## 1 Introduction
Finally, another drive regarding 3D printed parts in the shipbuilding industry lies in the continuously advancing era of artificial intelligence and condition-based maintenance, where 3D printed spare parts that have sensing capabilities (that is, 3D printed custom-made sensors) can play a key role, offering significant advantages compared to their traditional microelectronic counterparts, such as significantly reduced cost, remote and on-demand fabrication, simplicity, robustness, and customization [9]. Sensing devices play a vital role in ship operation and performance monitoring and are used to optimize the operation of specific subsystems to ensure acceptable performance, as well as for safety reasons, as in the case of gas leakage detectors on gas carriers, oil mist detectors, etc. Therefore, sensors installed in ships measure various quantities such as temperature, humidity, pressure, flow rate, exhaust gas purity, and shearing strain of the tail shaft as part of the power output calculation to achieve compliance with energy efficiency index regulations [10]; additionally, sensors can be utilized to calculate the displacement of the hull and the fatigue effect on the steel plates and brackets [11]. With regard to piezoresistive measuring devices, small-scale sensors have been proposed and employed in various applications over the years in marine and underwater environments where accurate pressure measurement is crucial [12]. These sensors are vital in tsunami detection [13], wave and tide gauges, platform levelling, and in-depth observation data from ROVs, profiling instruments, and towed arrays [14]. Additionally, piezoelectric sensors have been utilized to measure ocean wave heights and periods, as well as to detect underwater objects [15]. Furthermore, based on the work by [PERSON] and [PERSON] [16], who pioneered the development of the induced strain actuation mechanism for hull structure observation, the concept of vibration control for smart hull structures using optimally placed piezoelectric composite actuators was proposed [17]. Recent studies have suggested further advancements; for example, a piezoresistive pressure micro sensor, fabricated using Silicon On Insulator (SOI)-based micromachining technology, has been proposed as a novel solution for micro pressure sensors suited for harsh marine environments [18]. Additionally, a system of three triaxial propagation accelerometers has been employed to monitor crack propagation in wind turbines [19]. Lastly, piezoresistive sensors have potential applications as limit-switches [20], which are widely used in marine settings, particularly on cranes and davits where sensor hysteresis is a desirable property [21]. Over the past years, the size of all the above-mentioned sensing devices has decreased significantly because of the development of \"microsensors\" a product of the evolution of the semiconductor industry. However, this does come with some drawbacks. First, in the case of failure, on-site fabrication is not possible because of the complex structure and manufacturing procedure, which require high-end equipment and methods. Maintaining an adequate onboard quantity of spare sensors is very expensive; thus, only sensors required as \"critical\" spares are stored onboard (Section 10.3 ISM Code [22]). The type and quantity of critical spare parts to be kept onboard is imposed by classification societies, manufacturing companies, oil major forums (for oil and gas carriers), and ship-management companies, taking into account safety and acquired experience. However, in most cases, only metal parts and internal combustion engine hardware (bearings, liners, and cylinder covers) are included in the inventory, as parts directly connected with propulsion and electrical power generation that are considered as more critical [22; 23]. In case of failure, the ship owner or the management company has to face the high cost of the sensor and its handling/packaging (as it is a sensitive material), while on many occasions, there is a waiting period of significant time.