## 1 Introduction  
In the past decade considerable advances have been made in the technology of rapid manufacturing. Not only have the techniques per se evolved, but the range of available materials for the so-called 3D printing technologies (or additive manufacturing) has also widened [1]. Numerous works in the literature already refer to the next level of meta-structures which can be designed and manufactured under the provocative term \"4D printing\", referring to materials and structures with time-dependent response or geometry [2]. Another interesting feature of the wide range of additive manufacturing technologies is that sensors may be easily incorporated within the layered structure during its building phase, resulting in smart systems, i.e., objects with built-in sensing capabilities [3]. Several review articles (see [4, 5] for typical examples) classify such smart systems according to their manufacturing process, operating principle, and/or functionality. Different types of sensors, e.g., optical for interactive devices [6], or embedded wires for capacitive sensing applications [7, 8] have been suggested as potential smart systems. Mechanical strain sensing via various incorporated sensors in 3D printed systems has also been proposed [9; 10; 11]. Magnetite/thermoplastic composite in 3D printing as direct replacements for commercially available flow sensors [12], or custom-built devices (whiskers) with built-in magnetostrictive materials for specific flow measuring applications [13] have been successfully manufactured. Magnetic force sensors [14], or devices for sensing human tactile force and imperceptible skin deformation due to heart pulses [15] for biomechanical applications are reported as feasible options for 3D printed smart objects. Specifically, some applications related to stretchable sensors for biomedical purposes are also reported in [16]. On the other hand, several custom-built sensors designed to monitor structures for damage diagnosis purposes, have been proposed over the recent years, for instance [17; 18; 19; 20; 21; 22]. Such sensors are mostly designed to be patched onto the structure, so that the sensing element such as, fiber bragg gratings [18; 19], magnetoelastic strips [21], and PZT impedance [22] or MFC (macro-fiber composite) transducers [23] to name but a few, could measure specific dynamic properties of the structure under vibration. Magnetoelastic sensors have been under intense investigation for many applications in the past decade. Since they react to changes in imposed mechanical strain, by changing their magnetic constant, they can be used as wireless and passive sensors at the same time. Magnetoelastic sensors are usually made by rapid melt quenching, in the form of amorphous ferromagnetic ribbons. Most members of this family are iron-rich alloys, such as the well-known Fe\({}_{40}\)Ni\({}_{38}\)Mo\({}_{4}\)B\({}_{18}\) marketed under the commercial name Metglas(r)2826 MB. The authors have successfully used this material as a damage sensor for polymers and composite systems [21] and have gathered considerable experience in sensor application, data gathering, and processing over the last years. A main advantage of such sensors against all other types of relevant devices is that they require no wiring, electrical nor optical, and can be interrogated wirelessly and in a passive state. In general, the sensor is carefully configured in order to interfere as little as possible with the monitored structure's dynamics. Therefore, the sensor weight and volume are kept as low as possible, meaning that in some cases even thermal-spraying techniques to achieve deposition of sensing element (copper electrodes) onto the structure [24] may be used. A notable exception to this rule is found in [25], where the sensor placed on the structure is deliberately chosen of considerable weight so as to change at will the underlying dynamics.