Biomedical Engineering Reference
In-Depth Information
on the understanding of structure-activity relationship, one is able to mimic the
active domain of AFGPs and synthesize antifreeze glycoproteins by using ligation
and polymerization strategies. However, further optimization of the chemistry, as
well as new routes to mimic AFPs and functional analogues are needed to allow the
routine production of quantities of pure material on commercially relevant scales.
Keywords
Ice nucleation • Ice crystal growth • Antifreeze protein • Thermal
hysteresis • Heterogeneous nucleation • Nucleation kinetics • Ice nucleator •
Cryoprotection • Morphological modification • AFP mimics
2.1
Introduction
2.1.1
Freezing and Antifreeze in Biological Systems
Water is the solvent of all biological systems, and the necessity of maintaining a
liquid state at the cellular level for life is obvious. For poikilotherms, the limits of
cellular function and life are set by the temperatures at which phase transitions of
water take place. Thermodynamically, when ice becomes a stable phase, the freezing
of water may occur spontaneously. Nevertheless, normally the freezing point of
either water or the body fluids of the organism is lower than 0
ı
C due to kinetic
factors.
The understanding of the freezing of water in microsized water droplets is crucial
in climate change, agriculture, food industries, and life sciences [
1
-
5
]. Freezing is a
process of ice crystallization from supercooled water. In this process, water should
undergo the stage of ice nucleation, followed by the growth of ice [
6
-
9
]. Nucleation
is the key step to determine if or not ice will occur in the supercooled water, while
the growth process will determine how big and what shape the ice crystallites will
acquire. Actually, whether or not freezing takes place is determined to a large
extent by ice nucleation. In other words, there would be no ice crystallization if
ice nucleation does not occur. Therefore, to control freezing one should first control
ice nucleation and the growth if nucleation control fails.
Due to the fact that ice nucleation is the initial and one of the most important
steps in the freezing process, the question of whether ice crystallization is governed
by homogeneous or heterogeneous nucleation bears significant implications for
antifreeze, ice nucleation promotion, and freezing-related phenomena and activities.
It was considered that spontaneous nucleation of ice from water, in particular below
40
ı
C, belongs to “homogeneous nucleation” [
2
-
5
,
10
,
11
]. This implies that
the freezing of water is considered to be clean and dusts or foreign bodies exert
no influence on nucleation. The key questions to be addressed are whether the
conclusion is correct, and if not, under what condition homogeneous nucleation
will take place. The latest results indicate that under a well-controlled condition,
the influence of dust particles on freezing is still effective below
40
ı
C[
12
]or
heterogeneous nucleation of ice still occurs below
40
ı
C. According to the recent
