Moxibustion is a non-invasive Korean medical therapy using thermal and chemical stimulation on skin tissue by burning moxa [1]. Recognized by thermoreceptors, heat generated from moxibustion facilitates blood circulation and enhances nerve fiber activity, resulting in treatment effects [2,3]. Moxibustion is effective for a wide range of health conditions such as musculoskeletal disorders, gastrointestinal symptoms, fatigue, and conditions related to aging [4].

Despite these therapeutic effects, moxibustion has several disadvantages. Since it is difficult to control the magnitude of thermal stimulation other than by adjusting the number of substances burning, side effects such as burns, secondary infections, skin scars, and pain have been reported [5]. According to Lee et al. [6], burn injury was the most frequent side effect caused by indirect moxibustion. Han et al. [7] claimed that side effects like burns are the main reason patients refuse moxibustion treatment. In addition, the moxibustion combustion reaction varies depending on the storage conditions, as more moisture leads to an incomplete combustion reaction, which might increase carbon monoxide production [8]. Ventilation is needed to remove the smoke and odor from moxibustion combustion to reduce complications such as dizziness, coughing, dry throat, and headache [9,10].

Electronic moxibustion (EM) has recently been developed to overcome the limitations of the traditional moxibustion method and can stimulate the skin with a relatively constant temperature via electronically-produced heat, alleviating burns, smoke, and odor [3,11,12]. According to Chae et al. [13], there is no significant temperature difference between applying traditional combustible moxibustion and EM on skin surface temperature. Furthermore, Park et al. [11] reported on the proper treatment time when applying EM that is safe from causing burns. However, no study has yet analyzed the temperature change in deep tissue induced by EM by depth in comparison to traditional moxibustion.

Here we used silicon phantom to confirm the temperature distribution at various depths and compared the thermal change pattern between EM and traditional indirect moxibustion (TIM).

1. Materials

1) Phantom

The silicon phantom (Sylgard 184, Dow Corning, USA), made of Polydimethylsiloxane (PDMS), was used to measure the temperature change by depth. A 10:1 mixture of the base and curing agent was made at room temperature (25-26℃) and transferred onto the polycarbonate molds (5 cm wide, 5 cm long, and 6.3 cm high mold and 5 cm wide, 5 cm long, and 5 cm high phantom). We inserted needles to make space for the thermocouples at depths of 0, 2, 5, 7, and 10 mm from the surface and 3 mm (h) × 2 mm (v) from the center of the phantom. After degassing by a vacuum pump (MVP-12, Woosung Automa Co., Korea) and chamber, the phantom was cured in a forced convection oven (OF-02GW, Jeiotech Co., Korea) at 100℃ for 45 minutes. All measurements were made after solidifying the silicon phantom.

2) Moxibustion

We used adhesive indirect moxibustion (Taekeukttum, Haeng Lim Seo Won Medical Co., China) in the TIM group. Moxa used in TIM was made up of mugwort covered with a paper cylinder and circular plate (Fig. 1A). For the EM group, an electric moxibustion treatment device (Cettum, K-Medical Co., Korea) was used. This device is made up of two parts: the heating units and the charging equipment. A lithium-ion battery and connecting terminal are included inside the heating unit, enabling charging through electrical energy. By pressing the switch located on the top of the heating unit, thermal stimulation is transferred to the plate in contact with the skin (Fig. 1B).

Figure 1. (A) Traditional Indirect Moxibustion; (B) Electronic Moxibustion.

2. Methods

1) Experimental environment

We used an experimental setup consisting of an acrylic box (60 cm × 34 cm × 35 cm), a hot plate, and a tray containing water. The phantom was heated in a convection oven for 30 minutes to set the surface temperature to 30-31℃. A tray containing 38-42℃ water was placed on the hot plate, and the bottom of the phantom was immersed in the plate to distribute a temperature similar to that of skin and deep tissue. To minimize the influence of the external environment and to keep the internal air temperature at 27-29℃, the entire hot plate and tray were blocked with an acrylic box with a heating fan located inside the box (Fig. 2).

Figure 2. Diagram of the experimental setup. (A) acrylic box; (B) hot plate; (C) tray containing water; (D) silicon phantom.

