Targeted drug delivery has always been a keen interest of mine. Having seen first hand the effect of chemicals such as chemotherapy drug being delivered to the whole body rather than just the tumour, I am excited by the possibility of being able to prevent these drugs from affecting healthy tissue. Microbubbles have been used widely in targeted drug delivery and contrast imaging. However, their relative instability is a key disadvantage. Enter the nanodroplet: a particle of diameter ~200nm with a perfluorocarbon core and phospholipid shell. The smaller size allows them to be more stable and less likely to be removed by the immune system.

It has been shown that the superheated stability of nanodroplets enables them to undergo acoustic droplet vaporisation (ADV) to become a microbubble. ADV occurs due to an applied ultrasonic sound wave. Sound waves occur due to changes in pressure in the surrounding media. Areas of increased pressure occur as a result of the particles being compressed together. Areas of decreased pressure occur due to the particles spreading further apart in a phenomenon known as rarefaction. Rarefaction results in a decrease in pressure in the nanodroplet. If the pressure drop is enough then the required vaporisation temperature will reduce to the surrounding temperature allowing the nanodroplet to vaporise into a microbubble of diameter roughly 2.5-3.5 times the diameter of the original nanodroplet. If the peak negative pressure is large enough, the resulting microbubble is able to undergo inertial cavitation where the microbubble diameter doubles then violently collapses due to the inertia of the surrounding fluid which increases delivery of gases dissolved in the encapsulated perfluorocarbon by either increasing “leakage” from the microbubble or increasing permeability though the endothelium.

Overall, I feel nanodroplets are a promising new technology in the field of targeted drug delivery. They have the unique ability to solve the challenges posed by current microbubble therapy without completely redesigning the wheel.