  The benefits of ultrasound are its ease-of-use and its ability to precisely
deliver energy in opaque and complex media. However, most materials responsive
to ultrasound show a weak response, requiring the use of high powers, which are
associated with undesirable streaming, cavitation, or temperature rise. These
effects hinder response control and may even cause damage to the medium where
the ultrasound is applied. Moreover, materials that are currently in use rely
on all-or-nothing effects, limiting the ability to fine-tune the response of
the material on the fly. For these reasons, there is a need for materials that
can respond to low intensity ultrasound with programmable responses. Here it is
demonstrated that antibubbles are a low-intensity-ultrasound-responsive
material system that can controllably release a payload using acoustic
pressures in the kPa range. Varying their size and composition tunes the
release pressure, and the response can be switched between a single release and
stepwise release across multiple ultrasound pulses. Observations using confocal
and high-speed microscopy revealed different ways that can lead to release.
These findings lay the groundwork to design antibubbles that controllably
respond to low-intensity ultrasound, opening a wide range of applications
ranging from ultrasound-responsive material systems to carriers for targeted
delivery.
