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BIOLOGICAL MATERIALS
hardened form. Bone contains, for example, many rod-shaped inorganic mineral
crystals with typical 5 nm diameters, and lengths ranging from 20 to 200 nm.
The kinetics for the self-assembly of many of these films involved in
biomineralization can be approximately modeled as the initial joining together or
dimerization of two monomers
R + R + R ,
(12.7)
with a low equilibrium constant KD = CD/(cF)’, followed by the stepwise or
sequential addition of more monomers
(12.8)
with a much larger equilibrium constant K = Cn+l/CFC,,. These two constants
exercise control over the rate at which the reaction proceeds. For the case under
consideration, KD < K, the concentration of free or unbound monomers C, always
remains below a critical concentration Co = 1 /K, specifically, C, < Co. When the
total concentration of free and clustered (Le., bound) monomers CT satisfies the
condition CT < Co , then the free monomer concentration C, increases with increases
in CT. When a high enough concentration is provided so that CT becomes larger than
the critical value (i.e., CT > Co), then the aggregate forms and grows for further
increases in CT. In analogy with this model, self-assembly kinetics often involves a
slow dimer formation step followed by faster propagation steps, with KD << K.
There are many cases of multilayer thin films in biology, such as structural colors
in insects that change when the films are subjected to pressure, shrinking, or
swelling. For example, scale cells from some butterflies can produce iridescent
multicoloring affects due to optical interference of thin-film layers or lamellae
formed from the secretion of networks of filaments that condense on cell boundaries.
The biomineralization of mollusc cells begins by laying down a sheet of organic
material so that calcium carbonate can be deposited on its surface and in its pores,
and CaC03 layers can build up. Proteins from the mollusc shell containing high
concentrations of particular amino acid residues control the form of the calcium
carbonate layering, and these proteins can be altered to vary the layering morphology. Multiple layers either grow in sequences that are organic in nature or contain the
rhombohedral calcite form, or the orthorhombic aragonite variety of CaC03. It is
possible to imitate some aspects of these natural biomineralization processes for the
preparation of synthetic multilayer thin films, although the resulting films themselves
do not closely resemble those in molluscs. For example, consider a positively
charged substrate placed in a solution with a negative electrolyte, that is, a solution
containing negative ions that can carry electric current. The positive substrate
attracts the negative electrolyte, and the latter can adsorb on its surface, forming a
structure called a polyion sheet, as shown in Fig. 12.16. This sheet is rinsed and
dried, and then placed into another electrolyte solution from which it adsorbs a
second positive layer. The sequential adsorption process can be repeated, as
indicated in Fig. 12.16, to form a multilayer of alternating positively and negatively
BIOLOGICAL MATERIALS
hardened form. Bone contains, for example, many rod-shaped inorganic mineral
crystals with typical 5 nm diameters, and lengths ranging from 20 to 200 nm.
The kinetics for the self-assembly of many of these films involved in
biomineralization can be approximately modeled as the initial joining together or
dimerization of two monomers
R + R + R ,
(12.7)
with a low equilibrium constant KD = CD/(cF)’, followed by the stepwise or
sequential addition of more monomers
(12.8)
with a much larger equilibrium constant K = Cn+l/CFC,,. These two constants
exercise control over the rate at which the reaction proceeds. For the case under
consideration, KD < K, the concentration of free or unbound monomers C, always
remains below a critical concentration Co = 1 /K, specifically, C, < Co. When the
total concentration of free and clustered (Le., bound) monomers CT satisfies the
condition CT < Co , then the free monomer concentration C, increases with increases
in CT. When a high enough concentration is provided so that CT becomes larger than
the critical value (i.e., CT > Co), then the aggregate forms and grows for further
increases in CT. In analogy with this model, self-assembly kinetics often involves a
slow dimer formation step followed by faster propagation steps, with KD << K.
There are many cases of multilayer thin films in biology, such as structural colors
in insects that change when the films are subjected to pressure, shrinking, or
swelling. For example, scale cells from some butterflies can produce iridescent
multicoloring affects due to optical interference of thin-film layers or lamellae
formed from the secretion of networks of filaments that condense on cell boundaries.
The biomineralization of mollusc cells begins by laying down a sheet of organic
material so that calcium carbonate can be deposited on its surface and in its pores,
and CaC03 layers can build up. Proteins from the mollusc shell containing high
concentrations of particular amino acid residues control the form of the calcium
carbonate layering, and these proteins can be altered to vary the layering morphology. Multiple layers either grow in sequences that are organic in nature or contain the
rhombohedral calcite form, or the orthorhombic aragonite variety of CaC03. It is
possible to imitate some aspects of these natural biomineralization processes for the
preparation of synthetic multilayer thin films, although the resulting films themselves
do not closely resemble those in molluscs. For example, consider a positively
charged substrate placed in a solution with a negative electrolyte, that is, a solution
containing negative ions that can carry electric current. The positive substrate
attracts the negative electrolyte, and the latter can adsorb on its surface, forming a
structure called a polyion sheet, as shown in Fig. 12.16. This sheet is rinsed and
dried, and then placed into another electrolyte solution from which it adsorbs a
second positive layer. The sequential adsorption process can be repeated, as
indicated in Fig. 12.16, to form a multilayer of alternating positively and negatively