## 2 Materials and Methods
### 3D Printing of Electrically Conductive Circuits  
Prior to the 3D printing stage, nanocomposite conductive inks were prepared by a calendering dispersion process, based on a previously optimized 7-cycle method [35]. Here, the gap distance between rollers was reduced each cycle, keeping the speed of the rollers constant. Nanofilier content added was 0.5 wt.%. This CNT content, which was above the electrical percolation threshold, was high enough to ensure an efficient [PERSON]'s heating by applying a relatively low voltage. Moreover, this content was low enough to reduce, or even avoid, the UV shielding effect by CNTs, which hinders the photocuring reaction [24; 36; 37]. In this regard, an increase in the CNT content could cause a reduction in curing degree due to the increase in the UV-shielding effect, whereas a lower CNT content would induce a lower electrical conductivity of the nanocomposite and, therefore, a lower temperature reached by [PERSON]'s heating.  
After the dispersion process, the mixture was 3D printed by UV-Assisted Direct Write technology, using a BCN3D Plus 3D printer by BCN3D Technologies with the Paste Extruder module, modified to allow the use of disposable syringes instead of the original container. Direct Write technique consists of extruding a viscous paste through a printing nozzle, depositing the material layer by layer until the 3D geometry is completed. Moreover, two different devices for adapting the UV lamp to the 3D printer were designed and tested. First, Figure 1a shows the 1-stage printing design, where the UV light guide was pointing directly to the extrusion nozzle. Here, the UV lamp was turned on during the material extrusion. In this regard, ink extrusion and curing took place in the same stage. On the other hand, Figure 1b shows the 2-stage printing design, where the UV light guide was not pointed directly to the extrusion nozzle. Here, the material was extruded in the first stage without UV irradiation and then, once the whole layer was extruded, a second stage was carried out where the UV light guide was placed perpendicularly to the printed circuit and irradiated the specimen, following the same path that the syringe did in the first stage. In this regard, ink extrusion and curing occurred in different stages. Both devices were 3D printed on PLA by Fused Filament Fabrication (FFF) with a BQ Witbox equipment supplied by BQ.  
Figure 1: Alternative designs for housing the UV curing lamp to the 3D printer: (**a**) 1-stage printing device and (**b**) 2-stage printing device.  
The UV lamp used was the LC8 model supplied by Hamamatsu, whose wavelength ranges from 240 to 400 nm. The UV lamp was equipped with the A10014-50-0110 light guide and the E5147-04 condenser lens, also supplied by Hamamatsu. The intensity was set to 100% and a distance between substrate and UV light of 10 mm was established since the maximum irradiance can be obtained with these parameters, allowing a faster curing of the printed ribbons during the 3D printing process.  
The most relevant 3D printing parameters, defined with Slic3r software, are shown in Table 1.  
The geometry of the printed specimens was based on a 4 parallel lines circuit (Figure 2) constituted by 1 to 4 printed layers. Each layer was printed as a single continuous line in order to avoid flow disruption in the 3D printer and, therefore, possible dimensional distortions in the printed circuits.