State Fick's first law and its role in transdermal drug delivery.

• A. Fick's second law does not apply to diffusion.
• B. Flux is independent of concentration gradient in transdermal systems.
• C. The equation describes osmosis through a semipermeable membrane.
• D. J = −D(dC/dx); the flux depends on the diffusion coefficient and the concentration gradient, governing drug permeation through the skin in transdermal systems.

Answer: (D)

The key idea being tested is that drug flux through skin in transdermal delivery follows Fick's first law: the steady-state flux J is proportional to the negative concentration gradient, with the diffusion coefficient as the proportionality constant. In one dimension, J = -D dC/dx. This means how fast a drug moves across the skin depends on how steeply the drug concentration changes across the skin (ΔC over the skin's thickness) and on how easily the drug diffuses through skin tissue, captured by D. In a transdermal system, keeping a high drug concentration in the patch and a sink in the receptor side creates a gradient that drives permeation, and D reflects the drug’s ability to traverse the stratum corneum and underlying layers. Practically, this is often summarized as the permeability P ≈ D K / h, linking diffusion coefficient, partitioning into skin, and thickness to the overall flux J ≈ P ΔC under steady state.

Flux being independent of the gradient is incorrect because it is fundamentally driven by the gradient. Fick’s second law describes time-dependent diffusion, not the steady-state flux. Describing osmosis is outside this diffusion framework, as osmosis involves solvent movement due to osmotic pressure rather than solute diffusion through a medium.