A typical Transdermal drug delivery system consists of the following components.
Polymer Matrix.
Drugs.
Permeation Enhancers.
Pressure sensitive adhesives (PSA).
Backings Laminates.
Release Liner.
Other Excipients.
Polymer Matrix:
The Polymer controls the release of the drug from the device.
Possible useful polymers for transdermal devices are:
a. Natural Polymers:
cellulose derivatives,
Zein,
Gelatin,
Shellac,
Waxes,
Proteins,
Gums and their derivatives,
Natural rubber,
Starch etc.
b. Synthetic Elastomers:
polybutadiene,
Hydrin rubber,
Polysiloxane,
Silicone rubber,
Nitrile,
Acrylonitrile,
Butyl rubber,
Styrene Butadiene rubber,
Neoprene etc.
c. Synthetic Polymers:
polyvinyl alcohol,
Polyvinyl chloride,
Polyethylene,
Polypropylene,
Polyacrylate,
Polyamide,
Polyurea,
Polyvinyl pyrrolidone,
Polymethyl methacrylate,
Epoxy etc.
Drugs:
Desirable properties of a drug for transdermal delivery.
The drug should have a molecular weight of less than 1000 Daltons.
The drug should have affinity for both lipophilic and hydrophilic phases.
Extreme partitioning characteristics are not useful for successful drug delivery via the skin.
The drug should have a low melting point.
Drug should be potent, have a short half life and be non irritating.
Permeation Enhancers:
These are compounds that promote skin permeability by altering the skin as a barrier to the flux of a desired penetrant.
Penetration enhancers are added to a formulation to improve the diffusivity and solubility of drugs through the skin, thereby reducing the skin's barrier resistance.
These includes water,pyrrolidones,fatty acids and alcohols, alcohol and glycols, essential oils,terpenes and derivatives,sulfoxides like DMSO and their derivatives, urea and surfactant.
Pressure sensitive adhesives (PSA):
The fastening of all transdermal devices to the skin can be done by using a PSA.
The first approach involves the development of new polymers, which include hydrogel hydrophilic polymers, and polyurethanes.
The second approach is to physically or chemically modify the chemistries of the PSAs in current use (such as silicones, and acrylates).
Physical modification refers to the formulation of the base adhesives with some unique additives so that there is enhanced drug delivery and improved skin-adhesion properties.
Chemical modification involves chemically incorporating or grafting functional monomers to the conventional PSA polymers in order to improve drug delivery rates
Backings Laminates:
Backings laminates are selected for appearance, flexibility and need for occlusion.
Examples of backings are polyester film, polyethylene film and polyolefin film, and aluminium vapour coated layer.
Major areas of concern are the backing additives leaching out and diffusion of drugs or the compositions, through the backing.
An over emphasis on the chemical resistance often may lead to stiffness and high occlusivity to moisture vapour and air.
It causes the TDDS to lift and may possibly irritate the skin during long-term use.
Release Liner:
During storage, the patch is protected by a liner, which is removed and discarded before the patch is applied to the skin.
Since the liner is in direct contact with the TDDS, the liner must be chemically inert.
The release liner is composed of a base layer which may be non-occlusive (e.g. paper fabric) or occlusive (e.g. polyethylene, polyvinyl chloride) and a release coating layer made up of silicon or Teflon.
Other materials used for TDDS liners include, polyester foil and metalized laminate that protects the patch during storage.
The liner is removed prior to use only.
Other Excipients:
Various solvents such as chloroform, methanol, acetone, isopropanol and dichloromethane are used to prepare drug reservoirs.
In addition, plasticizers such as dibutyl-phthalate, triethyl citrate, polyethylene glycol and propylene glycol are added to provide plasticity to the transdermal patch.
Commonly asked questions.
Write in brief about the basic components of a TDDS.