2) EM and TIM treatment procedures

We applied moxibustion after confirming that the phantom surface temperature was maintained at 31-32℃ for 20 seconds. For the TIM group, the moxa was heated by igniting the upper portion using a torch lighter. For the EM group, a practitioner pressed the button on the top of the heating unit. The temperature was measured for 400 seconds in the TIM group and 1,200 seconds in the EM group. The procedure was repeated eight times for each group and the data were averaged. A new silicon phantom was used for every trial.

3) Temperature measurements and analysis

K-type thermocouples (Ø 0.5 mm, Shinsegae Sensor, Korea) were used to measure the phantom temperature at various depths. To reduce the disturbance of heat conduction by the thermocouples, the temperature was measured in four parts: the exposed surface (0 mm); 2 mm, 7 mm depth; 5 mm, 10 mm depth; and 3 mm (h) × 2 mm (v) deep from the center (Fig. 3). Furthermore, to ensure the internal environment was maintained, the temperature of the air inside the acrylic box and water in the tray were also measured.

Figure 3. Location of thermocouples inserted into the silicon phantom. Thermocouples of the same color were measured at the same time. h = horizontal; v = vertical.

Analog signals from the eight thermocouples were transmitted to the data processing program through a data acquisition device consisting of NI 9211 modules (National Instruments, USA) inserted into the NI cDAQ-9174 Compact DAQ chassis (National Instruments, USA). Transmitted electrical signals were converted into digital signals by LabVIEW (National Instruments, USA) and stored as graphs and numeric data. Temperature changes by time were measured in one-second intervals. The measured temperature data by depth were analyzed using Excel (Microsoft, USA).

3. Statistical analysis

All data are presented as mean ± standard deviation. Statistical analyses were carried out using STATA version 15.0 (STATA Corp, LP. College Station, USA). Graphical representations were generated using the GraphPad Prism 7 software (Graph Pad Software Inc., USA) and Excel (Microsoft, USA). The temperature change at each depth was compared with the mean value of the change (maximum temperature – base temperature). Changes in the values are indicated by Δ. Statistical comparisons between TIM and EM values were performed using either the Student’s t-test or the Mann-Whitney U test, per the data distribution. p-values < 0.05 were considered significant.

1. TIM

In the TIM procedure, a significant temperature increase was observed at all measurement points in the phantom compared to the base temperature with the maximum temperature (p < 0.0001, 0.0001). The maximum temperatures of TIM were 55.04℃ ± 0.92℃ (Δ23.79℃ ± 0.96℃) at the surface, 37.37℃ ± 0.37℃ (Δ6.18℃ ± 0.50℃) at 2 mm, 34.66℃ ± 0.35℃ (Δ1.87℃ ± 0.41℃) at 5 mm, 33.57℃ ± 0.46℃ (Δ1.34℃ ± 0.20℃) at 7 mm, 34.59℃ ± 0.50℃ (Δ0.51℃ ± 0.23℃) at 10 mm, and 36.13℃ ± 0.48℃ (Δ4.20℃ ± 0.59℃) at 3 mm (h) × 2 mm (v) deep from the center. The average time to reach the maximum temperature at the 0 mm, 2 mm, 5 mm, 7 mm, and 10 mm depths was 138 seconds, 150 seconds, 195 seconds, 248 seconds, and 339 seconds, respectively. As the measuring point deepened, a much lower peak temperature was observed with a longer time to reach the maximum temperature. As shown in Fig. 4B, the TIM group showed a sharp decrease immediately after reaching the peak temperature at the surface and 2 mm depths.

Figure 4. The temperature profile in various depths of the silicon phantom during EM (A) and TIM (B). EM = electronic moxibustion; TIM = traditional indirect moxibustion.

