[4]. In the future, vitrification may be used to trap radioactive waste
to prevent it from escaping into the biosphere [5, 6]. On the other
hand, the potential biological significance of vitrification has been
seriously contemplated for only 83 years [7].
The possibility of vitrifying water was postulated as long ago as
1860 [8]. The successful vitrification of small quantities of 0.1 M
CuCl 2 and even pure water by ultrarapid cooling was reported in
1980 [9], and in 1981 the vitrification of quenched 1 μm droplets
of pure water was claimed on the basis of an absence of visible ice
crystals in electron microscopic images [10]. But the apparently
successful use of vitrification to preserve biological viability was
most unambiguously achieved even earlier, in 1968, when human
erythrocytes were vitrified in a rapidly cooled aqueous solution of
~5.3 molar glycerol and remained intact after rewarming [11].
In the context of cryopreservation, vitrification is induced by
cooling, which in non-freezing aqueous solutions eventually elevates viscosity to ~10
13 poise, at which point the liquid is considered to have reverted to the glassy or vitreous state [2, 12]. In other
contexts, vitrification, or something very close to it, can also be
achieved by drying, and some organisms [13, 14] and many proteins [15, 16] can be preserved successfully in this way.
The ability of vitrification to preserve molecules, cells, tissues,
whole organs, and even some whole organisms has many agricultural, medical, scientific, and ecological ramifications. The application of vitrification to cryopreservation has been growing
exponentially since the early 1980s ([17–20] and Fig. 1 [21]) and
may eventually enable the preservation even of systems as complex
and massive as whole human organs for transplantation [17, 20–
23]. Given the broad potential biological relevance of vitrification,
an understanding of the basic principles of vitrification is becoming
increasingly important.
1.2 Basic
Terminology
and Concepts
The “glass transition temperature,” or T G , is the temperature at
which vitrification, the transition from a liquid-like state into the
glassy state, takes place on cooling; it is also the temperature at
which the glassy state reverts to a liquid-like state upon warming.
T G is usually defined on the basis of a change in heat capacity
detected by, for example, differential scanning calorimetry (DSC).
T G can be measured during either cooling or warming, but,
whereas passage through T G during cooling is called “vitrification,”
there is no generally accepted word that describes the reverse of
vitrification, i.e., the onset of liquid-like behavior as the temperature is raised from below T G to above T G . The terms “vitromelting,” [24] “vitrofusion” [25], and “liquefaction” [26] have been
suggested to describe this transition, but they have not been
adopted.
In aqueous systems that freeze, freezing concentrates the
remaining unfrozen liquid phase. In the theoretical case in which
28
Gregory M. Fahy and Brian Wowk
to prevent it from escaping into the biosphere [5, 6]. On the other
hand, the potential biological significance of vitrification has been
seriously contemplated for only 83 years [7].
The possibility of vitrifying water was postulated as long ago as
1860 [8]. The successful vitrification of small quantities of 0.1 M
CuCl 2 and even pure water by ultrarapid cooling was reported in
1980 [9], and in 1981 the vitrification of quenched 1 μm droplets
of pure water was claimed on the basis of an absence of visible ice
crystals in electron microscopic images [10]. But the apparently
successful use of vitrification to preserve biological viability was
most unambiguously achieved even earlier, in 1968, when human
erythrocytes were vitrified in a rapidly cooled aqueous solution of
~5.3 molar glycerol and remained intact after rewarming [11].
In the context of cryopreservation, vitrification is induced by
cooling, which in non-freezing aqueous solutions eventually elevates viscosity to ~10
13 poise, at which point the liquid is considered to have reverted to the glassy or vitreous state [2, 12]. In other
contexts, vitrification, or something very close to it, can also be
achieved by drying, and some organisms [13, 14] and many proteins [15, 16] can be preserved successfully in this way.
The ability of vitrification to preserve molecules, cells, tissues,
whole organs, and even some whole organisms has many agricultural, medical, scientific, and ecological ramifications. The application of vitrification to cryopreservation has been growing
exponentially since the early 1980s ([17–20] and Fig. 1 [21]) and
may eventually enable the preservation even of systems as complex
and massive as whole human organs for transplantation [17, 20–
23]. Given the broad potential biological relevance of vitrification,
an understanding of the basic principles of vitrification is becoming
increasingly important.
1.2 Basic
Terminology
and Concepts
The “glass transition temperature,” or T G , is the temperature at
which vitrification, the transition from a liquid-like state into the
glassy state, takes place on cooling; it is also the temperature at
which the glassy state reverts to a liquid-like state upon warming.
T G is usually defined on the basis of a change in heat capacity
detected by, for example, differential scanning calorimetry (DSC).
T G can be measured during either cooling or warming, but,
whereas passage through T G during cooling is called “vitrification,”
there is no generally accepted word that describes the reverse of
vitrification, i.e., the onset of liquid-like behavior as the temperature is raised from below T G to above T G . The terms “vitromelting,” [24] “vitrofusion” [25], and “liquefaction” [26] have been
suggested to describe this transition, but they have not been
adopted.
In aqueous systems that freeze, freezing concentrates the
remaining unfrozen liquid phase. In the theoretical case in which
28
Gregory M. Fahy and Brian Wowk