Drug Delivery Devices using Polymer Building Blocks
Conventional forms of drug administration generally rely on pills, eye drops, ointments and intravenous solutions. In recent years we have noticed an explosion in research on new and improved drug delivery systems, this may be due to advances in polymer science, biotechnology or the fact that it takes around 150 million dollars to create a completely new drug whereas it's far cheaper to improve delivery of an existing drug.
The advancement in drug delivery also may be due to the emergence or re-emergence of diseases that conventional methods simply do not work effectively on. Some new drug delivery methods simply improve efficiency of existing drugs i.e. increasing the life time of a particular drug or provide a constant predetermined rate of release of the drug.
At the start of the century sales of advanced drug delivery systems were approaching 20 billion dollars in the United Sates alone. This includes everything from liposomes, to transdermal patches, wafers, polymer-drug conjugates etc, but for the purpose of this article we will be concentrating only on drug delivery devices using polymer building blocks or based loosely around polymers.
Routes of drug administration and absorption
One method of classifying routes of drug administration is enternal and parenteral. Enternal involves the gastro-intestinal track in someway; these include oral, buccal and rectal administration of drugs. Parenteral means not through the alimentary canal usually referring to things such as IV, IM and SC but could also include inhalation and topical administration of drugs.
Along with the different methods of administering a drug there are three ways in which a drug can then be absorbed by the cells/tissue and they are:
* Paracellular route; passive diffusion of small hydrophilic molecules through junctions between adjacent epithelial cells.
* Transcellular route; passive diffusion of lipophilic molecules through the lipid bilayer and membrane-bound protein regions of cell membrane.
* Transcellular receptor-mediated transcytosis of molecules e.g. by surface bound ligands such as intrinsic factor-colobalamin complexes.
There are three factors that affect the absorption of particles through a membrane i.e. the intestinal membrane and they are; the size of the particle, the chemical composition and the charge on the particle.
Problems with current drug delivery
The problem with conventional drug delivery is that they do not usually provide one of the two following; rate controlled release of the drug and target specificity. Conventional drug delivery also only provides a sharp increase of drug concentration at potentially toxic levels, after that initial increase the drug concentration drops sharply. Oral drug delivery is currently regarded as the optimal means of achieving therapeutic effect as well as patient compliance. However this route is beset with problems such as the destructions of labile molecules by the gastro-intestinal tract as well as low levels of macromolecular absorption along with other problems.
Polymers containing dispersed medication such as microspheres provide controlled release of drugs over a predetermined period of time, As well as being used as drug carriers, polymers can also be used as drug encapsulates and can be used to protect an active agent during its passage through the body. The sue of these polymeric structures gives the drug an advantage whether its increasing the life time, controlling the rate of release or simply protecting the drug itself from being broken down by the body's natural activities i.e. stomach acids.
Polymer Based Drug Delivery Systems:
Micelles are formed when amphiphilic molecules are placed in water, they are made up of an inner core made from the hydrophobic segment of the molecule which is lipid solubilising. The outer surface of the micelle is made up of the hydrophilic segment of the molecule. The hydrophilic outer surface acts as a stabilising interface between the hydrophobic core of the micelle and the aqueous environment in which it resides.
In drug delivery the term “hydrogel” is typically reserved polymeric materials that can absorb a significant amount of water and other substances while maintaining a distinct three-dimensional structure.  Hydrogels have four key properties; degree of swelling, biocompatibility, permeability and swelling kinetics. The reproducible, controllable swelling kinetics of hydrogels mean that the permeability of the gels can be made to vary with time or even change in response to external conditions, enabling release of therapeutic agents in a prescribed fashion.