From Simulation to Reality: A Comprehensive Study on the Efficacy of a Rotating Monopolar and Bipolar Radiofrequency System through In-Silico Modeling and Pre-Clinical and Clinical Validation ()
1. Introduction
Collagen and elastin are the most relevant proteins that support the connective tissue of the skin, providing elasticity and a desirable appearance [1, 2]. The loss of these proteins is an undesirable process that results in clinical manifestations such as wrinkles, skin sagging and laxity since the skin becomes thinner and loses elasticity [1]. Aging of the skin or skin elastosis can be aggravated by genetic and environmental factors such as chronic exposure to ultraviolet radiation, smoking, air pollution and diet [3]. Traditionally, skin deterioration has been treated with invasive surgical procedures [4-6] which involve a certain risk for the subject during surgery, recovery time, post-operative care and side effects such as pigmentation. In addition, these invasive surgical treatments usually have a high financial cost.
Energy-based devices emerged as excellent candidates for performing skin treatments in a non- or minimally invasive manner. As the first candidates, non-ablative [7] and ablative [8, 9] lasers have been widely used in the aesthetic medicine sector, although their popularity has declined due to the complications associated with their difficulty of use and the side effects caused by absorption in the different skin chromophores [10]. From this situation, novel energy-based devices that use energy sources other than light have emerged, such as radiofrequency (RF) [11, 12]. RF technology has been found to provide optimal results through a technique that is simpler and more affordable for both clinicians and subjects, while also minimizing pain and side-effects and shortening recovery time [13, 14].
RF energy was first employed for therapeutic uses in the 1920s when Bovie developed a prototype electrosurgical instrument [15]. For more than a century, RF devices have been widely used in several medical fields, ranging from control of intra-atrial re-entrant tachycardia [16] to minimally invasive tissue destruction [17, 18] and pain reduction [19]. In the aesthetic medicine field, RF technology can be used to address skin elastosis, photoaging, cellulite or localized fat, among other medical and aesthetic indications [20, 21].
An RF device produces electrical energy at frequencies ranging from 3 Hz to 300 GHz. When the electrical current passes through the different tissues, it generates heat due to the electrical resistance (impedance) of the tissues themselves. The heat induces mechanical and biochemical reactions that affect the skin and the subcutaneous tissue. These biological effects include 1) vasodilatation [22]; 2) contraction of skin fibers, thereby increasing blood circulation and cellular oxygenation stimulation of the lymphatic system [23, 24]; 3) collagen remodeling [25, 26]; 4) fibroblast stimulation, producing new collagen and elastin and improving the extracellular matrix of the skin [27, 28]; 5) increased adipocyte metabolism [21, 29]; and even 6) the induction of adipocyte apoptosis [30]. In summary, RF primarily produces three effects: increased blood microcirculation, increased elasticity of connective tissue in the dermis, and shrinkage of fat cells in the subdermal tissue. In terms of the visual effect in subjects, RF produces skin tightening, a lifting effect and body contouring [31].
Unlike lasers, RF energy is not absorbed by skin chromophores and skin melanin, allowing it to be used safely on all skin types [32]. However, to ensure a completely safe and efficient effect of RF on the skin, a temperature of between 39˚C and 43˚C must be maintained for 5 - 10 minutes [25, 26]. It is generally advised to not exceed 45˚C and to use a limited time period to avoid any thermal injuries to the skin [33].
There is a wide variety of RF medical-aesthetic devices on the market that, based on the RF electrode configurations, can be classified as 1) unipolar [34]; 2) monopolar [35]; 3) bipolar [22]; 4) multipolar [36]; or 5) fractional [32, 37]. Most of these are considered non-invasive devices, except the fractional type which generates coagulation effects and tissue ablation. Among non-invasive medical-aesthetic RF devices, the most used and studied in the literature are the monopolar variety, where one electrode is in contact with the skin and the other serves as a grounding pad [38], and the bipolar variety, which incorporate two or more rows of parallel electrodes in the same applicator [39]. Electrode size and geometry are paramount for RF penetration [39], increasing in the bipolar type with the distance between the electrodes, and in the monopolar type with the size of the electrodes [12, 25]. Other important parameters for RF release are the electrical impedance of the tissue [38, 40], the RF power delivered, and the exposure time [11, 41]. In addition, the penetration depth of RF energy is inversely proportional to the square root of the frequency [22].
During RF application, two modes are used: static and dynamic. Static or stamping mode produces a high peak skin temperature with a short exposure time, and most of these devices employ contact cooling to prevent skin damage [35]. Great skin tightening results have been reported [20, 41]; however, these can be limited by pain and a high risk of burns and other side effects. On the other hand, dynamic or continuous mode produces a lower peak skin temperature and uses a longer exposure time by performing multiple passes over the treatment area [25], which is found to reduce pain and side effects.
