Ultrasound Transducers for Acoustic Rheometry and Microscopic Histotripsy

Abstract

Focused ultrasound is applied non-invasively using transducers. In this dissertation, custom transducers are designed and tested to track coagulation, observe cavitation-cell interactions, and cause other mechanical disruptions.

First, a histotripsy transducer is developed for use on a microscope stage. Histotripsy is mechanical ablation of tissue using high negative pressure to create cavitation bubble clouds. The custom transducer had 5 MHz center frequency and a focal area of 0.25 by 0.6 mm which was fully captured within the microscope’s field of view. Individual bubble clouds had a lifespan between 15 and 40 us. Cavitation was recorded in interconnected fibroblast cells in fibrin gel, which demonstrated reactions to the applied force. The first functional cell death assay immediately following histotripsy was also performed, corroborating that mechanical ablation is safe for surrounding cells while acutely damaging the target area.

Next, histotripsy was used for targeted removal of cells from glass to control tissue organization, called cell sculpting. Stem cell differentiation and other mechanosensitive cell patterning can benefit from the ability to locally remove plated cells. Using the same 5 MHz microscope transducer, the 2D cell removal area was precisely controlled. Interestingly, ultrasound transmission through thin glass also removed cells from within a microfluidic chamber.

Finally, a multichannel system is developed for improved resonant acoustic rheometry (RAR). This newly developed technique characterizes materials without contact and in small sample volumes. The custom system uses four 5 MHz transducers, allowing multiple simultaneous sample measurements from a well plate. Here, the RAR system was demonstrated for coagulation monitoring of human plasma with varied levels of anticoagulant medication. Clotting start time and clotting duration followed trends of anticoagulation levels.