Transdermal Patches in Pharmaceutics: Definition, Types, Formulation, Mechanism of Drug Release, and Therapeutic Applications\

1. Definition of Transdermal Patches

Transdermal patches (TDDS – Transdermal Drug Delivery Systems) are adhesive, medicated patches applied to the skin to deliver a controlled amount of drug into the systemic circulation through the dermal layers. These systems provide non-invasive, sustained, and controlled drug delivery, bypassing the gastrointestinal tract.

Pharmaceutical Definition:

A transdermal patch is a topically applied adhesive dosage form that delivers one or more active pharmaceutical ingredients through the skin at a controlled rate, achieving systemic or local therapeutic effects.

2. Types of Transdermal Patches

Transdermal patches can be classified based on design and drug release mechanism:

A. Based on Design

  1. Single-Layer Drug-in-Adhesive Patch:

    • Drug is incorporated directly into the adhesive layer.

    • Simplest design, drug diffuses through adhesive to the skin.

  2. Multi-Layer Drug-in-Adhesive Patch:

    • Contains a drug layer and separate adhesive layer.

    • Allows controlled or biphasic drug release.

  3. Reservoir Patch:

    • Drug reservoir is enclosed by a rate-controlling membrane.

    • Provides precise, sustained drug release.

  4. Matrix Patch (Monolithic System):

    • Drug uniformly dispersed in a polymeric matrix.

    • Drug diffuses gradually through matrix to skin.

  5. Vapour Patch:

    • Designed for volatile active ingredients (e.g., decongestants).

B. Based on Mechanism of Delivery

  1. Passive Patches:

    • Rely on concentration gradient for diffusion through the skin.

  2. Active Patches (Iontophoresis or Electroporation):

    • Use electrical current or ultrasound to enhance drug penetration.

3. Formulation of Transdermal Patches

The formulation requires selection of drug, polymeric matrix, adhesive, plasticizer, and backing layer to ensure stability, adhesion, and controlled drug release.

A. Components

  1. Drug / Active Pharmaceutical Ingredient (API): Suitable for transdermal delivery (lipophilic, low molecular weight).

  2. Adhesive: Ensures patch adherence to the skin; e.g., acrylic, silicone adhesives.

  3. Polymeric Matrix or Reservoir: Controls drug release rate.

  4. Backing Layer: Provides mechanical support and protection (impermeable film).

  5. Release Liner: Protective film removed before application.

  6. Permeation Enhancers: Chemicals that increase skin permeability (e.g., ethanol, oleic acid).

  7. Plasticizers / Stabilizers: Improve flexibility and shelf-life.

B. Methods of Preparation

  1. Solvent Casting / Evaporation Method:

    • Drug, polymer, and adhesive dissolved in solvent, cast into thin films, and dried.

  2. Hot Melt Extrusion Method:

    • Drug and polymer melted together, extruded into sheets, and cut into patches.

  3. Laminating Method:

    • Multi-layer patches assembled using heat or pressure.

4. Mechanism of Drug Release from Transdermal Patches

The drug from transdermal patches is delivered through several steps:

  1. Release from Patch Matrix / Reservoir:

    • Drug diffuses out of the adhesive or polymer matrix.

  2. Penetration into the Stratum Corneum:

    • Drug passes through the outermost layer of the skin (rate-limiting step).

  3. Diffusion through Epidermis and Dermis:

    • Drug moves by passive diffusion toward systemic circulation.

  4. Absorption into Capillary Blood:

    • Drug reaches blood vessels in the dermis for systemic distribution.

Mathematical Model:

  • Drug release often follows Higuchi kinetics (diffusion-controlled) or first-order kinetics depending on patch design.

5. Therapeutic Applications of Transdermal Patches

Transdermal patches are used for systemic or local therapies, particularly for drugs that require controlled and prolonged delivery:

  1. Cardiovascular Drugs:

    • Nitroglycerin patches for angina, clonidine patches for hypertension.

  2. Pain Management:

    • Fentanyl or buprenorphine patches for chronic pain.

  3. Hormonal Therapy:

    • Estrogen, testosterone, and contraceptive patches.

  4. Neurological Disorders:

    • Rivastigmine patches for Alzheimer’s disease.

  5. Smoking Cessation:

    • Nicotine patches for gradual withdrawal.

  6. Motion Sickness and Nausea:

    • Scopolamine patches applied behind the ear.

  7. Local Dermatological Applications:

    • Anti-inflammatory or anesthetic patches for localized skin pain.

6. Advantages of Transdermal Patches

  • Sustained and controlled drug release over hours or days.

  • Bypass gastrointestinal tract and first-pass metabolism.

  • Improved patient compliance with once-daily application.

  • Non-invasive and painless drug delivery.

  • Reduced systemic side effects for certain drugs.

  • Rapid discontinuation possible by removing patch in case of adverse effects.

7. Limitations

  • Skin irritation or sensitization at application site.

  • Limited to drugs with suitable lipophilicity and low molecular weight.

  • Adhesion issues in hot or humid conditions.

  • High cost compared to oral dosage forms.