Despite RF being a well-established technology that has been implemented in the market, these non-invasive devices present some limitations [40]. Most RF devices do not have an automatic temperature control system [42] and the heat is concentrated around electrodes which may lead to hot spots and non-uniform heating. The physicians must set the RF power, monitor the skin temperature and perform constant movements to provide more uniform and comfortable heating in accordance with the subject’s pain threshold and skin reaction. Consequently, treatment is highly dependent on professional experience and praxis. Otherwise, electric arcs can be produced when the contact between the RF electrode and the tissue is not suitable (on uneven body contours or due to irregular applicator lifting), causing severe skin damage [29, 43]. The use of capacitive electrodes versus resistive ones has been found to solve this limitation, but these RF devices may have less efficacy due to higher impedance [44]. Additionally, some RF devices incorporate vacuum suction systems in the handpieces [36]. Although the efficacy of energy delivery is controlled and optimized through the suction, the treatment time becomes longer.
The development, implementation and improvement of RF systems is typically constrained by a lack of knowledge about proper heating inside the tissue. Traditionally, heat dispersion is evaluated indirectly through pre-clinical and clinical studies at the final stages of device validation. However, these studies are not only time consuming but also costly. The use of engineering and mathematical tools in the early stages of RF device development would optimize the process by reducing time and improving the understanding of the application for better efficacy and safety. In-silico studies have proven to be an efficient tool in biomedical applications, particularly when measuring internal parameters of the human body is only possible using invasive methods [45] or when a priori understanding of the results of new designs is needed [46]. Recently, numerical simulations have been used to design other energy-based devices such as lasers and cryolipolysis tools for aesthetic indications [47, 48]. In this study, the full development and implementation of a novel RF device with rotative electrodes for medical-aesthetic purposes is described. The presented study spans the whole process from the in-silico design and study of monopolar and bipolar RF electrodes using COMSOL Multiphysics, right through to the clinical validation. This research aims to show the potential of in-silico studies to optimize RF device design. Numerical simulation provides precise insights into heat behavior (uniformity and depth) in human tissue, enabling more accurate and straight- forward device fabrication.
2. Materials and methods
2.1. Radiofrequency Device
Sculpt & Shape® (Sinclair, Spain) is a non-invasive RF device for skin tightening, body contouring and cellulite treatments. Among others, this device is equipped with three tips (Figure 1), whose technical characteristics are detailed in Table 1.
The RF tips include a 360˚ rotating technology to provide homogeneous heating and massage the tissue. The rotation speed for all electrodes is 40 rpm. Likewise, the target skin temperature can be configured between 39˚C and 45˚C for five treatments (wrinkles, facial skin tightening, body skin tightening, cellulite, and body shaping). A real-time temperature and impedance measurement system regulates RF power to achieve the target temperature and maintain it until the end of the treatment, or temporarily cut off the power delivery in the event of no contact between the skin and the electrode.
2.2. In-Silico Study
The RF tips were modeled, simulated and optimized using COMSOL Multiphysics® software (version 5.4, COMSOL Inc., Burlington, MA). The tips had been designed previously with SolidWorks® software (SolidWorks Corp., Massachusetts) and then imported to COMSOL Multiphysics. Several iterations of testing and improvement were performed. This study shows the final version of the design.
2.2.1. Model Geometry
The simulations recreated the application of RF electrical currents to a specific area of the body with rotational movements.
The meshed geometrical domains consisted of the biological tissues (dermis and adipose tissue) and the RF tip in contact with the tissue (Figure 2). The thickness of the skin and fat layers were 2.75 and 46.25 mm, respectively, for monopolar electrodes, and 4 and 51.5 mm for bipolar electrodes.
To present the results of the thermal maps, the XY-plane cut (Figure 2) was employed to determine the depth of heat penetration into the tissue and the XZ-plane cut (Figure 2) was used to show the superficial heat on the tissue.
The tips incorporate a rotating movement, which is implemented in the simulations through a deformed geometry feature. However, the translational movement that the operator would perform on the body area was not implemented.
Figure 1. Monopolar and bipolar RF electrodes used in this study.
Table 1. Technical characteristics of the tips.
Tip |
Description of electrode |
Type of RF |
Frequency (MHz) |
Shine |
Spherical anodized aluminum |
Monopolar |
1 |
Firm |
Rectangular and flat anodized aluminum |
Monopolar |
1 |
Sculpt6 |
Concave surface with 2 rows of 3 spherical stainless-steel electrodes |
Bipolar |
0.5 |
Figure 2. Geometrical layout of the in-silico model which consists of electrode, dermis and adipose tissue, as well as XY and XZ planes.
