Introduction of Polymers
Polymers are large molecules made up of repeating subunits called monomers. Due to their diverse structures and properties, polymers have emerged as a central component in the field of novel drug delivery systems (NDDS). These systems aim to enhance the efficiency, specificity, and safety of drug delivery, ensuring that drugs reach their intended sites of action in the body at optimal rates and concentrations.
Roles In NDDS:
Controlled Release: Polymers can form matrices that degrade over time, releasing the drug in a controlled manner.
Targeted Delivery: Surface-modified polymers can specifically target certain cells or tissues, enhancing the therapeutic effect and minimizing side effects.
Protection: Polymers can protect sensitive drugs from degradation in the body or the external environment.
Solubility Enhancement: Some polymers can improve the solubility of poorly water-soluble drugs, making them more bioavailable.
Applications:
Polymers are used in various drug delivery platforms, including microspheres, nanoparticles, hydrogels, liposomes, and implantable devices.
Their biocompatibility, versatility, and ability to modify drug pharmacokinetics make polymers indispensable in the development and optimization of novel drug delivery systems.
Classification of Polymers
Polymers can be classified based on their source, structure, mode of polymerization, molecular forces, and other properties.
The classification can be broadly broken down as follows:
1) Based on Source:
I. Natural Polymers:
Derived from natural sources, these polymers have been employed for various pharmaceutical applications due to their biocompatibility and biodegradability.
Examples: Cellulose, chitosan, alginate, gelatin, and proteins.
II. Semi-synthetic Polymers:
These are derived from natural polymers but undergo chemical modifications to improve their properties or to introduce new functionalities.
Examples: Carboxymethyl cellulose, hydroxyethyl cellulose, and ethyl cellulose.
III. Synthetic Polymers:
Produced from petrochemical sources, they are often designed to possess specific qualities suitable for particular drug delivery challenges.
Examples: Polyvinyl alcohol, polyvinylpyrrolidone, polylactic acid, polyglycolic acid, and their copolymers.
2) Based on Structure:
I. Linear Polymers:
These have a straightforward chain-like structure without any branches.
II. Branched Polymers:
As the name suggests, these polymers have a branched structure stemming from the main chain.
III. Cross-linked or Network Polymers:
These consist of monomeric units connected with strong covalent bonds, forming a three-dimensional network. These are typically used in hydrogel formulations in NDDS.
3) Based on Mode of Polymerization:
I. Addition or Chain-growth Polymers:
Formed by the sequential addition of monomeric units in a chain reaction.
Examples: Polyethylene, polystyrene, and polytetrafluoroethylene.
II. Condensation or Step-growth Polymers:
They form via the stepwise intermolecular reaction of monomers, usually resulting in the loss of simple molecules like water.
Examples: Polyesters, polyamides, and polyurethanes.
4) Based on Molecular Forces:
I. Elastomers:
These have weak intermolecular forces and can be stretched but return to their original shape upon the release of stress.
Examples: Natural rubber, polybutadiene, and polyisoprene.
II. Fibers:
Characterized by strong intermolecular forces such as hydrogen bonds or dipole-dipole interactions. They are used in the creation of sutures in the pharmaceutical world.
Examples: Nylon, silk, and wool.
III. Thermoplastics:
These have intermediate molecular forces and can be repeatedly softened on heating and hardened on cooling.
Examples: Polyvinyl chloride, polyethylene, and polystyrene.
IV. Thermosetting Polymers:
Once molded, they cannot be softened by heating due to their 3D network structure.
Examples: Bakelite and melamine-formaldehyde resins.