© 2011 Expert Reviews Ltd
Implantable devices for physiological monitor-
ing are used widely by clinicians and research-
ers to monitor health and to study normal and
abnormal body functions. These devices can
relay important signals (e.g., electrocardiogram,
glucose level and blood pressure) from implanted
sensors to external equipment to be analyzed or
to guide treatment. Implantable devices can
also be used to record neural signals in brain–
machine interfaces to control prostheses  or
paralyzed limbs .
Communication with implanted devices
is usually accomplished with a wired con-
nection or with wireless radiofrequency (RF)
telemetry. However, wires can break, become
infected or introduce noise in the recording
through movement artifacts or by antenna
effects. Complications with wires are fre-
quently reported with deep brain stimulation
devices  and with pacemakers and implantable
Wireless RF telemetry has been used in sev-
eral implantable medical devices to avoid the
complications of wired implants [5,6]. However,
wireless RF telemetry requires significant power
and suffers from poor transmission through
biological tissue. RF telemetry also needs a
relatively large antenna, which limits how small
the implantable devices can be and prevents
implantation in organs such as the brain, heart
and spinal cord without causing significant
damage. Other methods of wireless communi-
cation have been investigated to communicate
with implants, including optical  and ultra-
sound . However, these methods also have
low-efficiency transmission through the body
and would be difficult to miniaturize.
Intrabody communication is a recently devel-
oped alternative method of wireless communi-
cation, which uses the conductive properties of
the body to transmit signals. This article will
explain the major developments and the theory
of intrabody communication, describe chal-
lenges to putting the technology into practice,
and discuss how intrabody communication can
be used as the basis for a novel class of wireless
implantable medical devices.
The first report of intrabody communication
was in 1995 by Zimmerman et al. , where a
small signal (~50 pA) was transmitted through
the body and detected at a receiving electrode.
In this system, a single transmitting and a single
John E Ferguson1 and
A David Redish†2
1Department of Biomedical
Engineering, University of Minnesota,
Minneapolis, MN 55455, USA
2Department of Neuroscience,
6-145 Jackson Hall, 321 Church St. SE,
University of Minnesota, Minneapolis,
MN 55455, USA
†Author for correspondence:
Many medical devices that are implanted in the body use wires or wireless radiofrequency
telemetry to communicate with circuitry outside the body. However, the wires are a common
source of surgical complications, including breakage, infection and electrical noise. In addition,
radiofrequency telemetry requires large amounts of power and results in low-efficiency
transmission through biological tissue. As an alternative, the conductive properties of the body
can be used to enable wireless communication with implanted devices. In this article, several
methods of intrabody communication are described and compared. In addition to reducing the
complications that occur with current implantable medical devices, intrabody communication
can enable novel types of miniature devices for research and clinical applications.
Keywords: biotelemetry • cardiac implants • implantable device • intrabody communication • neural implants
• remote monitoring • wireless
Wireless communication with
implanted medical devices
using the conductive
properties of the body
Expert Rev. Med. Devices 8(4), 427–433 (2011)
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Expert Rev. Med. Devices 8(4), (2011)
receiving electrode were placed near the skin without touching
it, capacitively coupled to the body. Another set of electrodes at
the transmitter and receiver were also oriented away from the
body and were capactively coupled to the environmental ground,
serving as the signal’s return path (Figure 1A).
This type of telemetry, called capacitive intrabody communica-
tion, has primarily been used for surface-based communication
with both the transmitter and receiver electrodes placed on or near
the skin. The major limitation of this transmission method is its
reliance on capacitive connections to both the body and ground
and thus has not been used for communicating with implanted
devices. Several applications of capacitive intrabody communi-
cation have been developed for transmitting data to consumer
electronic devices [10,11].
The second type of intrabody communication, galvanic, was
first reported in 1997 by Handa et al. . A small alternating cur-
rent flowed from transmitting electrodes on the chest, through the
body, and was detected by receiving electrodes on the wrist. The
transmitting and receiving electrodes were in direct contact with
the body, resulting in galvanic coupling (Figure 1B). A major advan-
tage of this technology was its very small power requirement, only
8 µW. In addition, because no ground connection was required,
this type of telemetry could be used with implanted devices.
