Biomedical Engineering Reference
In-Depth Information
is usually kept at 40-60°C higher than the melting temperature before spraying so that the
overall operating temperature does not go beyond 150°C. The suggested spray rate for a
uniform coating is at 30 g/min. To ensure good coverage of the core, it has been reported
that the optimal size of the core shall be 0.1-0.75 mm in diameter [111].
Supercritical Fluid Coating
The supercritical state is defined as a state where both the pressure and temperature of a
substance are greater than its critical pressure and critical temperature [112]. Carbon dioxide
is the most commonly used supercritical solution for industrial applications because of its
moderate critical pressure (72 bar) and critical temperature (31.1°C) [113]. The supercritical
fluid coating works as follows: coating materials are dissolved in the supercritical solution
in a high-pressure vessel; insoluble core particles are dispersed in the supercritical solution
as well. A rapid expansion of volume of the vessel decreases the pressure quickly, and
supercritical solution becomes less “supercritical”; hence, the dissolved coating material
precipitates and coats onto the core particles. It is apparent that a key requirement of the
supercritical solution coating technique is that the coating material and the core particles
must have different solubility in the supercritical solution. Cosolvent can be added into the
supercritical solution to enhance the solubility of either coating material or the core.
Electrospinning
Electrospinning can be used for coating preparation. The distinct feature of electrospun
coating is that it is composed of fibers down to the size of submicron in diameter. In this
process, a high voltage (in the order of kilovolts) is applied across the spinning solution
(usually in a syringe) and a collector (distance between them is altered to reach differ-
ent electric field strength). The spinning solution is directed to the collector by the high
electric field and syringe pump. Yu et al. [114] used the electrospinning to fabricate a poly-
used the electrospinning to fabricate a poly-
vinylpyrrolidone fiber film that exhibits a fast dissolving process. Drug can be simultane-
ously loaded into the film by adding drug molecules into the spinning solution.
Another technique that uses electrostatic force is known as electrostatic spray powder
coating. Instead of using solution, in the electrostatic spray powder coating method, dry
powders of the coating material are charged at a high voltage (100 kV) [115], and the sub-
strate is grounded. The powder is carried by the air though an electrostatic powder gun
to the substrate. Once the powders are coated onto the substrate by electrostatic force, the
coating material is molten by heat so as to complete the coating process. The resistivity of
the core has to be less than 10
9
Ω
m, whereas the resistivity of the powder should be more
than 10
11
Ω
m [116]. A key parameter in this technique is the powder size of the coating
material, with 30-100 µm being the optimal.
[114] used the electrospinning to fabricate a poly-
114] used the electrospinning to fabricate a poly-
] used the electrospinning to fabricate a poly-
Precipitation Method
Precipitation coating method has been widely used for many different materials. Core is
dispersed in coating precursor solution. Precipitation of the precursor is induced by pH,
adding antisolvent or using supersaturation solution. Xu and Czernuszka [117] used pre-
used pre-
cipitation method to coat PLGA microspheres with a layer of HA for release rate control.
PLGA suspended water was added into a supersaturated solution of HA precursor. HA
would precipitate and deposit on the PLGA microspheres, and the onset of the experi-
[117] used pre-
117] used pre-
] used pre-
, and the onset of the experi-
ment would induce a drop in pH. It was reported that a more negative surface facilitated
s, and the onset of the experi-
