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• Chemical barrier
The horny layer is very impermeable to most chemicals and usually contributes to the rate-
limiting step in transdermal absorption.

• Radiation barrier
For skin exposed to sunlight, ultraviolet light of 290-400 nm is the most damaging.
Three main acute reactions follow irradiation:
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Ultraviolet light stimulates melanocytes to produce melanin, which partially protects the
skin.

• Heat barrier and temperature regulation
The stratum corneum is so thin over most body areas that it does not effectively protect the
underlying living tissues from extremes of cold and heat.
The skin, however, is the organ primarily responsible for maintaining the body at 37°C.

• To conserve heat, the peripheral circulation shuts down to minimize surface heat
loss; shivering generates energy when chilling is severe.

• To lose heat, blood vessels dilate, eccrine sweat glands pour out their dilute saline
secretion, water evaporates, and the removal of the heat of vaporization cools the
body.

• Metabolic function
Keratinocyte cells produce vitamin D3 due to exposure to UV radiation from the sun.
https://www.youtube.com/watch?v=tzzbyqFrl-8

2. Definition of TDDS

Transdermal drug delivery systems (TDDS) are defined as dosage forms which are when applied
to the skin, deliver the drug through the skin at a controlled rate to the systemic circulation

3. Rational approach to drug delivery to and via the skin

There are three main ways for delivery of topical dosage form:
We can manipulate the barrier function of the skin:
for example, topical antibiotics and antibacterials; sunscreen agents, and emollient preparations

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