Biomedical Engineering Reference
In-Depth Information
2
. The surface charge
density was sustained for more than 100 days, and it was stable up to
its glass transition temperature,
B
108
C. The capacitor plates con-
sisted of rectangular areas (10 mm
3
20 mm) covered with electrodes
(1 mm wide, 30
expected as power output is proportional to
σ
μ
m gap) and separated by an air gap (100
μ
m). For a
μ
μ
prototype having 200
m gaps) and oscillations
of 1 mm
p-p
at 20 Hz, 1 mW of power can be generated.
m wide electrodes (50
Boland et al. (2005) used fixed electret plates with liquid droplets in
between, called a liquid electret power generator (LEPG). The electret
plates were covered with Teflon, and the dielectric was made of liquid
droplets in addition to air. Polar liquids present high dielectric con-
stants producing large capacitance changes when air is replaced by
droplets. When the generator vibrates, the liquid droplets change the
capacitance of this arrangement producing energy. The prototype was
reported to produce 0.11
μ
W of power at 60 Hz, although it could pro-
μ
duce up to 10
W of power.
ZnO nanowires have also been suggested for energy generation (Liu
et al., 2008; Wang, 2008; Wang et al., 2008). Gao et al. (2007) indicated
that the use of flexible substrates would enable the use of piezoelectric
nano arrays for bendable power sources in implantable biosensors. A sin-
gle nanowire is known to generate 50 mV, thus arrays of nanowires could
produce enough from energy harvesting. Such nano arrays have reported
power densities of 100
W/cm
2
(Gao et al., 2007). Power densities
close to 83 nW/cm
2
for nanowires stimulated by ultrasonic waves have
also been reported (Liu et al., 2008).
200
μ
Energy generation has also used spherical geometries using electro-
magnetic transduction. Two devices having 1.5
4cm
3
were presented
by Bowers and Arnold (2008). The devices were held in the hand and
a pocket during walking and running tests. Electrical power as high as
1.4 mW was reported. A planar rotational electromagnetic device was
presented by Romero et al. (2011). The effective volume of the genera-
tor was reported as 2 cm
3
. The device was capable of producing up to
427
W
when running. This generator was evaluated at several body locations
and different walking and running speeds.
μ
W of power while walking on a treadmill and up to 540
μ
3.3
summarize the findings of energy harvesters for
body motion.
Figure 3.1
shows a review of energy generators by the
frequency of operation, whereas
Figure 3.2
provides a representation
Figures 3.1