Galvanic intrabody communication has been studied for a
range of medical applications including communicating with
implanted and surface-mounted devices. This article will focus
on galvanic communication; interested readers can find a recent
review of capacitive intrabody communication in .
In implant-to-surface communication, galvanic coupling is used
to send signals from an implanted device to electrodes on the
skin. This allows for easy placement and repositioning of the skin
electrodes to improve the quality of signal reception. However,
because the signal has to travel through the skin, which is less
conductive than many of the tissues inside the body, more signal
Human cadaver testing
Lindsey et al. tested a method of galvanic communication
between an implanted device and surface electrodes to monitor
and transmit information about anterior cruciate ligament graft
tension after surgery . Two platinum electrodes (each 0.38 mm
in diameter, separated by 2.5 mm) were used to inject current
into the leg of a human cadaver. Electromyography (EMG) elec-
trodes on the surface of the leg were able to detect the transmitted
signals. The signals tested were sine waves with frequencies of
2–160 kHz and currents of 1–3 mA, resulting in a minimum sig-
nal attenuation of 37 dB. The attenuation increased with smaller
currents, with longer distance to the surface electrodes, and with
decreased inter-electrode separation of the surface EMG elec-
trodes. In addition, the signal attenuation was sensitive to the
placement of the surface electrodes in relation to the joint line.
Because standard EMG electrodes were used to receive the signal,
they could be easily repositioned to improve the quality of signal
reception. However, the signal attenuation remained very high
(37–50 dB), making signal transmission with high signal-to-noise
Anesthetized animal testing
A more efficient implant-to-surface communication system was
developed by Sun et al. and tested in saline and an anesthetized
pig (Figure 1C) . The implanted transmitter was integrated in an
‘x-antenna’, where the electrodes were integrated in two parabola-
like surfaces that altered the current flow. The insulated sections of
the x-antenna caused the current to flow in larger paths around the
antenna and allowed for more current to be detected at the receiver
electrodes. In a saline test, signal delivery using the x-antenna was
found to only require 1% of the power of a traditional electrode
pair. However, the diameter of the x-antenna was 9 mm, and the
transmitter was designed to be implanted on the surface of the
brain in between the dura and the cortex, with the signal detected
by needle electrodes in the scalp. This system would be too large to
be implanted inside the brain without causing significant damage.
In implant-to-implant communication, signals are transmitted
from the implanted device to receiver electrodes also implanted
inside the body. The implanted receiver can then be connected
to equipment outside the body using a short wire or with wire-
less RF telemetry. In this way, less power is needed to transmit to
the implanted receiver electrodes than to electrodes on the skin.
However, the implanted receiver electrodes cannot be as easily
repositioned as skin-mounted receiver electrodes.
Tissue analog testing
A system for implant-to-implant communication was developed by
Wegmueller et al. and tested in a muscle-tissue analog (Figure 1D) .
The two electrodes of the transmitter galvanically coupled an alter-
nating-current signal into the body. The signal was then detected by
two receiver electrodes. Signals with frequencies of 100–500 kHz
were used in order to avoid common neural frequencies, and less
than 1 µA of current was used. Two different designs for the trans-
mitting and receiving electrodes were tested: pairs of exposed copper
cylinders (10 mm in length and 4 mm in diameter) and exposed
copper circles (4 mm in diameter). The electrode sites were spaced
50 mm apart for both the transmitter and receiver. The copper
cylinder electrodes could transmit sinusoidal signals with a loss of
approximately 32 dB over 5 cm, and the copper circle electrodes had
a loss of 47 dB over 5 cm. However, the electrodes were large and
significant signal loss was found with any misalignment between
the transmitter and receiver electrodes. The large signal losses were
caused by the four-electrode design; most of the transmitted current
returned to the transmitter and did not reach the receiver.
