Commercially, a large number of different, CMOS-manufactured µCs are available,

which can be configured by the user concerning the intended use and measurement se­

quence [18]. The chip architectures of microcontrollers can also be acquired as software

programs in languages commonly used for this purpose, such as VHDL, to tailor them to

specific user needs and have them manufactured as ASICs in semiconductor fabs.

21.5 Communication

As is common in wireless data transmission with conventional sensor and actuator sys­

tems, solid implants also use a radio module [18] and an antenna [38]. However, special

boundary conditions have to be taken into account when radiating out of the body, which

is derived from the dielectric properties of muscle and fat tissue. Both have relative di­

electric constants of εr = 58 and 11.6, which differ significantly from water or air.

Important preliminary decisions regarding range and signal level are thus made when

the communication frequency is specified.

Various frequency bands have found application in implants such as those at 125 and

134 kHz for RFID tagging of livestock and pets [39] or the MICS band around 403 MHz

(Medical Implant Communication Service), which is used for cardio implants and has been

approved by regulatory authorities for wireless communication from within the body.

RFID modules have a much lower power requirement than MICS radio modules. In return,

they show a low data rate and also range. However, this may not be a disadvantage for

metabolite monitors with a maximum of a few 1,000 measurement data per day, if the

readout device in form of a smartphone is brought close to the subcutaneously positioned

implant and data is exchanged at short distances using near-field communication NFC [8].

The antenna is also about four times longer for RFID than for 403 MHz radio modules.

This has a decisive influence on the overall size of the implant. The length of the antenna

must correspond to a certain fraction of the wavelength of the carrier frequency f, and λ/4

coils are often used. The miniaturization of the implant benefits from the fact that the an­

tenna length calculated in the air is reduced by 1/εr in body tissue. In muscle tissue, a λ/4

antenna for the MICS band therefore only needs to be 25 mm long. For implants the size of a

matchbox, such antennas are acceptable. For smaller implants, the antenna area must be

further reduced. Ref. [40] presents an antenna design where the edge lengths of the antenna

are 8.1 and 8.2 mm, respectively, which is already well suited for use in small implants.

21.6 Energy Supply

In the case of full implants, the energy supply can be realized in an internal and external

variant. In the internal variant, the implant is powered by a battery, which must not be too

large but must supply sufficient energy for the intended lifetime. Here, only three possible

battery types will be considered. The first is lithium-iodine batteries, which have a high

energy density and low self-discharge current. They are available in special versions for

implants and provide voltages of about 2.8 V and maximum currents of about 50 μA [41].

Due to the low current, they are rather unsuitable for many biosensor implants.

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Bioelectronics