Skin Research and Technology 2014; 0: 1–4 Printed in Singapore  All rights reserved doi: 10.1111/srt.12191

© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Skin Research and Technology

Effect of frequency on entrance and propagation pattern of high-frequency (radiofrequency) electrical currents in biologic tissues A. Taheri1, P. Mansoori2, K. E. Huang1 and S. R. Feldman1,2,3 1

Center for Dermatology Research, Department of Dermatology, Wake Forest School of Medicine, Winston-Salem, NC, USA, 2 Department of Pathology, Wake Forest School of Medicine, Winston-Salem, NC, USA and 3 Department of Public Health Sciences, Wake Forest School of Medicine, Winston-Salem, NC, USA

Background: Radiofrequency electrical currents have a tendency to move toward the center of the bulk of biologic tissues. Objectives: To evaluate the effect of the frequency of currents on their entrance and propagation pattern in biologic tissues. Materials and Methods: Three electrosurgical generators with 0.4, 1.5, and 3 MHz frequency outputs were studied. Current was applied using a metallic needle introduced into a piece of cow liver, with different amounts of energy delivered at multiple points. Cross-sections of the liver were then studied for tissue effect. The diameters of the coagulated areas at the deepest and most superficial parts were measured. The tendency of the currents for penetration in the deeper layers of tissue rather than in the superficial layers was assessed using the superficial diameter/deep diameter ratio.

Results: Diameter of coagulated area was larger around deeper parts than around superficial parts of the electrode. No correlation between frequency of current and the superficial/deep diameter ratio of the coagulation zone was found. Conclusion: Radiofrequency currents have a tendency to move toward the center of the tissue. Frequency of current over the range of 0.4–3 MHz did not show any effect on this tendency.

electrical currents, also called radiofrequency currents, have been used for decades in electrosurgery. Recently, these currents have been used for skin rejuvenation. Fractional heating of the dermis using radiofrequency devices is used for dermal collagen remodeling, skin tightening, and wrinkle reduction (1–4). Radiofrequency-based fractional devices use an array of multi-electrodepins that penetrate the skin and deliver a high-frequency alternating electrical current in radiofrequency range to the skin. This results in heating of areas which are directly targeted by the electrodes, leaving intact or slightly affected zones in between the targeted areas (1–4). High-frequency electrical currents tend to propagate and move toward the center of the bulk of the tissue (5). This means that when a penetrating electrode of a fractional resurfacing

device enters the skin, there will be more thermal coagulation in tissue around the deepest part of the penetrating electrode (the tip of electrode) in dermis than in superficial layers (epidermis) (6, 7). Radiofrequency-based fractional resurfacing devices use varying frequencies usually in the range of 0.3–5 MHz; however, to our knowledge, there is no study on the effect of frequency of current on the pattern of propagation of electrical currents in biological tissues and on the tendency of these currents to move toward the center of the bulk of the tissue. We hence conducted a study to evaluate the effect of the frequency of currents on the entrance and propagation pattern of high-frequency electrical currents in biologic tissues to find out if higher frequency currents have higher tendencies or lower tendencies for penetration into the deepest part of the tissue compared to lower frequency currents.

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IGH-FREQUENCY

Key words: skin – radiofrequency – tissue – electrical – current – high-frequency – frequency – propagation

Ó 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Accepted for publication 20 September 2014

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Methods In an in vitro study, we simulated, in large scale, a fractional radiofrequency skin rejuvenation electrode. Three electrosurgical generators with 0.4, 1.5, and 3 MHz frequency outputs in continuous (cutting) mode were used as the source of radiofrequency currents. A 2 cm long, thin, metallic needle with low electrical resistance was used to apply the current to a piece of cow liver larger than 10 9 10 9 7 cm in size. The needle was introduced into the liver at multiple points and different amounts of energy were delivered at each point while keeping the needle stationary. The procedure was performed at each point using 0.4, 1.5, or 3 MHz frequency currents. Therefore, each point received a certain amount of energy at a certain frequency. Cross-sections of liver, sliced parallel to the electrode, were then prepared and studied. Maximum diameter of the coagulated area at the deepest part (around electrode tip) and the diameter of the most superficial coagulated area were measured using a caliper (Fig. 1). The tendency of the currents for penetration in the deeper parts of tissue rather than in superficial layers was assessed calculating

Fig. 1. Using a high-frequency current and a penetrating needle, coagulation starts from the tip of the electrode (left). With higher energy, a rim of coagulated tissue forms around the needle in its all length; however, the rim is thicker around the tip of the electrode (right). The maximum diameters of the coagulated areas at the deepest (around electrode tip) and the most superficial coagulated areas were measured (arrows).