Anesthetized animal testing
A two-electrode system was developed by Al-Ashmouny et al.
and tested in an anesthetized rat (Figure 1e) . The system used
two electrodes in contact with the tissue, one for the transmitter
and one for the receiver. Both electrodes were made from 50-µm
Ferguson & Redish
diameter platinum–iridium wire. The
transmitter, an insulated complementary
metal–oxide–semiconductor chip less than
1 mm3 in volume, was implanted in the rat’s
brain and transmitted alternating-current
signals to the receiver electrode, which was
also implanted in the brain. Because the
transmitter’s circuit ground was insulated
from the tissue, the path for current return-
ing to the transmitter had higher imped-
ance than the path through the brain to the
receiver. Thus, there was a high-efficiency
transfer of the signal to the recording site.
Care was taken to use a charge-balanced
alternating-current signal in order to avoid
charge buildup or tissue damage at the elec-
trode. Using this setup, an encoded neural
signal was faithfully transmitted through
brain tissue with approximately 20 dB of
signal loss. A simultaneous microelectrode
recording showed no obvious disruption
in activity during signal transmission in
the anesthetized rat’s brain. The two-elec-
trode setup of this system allowed for high
efficiency transmission of the signal, but
made the system vulnerable to extra cur-
rent sinks in the system. If a low impedance
path to ground was present, such as contact
between the body and a circuit ground or
a grounded water pipe, the signal would
Galvanic coupling can also be used to com-
municate between devices mounted on the
skin. Surface-to-surface communication
allows for quick and easy positioning of
electrodes, fewer constraints on the size and
power demands of the transmitting devices,
and avoids surgical implantation. However,
because the sensors are on the skin, they
may be far from the sources of the signals
that are being measured and can result in
weak, distorted or indirect physiological
measurements compared with implanted
sensors. Nevertheless, these surface-to-sur-
face signals can be combined with signals
from implanted devices to create a network
of sensors across and inside the body.
Because of the convenience and noninva-
siveness of surface-to-surface systems, they
can easily be tested in humans. Many lab-
oratories have successfully used galvanic
Capacitive coupling 
Galvanic coupling 
Galvanic coupling 
Galvanic coupling 
Galvanic coupling 
Figure 1. Five types of intrabody communication. (A) Signal is transmitted from a Tx
to a Rx, both located on the skin, with the body capacitively coupled to the Tx and Rx
electrodes. The Tx and Rx are also capacitively coupled to the ground, but capacitance
between the body and ground reduces the efficiency of signal transmission. (B) Signal is
transmitted from a Tx implanted in the tissue to a Rx on the skin. The Tx and Rx
electrodes are galvanically coupled to the tissue. Most of the current passes between the
two Tx electrodes, but sufficient signal transmits across the tissue to be detected by the
Rx. (C) Using x-antennas to shape the current path, creating a higher impedance path
between the Tx electrodes, stronger signal is detected at the Rx than without
x-antennas. (D) Signals are detected by an implanted Rx, which reduces signal
attenuation and power demands compared with skin-mounted Rx electrodes. (E) By
using only one Tx electrode and one Rx electrode galvanically coupled to the tissue, the
path between Tx electrodes has higher impedance than the path to the Rx, resulting in
less signal attenuation. High-frequency, charge-balanced, alternating-current signals
prevent charge build up.
Rx: Receiver Tx: Transmitter.
Wireless implanted devices
Expert Rev. Med. Devices 8(4), (2011)
intrabody communication to transmit data between electrodes
attached to the skin [12,18–20].
One of the most difficult challenges for implanted device technol-
ogies to overcome is in providing implants with sufficient power
to record and transmit signals. However, there has been great
progress in understanding how to design miniature low-power
circuits for biological applications . The most common method
of powering larger implants such as pacemakers and deep brain
stimulation devices is via batteries. However, batteries are difficult
to miniaturize and remain the size-limiting component of many
implants. In addition, the lifetime of batteries limits the useful life
of potential implants. Battery replacement for implantable devices
often requires an additional surgery and can cause many complica-
tions. Alternatively, rechargeable batteries allow for longer useful
lifetimes but need an additional means of delivering power to
recharge, such as RF approaches, which suffer from low-efficiency
power transfer and require relatively large, aligned antennas.
