A second type is zinc-air button cells, which are used in cochlear and other implants.

They provide a voltage of 1.4 V, a maximum current of 30 mA, and have capacities in

the range of several 100 mAh. Their diameter is 11.6 mm and their height is 5.4 mm. The

third option is thin-film batteries. These batteries are characterized by a small thickness

(<0.3 mm). The largest thin-film batteries deliver ~5 mA/cm² for 2.7–4.2 V [42]. There

have been interesting developments in the field of energy harvesting in recent years.

Various concepts have been developed to obtain energy from bodily functions, such

as from thermal energy by using Peltier elements, from glucose, or kinetic energy via

piezoelectric layers [15]. At present, developments do not appear to have progressed to

the point where they are already being used in commercially available implants.

However, they could perform important functions in support of battery-powered systems

by enabling extended lifetime or further miniaturization.

In the case of the external energy supply, use is made of the impedance coupling of an

electromagnetic radiation field, which can be tapped in the implant with a coil. In the case of

the development of a subcutaneously placed glucose sensor, this is already being used [8].

The concept has also opened up significant miniaturization potential.

21.7 System Integration

For various applications, the use of semi-implants is currently still considered sufficient,

which can be placed easily and without great risk of injury to large blood vessels or

organs. For example, metabolites such as glucose can be determined well by measure­

ments in subcutaneous tissue, since their concentration there takes comparable values as

in blood with a slight delay. The advantage of semi-implants is that components not

directly required for bioelectronic function, such as µC, a radio module, antenna, and

power supply, are placed extracorporeally and are easily accessible. [43].

For full implants, RFID systems for animal identification are currently used with the

largest numbers, which have the shape of a cylinder with a length of just over 10 and

diameters of a few mm. With the small form factor, outpatient implantation with local

anesthesia becomes possible as is the case with RFID implants for pets. Migration of the

implant can be suppressed by various measures such as rough and hydrophobic surface

design that provokes adhesion or anti-migration caps.

The individual components of the implant, i.e., sensor/actuator chip, microcontroller,

front end, antenna, and power supply, must be integrated electrically, for which printed

circuit boards (PCB) are usually used as platforms. Due to the targeted miniaturization, the

packaging is always challenging. In particular, newly developed sensor chips to be in­

tegrated into such systems often pose a considerable challenge because all electrically

connecting components must be hermetically shielded against access by the surrounding

body fluid. Figure 21.5 shows the PCB and integration scheme of a glucose sensor implant.

A decisive aspect related to the topic of biocompatibility concerns of sterilization.

According to national and international specifications for medical technology products, a

technical system to be implanted must be sterilized. Both a medical device and the test series

of implantable biosensors to be used in a clinical study must be demonstrated that all test

objects are sterile. Sterilization is also required if the implant or prototype is to be used for

functional tests in animal models. Thermal, chemical, and irradiation processes can be

considered as sterilization methods. For the latter, irradiation doses on the order of 25 kGy

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