IEEE Solid-State Circuits Magazine - Spring 2014 - 49

ISM

-20

FCC Indoor (United States)
ECC (Europe)
Japanese Mask

-50

FCC Part15

PSD/dBm/MHz

WMTS

MICS/MedRadio

EIRP (dBm)

-30

UWB

-40
-50
-60
-70
-80
-90

0.01

0.1

10
100
f (MHz)

1e3

10e3 100e3

Figure 4: The equivalent isotropically radiated power for various
IMD-related standards.

These body models specify the
location of the tissues and refer to
the aforementioned database for tissue properties. Then the models can
be exposed to a simulated emission,
yielding an estimated field strength
distribution, which allows the specification of the telemetry system. Even
more importantly, such simulations
allow the estimation of absorption and
thus tissue heating. The most common
measure is the specific absorption ratio
(SAR), which is defined as the squared
induced current density divided by the
conductivity and the mass density.
The SAR is typically measured by
determining the temperature increase
upon a short EM exposure (to avoid
convection) and by multiplying the
result with the specific heat capacity.
The maximum recommended
SAR is specified in safety standards,
which we will discuss later.

Optical Properties
Photons traveling through tissue experience various interactions, which can
be described with four parameters:
absorption, scattering, the anisotropy
factor, and the refractive index. If we
simply assume a vertical exposure onto
the tissue, refraction can be neglected.
The anisotropy factor g describes the
preferred direction in which scattering
happens. In tissue, g typically ranges
from 0.8 to 0.95 and scattering happens mostly into the direction of photon travel; consequently, we talk about
highly forward-directed scattering.

4
6
8
Frequency/GHz

Figure 5: The indoor radiation limit [8] (courtesy of H.
Schumacher).

Photon absorption happens mostly
due to water content or due to hemoglobin and melanin; the "optical tissue
window" is found between these dominant absorption mechanisms, i.e., with
m between 600 nm and 1,300 nm.
In comparison, the absorption
coefficient of tissue is usually much
smaller than the scattering coefficient. Thus, scattering is the more
dominant process, but since it is
very much forward directed, the
beam spreads but it does not disappear; illustratively spoken scattering increases the distance traveled
by photons, and this increased
travel distance again increases the
probability of absorption.
Based on these parameters, several methods have been proposed to
model the light-to-tissue interaction,
including the Kubelka-Munk and the
multiflux approach, diffusion theory,
as well as the Monte Carlo method.
The latter is widely used if the tissue
geometry and structure is complex
and is based on tracing many photons
through a tissue layer stack. A simple
alternative to Monte Carlo simulation
is to reduce the effects of absorption,
scattering, and beam widening into
two numbers. First, we can use the
penetration depth as illustrated in
Figure 3. Obviously, several millimeters are achievable. A second prominent measure is the "full width half
minimum" (FWHM), which describes
the beam widening: obviously,
thicker tissue leads to a decrease of

the transmitted peak power density
in the main beam direction; but at
the same time, the FWHM, meaning
the opening of the beam, increases.
Consequently, going far away from
the light source results in a reduced
peak but increased width [13]; therefore, by taking a larger receiver area,
still most of the signal power can be
gathered but is limited in speed and
noise due to increasing the photo
detector area. This is obviously no
problem for optical power harvesting
but surely for optical data telemetry.

Regulations and Standards
Knowing what we can use to penetrate tissue for power and data
telemetry leaves us with the question: What we are allowed to use?
In the late 1980s and 1990s, there
were more than 100,000 incidents
reported involving cardiac-type medical devices; several thousands of
them were most likely caused by EM
interference. Therefore, regulations
have been formed, first the Medical Implant Communication Service
(MICS) band in the 1990s and later
the Wireless Medical Telemetry Service (WMTS) and the MedRadio band
for medical device communication.
Such regulations are needed for
two reasons: first, to limit the interference of what our transmitter could
emit to other devices in the vicinity,
and second, to keep safe limits of
what we expose to the human body
to avoid any adverse effects such as,

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Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Spring 2014