**5.1 Special requirements for the design of real-time systems**

Although the procedure (Paffi et al., 2010) for the design and characterization of the exposure system is the same regardless of the particular kind of analysis (real-time or offline), real-time acquisitions impose additional biological requirements to the system. In particular, when dealing with the electrophysiological recordings, they can be summarized in the easy access to the sample (i.e. through the microscope, the electrodes, and the perfusion apparatus) and the minimum coupling between the EM field and the acquisition equipment, in order to avoid artifacts.

For the correct placement of the electrodes, the sample must be illuminated and observed through a microscope. Moreover, in some experiments, the biological sample, e.g. brain slices, has to be perfused to preserve its structure and function. For these reasons, in order to easily reach the sample, the exposure system must be either an open structure or a closed one modified with holes.

The microelectrodes for electrophysiological recordings are usually made of glass filled with a saline solution with a metallic wire inside. Being the biological cultures exposed to CW or modulated signals, especially at the frequencies typical of mobile communication, the realtime acquisition implies the possibility of EM coupling between the electrodes and the electric field, with consequent artifacts on the recorded trace. Another possible source of artifacts is the interference between the EM field generated by the system and the whole acquisition setup. To minimize the coupling between the field and the electrode, the direction of the electric eld

Real Time Radio Frequency Exposure for Bio-Physical Data Acquisition 303

Otherwise animals are restrained inside plastic holders, to guarantee their relative position

Conversely, *in vitro* exposure systems have been classified (Paffi et al., 2010) in two different groups: off-line and real-time, depending on the kind of data acquisition they have been designed for. This latter classification has not been adopted for the *in vivo* setups, since realtime acquisitions are very uncommon for *in vivo* experiments, as pointed out in Section 4. Proposed classification is reported in Figure 5, together with the number of systems

belonging to each category and published in international journals since 1999.

Fig. 5. Classification of the exposure systems. For each category the number of systems

Among the 52 *in vivo* systems, those used for local exposure (19) are exclusively based on radiating structures. In particular, they are small antennas placed close to the target organ (brain, ear, eye) to induce significant and localized power absorption (Paffi et al., 2011). To reduce the cost of the experiment, a single antenna can be used to simultaneously expose several animals, if they are arranged in a sort of carousel around it (Schönborn et al., 2004). Generally, animals locally exposed are restrained within plastic holders to obtain a more accurate and precise SAR distribution, even though body-mounted antennas (Bahr et al., 2007) become necessary for the well being of animals when the exposure is prolonged.

For the 33 whole body exposure setups, the uniformity of SAR absorbed by animals of the same group and within each animal was a critical requirement (Paffi et al., 2011). This is particularly difficult to achieve especially for large-scale experiments. In this case, radiating structures (14 systems found in the literature) could be particularly suitable since a lot of bodies can be simultaneously exposed to a plane-wave equivalent field (Paffi et al., 2011).

published from 1999 is reported

and orientation with respect to the electric and magnetic fields.

must be almost orthogonal to the electrode. Moreover, to avoid interference of the eld with the laboratory equipment, in principle, the EM field should be confined in a closed structure. However, even open structures can be used, provided that the electric and magnetic fields sharply decay with the distance from the system.

Finally, the presence of metallic and/or dielectric objects (microscope objective, lamp, electrodes, perfusion apparatus), placed very close to the region where the sample is exposed (see Figure 4), may modify the field distribution and thus the whole behavior of the system. This kind of coupling must be carefully taken into account during the numerical optimization of the system in order to minimize it.

The different solutions adopted to meet such demanding requirements will be described in Section 6.

Fig. 4. Picture of a planar exposure system used for electrophysiological recordings from brain slices (Paffi et al., 2007). Arrows highlight the presence of different objects placed very close to the system surface during the experiments.