2. EM

The EM group showed a significant temperature increase at all measurement points in the phantom compared to the base temperature (p < 0.0001, 0.0001). The maximum temperatures of EM were 43.25℃ ± 1.95℃ (Δ13.00℃ ± 2.23℃) at the surface, 34.73℃ ± 0.44℃ (Δ3.36℃ ± 0.40℃) at the 2 mm depth, 34.72 ± 0.38℃ (Δ1.91℃ ± 0.49℃) at the 5 mm depth, 33.94 ± 0.57℃ (Δ1.45℃ ± 0.34℃) at the 7 mm depth, 34.47℃ ± 0.82℃ (Δ0.75℃ ± 0.28℃) at the 10 mm depth, and 33.08℃ ± 0.61℃ (Δ41.28℃ ± 0.57℃) 3 mm (h) × 2 mm (v) deep from the center. The average time to reach the maximum temperature at the 0 mm, 2 mm, 5 mm, 7 mm, and 10 mm depths was 126 seconds, 343 seconds, 403 seconds, 449 seconds, and 550 seconds, respectively. Similar to the results of the TIM procedure, at deeper the measuring points, a lower peak temperature was observed with a longer time to reach the maximum temperature.

As shown in Fig. 4A, EM maintained its maximum surface temperature for a longer time and showed a more gradual decrease. Surface temperatures above 41℃ were maintained from 34 to 305 seconds and above 40℃ were maintained from 31 to 329 seconds. Temperature changes from EM thermal stimulation showed relatively gentle upward and downward curves at all depths.

3. Between-group comparison of maximum temperatures (Fig. 5A)

TIM showed a statistically higher maximum temperature than EM at the phantom surface (0 mm) and 2 mm (p < 0.0001, 0.0001) depths, but there was no significant temperature difference between the two groups at the 5 mm, 7 mm, and 10 mm depths.

Figure 5. Maximum temperature (A) and change of temperature (B) according to the depth (cm) during EM and TIM treatment. Change of temperature (Δ) is defined as the maximum temperature minus the base temperature. *Indicates a significant difference (p < 0.01). EM = electronic moxibustion; TIM = traditional indirect moxibustion.

4. Between-group comparison of temperature change (Fig. 5B)

When comparing the change of temperature defined as the difference between the maximum and base temperatures in the two groups, there was a significant difference between the 0 mm and 2 mm depths (p < 0.0001, 0.0001), but no significant difference was found at depths below 5 mm.

Moxibustion is one of the main therapeutic interventions in Korean medicine that delivers thermal stimulation to acupoints and certain areas of the body [14]. In primary clinical settings, moxibustion covers a wide range of diseases, most commonly applied to gastrointestinal disorders, musculoskeletal disorders such as osteoarthritis, and abnormalities of the soft tissues and joints [15]. According to a survey on the clinical use of moxibustion, 91% of Korean medicine doctors were applying moxibustion, with the most common type being the adhesive-type indirect moxibustion using traditional moxa [6].

EM was recently developed to minimize side effects such as burns and overcome therapeutic complications such as smoke or smell arising from TIM [11]. Furthermore, there have been clinical studies investigating the efficacy of EM for breast cancer-related lymphedema [16], ankylosing spondylitis [17], shoulder periarthritis [12], knee osteoarthritis [18,19], lumbar stenosis [20] and traffic accident-induced lumbago [10].

As moxibustion implements thermal stimulation [21], there must be a balance between maintaining sufficient temperature for therapeutic effect and preventing excessive heat stimulation on the skin and tissue. Heat is perceived as a nociceptive stimulus through thermoreceptors. Therapeutic effects could be achieved by producing nerve fiber activities [22]. Local thermal stimulation of moxibustion leads to superficial hyperthermia of skin temperature, maintained at 40-45℃, which enhances the extensibility of collagen tissues and increases blood flow to affected areas by vasodilation [23,24]. Upregulated blood flow promotes tissue healing, revascularization, and fracture healing [25].

Chae et al. [13] reported no significant difference in body surface temperature after applying traditional moxibustion or EM for 2 minutes. Park et al. [11] reported on the proper treatment duration based on skin safety to prevent side effects by utilizing EM on the abdominal and back skin of rats. Furthermore, to substitute the thermal stimulation effect of traditional moxibustion, it is important to maintain heat conductance in deep skin tissues [26]. Myoung and Lee [5] measured temperature changes in rabbit tissue with an electronic thermal stimulation system and reported lower epidermal temperature and comparable subcutaneous thermal stimulation compared with traditional moxibustion. However, no study has investigated the three-dimensional thermal properties of EM due to research methodology limitations arising from the use of living tissues.

