Defense Intelligence Reference Document Biosensors And A Survey Of The Present Field

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Figure 26. An Illustration of the Retinal Neuroprosthesis Created by the Boston Implant Project
(http://www. bosto nreti na Ii mp la nt. org/ im pl ant. p hp -:>fontsi ze= norrna I&hi contrast)
The signal processing circuits are on the left while the power induction coil is the
circular structure. The MEMS stimulating microelectrode array projects out towards the
bottom of the picture.
The minimal size of the implant means batteries are impractical and forms of wireless
power by magnetic induction, ultrasound, or solar energy are required. In addition the
tissue surrounding the implant is usually very sensitive to temperature rise so the
implant must have very low power consumption to ensure it will not harm the tissue.
Power by magnetic induction is widely used because it allows relatively larger amounts
of power transfer compared to ultrasound and solar energy techniques. Parallel coils of
wire, one inside the body and one outside, exchange energy by magnetic field coupling
in accordance with Faraday's law of induction. Figure 27 is an artist's conception of the
magnetic induction method of power transmission. There are two coaxial coils that
couple magnetically, one on the inside of the body and one external. The energy of the
magnetic field is shown as the curved lines passing through the tissue.
A disadvantage of this method is that the coils are relatively large compared to the size
of micro-implants and so tends to define the minimum size. The coil size depends on
the amount of energy that needs to be induced and on the implant depth with larger
diameter coils being required.
This type of technology is giving way to improved methods of energy transmission into
tissue for implant power by using microscale devices that, although not strictly MEMS,
share many of their characteristics.
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