## 1 Introduction
### Skin-Mimicking Phantoms
The most common types of physical skin models are based on liquid suspensions, gelatinous substances, elastomers, resins, metals, and textiles with nano-fillers and micro-fillers [17; 19]. According to the literature written by [PERSON] et al. [17], the following materials can be used to mimick skin thermal properties: hydrogels, elastomers, metals and textiles. Metal-based skin models are most used to probe the thermal properties of clothing [30]. Textile-based skin models are used to mimick the sweating and mechanical and frictional behavior of the human skin [31; 32]. Elastomer tissue-mimicking phantoms are based on the use of silicones or polyurethane combined with additives like carbon black or titanium dioxide to tune the properties of the elastomers. Silicones and polyurethane have a long stability and can be tuned very specifically to obtain certain parameters needed for that application. The elastomer models can be molded into different anatomical shapes. Hydrogel-based skin phantoms consists mostly of gelatin,agarose models and polyvinyl alcohol gels [18]. A hydrogel-based phantom encapsulates water as a main component and forms a stiff matrix that limits water mobility. The human skin tissue-mimicking phantom used for our experiments is based on a hydrogel phantom, more specifically an agarose-based skin phantom. Agar is a gelatin-like substance made from seaweed polysaccharides [17]. Agarose skin models are not very stable in the long term and have limited shelf life, but they are versatile as well as easy to use and produce. The acoustic, thermal and physical properties are similar to those of human skin and thus widely used for the fabrication of thermal phantoms [19]. The thermal characteristics of the model can be altered by changing the composition of the mixture. Agarose skin phantoms are easily fabricated by heating a mixture of agarose powder and deionized water above 85 \({}^{\circ}\)C, mixing thoroughly and pouring the mixture into a mold. At room temperature, the model resembles a lightly opaque gelatinous substance [33]. In the literature, a range of different compositions of the agar phantoms are used. [PERSON] et al. used a 1% research grade agar and distilled water solution to simulate human brain soft tissue [34]. A human skin tissue phantom used by [PERSON] and [PERSON] is based on a composition of 4% solution of agar and distilled water to match the thermal properties of human skin [35]. [PERSON] et al. developed a gel phantom with a 0.6% agar solution dissolved in distilled water to quantify skin necrosis during cryotherapy [36]. The thermal conductivity of their 0.6% gel phantom is 0.58 (W/m \(\cdot\) K) (error = 0.0037). [PERSON] et al. conducted an experiment where the thermal conductivity and specific heat are varying depending on the agar-gel concentration [37]. The composition of the agar-gel was 0.5%, 1.5%, 2.5%, 3.5% and 5% with, respectively, a thermal conductivity in (W/m \(\cdot\) K) of 0.7749 \(\pm\) 0.124, 0.667 \(\pm\) 0.116, 0.591 \(\pm\) 0.145, 0.404 \(\pm\) 0.171 and 0.379 \(\pm\) 0.113. The specific heat c in J/kg \(\cdot\) K for the 1.5%, 2.5%, 3.5% and 5% agar mixtures is, respectively, 6465 \(\pm\) 452, 5827 \(\pm\) 218, 4887 \(\pm\) 477 and 4011 \(\pm\) 386. The thermal conductivity of agarose hydrogel resembles, according to [PERSON] [38], 0.65 (W/m \(\cdot\) K) and decreases slightly to 0.51 (W/m \(\cdot\) K) with the concentration increase from 0.5 wt% to 2 wt% of concentration.