Biomedical Engineering Reference
In-Depth Information
copolymer of methacrylic acid and ethylacrylate [1:1]) is a commonly used enteric coating
(dissolves above 6.0 [97]). The recommended coating level for Eudragit L30 D-55 to achieve
enteric properties is in the range of 4- to 6-mg polymer per cm
2
surface area of the core
[98] Eudragit FS 30 D is an anionic copolymer of methyl acrylate, methyl methacrylate, and
methacrylic acid. Eudragit SL and RL series are widely used in sustained release coating
in pharmaceutical industry.
Polyethylene Glycol
PEG is a water-soluble polymer, which has two hydroxyl groups at the end of the linear
structure. It exists in a wide range of molecular weight from a few hundred to tens of
thousands of Dalton. It is one of the most popular biomaterials because of its low toxicity.
PEG has been widely used to modify other materials in biomedical applications for better
solubility, prolonged circulation time in the circulation system, and adhesion to mucosa.
The process of conjugating PEG onto other polymer is known as PEGylation [99].
Polyesters
Poly-lactic acid (PLA) and poly lactic-
co
-glycolic acid (PLGA) are Food and Drug
Administration-approved biodegradable polyesters. Poly
ε
-caprolactone is degradable
polyester, which does not generate acid during degradation and has a slower degradation
rate as compared to PLA and PLGA [100].
Coating Techniques
Coatings are usually applied in aqueous or organic solutions; however, solvent-free coating
technologies are used as well. Different coating techniques can be found in both laborato-
Different coating techniques can be found in both laborato-
ifferent coating techniques can be found in both laborato-
laborato-
ries and industry.
Fluid Bed
Fluid bed coating is one of the most commonly used coating techniques. In this technique,
fluidized particle by turbulent air is coated by an atomized spray to produce a uniform
coating; however, it is limited to big particles, ranging from 0.5 to 1.2 mm [101]. This is
because smaller particles cannot have a stable fluidization state in the conventional fluid
bed [102]. The parameters to control the coating are as follows: (1) inlet temperature and
speed, (2) bed temperature, (3) spray rate, and (4) spray solution composition and distance.
The size restriction is because interparticular force overtakes gravitational force, which
negatively affects the flow properties so as to prevent forming a uniform coating [103]. There
are different types of fluid-bed systems, which can be classified by the type of the spray: top
spray, bottom spray (Wurster), and tangential spray. In the top spray configuration, air is
blown upward; coating materials are sprayed from the top of the chamber. In bottom spray
configuration, air is blown upward as well; however, air flow is stronger in the center of the
chamber. A Wurster insert is installed above the air distribution plate, and it ensures that
particles are lifted upward in the center and fall outside the insert. When particles return
to the bottom from the outside of the insert, they are blown upward again. The spraying is
down in the center, whereas particles are lifted upward. Particles will be dried when they
return back to the bottom. This process will be repeated for several times until the desired
thickness is achieved. After coating is applied, curing is needed for a uniform coating to be
