Biomedical Engineering Reference
In-Depth Information
1.4.2
Transcutaneous Power Transmission
by Near-Infrared Light
Once medical devices are implanted, they are expected to be used for a decade
or more. For their continuous use, they should be powered indefinitely by a
wireless method. To date, the following two methods have been considered to
be practical. One method is to equip an implanted device by a primary bat-
tery. Cardiac pacemakers currently in use are powered with lithium primary
batteries. The other method is to supply electromagnetic power through the
skin, allowing implanted devices to be powered indefinitely from outside the
body. In fact, power transmission by high-frequency electromagnetic induc-
tion is considered promising for driving artificial hearts [30]. However, these
methods have the following disadvantages. Implanted devices using the for-
mer method require an operation every time the battery is replaced by a
new one. The lifetime of batteries for cardiac pacemakers is in the range of
5-10 years. The latter method may cause electromagnetic interference with
surrounding devices. In medical facilities, there are a lot of instruments that
are susceptible to electromagnetic waves.
An alternative to these methods is to use near-infrared light as a medium
to transfer power [27,28]. High-eciency, noninvasive power transmission can
be expected from this method, because biological tissue exhibits a consider-
ably high transmittance to near-infrared light. To our knowledge, the first
experiment on near-infrared power transmission was reported in 1999 [27]. In
this experiment, a cardiac pacemaker was successfully powered by an implan-
ted photovoltaic cell illuminated with near-infrared light through the skin of
a guinea pig. The light source was an 810 nm near-infrared LD. The thickness
of the skin was 2 mm. This technique is not only noninvasive to tissue but also
almost free from electromagnetic interference with surrounding instruments.
A basic experiment was performed to investigate the eciency of power
transmission through tissue. Figure 1.22 shows the power density distribu-
tion of near-infrared light diffused by tissue. Samples of chicken were irra-
diated with a laser beam (140 mW,
λ
= 810 nm), the diameter of which
was determined to be 10 mm by using a circular aperture. Then the diffused
light was detected behind the samples with a PD having a detection area of
1mm
1 mm. The thicknesses of the samples were 5 and 10 mm. In Fig. 1.22,
the lateral axis represents the distance between the beam axis and the po-
sition of the PD. As shown in Fig. 1.22, the beam spreads with increasing
thickness of the tissue. The power transmittance can be defined as the inte-
gral of the power density over a suciently large area on the samples. From
Fig. 1.22, the power transmittance is as high as 30% and 20% for thicknesses
of 5 and 10 mm, respectively.
Figure 1.23 shows a rechargeable near-infrared power supply. It consists
of an Si single-crystal solar cell array, a rechargeable battery, and a voltage
regulator. The rechargeable battery is a polyacene capacitor (Kanebo). The
voltage regulator is a step-up dc-dc converter (Ricoh). The output of the
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