Biodegradable polymers in medical applications

Dental Devices.
Biodegradable polymers have found use in two dental applications. Employed as void filler following tooth extraction, porous polymer particles can be packed into the cavity to aid in quicker healing. As a guided-tissue-regeneration (GTR) membrane , films of biodegradable polymer can be positioned to exclude epithelial migration following periodontal surgery. The exclusion of epithelial cells allows the supporting, slower-growing tissue—including connective and ligament cells—to proliferate.

Biomaterials For Delivery Systems:
A range of materials have been employed to control the release of drugs and other active agents. The earliest of these polymers were originally intended for other, nonbiological uses, and were selected because of their desirable physical properties, for example:
Poly(urethanes) for elasticity.
Poly(siloxanes) or silicones for insulating ability.
Poly(methyl methacrylate) for physical strength and transparency.
Poly(vinyl alcohol) for hydrophilicity and strength.
Poly(ethylene) for toughness and lack of swelling.
Poly(vinyl pyrrolidone) for suspension capabilities.

To be successfully used in controlled drug delivery formulations, a material must be chemically inert and free of leachable impurities. It must also have an appropriate physical structure, with minimal undesired aging, and be readily processable. Some of the materials that are currently being used or studied for controlled drug delivery include:

Poly(2-hydroxy ethyl methacrylate)
Poly(N-vinyl pyrrolidone)
Poly(methyl methacrylate)
Poly(vinyl alcohol)
Poly(acrylic acid)
Polyacrylamide
Poly(ethylene-co-vinyl acetate)
Poly(ethylene glycol)
Poly(methacrylic acid)

However, in recent years additional polymers designed primarily for medical applications have entered the arena of controlled release. Many of these materials are designed to degrade within the body, among them major are:

Polylactides (PLA)
Polyglycolides (PGA)
Poly(lactide-co-glycolides) (PLGA)
Polyanhydrides
Polyorthoesters

Originally, polylactides and polyglycolides were used as absorbable suture material, and it was a natural step to work with these polymers in controlled drug delivery systems. The greatest advantage of these degradable polymers is that they are broken down into biologically acceptable molecules that are metabolized and removed from the body via normal metabolic pathways. However, biodegradable materials do produce degradation by-products that must be tolerated with little or no adverse reactions within the biological environment.

These degradation products—both desirable and potentially nondesirable—must be tested thoroughly, since there are a number of factors that will affect the biodegradation of the original materials. The most important of these factors are shown below—a list that is by no means complete, but does provide an indication of the breadth of structural, chemical, and processing properties that can affect biodegradable drug delivery systems.

Factors Affecting Biodegradation of Polymers:

Chemical structure.
Chemical composition.
Distribution of repeat units in multimers.
Presents of ionic groups.
Presence of unexpected units or chain defects.
Configuration structure.
Molecular weight.
Molecular-weight distribution.
Morphology (amorphous/semi crystalline, microstructures, residual stresses).
Presence of low-molecular-weight compounds.
Processing conditions.
Annealing.
Sterilization process.
Storage history.
Shape.
Site of implantation.
Adsorbed and absorbed compounds (water, lipids, ions, etc.).
Physicochemical factors (ion exchange, ionic strength, and pH).
Physical factors (shape and size changes, variations of diffusion coefficients, mechanical stresses, stress- and solvent-induced cracking, etc.).
Mechanism of hydrolysis (enzymes versus water).

Building on these type of developments, researchers will continue to evaluate these materials in medical field, taking advantage of the wide range of properties that can be obtained in polymers built with relatively few monomer units.

Contributed: Swapnil Liladhar Fegade, University of Mumbai, Institute of Chemical Technology