Biomedical Engineering Reference
In-Depth Information
Moreover, Chu et al. [
58
] have studied the synthesis of ACC films using an acid
polysaccharide as an additive by using the gas diffusion method. The thin ACC
film is formed directly onto TEM grids by floating the grids onto a crystallization
solution because of the rapid transformation of ACC into its crystalline phase.
Thus, they have successfully used TEM in combination with SAED to characterize
the mesostructure of the formed ACC films. Meldrum and co-workers [
59
]also
demonstrated that synthetic ACC can be produced with different short-range
structures by using the gas diffusion method, according to the solution additives
used. Their data suggested that the influence of these additives (Pasp or Mg) on ACC
formation can be apparent even in the structure of the first-formed precursor phase.
It would be useful to investigate the interplay of different additives in stabilizing and
directing the crystallization of synthetic ACC.
6.2.3
Hydrolysis of Carbonate Source Method
Faatz and co-workers reported a method in which the release of carbon dioxide
by hydrolysis of the carbonate source (dialkyl carbonate) [
60
]. When dialkyl
carbonates are hydrolyzed isothermally at a settled pH in an aqueous solution of
calcium chloride, CaCO
3
is precipitated slowly, on a timescale of minutes to hours.
The experiment was carried out starting with dilute solutions of calcium chloride
(0.01 M) in water and an excess of dimethyl carbonate. When the solution of
sodium hydroxide (0.5 M) was added, the solution started to become turbid after
90 s at 298 K. After 150 s, the precipitate was removed from the reaction mixture
by centrifugation, then washed with acetone and dried under vacuum. This result
demonstrates that ACC can be synthesized at room temperature in aqueous media
without any additives such as polyphosphonate or magnesium salts that can prevent
crystallization [
60
].
The size of the ACC particles can be controlled by using this method when a
double hydrophilic block copolymer (DHBC) was added in the reaction [
61
], which
is composed of blocks of polyethylene oxide and PAA. The optimally designed
DHBCs can control the particle size of ACC at concentrations corresponding to
a monolayer coverage of the liquid precursor droplets. The optimal concentration
in DHBC was found to be between 3 and 10 ppm, and ACC nanoparticles of 60-
100 nm in diameter could be formed.
In addition, the decomposition of urea by catalytically active Canavalia ensi-
formis urease in a solution of magnesium and calcium salts can be used to precipitate
uniform nanostructured ACC particles [
62
]. The reaction solutions containing
magnesium, calcium chloride, and urea were kept in tightly stoppered Erlenmeyer
flasks and saturated with nitrogen before a measured amount of urease solution
was injected. Then, the mixtures were kept at room temperature 2 min with gentle
stirring. The obtained ACC were sequentially filtered and washed with deionized
water to remove the excess of magnesium and calcium salts and urease. This
procedure prevents the formation of a new solid phase.