2.2.2. Physics and Thermal Equations
The equations used for the numerical simulations (dynamic electric currents, electrostatics, electromagnetic heating and bioheat transfer module) are included in Section 1.1 Supplementary Information (SI). Additionally, this section incorporates tables indicating the thermal parameters of the tissues and materials (Table S1), the electrical parameters of the tissues and materials (Table S2), and the thermal and perfusion parameters of the blood (Table S3).
2.2.3. Boundary Conditions
All information is provided in Section 1.2 SI.
2.2.4. Mesh Generation
All information is provided in Section 1.3 SI.
2.3. Ex-Vivo Study. Proof of Concept Tests
The simulated results of heating inside the tissue (XY planes in Figure 2) were compared with the results obtained with pig tissue (Figure S1). Pork belly pieces without the superficial skin and with an initial temperature of 15˚C to 20˚C were used.
The pieces of pig tissue were measured and cut to ensure uniform size and thickness. In addition, the pieces used with the bipolar electrode were sliced in half to compare the same plane as the simulations. A thin layer of glycerin gel1 was spread across the surface of the tissue touching the RF tip to improve contact.
The monopolar electrodes were evaluated in rotating mode. However, the bipolar electrode was used without rotation to avoid damaging the pig tissue during the test.
Thermal measurements were taken using a Ti200 infrared thermal imaging camera (Fluke Corp, Washington). It was placed in front of the tissue that was used to obtain cross-sectional temperature maps and determine the depth of heat penetration inside tissue. The experiments were performed at room temperature and carried out by the same person to avoid variations in the force applied to the tissue. A minimum of five independent experiments were conducted.
Thermal images were analyzed using Fluke Connect software (Fluke Corp, Washington). An isotherm area of 5˚C under the maximum temperature was considered to calculate the penetration depth of the heat inside the pig tissue. ImageJ software (National Institutes of Health, Maryland) was employed to convert pixels on the thermal image into millimeters of tissue. Data analysis was performed using Microsoft Excel 365 (Microsoft, New Mexico) and data were presented as Mean ± Standard Deviation.
2.4. In-Vivo Study. Proof of Concept Tests
The simulated results for surface heating (XZ planes in Figure 2) were compared with the results obtained from skin heating measurements conducted on facial and corporal areas. A total of 6 facial areas were heated with the monopolar Shine tip, 9 facial areas with the monopolar Firm tip, 10 corporal areas with the Sculpt6 tip at 0.5 MHz, and 9 corporal areas with the Sculpt6 tip at 1 MHz. All subjects who participated in the study were previously briefed and signed a consent form.
The skin was cleaned with mild soap and water. A thin layer of glycerin gel was then applied. No topical anesthesia was applied before the treatment. The target temperature was 42˚C. During the test, the handpiece was constantly moved in a uniform pattern to achieve a homogeneous heat while remaining in full contact with the body area being treated.
All tips have an internal infrared sensor that measures the temperature in real time. The data obtained was stored in the device’s memory and later extracted for analysis.
The times taken to reach the set temperature were compared. The sets of values obtained for each case were compared with the results obtained from the corresponding simulation.
2.5. Clinical Validation
The inclusion criteria included males and females between 18 and 75 years old with concerns about skin laxity, wrinkles, cellulite, or localized fat. Specific exclusion criteria included active dermatological disease or infectious processes in the treatment area; pregnancy; breastfeeding; implanted electronic devices; blood disorders or the use of blood-thinning or antiplatelet medications; a history of active or recent neoplastic malignancy; heat-stimulated conditions and orthopedic surgery such as replacement of the hip, femur, etc.
A total of 16 subjects (14 women and 2 men) were willing to be treated for skin tightening, body shaping, wrinkles, and improvement of the appearance of cellulite in sessions performed on both sides of the face, the neck, the décolletage, the abdomen, the legs, the buttocks, and the flanks. A total of 78 areas underwent treatment. 31 areas were treated with the Sculpt6 tip to address cellulite reduction and body shaping, 25 areas were treated for skin tightening using the Firm tip, and 22 areas were treated for wrinkles with the Shine tip.
Subjects were not required to follow a special diet or exercise plan. Prior to treatment, all subjects received a detailed and clear explanation of the planned procedure and signed an informed consent form. The complete confidentiality of the subjects’ data was guaranteed throughout the entire process.
