transdermal delivery

Transdermal delivery means getting a drug into the whole body by having it travel across intact skin — not to treat the skin itself, but to reach the bloodstream. The challenge is that skin evolved to keep things out, so this route is like smuggling cargo through a wall built to be impassable.

The main barrier is the stratum corneum, a thin outermost layer of dead, flattened cells packed in lipid like a “brick-and-mortar” wall. Most drug crosses by dissolving into those lipids and diffusing through, a slow passive process well described by Fick's first law: flux depends on the drug's concentration, its ability to partition into skin lipids, and how short the path is. Because flux is so limited, only potent, modestly lipophilic, low-molecular-weight drugs are realistic candidates without special help.

To widen what is possible, formulators add chemical permeation enhancers, or use physical methods such as iontophoresis (electric current), microneedles, or sonophoresis (ultrasound). Transdermal delivery is distinct from topical delivery, where the goal is to act locally in or on the skin rather than to be absorbed systemically; the same cream can be either, depending on intent and formulation. Its appeal is steady, prolonged, needle-free systemic dosing that bypasses the gut and liver.