Biomedical Engineering Reference
In-Depth Information
FIGURE 2-14
Nanofiber-based
energy scavenging
module. [Adapted
from (Chen, Shiuou
et al., 2010).]
It is therefore feasible to develop the mechanics to deflect the beam by 30 mm while
maintaining a reasonable safety margin. This consideration notwithstanding, the superior
flexibility of PVDF film makes it a better candidate for power extraction.
Because the total energy output is proportional to the volume of the film stressed, it
would appear that thicker films should be used. However, because they generate higher
voltages but form smaller capacitors, it is often preferable to use laminates made of from
many layers of thinner films separated by a compatible material such as polyester.
A 16-layer bimorph PVDF insole developed at the MIT Media Lab produced peak
powers of about 15 mW at heel-up into a matched resistive load, with an average of about
1.3 mW. Improved efficiency can be achieved by “tuning” the device so that its electrical
resonance is matched to the mechanical excitation frequency (Starner and Paradiso, 2004).
In the last few years, research has focused on using PZT or PVDF nanofibers to
produce smaller devices, as shown in Figure 2-14. The ultimate goal is to weave these
fibers into normal fabrics so that they can be used to produce power directly from a vest
or shirt (Chen, Shiuou et al., 2010).
2.3.4.2 Internal Devices
Zhong Lin Wang and his colleagues from the Georgia Institute of Technology have devel-
oped a nanogenerator that is able to scavenge power from involuntary movements such as
breathing or a beating heart. Zinc oxide nanofibers, which exhibit piezoelectric properties,
are bonded onto a 2
×
5 mm flexible polymer substrate before the complete unit is coated
in silicone to protect it from corrosion. When bonded to a rat's diaphragm using a tissue
adhesive, the device generated a current of 4 pA at 2 mV. The output increased to 30 pA
and 3 mV when a similar device was attached to a rat's heart.
Other researchers have implanted larger piezoceramic devices into muscles to obtain
power from normal body movement (Campbell, 2010).
2.4
SENSORS AND TRANSDUCERS
2.4.1 Resistive Displacement Sensors
2.4.1.1 Strain Gauges
Small displacements are generally measured using a strain gauge bridge configuration. A
strain gauge consists of a long narrow metal conductor such as a piece of metal foil (usually
constantan) mounted on a polyimide film or fine gauge wire stretched over a frame. If it
