Transdermal patches are medical devices that deliver medicine through the skin and into the bloodstream. They can provide steady dosing for hours or days, which helps avoid the peaks and drops that can happen with pills or injections. This technology is used for medicines such as nicotine, motion sickness drugs, pain medicines, and some hormones.
The design matters because the skin is a strong barrier that only certain drug molecules can pass through safely and predictably.
A typical patch has several layers, including a backing layer, a drug reservoir or drug-in-adhesive layer, a rate-controlling membrane in some designs, adhesive, and a removable liner. After the liner is removed and the patch is applied, drug molecules diffuse from a region of higher concentration in the patch toward a lower concentration in the skin and blood. The stratum corneum, the outermost skin layer, is usually the main barrier to diffusion.
Engineers control dose by changing drug concentration, patch area, membrane properties, adhesive chemistry, and wear time.
Key Facts
- Diffusion moves drug molecules from high concentration to low concentration.
- Fick's first law: J = -D dC/dx, where J is flux, D is diffusion coefficient, and dC/dx is the concentration gradient.
- Approximate delivery rate: rate = J A, where A is the patch contact area.
- Total delivered dose can be estimated by dose = rate × time when the rate is nearly constant.
- The stratum corneum is the main skin barrier for most transdermal drugs.
- Good patch candidates are usually potent, stable, and able to dissolve in both lipids and water to some degree.
Vocabulary
- Transdermal patch
- A medicated adhesive device that delivers a drug through the skin into the body over time.
- Diffusion
- The net movement of particles from an area of higher concentration to an area of lower concentration.
- Flux
- The amount of drug crossing a unit area of skin per unit time.
- Stratum corneum
- The thin outer layer of the epidermis that forms the skin's strongest barrier to drug entry.
- Rate-controlling membrane
- A patch layer designed to limit how quickly drug molecules leave the patch.
Common Mistakes to Avoid
- Assuming any drug can be put in a patch is wrong because many drugs are too large, too irritating, or unable to cross the skin well.
- Thinking a larger patch always means a stronger medicine is wrong because dose depends on drug concentration, patch design, skin permeability, and contact area.
- Cutting a patch without instructions is wrong because it can damage the rate-controlling system and cause too much or too little drug release.
- Applying a patch to damaged or irritated skin is wrong because broken skin can increase absorption unpredictably and raise the risk of side effects.
Practice Questions
- 1 A patch delivers nicotine at a steady rate of 0.80 mg per hour for 16 hours. What total mass of nicotine is delivered?
- 2 A transdermal patch has a drug flux of 0.025 mg/(cm^2 h) and a contact area of 40 cm^2. What is the drug delivery rate in mg/h, and how much is delivered in 24 h?
- 3 Explain why a transdermal patch is designed with multiple layers instead of simply placing liquid medicine directly on the skin.