2.5.1. Protocol
Anthropometric data (sex, age, body weight and height) were recorded. Photographs at 45˚ and 90˚ angles, as well as frontal and lateral views of the treatment area, were taken at every visit before starting the procedure.
Glycerin gel was applied to the treatment areas and no anesthesia was required. The applicator was continuously moved according to the direction of the lymphatic system to achieve homogeneous and gradual heating of the area while maintaining contact with the skin. The target temperature for the treatments was determined to be between 43˚C and 45˚C, depending on the subject’s tolerance. The device automatically regulated the RF power to maintain a constant temperature throughout the treatment.
Treatments were performed every 1 or 2 weeks. Facial sessions lasted between 12 and 40 minutes, while body sessions, depending on the areas treated, lasted between 20 and 60 minutes. Details of the clinical treatments are shown in Table S4. All treatments were performed in accordance with the treatment tables recommended by the manufacturer.
2.5.2. Efficacy and Safety Evaluation
Improvements in the appearance of skin texture and body contouring compared to baseline photographs were rated by three blinded and experienced researchers, who carried out the evaluation on each pair of pre- and post-treatment photographs. The Global Aesthetic Improvement Scale with a 0 to 4 range (0 = no improvement; 1 = mildly improved; 2 = improved; 3 = much improved; 4 = very much improved) was used. Likewise, subject satisfaction was evaluated on a 0 to 3 scale (0 = unsatisfied; 1 = low satisfaction; 2 = satisfied; 3 = very satisfied) 1 month after the last treatment.
Also, the safety was evaluated by asking the patients their perception of pain/heat during the treatment, through a scale with a 0 to 10 range (0 no pain, 1-4 mild pain, 5-7 moderate pain, 8-10 severe pain). Adverse effects are also reported during each session.
2.5.3. Statistical Analysis
A student’s two-tailed paired t-test was applied to assess the statistical significance of the efficacy evaluation. A probability value of 95% (p < 0.05) was considered statistically significant. Data were analyzed using Microsoft Excel 365.
3. Results
3.1. Comparison between Simulations and Ex-Vivo and In-Vivo Tests
3.1.1. Monopolar Electrodes
The thermal images taken of the skin of a subject during a real treatment were shown to be very similar to the simulation of the rotating tips. However, in the case of the Firm applicator, due to its rectangular shape, the absence of rotation generated a rectangular thermal image, which is transformed into circular heating when the tip rotates (Figure 3).
The temperature scales may differ between simulation and real treatment due to the difference between the initial values of the skin and ambient temperatures.
The heating inside the tissue has also been found to be very similar between the tests carried out with pig tissue and the simulations (Figure 4). An isotherm of 5˚C, from 44˚C to 39˚C (thin black line in the simulations) was drawn to calculate the heat penetration depth (Table 2).
3.1.2. Bipolar Tip
The effect of rotation is more notable in the case of the bipolar tip. Without rotation, all the heat is concentrated under the electrodes, while rotation homogenizes the heat under the tip (Figure 5).
Table 2. Penetration depth calculations from ex-vivo tests and simulations using the monopolar electrodes.
Tip |
Penetration depth (mm) |
Pig tissue |
Simulation |
Shine |
4.69 ± 0.96 |
4.37 |
Firm |
6.80 ± 1.63 |
10.58 |
Figure 3. Surface thermal maps from simulations and a subject’s skin using monopolar electrodes, with and without rotation. The time to reach maximum temperature for each test and simulation is shown.
Figure 4. Heat penetration depth simulated (left) and measured in ex-vivo pig tissue using monopolar electrodes (right).
Figure 5. Comparison of heat distribution with and without bipolar tip rotation. Results from simulations in the XY and XZ shear planes and testing on a subject.
The effect of frequency with the bipolar applicator is shown in Figure 6. The penetration depth simulated for 0.5 MHz was 25.00 mm, and for 1 MHz it was 20.15 mm.
Ex-vivo tests with pig tissue could not be performed with rotation because the tissue was damaged and did not allow for reliable results. Without rotation, heating was concentrated under the electrode (Figure 7), as shown by the tests conducted on the surface (Figure 5). Qualitatively, a greater penetration depth was also observed with the lowest frequency of 0.5 MHz.
3.1.3. Heating Dynamics
Figure 8 compares the times required by each tip to reach the set temperature (42˚C for these tests), which were obtained from the simulations and in tests with subjects. For each tip, a set of in-vivo tests was
Figure 6. Thermal distribution of the simulation result with the bipolar tip. XY-plane is shown with rotation.
Figure 7. Thermal distribution of the test on pig tissue with the bipolar tip. The rotation of the tip was not activated.
performed to provide a statistic which is compared with the single value obtained in the simulation. Monopolar electrodes took around 30 s to reach 42˚C, and about 100 s for bipolar one.
