Biomedical Engineering Reference
In-Depth Information
volume smaller than 10 cm
3
, while most devices were found in the
range of 100
W/cm
3
. From this chart, most electrostatic energy
harvesters were found to produce less than 100
1,000
μ
W/cm
3
, making this
transduction technique less competitive in comparison to electromag-
netic or electrostatic generation. Therefore, the power density chart
makes it easier to design systems for a given set of constraints.
μ
From the charts, electromagnetic and piezoelectric generators were
found to provide the largest amount of power per unit volume, with
electromagnetic devices having a relatively higher output. Most of the
evaluated harvesters were generators with linear displacement, whereas
human motion is three-dimensional, presenting linear displacements
and rotations. Thus, generator designs that harvest energy from more
than one direction or that harness the joint rotations could be better
suited for body motion. In this case, electromagnetic generation with
rotational architectures could have an advantage over piezoelectric
devices with linear designs. Only the impact-based piezoelectric design
from Cavallier et al. (2005) takes advantage of a three-dimensional
motion approach, but its power output was too low (less than 0.1
μ
W).
These charts can help to determine technologies and constraints for
different applications. Although devices with volumes over 10 cm
3
can
produce milliwatts of power, devices with dimensions on the order of
1cm
3
present potential applications due to their reduced sizes for
embedded or surgically implantable applications. Actual technology
limits power output for the latter to hundreds of microwatts.
As summarized in
Figure 3.3
, electromagnetic generators with rota-
tional designs were found to have high power densities (ETA
'
s watch,
Seiko
'
s watch, Wang et al., 2005a) in small volumes (less than 10 cm
3
).
Because the size of generators can be reduced by means of MEMS
technology, smaller energy harvesters for body motion can be fabri-
cated with high power density using rotational electromagnetic
generation.
