Biomedical Engineering Reference
In-Depth Information
the freezing point has been depressed [
44
]. This is because a change in morphology
can have different causes and can be accompanied by total protein inactivity [
5
,
84
,
95
]. The particular ice morphology observed is a direct consequence of the structural
details related to the adsorption mechanism of the AFP on the ice surfaces, such
that different modes of adsorption trigger different crystal habits consistently and
predictably.
2.4
Applications of AFPs
Any organic compound with the ability to inhibit the growth of ice has many
potential medical, industrial, and commercial applications. Although AFPs have
potential uses in all of these areas, this review highlights the medical [
96
,
97
]and
food [
98
] applications of AFPs.
2.4.1
Cryoprotection
Many complex processes occur when the cell ambient temperature is lowered near
to or below that of the freezing point. Physical cell rupture, necrosis, and cold-
induced apoptosis are presently the three distinct modes of cell death that occur upon
freezing [
99
]. Although all three processes are significant, the most common form
of cell death associated with cryopreservation is cell rupture owing to fluctuating
cell volumes and intracellular ice formation [
100
-
104
] and, consequently, in this
section cell damage will be emphasized because of membrane rupture followed by
intracellular ice formation.
Much effort has been dedicated to developing enhanced cryoprotectants and
preservation techniques after an accidental discovery that glycerol enabled fowl
spermatozoa to survive freezing at
70
ı
C. All cells are regarded as compartmen-
talized systems and the probability of ice nucleation is directly proportional to the
degree of supercooling and volume. Consequently, as the temperature is lowered, ice
nucleation is more likely to occur outside of the cell since the volume is greater and
the concentration of colligatively acting substances (salts, proteins, and so forth) is
lower than inside the cell. After nucleation occurs, extracellular ice growth results in
an increase in solute concentration in the diminishing extracellular volume. As the
concentration of these solutes increases, extracellular osmotic pressure increases.
The rate at which osmotic pressure increase occurs is directly proportional to the
rate of supercooling and this osmotic flux is compensated by the cell through
the regulation in the flow of water through the semipermeable cell membrane.
Fracturing of the cell membrane is possible when the rate of extracellular ice growth
is rapid. Upon the occurrence of the fracture, intracellular ice formation occurs
leading to the destruction of the cell. Debates have been going on by researchers as