Polymers are widely used in pharmaceutical dosage forms. Based on the solubility of polymers in water, they can be classified as water-soluble polymers, water-insoluble polymers, and hydrogels.
Pharmaceutical
polymers
Introduction
Polymers
are widely used in pharmaceutical dosage forms. Based on the solubility of
polymers in water, they can be classified as water-soluble polymers,
water-insoluble polymers, and hydrogels.
Water-soluble
polymers:
These polymers are used in many different
ways, so to increase the viscosity of the aqueous solutions; to improve and
maintain the physical stability of suspensions; to promote the adhesion of
solid particles of different types, leading to granulation in wet processes; to
form a flexible film on tablets during the coating of tablets; as adhesives for
buccal and bioadhesive drug delivery systems; as emulsifying agents; as
flocculating agents; and as components of sustained and site-specific drug
delivery systems. Water-soluble polymers used in the coating of tablets include
hydroxypropyl methylcellulose (HPMC) or polyvinyl alcohol (PVA) as a film
former, and polyethylene glycol (PEG) as a plasticizer. Water-soluble polymers
used for the stabilization of suspensions and emulsions include carrageenan,
hydroxypropyl cellulose (HPC), and xanthan gum.
Water-soluble
polymers can be cross-linked to give hydrogels.
For exam-ple, crospovidone is cross-linked polyvinylpyrrolidone (PVP), and
croscar-mellose sodium is cross-linked carboxymethyl cellulose (CMC) sodium.
Crospovidone and croscarmellose sodium are used as superdisintegrants in oral
solid dosage forms. The rapid and high water absorption capability of these
water-insoluble cross-linked polymers aids in the disintegration of compressed
dosage forms.
Water-insoluble
polymers:
These polymers are used to form membranes
and matrices for sustained-release and localized drug delivery systems.
Being water-insoluble, these polymers help delay, slow, or sustain the rate of
drug release. An example is the coating of a tablet with a sustained-release
water-insoluble polymer. For such an application, water-insoluble polymer is
mixed with a limited quantity of a water-soluble polymer, which dissolves in
contact with aqueous fluid, leading to the formation of pores in the membrane
through which the drug can diffuse out of the dosage form. Factors influencing
drug release from these systems include membrane thickness, drug solubility in
the membrane, and the porosity of the polymer matrix. Water-insoluble polymers
used in the coating of tablets include polymers that dissolve at basic pH but
not at acidic pH. Such polymers are called enteric polymers, and such coating
on the tablet is called enteric coating. Polymers used for the enteric coating of tablets include cellulose
acetate phthalate (CAP), hydroxypropyl methylcellulose phthalate (HPMCP), and
methacrylic acid—methyl methacrylate copoly-mers (Eudragit®).
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