Ultrasound-guided system for light focusing using microbubbles generated from polytetrafluoroethylene nanoparticles

Intrinsic lipid and protein components in biological tissues hamper the translation of light-related techniques to in vivo applications and highlight the need for strategies to improve photon trafficking. To meet this demand, we propose a method for the production of transient micro-sized bubbles to improve the local light intensity in the scattering medium by combining ultrasound and super hydrophobic polytetrafluoroethylene nanoparticles (PTFE NPs) with a mean size of 350 nm. The underlying idea was to convert the initial Rayleigh scattering effect into a Mie scattering effect to reduce loss during light propagation. The Monte Carlo simulation confirmed our hypothesis that only a specific size of microbubbles would increase light propagation. By fine-tuning the acoustic parameters (1.5 MPa, 100 cycles, and 10 Hz of pulse repetition frequency) and PTFE NPs' concentration (0.05 mg/ml), shell-less microbubbles were transiently and locally generated, resulting in a reduction in the scattering coefficient of the medium and improving the delivered light fluence by 6.2%. The experimental results further confirmed that only microbubbles of the proper size could repeatedly increase the light propagation. This method may be suitable for in vivo deep-tissue optical applications such as the delivery of PTFE NPs to deep tumor regions to increase the efficiency of photothermal therapy.