3.2. Clinical Validation
All subjects completed at least 7 sessions of RF treatments, with an average treatment time of 18 minutes for facial treatments and 40 minutes for body treatments; the treatment temperature range was 42 to 44˚C. Information related to the treatments and the characteristics of the subjects is presented in Table S4. Before and after images of representative treatments are shown in Figure 9.
The clinical investigators’ assessment using the GAIS resulted in a score of 1.83 out of 4, which is a 25% - 49% improvement over baseline. Moreover, subject satisfaction after 1 month was evaluated through personal questionnaires, which reported a value of 2.50 out of 3 (between satisfied and very satisfied). No side or adverse effects were reported.
The subjects rated the heat/pain experienced from the heating during the treatment at 6.6 ± 1.1 on a scale of 1 to 10. Minor adverse effects were observed, such as mild superficial first-degree burns (which disappeared within a few days) resulting from sparks due to the tip losing contact with the skin during treatment sessions. No major adverse effects were reported.
Figure 8. Heating dynamics. Time ranges to reach 42˚C measured in in-vivo tests and the simulated values for each tip.
Figure 9. Before and after treatment images. (A) and (B) face skin tightening treated with monopolar electrodes, (C) body shaping and (D) cellulite, both treated with a bipolar tip.
4. Discussion
The design of devices whose function is to obtain effects on humans is complex. In most cases, the effect is not observable and clinical tests are needed, which can be time consuming, expensive and risky for the individual since these devices are in the testing phase. It is normal for the results to not be optimal, and several stages of redesign and new tests are necessary, making the clinical validation process a long and very expensive one. In general, manufacturing companies opt for designs that replicate other existing products to ensure results and save time, which leads to very similar products and a sector with little innovation.
Simulation tools have been widely used in many sectors, such as electronics, mechanics, fluids, construction, automotive, aeronautics, robotics, and others, which has enabled much faster progress than in the medical sector, for example. This drastically reduces development times from years of trial and error to just a few months. And most importantly, allows for increased understanding and knowledge, as well as taking advantage of the time to innovate and introduce notable improvements in the new products.
The device presented in this study incorporates important innovations for the sector, such as the automatic rotation of the electrodes, which represents a considerable challenge since it compromises contact with the skin, which is essential to transmit the radiofrequency energy and produce the desired movement and heating effect. The aim was to treat the skin and fat in different areas of the face and body in a comfortable, safe, and effective way. Another objective was to provide a mechanical massage to promote lymphatic drainage and enhance the heating effect. Some of the questions that were raised included: What is the optimal geometry of the electrodes? Which technology is more appropriate, monopolar or bipolar? What parameters should be chosen for RF power and frequency? What materials should be used?
Numerical simulations allow for a rapid trial-and-error process, thanks to which a theoretically ideal design can be achieved that is then tested with real patients with a high probability of delivering the expected performance.
In this study, the thermal behavior of RF electrodes from the Sculpt&Shape device equipped with a 360˚ rotation system has been simulated with COMSOL Multiphysics software. The RF tips have been designed and optimized for treatments for different face and body areas and indications. The results of these simulations have been compared with ex-vivo and in-vivo experiments. The authors had previously used simulations with cryolipolysis and laser hair removal devices [48, 49].
For the facial area, monopolar electrodes have been designed with different sizes and shapes to treat both small areas, to achieve more superficial effects, as well as larger areas, to reach the deeper dermis. Monopolar technology and capacitive materials have been chosen because bipolar technology and resistive materials present a high risk of causing electric arcs due to loss of contact. The effect of RF frequency has also been studied, and a frequency of 1 MHz has been chosen for facial treatments because it is optimal for the depth of the facial skin. A lower frequency would heat deeper areas and may cause an undesirable effect on facial fat (the so-called skeleton face effect). The penetration depth obtained with the smaller electrode (Shine) was 4.37 mm, while the Firm electrode achieved 10.58 mm (Table 2). This is in line with the expectation that the monopolar applicator with a larger surface area would achieve a greater depth of penetration. Therefore, the Shine tip has been found to be more suitable for treating fine lines and small areas of the face, while the Firm tip would be used for larger areas and to achieve a deeper effect. It is worth mentioning that for the Shine tip, the results of the simulations and ex-vivo measurements were very similar. This is because the Shine tip is small and spherical in shape and when performing the test with the rotating tip, its center could be positioned at the edge of the pig tissue. On the other hand, due to the flat and rectangular shape of the Firm applicator, the center of the applicator was not on the edge of the tissue and therefore the cutting planes of the simulation and the pig tissue did not coincide. Beyond the variation values, the simulation provided relevant information regarding heat dispersion inside the tissue and possible biological effects.
