**3. Particle toxicity**

For magnetic particles to become widely used, the particles must have a negligible potential for toxicity [16]. This very high bar is reflected in the Food and Drug Administration's (FDA) classification of devices for the brain as class III, requiring a premarket approval (PMA) application. In comparison, devices for the peripheral nervous system are often treated as class II devices. Note that particles are often treated by the FDA as drugs, although in Europe, they may be treated as devices. Studies of ex vivo vital rodent brain slices have shown that the presence of magnetic particles does not cause a measurable disruption of function [17]. In fact, it is not unusual for humans living in industrial cities to have magnetic particles in their brains, with no known related diseases [18]. It is also very common for humans over 50 years of age to have radio-dense "calcifications" in the basal ganglia, again with no definite disease association [19]. Small animal studies have examined the toxicity of magnetoelectric particles with no adverse effects [20].
