Biomedical Engineering Reference
In-Depth Information
1.4
Transcutaneous Near-Infrared Light Power/
Information Transmission for Implantable
Medical Devices
1.4.1
Feasibility of Powering, Controlling, and Monitoring
Implantable Medical Devices Using Near-Infrared Light
Implantable medical devices assist or substitute for natural organs. Currently,
cardiac pacemakers and cochlear implants are already in practical use. They
are implanted to give signals to organs through electrical stimulation. In
the near future, implantable microelectronic mechanical systems will become
available, which will have more complete functions.
These devices need to be supplied with electric power and/or to commu-
nicate with instruments outside the body. In order to prevent infection, these
two requirements should be met by wireless methods. One promising method
is to use near-infrared light as a medium to transfer power [27,28] and infor-
mation [29-31]. High-eciency, noninvasive power/information transmission
can be performed by using near-infrared light, because biological tissue exhi-
bits a considerably high transmittance in the near-infrared region. Figure 1.20
shows a schematic diagram of implantable medical devices powered and con-
trolled by near-infrared light. In Fig. 1.20, the implanted devices are equipped
with photoelectric devices for receiving and emitting near-infrared light. For
power supply, as shown in Fig. 1.20a, near-infrared light is sent from outside
the body through the skin to a photodiode (PD) that powers an implanted de-
vice. Near-infrared communication has two functions, as shown in Fig. 1.20b:
controlling and monitoring implanted devices. For controlling an implanted
device, near-infrared light is aimed at a photodiode inside the body to send
signals. For monitoring an implanted device, light transmitted from a light-
emitting diode (LED) or a laser diode (LD) inside the body is detected by a
receiver outside.
Figure 1.21 shows the spectral properties of human skin [32] and photo-
electronic devices. As shown in Fig. 1.21, human skin shows a high transmit-
tance in the near-infrared region. The sensitivity of Si PDs, the most widely
used type of photodetectors, is high for wavelengths of about 900 nm. A light
source can be selected from a wide variety of commercially available LEDs
and LDs in the wavelength range of 700-1000 nm. The eciency of transfer-
ring power to an implanted photodiode is determined by the product of the
transmittance of the skin and the sensitivity of the photodiode at the wave-
length of a given light source. It follows from Fig. 1.21 that high-eciency
transmission of light can be performed in the range of 800-1000 nm. In the
following, techniques to transmit power and information are described.
