  Implantable bioelectronics often relies on an RF-wireless link for
communication and/or remote powering. Propagation through biological media is
very lossy, and previous work has shown that these losses can be separated into
three parts: the losses incurred by the propagating fields, the reflections at
media interfaces, and the coupling of the reactive near field and the lossy
body. The first two are unavoidable, but a clever antenna design can minimize
the near-field losses. A good physical understanding of this particular loss
phenomenon is thus very desirable. Unfortunately, previous work does not take
the implantation depth into account and is thus valid only for deep implants.
In this contribution, we propose approximate expressions to the near field
losses of antennas implanted in biological hosts considering the encapsulation
size, the frequency, the characteristics of the host medium, and the
implantation depth. They are obtained using a spherical wave expansion, and are
useful for realistic implantation scenarios. This is demonstrated by first
comparing the results obtained using these expressions with rigorous
computations for two canonical phantoms: a spherical phantom and a planar
phantom. Finally, the usefulness of the obtained expressions is illustrated in
the practical realization of a capsule-shaped implanted antenna.
