Biomedical Engineering Reference
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20 W and an average power output of 4.8 6 0.8 W. This generative
breaking was reported to use less than 1 W of metabolic power to pro-
duce 1 W of electrical power, while if used continuously more than
2 W of body work are needed to generate only 1 W of usable power.
A piezoelectric generator for powering artificial organs harnessing
the footfall during gait was presented by Antaki et al. (1995). This
generator consisted of two hydraulic cylinders placed in a shoe insole
containing lead zirconate titanate (PZT) piezoelectric stacks. The
hydraulic cylinders had pulse amplifiers beneath the toes and heel region
for transforming the low-frequency footfall into high-frequency pulses.
A 1/17 scale prototype was evaluated producing 150
675 mW for
walking (5.7 6 2.2 mWkg/L) and 675
2,100 mW (23.6 6 11.6 mWkg/L)
for simulated jogging, while up to 6.2 W could be expected from a 75 kg
individual.
The bending of the shoe has been studied as well. A flexible piezo-
electric generator was developed by Kymissis et al. (1998). They have
introduced the concept of parasitic power generation capturing the
energy that otherwise would be wasted or dissipated. For example, a
68 kg individual walking at 1 Hz with a 5 cm vertical displacement
represents 67 W of power employed (Starner and Paradiso, 2004).
Trying to harvest all the energy would severely interfere with the gait
process but using the deformation that a sports shoe suffers (less than
1 cm) seemed practical. Kymissis et al. (1998) presented two different
piezoelectric designs to be compared against an electromagnetic gener-
ator. The first configuration was made of a stack of polyvinylidine-
fluoride (PVDF) sheets shaped similar to a shoe sole to be stressed
under bending (the outside layers were stretched while the inner layers
were compressed). This arrangement provided 6 60 V with an average
power output of 1.1 mW. The second configuration was designed to
harness the heel strike using a unimorph strip (steel spring bonded to a
PZT piezoelectric material sheet). The steel was bent stressing the PZT
when there was a heel strike, generating peak voltages up to 150 V and
power outputs up to 1.8 mW. The electromagnetic design was made
from a lever-driven flashlight generator mounted in a shoe. A hinged
plate attached to the flashlight lever exploited 3 cm of walking stroke
producing an average power output of 230 mW, although interfering
with the normal gait. The piezoelectric shoe generators were also used
as Radio-frequency identification (RFID) transmitters sending a 12-bit
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