## 2 Materials and Methods
### THz Spectroscopy  
Analysis of the solutions (distilled water or blood serum samples) was carried out in special cuvettes, which were pre-printed on a 3D printer using Watson material (styrene butadiene copolymer, SBS) [35]. The selection of material for printing was made due to its sufficient transparency in the THz region of the spectrum [36], as well as high moisture resistance. When preparing for printing on the 3D printer, the standard preset provided by the \"Polygon X\" program was used, and the percentage of filling of model areas was specified as 100% (cast infill). Printing was carried out using fused deposition modeling (FDM). The principle of building according to FDM technology lies in the layer-by-layer growth of a product from a pre-melted plastic thread. Sample roughness is not automatically controlled. Cuvettes with obvious printing defects were not used for the study.  
An example of an average terahertz spectrum from 10 empty cuvettes is shown in Figure 1. As can be seen, there was little variation. The absorbance of the cuvette was the same for all printed samples. THz transmission spectra of mouse blood serum samples were measured in the range 0.2 to 2 THz by a T-SPEC spectrometer (EKSPLA, Vilnius, Lithuania). The spectral resolution was 1 GHz, the acquisition rate up to 10 spectra/s, and the dynamic range better than 70 dB.  
A cuvette was placed in the THz-TDS spectrometer so that the THz radiation beam passed through the cuvette center (see Figure 2a). THz radiation was focused onto the sample using parabolic mirrors. At the focus point, the THz beam diameter was about 3.5-4 mm. The thickness of the cuvette (Figure 2b) was 0.5 mm (Figure 2a). The position of the cuvette in the THz-TDS spectrometer is shown in Figure 2c. The measurements were carried out at a constant room temperature of \(21\pm 1\)\({}^{\circ}\)C.  
Figure 1: Example of an average THz spectrum from 10 empty cuvettes with standard deviation.  
The THz signal registered from the empty cuvette was used as a reference. Then, without changing the position, the cuvette was filled with either distilled water or a pre-thawed blood serum sample with a volume of 50 \(\upmu\)L, using an automatic dispenser. A new cuvette and dispenser tip were used for each sample. Sample spectral scanning was conducted along six spatial points of the cuvette with a step of 0.1 mm vertically and horizontally. Time averaging (over 256 spectra) was performed for each spatial point of the 2D scan. The signal transformation from a temporal to a frequency domain was conducted by TeraVil TRS-16 software (TeraVil Ltd, Vilnius, Lithuania). The impulse (time signal) and intensity spectra without a cuvette, with an empty cuvette, and for the cuvette with distilled water or blood serum sample are shown in Figure 3.  
Figure 3: THz spectra in (**a**) temporal and (**b**) frequency domains: without a cuvette (black dashed line); with an empty cuvette (green line); with the cuvette filled with distilled water (blue dotted line); or a blood serum sample (red line).  
Figure 2: (**a**) Scheme of the experiments and parameters of the cuvette, gray arrows show the path of the THz radiation beam; (**b**) the cuvette printed on a 3D printer; (**c**) view of the cuvette installed in the THz-TDS spectrometer.