## 2 Materials and Methods
### Experimental System  
Figure 2 shows a custom-made SS-OCT (swept-source) based on a Mach-Zehnder interferometer constructed with fiberoptic components such as couplers and circulators from (Thorlabs, Newton, NJ, USA). The SS-CCT was constructed with a swept light source (HSL-2100-ST, Santec, Aichi, Japan) with a center wavelength of 1310 nm, a scan range of 125 nm, a sweep frequency of 20 kHz, and an average power of 22.7 mW. The light from the source was split into two beams by a 50/50, 1 \(\times\) 2 optical coupler (TW1300R5A1, Thorlabs, Newton, NJ, USA) to illuminate the reference and sample arms separately. The split light was sent through the respective circulators placed in each of the arms (CIR-1310-50-APC, Thorlabs, Thorlabs, Newton, NJ, USA). The light reflected from the sample and the reference mirror, respectively, after passing through circulators was recombined by 1 \(\times\) 2 optical couplers (TW1300R5A2, Thorlabs, Newton, NJ, USA). In order to remove the source fluctuations, the interference signal was again split by a 50/50, 1 \(\times\) 2 optical coupler and then detected by a balanced detector (PDB470C, Thorlabs, Newton, NJ, USA). The reference mirror was mounted on a stepping motor stage (KXL06300-C2-F6A, Suruga Seiki, Shizuoka, Japan) to adjust the path length to obtain the interference signal. On the sample arm, light from the fiber was collimated by a fiber collimator and an OCT Probe unit (Santec, Aichi, Japan) was used to focus light onto the sample. The OCT probe unit also consisted of galvano mirrors (along the lateral directions x and y) that can be scanned to obtain 3-D volume scans. In this study, 2-D scans were acquired by scanning only one lateral axis. The interference signal acquired as a function of wavelength from the sweeping of the wavelength by the swept source was transformed to frequency, wave vector, or k-space, and then was Fourier-transformed to obtain the depth profile of the sample. All signal acquisition was achieved with custom-made software using LabView (LabView ver.2012 National Instruments, Austin, TX, USA). Interference is only obtained when the optical path difference between the object and reference light is within the coherence length, as calculated using Equation (1), which was 6 \(\upmu\)m. The lateral resolution of the OCT probe lens unit was 39 \(\upmu\)m. In the case of skin, for the conversion of the optical path length to free space length, an index of 1.43 was used [11].  
As for the impact of the measurement over the living cells, we used near-infrared light of 1300 nm, which can penetrate deeper, thus reducing localized absorption. In addition, the power incident on the skin was around a few 100 \(\upmu\)W, and the exposure continued for a  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Age (Average \(\pm\) SD)**} & \multicolumn{2}{c}{**Number (N = 20)**} \\ \hline Male & Female & Male & Female \\
22.6 \(\pm\) 0.89 & 26.8 \(\pm\) 1.64 & 5 & 5 \\
40.2 \(\pm\) 13.3 & 38.8 \(\pm\) 8.96 & 5 & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Subject details.  
Figure 2: SS-OCT experimental system. Inset: the dominant forearm of the subject attached to a USB heat pad set to 40 \({}^{\circ}\)C. Heating was applied for 5 min.  
few minutes. This is because, for each of the conditions--before heating, soon after heating, and 5 min after heating--two sets of OCT scans were obtained (8 s each) for each subject. As the wattage limits even for eyes are a few hundred uWs, we believed that no living cells would be affected.  
Figure 2 also shows an inset of the palmar forearm of the dominant hand of a subject. By scanning the probe beam at a rate of 12.5 frames per second, a total of 100 frames were acquired. Each frame consisted of 3840 (z) \(\times\) 1000 (x) pixels.  
\[\mathrm{l_{c}}=\frac{4\ln 2}{\pi}\frac{\lambda^{2}}{\Delta\lambda} \tag{1}\]