For the body area, the objective with the design of the RF tip was to also reach the hypodermis and provide a mechanical massage. A ball-based design and a concave cavity were chosen, which were optimized through simulation to treat large areas, achieve homogeneous heating, and offer a mechanical massage for improved comfort and effectiveness. To increase RF power transmission, resistive materials were chosen. Electric arcs have been suppressed by a real-time impedance measurement system that detects a lack of contact and momentarily cuts off the power. In addition, the effect of the RF frequency has been studied and a frequency of 0.5 MHz was determined for treatments whose target is localized fat, while 1 MHz was determined for skin treatments. Especially with bipolar applicators, the simulation has clearly shown the heating homogenization effect introduced by the rotation of the electrodes and how the absence of rotation produces very localized heating around the electrodes, which can cause hot spots and compromise the safety of the treatment (Figure 3). This uniform heating, both superficial and internal, ensures that treatments involving subjects are carried out in a safe manner, avoiding hot spots that can be painful and even lead to thermal injuries. This result, together with the internal RF power control of the equipment, allows for repeated treatments where the temperature can be maintained at therapeutic values during the indicated times to achieve the expected biological effects. Moreover, the simulations have demonstrated that heat penetrates deeper at 0.5 MHz than at 1 MHz, as widely reported in the literature [11, 22]. The simulated heat penetration depths obtained by the bipolar tip at 0.5 and 1 MHz were 25.00 mm and 20.15 mm, respectively.
Monopolar applicators usually provide deeper heating than bipolar applicators [50]. However, the bipolar applicator shown in this study produces deeper heating. This is due to its larger size and concave shape, which allow the tissue to be introduced into the applicator and for deeper layers to be targeted.
The results of the heating dynamics were also analyzed. Here, the tests on real subjects have shown a high dispersion, because there are many variables that influence RF power transmission into the tissue, such as the amount of gel used, the way the applicator is moved and pressed into the skin, and the characteristics of the human tissue. These variables affect the time needed to reach the target temperature during a treatment. However, the simulation tissue has fixed parameters and moreover no translational movements were considered. The significance of this study lies in the finding that both the simulations and the measurements taken during treatments have determined shorter times for the monopolar applicators, on average around 30 seconds, and longer times for the bipolar applicators, around 100 seconds.
The functionality of these applicators has been validated with the results shown in this study and with other studies carried out in external clinics [51].
The design and optimization of these three distinct electrodes tailored for specific face and body areas and skin layers demonstrate a thoughtful approach to treatment customization. This versatility is likely to contribute to the device’s effectiveness across a range of applications, addressing the diverse needs of subjects and clinicians alike. A commitment to creating a new device that ensures comfortable, efficient and safe treatments is essential.
5. Conclusions
This research has shown that the use of in-silico studies is able to reproduce the thermal behavior of human tissues, thereby providing an increased level of know-how of RF devices and optimizing and accelerating the design and fabrication processes.
The optimization of the Sculpt & Shape® novel RF rotative device has been described. In-silico studies performed with COMSOL Multiphysics software provided relevant information about the performance of the RF tips. The results showed the superficial and deep homogeneous dispersion of heat due to the release of RF energy into human skin tissue.
Ex-vivo studies performed on pig tissue for depth penetration analysis, in-vivo tests with individuals for superficial temperature evaluations, and finally, real treatments, have confirmed the outcomes of the simulations considering certain limitations associated with the differences between a simulation and a real case.
Ethics declaration
This experiment was conducted in accordance with the Declaration of Helsinki, and informed consent was obtained from the participants after they were given a thorough explanation of the main purpose and methods of the experiment both orally and in writing prior to the experiment.
S1. NUMERICAL SIMULATIONS AND IN-VIVO STUDY
S1.1. Physics and Thermal Equations
As previously mentioned, COMSOL Multiphysics® Software was used to solve both the heat transfer and electrical equations employed in these models. To study the interaction between the currents produced by RF and the tissue (dermis and adipose tissue), four physics are used: dynamics of electric currents, electrostatics, electromagnetic heating, and heat transfer in biomaterials.
1) Equations for Dynamic Electric Currents
In the frequency domain (AC current mode) and time dependent study types, dynamic formulas accounting for both conduction and displacement currents are used.
