Drug transport from blood to extravascular tissue can locally be achieved by increasing the vascular permeability through ultrasound-activated microbubbles. However, the mechanism remains unknown, including whether short and long cycles of ultrasound induce the same onset rate, spatial distribution, and amount of vascular permeability increase. Accurate models are necessary for insights into the mechanism so a microvessel-on-a-chip is developed with a membrane-free extravascular space. Using these microvessels-on-a-chip, distinct differences between 2 MHz ultrasound treatments are shown with 10 or 1000 cycles. The onset rate is slower for 10 than 1000 cycles, while both cycle lengths increase the permeability in spot-wise patterns without affecting microvessel viability. Significantly less vascular permeability increase and sonoporation are induced for 10 versus 1000 cycles at 750 kPa (i.e., the highest studied peak negative acoustic pressure (PNP)). The PNP threshold for vascular permeability increases is 750 versus 550 kPa for 10 versus 1000 cycles, while this is 750 versus 220 kPa for sonoporation. Vascular permeability increases do not correlate with αvβ3-targeted microbubble behavior, while sonoporation correlates with αvβ3-targeted microbubble clustering. In conclusion, the further mechanistic unraveling of vascular permeability increase by ultrasound-activated microbubbles in a developed microvessel-on-a-chip model aids the safe and efficient development of microbubble-mediated drug transport.

The blood vessel wall creates a barrier which can impair the transport of drugs from the blood to the underlying tissue. Lipid-coated gas microbubbles (diameter 1-10 μm) oscillate upon ultrasound application which can be used to locally enhance vascular permeability. However, the mechanism underlying this effect is poorly understood. Furthermore, it is yet to be discovered what ultrasound settings maximize the treatment outcome. This study aimed to create a microvessel-on-a-chip model to investigate the effects of ultrasound and microbubble treatment on vessel permeability and cell viability. Human microvascular endothelial cells were seeded against an extracellular matrix gel in the Organoplate® 3-lane and cultured for 4 days under bidirectional flow to form a 3D microvascular tube (300×220×2200 μm). The microvessels were treated with αvβ3-targeted microbubbles and 2 MHz ultrasound pulses of 10×10 or 10×1000 cycles, evenly spread over 30 s, and peak negative pressures ranging from 55-480 kPa. Controls included non-treated, microbubbles only, or ultrasound only. Permeability changes were investigated using 150 kDa FITC-dextran dye and fluorescent microscopy for 2 h. Cell viability was assessed using a WST-8 colorimetric assay which measures metabolic activity. Two hours after treatment, vascular permeability was only significantly higher for the microbubble and 480 kPa 10×10 cycles and 350 and 480 kPa 10×1000 cycles ultrasound treatments in comparison to all controls. In addition, within 5 min after treatment only the microbubble and 350 and 480 10×1000 cycles groups showed a clear leakage increase, suggesting an earlier onset of the treatment effect upon the 10×1000 cycles. Furthermore, the plateau of the leakage approached 100% for the 10×1000 cycles with microbubble groups whereas this was ~70% for the 480 kPa 10×10 cycles, indicating that the barrier loss was less with the short cycle’s treatment. The spatial leakage was unevenly distributed over the vessel which suggests that some vessel regions were more affected by the treatment than others. Finally, all treatments did not affect cell viability. These results show the potential of a microvessel-on-a-chip to investigate the mechanism and maximize the outcome of ultrasound and microbubble-mediated drug delivery treatments.