## 1 Introduction  
The progress made in additive manufacturing has allowed for many materials to be transformed into useful devices with the most ecological and economical procedures known to mankind as of yet [1]. Indeed, this technology, a legitimate child of Industry 4.0, and one of its main driving forces, is proving very useful in printing hybrid materials and also smart systems, which are materials with built-in sensing abilities [2] as shown recently. Certainly, the concept of embedding sensing elements into structural components has often been used for monitoring purposes, with recent examples of circular PZT (lead zirconate titanate (Pb[Zr(x)Ti(1-x)]O_{3})) patches incorporated in concrete cylinders reported in [3]. In that study, vibration measurements from the embedded PZT patch were successfully correlated with measurements from accelerometers mounted on the cylinder's surface. Since such measurements are useful for detecting increased stiffness of concrete due to aging, one may speak of concrete cylinders with integrated monitoring capabilities. At the same time, other studies attempted to evaluate problems from integrating sensing elements in various (mainly composite) materials. In [4], piezoceramic flexible patches (PFP) inside glass fiber reinforced plastic (GFRP) specimens produced heat during their operation thus reducing the sensing performance of the specimens. Again [5] concluded that integrating fiber Bragg grating (FBG) and strain gages via a hand lay-up process in sandwich composite material (with cork agglomerate core and fiberglass and epoxy resin skins) caused detector behavior in tensile, bending and creep tests. Consequently, using 3D-printing techniques to embed low-power, passive sensing elements (thus avoiding self-heat problems [4]) of limited dimensions into structural components, is the next step for creating smart materials with sensing capabilities. Many types of sensors and sensing systems can be 3D-printed nowadays and their numbers and applications are constantly rising [6; 7]. It is easy to predict that many structural components and devices and even larger structures will be enabled with built-in, ab initio embedded sensors, providing monitoring of structural data, or health status. The applications of such systems start from the biomedical device industry, with their wearable sensors trends [8], and finish up to smart aerospace materials endowed with sensing abilities [9]. A key application of embedded sensors regards the nondestructive condition monitoring of structures, and mainly of joints between structural members or components. In the past, different joint types were evaluated usually by dismantling the associated components and either testing them under static loading or inspecting them using traditional nondestructive testing and evaluation (NDT and E) techniques. For instance, in [10], butt joint structures on composite fuselage frames were inspected during static loading by means of acoustic emission (AE) techniques. Indices such as the amplitude and accumulative energy of the recorded AE signals were evaluated over a testing period, in order to identify five different damage stages along with the corresponding damage style of the specimen. A similar principle of (tensile stress) loading along with a multi-parameter analysis of multiple AE features was used for monitoring carbon-fiber reinforced polymer composite single-lap shear joints in [11]. In both studies, the monitored specimens were extracted from the structure and tested separately, meaning that normal operation of the structure had to be suspended. In [12], adhesively bonded sandwich joints (used in aircraft interior fittings, ceilings, bulkheads) were reviewed and commented upon with respect to possible defects potentially causing reduced structural strength. Inspection of such joints was carried out by X-ray tomography, ultrasonic resting and, interestingly, low-frequency vibrational testing. In principle, such methods may be used when the structure is \"off duty\" for inspection purposes.