# Additive Manufacturing for Sensors: Piezoresistive Strain Gauge with Temperature Compensation
## 1 Introduction  
Additive manufacturing (AM), well known for being well suitable in different fields such as aerospace [1] and biomedical [2], has been gaining popularity for the fabrication of unconventional (i) non-assembly mechanisms [3], (ii) 4D printed structures [4], and (iii) metamaterials [5]. Recently, a new wave of 3D-printed structures is becoming appealing: the fabrication of smart objects with embedded sensors. The key enabler for the fabrication of smart objects is the additive manufacturing of conductive materials used to create embedded sensing systems. Due to the possibility of extruding multiple materials simultaneously, AM seems to be the right technology to exploit for the fabrication of objects with embedded sensor systems [6]. In the case of fused filament fabrication (FFF) technology, piezoresistive materials can be exploited to fabricate sensors. Piezoresistivity describes the electrical resistivity change when a material is subjected to an external force. In order to exhibit piezoresistive properties, the FFF structure has to be electrically conductive. However, the extruded materials are usually thermoplastics. Therefore, they are not conductive. In this case,materials are made conductive by doping a polymeric matrix with conductive particles (such as carbon nanotubes) [7]. The piezoresistive effect of nanocomposite materials is mainly due to (i) the inherent piezoresistivity of the conductive nanoparticles, (ii) the modification of inter-nanoparticle distances (affecting tunneling resistance between adjacent nanoparticles), and (iii) the deformation of conductive networks formed by nanoparticles within the polymer [8; 9]. [PERSON] et al. [10] developed a multiaxial sensor made up of a conductive material and a flexible thermoplastic polyurethane (TPU) to detect a change of force along three axes till 4 N.  
[PERSON] et al. [11] fabricated in a single step a smart load cell composed of TPU and four embedded strain gauges (conductive material): after connecting the strain gauges in a full Wheatstone bridge configuration, a sensitivity of 0.008 \(\frac{\mathrm{\SIUnitSymbolMicro V}}{\mathrm{\SIUnitSymbolMicro g}}\) in the range [0, 100] g was found. By tailoring a new 3D printing strategy, [PERSON] et al. [12] fabricated, using a dual extruder machine, a porous sensor that can be easily integrated into biomedical devices (such as rehabilitative gloves) able to detect small applied pressure values. On the other hand, the authors pointed out a high viscoelastic effect due to the conductive plastic. Another example of a smart object is provided in [13]; they manufactured and tested in a single step, a soft pneumatic network (Pneunet) actuator with an embedded piezoresistive sensor placed in the bottom of the structure able to provide feedback (change in resistance) when the actuator is pressurized. A new promising field involving MeX technologies is the fabrication of an accelerometer [14; 15]: in particular, in a range of [10; 50]\(m\)\(s^{-2}\) excitation amplitude, a linear response has been found out. The possibility to reduce the current accelerometer size (37 mm \(\times\) 37 mm \(\times\) 37 mm) will enable the fabrication of small smart devices, which could potentially find application in the mechanic-electronic domain. Another class of 3D-printed sensors that is a candidate to be widely exploited in the future is the capacitive ones: innovative capacitive sensors fabricated using FFF technology have been recently proposed for the detection of force, strain, and temperature [16; 17; 18; 19; 20]. Several process parameter studies have been performed to reduce the gap in knowledge about the reduction of 3D-printed sensors. [PERSON] et al. [21] found the welding effect to be responsible for the increase in resistance and variability. The layer height and the total number of layers have to be reduced to reduce the aforementioned effect. [PERSON] et al.