## 2 Materials and Methods
### Characterization of Materials  
A number of methods were used to get information about the elemental composition, phase composition, and size characteristics of the powder produced from the feed material by spark discharge and the sintered material obtained through annealing of the deposited ink. The powder samples were taken for analysis from the collected powder so that the majority of the platinum droplets (10-50 \(\upmu\)m in diameter) were not brought into the samples. This was possible to execute due to the fact that large objects possessing low specific surfaces tend to be located at the bottom of the powder material. The estimated content of platinum droplets in the samples was less than 2 wt. %. Annealing of the deposited ink was carried out in air atmosphere in the muffle furnace as follows: (1) heating the furnace up to 750 \({}^{\circ}\)C at a constant rate of 10 \({}^{\circ}\)C/min, and (2) keeping the furnace at 750 \({}^{\circ}\)C for 2 h.  
The elemental composition was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with the use of a Liberty Series II ICP-OES (Varian  
Figure 1: Stencil of a microheater used for aerosol jet printing. International Inc., Zug, Switzerland). The oxidation state of the platinum was controlled by X-ray photoemission spectroscopy (XPS) with the use of a Theta Probe (Thermo Fisher Scientific, Waltham, MA, USA). The phase composition and crystallinity of the materials were estimated by analyzing X-ray diffraction (XRD) spectra measured with a D8-Advance diffractometer (Bruker, Bremen, Germany), while analysis of the XRD spectra was carried out with the use of the Rietveld refinement method, implemented in X'Pert HighScore Plus software (PANalytical B.V., Almelo, The Netherlands). The particles constituting the powder were characterized by transmission electron microscopy (TEM) with the use of a JEM-2100 (JEOL, Tokyo, Japan). The microstructure and elemental composition of the sintered material were examined by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX) with the use of a JSM-7001F (JEOL, Tokyo, Japan) equipped with a Bruker XLash 6/30 EDX spectrometer. The resistivity of the sintered material was calculated from the resistance of the printed lines by using Pouillet's law. The cross-sectional area of the lines used in the calculations was estimated from the data acquired with an S NEOX non-contact optical 3D profiler (Sensorfar, Terrassa, Spain). In the course of ink formulation, the viscosity of the ink samples was controlled with an SV-10 vibrational viscometer (A&D, Tokyo, Japan). The surface tension was estimated by the pendant drop method with the use of a DSA255 drop shape analyzer (Kruss GmbH, Hamburg, Germany). The concentration of platinum in the ink samples was controlled by Thermogravimetry-Differential Scanning Calorimetry (TGA-DSC) thermal analysis performed on an SDT Q600 (TA Instruments, New Castle, DE, USA). The ink samples were heated at a rate of 10 \({}^{\circ}\)C/min in air supplied at a flow rate of 250 mL/min. The size distributions of the colloidal particles were obtained by dynamic light scattering (DLS) with the use of a Zetasizer Nano ZS (Malvern Instruments, Malvern, United Kingdom). The packing density of the nanoparticles in the printed ink after removal of a binder (polymer with plasticizer) as a result of heat treatment at 400 \({}^{\circ}\)C was controlled by SEM with the use of a JSM-7001F (JEOL, Tokyo, Japan). The topography and microstructure of the annealed microheaters (heat treated at 750 \({}^{\circ}\)C) were examined by optical profilometry and scanning electron microscopy with the use of an S NEOX (Sensorfar, Terrassa, Spain) and an NVision 40 (Carl Zeiss, Oberkochen, Germany). The thermal radiation images of the annealed microheaters were obtained with a FLIR SCG55 thermal imaging camera (FLIR Systems, Wilsonville, OR, USA) and then processed in ThermaCAM Researcher Professional 2.10 software.