Biomedical Engineering Reference
In-Depth Information
formed. Hamed and Sakr [104] studied the effect of plasticizer and curing temperature on
drug release behavior: higher plasticizer level decreases the glassy transition temperature;
hence, a complete coating can be formed with a lower curing temperature. It was also
found that a suitable curing time is needed to complete the film forming process, which
reduces the drug release rate; however, if curing time is too long, drug in the core will
migrate to the coating, which results in a burst release. One of the most commonly encoun-
coating, which results in a burst release. One of the most commonly encoun-
tered problems in fluid-bed coating is agglomeration, which is caused by liquid bridges
between particles [105]. The formation of liquid bridges depends on the coating solution
and the size of the spray mist. It was found that adding NaCl could reduce the particle
agglomeration due to the reduction of viscosity of the coating solution by salting-out effect
[106]. A smaller mist size is preferred to suppress agglomeration, which can be realized
by a smaller spray-particle distance, a higher inlet temperature, and higher fluidization
air volume [139]. Similar to fluid bed, pan coater is commonly used in pharmaceutical
research and industry for film coating. In a pan coater, tablets are placed on a bed rather
than being fluidized in the fluid-bed coating system. Coating materials are still applied by
spray, and the tablets are tumbled to ensure complete coating on the tablets.
Compression Coating
Coating can be applied to the core by direct compression of the coating materials. Coating
materials are first introduced into a mold, and then the core is placed in the mold. Additional
coating materials are fed into the mold to cover the core. The compression is done by two
punches at a given pressure to form the coating. Lin et al [107] used compression coating
technique to coat ethylcellulose on an inorganic drug. Two compression forces were
evaluated: inner-core compression force and the coating compression force. It was found
that at a constant coating compression force (300 kg/m
2
), a higher inner-core compression
force could slow the dissolution of the drug, whereas at a constant inner-core compression
force (200 kg/m
2
), a higher coating compression force results in a longer lag time. Besides
ethylcellulose, another commonly used polymer in compression coating is acrylic-based
polymers (marketed under the brand of Kollicoat and Eudragit). The common problem
encountered in compression coating is the damage to the coating due to high compression
force. Dashevsky and coworkers [108] observed that drug was released faster with a higher
compression force.
Plasticizer Dry Coating
Coating polymers and plasticizer are sprayed onto the core materials from different nozzles
simultaneously. The coated polymer is then cured at elevated temperature (above glassy
transition temperature) to form a uniform coating. The function of the added plasticizer
is to lower the glass transition temperature of the coating material. Cellulose derivatives
can be easily coated onto particles using this method [109]. It has been observed that an
increase in moisture could increase the coating quality in terms of smoothness and integ-
integ-
rity [110].
Hot-Melt
For the hot-melt coating technique, the coating material is applied to the core at molten
state, and the coating is formed upon cooling. It has been suggested that the melting
temperature of the coating material to be around 85°C because the molten coating material
