Sylgard 170

The skin phantom structures included two layers of silicone polymers to mimic the epidermis and the underlying soft tissue^{22 (link)}. The latter (6 mm, thermal conductivity ~0.5 W/mK)^{23 ,24 (link)} was formed by mixing the base and curing agent of a commercial polymer kit (1:1, Sylgard 170, Dow Corning, the thermal conductivity of silicone made by Sylgard 170 is 0.48 W/mK)²⁵ , casting a sheet and curing at room temperature for 24 h. The former (100 μm, thermal conductivity 0.2 ~ 0.37 W/mK)^{23 ,24 (link)} was formed by mixing the base and curing agent of the same kit but at a different ratio (10:1, Sylgard 184, Dow Corning, the thermal conductivity of silicone made by Sylgard 184 is 0.27 W/mK)²⁶ , casting and curing at room temperature for 24 h. The casting process used a silicon wafer for the thin top film, an acrylic plate for the bottom film, and with spacers to control the thickness. Plasma treating the surfaces enabled a strong bond to form upon contact, to yield the final bilayer structure.

Regular cotton sewing threads (Ji-Meng-Se, Rizhao, China) in two plies with a linear density of 1160 Tex (g/km) were used to prepare the CTs. First, the cotton threads were carbonized at 800 °C for 1 h in N₂ atmosphere, and the CTs obtained were cut into 40 mm long pieces. Then two monofilament copper wires with a diameter of 0.24 mm as electrodes were soldered with silver paste (PELCO, TED PELLA) at the two ends of a piece of CT sample. Finally, the CT-with-electrodes assembly were encapsulated with polydimethylsiloxane (PDMS) elastomer by dip coating. PDMS resin (Sylgard 170, Dow Corning) was prepared by mixing PDMS base and catalyst in the mass ratio of 1:1. To cure the PDMS resin, the CT-with-electrodes coated with PDMS resin were heated at 80 °C for 30 min.

A 0.4 g
sample of Sylgard 170
(Dow Corning) Part A and Part B was added to a beaker containing 10
mL of the graphene–IPA dispersion and stirred under magnetic
stirring for 2 min. Further graphene–IPA was then added depending
on the required graphene loading. The mixture was gently heated to
40 °C, and the solvent was allowed to evaporate under continuous
stirring. Once almost all of the solvent had evaporated, the mixture
was transferred into Teflon molds (35 × 35 mm). The mixture was
left to stand for 12 h to ensure complete solvent evaporation and
then cured at 100 °C for 1 h in an oven. The final composite
was removed from the mold and measured ∼600 μm in thickness.
Details on composite characterization can be found in the Supporting Information