Transdermal and Topical Delivery

Transdermal and topical delivery apply a drug to the skin. Topical delivery aims for a local effect at or near the application site, while transdermal delivery aims to drive the drug through the skin into the systemic circulation. The principal barrier in both cases is the stratum corneum, the skin's outermost layer, which restricts passage to small, modestly lipophilic molecules unless enhancement strategies are used.

Definition

Transdermal delivery is administration of a drug across intact skin to reach the systemic circulation, while topical delivery applies a drug to the skin or mucosa primarily for a local effect; both are governed by permeation across the stratum corneum.

Scope

This topic covers the skin barrier and the routes of permeation across it, the physicochemical properties that allow a drug to be delivered transdermally, the distinction between local (topical) and systemic (transdermal) intent, and enhancement strategies such as chemical penetration enhancers and microneedles. It is a pharmaceutical reference and does not provide application or treatment guidance.

Core questions

• What makes the stratum corneum the rate-limiting barrier to skin permeation?
• Which physicochemical properties allow a drug to cross the skin passively?
• How do topical (local) and transdermal (systemic) delivery differ in intent and design?
• How do penetration enhancers and microneedles extend the range of deliverable drugs?

Key concepts

• Stratum corneum barrier
• Percutaneous absorption
• Topical versus transdermal intent
• Penetration enhancers
• Microneedles
• Transdermal patch
• Sustained transdermal release

Mechanisms

A molecule applied to the skin must traverse the stratum corneum, a layer of corneocytes embedded in lipid that strongly limits permeation. Passive transdermal delivery therefore favors small, modestly lipophilic, potent molecules, and a transdermal patch can sustain delivery over hours to days by maintaining a steady concentration gradient (Prausnitz & Langer, 2008). To extend delivery to molecules the skin would otherwise exclude, chemical penetration enhancers disrupt the stratum corneum lipids, and physical approaches such as microneedles create transient micron-scale pathways through the barrier (Prausnitz, 2004). Particulate carriers have also been explored to modify how drugs interact with and cross the skin (Kreuter, 2007). Topical products, by contrast, are designed to localize the drug at the application site rather than promote systemic absorption.

Clinical relevance

Transdermal delivery offers steady, non-invasive systemic exposure that avoids first-pass metabolism, while topical products concentrate a drug where it is applied, and recognizing the distinction supports appraisal of why a product is designed as a patch, gel, or cream. This entry describes delivery principles for reference and is not a basis for treatment or application decisions in any individual.

Evidence & guidelines

Reviews of transdermal delivery describe the skin barrier and the design of patches and enhancement strategies (Prausnitz & Langer, 2008; Prausnitz, 2004), and reviews of nanoparticle carriers provide historical context for particulate skin delivery (Kreuter, 2007). Standard pharmaceutics texts codify topical and transdermal dosage-form design (Aulton & Taylor, 2018).

History

The first transdermal patch products appeared around 1980, establishing the skin as a practical systemic route for selected small molecules. Subsequent work concentrated on overcoming the stratum corneum barrier for a wider range of drugs through chemical penetration enhancers and, from the late 1990s, microneedle technologies (Prausnitz & Langer, 2008; Prausnitz, 2004).

Key figures

• Mark Prausnitz
• Robert Langer
• Jorg Kreuter

Seminal works

• prausnitz-langer-2008
• prausnitz-2004

Frequently asked questions

What is the difference between topical and transdermal delivery?

Topical delivery applies a drug to the skin for a local effect at the site, whereas transdermal delivery is designed to drive the drug through the skin and into the systemic circulation for a body-wide effect.

Why can only some drugs be delivered through the skin?

The stratum corneum strongly restricts permeation, so passive transdermal delivery generally suits small, modestly lipophilic, potent molecules; other drugs require enhancement strategies such as penetration enhancers or microneedles.