has higher throughput and
better privacy and resilient features. We have validated the
predictions of the simulation using a prototype.
We have discussed the application of this communication channel,
especially in the medical domain. It allows the creation of onbody communication network between sensors, actuators and
other components. Moreover, it may be used as a sensing device
since it can register the physical interactions between people and
the interactions with certain objects. We suggest that this may
play a key role in tracking infections, especially in the hospital
environment.
Future Wearable Computers. International Workshop on
Wearable and Implantable Body Sensor Network.
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K., Sasaki, K., Hosaka, H., Itao, K. (2003). Development and
Performance analysis of an intra-body communication
device. International Conference on Transducers, Solid-State
Sensors, Actuators and Microsystems. IEEE.
[4] Fujii, K., Takahashi, M. and Ito, K. (2007). Electric Field
Distributions of Wearable Devices Using the Human Body as
a Transmission Channel. IEEE Trans. on Antennas and
Propagation.
[5] Gooley, T. A., Chien, J. W., Pergam, S. A., Hingorani, S.,
Sorror, M. L., Boeckh, M., et al. (2010). Reduced Mortality
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[6] Rosenthal, V. D., Maki, D. G., Jamulitrat, S., Medeiros, E.
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[7] Tibballs, J. (1996). Teaching hospital medical staff to
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We would like to further explore the properties of the proposed
channel. While the simulations provide encouraging results which
we validated with a prototype that we have built, a more
comprehensive study using the prototype is needed. Moreover, we
would like to test the effectiveness of the approach in a real
medical scenario.
[8] Rosenthal, V. D., Guzman, S. and Safdar, N. (2004). Effects
of education and performance feedback on rates of catheterassociated urinary tract infection in intensive care units in
Argentina. Infection control and hospital epidemiology.
9. ACKNOWLEDGMENTS
[10] Zimmerman, T., Smith, J. R., Paradiso, J., Allport, D.,
Gershenfeld, N. (1995). Applying Electric Field Sensing to
Human-Computer Interfaces. Conference on Human Factors
in Computing Systems, ACM Press.
This research was conducted while Trang Thai was visiting
Microsoft Research. The authors would like to thank Mike
Sinclair for his involvement in valuable discussions that led to the
measurement results of this paper.
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