Impact of far-infrared ray exposure on the mechanical properties of unwounded skin of rats

Far-infrared ray (FIR) has been applied to promote growth, modulate sleep, speed up the healing of wounds and accelerate microcirculation. The action of FIR on wounds has been well established in previous studies. However, whether the same action also works on normal skin is unclear. Therefore, the aim of this study was to evaluate the impact of FIR exposure on the tensile strength (TS) and composition of unwounded skin. In this study, 84 Sprague-Dawley (SD) rats were randomly divided into control and FIR groups. The dorsum samples were harvested for mechanical testing and histological observation at one, two, four and six weeks. The TS in the control group had no significant difference for all durations. However, a steep increase in the TS occurred between one and two weeks (P = 0.033) in the FIR group. The TS in the FIR group was found to be significantly higher than the TS in the control group at two weeks (P = 0.049). From histological observation, capillary dilation and increased inflammatory cells around the capillaries were observed at one week in FIR-treated groups. However, the mild inflammatory changes vanished after two weeks. In conclusion, our results showed that FIR may induce inflammatory changes and enhance skin TS in the short term, but the effect diminished with time.