  The development of methods to safely rewarm large volume cryopreserved
biological samples remains a barrier to the widespread adoption of
cryopreservation. Here, experiments and simulations were performed to
demonstrate that ultrasound can increase rewarming rates relative to thermal
conduction alone. An ultrasonic rewarming setup based on a custom 444 kHz
tubular piezoelectric transducer was designed, characterized, and tested with 2
mL cryovials filled with frozen ground beef. Rewarming rates were characterized
in the -20$^{\circ}$C to 5$^{\circ}$C range. Thermal conduction-based rewarming
was compared to thermal conduction plus ultrasonic rewarming, demonstrating a
ten-fold increase in rewarming rate when ultrasound was applied. The maximum
recorded rewarming rate with ultrasound was 57$^{\circ}$C per minute,
approximately 2.5 times faster than with thermal conduction alone. Coupled
acoustic and thermal simulations were developed and showed good agreement with
the heating rates demonstrated experimentally and were also used to demonstrate
spatial heating distributions with small ($<$3$^{\circ}$C) temperature
differentials throughout the sample when the sample was below 0$^{\circ}$C. The
experiments and simulations performed in this work demonstrate the potential
for ultrasound as a rewarming method for cryopreserved tissues, as faster
rewarming rates may improve the viability of cryopreserved tissues and reduce
the time needed for cells to regain normal function.
