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2. Drug
For successful development of a transdermal drug delivery, the following are the desirable
properties of a drug;
Physicochemical properties

▪ Non-ionic
▪ Low molecular weight (less than 500 Daltons)
▪ Adequate solubility in oil and water (log p 1-4)
▪ Low melting point (less than 200 ℃ )
▪ Potent (dose ideally less than 20 mg per day)

Biological properties
▪ The drug should be potent
▪ The half life of the drug should be short
▪ The drug must not induce a cutaneous irritant or allergic response
▪ Drugs degraded in the GIT or inactivated by the hepatic first pass are suitable candidates

for transdermal drug delivery
▪ Drugs that are to be administered for long period of time
▪ Tolerance of drug must not be developed

3. Backing layer

• It protects the polymeric drug matrix/reservoir from the external environment.

• It should have optimum elasticity, flexible and impermeable to drug diffusion to
prevent drug loss.

• It should be compatible with drug, polymer and excipients.

• Numerous materials can be used for patch backing layers, depending on the patch
design, size and length of intended use.

o For relatively short use small patches, an occlusive backing layer may be
selected, and this will hydrate the underlying skin which can improve delivery.
Example materials include polyethylene or polyester films.

o For larger and longer-term use patches, backing layers that permit some vapor
transmission are preferred, such as polyvinylchloride films. (why??)

4. Rate-controlling membrane
• Some patches contain semi- permeable membranes to control the rate of

drug diffusion from the reservoir (specific diffusion rate)
• For example, ethylene vinyl acetate is usually used as a rate controlling

membrane.

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