Perspectives on cavitation enhanced endothelial layer permeability

Traditional drug delivery systems, where pharmaceutical agents are conveyed to the target tissue through the blood circulation, suffer of poor therapeutic efficiency and limited selectivity largely due to the low permeability of the highly specialised biological interface represented by the endothelial layer. Examples concern cancer therapeutics or degenerative disorders where drug delivery is inhibited by the blood-brain barrier (BBB). Microbubbles injected into the bloodstream undergo volume oscillations under localised ultrasound irradiation and possibly collapse near the site of interest, with no effect on the rest of the endothelium. The resulting mechanical action induces a transient increase of the inter-cellular spaces and facilitates drug extravasation. This approach, already pursed in in vivo animal models, is extremely expensive and time-consuming. On the other hand in vitro studies using different kinds of microfluidic networks are firmly established in the pharmaceutical industry for drug delivery testing. The combination of the in vitro approach with ultrasound used to control microbubbles oscillations is expected to provide crucial information for developing cavitation enhanced drug delivery protocols and for screening the properties of the biological interface in presence of healthy or diseased tissues. Purpose of the present review is providing the state of the art in this rapidly growing field where cavitation is exploited as a viable technology to transiently modify the permeability of the biological interface. After describing current in vivo studies, particular emphasis will be placed on illustrating characteristics of micro-devices, biological functionalisation, properties of the artificial endothelium and ultrasound irradiation techniques.