Other non-RF methods to wirelessly power implanted devices
have been proposed but are only in very early stages of develop-
ment and will require many advances before they are practical.
Witricity, which uses magnetic resonance coupling, allows for
highly efficient energy transfer but requires large coils [22,23].
Ultrasound energy can be used to deliver power to implanted
devices, but the efficiency of power delivery is very small, approxi-
mately 0.06% . Energy scavenging  and optical energy 
have also started to be investigated but currently produce too little
energy to reliably power implantable devices.
Another approach is to design the implants as passive devices,
not requiring any onboard power source. In this approach, the
implant acts like a radiofrequency identification (RFID)-type
device and modulates the signal generated by an external source.
The signal then detected outside of the brain includes the infor-
mation transmitted by the implant. The interrogating signal
can be generated by radiative RF signals like a traditional RFID
device [26,27] or using volume conduction . This approach would
allow for the greatest degree of miniaturization since no battery is
required. However, early prototypes have used inductors, which
are difficult to miniaturize.
For a miniature implantable device, alternative approaches to
positioning the implant within the body are necessary. The easi-
est way to insert an implant is by injecting it with a hypodermic
needle. This technique is commonly used for implanting RFID
tags into the bodies of livestock for identification . For implan-
tation in the brain, the hard needle protects the implant from
the forces encountered when penetrating through dura and brain
tissue. However, the volume of brain tissue displaced is larger
than if the implant were moved alone. In addition, the positive
pressure from the syringe may cause damage to tissue. An alterna-
tive to a hypodermic needle is to use a vacuum-based tool, simi-
lar to the vacuum pickup tools used in placing microelectronic
components. In this setup, the implant is held to the tip of a
hollow tube by vacuum. Once inserted to the desired depth, the
vacuum is released and the tool is retracted, leaving the implant
Another approach to inserting implants is using magnetic guid-
ance, originally developed to guide catheters within the brain 
and for drug delivery of nanoparticles . In magnetic guidance,
several large external superconducting magnets control the move-
ment of permanent magnets integrated in the implant. This system
allows for control in three dimensions and for easy repositioning of
the implant. Nonlinear trajectories can even be used to avoid sensi-
tive regions of the brain, which would be impossible in a traditional
linear stereotactic approach. However, the implant must be mag-
netically sensitive, and a complex purpose-built system is required
to control the magnetic implant. Another potential concern is unin-
tentional movement of the magnetic implant after implantation due
to magnetic forces in the environment or from MRI.
Dissolvable silk films, which have recently been used to create a
mesh for electrodes placed conformably on the brain surface ,
could also potentially be used in implanting miniature wireless
devices. Silk films dissolve over time, leaving the implant com-
pletely unconnected to any wires or fibers. The silk structure
attached to the implant can also be used to move or extract the
implant during the first few days or weeks before the fibers dis-
solve. However, the mechanical properties of silk films require
further investigation and testing.
Another important challenge is to minimize the body’s response
to the implant. Upon recognizing a foreign implant, the body
mounts a complex response that occurs on both short and long
time scales [33,34]. This response can adversely affect both the
function of the implant and, more importantly, the health of the
tissue. Many approaches have been attempted to minimize the
tissue response that could also be applied to wireless implantable
devices, including careful selection of biocompatible materials
and coatings  and localized drug delivery .
It is also important to minimize the effects of intrabody com-
munication on the body, including localized heating caused by
power dissipation and unintended stimulation. To avoid the
localized heating that can occur with RF telemetry, intrabody
communication should use a low-frequency carrier wave, ideally
below a few MHz. Also, to minimize any unintended stimula-
tion, the frequency of the carrier wave should be above physio-
logically important frequencies, at least approximately 100 kHz.
This range of frequencies between the two bounds also has the
advantage of having good-quality transmission in biological
tissue [37–39] and is the frequency range of the tests described in
this article. Nevertheless, even at this middle frequency, care
must be taken to observe that the specific energy absorption rate
and the current density are below the values set in international
guidelines . Because intrabody communication is a new tech-
nology, potential tissue heating and unintended stimulation
should be closely monitored in future experiments, even if the
transmission is within accepted international standards.