Combining the time-harmonic equation of continuity (1) with Equation (2) and generalizing to handle current sources, yields the following Equation (3):
(1)
(2)
(3)
where J is the current density, σ is the electrical conductivity, E is the electrical field, Je is the external current density, j is the imaginary number, ω is the frequency used, ρ is the charge density, D is the electric flux density, ε0 is the vacuum permittivity, V is the electric potential, Q is the charge and P is the electric polarization vector.
For the transient case, using the transient equation of continuity (4) and generalizing to handle current sources, the resulting equation becomes (5):
(4)
(5)
2) Electrostatics Equations
Under static conditions, the electric potential, V, is defined by Equation (6). By combining this equation with the constitutive relationship (7) between the electric displacement D and the electric field E, it is possible to represent Gauss’ law as the following Equation (8):
(6)
(7)
(8)
This equation describes the electrostatic field in dielectric materials.
3) Electromagnetic Heating
The Electromagnetic Heating represents the electromagnetic losses, Qe. This electromagnetic heat will be the heat source used in the heat transfer equation, coupling the electric equations with heat transfer in biomaterials. It is determined by (9):
(9)
where the resistive losses Qrh are defined as (10) and, in this case, magnetic losses Qml are equal to zero.
(10)
where J is the current density and E is the electrical field.
4) Bioheat transfer module
The heat transfer equation for biomaterials is defined as (13) using (11) and (12):
(11)
(12)
(13)
where ρ is the density of the tissue, Cp is the specific heat capacity of the tissue at constant pressure, k is the thermal conductivity of the tissue, T is the absolute temperature of the tissue, q is the heat flux by conduction in the tissue, ρb is the blood density, Cp,b is the blood-specific heat capacity at constant pressure, ωb is the blood perfusion rate, Tb is the arterial blood temperature, Qmet is the metabolic heat source and Qe is the heat source provided by the electromagnetic heating.
To solve the equations, the thermal and electrical parameters of each of the tissues that make up the area to be treated must be known, as well as those of the materials used to manufacture the RF tips. The monopolar tips are capacitive electrodes fabricated from a resistive material (Aluminum 6082) but covered with a thin layer of an insulating material (Anodized Aluminum), thus, in addition to electrical resistance they also have a high electrical capacity. The bipolar tips are resistive electrodes made of a material that only has electrical resistance (Steel AISI 4340).
The thermal (Table S1) and electrical (Table S2) parameters of the tissues and materials used are shown, in addition to the thermal and perfusion parameters of the blood in the tissues (Table S3).
Table S1. Thermal parameters of the tissues and materials.
Thermal Parameters |
Material |
ρ (kg/m3) |
k (W/m·K) |
Cp (J/kg·K) |
Skin |
52 1030 [] |
53 0.376 [] |
54 3360 [] |
Fat (adipose tissue) |
55 900 [] |
56 0.204 [] |
57 2430 [] |
Electrodes (Aluminum 6082) [Supplier] |
2700 |
150 |
900 |
Electrodes (Stainless Steel) [Comsol] |
7850 |
44.5 |
475 |
Table S2. Electrical parameters of the tissues and materials (* = dependent on the frequency “f” in Hz).
Electrical Parameters |
Material |
σ(S/m) |
εr |
Skin [IFAC]* |
|
|
Fat (adipose tissue) [IFAC]* |
|
|
Capacitive layer (Anodized
Aluminum 30 um thick) [Supplier] |
10−12 (Ohm·cm) |
16.8 |
Electrodes (Stainless Steel) [Supplier] |
4.032 × 106 |
1 |
Table S3. Thermal and perfusion parameters of blood.
Blood Parameters |
Parameter |
Value |
Arterial blood temperature (Tb) |
310.15 (K) |
| 58 Specific heat, blood (Cp,b) [] |
3.22 [J/(g·K)] |
| 59 Blood perfusion rate for adipose tissue (ωb) [] |
4.2 × 10−4 (1/s) |
| 59 Blood perfusion rate for skin (ωb) [] |
0.0018 (1/s) |
| 58 Density, blood (ρ) [] |
900 (kg/m3) |
S1.2. Boundary Conditions
Thermal and Electrical Boundary Conditions Were Established
Thermal and electrical boundary conditions were established.
With regard to the electrical boundary conditions, first, the surfaces of application of an electric potential (electrodes) and the return surface (V = 0) can be distinguished. For a monopolar tip, the electrical potential application surface is the electrode surface, and the return surface is through a passive plate (for simplicity, a plane at infinity has been considered). In the case of the bipolar tip, the electrical potential application surface is the surface of one electrode row, and the return surface is the other row of electrodes. As an electric current source, a potential is applied. The equipment does not apply a specific power, but a potential difference, which, depending on factors such as the impedance adaptation or the electrical impedance of the tissue to be treated delivers a specific electrical power. The electrical power delivered heats the tissue depending on its electrical characteristics. Secondly, it is necessary to simulate the tissue edges. In this case, they are approximated by an infinite distance (infinite domains) to simulate that an electric current is applied to a tissue of dimensions much larger than the dimensions of the applicator. Lastly, all other external surfaces involved in the simulation are considered isolated from the outside, since in principle they are in contact with the air. The electrical initial conditions are 0 V in all the studied materials.
