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The higher concentration than solubility the excess solid drug will function as a reservoir and
help to maintain a constant drug concentration for a prolonged period.

Diffusion coefficient (D)
The diffusional speed of a molecule depends mainly on the state of matter in the medium.
In skin, the diffusion of molecules drops progressively and reaches its lowest values within the
compacted stratum corneum matrix.
For a constant temperature, the diffusion coefficient of a drug in a topical vehicle or on the
skin depends on the properties of the drug and the diffusion medium and, on the interaction
between them.
For example, some drugs may bind and become immobilized within the stratum corneum, and
this process affects the magnitude of D resulting in a lag time.
Temperature
The penetration rate of material through human skin can change tenfold for a large
temperature variation, as the diffusion coefficient decreases as the temperature falls.
However, adequate clothing on most of the body would usually prevent wide fluctuations in
temperature and penetration rates.

6. Criteria for selection of drug candidate for TDDS

• The dose should be low i.e. < 20mg/day.
• Half-life should be 10 hours or less.
• Molecular weight should be <400 Da.
• Partition coefficient Log Po/w should be between 1.0 and 4. 
• The skin permeability coefficient should be 0.5x10-3 cm/h.
• Drugs should be non-irritating and non-sensitizing to the skin.
• Oral bioavailability should be low.
• The therapeutic index should be low.

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