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Figure 2. In an ARS, bubble dynamics during and after ADV were dependent on the perfluorocarbon (PFC) liquid in the phase- shift emulsion. Longitudinal images, which were taken using ultra-high-speed microscopy, are shown for ARSs with emulsions containing perfluoropentane (C5F12; top), perfluorohexane (C6F14; center), or perfluorooctane (C8F18; bottom). In each series of images, a single droplet is shown during three stages: before (left), during (center), and after (right) ADV. During ADV, ultrasound caused formation of vapor in the PFC phase. Note the differences in bubble dynamics once the ultrasound is turned off (i.e., after ADV). A stable bubble was formed with C5F12 and C6F14. With C8F18, the generated bubble recondensed. with smaller emulsions. Additionally, unlike other appli- cations that often utilize lower molecular weight (and hence higher volatility) PFCs like perfluorobutane (i.e., C4F10) (Sheeran et al., 2016) or perfluoropentane (i.e., C5F12) (Mercado-Shekhar et al., 2019), emulsions in ARSs are typically formulated with higher boiling point PFCs like perfluorohexane (i.e., C6F14) or perfluorooctane (i.e., C8F18). These higher molecular weight PFCs offer better thermal stability by eliminating the potential for sponta- neous bubble formation. Higher molecular weight PFCs also yield interesting bubble dynamics during and after ADV that can be utilized for specific applications. For example, emulsions with lower molecular weight PFCs undergo irreversible vaporization, where a stable bubble is formed (Figure 2); this yields a complete release of a drug loaded within the emulsion. Comparatively, with higher molecular weight PFCs, the generated bubble recondenses; this yields partial release of a drug. Building Blood Vessels Using Bubbles Conventional treatments for ischemic cardiovascular dis- ease, which is characterized by insufficient blood flow, include bypass surgery and endovascular procedures. With the former, a healthy blood vessel is harvested from the patient’s body and surgically connected to circumvent a blocked blood vessel. With the latter, a catheter is used to remove the occluding material (e.g., atherosclerotic plaque) in the blocked vessel, and removal is sometimes followed by the installation of a stent in the vessel. How- ever, current treatments for cardiovascular disease are insufficient. For example, with critical limb ischemia, an advanced stage of peripheral artery disease character- ized by poor blood flow in the leg, 25% of patients are ineligible for current treatments because of other medical issues and 29% of patients will either die or undergo a major amputation within one year of diagnosis. Due to issues associated with standard interventions, alternative treatments are constantly being sought. One approach being investigated is the use of proteins to stimulate blood vessel growth. However, despite success in animal models, clinical translation has remained a challenge for multiple reasons. One critical limitation is that simply injecting the proteins into the body, in either the bloodstream or muscle, is ineffec- tive and can cause serious side effects. Incorporating Summer 2022 • Acoustics Today 17