Regarding the thermal boundary conditions, the initial temperature for biological tissue was set at 36˚C (body temperature) and the surface of the electrode at a temperature between 27˚C and 29˚C, taken from the experiments carried out with the different electrodes. All other exterior surfaces are considered thermally insulated, which means that there is no heat flow in their contours. The heat source is the electric current that flows through the tissue. Therefore, it is a source of heat by volume, and is the result of the application of the electrical module on the biological tissue.
S1.3. Mesh Generation
Four mesh structures were used: extra fine for the contact surface between the dermis and electrodes, very fine for the dermis, fine for the electrodes and coarse for the adipose tissue.
S1.4. Pre-Clinical Study with Ex Vivo Tissue
Figure S1. (A) Measurement of tissue thickness. (B) The tissue is placed on the passive plate if the test is conducted with monopolar tips. (C) Test with the monopolar Firm tip. (D) Test with the bipolar Sculpt6 tip; tissue cut in half to observe internal heating after the test.
S2. CLINICAL VALIDATION
S2.1. Clinical Treatments-Efficacy of S&S Tips for Skin Tightening and Body Contouring
Table S4. Patient information and clinical treatments.
Patient |
Gender |
Age |
Sessions |
Zone |
Area |
Tip |
Frequency (MHz) |
Time (min) |
P1 |
Female |
29 |
10 |
Body |
Legs |
Sculpt6 |
1 |
60 |
P2 |
Female |
43 |
7 |
Facial |
Neck |
Firm |
1 |
12 |
P3 |
Female |
47 |
8 |
Facial |
Face |
Shine |
1 |
24 |
P3 |
Female |
47 |
8 |
Facial |
Face |
Firm |
1 |
18 |
P4 |
Female |
22 |
9 |
Body |
Legs |
Sculpt6 |
1 |
60 |
P5 |
Female |
29 |
8 |
Body |
Buttocks |
Sculpt6 |
1 |
30 |
P6 |
Female |
51 |
10 |
Facial |
Face |
Firm |
1 |
18 |
P6 |
Female |
51 |
10 |
Facial |
Face |
Shine |
1 |
24 |
P6 |
Female |
51 |
10 |
Body |
Legs |
Sculpt6 |
1 |
60 |
P7 |
Male |
47 |
8 |
Facial |
Face |
Firm |
1 |
18 |
P7 |
Male |
47 |
8 |
Facial |
Face |
Shine |
1 |
18 |
P8 |
Male |
27 |
10 |
Body |
Flanks |
Sculpt6 |
0.5 |
20 |
P8 |
Male |
27 |
10 |
Body |
Abdomen |
Sculpt6 |
0.5 |
20 |
P9 |
Female |
29 |
8 |
Body |
Legs |
Sculpt6 |
0.5 |
60 |
P10 |
Female |
44 |
8 |
Facial |
Face |
Firm |
1 |
18 |
P10 |
Female |
44 |
8 |
Facial |
Face |
Shine |
1 |
24 |
P11 |
Female |
47 |
9 |
Facial |
Face |
Firm |
1 |
18 |
P11 |
Female |
47 |
9 |
Facial |
Neck |
Firm |
1 |
12 |
P11 |
Female |
47 |
9 |
Facial |
Face |
Shine |
1 |
18 |
P12 |
Female |
44 |
9 |
Body |
Abdomen |
Sculpt6 |
1 |
20 |
P12 |
Female |
44 |
9 |
Body |
Legs |
Sculpt6 |
1 |
60 |
P13 |
Female |
50 |
8 |
Facial |
Face |
Firm |
1 |
18 |
P13 |
Female |
50 |
8 |
Facial |
Neck |
Firm |
1 |
12 |
P13 |
Female |
50 |
8 |
Facial |
Face |
Shine |
1 |
24 |
P14 |
Female |
52 |
9 |
Body |
Legs |
Sculpt6 |
1 |
60 |
P15 |
Female |
60 |
8 |
Body |
Décolletage |
Firm |
1 |
10 |
P16 |
Female |
32 |
8 |
Body |
Abdomen |
Sculpt6 |
0.5 |
20 |
NOTES
1Sinclair internal reference: 100017252.