We calculated the thermal responds of skin tissue using the FEM software COMSOL based on coupling the data obtained from the bioheat transfer.
The skin is initially kept at constant temperature. At t = 0, the skin surface, initially at normal temperature, is suddenly taken into contact with a hot source of burning temperature generated by tonification. According to tonification method, after 20s, the hot source is removed. Then, make the temperature keep the one at 20s for 5 seconds. Then the temperature fields are obtained by using FEM, which are subsequently used to calculate the corresponding thermal damage and thermal stress.
The calculate temperature distribution in skin tissue is plotted in Fig. 6(a), which shows that the temperature decreased exponentially along the skin depth, with a sudden decease at the Dermis-fat interface due to the large difference between the absorption coefficients of the dermis and fat. And the temperature histories at the ED interface was shown in Fig. 6(b). The temperatures at the ED (TED) interface increase quickly after the application of the data of burning temperature. Upon temperature peak, TED decreases instantly, whereas TED decreases slowly after 20s.
Figure 7(a) plotted the corresponding thermal damage distributions along the skin depth, which was similar to that temperature. The thermal damage at the ED interface increased almost linearly during the temperature raring, as shown in Fig. 7(b). Once after the peak temperature, the thermal damage changes little, for thermal damage is a process of accumulation.
The corresponding thermal stress fields were presented along the skin depth in Fig. 8(a), which was shown that the stratum corneum layer played an important role in the mechanical behaviors of skin. The thermal stress at ED interface varied with the change of temperature, as shown in Fig. 8(b).
The skin is initially kept at constant temperature. At t = 0, the skin surface, initially at normal temperature, is suddenly taken into contact with a hot source of burning temperature generated by sedation. According to sedation method, after 15s, the hot source is removed and the skin was cooled by natural convection of environmental air for 5s. Then the temperature fields are obtained by using FEM, which are subsequently used to calculate the corresponding thermal damage and thermal stress.
Figure 9(a) given the temperature distribution along the skin depth at 8s, 12s, 16s, and 22s, while the temperature history at the ED interface was presented in Fig. 9. The temperature risen abruptly comparing with tonification and the peak temperature was higher than tonification. After the peak temperature, the temperature decreased sharply due to air cooling at the skin surface in the sedation method.
The thermal damage changes little once the heating is removed. The distribution of thermal damage along the skin depth is similar to that of temperature which we used, as shown in Fig. 10(a). Thermal damage at the ED interface increases almost linearly during temperature rising, but the rate is higher for the tonification, as shown in Fig. 10 (b).
The corresponding thermal stress fields were presented along the skin depth in Fig. 11 (a), and were similar with tonidication. Figure 11(b) given the thermal stress at ED interface, by comparing it and Fig. 8. (b), it can be seen that thermal stress due to sedation is larger than that due to tonification. Also, the thermal stress is attributed to the different cooling methods used in sedation and tonification, where active cooling is often applied in sedation.
According to the above-mentioned analysis, three points have been worked out as follows:
First, for thermal transfer, as the Fig. 6(b) and Fig. 9(b) shows, the peak temperature in ED interface of sedation is higher than that of tonification. The threshold of the sensor cells for temperature pain is 43oC, so sedation has larger stimulus to skin tissue than tonification.
Second, for thermal damage, as the Fig. 7 and Fig. 10 shows, the damage caused by sedation is larger than that by tonification. So, sedation has larger stimulus to skin tissue than tonification.
Third, for thermal stress, as the Fig. 8(b) and Fig. 11(b) shows, thermal stress in ED interface caused by sedation becomes much larger than that by tonification. The mean mechanical threshold of the nociceptors in the skin is in the range of about 0–0.6 MPa , so sedation has larger stimulus to skin tissue than tonification.
Thus, in view of analysis above, the pain stimulation generated by sedation is much larger than that by tonification , which is in accordance to ancient literatures of traditional medicine.
The study of tonification and sedation in moxibustion provides a more scientific basis for physicians by using the FEM. Therefore, it is hoped that that this study will contribute to the application of, improvements in, and standardization of, tonification and sedation.
However, this study did have several limitations. Its main limitation is the assumption that the skin-tissue properties are constant. Collagen is the major component of the skin, and its thermal and mechanical properties vary with temperature. However, there have been relatively few studies regarding these changes, which limit the research on moxibustion treatment significantly. Moreover, Nishitani has suggested that other biochemical responses, such as chemical stimuli, are generated during moxibustion treatment .