Ferguson & Redish
Expert commentary & five-year view
Several approaches to communicating with implanted medical
devices using the body as the transmission channel have been pro-
posed and tested. Each of these methods offers some insight in how
such a communication system can be realized. Intrabody communi-
cation offers several advantages over wires and RF wireless telemetry
for communicating with implanted devices. However, intrabody
communication is a new technology and several challenges, espe-
cially improving power delivery and thoroughly evaluating safety,
need to be addressed before it is implanted in humans and used
for routine clinical applications such as physiological monitoring.
In the near future, the likeliest users of intrabody communica-
tion will be biomedical research laboratories that will investigate
the capabilities of the technology and develop applications for
small animal studies, where miniature implantable sensors are vital
for many research questions. Further in the future, a novel form of
physiological monitoring can be envisioned, where multiple ultra
miniature implants are injected into various locations in the body.
These implants can be interrogated using an RFID-type telemetry
system. By making each implant sensitive only to a specific fre-
quency range, the implants can be made individually addressable
and be used in a body-wide network. Such a system of implant-
able devices would allow for flexible positioning options without
the restrictions and problems of wires and could enable access to
tissues sensitive to movement such as the heart and spinal cord.
One especially exciting potential future application is a network
of injectable, miniature wireless neural implants (Figure 2). By being
wireless and miniature, they would allow researchers to have com-
plete freedom in selecting the locations of neural recording sites.
Since most neurological diseases affect multiple brain regions,
being able to monitor neural activity and observe intra-region
communication is likely to be important to our understanding of
dysfunction. For example, multiple injectable neural recording
implants in and around the focus of seizure activity would be
beneficial in surgical planning or monitoring for epilepsy patients.
Because of the body’s conductive properties, it can be used as
a communication channel to transmit power or information to
or from an implant. By eliminating wires, miniature devices can
be implanted in multiple structures without restrictions in their
positions or be implanted in fragile structures, such as the heart
or spinal cord, that would be damaged with moving wires. In
addition, the miniature devices could simplify surgical procedures
and would help minimize the surgical complications common in
implants that use wired connections. Low-power, ultra-miniature
implantable devices that use intrabody communication have the
potential to enable many exciting applications in the future for
both biomedical researchers and clinicians.
The authors would like to thank Jadin Jackson, Andrew Papale and Chris
Boldt for helpful discussions.
• Implantable medical devices are important tools for researchers and clinicians, but the wires connecting the implants to external
circuitry are common sources of complications (e.g., wire breakage, infection, tissue damage and electrical noise).
• Wireless radiofrequency telemetry is also being used for communicating with implants, but its transmission efficiency is very low
through biological tissues, and it has large power demands. In addition, the antennas are too large to fully implant in structures such as
the brain and heart without causing significant damage.
• Intrabody communication, which uses the body as a conductor, allows for a miniaturizable and power-efficient means of wirelessly
communicating with implants.
• Shaping the current flow through the body with high- and low-impedance paths improves the efficiency of signal transmission.
• Issues such as safety, insertion methods, tissue response and power are important practical considerations in the development of
implantable, wireless neural devices.
Figure 2. A possible future vision for wireless, miniature
implantable devices for neurological monitoring
applications, different from any currently available
technologies. Several implants (N1–N5) are injected into the
brain and spinal cord. The implants are tuned to specific
frequencies (f1–f5) and thus are individually addressable. The
receiver, the waystation, allows for communication between
multiple implants and external devices, and, because it is
implanted, it improves the transmission efficiency. This
technology could enable the development of novel tools for
neuroscience research and clinical care.
Background image by Patrick J Lynch. License: GFDL.
Source: Wikimedia Commons.
Wireless implanted devices
Expert Rev. Med. Devices 8(4), (2011)
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involvement with any organization or entity with a financial interest in
or financial conflict with the subject matter or materials discussed in the
manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
Financial & competing interests disclosure
This work was supported by a grant from the University of Minnesota
Institute for Engineering in Medicine (IEM) and an NIH T32-EB008389
training grant. The authors have no other relevant affiliations or financial
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