Theoretical calculation and experimental demonstration of differential heating for conductive nanoparticles in lossy biological media under radio frequency irradiation

Nanoparticles that strongly absorb radio frequency (RF) energy are desirable for techniques that require a wireless heat source deep within tissue, such as hyperthermia. While many studies have focused on the RF absorption of spherical metallic nanoparticles, opportunities afforded by high-aspect-ratio nanomaterials have not been sufficiently explored. We use the electrostatic approximation to calculate the relative-absorption ratio of metallic nanoparticles implanted in various biological tissues from 1 MHz to 10 GHz. We find that high-aspect-ratio prolate spheroids (approximating nanowires) offer powerful absorption of RF compared to the surrounding tissue, while oblate spheroids and spherical nanoparticles offer minimal relative absorption. These results inform our subsequent experiments with conductive carbon nanotubes (CNTs). Our sonication-free preparation preserves the high aspect ratio and local concentration of "pristine" CNTs. We demonstrate a 4.5-fold increase in heating of CNTs under 2 GHz irradiation compared to a saline solution, and we show localized differential heating of pristine CNTs injected into a tissue phantom. This work provides a generalized theoretical platform for determining the relative RF absorption of sub-wavelength particles compared to a tissue background.