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the ratio of the superficial diameter/deep diameter. This ratio was compared visually between the three frequencies using linear fit lines. The temperature of the liver, including the center, was maintained at room temperature for the entirety of the study. The return electrode was placed underneath the liver far from the needle.

Results Each frequency was tested on 23 points. Using all three frequencies (0.4, 1.5, and 3 MHz), thermal coagulation of tissue was started from the tip of the electrode. With higher energy levels, a rim of coagulated tissue formed along the entire length of the needle; however, this rim was thickest around the tip of the electrode (Fig. 1). By visual inspection of the best-fit-lines of the ratios of superficial diameter/deep diameter of coagulated tissue for each frequency, there was no evidence of a clear dose–response between the frequency and superficial/deep diameter ratio (Fig. 2). At 3 MHz frequency, the current showed higher superficial diameter/deep diameter ratio comparing to other frequencies (Fig. 2). However, superficial diameter/deep diameter ratio was lower with 1.5 MHz frequency than with 0.4 MHz frequency. Therefore, a linear correlation was not found between the frequency and

Fig. 2. Superficial diameter/deep diameter ratio of coagulated tissue. At 3 MHz frequency, the current showed higher superficial diameter/deep diameter ratio at each point on the linear fit line compared to other frequencies. However, the superficial diameter/deep diameter ratio was lower using 1.5 MHz frequency rather than 0.4 MHz frequency. Still these ratios were very close to each other.

Propagation of electrical currents

the superficial diameter/deep diameter ratio. The mean superficial diameter/deep diameter ratio was not significantly higher in 3 MHz than in 1.5 MHz frequency (0.63 and 0.59, respectively; P = 0.053, student t-test).

Discussion In a long conductor with a small cross-section/ length ratio such as a metal wire, high-frequency electrical currents tend to move on the surface of the conductor, avoiding the center, a phenomenon called “skin effect” (8). The higher the frequency, the more superficial the current flows. Detailed discussion of the mechanism of skin effect is beyond the scope of this article. However, there are conflicting reports on the pattern of propagation of alternating electrical currents in biological tissues (9). This study confirmed the results of our previous study and showed that in a piece of liver, which is not a good conductor and does not have a small cross-section/length ratio for passing the current, a high-frequency electrical current tends to move toward the center of the bulk of the tissue rather than moving on the surface, a pattern different from what happens in a metallic wire (5). We also found no evidence of any relationship existing between the frequency of current and the depth of its maximum penetration in tissue. When an electrode of a fractional radiofrequency device enters the skin, regardless of the frequency used, maximum heating effect will be around the tip of the electrode in dermis. This phenomenon can save epidermis during dermal heating and reduce the risk of post-inflammatory dyspigmentation. In addition, the penetrating electrodes can be insulated on the proximal part, while the distal part is in contact with deeper dermis, thus saving the epidermis more efficiently during heating of the dermis (10). Compared to laser-based fractional resurfacing devices that provide relatively more effect (and also side effects) to the epidermis than to the deep dermis, radiofrequency devices may be a better choice when our goal is releasing energy into the dermis to induce dermal collagen remodeling, tighten skin, and reduce wrinkles (6, 7). Easier post-operative care, shorter downtime, and lower risk of dyspigmentation is expected when the epidermis remain intact during such rejuvenation procedures (6, 7).

In this study, we used a monopolar single electrode model of fractional resurfacing devices and evaluated 0.4, 1.5, and 3 MHz alternating currents. Most of the available fractional resurfacing devices use multi-electrode-pins. Some of them use bipolar mode or higher frequencies. Therefore, we cannot generalize the results of this study to all fractional resurfacing devices. We are not sure if the tissue results of multi-electrode devices are the same as a monoelectrode device. Some fractional resurfacing devices use bipolar mode. Because in bipolar mode active electrodes are near each other, the pathway the current passes is probably the nearest path. Therefore, in bipolar devices, we do not expect the current to have a tendency for propagation in deeper tissues unless the penetrating electrodes are insulated on the proximal parts.

