And what happened to Hitachi's 0.4-millimeter by 0.4-millimeter µ chip (mu-chip), announced around 10 years ago?
The first question is difficult to answer, as I'm not sure what makes a chip suitable for implantation in a human or animal. If you were going to inject a subcutaneous RFID transponder—one placed into the subcutis (the layer of skin directly below the dermis and epidermis)—then you would want to make it small enough to be injectable, but large enough that you could read the information on the chip. The smaller transponders are, the shorter their read range. At present, low-frequency (LF) tags placed in ampoules for injecting are about 2 millimeters (0.08 inch) in diameter by 10 to 12 millimeters (0.4 to 0.5 inch) in length.
As for your second question, nothing happened to Hitachi's µ chip (see Hitachi Unveils Smallest RFID Chip, Hitachi Unveils Integrated RFID Tag, Hitachi Shrinks Smallest RFID Chip and When Will RFID-Embedded Paper Be Available?). It is still being sold.
I don't think the µ chip would be a good choice for implantation in the human body. One reason is that it is not available in an ampoule—and even if it were, you would not be able to read it under the skin. You would have to practically touch the tag in order to read it, since the antenna is so small. And it would probably float around inside the body. It might even emerge from a tear duct, like some miniaturized submarine from a 1960s science fiction movie.
—Mark Roberti, Founder and Editor, RFID Journal
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