However, thermal stimulation exceeding the threshold of tissue damage can cause burns or blisters, common side effects of moxibustion. Habash et al. [27] reported that tissue necrosis, coagulation, and protein denaturation can occur in thermal stimulation of 47-50℃ over 10 minutes, while Park et al. [11] demonstrated burn and blister formation from thermal stimulation with EM at 47℃ for more than three minutes. Therefore, thermal stimulation of moxibustion should be kept in the range of 40-47℃ in biological tissues to gain a better therapeutic effect without possible side effects.

This study aimed to investigate distributions and dynamics of EM thermal stimulation at various depths and compare these to TIM using a silicon phantom, which is set similar to the skin tissue under a controlled experimental environment.

The heat generated from TIM and EM is transferred from the phantom surface to the deep phantom tissues, causing a statistically significant temperature increase. This heat transfer process involves convection, conduction, and radiation. During thermal therapy, the radiation energy generated by moxibustion is scattered and absorbed, resulting in a gradual loss of heat as the skin layer deepens. Thus, temperature decreases and the time to reach this maximum temperature increases in correlation with the depth of the measuring point of the phantom peak.

Measured at the surface of the phantom, the peak temperature after applying TIM (55.04℃ ± 0.92℃ [Δ23.79℃ ± 0.96℃]) was significantly higher than after EM (43.25℃ ± 1.95℃ [Δ13.00℃ ± 2.23℃]), with both groups reaching their highest temperature at around two minutes. The temperature distribution curve suggests that, after applying TIM, the surface temperature of the phantom increases rapidly to the highest temperature that carries a burn risk ( > 47℃) and then decreases sharply again. For the experimental setting with EM, after reaching the peak surface temperature, a more gradual temperature decline was observed. This trend was constantly observed at depths of 2 mm, 5 mm, 7 mm, and 10 mm with EM, demonstrating a gradual decrease after reaching the maximum temperature. Thus, the superficial hyperthermic temperature (40-47℃) with therapeutic effects could be achieved for a longer time with a lower risk of burns using EM. Overall, TIM provides short and strong thermal stimulation by combustion, with a higher temperature than EM near the epidermis and an increased risk of burns and tissue damage. EM stimulates the tissue for a longer time at the therapeutic temperature and has a lower risk of side effects.

Our research has several limitations. Although our experimental settings tried to keep the phantom's temperature constant and Sylgard-184 is widely used to simulate human tissue properties [28,29], we could not reproduce the thermoregulatory mechanism of biological tissue, such as blood circulation and perspiration. Furthermore, the composition of multiple skin tissue layers including the epidermis, dermis, subcutaneous fat, and muscle could not be artificially duplicated. In addition, although this study was conducted under the experimental settings blocked by an acrylic box, external environmental factors such as airflow were not taken into consideration. An actual clinical setting using moxibustion was not established, as the practitioner can adjust the position of TIM administration in cases of excessive heat.

Despite these limitations, this study is the first to measure the temperature dynamics of EM at different depths of the phantom, which is compatible with human skin tissue in a controlled environment compared with TIM. To enhance our understanding of the thermal properties and safety profile of EM, further research requires the development of a phantom that can more accurately simulate skin layer anatomy and biological responses to thermal stimulation, as well as experimental methods and analysis techniques that allow for an analysis of the conduction, convection, and radiation of thermal energy in EM procedures.

This study aimed to analyze temperature distributions measured at various depths of silicon phantom during EM and TIM operation. When measured at the surface and a depth of 2 mm, applying TIM compared to EM resulted in a significantly higher temperature rise, but at 5 mm, 7 mm, and 10 mm, the temperature escalation between the two groups was not statistically different. When TIM was applied to the phantom, the maximum surface temperature was 55.04℃ ± 0.92℃ (Δ23.79℃ ± 0.96℃), which exceeds the threshold of tissue damage (50℃) and presents a burn risk. The maximum surface temperature when using EM was measured at 43.25℃ ± 1.95℃ (Δ13.00℃ ± 0.79℃), which had a relatively lower risk of side effects. Furthermore, after reaching its peak surface temperature, a rapid decrease was observed with thermal stimulation from TIM, whereas EM showed a gradual decrease after maintaining the surface temperature between 40-45℃ for a longer period. This indicates a possible therapeutic benefit of EM over TIM.

This research was supported by a grant from the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (grant number HI19C0142).

The authors declare no conflict of interest.