Transdermal delivery systems (TDS) are designed to deliver drugs through the skin, providing a non-invasive and controlled method for medication administration. Here's a breakdown of their work:


1. Drug Formulation

The drug is incorporated into a patch or other topical form, typically with the help of permeation enhancers or carriers that facilitate its passage through the skin layers.

2. Application to the Skin

The transdermal system, usually in the form of a patch, is applied directly to the skin. The drug then begins to diffuse through the skin’s outermost barrier, the stratum corneum.

3. Skin Penetration

The skin has multiple layers:

  • Stratum corneum: The outermost layer made up of dead cells. It acts as the primary barrier.
  • Epidermis: Below the stratum corneum, consisting of living cells but without blood vessels.
  • Dermis: Contains blood vessels and facilitates drug absorption into systemic circulation.

The drug first penetrates the stratum corneum either by:

  • Transcellular route (through the cells),
  • Intercellular route (between the cells), or
  • Through hair follicles and sweat ducts.

4. Drug Release and Absorption

After crossing the stratum corneum, the drug diffuses through the epidermis and reaches the dermis, where it is absorbed into the blood vessels. The drug is then transported via systemic circulation to the target site in the body.

5. Controlled Release

Transdermal patches are designed to release the drug in a controlled manner over time, providing a sustained therapeutic effect. The release rate can be influenced by the composition of the patch, the use of rate-controlling membranes, and the nature of the drug itself.

6. Advantages of Transdermal Delivery

  • Bypasses the gastrointestinal tract, avoiding degradation by digestive enzymes or first-pass metabolism in the liver.
  • Non-invasive, painless, and convenient for patients.
  • Sustained and controlled release helps maintain steady drug levels in the bloodstream.
  • Improved patient compliance as patches are usually long-lasting (hours to days).

7. Limitations

  • Limited to drugs that are potent enough to be effective in small doses.
  • Only suitable for drugs that can permeate through the skin.
  • Skin irritation or allergic reactions can occur with prolonged patch use.

Transdermal systems are widely used for drugs like nicotine, fentanyl, and hormone replacement therapy due to their effectiveness and ease of use.