Conclusion High-frequency electrical currents have a tendency to move toward the center of the bulk of tissue. Change in the frequency of current over the range commonly used for radio frequency skin tightening did not show any effect on this tendency.

Acknowledgements The authors thank Avan Teb Co. (Dezfoul, Iran) for providing electrosurgical generators for this study. The authors also thank Avan Teb Co. (Dezfoul, Iran), Kavandish System (Tehran, Iran), and Tektran Inc. (Edmonds, WA, USA) for contributing scientific materials for this study.

Funding Electrosurgical generators for this study were provided by Avan Teb Co. The Center for Dermatology Research is supported by an unrestricted educational grant from Galderma Laboratories, L.P.

Conflict of interest Arash Taheri, Parisa Mansoori, and Karen Huang have no conflicts to disclose. Steven R. Feldman is a consultant and speaker for Abbott Labs, Amgen, BiogenIdec,

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Bristol Myers Squibb, Galderma, Genentech, GlaxoSmithKline, Janssen, Photomedex, Stiefel, and Warner Chilcott. Steven R. Feldman has received grants from 3M, Abbott Labs, Amgen, Astellas, Aventis Pharmaceuticals, BiogenIdec, Bristol Myers Squibb, Galderma, Coria,

References 1. Hantash BM, Ubeid AA, Chang H, Kafi R, Renton B. Bipolar fractional radiofrequency treatment induces neoelastogenesis and neocollagenesis. Lasers Surg Med 2009; 41: 1–9. 2. Hruza G, Taub AF, Collier SL, Mulholland SR. Skin rejuvenation and wrinkle reduction using a fractional radiofrequency system. J Drugs Dermatol 2009; 8: 259–265. 3. Lee HS, Lee DH, Won CH, Chang HW, Kwon HH, Kim KH, Chung JH. Fractional rejuvenation using a novel bipolar radiofrequency system in Asian skin. Dermatol Surg 2011; 37: 1611–1619. 4. Seo KY, Yoon MS, Kim DH, Lee HJ. Skin rejuvenation by microneedle fractional radiofrequency treatment in Asian skin; clinical and histological analysis. Lasers Surg Med 2012; 44: 631–636.

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Genentech, GlaxoSmithKline, Janssen, Novartis, Ortho Pharmaceuticals, Pharmaderm, Photomedex, Roche Dermatology, Stiefel, and Warner Chilcott. Steven R. Feldman has received stock options from Photomedex.

5. Taheri A, Mansoori P, Sandoval LS, Feldman SR, Williford PM, Pearce D. Entrance and propagation pattern of high-frequency electrical currents in biological tissues as applied to fractional skin rejuvenation using penetrating electrodes. Skin Res Technol 2014; 20: 270–273. 6. Brightman L, Goldman MP, Taub AF. Sublative rejuvenation: experience with a new fractional radiofrequency system for skin rejuvenation and repair. J Drugs Dermatol 2009; 8(11 Suppl): s9–s13. 7. Peterson JD, Palm MD, Kiripolsky MG, Guiha IC, Goldman MP. Evaluation of the effect of fractional laser with radiofrequency and fractionated radiofrequency on the improvement of acne scars. Dermatol Surg 2011; 37: 1260–1267. 8. Miranda EN. A simple model for understanding the skin effect. Int J Elect Enging Educ 1999; 36: 31–36.

9. Christie RV, Binger CA. An experimental study of diathermy: IV. Evidence for the penetration of high frequency currents through the living body. J Exp Med 1927; 46: 715–734. 10. Hantash BM, Renton B, Berkowitz RL, Stridde BC, Newman J. Pilot clinical study of a novel minimally invasive bipolar microneedle radiofrequency device. Lasers Surg Med 2009; 41: 87–95. Address: A. Taheri Department of Dermatology Wake Forest School of Medicine 4618 Country Club Road Winston-Salem, NC 27104 USA Tel: +336 716 1763 Fax: +336 716 7732 e-mail: [email protected]

Effect of frequency on entrance and propagation pattern of high-frequency (radiofrequency) electrical currents in biologic tissues.

Radiofrequency electrical currents have a tendency to move toward the center of the bulk of biologic tissues...
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