from structure conformation into biomimic preparation, from theory analysis into
biomimetic synthesis, from cell mediation to gene mediation ((Davis 2004);
(Ameye et al. 2001); (Hunter 1996); (Choi & Kim 2000); (Ogasawara et al.
2000); (Sarikaya et al. 1999); (Mann 1996)).
Calcium-based materials are the most important inorganic phase of biomineralization systems. Calcium carbonate is one of the most important biominerals that
have been a research focus for decades. There are three crystal forms of anhydrous
calcium carbonate: calcite, aragonite and vaterite, and their crystal systems are
rhombohedral, orthorhombic, and hexagonal, respectively (de Leeuw & Parker
1998). In aqueous system at 25
C, they have decreasing stabilities and increasing
solubility limits. Their solubility constant (K sp ) values are 10
À8.48 , 10
À8.34
, and
10
À7.91 (Plummer & Busenberg 1982), respectively. Geological minerals of calcium carbonate are almost all calcite with a few aragonites, since its Gibbs free
energy is the lowest of the three. In biominerals, calcite and aragonite are the most
common forms of calcium carbonate crystals, which exist mainly in mollusk shells
and bird eggs (de Leeuw & Parker 1998). Vaterite is metastable, the most unstable
crystal form of calcium carbonate, and would automatically transform into calcite
or aragonite in aqueous solution. But in natural systems like carp asteriscus (Li &
Feng 2007) and lackluster pearls (Hang 1994) in fresh water, it was found that
vaterite could exist stably. Based on their unique characteristics, vaterite and
amorphous calcium carbonate (ACC) have become hot research spots in recent
years. In order to understand how protein matrices mediate calcite, aragonite, and
vaterite in living organisms, and how complicated microstructures are formed,
many experiments were processed in vitro to simulate biomineralization.
6.4.1 Effects of Additives on Calcium Carbonate Mineralization
6.4.1.1 Soluble Matrices (SM) in the Solution as Additives
In most biomaterials, inorganic components are dominant with over 95% of the
mass or volume. Protein matrices, microelements, polysaccharides, and others
share the rest 5%. Studies for years have proved that proteins are the most important
influence factors in the formation of calcium carbonate crystals. One of the most
commonly used method to study the effect of proteins on mineralization of calcium
carbonate crystals is to extract, separate, add them into simulation systems, and to
study their functions under different conditions in vitro.
In previous studies, organic matrices were extracted by their solubility in sodium
ethylenediaminetetraacetic acid (EDTA), and were classified as insoluble matrix
(IM) proteins and soluble matrix (SM) proteins. IM and SM are found to have
different roles in controlling the polymorph and morphology of calcium carbonate.
IM proteins are normally structure molecules, and provide SM joint sites as
substrates. In the mineralization process, IM could control crystal growth, which
influences the size and orientation of the crystals. SM proteins are mainly composed
170
Q. Feng
biomimetic synthesis, from cell mediation to gene mediation ((Davis 2004);
(Ameye et al. 2001); (Hunter 1996); (Choi & Kim 2000); (Ogasawara et al.
2000); (Sarikaya et al. 1999); (Mann 1996)).
Calcium-based materials are the most important inorganic phase of biomineralization systems. Calcium carbonate is one of the most important biominerals that
have been a research focus for decades. There are three crystal forms of anhydrous
calcium carbonate: calcite, aragonite and vaterite, and their crystal systems are
rhombohedral, orthorhombic, and hexagonal, respectively (de Leeuw & Parker
1998). In aqueous system at 25
C, they have decreasing stabilities and increasing
solubility limits. Their solubility constant (K sp ) values are 10
À8.48 , 10
À8.34
, and
10
À7.91 (Plummer & Busenberg 1982), respectively. Geological minerals of calcium carbonate are almost all calcite with a few aragonites, since its Gibbs free
energy is the lowest of the three. In biominerals, calcite and aragonite are the most
common forms of calcium carbonate crystals, which exist mainly in mollusk shells
and bird eggs (de Leeuw & Parker 1998). Vaterite is metastable, the most unstable
crystal form of calcium carbonate, and would automatically transform into calcite
or aragonite in aqueous solution. But in natural systems like carp asteriscus (Li &
Feng 2007) and lackluster pearls (Hang 1994) in fresh water, it was found that
vaterite could exist stably. Based on their unique characteristics, vaterite and
amorphous calcium carbonate (ACC) have become hot research spots in recent
years. In order to understand how protein matrices mediate calcite, aragonite, and
vaterite in living organisms, and how complicated microstructures are formed,
many experiments were processed in vitro to simulate biomineralization.
6.4.1 Effects of Additives on Calcium Carbonate Mineralization
6.4.1.1 Soluble Matrices (SM) in the Solution as Additives
In most biomaterials, inorganic components are dominant with over 95% of the
mass or volume. Protein matrices, microelements, polysaccharides, and others
share the rest 5%. Studies for years have proved that proteins are the most important
influence factors in the formation of calcium carbonate crystals. One of the most
commonly used method to study the effect of proteins on mineralization of calcium
carbonate crystals is to extract, separate, add them into simulation systems, and to
study their functions under different conditions in vitro.
In previous studies, organic matrices were extracted by their solubility in sodium
ethylenediaminetetraacetic acid (EDTA), and were classified as insoluble matrix
(IM) proteins and soluble matrix (SM) proteins. IM and SM are found to have
different roles in controlling the polymorph and morphology of calcium carbonate.
IM proteins are normally structure molecules, and provide SM joint sites as
substrates. In the mineralization process, IM could control crystal growth, which
influences the size and orientation of the crystals. SM proteins are mainly composed
170
Q. Feng