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Yang et al. Soft Sci 2024;4:9 https://dx.doi.org/10.20517/ss.2023.43 Page 3 of 26

certain time. Also, the ice nucleation rate and growth rate highly depend on the degree of supercooling.
There is a most promotive temperature for these two processes, respectively. Moreover, As the temperature
drops, the dynamic viscosity of the solution will increase, and the diffusion coefficient will, thus, be
[20]
decreased, which is unfavorable to ice growth . When the temperature drops below a certain point, neither
of these two processes is favorable, rendering the system incapable of crystallization . Hence, as long as the
[21]
cooling rate of this system is fast enough so that its temperature drops below its glass transition temperature
(T ), no ice crystal will form in this system, and it will turn into a glass state. The T is also believed to
g
g
[22]
increase with a higher cooling rate , which means the system can more easily transit into a glass state with
a higher cooling rate. Therefore, it is comprehensible that high-concentration CPAs are always essential to
help realize vitrification: (1) They could decrease the melting point and increase the T of the system,
g
making it easier to pass through the temperature region that is favorable for ice nucleation and growth; (2)
High-concentration CPAs could increase the viscosity of the system, hindering the molecular locomotion
from the solution to the surface of ice nuclei.

From this perspective, it is comprehensible that high-concentration CPAs are essential to increase viscosity
and suppress ice nucleation and growth. However, the resuscitation of vitrified biospecimens is rather
challenging than cooling because the ice nuclei forming during the cooling process tend to expand and
burst during rewarming processes. Cooperatively, the cooling and warming processes both need to be rapid
enough to ensure the phase change to overcome ice formation. Usually, for a certain CPA recipe, the critical
[23]
warming rate (CWR) is several times higher than the critical cooling rate (CCR) . The gold-standard water
bath rewarming could hardly meet the criteria. Therefore, the idea of utilizing specific stimuli-responsive
material-mediated heating effects to heat up cryopreserved biospecimens was proposed. Manuchehrabadi
et al. developed and promoted a nano-warming strategy, which enabled ultra-fast rewarming of vitrified
biosamples. They took advantage of the inductive heating effect of iron oxide nanoparticles (IONPs) to
rewarm vitrified biosamples uniformly and rapidly and successfully maintained their viability in both 1-mL
and 50-mL systems . In the same year, a laser-induced photothermal (PT) conversion of gold nanorods
[24]
(GNR) was demonstrated to provide an ultrahigh warming rate and facilitate the vitrification of zebrafish
[25]
embryos . In 2020, they studied the rapid rewarming of tissues based on the inductive heating of thin
[23]
metal forms . These were encouraging advances; they not only displayed an ultra-rapid rewarming rate
that exceeded CWR by orders of magnitude but also showed the possibility of vitrification protocol applied
to large-volume specimens. However, this method still raises the issue of non-intimate contact between
solid metal forms and soft biological tissues [23,26] . Additionally, the process of synthesizing noble metal
nanoparticles is inherently complex and expensive. Because of the aforementioned reasons, there is an
urgency to explore a soft, flexible, easy-to-mold, and biocompatible material to supplement the vitrification
technique.

Liquid metals (LMs) with low melting points have been drawing increasing interest in the past decades.
Mercury is commonly used in routine life to be applied in thermometers, and its alloys and compounds
have also been developed as dental fillers, cosmetic additives, and medicines . Alkalis alloys also have
[27]
subzero melting points and have been applied as a heating source in hyperthermia therapy for tumors [28,29] .
However, these chemically unstable, hazardous substances require extra elaborative storage and operation,
which restricts the application scope in the biological field. Recently, several studies have spotlighted
gallium (Ga)-based LMs with superior chemical stability, low vapor pressure, and great biocompatibility .
[30]
The laboratory of the authors has been focused on studying fundamental behavior, characteristics, and
applications in multiple fields. Based on previous research on cryobiology, our laboratory has been
exploring the possibility of employing LMs in cryobiology to realize better outcomes. Thus, this review will
only give attention to Ga-based LMs.