Selective and localized radiofrequency heating of skin and fat by controlling surface distributions of the applied voltage: analytical study

At low frequencies (hundreds of kHz to a few MHz), local energy absorption is proportional to the conductivity of tissue and the intensity of the internal electric field. At 1 MHz, the electric conductivity ratio between skin and fat is approximately 10; hence, skin would heat more provided the intensity of the electric field is similar in both tissues. It follows that selective and localized heat deposition is only feasible by varying electric fields locally. In this study, we vary local intensities of the internal electric field in skin, fat and muscle by altering its direction through modifying surface distributions of the applied voltage. In addition, we assess the long-term effects of these variations on tissue thermal transport. To this end, analytical solutions of the electric and bioheat equations were obtained using a regular perturbation method. For voltage distributions given by second- and eight-degree functions, the power absorption in fat is much greater than in skin by the electrode center while the opposite is true by the electrode edge. For a sinusoidal function, the absorption in fat varies laterally from greater to lower than in skin, and then this trend repeats from the center to the edge of the electrode. Consequently, zones of thermal confinement selectively develop in the fat layer. Generalizing these functions by parametrization, it is shown that radiofrequency (RF) heating of layered tissues can be selective and precisely localized by controlling the spatial decay, extent and repetition of the surface distribution of the applied voltage. The clinical relevance of our study is to provide a simple, non-invasive method to spatially control the heat deposition in layered tissues. By knowing and controlling the internal electric field, different therapeutic strategies can be developed and implemented.