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This phenomenon is now reaching the particular case of
pacemakers, where power consumption and theoretical
generated power from a reasonably sized energy harvester
are both reaching a value of several tens of microwatts.
Goto et al. [12] have already proven the feasibility of using
an energy harvesting system to power a mongrel dog’s
pacemaker. In their work, they removed the powergenerating
mechanism from a SEIKO kinetic watch and
encapsulated it in a polyvinyl case. SEIKO’s energy
harvesting system is based on a rotating eccentric mass
transmitting its energy through a gear train to a rotor that
generates a voltage electromagnetically. This device is then
placed on the right atrioventricular wall of the dog’s heart.
Although the extracted energy (13 µJ/heartbeat) was lower
than the pacemaker consumption (50 µJ/heartbeat), the
feasibility of an energy harvester powered pacemaker is
envisioned. Tashiro et al. also addressed this subject in [13]
where they present an experiment of an electrostatic system
harvesting enough energy from the motion of a canine heart
wall to power a pacemaker. However, this system is so
cumbersome that it would be impossible to implant and it
had to stay on a simulation table for this proof of concept
experiment.
II. INERTIAL ENERGY HARVESTERS
The vast majority of up-to-date energy harvesters are
based on inertial power generation [6, 14, 15]. This focus is
due to two main reasons: vibrations are widespread in our
environment and acceleration is inherently transferred
through packaging, which greatly helps sealing and
integration. These considerations apply for harvesting the
vibrational energy near the heart. While some of the
conventional energy harvesting technologies are not
applicable such as photovoltaic or thermoelectric
conversions as the body is mostly opaque and
thermoregulated, heart beats are providing a continuous
source of vibrational energy. Additionally, the inertial
harvesting device can be properly encapsulated in a rigid
package which helps biocompatibility and integration.
Under the assumption that the transducing force
(electromagnetic, electrostatic or piezoelectric) [6, 15] is
acting as a viscous damper, a typical inertial energy
harvesting system can be modeled as in Fig. 2, following
the analysis of [16].
Fig. 2. Mechanical system of an inertial energy harvester with viscous
damping transduction.
11-13
May 2011, Aix-en-Provence, France
In this figure, m represent the proof mass, k represents
the stiffness of the spring attaching the proof mass to the
frame, b m and b e respectively the mechanical and transducer
(electrical) viscous damping constants, y(t) the
displacement of the frame and z(t) the relative displacement
of the proof mass. The equation of motion can be written as:
. (1)
If the excitation is harmonic at an angular frequency ω, we
can analytically find an expression for the displacement and
the mean power of the transducing force [14] is expressed
as:
, (2)
where ω n represents the resonant angular frequency ⁄ ,
and ζ e and ζ m represent the normalized electrical and
mechanical damping ratios , ⁄ 2√, and Y 0 is the
frame motion amplitude. This expression shows that in
order to maximize the output power, the system resonant
frequency should match the excitation frequency as closely
as possible. Additionally, the electrical damping ratio
should be equal to the mechanical damping ratio and they
need to be as small as possible. However, one should be
careful of the displacement range that will greatly increase
when the damping decreases.
As the excitation is rarely purely harmonic, the response
to the whole excitation spectrum has to be analyzed. If the
spectrum has a narrow bandwidth and is not subjected to
shift, then a high quality factor harvester centered on the
same frequency can generate a high mechanical
amplification hence a high power as expressed in (2). The
limitation comes then from the travel range, as the
amplitude of the mass movement is largely amplified by the
same quality factor. Hence, high quality factor inertial
harvesting systems are best suited for narrow and stable
spectrum, low amplitude excitation. This is typically
interesting for industrial applications or for machines that
vibrate at specific known frequencies, such as the electrical
grid frequency. When the excitation spectrum is wide or has
an unsteady peak, energy harvesters should be damped
further in order to provide mechanical amplification for a
broader range of frequency, even though the amplification
magnitude is lower. This principle has been applied by
Despesse et al. in [17] where highly damped electrostatic
harvesters able to harvest wide spectrum vibrations such as
cars, drill or metallic stairs vibrations are presented.
III.
HEART ACCELERATION ENERGY HARVESTER
A. Heart acceleration spectrum
To predict the amount of energy that can be harvested
from the heart acceleration and to determine the harvesting
device and transducer characteristics, the acceleration
spectrum of the heart has to be measured. Therefore we
have implanted different types of accelerometers (one- or
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